Ind. Eng. Chem. Res. 2002, 41, 1329-1334
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SEPARATIONS Growth Kinetics of Thiobacilli Strain HSS and Its Application in Bioleaching Phosphate Ore Chen Maochun, Zhang Yongkui, Zhong Benhe, Qiu Liyou, and Liang Bin* Multi-phases Transfer and Reaction Engineering Laboratory, Department of Chemical Engineering, Sichuan University, Chengdu 610065, P. R. China
The growth kinetics of the bacterium Thiobacillus thiooxidans (HSS) was investigated and its metabolic acidic solution was used to bioleach phosphate rock. The bacterium HSS adsorbs on the surface of elemental sulfur particle and oxidizes sulfur to sulfuric acid to provide itself energy. The equilibrium between attached cells on sulfur surface and unattached cells in solution can be described with the Langmuir isotherm equation. The maximum adsorption capacity XAM is 7.310 × 1012 cells/kg of sulfur and the equilibrium constant KA is 2.9110 × 10-14 M3/cell. In a batch growth process, the specific growth rate of the bacteria on sulfur, µA, and the growth yield, YA, are 2.874 day-1 and 8.455 × 1014 cells/kg of S, respectively. The bacterium shows the highest activity to oxidize sulfur at 30 °C. The dilute microbiological acid solutions produced by HSS were used to dissolve fluorapatite ore. The digestion conversions of phosphorus ranged from 24% to 100%. Fluorine ion contained in fluorapatite ore shows a significant effect on the biological activity of HSS. 1. Introduction Commercial phosphoric fertilizer production is dominated by the acidolysis of fluorapatite. Fluorapatite reacts with sulfuric acid to be converted to hydro-soluble phosphoric compounds, which are easy to be absorbed by plants. Dissolving phosphate rock consumes large amount of sulfuric acid. To produce 1 ton of ammonium phosphate, 3.6 tons of sulfuric acid is needed.1-3 In a typical phosphoric fertilizer plant, devices for sulfuric acid production cost a large part of investment capital and the operation cost for sulfuric acid production is relatively high. Using a metabolic acid solution of microorganisms to digest the fluorapatite can avoid a high investment in the sulfuric acid production. On the other hand, it is ideal to produce slow-release fertilizer by mixing microorganisms and phosphate rock together or to produce bioreagents, which can digest the insoluble phosphate components in the earth. The problem is which kind of microorganism can be used to digest the phosphate rock and what condition should be kept to raise the microorganism. In recent years, utilizing microorganism to dissolve phosphate rock has attracted great attention. Halder4 found that some organic chemoheterotrophic bacteria, such as Rhizobium, Sauharomyces, etc., grow on organic compounds (such as glucose) and produce an organic acid solution, which can be used to solubilize phosphate ore. The process is not economically attractive because the organic nutrients, such as glucose, normally are not cheap. Vassiliv5 employed waste molasses and sweet roots as substrates, in the presence of yeast Aspergillus * To whom correspondence should be addressed. Telephone: +86-28-5405201. Fax: +86-28-5461108. E-mail:
[email protected].
niger, for the solubilization of phosphate ore. Unfortunately, the phosphate conversion was low. In the proposed processes, microorganisms were used to degrade organic compounds to produce organic acids, which react further with phosphate ore. The pH value of metabolic solution is usually in the range of 4.0-5.0 and the rate of these acid digestion reactions is rather slow. The rock amount processed by unit volume of solution is usually below 10 g/L, and typical phosphate conversions were lower than 40%. Thiobacillus ferrooxidans and Thiobacillus thiooxidans are the most important microorganisms involved in the bioleaching of a number of sulfide minerals. They oxidize metal sulfide and/or sulfur to form dilute metal sulfate solution and/or sulfuric acid in order to provide their living energy. In the metallurgy of noble metal, their selective oxidation ability to sulfide compounds or sulfur makes it possible to leach low content mineral in an energy saving process. Insoluble salts are converted into hydro-soluble sulfates or digested by their metabolizing acid in common bioleach process. Costa and Medronho6 employed T. ferrooxidans to bioleach phosphate ore. Phosphate conversion ranged from 12% to 100%. Bacterium T. ferrooxidans consumes Fe2+ and S2- as its energy source. But the ferric ions, existing in T. ferrooxidans metabolic acidic solution, are likely to combine with phosphatic ions to form precipitates, which results in the degradation of effective phosphorus. Bacterium T. thiooxidans uses elemental sulfur or reduced sulfur compounds as its energy source. It produces dilute sulfuric acid without ferric ions. Its metabolic solution is suitable to dissolve phosphate ore. In this work, the metabolic activity of T. thiooxidans will be investigated and the metabolic solution will be tested to dissolve the fluorapatite ore. Our research will
10.1021/ie010064y CCC: $22.00 © 2002 American Chemical Society Published on Web 02/08/2002
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focus on the influence factors of the growth rate of the T. thiooxidans. At last, a bioleaching test will be conducted in the presence of bacterium T. thiooxidans. 2. Materials and Methods 2.1. Bacteria Strains. Bacterium HSS was collected from a hot spring near Chengdu, in the southwest of China. It belongs to T. thiooxidans. 2.2. Materials. A fluorapatite ore from Xifeng (Guizhou, in the southwest of China) contains 34.12% P2O5 and 3% F. The ore was milled to a size of -200 mesh (
