CONTROL OF COPPER ELECTROLYTE IMPURITIES USING SHORT BED ION EXCHANGE ABSTRACT

Transcription

1 CONTROL OF COPPER ELECTROLYTE IMPURITIES USING SHORT BED ION EXCHANGE Mike Sheedy and Paul Pajunen Eco-Tec Inc Squires Beach Road Pickering, Ontario, Canada L1W 3T9 ABSTRACT This paper discusses the short bed ion exchange process for the removal of metal contaminants from copper electrolyte bleed streams. Processes for the selective removal of sulfuric acid, nickel, arsenic, antimony, bismuth, and iron will be discussed. Considerable importance is given in controlling nickel and iron content in electrolyte by adopting the short bed ion exchange process for low cost acid purification system. This novel method coupled with an effluent treatment plant ensures recovery of sulfuric acid, removal of nickel and iron from electrolyte, production of disposable nickel sludge besides ensuring zero discharge to environment. With a more suitable impurity management system, copper refineries are able to achieve and sustain the cathode quality superior to ASTM standard. Case studies outlining the performance of such systems are presented.

2 SHORT BED ION EXCHANGE The short bed ion exchange process has been extensively used in the metal finishing industries since the early 1970 s (1,2). Short bed systems have also been sold to primary metals producers for the separation of sulphuric acid from Cu refinery bleed streams (3) and for the recovery of nickel and cobalt from plant effluent streams. This unique technology optimizes the ion exchange process and offers a number of significant advantages over conventional ion exchange. The principal features and benefits of this technology are presented below. Fine Mesh Resins Short bed technology uses resin beads with much smaller diameters than conventional ion exchange systems. Reducing the size greatly improves the exchange kinetics. This allows operation at higher flowrates and reduces the length of the mass transfer zone. This is particularly important for chelating resins, which have very slow exchange kinetics. Work by Price(4) indicates that the exchange rate is inversely proportional to the square of the particle diameter. Thus, halving the particle size increases the exchange rate by 400%. The resulting higher flowrates significantly reduce the volume of resin required. This is particularly important when using expensive chelating resins. Finer particles also aid in fluid displacement which allows a reduction in rinse volume requirements. Figure 1 Conventional IX Resin Short Bed IX Resin Short Depth Resin Beds During the operation of a fixed bed process exchange takes place only in the fraction of the bed occupied by the mass transfer zone. Upstream the resin has been exhausted, while downstream the resin remains in the regenerated form. Thus, in a conventional column the majority of the resin is inactive. The short bed process reduces the depths of these inactive regions and makes more effective use of the remaining resin. It should also be noted that the increased kinetics reduce the depth of the mass transfer zone to m.

3 Conventional Profile Fine resins reduce MTZ Short Bed Figure 2 - Use of Short Bed Depth Counter-Current Regeneration Counter-current regeneration introduces the regenerant into the resin bed in the direction opposite to the feed solution. This technique is a well known chemical engineering principle which in this case, helps to reduce the amount of regenerant required, maximizes the concentration of the recovered metal or acid and ensures complete regeneration minimizing leakage. Fully Packed Bed In a short bed the vessel is completely packed with resin leaving no freeboard. In a conventional column, mixing of the various solutions being processed occurs in the freeboard space. The resulting dilution is very undesirable when processing concentrated solutions. A packed bed eliminates this dilution and also helps to maintain the concentration profiles developed within the resin bed. This ensures the benefits of counter-current regeneration are fully realized. While these features offer many benefits the following disadvantages should be noted. Unfiltered feed solutions will quickly foul fine mesh resin and a packed bed does not allow fluidized backwashing of the resin to remove any entrained solids. Thus it is necessary to properly prefilter all solutions fed to a Recoflo bed. Also, fine mesh resins are not standard materials and as a result are more expensive on a per unit basis and can be more difficult to obtain.

