Sulfidation behavior of Zn and ZnS crystal growth kinetics for Zn(OH)2–S–NaOH hydrothermal system

• The pH-dependent hydrothermal sulfidation procedure was proposed.
• The sulfidation process is analyzed with a new variant of the shrinking core model.
• The growth of ZnS crystals proceeds via Ostwald ripening.

Recently, hydrothermal sulfidation has received much attention for its potential application in the recovery of heavy metals from waste. To achieve a better understanding of the hydrothermal sulfidation process, the sulfidation behavior of Zn and ZnS crystal growth kinetics for the Zn(OH)2–S–NaOH hydrothermal system was studied in this article. Based on discussion of the thermodynamics, the behavior of S0 in the S–H2O system and the behavior of Zn(OH)2, the reaction steps of hydrothermal sulfidation were proposed. The sulfidation process was then analyzed by a new variant of the shrinking core model in which both the interfacial transfer and the diffusion across the product layer affect the sulfidation efficiency, and its apparent activation energy is 21.4 kJ/mol. The crystalline size data of ZnS are fitted by a standard Ostwald ripening model consistent with crystal growth controlled by the volume diffusion of ions in solution. The deduced activation energy for ZnS crystal growth is 39.8 kJ/mol.

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