# Stripping gold plate from AL/TI



## Anonymous (Mar 9, 2008)

Hi all, 

I've got several boxes full of some sort of electronic (semiconductor style but not silicone substrate) scrap wafer discs. A guy that I ran across at a storage unit auction was starting to toss these out; sparing the bicycles and furniture he had won.
I immediately recognized the Gold plate layers on the majority of the wafers and bought the load from him for $25. Turned out to be about a half a ton of materials.

The wafers are layered with Aluminum, Titanium,Some with copper, -Top layer Gold. 

Ive experimented with an AR solution (You guessed it ; Shor) to dissolve the metals on a sample batch which yielded a gob of TI/AL sludge to clog my filters. At precipitation; along with a black/brown mud at the bottom of my container, Apparently the gold also thinly plated out the entirety of the sides of the container (2 quart Plastic Pitcher) and left a floating gold film covering the top.

After several rinsings; I'm still testing a lot of contaminates and fear that I'm throwing a lot of pm out with the wash since particles are very fine I'm getting a lot of brown water in the rinse.

Moment of truth here; I have no idea what I'm doing. 

Is there Some way to dissolve the TI/AL layers so that I can just capture the gold in a filter process?


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## Anonymous (Mar 9, 2008)

From some quick readings through google search; Supposedly HCL should attack both aluminum and titanium. So why all the shiny sludge sullying up my stew?


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## lazersteve (Mar 9, 2008)

Jeff,

Welcome to the forum.

It seems like you could heat the wafers in HCl to remove the base metals and then go at the residue with AR.

A word of advice, don't use plastic when precipitating, always use very clean pyrex.

Steve


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## Anonymous (Mar 9, 2008)

I was thinking along the same lines. 
>Thanks for the tip on the plastic; Hard earned lessons as I go.
will post some pics of my raw materials tomorrow.


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## Lou (Mar 9, 2008)

HCl will have slow action on the titanium if it's an alloy.

These discs sound like they have gold sputtered onto them. That could mean very thick gold deposits. 

I would handle these with cyanide (to strip off the gold), or perhaps electrolytically. 


Hot NaOH will readily dissolve the aluminum, and do a number on the titanium. After rinsing, go after the rest of the titanium with HCl or a fluoride salt and dilute sulfuric acid.


Lou


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## Anonymous (Mar 9, 2008)

if so .. they've been through a mill process. The surface is very smooth and the devices are fully exposed save a layer across the solid conductors. The wafers are only 4" in diameter and even though many are flood filled; only a pattern in the middle has been fully etched. 

Some bonuses though. Gold bumps on a few; Used a straight edge razor and isopropyl to shave 10 grams off of 50 of them. :wink:

I'll pass on the cyanide though, thanks. 

small accident with HCL vs Cyanide....I vote HCL :lol:

Hot Draino, Huh?


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## Lou (Mar 10, 2008)

10 g of Au from 50 of them?

Not bad at all!! And you have 1000 lbs of these...you could be in the money.


Congrats. Wishing I was you,

Lou


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## goldsilverpro (Mar 10, 2008)

I would try using the standard concentrated sulfuric cell. I've had good luck with H2SO4 stripping gold from aluminum, with no attack on the Al. Probably could strip the Au in seconds. I would guess similar success with Ti.


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## Anonymous (Mar 10, 2008)

how about and air powered parts cleaner, you know the type with abrasives in it? could grind of the layer then just pan down to concentrate and recover.

I was thinking about this after watching smash lab so please take it easy on me. I love smash lab and master blasters for that matter.


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## Anonymous (Mar 11, 2008)

Yes I do have a tremendous project ahead of me and I'm working slowly now with small batches from the first box to develop a good process that I'm comfortable with. 

Ive worked with electronics and manufacturing process development for 15 years and have learned to consider and evaluate all suggestions especially in an area I'm vaguely familiar with. I really appreciate everyones support here with this.

Not quitting my day job yet though, I actually like the company and the people I work with right now. This project will certainly give me an excellent hedge against the ongoing economic uncertainty. 



Oh, and yes I gave power grinding a shot; the wafers are simply too thin and shatter too easily for any brute force procedures.

-Jeff


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## Anonymous (Mar 11, 2008)

jeff, 

not sand blasting, use a bucket of sand with air blown in the bottom. It is like and abrasive fluid bath.


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## goldsilverpro (Mar 11, 2008)

Again, have you tried the sulfuric cell?. If the wafers are conductive, it should work.


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## Anonymous (Mar 11, 2008)

I checked out Steve's cell video. That precise setup may not be appropriate. 

Deeper dish definitely and instead of clips perhaps a small conductive cage to sandwich the wafers and broken pieces in. Will have to have another well to neutralize the acidic cage in before loading and unloading. Sounds workable, are other metals drawn along with the Au?

As long as I can draw off the top AU layer and isolate it from the other metals , I really don't care if the other base layers dissolve or not.

