Analysis of Dry Powder Mixing Processes Using a Fine Cohesive Pigment

A novel extension to a previous method has been proposed to evaluate the nature of dry powder mixing processes using a mixing sensitive colouring agent. The method proposed measures the change in the colour (hue) and colour intensity of the powder blends over mixing time. To evaluate the extent of fine particle dispersion and de-agglomeration, cohesive and free flowing lactose (white) and cohesive micronized iron oxide (dark red) were blended as a bulk and tracer respectively. Measuring the change in hue and intensity of micronized iron oxide in a lactose bulk enables the analysis of two distinct mixing behaviours, namely the deagglomeration of the pigment (transition from red to orange) and the dispersion of a given sized aggregate through the bulk material (increase in the intensity of the blend’s hue). Blends were manufactured using two different types of powder mixers at a range of processing conditions. Samples were withdrawn from the mixers at regular time intervals and the hue and intensity analysed using a colourimeter. Hue values were assembled to create a series of process specific curves. The colourimetry data was able to clearly separate and group the mixers into low and high levels of shear and/or mixing intensity over a range of conditions, and suggests that the gradual dispersion and de-agglomeration can be monitored independently using this method. These initial results suggest that the iron oxide tracer method could provide the basis for a novel quantitative approach to producing equivalent powder blends as a function of mixer type, scale and operating conditions.