Folia Microbiol. 39 (2), 137-140 (1994)

Fungal Leaching of Nickeliferous Laterites P.G. TZEFERIS*

Laboratory of Metallurgy, Department of Mining-Metallurgical Engineerin& National Teclmical University of Athens, 106 82 Athens

Received October 22, 1993 Revised version January 4, 1994

ABSTRACT. Extraction of nickel by microbial leaching of Greek laterites is feasible by using of Aspergillus and Penicillium. The effectiveness was found to depend on the ability of the microorganism to produce hydroxycarboxylic acids, especially citric acid, as well as other metabolites. The nickel recoveries achieved were as high as 60 %, in 48 d, when the ore was leached in the presence of the living fungi, in a sucrose medium, and as high as 70 %, in a much shorter time, when the ore was leached by their metabolic products after pH adjustment by means of sulfuric acid. The use of much cheaper, factory grade, Greek beet molasses as a growth medium proved promising, giving the possibility of making the process more attractive in economic terms.

Nickel ore deposits are found either as sulfides or as laterites. The importance of lateritic ores for the future supply of nickel becomes obvious when one considers that about 80 % of the presently known nickel reserves are associated with lateritic types of ore (Roorda and Hermans 1981). Besides, sulfide ore reserves are being depleted rapidly and new reserves are more difficult to locate. Mining of lower-grade ores as well as the stringent environmental restrictions on sulfide processes will result in significantly higher operating costs for the sulfide processes (Scott 1976). Extraction technologies developed for the treatment of lateritic ores involve both pyrometallurgical and hydrometallurgical techniques. Unfortunately, most of the current industrial practice is energy-intensive and the majority of installations were conceived at a time when fuel oil was believed to be abundantly available (Queneau 1970; O'Kane 1979).

It has thus become necessary to develop new hydrometallurgical methods. A microbial process could possibly prove economic, more competitive and environmentally safer than the existing processes (Karavaiko et al. 1988; Lundgren et al. 1986).

The aim of the present study was to determine whether various heterotrophic microorganisms can be effectively used for the extraction of nickel from Greek laterites, as Greece has extensive laterite domestic sources and also is the only EEC country producing nickel (pyrometallurgically) from domestic ore sources. Other factors examined included growth media, growth conditions (growth media composition, inoculum volume, aeration, pH), leaching techniques and leaching conditions (pulp density, temperature, pH, stirring speed etc.). The responses studied were nickel and cobalt recoveries and iron codissolution.

M A T E R I A L S AND METHODS

Two types of Greek laterite ore were tested, a low-grade limonitic ore, containing 0.73 % nickel and 13.52 % iron, and a garnieritic ore with 4.27 % nickel and 5.72 % iron.

Strahts used. Aspergillus sp. (A1, A2 and A3) and Penicillium sp. (P2, P14, P24 and P6) were obtained from English (Commonwealth Mycological b2stitute, London), German (Institute o f Geo- sciences and Natural Resources, Hannover) and Greek (Technical Educational blstitute, Athens) col- lections. Also, one Penicillium strain (F1) was isolated from an actively laterising Greek ore, with char- acteristics likely to be beneficial to the leaching process.

All strains were tested for their ability to (i) produce organic chelating acids and (ii) tolerate high nickel and hydrogen ion concentrations as in an actual leaching process.

The following substrates were used for growth and bioleaching experiments: 1. Glucose medium was used for both growth and screening leaching experiments. It contained

(in g/L): glucose 150, NaNO3 3, MgSO4"7H20 0.5, K2HPO4 1, KC1 0.5, FeSO4-7H20 0.01. 2. Sucrose medium was chosen specifically for citric acid production (Shu and Johnson 1947,

1948; King 1985). It contained (in g/L): sucrose 150, (NH4)2CO3 1.8, K2HPO4 0.25, MgSOa'7H20 1.2,

"Address for correspondence: 45, Gythiou Str., 185 44 Piraeus, Greece.

