Silica is not soluble either in water or in acid and silica is usually in the form of colloids. Silica is found in almost all rocks and susceptible to weathering. The main natural source of silica is mineral quartz. Anthropogenic source of silica is relatively very small.
The presence of silica in water does not make problem for human kind because it is not toxic to living things. However, for water which is designated for industrial use, the presence of silica can cause problem on piping system and other equipment because it can form deposit of silica.
The deposit of silica in boiler water which occurs is as follow:
Analcite (Sodium alumina silicate - Na2O.Al2O3.4SiO2.2H2O) is formed as result of the aluminum content in boiler water through feedwater. Aluminum which is usually delivered is aluminum that is used in pre-treatment in which less supervision occurs in its implementation. Little aluminum content in feedwater can cause high deposit of silica in boiler water. Therefore feedwater treatment must be controlled to eliminate aluminum and silica.
Acmite (Sodium Ferrous Silica - Na2Fe2O3 4SiO2) and crust Fe - Si can be formed from corrosion result. Acmite can be found easily in connection of boiler parts where it easy to occur corrosion.