While there are over 45 different nickel minerals, only a few of them are of economic importance to the minerals industry. The principal nickel mineral in nickel sulphide ores is pentlandite ((Ni,Fe)8S9) while the less common nickel sulphides include millerite (NiS). violarite (Ni2FeS4), polydymite (Ni3S4), siegenitc {(Co,Ni)3S4), and heazlewoodite (Ni3S2). As well, often present are nickel arsenides such as gersdorffite (NiAsS) and niccolite (NiAs).
Pentlandite is recovered primarily by froth flotation and the flotation behaviour of pentlandite is well known and defined. In the literature, however, there is very little data on the flotation behaviour of millerite. Millerite is a relatively rare sulphide mineral and is mostly found in hydrothermal deposits as a replacement for pentlandite. Bulatovic claims millerite, gersdorffite and niccolite have similar flotation properties to pentlandite but tend to oxidise more readily in the presence of oxygen. As well, he claims nickel sulphide minerals in general can be depressed with lime at pH values greater than 9.5 whereas millerite is not depressed until pH 10.0. Evidence was presented to show a combination of mercaptan and dithiophosphate collectors performed better than xanthate collector alone on a Canadian sulphide ore containing nickel present as violarite (70%) and millerite (30%).
Two micro-flotation test studies on closely sized fractions examined the collectorless flotation of millerite. one in acidic conditions and one in alkaline conditions. In acidic conditions (pH 2) millerite was found to float strongly without collector but in alkaline conditions (pH 7.5-7.8) the recovery of millerite without collector was found to be not much higher than that expected from entrainment.
In terms of the relationship between the Notability of millerite and that of arsenides commonly found in sulphide nickel ores, such as niccolite and gersdorffite it is important to note that arsenic is a penalty element for many nickel smelters where it can cause environmental problems when volatile arsenic compounds are emitted during processing. Consequently, financial penalties are imposed by smelters to treat nickel concentrates with high arsenic content. In high-arsenic nickel ores, therefore, often a separation between nickel sulphide minerals, such as millerite. and nickel arsenides is desirable.