After more than a century of intensive exploitation of soils, New Caledonia now has many mineral but low . It concentrates 30% of the world laterite resources. Therefore Vale in New Caledonia has chosen to use the hydrometallurgical process , more suitable for treating low grade laterites .
Hydrometallurgy is used since the early 20th century for zinc , copper and aluminum. In the 50s , this process is suitable for treating lateritic nickel ores.
Several steps are used to extract nickel and cobalt from laterite and the Great South produce nickel oxide and cobalt carbonate:
The pulping ore
After the rejection of large rocks, earth and rock ore from the mine operation ( Limonite and Saprolite ) are pulped by adding water and stirring . The ore slurry is transported through pipes to the factory, located below the mine.
The pulp is then decanted to obtain , first , a slurry of ore, said thickened pulp , and , on the other hand , water , which is recycled to the mine to produce again the pulp .
The extraction of metals from the ore pulp ( leaching )
The thickened pulp is then treated by addition of acid : this will have the effect of extracting metals from solid minerals and liquefy . The thickened pulp is gradually heated up to 260 ° at a pressure of 45 bar which prevents the water from reaching the boiling point.
It then undergoes a process of leaching , called leaching. It is injected in an autoclave at 270 ° C where it is brewed with sulfuric acid. Certain metals such as nickel and cobalt are dissolved , while the iron content in the pulp , for example, remains in a solid state in the form of hematite (iron oxide ) . Each autoclave processes per hour, 200 tons of ore and consumes 67 tons of acid. After this step , the pulp is now ” leached ” and it is gradually cooled at atmospheric pressure.
Producing a solution after extraction of solids leached pulp
The leached pulp is decanted into thickeners operation during which the solids settle to the bottom of the thickener . Thus, these solid , stripped of their nickel and separated from the solution is discharged to the treatment facility discharges Vale New Caledonia . The decanted solution now contains Nickel, Chrome, Zinc , Cobalt , of Aluminium , Copper , Magnesium, Manganese , some traces of iron and a little sulfuric acid, not consumed during leaching .
The progressive purification of the solution
The next step is to eradicate acid still present in the mixture : thus adding limestone and lime to the solution. By reacting with acid, these bases will produce gypsum , more commonly known as plaster. Aluminum, chromium, iron and most of the copper contained in the initial solution then solidify , due to the disappearance of the acid, and mix with gypsum. Gypsum enriched with these minerals is then discharged to the treatment facility discharges Vale New Caledonia to be processed .
Nickel and Cobalt are finally isolated Copper , Zinc , Magnesium and Manganese through a series of chemical reactions that take place in the solvent extraction columns. Copper, Zinc, Magnesium and Manganese are also sent to the waste treatment center at the end of these operations.
Separation of Nickel Cobalt
A second solvent extraction unit separates the nickel and cobalt in two separate solutions .
Production of Nickel and Cobalt solid
Nickel solution obtained is then passed in ovens at a temperature of 800°C. The water evaporates and nickel oxide grains are formed. This is the main product that will sell Vale New Caledonia . The solution is seen Cobalt add sodium carbonate , causing the precipitation of cobalt carbonate , the second product for export .