Immobilization of Arsenic through pH Control during Bioleaching of Nickel Flotation Concentrate

Abstract

Bioleaching is a biohydrometallurgical method that utilizes the ability of acidophilic microorganisms such as Acidithiobacillus ferrooxidans, At. caldus, or Leptospirillum ferrooxidans to oxidize mineral sulfides, iron and sulfur and thereby solubilize valuable metals from low-grade or refractory ores and concentrates at very low pH (< 3). Arsenic is a toxic element found everywhere in the Earth’s crust and a harmful side-product of hydrometallurgical processes including bioleaching. Arsenic is currently removed from wastewater streams of the mineral processing industry by co-precipitation and sorption with iron oxides by using lime neutralization, i.e. raising solution pH. The aim of this thesis work was to assess the simultaneous immobilization of arsenic and selective bioleaching of nickel and cobalt from nickel flotation concentrate (NFC; Mondo Minerals LLC, Sotkamo, Finland) by maintaining a constant elevated process pH of 3.0.

Bioleaching of NFC was performed in aerated continuously-stirred tank reactors (CSTRs) as 30 L batch mode and 20 L semi-continuous mode (10 d retention time) processes at 21 °C. The aim of the batch mode experiment was to assess the effect of scale-up from previous shake-flask experiments and efficiency of bioleaching and arsenic immobilization with different pulp densities (5, 10, 15 % p.d.). The purpose of the semi-continuous mode experiment (5 % p.d.) was to assess parameters required for developing a continuous mode process. Finally, a shake-flask experiment (100 mL, 150 rpm, 27 °C) was conducted without NFC with the aim to assess the role of bioleaching microorganisms in arsenic speciation and immobilization.

In the 30 L batch mode experiment, bioleaching was conducted for 94, 58, and 79 days with 15, 10, and 5 % p.d., respectively. Final nickel yields were 73, 66, and 76 %, cobalt yields were 78, 71, and 100 % with 15, 10, and 5 % p.d., respectively. Maximum leaching rates (250 mg-Ni/L/d and 18 mg-Co/L/d) were achieved with 15 % p.d. Soluble arsenic level remained low (< 1 mg/L, < 0.4 % yield) with all studied pulp densities after 21 days of operation. Leach residue analysis by X-ray diffraction (XRD) revealed that iron sulfates were the predominant compounds, ranging from 55 to 70 % (w/w) with all pulp densities studied. Formation of goethite (α-FeOOH, an effective adsorbent for arsenic) increased with increasing pulp density. Bacterial community analysis from samples collected at the end of the experiment showed that At. ferrooxidans dominated the population with all studied pulp densities. Additionally, Alicyclobacillus sp. was present in the 5 % p.d. reactor.

Bioleaching in the 20 L semi-continuous mode experiment was conducted for 38 days. Dissolved arsenic remained below 0.4 % throughout the experiment. After 14 days of operation and after reaching the maximum yields of 12 and 22 % for nickel and cobalt, respectively, the metal yields started to decline until the end of the experiment. The 10 d retention time was possibly too short for the microbial community to establish a high bioleaching rate. Further, mass exchanges for the reactor were performed daily as a single 2 L exchange. Such abrupt changes in the reactor environment likely affected the microbial community development. A more continuous material flow and use of multiple reactors in a cascade are suggested for further studies. Leach residue analysis showed that the goethite content was approximately 5 times higher than in the 30 L batch mode experiment [~ 25 vs. 5 % (w/w), respectively]. More information is needed on the conditions that facilitate goethite formation during bioleaching of NFC.

In the shake-flask experiment, solutions with 25.1 g/L ferrous sulfate, 20 mg/L arsenite [As(III)] and 10 % (v/v) inoculant from the 30 L batch experiment as well as abiotic controls were incubated for 28 days. Two starting pH values were used, i.e., pH 1.7 and 2.5. Solution pH in the biotic flasks became and remained similar after 6 days. In all flasks, arsenite was completely oxidized to arsenate [As(V)] in one day. After 6 days of incubation, the amount of soluble arsenic in the biotic solution with starting pH 2.5 decreased to only 4 mg/L, majority of which (75 %) was arsenite. The concentrations of soluble arsenite and arsenate remained low until the end of the experiment, finishing at 4 mg/L As(III) and 1 mg/L As(V). Arsenite was also the dominant (~81 %) oxidation state in the biotic solution with starting pH 1.7 after 6 days, although all of the arsenic was in solution. The concentrations of arsenite and arsenate both decreased slowly towards the end of the experiment, finishing at 12 mg/L As(III) and 3 mg/L As(V). Thus, the starting pH had a significant effect on arsenic immobilization. The role of microorganisms was crucial in arsenic immobilization. However the main mechanism of immobilization remains to be determined. Possible mechanisms include the following: sorption on biomass or coprecipitation and sorption on ferric oxides generated by microbial oxidation.

In conclusion, this thesis work showed selective bioleaching of NFC for nickel and cobalt and efficient immobilization of arsenic in batch and semi-continuous mode by maintaining a constant pH of 3.0. The highest leaching rates of nickel and cobalt were achieved with the highest pulp density studied, which encourages to further increase the pulp density in future studies. This work also demonstrated the importance of sufficient mixing: Settled and partially unleached material was present on the bottom of the 15 and 10 % p.d. reactors at the end of the 30 L batch mode experiment. Similarly, unleached material was present on the bottom of the 20 L, 5 % p.d. reactor. Finally, the major role of microorganisms in arsenic immobilization in a biomining environment was confirmed. The mechanisms of arsenic immobilization and the role of microorganisms in arsenic immobilization should be looked into in future studies.