THE LEACHING BEHAVIOR OF NICKELIFEROUS PYRRHOTITE CONCENTRATE IN HOT NITRIC-ACID

This paper describes the leaching behaviour of nickeliferous pyrrhotite concentrates in hot nitric acid. The effect of the following parameters was investigated by applying a conventional leach procedure, where pyrrhotite was leached in a solution containing excess HNO3: temperature, solids content, [HNO3] and particle size. The reaction products were, Fe(NO3)(3), Ni(NO3)(2), elemental sulphur, SO42- and NO gas. Sulphate formation has a strong negative effect on the efficiency and economics of any nitric acid based leach process and special attention has been given, therefore, to the mechanism of formation of SO42-. Sulphate formation can be minimized by applying a so-called controlled leach technique. This technique involves activation of the dissolution reaction of pyrrhotite by addition of a small excess of HNO3, followed by slow addition of HNO3 keeping the acid concentration at a constant low level. By applying this technique the efficiency and kinetics of the leach are improved as well. Finally a method for recovery of Ni as a Ni-sulphide concentrate is discussed and a conceptual flowsheet is proposed, in which Fe will be produced as well grown and clean hematite, sulphur as elemental sulphur and Ni as a Ni-sulphide concentrate.