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Filter cake is hex chrome not trivalent

we are a materials recovery firm that handles ni/cr ECM filter cake for a client (65% water, 10%salts, 25% metal containing solids by weight. We would like to dry the material to reduce bulk and dead weight freight (water). but the chrome fraction presents as hex chrome. is their an effective/practical way to convert hex cr to tri cr? Thanks for your help.

Robert Raizk
business owner - Wilmington Ohio U.S.
March 24, 2011

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You might try combining the wet cake with ferrous sulphate ferrous sulphate [affil links], allowing it to stand for a few hours, and then drying.

Dave Wichern
Consultant - The Bronx, New York
March 28, 2011

Dave, thanks for your response. Any idea how much ferrous sulphate ferrous sulphate [affil links] per ton? Source, cost of ferrous sulphate ferrous sulphate [affil links]? Thanks.

Robert Raizk
- Wilmington, Ohio, USA
April 7, 2011

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Unfortunately, it takes a large amount of ferrous sulphate ferrous sulphate [affil links] compared to hexavalent chromium, and a long, long time at neutral or alkaline pH for ferrous sulphate ferrous sulphate [affil links] to work.

The best solution may be to work with the generator to have them reduce the hex chromium to trivalent before flter pressing. This is usually done with sodium metabisulfite at a pH below 3. Another possibility is to use alkaline sodium sulfide or an organic sulfide at neutral pH. This assumes that a sulfide containing sludge is compatible with your recovery process.

Lyle Kirman
consultant - Cleveland Heights, Ohio
April 8, 2011

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sodium hydrosulfite [on eBay or Amazon] will reduce hexavalent chromium in alkaline conditions.
If considering use take care as the powder is spontaneously combustible in moist conditions.

Geoffrey Whitelaw
- Port Melbourne, Australia
April 11, 2011

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Mr. Kirman is right, though I think that limiting factor is the solubility of the ferrous ion at the likely pH of the filter cake, rather than the speed of the reduction. Fe2+ will precipitate, and unless some kind of machinery is used to grind the ferrous sulphate ferrous sulphate [affil links] together with the sludge, intimately, the reaction will not proceed to completion.

If this equipment is available, I still think ferrous sulphate ferrous sulphate [affil links] is worth a try.

Geoff beat me to it, suggesting NaS2O4 as a reductant that works at high pH levels. His caveat regarding its spontaneous flammability is important and is to be heeded. I would apply it as a saturated solution to the solid filter cake, then mix very well, then dry. Though I do not know, I doubt that the sludge/dithonite mixture will burst into flame. A well designed lab test to confirm this is essential.

Dave Wichern
Consultant - The Bronx, New York
April 12, 2011