Mechanical Agitation of Slurries

Mechanical Agitation of Slurries

Martin Marietta Magnesia Specialties manufactures a high-purity magnesium hydroxide slurry which is used in a wide-variety of applications. With proper storage and handling, utilizing a magnesium hydroxide slurry in your process can be effortless and trouble-free.

A slurry must be agitated to prevent sedimentation on the bottom of the storage tank. Mechanical agitation is the preferred method of agitation to guarantee a homogenous product and prevent slurry from settling on the bottom of the tank. An axial flow pattern will push the slurry towards the bottom of the tank and sweep the bottom of solids.

Vertical slurry tanks are recommended and usually have a height to width ratio greater than or equal to one. Ideally a tank will be sized to store approximately one month’s worth of slurry. Steel tanks are most common, but HDPE and fiberglass tanks are also acceptable, provided they are designed for a specific gravity of 1.52 and can support the weight of a mechanical agitator. Dish bottom tanks are ideal for agitation but cost significantly more than the acceptable flat-bottom design. Slurry must never be diluted in storage. Freeze protection must be provided to keep the slurry above 40°F.

Gentle mixing is recommended. Putting the mixer on a timer for 30 minutes every two to three hours is recommended to avoid heat buildup in the tank and prevent evaporation of water from the slurry. Mixers are typically vertical top-entry mixers, with an impeller diameter 30% to 45% of the tank diameter. Mixers may have one or two sets of impellers, and the bottom impeller is typically <30% the depth of the tank. Keep in mind that the tank should be sized so that the slurry is never drawn below the level of the bottom impeller. Baffles are placed around the walls of the tank to prevent the formation of a vortex.

In Slurry Systems Handbook, Baha Abulnaga writes “The science of mixing and keeping solids in suspension is highly empirical. The engineer should take into account existing similar installations” (Abulnaga, 2002). Martin Marietta Magnesia Specialties Application Engineers can work with you and your manufacturer’s representative to design the ideal system for your application.

• Abulnaga, B. (2002). Slurry Systems Handbook. New York: McGraw-Hill.