Selective Metal Recovery

Chapter 1

1. Introduction

This chapter emphasis the topical subjects in the area of selective metal recovery from wastewater with low metal ion concentration. The main drivers are based on environmental concerns, legislation and economic motivations.

During the industrial revolution, engineers were practicing their profession without addressing the environmental issues with the same depth which is now required. The standard production philosophy was to increase the depletion of the raw materials including metals ores and increase amounts of different forms of waste. Kiely, (1997) listed some industrial producing metals wastes in table 1-1. This philosophy was valid until the demand for the productions increased which required increasing the supply. This has caused deterioration of the environment by decrease of the resources since the environment has limited materials that are converted into products. Also, the disposal places or landfills of waste are also decreased, (Environmental Protection Agency, 2003).

The toxic substances produced by metal industries and derivatives, such as metal plating facilities, mining operations, and tanneries, can directly affect an organism's health. Hettiarachchi and Pierzynski, (2004) stated that lead, Pb, is toxic to human and affects virtually every system in the body. Many environmental sources, such as contaminated soil, household dust and industrial wastes, are typical sources of Pb exposure for humans (Karabulut et al., 2000). According to Kiely, (1997), heavy metals are one of the toxic components where their toxicity depends on two factors. These factors are:

Concentration

some heavy metals, such as copper and zinc, required in trace concentration because they can be toxic in large quantities. On the other hand, metals like lead; aluminium and mercury have no known physiological role since they are highly toxic to organisms. Generally, it is important that all these metals have to be in a form that can be taken into the organism before they become toxic, (i.e. bio-available), (Karabulut et al., 2000). This depends on the chemical form of the metals.

Chemical forms

A complete examination of the form of the metal must be established in any pollutant before its toxicity can be determined. This conclusion is based on a research carried out by Howells, (1999) who studied stream pH levels in which a particular form of aluminium called labile monomeric aluminium can occur. This component is believed to be toxic to fish in streams.

Table 1‑1: Some industries producing metals waste.

Industrial type	Hazardous substance
Batteries Chemical manufacturing Electrical/electronic Printing Electroplating Textile Pharmaceutical Paints Plastics Leather	Cd, Pb, Ag, Zn, Ni Cr, Cu, Pb, Hg Cu, Co, Pb, Hg, Zn, Se As, Cr, Cu, Pb, Se Co, Cr, Cn, Cu, Zn, Ni Cr, Cu As, Hg Cd, Cr, Cu, Co, Pb, Hg, Se Co, Hg, Zn Cr

The metals listed in table 1-1 have maximum permissible values in different environment and there are still proposals to achieve values below these limits, (Crommentuijn et al., 1997).

Gungor and Surendra, (1999) pointed out three main reasons for the recovery of material and recycle of products to carry out. These are: hidden economic value of solid wastes, market requirements, and governmental regulations. Therefore, metal recovery from waste streams is becoming an increasingly important topic due to the growing economy in many places in the world, such as China and India. It is also considered as a long strategic importance for the modern world.

Fortunately, heavy metals are not degradable and tend to accumulate in waste streams making it possible to be recovered.` Sustainable development is defined as “development that meets the needs of the present without compromising the ability for future generations to meet their own needs”. Sustainable development is based on three important factors: economic development, social consequences and development, and environmental protection. The sustainability of production and processing systems including technology for recycling are important to cover the regulations of waste materials, which have been put in order to identify the extent of the environment problems and take corrective actions. As a consequence of that, two forms of optimal primary objectives have been employed in the real industries, (Gungor and Surendra, 1999). These are:

• Creating environmentally friendly products (i.e. green products).
• Developing techniques for product recovery and waste management.

Technical and refining charges vary with supply and demand in the concentrate market, but can differ with movements in the metal market. Consequently, the technical and refining share of the metal price varies considerably. Treatment processes for metal recovery include chemical precipitation, membrane filtration, ion exchange, carbon adsorption and electrowinning. Some other techniques like smelting are now becoming very expensive with long costly operating conditions. In addition, wastes containing heavy metals are treated to remove them from water resulting in a solid waste containing relatively high concentration of metals to make these solid materials toxic with the consequent expensive cost of disposal.

There has been considerable interest in the development of cost effective technologies for the recovery of metal ions from effluent streams. Selective recovery of metals is seen as one method of improving the economics of the operation. This is because the technology recovers relatively high purity metals that have considerable value and can be reused at the same time. Also, the remaining waste of such a technique should be detoxified so making it cheap to dispose or to become recyclable. However, there is still a need to develop cost effective, environmentally friendly process for metal recovery at low concentrations.

1.1. The Project Aim:

Despite the importance of removing heavy metals for the environment protection, some of these metals, such as lead, are not a highly priced metal and that could cause the project economic losses.

The aim of this project is to investigate the viability and utility of integrated metal recovery processes from relatively low concentrations. Also, the potential for reprocessing wastes on site and at a relatively small scale is required. A number of objectives are: Assessment of current technology and its ability to reprocess real industrial wastes, assessment of selected targets for reprocessing such as metal electroplating and experimental investigation of specific targeted processes

Zinc metal has important markets in the brass and construction industries and in chemicals and forms an important nutritional element. Zinc is used in galvanised steel, protective coatings for steel, and die-casting. Zinc compounds are used for luminous dials, cosmetics, plastics, rubber products, soaps and inks. The increased demand for galvanised products is reflected in the growth in demand for zinc. Meanwhile, the higher zinc prices encourage increased mine output and waste treatment. Therefore, the project presents experimental examination of electrowinning of this metal and polymer enhanced ultrafiltration processes of zinc solution. It is also necessary to construct a design model for an integrated zinc recovery plant from low concentration solutions and compare different extraction percentage values to determine the optimum extraction value as function of profit under a given set of conditions.

1.2. Structure of the Thesis:

Chapter 2 is a survey of the relevant literature. This consists of several topics, viz. current metal recovery technologies used by industry, an overview of metal in the environment and methods of treatments used to extract metals from the environment. Also, review of previous experimental work is discussed for metal recovery through operating conditions, design and relevant operating strategies.

Chapter 3 presents the experimental methodology of both electrowinning and polymer enhanced ultrafiltration processes, and explains the construction of each process and material employed for these particular systems. The proposed zinc recovery process is also explained.

The results of the zinc electrowinning for both the acidic and alkaline system design and the optimisation procedure are showed in Chapter 4. It is found that alkaline electrowinning can provide better performance and less energy usage than acidic process. Alkaline cell thence selected in order to carry on examining the effect of varying the parameters and code a model that could predict the performance of such a process. Further suggested development improve the deposition efficiency of zinc is also explained.

In chapter 5, the results of polymer enhanced ultrafiltration experiments are presented. Optimum operation and number of washing times is concluded in the second section of this chapter. Also, this chapter presents the mathematical simulation and of the volume concentration factor relationship as a function of permeates flux and this is used to evaluate the optimum flux with the strategy of achieving the minimum energy usage possible.

The principle findings regarding the zinc recovery system design and the optimum operation of the plant are discussed in Chapter 6, whereas Chapter 7 draws conclusions from the results obtained and describes ways in which the project may be extended in the future.

Chapter 2

2. Literature review

Many publications have considered heavy metals recovery and reprocessing principles and technologies. Directly relevant work is reviewed here to provide general background to the viability and utility of individual and integrated metal recovery processes.

2.1. Metal Recovery from Mining Ores:

2.1.1. Overview:

Mining is the first and main source of metals and most of metals could be found in complex form called ores. Mining is important to producing and developing countries. In other words, it has a huge contribution to the economy and development. Mined materials are needed to construct roads and hospitals, to build automobiles and houses, to make computers and satellites, to generate electricity, and to provide many other goods and services.

The life cycle of mining begins with exploration, continues through production, and ends with closure and postmining land use. Most metallic ore deposits are formed through the interaction of an aqueous fluid and host rocks. At some point along the fluid flow pathway, the fluids encounter changes in physical or chemical conditions that cause the dissolved metals to precipitate. The focus has traditionally been on the location of metal depositions, that is, the ore deposit. Consequently, the process of mining commonly exposes ore to more rapid oxidation, which naturally affects the environment. Therefore, understanding the movement of fluids through the Earth, for example, through enhanced hydrologic models, will be critical for future mineral exploration, as well as for effectively closing mines that have completed their life cycle, (Hayes, 1993).

After a mineral deposit has been identified through exploration, the industry must make a considerable investment in mine development before production begins. Further exploration near the deposit and further development drilling within the deposit are done while the mining is ongoing. These are comminution (i.e., the breaking of rock to facilitate the separation of ore minerals from waste), combines blasting (a unit process of mining) with crushing and grinding (processing steps), (Hayes, 1993).

2.1.2. Mining Ores:

Ore can be defined as a mineral or association of minerals that may be worked commercially for the extraction of one or more metals. In addition, ore is a mixture of minerals one or more of which can be economically exploited to become a source of supply of a particular material. On the other hand, all metals that have no economical value to the process under consideration are referred to as gangue. Generally, several like gold, silver, platinum and copper could be found in any quantity in nature as pure metals. However, some of these could consist of very small particles dispersed in the surrounding rock. Other metal ores could contain relatively high percentage of gangue materials and relatively small percent of metals. For example, zinc mining sphalerite contains 67 percent comparing to 10 percent or less of zinc after mining, (Kuopanportti et al., 1997).

Table 2‑1: Copper ores

Ore Type	Ore Name	Chemical Structure
Sulphide ores Oxidation ores	Chalcopyrite Chalcocite Covellite Bornite Cuprite Carbonate Malachite Azurite Sulphate Brochantite Silicate Chrysocolla	CuFeS2 Cu2SCuS Cu2S.CuS.Fe.S Cu2O CuCO3.Cu(OH)2 2CuCO3.Cu(OH)2 CuSO4.3Cu(OH)2 CuSiO3.2H2O

Each metal could be found and extracted in relatively high percentage comparing to other metals in the same ore. Tables 2-1 to 2-7 present the type of ores for each metal. Metals like cobalt do not exist in sufficient concentration in ores to make their extraction possible at an economic price. Nevertheless, their recovery as a by-product could be possible within reasonable cost. Cobalt usually associated with other metals such as copper, lead and nickel. Table 2-6 presents the type of ores that contains cobalt associated with the above metals.

Table 2‑2: Zinc ores

Ore Type	Ore Name	Chemical Structure
Sulphide Oxidation	Sphalerite Calamine Smithsonite Zincite Marmatite	ZnS Zn(OH)2.SiO3 ZnCO3 ZnO (ZnFe)S

Table 2‑3: Tin ores

Ore Type	Ore Name	Chemical Structure
Oxide Sulphide	Cassitrite Stannite	SnO2 SnS2.FeS.Cu2S

Table 2‑4: Nickel ores

Ore Type	Ore Name	Chemical Structure
Hydrous Sulphide	Garnerite Pentlandite	((NiMg)SiO3.H2O (FeNi)S

Table 2‑5: Associated cobalt ores

Ore Type	Ore Name	Chemical Structure
Sulphide Arsenide	Linnarite Carrollite Smaltite Cobaltite	Co3S4 CuCo2S4 CoAs2 CoAsS

Gold and silver could be mainly found in native form or alloyed, more or less with each other. Gold could be associated with sulphides such as pyrite and aresenopyrite. Tables 2-6 and 2-7 present the type of ores for both gold and silver receptivity.

