Pyrometallurgical Recovery Of Metals From Electronic Waste Environmental Sciences Essay

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1st Jan 1970 Environmental Sciences Reference this

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Veldbuizen and Sipple (1994) acknowledged that materials entering into the reactor are immersed in a molten metal bath where the temperature is around 1250 degrees, which is churned by a mixture of supercharged air. The combustion of plastics and other inflammable constituents in the feeding process reduce the cost of energy. In the reactor, all impurities including iron, zinc and lead are converted into oxides and then converted into slags to silica by the agitated oxidation zone. These slags are cooled and milled to get more and more metals before the disposal (Cui & Zhang, 2008).

The diagrammatic depiction of the plant and the recycling process of E-waste are given below. These two diagrams will explain how discarded waste of PCs and laptops are recycled and disposed off. The copper matte containing precious metals is removed and transferred to the converters. After the progression in the converters, liquid blister copper is refined in anode furnaces and cast into anodes with almost absolute purity of 99.10 percent. The residual 0.9 percent holds the valuable metals such as gold, silver, platinum and palladium. There are also some other recoverable materials like nickel, selenium, tellurium etc. Afterwards, electro-refining of the anodes recovers these marketable metals (Cui & Zhang, 2008).

Fig-9 shows that E-waste can be fed into the process in different steps depending upon their purities. High copper containing scrap is fed into converting process directly but low grade E-waste is fed into Kaldo Furnace. The needed oxygen is supplied by the oxygen lance for the purpose of combustion along with oil oxygen burner. The off-gases require an additional combustion air of about 1200 degrees after combustion. Thermal energy is recovered by a steam network through standard gas handling system.

Fig: – 8 Pyrometallurgical Recover Process at Recycling Plant

(Source: Cui & Zhang, 2008)

Fig: – 9 Recovery of Precious Materials

(Source: Cui & hang, 2008)

The installation of off-gas emission control system in an IsaSmelt Furnace (a new method of E-waste recycling and metal recovery) is an example for recovering thermal energy by a steam network through standard gas handling system. Here the hygienic gases and process gases are cooled to recover the energy and are cleaned using techniques like bag house filters, electro filters etc.

2.12.3 Recovery of Precious Metals Using IsaSmelt Furnace:

The smelting (the process of melting to recover metal from its ore) process in the recycling of E-waste is done in IsaSmelt Furnace (Cui & Zhang, 2008). Like Pyrometallurgical Process one of the most important advantages of using this method is that the need for energy source and reducing agent for the smelting process will be quarterly substituted by the burning of the organic substances like plastics that are present in the E-waste (Hageluken, 2007). The smelting process separates precious metals in copper bullion from other metals that are present in a lead slag, which is further treated in a BMO. The recovered copper bullion is used in the next process of valuable metals recovery through copper-leaching (a method used for the recovery of copper from its ore) and electro-winning (method of removing impurities from the metals) processes.

There are three core processing steps in the BMO:

The Lead Blast Furnace: to decrease the oxidised lead slag from the IsaSmelt together with high lead containing third party raw materials.

The Lead Refinery: the mixed lead bullion, collecting most of the non-precious metals is further treated here and the process is called Harris process.

Special Metals Plant: pure metals are recovered in this plant.

(Cui & Zhang, 2008)

The picture of the emission control system is as under and could be adapted by the recycling centres to reduce the level of emission. Hageluken (2007), in his report accepted that the emissions from the plant are much below the limits set by the European agencies and government.

Fig: – 10 IsaSmelt Furnace Fitted With Emission Control System

(Source: Cui & Zhang, 2008)

Apart from recycling methods like Pyrometallurgical Process and method using IsaSmelt Furnace, bio-metallurgical process is another method of recovering precious metals from E-waste. Due to low cost and high specificity for the target elements, bio-metallurgical processing is attractive and presently limited to only rich countries (Cui & Zhang, 2008). Brand et al, (2001) displayed how Fungi (Aspergillus Niger, Penicillium Simplicissmum) and Thiobacillus bacteria can enable metal filtering from electronic scrap. To recover gold, copper etc from E-waste, Creamer et al (2006) engaged Desulfovibrio Desulfuricans.

