LAUSR

Abstract Thermal processing methods aimed at the recycling of phosphate from leftover materials oftentimes result in the formation of silicon-substituted apatites alongside aluminum-silicate phases. In order to improve the recovery of the apatites obtained in such a way, an enrichment technique is required. The present research work studies the separation of a silicon-substituted apatite (silicocarnotite, Ca5(PO4)2SiO4), from an aluminum-silicate phase (gehlenite, Ca2Al2SiO7), via a straightforward liquid-liquid separation process using an anionic surfactant (sodium dodecyl sulfate –SDS–), a polyelectrolyte (chitosan) and kerosene. Analysis of the thermal behavior of silicocarnotite and gehlenite, suggested these substances can be formed simultaneously at temperatures higher than 1200 C°. Secondly, zeta potential measurements for these species showed silicocarnotite has the point of zero charge (PZC) at around pH = 5.5, whereas gehlenite remains negatively charged in the entire range of pH tested (2.5–11). Results for the liquid-liquid separation process indicated that SDS alone promotes the agglomeration of silicocarnotite in the organic fraction at pH values lower than 5.5. This can be explained due to be the preferred interaction between the net positive charge on the surface of silicocarnotite particles and the negative charge present in SDS. In sharp contrast, it was found that SDS alone does not exert any influence on gehlenite agglomeration as predicted by the zeta potential analysis. When the separation process was tested onto a 1:1 mix of silicocarnotite and gehlenite, a chitosan dose of 100 g/tonne alongside a dose of 1.44 kg SDS/tonne, promoted the largest capture of material in the organic fraction (around 60% of the initial material), and caused the amount of P2O5 content in the organic fraction to be roughly twice the amount that of the aqueous fraction. Consequently, these results put forward the liquid-liquid process developed as a possible separation path for the target species.