Recovery of Cobalt from Slag in a DC Arc Furnace at Chambishi, Zambia
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Recovery of cobalt from slag in a DC arc furnace at Chambishi, Zambia by R.T. Jones*, G.M. Denton*, Q.G. Reynolds*, J.A.L. Parker†, and G.J.J. van Tonder† copper deposits in the Democratic Republic of the Congo (DRC) and Zambia. In addition, it is Synopsis estimated that there are between 2.5 and 10 million tons of hypothetical and speculative The 20 Mt reverberatory furnace slag dump at Nkana in Zambia cobalt resources in deep-sea manganese contains a substantial quantity of cobalt. A 40 MW DC arc furnace nodules and crusts on the ocean floor. has been built at Chambishi for the purpose of recovering cobalt Worldwide production of cobalt is currently from the slag. The technology was jointly developed by Mintek and Anglovaal Mining Ltd (Avmin), and the furnace was designed by over 30 kt/a (compared to world nickel Bateman Titaco. production of around 700 kt/a). Demand is Testwork at the 150–250 kW scale was conducted at AVRL, and expected to rise along with production of larger scale piloting was conducted at Mintek (in partnership with superalloys (increasing at perhaps 7% per Avmin) in a 3 MW DC arc furnace. Approximately 840 tons of annum) used for jet engines and gas turbines, Nkana dump slag (ranging from 0.66% Co) was processed at power as well as catalysts for oil refining (about 4% levels around 1-2 MW. Good overall cobalt extraction was achieved, per annum), and especially rechargeable and approximately 100 tons of cobalt-bearing alloy was produced batteries (growing about 40% per annum). (containing 5 to 14% Co). This testwork demonstrated that the Cobalt supply is expected to continue to Nkana dump slag could be processed in a DC arc furnace of suitable increase over the next few years, primarily design, to produce a cobalt-bearing alloy suitable for further hydrometallurgical processing. from new nickel mines, where cobalt is Bateman Titaco were contracted to design and build the produced as a by-product. furnace, and the design team contained representatives from For many years, cobalt production was Avmin and Mintek. The project was conceived as a fast-track dominated by Zaire (now the DRC). At its peak exercise, and many activities had to be run in parallel. Notable in 1986, state-owned Gecamines produced features of the furnace include an ABB power supply, a Concast 14.5 kt/a of cobalt. However, as a result of the conductive hearth, the refractory design, and copper cooling in the conflict in that country, production dropped to sidewalls. below 5 kt/a by 1993. Power to the furnace was switched on during January 2001. In recent years, there has been a great deal of interest expressed in the recovery of cobalt (often together with nickel and copper) from a variety of slags. A process to carry this out, using a DC arc furnace, has been under Introduction development at Mintek since 1988, and has successfully been demonstrated at pilot scale1. Cobalt has been in use since at least 2250 BC, The process involves the selective when the Persians used it to colour glass. It carbothermic reduction of the oxides of cobalt, was not until 1735, however, that Swedish nickel, and copper, while retaining the scientist G. Brandt first isolated metallic cobalt, maximum possible quantity of iron as oxide in and it was not until 1780 that it was the slag. Early pilot-plant testwork1 at Mintek recognized as an element. Today, cobalt is demonstrated recoveries of 98% for nickel and used mainly in high-temperature steel alloys, over 80% for cobalt, at power levels of up to magnetic alloys and hard-facing alloys 600 kW. resistant to abrasion. Jet engines in current 747 aircraft are estimated to contain 180 kg of cobalt apiece. Identified world cobalt resources are about * Mintek, Private Bag X3015, Randburg 2125. † 11 million metric tons. The vast majority of Bateman Titaco, P.O. Box 32102, Braamfontein 2017. these resources are nickel-bearing laterite © The South African Institute of Mining and deposits, with most of the rest occurring in Metallurgy, 2002. SA ISSN 0038–223X/3.00 + nickel-copper sulphide deposits hosted in 0.00. This paper was first presented at the SAIMM mafic and ultramafic rocks in Australia, Conference: Copper cobalt nickel and zinc recovery, Canada, and Russia, as well as in sedimentary 16–18 July 2001. The Journal of The South African Institute of Mining and Metallurgy JANUARY/FEBRUARY 2002 5 ▲ Recovery of cobalt from slag in a DC arc furnace at Chambishi, Zambia Theoretical background to the smelting process Mineralogical studies have shown that cobalt is present as CoO in copper reverberatory furnace slag. Copper in the slag is mainly