Plaisir d’amour ne dure qu’un moment; chagrin d’amour duré toute la vie. Classic French Ballad
The papers in this issue focus on sulfuric acid. Sulfuric acid has become an integral part of the mining industry and today there are more sulfuric acid plants operated in the mining industry than in the chemical industry in South Africa. This revolution took place in the 1950s with the advent of the uranium extraction and recovery process. The first plant in the mining industry was, I believe, at Zincor, the raw material being derived from the roasting of sphalerite, the sulfide of zinc. Most of the large uranium plants had their own sulfuric acid plants using pyrite as the source of sulfur.
Many of the plant and consulting metallurgists had to have some rapid training in the chemistry and operation of a chemical plant. Such training has served the mining industry well since there are few metallurgical plants that are not involved in the production of this most prevalent acid. Indeed, during the last century there have been a number of cycles of famine to feast and there was a stage when the mining and industrial world was desperately searching for new uses for sulfuric acid. This came about as a result of the antiacid rain campaign across the world, which forced base metal smelters and other industries emitting sulfur dioxide into the atmosphere to convert this to sulfuric acid or bear the cost of absorbing the sulfur dioxide in lime and dumping calcium sulfate.
The best and most profitable use of excess sulfuric acid was to make phosphoric acid. Since almost all phosphoric acid is made from apatite, containing calcium, the position is now that a great deal of the sulfuric acid made in the world today is ending up in small mountains of waste calcium sulfate, or phosphorgypsum, which is the better known name. Even if the acid is used for leaching of uranium or base metal ores, it ultimately ends up as gypsum since it is necessary to precipitate the metal sulfates and any unused acid with lime (the lowest cost alkali) to prevent pollution.
The dreaded acid mine drainage has for over a century been neutralized with lime to produce waste gypsum which is then deposited on a dump or slimes dam. There is a misconception that these mountains of gypsum are harmless in terms of water pollution. After all, gypsum is innocuous and nontoxic, and pretty well insoluble. These perceptions are not correct.
Gypsum is not, in fact, insoluble. Calcium sulfate di-hydrate (gypsum) has a solubility of 2 000mg/l. well above the total dissolved solids (TDS) specification for domestic water.
This means that every time rain falls on the pristine looking white dumps of gypsum an amount of calcium sulfate is washed into the water streams and in the Vaal catchment area, for example. I have a shrewd suspicion that in order to meet the specifications of domestic water, a large amount of pristine Lesotho mountain water is needed to flush these TDS values
downstream to the lower reaches of the Vaal River.
This was the conclusion of some computer simulations at the end of the last century and I suspect that the powers that be are reluctant to repeat these calculations as the position is far worse today. It is now being suggested to spray such ‘gypsiferous’ waste water onto pastureland believing it acts as a soil improver. This is not a sustainable solution and indeed only increases the surface area available for distribution of gypsum into rivers.
Unfortunately waste gypsum almost invariably contains toxic metals and other elements such as fluorine, particularly when derived from fertilizer production. These impurities make such suggestions even less acceptable. There is a paper in this issue dealing with a possible way of removing fluoride from effluents. Moreover, the precipitation of metal sulfates
using lime is also unsustainable and over the decades, a leakage of toxic metals from existing slimes dams is commonplace and increasingly frowned upon.
The position thus seems to be the more sulfuric acid we produce to dissolve valuable constituents from minerals, the greater is the risk of establishing an inland time bomb to haunt us.
In the light of all the predictions that we are going to depend more and more on acid leaching of low grade ores for our base metals and uranium recovery, it is becoming necessary to solve the problem of sulfate disposal. Fortunately there are several conceptual solutions possible, but so far none of them has been translated into sustainable commercial activities. Thus there is a call to establish a nationwide portfolio of long-range development research to explore the critical paths to find the best solutions for the different environments. But there are some intriguing suggestions. There is the biological approach such as the ‘BioSure’ process developed in South Africa at Rhodes University, sponsored by the Water Research
commission and the East Rand Water Board and commercialized at the Grootvlei mine in a 10 Ml/d plant.
This has, I understand, been abandoned. But I am convinced that more bites at this cherry are well justified.
Another approach tried several decades ago is worth revisiting. This is to use gypsum converted to a low-cost versatile ‘DIY’ building material. This was a big improvement on the previous commercial plant to produce cement and recover sulfuric acid which was abandoned some 20 years ago as being hopelessly uneconomic.
Now that water position is becoming critical and we must keep on trying. We must persist with the only strategy for effluent treatment, which is the zero waste option where all waste products are converted to a saleable by-product. There is a host of potential other uses for calcium sulfate and the alternative by-products derived from the acid neutralization, which could well be economically saleable.
These can and should form the basis of a national priority R&D effort. Space does not permit the detailed discussion of these possibilities but suffice it to say that such a nationwide effort would encompass virtually all our national research institutions and invoke all the priorities prescribed by the Department of Science and Technology: poverty alleviation, carbon dioxide emissions, green energy and water pollution. In terms of science buzzwords, it would include nanotechnology, catalysis, the hydrogen economy fuel cells and
Sulfuric acid production is with us for a long time and will bring many quick benefits to mining and metallurgy. The evergreen French ballad provides an apt analogy.
The pleasures of love last but a moment.
The vexations remain for a lifetime.
Please note that my spelling is not that bad! I have been advised that the international committee on chemical nomenclature has decreed that the spelling of sulfur, sulfuric and sulfates shall be with an ‘f’ across the whole English speaking world.
This was painful for me. But, fortunately the Publication Committee has agreed that either spelling can be used provided it is consistent.
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