138 P,G. TZEFERIS Vol. 39

Fe 2+ 10/zg/L, Z n 2+ 10/zg/L, Cu 2§ 10/zg/L; pH 4.3. Concentration for trace elements (iron, zinc and copper) was selected after optimization procedure conducted by means of a factorial analysis to maxi- mize the yield of citric acid (Tzeferis 1991).

3. Industrial-grade Greek beet molasses (47 % sucrose) was also used in an effort to reduce the overall cost of the bioteaching process. This substance had a very high ash content of more than 11.4 %. The media based on molasses contained (in g/L): molasses 150, (NH4)zHPO4 1.6, Cu 2+ 1.3 %; pH 5.0. Cooper is reported as antagonistic towards iron, thereby it was added to reverse the inhibitory effect of iron contained in molasses for citric acid production (King 1985). For elimination of ash and metal content, molasses was treated before sterilization with 1.5 g/L tetrapotassium hexacyanoferrate (Tzeferis 1991).

All fungus strains used, prior to leaching, were adapted to nickel (up to 0.4 % Ni) and laterite environment (up to a 10 % pulp density) by progressive acclimatization.

Two bioleaching techniques were used: (a) leaching in the presence of microorganisms at 30 ~ using saccharide media such as glucose, sucrose and molasses and (b) chemical leaching at 95 ~ by a solution containing the metabolic products from the cultivation of the strains at 30 ~ in the above mentioned saccharide media.

Metal concentrations in clear supernatants were measured by atomic absorption spectropho- tometry.

Citric and oxalic acids produced by fungal metabolism were determined using biochemical analytical methods.

Metals taken up by the fungal biomass were estimated by a dry-ashing technique (to decom- pose the biomass) followed by leaching with slightly acidic water (to remove nickel which had been associated with the biomass).

RESULTS AND DISCUSSION

One-phase microbial leachhlg

Leaching the ore in the presence of a commercial Aspergillus citric acid producing strain (A3) as well as in the presence of a Penicillium species (P2) nickel recoveries reached 55-60 % in almost 50 d (Table I). Losses of soluble nickel in the fungal biomass were found to be 3.5 and 10.8 %, respec- tively. The cobalt recovery achieved was around 50 % for both strains. Maximum iron codissolution

Table I. Nickel recovery (%) using a single-phase bioleaching technique and various saccharide media a

Strain A1 A2 A3 P2 P6 P14 P24 F1 Control

Glucose med ia b

Low-grade laterite 18.0 9.6 16.2 9.5 15.6 7.5 7.2 12.2 0.7 High-grade laterite 18.5 21.5 15.4 7.5 14.3 7.54 7.6 11.2 1.2

Sucrose m e d i a c

Low-grade laterite 12.8 12.3 54.9 57.9 24.5 7.5 8.2 5.8 1.2 High-grade laterite 13.9 8.9 15.3 15.3 14.7 5.6 9.0 5,0 1.8

Molasse m e d i a d

Low-grade laterite 25.4 9.9 10.8 37.3 12.4 16.4 35.5 1.2

~Femperature 30 ~ pulp density 10 %, agitation frequency 6.5 Hz. bGlucose 15 %, initial pH of substrate: 3.0. CSucrose 15 %, initial pH of substrate: 4.3. dMolasse 15 %, initial pH of substrate: 5.0.

was 30 %. Chemical analyses of the leach liquors showed the presence of significant amounts of citric (Table II), oxalic and other organic acids, indicating that leaching could be ascribed to the production of certain metabolic products of fungal activity. This conclusion was confirmed by the results of chemi- cal leaching by synthetic solutions of citric, oxalic and other organic acids which showed that citric acid

1994 FUNGAL LEACHING OF LATERITES 139

was the most effective for nickel leaching, presumably facilitating Ni dissolution by chelation, while oxalic acid was mainly responsible for iron codissolution (Tzeferis and Agatzini 1994). The different leaching behavior of the laterite ores used was attributed to their different chemical and mineralogical composition. Use of factory-grade Greek beet molasses gave lower nickel yields, in the range of 30-35 %. Considering, however, the much lower prices of molasses in Greek industrial market, com- pared to sucrose and glucose media, it is worthy of further studies.

