Basic Chrome Sulfate

The following information has been modified from an piece written by CW Harland (Farmers Feriliser Ltd) and revised January 1996 by Jenny Simpson. The full unedited piece is available online at: http://www.nzic.org.nz/ChemProcesses/production/1I.pdf

CHROMIUM SULFATE TANNING POWDER
Chromium sulfate is the most important ingredient used in the tanning of leather. It is manufactured from the simple reduction of Cr (VI) to Cr (III) by the addition of an excess of sulphur dioxide:

This reaction is carried out in a steam-heated vessel, and then the excess SO2 is removed in a second reaction tower. Finally, the liquid product is spray-dried to form a powder which is then bagged and sold.

THE MANUFACTURING PROCESS

The production of basic chrome sulfate is based on the chemical reaction in which hexavalent chromium (ie. +6 oxidation state) in the form of sodium dichromate, is reduced to the trivalent state (+3) by sulfur dioxide.

Step 1 - Reduction of hexavalent chromium Anhydrous sodium dichromate powder is reduced by `burner gas' plant. Burner gas contains about 9% sulfur dioxide. The sodium dichromate is dissolved in water to make a strong solution which is pumped continuously over reaction towers filled with ceramic packings where it reacts with the sulfur dioxide bearing gas. Although the reaction is exothermic, steam injection is required to maintain the temperature and hence the reaction rate. The product is 100% reduced to the +3 oxidation state and has a basicity of 33%. The terminology is often confusing as this product is referred to as being 33% reduced.

This compound is still freely water soluble. The sodium sulfate shown on the right hand side of the equation is part of the product and is normally not separated, and merely remains as an inert diluent.

An excess of sulfur dioxide is required to ensure the chromium is fully reduced to the +3 oxidation state. A second reaction tower is used to remove the residual sulfur dioxide.

As well as the 33% basicity products, 42% and 50% basicity products are manufactured by the addition of soda ash solution. These products are `masked' by the addition of sodium formate, which alters the characteristics of the tanned leather.

Step 2 - Drying the product
The viscous green liquid produced by the foregoing process is converted to a dry powder by means of a spray drier.

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