Granulometric distribution of natural and flux calcined chert from Ypresian phosphatic series of Gafsa-Metlaoui basin compared to diatomite filter aid

The cherty rocks of the Chouabine Formation of the Gafsa-Metlaoui basin (south-western Tunisia), that is composed by biogenic silica, are treated using thermal treatment at 1000°C with flux calcination method in order to prepare a specific filter aids of melting sulfur filter used for the production of sulfuric acid. This work presents the effect of heating on the granulometry of chert. The mineralogical composition of natural starting chert is composed by opal CT (cristobalite/tridymite) and by the mineral mixture of quartz, smectite clay minerals, palygorskite-sepiolite fibrous clay minerals, calcite and hematite. After thermal treatment, at 1000°C, the crystallinity of chert increases significantly and the opal-CT, transforms mostly to opal-C. The comparison of infrared spectra of untreated and treated cherts shows systematic variation of the wavenumber and the intensity of the absorption bands, due to the apparition of 619, 795, 1094 and 1202 cm−1 absorption bands of cristobalite in the infrared spectrum of chert treated sample, which is similar to diatomite. Granulometric analysis show that the natural untreated chert sample displays unimodal distribution, whereas the treated chert sample display bimodal distribution, as same as diatomite. Thus, a new mode appears systematically, between 0.1 μn and 1 μm, for all thermal treated samples with alkaline flux as observed for diatomite.