Table 2‑6: Associated gold ores

Ore Type	Ore Name	Chemical Structure
Combined ores (with Tellurides)	Calaverite Sylvanite	AuTe2 ((AuAg)Te2)

Table 2‑7: Associated silver ores

Ore Type	Ore Name	Chemical Structure
Sulphide Chloride Combined ores (with Tellurides)	Argentite Chloride Hessite Naumannite	Ag2S AuCl Ag2Fe ((AgPb)Se)

Metals amounts in ores vary depends on the location from where the ore was explored. Ores go through several treatments (physical or pre-treatment processes) before extracting metals form them. The first step is to reduce the size of ore by crushing, grinding and flotation, which will be discussed in more details in this chapter.

2.1.3. The Physical Treatment of Ores for the Enrichment of Metals:

The physical processing involves separation of mineral grains from an ore in order to recover particular minerals. This includes recovery of elements contained within those minerals and removal of unwanted materials from mining feed. The process does not result in any change to the chemical properties of the individual phases present in process streams. Physical process could be performed in tow individual stages. These are: size reduction of the particles and selection of the desired component from the mixture.

The process of flotation in the second stage is one of the most widely employed techniques. The process has shown an effective performance in selecting the desired particles from ores mixtures. The desired particles could move with gas bubbles to the surface of liquid forming a forth, (See figure 2-1). Both froth and the suspended particles are separated from the slurry by skimming in the froth from the surface of the liquid. The mineral particles remain in the slurry then removed in a separated process stream, such as hydrometallurgical units. The valueless gangue materials cannot be attached to the bubbles and can be discarded.

Generally, most of sulphide ores, which are the most common and usually found on the sites, are concentrated by flotation. Meanwhile, oxidized minerals could be floated with less recovery at certain grade of concentration. It is also important to separate specific mineral from other mineral in the ore. Consequently, several reagents could be employed in order to achieve the task. There are specific kinds of reagents used during the flotation. The first types, which bind undesired minerals in order to prevent them temporary or permanently to be floated, are called depressants. However, few minerals do not need depressants since they remain un-floated. There are other types of reagents called activators. These reagents act as opposite behaviour of depressants and are required to liberate the depressed minerals from the slurry in order to separate the minerals desired subsequently. The modifying agents are utilized to adjust the environment of the flotation processes. They act as pH regulators, controlling the alkalinity of the pulp and counteracting interfering effects of negative substances and soluble salts. These agents could effect on the rate of floatation and should be used depends on the process requirements.

Fairthorne et al., (1997) conducted a research to study the interaction performance of thionocarbamate, (O-isopropyl-N-ethyl thionocarbamate (IPETC) and O-isobutyl-N-ethoxycarbonyl thionocarbamate (IBECTC)) and thiourea, (butyl ethoxycarbonyl thiourea (BECTU)) reagents with sulphide minerals, chalcopyrite, pyrite and galena ores, in order to find out the suitable and more efficient selective agent, (or collector), for each ore. They used conductivity water that is produced by reverse osmosis, two stages of ion exchange, High purity nitrogen and two stages of activated carbon prior to final filtration. Potassium chloride, (0.01 mol/dm3 KCl), was used as a conditioner in their experiment and the mineral dispersion was transferred to a water-jacketed vessel and conditioned for 20 minutes in the present of N2 and at several pH values. The minerals then were passed through a modified microflotation Hallimond tube and floated with nitrogen gas. They concluded that the above reagents are more selective for chalcopyrite than for galena and pyrite. In addition, the flotation using these collectors increases with the collector concentration and decreasing pH values.

Similar work has been published by Shen et al., (2001) aimed to study the flotation mechanism of flotation process of sphalerite and pyrite using sodium sulfite as depressants. Moreover, the effect of sodium sulphite on copper activated pyrite flotation was documented in their paper alongside the separation of sphalerite in mildly alkaline conditions, (i.e. pH = 8.5), as it corresponds to the pH values used in the copper circuits. The work concluded that the separation of sphalerite from pyrite could be better when adding sodium sulphite with oxygen conditioning gas. The authors also recommended adding xanthate immediately after sodium sulphite addition. The UV-visible infrared and XPS spectroscopy analysis employed in their study showed that the selective depression of the flotation of pyrite with sodium sulphite is the result of an increased oxidation of the pyrite surface, mainly as copper hydroxide which inhibits collector adsorption.

Farther work conducted by Guang-yi et al., (2006) who compared the collecting performance of ethoxycarbonyl thionocarbamates, (ECTC), with dialkyl thionocarbamates and xanthates in order to separate copper sulphide minerals from iron sulphide minerals. Copper bearing minerals were chalcopyrite, chalcocite, covellite, bornite and the main iron bearing minerals were pyrite and marcasite. The operating pH was 8.5. The experiment resulted poor xanthates selectivity due to its ability to react with metal cation on the surfaces of copper and iron sulfide minerals through forming normal covalent bonds. Dialkyl thionocarbamates were more selective for copper sulphide flotation, particularly against gangue iron sulphides. The interaction of ECTC for copper sulphide minerals and their selectivity against iron sulphide minerals showed better performance at the same time. This means this collector could be the best choice for floatation of copper ores.

Kelebek, (1996) suggested the use of sulphur dioxide in combination with diethylnetriamine, (DETA), at moderate pH in order to depress pentlandite and pyrrhotite from pyrrhotite, pentlandite and chalcopyrite ores mixture. The last ore is believed to have high floatable nature and the authors concerned improving the separation process of chalcopyrite form the other two ores first. This means no collectors are needed when separating this particular ore. However, the separation of the other two could be achieved with retreatment of tailings. This required more grinding since these minerals are intimately associated with each other. Collectors were also needed when splitting these ores from each other.

It can be concluded from the above literature that the success of floatation processes depends directly on the type of ores, depressors and conditioning reagents. The above researches where some examples to show how to manipulate and adjust the process conditions depending on the ore type and hence, the collectors employed. Most of these researches run their experiments in a roughly similar pH value, which is within the range of 8-9. However, further work is required in order to standardise the above factors to suite a particular floatation system and suit the recovery of specific metal ores. However, this kind of a process involves the use of expensive complex components that might harm the environment.

2.2. The Current Industrial Technologies of Metal Recovering by reprocessing:

Basically, most of the methods employed in this stage have started during the mining revolution. After the physical process, minerals go to the most important stage called the chemical treatment, where some change to the physical or chemical properties of the individual phases could occur. This could involve several operations that might lead to a high metal purity. However, some of these processes could not be valuable since the cost effect and the environment problems are a huge concern nowadays.

There are several techniques, such as smelting and hydrometallurgy processes, used in the latest technologies in order to extract metals from different environments. The broad concept of reprocessing of waste and consumed products can be applied to many situations in industry making the current processes safer and more viable. For example, the importance of recycling and reprocessing batteries and consumed electronic devices cannot be ignored.

2.2.1. Smelting or Pyrometallurgical Processes:

Pyrometallurgical processes are Smelting, (or melting and separation), based processes employed to remove metals. The high temperature processing is widely used in the treatment of metals to achieve chemical and structural changes. Smelting processes are done using several type of furnaces, which are refractory structures designed to stand the effect of the heat set free by the combustion of fuel and the corrosive effects of slag and metals. The processes showed a high degree of production rates. However, the high temperature process could cause major environmental problems that come from gases evolved during the process, such as sulphur dioxides (SOx), nitrogen oxides (NOx), carbon dioxide (CO2) and particulate solids. To avoid that, it is suggested the use of gas cleaning processes, which their efficiency depends on the size of the particles to be removed. However, this could perform low efficiency when removing smaller particles and increases the process capital and operating cost, (Kuopanportti et al., 1997) and (Dennis, 1965). Also, high concentration of metal in ores is essential to smelt economically both financial and environmental cost but it is not suitable for treating ores with small metal concentrations.

The approach of combined hydrometallurgy and pyrometallurgy process was proposed during 1990's in order to reduce the cost of the smelting process. Rabah, (1998) published a paper explaining how pyrometallurgical treatment involves melting with a carbon/alkali borate flux at 1150-1300°C. This is done after utilizes hydrogen peroxide in ammonia as leachate solution to dissolve copper and lead selectively. However, the process could still be costly nowadays and might increase the risk of producing toxic solutions.

2.2.2. Hydrometallurgy Processes:

The process could be defined as the use of chemical treatment with aqueous and organic solutions. The process involved the selective transfer of species in liquid/liquid and liquid/solid systems. The process is more environmental friendly and less expensive than smelting process. The hydrometallurgical treatment procedures of ores or other metal source is shown in figure 2-2, (Dennis, 1965).

Hydrometallurgical processes starts from the process of leaching, which involves the extraction of a metal from a mineral or concentrate by means of a suitable solvent followed by the removal of the resulting solution from unwanted material. It can be seen from figure 2-2 that the essential operation in leaching is only one in a series of operations, which could be formulated as following, (Dennis, 1965): preparation of ore involving crushing and grinding and/or roasting, leaching the metal from the ore, separation of the metal solution from the gangue material and recovery of metal from solution.

Sadegh Safarzadeh et al., (2007) presented a review of cadmium recovery from different resources, such as obtaining zinc and zinc-lead ores and Ni-Cd batteries, using different schemes of hydrometallurgical methods. They also outlined steps starting with leaching the media by sulphuric acid to dissolve metals. Thus, precipitating cadmium from its solution by adding zinc powder, leach cadmium from the mixture again using sulphuric acid and the solution obtained can be used as electrolyte in electrowinning cell.

The above procedures can be applied on different kind of environment that contains cadmium. Hydrometallurgy processes of recovering metals from their solutions after leaching were carried on using chemical precipitation, solvent extraction ion exchange or ultrafiltration.

1) Chemical Precipitation:

Chemical precipitation involves the formation of an insoluble solid using specific reagent. This process is not widely employed as the sole recovery method. The process of cupper separation is carried out industrially in long canals filled with scrap via the following reaction:

The above reaction can be slow and long intervals are usually required before the final step recovery of copper can take place. The purity of metal recovered by this method could be low, i.e. ranged from 50 to 75 per cent, (Dennis, 1965). In addition, the separation of these metals could be difficult in filtering and smelting the precipitate.

Hsien Lee et al., (2006) stated that chemical precipitation method, which is used to treat wastewaters containing heavy metals in Taiwan, could remove heavy metals form wastewaters efficiently but the resultant heavy metal sludge is classified as hazardous solid waste that is another environmental problem. Safari and Bidhendi, (2007) indicated that the effect of lime could increase the pH of the leachate in the mechanism for removal of zinc and magnesium metals by the metal precipitation. However, thbis method is not suitable to recover gold and silver metals.

