THE BEIJING AXIS China Business in Perfect Strokes

Copper has historically been recovered by the pyrometallurgical technique of smelting concentrates of copper rich ore. With the advent of solvent extraction and electrowinning (SX/EW) in the 1970s, it was then possible to economically recover lower grade ores consisting of oxide copper minerals using various leaching techniques such as stirred tank, dump, heap or vat leaching. Since the late 1990s it has also been possible to recover lower grade secondary sulphides through hydrometallurgical techniques involving SX/EW.

Currently approximately 80% of world copper production, equating to 12 million tonnes (2008), is produced by smelting chalcopyrite concentrates. The remaining copper production comes primarily from large-scale heap leaching of low-grade copper oxide ores followed by SX/EW. A select few operations treat secondary copper sulphides via pressure leaching methods, followed by SX/EW.

Smelting technology is well developed and recent innovations in flash smelting have made the process highly efficient. Modern smelters are capable of producing high quality copper suitable for industry, fix iron in stable slags suitable for disposal, convert sulphur into sulphuric acid for sale and allow the recovery of precious metals from anode slimes. However, smelters have very high capital and operating costs, long construction times and are not economical unless on a very large scale. They can also emit significant levels of SO2 gas that is a primary cause of acid rain. These factors combine to make permitting requirements increasingly more difficult. Smelters are also unable to effectively treat concentrates with deleterious impurities such as arsenic and bismuth, which are becoming more common as reserves of pure copper ores are depleted. This makes the construction of new smelting facilities less attractive. The only new smelter constructions are likely to be located in third world countries where environmental controls are not as stringent.

As smelters become less able to treat the ores now available and permitting becomes more difficult, operators are seeking alternative treatment methods. The focus of these investigations over the last 20 years has centred on hydrometallurgical processes incorporating a range of different leaching techniques followed by the now well proven SX/EW process.

Trends in production methods and capacities

The International Copper Study Group (ICSG) has broken down the mine production of copper via concentrate and SX/EW routes over the last 40 years. The increase in production from SX/EW process routes has been most rapid between the mid-1990s through to the mid-2000s, which gave hope to the mining industry that hydrometallurgical routes may take over traditional smelting routes. Unfortunately that momentum growth has not been retained since the mid 2000s as the copper hydrometallurgical process has struggled to compete economically with smelting.

With the advent of solvent extraction and electrowinning came the recognition that copper could be recovered economically and with better environmental outcomes. (Leaching of scrap and SX/EW allows for the economic refining of recycled copper.) The increasing environmental awareness of society and a rising copper price have seen secondary refinery and SX/EW production increase to the current level of 15% and 17%, respectively, in 2008.

The most significant increase in refinery capacity between 1980 and 2009 has taken place in the Asian and South American regions. Asia’s refining capacity has increased over 360% from 3500ktpa to 9000ktpa whilst the South American capacity has increased by a full 420% from 1000ktpa to 4200ktpa capacity.

Due to operational constraints, plant shutdowns and supply input restrictions/interruptions, the refinery capacities shown are not always fully utilised. Actual refined production by region shows production in the Asia region increasing from 3000ktpa to just under 8000ktpa, a 266% increase, whilst actual production in the other regions has remained constant or dropped slightly over the same period.

By country Chile is the leading exporter of refined copper, making up 38% of world production, while China is the largest importer of refined copper at 20%. The top 4 export countries for refined copper, Chile, Zambia, Japan and Peru make up over 55% of world exports while the top 4 importers of refined copper, China, Germany, the US and Italy make up 49% of the import markets.

Thus smelting and electrorefining of copper sulphide concentrates are still the most widely used methods for producing copper metal. This is despite predictions some 30 years ago that hydrometallurgical routes were certain to make conventional smelting obsolete. However, copper oxide resources have a finite life and little additional resources are being found to replace ore consumed so the development of a commercially viable chalcopyrite hydrometallurgical process still remains a key objective for many mining companies, otherwise copper recovered by these means will only start to decline.

