Recovery of calcium sulfide from Tunisian phosphogypsum and olive mill wastewaters: a thermogravimetric and kinetic study

Abstract

Large amounts of phosphogypsum (PG) and olive mill wastewaters (OMW) are rejected regularly in the environment. This paper deals with the valorization of both PG and OMW from Tunisia in order to recover calcium sulfide. It consists in studying the reduction of the former using the latter as a source of carbon, in nitrogen atmosphere with or without H2O vapor. Experiments were carried out by thermogravimetry in non-isothermal mode, and the solids were identified by X-ray diffraction. Raw (OMWR) and preheated samples (OMWP) of OMW were used, and the PG over OMW mass ratio was varied to find the stoichiometric proportion of the reaction between CaSO4 and carbon. It was found that in pure N2 atmosphere the use of OMW as a carbon source allows to significantly reduce both the onset reduction temperature and the reduction temperature interval: the PG reduction takes place in the temperature range of 600–750 °C (instead of 750–1080 °C with pure carbon) leading to CaS formation. Utilizing Kissinger–Akahira–Sunose, Flynn–Wall–Ozawa, Kissinger, Satava-Sestak and master plots methods, activation energies, pre-exponential factors and the mechanism models of the reduction reaction were determined. The best models for PG + OMWP at 1.42 mass ratio and PG + OMWR at 0.23 mass ratio mixtures are Avrami-Erofeev A3 and A2 mechanisms, respectively. The kinetic models corresponding to the latter mixtures are: $$\frac{{{\text{d}}\alpha }}{{{\text{d}}t}} = 2.68~ \times 10^{{13}} {\text{~exp}}\left( { - \frac{{242.70~ \times 10^{3} }}{{RT}}} \right) \times 3\left( {1 - \alpha } \right)\left( { - \ln \left( {1 - \alpha } \right)} \right)^{{{\raise0.7ex\hbox{$2$} \!\mathord{\left/ {\vphantom {2 3}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{$3$}}}}$$ and $$\frac{{{\text{d}}\alpha }}{{{\text{d}}t}} = 1.32~ \times 10^{{12}} ~\exp \left( { - \frac{{214.92~ \times 10^{3} }}{{RT}}} \right) \times 2\left( {1 - \alpha } \right)\left( { - \ln \left( {1 - \alpha } \right)} \right)^{{{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 2}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{$2$}}}}$$ , respectively. In wet atmosphere, the conversion rate of this reaction decreases by about 16% and the determined activation energies in that atmosphere were higher than in pure N2 one, supposing that the reduction reaction was influenced negatively by the wet atmosphere.