Cobalt and nickel recoveries from laterite tailings by organic and inorganic bio-acids

ta ez # n a or hi -ox resi by cultures of A thiooxidans or Aspergillus niger respectively were used to study tribute Hydrometallurgy 94 (2008) 18–22 Contents lists available at ScienceDirect Hydrome j ourna l homepage: www.e lse comprises the top lateritic layer, is a homogeneous ore consisting mainly of goethite associated with nickel (Gleeson et al., 2003; Golightly, 1981). The Cuban nickel deposits are recognized as one of the biggest deposits in the world and represent one of the most important sectors of the Cuban economy. Cuba has an estimated 37% of the world reserves of Ni and ranks first in the world reserves of Ni and second in world reserves of Co. (i) acidolysis, (ii) complexolysis, (iii) redoxolysis, and (iv) bioaccumu- lation (Burgstaller and Schinner, 1993). The nickeliferrous laterites are usually treated by pyrometallurgy or by hydrometallurgy. These processes generate huge volumes of tailings with considerable amounts of metals like Ni (0.25%) and Co (0.09%) that are partially oxidized. Sulphur-oxidizing bacteria are considered to be one of the most useful microorganisms for the bacterial leaching of sulphide ores. Acidithiobacillus ferrooxidans and Bioleaching of oxide and silicate ores microorganisms which are able to produc ⁎ Corresponding author. E-mail addresses: [email protected] (O. Coto), galizia [email protected] (I. Hernández), macoto10400@yah [email protected] (E. Donati). 0304-386X/$ – see front matter © 2008 Elsevier B.V. A doi:10.1016/j.hydromet.2008.05.017 climate and abundant three layers, namely the layer. Limonite, which by heterotrophic microorganisms generally involves an indirect process with microbial production of organic acids, amino acids and other metabolites. Four mechanisms have been identified: rainfall. Lateritic deposits usually consist of limonite, the saprolite and the garnierite They were formed during laterizat ion, a weathering process of ultramafic rocks that is favoured by warm 1. Introduction Laterites are oxide ores widely dis 7% Ni and 16% Co were leached after 3 days although there was a significant metal precipitation later. In one- stage batch experiments using A. thiooxidans and elemental sulphur as energy source higher percentages of metal solubilization (100% Ni and 80% Co) were reached after 15 days. In two-stage batch experiments with sulphuric bio-acid the metal recoveries were much higher (79% Ni and 55% Co) than those obtained with citric bio-acid but lower than those reached in one-stage batch experiments. Since the mineral composition of raw material is a crucial parameter to select the suitable leaching agent for oxide ore, the treatment of residual nickel-ferrous material from the CARON process with bio-sulphuric acid produced by cultures of A. thiooxidans could be an attractive alternative in the development of a sustainable technology for the Cuban mining-metallurgy industry. © 2008 Elsevier B.V. All rights reserved. d in the tropical regions. citric acid, oxalic acid, gluconic acid, tartaric acid, and pyruvic acid. Aspergillus and Penicillium are the most important filamentous fungi for the extraction of metals from oxide ores (Bosecker, 1986, 1989, 1997; Franz et al., 1991; Coto et al., 2001, 2003, 2005). Metal leaching experiments with direct inoculation of those microorganisms. Using A. niger and sucrose as carbon source, Aspergillus niger Laterite tailings and organic acids produced the extraction of Co and Ni from laterite tailings. The results are compared with those obtained in leaching Cobalt and nickel recoveries from laterite Orquidea Coto a,⁎, Federico Galizia b, Ianeya Hernánd a Departamento de Microbiología, Facultad de Biología, Universidad de la Habana, Calle 25 b CINDEFI (CONICET-UNLP), Universidad de La Plata, Calles 47 y 115 (1900), Argentina A B S T R A C TA R T I C L E I N F O Available online 22 July 2008 Keywords: Cobalt and nickel leaching Acidithiobacillus thiooxidans Cuban serpentines are know treated by pyrometallurgy residues, which still contain the chance to use sulphur sulphuric acid to leach the requests heterotrophic e organic acids, mainly @biol.unlp.edu.ar (F. Galizia), oo.com (J. Marrero), ll rights reserved. ilings by organic and inorganic bio-acids a, Jeannette Marrero a, Edgardo Donati b 462, (10400), Cuba s one of the richest deposits of Ni and Co in the world. These ores are usually by hydrometallurgy. These processes generate huge volumes of mining gh amounts of Ni (0.25%) and Co (0.09%). Since metals are partially oxidized, idizing bacteria (Acidithiobacillus thiooxidans) which are able to generate dues from CARON process has been evaluated. Thus, in this work, inorganic tallurgy v ie r.com/ locate /hydromet A. thiooxidans