2) Solvent Extraction:

Solvent extraction is to remove unwanted substances from a metal component and to transfer the purified metal to an environment more suitable for isolation. This is based on the use of an immiscible solvent in which the component is completely dissolved. The component can distribute itself between the appropriate organic solvent and aqueous media, depends on the relative solubility in the two phases.

It is important to select the correct solvent in order to extract particular metal. Furthermore, the solvent must have a high selectivity for particular metal. Otherwise, it will be likely to be encountered in effecting the subsequent recovery of pure metal from solution, (Fairthorne et al., 1997). The presence of gangue materials is another issue which is important to be considered in choosing the solvent.

Dilute sulphuric acid, (H2SO4), is one of the most widely used solvents. This solvent found to be efficient and used extensively in the leaching of zinc, oxidised copper ores and cobalt. Ammonia at ordinary temperatures and pressure could be used to recover copper from oxidised ores. On the other hand, the above solvent is employed at elevated temperatures and pressures for leaching copper-nickel-cobalt-ores. TBP (tri-n-butyl phosphate), LIX, (a hydroxyl-oxime complex chelating compound), and Cyanex, which are some commercial sulphur-containing extractants, are widely used in solvent extraction for separation of various zinc metal ions.

Gilchrist, (1989) stated that about 10-15% of chelate would be dissolved in the kerosene and Sarangi et al., (2007) reported the separation of metal ions using solvent extraction in order to develop a process for separation of iron, copper and zinc ions present in the solution using TBP, LIX 84I and Cyanex 923. The recovery of iron from the leach liquor was conducted in two counter-current stages at equal phase ratio, (See figure 2-3).The result of 0.004 kg/m3 iron in the raffinate could be removed by stripping of the iron loaded organic phase with distilled water using TBP, LIX 84I and Cyanex 923 in kerosene. Hence, copper was separated from the Fe-free raffinate in three stages at A:O ratio of 1:2, where A is the aqueous flow volume and O is the organic flow volume. This could be achieved by using 70% LIX 84I in kerosene. Hence, Zinc was separated from the Fe- and Cufree raffinate using 0.05M Cyanex 923 in kerosene at A:O ratio of 2:1. The quantitative stripping of iron loaded TBP, and zinc loaded Cyanex 923 were also carried out with distilled water but stripping of the copper loaded LIX 84I was carried out with H2SO4 solution.

Another work has been conducted by Kongolo et al., (2003) concerning the extraction of cobalt and zinc by simultaneous solvent extraction followed by their selective stripping from the organic phase. The method mainly consists of selective copper extraction with LIX 984, iron removal by precipitation with CaCO3, simultaneous cobalt and zinc extraction with D2EHPA followed by their separation by selective stripping with H2SO4 of different concentrations. More than 95% copper has been recovered from the pregnant solution typically containing 1.0 g/l Co, 2.0 g/l Cu, 12.60 g/l Zn and 8.4 g/l Fe. Cobalt and zinc recoveries were on an average of 90% each in their respective individual solutions.

Alam et al., (1997) published a paper aimed to compare the solvent extraction performance of SFI-6R (dihexyl sulphide), MSP-8 (di (2-ethylhexyl) monothiophosphoric acid), Cyanex 302 (di (2,4,4_trimethylpentyl) monothiophosphinic acid) and Cyanex 301 (di (2,4,4-trimethylpentyl) dithiophosphinic acid) to recover silver from chloride media. The above solvents are found to be efficient because the nature of silver as a soft acid. However, no further work concerns the performance of some of the above solvents when extracting other metals.

It can be seen from the above review that solvent extraction could be a good choice for the recovery of metals from different environments. However, many hazardous solutions could be produced and the need to use high cost reagents.

3) Ion Exchange:

Ion exchange is a chemically driven separation process. It is a suitable method for separation of low concentration of ionic materials from dilute rinsewater and minerals. The principles of the process is similar of solvent extraction but the non-aqueous phase is a solid substance of extensive surface area on which the metal can be collected via chemi-sorbtion process. Furthermore, the process of ion exchange can be outlined in three stages, (Kuopanportti et al., 1997) and (Tzanetakis and Scott, 2004): the absorption stage, in which the metal ions in solution undergo exchange with ions of the same sign in the resin contained in a column, so that as solution percolates down the column the resin becomes saturated and the solid becomes depleted in metal ions, removal of the retained ions in the resin by a suitable solution known as the eluate and washing of the resin free of eluate.

Agents employed in this method can be classified into natural inorganic and organic, such as clay, zeolites and ultramarines, and synthetic resins. Synthetic ion-exchange resins have long been used in commercial scale applications for the softening or demineralisation of water. However, such resins have suffered from a lack of selectivity. Therefore, many researches have been directed towards improving this selectivity.

Simpson and Laurie, (1999) studied the removal of zinc from zinc-rich industrial waste liquor and from model zinc-rich solutions using of ion exchange resins chosen to cover a range of the different types available, i.e., strong and weak acid cation exchangers. The study recommended the use of ion exchange resins over solvent extraction in connection with zinc electrowinning since the removal of the carry-over of any organic reagents or impurities to the electrowinning cell is reduced down to an acceptable level. This approach came after a previous study done by McLay and Reinhard, (1996) aimed to review the technologies employed in finishing industry. They identified problems associated with operating ion exchangers in that it is difficult to separate recovered metal salts and excess re-generant acid in the plating bath.

The cation exchange process appears as the main mechanism responsible for Cd, Zn and Ni sorption. The use of zeolite sorbant depends on the removal situations. Moreno, (2001) used specific kind of synthetic zeolite, NaP1, generated from hydrothermal alkaline activation of fly ash from a power station of Teruel in Spain. The work concluded that each targeted metal was recovered under individual pH value in their experiment for each resin. The work concluded that precipitation of metal hydroxides is much more likely on synthetic zeolite than on natural zeolite. This is because natural zeolite tends to decompose irreversibly in acid solutions, (Dennis, 1965). Consequently, the use of synthetic zeolite could be costly.

2.3. Selective Separation Techniques:

Several chemical separation methods, such as physicochemical treatment, (i.e. flocculation, precipitation and filtration), ion exchange resins, vacuum evaporation, and solvent extraction, electrowinning and membrane technologies, could achieve selective separation of a specific metal ion from a mixture, (See figure 2-4). Electrowinning process is consider to be the last unit in a complete system that recovers high degree of purity of metals from waste streams, (Simpson and Laurie, 1999).

Simpson and Laurie, (1999) pointed out three key factors required for any separations developed to be commercially viable. First, the method should be robust enough to cope with variations in impurity ion levels between batches of waste liquor. Second, the separation process should produce the metal in a form that is commercially useful. Third, the overall process should be cost effective, taking into account the balance between the cost of recovery against the cost of conventional disposal and the market value of the recovered metal. This section presents some of the most efficient techniques involved in the recovery of heavy metals.

2.3.1. Electrometallurgy:

Any electrochemical reaction is a heterogeneous chemical process involving transfer of charge to or from an electrode that could be metal or semiconductor.For many years, electrochemical processes scope for electrolytic recovery and refining of metals has been established, (Janssen and Koene, 2002) and (Juttner et al., 2000). Generally, these processes deal with the conversion of electrical energy into chemical energy with the utilization of thermal energy derived from electrical energy, (Kuopanportti et al., 1997). It is well known that in the extraction process the metal is plated from its solution on to the cathode, whereas the anode being an insoluble conductor.

Electrolysis is maintained by simultaneous processes at the anode and cathode immersed in electrolyte and in contact via an external circuit, (see figure 2-5). Positive charge ions (cations) pass towards the cathode and cations take up electrons and become atoms. The non-metalic negative ions (anions) go towards the anode and the anions lose electrons and become neutral radicals. Ions carry a number of charges equal to their chemical valency. An example of this process can be represented by the following electrochemical equations for the dissociation of a copper sulphate solution:

Generally, electrolysis treatments are employed as final step in many metal recovery processes involves where metal powder could be produced as final product. For instance, cathode nickel can be produced from a variety of electrolytes, such as chloride and sulphate which are an acid system used in hydrometallurgical processing and electrowinning, (Szlag and Dilhoff, 2000) or a mixed chloride/sulphate, which is used for electrorefming the matte from traditional matte-smelting operations, (Oztekin and Yazicigil, 2006).

Several authors described electrowinning units as open cells of concrete tanks linked with rubber or plastic, (Kentish and Stevens, 2001), (Kuopanportti et al., 1997), (Dennis, 1965) and (Fornari and Abbruzzese, 1999). The lines of cathode and anode plates are placed alternately with separation spaces of 5-15 cm, (Kuopanportti et al., 1997). It is important to use insoluble anodes and the compound of the targeted metal decomposes with the deposition of pure metal at the cathode. Operating conditions depend on the impurity content of the electrolyte.

2.3.1.1. Electrometallurgy classification:

It is important to understand the principles of electrometallurgy as one of the technical approaches employed in metals recovery and processing in the industry.represents electrometallurgical methods and their procedures.

• Electrowinning:

the process involves the extraction of metals by electrodeposition from aqueous solution or melts of their salts. It is used for extraction of electronegative elements that cannot be electrodeposited from aqueous solutions, such as aluminium and magnesium, as well as very pure copper, zinc and cadmium by electrodeposition from aqueous solutions of the metal salts, (Pletcher, 1982).

• Electrorefining:

is the purification of metals by electrolysis. The impure metals are dissolved anodically and pure metal is deposited cathodically, while the impurities are left as anode sludge or as ions in the solution. Many metals are electrorefined, such as copper because of conducting application and precious metals because of theirs cost. Electrorefining is also a part of processes in recycling of metals. However, large electrolytic plants for metal production are heavy consumers of electric energy, (Kentish and Stevens, 2001) and (Kuopanportti et al., 1997).

• Electroplating:

can be defined as a treatment that modifies the surface of a metal or occasionally a non-metal, without changing its bulk properties in order to improve the appearance of a surface, to decrease the corrosion and abrasion resistivity, etc. Metal objects are often electroplated. In addition, the most important application of electroplating technology is the manufacture of electronic components (circuit breakers and contacts). Electroplating can be performed from molten salts, non-aqueous solutions but most frequently from aqueous solutions. This depends on the nature of electrodeposited metal.

• Electroforming:

this process deals with the manufacture of articles by electrodeposition. If deposit is good from electroplating point of view except adhesion, and can be removed from the cathode as an entity in itself, it has been electroformed. Electroforming is a branch of electroplating technology, but involves some additional steps, as for example the production, preparation and extraction of the master.

The main requirements in metal electrorefining and electrowinning are to produce pure and compact deposits. This is done at lower current densities. Powder electrodeposition can also be treated as kind of electrowinning or electrorefining, which produces the metal deposits in forms suitable for sintering and various different applications. Electrowinning method is the main concern in this project and can be employed to achieve the purpose of the work. Therefore, it is required to understand more about the basic principles of electrode reactions of the above methods.

