Bacterial removal of iron impurities to increase the quality and value of industrial minerals

Abstract

Kaolin, silica sand and high grade carbonates are industrial minerals widely and extensively used as fillers and coating agents in the manufacture of paper, ceramics, glass, paint and cosmetics. When mined, kaolin and silica sand are generally not pure, often associated with iron hydroxide and oxy-hydroxide impurities (ranging between 0.074 up to 44.2mg Fe per gram of mineral) usually in the form of Fe3+-phases adsorbed onto the mineral surface, covering the entire mineral surface, or admixed as a separate iron bearing phase. The ironbearing impurities associated with calcium carbonate are mostly ferrous (Fe2+) in the form of siderite (FeCO3). In all cases, the presence of iron affects the colour and the physical properties of the mineral, and so lowers the industrial value and limits the application and uses. Due to the environmental, economic and operational disadvantages associated with conventional physical and chemical refining processes, this thesis considers a microbial refining process in a closed non-growth system as an alternative for iron removal from these minerals. Bioreduction was initially optimised using 80ml microcosm batch experiments and subsequently up-scaled to a 4.5L closed system bioreactor with continuous monitoring of pH, Eh and temperature. The anaerobic removal of Fe(III)-bearing impurities from these minerals was investigated using different iron-reducing Shewanella strains (S. putrefaciens CIP8040, S. putrefaciens CN32, S. oneidensis MR-1, S. algae BrY and S. loihica) in the presence of anthraquinone 2,6 disulphonate (AQDS) serving as electron shuttling mediator. The efficiency of natural organic matter (NOM) was also investigated as a substitute for AQDS. In the microcosm experiments, up to 4.4% of the insoluble Fe(III) in kaolin (as determined by XRF) was successfully reduced to more soluble and leachable Fe(II) by the five different iron reducing bacteria (except S. loihica) after 5 days. This was equivalent to 46.6mg of bioreducible Fe2O3 per 100g of kaolin. After bioreduction, the colour value of the biotreated kaolin mineral improved, with a substantial increase in brightness from 76.1% to 79.7% and whiteness from 57.7% to 67.8%. For the silica sand, up to 17.6% of the iron bearing impurities (~117mg of bioreducible Fe2O3 per 100g of silica sand) was successfully removed after 15 days. In both cases, addition of AQDS as electron transport mediator enhanced the rate and extent of bioreduction by facilitating the exchange of electrons between the ironreducing bacteria and the iron-bearing phase in the mineral. Natural organic matter also increased the rate and extent of Fe(III) reduction when compared with experiments without electron shuttling mediators. Because iron-bearing impurities present in the carbonate (chalk) were mostly ferrous, biotreatment by iron reduction was unable to improve significantly the iron content and colour properties in this mineral. Another microbial bioleaching method was tested for the carbonate using Desulfovibrio desulfuricans. Desulfovibrio desulfuricans, which is a sulphate reducing bacterium, was able to leach Fe from the chalk but the Fe immediately precipitated as a different iron phase (probably iron sulfide) giving a dark coloration to the chalk. The colour properties did not improve, with the colour value decreasing below the initial value after sequential centrifugation. The 4.5L batch bioreactors designed to upscale the bioleaching process successfully supported the bioreduction of Fe(III)-oxide in both kaolin and silica sand. Up to 5.9% of the total iron bearing impurities of the same kaolin material was removed (equivalent to 62.2mg of bioreducible Fe2O3 per 100g of kaolin material) with appreciable improvement in brightness from 75.79% to 79.06% and whiteness from 55.69% to 66.01%), similar to what was observed in the small scale microcosm study. Bioleaching of silica sand (King’s Lynn) in the batch reactors successfully removed up to 53.9% of Fe-bearing impurities (~89.03mg of Fe2O3 per 100g of silica material) on day 4. The bioreduction did not cause any unfavourable modification in the mineralogy of either kaolin or silica sand, and no apparent crystalline byproduct was formed after biotreatment.