MS NH3 202

where M = nickel, cobalt, iron, copper, zinc, and n = 2 to 6.

The leaching process takes advantage of the differing stability of the ammine complexes. The iron ammine complex is the least stable and is completely hydrolyzed to a hydrated iron oxide, Fe2O3H2O (hematite), which is reprecipitated. The leach slurry passes to a lamella thickener where the leach liquor is separated from the solid residue.

In fact, the actual reaction chemistry of the leaching process is much more complex due to the behavior of sulfur in alkaline solutions. The reaction sequence of the oxidation of sulfur to form sulfate, sulfamate, and a range of ammonium thiosalts may be represented as follows:

s2" so3nh.

where n = 2 to 6. At any given time, the leach liquor will contain sulfur in several forms. The amounts of these species present depend on the leach conditions and duration (Fig. 7.). The leach solution typically contains 60 to 70 g/L Ni, 6 to 9 g/L Co, 1 to 2 g/L Cu, 130 g/L NH3, 200 g/L (NH4)2SO4, and varying amounts of other sulfur oxyanions.

Fig. 7 A representation of the leach solution composition as determined in batch leaching tests

Before it is possible to reduce the nickel from the leach solution as the pure metal, several solution purification steps must be completed.

Cobalt Separation. The leach liquor is transferred to the nickel-cobalt separation plant. Here cobalt is separated from the nickel as pure crystalline cobaltic hexammine salt. The Sherritt cobalt refining process is further discussed in the article "Production of Cobalt-Base Powders" in this Volume. After the removal of cobalt, the nickel-rich, or "pregnant," solution proceeds through the copper-removal circuit.

Copper Removal. Copper is removed from the leach liquors and pregnant solution from cobalt separation in a unit operation referred to as the "copper boil." In the copper boil, the combination of lowering the ammonia level by boiling to distill off the free ammonia and the addition of elemental sulfur and sulfur dioxide at elevated temperature results in the precipitation of copper sulfide. The copper is a mixture of cupric sulfide (CuS) and cuprous sulfide (Cu2S) and is prepared s by the reaction of soluble copper with thiosulfate (-3 ):

+H20

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