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Study o f the b io leach ing o f a nickel conta ining b lack-sch is t ore

M. Riekkola-Vanhanen and S. Heimala

Outokumpu Research Oy, P.O.Box 60, SF-28101 Pori, Finland

Bioleaching tests of nickel from a complex black schist mineralisation in Finland have been performed. The ore consists of pentlandite, violarite, sphalerite, chalcopyrite, pyrite and pyrrhotite. Its most important metal is nickel, which is present as pentlandite and violarite. The mixed bacterial culture used was derived from composite water samples from the area of the orebody. It was a mixture of ferrous iron and sulphur oxidising acidophilic bacteria with Thiobacillusferrooxidans as the major constituent. Shake flask tests were conducted in order to determine the amenability of the ore to bioleaching. During the bench scale testing phase it was confirmed that the pH value of the leaching solution should be about three in order to prevent the leaching of the high amount of silicates in the material. The experiments were successfully scaled up to pilot scale by using columns containing 900 kg ore with a grain size of 70 % 0.5 - 2 mm. The best leaching rate obtained indicated that 92 % of the nickel would have been leached in less than 300 days.

The mineralogical analysis of the residue showed that all remaining nickel sulphides were inside large silicate particles, where the leaching solution could not gain entrance. Most of the iron was precipitated in the reactor and did not interfere with the leaching. Further pilot scale leaching tests are going on in our laboratory in order to define the significance of the agglomeration of the ore, aeration of the columns and grain size.

1. INTRODUCTION

Black schist rocks are common in the Eastern parts of Finland. Appreciable concentrations of sulphide minerals have been found in these schists. Such an anomaly, located near the town of Sotkamo, has been investigated in more detail since the late seventies. The surveying revealed a mineralisation comprising about 300 million tons with about 8% S and 0.3 % Ni as the most valuable metal. As the occurrence is situated at the surface with practically no overburden and can easily be mined as an open pit, economical exploitation could be possible in spite of the low content of valuable metals.

It was quickly apparent from the ore dressing tests that a satisfactory nickel concentrate was not achievable due to the complexity of the minerals. This meant that the material could not be treated in the normal way of flotation and smelting of the concentrate. A hydrometallurgical process to treat the material was developed at Outokumpu Research at the end of 1970's (1). The decision to start processing was postponed because of the low nickel prices at that time.

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Bioleaching has also been considered as a possible method to treat this material, Bioleaching experiments with the black schist ore were already started in Outokumpu Research fifteen years ago. The results obtained in the latest pilot-scale bioleaching test are presented in this paper.

Many bioleaching tests of the black schist ore have been made both in shake flasks and agitated reactors (2). In most tests the temperature was about 30~ and the grain size 85% - 0.074 mm. In 7 - 21 days 99% Ni, 100% Zn, 90% Co and 30-40% Cu were dissolved depending on the pulp density used. According to these experiments, bioleaching of nickel from the black schist ore seemed to be easy.

Several percolation tests were made by using different conditions. The leaching rates were, however, quite slow: about 40 % nickel had been leached in 200 days. The amount of ore used in these tests varied from 5 to 50 kg. A new irrigation method of the material in the column was developed and tested with 5 kg ore, 50% 0.1 - 0.5 mm. With this system 95% nickel was dissolved in 180 days.

The black schist ore contains much silica and the earlier chemical leaching experiments indicated that below pH 2.5 part of the silica minerals dissolved and formed a gel which hindered the dissolution of valuable minerals. Bioleaching is normally done at pH values under 2.5 in order to keep ferric iron in solution. Ferric iron is an important oxidising agent in the bacterial leaching of sulphide minerals.

Tackaberry et al. (4) observed in their work to recover nickel from pyrrhotite-based tailings, that at low pH values (<2) the nickel recoveries were low. But in contrast, where the pH stabilised at values greater than 2.5 to 3 and where Thiobacillusferrooxidans was the primary strain, the nickel recoveries were significantly increased. High acid strenghts have also been implicated in the suppression of nickel dissolution during bacterial leaching of pentlandite (4). These findings confined that it is possible to bioleach nickel in the pH area where ferric iron precipitates.

