Sections
- Introduction
- Pal and caron processes
- Sherritt Gordon process
- Nickel Chloride process
- Extraction by chelate formation
Introduction
Cobalt and nickel are found together in nature in a large number of deposits. Mining of these metals, together with others technologically associated with electronic advances, is being reactivated to guarantee growing supplies; especially for the fields of batteries, energy accumulation and the decrease in the volume of these essential components for our mobiles, computers, hybrid and electric vehicles. The international price of cobalt shot up 127% in 2017, that of copper increased by 30%, tungsten was up 27%, while the price of lithium has almost doubled since 2015; in conjunction with these figures, certain countries are positioning themselves in certain deposits, such as China in Katanga. These extraction processes have to evolve from optimised technologies to deal with deposits with a lower metal content. Cobalt and nickel are two elements that are well positioned in new technologies, especially those with an energy link.
Cobalt is part of the steel superalloys that work at high temperature with a high yield strength: e.g. magnets (e.g. Alnico, Fernico and Cunico), enamels, coatings, electrodes, batteries and structured cabling for tyres. The most important cobalt minerals are smaltite CoAs2 and cobaltite CoAsS; however, from a technical point of view, the main sources of cobalt are “speiss”. These materials are a mixture of arsenides that contain appreciable amounts of nickel, cobalt, iron and silver. The main cobalt reserves are found in the Congo, Russia, Peru, Canada, Finland, Chile, Burma, Morocco and Zimbabwe. Normally, arsenic is also a constituent of these minerals. The initial treatment of these arsenides is crushing and concentration by flotation or gravity. The parts enriched with nickel, cobalt, and often also iron, are mixed with metallurgical coke. This coke is obtained from bituminous coal in ovens with an oxygen-free atmosphere, removing the volatile content and obtaining a porous coal suitable for these metallurgical treatments. The ore is mixed with calcium oxide and silica, producing a slag. This makes it possible to separate silver, a mixture of materials rich in copper, nickel and cobalt and a scorifiable residue. This mixture rich in copper, nickel and cobalt is what is technically called “speiss”. Getting from this mixture to the pure metal is a complex process.
Nickel is normally found in nature combined with arsenic, antimony and sulfur in the form of sulfides. Approximately 65% of nickel is used in the manufacture of austenitic stainless steel and another 21% in the manufacture of superalloys. The rest is used for the manufacture of other alloys (Alnico, mu-metals, monel, nitinol) and catalysts. Canada, Cuba and Russia produce 70% of the nickel in the world. Bolivia, Colombia and New Caledonia also have significant deposits. The mineral sources of nickel are millerite, NiS, as well as deposits of NiSb, NiAs2, NiAsS, NiSbS and garnierite Si4O13[Ni, Mg]2•2H2O. The most important deposits from a commercial point of view are those of garnierite, which is a magnesium and nickel silicate of variable composition and combined with pyrrotin (FenSn+1) that contains 3-5% nickel. Nickel alloyed with iron is also found in meteorites.
Pal and caron processes
In general, nickel is obtained by roasting nickel sulphides in air to obtain NiO. This is then reduced with carbon to obtain nickel metal. The nickel is purified by combining carbon monoxide with impure nickel at 50°C and atmospheric pressure, or from the mixture of nickel and copper, under more complex conditions, obtaining Ni(CO)4, which is volatile. By thermal decomposition at 200°C, pure nickel is recovered at high purity.
When talking about nickel and cobalt metallurgy, different types of deposits must be distinguished. Firstly, there are the limonite laterite deposits. They are soils located in warm regions noted for their low silica concentration and high oxide content. These materials are usually treated with hydrometallurgical methods: the CARON process (leaching generated by ammonium carbonate) and the PAL process (high pressure acid leaching). The PAL metallurgical process involves preheating the ore and leaching with concentrated sulfuric acid at high temperatures and pressures. The chemical species of nickel and cobalt by hydrometallurgical chemical process are soluble sulfate salts, which are recovered from dissolution in a countercurrent decanting circuit (CCD). CCD involves washing the residue and recovering soluble nickel and cobalt. The remaining acid is neutralised using a calcium carbonate suspension, which produces a calcium sulfate precipitate. Hydrogen sulfide can be injected to precipitate nickel and other sulfides. After this, there is another leaching stage to remove iron and copper, and finally nickel is precipitated by the addition of ammonia, ammonium sulfate and hydrogen. One of the most used processes to treat nickel and cobalt “speiss” is that used by the company, Sherritt Gordon Mines Ltd, from Fort Saskatchewan, Alberta, Canada.
