SX-EW Still Winning

SX-EW Still Winning

Casteel, Kyran

Mature but still improving, solvent extraction-electrowinning technology continues to gain ground

It is half a century since the application of solvent extraction (SX) technology to primary metals separation, purification and recovery started. Coupled with electrowinning (EW) of metal cathode, SX matured to have a major impact in the 1990s, especially on copper production from oxide deposits but also in the nickel, cobalt and zinc industries. In the present century, SX-EW continues to have a positive effect on the economics of metals production through application to low- and relatively low-grade sulphide ores as well as oxides and use in several relatively new mining regions.

This article looks at today’s more significant providers of the required technology, how these suppliers have interacted in recent and current projects, and at a new application in vanadium processing.

Key Suppliers

Mining companies, large and small EPCM contracting firms and specialist manufacturers contribute proprietary and generic processes and equipment to the technology pool. R&D centers, consultants and process testing companies contribute to project development, and collaborative academic research organized on international and national lines is increasing.

Companies presently supplying a large part of the process know-how and equipment required for SX-EW operations can conveniently be separated into those furnishing proprietary or generic core process units and others that have devised problem-solving bolt-on “peripheral” equipment. EPCM contractors typically integrate these various components but may also supply their own proprietary technology.

The core component of an SX plant is the mixer-settler technology. Antecedents of the EPCM contractor Aker Kvaerner developed proprietary mixer-settlers more than 30 years ago and in terms of installations this company is the copper market leader. An alternative design developed by Krebs et Cie in France is available from Technip. But Outokumpu Technology claims the largest fraction of world electrowon copper output coming from a single mixer-settler design-the VSF system.

Probably the widest range of designs is offered by another EPCM group, Bateman, which offers the Bateman reverse settler, compact settler, IMI turbine-pump mixer, and Bateman pulsed column-as used for uranium extraction at Olympic Dam. Latest addition is the vortex-ring mixer developed by Mixis in Canada, said to use much less power than conventional impellers.

Now offered by SPX Process Equipment, the Lightnin mixer settler range has established a good reputation, particularly with companies using generic design engineering to build SX plants. Major EPCM companies undertaking projects using generic and/or bought-in SX-EW technology include Bechtel, Fluor and SNC-Lavalin, while the more specialized “boutique” Australian firms GRD Minproc and Ausenco have built considerable reputations in the world market, too.

Another significant developer-supplier of complete SX-EW plants is the Canadian company SX Kinetics, which specializes in small and pilot scale facilities typically for research organizations and early-phase project development. The firm has been involved in more than 70 SX projects in 14 countries, covering 26 metals. Companies offering test work facilities for SX-EW project developers include SGS Lakefield and AMMTEC, while ALTA Metallurgical Services and Post Mixing Optimization and Solutions are prominent among consultants with long experience in the field.

Three mining-metals companies have made available electrorefining and electrowinning technology advances developed for their own operations and have continued development in collaboration with users. At Townsville in Australia, Kidd Creek in Canada and at the Kokkola and Harjavalta operations in Finland, Mount Isa Mines-now part of Xstrata-Falconbridge and Outokumpu, respectively, developed tankhouse processes that are generally similar but differ in detail. The ISA Process, Kidd Process and Outokumpu Process are marketed by Xstrata Technology, EPCM Services and Outokumpu Technology.

The step-change component developed for each process is the permanent stainless steel cathode, which eliminated starter sheets and reduced the incidence of short circuits between electrode pairs. Today, these cathode designs differ mainly in structure. Handling these cathodes and stripping the deposited metal required the parallel development of mechanized and subsequently automated equipment. The Townsville copper cathode stripping machines introduced in 1978 were based on zinc cathode stripping equipment at a Mitsui Mining & Smelting plant in Japan.

As well as Xstrata and EPCM offering permanent cathodes and handling equipment, there are also companies specializing in cell structures in various materials, such as Corrosion Technology International in the United States. Also important is the electrical gear required to power the EW process. Major electrical equipment manufacturers such as ABB do offer suitable rectifiers etc., but the British company G Corner has built up a worldwide reputation through 40 years as a specialist supplier of DC Bus Bar and related systems.

In Canada, MagPower Systems is working on hydrogen inhibitors that can reduce power consumption and raise EW efficiency. Mist control systems and agents are another specialist area while cathode quality control has recently received attention. BHP Billiton has worked with Computer Sciences Corp. in Australia on the application of radio frequency identification (RFID) technology for tracking the progress of cathodes, installing a trial system at the Cerro Colorado operation in Chile. The Chilean based organization Apliq has developed electronic camerabased systems for cathode inspection, plus CEPA-a Copper Electrowinning Process Analyzer that integrates and analyzes information from inspection systems.

