Atmospheric pressure leach process for lateritic nickel ore

1. An atmospheric leach process in the recovery of nickel and cobalt from lateritic ores, said processing including the steps of: a) separating the lateritic ore into a low magnesium containing ore fraction, and a high magnesium containing ore fraction by selective mining or post mining classification; b) separately slurrying the separated ore fractions; c) leaching the low magnesium containing ore fraction with concentrated sulphuric acid as a primary leach step; and d) introducing the high magnesium content ore slurry following substantial completion of the primary leach step and precipitating iron as goethite or another low sulphate containing form of iron oxide or iron hydroxide, wherein sulphuric acid released during iron precipitation is used to leach the high magnesium ore fraction as a secondary leach step.

2. A process according to claim 1 wherein the iron is precipitated as goethite.

3. A process according to claim 1 , wherein the low magnesium containing ore fraction includes limonite ore containing less than about 6 weight % magnesium.

4. A process according to claim 1 , wherein the high magnesium containing ore fraction includes saprolite ore having greater than about 8 weight % magnesium.

5. A process according to claim 3 , wherein the low magnesium containing ore fraction also includes medium level magnesium content smectite or nontronite ore.

6. A process according to claim 4 , wherein the high magnesium containing ore fraction also includes medium level magnesium content smectite or nontronite ore.

7. A process according to claim 1 , wherein the separated ore fractions are slurried in sodium, alkali metal and ammonium free water at solids concentration greater than approximately 20 weight %.

8. A process according to claim 1 , wherein the primary leach step is carried out in a first reactor at a temperature of up to 105° C. or the boiling point of the leach reactants at atmospheric pressure.

9. A process according to claim 8 , wherein the sulphuric acid is preferably in a concentration of from 100 to 140% of stoichiometric proportions.

10. A process according to claim 1 wherein the high magnesium content ore slurry is introduced in a second reactor for completion of the secondary leach step at a temperature of up to 105° C. or boiling point of the leach reactants at atmospheric pressure.

11. A process according to claim 10 , wherein goethite, hematite or gypsum containing seeds are added to the second reactor immediately after the introduction of the high magnesium containing ore to initiate or assist iron precipitation.

12. A process according to claim 11 , wherein the dose of seeds is added in an amount of up to 20 weight % of the total of the low magnesium containing ore and high magnesium containing ore weight.

13. A process according to claim 1 , wherein the redox potential during the primary leach step is controlled to between 800 mV and 1000 mV (SHE).

14. A process according to claim 13 wherein the redox potential in the primary leach step is about 835 mV (SHE).

15. A process according to claim 13 , wherein the redox potential is controlled by injecting either sulphur dioxide gas, or sodium-free metabisulphite or sulphite into the slurry.

16. A process according to claim 13 wherein the redox potential in the secondary leach step is between 700 and 900 mV (SHE).

17. A process according to claim 1 , wherein the dry ratio between the high magnesium ore and low magnesium ore is from about 0.5 to 1.3.

18. A process according to claim 1 , including the further step of neutralisation of the leach solution after the secondary leach step by the addition of a limestone slurry to complete iron precipitation as goethite.

19. A process according to claim 18 , wherein the end point of neutralisation is to raise the pH to 1.5 to 3.0 as measured at ambient temperature.

20. A process according to claim 1 , including the further step of precipitating the remaining iron after the secondary leach step as jarosite by the addition of a jarosite forming ion.

21. A process according to claim 20 , wherein the jarosite forming ion is sodium, potassium or ammonium ion.

22. A process according to claim 1 , including the further step of reducing the remaining iron after the secondary leach step, to the ferrous state by the addition of a suitable reductant.

23. A process according to claim 22 , wherein the reductant is sulphur dioxide.

24. A process according to claim 1 , wherein the nickel and cobalt is recovered by way of either sulphide precipitation using hydrogen sulphide or other sulphide source, mixed hydroxide precipitation, ion exchange or liquid-liquid extraction.