2.3.1.2. Metal Recovery by Electrowinning:

Electrowinning has been addressed in the literature and widely employed to treat solutions contaminated with metals, such as mining ores, electroplating processing, mining tailing and consumed electronics. Pletcher, (1982) described several electrochemical processes, which were employed in order to remove and recover heavy metals form dilute solutions within the range of 1 - 1000 ppm. The feed to these processes might be resulted from different chemical operations such as plating or photography rinse water, etching processes, or mine workings. The author also recommended these processes since they have a major advantage in that the metal could be recovered at most valuable form.

The author described eco-cell process use and development. The process basically consists of a rotating cylinder cathode surrounded by an anion exchange membrane and concentric anode. The cell is also designed with a high ratio of electrode surface area to catholyte volume. Pletcher, (1982) also pointed out that the above cell design plus the use of conditions that are highly turbulent, which could be introduced by the rotation of the cathode, are important factor in order to achieve an acceptable rate of metal removal. A number of cells could be run in series in order to achieve complete stripping of metal ion. However, the last few cells might only be responsible of the remove of 10% of the metal ions while costing the same as the first two cells to install and operate. This economically could cause a high capital investment and running cost.

Another kind of cells discussed by Pletcher, (1982) called Chemelec cell. This cell designed with a series of closely spaced gauzes or expanded metal electrodes; alternately beds of non-conducting beads separate anode and cathode. This type of cells was designed in order to remove several metals from wash water resulted from electroplating industries. However, this unit is not suitable for the treatment of solutions with very low concentration of metals.

Pletcher, (1982) has also mentioned another approach in his book. A process based on the fluidised bed cathode that has been developed by Akzo Zout Chemie company. Cathode is employed as a turbulence promoter in this unit. The process is capable of maintaining high apparent current densities and rate of metal removal. This process has a high mechanical strength and resistance to chemical attack but a low hydrodynamic permeability and electrical resistance.

Pletcher, (1982) emphasised that each application in the table require different deposit thickness and characteristics hence a different bath formulation and electrolysis conditions. These could be temperature, electrical current, additives, and the type of application. However, this process could result different kind of inert electrolyte, which contains different type of heavy metals, although the amount of the metal content could be relatively small comparing with other types of metals sources.

Many authors have considered the removal of metals from electroplating wastewater with respect to the economic reviews using several techniques. Applications of electroplated metals are presented in table 2-8. Further improvements have been added on an electrowinning cell to improve its performance, (Pickett, 1979). Bolger and Szlag, (2002) mentioned a problem associated with the production of hydrogen ions at the anode due to water electrolysis. Amara and Kerdjoudj, (2002) gave a general view of the problem. They stated that some cations are able to be transferred freely when their mobility is high and, as a result, the cell becomes more ineffective because of competition between ions and predominating flux of hydrogen ion.

Bolger and Szlag, (2002) suggested two options in order to neutralise hydrogen ions. First option is to pass the electrolysis solution through a weak base anion exchange column to replace the sulphate and chloride anions in the rinse water with hydroxide ions. However, Bolger criticized this option in that it could bring more expense to the treatment when recovering base rinse water. Nevertheless, this idea has been reviewed by Szlag and Dilhoff, (2000) who emphasised on the environmental and economic positive outcome of this technique. The above approach could be efficient when treating other metals rinses, such as gold and silver.

Table 2‑8: Some applications of electroplated metals

Electrodeposited metal	Application
Tin	Protective coating for steel cans used for food packaging. Electrical contacts for soldering
Nickel	Protection and decoration of household items. Protection and repair of engineering components. Undercoat for Cr plating. Protection of chemical plant.
Copper	Contacts and circuitry in the electronics industry. Undercoat for Ni and Cr plating. Decoration of consumer goods.
Chromium	Decoration and protection of household items, car components, screw, etc. water-resistance surfaces in tools, machine parts and valves.
Cadmium and zinc	Corrosion protection of steel and iron base alloys
Silver and gold	Decoration. Mirrors and reflectors. Electrical contacts

The second option involves the addition of either anion exchange membrane, (AEM), or cation exchange membrane; (CEM). This idea is becoming more popular in laboratory and industrial scales. McLay and Reinhard, (1996) defined the ion exchange membrane as ion permeable and selective, permitting ions of a given electrical charge to pass through. Moreover, anion exchange membranes allow only anions, such as sulphates or chlorides, to pass through. Similarly, cation exchange membranes allow only cations, such as copper or aluminium, to pass through. Amara and Kerdjoudj, (2002) introduced cathodic exchange membrane, CEM and anodic exchange membrane, AEM used in their experiment. CEM contains sulfonic acid groups in a polymer matrix, and an AEM containing quaternary amine groups in a polymer matrix were used for the electrodialysis. The conclusion of their work is that a number of advantages, such as a good water solubility, high content of functional groups and chemical stability. Also, the electrodialysis process using membranes selective to monovalent ions is strongly recommended for the treatment of effluents containing nitrate or chloride ions.

Bolger and Szlag, (2002) and Oztekin and Yazicigil, (2006) suggested employing AEM as a second option in order to separate the cathode and the anode in the cell. The principle is to place nickel on the cathode and excess anions in the catholyte migrate across the anion exchange membrane to the anode compartment to maintain electroneutrality. The authors summarised the benefit of this option in that dual electrode use could be permit, recovering the metal from the rinse water at the cathode and generating an acid mixture at the anode that can be used in cleaning metal parts before plating. The above-modified cell can be shown in figure 2-7. Amara and Kerdjoudj, (2002) supported the above approach in that neutralisation of effluents and concentration of acids in the concentrate compartment may be achieved completely by electrodialysis when the current density is high (using a low resistance membrane) and the experiment duration is long.

Oztekin and Yazicigil, (2006) compared electrodeposition method performance with other methods conducted by Johnson et al., (1972) and concluded that chelating agents could be oxidised into many components. This makes the tradition hydrometallurgical methods less efficient in order to separate metals from chelating agents. Consequently, there was a need to employ CEM membrane for better recovery.

Lemos et al., (2006) found out that CEM membrane could be used to avoid the anodic oxidation of the cyanide ions released during cathodic reduction of the copper or gold cyanide complexes. In addition, Lemos et al., (2006) conducted several electrowinning tests with copper in a flow-by cell with a reticulated vitreous carbon, RVC, cathode separated of the anode by a cation exchange membrane. They concluded that the above approach could perform the recovery of 99.6% of copper with the liberation of 99.4% of the cyanide as free ion for recycling. However, the low current efficiency applied caused an increase in resistivity of the RVC cathode, which promotes an uneven current distribution throughout the cathode thickness. This would lead to dramatic increase in energy consumption. Stavart et al.,(1999) emphasized on the importance of the nature of the separator between the anodic and cathodic compartments and that it has to be optimised when the process requires it. Cifuentes and Simpson, (2005) stated that the use of anion membranes was a good option to permits the separation of a ferric anolyte, which is generated when using graphite bar anodes that could transfer the ferrous to ferric ion reaction, from a copper electrowinning type catholyte. This means that this kind of membrane might perform better when separating ferric ions from other non-ferric once.

According to McLay and Reinhard, (1996) and Amara and Kerdjoudj, (2002)The advantages of the above option are: low energy consumption, the ability to produce a highly concentrated stream for recovery and the ability to recover only ionic materials. This means all the undesired impurities could be retreated and rejected.

Although, the above technology could dramatically improve the efficiency of electrowinning cell, this kind of separation requires clean feed, careful operation and periodic maintenance in addition to the energy consumption.

2.3.1.3. Cell Components and Design:

Generally, an electrochemical circuit consists of a current source, metallic connecting wires, an electrochemical cell, ohmic resistance, current and voltage measuring instruments and circuit breaker. A steady current flow in such a circuit can only be maintained if there is a change of charge carrier at the metal electrolyte interface by a chemical transformation involving the transfer of electrons across the interface.

There are several rules to be followed when designing a cell. These can be listed as following, (Greef et al., 1985):

• The process should meet the requirements in simplest and cheapest way possible. Therefore, all kind of separators, solution flow and stirring should be avoided.
• The unit must operate alongside other operations, such as leaching.
• In case of the need to employ electrolyte flow or stirring, it is recommended to consider the transport regime at early stages.
• The potential distribution over the electrode surface should be uniform. This could be achieved by simple electrode geometry.
• The call should have a small gap between the cathode and anode.
• The material construction in the cell should be compatible with the electrolyte and the products.

It should be pointed out the importance of deciding metal type in the feed used and product forms and purities before selecting the suitable cell for particular products. Once the above are established, the characterisation of both the electrolytes and electrode could be specified, (De Francesco and Costamagna, 2004). This of course is accompanied by specifying the cell design limitations.

1. Electrolysis:

Electrolysis can be defined as the conductors in which the passage of a current accompanied by a simultaneous migration of material. Conduction in electrolytes is elementary electronic charges, (electron), that move and give rise to the electric current. Several parameters could effect on electrolysis, (Pak et al., 2001). These could be listed as following:

• Electrode potential: this determines the absolute rate of electron transfer reactions and its type.
• Electrode material and structure: electrode material should be totally stable in the electrolysis medium but allow the desire reaction with high current efficiency at low overpotential.
• The concentration of electroactive species: this is another major factor that determines the maximum feasible current density and optimum time yield. In other word, the higher the concentration is, the more current density obtained.
• Type of electrolysis medium: this parameter is related to the choice of solvent, electrolytes and pH and complex agents, additives and reagents.
• Temperature and pressure: it is desired to operate an electrochemical cell under atmospheric pressure. In addition, it is not recommended to vary the pressure, (Kargl-Simard et al., 2003). On the other hand, the effect of temperature could effect on the performance of the cell depends on other factors.
• Mass transport regimes: different kind of fluid mechanics could be involved including convection and diffusion in unstirred to high turbulent conditions, which could be produced by rapid stirring and pumping. However, all forms of pumping and stirring could add more cost to the system and might produce problems of interaction of anode and cathode products, (Kargl-Simard et al., 2003).
• Cell design: the principle factors determining the electrolysis performance could be the presence or absence of a separator and its type, the mass transport regime, the arrangement of electrodes and the anode-cathode gap and the potential distribution at both electrodes and materials of construction.

The above parameters should be considered when designing any kind of electrochemical cell at experimental or industrial scale. Each parameter has different way to be defined and/or estimated.

2. Electrodes:

Electrodes can be defined as the metallic end of external circuit and are immersed in the electrolytes. Ions which are material carriers of the electrical charges could be divided into cations which are positively charged and migrating towards the cathode, which is the negatively charged electrode and anions which are negatively charged and migrating towards the anode, which is positively charged electrode.

