Kinetic and Physicochemical Features of the Decomposition of Arvatensky Serpentinite by Nitric Acid

Serpentinite attracts considerable interest as a promising raw material for the production of magnesium and silicon oxides, which are widely used in the synthesis of both high-purity and applied materials. According to its mineralogical composition, serpentinite is classified as a magnesium-enriched ore. Studies on extracting magnesium from serpentinite by chemical methods are therefore of particular importance. This work examines the decomposition of serpentinite from the Arvatensky deposit by nitric acid over a wide range of acid concentrations (20–50%), stoichiometric ratios (70–120%), temperatures (40–120°C), and contact times (5–300 min). Based on chemical analyses of the residual solids and filtrates, the degree of transition into the liquid phase (Strans) and the decomposition coefficient (Kdecomp) of serpentinite components (MgO, CaO, Fe2O3, and Al2O3) were calculated. Experimental results show that Kdecomp, which reflects the extent to which mineral components are broken down, is most strongly influenced by nitric acid concentration, temperature, mixing time, and acid ratio. Optimal serpentinite decomposition conditions were found to be: HNO3 concentration 30%, HNO3 stoichiometric ratio 100%, solid-to-liquid ratio 1 : 5, process temperature 90°C, and contact time 180 min, under which an acceptable Kdecomp value for MgO of 96.36% was achieved. The kinetic constants of serpentinite decomposition were calculated assuming first-order kinetics at 313–363 K. The apparent activation energy (Ea) of serpentinite ranges from 18.75 to 9.19 kcal/mol (78.38 to 38.41 kJ/mol), with an average value of 13.52 kcal/mol (56.53 kJ/mol). X-ray diffraction showed that the rate-limiting stage of serpentinite dissolution is external diffusion, indicated by the increased amount of amorphous silica. Based on the kinetic-constant values, serpentinite from the Arvatensky deposit can be regarded as raw material with sufficient reactivity, which is essential for establishing optimal conditions for maximum magnesium extraction and processing of magnesium-containing ores in hydrometallurgy.