Under the extreme temperatures and high concentrations of dissolved solids in the water used for producing Energy, the scale producing salts and compounds precipitate in to hard crystaline solids on the surfaces of heat exchangers, cooling towers, pipelines, etc., causing inefficiency and much down time. Operations must be shut down often for descaling. This of course, results in lost production and increased operating costs.We are concerned in our study with silica and calcite scaling.


Silica (Amorphous):


Amorphous silica is slightly soluble in water. From 0 to 2500 C and at the vapor pressure of solution, the solubility depends only the temperature:

T = 731/ (4.52 - log C ) - 273.15 ; C = {mg of silica /kg of water}

SiO2 + 2H2O = H4SiO4

H4SiO4 = H+ + H3SiO4-

Increase in salinity causes a decrease in solubility.

Solubility increases with an increase in pH of the solution.


Three main conditions, namely temperature, salting and boiling effects, control the silica scaling from solution.

Silica scaling exists only at temperatures below the saturation temperature for the equilibrium solubility of amorphous silica.

Therefore as temperature decreases, deposition increases. But in the case of boiling, the concentration of solutes in the solution increases due to loss of water. This favors the deposition of silica greatly.



Although slightly soluble in water, calcite dissolves readily in water containing dissolved carbon dioxide, to form calcium bicarbonate.

Ca2+ + 2HCO3- = CaCO3 (solid) + H2O + CO2

CaCO3 + H2O + CO2 = Ca (HCO3)2


When water evaporates from a solution of calcium bicarbonate and the dissolved carbon dioxide escapes into the atmosphere, the reaction is reversed and calcite is deposited.

Calcite scale increases with: