Dewatering of Rolling Mill Sludge
From A Tin Mill
HANS W. FORSTER, Product Manager
Solids Disposal Division
Beloit-Passavant Corporation
Birmingham, Alabama 35201
The chemical composition and relatively low solid concentration of sludges generated
from Tin Mills have historically been difficult to dewater to any reasonable degree.
The greatest single constituent contributing to the difficult dewatering job are the ever-
present oils and greases. These stem from hydraulic systems as well as lubricating systems
and coating systems. The remainder of solids in the sludge are ferrous compounds,
chromium, phosphate, sulphur, etc.
Dewatering of these sludges has been attempted by the well known devices, as vacuum
filters and centrifuges.
A very large plant in the Middle West has employed centrifuges for several years but
was unable to concentrate the sludge to more than 18-20 percent solids, hardly a product
which can be readily handled. Vacuum filters, likewise, will tend to dewater this material up
to a point; however, in no case was the degree of solid concentration in the resulting cake
high enough to permit recycling of the resulting cake through the roasting process.
In addition, conditioning cost of vacuum filtration and centrifuging in particular, have
been high. Polymers appear to be the only conditioning agent suitable for this dewatering
technique and the required admixture high.
Changes in solid concentration of the incoming feed present still another obstacle to
successful dewatering of this sludge. Although solid concentrations may run as high as 8
percent, they can go down to well under 4 percent.
The limitation in pressure differential and solids concentration on the vacuum filter
rules thistype of machine out; the relatively short retention time in the machine proper of a
centrifuge and the limited "G" forces produced no greater solid concentration than those
mentioned above.
Pressure filters were then investigated because of their ability to accept great changes in
sludge characteristics as well as sludge concentrations. In addition, the sludge accumulated
in the pressure filter is "captive", thus it cannot slough off.
Furthermore, the pressure differential of 225 psig results in filter cake concentrations
anywhere  from 50-60 percent solids.
As in other systems, the sludge feed for a pressure filter must, likewise, be conditioned
in order to render it filterable. In this particular case, it was found that the addition of
hydrated lime at a rate of 8 percent of dry solids in the sludge produced acceptable filtration
rates and reasonable filter cycles.
It is to be noted that the treatment plant for which the dewatering system was planned,
did not even exist at the time of testing, thus — all test work was conducted at another plant
site and was assumed that sludge conditions would be similar at the new plant site. This
assumption proved to be correct with the exception that the oil and grease content of the
sludge was somewhat higher than anticipated. This, however, did not influence — in any
way, the actual operation.
After all test data had been analyzed, design parameters determined and the plant
actually designed, the client suggested another series of tests, however, this time to use
323