Probably the main process step where specialist companies have come into their own is solution purification, although some of the core technology suppliers are active here too. For example, Bateman developed the pinned-bed clarifier for removing suspended solids from pregnant leach solutions.

The U.S. company Spintek Filtration has built a significant position in the supply of equipment for recovering organic extractants from either the electrolyte or raffinate streams in SX plants. The range includes solvent extraction filters, CoMatrix towers and Matrix towers. Smith & Loveless also enjoys a prominent role in the electrolyte filtration field.

An alternative means of removing entrained organics is column flotation. Both conventional columns such as those developed by CESL and Xtrata Technology’s Jameson Cell have been modified for this purpose. Xstrata Technology reports that sales to Bwana Mkubwa in Zambia and a 7.5-m-diameter unit treating raffinate at Olympic Dam have raised the total Jameson SX population to 40.

Many SX-EW plants have centrifuges to remove crud from the circulating fluids. Recently, Schwarz Global Consulting, the South African distributor of solid bowl centrifuges manufactured by Germany’s Flottweg, has reported the successful use of the Tricanter three-phase separation centrifuge in this application.

Finnish Competence

Today, the supplier able to offer the most comprehensive range of commercial scale SX-EW process know-how and equipment is Outokumpu Technology. Outokumpu built its first SX plant in 1977 for recovering molybdenum at the Kokkola works in Finland now owned by OMG Kokkola Chemicals. Between 1981 and 1994, the company extended the use of SX at Kokkola to recover metals (Re, Zn, Ni), separate metals (Co, Ni, Mg) and remove impurities (anions, Mn, Co, Mg). In 1995 the operating company added a cobalt removal unit at the Harjavalta plant, and in the same year the Technology arm supplied its first large primary SX plant to the Zaldivar copper project in Chile.

Since then Outokumpu Technology has worked on SX plants with several mining companies and various EPCM contractors. Test work at Chuquicamata led to selection of Outokumpu’s third-generation Vertical Smooth Flow (VSF) SX technology for Codelco’s new copper operation, Radomiro Tomic. This opened in 1997 and SX installations for Phelps Dodge Morenci, OMG Kokkola and an expansion at Radomiro Tomic followed in 1998-2001.

With the 2001 Kokkola job, Outokumpu Technology also made its first electrowinning tankhouse sale, becoming the only organization able to install an electrowinning plant independent of other firms. Since then, full SX-EW packages have been installed at Oxiana’s copper-gold operation in Laos and the Industrias Penoles Milpillas project in Mexico. Engineering and delivery of packages for Gumeshevsky in Russia, Erdenet Mining Corp. in Mongolia, and Inmet Mining’s 70% owned Cobre Las Cruces project in southern Spain are presently under way. And Outokumpu has also supplied electrorefining technology to customers in Russia and China.

A key feature of VSF solvent extraction technology is the separation of the pumping and mixing processes. This ensures low entrainment values because the mixing intensity can be maintained at the optimum level even with variable flow rates. Low entrainment reduces the need for extra equipment to separate and remove trapped materials. Outokumpu Technology says its dispersion overflow pump (DOP) units have been operated at flows of up to 5,000m^sup 3^/h and their impact on stage efficiency is considerable, with performance figures approaching 100% typically found in any VSF design.

The mixing unit typically consists of two cylindrical mixer tanks baffled and equipped with double helical SPIROK stirrers that maintain a vertical circulation throughout the mixers at low rotation speeds. An uptake channel is used to connect the last mixer tank with the settler and the dip cover structure of the mixer tank prevents air entering the dispersion. SPIROK enables mixers to operate with minimal pumping function, ensuring gentle, uniform mixing and minimal crud formation.

The deep-dense-dispersion (3D) settlers are typically equipped with three non-jetting picket fences, as well as a guiding fence in the feed end of the settler. Outokumpu Technology says the third picket fence is especially useful in SX plants requiring clean solutions with low entrainment values as well as flexibility with regard to maximum solution flows. Because of the deep, dense dispersion layer at the feed end of the settler, higher feed flows can be handled more flexibly and stage efficiency is higher.

The performance of VSF mixer-settlers has been excellent, Outokumpu claims. VSF technology normally produces copper electrolyte containing less than 5 ppm of organic entrainment, well within the requirements for straightforward electrowinning. Plants using the technology are among the most profitable in the world and now yield almost 25% of the world’s electrowon copper. Not only is it designed to run at minimum operating costs, ensuring profitability even during copper price downturns, but its scalability makes the technology exceptionally well suited to larger plants, as it delivers low entrainment values and very stable performance even at high flow rates. Equally high flow rate per train means that fewer trains are required, allowing a more compact plant layout, reduced space requirement and hence savings in investment and operational costs.