2. MATERIALS AND M E T H O D S

The black schist ore used in this study contained pyrrhotite (Fel-xS), pyrite (FeS2), sphalerite (ZnS), pentlandite [(Ni, Fe, Co)9Ss], violarite [(Ni,Fe, Co)3S4], chalcopyrite (CuFeS2) and graphite. The main silica containing phases were quartz, mica, anorthite and microcline. The bulk sample used contained Ni 0.33 %, Zn 0.56 %, Cu 0.23 %, Co 0.02 %, Fe 7.1%, S 5.2 %, C 8.4 % and SiO2 55.7 %. Pentlandite and violarite contained 90 % nickel, the rest was in the pyrite and pyrrhotite. Cobalt was distributed in the pyrite (2/3) and in the Ni-containing minerals (1/3). Zinc was in the sphalerite and copper in the chalcopyrite. About 2/3 of the iron was in the pyrrhotite and the rest in the pyrite.

A mixed culture of iron- and sulphur- oxidising bacteria, designated as SB/P-II (2) was used throughout this work. The bacterial consortium was originally produced by combining several mine water enrichment cultures from the site of the orebody and was then maintained with the ore as the sole substrate. It was established that SB/P-II contained active Fe2+-, elemental S- and FeS2-oxidising bacteria capable of growing at pH values 1 - 4. Thiobacillusferrooxidans was the main constituent.

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The culture was grown in a mineral salts solution (0.4 g/1 each of K2HPO4, (NH4)2SO4 and MgSO4*7H20) at pH 2.0 containing 10 % (w/v) black schist ore at 30~ Cells were harvested by centrifugation, washed twice and resuspended in 0.01 N H2SO4. The cell density was estimated from the inoculum microscopically with phase contrast illumination, with the aid of a Thoma counting chamber. The cell density in the inoculum was approximately 109 cells/ml.

Redox-potential values were measured with a Pt electrode against an Ag/AgCI reference electrode. The liquid samples taken at suitable intervals were filtered, and analysed by atomic absorption spectroscopy. The solid samples were first air dried and then dissolved with HNO3, HCI, HF and HC104 on a hot plate or in a microwave oven, diluted to volume with distilled water and analysed with atomic absorption spectroscopy. The silica analyses were done by fusing the sample with sodium hydroxide, dissolving the melt in HC1 and finally colorimetrically from the resulting solution. Sulphur and coal were analysed with the Leco CS-244 analyser.

The solid samples were also examined with an optical microscope. The mineralogical composition of the leach residues was analysed by x-ray diffraction. XRD analyses were conducted with a Siemens D500 diffractometer. Electron probe microanalyses were done with a Camebax micro beam/Link AN 10000.

2.1. Pilot tests The pilot bioleaching equipment at the pilot plant of Outokumpu Research Oy was

constructed in a new way in order to enable us to irrigate the ore in the same way as in the bench-scale tests earlier. In this method the leaching solution from the solution container is quickly pumped upwards into the column so that the solution surface rises over the ore bed. The particles are sorted and the smallest ones come to the surface. The solution is allowed to flow freely back into the container.

Two columns were filled each with 450 kg freshly excavated ore, which was crushed with a roll crusher to 70% 0.5 - 2.0 mm. The coarser particles were loaded to column 2 and the finer ones to column 1. The height of the columns was three meters and their diameter 60 cm. 350 1 water was added into the solution container. Air moistened with water was blown into both columns (300 I/h) from the bottom of the columns. The solution container was also aerated (150 l/h). After one hundred days 0.5 % CO2 was added to the airflow.

In the beginning the solution was risen once a day and later two times a week through the first column above the surface of the bed and allowed to flow back into the container. Then the procedure was repeated with the other column. Aeration of the columns and the container was shut down for the irrigation time. After five days the pH value of the solution had decreased to 3.2 and the bacterial inoculum was added. Nutrient salts (0.4 g/l each of K2HPO4, (NH4)2SO4 and MgSO4*7H20) were also added. The amount of the inoculum was only 2% of the solution volume, because it was thought that the freshly excavated ore already contained enough bacteria.

Four electrode packets each containing pentlandite, pyrite, pyrrhotite, chalcopyrite and chalcocite mineral electrodes were prepared. A platinum electrode and an Ag/AgCI reference electrode were also added to the packets. The packets were placed in the columns through small holes in the sides, in the upper and lower parts of both columns. Preparation of the mineral electrodes has been described earlier (3).

