Electrometals Technologies Limited A C N 000 751 093 28 Commercial Drive Ashmore Queensland Australia 4214 Telephone: 61 7 5526 4663 Facsimile: 61 7 5527 0299 Email Address: EMEW@electrometals.com.au EMEW ELECTROWINNING Treatment of copper sulphate pickling and etchant solutions P.A.Treasure May 2005 1
SUMMARY Copper bearing waste solutions are produced by many companies and industries around the world. As well as representing a lost resource in the value of the contained copper, the costs of disposal of these solutions is growing to a point where on site treatment is becoming essential. The composition of these waste solutions varies widely and the method for treatment varies accordingly. The purpose of this paper is to present the case for the EMEW electrowinning technology for treatment of industrial sulphate based waste streams. Sources of such solutions include, but are not limited to, the following: Copper Rod and Wire manufacture Copper parts manufacture Copper plating shops (sulphate or cyanide) Computer chip manufacture The EMEW technology, developed by Electrometals Technologies Limited of Australia, offers (through its high efficiency and simplicity) this market a new generation of electrowinning technology which is simple and affordable. There are a variety of techniques currently used to dispose of or treat these solutions, including: 1. Chemical precipitation and dumping 2. Ion exchange (IX) 3. Production of secondary copper chemicals (eg copper sulphate) 4. Small proprietary electrowinning cells Electrolytic recovery of copper from these solutions has distinct benefits over the other methods: a) The treatment circuit is simple and easier to manage b) The result is a usable or saleable product c) The process returns to the circuit the acid that is initially required. 1. If a waste generator is producing over 20 kg s per day of copper in a pickling or etchant solution, the costs of disposal, cost of acid and loss of value in contained copper suggest that an on site treatment facility is becoming essential. 2. It is clear that the only technique which allows simplicity and direct return of value for the pickling stream treatment is electrowinning. And further that conventional open tank electrowinning can present some difficulty in maintaining copper at a low concentration. 3. In general terms the solution composition and copper concentration in these solutions are outside of the general capabilities of a conventional tank electrowinning cell. The conventional design utilised in, for example, the mining industry is incapable of sustaining high efficiency and good product quality in the range of composition required in these applications. 4. For electrowinning to be efficient in this setting, modifications to the design of cell are required, in order to dramatically improve the rate of supply of metal ions to the cathode. There are some cell designs on the market which achieve this aim, through relatively complex mechanical or cathode design means. They are, however, costly and would appear suited only to relatively small applications. The EMEW technology is shown through this paper to offer significant operating and capital cost savings, both over other electrowinning cells on the market and the alternate treatment methods listed above. It represents a new generation in electrowinning which offers the benefits of simple single step recovery of copper from waste solutions at an affordable price. 2
1.0 INTRODUCTION The EMEW technology has been employed over a number of years for recovering copper from a variety of sulphate based solutions, and is commercially applied in the following environments: 1. Recovery of copper from refinery waste solutions 2. Primary copper recovery from copper oxide ore leaching 3. Stripping of copper from pickling/etching solutions The process requirements and financial drivers behind each of these applications are significantly different. A detailed description of the benefits that the EMEW technology brings to each is too extensive to cover in a single paper. The purpose of this document is to present the case for EMEW for treatment of industrial sulphate based waste streams. Sources of such solutions include, but are not limited to, the following: Copper Rod and Wire manufacture Copper parts manufacture Copper plating shops (sulphate or cyanide) Computer chip manufacture In these applications the waste copper bearing solution is generated from washing of copper parts prior to fabrication into components, or from etching of copper sheets to provide electronic pathways on computer chips and board. Basically, sulphuric acid is used to remove oxide layers from pure copper (pickling), or to purposefully dissolve the metal (etching), thus creating a waste copper sulphate solution. There are a variety of techniques currently used to dispose of or treat these solutions, including: 1. Chemical precipitation and dumping 2. Ion exchange (IX) 3. Production of secondary copper chemicals (eg copper sulphate) 4. Small proprietary electrowinning cells Each of the first three requires more than one step in the processing chain and, generally, produces an intermediate product of reduced value. They can be relatively costly systems, which require careful supervision to achieve the required results. Electrowinning results in the recovery of the copper in a solid metallic form through the process of electrolysis. The product is of high quality and is either reusable by the generator of the waste or can be sold at market value. As a one step process it is less complex (and therefore less costly) than the other techniques and, through continuous removal of copper from a bath or etching solution, results in significant extension of the effective life of the inventory of solution. In addition a key element of the process it results in regeneration of the acid required for the pickling and/or etching steps. Electrolytic recovery of copper from these solutions therefore has a number of benefits: a) The treatment circuit is simple and easier to manage b) The result is a usable or saleable product c) The process returns to the circuit the acid that is initially required. 3
