Reducing hex to tri in solution is usually done by adding sulfuric acid to lower the pH to 2 to 3 and then using sulfur dioxide gas or sodium bisulfite to reduce the hex to tri. Raising pH by adding caustic causes the tri chrome to precipitate. Other processes include use of ferrous sulfate and iron (some report that this process can be done under alkaline conditions) and the use of sacrifical iron anodes at neutral pH. The big drawback is the amount of iron hydroxide sludge that is formed.
But your problem is more mysterious. Why is a fiberglass producer generating hex chrome "in slag"? Chrome waste is not a routine waste from the fiberglass production process. Is he hard chrome plating his spinner heads onsite? If so, why is the chrome ending up in "slag." Slag implies a furnace or metal melt process. For for his spinner heads, can he send them offsite for plating? This transfers the waste generation offsite to someone who is set up to handle it. This is the most common way to get hard chrome plating out of the shop. Alternatively, there are replacements for hard chrome on the market that may work but it will be an R&D effort for your client.
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