Powder mixing Convection

Portillo PM, lerapetritou MG, Muzzio FJ Characterization of continuous convective powder mixing processes. Powder Technol 182 368—378, 2008. 

Afterwards, the PP powder is fluidized by a mixture of ethylene, propylene and inert gas (propane) in the fluid bed reactor, FBR, which is cooled by convective gas flow. The monomer conversion per gas circulation is limited to a few percent, because of the adiabatic temperature rise of more than 10 K per % conversion. Neglecting the heat loss through the reactor wall and through the product withdrawal, and assuming a well mixed reactor, the heat removal capacity of a steady-state FBR limits the productivity and can be expressed by... 

Continuous rotary Convection Direct/indirect Direct Indirect Conduction Crystals, coarse powders, extrudes, preformed cake lumps, granular paste and fillers, cakes back-mixed with dry product Chemical ores, food products, clays, pigments, chemicals Carbon black... 

An analysis of powder batch mixing suggests that there is little difference in the unit running costs of conventional convective and tumbler mixers. Labour costs were the dominant cost component in determining the overall costs of mixing in the majority of the cases examined and any reduction in labour utilization produces a marked improvement in unit costs. 

Convective mixing portions of the powder mixture move in relation to each other... 

In shear mixing, shear stresses give rise to slip zones and mixing takes place by interchange of particles between layers within the zone. Diffusive mixing occurs when particles roll down a sloping surface. Convective mixing is by deliberate bulk movement of packets of powder around the powder mass. 

Numerous distinct mechanisms for both mixing and de-mixing of granular materials have been cataloged, including convection, diffusion, shear, and percolation, and in most applications several mechanisms act concurrently and interact in complex ways. For example, details of loading of powders into blenders of... 

The molten salt method is seldom used except for minerals of very high SG. The grains are mixed with the powdered salt in a test tube, which is then kept a little above the melting point of the salt for a short time with carefully controlled heating to minimize convection currents. After separation is complete, the tube is cooled and broken, and the column of salt is cut into a lower and upper half, from which the separate fractions can be recovered by dissolving out the salt with water (Williams [1961]). 

---