From a process point of view, therefore, electrolysis is the most attractive means of treating or maintaining and inventory of pickling or etching solutions. Conventionally available electrowinning systems are, however, either o o o Limited in their process capabilities (can not remove copper efficiently down to the levels in solution required), or Relatively costly, as complexity is added to the circuit or mechanism to achieve low copper levels in solution Typically a conventional electrowinning cell will struggle to maintain a low copper concentration. In comparison EMEW electrowinning can economically achieve constant bath conditions below 5 g/l of copper whilst maintaining very high cathode purity, in the case of copper pickling solutions this purity is greater than 99.99%. There are some electrowinning systems available on the market, and a number of others have been introduced but not widely accepted. Those that remain in use utilise relatively complex methods to increase the efficiency of electrowinning either specially manufactured electrodes (for example the Retec Cell) or mechanical assistance (Chemelec Cell). These requirements lead to comparatively high capital and operating cost per unit of copper recovered. Electrometals Technologies Limited has developed an electrowinning cell which has dramatically improved the efficiency of the electrowinning process, through a relatively simple modification of the hardware in which the process is performed. The high performance of the EMEW technology is simply achieved through a hydraulic mechanism which presents metal ions to a cathode in a far more effective and direct manner than in a conventional unit. The EMEW cell is constructed from a pair of concentric tubular, rather than planar, electrodes. The ends of the assembly are fitted with plastic end caps, thus forming a closed chamber through which target solution is be pumped at high rate. The resulting high flow and efficient mixing results in forced and continual supply of metal ions to the surface of the cathode which increases dramatically the efficiency of the metal electrolysis. The technology has been demonstrated and proven in numerous process environments. Its growing place in its target markets is assured by a combination of physical, process and economic factors. Its particular benefits in treatment of sulphate based copper waste streams include: Lower capital cost per unit of metal recovered Low operating costs Highly versatile in process performance Complete capture and control of hazardous solutions and gases 4
2.0 REVIEW OF AVAILABLE TECHNOLOGIES/METHODS There are a variety of techniques currently in use for disposal and/or treatment of copper bearing waste solutions. They commonly involve one, or a combination of, the following methodologies: 1. Chemical precipitation and dumping 2. Ion exchange (IX) 3. Occasionally, solvent extraction 4. Production of secondary copper chemicals (eg copper sulphate) 5. Electrowinning The drivers behind the choice of system are naturally: Economic cost of establishment of facilities and potential recovery of value. Environmental removal of contaminants from the circuit and responsible final disposal. Scale of operation sustainability of capital and operating costs of treatment system The advantages/disadvantages of each system can be summarised as follows: 2.1 Chemical precipitation and/or dumping There are waste treatment companies around the world who will take responsibility for removal of waste solutions, treatment and disposal of the precipitated products in hazardous waste landfills. Although this takes the problem off the hands of the waste generator, costs are high: All acid in the waste solution is lost and must be replaced Fees charged by waste contractors are high There is no recovery of valuable metal from the waste solution In cases where the volume of waste generated is relatively low, some manufacturers have installed simple neutralisation plants at their site. This can achieve a saving in removal costs (through reduction in volume of the waste generated), but it requires a number of steps, specialised equipment, purchase of reagents and a degree of process control, potentially beyond the capabilities of a small operator. It is noted that one such operator in Australia made a switch to EMEW copper electrowinning approximately four years ago. Since that time he has not once had to replace his pickling solution inventory. 2.2 Ion Exchange Ion exchange is a good technique for removal of low concentrations of metal ions from solution and resins selective for copper have been on the market for many years. Recovery of copper ions from the solution results in regeneration of acid in the pickling solution Ion exchange resins operate by exchanging a H+ ion for a cation in the waste stream, or in the case of anion resins, an OH- ion for an anion in the waste stream. When most sites have exchanged their base ion, the resin must be regenerated. During the regeneration phase, an acid is passed through the cation resin (a base is passed through the anion resin) and cations previously removed are exchanged for the base H+ ion. Metal ions present in the regenerate (in concentrations of a few grams per litre) are commonly removed using electrowinning, or the regenerant may be sent to a conventional precipitation treatment system. Ion exchange is a versatile technology, but is relatively high cost due to: 5
