Solubility and dissolution kinetics of quartz in NH3–H2O system at 25 °C

Understanding of the solubility of quartz in aqueous ammonia solutions is crucial to our knowledge of many hydrometallurgical processes. The solubilities of natural quartz and ground quartz with particle size from 45 μm to 75 μm were measured in the NH3–H2O system at 25 °C, with ammoniacal concentrations up to 2.93 mol/kg H2O. Solubilities were determined by molybdate blue method. Quartz and amorphous silica solubilities in the NH3–H2O system were also calculated by geochemical modeling code. The experimental and predicted results indicate that the two minerals have fundamentally different dissolution behavior. Natural quartz solubility undergoes large enhancement with ammonia, rising rapidly from a silica concentration of 6.83 mg/L in pure water and leveling off at 419 mg/L in 2.92 mol/kg NH3 solutions. Ground quartz solubility lies between natural quartz and amorphous silica, which shows monotonically increasing from mSiO2 = 690 mg/L in 0.26 mol/kg NH3 solution to 1530 mg/L in 2.74 mol/kg NH3.The rate of dissolution of ground quartz far from equilibrium was also studied in the NH3–H2O system. The effects of particle size and duration time on dissolved silica were investigated. The results showed that ammonia concentration has no great impact on the rate of dissolution of ground quartz with particle size of 45–75 μm. Our findings clearly imply the degree of particle surface damage by grinding and the reaction time are the most important factors for the ammonia leaching of quartz.

Research Highlights
► The solubilities of natural and ground quartz in ammonia solution at 25 °C were reported.
► The exponential increase in quartz solubility with increasing NH3 concentrations is expected.
► Ground quartz solubility shows a linear increase with growing ammonia concentrations.
► Ground quartz has a higher solubility than natural quartz due to the easily soluble layer.
► The degree of particle damaged and reaction time are the key factors for ammonia leaching of quartz.