2.13 Batteries: Recycling & Material Recovery:

Batteries are one of the end products of E-waste recycling because it remains intact during the E-waste recycling process (SWEEEP, 2010). Land filling or Incineration of these batteries can cause severe health and environmental hazards; when the casing of these land filled battery corrodes it can result in air, soil and water pollution because it contains toxic substances like lead, mercury, cadmium (human carcinogen: substance that causes cancer) etc (Frick & Knudsen, 2002). Similarly incineration of batteries will result in air pollution and other health hazards.

On the other hand proper treatment and recycling of these batteries will be beneficial both for the environment and for the economy because it contains valuable materials like steel, silver, nickel, zinc, manganese, gypsum etc which can be recovered and the harmful substances like lead, cadmium, mercury can be safely separated and can be reused as secondary raw material (G&P, 2010).

UK generates 20,000 to 30,000 tonnes of waste batteries every year and out of it less than 1,000 tonnes are recycled (Resource Management & Recovery, 2003). According to waste battery regulations in Europe (European Batteries Directive, 2006/ 66/EC), UK is needed to recycle 10% of portable batteries in 2010, but in UK, only 3% of portable batteries are currently being recycled.

2.13.1 Methods of Battery Recycling:

According to Espinosa, Bernardes & Tenorio (2004), it is imperative to know the composition of batteries in order to promote its recycling because the chemical compositions of different batteries are different, so same methods cannot be used for recycling different batteries. But unfortunately there is no relationship between the size or shape of batteries and their composition. There are principally three methods for the recycling of batteries:

Pyrometallurgy

Hydrometallurgy

Separation of components through unity operations of mining treatment

Out of these three methods the most commonly used method by all the recycling companies in UK and Europe are Pyrometallurgy and / or Robust Pyroprocess (another method) respectively because of their simplicity and high efficiency / capacity (even though the energy requirements for these processes are high) (Jan Tytgat, 2010). The Pyrometallurgy and Robust Pyroprocess methods of recycling will be discussed in detail in chapter four.

2.13.1 Hydrometallurgy Method of Battery Recycling:

In the past two decades, the most active research area on recovery of metals from battery is recovering precious metals by Hydrometallurgical process (Horn & Holt, 1990). In this method the unsorted batteries are fed directly to the furnace. Inside the furnace the organic components will get decomposed to form vapours (Sequeira, 1994). For example mercury compound will get decomposed to form mercury vapour. These vapours are then passed through a condenser to recover mercury and other condensable liquids.

The condensate will get fractionated by the centrifugation process into mercury, waste water etc. The waste water will then passed through an aluminium cementor to recover the residual mercury in the form of an aluminium-mercury alloy. The remaining waste from the cementor is send to an evaporator to generate some steam or water and a salt mixture (disposable) in order to prevent the excessive salt build up in the process. The residual gases and other organic vapours are oxidized in an afterburner and expelled to the atmosphere in the form of water vapour and carbon dioxide after a careful treatment to remove the remaining amount of mercury since this mercury may contain traces of hazardous substance like cadmium (Sequeira, 1994).

The remaining solids from the furnace are shredded and leached (acid or caustic leaches) before the magnetic separation of iron and nickel from other non magnetic solids. The leaching solutions which contain oxides of carbon, zinc, manganese etc are then subjected to separation and purification procedures such as precipitation of impurities, solvent extraction, ion-exchange etc. Consequently, the solutions are treated by electro refining process for metal recovery (Sequeira, 1994). It is estimated that for recovering a tonne of copper this process requires around 3,400KWh of energy (Liew, 2008).

Precipitation of impurities is nothing but the impurities present in the solution will become a precipitate (solid) during the chemical reaction, which can be removed later by filtering. Thus the remaining liquid (supernate) above the solid containing precious metals can be separated and precious metals can be recovered by electro refining. Electro refining is a method of purifying metal by electrolysis. Here the impure metal and cathode will be immersed in a solution (electrolyte) containing cations and electric current will be passed between the impure metal and cathode. As a result of it the pure metal will get deposited on the cathode and can be separated.

