Continuous mixing of solids

The most important literature on theoretical aspects of mixing solids was reviewed.

Only when the mixed materials showed no segregation it was possible to analyse the mixing process quantitatively. In this case the mixture could be described by the 'χ' Square test. Longitudinal mixing could be studied by estimating the spread in residence times.

In two common types of mixers the mixing was studied of two kinds of granular materials, differing only in colour of granules. (Colour had no influence on mixing.)

In a continuously operating semi-technical 'Spaans' screw and ribbon mixer the mixing proceeded rapidly but irregularly. The minimal mixer length necessary for apparent completion of mixing decreased at higher speeds of rotation. The performance of a more widely pitched. screw and ribbon element was slightly better. The spread in residence times was appreciable. Longitudinal mixing was slightly more intensive at higher speeds of rotation, wider screw and ribbon pitches and greater mixer lengths.

In a mixer consisting of a simple, rotating, slightly inclined, partially filled, hollow cylinder mixing proceeded more gradually. Mixing was 'diffusive'. The mixing rate increased with increased rotational speed, increased wall roughness and decreasing feed rate. Longitudinal mixing proved to be very limited.