Purification of molybdenite concentrates

Abstract: A molybdenite flotation concentrate was refined by selective removal of
copper impurity with minimum dissolution of molybdenum in the concentrate.
Copper is present in the concentrate mainly as copper sulphide,
chalcopyrite. Investigations were carried out on the removal of the sulphide
by their selective leaching in sodium cyanide, ferric chloride and ferric
sulphate solutions.

Approximately 80 to 100% of the copper concentration was removed under
optimum conditions with sodium cyanide and ferric chloride solutions with
little dissolution of molybdenum while ferric sulphate solution was
ineffective due to a number of factors such as passivation of chalcopyrite,
temperature and redox potential.

Leaching with sodium cyanide was carried out with stoichiometric
concentration of the salt, 20% solids, at ambient temperature and pH above11
with oxygen as oxidative gas, for 53 and 74 hours. Ferric chloride leaching
was carried out at 35% solids with 10% ferric chloride, 1% cupric chloride,
20% calcium chloride all by weight of solution at an average temperature of
80oC and pH about zero for 4 hours. Ferric sulphate leaching was carried out
with stoichiometric concentration of ferrous sulphate as the starting
solution in which ferric sulphate solution was generated by oxidation with
addition of oxygen and sulphur dioxide in ratio 10:1, 10% solids and pH
below 0.5 at 65oC for 5 hours.
The intermediate pregnant solution samples and final leach solutions from
the tests were analyzed for dissolved copper with the aid of atomic
absorption spectrophotometer, (AAS) and the purified concentrate (solid
residue) was analyzed for molybdenum, copper and other important elements
with different analytical techniques such as: (i) AAS for molybdenum,
calcium, bismuth and low level copper, iron and lead. (ii) Solution X-ray
for high-level copper, iron and lead. (iii) Fire assay for gold and silver.
(iv) Leco-owen for sulphur. (v) Field ionization mass spectroscopy, (FIMS)
instrument for mercury and (vi) Selective electrode for chloride ion.