|Mining in Africa Today - Strategies and Prospects (UNU, 1987, 91 pages)|
With the exception of uranium, mining and metallurgical technologies are relatively stable, standardized and widely diffused. The strong growth recorded in copper and bauxite production since the Second World War has involved the scaling up of equipment and plants rather than the conception of new technical processes; the contemporary expansion of the steel industry has been largely founded on basically unchanged technology. The classical production route of steel from pig iron, however, not only utilizes new equipment such as the oxygen converter, but an altogether new technology has been developed which allows the direct transformation of iron ore by hydrocarbons. The greatest technical changes are associated, of course, with uranium processing, and from the start, both military and civilian uses developed on the basis of an extremely sophisticated technology, founded on the most recent scientific knowledge.
For all minerals, the scale of production has increased especially in the extraction of the ore. Historically, mining activity began and expanded with the exploitation of underground deposits, usually of high grade ore. For example, the first copper deposits to be exploited were small, stratified concentrations of high quality ore, with from 10% to 15% metal content, but later, the growth in world demand led to the exploitation of much larger open-pit mines with lower grade deposits. Today, more than half of copper output is from such fields in which the metal content does not exceed 6%. The same evolution can be observed for iron ore production; for example open-pit mines provide 97% of US ore output. Four-fifths of the world's bauxite is also produced from open-pit mining and it increasingly characterizes the extraction of uranium ore.
This move to open-pit mining has resulted in a considerable increase in the scale of production. The current optimal size is about ten million tons each of iron ore and bauxite, and five million tons of copper and uranium per annum. As a result the unit costs of ore extraction have decreased, thus reducing the minimum efficiency level of the mining. In the early 1960s, for example, to be exploitable, the minimal acceptable mineral content of copper ore was 0.8%, but a minimal metal content of 0.3% is now considered to be adequate. Another example of this trend is the Rossing uranium mine in Namibia, which has been opened despite a metal content of only a few hundred grams per ton of ore, while the normally accepted grade is about one kilogram per ton. Development of open-pit, large-scale mining of low grade fields, however, would have been impossible without some progress in the technology of ore upgrading, which prepares the minerals for processing.
The production scale for smelting and refining rose with that of mining, but different technical thresholds characterize the successive stages of the metallurgy industry. The production scale of smelters varies between 10,000 tons (as in Santa Rosalia in Mexico or in Moubhander in India) and 300,000 tons (as in Chuquicamata in Chile). The economies of scale begin to be realized when the size of the plant reaches 40,000 tons per year and seem to be exhausted beyond the threshold of 100,000 tons for the smelters and 150,000 tons for the refineries. The big mining enterprises' plants are, however, usually much larger. Codelco's plant on the Chuquicamata field in Chile has, for example, a production capacity of 300,000 tons, as does Zimco's plant on the Rokana site in Zambia. The US firms Anaconda and Kennecott have plant capacities ranging from 170,000 to 270,000 tons. But the growth of production scales ended during the 1970s. A new smelter built in that decade at Kolwezi, in Zaire, has a capacity of 100,000 tons, which is lower than that of the Lubumbashi smelter that was built at the beginning of the twentieth century!
The optimal size of alumina smelters is larger than that of copper foundries, because the economies of scale can be realized at 30,000 tons and are exhausted only after 300,000 tons. All US plants have production capacities higher than 300,000 tons, and the average capacity for European and Japanese plants varies between 100,000 and 200,000 tons. The optimal capacity of refineries is half that of smelters because two tons of alumina are required to produce one ton of aluminium. Here, too, production scales have not increased since the 1970s, because to build larger plants did not further reduce cost; but new investments often correspond to very large plant sizes. Some alumina plants, indeed have a very high production capacity: 500,000 tons for that of Madhya Pradesh in India, and even of 800,000 tons for that of Kalimantan in Indonesia.