are capable to oxidise metal sulphides to sulphates and thus releasing the metals in those cases in which the respective sulphates are soluble (Ehrlich, 1999; Rawlings, 1997; Rohwerder et al., 2003). Bioleaching processes in two stages have been studied and are considered as attractive and profitable processes for the dissolution of metals from laterite ore (Coto et al., 2001). The aim of this work is to compare the leaching of cobalt and nickel from laterite tailings in one- and two-stage batches using organic and inorganic bio-acid. (leaching step), tailings (10% w/v) were put in contact with those liquors containing organic or inorganic bio-acids. The leaching ex- periments were carried out in 500 ml Erlenmeyer flasks with final volume of 100 ml incubated at 200 rpm and at 60 °C. 2.4. Chemical leaching Citric acid (0.1 M) and sulphuric acid (0.1 M) were used to evaluate chemical leaching of the tailings. The leaching solutions were prepared from analytical reagents. A consecutive leaching using citric and sulphuric acid was carried out. For that, after a first chemical leaching using citric acid (or sulphuric acid) for 7 days (200 rpm, 30 °C), the residue was treated with the other acid under the same conditions. 2.5. Analytical methods Metal concentrations in the liquors were measured by atomic absorption spectrophotometry. Acid concentration was determined 19O. Coto et al. / Hydrometallurgy 94 (2008) 18–22 Comparatively chemical leaching was evaluated by use of commercial sulphuric and citric acid. 2. Materials and methods 2.1. Laterite tailings Chemical analysis and mineral composition of the laterite tailings used throughout this paper are given in Table 1. Those tailings are residues from the CARON process which is the main technology most laterite processors used to extract nickel and cobalt. In this process, after roasting at up to 800 °C, an ammonium carbonate solution dissolves the metals and separates them from the ore. After this procedure, the residues (tailings) still contain highly recalcitrant oxides with low amounts of different metals as it can be seen in their composition. 2.2. Microorganisms The strain Aspergillus niger O5 belongs to the microbial collection of the Laboratory of Metal Biotechnology, University of Havana, Cuba (Coto et al., 2001). The strain was grown on solid Sabouraud medium for 5 days at 30 °C. 2 ml of 107–108 spores ml−1 was used as inoculum. The amount of spores was counted in a Thoma counting chamber. A. thiooxidans DSM11478 was grown in 0 K (iron-free 9 K) mediumwith 1% w/v sulphur as energy source and initial pH value of 3.0. Cells were harvested by filtration through blue ribbon paper followed by centrifugation (20 min, 2000×g) and suspended in fresh medium. 10 ml of this suspension (with about 1×109 cells/ml) was used as inoculum for 100 ml final volume. 2.3. Leaching methods Two different methods were used in the bioleaching experiments. In the one-stage batch, tailings (5% w/v) were added directly to the cultures inoculated with heterotrophic or autotrophic cells. A. niger cells were grown in medium 3a containing 0.3 g/l yeast extract, 0.5 g/l Table 1 Chemical and mineralogical composition of laterite tailings Metal Concentration (ppm) Ni 2500 Co 890 Fe 458,600 Mg 198,600 Mn 8150 Cr 9900 Main phases Fayalite: (Fe,Mg)2SiO4 Donathite: (Fe,Mg)(Cr,Fe)2O4 Magnetite: FeFe2O4 Trevorite: Ni(Fe)2O4 KH2PO4, 0.250 g/l MgSO4, 0.125 g/l MnSO4, and 140 g/l sugar cane as energy source at initial pH value 5.0 (Coto et al., 2003). In the case of A. thiooxidans, cells were inoculated in 0 K medium at initial pH 6.0 with 2% w/v elemental sulphur as energy source. After 48 h, different amounts of tailings were added in order to reach different pulp densities (2.5%, 5%, and 10% w/v). Abiotic controls were carried out replacing the inoculum by sterile medium. In the two-stage batches, organic acids were obtained from cultures of heterotrophic cells after 7 days of incubation in medium 3a (see above) at initial pH value 5.0 and inorganic bio-acids from cultures of sulphur-oxidizing bacteria in 0 Kmedium containing 1%w/ v sulphur and initial pH value 3.0. In both case, the biomass was separated from the liquor by filtration through Filtrak paper. Those cultures were carried out in 500 ml Erlenmeyer flasks with final volume of 100 ml incubated at 200 rpm and 30 °C. In a second step titrimetrically (AOAC, 1995) using 0.1 N NaOH and phenolphthalein as indicator. Organic acids present in the liquors during the bioleaching (batch system) were determined by HPLC Waters, column symmetry Waters C18 5 μm (4.6mm×150mm)with a solution of 97% phosphoric acid 0,005 M and 3% of acetonitrile (pH 2.3) as the mobile phase at isocratic flow rate of 1 ml/min. 3. Results and discussion 3.1. Bioleaching in batch culture