Ion exchange resins are expensive and limited in their metal loading capability. The recovery and regeneration process can be quite complex and require automation. Additional capital costs are incurred if a metal recovery system is used as a second step If metal recovery is not implemented, operating costs in acid, metal loss and disposal are high Ion exchange systems require a reasonably sophisticated level of technical supervision. 2.3 Solvent extraction Solvent extraction is a reasonably well developed methodology for removal/recovery of a single metal from solution and is widely used in the mining and resource industry. Again, it has drawbacks in the fact that expensive reagents are required, capital cost is relatively high and a secondary system needs to be added to achieve recovery of the contained copper. 2.4 Production of secondary copper chemicals A number of the larger generators of acidic copper waste solutions utilise crystallisation to produce a copper sulphate material for sale on the open market. It is unlikely that full recovery of value is achieved for the acid and copper contained, but the result from a relatively clean copper sulphate waste solution is complete disposal of both the copper and acid, in saleable form. Capital cost of a crystalliser and associated equipment is relatively high and most likely can only be justified in a large stream environment (for example a copper rod and wire manufacturer). In addition, the market for copper sulphate can be fickle and dumping of secondary material often results in marked reduction of value of the product for extended periods of time. Technical and control requirements in a sulphate plant are also relatively high. 2.5 Electrowinning In a situation where the target waste solution is relatively clean (no major build up of contaminants) the most simple and effective technique for removal of the copper in a directly saleable form is through electrowinning. The process entails passing the target solution between two electrodes (positive and negative) and attraction of the target metal, by electrolysis, to the cathode where it plates as a solid metal product. The advantages of this method include: 1. A single step process which does not use any reagents 2. Direct recovery of a saleable or reusable commodity 3. Direct regeneration of the acid in the pickling solution 4. High level of technical expertise is not required The most effective use of electrolysis in a pickling circuit is to install the facility in-line, with the size of the electrowinning plant being selected to match the rate of uptake of copper into solution during pickling. Copper concentration can be kept at a constant level in the recycling solution and, importantly, acid (the reagent that actually performs the pickling process) is continually regenerated. There are only a small number of proprietary electrowinning cells on the market and most are suited only to small applications, due to prohibitively high cost in larger installations. The EMEW technology brings a more versatile electrowinning cell which is applicable at all scales of operation from a few kilograms to in excess of 2 tonnes of copper per day in the target solution. 6
3.0 ELECTROWINNING AND EMEW IN THIS INDUSTRY As noted above, the most appropriate position for a copper electrowinning circuit in a recycled pickling, or etching, solution is in-line in the solution recycle loop. The electrowinning plant can be sized to recover copper at the rate at which it is accumulated by the washing or etching process and thereby maintain constant conditions (copper and acid) in the recycle circuit. In order to maintain the highest efficiency possible in the pickling or etching process, acid concentration needs to be maintained at a maximum practical level which, in turn, requires copper to be maintained at a minimum. The size, cost and efficiency of an electrowinning circuit is dependant on a number of variables, including: 1. Composition of the target solution 2. Concentration of copper in solution 3. Concentration at which the operator wishes to maintain copper (and acid) 4. The design of the electrolysis cell 5. The efficiency of supply of metal ions to a cathode (mass transport) Conventional electrowinning cells generally comprise an open top rectangular tank, containing serially arranged flat cathodes and anodes. The target solution is pumped in one end of the tank and exits the other, whilst power is applied to the electrodes. These systems are reasonably simple in design, but have severe limitations in their performance capability, notably in their inability to economically maintain copper at low concentrations. During electrowinning, a zone of depletion in metals ions builds up