Quartz bleaching by phosphoric acid: An investigation on the relationship between Fe rejection rate and colour response

Advances in high-tech industries have led to increasing the demand for highly pure quartz. Research on the beneficiation of quartz has been conducted to meet market requirements by different processes including hydrometallurgical ones. Even though phosphoric acid (PA) has a capacity to complex iron, and therefore has the potential to bleach quartz, this reagent has not attracted sufficient attention. This study was conducted to investigate the relationship between Fe remaining in concentrate by PA bleaching and colour responses of quartz ore sample employing Box-Behnken Design, a common designs of Response Surface Methodology. Effects of bleaching parameters (PA concentration 0-2 M, temperature 65-95 degrees C, bleaching time 45-180 min.) were investigated in agitated pulp condition at 400 rpm. The efficiency of the process was determined by colorimetric analysis and chemical analysis (ICP-MS). Experimental results revealed that the linear effects of bleaching parameters on Fe removal by PA were statistically significant. The lowest Fe remaining in concentrate, 79 ppm, was observed in two different conditions tested at the highest initial PA concentration (2 M) at 95 degrees C, over 90 min, and at 85 degrees C, over 180 min of bleaching. The increase in acidity resulted in the higher Fe rejection rate and the lower yellowness. Significant improvements in whiteness and yellowness were obtained by PA bleaching, whereas redness could not be improved considerably. This finding has been attributed to the presence of free dark coloured particles, and the resistance of redness source minerals like hematite to react with PA ions.