Outokumpu has also long been a leading developer, operator and supplier of specific tankhouse machines. This experience has now been applied in a range of new technology, specially designed for the metals processing industries and offering improved occupational health for operators as well as protection for the environment. And, Outokumpu Technology says, it can provide everything from individual pieces of equipment to total electrowinning solutions, for customers operating small manual plants or large fully automated mechanized plants.

The highly automated tankhouse concept significantly reduces operating costs and increases production reliability and product quality. The processes have been designed as a seamless whole with optimized process interfaces and equipment integration. The layout can therefore be planned to minimize space and maximize efficiency. Tankhouse machines are also compatible with most other tankhouse technologies, allowing for easy upgrade and modernization projects.

Outokumpu Technology claims its cathode structure will ensure superior strength and performance, maximizing productivity and minimize maintenance. The strong edge strips reduce maintenance requirements and problems in stripping. The suspension bar, with copper core, has a full-length stainless steel jacket, so that copper is only exposed at the busbar contact areas. This provides high mechanical strength, unsurpassed acid-corrosion resistance, and the option to replace a damaged plate and reuse the bar. The large copper cross section in the suspension bar ensures low electrical losses throughout the lifetime of the cathode.

The design of Outokumpu Technology’s electrolytic cells for electrowinning is based on the proven 84 cathode jumbo cell technology, resulting in a smaller tankhouse area, which in turn leads to lower construction costs and shorter construction times. The cells can be equipped with hoods and scrubber, which lead to almost 100% sulphuric acid capture and results in lower operational costs, through acid recycling and savings in corrosion-related maintenance as well as improved occupational health and safety. The impact on the environment can be minimized so that environmental regulations can be met, even over the long term. The dust protection provided by the hoods and scrubbers also increases cathode quality and improves the cells’ and electrodes’ electrical connections.

Outokumpu also supplies an innovative double contact busbar system, in which anodes and cathodes have an electrical contact on both sides. The principal benefit of this technology is claimed to be more even current distribution, which in turn leads to reduced energy consumption, a more even cathode weight distribution and fewer short circuits. Fewer short circuits means higher current efficiency and better cathode quality as well as longer insoluble lead anode life as less damage occurs to the anode surface. This also extends cell maintenance cycle times.

21st Century Mix

The way in which SX-EW customers are utilizing EPCM contractors and proprietary technology can be partially illustrated by some ongoing and recently completed developments, including two of the largest copper SX-EW projects scheduled for completion this year and underway in Chile: the ongoing Minera Escondida (BHP Billiton 57.5%) and the new Minera Spence (BHP Billiton 100%) operations.

At the Escondida Sulphide Leach Project (SLP) throughput is the name of the game. Having started copper production in 1990 with a relatively conventional but subsequently much expanded concentrator, Minera Escondida added a 125,000-mt/y oxide leach-SX-EW plant in 1998-99. In 1997, the company had also started conceptual studies for marginal-grade sulphide ore leach-SX-EW and Fluor completed the final feasibility study in 2003. Together with subsidiary trade-off studies to optimize the project economics and accommodate major developments in the overall Escondida mining plan, this study formed the basis for Fluor’s EPCM contract. The SLP was approved in April 2004 with a budget of $870 million.

Run-of-mine ore from both the Escondida mine and the new Escondida Norte pit commissioned in mid-2005 is being trucked to the dump leach pad by a fleet of six 360-mt Caterpillar 797Bs. This pad is 4.9 km long by 2.0 km wide and will attain an ultimate height of 126 m. The ore averages 0.50% TCu and the pregnant leach solution averages about 3 g/l Cu, lower than other copper SX feeds. To meet the project’s designed production rate of 180,000 mt/y cathode copper, the SX plant must treat about 9,000 m^sup 3^/h. With a system similar to the one used at the oxide treatment plant, this would require at least eight SX trains.

Putting Outokumpu Technology’s claim for the scalability of the VSF mixer settler to the test, the SX plant was specified with just two trains handling 4,500 m^sup 3^/h each (as compared with the 2,000 m^sup 3^/h units previously supplied for Radomiro Tomic and Morenci). This would significantly reduce cost per unit treated. The two trains are designed in a 3-stage series-parallel extraction configuration, with one wash stage and one stripping stage. The SX plant contract was awarded to Outokumpu Technology in October 2004 and the plant is the largest VSF installation in the world. In combination with the oxide plant it makes Escondida the largest SX operation in the world.

