Process for sulphide concentration
The present invention relates to a gravity/flotation circuit where a mineral stream, such as a flotation rougher or cleaner concentrate, undergoes a two stage size separation wherein: i) the screen of the first stage provides a coarse stream of particles greater than 110 microns and a fines stream of particles less than 110 microns; and ii) the fines stream of less than 110 microns is subjected to the second stage separation whereupon a cyclone provides an ultrafine/slimes stream of p80 less than about 25 microns and a middlings stream of p80 greater than around 25 microns. The middlings stream is subjected to flotation to recover nickel sulphide which is sent to final concentrate, and reject or depress magnesia which undergoes gravity separation to concentrate the MgO. It has been discovered that a large proportion of the MgO minerals in the concentrate are contained in the 30 to 100 micron size fraction and that they are well liberated making physical separation possible. Furthermore, the nickel sulphide minerals and magnesia minerals have a significantly different specific gravity which can be exploited using gravity separation equipment to achieve magnesia rejection.
1. A method of processing a mineral stream containing valuable nickel sulphide and a solid contaminant in the form of magnesia (MgO) that includes the steps of:
(a) separating a feed stream in a first stage on a screen into a coarse stream of particles greater than 110 microns and a fines stream of particles less than 110 microns;
(b) separating the fines stream in a cyclone in a second stage and producing an ultra fines/slimes stream of P80 less than 25 microns and a middlings stream of P80 greater than 25 microns;
(c) subjecting the middlings stream to flotation and recovering nickel sulphide which is sent to final concentrate and rejecting or depressing MgO in a tails stream; and
(d) treating the rejected/depressed MgO-containing tail stream by gravity separation and concentrating the MgO.
2. A method as defined in claim 1 wherein the gravity separation in step (d) is conducted in at least two stages such as a rougher and a scavenger stage arranged in series.
3. A method as defined in claim 1 wherein the coarse stream from step (a) is ground and then floated to reject more of the MgO.
4. A method as defined in claim 1 wherein the middlings stream is treated with an alkali and/or depressant to enhance flotation of the nickel sulphide in step (c) to the final concentrate and depress the MgO to thereafter undergo the gravity separation.
5. A method as defined in claim 1 wherein the ultrafine/slimes stream is subjected to flotation in a further step to reject any of the MgO to upgrade the concentration of nickel sulphide.
6. A method as defined in claim 5 wherein said ultrafine/slimes stream is treated with an acid and/or activator to enhance flotation of the nickel sulphide to a final concentrate and depress the MgO to a concentrated contaminant or tails stream.
7. A method as defined in claim 1 wherein the MgO is rejected to a concentrated contaminant or tails stream which is subjected to flotation in a further step to recover residual valuable minerals from the mineral concentrate.
8. A method as defined in claim 7 wherein the concentrated contaminant or tails stream is ground to liberate at least some of the residual nickel sulphide prior to said flotation.
9. A method as defined in claim 1 wherein the fines stream from step (a) is from between about 30 to 110 microns.