Mineralogical study and properties of natural and flux calcined porcelanite from Gafsa-Metlaoui basin compared to diatomaceous filtration aids

The purification of melting sulfur by Tunisian Chemical Group factories was performed with filtration by siliceous earth filters such as French clarcel, Algerian kieselguhr, Spanish perlite and Spanish diatomite, i.e. calcined diatomites which have biogenic origin, except for perlite, which is volcanic glass. The porcelanite rocks of the Ypresian phosphatic series from the Gafsa-Metlaoui basin also have a biogenic origin and are formed in Si-rich depositional environment, at low diagenetic conditions. This work examines the thermal treatment of these porcelanite rocks to obtain calcined siliceous material with physical–chemical characteristics comparable to the diatomaceous earth filters. The porcelanite samples used in this work come from the Mides CPG trench, where the Ypresian porcelanite beds are 10m thick. The flux calcined porcelanite was obtained by heating the raw samples at 400, 800 and 1000°C with 5% Na2CO3 flux. The raw porcelanites from the Gafsa-Metlaoui basin had high SiO2 content, similar to the usual filtration aids. The natural porcelanite is dominated by opal-CT and minor clay minerals (smectite, palygorskite and sepiolite), hydroxylapatite, quartz, calcite and hematite. During heating up to 800°C, the opal-CT progressively turned into opal-C. After 1000°C, the calcined porcelanite samples had similar properties as the calcined industrial products derived from the Spanish and French diatomite raw materials. Hence, the short range ordering of opal CT increased significantly as cristobalite-like structure formed. The thermal treatment of porcelanite rocks up to 800°C decreased the specific surface area and increased the permeability coefficient of the calcined porcelanite samples to values comparable to calcined industrial filtration aids. ► Porcelanite of the Gafsa-Metlaoui basin is dominated of opal-CT. ► Opal-CT is associated with smectite, palygorskite and sepiolite. ► SiO2 content of the raw porcelanite is high, like industrial filtration aids. ► Thermal treatment of porcelanite reduces BET and increases permeability. ► BET and permeability of calcined porcelanite and diatomite are similar.