To Circulate or Not to Circulate: That is the Question
This article examines these processes in a way that allows the selection of the optimum fine media mill process for a specific end use.
The circulation process is not new. I worked for a major finishes manufacturer who used the circulation process with sand mills as the media mills in the 1950s at one manufacturing site. This process was chosen because the plant lacked enough space to install the two mixing tanks that were normally installed with each sand mill to allow multiple pass processing. In this case, the comparatively slow product feed rates available with these mills resulted in inordinately long grinding cycles when compared with the same formulations produced by way of multiple pass processing at other locations. After 10 or 15 years of operation, the equipment configuration at this site was revised to add the second mixer required for multiple pass manufacture of dispersions.
Hydraulic media packing takes place in all continuous fine media mills, despite the fact that a properly designed media mill is configured to circulate the media throughout the mill and, particularly, away from the exit end of the mill against the linear flow of product from the mill inlet to the mill exit. Media mill design normally creates a minimal cross-sectional area between the mill shell and the agitator at the point where the disc or pegs are located. This area is the point of greatest linear flow of product from the inlet end of the mill to the exit end of the mill. If this product linear flow rate exceeds the rate at which the media is being pumped back against the product flow, the media will concentrate at the exit end of the mill. When the local concentration of media at this end of the mill reaches about 94%, the media interlocks into a nonflowable configuration and stops moving. Succeeding interlocked media layers will build up in the mill with further operation until this static media bed covers the mill screen or rotary gap separator. Mill operating pressures will rise with increasing speed as the separator is blocked and the increased concentration of media also acts as a brake and increases the mill power requirement. This nonflowable media configuration can also occur at the shell wall without blocking the media separator and will eliminate the flow of product past the shell cooling surface at the exit end of the mill. In this case, the symptom of hydraulic media packing is the rapid and sudden rise in the product exit temperature.