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The aim of the project is to develop a highly innovative process for the partial removal of silica from bauxites by microorganism assisted leaching.
Research has been carried out in order to develop a process to upgrade European bauxite deposits by removing the silica fraction associated with the aluminosilicates by microorganism assisted leaching, or by leaching with microbially produced acids. Heterotrophic bacteria related to the species Bacillus circulans and Bacillus mucilaginosus, known as silicate bacteria, have been reported as capable of leaching silicon dioxide from aluminosilicate minerals. Extraction of silica of up to 80% has been reported by the above bacteria.

The removal of silica from subgrade bauxites from Greece and Italy was studied. The appropriate B circulans strains were selected, grown and tested on pure mineral samples as well as on bauxites. The initial experiments have shown that silica removal with B circulans, does not proceed to any useful extent, contrary to the results reported in the literature. It was then decided to study in parallel the removal of silica by the fungus Aspergillus niger, known to produce a mixture of citric and oxalic acid through its metabolic activity. These tests showed that up to 20% silica can be removed, with simultaneous alumina and iron removal.

Using microbially produced mixtures of oxalic and citric acid by the fungus Aspergillus niger, useful results regarding kaolinite destruction and silicon dioxide and bauxite dissolution have been obtained; partial dissolution of the contained boehmite and almost complete dissolution of hematite were also observed.

Finally, using microbially produced citric acid oxalic acid mixtures, 16% silicon extraction and 8.5% aluminium extraction was observed under conditions of 70 C, retention time 3 days, and pulp density 1% weight by volume. The removal of 85% of iron was also remarkable.
It has to be noted that when leaching bauxite by means of organic acids, reprecipitation of silica from the solution takes place because of its low equilibrium solubility. An alkaline wash of the solid residues after leaching is required in order to redissolve the precipitated silica.
Silica is a major impurity in bauxites that are to be treated by the Bayer Process for alumina production, and should normally be less than 5% to 7%. For this reason, over 7 million tons of bauxites are imported annually in the EEC, while, at the same time, several indigenous bauxite deposits cannot be economically processed because of their high silica content.

Heterotrophic bacteria related to the species Bacillus circulance and Bacillus mucilanginosus, known as silicate bacteria, have been reported as able to leach silicon oxide from silicate and aluminosilicate minerals. The bacterial action is connected with the formation of mucilanginosus capsules consisting of exopolysaccharides as well as with the production of various metabolites, such as organic acids and amino acids.

The presence of silica in bauxites is due to various silicate and aluminosilicate minerals, as well as quartz. Therefore, leaching of bauxites with the silicate bacteria is expected to lower their silica content.

Subgrade bauxite deposits from Greece and Italy will be used in this study.

The work to be carried out involves the partial removal of silica from bauxite ores by means of microorganism assisted leaching. The bauxite ores to be studied are those of interest to the industrial partners and include high silica bauxite from Greek deposits (Parnassus - Ghiona area) and high silica bauxite from Italian deposits (Nurra area, Sardinia).

The work programme will be implemented in the following phases, each of which will contain the indicated areas:
selection of the appropriate deposits, collection and preparation of representative samples;
chemical and mineralogical analyses;
collection of appropriate strains, cultivation of strains and cultures;
microbial leaching of individual minerals;
microbial leaching of the selected bauxite samples;
and process feasibility.


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National Technical University of Athens
EU contribution
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42,Patission Street
10682 Athens

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Participants (3)