Controlling the level of impurities in copper refinery electrolytes is essential to achieving acceptable cathode purity. Purge of electrolyte to waste is not an entirely satisfactory remedy, since the cost of neutralization, hazardous solid waste disposal, and replacement acid can be appreciable. Recovery by evaporation/crystallization is also not ideal because of high capital, operating, and maintenance costs.
Based on a phenomenon called acid retardation, the APU® process employs ion exchange resins capable of sorbing sulphuric acid while excluding most metal salt impurities. The resulting de-acidified metal salt by-product solution can be disposed of or further treated to recover its metal content. Purified acid is recovered from the resin for reuse by water elution. The APU utilizes a novel ion exchange technique called Recoflo® which features short resin beds (7.5 cm to 60 cm), fine resin particles (75 to 150 µm), and counter-flow regeneration to improve separation efficiency. A number of patented innovations aimed at improving flow distribution have further enhanced the performance of the APU process. Extensively employed for recovery of waste pickling and anodizing acids since 1977, the APU system is now being used for removal of contaminants such as antimony, bismuth, and nickel from copper electrolytes.
The APU equipment is extremely compact, a typical resin bed for this application being only about 1000 mm to 1500 mm in diameter by 600 mm in height. The pre-assembled, skid-mounted equipment is inexpensive to install and does not take up a lot of plant space (see figure). A major advantage of the fully automated system is that since no chemicals and almost no energy is required, the operating costs are extremely low. In addition, almost no operator attention is required. Since the process operates at ambient temperatures, inexpensive plastic piping and components can be employed, thereby avoiding corrosion problems.
Successful piloting of the process in a copper refinery was initially done in 1989 at Kennecott in Utah. The first industrial-scale APU unit for this application was installed at the Kidd Creek Division of Falconbridge in Timmins, Ontario, in 1995, following additional, successful laboratory and field pilot plant trials. The unit typically recovers 90% to 95% of the free sulphuric acid content. Higher acid recovery efficiencies are possible, however, it was found that if the pH of the by-product was raised too high, some of the metals precipitated out, clogging the resin bed. The system typically removes 70% to 80% of the copper, nickel, and iron and about 50% of the antimony and bismuth from the electrolyte in a single pass. Arsenic, on the other hand, existing largely as arsenic acid, is taken up by the resin and not separated from the sulphuric acid to any significant extent. Other ion exchange processes are currently being developed for arsenic removal. At Falconbridge, the de-acidified by-product is neutralized with soda ash to recover nickel carbonate. Because more than 90% of the free sulphuric acid is recovered, the quantity of soda ash consumed has been substantially reduced since installation of the APU, providing a rapid return on initial investment.
Pre-filtration of suspended solids prior to APU treatment is an important consideration. Good success has been obtained with a proprietary, back-washable, dual-media filter employing a very fine layer of high-density granular media below a layer of coarse crushed anthracite. In order to minimize acid losses, the filter vessel is drained of electrolyte using compressed air, prior to water backwash. Similarly, to avoid dilution, the water is drained from the vessel after backwash, before returning to service.
Additional APUs have recently been installed at refineries at Sterlite Copper in India and Phelps Dodge in El Paso, Texas. The Phelps Dodge APU installation, shown in the figure, replaced a sulphuric acid evaporator/crystallizer. A number of other pilot plants are currently being evaluated in other copper refineries. The process is also being considered for a similar application in zinc refineries. The system now has several years of operating experience and is proving to be a reliable and economical means of controlling impurities in copper electrolytes.