The most common electrode material used in the real world of electrometallurgy is presented in table 2-9, (Pletcher, 1982). Electrodes also could take different ships, i.e. flat plate, rotating cylinder, low- and high-porosity three-dimensional, (Panzer and Elving, 1972) and (Panic et al., 2006b). It is known that electrode material may be involved in the electrochemical reactions. It could be consumed or deposited from the solution or it may remain inert to provide an interface at which the reduction may occur. Consequently, it is required to minimise the power consumed by the process, (Panic et al., 1999). To achieve that, electrodes should have several properties of good electrical conductor material, low activation overpotential at the electrode and high exchange current density for the produced species, passivated electrode during cell operation, i.e. it should not react to form any compound that could reduce the efficiency of the cell on the electrode surface, easily manufactured, resisitant to corrosion by elements in the cell, have good dimensional stability and strength and low cost.

Table 2‑9: Electrode materials

Cathodes	Anodes
Hg, Pb, Cu, Ni Graphite and other forms of C sometimes treated thermally or with organics to modify porosity, density, and corrosion resistance. Steels Coating of low hydrogen overpotential materials on steel (Ni, Ni/Al, Ni/Zn)	Pt, Pt/Ti, Ir/Ti Graphite or other forms of C Ni in alkaline media Dimensionally stable anode (RuO2 on Ti) Oxide coating (CuxCO3-xO4, Iro2 on Ti) Magnetite Fe3O4

The choice of the anodes for the direct processes is complex since the reaction mechanism is strongly dependent on the electrode properties. In the non-selective scheme, the electrodes for the complete combustion of the pollutants require that the oxygen transition is slower than hydroxyl production. Anodes for the complete combustion should not have oxygen vacancies (i.e. a high oxidation level or oxygen excess, also in combination with doping with other metals); on the contrary, the presence of oxygen vacancies is preferable for the anodes for the selective process. For these reasons tin oxide (SnO2) shows a non selective behaviour and platinum and titanium coated with IrO2 are selective electrodes, (Kentish and Stevens, 2001), (Doulakas et al., 2000), (Armelao et al., 2003) and (Panic et al., 2006a).

2.3.1.4. The Nature of Electrochemical Reactions:

The oxidant and reductant in electrolyte solution reach the electrodes via one of three mass transfer processes, (Koene and Janssen, 2001), (Crow, 1988) and (Milazzo, 1962). These are:

• Migration: this process occurs due to electrical charges of the species. It involves the movement of cations and anions through the solution under the influence of an applied potential between two or more electrodes placed in that solution.
• Diffusion: where an electrode reaction decreases the concentration of oxidant or reductant at an electrode surface and produces a concentration gradient on the surface. This process occurs for both charged and uncharged species.
• Convection: this involves the movement of the fluid by thermal and stirring equipments.

When the metal ions in the solution are the same type as in the electrode metal lattice and electron transfer reaction can occur at metal electrolyte interface and lead to generate a potential difference. The electron takes place until the process reaches a dynamic equilibrium state. The above process starts with either the deposition of ions from solution onto the metal electrode, (where electrode is more positive than the solution at equilibrium), or with the dissolution of the metal electrode, (where electrode is more negative than the solution at equilibrium).

The nature of electrode reaction can be explained by considering two completely stable and soluble species, O, (the oxidized state that accepts n electron), and R, (the reduction state or the donor of electrons), in an electrolyte medium containing an excess of electrolyte which is electroinactive. The following reactions could occur:

It is required to maintain a current in order to supply reactant to and remove the product from the electrode surface and for the electron transfer reaction at the surface to occur. Therefore, the above electrode reaction could be carried out in three steps as following:

In addition, it is important to point out that the rate of reduction and/or cathodic current could be determined by the rate of overall reaction, which depends on the rate of the slowest step.

Meanwhile, electrode reaction could involve multiple electron transfer plus three other steps. These steps are as following:

• Chemical Reactions: the species resulted from electron transfer step are not stable in the electrolysis medium since they act as an intermediate in order to cause chemical change. Two possible types of reactions could occur, homogeneous process where species R is transported away from the surface and heterogeneous process while R is adsorbed on the surface.
• Adsorption: this process is considered to be very important in electrode reaction. It is responsible for the success of electrochemical processes. In electrocatalysis, such as platinum, Pt, adsorption of intermediate on the surface could provide alternative lower energy pathways.
• Phase Formation: electrowinning processes involve the formation of new phase. This step requires both nucleation and subsequent growth, where as crystal growth could involve surface diffusion and lattice growth.

The reversible electrode potential of a reaction at equilibrium state could be described mathematically using Nernst equation. The equation could be written for reaction 3.1 as following, (Farrington and Daniels, 1979):

where is the equilibrium electrode potential, is the standard electrode potential, are the activities of the electron acceptor and donor respectively, is bath temperature, is Faraday's number, (96500 A/s) and number of electrons transferred in the cell reaction.

According to the above equation, when the cell is connected to a high impedance voltmeter, the equilibrium concentration of elements in the more negative electrode will be larger than it is in the more positive one. On the other hand, if the cell is connected with a low impedance amperometer, oxidation process will take place on the more negative electrode and reduction process will occur on the more positive electrode and become electron source. This transformation at the two interfaces provides electrons available for external use. This could result a complete removal of positive ions.

The reversible reduction potential calculated form Nernst equation assumes each electrode to be in equilibrium with its local environment. In addition, Nernst equation stats that it should be possible to decrease metal ion concentration in solution to an arbitrarily low level, if the potential E of the metal/metal ion electrode is maintained sufficiently negative with respect to the standard potential. However, the equation does not provide any information regarding the reaction rate in the cell at any potential.

2.3.1.5. Electrowinning Process Conditions:

The degree of performance of the method has been the main concern of many researches, (Szpyrkowicz et al., 2005) and (Szpyrkowicz et al., 2000). This included the electrowinning cell design and operating conditions. For instance, it has been reported that for the recovery of cupper, purity of copper could reach around 99.5% when electrolyte temperature between 40 to 60 degrees, current density of 15 to 150 mA/cm2, (the intensity of electrolysis current over the electrode surface area), cell voltage of-1.9 to -2.5 V and current efficiency of 80 to 96%, (Kuopanportti et al., 1997). However, the above conditions could be changed depending on metal type and the degree of purity and recovery required. Also, other conditions, such as type of electrolyte solution, (acidic or alkaline), pH for acidic type, initial concentration of metals in resin water, type of anode and cathode material and process kinetics have been involved in studying the performance of this process.

Ramachandran et al., (2004) explored the process of zinc recovery from zinc ash using iridium oxide, IrO2 catalytic anode. The performance of both IrO2 catalytic anode and traditional lead anode where compared in their work. Ramachandran et al., (2004) stated that zinc ash contains a large amount of chloride, (around 3.0 g/l) which could result a heavy contamination of the zinc sulphate solution with chloride ions. This would affect the traditional lead anode during electrowinning process. Consequently, the authors explained the high performance of IrO2 catalytic anode in that this anode can resist the attack of chlorine and oxygen evolved as a part of anodic reaction. However, their work did not consider the effect associated with hydrogen liberated as a side reaction. This might cause problems regarding the morphology and reduce the recovery percentage of zinc.

Another interesting work has been conducted by Sharma et al., (2005) concerning the recovery of cobalt, Co from its sulphate solution by electrowinning. They studied the effect of hydrogen generated on the process criteria. They stated that when the pH of the electrolyte drops, the Co strength of the electrolyte will deplete. Sharma suggested a method called restoring pH in order to regenerate the bath with respect to the pH and Co concentration. This could be done by adding cobalt hydroxide, Co(OH)2. However, other side reactions, such as generation of Co3+, could occur as following:

This problem has also been pointed out by Lakshminarayanan et al., (1976) and Pradhan et al., (2001) who stated that most of Co3+ ions could deposit at the anode as a black oxide or hydroxide powder which is hard to abandon even a small amount of the above components once they reaching the cathode. This could affect the nature of the deposit. Despite that, Sharma et al., (2005) recommended neglecting the above reaction because the amount of Co deposited at the anode was only a small fraction of Co deposited at the cathode.

Based on the above discussion, Sharma et al., (2005) carried out studying the effect of cobalt ions concentration, concentration of sodium sulphate, temperature, pH, and cathode current density on cobalt recovery form synthetic solutions containing cobalt sulphate and sodium sulphate.

1. Effect of Metal Initial Concentration:

Several papers concerned the effect of metals concentration on the efficiency of electrowinning cell. Sharma et al., (2005) started their study by varying cobalt concentration, (from 20 g/l up to 70 g/l), at sodium sulphate concentration of, 15 g/l, temperature of 60oC, pH4, and current density of 400 Am/m2 during acidic electrowinning. The experiment showed that increasing cobalt initial concentration could increase the current efficiency and decrease energy required for the deposition. This is due to higher conductivity occurred by the metal. However, low concentration of cobalt could result less deposition of cobalt due to high hydrogen ions evolution rate.

The above results were also achieved by previous work conducted by Elsherief, (2003) who concluded that the cathodic efficiency of cobalt deposition from solutions containing cobalt was increased when initial concentration increased. In addition, current efficiency increased from 94% to 97% with increase in cobalt concentration to 60 g/1 at 20oC. Elsherief, (2003) gave more explanation in that as the cobalt concentration is increased, the area covered by the anodic curve is increased, and the charge related to anodic stripping is increased. Oztekin and Yazicigil, (2006) concluded the same and added that the recovery efficiency of the process could also vary depends on the type of targeted metal.

Holm and O'Keefe, (2000) aimed in his work to determine the effects nickel initial concentration on the current efficiency. Nickel concentration was varied from 20 to 60 g/1 for electrolyte containing 150 g/1 sodium sulphate at constant pH2.5 and 40°C. The work reported poorer deposit quality when nickel concentration was 20 g/1 compared to the deposit from 60 g/1 Ni electrolyte.

It can be seen from the above review that increasing metals initial concentrations could increase the performance of electrowinning cell. However, it is still required to introduce other approaches in order to recover metals that exist in small concentrations in their solutions. McLay and Reinhard, (1996) suggested two options in order to reduce the effect of electrode polarization. First, by adding heat to the process but this is not recommended because it could increase the cost of the process. The second option could be performed by delaying plate-out of metal from these low concentration solutions. This could be done alongside strong air agitation to provide adequate mechanical mixing. However, McLay and Reinhard, (1996) again criticised this option because it could remove heat from the system, thus reducing operating rates. Air agitation may also add to the load on air pollution control equipment.

2. Effect of Temperature:

Many papers emphasised on the effect of bath temperature on the performance of electrowinning cell. Early work has been conducted by Fratesi et al., (1997) to study the deposition of Zn-Co alloys from a chloride bath plotted. They found out that an increase in temperature caused a dramatic decrease in cobalt amount recovered when applying high current density. According to the results achieved by Elsherief, (2003), increasing the electrolyte temperature should enhance the deposition reaction but the author also found out that the hydrogen evolution reaction occurs more readily at higher temperature. This could support conclusion achieved by Fratesi et al., (1997). Holm and O'Keefe, (2000) concluded that higher temperature has the most impact on the quality of nickel deposition when maintaining solution pH and nickel concentration.