4 Figure 3 Fully Packed Short Bed ACID SEPARATION Short bed technology has been adapted to acid separation and is provided as an acid purification unit, APU. This process uses ion exchange resins that selectively absorb free acid but reject dissolved metal salts. The absorbed acid can be removed by passing water over resin. While the use of a specific type of ion exchange resin is required the process is strictly speaking not ion exchange. The acid is not exchanged onto a specific site and the process capacity is not limited by the resins ion exchange capacity. The process is driven by the acid concentration difference between the interior of the resin phase and the surrounding fluid. H 2 O H 2 O NiSO 4 NiSO 4 H 2 SO 4 H 2 SO 4 Figure 4 Acid Sorption and De-sorption

5 Typically only a small volume of solution, less than the volume of the ion exchange resin bed can be processed during each cycle. This restriction coupled with the standard engineering design for most ion exchange processes delayed the successful commercialization of the acid retardation process until the late 1970 s (1). Since short bed technology uses a packed column and counter-current regeneration the problems associated with dilution and control faced by conventional designs are overcome. Resin phase has limited capacity for acid Liquid phase of resin absorbs acid. Resin is ~45% water Resin bed void space 38% of column Theoretical Capacity (feed processed per cycle) = (1-0.38) x 0.45 x 100 = 28% volume of APU column H 2 O H 2 SO 4 Figure 5 APU Capacity In 1979 the first APU s were installed in the aluminum industry to recover sulfuric acid used in anodizing operations(1). Since then, the APU has been used for the recovery of many different acids in many different industries, including the treatment of hydromet streams (3,5). To date, over 300 units have been installed worldwide. The majority of these units have been installed in the metal finishing industries. APU OPERATING CYCLE The first step in the APU process is to filter the feed through a multi-media sand type filter. This is then followed by cartridge polishing filters located on the unit. The APU operating cycle, which normally lasts about 4-5 minutes, is divided into an upstroke and downstroke shown in figure 6. During the upstroke, feed solution is pumped up through the APU resin bed. Initially the interstitial void volume in the resin bed is filled with water from the previous cycle. The incoming filtered feed displaces this water, which is then collected as the water void, which is transferred to the water tank and used along with additional make-up water during the downstroke. As the feed continues to pass over the resin, the acid is absorbed into the bead and thus its passage through the resin bed is retarded. The metal salts are rejected by the resin and breakthrough into the effluent or byproduct stream ahead of the acid. The upstroke is completed just as the acid begins to breakthrough into the byproduct.

6 During the downstroke, water is pumped downward through the bed. Initially the bed void is filled with unseparated feed solution. This feed void is displaced by the incoming water back into the feed tank for processing during the next cycle. As the water continues to pass through the bed, the majority of the metal is displaced and the purified acid removed during the upstroke is de-sorbed and collect as the product. The purified acid product concentration may range anywhere from % of the feed value depending on the operating conditions and the feed concentration. By adjusting the volumes and flowrates of these various process steps it is possible to optimize APU performance depending on the process objectives. Typical process capabilities range from 70-95% acid recovery and 50-90% metal removal. Upstroke Water void byproduct water makeup water feed Downstroke water acid makeup water acid void feed product Figure 6 APU Operating Cycle

7 CASE STUDY #1 FREEPORT-McMORAN EL PASO, TEXAS The El Paso copper refinery has been in operation since 1930 and by 2006 had reached a production capacity of 430,000 tons of cathode copper per year. As the city of El Paso has grown around the refinery, attention to unique environmental considerations specific to this location are of prime importance in all endeavors of operation. The entire facility is a zero discharge operation with only a small amount of potable water allowed to be sent out to the local POTW. For many years, discard electrolyte solution from the tank house was treated via a nickel circuit for nickel recovery. The technology employed was vacuum evaporation for the crystallization of nickel sulfate. These crystals would be separated by a centrifuge and packaged for market. The resulting waste solution was a black acid liquid material that needed to be transported off site by rail car. Fresh, concentrated sulfuric acid was used to make up the acid losses encountered in the tank house as a result of this method. In 2002, the age and condition of this equipment was such that it needed to be replaced or intensely refurbished. System Operation The system selected and installed to replace the existing evaporator/crystallizer system involved the use of an Acid Purification Unit (APU) to deacidify the liberator electrolyte bleed solution in order to recycle the recovered sulfuric acid followed by two stage precipitation to obtain a purified, salable nickel carbonate byproduct material. APU Figure 7 Tankhouse Bleed Electrolyte Treatment System

