# silicates contaminating gold



## yvonbug (Nov 28, 2007)

I'm BACK... In the past I turned in some gold once or twice, and the guy I was selling it to would melt it and he'd be able to tell right off if there was any contamination in it. One time he showed me what silicates looked like. It had a crosshatched pattern on it. Well, just lately I've gotten into some of my computer scrap, (which is all I ever process, by the way) I have these silicon wafers (lots of them), that have a lot of gold on them. Like the whole backside of them and in between. How would I process something like that? And get rid of all the silicon? -Y- :twisted:


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## Lou (Dec 1, 2007)

One nasty way to get rid of silicon, silicon dioxide glass, and ceramics but leave the gold (and most other metals behind) is HF, hydrofluoric acid. It dissolves glass and many minerals but it's exceedingly dangerous to use--a few drops of the concentrated stuff on your skin will really ruin your day and probably send you to the hospital.

You work with it in plastic HDPE containers. The acid actually fluorinates the polymer to something akin to PTFE (aka teflon, polytetrafluoroethylene) which is really inert.

Or you could try crushing them up and using cyanide to complex with gold. That'll leave the silicon behind. Or you could try bleach and muriatic acid. There's a bunch of ways to skin the cat, just depends on what's easiest for you to do and how well you're set up to do it.


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## yvonbug (Dec 1, 2007)

Isn't that the stuff that if it gets on your skin, you don't know it until it gets to your bones and starts disolving them? Yeah, I'm not set up for that, thats for sure. Bleach and muratic? Whats that about? That I might be interested in. Thanks for responding!-Y-


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## goldsilverpro (Dec 1, 2007)

Unless you are a chemist, I would not use HF. Too damned dangerous if you don't know what you're doing. I've used it for certain applications, but certainly don't recommend it for the amateur.


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## tlcarrig (Dec 1, 2007)

The product "WHINK" at Wally World in the isle with CLR and Draino is HF acid. I don't know what concentration but it must be weak. HF is VERRRY DANGEROUS. Previous poster is correct that it will kill you before you know it. Do a Wikkipedia on it. Yea, I bought some but I won't use it very much until I find out its effectiveness.


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## Lou (Dec 1, 2007)

I have some of the 70% stuff which fumes. Not nice at all. It will boil when you put a drop in a test tube. To work with it safely, you need a hood, damn good gloves, and calcium glutaconate jelly. But it does work, and it works well. Nice stuff for pretreating titanium before plating...


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## Irons (Dec 1, 2007)

I've worked with 70% HF and it's not for someone who doesn't know what the dangers are and doesn't have the proper equipment.

A lung full will give you a very painful death. HF burns are the most painful chemical burns i've experienced. Just a drop on your skin will have you begging for someone to give you a dummy slap.


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## yvonbug (Dec 2, 2007)

I'm not messin' with it.-Y-


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## yvonbug (Dec 2, 2007)

Actually, that stuff is not even in MY little book of life. Don't even want it around me. Guess I'll just have to look at all the pretty gold on the disks and wish. oh well, maybe I could sell 'em to somebody who's willing to take that risk. I might work with some nasty stuff, but, to me HF is too much. -Y-


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## goldsilverpro (Dec 2, 2007)

Back to the original question.


> I have these silicon wafers (lots of them), that have a lot of gold on them. Like the whole backside of them and in between. How would I process something like that? And get rid of all the silicon?



I don't understand what you mean by "in between" - please explain.

A refinery I worked for got these gold backed silicon wafers by the drumloads from companies in the Silicon Valley. I think the gold is evaporated or sputtered onto the silicon. I don't remember the gold thickness but I don't think it is very thick. It would certainly add up in quantity. I also don't remember how we processed them. We probably used cyanide, since it was common back then. It was also cheap, fast, safe, and efficient.

Since the government has taken away the marvelous cyanide, you are stuck using some form of acid chloride. This would include the standard aqua regia, HCl/bleach, and HCl/peroxide processes. None of these should attack the silicon. They should only attack the gold. You would then drop the gold from the solution, as normal. 

The problem with working with these wafers, when whole, is that they tend to stick to each other when wet. The acid then has to penetrate through the edges of the stuck together wafers and, this can be a slow process. This can be sped up by first breaking up, or grinding up, the wafers. 

If you grind up the wafers, there may be a potential health hazard with the dust, if you don't wear a proper mask. I would read the stuff on this link.
http://www.msha.gov/S&HINFO/SILICO/SILICO.HTM

When you breathe silica (silicon oxide) dust, there is the possibility of getting silicosis, a lung disease. Although we're talking about pure silicon and not the silicon oxide covered in the link, I think I would take the same precautions, were it me. Better safe than sorry.


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## yvonbug (Dec 3, 2007)

What I meant by in-between was where the wafers were done in layers. And alot of the wafers have already been cut up into itsy bitsy pieces. I;ve included some pics. I have alot of these., but not enough to fill a 55 gal. drum. Some of them have a gold backing that is dull, no shine, and it looks kinda thick. And I have alot of these that are not cut up, also. Actually, if I put them all together, they would probably just fill 1 1/2 5 gal. buckets. Small time. -Y-


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## draftinu (Mar 16, 2008)

That sure is some nice looking stuff


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