with heterotrophic microorganism Results in Fig.1 show that the fungus A. nigerO5was able to growat a tailings pulp density of 5% w/v. The pH value decreased for the first 3 days of bioleaching from 5.0 to 3.0, surely due to the production of organic acid metabolites by the fungus. After this time, there was no significant change of pH. Chemical analysis of the culture by HPLC indicated the presence of both citric acid (0.025 M) and oxalic acid (0.11 M). A. niger cells are able to accumulate citric acid in a medium rich in carbohydrate but deficient in phosphate and trace elements like ferrous iron, zinc and manganese(II) (Pondey et al., 2001). However, the amount and the type of organic acid produced by the fungus depend upon pH, nutrients, temperature, presence or absence of ore, and type of strain (Coto et al., 2001, 2003, 2005). A. niger O5 was selected for our experiments because of its capability of producing only citric acid when cultivated in amediumwithout ore (Coto et al., 2001), although the presence of ferrous iron andmanganese(II) (e.g. from the leaching of tailings) can affect considerably the biosynthesis of citric Fig. 1. Kinetic of cobalt and nickel solubilization, citric acid concentration and pH in one batch system during the bioleaching of laterite tailings (5% w/v) using Aspergillus niger O-5 (medium 3a, 14 g/L sugar cane, pH 5.0, 200 rpm, 30 °C). ■: Co (%), ♦: Ni (%), ▲: pH, □: organic bio-acid production. could explain this behaviour. Fig. 2b and c show that sulphuric acid produced by A. thiooxidans is an excellent leaching agent. As expected, the highest percentage of metal extraction was obtained at a 2.5% w/v pulp density. After 12–15 days, 80% Co (Fig. 2b), 100% Ni (Fig. 2c) and almost 100% Mn (data not shown) were leached. At higher pulp em during the bioleaching of laterite tailings using A. thiooxidans DSM11478 (medium 0 K, ed) different amounts of laterite tailings were added (♦: 2.5%w/v, ■: 5% w/v, ▲: 10% w/v). 20 O. Coto et al. / Hydrometallurgy 94 (2008) 18–22 acid. Precisely, our results show that cells produced mainly oxalic acid in the presence of the tailings. Organic acids excreted into the liquid phase can dissolve heavy metals by direct displacement of metal ions from the orematrix by hydrogen ions (acidolysis) and by the formation of soluble metal complexes and chelates (complexolysis) (Bosecker, 1986; Burgstaller and Schinner, 1993; Rezza et al., 2000). However, oxalic acid is not a very efficient leaching agent becausemany oxalates have a very low solubility in water. In our case, cobalt(II) and nickel(II) oxalates are insoluble while iron, manganese(II) and magnesium oxalates have a low solubility in cold water. Fig. 1 shows an initial dissolution of metals due to the excretion of organic acid (particularly citric acid) into the culture broth. However, after 9 days therewas a decrease of metal concentration in solution. At the end of the experiments (15 days) only negligible nickel and cobalt concentrations were detected. Although it cannot be confirmed yet, uptake of metal by the fungal biomass (Franz et al., 1991), electro- sorption (Valix et al., 2001) or precipitation of metal oxalates (Alibhai et al., 1991) could explain the absence of heavymetals at the end of the experiment. Thus, our results suggest that bioleaching of laterite tailings with A. niger O5 in a one-stage system is not efficient for metal dissolution. 3.2. Bioleaching with sulphur-oxidising bacteria Laterite leaching by A. thiooxidans was initially investigated by inoculating flasks containing 0 K medium (initial pH 3.0), elemental sulphur (1%, 3% or, 5% w/v) and, laterite tailings (10% w/v). In these experiments (data not shown), there was an immediate increase in pH (from 3 up to 7) indicating there was no cell growth and conse- quently metal dissolution was negligible. Taking into account these results, the method was changed in a second series of experiments. A. thiooxidans cells were inoculated into 0 K medium with elemental sulphur (2% w/v). After 48 h of incubation at 30 °C, the pH in the Fig. 2. Kinetic of cobalt (B) and nickel (C) solubilization [%] and pH (A) in one batch syst 2% w/v sulphur, pH 6.0, 200 rpm, 30 °C). After 48 h of incubation (pH about 1 was reach □: abiotic control (inoculum was replaced by sterile medium). medium decreased to 1 due to the production of sulphuric acid by the metabolic activity of the cells. At this moment, different amounts of laterite tailings were added to reach three different pulp densities (2.5%, 5%, 10% w/v). The initial pH in the three cases was 1.5. As shown in Fig. 2a the pH values increased from 1.5 to 2.2 or 3.0 depending on the amount of tailings added. The dissolution of oxides by acid attack Table 2 Content of the solid residue after treatment of the laterite tailings in shaking conditions (200 rpm) at 60 °C, 23 h by using organic bio-acid from A. niger strain O5 Samples % Ni Co Fe Mn Mg Laterite tailings before leaching 0.250% 0.089% 45.86% 0.815% 3.72% Solid residue after leaching 0.309% 0.063% 44.68% 0.521% 3.185% % leached – 30% 2.6% 36.1% 14.4% densities the metal recoveries decreased although they were high (about 80% nickel and more than 70% cobalt for the other two pulp densities). The low operational costs for using sulphur-oxidizing bacteria and the high metal recoveries could be important for the final treatment of laterite tailings. 