The EW plant is not from Outokumpu but incorporates the latest features used at the oxide operation-EPCM cathode harvesting machines, SAME mist capturing system and the cathode quality control system developed by Apliq. The overall electricity supply system was provided by Areva.

As well as the leach-SX-EW elements, the SLP includes a process water supply system based on a 525-liter/sec seawater desalination plant from Degremont at the Minera Escondida port in Coloso. Bechtel worked on the system pre-feasibility study.

BHP Billiton comments that increases in raw material and labor costs, coupled with US dollar devaluation, are affecting capital costs across the entire industry but the company continues to report that the SLP is on time and budget for completion later this year.

Pre-feasibility for the Minera Spence project was completed in 2001 and revised with trade-off studies by Aker Kvaerner, starting in April 2003. The project was approved in October 2004, with a budget of $990 million and Aker Kvaerner was awarded the EPCM contract in January 2005. Proven and probable ore reserves totalled 310 million mt with an average overall grade of 1.14% Cu and a cut-off of 0.30% Cu. Design capacity is a nominal 200,000 mt/y copper cathode based on a mined grade of approximately 1.3% Cu.

The operation is an open-pit mine and a fleet of 240-ton Caterpillar 793C trucks will haul ore to the primary crusher. A conveying system will supply bio- and chemical heap leaching pads. The oxide and sulphide ores will be treated in separate SX-EW plants but will supply electrolyte to a single automated electrowinning plant. Aker Kvaerner is supplying the SX technology and is using Xstrata Technology stripping machines and Kunz cranes in the three tankhouses. The automation technology is from Siemens. G Corner was contracted to provide electrical supply equipment worth $2.6 million. ALTAmet Metallurgical Services has been the consultant for SX plant fire protection.

Oxiana Resources’ Sepon Copper/Gold Project required the Khanong Development Group (KDG) to engineer, procure, construct and commission a world-first copper processing facility to produce 60,000 mt/y LME “A” grade copper cathode from secondary sulphide minerals, and also to double the capacity of the existing gold plant to 2.5 million mt/y ore throughput. Minor complications: the site is located in the jungle of Laos over 1,000 km from the nearest port, and on the Ho Chi Minh Trail, requiring unexploded ordnance to be cleared before work could commence. But KDG, which is a joint venture between Bateman and the Australian firm Ausenco, completed the copper facility two weeks ahead of schedule in March 2005 and under budget.

The remote location requires the acid and ferric reagents for the patented copper-leaching process, which is able to accommodate acid-hungry high-carbonate oxide ores, to be generated autogenously on site. The two-step leaching process recovers more than 90% of the copper, which is recovered after clarification of the liquor and passage through the solvent-extraction and electrowinning circuits, which were supplied by Outokumpu Technology.

Bateman says the key to the overall flowsheet is the simple method of ferric generation that allows an aggressive approach to copper recovery in the first, atmospheric-leach, circuit. Following counter-current decantation, the remaining elemental sulphur, copper sulphides and pyrite are recovered by acidic flotation, with the copper-rich portion of the concentrate treated in medium-pressure autoclaves at 220°C. The autoclaves generate reagents that are recycled to the atmospheric-leach circuit.

SX-EW technology has made an important contribution to the re-invigoration of the Central African Copperbelt, once the head-to-head competitor with Chile in the copper market. The process was started by the enterprising Canadian firm First Quantum Minerals (FQM) in 1998, initially treating only dump material at Bwana Mkubwa to yield cathode copper and surplus sulphuric acid. In 2000, the company discovered the Lonshi deposit just over the border in DRC and started mining it in August 2001 to supply the accordingly modified SX-EW plant at Bwana Mkubwa. Engineered by GRD Minproc, this was the first SX plant to use Bateman pinned-bed clarifiers to reduce suspended solids in the pregnant leach solution.

In 2001, FQM also acquired an 80% interest in the Kansanshi copper-gold project and the following year developed concepts for two development phases. GRD Minproc completed a definitive feasibility study for the first 16-year phase and development started in 2003. Later an independent operations review led to the acceleration of the mining schedule and higher production rate targets. FQM and GRD started prioritized commissioning November 2004 and commercial copper cathode production commenced in April 2005.

Kansanshi is mining approximately equal amounts of oxide plus mixed ore and sulphide ore. The milled oxide/ mixed material is cycloned to separate feeds for gravity gold recovery and copper flotation. The sulphide mill product is separated into a flash flotation feed and a conventional flotation feed. The flotation concentrates are filtered for sale to smelters, including the Mufulira unit owned by Mopani Copper Mines in which FQM has a minor stake. The milling, thickening and flotation equipment (but not the SX or EW systems) was supplied as a package by Outokumpu Technology and FQM has just ordered a 12,000-kW SAG mill plus ball mills for its new Frontier project which straddles the DRC-Zambia border.

