Arseenipitoisten ja vaikeasti hyödynnettävien rikasteiden biohapetus ja -liuotus

Bioleaching and bio-oxidation are used to extract metals from low-grade, low-quality and complex ores and ore concentrates such as arsenic-containing and refractory gold concentrates. In this thesis, the bio-oxidation of a refractory gold flotation concentrate (from the Agnico-Eagle, Suurikuusikko mine site, Kittilä, Finland) and the bioleaching of two nickel- and cobalt-containing flotation concentrates (from Mondo Minerals Oy Talc Mining Site, Sotkamo) were studied. The aim was to use bio-oxidation as a pretreatment for refractory gold concentrate to liberate the gold which is encapsulated inside of a sulfide mineral and to produce a leach residue amenable to cyanide leaching. The aim of bioleaching was to continue the previous work and test the effect of different pH on the dissolution of nickel, cobalt, iron and arsenic from nickel concentrates. The bio-oxidation experiments were performed in shake flasks at the pH values of 1.8–3.2 and the pulp density was varied from 1 % to 10 %. Preliminary bio-oxidation experiments in shake flasks using laboratory cultures showed that gold flotation concentrate was amenable to bio-oxidation. When using laboratory cultures, the highest yields for iron and arsenic were 45 % and 60 %, respectively, after 100 days of bio-oxidation. Bio-oxidation with microorganisms enriched from the gold mining site enhanced the yields of iron and arsenic: during the enrichment phase iron and arsenic yields were 34 % and 65 %, respectively, after only 7 days of bio-oxidation. The effect of different starting pH on the dissolution of iron and arsenic was studied in shake flasks with Kittilä enrichment culture: the decrease in starting pH markedly increased the dissolved iron and arsenic concentrations. The bio-oxidation activity was seen as a decrease in pH and high redox at starting pH 2.5 and 3.2; however the iron, arsenic and sulfate concentrations remained low, suggesting a formation of iron, arsenic and sulfate precipitates. The highest yields (72 % Fe, 94 % As) were achieved with starting pH 1.8 after 29 days of bio-oxidation. The increase in pulp density decreased the iron and arsenic yields. Bio-oxidation was performed in batch reactors (pH maintained at 1.8, 5 % pulp density, 27 oC) to produce a leach residue for the cyanide leaching of gold. The iron and arsenic yields in batch reactors after 35 days of bio-oxidation were 20 % and 33 %, respectively. At the end of the experiment, sulfate and iron concentrations decreased, suggesting a formation of precipitates. The gold recovery from cyanide leaching was 36 %. The bioleaching experiments were conducted in shake flasks in the pH range of 1.8–3.0 (1 % pulp density, 27 oC). During bioleaching of nickel concentrate, the highest yields (nickel 60 %, cobalt 49 %) were achieved with the starting pH 1.8 when also iron (28 %) and arsenic (33 %) yields were the highest. At starting pH 3.0, all metal yields (nickel 31 %, cobalt 31 %, iron 12 % and arsenic 11 %) were lower than with starting pH 1.8. Bacterial community analysis showed a diversity of acidophilic, iron- and sulfur oxidizing bacteria in all cultures used in bioleaching and bio-oxidation. Acidithiobacillus ferrooxidans and Alicyclobacillus sp. were present in all bioleaching and bio-oxidation cultures. Also Acidithiobacillus caldus and Thiomonas cuprina/arsenivorans were present in some of the cultures. In conclusion, this work showed that Kittilä concentrate was amenable to bio-oxidation and a decrease in pH increased the dissolution of iron and arsenic. The highest concentrations of dissolved iron and arsenic were obtained using indigenous Kittilä enrichment culture at pH 1.8 which resulted in the highest yields of iron, arsenic and sulfate. Bioleaching at pH 1.8 increased the dissolution of iron, nickel, cobalt and arsenic. The culture used to bioleach gersdorffite had a low activity. This work shows that pH has a marked effect on the dissolution and precipitation of iron, arsenic and sulfate in bioleaching and bio-oxidation. The dissolution and formation of iron-, arsenic- and sulfate-containing precipitates in bioleaching and bio-oxidation will be studied in the future experiments. /Kir1