What is Silica?
To answer this question, it is important to first define what silicon is. That said, silica is a form of ore derived from silicon, hence the importance of defining it.
Silicon, with the symbol Si, is an element of the periodic table in the carbonaceous category. Constituting about 27.7% of the earth's crust, silicon derives its name from the Latin terms "Silex" & "silicis", which respectfully mean "flint" and "hard stone". Although pure silicon is far too reactive to be found in its pure state naturally, it is found in virtually every type of rock, sand and soil.
Silicon atoms are predominantly covalently bonded and are very often intertwined with oxygen atoms. The general arrangement of the silicon atoms tends towards the formation of siloxanes. These macromolecular siloxanes generally have a mass large enough to be categorized as polymers. When the silicon atom formations do not have these qualities, they are generally in aqueous or oily form.
- As an aside, siloxanes are a group of organosilicon compounds composed of Si - O - Si - O - Si backbones with R side chains that attach to silicon atoms. For example, among the most common siloxanes, polydimethylsiloxane (PDMS) is the most common polymer in the silicone industry (glue tube).
Anyway, in the water treatment sector, it is generally in its mineral form that we find the presence of silicon.
Mineral Forms
Since the purpose of this article is not to present the mineral derivatives of silicon, we will only discuss two of them: silicates and silica.
Silicates
They constitute a family of minerals that includes some 600 different mineral species. What they have in common is their basic structure, an atomic architecture (SiO4)4-
To form a silicate, a silicon dioxide anion located in the centre and surrounded by oxygen atoms associates with metal cations (aluminum, magnesium, iron, etc.). This arrangement allows the metallic neutrality of silicates.
In the category of silicates, there are several families allowing their identification. We can think of the neosilicates, the sorosilicates, the cyclosilicates and many more.
Silica
Crystalline silica, silica or silicon dioxide goes by many names and is a mineral commonly found in nature. It is particularly found in sands, mortars and concrete, but it can also be found in glass, pottery, ceramics, bricks and much more.
As its scientific name suggests, silica is made up of one silicon atom and two oxygen atoms (SiO2). As with silicates, there are several varieties of silica. Three of the most common varieties are quartz, tridymite and cristobalite.
Finally, pure silica minerals have a glassy appearance and are translucent. Besides, they do not have the quality of a conductor.
Silica and Water
The presence of silica in water can be caused by human/industrial activities since some silicon derivatives are used as conditioners, detergents, or corrosion inhibitors.
Silica can also spread naturally, mainly due to soil and mineral erosion. This occurs when water flows over or through different types of soil.
- For convenience, we will not elaborate on the different types of soils, but if you are interested in the subject, here is an article that might touch your curiosity: Mining Water Input: The Impact of Different Contaminants.
When they come from natural sources, the traces of silica in water are mainly due to the hydrolysis of Si- O - Si bonds. When this dissolution reaction takes place, it is in the form of silicic acid that silica is found.
SiO2 + 2H2O -> SiO2 + H4SiO4
Typically, the concentration of silica in natural waters ranges from 5 to 25 mg/L, but it is not unusual to find concentrations exceeding 100 mg/L.
As a solution, silica is known to be highly affected by its environment. Factors such as temperature, crystallinity and pH will have an impact on the type of degradation. Depending on its environment, three silica derivatives can be formed.
- Monosilicic acid
- Polysilicic acid
- Colloidal silica particles
We distinguish silica in two categories when it is dissolved in water: Reactive silica and non-reactive silica.
Reactive Silica
Reactive silica is dissolved in water and is virtually unionized and has not been polymerized into a chain. Although reactive silica has the characteristics of an anion, it is considered to be part of the total dissolved solids (TDS). It’s this factor that must be considered when creating a Reverse Osmosis treatment projection program.
Non-Reactive Silica
There are two types of non-reactive silica: polymerized silica and colloidal silica. Colloidal silica is a mixture of siloxane (Si - O - Si) and silanol (Si - OH). The way they are made makes colloidal silica a very hydrophilic substance and having the ability to form many hydrogen bonds. Having a size of almost 0.008 microns, this type of silica can be extracted by reverse osmosis and its presence can be identified by an SDI test.
- To learn more about the SDI test, visit this article: What is an SDI test and how to understand it?
Although osmotic membranes have the ability to extract these colloidal clusters, they will usually tend to clog quickly due to the formation of silica on the membrane edge.
When colloidal silica is not properly extracted, if it comes into contact with high temperature or high-pressure equipment, the colloids break up and can accumulate in another form on the surfaces of the equipment.
Risk associated with the presence of Silica in Water
When consumed in water, there is no evidence that any level of silica in water is harmful or hazardous to health. Obviously, an absurd concentration of silica or other dissolved materials in water can be harmful in one way or another. On the other hand, if this is the case, the water would not be interesting to drink (colour, smell, etc.).
- It is important to note that although its ingestion is not harmful to health, inhalation of silica has been shown to cause lung disease.
Risk to Equipment
The presence of silica alone does not tend to cause problems with equipment. Besides increasing the difficulty of water treatment since silica tends to clog water treatment equipment, it is usually when systems are operated under high pressure or high temperature that problems occur.
Generally, it is in the form of steam that silica becomes the most problematic. In high-pressure/high-temperature boilers, the presence of silica can cause downstream problems when it gets into the vapours. Typically, the higher the temperature/pressure of the boiler, the higher the concentration of silica in the vapour. When this occurs, deposits form inside the system or on the turbine blades and decrease the efficiency of the equipment while increasing the risk of breakage or accidents.