against the surface of the cathode (the ions that were there have migrated to the cathode), which hampers the ongoing process of electrolysis. In a tank cell the result is that there is a limit to the current that can be applied and only relatively high concentration solutions can be treated. Productivity of the cell is limited and copper concentration can not be maintained at low levels. There are some electrowinning cells on the market which have overcome this phenomenon through: a) Increasing the cathode area in the cell, and thus capital cost. b) Providing mechanical methods of agitating the solution in the cells, thus increasing risk c) Continually moving (rotating the cathode), thus increasing complexity Therefore, although these methods have proven effective in allowing stripping (and maintaining) copper at low levels, the mechanisms and designs which have been used have resulted in cells of complex design - which can only be viably applied at relatively small scale. Their cost is prohibitive in medium to large scale operations. The EMEW Technology overcomes the inherent problems and inefficiencies in electrowinning in a very simple manner without moving parts in the cell, or specially designed cathodes. EMEW is not a new process technology. It represents simply a modification of the hardware used in electrowinning, which achieves 'factorially' higher efficiency in mass transport of metal ions than a conventional EW cell. It is simple, modular and of relatively low unit cost. Across most applications examined, tested and installed, application of the technology leads to lower capital and operating cost. The high performance of the EMEW technology is simply achieved through a hydraulic mechanism which presents metal ions to a cathode in a far more effective and direct manner than in a conventional unit. The very high mass transport capabilities of EMEW derive from a radical redesign of the vessel in which electrowinning is performed, and in the method of circulation of the target solution. The essential difference is that the EMEW cell is constructed from a pair of concentric tubular, rather than planar, electrodes. The ends of the assembly are fitted with plastic end caps, thus forming a closed chamber through which target solution can be pumped at high rate. The resulting high flow and efficient mixing result in forced and continual supply of metal ions to the surface of the cathode. 7
The EMEW technology has now been demonstrated and proven in numerous process environments, both in mining and general industry. Its growing place in hydrometallurgy is assured by a combination of physical, process and economic factors: Can easily maintain copper concentration below 5 g/l. It is extremely simple to operate and has no moving parts. The cell is modular and portable, facilitating relocation and expansion. Capital cost is lower. There is no requirement for regular replacement of cathodes The technology s capability of processing low grade streams is well proven. Its efficiency permits significantly higher current density than conventional units. The technology operates over an extremely wide range of metal concentration. The cell is more tolerant of contaminants in solution (for example iron and chlorides) Without any major change to the hardware, it is capable of extracting a variety of metals. It is capable of selective electrowinning of metals from complex solutions. The closed nature of the cell prevents acid mist in the plant. It allows complete control of the gaseous products from electrowinning. Flow Sheet Setting The following diagram provides an example of the recommended flow sheet positioning of an EMEW electrowinning facility in a copper rod manufacturing line. 5 g/l Cu 150 g/l H2SO4 5 g/l Cu 150 g/l H2SO4 EMEW Circuit COPPER CATHODE The advantage of the EMEW circuit in this position is that the pickling solution is continually recharged, copper is recovered as a high quality product (>99.99% Cu) and acid is rigorously maintained at an optimal level for the pickling process 8
4.0 EMEW PLANT SIZING Being modular in nature and capable of sustaining high current efficiency in copper recovery, it is easy to select an EMEW plant sizing based on the copper accumulated in a pickling or etching circuit. Performance in this function is extremely predictable. The following table provides examples of plant sizes which have been formally modelled by Electrometals for a variety of applications: APPLICATION 1 2 3 4 5 6 Copper production kg/day 5 50 130 273 441 1700 Acid concentration g/l av 50 100 100 190 30 150 Cu concentration (current) start g/l 30 35 35 35 15 35 Litres if disposed m3/yr 166 1,428 3,714 7,808 29,400 48,571 EMEW circuit Cu maintained at g/l 5 5 5 3-5 5 5 Current density a/m2 300 400 400 400 400 500 Current efficiency % 90 90 90 85-90 90 90 Number cells cells 2 10 24 60 90 270 Cathode area m2 1 5 12 30 45 135 Note. There is 0.5 m² of cathode area in each EMEW plating cell. Application Client 1 : Copper parts manufacturer, Australia 2 : Copper plating facility, Brazil 3 : Control of copper levels in coin plating facility, Europe 4 : Medium sized copper rod manufacturer, Europe 5 : PCB sulphate etchant, Taiwan 6 : Large copper rod manufacturer, US 9
