Basic knowledge about silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula SiO2, most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and most abundant families of materials, existing as a compound of several minerals and as synthetic product. Notable examples include fused quartz, fumed silica, silica gel, and aerogels. It is used in structural materials, microelectronics, and as components in the food and pharmaceutical industries.Inhaling finely divided crystalline silica is toxic and can lead to silicosis, bronchitis, lung cancer, and systemic autoimmune diseases, such as lupus and rheumatoid arthritis.
What is colloidal sol?
A sol is a colloidal solution suspension of very small solid particles in a continuous liquid medium. Sols are quite stable and show the Tyndall effect. Examples include blood, pigmented ink, cell fluids and paint.Artificial sols may be prepared by dispersion or condensation. Dispersion techniques include grinding solids to colloidal dimensions by ball milling and Bredig's arc method. The stability of sols may be maintained by using dispersing agents.Sols are commonly used in preparing sol-gels.
How does colloidal silica differ from fumed, fused, or precipitated silica?
Colloidal silica varies from other types of silica in several significant ways. The most noticeable difference is that it's in liquid form, as opposed to powder. In addition, it has the widest ranging surface area, and its aggregate size can be as small as the actual size of the primary particle.
What's the difference between sodium silicate (water glass) and colloidal silica?
Colloidal silica consists of dense, amorphous particles of SiO2.The building blocks of these particles are randomly-distributed [SiO4]-tetrahedra. This random distribution is what makes amorphous silica different from crystalline silica - ordered on a molecular level. Sodium silicates are alkaline solutions with pH ranges of 12-13, compared to 9-11 for colloidal silica. Sodium silicates are also composed of silicate monomers, as opposed to colloidal silica composed of polymeric silicates.The composition of sodium silicates have a SiO2/Na2O ratio of approximately 3.4, whereas colloidal silica generally has a SiO2/Na2O ratio greater than 50. Finally, the viscosity of sodium silicates is much higher - closer to that of a syrup, while colloidal silica have viscosity close to that of water.
What is colloidal silica?
Colloidal silica is silica particles that are suspended in a liquid. The liquid is denser than water and has been stabilized electrostatically to allow the particles to stay suspended in the solution. The silica particles are also very small and do not have a large density. The range has to be approximately from thirty to a hundred nanometers. If the particles are too large, they will settle out of the solution. If the silica particles are too small, they are difficult to stabilize in a solution.
Preparation of colloidal silica is a multi-step process. Beginning with an alkali-silicate solution, the solution is neutralized which will cause silica nuclei to form. These particles are extremely small, usually just a few nanometers in diameter. The greatest control over the solution happens in this period.
In order to change the particle size and solution, there are two points of control. The pH and the sodium content of the solution will determine the final mixture. If the pH is reduced to below seven, making it acidic, or salt is added, the silica units will begin to fuse together, forming larger particles. These particles are called silica gels and are not stable in a solution as they can dissociate again if the solution is altered.
If the pH remains basic or neutral, then the silica particles will stay separated and actually begin to grow. After the size of the particles is maximized, the suspension is stabilized by adjusting the pH which will stop the silica particles from changing and clumping together. The solution can be adjusted by evaporated, depending on the density needed. The particle size will determine how dense a solution can become before it is unstable and the silica particles begin to precipitate from the solution.
Uses of colloidal silica vary. In paper-making plants, colloidal silica is used to drain the liquid from the paper. By allowing the water to leave the paper rapidly, the starch in the paper will remain, increasing the strength of the paper. This solution has many industrial applications as a moisture absorbent in many plants and factories. In some cases, it has been used to increase the friction of a surface. After a floor has been waxed, the surface is slippery and can be dangerous. By coating a waxed floor with colloidal silica, the friction is increased which will prevent accidents. Colloidal silica has many uses and functions and is an important product.
How do I measure specific surface area?
Particle surface area can be determined through titration.
