chromium reducing in clinker

PCA R&D Serial No. 2983

Hexavalent Chromium in Cement
Manufacturing: Literature Review by Linda Hills and Vagn C. Johansen

©Portland Cement Association 2007
All rights reserved

KEYWORDS
ACD, alkali, allergic contact dermatitis, cement, chromate eczema, chrome, chromium, clinker,
Cr (VI), Cr (III), ferrous sulfate, finish mill, health, hexavalent, kiln atmosphere, manganese sulfate, Occupational Safety & Health Administration, OSHA, oxidation, portland cement, raw materials, reducing agent, refractory brick, solubility, stability, stannous chloride, stannous sulfate, storage period, trivalent.

ABSTRACT
With regard to chromium and health and safety aspects, the water-soluble compounds of chromium in cement are most relevant, specifically compounds of the form Cr (VI).
Chromium in the cement can originate from: 1) raw materials or fuel, 2) magnesia-chrome kiln refractory brick, if used, 3) wear metal from raw mill grinding process, if chromium alloys are used, and 4) additions such as gypsum, pozzolans, ground granulated blast furnace slag, mineral components, and cement kiln dust.
The cement process, specifically kiln conditions, can influence how much Cr (VI) will form. In the kiln, oxidizing atmosphere will play the largest role, with more oxygen in the burning zone leading to increased Cr (VI) formation. Alkali concentration is also of importance, since Cr (VI) in clinker is primarily in the form of chromates. In the finish mill, thermodynamically favorable conditions for oxidation to Cr (VI) exists, including high air sweep, moisture from gypsum dehydration, cooling water injection, and grinding aids, along with the high pH of the cement.
Several materials have been used to reduce the level of soluble Cr (VI) formation. The most widely used material is ferrous sulfate; other materials include stannous sulfate, manganese sulfate, and stannous chloride. Some of these materia2ls have limitations such as limited stability,

References: Directive 2003/53/EC was implemented January 2005 and is binding in the UK and other EU member states (European Parliament 2003) (Mishulovich 1995) although during cement hydration, these forms disproportionate to Cr (III) and Cr (VI) (Johansen 1972) *Affiliated Consultants, CTLGroup, 5400 Old Orchard Road, Skokie, IL 60077 USA [phone], www.ctlgroup.com sulfate, chromic chloride, and chromic potassium sulfate (APCA 1998). Compounds with Cr (III) are stable, and therefore the form found in quarried materials, and most dichromate, zinc chromate, calcium chromate, lead chromate, barium chromate, and strontium chromate (APCA 1998) oxidizers and unstable (Mishulovich 1995). It’s solubility in water is related to reported health risks, as described further below. form the allergen that causes sensitivity to certain people (Chandelle 2003). This allergic problem only occurs in certain individuals who are particularly sensitive; once sensitization is Most quarried raw materials contain no water soluble chromium as Cr (VI), and chromium is usually in oxidation state Cr (III) (ATILH 2003) dust are reported in the range of about 0.5 ppm and 0.3 ppm respectively (ATILH 2003).