Designing a Magnesium Hydroxide Slurry Mixing Storage System:
There are numerous advantages for using Magnesium Hydroxide rather than caustic soda or slaked lime for various applications. At this juncture, we'll assume that your chemical supplier has already provided you with the details for using Magnesium Hydroxide for your specific processing needs where the scope of this article will focus primarily on the mixing application optimums for storage along with the equipment required.
Magnesium Hydroxide can be prepared using labor intensive 80# bags or an automated system using super sacks of Magnesium Hydroxide powders, with the addition of water, but in most cases Magnesium Hydroxide is typically delivered pre-mixed by tanker truck. The slurry is then pumped into your on-site storage tanks for use with your specific waste treatment processing. Delivery costs vary, where facility storage capacity can result in a significant cost savings. Preparation from Magnesium Oxide can be a volatile exothermic reaction that requires significantly more power per unit volume for make-up as compared to simple storage so this is generally not an option. In short, most do not want to get involved with Magnesium Oxide special processing requirements and prefer the stable Magnesium Hydroxide pre-mixed slurry.
Storage Tanks: Carbon Steel, fiberglass and poly tanks are preferred for storage of magnesium hydroxide, where aluminum is not recommended. Storage tanks should be installed as close to the application to prevent potential plugging. Vertical tanks, with height to diameter ratios of 1.0 to 1.2 are preferred over horizontal tanks since they take up less space, are easily supported on a concrete slab. Horizontal tanks also generate flow patterns that result in quiescent or dead mixed zones causing areas of solids assimilation, which is not recommended for slurries. A rule of thumb is to have a tank capacity of 1.5 times the required amount to accommodate the contents of the tank car or truck, plus rinse water. Tanks having mixers installed vertical-on-tank-centerline should have anti-swirl baffles installed 120 degrees apart, 1/16 to 1/12 of the tank diameter, with either full length (straight-side) baffles or 1' above the floor bottom. An angular offset mounting arrangement, without the need for tank anti-swirl baffles is also quite popular for use with poly tank designs. If the tank is installed within a building that is heated above 4o centigrade, no special insulation is required. Below these temperatures provisions for both insulation and heating (steam infusion or electrical tape) should be considered. For high ambient temperatures, to prevent evaporative losses, misting nozzles should be considered.
For economical reasons, it is therefore quite common to see as many as three (3) magnesium hydroxide storage tanks on site to handle either a full or partial tanker load at one time. To accommodate the limits of a portable mixer design, 2,500 gallon designs are quite common. Some also start with one tank, and then economically add more storage tanks over time. There are economical advantages to 4,000 gallon tank designs.
From a mixing sense, Magnesium Hydroxide is considered a special slurry application in that over time, if the slurry is left unmixed, it will dewater, where the solids phase will become a thick viscous mass. Unless specifically designed for these conditions, start-up of a mixer under these conditions is not recommended. Overload or mixer damage may result under these conditions. The viscous mass analogy is somewhat similar to pouring ketchup from a new bottle. Ketchup is shear-thinning, were once it is shaken or mixed, it's viscosity lowers significantly, where it then becomes pour-able. After weeks of sitting motionless, some say you could almost walk on top of the magnesium hydroxide mass (just to get an idea of how thick it can become).
Once mixed, even at high concentrations, magnesium hydroxide acts and flows like a low viscosity water-like fluid. The good news is that in most instances, it can take weeks for this thick viscous mass condition to develop, where a typical maintenance shut down of a couple days typically wouldn't generally influence the slurry. It is for this reason that timer systems, such as hand-off-auto switches are installed or are included with these mixing systems. If set on auto, the mixers operate for a specified period each day, for example, followed by a substantial timed shut down period. The switching device also allows you to completely shut down the mixer or to continuously operate the mixer. The later can be useful upon receipt of a new shipment, where a dried bottom layer would then be eroded and resuspended into the new batch. Long shutdowns should be avoided, but if unavoidable, sparging with air around the impeller is recommended prior to restarting the mixer. Failure to loosen the mass around the impeller may result in an inappropriate power surge and/or damage to the mixer resulting in premature failure. Continuous agitation for 6 to 8 hours is recommended after extended shutdown periods.
As for the mixer design, the use of an upper impeller is definitely recommended. In the event of dewatering, the upper impellers purpose will be to capture and entrain the dewatered top phase and pump the water phase down into to lower regions of the storage tank to quickly achieve a uniform slurry. Without the upper impeller, the lower viscosity water phase may lie on the surface and take an infinite amount of time to reincorporate into the batch.