Dewatering of Metal Hydroxides
WILLIAM L. SCHWOYER, Research Associate
LIONEL B. LUTTINGER, Senior Research Chemist
The Permutit Research and Development Center
Princeton, New Jersey
INTRODUCTION
Industrial processes rarely generate pure metal hydroxides as such. An exception, acid
mine drainage, which might be classified as a natural phenomenon, constitutes a large
volume of dilute, pure waste in the form of iron hydroxide. Russellman (1) mentions that
there are 3600 clarification water treatment plants alone in the U.S. All of these plants use
either iron salts or alum to coagulate the turbidity of surface waters, in order to render them
suitable for drinking purposes. Materials such as zinc chloride are used as lubricants for
spinning synthetic fibers or catalysts in roasting certain cellulosic materials and
automatically find their way into waste streams as suspensions of the hydroxides. More and
more industrial processes are utilizing the coagulating ability of the hydroxides of iron,
aluminum, zinc, and magnesium in order to treat the organic wastes resulting from various
processes. Such uses result in complex hydroxide-containing sludges. Therefore, the total
sum of the many combinations of hydroxides with crystalline, oily or organic wastes as
generated in various industrial applications, results in probably the largest single category
of sludges today, with the possible exception of waste activated type sludge.
Waste activated sludges, while varying to a degree, do not have the diverse composition
of the hydroxide sludges, and therefore, are comparatively simple to treat. Hydroxide
sludges, which can vary from the pure hydroxides to a hydroxide/oil mass or
hydroxide/crystalline semi-solids, require a variety of conditioning methods in order to
effect the liquid-solids separation required for the successful treatment of a waste stream.
The method of treatment will depend not only upon the complexity of the sludge, but also
upon the hydroxide present. Several investigations indicate that magnesium hydroxide
renders water softening sludges difficult to dewater. Magnesium hydroxide does represent
one of the more difficult species to treat.
Because of the diverse nature of hydroxide sludges, continuous mechanical gravity
dewatering has been found to be among the most effective methods for the liquid-solids
separation process. The driving force for the separation of the liquid and solid phases, in the
gravity dewatering system is the weight of the water itself. The device used for the tests
discussed in this paper, The Dual Cell Gravity Solids Concentrator, employs a maximum 4"
hydrostatic head for the separation.
Continuous mechanical gravity dewatering depends upon the rapid drainage of the
water through the agglomerated solids in suspension. In order to accomplish such drainage,
conditioning of the sludge, generally by use of a polyelectrolyte, is required. The
polyelectrolyte flocculates the solids into larger particles in a clear liquid phase. These
particles have sufficient integrity to withstand the mechanical handling and are large
enough in size to form a porous bed through which the water can drain. This can be better
understood from Figure 1, which is a cut-away view of the Dual Cell Gravity Solids
Concentrator.
The dual cell unit derives its name from the two cells formed by a continuous
monofilament nylon filter belt which is run over two sets of rims, divided by a drive
sprocket. In operation, the conditioned sludge enters the right hand or dewatering cell,
where the solids rapidly settle on the screen and the water drains through. The partially
dewatered solids are then carried over the dividing hump into the left or cake forming cell,
1120