Particle growth of nano-silica below the isoelectric point

Nano-silica is one of the most used nano-materials, and its use is rising 5.6% per year to reach a projected 2.8 million metric tons in 2016 with a value of $6.4 billion. The current production methods involve steps with high temperatures resulting in environmentally unfriendly and expensive process. The production of nano-silica by the dissolution of olivine is an interesting alternative to the existing commercial methods because of the good quality of the resulting silica and low energy requirements and CO2 emissions. Olivine nano-silica is produced in a very acidic environment, much more acidic than its isoelectric point. The particle growth of silica under these acidic conditions has only been studied by a few researchers. The main aim of this study is to clarify the mechanism and kinetics of the silica particle growth under these conditions. The silica particle growth was studied with Norwegian olivine in a vigorously stirred reactor at 20 °C with sulfuric acid. The concentration of hydrogen ions was determined at regular time intervals, from which the amount of silica produced was calculated. The silica particle size was measured with the Malvern ZS after passing the samples through a filter of 1 ¿m. The cluster size of olivine silica grows up to 420 nm after 4300 minutes of reaction at 20 ¿C in 2M H2SO4 solution. The mechanism of the growth of silica is likely due to the nucleation of primary particles and subsequent aggregation of these primary particles. Olivine nano-silica does not gel. This behaviour can be due to three reasons: 1) silica polymerizes fast at 90 ¿C in 3M H2SO4 solution, forming particles above 100 nm in a short time; 2) the concentration of small silica colloids is low; and 3) the vigorous mixing in the reactor to keep the olivine particles suspended.