The treatment of nickel ores
Nickel is used to produce metal alloys, including stainless steel and the alloys used to manufacture coins. In January 2009, BHP Billiton announced the closure of the Ravensthorpe nickel mine in the south of Western Australia. The mine had only been open for a year. Despite the stocks of nickel ores at the site being estimated to be capable of allowing for 25 years of production, the mine was declared economically not viable.
The background
Read the article 
to find out about the issues relating to the closure of the Ravensthorpe mine.
The following information focuses on the processes involved in the production of nickel. Read the information and complete the questions to test your understanding of the concepts of equilibrium, rates of reaction and chemical calculations. You should also gain an appreciation of the factors that go towards designing a successful industrial chemical process.
The chemistry involved in nickel extraction
Laterite ores contain low quantities of nickel, often less than 1% and are normally found close to the surface. The nickel is often present in ores of other metals, for example, goethite, which is an iron-containing material with a formula of FeO(OH).
Other ores of nickel are found deeper in the earth. These generally have a higher nickel content, but still only 2-3% of the mass of the ore.
The technique chosen to be used at Ravensthorpe to process the ore was called high pressure acid leaching (HiPAL), with acid used to dissolve the metal-containing minerals.
A simplified flow chart of the process used to treat the nickel ores and produce nickel is shown below. Click on each stage of the process to see what happens next.
The steps in the process are:
- Laterite ore
- HiPAL
- Intermediate product
- Re-dissolving
- Solvent extraction
- Electrowinning
- Nickel
The HiPAL process is carried out at high pressure, using sulfuric acid (H2SO4) of concentrations of over 90%, with temperatures of around 250°C at pressures that can be up to 9000 kPa.
This allows the nickel-containing minerals to be separated from other substances, such as iron compounds. The iron is precipitated as basic iron sulfate (eg Fe3(SO4)2(OH)5 ). This can then be converted to haematite (Fe2O3).
After the reaction with the acid, the material is neutralised by using a slurry of crushed limestone (containing CaCO3) mixed with water. The dissolved nickel ions are then precipitated by producing insoluble solids. This can be done by adding hydrogen sulfide gas (H2S) to precipitate solid sulfides (eg NiS). This step will take place at about 120°C and 1000 kPa. This process is very selective and results in a high percentage of nickel in the product.
Alternatively, magnesium oxide (MgO) can be used which results in the precipitation of hydroxides (eg (Ni(OH)2 ). This process can be carried out at 50°C and at normal pressures. Because the technique is less selective than the sulfide precipitation, an additional step, which involves leaching with ammonia solution, is used to improve the purity of the nickel-containing product.
These solids are called the intermediate product. Further treatment is used to increase the purity of the nickel compounds.
Solvent extraction can be used to separate the various components during the process, with the solvent chosen to dissolve only certain materials. The solvents used are fairly complex. An example of such a substance is Cyanex 272, which is excellent at separating nickel compounds from mixtures containing sulfates, as is the case in the HiPAL process. The chemical name and structure of Cyanex 272 is shown below.
bis(2,4,4-trimethylpentyl)phosphinic acid
Once the concentration of nickel ions in the solution is high enough, and most of the impurities have been removed, the nickel can be finally extracted by electrowinning (using an electrolysis cell).
Because ores from different parts of the country will contain different amounts of the various minerals, a number of variables have to be investigated when considering the best way to process an ore. This research involves small scale testing of the processes in laboratories. Much of the research into laterite ore processing has been carried out in pilot plant facilities in Perth, Western Australia.
Sources:
1. SGS Lakefield Oretest Pty Ltd. Malaga, Western Australia
2. J.H. Kyle and I.J. Corrans, 1998, Metallurgical Testing of Nickel Laterites Oretest Pty. Ltd. Kewdale, WA.
Equilibrium reflection
Now complete a reflection activity that will assess your knowledge of the concept of equilibrium.