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3. RESULTS AND DISCUSSION

3.1. Operation of the columns In the beginning of the test it took 11 minutes to raise the solution through the columns and

22 minutes for the solution to flow down. The calculated velocity of water was 3 mrrds which is enough to get the ore to float. At the end it took seven minutes for the solution to be raised, but the flow downwards lasted about three hours. There were also at that time some sintered parts in the columns. Microscopical examination of samples taken from the sintered parts revealed that iron oxyhydroxide sulfates together with some fine sludge had precipitated in big particle interstices consolidating it. The solution flow was then suppressed in these areas.

After 150 days the dissolution of especially nickel slowed down. It was thought that one possible reason might be the quite low amount of bacteria in solution. A bigger bacterial inoculum, 5 % of the solution volume was added on the 186th day. The leaching rates of Ni, Zn and Co increased quickly.

The leaching of nickel seemed to slow down or end when the nickel content in the solution reached about 3 g/1. Therefore half of the solution was removed from the container and replaced with the same amount of nutrient solution. This accelerated the leaching again. The last dilutions were made in order to wash the precipitated metals out of the colums. The temperatures measured varied between 17 and 23~ during the test. The mass loss of the material was only 0.5 %.

The pH and redox potential values measured during the black schist bioleaching are presented in Figure 1. The pH value came down to 3.2 during the first five days due to the dissolution of pyrrhotite. The bacterial inoculum was added on the fifth day, it is the day 0 of the experiment. After that the pH value remained at 3.1 - 3.4 , suggesting that the acid production and acid consumption were in balance. At the end of the test the pH value was adjusted to 2.5 with sulphuric acid in order to see, if the leaching rates would accelerate. The amount of acid consumed was only 4 kg/t of ore. The redox potential increased from the initial value of 300 (inoculation day) to the 500 mV vs SCE level.

3.3. Electrode potentials Mineral electrodes have already proved to be useful tools for process control (3). Mineral

electrode measurements represent the sum of the oxidative and reductive reactions. The mixed

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potentials measured can be used to evaluate the reactions of the minerals with the aid of Pourbaix-type diagrams.

Electrode potential values of pentlandite, pyrrhotite, pyrite, chalcopyrite and chalcocite measured from different places in the columns are presented in Figure 2. The influence of the way to irrigate the columns can be seen in the graphs. The oxygen-rich solution after aeration was risen to the first column and the main part of the dissolved oxygen was consumed, the solution flowed back to the container and was risen to the second column. In spite of the fact that both columns were aerated, when they were not irrigated, there was less oxygen available in the second column. This can be seen especially well in the potentials of the pentlandite. In the beginning the potentials rose in the bottom of both columns to 400 mV and in the top to 200 mV vs SCE. After the first 50 days the potentials decreased to the 100 mV level in the first column, to -300 mV on top of the second column and to -100 mV in the bottom of the second column. The value of 200 mV defines the potential at which there is a change in the mechanism for the oxidative dissolution of pentlandite (5). -100 mV is the equilibrium potential for the reduction of sulphur to sulphide (5). The tests done in our laboratory show, that the potentials measured with a pentlandite electrode in a solution containing bacteria are lower than those without bacteria. The leaching activities are, however, much bigger in the solutions containing bacteria.

At the start the potentials of pyrite rose from the level 200 to 500 mV vs SCE as soon as bacteria were added. The chief differences observed were mainly due to the fluctuating amounts of elemental sulphur and ferric iron on the surfaces of the electrodes. The 500 mV level is well within the area of pyrite dissolution (3). With pyrrhotite the levels also rose in the beginning from about 100 to 450 mV vs SCE, but fell quite quickly down during the first hundred days. In the bottom of the first column the potentials stayed at the 200 mV level, but the potentials elsewhere were under 100 mV. It seems that the particles were almost completely covered.

Chalcopyrite showed the same phenomenon at first, the potential rose from 200 to 450 - 500 mV vs SCE. After that the potential stayed at the 350 mV level. This is about the equilibrium determined by CuS/CuS2 (6). In the bottom of the second column the potential started to decrease after the 170th day to the 0 mV level. There was probably a cover on the particles, under which there were bacteria in low oxygen conditions. The potential of chalcocite stayed the whole time on the 200 mV vs SCE level. This is the value where chalcocite decomposes to covellite and Cu2+. On top of the second column the potential fell down after 400 days. One possible explanation is the depletion of the electrode material.

