MOLYBDENUM-COPPER SEPARATION BY FLOTATION
Molybdenite belongs to the minerals group of easy flotation which is related to its crystal structure. After grinding molybdenite particles present laminar structure that favors natural hydrophobicity. For this reason, its elevated hydrophobic capacity allows recover successfully molybdenum from ores with low grades.
The separation molybdenum-copper has been practiced for a long time. The predominant process is to depress copper sulphides and capitalizing on the easy floatability of molybdenite. There several depressants for copper minerals such as sodium cyanide, mainly if copper is present as chalcopyrite. Others reagents are certain types of sulphides such as sodium sulphide, sodium hydrosulphide or phosphorous pentasulphide when there is a mixture of copper sulphides: bornite, chalcocite, and chalcopyrite. Other methods, no very common consist in employing sodium peroxide or sodium hypochlorite.
The depressant agent most employed is sodium hydrosulphide (NaHS) which is chosen considering environmental aspects. When the reagent is added, the potential of copper sulphides is reduced to the point where they are unfloatable by xanthates. Use sodium cyanide implies more time for depressing copper sulphides and also serious problems of oxidation. Depression starts at oxidation-reduction-potential (ORP) of -250 mV and when the flotation is stable, the ORP is -450 to -500 mV.
It is very important observe the slurry and try to understand what action is doing the depressant. Thus, even with small or big dosages of NaHS, there is certain amount of oxygen in the slurry which reduces sodium hydrosulphide efficiency. The presence of dissolved oxygen trends to affect the flotation process in these ways: first, formation of sulphoxy species such as thiosulphate ions (S2O3-2), sulphite ions (SO3-2), and sulphate ions (SO4-2). As a result, HS- ions are consumed by oxygen requiring additional quantity of NaHS. Second, it is known the adverse effect in the performance of ORP electrodes because lectures are erroneous. Third, oxygen can react with the freshly formed sulphide layer favoring a bad reaction between sodium hydrosulphide and copper sulphides because oxygen can act like a sponge absorbing sulphide ions.
Consequently, air addition promotes ion sulphide oxidation to compounds less active. For this reason is common employ nitrogen as flotation gas during roughing and cleaning flotation stages. Also, NaHS addition allows obtain sulphides with fresh surface because eliminates collector, making easy its depression. With these conditions, molybdenite is ready for its recovery by flotation.
Some times, it is useful a little addition of fuel oil, mainly when molybdenite can be associated with certain iron minerals.
As pH regulator can be employed CO2 which forms a weak acid in the slurry. Other use is to modify froth texture.
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