Chromium Oxide aka Green Rouge

[grainsofgold]

Chromium Oxide aka Green Rouge is often used by jewelers. I have always heard from refiners that using this as a polishing compound can cause problems in refining gold.

Can someone who is familiar with Chromium Oxide please comment on this and detail what steps if needed are used to work around the Chromium in using AR methods of refining?

Thank you in advance-

 

[Harold_V]

One of my customers used it. His polishing wastes were green after incineration, instead of the usual plum purple with which you may be familiar.

How I dealt with it was simple. I ignored its presence. After thorough incineration, his polishing wastes were screened, then given a boil in HCl. After rinsing, the material was then processed with AR. I had no problems. I ran his material for years.

Can you be specific in describing any problems that one might expect?

Harold

 

[lazersteve]

For comparison of color here's a photo of the plum and the green after processing.

The purple in the photo appears lighter than it is in real life.

Steve

 

[grainsofgold]

Thank you for the quick reply.

Harold, I assume that with the HCL wash this got rid of the Chromium oxides. Maybe this is why refiners do not care for green rouge polished material as it requires an additional step instead of being able to go right in with the AR.

Maybe someone can explain Chemistry wise what all goes on in relation to the chromium.

Thank you-

 

[goldsilverpro]

According to the CRC Handbook, the green chromium oxide, Cr2O3, is not soluble in acids or alkalies. Therefore, it won't dissolve in HCl.

 

[4metals]

I've rarely refined prepared sweeps, we just assayed and shipped to a smelter when we had enough for a production run. I have however assayed a lot of sweeps with green rouge, and they do not fuse with normal assay flux. I had posted the formulas for green rouge in the post about building a small refinery / assay lab.

I once had a jeweler for a customer who designed a ring that was chrome plated with karat gold inlays, I guess it never sold because we got in many thousands of the rings for refining. Since the rings were all the same and he had a good idea of the gold content we didn't melt the rings we just put them up in acid to recover the gold. The metal digested and the gold precipitated like typical karat scrap so the chromium didn't interfere with the acid refining process.

 

[grainsofgold]

Thank you all for the replies.

Apprarently the green rouge traps gold and is very difficult to separate out. I know a commercial refiner on the West Coast and called him today about this. He is usally tight lipped on refining but when I asked him about this he was very open with me as he does not process it in house. His method was to incenerate the green rouge material , sample it for PGMs and sell it off to someone that could actually spearate it out which is a difficult thing to do.

Harold, I do not doubt for a second that you were able to recover Gold out of the green rouge material, but perhaps there was some left unclaimed in the spent material.

Maybe someone smarter than I can comment on this.

Thanks again for the insights.

 

[Harold_V]

Harold, I do not doubt for a second that you were able to recover Gold out of the green rouge material, but perhaps there was some left unclaimed in the spent material.

To be clear, there is always something left over when extracting values from polishing wastes by the AR procedure. The point is, I got excellent results, pound for pound. After running these wastes for years, you get somewhat familiar with what you can expect. The results from these lots were very much in keeping with the results from other lots.

A practical way to look at this matter is the cost of extraction versus the values left behind. If it costs $100 to recover $50 worth of metals, seems to me the wise solution is to take what comes easy and fast and be done with the job.

I'm not convinced that the presence of the chromium is troublesome when processing with AR.

So there is no confusion, all of the polishing wastes I processed, with the exception of that which came from the bench of a silversmith, were given the preliminary wash in HCl. That serves two purposes. One of them is to remove base metals, with the other being to eliminate substances that make filtration difficult for the resulting solution of values. It had nothing to do with the nature of the polishing compound. As I said, I simply ignored the material. Seemed to work just fine.

If the material is not dissolved in acids, I'm having a bit of trouble understanding how it could complicate recovery. I fully understand how it might be troublesome when melting.

Harold

 

[4metals]

Perhaps the green rouge traps the gold very much like the carborundum in grindings traps the gold. If you try and melt grindings it is very difficult to flux the gold out of the carborundum just like it is difficult to flux the chromium out of the green rouge assay.

But the carborundum doesn't adversely effect recovery of the gold in acid, just as the chromium doesn't effect the gold recovery in sweeps. The possible exception is if the chromium completely encrusts the gold particles (or some of them) causing behavior like silver chloride encrusting karat gold. As GSP has pointed out the chromium oxides are insoluble in Harolds pretreatment with HCl so if they are encrusting the gold it will not digest. It is my guess that the lions share of the gold is recovered by Harolds method.

 

[Lou]

Heating it with a concentrated lye solution will handle it.

 

[goldsilverpro]

Lou,

Have you done that? The CRC says Cr2O3 is not soluble in alkalies.

 

[Lou]

Let me rephrase that:

heating it with lye will dissolve it and make it water soluble. This is done in a cast iron vessel with a lid.

Additionally, this material will dissolve in hot, concentrated sulfuric acid if it is first heated and held at a dull red heat.


EDITED to make it clearer.

 

[Anonymous]

HCL dissolves stainless, right? Isn't it chromium oxide that passivates it?

Jim