How to recover gold from large pieces of metal?

[JustRandy]

I have some large (larger than a beaker) pieces of metal (stainless mainly) with gold stuck to them. I've scraped off what I could, but there remains a significant amount. What is the best way to recover this? I see a lot of tutorials about using acids or electricity, but I haven't seen anyone comment on using a torch. Could I just torch the gold off? Gold melts around 2000, steel melts around 2500, and a torch is 3500. So, wouldn't the gold just drip off under its own weight? Or would some other force keep it attached to the steel?

If you're curious, it was part of a machine that evaporated gold for the purpose of gold plating. Everything in the machine is covered in gold.

 

[qst42know]

No one mentions using a torch because it doesn't work.

Large pieces require large stripping cells or the pieces could be cut down to size. Different techniques are used to suit the job at hand.

 

[g_axelsson]

I have an idea, I haven't tested it myself but I've been thinking of it for a while. I've got some massive IR aluminum reflectors which are gold plated and also some large microwave tubes I wanted to process.

My idea is to use a version of the sulphuric acid cell with a brush or a cloth around the cathode. You just brush the object slowly, the gold is deplated as you go. Then it is a matter of wiping off the gold dust/acid from the surface and the brush.

Kind of the same setup that exists for plating gold with a brush but in reverse.

Great care has to be taken as always when working with concentrated sulphuric acid.

/Göran

 

[JustRandy]

No one mentions using a torch because it doesn't work.

Large pieces require large stripping cells or the pieces could be cut down to size. Different techniques are used to suit the job at hand.

Why doesn't it work?

 

[butcher]

With a torch the gold and iron and other metals in the stainless steel will melt together and actually make it harder to get your gold off, when two metals are melted together one metal can dissolve the other metal into the melt, the melting point of these metals change, as they are no longer a single metal but form an alloy of metals with a different melting point than any of the metals involved, when they solder components on circuit boards the solder has a very low melting point, and gold has a very high melting point, but if soldering gold plated copper pins the solder at around 700 deg F will dissolve some gold into the solder.

Others may be able to explain this better,

If this would work we could just melt and separate metals by their melting or freezing points of molten metals, or melt skim off the lighter or easier melted metals, or cool the melt to a bar and cut this bar at the point where the metals separated by density or when cooling to solid at different temperature (which will not work either).

Actually I do not think we would have the alloys we have if this would work, as metals can also have different boiling points and if these did not change when melting we would probably boil off the easier melted metal by the time the higher melting metal became molten, and would not be able to form an alloy, or the metals in the alloy would separate as they cooled and then we would not have an alloy, but a worthless tool that is too soft on one end, brittle in the middle, and so hard on the other end it would shatter if struck on something, then we would have never have been able to make bronze swords, or other tools and history would be much different.

 

[g_axelsson]

Correct in almost all, Butcher, but you can boil off one metal from another. A common example is using a retort to boil off mercury from amalgam, leaving the gold behind.

... and I think solder will dissolve all gold it covers on the pin unless it is really heavy plated.

Except from that, a very good explanation.

/Göran

 

[butcher]

g_axelsson, thanks.
I understand one metal can boil off from another, but this normally happens with metals that do not alloy with each other, or when one metal oxidizes easier than other in the heat,or by conditions of the environment of the heating process, other special conditions, and I also understand, we can use some of the the things that in general do not work for most metals, like purifying molten lead by skimming oxides or separating silver from lead. or that zone heating can be used to seperate metals in a wire and so on.

Yes there are exceptions to every rule, I was more focused on the general rule not the few exceptions.

 

[g_axelsson]

That is just wrong. None of your examples were boiling metals.

In the old way to separate silver from lead you use crystallization of silver in molten lead, keeping it just under the melting point of lead. Not even close to the boiling point.
The same thing applies to when you oxidize metals, that is not boiling. It uses the fact that oxides and molten metals usually doesn't mix that well and we get separation in two phases.

The reason we do not see more examples of boiling metals is that metals boils at so high temperatures and you often need an oxygen free environment so it doesn't oxidize before boiling. One other example I can think of is melting brass. When brass is melted some of the zinc starts to boil away and catch fire when it comes in contact with oxygen on top of the melt. To compensate some pure zinc is usually added to the melt.

There is no difference between molten metals and other liquids, they all follows the same physical laws. The only difference is that it is so much harder to boil most metals.

/Göran

 

[butcher]

I was not making any specific examples of boiling one metal from another; I was trying to give a few instances where metals are separated from another metal using heat.

If you wish to have an instance of where boiling off of one metal from another, in welding you can boil one metal from another, welding galvanized iron will boil off zinc as fumes, of course this is not a separation method.

