The kinetics of the olivine dissolution under the extreme conditions of nano-silica production

• Experimental conditions: pH = −0.7 to 1, olivine 250 mg/L, ∼70 °C.
• Silica layers on olivine surface were rarely found under the conditions used.
• The limiting step of the process is the surface reaction.
• Kinetic parameters: Ea = 78.5 kJ/mol and a reaction order of 0.5.
• The kinetic model of the olivine dissolution is robust with an error of 5%.

This article addresses the kinetics of the dissolution of olivine for nano-silica production at extreme conditions. The extreme conditions are pH values between −0.7 and 1, temperature between 50 and 90 °C, solid content around 250 g/L and percentage dissolved between 80% and 99%. This work is structured in 3 parts: (1) chemical and mineralogical characterization of the dunites employed; (2) mechanism of the olivine dissolution focusing on the possible resistances to the transport; and (3) determination of the kinetic parameters kT and n.The results shown here demonstrate that: (1) the limiting step of this process is not the diffusion through a silica layer but the surface reaction; and (2) the dissolution of olivine under the olivine nano-silica production conditions is well described by:r=A·exp-EaR·T·aH+n=856·exp-78.5R·T·aH+0.5,where r is the dissolution rate (mol cm−2 s−1), A the pre-exponential factor (mol cm−2 s−1), Ea the activation energy (kJ/mol), R the gas constant (8.314 · 10−3 kJ mol−1 K−1), T the temperature (K), aH+ the hydrogen ion activity and n the reaction order. The average error of the reaction rate calculated using these parameters is 5.5% for dunite CRS-US. In addition, this model is successfully applied to the dissolution of other commercial dunites and for bigger reactor volumes. Therefore, this model can be considered to be robust, and it can be used in the industrial production of olivine nano-silica.

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