Apurva Patel, Prof. Nimish Shah
Abstract: Copper is one of the most used metals in recent developments and demand of this red metal is increasing with passing of each day. Production of copper is 12 million tons per year and copper reserves are expected to run for 25 years with the estimated world copper reserves of 300 million tons. Recovery of copper from metallurgical waste is a trend that is being followed from beginning of industrial age and has many developments over a large time frame. Out of all the copper used in existing process, 2 million tons of copper is utilized with recycling of copper waste. India has limited copper ore reserve contributing about 2 percent of world reserves. We can say that copper has a large amount at our reserves but excavation is not as simple as it seems. Copper content in the raw mines is ranging from 0.5 to 1 percent. Even after recovery of copper there is large waste generated at the end of the process. Copper content in the waste is up to 0.3 percent at the discharge. Ultimately around hundred times of waste is generated for recovery of one part of copper. That pushes forward the need of recycling copper from metallurgical waste to cater the need of increasing copper demand. Copper recovery from high copper containing metallurgical wastes like brass industries are generally dealt with smelting process. In such case large amount of energy is utilized to just melt down all the material. This process has a limitation of copper content i.e. if copper content is low then all the energy is utilized in melting of undesired material. Demand for electroplating of copper has increased significantly. Low efficiency or improper process handling causes remarkably high copper content in waste discharge, which is over the range of discharge criteria of heavy metals. So to control the increasing price of metals and to limit the use of fresh copper, recycling must be done so the recovery from waste also gives the advantage of being in range of the allowable government legislations. Though these hazardous heavy metals in electroplating waste having concentration high enough to give harmful impacts to environment but convincingly low concentration that is not enough to recover these metals effectively. In this paper, an overview of different methods for copper recovery is illustrated and justified the selection of different methods over different copper content of various sources.
Keywords— Copper extraction, Copper recovery, Electroplating, Recycling,
Increasing demand of copper gives elevated chances for generating copper waste from different industries. There are thousands number of industries existing which includes utilization or processing of copper. In this paper, review of several most copper containing waste and most optimum copper recovery methods are described. Waste source is targeted which gives better possibilities of copper recovery and ease of operation. Several metallurgical source like; bronze scrap, copper converter slag, electroplating waste, and pickling solution is included in the study.
II. Different copper source and recovery methods
- Recovery from copper slag
There are different verities of slag produced from smelters for non-ferrous production. Major emphasis is given to copper slag as it has equal to or higher copper content compared to raw copper ore. Generation and utilization of copper slag has higher environment impacts compared to steel and iron slag as they contain remarkable quantity of heavy metals with higher solubility. Chemical composition of copper slag varies with different origins. Chemical composition is given as per work of Shen & Forssberg in table 1.
Chemical composition of copper slag
There are specific three methods to recover copper from copper slag; Floatation, Leaching and Roasting
Floatation: Barnes has given industrial floatation process at Mount Isa Mines Limited to recover copper from copper slag. Grinding operation is applied until obtain the granular size of 80%- 74 µm before floatation. Floatation is also feasible for magnetite present in the raw copper slag, so hydroxy ethyl cellulose is used in the process as a depressant of magnetite impurities. MIBC is used in the process as froather agent and sodium sec-butyl xanthate is used as a collector of copper from the waste. The result of this process gives concentrate grade copper with high percentage as 42.54%. Overall yield of such process is 82%. This experiment is observed for copper slag containing 3.7% copper. In this experiment most of Co is observed with floatation tail.
Mainly, copper slag floatation is somewhat similar with sulfide ore floatation because of the fact that only metallic copper and sulfide minerals from the copper slag can only be effectively floated. In other slag copper is usually observed under oxide state and Co and Ni are also in oxide state because of its homogeneous distribution in the slag. So the stated method will not be utilized effectively with Co, Ni and oxide copper state. Therefore the span for the floatation process is reduced in size as less quantity of Co, Ni must be present in the slag or copper must not be in the form of oxide.
Leaching: Basire and Anand described leaching thoroughly about its use and study over some leachants mainly hydrochloric acid, ferric chloride, ammonia, and sulfuric acid. In the initial era cyanide was also used but it was terminated because of its harmful effects to environment. Leaching is positively influenced by addition of H2O2, or leaching with Cl2/Cl– system, or pressure leaching. Figure 1 shows effect of H2O2 on leaching of copper based on the experiments of Base metal recovery.