It is recognized that to avoid the risks and complications related to the presence of silica or its derivatives in a high pressure/high temperature boiler, concentrations of 0.02 ppm should not be exceeded. Ideally, it is recommended to be well below this limit. As an example, if a boiler system evaporates 1000 tons of water per hour and has a concentration of 1 ppm of silica, that is 24 kg of silica per day that would accumulate inside the system.
How to extract Silica an its Derivatives?
Now that we understand how and why silica is problematic with respect to various industrial equipment, let's focus on understanding the techniques used to extract it.
To do so, it is important to specify that the tactics used can vary according to the needs and the different situations. For example, for a medium-pressure boiler with a clean water supply, the treatment will be simpler than for the same boiler with highly contaminated water.
The addition of a reverse osmosis system is very often the ideal solution for the reduction of silica and other contaminants in the water. Depending on the quality of the feed water and its silica concentration, pre-filtration steps may need to be installed. Different pre-filtration techniques can be used: softening, coagulation, precipitation/aggregation, ultrafiltration, ion exchange or media filter.
Reverse Osmosis
As for many uses, reverse osmosis is often the best choice for contaminant removal. It is the same in the case of silica since we are talking about a rejection rate of contaminants varying between 95 and 99%. The difficulty with the extraction of this contaminant via reverse osmosis is that silica is the main source of fouling of osmotic membranes.
As the reverse osmosis process treats the water, the concentration of silica increases on the concentrate side. At a certain level of saturation, the chances of silica or metal silicate deposition on the surface increases significantly. When accumulation occurs, the efficiency of the treatment can be affected due to a loss of permeability of the membranes.
In addition to causing performance deterioration, silica accumulation on membranes can lead to other problems. These include a complete shutdown of the system due to high pressure or increased operating costs of the system since the energy required to perform the treatment is increased due to membrane fouling.
In order to avoid these problems, a pretreatment step may be necessary, or the adjustment of the RO system may be sufficient. This can be done by adjusting the recovery rate of the reverse osmosis to reduce the saturation of the concentrate. By doing this, the percentage of water recovered for treatment is decreased, which increases the cost of treatment per gallon, but the chances of problems associated with silica are equally decreased.
Split-Bed Ion Exchange
Ion exchange systems can be very effective for silica extraction. With the right arrangement of resins, it is indeed possible to completely remove the silica from the water. This is why these systems are used for the treatment of water for the supply of high-pressure boilers. In order to optimize the operating costs of this type of system, silica concentrations in the water should not exceed 10 ppm.
The "separate bed" principle consists of two resin tanks that are installed in series. For silica extraction, the first tank consists of a cationic resin bed that must be regenerated with an acid solution. The passage of the silica through the cationic tank alters its composition and "converts" it into silicic acid. Once this conversion is done, the silicic acid passes through the anionic resin bed which will allow the extraction of the acids. Between the two resin tanks, it is common to find a decarbonation unit to allow the extraction of carbon dioxide in aqueous form. This step is important because the anionic resin could be fouled with CO2 before being able to extract the silicic acid from the water.
The problems that can result from too high a concentration of silica in the water to be treated by an ion exchanger basically consist in the fouling of the resin. In other words, if the concentration of silica is too high for the size of the equipment, build-up can occur and affect treatment efficiency. To remedy this, resin regeneration must always be done properly, and sensors can be installed on the system.
Polishing
Since silica is a weakly ionized molecule, during treatments such as ion exchange, residues of other molecules will be extracted before silica. This is why it is common to find residual forms of silica in water treated by ion exchange or reverse osmosis.
For the complete extraction of silica traces, a polishing step is necessary. Polishing consists of an additional treatment of the water in order to extract the most recalcitrant particles, such as silica.
For this type of contamination, two technologies are known to be effective and advantageous: the mixed bed ion exchanger and electrodeionization.
Mixed Bed Ion Exchanger
The principle of this technology is exactly the same as that of the separate bed ion exchanger. The difference is that here there is only one tank filled with a mixed resin bed. This means that both anionic and cationic resins are present in the resin bed.
It is important to note that the use of this technology for silica extraction can only be used for water polishing, as too high concentrations of contaminants would prevent the capture of silicon ions.
On the other hand, if a mixed bed ion exchanger is installed downstream of a reverse osmosis system or a separate bed ion exchanger, the contact point between the anionic and cationic resins allows the removal of silica residues that may have resisted the previous treatment.
Electrodeionization
Without going into the details of how EDI modules work, they operate on the same principle of ion exchange that is found in resin bed treatment units. What makes EDI modules a special technology is that the resin bed is regenerated by the emission of a continuous electric current.
Since electrodeionization works on the same principle as standard ion exchange, it has the ability to remove all traces of residual silica from the water.
Although very effective and inexpensive to use, these systems are very expensive to purchase and are only useful for water polishing. The advantage of opting for electro-deionization is mainly associated with the fact that no intervention is required to operate. This reduces the risk of accidents related to the handling of chemicals used for regeneration. In terms of cost, EDI modules are generally economically advantageous when the water flow rate is between 20 gpm and 250 gpm.
Silica Extraction : A Possibility
As discussed above, silicon and its derivatives can cause a variety of problems when in the presence of high temperature or high pressure. Being difficult to extract, the presence of silica in water must be addressed conscientiously to avoid choosing a treatment technology that is not effective or optimized for your situation.
Although there are other methods of silica extraction, the technologies presented are those with the greatest advantages and are the most versatile.
We hope your questions about silica processing have been answered. If you have any further questions or problems, please do not hesitate to contact us.
In the meantime, we invite you to consult these articles:
[1] When the rejection rate of an osmotic membrane is below 95%, it is usually time to change it.
[2] In a reverse osmosis system, the concentrate refers to the reject after treatment. In other words, the pure water passes through the osmotic membrane and the concentrate is the effluent that cannot be used.