8 The process, as depicted in Figure 7, is such that electrolyte bleed solution from the existing liberator cells is delivered to the system. A backwashable Media Filter is used to remove suspended solids from solution. Filtered solution is then transferred to the APU feed tank. Feed solution is pumped through the APU where acid is sorbed by the ion exchange resin material while the metallic salt impurities pass through to the byproduct. This byproduct stream consists of the metallic salts and a small amount of free sulfuric acid. Water used for regeneration of the ion exchange resin is pumped down through the APU resin bed from the APU water tank so that the sulfuric acid is desorbed as a product solution. This purified solution exits from the APU and is collected for reuse within the tank house. System Performance Typical process specifications around the APU subsystem are shown in Table 1. Up to 3.2M 3 /h of liberator bleed solution is processed resulting in a nickel removal rate of up to 36 kg/h. The APU shown in figure 8 is 1.4M in diameter and 0.6M in depth with a resin volume of 0.9M 3 and can treat a maximum flowrate of 4.2M 3 /h. Stream [H 2 SO 4 ] [Ni] [Cu] Flow M 3 /h Feed Acid Product Metal Byproduct Water Loss / Removal 12.75% 75% 75% --- Table 1 APU System Performance Typical performance indicates an acid recovery rate of greater than 87% and a nickel removal rate of greater than 75% through the APU system. Other metallic contaminants such as copper and iron are removed at the same efficiency. The majority of the arsenic follows the sulfuric acid back to the tank house (typically only 10 15% removal). This perceived poor removal efficiency associated with arsenic is of much benefit in enhancing quality and productivity issues within the tank house. Removal efficiency for contaminants such as antimony and bismuth is on the order of 50%.

9 Figure 8 APU System at El Paso Refinery CASE STUDY #2 STERLITE INDUSTRIES CHINCHPADA, SILVASSA Sterlite Industries operates copper refinery at Silvassa, India using ISA PROCESS with a capacity of 1,800,000 TPA. The anodes are received from its custom Smelter at Tuticorin, Southern India. The greatest challenge for Sterlite is to efficiently produce cathode quality at par with the international producers. Considerable importance is given in controlling nickel and iron content in the electrolyte by using a low cost acid purification system. Electrolyte impurity management at Sterlite Copper, figure 9 basically involves the following four steps. 1. Control of bismuth and antimony in electrolyte by altering the anode and electrolyte chemistry 2. Control of arsenic by purification cells (Electro-winning) 3. Control of nickel, iron and other soluble impurities by acid purification unit. 4. Control of tellurium by cementation on copper.

