Journal of Biotechnology, 31 (1993) 115-123 115 © 1993 Elsevier Science Publishers B.V. All rights reserved 0168-1656/93/$06.00

BIOTEC 00945

Bioleaching of minerals - a valid alternative for developing countries

Fernando Acevedo and Juan Carlos Gentina School of Biochemical Engineering, Universidad Cat61ica de ValparaEso, Chile

Sergio Bustos Sociedad Minera Pudahuel, Santiago, Chile

(Received 21 August 1992; revision accepted 3 May 1993)

Summary

The feasibility of applying bacterial leaching of minerals in developing countries is analyzed. There are several examples that show the economic viability of this technology. The Chilean experience in this area is presented as a case study. The concerted effort that was made during the last decade, allowed a technology transfer from experimental stage to commercial exploitation of copper ores.

Bioleaching; Biohydrometallurgy; T. ferrooxidans; Technology development

Introduction

The bioleaching of minerals is a simple and effective technology for the processing of ores that contain acid insoluble sulfides and oxides. These insoluble species can be solubilized by oxidation mediated by the action of certain microor- ganisms of the genera Thiobacillus, Leptospirillum and others. The participating bacterial flora obtain metabolic energy from the oxidation of sulfides, some oxides

Correspondence to: Fernando Acevedo, Escuela de Ingenierla Bioqulmica, Universidad Cat61ica de Valparaiso, Casilla 4059, Valparaiso, Chile. Fax 56-32-233393.

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and ferrous ions. In the latter case the mineral is oxidized by ferric ion rendering ferrous ion which in turn is reoxidized by the bacteria. The metals contained in the soluble sulfates and oxides can be recovered by cementation or electrodeposition.

Bioleaching can be carried on in heaps, dumps and tanks or by in-situ and in-place methods. The fundamentals and applications of bacterial leaching have been extensively covered in the literature (Karavaiko and Groudev, 1985; Lawrence et al., 1986; Norris and Kelly, 1988; Salley et al., 1989; Acevedo and Gentina, 1989; Karavaiko et al., 1990). Bacterial leaching is thought to be a technology that requires moderate capital investment, has low operating costs, is appropriate for low-grade ores, and the secondary recovery of metals that otherwise are discarded, does not produce atmospheric pollution and does not require sophisticated equip- ment and operating procedures (Da Silva, 1981; Gentina and Acevedo, 1985; Acharya, 1990). Its field of application has broadened in the last decades and now includes the processing of concentrates and medium- and high-grade ores. The main limitations of bacterial leaching are related to its low reaction rates and the difficulties in exerting close process control in all modes of operation except tank leaching.

Since most of the commercial applications of bacterial leaching are concerned with dumps of material previously considered waste, their design conditions are not optimal with respect to the microbial component of the process (Warhurst, 1985). So at the present time, the developed countries cannot offer an optimized and tested technology to the developing world. This fact created a challenge to countries in Latin America, that led to interesting efforts such as the Andean Pact Copper Project (Warhurst, 1985), the UNDP bacterial leaching national project in Chile (Anonymous, 1987) and the development of a complete bacterial process by Sociedad Minera Pudahuel, Chile.

Economics of bioleaching

The economics of bacterial leaching processes have to be carefully assessed in order to determine their commercial feasibility. Unfortunately the relevant infor- mation on this specific matter is scarce. A useful methodology for the economic evaluation of the exploitation of small mines is given by Lewis et al. (1983). Capital investment for some commercial heap, dump and in-situ leaching has been reported by Warhurst (1984). A recent paper (Montealegre and Bustos, 1991) states that the bacterial treatment of secondary copper sulfides requires an investment of the order of US$2500 per metric tonne of cathodes per year, with a production cost of US$0.88 kg -1 for a plant that produces 50000 t a -1 of cathodes. Acharya (1990) examines different alternatives for the bacterial leaching of copper and gold, and concludes that copper dump leaching is economic even for lower-sized ore bodies (50 million t, 3-8 million t a -1 of ores). Heap leaching requires larger sizes in order to become economical. In the case of gold, the analysis favors heap leaching for lower-grade ores (3 g t -1 of gold), while vat leaching is economic for higher grades (5 g t-1 of gold). It must be stressed that

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the author did not consider tank leaching of gold concentrates, which is likely to be the alternative of choice.

B a c t e r i a l l e a c h i n g o f c o p p e r o r e s i n C h i l e

Chile is one of the main copper producers of the world, exceeding 1 600 000 t a -1 in the past years. About 75% of the annual production correspond to CODELCO-CHILE's mines, the National Copper Corporation, a state enterprise (Williams, 1990).