5.0 PLANT PERFORMANCE The following chart presents an EMEW copper depletion profile for a copper rod pickling solution (at a constant current density of approximately 500 a/m2). Copper depletion profile 35 30 25 g/l Cu 20 15 10 5 0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 Amp hours per litre of solution This graph illustrates a number of key characteristics of the EMEW technology and its markedly enhanced performance: It can sustain high current density in recovery of copper from these solutions the consequence of which is the minimisation of cathode area for a given production rate Current efficiency remains high even at high current density and down to very low concentrations of copper the consequence of which is that copper concentration can be minimised in the pickling solution inventory at low levels (and acid concentration at high levels) The rate of copper recovery in the EMEW cell remains constant and predictable over a very wide range of copper concentration the consequence of which is that, regardless of variations in copper concentration in solution, production rate from the plant will remain constant. In essence, once set and running, the EMEW plant will perform very consistently and requires very little monitoring. Added to these benefits is the fact that the copper produced is of very high quality and, dependant on the actual composition of the solution, can exceed 99.999% Cu which may command a premium value. 10
6.0 CAPITAL AND OPERATING COSTS The capital cost of an electrowinning circuit will naturally depend on whether the plant is provided on a turnkey basis or only selected portions thereof. It is noted that an EMEW facility is provided fully engineered, on a plug and play basis complete with rectifier, pump and all electrical and solution reticulation requirements. Little site capital works are generally required, and Electrometals will install and commission the facility to ensure that there is no interruption to the established labour team. The following chart compares the capital cost of EMEW against two other ready-built units on the market (types A and B), and against a quote obtained from an engineering company to construct a specially constructed unit (type C). Capital cost various electrowinning technologies 1,400,000 1,200,000 US$ 1,000,000 800,000 600,000 400,000 EMEW Type A Type B Type C 200,000 0 0 100 200 300 400 500 Production rate (kg copper per day) The prices indicated in the graph are indicative prices obtained through discussion with users of the electrowinning systems in question. They will be subject to change over time, and will differ if special engineering is required for a particularly site. It is quite clear, however, that the EMEW technology is significantly cheaper than other commercially available units, aside from at very low scale (around 5 lb of copper produced per day). This is the direct result of the simple and modular nature of the technology. Operating costs in an electrowinning system will change from site to site varying in accordance with local labour and power charges. The following table provides an estimate of operating cost for an EMEW circuit for the applications/clients mentioned previously, based upon the following assumptions: Power cost : US$0.05/kWh **Copper harvesting manpower : 1 hour per 5 m² of cathode area per day **Plant maintenance labour : 2 hours per 5 m² of cathode area per day Technical supervision : 1 to 2 hrs per day Maintenance plant : US$150 per cell per year Labour cost : US$20/hr ** These figures are significantly lower in larger scale facilities 11
APPLICATION 1 2 3 4 5 6 Copper production kg/day 5 50 130 273 441 1700 Power $/day 4.44 16.83 39.92 57.82 98.37 328.68 Labour $/day 0.5 2.5 6 15 22.5 67.5 Supervision $/day 5 25 25 25 25 100 Maintenance parts $/day 0.83 4.17 10.00 25.00 37.50 112.50 Total US$/lb Cu 0.98 0.44 0.28 0.20 0.19 0.16 There is little published information on the operating costs of other systems, but due to their complexity and requirement for frequent change of electrodes, it is expected that they will be significantly greater. Some published estimates of the operating cost of small electrowinning units are as high as US$13 per pound of metal recovered. Given the low operating costs of the EMEW system, payback of the capital required for its installation can be very short. The value proposition for installation of a treatment facility is driven by the facts that: 1. Disposal costs of waste solution are avoided 2. The costs of continual purchase of fresh acid are avoided 3. High value is obtained for the recovered copper It is obvious that, aside from very small plants, incurring the costs of disposal and dumping of the exhausted pickling solution is not an option. At the varying scales of operation examined by Electrometals, the annual cost of dumping (at an estimated US$0.05 cents per litre and providing for cost of replacement acid and lost copper revenue) would be of the order of: Copper production kg/day Total potential cost US$/annum 5 13,160 50 122,671 130 318,946 273 688,040 441 1,903,062 As previously noted, there are a number of potential process options, and variants thereof, which will each derive a different financial result. Detailed analysis of each of these variants is outside of the scope of this study. However, the following table and chart provide an example comparison between production of copper sulphate and EMEW electrowinning based on the following estimates: Operating cost in sulphate production US$0.4 per pound of contained copper Value of copper sulphate US$600/tonne of sulphate pentahydrate Sulphuric acid cost US$50/tonne Value of copper produced US$2,300/tonne 12
