Silica Removal Processes in Wastewater Treatment
Coagulation Silica Removal
Coagulation silica removal is a physical-chemical method that removes silica through adsorption or coagulation by certain metal oxides or hydroxides. It is a non-deep silica removal method: in general, coagulation plus filtration can remove about 60% of colloidal silica, while coagulation plus clarification filtration can remove about 90%.
1. Magnesium Dose Silica Removal
In practice, magnesium reagents are usually applied together with lime to ensure the desilication effect. The performance of magnesium dose silica removal depends on:
pH value: the optimum pH for magnesium dose silica removal is 10.1-10.3. Lime must be added to maintain the pH; besides adjusting pH, lime also removes part of the silica, temporary hardness and carbon dioxide.
Coagulant dosage: a coagulant is usually added together with the magnesium reagent. A suitable coagulant improves the properties of the magnesium oxide sludge and raises the desilication efficiency. Iron salts are generally used, at 0.2-0.35 mmol/L.
Water temperature: raising the temperature accelerates silica removal and improves the effect. At 40 °C the residual silica in the effluent can be reduced below 1 mg/L.
Residence time in the clarifier: at 30 °C the actual residence time should exceed 1 h; at 40 °C about 1 h; at 120 °C 20-30 min.
Raw water quality: higher raw water hardness benefits magnesium dose silica removal. The silica compound content of the raw water affects the magnesium dose ratio (mg MgO/mg SiO2-3): the ratio decreases as the silica compound content increases and increases with the proportion of colloidal silica, generally ranging from 5 to 20.
2. Aluminum Salt Silica Removal
The main conditions determining the removal of dissolved silica by aluminum salts are:
Temperature: the optimum temperature for aluminum salt silica removal is 20 °C.
Contact time: after 30 min of contact between the aluminum salt and silica-bearing water, most of the silica can be adsorbed and removed.
pH value: the optimum pH range is 8-9.
Crystal state and physical properties of the aluminum salt: if the aluminum salt precipitate forms outside the solution - especially after drying - its desilication effect is greatly weakened. The influence of the crystalline state on SiO2 removal follows the order AlO(OH) > Al2O3·3H2O > Al(OH)3.
For colloidal silica, the optimum pH range for aluminum salt removal is 4.1-4.7; approximately 40 mol of colloidal silica can be removed with only 1 mol of aluminum salt.
3. Iron Salt Silica Removal
Iron hydroxide can adsorb dissolved silica; its most effective pH is generally considered to be 9, and amorphous ferric hydroxide adsorbs better than its crystalline form. Removing 1 mg of SiO2 requires 10-20 mg of iron sulfate. When silica-bearing water is treated with iron salt flocculants at ambient temperature, the residual dissolved silica can be reduced to 3-5 mg/L. It has also been reported that treating water containing 20 mg/L of silica with suitable doses of ferric chloride, sodium aluminate and calcium oxide achieves a silica removal rate of 70-80%.
4. Lime Silica Removal
Treating raw water with hydrated lime removes temporary hardness and carbon dioxide at 40 °C while also removing part of the SiO2, reducing the residual silica in water by 30-35%. At 110-115 °C, using slaked lime for silica removal pretreatment of boiler feedwater (as applied at the Fujian refinery), the silica removal rate can reach 80% owing to the formation of CaSiO3 precipitate.
In recent years, coagulation technology for water treatment has advanced in two respects. First, attention has been paid to the combined use of coagulants, exploiting synergistic effects to achieve the best coagulation and settling results. Second, inorganic polymer coagulants such as polyferric sulfate and PAl-3 have been developed and put into industrial application. These inorganic polymer coagulants have a wide application range and low price; compared with traditional aluminum and iron salts, they omit the hydrolysis and polymerization reactions, which not only accelerates the coagulation process but also reduces interference from many factors affecting coagulation, giving more stable silica removal.
Reverse Osmosis Silica Removal
Reverse osmosis is the reverse of the natural phenomenon of osmosis. RO can remove both colloidal and dissolved silica and is suitable for purifying boiler feedwater, recovering part of cooling tower blowdown, and producing ultrapure water. According to the literature, the overall desalination efficiency of RO can reach 93%, with SiO2 removal of about 80%. A Japanese patent reports that for mixed water with a silica content of 5-20 mg/L, membrane permeation reduced the silica content of the effluent to below 1 mg/L.
Since the 1980s, reverse osmosis has become an important treatment method for boiler feedwater, often used ahead of ion exchange systems for preliminary desalination to reduce the load on the ion exchange system.
Ultrafiltration for Colloidal Silica Removal
Like reverse osmosis, ultrafiltration is driven by a pressure differential: the water to be treated is forced under pressure through a polymer membrane that allows water and low-molecular-weight solutes to pass while macromolecular and colloidal substances are retained, thus achieving separation. The membrane materials and equipment used in UF are similar to those of RO, and its separation mechanism is mainly the sieving effect. When ultrafiltration is used to remove colloidal silica, the membrane pore size should not exceed 100 nm, otherwise not all colloidal silica can be retained. Relevant tests show that the operating pressure should not exceed 7×104-7×105 Pa.
Ultrafiltration has no desalination capability and has almost no removal effect on dissolved silica.
Flotation for Colloidal Silica Removal
Cassell et al. studied water treatment by microbubble flotation and found that this method removes all colloidal substances in water. Before flotation, the water is conditioned for 10 min with 1.0×10-3 mol/L Al(NO3)3. The collector is lauric acid (25 mg/L) and the frother is ethanol (2.5 mL/L). The desilication effect is closely related to pH: in the range of pH 8-11, more than 90% of colloidal silica can be removed after 5 min of flotation. The addition of aluminum salt is also essential - colloidal silica alone cannot be floated. Cassell considered that the key to successful microbubble flotation lies in meeting three conditions: adding a small amount of electrolyte and adjusting the pH so that the colloidal particles agglomerate; adding a suitable collector; and forming a stable froth layer.
Process Selection
Silica compound treatment cannot be carried out in isolation; the removal of all suspended matter, colloidal matter, dissolved harmful gases and ions in the feedwater must be considered together to ensure that every index of the system effluent meets industrial production requirements. In general, coagulation treatment is shallow and it is difficult to reduce the silica compound content of the effluent below 1 mg/L. If the water quality requirements of the user system are high, reverse osmosis, electrocoagulation or ion exchange must be arranged after coagulation to improve effluent quality. The specific treatment process flow should be selected according to the raw water quality and the technical and economic merits of the different processes.