3.4. Dissolution of metals The dissolution of metals calculated with the aid of solution analyses is presented in Figure 3.

Metal sulphide leaching by micro-organisms is known to be electrochemical in nature. The sulphide minerals can be presented as an electrochemical series, where the minerals are ranked according to their electrode potentials (3). Therefore the minerals of this black schist ore are leached in the order pyrrhotite, sphalerite, nickel sulphides, chalcopyrite and pyrite. During the first 180 days the dissolution ofNi, Zn and Co was low. Almost no copper was dissolved. The addition of more bacteria on the 189th day improved the leaching rates. The leaching of nickel seemed to slow down or end when the nickel concentration in the solution reached about 3 g/l.

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Figure 3. Dissolution of metals calculated according to the results of the solution analyses.

Figure 4. Dissolution of nickel. The thin line is the leached percentage and the thick one the concentration in solution.

When the solution was diluted, nickel leaching was re-activated (Figure 4). When the nickel level in the solution is high, it is possible that some nickel starts to precipitate on the surface of the pyrrhotite. This can be prevented by changing the kinetics of the process, e.g. by diluting the solution. The more dilute leaching solution had less influence on the leaching of the other minerals. Copper leaching did not start until the 250th day. The decrease of the pH value to 2.5 did not accelerate the leaching. In the last washing stage of the columns, especially high amount of the zinc was dissolved.

During the active leaching time, after the addition of more bacteria, the iron concentration in solution was only 10-20 mg/1. When the pH decreased to 2.5, the iron values rose tenfold. The effect of iron precipitation on the nickel leaching rate was evaluated by calculating the ratio of dissolved nickel and iron in the solution. No correlation could be seen in these ratios. The amount of iron in the solution does not seem to influence the leaching rate of nickel. Ahonen and Tuovinen (7) also came to the same conclusion in their work with complex sulphide ore.

When the dissolution of the metals from the ore was calculated with the aid of the solution analyses (Figure 3), it was found that 90 % nickel, 59 % zinc, 65 % cobalt and 13 % copper had dissolved. The analysis of the solid phases is presented in Table 1. The final analyses at the end of the test are means of six samples representing the whole columns. The recoveries of the metals were 92 % for nickel, 80 % for zinc, 65 % for cobalt and and 66 % for copper.

Table 1. Chemical composition of the solid samples before, in the middle and after the test.

Solid material Original material Column 2 68 days Column 1 297 days Column 2 297 days Column 1 474 days Column 2 474 days

Ni% Zn% Cu% Co% Fe% Mn% Ca%Mg%SiO2% S% C% 0.33 0.56 0.23 0.02 7.1 0.23 1.5 1.6 55.7 5.2 8.4 0.28 0.51 0.11 0.02 6.9 0.16 0.32 0.11 0.02 6.7 0.12 1.0 1.4 51.7 4.9 8.3 0.17 0.33 0.11 0.02 6.6 0.12 1.1 1.4 52.3 4.8 8.1 0.03 0.10 0.08 0.01 7.2 0.14 0.80 1.6 51.8 3.6 7.8 0.03 0.12 0.08 0.01 7.3 0.14 0.87 1.6 51.6 3.8 7.7

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These values are more reliable than the values of these solution analyses, because in the calculation of the results there can be many errors originating from the difficulty to control the solution volume accurately. The biggest discrepancies are in the recovery of copper and zinc. The leaching of copper sulphides proceeds through the formation of polythionates and thiosulphates. These form complexes which adsorb easily on all surfaces. This phenomen may be the reason for the copper aberrations. The difference in zinc is probably due to the precipitation of zinc in the tubes leading to the solution container. That precipitate had to be removed quite often during the test. According to the x-ray diffractograms it contained ZnSO4*6H20, MgSO4*6H20, MgA12(SO4)4*22H20 and CaSO4*2H20. In many earlier column tests the leaching rate of nickel gradually decreased. The decreasing rates may be partially attributed to the formation of a product layer on the mineral surface which created a diffusion barrier to the interfacial fluxes of reactants and products. With the type of reactor used in this test there was an abrasion between the particles each time the columns were irrigated and the layers were at least partly removed. The anticipated bioleaching time would have been about 300 days, if the leaching rate had all the time been the same as during the days 189 - 420. This means that enough bacteria should have been added already in the beginning and that the nickel content in solution should also have been controlled from the beginning of the test.