3.3. Semi-continuous leaching (two-stage batch) with organic and inorganic bio-acid Metal extraction in two-stage batches using organic bio-acid pro- duced by A. niger O5 is shown in Table 2. In this case, the results were calculated from the metal content in the original ore and in the residue after the leaching. Cobalt extraction (30%) was higher than that obtained in the one-stage leaching (Fig. 1). Little dissolution of iron and magnesium were observed. In addition, there was no nickel dissolution as it is shown by the final nickel content in the residue; this nickel content was even higher than the initial content probably due to the decrease in the total solid material. Even when the second leaching step was carried out at 60 °C, the metal recovery was low suggesting that the leaching of tailings using A. niger cells is not adequate, probably because the concentration of the best leaching agent (citric acid) and the acidity (pH value about 4.0) in these cultures were not high enough. Fig. 3. Kinetic of cobalt and nickel solubilization and pH during the leaching of laterite tailings (10% w/v, 200 rpm, 60 °C) using inorganic bio-acid previously produced by A. thiooxidans DSM11478. ♦: Ni , ■: Co , □: pH. Fig. 3 shows the results of the bioleaching experiments using sulphuric acid previously produced by A. thiooxidans. The nickel and cobalt recoveries (79% and 56%, respectively) were lower than those 60 °C and the maximum of metal extraction was reached within 25 h for cobalt and 85 h for nickel, while it took about 8–15 days for cobalt and 15–35 days for nickel, depending on the pulp density, when Fig. 4. Nickel and cobalt recovery in consecutive chemical leaching of laterite tailings at different pulp densities using citric and sulphuric acid. The grey bars indicate the recoveries after the first step of leaching, the white bars the total recoveries after both leaching processes. The first leaching (7 days, 200 rpm, 30 °C) was carried out using the acid indicated in the first place in the label; thereafter the residue was leached (7 days, 200 rpm, 30 °C) with the acid placed in the second place in the label. 21O. Coto et al. / Hydrometallurgy 94 (2008) 18–22 obtained when the tailings were directly added to A. thiooxidans culture growing on sulphur. Due to the absence of cells, sulphuric acid was depleted as it is shown by the increase in pH and the decrease in metal dissolution. In the one-stage experiment – described in Section 3.2 – cells were able to produce sulphuric acid continuously. However, using the two-stage method it would be possible to increase not only the pulp density (because there would not be inhibition of microbial activity), but also the temperature in the second step allowing faster dissolution. In our experiments, the second step was carried out at Fig. 5. X-ray diffraction patterns of laterite tailings before and after treatment with citric ac (FeSO4 ·6H2O); Fa: Fayalite ((Fe,Mg)2SiO4); H: Hematite (Fe2O3); G: Goethite (FeO(OH)); M: M arbitraries. tailings were directly added to the A. thiooxidans cultures (Fig. 2). The results described in this section confirm that inorganic acid produced by A. thiooxidans is more efficient as leaching agent than organic acid produced by A. niger and could be effectively used for the treatment of the solid residue from the CARON process. In addition, the two-stage method could be more adequate for an industrial process in order to increase not only the leaching temperature (reducing the time of the process) but also the amount of tailings per unit of volume being treated at the same time. id 0.1 M and sulphuric acid 0.1 M. D: Donathite ((Fe,Mg)(Cr,Fe)2O4); F: Iron(II) sulphate agnetite (Fe3O4); Mh: Maghemite (Fe2.67O4); T: Trevorite (Ni(Fe)2O4). Units of Y-axis are 3.4. Chemical leaching In order to analyse whether citric and sulphuric acids are able to low production of citric acid and an important production of oxalic acid are responsible for that behaviour. The highest metal recoveries (100% Ni and 80% Co) were achieved when tailings were directly added to A. thiooxidans cultures growing on sulphur, but the leaching took several days. Using a two-stage method with