DIAMONDS IN AFRICA
DEMOCRATIC REPUBLIC OF CONGO
The exploitation takes place in the open pits excavated by steps. Excavators in cooperation with German made stacker (Lubecker Maschinen Geshaft) removed the overburden. The pay-gravel is extracted by the mechanical shovel (Bucyrus) with the help of bulldozers and scrapers, and transported by more than 2 km of 0,80 m. wide conveyor-belts or wire-belts.
At the excavation site first concentration take place, the eluvial material, after scrubbing and sifting to <25 mm, goes on diamond pans. The kimberlite is treated directly by heavy medium process.
The second concentration takes place in a central factory build in 1958 that treats 400 t/h using heavy medium process. A first trommel with the scrubber rejects the 150-250 mm, a second trommel perfects the scrubbing, and a third trommel separates <25 mm. fraction.
The vibrating screens separate 2 categories 0,8 - 2 mm and 2 -25 mm that are treated in 2 different circuits.
The grains 2 - 25 mm, after homogenization and a control of humidity on special sifters, pass in the cones filled with heavy "medium" 2,8 g/c m3 (suspension of a ferro-silicon). The debit of the heavy minerals that sink is controlled to the forefront of the cone by an adjustable magnetic valve whose field is regularly pulsed. There is a circuit of recuperation of the medium that is clarified, cleaned of its impurities and re-densitied, naturally.
The grains 0,8 - 2 mm, are also treated after drainage by heavy medium separation with little different devices.
All the installation is automated and is controlled from a central station from where one can follow and can control every device, or modify the flow-sheet, etc.
The older factory of final sorting abandoned the grease tables to move toward the controlled grinding of the concentrates. It does separations using high density liquid such as the sulfonate of Pb (d = 3,2 to 3,6 g/c m3) maintained steady to 100° C. Thus, 30,000 carats of small diamonds are recovered per day that it had been difficult to separate of fine concentrates by another method.
The power is supplied by a hydroelectric plant of 8,000 Hp on the Lubilash River close to the confluent of the Bushimaïe River. There is a plan to construct second power station more down-stream.
The society employed about 130 to 150 Europeans, and 3,600 Africans; these strengths have been reduced because of the advanced mechanization these last years: in 1957, they reached 209 Europeans and 7,520 Africans.
The grade of exploited gravel is of 8 to 10 cts/m3. It reached 12,28 cts/m3 in 1961, probably because richest parts of the layer were exploited "to save furniture" at the time of the political unrests.
The total (overall) grade of gravel and overburden was from 2,8 to 4,9 cts/m3, and was increasing year after year at the same time as the thickness of overburden was decreasing in relation to the one of gravel.
The overburden to gravel ratio passed from 2,06 in 1957 to 1,4 in 1959 and 1960, and 0,89 in 1961, at the same time as the total (sterile + gravel) grade passed from 2,82 to 4 and 5 cts/m3 do The perfecting of the concentration and sorting have maybe also, thanks to a better recuperation, influenced on this marked raising of the total grade The cut off grade of the exploitation would be about 2 cts/m3, evidently more elevated than the normal because of the weak value of diamonds recovered.
The exploitation of the kimberlite itself takes, it is necessary to add, a more and more important place in the production of Bakwanga (1973).
Value of the diamonds
The "value export" given by statistics is maybe of 9,90 F/cts a little lower to the real price that would be of US$2 to 3 by carat. Price 1962 = 105 F. Belgians (valued). It is certain that the synthetic diamond of an analogous quality otherwise superior to the one of the boart of Bakwanga, can make at last a strong competition, but until now it seemed that the prices remained comparable, in all non competitive cases for the natural produce. The sale of the production is assured by de Beers - itself a big potential producer of synthetic diamond - that tried certainly to maintain a balance between opposite interests. Maybe the synthetic whose quality improved considerably would have now a tendency to take over the market.
The evolution of the eluvial production in Bakwanga (Bushimaïe)
The eluvial production of Bakwanga (a weak part is alluvial) shows a fantastic growth, since 1926 to 1965, it passed from 740,000 to 12,500, 000 carats per year (rounded numbers). In detail this progression is very irregular. One notes a first decrease of 1932 (3 M carats) to 1933 (0,6 M carats) due to the continuations of the world crisis of 1929-1930. In 1935 it rises to 3,4 M carats, and in 1940 to 8,9 M carats. The second decrease to 3,4 M carats in 1943. In 1940, the stock of crushing boart passed 30 M carats: the American war effort absorbed it quickly and the production again pushed to 9,86 M carats in 1945, to fall again to 5.5 M carats in 1946. The growth takes up again to 5,2 M carats in 1948, to reach 16 M carats in 1958 (having tripled therefore in 10 years).
Thereafter it has been constrained, except the record year in 1961 with 18,142,871 carats that bursts the production ceiling. Currently it is limited voluntarily around 12 M to 15 M carats. Actually to it is added of a strong illicit production: 5 M carats per year passing in contraband, during several years, through a market in Brazzaville without counting what passed to the Rwanda, which could amount to 2 M carats per year. In 1969, the government of the DRC stated to have "recovered" 7,785,025 carats of "illegal" diamonds, the number that appears incredible. This production of Bakwanga, formed to 83% of very mediocre quality crushing boart, is therefore extreme sensitive to the world economics. Bakwanga is too rich and has a tendency to flood the market, during a strong expansion. At the same time its diamonds are less and less competitive in front of the synthetic whose quality has been improving constantly.
The official production of the mines of Bakwanga, from 1907 to 1970, was 390,850,000 carats (rounded number). To that it is necessary to add 20 or 30 M carats of contraband diamonds. In all, more then 80 metric tones of diamonds have been extracted of only one huge layer, about 10 years of the present world production! One would be wrong however to disregard the jewellery part which was 2 to 3% of the total, representing every year from 300,000 to 450,000 carats of gems. In all, with the Kasaï, Congo produced more 16 M carats of jewellery diamonds.
Congo and the world production
The proportion of Congolese diamond in weight in relation to the world production settles to about:
This proportion has tendency therefore to become lower for the following reasons:
Rafal Swiecki, geological engineer email contact
This document is in the public domain.