Figure 1. Kinetics of copper recovery
Graph showing metal recovery with highly oxidising agent like H2SO4 for copper slag. Experiment is carried out with 10% solid in solution and particle size less than 100 µm. Experiment is carried out at 70 oC and pH maintained at 2.5 with applied H2O2 at 35 L/(h.t)
Pressure leaching has broadly described by Anand, shows that with pressure leaching and use of dilute H2SO4 recovery of copper about 90% achieved from copper converter slag from the initial concentration of 4.03% Cu.
Roasting: Roasting is actually one intermediate step which involves the process for converting the copper in desired form that can be easily separated from the raw material. After application of roasting, Leaching or floatation must be used to achieve desired separation. If we narrow down the process criteria then we can say a lot more specific term as sulfate roasting instead of roasting. In this process conversion of cupper cobalt ant nickel is taking place and transformed into more feasible soluble sulfates. Raw material is processed at 200-600 oC by addition of sulfide or sulfate agents. Then these soluble sulfates are dissolved in water and easily separated from slag. Some of the agents used in the process are, (NH3)SO4, H2SO4, H2S, pyrite etc. Sulfurization reactions are summarized as bellow.
Cu2O + H2S ïƒ³ Cu2S + H2O …(1)
2Cu + H2S + ½ O2 ïƒ³ Cu2S + H2O …(2)
Sulfides of copper are then easily converted to soluble copper sulfate with roasting at 600oC. Ziyadanogullari used this method to treat copper slag containing 2.4% copper. By sulfurization in closed system with 140oC for 1 hour and then heating and roasting with 600oC for 360 minutes gives better result for recovery of copper up to 99.2%.
- Copper recovery from bronze scrap
Bronze is the mixture of copper, lead and tin. Vast numbers of studies are going on as well as succeeded for recovering copper from brass scrap. Ludovicus produced black copper by reducing scrap at 1300oC after melting at 1180oC. After recovery of black copper oxygen gas is supplied in ample amount with a view to oxidize all metal impurities and obtained anode grade copper. Morsi and Rabah have studied different parameters for recovery of copper by melting the bronze scrap. They obtained bronze scrap which comprise of chips, turnings and pieces ranging from 1-5 mm in size from El-Maady Co. for Engineering Industries, Cairo, Egypt. Experimental work uses a computerized heating mechanism with silicon carbide-heated muffle furnace. With operating temperature of 1600oC slag generated contains tin and lead that is continuously skimmed off with a view to eliminate the possibility of joining the slag to molten copper. Slag generated from the process also has small copper content which is recovered by acid leaching to increase overall yield.
Experiment is carried out by Morasi and Rabah at different temperatures and copper content with overall yield is measured at 15 and 30 minutes.
Figure 2 Effect of temperature on obtained copper alloy
Figure 2 enlightens the behavior of copper residue and composition of the same. Experiment is carried out at 1150oC, 1200oC, 1250oC and 1300oC and at time interval of 15 and 30 minutes. As we can see with increasing time recovery is increased but behavior of recovery is same. Overall yield is decreased with increasing time because more amount of metal is being dragged with slag formation. Removal of lead and tin from slag is favored by addition of copper sulfide with provision of air which convert impurities into sulfides and pushes forward the percentage amount of impurities into slag and increased recovery of copper in residue. Addition of 10-20 percent copper sulfide reduces up to 10 percent lead and 8 percent tin in copper residue. Recovery of copper is 96.2% with combination of acid leaching from the generated slag.
- Recovery of copper from pickling solution.
Like every common metal, brass also has a tendency to corrode. Corrosion of brass takes place by contacting the outer surface to air. Corrosion of brass looks like black coating on the outer surface and it is called tarnish. This coating in non beneficial in any condition so it must be removed. Removal of corrosion includes dipping brass metal in dilute sulfuric acid solution, which is used for a long time before it finds its way to discharge. This utilization over a long period of time enriches the dilute solution with valuable metals such as copper, zinc, chromium etc. This heavy metal enriched mild acid solution is called waste pickle solution. Because of its environmental hazards it cannot be disposed off without removing toxicity. Recovery of copper is a coincidence that will make disposal waste under legislative limits and will provide economical benefit by recovering valuable metals.
