Fluidised mixers

Optical techniques were developed - both in situ and on line - over a broad range of applications and various types of light sources. Main examples are solids concentration measurements in particle fluidisation [6,7], as well as gas-solid flow [8], or sample composition in dry blending operations, this latter beginning from the pioneering work of Harwood et al. [9] up to more recent advances in this scientific field [10-12], The main interest of these techniques lays in the fact that optical probes are easily available on the market, so that qualitative monitoring of mixers is possible at an industrial scale. However, these probes only provide local information of the mixture (typically 1/40 of a tablet), so that in essence, they still sample the powder flow in a way that may be intrusive and not always representative of the overall stream. 

For the semi-dry process, dry starch is fluidised in a special mixer to which are added the various reagents in dilute form. Water needs to be present to carry the chemicals into the starch granule. The process is continuous with the product being dried in line, sieved and bagged. 

Mixers of this type provide a convenient method for surface application of treatments to fillers, in order to modify their interfacial properties. Since filler is in a fluidised state and may be cold or heated, surface treatment can be introduced through an opening on the mixer lid to achieve rapid and uniform coating. 

Blends of 40 wt% stearate coated calcium carbonate filler with polypropylene (PP) pellets were prepared in a 24 litre high speed mixer run at half speed of 1200 rpm under vacuum to prevent fluidisation and also remove any moisture present. Butyl stearate (0.1 wt%) was included to minimise filler separation during extruder feeding. Mixer discharge temperature was 110 °C. Figure 13.1 shows PP granules coated with filler. 

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