by Alexander VOROBYOV, Dr. Sc. (Tech.), Russian University of Peoples' Friendship; Tatyana CHEKUSHINA, Cand. Sc. (Tech.), RAS Institute of Problems of Integrated Development of Mineral Resources
The progress of mining industry has always depended on the level of its technology. In coal mining, for example, the "pickax-basket" era was replaced with that of the "pneumatic drill-trolley" and then with "mining combine- transporter belt". It was only at the end of the 20th century that a fundamentally new physico-chemical technology was developed providing for the extraction of only the "productive share" of mineral deposits, leaving the barren rock behind.
Articles in this rubric reflect the opinion of the author. -Ed.
Volumes of extracted minerals (1) and their utilization (2) in the USSR from 1980 to 1990.
Since time immemorial people have been trying to use mineral resources with maximum efficiency and at minimal labor expenses. Today we witness the results of this strategy on a global scale.
Most of the damage to the biosphere is caused by what we call the extractive industry and primary processing of raws. Until the middle of the last century progress in these fields had been fuelled only by considerations of boosting the output with total disregard for the state of the environment. But the dwindling mineral resources and mounting ecological damage has made it necessary to develop new "resources- saving" technologies, taking into account environmental protection requirements. These include underground lixiviation, or leaching of metals and the smelting of sulphur, gazification of coals and "well extraction" of soluble salts. And that has been just the beginning.
The world consumption of mineral raws is doubled every 10 - 15 years. From the year 1900 to 2000 coal output reached 170 bin t, the output of iron ore-35 bin t, and in the last quarter of the 20th century the output of metallic ores reached 15 bin t (including tens of millions tons of lead, zink, aluminum, many thousands of tons of other non- ferrous and rare metals and hundreds of tons of gold). And that involved the mining and processing of one to two percent more of the enclosing rock.
As experts point out now, all of the richest and most accessible mineral deposits have now been exhausted which means that we have to mine other ones with low metal contents.
Over the past 35 years this level dropped down by 2 times for iron, by 2.5 times for copper, by 4.5 times for lead and by 6 to 8 times for some rare and noble metals. These figures, not to mention the mounting volumes of extraction of natural raws, signal a marked increase in the volumes of solid and liquid wastes. For example, from 1945 to 1991 on the territory of the Soviet Union they were estimated at some 150 bin t mainly at the industries of the Urals (more than 600 mln m 3 ), Krivorozhsky Basin (some 400 mln t of ferruginous quartzites), Kursk Magnetic Anomaly (some 1 bin m 3 of overburden rock), and in the north-eastern regions-more than 1.5 bin m 3 mainly from gold extraction from placers.
In the course of extraction of minerals the volumes of wastes change within considerable limits depending on the kind of the mineral. Thus for one ton of coal this volume is greater by three times; for iron this value is twice bigger, for non-ferrous metals it rises up to 100 - 150 t, and for rare, noble and radioactive metals it reaches 5 - 10 thous.!
A somewhat similar situation is observed in the processing of raws: 11 of iron produces 0.5 - 0.7 tons of wastes, 1 t of nonferrous metals-more than 50 - 60, and for rare and radioactive metals this value increases to 10 - 100 thous. t.
The above statistics make it abundantly clear that time has come for significant changes in mining operations.
As experts point out, the main snag of the existing technologies consists in the fact that it is only ore which is being processed during the mining of deposits, that is only the vein rock with an increased content of valuable components. At the same time the metal-bearing rocks, which sometimes surpass the ores by the absolute amounts of useful materials, are piled up for long-time storage. Such dumps occupy considerable areas and provide a source of environmental pollution. The volume of utilization of wastes of the mining and processing complexes does not exceed 6 - 10 percent. As a result, colossal damage is done to the environment.
Having said that, what should be the basis of a new concept of utilization of mineral resources?
In our own view this calls, above all, for geochemical assessments of mineral deposits which should take into account not only the peculiarities of their development, but also the conditions of raws reproduction or replenishment, i.e. artificial (technogenic) ore formation. As the saying goes-it is time to turn wastes into profits. In some cases the appropriate technologies have already been developed or are at the development stage. Their practical introduction, however, is delayed because of the three main reasons.
It is still possible to exploit natural resources with an eye on the richest deposits of mineral raws which are being discovered by geologists. This approach remains the most attractive economically, although it fails to take into account the full measure of ecological harm caused by our mining and processing industries. Today, just as before, profitability remains the factor of decisive importance.
