Silica is present in two forms: reactive silica (single unit), and colloidal silica (multiple units).
- Colloidal silica has virtually no ionic character, but has a relatively large size. Colloidal silica can be removed by fine mechanical barriers, such as ultrafiltration.
- The single-unit reactive (ionic) silica is much smaller than colloidal silica. As a result, most mechanical removal techniques such as flocculation, clarification, filtration and flotation are not capable of removing this species. Technologies that allow removing silica are reverse osmosis, ion exchange and continuous deionization.
Lime softening
Lime softening is one of the most common methods for removing silica from water such as make up to cooling towers, make up to boilers or boiler blow down water. Lime softening utilizes the addition of lime (calcium hydroxide) to remove hardness (calcium carbonate and magnesium carbonate) ions by precipitation. Silica particles are absorbed in flakes of magnesium and calcium hydroxides. These flakes (floc) can then be sent to a clarifier or filter where the separated silica can be disposed. As the percentage of silica in the water changes, so must the percentages of chemicals that must be added.
In many cases, heavy metals will be present in the sludge (settled floc) and will require the extra expense of disposal into a hazardous waste facility. Lime softening typically requires a fairly sizable capital investment and can be costly due to the quantity of chemicals used and disposal costs of large amounts of residuals produced, especially if hazardous waste facilities are needed.
Ion exchange
According to Wikipedia, ion exchange is, “An exchange of ions between two electrolytes or between an electrolyte solution and a complex. In most cases, the term is used to denote the processes of purification, separation and decontamination of aqueous and other ion-containing solutions with solid polymeric or mineralic ion exchangers.”
Ion exchange works very well for the removal of silica; however, the disadvantage of this process is that the resin exchangers need continual recharging and replacement, adding significant ongoing costs to the process. In addition, if there are heavy metals within the exchange resins with the silica, these concentrated ion metals usually do not pass the TCLP leachability test and must be disposed of in a hazardous classified landfill, adding further expense.
Reverse osmosis
Reverse osmosis (RO) is a membrane-technology filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side. To be “selective,” this membrane should not allow large molecules or ions through the pores (holes), but should allow smaller components of the solution, such as the solvent, to pass freely.
Using reverse osmosis or any membrane technology to remove silica can be problematic. Silica is like glass and will cut holes in the membrane and will decrease the efficacy of a membrane because of the abrasiveness of the particles. Heavy metals foul membranes, leading to failure. Membrane systems should have a pretreatment regimen to get rid of the constituents that foul membranes. If pretreatment is done prior to membrane treatment, then membranes will perform optimally without expensive replacement.
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