How to remove Silica from Industrial process water?
Silica is a catch-all term for all reactive and inert forms of silicon's chemically resistant dioxide SiO2 or silicates. It is made up of silicon and oxygen, as well as one or more minerals or metals. Silica, such as quartz, is found in the majority of the earth's minerals. Silica is a hard, glassy-like mineral found in water as a result of the bedrock it passes through, such as sandstone and granite.
Because silica is one of the most common substances found on Earth, it is not surprising that it is almost always present in source water, process water, and wastewater streams. Proper silica level management is critical for industrial facilities to ensure optimal function and performance life for equipment.
Why is silica removal required?
The most common reason for removing silica from a process or wastewater is to protect industrial equipment from premature wear or damage. This is because silica content can accumulate on surfaces as hard mineral deposits known as scale. Scale deposits can reduce heating and cooling efficiency and make it difficult to operate systems within a given pressure range. As a result, silica scaling has the potential to reduce the efficiency, performance, and safety of equipment such as boilers, turbines, piping, and filtration units.
There are several treatment options for removing silica from water. The removal process that is most appropriate depends on whether the silica is dissolved or colloidal. Reverse osmosis is the best method for removing dissolved silica, while ultrafiltration is the best method for removing colloidal silica. Ion exchange and membrane filtration are the two most common methods.
Ion exchange (IX) is a process in which a stream is passed through a resin substrate, allowing ionic contaminants to be removed and replaced with similarly charged particles. Managing IX for silica removal can be a delicate process. However, when used correctly, IX offers the most complete removal of ionic silica when compared to other treatment technologies.
Keep in mind that if IX is the best option for your process and the feedwater contains a high concentration of silica, your ion exchanger must be larger because you must be careful how much silica you load onto the ion exchanger to avoid fouling the resins.
An IX system must be maintained through a resin regeneration step during normal operation. The improper use of caustics during regeneration cycles is the leading cause of silica scaling on IX resins and equipment. To avoid resin fouling and leakage, facilities should carefully monitor IX treatment cycles and adhere to manufacturer recommendations for properly regenerating IX resins.
When it comes to membrane filtration and higher silica loads, you simply need to ensure that your membranes are flowing freely and not clogging—you will need to clean them more frequently. When it comes to using membranes to treat silica, concentration is critical. If the proper parameters aren't set and care isn't taken to minimize fouling, cleaning your membranes can be difficult, and you'll have to replace them more frequently.
If the silica concentration becomes too high, you must proceed to the precipitation step, which usually requires the use of chemicals and a pH change to clump everything together and create a floc. In many cases, you will need to use lime softening, which will remove silica along with the lime and alkalinity.These steps can occasionally cause problems, especially if the process requires a temperature change, as is the case with hot lime softening. This temperature change affects the value solubility of calcium carbonate, which affects your silica treatment. Because of the energy costs associated with heating the water, most facilities will try to avoid this if at all possible.
Which is preferable for your process?
The best solution, as with any problem your facility is attempting to solve, often depends on the circumstances and what you're attempting to achieve. The same is true for silica removal, in part because water sources, silica levels, and forms vary so greatly, and also because, as mentioned in the introduction, silica exists in natural water sources in a variety of forms, including dissolved material, ions, polymerized (very large molecules), and colloidal form (which is more of a particle than a molecule).
The form of your silica is usually the deciding factor between ion exchange and membrane filtration, and the concentration will determine to what extent. If you have colloidal silica, for example, ion exchange may not be the best option because an ion exchanger removes ionic material from the water rather than suspended solids. You can only process a certain amount of incoming silica before it begins to coat the membrane surfaces when using membrane filtration to remove silica.
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