To keep world production leadership, Chile has to expand its mining opera- tions. Chilean copper reserves have been estimated in 184 500000 t, but 30% of them are contained in low-grade ores. Besides, at several mine sites thousands of tons of acid-leached ores, tailings and non-exploited low-grade ores have accumu- lated in huge quantities through the years.

These materials can be incorporated to production if an appropriated low cost technology is available. In this sense bacterial leaching emerged during the last decade as a promissory technology.

Up to the decade of 1960, bacterial leaching was not well known either in the academic or in the industrial sector. Early in the 70's, a growing interest on this technology started and the first isolated attempts of research activities were done (Arrieta and Grez, 1971; Gonzalez et al., 1974; Anonymous, 1974; Gonzalez and Cotor~s, 1978; Rodrlguez-Leiva and Pichuantes, 1978; Henrlquez et al., 1978). Both ~ectors worked independently, a situation that gradually changed when at the beginning of 80's some universities organized courses and workshops with the participation of several well known international experts.

In the mid-80's, a national research project finan.ced by UNDP was approved. The participants were one of the divisions of CODELCO-CHILE, two research ir~titutes and three universities, involving basic research (microbiology, molecular biology, genetics), applied research in engineering science (biochemical, chemical, mining and metallurgic engineering) and industrial application (field engineering). The project lasted two years and a half, and was extended for a new period of two years that ended on December 1990.

The main objectives of this project were to increase the knowledge on the genetics, physiology and biochemistry of the leaching bacteria, to gain a deeper insight in the nutritional and physical-chemical requirements in relation with process conditions, to perform studies at pilot level, and to train personnel in basic and applied microbiology and extractive metallurgy.

Besides publications and presentations to congresses and symposia, the net benefits of the project can be summarized as follows: - The training of a significant number of scientists and engineers. Most of them

are currently involved in bioleaching research while others give technical assis- tance to the mining industry.

- The agreement a~0ng industry and academia about the importance and rele- vance of bacterial leaching to the country.

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- The feeding of industry with new information about the bacterial process and associated technologies, making them available for their own development.

B T L p r o c e s s

An important breakthrough in the bioleaching technology in Chile has been the process developed by Sociedad Minera Pudahuel (SMP), named the Bacterial Thin Layer Leaching process (BTL), currently used at their plants of Lo Aguirre and La Cascada.

Sociedad Minera Pudahuel began to exploit the Lo Aguirre ore deposit in 1980, producing an average of 14500 t per year of cathodic copper. At that time, the ores contained copper mainly in the form of oxides (malachite and crisocolla) and were extracted by acid heap leaching, using the Thin Layer (TL) process patented by SMP (Domic, 1984; Rauld et al., 1986). Briefly, their main characteristics were: - A 'curing' stage, consisting in the addition of concentrated sulfuric acid, which

allows a fast and efficient initial conditioning of the gangue. This stage provides an almost homogeneous acid distribution inside the heap, creating at the same time new reaction zones.

- Agglomeration of the ore and irrigation in a non-flooded system (trickle-bed systems). These factors result in a homogeneous solid bed with excellent conditions for liquid and gas permeability, avoiding channelling and allowing efficient operation at different heights and solution flow rates. After 18 d of extraction, the solubilized copper was recovered in a solvent

extraction stage (SX) followed by electro-winning (EW). During the first years of operation, the insoluble copper content in the ore

ranged between 0.3-0.55%, mainly chalcocite and bornite. Around 40% of the sulfide copper was recovered in the TL process by means of the natural ferric ion oxidation occurring in the leaching solution circuit.

The increasing amount of insoluble copper in the ore through the years induced SMP to test and develop a secondary bacterial leaching stage for the copper remaining in the tails of the acid leaching. This system started operating in 1986

TABLE 1

Percent of copper recovery during the primary and secondary heap leaching stages

Species Primary stage Secondary stage

Recovery Time Recovery Time (%) (d) (%) (d)

Oxides 80 18 90 150-210 Cu zS 40 18 80-85 150-210 CusFeS 4 30 18 70-76 150-210

The ore was 100% under 6 mm (1/4 inch). Data from Montealegre and Bustos (1991).

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TERTIARY

woter su If uri,: aci, 3N

ELECTROWI

PQffinate

Cu CATHODES

Fig. 1. Block diagram of the BTL process.

(Montealegre and Bustos, 1991). Table 1 shows the recoveries of soluble and insoluble copper during the primary and secondary leaching stages. The secondary leaching stage took place in 5-6 m high heaps irrigated with an acid solution, keeping the pH of the solution according to the bacterial requirement.