10 Bi, Sb PRECIPITATION IN ELECTROLYTE Normal Cells Leaching Liquor from slime Plant Primary Liberator Cells Tellurium Free Leached Electrolyte Liberator cell Outlet Electrolyte 3 x 3 De- Copperisation Cells Copper Chips DE-TELLURISATION De-Copperised Liquor As rich cathode /sludge for recycle ARSENIC REMOVAL Water Heavy Metal Nickel rich Acid Purification Unit Precipitation rich Effluent Effluent Evaporator Recovered Acid to cellhouse Metal Sulfide/Hydroxide NICKEL/IRON REMOVAL NiSO4 + Na2SO4 Crystals for sale Figure 9 Impurity Control Sterlite After carrying out an extensive study of all the systems available for nickel and iron control Sterlite decided to treat the de-coppered bleed solution using an APU, followed by sulfide precipitation and an evaporator crystallizer. The APU system was commissioned in June of 2001 and has been in continuous operation since. A column 0.9M in diameter and 0.6M in depth was supplied and is capable of treating a maximum flowrate of 1.9M 3 /h. At Sterlite, the bleed electrolyte after decopperising in purification cells is passed through this resin bed. The de-acidified metal rich stream containing impurities such as nickel, iron, bismuth, antimony is ph adjusted and sent to the precipitation step where residual Cu, Fe, Sb, Bi, and As are removed using sodium sulfide. The nickel bearing effluent is then sent to an evaporator/crystallizer to produce nickel sulphate. The recovered sulfuric acid is sent back to the electrolyte system or used in the slime leaching section depending on requirement. Table 2 gives the separation efficiencies of APU and table 3 gives the typical composition of APU feed, product and byproduct streams.

11 Acid /Impurities % going to Product (Recycled to system) % going to Deacidified byproduct 1 Sulfuric Acid % % 2 Copper 25 % 75 % 3 Nickel 25 % 75 % 4 Iron 34 % 66 % 5 Arsenic % % 6 Antimony 50 % 50 % 7 Bismuth 50 % 50 % Table 2 APU Separation Efficiency Streams H 2 SO 4 Cu Ni Fe As Bi Sb APU Feed APU Product APU ByProduct Table 3 APU Stream Compositions CASE STUDY #3 KGHM PILOT PLANT TESTING KGHM was interested in evaluating new liberator electrowinning technology in conjunction with an APU. Internal pilot plant tests were conducted at Eco-Tec s development laboratory in Canada. The results of this test are shown in table 4. Subsequently a larger fully automated pilot plant capable of processing up to 145 dm 3 /h was tested at the Glogow refinery. Some of the results from these tests are shown in table 5.

12 Feed As Received Relative Free Acid Ni As Cu Fe Sb Bi Stream Volume Feed < Product < ByProduct < Metal Removal/Acid Loss (%) % Mass Balance Table 4 KGHM Pilot Plant Testing by Eco-Tec Inc Volume Free Acid Ni As Cu Fe Sb Stream L (%) Feed Product Byproduct Removal/Loss (%): Table 5 KGHM Pilot Plant Testing Glogow Refinery Summary Case studies of two full scale installations and a field pilot plant test of the short bed acid purification unit, APU have been presented. Copper refinery tank house bleed electrolyte treatment systems using an APU have been installed and in operation since the summer of 2004 at the Freeport McMoran El Paso copper refinery operation and since 2001 at Sterlite s Chinchpada, Silvassa refinery. The systems are comprised of an acid purification subsystem utilizing an APU system in conjunction with subsequent processes for the product of marketable nickel carbonate or sulphate. Implementation of these systems have deemed to be a success based not only on cost savings associated with energy reduction, sulfuric acid recycle, and sale of nickel salts but also environmental improvements, productivity enhancements, and quality gains as a result of the tank house return of sulfuric acid and arsenic.

13 REFERENCES 1. C.J. Brown, D. Davy and P.J. Simmons, Nickel Salt Recovery By Reciprocating Flow Ion Exchange, Paper presented at the Annual Technical Conference of the American Electroplaters Society, USA, June C.J. Brown, D. Davey and P.J. Simmons, Purification of Sulphuric Acid Anodizing Solutions Plating and Surface Finishing, 66 (54), January M. Sheedy, Acid Recovery and Purification Using Absorption Resin Technology, Paper presented at the 126th TMS Annual Meeting, Orlando, Florida, USA, 9 February S.G. Price and D.J. Hebditch Diffusion or Chemical Kinetic Control in a Chelating ion Exchange Resin System Ion Exchange for Industry, M. Streat Ed., Ellis Horwood Ltd., Chichester, West Sussex, England, 1988, Michael Sheedy : JOM, Vol.50, No.10, October 1998, pp 66-69