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Over a century of bioleaching copper sulphides at Andacollo.

Over a Century of Bioleaching Copper Sulphides at Andacollo

One could say "there is nothing new about copper being extracted by cementation" and of course one would be right. This process was already known by Chinese scientists more than 2,000 years ago, as indicated by a book written by the king Liu An before 120 B.C..[1] However, if the following aspects are considered: * the minerals being leached are copper sulphides, not oxides; * the mine and plant have been using the leaching/cementation process for more than a century; and * the leaching of sulphides is enhanced by bacteria of the thiobacillus genus; then the case might have some importance.

What we are talking about is an in situ leaching process taking place on top of one of the major Chilean porphyry copper deposits: Andacollo. The deposit is leased to small miners by ENAMI (for more details of this very special company see MM, October 1987, pp 322-331). These miners exploit the deposit at different sites, thus defining the so-called "Andacollo Mining District".

Andacollo is a porphyry copper deposit located some 55 km southeast of La Serena at an elevation of 1030 m, within a semi-arid hilly landscape. Mining activity began in pre-colonial times when the Inca empire expanded southwards in the mid-fifteenth century. First the Incas and later the Spaniards exploited Andacollo for gold, mainly from auriferous gravels. At the end of the nineteenth century mining activity was renewed, with copper being mined by the Chileans from one of the currently oldest mines of the Andacollo mining district: La Hermosa ("the beautiful one").

The early hydrometallurgical procedures at La Hermosa came about as a result of the discovery that the mine waters were rich in copper sulphate and therefore that copper could be extracted by cementation, a process locally devised by the manager of La Hermosa around 1883.[2] Even if at that time the mineralized hill was not being sprayed with acid solutions (the process largely depended on rain water infiltration of CuS[O.sub.4]-rich groundwaters) the date can be regarded as the beginning of the present in situ leaching/cementation process, which has thus been operated for over 100 years.

Sulphuric acid make-up began in 1907[3] and from the 1940s onwards the process became fully controlled.

Geology

Andacollo should not be regarded as a typical Chilean porphyry copper deposit as it does not lie within the main Cenozoic porphyry copper belt. It is older than the rest (late Cretaceous) and has low Mo values and relatively high Au grades. The geologic history of this deposit[4] begins with the intrusion of stocks of tonalitic composition (the Andacollo porphyry, Upper Cretacous) within a layered sequence of andesite and dacites, which induced pervasive hydrothermal alteration (K silicate, quartz sericite) and mineralization (mainly pyrite-chalcopyrite plus some minor quantities of molybdenite, bornite, cubanite and pyrrhotite). Supergene enrichment was very important in creating the economic ore, a 40 m thick layer containing chalcocite, chalcopyrite and pyrite, grading 0.9% Cu. Drilling carried out by ENAMI and Noranda in the late 1970s resulted in the following figures: 249 Mt grading 0.62% Cu, 0.25 g/t Au and up to 0.015% Mo. At present, the Dayton Development Corporation has drilled part of the volcanics outlining some 29 Mt of 1.2 g/t Au.[5]

In-situ bioleaching at La Hermosa

The current operation at La Hermosa involves an area of 120 m x 30 m located on the northwest side of the pit, which is being irrigated through polyethylene pipes at a rate of 500 [m.sup.3]/d. The recycled solution is acidified to pH 3 and pumped from the acid make-up pond by two pumps (each 25 hp) with a capacity of 300 1/min. Consumption of sulphuric acid amounts to 4.7 kg [H.sub.2][SO.sub.4] per kg of leached Cu.

Bacterial concentration in the pregnant solution amounts to [10.sup.6] organisms/ml (mainly thiobacillus ferro-oxidans[6]). Since the leachable minerals are chalcocite and some minor amounts of chalcopyrite and pyrite (supergene enrichment zone), reactions 1 and 2 are likely to be occurring within the mineralized body. These show the two-step total oxidation/leaching of chalcocite ([Cu.sub.2]S) and covellite (CuS), markedly enhanced by bacterial activity.

