In cation exchange processes, the cation exchange resins used are mostly cross-linked organic polymers. These resins contain reactive acidic groups that can be exchanged by cations, such as sulfonic acid groups (-SO₂H), carboxyl groups (-COOH), and phenolic groups (-OH). Since sulfonic acid is a strong acid, resins containing sulfonic acid groups (R-SO₃H) are strongly acidic cation exchange resins, while R-COOH and R-OH are weakly acidic cation exchange resins. The H⁺ ions on the acidic groups in these resins can ionize, thus allowing them to exchange with other cations.
In anion exchange resins, strongly basic resins are the most widely used. They can exchange both strong and weak acid anions and can be used in acidic, alkaline, and neutral solutions. Weakly basic anion exchange resins do not dissociate in alkaline solutions and therefore can only be used in acidic solutions. Expired anion exchange resins can be regenerated by rinsing with a 4% NaOH aqueous solution to restore the resin to its original state for reuse.
The silica sol after anion exchange through an anion exchange column has a pH < 6 and is a slightly milky white transparent colloidal solution. The silica sol after cation and anion exchange is a highly reactive dilute silicic acid, which is easily gelled by impurity ions. Therefore, stabilizers must be added to maintain its pH between 8.0 and 9.5. Stabilizers used include NaOH, NH4OH, and NaSiO3. After adjusting with stabilizers, the sol is concentrated under normal pressure or reduced pressure to obtain the final product. In addition, there are methods for producing high-concentration silica sol with a SiO2 content higher than 50%. This type of silica sol can be formulated into a water-soluble solid silica gel, which can be dissolved in water to form a stable silica sol solution for use.