Ask an Expert: How Do I Efficiently Mix Powders Into Liquids?

Most tablet coatings are solid suspensions or solutions, which may contain various ingredients such as natural or synthetic resins, polymers, gums, plasticizers, fillers, pigments, and flavors. Dispersing dry raw materials into liquid presents several processing challenges due to some powders' tendency to quickly form stubborn agglomerates. In addition, lightweight and dusty solids often require time-consuming clean-up in the mixing area and impact plant safety overall. Slow and careful addition of powders into a batching tank helps to reduce both lumping and dusting issues, but inevitably that lack of speed becomes impractical in large-scale production. Selecting the proper mixing equipment and implementing the appropriate processing techniques is critical in achieving the desired film properties, clarity, color, particle size distribution and viscosity. Two recent technologies can help:

Improve powder wet-out

Certain solids not only require high speed mixing but also an effective method of combining them with the liquid phase. For instance, many gums and thickeners are difficult to disperse because they resist wetting out. When mixed with a slow-speed agitator, they float stubbornly for hours on the surface of the batch. Even when processed using high-speed devices such as rotor/stator mixers and saw-tooth dispersers, some powders still take a long while to hydrate completely. Individual solid particles, instead of being wetted by the liquid, group together as agglomerates and form “fish eyes,” which

are very hard to break apart. Fish eyes are agglomerates of partially hydrated powders characterized by a tough outer layer that prevents the complete wetting of particles within the interior. Some operators are left to contend with carefully sifting and slowly adding solids into the batch. On the other hand, some formulations cannot tolerate very slow powder addition because as the batch starts to thicken, the remaining solids become even more difficult to mix into an already viscous solution. In extreme cases, solids are intentionally overdosed and the undispersed agglomerates are simply filtered out. Moreover, while extended agitation breaks lumps in the batch it can also over-shear already hydrated or dispersed particles, resulting in a permanent viscosity loss and damage to the product.

A high shear mixer with built-in powder induction capability is worth considering, preferably a design that does not rely on a separate eductor. One such technology is the ROSS Solids/Liquid Injection Manifold (SLIM), available on both batch and inline models. The SLIM features a rotor/stator assembly that draws solids into a high shear zone for rapid mixing with the liquid vehicle. 

Dry ingredients like hydroxypropyl methylcellulose (HPMC), sodium carboxymethyl cellulose (CMC), hydroxyethyl cellulose (HEC), alginates, gums, talc, titanium dioxide and fine powdered colorants are wetted out more efficiently in a SLIM mixer compared to the conventional batching method where powders are added to the top of vigorously agitated liquid.

Processing advantages of the SLIM technology:

• Simple and straightforward operation. The operator turns on the mixer and it starts inducting powders. No eductors or vacuum pumps to deal with.
• Shorter cycle times. SLIM users switching from conventional impellers and agitators reduce their overall cycle time by as much as 80%.
• Increased yield and higher quality dispersions. By dispersing agglomerates and eliminating floating powders, the SLIM maximizes the functionality of all solid raw materials. 
• Improved operator safety. The inline SLIM is usually installed at floor level, so operators need not climb up a mezzanine carrying bags of powder. A “hose & wand” attachment may be used to induct lightweight powders straight from the container without creating a dusty environment.
• Cleaner mixing. SLIM Mixers are easy to clean in place and support fast changeover. Inline models are 3A-approved.
• Flexibility. The portable inline SLIM unit can service multiple process lines virtually regardless of vessel size or shape. 

The SLIM rotor not only turns at high speeds but also generates a powerful vacuum that combines solids and liquids sub-surface – at precisely the point where intense agitation takes place. A very uniform consistency is achieved in just a few turnovers so that viscosity loss due to over-mixing is easily avoided. In addition to the short cycle time, issues like floating powders, foaming and excessive dusting are also eliminated. Particle size distribution is an important mixing parameter because large agglomerates can clog spray nozzles in the coating operation.

Consider ultra-high shear options

Ultra-high shear mixers are rotor/stator devices that feature complex geometries and extremely close tolerances, subjecting the product to thousands of intense shearing events in each pass and turning at tip speeds of over 11,000 ft/min. The level of dispersion achieved is generally superior to traditional colloid mills. In certain applications including tablet coatings that require aggressive shear to reduce the size of agglomerates, ultra-high shear mixers present as potential alternatives to expensive high pressure homogenizers because they deliver comparable size reduction at much higher flowrates and require simpler maintenance. The ROSS X-Series ultra-high shear mixer, for instance, is easier to clean and disinfect in place. Based on user experiences, the shorter cleaning time equates not only to a faster changeover procedure but also to longer intervals between cleaning cycles (longer production runs). In addition, a comparably-sized X-Series costs less than a high pressure homogenizer while being less sensitive to clogging and changes in viscosity.