Other reactions that would take place within this environment are those numbered 3 and 4, which show the oxidation of pyrite ([FeS.sub.2]) to iron sulphate (reaction 3) and the second step, involving the oxidation of ferrous to ferric sulphate (reaction 4). This oxidation (ferrous to ferric sulphate) in acid solution is about one million times faster if bacteria are present.[7] Reaction 3 is very important because ferric sulphate plays an important role in the leaching of other sulphides such as chalcopyrite (CuFe[S.sub.2] -- reaction 5). Finally, reaction 6 shows the typical oxidation of chalcopyrite within an acid oxidizing environment, a reaction in which bacteria act as powerful catalysts.

The copper sulphate solution (pregnant solution with 0.15-0.2 g/l Cu) is collected from two different sources: a) the pond formed at the bottom of La Hermosa pit; and b) two wells. The pregnant solution is pumped from there to the cementation tanks where scrap iron is added to obtain cement copper.

Cementation

There are six 7 m x 7 m x 2 m deep cementation tanks, of which only three are in use at any one time. The cementation process takes 15 days, during which the pregnant solution passes from tank 1 through tank 2 to tank 3. Final cement copper grades 60% Cu in the first tank, 55% Cu in the second and 46% Cu in the third. Scrap iron consumption amounts to 2.5 kg Fe per kg of Cu. This iron consumption is relatively high, but it is, nevertheless, within the "normal" range for this type of process (1.5-2.5 kg Fe per kg of Cu). It should be noted that the theoretical value of 0.88 kg Fe per kg of Cu (equation 7) is never achieved in commercial operations because the actual reactions taking place (equations 8, 9 and 10) increase the total consumption of iron during the cementation process.[8]

The spent solution (containing some 25 mg/l Cu) is recycled, being pumped from the cementation tanks to the acid make-up pond, where it arrives with a pH of about 4.

Production

Copper recovery is about 85%. The cement copper finally produced, which averages 53% Cu, is sent by road to ENAMI's Hernan Videla Lira smelter at Paipote (MM, October 1987, p.327).

Total production costs amount to $US1,285 per tonne of copper, as shown in Table 1. Production over the last four years has amounted to a total of 1,100 t of copper.

Table 1: Copper production costs
Item                    $US/tonne
Scrap iron                    195
Sulphuric acid                399
Energy                        150
Salaries (32 workers)         225
Maintenance                   109
Rent (to ENAMI)               191
Other                          16
Total                       1,285


[Mathematical Expression Omitted]

PHOTO : In-situ leaching in the La Hermosa open pit.

PHOTO : Handling scrap iron in the cementation vats.

PHOTO : Fig. 1: The Andacollo porphyry copper deposit: location and geology.

PHOTO : Fig. 2: Operational flowsheet of La Hermosa: in-situ bioleaching and cementation.

References

[1]Rossi, G. (1990) Biohydrometallurgy, McGraw-Hill Book Co. GmbH, Hamburg. [2]Marcial Aracena, F. (1884). La industria del cobre en las provincias de Atacama y Coquimbo. Imprenta del Nuevo Mercurio, Valparaiso, Chile, pp 185-203. [3]Yunge, G. (1909) Estadistica minera de Chile en 1906 y 1907. Imprenta, Litografia y Encuadernacion Barcelona, Santiago, Chile, Tomo 3, pp 208-212. [4]Llaumet, C., Olcay, V., Narin, A., Marquardt, J., & Reyes, R. (1975) El yacimiento de cobre porfidico Andacollo, provincia de Coquimbo, Chile. Revista Geologica de Chile, 2: pp 56-66. [5]Bernstein, M. (1990) The geology of copper and gold ore deposits in Chile. Mining Magazine, September 1990, pp 155-168. [6]H. Maturana, personal communication. [7]Gibbs, H. E., Errington, M., & Pooley, F. D. (1985) Economics of bacterial leaching. Can. Metall. Quarterly, 24(2), pp 121-125. [8]Wartman, F. S., & Robertson, A. H. (1944) Precipitation of copper from an acid mine water. U.S. Bureau of Mines, Report of Investigation 3746, Washington, D.C.

A. Concha, R. Oyarzun, R. Lunar and J. Sierra, The authors are with the Dept. de Cristalografia y Mineralogia, Facultad de C.C. Geologicas, Universidad Complutense de Madrid, 28040 Madrid, Spain.
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Title Annotation:porphyry copper deposit in Chile
Author:Concha, A.; Oyarzun, R.; Lunar, R.; Sierra, J.
Publication:Mining Magazine
Date:Nov 1, 1991
Words:1403
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