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:: A Real Investment Opportunity in Ore Treatment
Correspondence from P. Gray to MEI
Over recent years I have come to believe that the treatment of the multi metal sulphide ores can provide a huge opportunity for profitable investment. It is time that a mining entrepreneur with foresight has a look at this possibility.
The ores which are the primary source of zinc and lead and an important secondary source of copper, silver and gold, have complex sulphide matrices. This complexity is now the cause of substantial economic wastage in existing treatment routes. Recovering most of this wastage as earnings would be a rewarding return on investment. Scientific progress over the past decades gives a new lead to alternative processing routes from ore to refined metals. Routes now in service were pioneered and established many years ago when knowledge, ore and operating conditions were different.
I have accumulated data from operating mines around the world on their recent metallurgical and economic performance. A selection of this is shown in the chart. The mines included were all active producers in the 1980s and 90s and are located in several Continents. I have data also from many more mines and the pattern is very similar.
In the chart a total dollar value of content is assigned to the run-of-mine ore assuming the current market price and 100% recovery. The percentage of this which was actually recovered by sale of products is shown in yellow. The wastage is divided between values going to mill tailings (light blue) and values for which little or no payment is received by the mine from the smelters (dark blue). The best economic recovery is 84% for a mine working a simple lead rich ore to the lowest at 60%. The average for such mines is around 70%.
If one bears in mind that a typical total value in such ores is $US120 to 220 and that an ore throughput for the mill can be from 500,000 to several million tonnes per year, the total revenue wastage at any one mine is anywhere from 20 to 160 million dollars a year. Recovery of some of this is a handsome world wide target for investment, with global mined zinc production now above 8 million tonnes per year.
What is the fundamental cause of this wastage? With the greatly improved mineralogical instrumentation we now have we can see very clearly that trying to concentrate mineral elements to high recovery and meet the concentrate specifications set by smelters (especially the necessarily picky electrolytic zinc plants) is not on. Mineral liberation and separation by differential flotation is unable to cope efficiently with fine mineral intergrowths or sub micron inclusions of one mineral inside another; ores of this type are now common.
Sulphide flotation has probably been researched more thoroughly than any other branch of ore treatment technology and improvements in recovery for some time past have been only marginal. The detailed mineral structure evidence we now have make it clear that we must destroy the minerals at the start of the ore treatment route. Hydrometallurgical methods are attractive in that elements in solution are separable by a number of modern techniques. For the sulphides, however, hydrometallurgy is faced with the difficulty of dissolving some of the values (e.g. lead, silver and gold) and the heavy power operating and capital costs for recovering metals from dilute solutions.
It seems to me that the only way to achieve a major step-up in recovery and separation of unwanted elements is to destroy the matrix thermally by melting. One could envisage using rougher flotation to separate the bulk sulphides from the barren host rock and then treat the bulk (including iron sulphides) as we, in principle, already do to recover copper or nickel from a matte phase. Oxidation of sulphides does, of course, also provide a great deal of heat energy for use in reduction reactions.
Can one engineer this sort of route to be both efficient in recovery of values and in cost? Following the suggestion of Professor Warner of the University of Birmingham to use matte smelting accompanied by zinc metal volatilisation. From this suggestion, in ZML we have carried out extensive design work on the plant engineering and economics. We are now in no doubt that the system can be engineered without resorting to plant practices that are not already well known and used. We can also deduce that the capital and operating costs for such a plant would not be more than present production installations and could well be substantially less.
We are also aware, although this is somewhat outside our expertise, that such a processing route could improve profitability by enabling operating mines to lower their cut-off grades and exploration geologists to look for deposits of high value which have hitherto been too contaminated to be used by current technology. Metal production integrated more closely to the mine would increase added value in exploitation of national resources.
More about this possible processing route can be found on the website address (1) and in the paper presented at the AIME Zinc International Meeting in Pittsburgh in October 2000 (2).
Developing a new route such as this will require some risk money and time, not to say courage and foresight, but the rewards could be immense. Step forward those looking for riches
The influence of new technology at Sulphide Ore Mine Sites on metals production and recoveries, with its commercial significance, H.Fletcher and P Gray, Lead-Zinc 2000 Symposium in Pittsburgh Penn October 2000. Proceedings published by AIME, Warrendale, Penn, pp659-675
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