Solvent Extraction is the process of separating liquid mixtures by making use of solubility differences of the different components (Cox & Rydberg, 2004). Ion-Exchange is an electrochemical process in which an unwanted chemical component is removed from solution by replacing it with a more attractive one (Friedrich, 1995).

The main end products of this process are carbon and graphite which can be removed later by filtration. Compared to Pyrometallurgical process Hydrometallurgy is more exact and predictable but its operating cost is high and it is more complicated (Sequeira, 1994). This method is mainly used for the recycling of lithium-ion, nickel cadmium, zinc oxide and mercury oxide batteries.

Fig:-11 Hydrometallurgy Method of Battery Recycling

(Source: Sequeira, 1994)

2.13.2 Separation of Components through Unity Operations of Mining Treatment:

Separation of components through unity operations of mining treatment method is also used for battery recycling (e.g. nickel cadmium batteries). Using this method more than 2kg of nickel plates present in a nickel cadmium battery can be recovered. A compound with a high amount of cadmium can also be obtained and can be further treated (cadmium distillation) to recover the material. This method is not used frequently because of its high operating cost and complexity (Espinosa, Bernardes & Tenorio, 2004).

2.13.3 Recycling Processes under Pyrometallurgy & Hydrometallurgy:

There are several battery recycling processes that works by the Pyrometallurgy and Hydrometallurgy principles. Sometimes these processes are designed for specific kind of battery, but there are some in which batteries can be recycled together with other types of materials. The processes are as follows:

Sumitomo: It is a Japanese process which is completely based on calcinations (method of converting metals to its oxide at high temperature) at about 1000 degree centigrade in a furnace (Tedjar et al, 2010). Its cost is very high and it is used to recycle all types of portable batteries e.g. lithium-ion battery. The residues formed as a result of calcinations is crushed and screened. The residual powder will contain oxides of different metals. It is not suitable for recycling nickel cadmium batteries.

Fig:-12 Sumitomo Method of Battery Recycling

(Source: Sequeira, 1994)

Recytec: It is the Swiss process that combines pyrolysis (thermal treatment), gas treatment, shredding, washing, electrolysis (for non-ferrous substances) followed by magnetic separation and other physical treatments (Sequeira, 1994). It is used for recycling all types of portable batteries and also fluorescent lamps and mercury containing tubes.

Initially pyrolysis is done at 550 degree centigrade in a reducing atmosphere and the waste gases produced during this process are passed through a condenser for purification. The solids that left after the pyrolysis are shredded and washed with water to mobilise different salts and oxides. The oxides of manganese and zinc get dissolved in an acidic leaching procedure and these are simultaneously separated by an electrodeposition process. Ferro-magnetic materials are separated by magnetic separation process from other non magnetic substances like graphite. The materials separated by the magnetic separation process then enter into an electrochemical system and from here the materials are separated by anodic dissolution method. The method of anodic dissolution offers 99% purity of the metals recovered.

Fig:-13 Recytec Method of Battery Recycling

(Source: Sequeira, 1994)

This process does not recycle nickel cadmium batteries. The initial investment for this process is smaller than that for the Sumitomo process, but its operating cost is high (Espinosa, Bernardes & Tenorio, 2004). Another advantage of this process compared to Sumitomo is its excellent recycling efficiency of 95% i.e. only 5% of secondary waste.

TNO: It is a Hydrometallurgical Dutch process for the recycling of scrap batteries. This process developed two recycling alternatives one for alkaline household batteries and the other for nickel cadmium batteries. The alternative for household batteries was not commercially implemented (Espinosa, Bernardes & Tenorio, 2004).

In this process the waste batteries are first shredded into small (fine) fractions. Many metal and plastic parts can be retained from these fine fractions. These fine fractions are then subdivided into two fractions: magnetic and nonmagnetic. Then both these fractions will be leached with hydrochloric acid to dissolve the cadmium content in it. The magnetic particles like iron and nickel will be separated by magnetic separation process after the leaching process.