Probably the growth of production has been the most spectacular in the steel industry. At present the average capacity of steel plants varies between 500,000 and several million tons. A steel complex of 800,000 tons actually requires only half the investment per ton that is necessary for a plant of 200,000 tons capacity. In the classical production route, especially for flat products (strips, sheets and plates), the economies of scale are very important, but much less important in the new process of direct reduction, in so far as it is possible to increase the production capacity simply by adding equipment for ore reduction, and electric furnaces. In common with copper and bauxite, the extent of the steel industry tended to stabilize during the 1970s. Recent technological progress allows for greater flexibility of production scales and lowers the minimal technical size of plants.
Increased production made necessary the introduction of techniques for upgrading the ores before processing them into metal products. These techniques were first designed for copper, which, compared to other minerals, has the lowest metal content. Iron ores have different grades, but very rarely lower than 40%, and the grade for bauxite ores usually varies between 45% and 60%. Copper fields, however, are qualitatively much poorer, whether of porphyric deposits with a maximum metal content of 7% or stratified deposits with a maximum metal content of 15%; the average grade of most deposits is still lower. Yet progress in ore-upgrading technology has been more important for iron ore. Increasing the size of blast furnaces and the application of stricter operational standards resulted in the development of techniques for preparing and upgrading the ores before they can be used to produce pig iron and steel.
Progress in ore-upgrading technology made it feasible to exploit low grade deposits, and this, along with increased production, reduced the unit costs of mineral extraction. The mineral fields located in the developed countries usually produce lower grade ore. Such technology, therefore, reduces Western countries, dependence upon Third World fields. In France, West Germany and Britain there are large deposits of very poor haematite iron ore which would have been very difficult to exploit in the absence of ore-upgrading techniques; the same is true of the huge taconite iron ore resources in the United States.
When the ore is extracted it must then be upgraded or concentrated, and processed into metal. This stage is defined as metallurgy, while mining is concerned with extracting the ore.
The metallurgy of copper, bauxite and iron ore are founded on the same principle of progressive ore refining to obtain the metal. (Uranium technology, however, is very specific.) Both copper and bauxite metallurgy encompass two distinct phases: smelting the ore in order to obtain a product with very high metal content; and the electrolytic refining of this product to obtain pure metal. Unlike in the steel industry, no major innovation has been introduced in the copper or bauxite industries since the Second World War. The furnace is the central element of the smelting plant, and different types of furnace equip the different types of smelters. Three furnaces, which have been introduced at different times, at present represent the three technical processes of smelting. The oldest is the blast furnace, quite obsolete today but still in operation in about ten smelters in Africa, Canada and Japan. It has been progressively replaced by the reverberatory furnace, which is now the one most widely used. The most recent - the electric furnace - is in operation in only 15 plants, including those which process mixed ores of copper and nickel. Its diffusion has, of course, been limited by the rising price of energy in the recent period, but its use could develop, in so far as it is less dependent on considerations of minimum size.
The processing of bauxite into alumina is based on the Bayer process which was introduced as early as 1897. Between two and three tons of bauxite ore are needed to obtain one ton of alumina. The Bayer process works differently according to the type of ore; briefly, there is a European process for European bauxite and an American one for the types of ore found in the Caribbean, South America, Australia and Africa. This heterogeneity of the mineral is a source of technical rigidities: the smelters are designed for one specific type of ore and adaptation for other types entails additional costs.
In copper and bauxite metallurgy, smelting the ore gives a semi-finished product that has to be refined in order to obtain the pure metal. Smelting copper ore, after it has been concentrated and upgraded, produces the blister, with a metal content of 98.5% against 20% to 30% for concentrates. The copper blister is first pre-refined to get copper anodes, which have a metal content of 99.5%; these anodes are then refined by an electrolytic process to obtain copper cathodes, which have a metal content of 99.99% and represent the copper metal in its pure form.
Aluminium is obtained by the electrolytic refining of alumina. Two tons of alumina are usually necessary to produce one ton of aluminium. Refining is based on the Hall-Heroult process, which was developed simultaneously in Europe and in the USA. Although the energy consumption has been reduced by improvements to the basic process, about 14,000 kWh (and even more in the old plants) are still needed to produce one ton of aluminium.
