Increasing concern with the emission of sulphur dioxide from smelters
and roasters has caused renewed interest in processes which can use waste
sulphur dioxide, such as the sulphation of nickel laterite ores. The
process described here proposes to by-pass the production of sulphuric
acid and react laterite directly with a sulphur dioxide containing
atmosphere. A thermodynamic analysis identified operating windows in
which nickel sulphate forms while iron oxide (the major impurity in most
laterite ores) remains stable. The initial assumptions included unit
activity of all species except nickel oxide. The lowest possible nickel
oxide activity (while maintaining selectivity) increased with increasing
temperature, indicating that selective sulphation is favoured by low
temperatures. The temperature of interest was found to be between 500
and 800C. Calculations show that a feed gas containing ten per cent
SO2, five per cent 02, balance N2 requires a temperature greater than
650C for selective sulphation of nickel. Laboratory experiments showed
that the assumption of unit activity of ferric sulphate was unrealistic
above about 680C due most likely to the formation of solid solutions
which decreased the ferric sulphate activity. Results are presented which
illustrate the limiting equilibrium sulphur trioxide partial pressures at 700
and 730C below which selective sulphation occurs. Cobalt extraction
was usually between 80 and 95 per cent under conditions yielding
selective sulphation, whilst nickel extraction was about 50 to 75 per cent
depending on temperature and choice of upper limit of iron extraction.
An operating diagram is presented for a limonite ore and shows that the
optimum reaction conditions are 680C and an input gas containing 20
per cent SO2. Under these conditions nickel, cobalt and iron extractions
are 73, 95 and two per cent respectively.