Sub simmering the raw material in muriatic for an hour works pretty well. Lotsa fumes, but decent filtering and isolation at least on a small scale.will continue with this batch next week on my days off.

-Think I'll pass on the sand ; thanks though. -Considered added contamination and would still have to undergo Acid process to isolate AU.


-Jeff


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## Anonymous (Mar 11, 2008)

Would definitely have to experiment with the cage theory; seems I would have to somehow force the current through the AU layer.With an exposed cage the current would bypass the wafers and just conduct across the acid mixture. 

A Series of Long Clamps?

-Jeff


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## Harold_V (Mar 12, 2008)

I have not operated a cell, but I have witnessed one in operation. 

If your items are conductive (they are, apparently), they can be stripped in a basket made of heavy copper screen. They may require being disturbed once, to insure that all surfaces get stripped. Flat pieces present problems when parallel surfaces are in intimate contact. 

Stripping by a sulfuric cell is interesting----it conducts because it's dissolving gold or other values. It does not dissolve base metals, so conductivity falls off as the values have been removed. It is one of the best possible ways to recover gold plating, assuming the material lends itself to the operation. 

You should be paying attention to GSP in this instance. He has considerable experience with stripping cells. 

Harold


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## goldsilverpro (Mar 12, 2008)

As Lou said, cyanide would be the best. We received gold backed silicon wafers by the drum full and tumbled them in cyanide. Unfortunately, you don't have cyanide.

The sulfuric cell should strip them quickly and cleanly. I envision making a small steel rack that would allow you to separate them (maybe an inch apart) and run a number of them in a row at one time. They shouldn't need clamping - contact with the rack should suffice. It may need an occasional jiggle. Most likely, you will have to separate them. If you run them in bulk, they will stick together and prevent gold dissolution.


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## Anonymous (Mar 12, 2008)

A steel wafer boat/rack sounds workable Any thoughts on an appropriate tub? An aquarium perhaps? Bear in mind Possible acid effect on silicon seals.
-Jeff


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## goldsilverpro (Mar 12, 2008)

A steel tank would work great. Use it as the cathode.


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## Higashi (Aug 24, 2010)

goldsilverpro said:


> A steel tank would work great. Use it as the cathode.



Would stainless steel also work?

I read that some cell (not sure which one) don't work well with SS. Read too much, got mixed up.


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## goldsilverpro (Aug 24, 2010)

I tried to use stainless tanks (vessels) a couple of times and ended up with holes in the tanks. The sulfuric seems to get after the nickel and/or chromium. I always used mild steel tanks as the cathode with little or no attack after a couple of years. Just make sure the welding rod used to weld up the tank doesn't have Ni or Cr in it.


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## Higashi (Aug 24, 2010)

goldsilverpro said:


> I tried to use stainless tanks (vessels) a couple of times and ended up with holes in the tanks. The sulfuric seems to get after the nickel and/or chromium. I always used mild steel tanks as the cathode with little or no attack after a couple of years. Just make sure the welding rod used to weld up the tank doesn't have Ni or Cr in it.



Eww.. that's bad.

What's the chemistry behind this... why does it not attack copper, but it does on Ni and Cr, yet in the same time dissolves Au.
forgive me for being blunt, last i ever try to understand anything chemistry was for CHEM101 in college, which was like over 10 years ago.


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## Barren Realms 007 (Aug 24, 2010)

Higashi said:


> goldsilverpro said:
> 
> 
> > I tried to use stainless tanks (vessels) a couple of times and ended up with holes in the tanks. The sulfuric seems to get after the nickel and/or chromium. I always used mild steel tanks as the cathode with little or no attack after a couple of years. Just make sure the welding rod used to weld up the tank doesn't have Ni or Cr in it.
> ...



It will attack copper.


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## goldsilverpro (Aug 25, 2010)

Barren Realms 007 said:


> Higashi said:
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> > goldsilverpro said:
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(1) If the sulfuric is strong enough and (2) if you don't run the current too high and (3) if you don't let the solution get hotter than about 110-120F and (4) if you harvest the gold when you should, it won't attack copper.


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## Oz (Aug 25, 2010)

The #1 mistake that most make with sulfuric cells is that even if they start with properly concentrated sulfuric acid they end up with it being too dilute without knowing it. Concentrated sulfuric will draw visible water vapor off of your damp gloved hand at 6 inches. In other words it dilutes itself by taking on water from the air.


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## Barren Realms 007 (Aug 25, 2010)

goldsilverpro said:


> Barren Realms 007 said:
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> > Higashi said:
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Thats a lot of if's for new people to keep under control. 
Mine does a good job with little to no problem's.


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## Higashi (Aug 25, 2010)

Barren Realms 007 said:


> goldsilverpro said:
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> > Barren Realms 007 said:
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you use stainless steel parts?


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## Barren Realms 007 (Aug 25, 2010)

Mostly plastic.


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