What is Silicone Dioxide?
Silicon dioxide, also known as silica or silica acid, is an oxide of silicon. The molecular formula for silica is SiO2. Unaltered, silica is a translucent off white or grey powder, and has no odor. It is a very hard substance and has a very high melting point… making it ideal for many industrial applications. It can be irritating to the eyes and skin, when contact occurs. When inhaled, silica can cause severe respiratory irritation. It is not water soluble.
Silicone Dioxide is a common byproduct of VOC abatement for commercial paints and coatings. Most VOCs form CO2 and water vapors, when destroyed in a combustion device, such as an RTO, TO, TRO, etc… Some paint and coating formulations contain silicone pigment dispersants, siloxane defoamers, and silane coupling agents. These VOCs are unusual, because they create SiO2, as well as CO2 and water vapors. This is an issue, because many paint and coating formulations are proprietary, and they do not list all ingredients, especially if they are not flammable or combustible. When the components of a paint are unknown, the exhaust stream’s contents will be unknown, which makes abatement a guessing game.
Because the silica formed by gas-phase reactions of vaporized silicone is so fine, it appears like smoke, similar to the fumes released from smoke stacks. This causes the SiO2 byproducts to become sticky, and accumulate, rather than being carried away as a smoke stream. After time, this sticky byproduct will gather enough accumulation, that it blocks the airflow through the media. This sticky accumulation also interferes with heat transfer, and can eventually cause an RTO to shut down, completely.
Maintenance issues among Regenerative Thermal Oxidizers in the paint and coating industry are incredibly common, because of this media blockage issue. But, there is a solution that allows paint and coating streams to be abated, without the accumulation of SiO2 byproduct build-up, and the associated operational problems.
The SiO2 Abatement Solution
There is a type of media, used in Regenerative Thermal Oxidizers, that was designed especially for the paint and coating industry application, to negate this troublesome maintenance nightmare. MLM-SR, or Multi-Layer Media SR, is a patented type of media, made of specially designed layered ceramic. It is an extremely low pressure drop media, which provides a wide-open area, for complete particulate plugging resistance. It works by never allowing the SiO2 byproducts to collide with the heated media, instead letting it flow smoothly through. This stops it from igniting, and becoming that problem sticky substance, that will accumulate and block the media. This is called fouling. The fouling of media can be expensive, over time; using MLM-SR media increases the maintenance interval of the media by a couple of years, saving a substantial amount of money and downtime. Another RTO specification that can increase the life of the media is to increase the size the bed size of the RTO. Doing this can extend the maintenance life of the media by 2-3 years. The specialized MLM-SR media, combined with the larger bed, and direct fuel injection, is the perfect solution for silicone based paint and coating applications.
Silica emissions are not heavily regulated, unless the smoke stack produces too much visible smoke. However, this is not generally an issue, when using an RTO with MLM-SR media. Instead of accumulating, the silica particles are emitted in a smoke stream, that will eventually be captured by rain, and disposed of in the soil. Silica is a natural component of soil, so this is not an environmental concern. This is because this is amorphous silica; crystalized silica is different, and generally found only in mining applications. Crystallized silica can cause health problems, from prolonged exposure. Amorphous silica, or “fumed silica,” is not considered a carcinogenic by most regulating agencies, including OSHA, IARC, and NTP. It is so harmless, it is often used as a thickening agent in makeup, personal care products, and food. However, it should be noted that silica can become airborne, because of its fineness, and can cause minor irritation to people with respiratory sensitivities. The average person’s lungs can clear normal amounts of airborne silica, without issue.
It is important to note that MLM-SR is not a cure-all for silica accumulation in RTOs. It is specially designed to treat vaporized silicone compounds, that are not able to be abated by pre-filtering or scrubbing; it is not a substitute for upstream treating.
MLM-SR is available in a number of sizes, to meet specific needs. Fouling issues are typically very site specific, so it is important to have a problem RTO evaluated by an expert, to determine the appropriate MLM-SR sizing and amount. Gulf Coast Environmental Systems has a team of these experts, to help you maximize the use of your RTO, and reduce fouling issues. To speak with a pollution control expert, today, click here.