Sharma et al., (2005) had the same results when increasing the temperature from 27 to 70oC under constant electrolyte composition of 60 g/l cobalt and 15 g/l sodium sulphate. They found out that higher temperature up to certain point provide higher quality and more adherent deposition. At high temperature, high current efficiency might be due to increased ionic mobility. However, the researchers emphasised on the fact that optimum temperature should be specified because of the generation of some kind of stress that could result detachment of the deposit at very high temperature. Therefore, most of the above mentioned work recommended the optimum temperature to be within the range of 50-60oC for high quality deposition.

Another work conducted by Cifuentes and Simpson, (2005) reviews the effect of temperature on several kinetic parameters for the copper electrowinning reaction. They present two equations derived in order to estimate the relationship between current densities of both cathodic and anodic reactions as following:

where i0c, i0a are cathodic and anodic exchange current densities, A/m2, , are cathodic and anodic limiting current densities, A/m2, , are cathodic and anodic charge transfer coefficients, , are cathodic and anodic overpotentials, V, F is the Faraday's constant, 96,500 C/eq, R is gas constant, 8.314 J/mol K and T is temperature, oK.

The above equations showed that both the exchange current density for the Cu2+/Cu0 reaction and the limiting current density for the Cu2+/Cu0 reaction increased by increasing the temperature. However, the charge transfer coefficient for the Cu2+/Cu0 reaction does not significantly change with temperature. The authors recommended employing temperature-dependent mathematical model derived in their work for the operation of the elecrtowinning processes in order to study the temperature effect on that unit. However, further work is required to study temperature effect when recover other specific metals, such as cobalt and nickel.

3. Effect of Electrolyte pH and Electrolyte Type:

This parameter could be strongly related to the type of electrolyte employed in electrowinning method. This factor has an important effect on the process. The study of the effect of pH parameter is important concerning the initial concentration of metal(s) in the electrolyte. Furthermore, the type of electrolyte plays an important role when initial concentration of metal is very low. Other reasons, such as the hydrogen generation, which could effect on the recovery performance, are also important to consider in this study.

Many papers have considered the behaviour of electrolyte solution and the effect of its pH during the recovery process. Sharma et al., (2005) studied the effect of pH on the current efficiency of the unit, by varying the value of pH from 2 to 5. Therefore, their work concluded that when pH was increased, the efficiency was increased. However, the efficiency of the unit was stabilised when pH value was between 3 up to 4 but when pH greater then 5, the efficiency was dropping to the minimum value. Hence, at lower pH, the higher hydrogen ions concentration could reduce current efficiency of the unit. This was confirmed by Fornari and Abbruzzese, (1999) who stated that decreasing of pH could also increase energy consumption for unit weight of metal by more than 3.4%. When pH was greater than 5, small amount of hydrogen ions discharged at the cathode, (Sharma et al., 2005). This caused produce metal hydroxide which could result very poor quality plating. Similar results were also achieved by Casas et al., (2000) who referred to the fact that in sulphuric acid solutions, mainly copper sulphate complexes could generate. This complex is more stable than copper hydroxide ones. (Fratesi et al., (1997) agreed with the fact that increasing pH value could result the formation and precipitation of nickel hydroxide on the cathode, which decreases the current efficiency.

Lupi and Pasquali,(2003) found that the optimum electrolyte pH is within the range of 3 - 3.2. The authors also stated that pH could be more effective at lower current densities. Jeffrey et al., (2000a) and Bolger and Szlag, (2002) recommended the same. Jeffrey et al., (2000a) stated that when increasing pH; the current efficiency could be increased due to both the enhancement of cobalt deposition and the reduction in hydrogen evolution. However, the pH of the solution cannot be increased too much, as the solution resistance becomes high.

Tsuru et al., (2002), Yin and Lin, (1996) and Lupi and Pilone, (2001) recommended the use of boric acid and ammonium sulphate as buffer to control electrolyte pH. They found that the pH adjacent to the plating metal surface was maintained at a value close to that in the bulk solution, so that the deleterious effects of hydrogen co-deposited during electroplating on the development of high internal stresses in the resulting nickel films are greatly reduced. However, Ji et al., (1995) criticised this approach in that other components may be formed and could interrupt with metal deposition reaction.

4. Effect of Current Density:

Many electrowinning processes are run under the galvanostatic scheme, (under constant current density). Faraday's law is used in calculating current density for the deposition process. The mathematical formula of the law is as following

where n is the number of moles of electrons transfer per mole of metal deposited, F is faraday's number, (96500 A/sec), M is atomic mass of the metal deposited, A is surface area, dm is the change of electrode mass and dt is time difference.

Current density calculated could be employed in the design of electrowinning cell. However, it is difficult to estimate the real amount of metal deposited particularly when other side reaction, such as evolution of hydrogen and chlorine gases, occurs.

Sharma et al., (2005) found out that the effect of varying current density could be clearly apparent when recovering cobalt. The percentage recovery of cobalt under certain fixed experiment parameters was increased when increasing the current density up to certain value. However, beyond that value, the efficiency was decreased due to the generation of hydrogen gas. Elsherief, (2003) concluded that hydrogen evolution could cause a loss of useful current density. Hence, he suggested maintaining pH at 4 to solve this problem. Also, Amara and Kerdjoudj, (2002) pointed out that decreasing pH could decrease current density. This could decrease the efficiency of the process. Consequently, there is a need to use other methods, such as adding boric acid as a buffer in order to control pH without affecting the optimum current density. Oztekin and Yazicigil, (2006) found in their study that increasing current density at different pH values could increase the cupper recovery. In addition, current density could effect on the structure of metal deposition. This supports a previous study conducted by Fratesi et al., (1997) whom concluded that increasing current density might give more uniform structure to the targeted metal.

Jeffrey et al., (2000a)suggested the use of a rotating electrochemical quartz crystal microbalance, (REQCM), which has been highly recommended in previous work conducted by Jeffrey et al., (2000b) in order to distinguish the current density, which is due to metal deposition and hydrogen evolution since, hydrogen evolution often occurs simultaneously with the deposition reaction during metal deposition. The above device could be employed to evaluate the electrode mass change at several intervals. Jeffrey et al., (2000b) stated in his paper that REQCM could also achieve good results when removing copper from cyanide solutions the above process could be complicated by the fact that there are several possible discharging species, including , and . Moreover, the equilibrium of these possible dischargeable species in the diffusion layer may be different from that of the bulk solution.

5. Effect of Impurities and Other Chemical Additives:

The effect of the present of one common plant impurities, such as nickel, copper, iron, lead and zinc, has been widely concerned by many researchers. Jeffrey et al., (2000a) studied the effect of zinc presence on cobalt deposition process. They found that when zinc concentration exceeded 10 pop, the current density increased. Also the amount of hydrogen evolved decreased. Jeffery described the presence of zinc as a code posited metal with copper since this leads to suppression of hydrogen evolution. However, intensive increase in zinc concentration could hinder cobalt deposition. The same conclusion achieved by Fratesi et al., (1997) when recovering cobalt from zinc-cobalt alloy.

Lecheries, (2003) also studied the effect of zinc cupper and iron on cobalt recovery. He achieved similar conclusion stated by Jeffrey et al., (2000a) regarding the presence of zinc. Moreover, Elsherief, (2003) also found that increasing the copper amount increases the efficiency of the deposition with no certain copper concentration limitation. On the other hand, it is recommended to remove iron from deposited solution since it reduces the current efficiency of the cell and increases hydrogen evolution. The effect of chemical additives on metal deposition has also been studied in the literature. Jeffrey et al., (2000a) used boric acid and sodium lauryl sulphate, (SLS), in his study. Both substances are found to reduce hydrogen evolution at low current density. However, increasing current density could result less electrowinning potential and less efficiency when these additives are present, (Tsuru et al., 2002) and (Fratesi et al., 1997).

Other kind of additives called chelates like citrate, nitrilotriacetic acid, (NTA) and ethylenediaminetetraacetic acid, (EDTA), could also effect on metal elecrtowinning. Oztekin and Yazicigil, (2006) concluded that the recovery of cupper could be varied depends on the chelates type. These chelates might improve the efficiency of electrowinning since they have the ability to reduce side reactions. Sharma et al., (2005) recommended the addition of sodium sulphate, Na2SO4, to increase the efficiency. This could increase current efficiency and lower hydrogen evolution, which means higher metal recovery, plus decrease in energy consumption. However, the results showed no effect on the above parameters when increasing the concentration of this component more than certain level.

2.3.2. Separation by Absorption:

Adsorption processes have been widely used to achieve a better metal recovery from electroplating wastewater, (Al-Asheh and Duvnjak, 1997), (Taty-Costodes et al., 2003), (Balkose and Baltacioglu, 1992), . It has been recommended the use of sobants that are obtained from natural materials available in large quantities, or certain waste products from industrial or agricultural operations (Moreno et al., 2001) and (Peternele et al., 1999). This is because of their low cost and can be disposed of without expensive regeneration. However, a sorbent can be assumed as low cost if it requires little processing, is abundant in nature, or is a by-product or waste material from another industry.

Bailey et al, (1999) presented a review paper with a list of sorbent used in the literature and provided a summary of available information on a wide range of a low cost sorbents. Sciban et al, (2007) studied the adsorption of copper; zinc and cadmium form electroplating effluent using wood sawdust, (a solid metal obtained from mechanical wood processing). The authors mentioned the advantages of this sorbant in that it has an extremely low cost, widely available and has a good mechanical stability. The work showed an increase in the efficiency of sawdust adsorption with the increase in the amount of the sawdust. Also, the efficiency of the adsorption depended on the type of the metal recovered and the amount of that metal. However, the selectivity of metal removal using this method did not show satisfactory results. Bailey et al, (1999) also related the sorbant low cost to the degree of processing required and local availability. Hence, improved sorption capacity may compensate the cost of additional processing. Meena et al, (2005) stated that the presence of agricultural materials may provide a good performance in removing heavy metals from effluent solutions. This is due to the adsorption on surface and pores and in addition to complexation by these materials. Sorption data could be effectively described by Langmuir model.

The removal of heavy metal ions using carbon aerogel adsorbent has been proposed by Meena et al, (2005). The work also concerned the effect of other factors, such as concentration, pH, contact time, temperature and etc. pH found to be a most effective variable in this kind of adsorption. Moreover, the experiment showed almost 100% adsorptive removals of heavy metal ions under certain condition. However, the work was aimed mainly to reduce these metals and did not concern the recovery in order to reuse these metals. Furthermore, carbon aerogel could be expensive to manufacture and use.

