Mixing Free-Flowing Powders

The mixing of free-flowing powders is a necessity in many industries. An effective mixing geometry is the starting point for many of these processes. Some geometries mix well; others do not.

To judge mixing effectiveness, the number of revolutions to mix (NRM) becomes important. The NRM is obtained by multiplying the processing time by the rotational speed. No magical formula to calculate NRM exists because equipment, batch volumes and feeding methods vary so much from plant to plant. Therefore, NRM must be determined from process data and typically ranges from 5 to 3,000 in total for effective mixing.

Two equipment geometries that provide effective mixing of powders are the helical-ribbon blender and the slant-cone tumbler. Both provide the 3-D motion necessary for effective mixing.

The slant-cone tumbler is a version of the more common double cone mixer. The helical-ribbon blender is touted as the better of the two, but this could be a matter of operation. Any equipment used improperly will perform poorly.

Another geometry is the V mixer. A straight V mixer without modification does not provide good cross mixing between the two chambers that make up the V. However, baffles can be added to improve mixing by causing cross flows between the two chambers.

Plants can take a number of steps to help minimize powder,"powder mixing problems. One process guideline is to feed the mixer in an appropriate manner. Minor ingredients should not be added first to a completely empty vessel. This precaution prevents the material from getting into the nooks and crannies of the vessel, which are difficult to reach by mixing. Different feeds should be layered in to avoid this problem.

Mixing during the addition of material is always a good idea if it can be executed because it prevents stratification. Tumbler mixers should be filled to only 45 percent to 55 percent by volume to allow mixing of the material. Tumblers will not mix at all if filled to 100 percent.

Agglomeration, demixing and static cling are processes that oppose mixing. Agglomeration prevents the spread of materials.

Demixing can occur through a number of different mechanisms. Differences in size can lead to demixing ," smaller particles sieve through larger particles. In a box of cereal, fines go to the bottom of the box by this mechanism.

Fines with very large particles might require the addition of a binder to keep the material mixed. Static cling can prevent material from flowing into the process to begin with and can cause dust explosions. By changing the material of construction, adding a liner or increasing the humidity, plants could minimize static cling.

Two true stories come to mind in the powder,"powder mixing area. One involves two competing pharmaceutical companies. One company knew about the NRM concept. It validated its powder," powder mixing step with a half-hour mix time ," i.e., NRM of its equipment and batch volume divided by the rotational speed. The company's competitor, which was not familiar with the NRM concept, cycled its powder,"powder mixing step over 18 hours. Many mixing and demixing cycles occurred during that time. Unfortunately, whether the company's mix/demix cycle lengths remain constant during processing could not be guaranteed. As a result, the batch might be in an unmixed state after 18 hours, creating bad product from time to time.

The other story involves demixing upon emptying and transport. In this case, a small-sized active material was added to large chips. The material was mixed well in a helical-ribbon trough blender. Unfortunately, the material demixed as it was conveyed pneumatically to a bagging plant. The much smaller particles sieved through the much larger particles.

The problem was discovered much later when the bags were sampled again, and no active material was found on the chips. To prevent this from happening in the future, a binder was added during the mixing step, gluing the particles together.

Sometimes, powder,"powder mixing becomes really difficult using mixing equipment. For example, 0.05 percent or less active material in a large batch might never get mixed in uniformly. In these cases, mixing could be performed in stages. Another option is to dissolve the active material in liquid, add in the carrier solid and spray dry.

The mixing of free-flowing powders can be very tricky. However, the guidelines above should help. If you have an interesting case history on this subject, write to me at [email protected].

Tatterson is a technical editor for Chemical Processing. He is a professor at North Carolina A&T State University in Greensboro. Contact him at gb[email protected]. He also teaches short courses for the Center for Professional Advancement, www.cfpa.com.