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Powder blend consistency can be difficult to maintain during processing. This article provides an overview of common segregation mechanisms, highlights key sampling techniques, and offers practical guidance on preventing and solving segregation problems.
Bulk solids handling in the chemical process industries (CPI) routinely involves mixing and blending specialty chemicals, plastics, fertilizers, dry powdered detergents, glass or ceramic powders, and even explosives. Although CPI facilities perform these processes daily, many lack an effective, fundamental approach to maintaining material or blend consistency. As a result, particle segregation may occur.
Segregation is defined as the separation of particles into distinct zones by particle size, shape, density, resiliency, or other physical characteristics, such as static charge. Blending and segregation (the process of de-mixing) can be considered competing processes. A mixture that is well-blended can still segregate during subsequent storage, conveying, or packaging.
Segregation can cause variations in a bulk solid's chemical composition, performance, appearance or color, stability or reactivity/explosivity, taste, or dustiness. For example, a granular lawn fertilizer that has segregated can damage grass due to over-application of nitrogen, phosphorous, or potash. Or during routine drug production, an active pharmaceutical ingrethent (API) can segregate from the excipients (e.g., microcrystalline cellulose, lactose, or dicalcium phosphate), causing variations in tablet weight or drug potency and effectiveness.
At the most basic level, a powder will segregate due to:
* physical and chemical property variations among particles
* forces induced on the particles
* fill and flow sequence.
Particle size and distribution can have a large impact on a blended material's propensity to segregate (/). Research by Williams indicates that a material or blend with a mean particle diameter larger than 100 µ?? and variation in particle diameter of more than 30% can be highly prone to segregation. For example, dry laundry detergent can segregate because of significant particle size differences between the major component (phosphates or carbonates) and the minor components (bleach and enzymes). Particles in a blend should have the same size (i.e., be "mono-sized") to reduce separation tendencies. This is often impractical, however, for most bulk solids or mixtures.
Segregation can occur even in blends of mono-sized particles when the particle densities differ. Imagine a binary mixture of golf balls and ping pong balls in a container. Although the...