70    REMOVAL OF CHROMATE FROM SALT CAKE
Wei-chi Ying, Scientist
Robert L. Zeller III, Senior Research Scientist
Russell Morgan, Manager
Occidental Chemical Corporation
Grand Island, New York 14072
Dianliang Xu, Associate Professor
Shanghai Jiao Tong University
People's Republic of China
INTRODUCTION
Occidental Chemical Corporation operates the largest facility for production of chromium chemicals in the Western hemisphere and is one of the largest producers in the world. The plant is located at
Castle Hayne, North Carolina and produces chromic acid, sodium bichromate, and sodium sulfate
(salt cake). The Castle Hayne plant became operational in 1971 and represented a significant technological advancement in chromium chemicals manufacture (Figure 1) and is still at the leading edge of
that technology. Chromite ore, sodium carbonate, and a small amount of lime, are blended and
roasted in a kiln at temperatures exceeding 1200°C to oxidize the Cr(III) in the ore to Cr(VL). The kiln
end Product is quenched and leached to form raw sodium chromate liquor. The raw liquor contains
soluble iron, aluminum, and other components from the ore, and is purified through a number of pH
adjustment and filtration steps. The purified sodium chromate liquor is reacted with recycled sodium
bisulfate to form sodium bichromate and salt cake. Evaporation of the sodium bichromate results in
the precipitation of salt cake crystals, which are washed, dried, and packaged. The salt cake crystals
are used primarily by paper manufacturers in the kraft pulping process. The concentrated bichromate
liquor produced can be sold as product or processed further to generated sodium dichromate crystals.
However, the majority of the concentrated sodium bichromate solution is reacted with sulfuric acid to
form chromic acid crystals and sodium bisulfate. The chromic acid crystals are separated by filtration, melted, and then flaked to produce a purified product. The filtrate from this process, primarily
comprised of sodium bisulfate, is recycled for reaction with sodium chromate.
Salt cake typically contains 500 to 700 ppm Cr(VI) as Na2Cr04 Chromate removal is essential for
increasing the marketability and value of this product. A variety of chromate removal technologies,
including reduction/precipitation and separation of chromate, have been investigated. This paper
Filtrate (NaHSO,)
Ore
J-
Sodium
Chromate
H2SO,
Sodium
Bichromate
Salt Cake
Chromic
Acid
Cr Removal
Purified
Salt Cake
Solution
Figure 1.    Process flow diagram for production of chromium chemicals.
47th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118
Printed in U.S.A.
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