Physico-Chemical Studies of the Formation of the Main Minerals of Cement Clinker and Zinc Sublimation Using Thermodynamic Modeling

The paper presents the results of thermodynamic modeling of the processes of formation of the main minerals of Portland cement clinker (alite C3S, belite C2S, tricalcium aluminate C3A and fourcalcium alumoferrite C4AF) when tailings containing zinc oxide (ZnO) and zinc carbonate (ZnCO3) are introduced into the feedstock. The study was performed using the HSC Chemistry 6.0 software package and the SGTE database in the temperature range of 500–1500 °C. The relevance of the work is due to the need to dispose of large-tonnage zinc-containing man-made waste as a secondary raw material. The standard clinker formation reaction is compared with model reactions involving ZnO and ZnCO3, and the sequence of transformations of zinc-containing compounds in the Zn–C-O system is studied. It is shown that at 500 °C the Gibbs energy of the clinker formation reaction with ZnO is +965.9 kJ, whereas for the reaction with ZnCO3 it is +839.6 kJ. The temperatures at which the condition ΔG ≈ 0 is reached are about 1057 °C for the reaction with ZnO and 906 °C for the reaction with ZnCO3. At a temperature of 1450 °C, the reaction with ZnCO3 is characterized by a more negative value of ΔG (−900.0 kJ) compared with the variant with ZnO (−540.0 kJ). Additional analysis showed the sequence of transformations ZnCO3 → ZnO → Zn(g). The reduction of ZnO by carbon becomes thermodynamically possible at about 950 °C, while the reaction of ZnO with CO reaches the condition ΔG ≈ 0 at about 1317 °C. The direct dissociation of ZnO with the formation of Zn and O2 remains thermodynamically unfavorable in the range of 500–1500 °C. The results obtained indicate the fundamental possibility of combining clinker formation and zinc distillation during high-temperature tailings processing, followed by the capture of the zinc-containing gas phase.