Hydrometallurgical recovery technologies

As noted above, there are increasing incentives to develop hydrometallurgical process routes which can handle primary sulphide concentrates that may have significant levels of contaminants such as arsenic and bismuth. To be competitive, hydrometallurgical processes must meet the following objectives:

• Produce high quality copper metal
• Convert the sulphur present in chalcopyrite to a marketable form (sulphur or sulphuric acid) or a form suitable for long-term disposal (gypsum)
• Convert the iron present in chalcopyrite to a form suitable for disposal (goethite or hematite)
• Convert impurity metals such as selenium, mercury and lead to a form suitable for long-term disposal or recovery
• Allow recovery of gold, silver, platinum group metals(PGMs) and base metals such as nickel, cobalt and zinc sometimes associated with chalcopyrite in marketable forms (metals or concentrates)
• Be able to treat concentrated and/or whole ore sources containing impurity (penalty) elements such as arsenic and bismuth to a environmentally acceptable form

Over the years over 30 technologies have been developed, evaluated and tested for the recovery of copper from ores and/ore concentrates using these lixiviants as a basis of forming a flowsheet. Some of these processes have proven to be commercially successful, others have failed to be successfully commercialised while others are still at the early development/evaluation stage.

The technologies which represent the most realistic chances of becoming a commercial reality are detailed below, highlighting the key specifics of each technology and the stage at which each process is at in the commercialisation process.

Activox® is a combination of fine grinding and pressure oxidation. It operates under moderate pressure and at elevated temperatures, but under milder conditions than those required by conventional Total Pressure Oxidation processes. The process was demonstrated at pilot scale on a nickel concentrate at Tati Nickel in Botswana but since 2007 this operation has been on care and maintenance.

The Albion Process was developed by MIM (now Xstrata) in 1993. It consists of a hot oxidative leach of finely ground concentrate at atmospheric pressure, but is not reliant on autoclaves or bacterial cultures. The process’ ability to oxidise pyrite allows the overall acid balance to be met internally reducing the need to import and/or neutralise the sulphate formed. The process has been commercialised for gold with two current projects under construction – Envirogold’s Las Lagunas tailings treatment project in the Dominican Republic and European Goldfields’ Certj project in Romania while various pilot programs and studies have been completed on copper concentrates.

The CESL Copper Process developed by Teck Cominco involves the oxidation of sulphide concentrates at elevated pressure and temperature in the presence of catalytic chloride ions. In addition to copper, impurity metals such as nickel, cobalt, and zinc are oxidized during the pressure oxidation process. Depending on the concentrate metallurgy, it may be economically feasible to recover some of the more valuable impurity metals from the process solution.

The leach filter cake, containing oxidized copper, hematite and elemental sulphur is repulped with recycled raffinate from Solvent Extraction. The pH of the slurry is controlled in order to efficiently leach the copper from the pressure oxidation filter cake.

Impurities are removed from the copper-rich solution by solvent extraction, a conventional process used throughout the world. The purified solution is then electrowon, producing copper cathodes of LME Grade A standards. CESL have constructed and operated a 10000tpa demonstration plant at Carajas in Brazil since 2009.

The Galvanox™ process developed by the University of British Columbia and marketed through Bateman Engineering utilises ferric sulphate to leach copper from concentrates containing a mixture of copper sulphides, but particularly those containing chalcopyrite (CuFeS2). The process takes advantage of the galvanic couple between pyrite and chalcopyrite to ensure rapid oxidation of chalcopyrite under mild conditions in the acidic iron (ferric sulphate) solution, without the need for microbes, ultrafine grinding, or chemical additives such as chloride, nitrate, or surfactants.

The process offers several other potential advantages over existing processes including the fact that the leach liquor is a pure sulphate medium with a relatively low propensity to corrode the process vessels, equipment and piping. Elemental sulphur rather than sulphate is generated, facilitating waste treatment, and the process also operates below the melting point of sulphur and therefore requires no surfactants.

Numerous bench scale testwork has been carried out on concentrates from all copper producing regions throughout the world and pilot plant testing has been completed on two concentrate samples since 2007.