Alvarez-Ayuso et al, (2003) studied the sorption behaviour of natural and synthetic zeolites with respect to heavy metals, such as Cr, Ni, Zn, Cu and Cd, commonly present in metal finishing wastewaters. Four wastewater samples were used from an electroplating wastewater. These are: acid zinc electroplating process sample, cyanide zinc electroplating process sample, typical chromium electroplating process sample after reduction of Cr(VI) to Cr(III) with Na2S2O5 and typical nickel electroplating process (Watts bath) sample.

The selection of the type of the unit depends on the efficiency and economic outcome of the unit. Moreover, absorption processes can only be used within a given concentration range but not to treat large volumes of effluents containing low concentrations of metal cations, (Yordanov and Roundhill, 1998) and (Baticle et al., 2000). The following section discusses one of the most efficient units that could achieve the above task with low cost.

2.3.3. Polymer Enhanced Ultrafiltration, (PEUF):

The concept of PEUF has been well documented in the literature, (Singh et al., 2000), (Jones, 2005), (Coulson et al., 1999), (Juang and Liang, 1993b), (Canizares et al., 2002) and (Aliane et al., 2001). The improvements in commercial UF technology over the last l0-15 years have made the use of water-soluble polymers with UF a more reliable and cost-effective approach for separations. The process works in similar manor as the absorption method described by Bailey et al., (1999), Sciban et al.(2007) and Meena et al., (2005) but using suitable polymer that can selectively bind metal ions, (Kaliyappan and Kannan, 2000) and (Kaliyappan and Kannan, 2000).

The appropriate complexing agent must complex specific metal ions satisfactorily in order to ensure the complete rejection of the metal before the membrane. However, the complexation must be reversible so that the complexing agent can be easily regenerated for recycling. Some polymers can be highly selective to particular metals. e.g. the use of molecular imprinted polymer, (MIP) technique to produce.

The main steps of this processcan be shown in Figure 2-8. The first step called complexation where water-soluble polymer with the ability to selectively chelate metal ions is added to metal ion solution. This generates a complex of targeted metal ion with the polymer under specific conditions, such as pH. The macromolecular complex and the polymer than retained by the membrane while the non complex metal ions pass through the membrane to the permeate stream. The next step involves dissociating the polymer-metal ion complex. This can be done by acidification of the component in order to lower solution pH so that the polymer gets protonated and the metal complex breaks up. Secondary ultrafiltration separates the polymer form metal ion and permeate produced from this stage will be a concentrated metal ion solution suitable for recovery of metal by electrowinning.

Polymers could be highly selective for copper ions over zinc ions, (Singh et al., 2000). However, Jones, (2005) mentioned that the investigation of PEUF using various commercially available water-soluble polymers was required since there are no chelating agents developed specially for this type of separation. Polyethylenimine, PEI has been used as basis polymer for the production of numerous chelating derivatives by the addition of chelating groups, (Jones, 2005). It is believed that PEI has a number of advantageous that are required for a successful polychelatogen. These include high density for functional groups, good water solubility, chemical stability and metal selectivity, (Singh et al., 2000) and (Geckeler and Volchek, 1996). Consequently, PEI is used for the experimental work performed in this project.

Several kinds of PEUF modes can be used. These are: Batch, Permeate recycle, Washing method with variable ionic strength, Washing method with constant ionic strength and Enrichment methods. Jones, (Jones, 2005) has recommended the use of either batch or enrichment technique in order to treat wastewater, which is subjected to this work.

2.3.3.1. PEUF Parameters:

Some parameters have been shown to influence metal ions separation. The parameter of ionic strength of the solution effects in a way that precipitation can occur where higher ionic strength as the solubility of the polymer decreases. The formation of metal salts should be avoided since it inhibits complexation of target metal ions by compression of the electric double layer. This could inhibit the complexation of targeted ions. It also reduces the overall thickness of double layer. The sensitivity of water-soluble polymer can be expressed by the following equation, (Coulson et al., 1999):

where, is the ionic strength of solution, (mol/l), represents the concentration of ion in solution, (mol/l) and the ion valence.

It is importance to point out that the electric double layer can reduce electrostatic attraction between metal ions. Increasing polychelatogen/metal ion loading, or loading ratio can increase retention. The loading ration can be shown in the following relationship:

Increasing operation pH in the acidic region could increase the metal retention in the presence of PEI, (Jones, 2005) and (Juang and Liang, 1993b). However, metals could form insoluble hydroxide of sufficient sixe to be retained by the membrane when pH exceeds certain value. Nevertheless, pH sensitivity of metal - polymer complexation can vary significantly between different metals. Based on the above, Jones, (2005), Baticle et al., (2000), Canizares et al., (2002), Juang and Liang, (1993a), and Aliane et al., (2001) suggested the control of pH which could offer the possibility of selectively binding metal ions. Aliane et al., (2001) listed a table which shows the retention values of various metal cations over a range of pH using PEI as polychelatogen, (See table 2-10).

Table 2‑10: Retention of metal ions by PEI for various pH values. PIE concentration was 1% and initial metal concentrations were 10ppm.

pH	Retention, %
	Co	Ni	Zn	Cd	Cu	Mg	Na
2.1	1.2	0.1	1.0	0.1	86.3	-	-
3.2	16.3	21.3	0.5	0.7	99.0	-	-
4.0	93.1	99.0	-	98.1	99.0	-	-
5.3	99.0	99.0	99.0	99.0	99.0	0.1	0.1

High cross flow feed rate could also reduce concentration polarization but improves permeate flux rates. However, a high feed velocity invokes high shear which in turn degrade polychelatogen. According to Geckeler and Volchek, (1996), this could result in a reduction of molecular weight and permit the polymer to pass through the membrane.

After the metal ions are complexed with the polymer and selectively separated from non targeted ions, it is required to decomplex these ions and regenerate the chelating agaent without excessive damage. Two methods have been reported in the literature, first the electrolysis by Geckeler and Volchek, (1996) where ions could be separated in cells similar to the one discussed in section 2.3.3.6. However, Juang and Chiou, (2000) criticised this approach because of the anodic oxidation that could cause excess damage to the polymer. The second method has been recommended by Baticle et al., (2000), Geckeler and Volchek, (1996) and Jones, (2005) as an efficient strategy. Chemical decomplexation operation depends on chemical used to adjust pH. It is known that different metal ions can be eluted at different pH values with different chemicals. Consequently, this can offer the possibility to selectively recovering metal ions during decomplexation, (Jones, 2005).

The retention can be defined as the fraction of a solute in the feed solution retained by the membrane. This can be show by the following equation:

The retention of metal ions can be used as an indication of the degree of the folmation of macromolecular metal-polymer complexes with the assumption that only metal ions complexed with polychelatogen are retained by the membrane while non-complexed metal ions pass through the membrane, (Jones, 2005).

Langmuir and Freundlich equation has been employed to describe the complexetion process via retention values, (Jones, 2005), (Moreno-Villoslada and Rivas, 2002) and (Moreno-Villoslada and Rivas, 2003). The investigation showed the retention data fitted the linear form of each isotherm with coefficient of correlation, R2 greater than 0.9. Therefore, the isotherm could be employed to model this kind of processes. However, Jones, (2005) concluded that Langmuir equation provide the better description of the system.

2.4. Metal Recovery from Wastes and the Environment

Many technologies have been designed and developed to suite the type of metal (base, precious or strategic) to be recovered or removed, and the environment (water or soil) and also metals from wastes and consumed products. Therefore, authors have addressed these technologies that suit the phase and type of waste inputs. The above environments will be discussed in the following sections.

2.4.1. Recovery of Metals from Soil:

Soil is one of the environments that could bid relatively large amount of metals form industrial wastes. The migration of heavy metals into soil depends on the solubility of the original metal bearing and the weathering products. For example, the wastewater discharged for electroplating processes and spent mining tails and also waste products accumulated in many lands can greatly influence the availability of these metals in soil. He et al., (2006) mentioned that the estimated amount of the leached ratio of the heavy metals from methanogenic landfill were only 0.13%, 1.8%, 0.15%, and 0.19% of Cu, Cd, Pb, and Zn, respectively. These values where obtained from two wells drilled in Hangzhou Tianziling landfill, in China. Flyhammar, (1995) found in his research that the leachate from methanogenic landfill in Sweden contains low concentrations of heavy metals.

Many papers have considered the recovery and removal of heavy metals from polluted soil environment. According to Hodson et al., (2000), decontamination techniques, such as solidification and stabilisation off-site, and soil washing are expensive and disruptive. Consequently, another site decontamination way called phytoextraction has been suggested by Cunningham and Berti, (2000) to translocated the lead soil to the top of the plant, and removed by plant harvesting. Hettiarachchi and Pierzynski, (2004) criticised this proposal in that low solubility of lead, Pb in soil may cause unavailable of Pb for plant uptake, poor Pb translocation from roots to tops, and the high degree of Pb toxicity to plant equipments and tissue, (Demirbas, 2004). Hettiarachchi and Pierzynski, (2004) found out that in situ (i.e. in the original place instead of being moved to another place) stabilisation seems to be the most economical and attractive alternative currently available for remediating Pb contaminated sites.

Hettiarachchi and Pierzynski, (2004) also presented a review of soil lead bioavailability, (i.e. the portion of Pb in a soil that is available for absorption into living organisms, such as the human blood stream), which depends on the solubility of lead solid phase and other site-specific soil chemistry. They suggested the use of the physiologically based extraction test, (PBET) known as vito, in order to predict the bioavailability of lead metal from a solid mixture. The test was performed earlier by Ruby et al., (1996). It included the influence of other parameters such as simulated gastric solution and appropriate temperature. The review also discussed several in situ stabilization methods of reducing Pb bioavailability, such as the process of adsorbing Pb onto surfaces of freshly precipitated manganese and iron oxides. McKenzie, (1980) emphasised on the fact that immobilisation of metals ions by synthetic manganese oxides and three synthetic iron oxides was due to strong specific adsorption. However, Hettiarachchi and Pierzynski, (2004) found out that this process could be irreversible and other kind of complex components could be formed and suggested the use of phosphor, P in situ stabilization of soil lead. The above techniques showed a good performance in terms of recovering lead from soil.

2.4.2. Recovery Metals from Water:

Water is one of the most important entities in the environment. Water has been used intensively during the industrial activities in order to improve the quality of products. Consequently, water pollution has been one of the biggest environmental issues and many researchers have concerned the removal of pollutants form this environment.

Heavy metals have been released into the environment in a number of different ways, such as coal combustion, automobile emissions, mining activities, sewage wastewaters, the utilization of fossil fuels, electroplating processes, etc, and metals like Pb, Cd, Cu, Hg, Cr, Ni, and Zn are found in large amounts in water. Therefore, Removal, separation and enrichment of heavy metal ions in aqueous solutions play an important role for the environmental remediation of wastewater.

Selective separation is one of the most widely hydrometallurgical techniques used in the recovery and removal of heavy metal ions from water. It has been given a huge interest and concern by many publications because it somehow covers both environment and economic requirements. In addition, it showed the economically feasible alternative method for removing trace metals from wastewater and water supplies, (Van Erkel, 1992).