Sherritt Gordon Process
The nickel and cobalt metallurgy processes begin with an initial treatment of the mineral, re-concentrating it through crushing and flotation/gravity and obtaining a “speiss”, rich in cobalt and nickel. The process begins by adding the sulfide-associated mineral to a reactor with sulfuric acid and pressurised air. This procedure removes the sulphides to obtain nickel (II) sulfate and cobalt (II) sulfate in solution. In this first dissolution stage, the sulfide is removed.
NiS + 2O2 ⟶ NiSO4
CoS + 2O2 ⟶ CoSO4
The leaching process takes place with NH3, the pH is adjusted and an initial precipitation of iron occurs.
NH3 + H2O ⟶ NH4OH
Fe+3 + OH ⟶ Fe(OH)3
Precipitation of Fe3+ occurs in the form of Fe2O3 and SiO2. Iron is usually associated with cobalt and nickel sulphides and must be separated. By regulating the pH to values close to 7, it precipitates. Subsequently, under the same conditions of pressurised air and ammonia, oxidation of Co2+ to Co3+ occurs.
Cobalt (II) in aqueous solution in the presence of ammonia easily oxidises to Co (III) with the formation of a complex. The majority of complexing agents are ligands from weak acids, i.e. Brönsted bases and therefore the pH value is a critical factor for the formation and stabilisation of the complex. The effective concentration of the ligand in the solution, determined by the pH, affects the dissolution of the complex. In general, the complex dissociates less at high pH values, as the free ligand predominates at those pH values.
However, the oxidation of Co (II) to Co (III) is not easy since Co (II) compounds are much more stable and the coordination compounds of Co (III) hardly exchange ligands, unlike those of Co (II). So the chemistry requires time and you cannot provide heat energy to the system, as almost always you get a mixture of the two complexes, those of Co (II) and those of Co (III). For this reason, the reactor is brought to temperatures of 80°C and air pressures of 9 atmospheres. An explanation can be given by considering the data in the following table;
In this part of the leaching process, the temperature input is necessary for the formation of [Co(NH3)6]3+.
The formation of Co3+, as indicated by the Pourbaix diagram, is promoted by basic pHs and potentials greater than 1.2.
In the next step, sulfuric acid is added to produce nickel and ammonium sulfate (NiSO4 (NH4)2SO4.H2O). This has a green colour and is poorly soluble in water; thus, precipitation is favoured. In this step, this salt is evaporated and crystallised repeatedly to increase the purity of the crystals. These nickel and ammonium bisulfate crystals are treated with a concentrated solution of NaOH to form Ni(OH)2. Nickel hydroxide is dissolved by sulfuric acid to form nickel sulfate, which is reduced by electrolysis: Ni2+ to Ni0. The ammonium sulfate solutions allow the recovery of ammonia by stripping.
The liquid solution containing [Co(NH3)6]3+ is reduced from Co3+ to Co2+ by Co0 powder (cobalt metal). Finally, Co2+ is reduced to Co0 by hydrogen, obtaining metal cobalt powder.
Below is a diagram showing the sequence of the different Sherritt Gordon process operations.
NICKEL CHLORIDE PROCESS
At present, the large cobalt and nickel ores are in the process of clear depletion and the main ores of these two metals are constituted by minor mineral concentrations in ores formed by different metals. This requires the mineral extraction processes to be modified in each case.
Thus, for “speiss” with low grade cobalt and nickel, treatment in chlorinated medium occurs:
Co2+ + Cl2 + ZnO (pH4; pH regulator) ⟶ Co3+
Ni2++ Cl2 + ZnO (pH4; pH regulator) ⟶Ni3+
By properly controlling the pH and the reduction potential, especially at basic pH according to the chemical species distribution diagram, Ni (II) can be kept in solution and Co(OH)3 precipitated:
Cobalt hydroxide can be converted into cobalt oxide by calcination. This cobalt (III) oxide may have zinc oxide as an impurity, so this must be treated at a weakly acidic pH to separate it from cobalt. Finally, cobalt hydroxide can be converted by calcination of cobalt (III) oxide.
EXTRACTION BY CHELATE FORMATION
Another of the methods most used for the treatment of concentrated metal mixtures is the treatment with extracting agents. In this process, a concentrated solution is treated by leaching sulfuric acid in species such as nickel, copper and iron. First, the high concentration of sulfuric acid is treated with amine to reduce its concentration. Secondly, the pH is adjusted to between 3 and 4 which facilitates the precipitation of iron. The copper and nickel remain In solution. The copper is solvent extracted. The types of solvent used in this process are organic ones such as oximes, diethyldithiocarbamate, butyl acetates and ketoximes. These solvents form organometallic chelates with the metallic species in question, in this case copper, which are soluble in the organic phase.
The aqueous phase is enriched with NaCN to facilitate the formation of nickel complexes and accentuate the difference between the two phases, so that the two species are stabilised in their respective phases.
Once the nickel and copper solutions are separated, the metals can be recovered electrolytically.
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