87    STEEL MILL SCALE DE-OILING: A NEW WASTE
MINIMIZATION PROCESS
Natalie R. Blake, Senior Engineer
Kristine S. Siefert, Senior Research Chemist
Nalco Chemical Company
Naperville, Illinois 60566
INTRODUCTION
In 1988, our Waste Minimization Marketing identified a new market for our products in steel mill
scale de-oiling. There appears to be no existing process in the U.S. for chemically cleaning mill scale,
though solvent extraction and steam stripping have been proposed as alternate technologies. The use
of high shear mechanical treatment, combined with non-volatile surface-active chemical treatment
represents a new process for cleaning mill scale. A patent application has been submitted for this
unique technology.
In high temperature steel-making operations, such as in the hot strip mill, the hot surface of steel
sheets, bars, blooms, and slabs becomes oxidized, is cooled, and cleaned with high pressure water
spray.1 A layer of iron oxides is continually being formed on the surface of the steel during exposure
to air at elevated temperature. This layer is referred to as "mill scale." The scale is composed primarily
of oxides of iron, FeO, Fe203 and Fe304. Scale must be removed from the steel to insure satisfactory
results in downstream processes involving deformation of the material, such as rolling or drawing.
As hot steel is processed, the layer of mill scale is broken away and replaced by a fresh layer of iron
oxides. The formation of new scale takes place every time the shape or size of the hot steel is changed.
As shown in Figure I, the scale is removed from the steel with high pressure water, and the solids and
water are collected in a sewer system. As the result of leaks from equipment hydraulics, direct use of
oils in some milling operations, and accidental dumping of oils and lubricants, mill process waste
water can contain a high level of dispersed oil which also contaminates the mill scale surfaces. Waste
water flows into one of a number of pits where floating oil is removed by skimming, and solids can
settle to the bottom. These mill scale solids, coated with oil, are periodically dredged or continuously
scraped from the pits.
A Midwestern steel mill produces approximately 500 tons per day of mill scale from its normal
steelmaking operations. Currently the mill uses only a small fraction (less than 10%) of the less oily
scale in its sinter plant. The remaining, larger fraction of high oil content scale is not used because its
use in sintering would result in unacceptable hydrocarbon emissions and fouling of the filter bags and
fans in the baghouse. Iron ore is currently being purchased to replace unusable mill scale as an iron
source. The steel industry as a whole is faced with increasing amounts of mill scale that cannot be used
in sintering operations, resulting in both higher raw materials costs and higher disposal costs. This
mill previously used a patented kiln process which vaporized the oil, but the resulting hydrocarbon
emissions exceeded allowable air quality levels, and the kiln process was later discontinued.
EXPERIMENTAL
Process Requirements
It was difficult to define the Midwestern steel mill's requirements with regard to final oil content of
the treated mill scale. Even mill personnel were not sure what level of oil would allow use of a larger
fraction of the mill scale without causing problems in sinter plant operation. In addition, early
problems were encountered with differences between our oil analysis methodology and the mill's
methodology. A final oil content of 0.2-0.3 wt% was eventually chosen as the target, as determined
by solvent extraction.
The de-oiling process should be capable of dealing with a range of oil concentrations, scale composition, and different particle size distributions, since mill scale is quite heterogeneous in composition.
45th Purdue Industrial Waste Conference Proceedings, © 1991 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
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