Adsorption of heavy metals by different kind of agricultural materials, such as palm kernel husk, (Hawthorne-Costa et al., 1995), modified cellulosic material, (Elik et al., 2004), corn cobs, (Lalvani et al., 1997), residual lignin, (Gharaibeh et al., 1998), apple residues, (Raghuwanski et al., 2003), olive mill products, (Garcia et al., 2003), pine bark, (Consolin Filho et al., 1996), sawdust, (Zhang et al., 2000), etc., have been reported for the removal of toxic metals from aqueous solutions. Doyurum and Celik, (2006) investigated the removal of Pb and Cd from wastewater using olive cake as an adsorbent. They described an olive cake in which is a waste of olive factory and usually used heating, fertilizer and feeding material. According to Garciaetal., (2003), the structure of an olive cake contains organic compounds like lignocellulosic material, polyphenols, amino acid, protein, oil, and tannins.

Meena et al., (2005) stated that adsorption of heavy metals by these materials might be attributed to their protein, carbohydrates and phenolic compounds, which have metal binding functional groups, such as carbonyl, hydroxyl, sulphate, phosphate, and amino groups. Hence, Doyurum and Celik, (2006) emphasised on the fact that olive cake has an extensive surface area and may be used as an adsorbent material.

Meena et al., (2005) investigated the feasibility of using carbon aerogel as an adsorbent material. However, the carbon aerogel used in their experiment has been characterised for relevant parameters of initial concentration, adsorbent dose, contact time and pH before conducting the experiment. This technique might show lower percent removal of many metals, such as Cd and Ni, at high initial concentrations. Furthermore, the use of carbon aerogel could be relatively expensive.

Peternele et al., (1999) found that adsorption increases with increasing temperature when using Sugar cane bagasse, which was extracted sequentially with n-hexane, ethanol and water in a soxhlet system. Their work was based on a research conducted by Consolin Filho et al., (1996) who suggested the use of lignin from sugar cane bagasse as copper adsorbent with low cost. Peternele concluded that cheap materials such as lignin from sugar cane bagasse can be easily modified in order to obtain new materials able to adsorb heavy metals ions. However, the absorbent used in their experiment can adsorb Pb selectively rather than Cd under special conditions (pH 6.0, 30°C and ionic strength of 0.1 mol dm-3), when both ions are present in mixture.

Although adsorption process could perform high level of water cleaning with less cost, the process can not considered to be efficient in recovering high purity of an individual metal, which is an important aim of this project. Therefore, the extractions of metals by PEUF and electrochemical processes provide high purity metal recovery as previously discussed.

2.4.3. Metals from Industrial Products:

In General, all industrial products reach the end of their product life cycle for a number of reasons, such as market demands, technology innovation and development driving changes. Consequently, old products could be replaced by functionally richer technology. This might lead to accumulate the old products like batteries, computer hardware and electronics as wasted products. The above products contain relatively large amount of heavy metals that could be recovered

Huisman et al., (2004) stated that the end-of-life processing could serve several goals. These are: reduction of materials going to landfill in order to minimising landfill volumes, recycling of materials in order to keep maximum economical and environmental value and prevent new material extraction and reduction of emissions of environmentally relevant substances; including leaching from landfill sites and incineration slag, etc. Huisman et al., (2004) also reported that cellular phones could contents relatively high amount of precious metal, (between 320 ppm and 385 ppm of gold, 187 ppm to 222 ppm palladium and 500 ppm to 800 ppm of other metals). On the other hand, the precious metal content of the cordless phones varies from 8 ppm to 183 ppm for gold and 23 ppm to 135 ppm palladium.

Nogueira and Delmas, (1999) listed the main features of hydrometallurgical process in sulphate medium for spent Ni-Cd battery recycling. These are: mechanical processing by shredding and separation of battery fractions, leaching of electrode fractions with sulphuric acid as cheap reagent and less corrosive than other acids, cadmium, cobalt and nickel separation by solvent extraction, metals recovery from the separated streams by electrodeposition or crystallisation as sulphates and separation of case materials like nickel, iron and plastic.

The following subsections will mainly focus on the current technologies employed in the literature in order to recover heavy metals form certain end of life products that might contain various amount of metals used in the past life of these products.

2.4.3.1. Metals from Batteries:

Batteries are classified as primary and secondary. This classification depends on particular type of their application. Primary batteries were predominantly zinc-anode-based system. This kind has been developed to the original made (carbon-zinc) system. It has been widely spread and other metals have been added to form more efficient primary type, such as zinc oxide, alkaline manganese dioxide and silver oxide. Primary batteries are used extensively in a wide range of applications, such as road hazard lamps and intruder alarm circuits. The secondary type has higher gravimetric density than the primary type. This kind of batteries have been designed in order to provide the advantages of being recyclable, extended shelf life prior to use, remote activation, adjustment in load current with minimum voltage variation and operation under extreme temperature conditions, (Barr and Hestrin, 1999) and (Tzanetakis and Scott, 2004).

The recovery of heavy metals from batteries has been intensively discussed in the literature, (Linck, 1998). Nogueira and Delmas, (1999) proposed the method of solvent extraction for the recovery of cadmium, cobalt and nickel form batteries in pure and marketable forms from sulphate solutions. The paper also reviewed previous publications done by several authors in order to recover these metals from different type of batteries. Processes such as the SAB NIFE, (Hanewald, 1998), the SNAM, (Bartolozzi et al., 1995) and the INMETCO, (Lyman, 1994) are considered to be pyrometallurgical process. According to Nogueira and Margarido, (2004), these processes are in fact very efficient concerning the management of Ni-Cd residues since they are capable of solving the main related environmental problem (i.e. the release of the contained cadmium) but has less potential to recover cobalt and nickel. Furthermore, the above processes are strongly energy consuming and some emissions of dust and gases would be expected.

Alternatively, several hydrometallurgical processes have been developed for the treatment of spent primary and secondary batteries. Solvent extraction is therefore used as the most efficient separation technology for the treatment of wasted batteries, (Pletcher, 1982).

TNO is a primary process described by Van Erkel (1992) employed in order to remove Ni and Cd from Ni-Cd batteries using hydrochloric acid leaching followed by the separation of cadmium chloride complexes using tributyl phosphate, TBP as solvent. The process followed by recovering nickel and cadmium separately by electrowinning. Another approach proposed by Bartolozzi et al., (1995) in order to perform a complete hydrometallurgical process. He stated that electrode powder could be mechanically separated from the external case and the metal supports, and afterwards, the powder leached with acid and then electrolysed. The above approach showed good results in order to recover about 99% of cadmium. However, nickel recovered as NiCO3 was of the purity of 98% of the nickel, which is less than the purity expected. Di (2-ethylhexyl) Phosphoric Acid, (DEHPA) solvent was used by Lyman (1994) in order to investigate the recycling of nickel-metal hydride batteries.

Zhang et al. (2000) and (1998) also presented hydrometallurgy treatment alternatives for the recovery of metal values from lithium-ion and nickel-metal hydride spent batteries, which are used as electrical supplies for mobile phones. Hydrochloric acid, HCl, was found to be the most suitable solvent among the other three reagents used in their experiment. In addition, leaching extraction efficiency of around 99% of cobalt and lithium could be achieved when a solution of 4 M HCl was used at a temperature of 80°C and a reaction time of 1 h.

It can be seen that the process of heavy metals recovery from batteries involves several steps in order to achieve maximum purity of these metals for future reuse. The type of the solvent used also depends on many factors, such as temperature, pH, initial concentration, and type of the recovered metal. The recovery of heavy metals from batteries became very important issue since the target of high purity or economic improvement and environment care could be achieved.

2.4.3.2. Metals from Electronic Wastes:

Electronic waste or E-Waste is one of the fastest growing components of municipal trash. Therefore, many authors, organisations and individuals raise warning alarms concerning the fast grow of E-waste. According to the SVTC, consumer electronics in the United States already account for 70% of the heavy metals, including 40% of the lead, found in landfills, (Pletcher, 1982). The impact of this kind of wastes could bring an absolute negative outcome to the environment. Nevertheless, these wastes could be treated in order to recover some those metals for further use.

Yadong Li et al., (2006) pointed out the fact that E-waste already constitutes 1% of the municipal solid waste. Therefore, the recycling and reuse of the obsolete electronic products is strongly encouraged. However, only 9% is recovered for recycling and the vast majority ends up in landfills. According to Smith et al., (1996) the cathode ray tubes (CRTs) in computer monitors and TVs contain about 8% lead (Pb) by weight. Lee et al., (1998) and Lee and Hsi, (2002) concluded that this kind of products could also contain small amounts of other toxic substances, such as barium Ba, cadmium Cd, chromium Cr, copper Cu, and zinc Zn.

Personal computers (PCs) are the most significant component in E-waste stream. Printed wiring board (PWB) is the foundation both literally and figuratively for virtually all electronics in the world. It is the platform upon which electronic components such as integrated circuit chips and capacitors are mounted. The PWB, or printed circuit board (PCB) provides both the physical structure for mounting and holding electronic components as well as the electrical interconnection between components. A PWB consists of a non-conducting substrate (typically fibreglass with epoxy resin) upon which a conductive pattern or circuitry is formed. Copper is the most prevalent conductor, nickel, silver, tin, tin-lead, and gold may also be used as etch-resists or top-level metal.

Pyrometallurgical and hydrometallurgical processes have been employed and applied to achieve metal recovery from waste PWBs. Kinoshita et al., (2003) reviewed these processes and their efficiencies to recover metals from PWBs. He criticised methods proposed by Iji and Yokoyama, (1996a) and Iji and Yokoyama, (1996b) whom suggested carbonising or thermally decomposing of resin substrates and recovering the remaining metals by a physical method. Physical methods might be considered when treating large quantities with no concern of energy consuming.

Hydrometallurgical process has been widely applied in the field of metal recovery from industrial wastes, due to its advantages of being flexible, environment friendly and energy saving features (Alvarez-Ayuso et al., 2003). According to Kinoshita et al., (2003), hydrometallurgical processes have been efficient in terms of selectively leaching of nickel and copper from non-mounted printed wiring boards, along side recovering solid flakes of gold at high purity using nitric acid. In addition, the work achieved the optimum leaching performance, which was evaluated under varied experimental parameters of concentration, temperature and solid-liquid ratio.

Cathode ray tubes (CRTs) in television and computer monitors are one of the most common components of discarded electronics in the solid waste stream. CRTs in computer monitors and televisions may contain barium, cadmium, copper, lead, zinc, and several rare earth metals. Consequently, CRTs present a disposal problem because of their growing magnitude in municipal solid waste (MSW). Jang and Ownsend, (2003) published a paper concerning leaching tests using MSW leachates from lined landfills in Florida in order to examine lead leachability of PWBs from computers and colour CRTs from computer monitors and televisions. The work was based on previous attempt performed by Musson et al., (2000) who concluded that the cost of hazardous waste management is much greater than MSW management and recycling becomes a more cost-effective alternative.

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