In Revolution and Reformation how can a people who have struggled long years under oppression throw off their oppressors and establish a free society? The problems are immense, but their solution lies in the education and enlightenment of the people …. Thomas Jefferson
The new year issue is starting off well with a revolution. This word is to the old guard aristocracy a frightening term and inspires fearsome thoughts of the many heads that rolled from the guillotine in the notorious French revolution. The revolution in our case also has to do with rolls, as in the paper: HPGR—revolution in platinum? By C.M. Rule, D.M. Minnaar and G.M. Sauermann In this case, the rolls refer to the high pressure grinding rolls, which were the basis of some very successful testing at two of the Anglo Platinum mines.
I was first made aware of the early tests on HPGR at the AAC research laboratories some 15 years ago— was immediately impressed at the theoretical potential of liberating the valuable minerals in conglomerate gold reefs the direct breakage at the solid phase boundaries rather than the wearing away of the constituents from the abrasive action, which is the characteristic of autogenous and ball milling. At the time, there was a lot of work going on to squeeze out a few more fractions of a percent in gold extraction. It was suspected that the ductile metallic gold particles were being ‘smeared’ onto surfaces of reducing minerals in the ore. Unfortunately I was not able to be involved in the final outcome of this approach but the mechanism of being able to apply a force on opposite sides of a particle to cause a compressive breakage at mineral phase boundaries, as well explained in the paper, seemed to me be a fundamentally correct approach.
Now some 15 years later it looks as though this concept has been well demonstrated as a practical proposition and there is almost certainly going to be a dramatic growth in commercial applications. A great deal of work has gone into the practical engineering aspects over twenty years, which is not unusual in converting concepts to profitable engineering usage. All those involved are to be congratulated on a fine achievement and some carefully executed experimental work to demonstrate with hard results the cost benefits of this new equipment. I have no doubt that the important cost savings will be realized. An increase in the use of HPGRs will be forthcoming globally. This is a great opportunity for South African equipment manufacturers to create the job opportunities and added value of manufactured products for the world’s mineral industries. But the best of revolutions must result in more than cost savings.
In the words of Jefferson, a revolution must lead to reformation in a new area of enlightenment, which in our scientific environment implies a lead into new fundamental advances and new freedom in creative thinking. In this HPGR development, the use of the word revolution is perhaps justified. I can see this happening in terms of new thinking in mineral dressing and extractive metallurgy, and not the least in gold and platinum processing, and even more so in the processing of the UG2 reefs of the Bushveld Igneous Complex. The badly needed reformation is to move away from the basic approach of converting all the material from underground to face powder in an attempt to achieve maximum recovery. In many of the narrow tabular ore bodies, the valuable constituents are gold or platinum.
Today even uranium is present in a narrow band of mineralized reefs, which volume and tonnage wise are a small percentage of the barren gangue minerals brought from underground. Leaving out the concept of selective mining, one must still consider the possibility of ‘selective’ crushing leading to ore dressing and physical separation of the mineralized portions at an intermediate stage of comminution Now with the HPGR this is a distinct possibility. It seems possible to undertake secondary crushing in a way to liberate the mineralized constituents from the barren bulk material. In the case of the UG2 reefs, for example, we know that the PGMs, whether in the so-called triplets or in the main tabular reefs, are exclusively associated with the chromite mineralization.
There are no PGMs in the surrounding rock structures away from the chromite bands. Using the ‘revolutionary’ crushing there should be no problem to liberate the chromite bands from the gangue at a secondary crushing stage. What is then needed is a physical method to identify and achieve a separation of the chromitic material from the gangue. There are several possibilities that have been proposed, even to the extent of a rapid optical sorting technique now becoming commonplace in many industries. Even more interesting fundamentally, it is also known that the PGM’s are in the interstitial spaces between the individual grains of chromite.
No PGMs are to be found within the crystal structure of the chromite mineral. So the next obvious question is whether the concepts inherent in the HPGRs can be applied in a tertiary roll crushing stage at an order of magnitude smaller roll separation distance. What about trying, say, one millimetre rather than, say, the centimetre roll spacing apertures used at present. This might demand even more precision in roll dimensions and positioning. A challenge but a long way off being impossible with modern computerized engineering methods. Since this is getting down to the size where complete dust collection is imperative wet crushing might be necessary.
Once complete liberation the final separation of the interstitial mineralization from the chromite crystals is no great a challenge where several methods can be proposed including some chemical recovery techniques. In fact there is another paper in this issue from the Hazen Laboratory in Golden, Colorado which has been involved in South African metallurgy for half a century. This is the ConRoast process for calcining the material from the UG2 reefs. The cardinal point is that one is now working at several orders of magnitude smaller in terms of quantities of material to be handled and many extreme treatments such as calcinations or high redox potential chlorination become plausible economically. The breakage of heterogeneous assemblies of minerals forming rocks is as old as mining itself and one is sceptical of claims of revolutions in doing this. However the vast tonnages and the cost can create log jams in thinking.
When a breakthrough in developing more sophisticated equipment which does prove to be more than justifiable economically but also offers a theoretically fundamental improvement in mineral liberation we must think deeper. This could well be termed a revolution leading to a reformation in mineral processing concepts. Can we afford not to pursue such possibilities? Do we have the organizations with the ‘mission’ and the expert manpower to lead the reformation? In these times of economic downturn, I am afraid that much education and enlightenment of our financial decisionmakers might be necessary. R.E. Robinson January 2009
- Written by R.E. Robinson