A method is provided for treating silica sand scrubs (SSS) generated and accumulated as waste in the chloride manufacturing process of titanium dioxide pigment. A hydrothermal process is used to produce sodium silicate solutions of modulus 3.0 to 3.8, and precipitated silicas. In some embodiments, the process uses two specific principal reaction stages. A sodium silicate solution having a low SiO.sub.2:Na.sub.2O molar ratio, in the range from 2.0 to 2.8, is first produced by reaction of the SSS, as a cost-effective SiO.sub.2 source, with aqueous caustic soda. The conversion of this intermediate sodium silicate solution of low modulus to a high SiO.sub.2:Na.sub.2O molar ratio is made possible by using a SiO.sub.2 source that is prepared as precipitated amorphous silica from the intermediate sodium silicate solution produced above.