SCAI,E-UP CONSIDERATIONS IN COSMETIC MANUFACTURE 267 the pilot plant experiments. In general, if viscous or other psuedo-plastic materials are used, gear drive portable mixers are commonly used up to 3 horsepower. If slower speeds are required, slow speed turbine type equip- ment is utilized. Above 3 horsepower, turbine units are normally used. SOLID SUSPENSION In giving process requirements for suspension operations, it is desirable to distinguish between the characteristics of the solids, whether they have a finite settling velocity or not. It is also desirable to distinguish between a suspension of solids and a requirement for breaking up agglomerates of solids. If the solids are difficult to wet out, this should also be noted. In the discussion below, three separate categories are treated: 1. Suspension of particles of essentially zero settling velocity. 2. Suspension of solids with a finite settling velocity. 3. Dispersion of solids and agglomerates. Essentially Zero Settling Felocity In these processes, the major requirement is to produce a uniform dis- persion throughout the entire medium. For batch operation, it is usually a matter of determining the time required to obtain a uniform suspension. [f the solids are difficult to pull in from the surface, an impeller is often placed near the surface to aid in this operation. However, this also induces air into the system. Once uniformity is achieved, there is no further ad- vantage in keeping the mixer on. Propeller type mixers come in sizes up to 3 horsepower and can handle viscosities up to 5000 cps. in tanks up to 500 gallons, and tanks up to 500(3 gallons in volume with waterAike fluids. Normally gear drive mixers at speeds from. 280 to 420 r.p.m. are used in these operations. Tank propor- tions of liquid depth equal to tank diameter are used, but other proportions may be used if required. If the power requirement is above 3 horsepower, turbine units are nor- mally used. These are equipped with either fan turbines or fiat blade tur- bines. Finite Settling Felocities In this case, solids are picked up in suspension rapidly but may settle out if the mixer is turned off. The degree of uniformity produced is a func- tion of the power applied to the system, but if the settling velocities of any of the particles are above 1 f.p.m., there may be gradients in concentration throughout the tank unless relatively high power levels are used. If the settling veolicities are below 1 f.p.m., the solids will be essentially uniform throughout the entire tank. As in the case above, gear drive propeller units are commonly used in

268 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS applications requiring 3 horsepower or less, while turbine units are used for the more difficult applications. One problem which often occurs is to produce a uniform suspension in the tank and maintain this suspension while material is drawn off to a filling or packaging machine. Sometimes this can be done by putting dual ira- pellets in the system and placing the lower impeller very near the bottom of the tank. Solids Dispersion In case there are agglomerates to be broken up, it is desirable to use higher shear rates. In these cases, direct drive 1750 or 1150 r.p.m. propeller type units are used. If additional dispersion is required, units operating in the 3000 to 4000 r.p.m. range may be employed with an impeller designed for high speed operation. These high speed shear rates will break down ag- glomerates in the system. It is essential in describing the mixing operation to state whether agglomerates are to be dispersed or whether it is primarily a matter of suspending agglom- erate particles. Alir Enlrainment There are times when the en- trainment of air in the batch is of particular concern. In case it is desired to keep the degree of air entrainment to a mini- mum, at least one impeller di- ameter of coverage should be provided on the upper impeller. If the •ank is equipped with wall baffles, baffles which ex- tend across the surGce are also often helpful. Figure 6 illus- trates the arrangement of these baffles. In case there is a variable liquid level, it is sometimes Figure 6.--Arrangement of baffles across the surface to prevent air entrainment. desirable to use multispeed units, or to arrange for additional bail]es at the lower liquid level. Cry slal Characteristics If the particles are sensitive to fluid shear, then the fluid shear component in the system must be carefully controlled. It is usually desirable to in- vestigate the effect of fluid shear on crystal growth on small laboratory