Can comminution be eliminated from mining (posting of 20th May ) has generated a tremendous amount of interest, both here and on Minerals Engineers on LinkedIn. The interesting thing is that the initial discussion was inspired by the abstract of one of the keynote lectures at next year's Comminution '14, and at this stage we have no idea what the presenter, Alan Muir, will be putting forward as an alternative!
So will comminution eventually be phased out of mining operations? Most of the comments suggest that comminution will evolve and will be around for ever, but my bet would be that this, perhaps the least energy efficient of all major industrial processes, will eventually become extinct, but what will replace it?
Most probably far-off processes that we have not even dreamt about; but could any existing techniques provide a replacement? In situ leaching is a possibility, and this is currently used to extract water soluble salts such as sylvite and halite. Just under half of the world's uranium is produced by in situ leaching, most of the uranium produced in the USA and Kazakhstan being by this method. In situ leaching has also been used to dissolve oxidised copper minerals such as malachite and azurite.
But of course not all ores are amenable to leaching, and there is the inherent problem of contamination of ground water, as well as the very high cost of the reagents, which makes it prohibitive for many ores. So attempts to remove comminution from the flowsheet must follow a 'horses for courses" approach.
The mineralogy of many of the polymetallic ores containing economic amounts of copper, lead and zinc is a complex assembly of finely disseminated and intimately associated chalcopyrite, galena and sphalerite in a gangue consisting predominantly of pyrite or pyrrhotite (often 80-90%), quartz and carbonates. Extensive fine grinding is usually needed, usually to well below 75 microns. A classic example of the difficulty in treating these ores is the huge zinc-lead-silver deposit at McArthur River in Australia, one of the world’s largest zinc-lead deposits. Discovered in 1955, for 35 years it resisted attempts to find an economic processing route due to the very fine grained texture of the ore. However, the development of the IsaMill, together with an appropriate flotation circuit, allowed the ore to be successfully processed, and the mine was finally opened in 1995. The concentrator produces a bulk lead-zinc concentrate with an extremely fine product size of 80% minus 7 microns.
Grinding of complex massive sulphide ores consumes vast amounts of energy, and extremely fine mineral dissemination leads to relatively low concentrate grades, and high metal losses, not only in the flotation tailings, but into the ‘wrong’ concentrates, penalties often being imposed for the presence of zinc and lead in copper concentrates.
So is there a technique currently available that could eliminate the comminution step in the treatment of these important sources of base metals? Well, yes there is, and not only could it remove the comminution stage, but also the difficult and inefficient flotation stage! It may seem economically impossible, but it has been proven at pilot stage to be viable.
|Amanda with Prof. Noel Warner in 2010|
Polymetallic smelting of concentrates is established practice and is used in the copper-nickel industry to produce matte from bulk Cu-Ni-Co sulphide concentrates. The Imperial Smelting Furnace has been used for well over half a century for treating bulk lead-zinc concentrates, and the KIVCET process has been used to smelt complex Cu-Zn-Pb sulphides. But what was to become known as the Warner Process was more radical, in that it was the bulk ore that was smelted in a single furnace, the enormous amount of energy required to do this being recovered from the molten slag. Expensive comminution was avoided, apart from some preliminary crushing, and the inefficient flotation step was also by-passed. Pilot plant runs using McArthur River ore showed that zinc and lead recoveries could be well over 90% and with the adoption of innovative energy recovery technology, the thermal requirements could be satisfied by the inherent energy content of the ore itself. Large amounts of energy are consumed in melting the gangue minerals but dry granulation of the molten slag can transfer the slag energy into a carrier gas thereby allowing sensible heat to be passed back to the front-end of the process in an ore preheater. Under these conditions only the thermal losses have to be added and the energy demand is then more modest. The Birmingham team showed that the energy requirements of direct ore smelting can be competitive with conventional mineral processing, particularly for ores containing sulphides.
A comprehensive description of the Warner Process can be found in Minerals Engineering Vol. 2 No. 1 (1989), and a later paper in Vol. 22 Issues 9-10 (2009). Despite its attractions, the process has never been used at full scale, but maybe the time is approaching when it should be looked at more closely.
I have no doubt that comminution and concentration techniques will continue to evolve, but will there be a time when they lose the battle, when the remaining ores are so finely disseminated and intergrown that they can no longer be treated by physical methods? Is no mineral processing the future of mineral processing, and will the future be direct hydrometallurgical and pyrometallurgical routes? I look forward to your opinions.