Development of the particle and pore structure of 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 the isoelectric point of the silica. The particle and pore structure development of silica under these acidic conditions has only been studied by a few researchers. The main aim of this study is to develop a model about the particle and pore formation of silica in very acidic environments. Nano-silicas prepared via the olivine route were analyzed to determine their pore structure and SSANMR. The pore structure was determined using the nitrogen physisorption, NMR and PALS (positron annihilation spectroscopy) techniques. The development of the nano-silica structure during the olivine process can be described by the following steps: 1) initially, soluble silica nucleates and primary particles of around 2 nm are formed; 2) the silica particles grow via a condensation route, forming linear chains; 3) as the growth continues, the silica particles keep increasing in size, resulting in 3D networks; and 4) with time, the aggregates become bigger and more compact. The final result of the olivine process is agglomerates as big as 20 ¿m. In addition, internal pores (also called closed pores) are developed with the reaction conversion degree as the result of condensation of two blocks of silica particles.