The oxide/mixed flotation tails are thickened for agitated leaching in four vessels which supply the SX-EW system. This also incorporates two Bateman pinned-bed clarifiers and the company has since supplied four more units to metals producers, including one for Phelps Dodge Morenci and one for uranium clarification. G Corner supplied the electricity supply systems for three EW tankhouses.

At least in part as a response to the anticipated medium term shortage of copper smelter capacity in Zambia, FQM engaged GRD Minproc and Bateman’s Western Australian office to work on a second leach-SX-EW project for Kansanshi, which will treat sulphide flotation concentrate grading a nominal 29% copper and 8 g/mt gold. Ammtec, also based in WA, undertook the test work, directed by First Quantum and Bateman. In December 2005, Bateman completed a scoping study for this high-pressure oxidation and leach (HPL) scheme to treat, initially, 105,000 mt/y concentrate. The process will utilize two autoclaves and an oxygen plant acquired from Turquoise Ridge in Nevada, USA, plus an additional 35,000 mt/y SX-EW facility.

Construction started in 2005 and the commissioning program should start in third quarter 2006, First Quantum recently reported. The autoclaves can handle up to 200,000 mt/y concentrate given an expansion of the oxygen plant. Bateman, GRD Minproc and First Quantum estimate project capital cost at US$72 million, with the use of the second-hand equipment saving up to 12 months time. The project should also improve the economics of the existing oxide treatment by meeting part of the leach acid demand.

Another project that has been reworked to improve the economics is Inco’s Goro nickel-cobalt venture. Although this exercise has not changed the process flow sheet much from that originally devised by Bechtel in joint venture with Technip, the new EPCM team of SNC-Lavalin and Foster Wheeler is using Bateman Litwin SX equipment rather than Technip’s Krebs mixer-settlers. The process involves a primary SX circuit that extracts nickel, cobalt and zinc; an ion exchange circuit that removes the zinc to yield a nickelcobalt solution; and a secondary SX circuit that removes cobalt from this solution. The two SX circuits have 19 pulsed columns (for extraction, scrubbing and stripping) and two reverse flow mixer settlers for scrubbing. Start-up is scheduled for January 2007.

And Next

A significant part of recent and ongoing SX development effort focuses on extending the application range. With electrowinning new applications in, for example, titanium production are also of interest although, driven by the energy cost factor, improvements to the process are probably of wider concern.

Vanadium recovery is one application where solvent extraction is poised to make a breakthrough. Within the Anglo American constellation, Highveld Steel and Vanadium Corp. (Vanchem) is a major supplier of vanadium products to international markets. Significant premiums can be obtained for high-purity chemicals such as ammonium metavanadate (AMV) and vanadium pentoxide. On the other hand, explains Anglo Research solvent extraction fundi Kathy Sole, vanadium’s numerous oxidation states and narrow regions of species stability have thus far frustrated the development of a robust refining process.

So there is good reason to improve the current process, which involves conventional roast leaching of a titaniferous magnetite, precipitation of impurities from the leach liquor to yield a desilicated liquor and processing of this desil liquor to AMV; the latter can be calcined to make vanadium pentoxide. Accordingly, Anglo Research (AR) has been working with Vanchem and recently brought together many years’ work to pilot a vanadium SX circuit. AR carried out the development work and a pilot trial was then carried out on-site. This pilot plant operation was led by the AR team but also involved seven Vanchem metallurgists familiar with practical issues relating to full-scale operation, knowledge that proved beneficial during the on-site trial.

Selection of a suitable tertiary amine SX reagent for processing the desilicated liquor to a high-purity vanadium electrolyte followed evaluation of both SX and ion-exchange options. Another essential factor in achieving success was the use of more sophisticated monitoring equipment than had previously been available, says Sole: the vanadium SX process is sensitive to pH in both the extraction and stripping circuits but the use of pressurized gel-based pH probes linked to automatic dosing controllers facilitated close control of pH stability.

Approximately 1 m^sup 3^/day of desilicated liquor was processed through the pilot circuit over a four-week campaign in February 2005, and the loaded strip liquor returned to Vanchem for conversion to AMV. The objective of proving the process in continuous operation-with respect to chemistry, operability, and process control-was successfully met and the design and operating parameters for a full-scale circuit were confirmed. The team obtained sufficient mass-balance data for the design and costing of a commercial circuit.

By Kyran Casteel, European Editor

Copyright Mining Media Mar 2006

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