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By Rick Isherwood and Dr. Hans-Joachim Jacob
Highly dispersed fumed silicas — such as Aerosil, Cab-O-Sil, HDK and others — are used as thickening and thixotropic agents in many products. Precipitated types or silica gels (including Acemat, Gasil, Syloid and others) are not as light as the fumed silicas, but the handling of these powders involves similar issues: Both are very dusty! This article examines how induction with Ystral machines eliminates silica dust, leading to more efficient production and a cleaner work environment.
Significant problems occur when adding highly dispersed fumed silicas and silica gels onto the surface of liquids. Primarily these problems are a result of the powder transfer into the vessel containing the liquid. Very often the silica powders are manually poured into the vessel, causing unacceptable dust in both the vessel and work environment. As a result of this dust, the walls of the vessel and the shaft of the mixing element become covered with partially wetted crusts, also called agglomerates. These agglomerates reduce the quality of the final product and are also not economical due to the powder waste.
The dust problem
In terms of dust escaping into the environment, if there is a dust exhaust system installed on the vessel, an uncontrolled part of the very light powder goes into the filters and is unrecoverable, again resulting in product waste. If there is no dust exhaust, the dust covers the operators’ bodies, drying out the hair and skin. If a good mask is not worn, the powder goes into the lungs. Although the powder itself is not critical, in combination with other airborne items such as solvent vapor, it can cause serious medical issues.
Some operations working with highly dispersed silica fumes and silica gels try to reduce the amount of dust created by using pneumatic conveyors to transport the powder into the vessel. This transport process can actually result in dusty air being brought into the vessel. If the vessel is open, more dust can enter the general atmosphere. If the vessel is closed, all the dusty air, along with the vapor of the solvent, has to be evacuated from the vessel through filter systems. As the specific weight of the highly dispersed silica is very low, conveying this product pneumatically suspends more dust in the air resulting in product loss and clogged filters.
Pneumatic conveying also does not solve the problem of the powder crusting on the sides of the vessel and on the shaft of the mixer. This silica powder is so light that it will float for an extremely long time, and will therefore, take a very long time to be wetted out and dispersed. While this wetting and dispersion is occurring, the exhaust filters are always running and collecting vapor and dust — a very expensive option.
Many companies use batch mixers, cowls, blade dissolvers or batch dispersers to add the silica powder to the liquid. These are not effective as they do not wet and disperse the primary particles completely.
Silica powders have a very large surface area. One gram of Aerosil 200, for example, has a surface area of 2,000 ft² (200 m²); one bag with 22 lb. (10Kg) has a surface area of 20 million square feet (2 million square meters). If using a conventional stirrer or dissolver, umpteen millions of square feet of powder surface will be wetted completely by tipping it on only 20 to 30 square feet of liquid surface, resulting in agglomerate production. Even long dispersing times after powder addition could never reach the quality possible when using the correct powder addition and dispersion method.
Another problem in the addition of silica powder onto the liquid surface is the fact that the powder is exposed to the vapor of the liquid before wetting. Highly dispersed silicas have a high affinity to bind solvents. If silica powder is already bound to a solvent, it has reduced effects later in the resin, the end product. Therefore more powder is required to get the required thixotropy in the final product, increasing costs.
Ystral has developed two approaches to solve the dust issues during induction of highly dispersed fumed silica and silica gels into liquids. The first approach offers machines installed inside the vessel: the TDS-Induction Mixer and the TDS-Dispermix. When these machines are installed inside a vessel, they create a vacuum in their mixing head and induct and mix under the liquid surface. They are limited to low viscous liquids (max. 1000 mPas) and have a reduced or medium shearing effect.
The second approach is the Conti-TDS, an external inline dispersing and powder induction machine. It is not dependent on the size of the vessel or viscosity and can be connected easily to any existing vessel. The machine inducts by creating a vacuum in its high shear dispersing zone. The result is dust- and loss-free induction into the liquid, complete wetting and dispersion – all via the same machine.
Because the powder is inducted into the liquid directly — it is not exposed to the solvent vapor and potential binding risks, resulting in 100% of the product being used to build up the required thixotropy. In addition, the wetting carried out inside the Conti-TDS is under high shear and vacuum where the liquid surface is increased some million times, placing it in the same range as the surface of the powder. The powder is inducted directly into the liquid and is immediately wetted and dispersed. This causes a much higher efficiency and an agglomerate-free dispersion. The quality of the final product is always better than with any traditional mixing and dispersing method.
The advantages afforded by these solutions include:
• Elimination of dust
• Reduction of product waste and product loss
• Maintaining a safer work environment
• Higher quality, better dispersion compared to conventional methods
• Higher safety standards! Powder does not have to be brought into the solvent-containing atmosphere by the operator
• No partially wetted crusts and lumps in the vessel
• No agglomerates in the product
• Higher viscosity, higher thixotropy effect
• Same results with lower concentrations possible
• Flexible batch sizes
• One machine can operate with multiple vessels
• Up to 90% reduction of process time and costs
• Short amortization time
Rick Isherwood is president, Powder Technologies, Inc. Dr. Hans-Joachim Jacob is a process engineer with Ystral.
Powder Technologies (PTI) is a leader in powder-processing applications for the food, cosmetics, pharmaceutical, chemical and coatings industries, offering equipment design and fabrication for size reduction, screening and classifying, mixing, and feeding equipment for dry-bulk processing and powder inducting, wetting and dispersing systems. PTI serves as the exclusive distributor in North America for Ystral, an experienced, highly regarded processing-equipment manufacturers with a proven commitment to design and fabrication. PTI can also assist with feasibility and engineering studies, process and plant design, installation, start-up, training, and maintenance and parts inventory.