These separated iron and nickel particles will contain cadmium and this cadmium can be removed by extraction process with the help of TBP. The cadmium salt from the extract will be further removed by acid extraction. The acidity of the so formed cadmium chloride is then adjusted to precipitate residual iron as ferric hydroxide and it is separated by the filtration process. By using the electrolysis process metallic cadmium will be recovered and the remaining solution is discarded (Sequeira, 1994).

Accurec: It is a German Pyrometallurgical process to recycle batteries mainly used for nickel cadmium batteries. This process has got several advantages like: it is just a one step process compared to other processes, the emission of green house gases to the atmosphere is less (>0.01g/h), it is energy efficient and a highly secured process (Accurec, 2010).

Fig:-14 Accurec Method of Battery Recycling

(Source: Accurec, 2010)

Snam-Savam: French process for nickel cadmium battery recycling, totally based on Pyrometallurgy method (Espinosa, Bernardes & Tenorio, 2004). This is a closed furnace battery recycling technique in which cadmium is distilled at 850-900 degree centigrade. This method offers 99.9% purity of the recovered materials (Sequeira & Moffat, 1997).

Sab Nife: Swedish process for nickel cadmium batteries. This method is also based on Pyrometallurgy. In this method the cadmium is distilled at high temperature (850-900 degree centigrade) in a reducing atmosphere. Then the cadmium is chemically leached with the help of sulphuric acid and finally cadmium is recovered from the leachate by electrolysis (Sequeira & Moffat, 1997).

Atech: This process is based on the physical treatment of discarded batteries; it is having comparatively lower cost than the other types of recycling processes but the purity of the recovered materials will not be high. It is used for recycling all types of portable batteries.

Recycling Companies does not rely on any single method or process but it uses the combinations of many methods and processes for the recycling of various types of batteries, as it is evident that there are many types of batteries having different shapes, size and chemical composition. So, the company uses the method and process which are suitable for the particular set of batteries.

Table — 11: Valuable Materials that can be recovered by Battery Recycling:

Name of the Battery

Materials that can be Recovered

Lead Acid

Lead, Polypropylene, Gypsum

Zinc Based

Steel, Zinc, Manganese

Nickel Cadmium

Nickel, Steel, Cadmium

Lithium-Ion

Cobalt, Steel

Silver Oxide

Silver, Steel

Mercuric Oxide

Mercury, Steel

(Source: G&P, 2010)

These recovered metals and materials has got many applications like: lead and cadmium can be used for making batteries again, steel and nickel can be used in the steel industry, polypropylene can be used for making battery cases, gypsum can be used for agricultural purposes, cobalt and silver can be used in electronic and photographic industries etc.

Overall E-waste is very unsafe for the environment and for the human health. On the other hand if proper recycling centres are established, the economy as a whole would be benefited and there would be fewer burdens on natural resources like gold, copper, aluminium, silver and other precious metals and materials. In this way the reuse rate would be increased and less emission of green house gases would occur. It would be beneficial for the environment, human health and national economy of the UK to establish proper and dedicated recycling centres to improve the condition. UK especially needs it, as it has the highest number of computer users. As per estimated data there are at present 360 million computer users in UK (Internet World Statistics, 2009). One can easily make estimation of the volume of E-waste, keeping in mind the maximum life of computers and replacement in UK.

2.14 Conclusion:

From the review of the available literature, it can be argued that E-waste is seriously very dangerous for the environment and for the human health as well. The whole world needs to take proper initiative for handling E-waste and there should be dedicated recycling centre for it. Although, there are initiatives that have been taken but those are not enough as the quantity of E-waste is growing day by day. The governments across Europe and the government of UK have enacted laws and directives for mitigating the ill-impact of E-waste. After the earth summit in 1992, two regulations have been enforced to mitigate the ill-effect of E-waste i.e. the Swiss ORDE regulation and EU WEEE regulation. Under both the regulations, the list of electronic items is by and large same and contains almost the same list.

