Ann: Further Testwork to Enhance Opuwo Scoping Study, page-429

THERMAL DECOMPOSITION (PYROLYSIS) OF PYRITE CONCENTRATE
The pyrite mineral is thermally decomposed into pyrrhotite and elemental sulphur by heating to 650–700°C. A nitrogen atmosphere
is used to prevent any oxidation. The off-gas is collected, and cooled to recover the sulphur. In the PFS testwork program, 100 kg of
concentrate grading 3326 ppm cobalt was processed in a custom built rotary furnace. Variations in operating conditions were tested,
with the best results showing that >95% of the pyrite could be converted into pyrrhotite along with the simultaneous recovery of 40%
of the head sulphur. The calcine was then passed through a magnetic separator to prepare a magnetic fraction containing pyrrhotite
for leaching, and a non-magnetic fraction containing unreacted pyrite for recycle to the concentrator circuit.

LEACHING AND PRODUCTION OF MIXED HYDROXIDE PRECIPITATE
The artificial pyrrhotite is leached in a low-temperature (130°C) and pressure (10–15 bar) autoclave. The resulting leach residue is
screened, and the coarse fraction is sent for sulphur recovery by distillation or remelting. The fines fraction is discarded as tails from
the process plant. The resulting leach solutions are treated to remove iron, copper and zinc before precipitating the cobalt as a
mixed hydroxide (along with nickel and manganese).

Leaching is similar but higher than CLAs previously stated temp and pressures

The furnace is the expensive bit. Maintaining 700deg C in a large volume is not cheap.