Iron metallurgy is also based on the reduction of the ore to obtain pig iron, which has a much higher metal content. The pig iron is then processed in a converter in order to obtain steel - unlike alumina and copper blister which are electrolytically refined. Processing the ore into pig iron takes place in a blast furnace; this type of furnace has been greatly improved since the nineteenth century although its basic principle remains the same. Processing pig iron into steel, however, underwent decisive innovations with the introduction of new types of converters. Over the last 20 years the oxygen converter, now very widely used, replaced the open-hearth furnace, which in turn had replaced the Thomas converter. One peculiarity distinguishes iron metallurgy from that of copper and aluminium, and this has implications for the spatial location of processing: the technical characteristics of iron metallurgy require that smelting and refining must be located on the same site, because a blast furnace cannot work without the proximity of a steel mill and vice versa. Unlike copper blister or alumina, pig iron is not a commodity that can be sold on the national or international market. This technical rigidity explains why the iron ore is processed into pig iron on a large scale in the developed consuming countries where both smelting and refining activities are located. On the other hand, a new iron metallurgy has been recently developed, based on the direct reduction of the ore by hydrocarbons and bypassing the pig iron production stage. This technology was first developed in Mexico in the late 1950s, based on natural gas, but it has since been assimilated by the big Western steel firms which developed similar processes based on gas, oil or coal. Apart from these direct reduction processes, the diffusion of which on a world scale is still severely controlled by a few big enterprises, the metallurgical techniques for copper, bauxite and iron are no longer barriers to entry for new investors, even from the Third World; these technologies are quite old and have long been standardized. Obviously this is not the case for uranium processing technology.
Roughly, five technical routes are involved in processing uranium, each one being defined as a combination of a fuel, a thermic agent and a regulator element. The fuel can be either natural or enriched uranium; or plutonium; or a combination of the two. Natural uranium is mainly composed of uranium 238 (99.3%) and very small quantities of uranium 235 (0.71%). Only the latter is a nuclear fuel, so the process of enriching natural uranium means increasing the proportion of uranium 235 to between 2% and 4% The thermic agent heats up when the reactors are switched on and drives an electric turbine. Depending on the processes, this thermic or thermo-drive agent can be either boiling or pressurized water or a gas. Finally, the regulator element, which can be light (ordinary) water, heavy water or graphite, moderates the nuclear fusion so that the reactor does not race.
Nuclear power stations employ a similar process in the production of electricity. Each nuclear station is equipped with a reactor where the nuclear fusion takes place, and a turbine which transforms the heat into electricity. The most widely used nuclear process uses light water and enriched uranium. Within this technical route, the US firms, Westinghouse and Combustion Engineering, use pressurized water as a thermo-drive agent, while another US enterprise, General Electric, uses boiling water. These firms have granted licences to European and Japanese enterprises, but in most cases, domestic programmes of research and development have resulted in a redevelopment of US technology.
This dominant process based on light water and enriched uranium is challenged by another technique that uses heavy water and natural uranium, which has been developed mainly in Canada, and to a lesser extent in France and Britain. Heavy water is obtained by replacing the hydrogen atoms of ordinary water by deuterium, which is a 'heavy hydrogen'. A by-product of this second technique is plutonium, which can have military uses.
Two other nuclear techniques are much less diffused on the world scale: the graphite-gaz process, developed on a small scale in Britain and France; and the HTR process, developed by the US Gulf Oil group. The HTR process allows the production of heat for industrial use, for instance for coal gasification, or sea-water desalination. The first uses natural uranium while the second uses enriched uranium. Lastly the fifth technique, which has been developed more recently, is that of the breeder reactors. These reactors are without a regulator element, and use enriched uranium or plutonium or a combination of both. This technique, which was developed in the USSR, France and Britain, has the peculiar property of producing more nuclear materials than it uses.