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 advantages 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 both 7,500
& 5,600 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 relatively 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. 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 significant amount of time
to reincorporate into the batch, causing undo strain upon the mixer design.
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