Table II. Citric acid production vs. nickel extraction

Strain Citric acid concentration, g/L

Nickel in bioleaching in fermented recovery

solution sucrose media %

P2 57.2 21.0 61.5 A3 61.5 42.0 52.8 P6 20.4 3.5 24.6 F1 0 0 5.7 A2 8.4 3.5 12.8

Two-phase microbial leaching

Leaching of the limonitic ore sample at 95 ~ by metabolic products, produced as a result of cultivation of strains P2 and A3 in sucrose-based media at 30 ~ gave nickel re- coveries of up to 25 %. This recovery in-

creased to 72 % when sulfuric acid was added to the metabolic liquid to produce a free acid concentration of around 0.5 g/L (Table III). Leaching of the above mentioned ore sample by metabolic products of strains P2 and F1 in 15 % molasses medium after acidification of the fermented liquid to [H +] = 0.5 g/L with sulfuric acid, gave nickel yields of 54 and 62 %, respectively. The molasses medium used in those tests had been pretreated with tetrapotassium hexa- cyanoferrate (K4[Fe(CN)6] solutions to remo-

Table Ill. Nickel, iron and cobalt recoveries (%) from low- grade ore using the two-phase leaching technique a

Strain pH control b Ni Fe Co

Leaching with metabolites from sucrose-fermented media c

P2 d with b 71.7 21.6 51.2 without 24.5 16.0 17.9

A3 e with b 67.4 24.4 55.6 without 22.5 3.31 15.5

Leaching with metabolites from hexacyanoferrate- pretreated molasses media f

P2g with b 54.3 15.8 44.2 without 15.6 7.7 9.2

F1 h with b 62.3 16.2 48.9 without 18.2 10.9 12.9

Leaching with sulfuric acid b 45.2 11.8 39.4

aTemperature 95 ~ pulp density 10 %, agitation frequency 6.5 Hz, time 5 h.

b[H + ] = 0.5 g/L. CSucrose 15 %, temperature 30 ~ aeration rate 150 mL/min,

fermentation time: 2, 4, 6, 8, 10, 12 d. dCitric acid yield (maximum, 8 d fermentation): 27.4 g/L. eCitric acid yield (maximum, 8 d fermentation): 43.8 g/L. fTetrapotassium hexacyanoferrate 1.5 g/L. gcitric acid yield (maximum): 10.2 g/L, oxalic acid yield:

5.6 g/L. hCitric acid yield (maximum): 15.0 g/L, oxalic acid yield:

7.0 g/L.

ve the ash and toxic trace metals contained and thus increase its ability for citric acid production by fermentation. Nickel recoveries obtained by molasses media (up to 62 %) compared to nickel recov- eries in pure sucrose media (up to 72 %) and leaching with sulfuric acid (45.5 %), all under the same leaching conditions, could possibly prove a promising alternative despite the cost of the necessary pre- treatment of molasses. The price of untreated molasses on the Greek industrial market is estimated approximately 0.05 USD ($) per kg, considerably lower than the price of industrially produced sulfuric acid and at least twenty times less than the price of food grade sucrose.

The financial support of the commission of European Communities DGXII under contract No.MAIM.0017.C(H) is gratefully acknowledged. Thanks are also expressed to Dr. S. Agatzini (National Technical University of Athens, Athens), Dr. E.T. Nerantzis (Technical Educational Institute, Athens), Drs. D. Leak, K. Alibhai and A. Dudeney (Imperial College of Science and Technology, London) for their helpful collaboration.

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