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Removal of hex chrome in solid state

I work for a company that develops Chromic oxide (Cr2O3) powders to be plasma spray coated on metal surface. The Chromic oxide powder generally contains some amount of hex chrome (~100 ppm). Although most of the questions on this site refer to the removal of Cr(IV) dissolved in a solution, I was wondering if there is a way to remove/reduce the hex chrome using a solid state reaction? If yes, what are the potential technical and commercial limitations of the process?

Lallan Pandey
engineer - Stony Brook, New York
2007

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2007

Thermal decomposition may work, reducing chromate via evolution of O2 gas. Chromic acid decomposes to Cr2O3 + O2(g) by heating to 250 °C in vacuum or ~500 °C in air.
Alkali metal impurities may necessitate higher temperatures and either vacuum or a hydrogen atmosphere.

The decomposition of CrO3 occurs with considerable splattering, as does desorption of moisture upon rapidly heating, so use a trap or or heat slowly to protect your vacuum system.

The plasma coating process probably creates some hex. Cr; a thermal anneal may help.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.

Just guessing. Maybe you could "wash" your powder mixture to dissolve the hex chrome compounds, then separate the clean powder from the liquor and treat it in a conventional way.

Guillermo Marrufo
Monterrey, NL, Mexico
2007

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Dear Ken and Guillermo,

Thank you both for your valuable suggestions.

From whatever little knowledge I gathered from the available literature, I also had very similar ideas and was currently trying both of these methods. Ken's suggestions of temperature and heat-treatment environment are particularly helpful as these are not easily available (I am not sure if some people routinely use these heat-treatments).

I will get back for a discussion after getting the results from these experiments.

Best regards,

Lallan Pandey
- Stony Brook, New York
2007

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