Comparison EMEW Electrowinning vs. Copper Sulphate Production. Summary of net revenue COPPER PRODUCTION (kg/day) 5 50 130 273 441 Copper Sulphate Production Tonnes/annum Cu sulphate 7 70 182 352 617.4 Revenue (US$/annum) 4,200 42,000 109,200 211,200 370,440 Annual operating cost (US$) 1,543 15,428 40,113 77,581 136,075 Replacement acid (US$/annum) 1,218 10,993 28,581 60,085 96,957 Net Result - US$/annum 1,439 15,579 40,506 73,534 137,408 EMEW Cathode Production Revenue (US$/annum) 4,025 40,250 104,650 202,400 355,005 Operating cost (US$/lb Cu) 0.98 0.44 0.28 0.20 0.19 Acid saving (US$/annum) 1,218 10,993 28,581 60,085 96,957 Net Result - US$/annum 1,472 34,268 104,909 222,936 387,783 EMEW BENEFIT 33 18,689 64,403 149,402 250,375 2% 120% 160% 203% 182% This analysis has been conducted at a broad level only, but is sufficient to illustrate the result between the two treatment methodologies the sulphate route (regardless of capital cost) requiring continual replacement of acid into the pickling circuit and having inherently higher operating cost than the EMEW circuit. Comparison between EMEW and Sulphate 450,000 400,000 350,000 US$ per year revenue 300,000 250,000 200,000 150,000 100,000 50,000 0 0 100 200 300 400 500 kg per day of copper Sulphate EMEW 13
7.0 CONCLUSION The following facts are abundantly clear: 1. If a waste generator is producing over 20 kg per day of copper in a pickling or etchant solution, the costs of disposal, cost of acid and loss of value in contained copper dictate that an on site treatment facility is essential. 2. There are a number of potential treatment methods for these solutions, but clearly the best result is gained from a simple one step process which is relatively low in capital cost and easy to operate. 3. It is clear that the only technique which allows simplicity and direct return of value for the pickling stream treatment is electrowinning, in that: a. It is one step in nature b. It directly regenerates the acid used for pickling c. It produces and easily saleable product with an immediate market 4. In general terms the solution composition and copper concentration in these solutions are outside of the general capabilities of a conventional tank electrowinning cell, as they struggle to economically maintain copper at low concentrations. The conventional design utilised in, for example, the mining industry is incapable of sustaining high efficiency and good product quality in the range of composition required in these applications. 5. For non EMEW electrowinning to be efficient in this setting modification to the design of cell is required, in order to dramatically improve the rate of supply of metal ions to the cathode. There are some cell designs on the market which achieve this aim, through relatively complex mechanical or cathode design means. They are, however, costly and would appear suited only to relatively small applications. 6. The EMEW technology, through a simple redesign of the vessel in which electrowinning is performed has achieved the required aim in a very cost effective manner. Comparison between capital and operating costs between EMEW and other electrowinning technologies on the market shows very significant savings. Comparison between EMEW electrowinning and copper sulphate production shows a significant increase in financial return in the former. 14
APPENDIX A Use and effect of Hydrogen Peroxide Pickling baths remove the surface oxides and a very thin layer of copper metal from the surface of the product as it cools whilst nearing the end of the line. The final stage of the bath coats the product in wax to prevent oxidation. In some applications there is a need to etch deeper into the copper product. This is often achieved through the addition of peroxide Peroxide addition The sulfuric acid in the pickling bath is capable of removing cupric oxide (CuO) from the surface of the copper product. The cuprous oxide (Cu 2 O) is more difficult to remove with sulfuric acid and the copper metal will not be removed with sulfuric acid. Hydrogen peroxide is added to the bath to remove the cuprous oxide rapidly and to etch the copper metal. The generally accepted mechanism for copper removal with acidic peroxide is described below. 1. Cu + H 2 O 2 CuO + H 2 O 2. CuO + H 2 SO 4 CuSO 4 + H 2 O Glycol addition In general, where Peroxide is added, pickling baths will operate with 0.4-0.8% free hydrogen peroxide. The peroxide decomposes rapidly at the elevated temperature of the pickling bath (60 o C / 140 o F). Propylene glycol is added at 1:6 glycol:peroxide to slow the peroxide decomposition at this temperature. Electrowinning copper in the EMEW cell in the presence of peroxide is possible however the current efficiency is marginally reduced by the redissolution of copper on the cathode according to the above equation. The peroxide is destroyed in the EMEW cell during electrowinning, however the cathode can be eroded during the early part of deposition and poor morphology may result. For this reason it is recommended that the peroxide be decomposed completely before the pickling solution is fed to the EMEW cell for copper recovery. The simplest means of decomposing the peroxide is to heat the solution above 76 o C (170 o F). A typical solution of 0.8% peroxide will be decomposed to less than 0.03% peroxide (recommended maximum for EMEW feed) in 5 hours at a temperature at or above 76 o C. 15