Further increase of the insoluble copper content, amounting to 75% of the total copper of the ore (around 1.7% CUT), and several problems detected in the operation of the secondary leaching stage led to the development of the concept of the one-stage leaching or Bacterial Thin Layer (BTL) process. Fig. 1 shows a block diagram of the BTL process. This process was successfully tested in experimental heaps of about 30 000 t during 1988. Total copper recoveries of 80% were attained after leaching times ranging from 150 to 230 d. Fig. 2 compares the percent of copper extracted through time, with and without the presence of bacteria in the leaching solution (Montealegre and Bustos, 1991). Figl 3 compares the results of BTL derived from experimental runs in pilot pads and from industrial-scale heaps (Bustos et al., 1991).

Since January 1989, the copper from Lo Aguirre mine has been recovered by the BTL process at a rate of 3000 t d -1 of ore, with leaching times of 6 to 12 months. The total copper recovery depends on the mineralogical composition of the ore and for Lo Aguirre is around 80%, and the production has been of 14000 t

100

*~ a0

L..

0 I . I IlJ

t_ t.O IIJ

20 U

100

0~ so

60 o

~0

20 8

0

0 200

~ ita

without bocterio

I I I I / I

/,0 80 120 150 Time (d)

Fig. 2. Effect of the bacteria in the one-stage TL leaching of secondary copper sulfides (Montealegre and Bustos, 1991).

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0 2 /, 6 8 10 Time (m)

Fig. 3. Copper recovery kinetics in experimental pilot pads and industrial heaps using the BTL leaching process. Curves 1 and 4: pilot pads; curves 2 and 3: industrial heaps.

a-1 of high-grade copper cathodes, coming mainly from the leaching of sulfide ores.

Current investments

Presently two agreements have been signed to use SMP's BTL process in copper mines in northern Chile. One of them is Cerro Colorado at 2600 m above sea level. Est imated reserves are of 80 million t with a copper grade of 1.4%. Its production rate is planned to be of 45 000 t a-1 of copper cathodes. The other deposit is Quebrada Blanca at 4200 m above sea level. Its reserves are of 75 million t, 1.5% copper grade, and will produce 75 000 t a -1 of copper cathodes (Jo et al., 1991).

TABLE 2

Composition of the Cerro Colorado ore

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Chemical composition (%) Total copper 1.47 Oxide copper 0.14 Sulfide copper 1.33 Total iron 2.42 Soluble iron 0.09 Molybdenum 0.02

Mineral composition (%) Chalcopyrite 0.1 Chalcocite 96.7 Covelite 3.2 Bornite 0.1

Data from Jo et al. (1991).

The BTL has been successfully tested in pilot plants located in Cerro Colorado, Quebrada Blanca and Lo Aguirre. The Cerro Colorado pilot tests were run in ten 36 t pads from January 1989 to March 1990. The composition of the mineral is given in Table 2.

Copper recovery was around 90% after 7 to 11 months. The pads effluent solution contained about 106 cells per ml, 2-3 g 1-1 of iron and 20-30 g 1-1 of sulfate ions. Temperatures measured inside the pads varied from 12 to 27°C.

The pregnant solution was treated by solvent extraction and the copper recov- ered by electro-winning. The raffinate contained 0.3 g 1-1 of copper and was recycled to the pads.

The Quebrada Blanca pilot test program was carried out from January 1990 until December 1991 at the mine site. The extreme ambient conditions imposed the main questions in the experimental work. A complete phenomenological model was developed for the simulation of the thermal behavior of a BTL leaching-SX-EW plant, thus allowing SMP the selection of the best operating conditions for the design of the process.

At present both projects are under construction and will be started up by 1994. Besides these two projects, CODELCO-CHILE at Chuquicamata is building a

low-grade copper sulfide ore dump, which will be irrigated according to a strategy previously defined at a large pilot level (Pincheira and Heller, 1986). Also, at El Teniente, another mine own by CODELCO-CHILE, an in-place bacterial leaching process of a copper sulfide ore body (with less than 0.7% of Cu) is in operation-. Currently its production is 7000 t a -1 of copper cathodes and it is planned to increase it up to 30 000 t a-1 (Ovalle, 1988; Montoya, 1990).

In Chile, it is expected that in the next 3 -4 years several industrial projects based on bacterial leaching will enter in operation, rendering a production of 250 000 t a-1 of copper cathodes, approx. 16% of the total present production.

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Conclusions

Since the late 60's the industrial development and the number of scientists and technologists trained in the subject of bacterial leaching have increased notably.

The join efforts of personnel from the university and from the mining compa- nies have been positive for the progress of bacterial leaching.

The expertise gained with the leaching of copper sulfides has opened the possibility of working with other minerals, such as refractory gold.

The technology developed in Chile to benefit secondary sulfides (covelite, calcosite, bornite, etc.) is economically viable.

The heap leaching technology is not well suited for the exploitation of chalcopy- rite at a commercial level. Further development are needed in this area.

Bacterial leaching of chalcopyrite bearing minerals is practical using dump or in-place systems, because the low recovery rates of copper are compensated by the low cost of the process.

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