4. Conclusions
Difference in bulk density between the salt and food powders displayed a very big influence on the mixture quality (MQ) of the binary mixes. Very good MQ was achieved for bulk density ratios less than 3.5. However, poor MQ with visually observed segregation was obtained for mixes with bulk density ratios greater than 6. This was most likely due to the low bulk densities of the non-salt powders, that is, thyme and oregano, although particle size and differences in particle density may also be contributing. The low bulk density of these powders was not due to unusually low particle densities, but due to unusually large porosity or voidage between the particles, which was mostly likely caused by the irregular shapes of the thyme and oregano. Consequently, differences in particle shape can influence MQ. The large voidage allowed the more regularly shaped salt particles to move downwards through the voids and this created a strong segregation mechanism that counteracted the mixing mechanism in the paddle mixer.
Differences in particle size had an effect on MQ but it was not as large as initially expected. Size ratios above somewhere in the region of 5 led to poorer MQ but these were just at the limit of satisfactory MQ with a CoV of 0.06, even for the extreme size ratio of nearly 16. This may in-part be due to finer particles coating the larger particles due to their cohesiveness, thus counteracting the size segregation tendency. Overall, in this study, the differences in bulk density and shape had a much stronger influence on MQ than differences in particle size and particle density. Of course, dry powder mixing is a complex operation and there are many factors that come together to determine the MQ of a mix, including mixer type and operation along with the many powder properties highlighted in this study such as bulk and particle densities, particle size and shape and cohesiveness.