Changing volumes of overburden rock in the mining of ore deposits in the world. 1-copper, 2-aluminum, 3,4-tungsten-molybdenum, 5-rare-metal, 6-lead-zink, 7-mercury-antimonial, 8-fluorite, 9-tin.
Metals content within the borders of a conventional deposit. a-plan; 1-ore deposit, 2-dispersion halo, 3-contour of site, 4-metal-beating rock; b-metals contents along profile I-I; 5-background.
Finally, in the industrially developed countries relatively much attention is given to the protection of the environment by way of siting "dirty" industries in underdeveloped states, thus turning them into a kind of ecological colonies.
But the general international situation is constantly changing in this respect, and these changes are, regrettably, not for the better. One should also bear in mind that transition to new geotechnologies is a slow process and one has to prepare in advance for a critical situation with the natural resources which is bound to take place in a not too distant future.
And it is possible to improve the initial properties of mineral raws, above all those with low levels of the useful components. This can be done in several ways. Rock and ores can be processed on the spot of their natural occurrence mechanically, chemically or with the use of microbes and electromagnetic fields, and different other solutions of activating geochemical processes.
In Russia, for example, about one third of gold in native deposits is represented by ores with finely dispersed (microscopic) dissemination for which there exist no effective and industrially accepted processing technologies (such deposits are classed as "reserve" ones). Their partial stripping, or opening, is desirable, letting nature itself conduct the pre-processing of raws. And one more thing. More than half of what we call reserve deposits of our country are represented by ores with an increased content of organic compounds, mainly carbonaceous matter and admixtures of arsenic, copper, antimony, etc. In such cases it would be expedient to reduce their initial levels in the deposits themselves "on the spot" before mining.
The proposed strategy provided for a redistribution of the useful components in the mountain massif so that accumulations of "workable" raws are produced. Of special importance is the time factor, because the process of such enrichment can last for decades. Industrial methods with the use of steam, gases or artificial reagents would be unprofitable (even more so where there are no favorable conditions, such as plenty of hot water and certain kinds of solutions). It would be more effective to take advantage of natural compounds "generated" directly in the mountain massif (like in sulfide oxidation).
Of great importance in what we call technogenic ore formation are geochemical barriers. In the latter half of the 20th century their theory was formulated by Alexander Perelman, Dr. Sc. (Geol. & Mineral.). What we are dealing with here are zones in the earth crust with sharp changes in the physical-chemical parameters resulting in the accumulation of the valuable component. Such barriers can be hydrodynamic, sorption (assimilating), oxidative, reductive, alca-line, evaporatory and also radiation-chemical (identified by one of the authors of this article-A. Vorobyov). In the natural conditions they most often "coexist" in various combinations and determine a whole range of processes.
Say, the hydrodynamic process is most effective in the redistribution of weakly soluble noble metals (silver,
Section of technogenic deposit at Sadonsky mine. 1,6-markings, 2-lean ores, 3-caving rock, 4, 5-tectonic faults, 7-atmospheric and surf ace waters, 8-metal- bearing rock, 9-mining workings.
platinum, gold) and readily soluble ones (copper, uranium, zink, lead, etc.). In the latter case one can produce with the help of explosions what we call filtration nonuniformity or inhomogeneity, thus producing a more penetrable medium in the leaching zone (say of over 30 cm/h), and in the zone of concentration of the useful component keeping this value within 5 cm/h and less. In addition to a sharp change of the rate of flow of metal-bearing waters, there occur significant changes in the acid-alkaline (pH) properties and in the oxidation-reduction (Eh) potential of solutions. As a result geochemical reactions become more active, forming technogenic accumulations of minerals. With hori-zontal-laminary rock mass the maximum concentration of the deposits produced will gravitate to the bottom of the low-permeability horizons (levels), with the less abundant ones being located below; and even lower down there is a zone of barren (lean) ore im-preguations.
These processes can last for years and even decades, as long as the "migration" of solutions continues. The scale of technogenic ore deposition depends on the volumes of water channelled towards the hydrodynamic barrier, its contrast and the initial metal content in the leached mountain massif.
The authors of the article observed such phenomena at the Sadonsky
Mine (Northern Osetiya-Alaniya) where they occurred in a natural way and without the use of special geotech-nologies. There, on a vast area of exhausted patches of deposits, more than 100 mln t of solid wastes are buried in which lead content reaches 0.32 percent, and zink-0.68 percent. Consequently, the total resources of these metals at the site amount to 300 and 790 thous. t respectively. Mineral masses stored in cavities possess filtration nonuniformity, causing the appearance of hydrodynamic barriers. As a result there accumulate at certain sections what we call secondary deposits of lead and zink ores.