attributed to the presence of copper-rich sulphides. The cobalt oxide, and, to a lesser extent, the copper oxide associated with the silicate/oxide phases, is reduced by Fe from the alloy to form metallic Co (and Cu), resulting in the formation of FeO in the slag. The CoO in the slag is associated primarily with Fe2SiO4, and analysis by scanning electron microscopy showed some Fe2SiO4 particles with no detectable Co or Cu, thus demonstrating that it is, in principle, possible to remove all the Co and Cu from this phase. Because the cobalt is present in the slag in oxidized form, recoveries would be very low if conventional slag cleaning Figure 1—DC arc furnace building at Chambishi (typically using an AC slag resistance furnace) was used, as this relies largely on a gravity settling mechanism, whereby entrained sulphide and metallic droplets are simultaneously collected. Other conventional recovery techniques, such as More recently, attention has been focused on the Nkana slow cooling of the slag, followed by milling and flotation, slag dump on the Zambian Copperbelt. More than six decades are also inappropriate for the recovery of oxidized metals of copper mining and smelting at the site near Kitwe, 250 km dissolved in slag. Conditions need to be sufficiently reducing north of Lusaka, have left behind about 20 million tons of to recover a significant fraction of the cobalt present. These slag grading between 0.3 and 2.6% cobalt. This dump, conditions can be achieved very effectively by means of the covering a square kilometre to a depth of about 30 m, is addition of a reductant (such as carbon) to a DC arc furnace. probably the world’s largest cobalt resource that is situated When carbon is added to the slag, the various metallic above ground. Many attempts have been made over the years to recover cobalt from this dump, and much research has elements reduce to different extents, at a given level of been carried out in this field. Anglovaal Mining Limited carbon addition. This behaviour allows a reasonable degree (Avmin) of South Africa purchased the reverberatory furnace of separation to take place during smelting. The intention in slag dump at the Nkana smelter and the Chambishi Roast this part of the process is to separate the valuable non- Leach Electrowinning (RLE) plant in 1998 for US $50 ferrous metals from the iron and the gangue constituents million. In July 1999, plans were announced to expand the present in the slag. The desirable area of operation is clearly Chambishi RLE plant to incorporate a US $100 million somewhere in the region where the recovery of cobalt is high, smelter facility to process the slag, increasing the Chambishi and the recovery of iron to the alloy is still reasonably low. production by 4 kt/a of cobalt and 3.5 kt/a of copper. This process needs to operate at a temperature above the The overall management of the project was handled by liquidus temperature of the alloy containing the Co, Cu, and Kvaerner, with Dowding Reynard & Associates providing the Fe. Of these elements, Fe has the highest melting point of equipment for material handling and feed preparation circuits around 1540°C. For present purposes, let us assume an for the solid dump slag, Bateman Titaco providing the DC arc operating temperature somewhere between 1500 and furnace, Atomising Systems (through Bateman IST) 1600°C. supplying the water atomizer, and Hatch providing the The interchange between Co and Fe can be seen by leaching equipment. studying the liquid reaction between slag and alloy: Chambishi Metals plc (owned 90% by Avmin, with ZCCM CoO + Fe = Co + FeO [1] holding the remainder) recently commenced the commis- At equilibrium, the degree of separation between Co and sioning of its new slag processing facility. Avmin have Fe can be indicated by the equilibrium constant, K, which is constructed a new wing at the Chambishi cobalt refinery, and strictly a function of temperature only. Over the temperature the smelter and leach plants are expected to ramp up to full production during 2002. The new treatment facility will range of interest, K has a value of approximately 30. aa⋅ blend, smelt, leach, and electrowin cobalt and copper from K = Co FeO ⋅ [2] Nkana slag. The additional section is expected to add about aaCoO Fe 3.6–4.1 kt/a of cobalt, and 2.7–4.5 kt/a of copper to Chambishi’s output. The expanded plant will have the The activities a may be expressed in terms of activity γ capacity to process around 7 kt/a of cobalt, and 15 kt/a of coefficients and mole fractions x. copper. The 20 Mt of cobalt-rich slag from Nkana will pass γχ⋅ γ χ K = Co Co FeO FeO through the new plant prior to being refined into cobalt and γχ⋅ γχ [3] CoO CoO Fe Fe copper cathodes in the existing plant.