Total Pressure Oxidation (TPOX) involving the use of high pressure and temperature autoclaves has successfully been commercialised at both Freeport McMoran’s Baghdad Operation in Arizona and First Quantum’s Kansanshi Copper Operation in Zambia. Both these operations have the unique ability of integrating the TPOX route with their existing operations due to the excess sulphuric acid produced by the process of oxidising all the sulphide feed into the autoclave and converting it into sulphuric acid used elsewhere. Whilst extremely high copper recoveries of >99% into solution are achievable via TPOX techniques which are subsequently treated by conventional SX/EW techniques, the cost of the TPOX portion is relatively high on a $/t Cu produced basis. TPOX processes are also prone to relatively high operating costs when compared to alternate technologies due to the high purity of oxygen addition required and need for limestone/lime to neutralise the residues should an alternative method not be available such as treating a low grade oxide ore via heap and/or tank leaching.

By means of a concentrate chloride leaching, solution purification and metal recovery by hydrogen reduction system, Outotec has produced (on a plant demonstration scale) a high quality copper wire rod known as the Hydrocopper® process. The concentrate leaching is completed in atmospheric counter current operation at a temperature of 80-100°C, using stirred tank reactors and thickeners.

Divalent copper leaches the copper of chalcopyrite and other sulfides. Ferrous iron is further oxidized by air or oxygen into ferric, which precipitates as goethite or hematite. Other sulfides present in the concentrate are also dissolved. Once all the copper has been leached, gold begins to dissolve as chloro- and bromo-complex with gold recovered in the last leaching stage. Half the solution from the concentrate leaching is fed into the oxidation reactors, and copper is oxidized into cupric, using chlorine gas.

Copper is precipitated as copper (I) oxide, using sodium hydroxide. After precipitation, the slurry is filtrated with the filtrate and pure NaCl solution directed to chlor-alkali electrolysis, which is a process used to produce caustic soda, chlorine and hydrogen used within the Hydrocopper circuit.

The wet cuprous oxide (Cu2O) from the belt filter is fed into a reduction furnace where it is dried and reduced to metallic copper with hydrogen before being discharged and fed into a melting furnace prior to casting products such as copper wire.

Geobiotics offer an approach combining bacterial leaching with low capital and operating cost heap leaching techniques to treat refractory sulphide gold concentrates and to copper, nickel, cobalt, zinc and polymetallic base metal concentrates. The process is particularly suited to treating dirty and low grade concentrates which attract high smelter penalties and/or low net smelter returns or those totally unacceptable for smelting altogether.

Geoleach has been successfully tested with both laboratory and field scale testing, including a demonstration scale heap leach pad at the Quebrada Blanca mine located in Chile and at Caraiba Copper in Brazil for copper concentrates.

Secondary sulphide treatment routes

Three copper hydrometallurgy processes have been developed and operated successfully on industrial scale for the treatment of secondary sulphides, mainly chalcocite, in either a whole ore or a concentrate form. These processes are the stepping stones to the treatment of primary sulphides.

Successful treatment methods for secondary sulphides include:

• The Mount Gordon Leach process at the Western Metals (now Aditya Birla)
• Mount Gordon operations site in Queensland, Australia
• Sepon Copper Process at Minmetals Sepon Copper Project, located in Laos Ferric atmospheric tank leaching process at the Las Cruces Project in Spain

Bateman has been involved in the study and development of many of these processes over the last 20 years including the successful project completion of the Mount Gordon project, Sepon Copper project and Kansanshi total pressure oxidation projects which produce over 150,000tpa of copper by these new hydrometallurgical methods. Bateman actively pursues new copper technologies and has the exclusive marketing rights to the exciting Galvanox™ technology from UBC and has recently completed a demonstration plant for Nippon Mining and Metals (NMM) based on the patented N-Chlo technology held by NMM.

Linus Sylwestrzak (Leaching Technology Specialist, Bateman Engineering)

Note: This article is republished here courtesy of Bateman Engineering.