Nickel ores from three different mines, Digger Rocks, Cosmic Boy and
Flying Fox, are treated at the Forrestania concentration plant. The ore
types vary from oxidised supergene ores to primary
pyrrhotite+pentlandite ores. Mineralogical investigations on the ore and
process samples have been carried out in different stages of process
development. One of the studies was done with the samples collected
from the flotation circuit in 19 June 1994 when the Cosmic Boy ore was
processed. The process products were screened, and the size fractions
were analysed for the main elements by appropriate analytical methods.
Mineralogy of the products was studied microscopically and composition
of the sulphides was analysed with an electron microprobe. Liberation of
the main sulphides in the flotation feed was determined with the
point-counting method, and weight percentages of the minerals were
calculated using chemical and mineralogical data. The typical sulphide assemblage of the Cosmic Boy ore is violarite,
pentlandite, pyrite, pyrrhotite and chalcopyrite. Violarite dominates over
pentlandite (ratio being 70 per cent to 30 per cent), and pyrrhotite is
strongly replaced with pyrite and magnetite. One of the most common
mineralogical features of this ore type is secondary magnetite, which
occurs as narrow rims in porous violarite or in association with secondary
pyrite. Serpentine, talc and chlorite are the main gangue minerals. In the milled feed (76 per cent -74 m) locked particles between nickel
and iron sulphides are common. About 41 per cent of total pentlandite
occurs as pure liberated particles, and 33 per cent as locked grains with
violarite. Liberation degree of violarite is 55 per cent, and the most
common type of locked particles is violarite+magnetite, carrying about 20
per cent of total violarite. To achieve a recovery of 80 per cent for
pentlandite, all the particles containing at least 50 vol-% pentlandite
should be floated into the nickel concentrate. Correspondingly recovery
of 80 per cent can be reached for violarite if all the particles containing at
least 65 vol-% violarite are floated.