Is colloidal silica dangerous to the environment?
Since colloidal silica products consist of amorphous silica and water, they rank as one of the most environmentally-friendly, industrial chemical products.
Does it pose any particular health hazards?
Colloidal silica products are aqueous dispersions of amorphous silica. Colloidal silica is not classified as harmful, but as mildly irritating. Because the products can have a drying effect on the skin, protective gloves should always be used. In case of skin contact, wash the area of contact with plenty of water. The use of safety glasses is always recommended. In case of eye contact, rinse with large amounts of water and seek professional medical advice. For further information, please reference the Safety Data Sheets for each product.
Storage recommendations
1)Shelf life
The maximum shelf life of a colloidal silica product varies between the different products and is referenced in the product data sheets. Storage of the product longer than the shelf life or in unfavorable conditions could affect the performance of the product. Avoid strong UV-light as this might reduce the shelf life due to organic growth.
2)Temperature
Colloidal silica is a freeze sensitive material. If allowed to freeze, the product will irreversibly agglomerate or gel. If product has been frozen, it will most likely be rendered useless and must be disposed of. If exposed to high storage temperature for extended periods, the shelf life of the product may be shortened. To reduce temperature effects to colloidal silica products and maximize shelf life, they should be stored at a recommended storage temperature of 20°C (68°F). Ambient conditions of +5°C to 35°C (+40°F to 95°F) are generally also acceptable, but please refer to the product data sheets for individual product storage recommendations.
General handling
1)Opened containers
The shelf life after opening is very much dependent on storage conditions and handling. Keep the lid on packaged goods tightly sealed to avoid contamination and oxygen addition that could lead to organic growth. If using an IBC container as an intermediate storage tank, please refer to the tank cleaning information on next page.
2)Mixing with other chemicals
One of the most useful properties of colloidal silica is its high surface area. When handling colloidal silica it is important to understand that the high surface area also may cause the product to gel or agglomerate if exposed to unfavorable conditions. Our Colloidal Silica is generally very stable but mixing with even small amounts of e.g. salt, strong acids or alkaline materials could trigger an agglomeration reaction. Also dilution of colloidal silica with hard fresh water or addition of glycol ether to the product can significantly reduce the storage stability and therefore the mixture should be used within a few days.
When intending to mix colloidal silica with other chemicals we recommend to always performing a lab scale evaluation first, ensuring that neither undesired gelling nor organic growth will start, as that will reduce the performance of the product.
3)Disposal of product
If the product has started to agglomerate, due to contamination, exposure to heat or due to other reasons, we recommend disposing of the residues in the tank. Do not blend with fresh product as the total volume then can be affected.
Dosing equipment
When installing equipment for colloidal silica, it is important to avoid trapping product in low points in pipes or hoses to reduce the risk of organic growth. If allowed to dry, colloidal silica can form silica deposits that are very difficult to remove. Avoid leaving pipes filled with product when not in use as dried silica solids can result in blocked pipes, stiff valves etc. Pipe systems, instruments, pumps and other wetted equipment should be made of stainless or acid resistant stainless steel. Compatible gasket materials are aramid, graphite, PTFE or FPM/FKM. A compatibility list of construction materials is available upon request.
Inspection and cleaning
1)Inspection
IBC containers used as intermediate storage tanks shall be inspected annually to track build-ups of agglomerated or gelled colloidal silica on the walls and the bottom of the container. Solids and gels can adversely affect flow and are excellent breeding grounds for organic growth. If there is any suspicion of organic growth in the IBC container, this could be indicated by flocculus floating on the surface or a biological odor. Regular sampling and testing for bacteria is recommended. Pipes, pumps, filters and valves that have contained colloidal silica must also be checked on an annual basis.
2)Cleaning
The simplest method of cleaning pipes, valves and pumps is to rinse them with water directly after use. Pay special attention to dead zones in the piping system. IBC containers that are used as intermedi
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