sulphuric acid previously produced by A. thiooxidans 22 O. Coto et al. / Hydrometallurgy 94 (2008) 18–22 which of them is more efficient, chemical leaching experiments were carried out using 0.1 M solutions of those analytical reagents. That concentration was selected because it is approximately reached in cultures of A. niger and A. thiooxidans growing in the absence of the mineral. The results are represented in Fig. 4. At all pulp densities cobalt extraction was much higher than nickel extraction; these results are opposite to those obtained in bioleaching processes probably due to the complex effects of other ions present in the media. As it was expected, the lower the pulp density the higher metal recoveries were obtained. However, the second leaching improved the metal recovery and was more efficient at higher pulp density suggesting that the metal recovery stopped in the first leaching step when the acid was exhausted or when the concentrationwas not high enough to leach the total solid material. Total metal extraction was independent of the order inwhich the acids were used, at least within the range of experimental errors. These results indicate that both acids are adequate agents for leaching the laterite tailings. X-ray diffraction patterns of laterite tailings before and after the chemical leaching with citric or sulphuric acid are shown in Fig. 5. The main mineralogical species in the untreated tailings are fayalite and maghemite and, less abundant, donathite and trevorite. These species contain the more abundant metals in the tailings, mainly iron and magnesium, but also chromium and nickel. Species with cobalt, surely in low amounts, could not be detected by X-ray. After the treatment, intensities of the peaks of fayalite, maghemite and donathite intensively decreased indicating that these minerals were partially solubilized during the leaching; that was confirmed through the high concentrations of iron and magnesium in solution (data not shown). Comparing the diffraction patterns after the leaching with citric and sulphuric acids, it can be suggested that the mechanism of attack should be the same (surely acid attack) in both cases. After the leaching processes new peaks corresponding to magnetite, goethite and even hematite were detected. Those species correspond to more oxidized species of iron which is the most abundant metal in the tailings. The presence of those species after the leaching suggests a partial oxidation of iron in solution favoured by the high tempe- rature and the consequent precipitation of ferric (or ferric/ferrous) iron oxides. 4. Conclusions The mineralogical composition of raw material is a crucial para- meter in the processes of bioleaching of laterite ore. Usually laterites are leached using organic acids; however our results show that nickel and cobalt can be leached from laterite tailings also by sulphuric acid treatment. In chemical leaching, sulphuric acid was as efficient as citric acid. In both cases, higher acid concentrations allowed higher metal solubilizations. Both acids seem to leach laterite tailings via acid attack although some complexation effects may occur when citric is used. The bio-production of these acids could be important in order to decrease the cost of the leaching processes. However, A. niger cells were not able to dissolve nickel and cobalt when they were inoculated directly into medium containing the tailings. In addition, no dissolu- tion of Ni and Co was observed when the culture filtrate after fungal growth was used to leach the tailings in a second step. Probably the cells, the recovery was lower (79% Ni and 56% Co) but the process took just a few hours. These results can constitute the basis to design a treatment process for recovering the rest of metals present in the Cuban laterite tailings coming from the CARON industrial process. Acknowledgements This work was supported by the Cooperación Argentino–Cubana (SECYT-CITMA) through the grants CU/PA04-BVIII/039, ANPCyT (PICT 25300) and CONICET (PIP 5147). Authors gratefully acknowledge the BIORECA (CYTED) and Dr. Gisela Pettinari (CIMAR-UNC) for X-ray diffraction assays. 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Minerals Engineering 14 (12), 1629–1635. dissolve themetals present in different mineral species and to find out Cobalt and nickel recoveries from laterite tailings by organic and inorganic bio-acids Introduction Materials and methods Laterite tailings Microorganisms Leaching methods Chemical leaching Analytical methods Results and discussion Bioleaching in batch culture with heterotrophic microorganism Bioleaching with sulphur-oxidising bacteria Semi-continuous leaching (two-stage batch) with organic and �inorganic bio-acid Chemical leaching Conclusions Acknowledgements References