The growing market for PCs and its penetration and replacement markets in developed countries like UK and high obsolescence rate have made WEEE as one of the fastest growing waste streams. Puckett & Smith (2002) roughly estimate the chemical and non-chemical substance present in E-waste and it is really very shocking as the quantity projected could be extremely harmful for the environment and human health. Precisely, it could be health hazard as the chemicals present in E-waste are lead, cadmium, mercury, plastics etc (Culver, 2005). If these chemical substances and metals are burnt, it will have severe impact on the environment and on the human health.

Widmer et al, (2005) say that these E-waste contains highly toxic chemicals and the paradox is that the UK and EU is bound to dispose E-waste within their own geographical area as they are obliged to follow the directives because the directives of the Basel Norms prohibits them to trade with Non-OECD countries.

The UK government is very serious for the environmental protection and it explores what actions might be taken and poses questions, on which comments and suggestions are invited from a range of players including the general public. Phillips et al (1998) argues that there is an urgent need for the effective protection of the environment and prudent use of natural resources, so that the sustainable development programme could not be hampered. The management of E-waste is acknowledged as essential to the sustainable development in UK.

Earlier, the E-waste has used to be disposed through land fill and incineration. Landfill is / was not an effective way to dispose E-waste as it is not bio-degradable and used to harm the fertility of the land. Incineration is also very dangerous as it can emit a lot of harmful gases and substances. Spalvins et al (2008) and Dagan et al (2007) argued against the disposal of E-waste along with MSW as the toxic characteristics could not be mitigated through landfill and could challenge regulatory compliance.

To overcome this serious threat government has introduced the concept of EPR both legally and operationally. It involves transaction cost, collection cost, recycling cost and other types of costs. The manufacturers also have to negotiate with recycling centres and to identify the collection centres so that E-waste could be collected efficiently and economically. The manufacturers are now bound to make such electronic products so that at least 70% to 80% (by weight) of the materials could be collected from recycling and the reuse rate of materials must not less than 50% by weight so that natural resources could be protected.

There are currently various models have been adopted for the management of E-waste. Among that the four phase model is very effective. Another extension of the EPR is the ARF. It is collected at the time of purchase of electrical equipments and also from the end users, so that the recycling process could be financed once the product is discarded by the end users.

The discarded E-waste especially the personal computers scrap are valuable in the sense that it carries metals about 70% of the weight of computers and are recyclable. This is the major driving force behind the recycling of E-waste as every company want to minimise the input cost and maximise the profitability. Earlier Pyrometallurgy is used for the recovery of valuables metals from the scrap E-waste. However there are some weaknesses in this method. It was not very friendly for the environment as there is more emission of green house gases. Cui & Zhang (2008) argue that the retrieval of energy from E-waste pave the way for using plastics in E-waste. Now it has been evident that thermal processing of E-waste delivers an approach for recovery of energy from E-waste if a wide ranging emission control system is installed.

The batteries that are the end products of E-waste recycling can be further recycled to recover and separate precious materials like steel, silver, nickel etc and toxic substances like lead, cadmium etc respectively. The recovery and separation of these metals and materials will further contribute to the economy of the country and sustainability of the environment. There are several methods and processes for battery recycling and no same method or processes can be used for different batteries since they differ in their chemical composition. The process of Pyrometallurgy and Robust Pyroprocess are preferably used by the recycling companies in Europe.

From the available literature it can be argued that most of the above mentioned recycling process / methods offer almost 90% to 95% purity of the recovered metals and materials which is an advantage. On the other hand these methods have got many disadvantages. Most of these processes / methods that works under the principle of Hydrometallurgy are complicated as it needs to undergo many other sub-processes in between (except few processes like Accurec), e.g. it needs to undergo shredding process before the real recycling. Hydrometallurgy processes are most of the time dedicated to only a particular type of battery chemistry / small range of different chemistries (Jan Tytgat, 2010).

Higher energy requirement is another disadvantage of these methods (mainly for processes that works under Pyrometallurgy principle). For example most the methods required energy of almost 1000 degree centigrade or more. As result of it the operating cost is high. Discarding the end product which contains valuable metals (which can still be recovered) is another disadvantage. Discarding of the remaining solution after the electrolysis of cadmium in the TNO process is an example for this. As mentioned above the loss of remaining 5% to 10% purity of recovered materials is also an important matter that needs to be considered

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