A modification in the flotation process of a calcareous–siliceous phosphorite that might improve the process economics

•Low-grade phosphate ore was upgraded using fatty acid flotation process.•The main gangue minerals are calcite and quartz.•Acidic flotation of calcite was conducted without phosphate depressant.•Phospho-concentrate assaying 30.54% P2O5 was obtained. The amenability of a low-grade Egyptian phosphorite to flotation for separation of both calcareous and siliceous gangue minerals by just pH control was investigated. The ore, assaying 19.39% P2O5, 16.1% L.O.I. and 12.41% A.I. is mainly composed of francolite and hydroxy apatite minerals consolidated into three different phosphatic varieties according to texture and origin, i.e. coarse phospho-chem, sharp-edged phospho-clast and fine cementing phospho-mud. This was endorsed by microscopic investigation of thin sections. X-ray diffraction analysis of the ore sample showed that the main gangue minerals are calcite and quartz with minor dolomite and some gypsum. Anionic flotation of calcite, under pH4.5, was successfully conducted on the −0.25+0.074mm phospho-chem fraction without any use of phosphate depressants. This was followed by direct flotation of phosphate after raising the pH to 9. Mechanical cleaning of the phospho-concentrate was carried out, without any addition of the collector to get rid of the entrained silica. About 3kg/t of oleic acid was required for the whole process which was added step-wise 0.5kg/t each except for the first step which was 1.0kg/t to activate the flotation pulp. Phospho-concentrate assaying 30.54% P2O5, 8.7% L.O.I. and 5.76% A.I. with a P2O5 recovery of 64.34% was finally obtained without the use of expensive depressants, e.g. phosphoric acid or sodium silicate. A trial to explain the results in view of others’ findings and in terms of the ore mineralogical characteristics was shown.