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Factors affecting bio-oxidation at high solids concentrations

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dc.contributor.advisor Hansford GS, Prof en
dc.contributor.author Bailey AD en
dc.date.accessioned 2016-09-22T09:10:37Z
dc.date.available 2016-09-22T09:10:37Z
dc.date.submitted 1994 en
dc.identifier.uri http://hdl.handle.net/20.500.11892/45034
dc.description.abstract Bacterial oxidation is currently finding significant application for the oxidative pretreatment of refractory gold-bearing sulphides. Plants processing sulphide concentrates have commonly been operated at solids concentrations of between 18 and 20 per cent (m/v) (i.e 180 and 200 kg.m-3). At higher concentrations, a decline in the bio-oxidation rate has been observed. Other metallurgical processes, such as chemical leaching and cyanidation, are performed at higher solids concentrations of between 40 and 50 per cent (400 and 500 kg.m-3), providing an incentive to increase the solids concentration at which bio-oxidation plants are operated. A review of literature indicated the following factors to be potential causes of reduced bio-oxidation rates at high solids concentrations: oxygen and carbon dioxide mass transfer; a low bacteria-to-solids ratio; mechanical damage of the bacteria; and the build-up of inhibitory oxidation products. Interaction of these factors in the completely-mixed reactors that are commonly used for bio-oxidation, has confounded the interpretation of the effects of individual factors. Analysis of literature data revealed a link between the sulphide grade of a particular material and the highest solids concentration at which the bacterial oxidation rate was maximal. The oxygen demand is directly proportional to the sulphide concentration in the reactor. Correlations were used to predict the oxygen transfer potential in the experimental reactors and it was found that as long as the oxygen transfer potential exceeded the oxygen demand, the biooxidation rate was proportional to the solids concentration for a specific material. When the oxygen demand equalled or exceeded the oxygen transfer potential, then the bacterial oxidation rate was limited by oxygen availability. The sulphide grade is characteristic of a particular ore or concentrate and from the data analysis oxygen availability appeared to be the underlying reason why low grade materials could be oxidised at the maximum specific bio-oxidation rate at far higher solids concentrations than high-grade materials. The experiments performed in this study were designed to further investigate the apparent relationship, identified by analysis of literature data, between sulphide grade and the solids concentration at which the bacterial oxidation rate was maximal. The effect of both solids concentration and sulphide grade on the bio-oxidation rate was investigated and related to the oxygen availability in the reactor. Two, high grade pyrite concentrates (sulphur > 28%) and a low pyrite content ore (2% sulphur) were used for the testwork. Inert silica was mixed with the high-grade concentrate to simulate intermediate sulphide grades. A novel fluidised bed reactor as well as batch stirred tank reactors were used. The fluidised bed reactor (FBR) allowed the solids and liquid environments to be controlled independently, enabling specific factors, thought to limit the bio-oxidation rate, to be isolated for investigation. The FBR effectively circumvented the problem of interaction between the various factors that is encountered in completely-mixed reactors. In the FBR it was possible to ensure the availability of sufficient oxygen, even at high bed solids concentrations. In contrast to observations in stirred tank reactors, the specific bio-oxidation rate of high sulphide content material in the FBR was constant up to 45 per cent solids (450 kg.m-�), the maximum concentration tested. When the solids loading (and consequently the oxygen demand) was increased, however, it was found that the moment the oxygen demand exceeded the oxygen supply rate, the bio-oxidation rate was limited. This confirmed the hypothesis that had been proposed after analysis of the literature data. The FBR has emerged as a useful tool for determining the specific bio-oxidation rate under controlled conditions. Batch stirred tank reactor (STR) studies further consolidated the relationship between the sulphide grade and the oxygen demand, as a key factor in determining the highest solids concentration at which a reactor can be operated at the maximum specific bio-oxidation rate. The bio-oxidation rate of the low-grade ore (1.24% S) was found to be proportional to the solids concentration up to 60 per cent solids (600 kg.m-�), the highest solids concentration tested; considerably above the concentration at which the rate usually decreases when high sulphide content material is processed. This observation was due to the low oxygen demand in the reactor even at 60 per cent solids. Furthermore, it was demonstrated that the addition of inert solid particles to 10 per cent (100 kg.m-3) pyrite concentrate, up to a maximum total solids concentration of �0 per cent (�00 kg.m-�), did not affect the specific bio-oxidation rate in the STR. It was also apparent from the STR results that mechanical destruction, or trauma, of the bacteria was not a primary cause of low bio-oxidation rates at high solids concentrations. The experimental results obtained from both the FBR and STR testwork confirmed that it is not the solids concentration per se that affects the biooxidation rate at high solids concentrations, but rather the magnitude of the oxygen demand in relation to the oxygen transfer potential of the system. The highest solids concentration at which bio-oxidation can be conducted at the maximum specific oxidation rate is limited by the oxygen transfer potential of the system. The bacteria were shown, in FBR reactor tests, to be capable of oxidising all of the ferrous iron generated during the processing of high pyrite content solids at a concentration of 50 per cent (50 kg.m-�). Ferric iron concentrations in excess of �0 g.l-1, however, decreased the ferrous oxidation rate of the bacteria. Results from FBR tests in which the free cells in the system were suddenly depleted, indicated that the attached bacterial population was more significant to bio-oxidation than the free bacterial population. If there was a sudden depletion of the free bacteria, then attached bacteria were found to leave the mineral surface to replenish the solution, maintaining both the redox potential and the bio-oxidation rate. The bio-oxidation rate was, however, adversely affected when the attached bacterial population was low. en
dc.language English en
dc.subject Chemical engineering en
dc.subject Chemical engineering en
dc.title Factors affecting bio-oxidation at high solids concentrations en
dc.type Doctoral degree en
dc.description.degree PhD en


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