Another example is the technogenic deposit formed at the Kholstinsky Mine (North Caucasus). Since the start of its operation in 1956 more than 80 thous. t of waste rock has been accumulated with the contents of lead of 0.5 percent and of zink of 0.7 percent with very different permeability. In addition, layers and sinters of secondary, formed in artificial conditions, zink gel were discovered in some places at other mines.
Experimental studies of sedimentation of polymetals (lead, zink, copper) have proved the possibility in principle of the formation of technogenic ores and emanation of each of these metals depending on the changing migration conditions. Thus, in neutral and alcaline conditions, lead and especially zink are "mobile" and copper precipitates. Processes of this kind occur in two ways.
In a mountain massif it is possible to create, or increase geochemical barriers (for example, by pumping through holes solutions with certain substances and microorganisms). -Another method provides regulation of solutions, passed through zones with sharply different properties and metal contents (this approach is more preferable in the "restoration" of mineral resources at sites with complex structure, such as when there are interlayers of rock with contrast composition and chemical properties: sulfides and carbonates, acid and basic ones, graphite- and pyrite-bearing shales, etc.).
Surveys of worked-out sites on mines of the Sadonskoye field proved that phenomena of this kind occur in all places, but are now at different stages which can be easily identified by the composition of mine water flowing from caved workings. Such early stages are characterized by high levels of zink and low levels of iron, and the picture is just the opposite for late workings.
More than a quarter of a century ago Prof. A. Perelman proposed identifying
"useful" technogenic anomalies which help improve the environment (say, replenishment of the deficit of certain chemical elements in soils, subterranean and surface waters) and also harmful ones, detrimental for the environment of people, animals and plants.
Thanks to the progress of modern-day physical-chemical technologies it has become possible to "neutralize" such harmful anomalies. But to put this into practice one has to take into consideration not only some concrete features of deposits and geochemical barriers, but also some broader characteristics of different geographical zones.
In humid landscapes natural waters are saturated with metal-organic complexes. In arid conditions the levels of soluble organics are considerably reduced because of increased amounts of inorganic compounds with different electrostatic and electrolytic characteristics.
In the course of migrations and precipitation on geochemical barriers of toxic-heavy and non-ferrous-metals there occurs a transformation of the original mineral forms. For example, in stray fluxes, or streams, near the Udokanskoye deposit (Transbaikalia, zone of the Baikal-Amur railway) copper is chiefly present in the form of positively charged particles (cations). Some 300 - 600 meters away their levels drop sharply (up to 25 percent) with a simultaneous increase of concentrations of complex copper-bearing compounds. This is mainly explained by the changing pH and Eh factors in water solutions.
For example, copper hydroxide carried to the area near the deposit, undergoes a number of metamorphoses in the solid phase including water-soluble, exchange, carbonate, organic, amorphous and silicate "phases". Some time after the pollution, or contamination, of soil the main form of the presence of copper is the carbonate (60 percent), then the amorphous (20) and organic (10); the silicate one shows insignificant (6 percent) propagation and the rest are insignificant. Different copper compounds also accumulate depending on the nature of geochemical barriers: on sorption (absorptive) ones there usually prevail organic forms, and on alkaline- amorphous ones.
The knowledge of the aforesaid regularities makes it possible to implement effectively measures for natural protection, produce technogenic mineral deposits. Such, in our view, are the promising trends in the development of mining in the 21st century.
Preparations for a transition to new geotechnologies have to be started now. Projects of future mining and concentration enterprises have to take into account the geological and landscape-geochemical conditions of not only the given area, but also of the nearby territories and water areas. This should make it possible to regulate the inflow of harmful wastes by altering the rates of biological assimilation. It should be possible to increase in ground water and soils the levels of chemical elements which are scarce in the given region, while reducing the levels of "excessive" ones.
Before storage, all kinds of mineral wastes should be turned into optimal geochemical forms not only for reasons of environmental protection, but with a view to building up technogenic deposits of useful minerals. In particular, compounds of toxic elements should possess the lowest possible thermodynamic activity, and the elements in deficit should be in an easily soluble form.
The introduction of progressive geotechnologies, needless to say, will run into some formidable problems. They stem from the fact that not only their development, but putting them into industrial practice will call for considerable expenses while, more often than not, the economic benefits can only be expected in a more or less distant future, probably decades away. But there is no denying the fact that all such efforts will be rewarded in terms of ecological benefits, preservation of natural resources and a healthier environment.
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