Southern African Institute of Mining and Metallurgy
Abstract
Four sulphide minerals – pyrite (FeS2), pyrrhotite (Fe1–xS), pentlandite
([Ni,Fe]9S8), and chalcopyrite (CuFeS2) – contain the base metals and
most of the iron in concentrates of platinum group metals (PGMs). In the
pyrometallurgical processing of PGM concentrates these sulphides form a
matte during smelting, and iron and sulphur are removed from the matte
during the converting process. This paper discusses the roasting of
Merensky concentrate in air before smelting, with the purpose of reducing
the matte load to the converter.
Roasting tests were conducted in a bench-scale rotary kiln at temperatures
from 350°C to 700°C. The concentrate tested contained 17.4%
sulphur and consisted of 23% pyrrhotite, 16% pentlandite, 11%
chalcopyrite, and 2% pyrite. The particles were fine (d50 = 22 μm), and all
the sulphide particles were liberated. Roasting in air at 550°C and 650°C for
20 minutes removed respectively 60% and 70% of the sulphur. The iron in
the sulphides was oxidized to Fe3O4 (magnetite) at temperatures below
500⁰C and to Fe2O3 (haematite) at temperatures above 550⁰C. At 700⁰C the
bed sintered and copper oxides formed. At temperatures below 450°C
oxidation was incomplete: pyrrhotite remained and only 30% of the
sulphur was removed. Smelting tests were conducted to assess matte fall
and the deportment of copper and nickel to matte. It was evident that
roasting resulted in lower matte falls (a drop of approximately 60%)
compared with matte falls from unroasted concentrate. The iron and
sulphur levels in the matte were reduced to below 3.5% and 22% respectively.
This paper also briefly describes the mechanisms by which pyrrhotite,
chalcopyrite, and pentlandite are oxidized during roasting. For
chalcopyrite, the mechanism proceeds through an intermediate solid
solution phase, which extends from Cu1.02Fe1.04S2 to Cu2.04Fe0.72S2 to a
copper-rich solid solution of bornite (Cu4Fe1.4S4–Cu2S). The oxidation of
pentlandite proceeds through a monosulphide solid solution
(Ni0.39Fe0.53S–Ni0.74Fe0.15S) to a solid solution of heazlewoodite
([Ni,Fe]3±xS2). These mechanisms are explored in relation to chemical
thermodynamics and microstructures.This paper
was first presented at the, Platinum Conference
2014, 20–24 October 2014, Sun City South Africa.The National Research Foundation of South Africa (Grant
number TP1208219517).http://www.saimm.co.za/journal-papersam201