3.5. Mineralogical analysis of the solid phases At the end of the test six samples representing the whole material were taken from both

columns. The samples were washed two times with water and air dried. All these samples appeared to be identical. The amount of sulphide minerals was so small that they could not beidentified in the x-ray diffractograms. Gypsum CaSO4 and iron hydroxysulphate Fe3(SO4)2(OH)5*2H20 appeared as secondary precipitates. The gangue material consisted of quartz, graphite and various hydrated silicates: SiO2, C, KAiSi3Os, KMg3(Si3AIOlo)(OH)2 and CasSi6016(OH)2*8H20. When the polished specimens were examined with an optical microscope it could be seen that nearly all remaining nickel sulphides were inside silicate

Figure 5. Microanalysis of a sulphide particle inside a silicate particle. 1 = Fel-xS, 2 -

(Fe,Ni)9Ss, 3 - A1,K-silicate and 4 = SiO2. Magnification 200x.

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Figure 6. Microanalysis of a partly dissolved pyrrhotite particle containing pentlandite lamellae. Magnification 210x.

particles, where the leaching solution could not gain entrance (Figure 5). Pentlandite is frequently intimately associated with pyrrhotite, in which pentlandite occurs exsolved as fine lamellae. A microanalysis picture of a partly dissolved pyrrhotite particle is shown in Figure 6. It can be seen that the leaching had advanced parallel to the direction of pentlandite lamellae, but not all nickel had been dissolved. The EDS-analyses from 44 random undissolved Ni-sulphides showed that the more sulphur rich samples were already near the MeS2-type. Half the sulphides were violarites (Fe,Ni)3S4, and the rest pentlandites with the Ni/Fe ratio at about one. It could be seen from the structures of the violarites that they were secondary precipitates, were thus formed in the bioleaching process and were not part of the original ore.

4. CONCLUSIONS

The black schist ore can be bioleached at pH values of about three, where the silicates do not dissolve and cause inconveniences by gelling the material. Most of the iron was precipitated inside the reactor and did not interfere with the leaching. Efficient leaching did not start until a big enough inoculum had been added. The leaching of nickel seemed to slow down or to stop when the nickel content in the solution reached about 3 g/1. This decrease in the leaching rate could be prevented if the leaching kinetics were effected by diluting the leaching solution.

With the irrigation method used there was an abrasion between the particles each time the solution was raised over the ore bed. The product layers on the mineral surfaces seemed to be at least partly removed and did not influence the leaching. The particles were also sorted each time the columns were irrigated. The small particles continuously rose to the surface and did not restrict the circulation of air and solution.

Nearly all the remaining nickel was as sulphides inside silicate particles where the solution could not gain entrance. Half of the nickel sulphides present at the end were violarites, which had been formed during the bioleaching process.

REFERENCES

1. Fugleberg, E. Nermes, S. Heimala, V. Hintikka, S-E. Hultholm., J. Jarvinen, A. Lilja, B. Nyman, J. Poijarvi, L. Rosenback and M. Saari, XVth International Mineral Processing Congress, Gedim (Ed.), tome III, Reboul Imprimerie, St-Etienne, (1985).

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2. Niemela, M. Riekkola-Vanhanen, C. Sivela, F. Viguera and O.H. Tuovinen, Appl.Environ.Microbiol., (1994) 1287.

3. Riekkola-Vanhanen and S. Heimala, Biohydrometallurgical Technologies, A.E. Torma, J.E. Wey and V.I. Lakshmanan (Eds.), Vol. I, TMS, Warrendale, PA,(1993).

4. Tackaberry, V.I. Lakshmanan, G.W. Heinrich, M. Collins and R.G.L. McCready, Waste processing and recycling in mining and metallurgical industrie. S.R. Rao, L.M. Amaratunga, D.A.D. Biateng and M.E. Chalkley (Eds.), Proceedings of the international symposium, CIM, Montreal, (1992).

5. Warner, N.M Rice and N. Taylor, Hydrometallurgy, 31 (1992) 55. 6. Torma A.E., Hydrometallurgy and electrometallurgy of copper, W.C. Cooper, D.J. Kemp,

G.E. Lagos and K.G. Tan (Eds.), Proceedings of the international symposium Copper 1991- Cobre, Pergamon Press, New York, (1991).

7. Ahonen and O.H. Tuovinen, Hydrometallurgy, 37 (1995) 1.