Pickling solution has a base of mild sulfuric acid and has the highest percentage around 45.1 g/L of H2SO4 in the same. It has around 25 g/L Zn and 35 g/L Cu(II), and other negligible impurities like chromium, iron and nickel. First using equal volume of TEHA (tri (2-ethylhexyl) amine) extractor acid is being removed with formation of immiscible layer over organic layer and can be easily removed from the mixture. Copper and zinc may be present in the sulfate form as no part of any metal is observed in the extracted acid and TEHA has less affinity towards sulfates of copper and zinc. This acid free pickle liquor is used with other extractors like Versatic 10 acid and Cyanex 272[bis-(2,4,4-trimethylpentyl)-phosphinic acid] for recovery of copper and zinc. Kerosene is used as diluents in the solvent extraction. pH of Versatic 10 acid and Cyanex 272 is having higher influence on extraction. Increasing Ph resulting in increment of metal extraction and at 5 pH and 30% Versatic acid concentration all the copper is extracted. Whereas, zinc extraction is observed above pH 5 and is completely extracted in organic phase at pH 7.0.
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III. Copper recovery with electroplating
With a specific type of waste that has copper in isolated pure form this technique can be used. A new idea is generated for separation of copper from waste like alloy waste brass industries waste, electrical waste etc. Tank is filled with electrolyte like zonax, copper sulfate etc. The idea is to provide continuous separation from raw material and to facilitate collection of nearly 100 percent pure copper. New type of vessel needs to be developed which has stainless steel base that can be acting like cathode and a receiver anode needs to be in center of tank. Outside walls must be made up of insulating material or at-least needs to have insulation over the wall to isolate human contact by accident during the process.
Figure 3 electroplating for copper recovery
Figure is showing the possible assembly of electroplating mechanism for semi continuous copper recovery. Copper in the raw material comes with contact of electric field applied across the length of the assembly. Copper in the electrolytic solution first separated and settled on anode. By this, copper-ion deficiency is generated in the solution. To mitigate the deficiency, copper from the raw material comes in to electrolytic solution and completes the chain reaction.
Pure copper sulfate and zonax solutions are not conductors of electricity. For ease of operation pure water needs to be added to convert ions from the solution. Because of water addition now electrolytic solution has H+ ions and by getting enough electricity they will convert into hydrogen gas and applied electricity cannot be used efficiently. This apparatus can be operated within range of 2-12V. Above this range H2 will start to consume additional energy. Higher the applied amperes better the rate of deposition of copper from the raw material. Rate of deposition can be calculated by equation given by Michal faraday, i.e. W=(I.T.A)/(Z.F) where, W is the weight of copper deposited, I is the amount of current applied (amp), T is the amount of time for which current supplied, A is the atomic weight of substance, Z is vacancy and F is faraday constant 96,500 coulombs.
For selection of electrolyte free electrons play vital role. General electrolyte as copper sulfate has higher efficiency for electroplating then zonax, but zonax is mono-valance electrolyte and less electricity is utilized for almost double copper extraction compared to copper sulfate.
For efficient recovery of copper from various source first type of source and form in which copper is present must be determined. Floatation is not the most accurate and efficient method for separation of copper but it is widely used for primary separation of copper and for concentrating the raw copper for other applicable process like smelting. Leaching of copper is the most widely used and efficient copper removal process but it must be followed by electroplating of iron addition process to obtain pure copper. Some parameters like pH and temperature of leachant must be observed and needs to be carefully maintained. Roasting is also a two step process in which copper is converted to more feasible sulfate form and can be easily extracted by leaching. Bronze scrap has more copper content in the structure and needs to be carefully processed to obtain large number of copper content recovery up to 96% and other recovery by leaching from slag generated can increase the overall yield. Another phenomenon has been studied for pickling solution which has serious pollution problem at the disposal and economic problem at treatment. For treatment acid must be removed in the pretreatment and then copper along with valuable metals can be efficiently recovered with Versatic 10 acid. In the end new method has been observed for semi continuous operation for copper recovery using specific electrolyte and modified operation tank.
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