Many metals will boil off from a melt, metals very high in the reactivity series can oxidize easily these can also boil away as fumes in a melt.

Some metals are distilled for recovery because they can boil off from the other metals easily when in solution like osmium.

I do not see where what I said was wrong, unless I am using some wrong terminology to describe what I was trying to say, if you can help with this please do so.

 

[JustRandy]

With a torch the gold and iron and other metals in the stainless steel will melt together and actually make it harder to get your gold off, when two metals are melted together one metal can dissolve the other metal into the melt, the melting point of these metals change, as they are no longer a single metal but form an alloy of metals with a different melting point than any of the metals involved, when they solder components on circuit boards the solder has a very low melting point, and gold has a very high melting point, but if soldering gold plated copper pins the solder at around 700 deg F will dissolve some gold into the solder.

Others may be able to explain this better,

If this would work we could just melt and separate metals by their melting or freezing points of molten metals, or melt skim off the lighter or easier melted metals, or cool the melt to a bar and cut this bar at the point where the metals separated by density or when cooling to solid at different temperature (which will not work either).

Actually I do not think we would have the alloys we have if this would work, as metals can also have different boiling points and if these did not change when melting we would probably boil off the easier melted metal by the time the higher melting metal became molten, and would not be able to form an alloy, or the metals in the alloy would separate as they cooled and then we would not have an alloy, but a worthless tool that is too soft on one end, brittle in the middle, and so hard on the other end it would shatter if struck on something, then we would have never have been able to make bronze swords, or other tools and history would be much different.

I assumed that gold didn't react with much of anything. So it seemed like I could just heat it up to liquid state and it would not want to bond to anything. But you're saying it would actually burrow its way inside the steel and become an alloy like carbon would? Carbon reacts with seemingly everything. Carbon I can see, but gold?

I have some experience with solder. I heat the thing I want the solder to stick to, then touch the solder to it, and it "wicks" in. If the metal is not heated enough, the solder will just form a ball and not stick. To remove the solder I heat it and use a suction device. My understanding is that the solder doesn't bond to the metal, but freezes around the metal. Heat it up and you can easily fling it off.

But I guess the "why" isn't important. If you guys are all in agreement that it won't work, then I guess it won't work. It surprises me, but apart from this one project this is more metalurgy than I'll ever need to know lol

 

[butcher]

Iron and gold alloy very easily this is where a lot of gold in the ore is, bonded with iron, if it is not free gold.
Solder like lead can be melted in an iron pot, the oxidation on the iron can keep lead from wetting and sticking to the iron,
If you tried to melt gold in an iron pot your gold would form an alloy with the iron.
Lead is used in an assay to collect gold, as the melted lead oxide melts to form lead it absorbs or collects the gold from the melt.
Carbon can reduce metal oxides back to elemental metal in a melt.

18K gray-white gold alloy 75% gold, 17% iron, 8% copper, here is a jewelry alloy of gold and iron.

Get a gold plated steel spoon or a silver plated steel spoon or plate from your local second hand store take this in your back yard with a Mapp gas torch. tell us how much gold of silver you can get to drip from it by using the torch.

 

[JustRandy]

Iron and gold alloy very easily this is where a lot of gold in the ore is, bonded with iron, if it is not free gold.
Solder like lead can be melted in an iron pot, the oxidation on the iron can keep lead from wetting and sticking to the iron,
If you tried to melt gold in an iron pot your gold would form an alloy with the iron.
Lead is used in an assay to collect gold, as the melted lead oxide melts to form lead it absorbs or collects the gold from the melt.
Carbon can reduce metal oxides back to elemental metal in a melt.

18K gray-white gold alloy 75% gold, 17% iron, 8% copper, here is a jewelry alloy of gold and iron.

Get a gold plated steel spoon or a silver plated steel spoon or plate from your local second hand store take this in your back yard with a Mapp gas torch. tell us how much gold of silver you can get to drip from it by using the torch.

So I typed in "gold iron alloy" in google hoping to read a bit about it and the first thing that popped up was http://www.newscientist.com/article/mg20727745.400-first-goldiron-alloy-shows-power-of-magnetic-attraction.html

"GOLD readily forms alloys with the precious metals silver and palladium, but it normally blends with cheap iron about as well as oil mixes with water. That has now changed, with the creation of a gold-iron alloy that is held together by magnetism. "

 

[butcher]

Some what true but it also depends on what conditions iron and gold are melted together, in air iron can oxidize easy to iron oxides these will not melt back to iron without great difficulty and going to a lot of trouble, but this does not mean that melting gold and iron together is a good idea they will alloy and you will make your gold harder to get back, or that you can melt plated gold of an steel item and seperate these metals by melting temperatures. Yes there are other metals that alloy with gold much easier than iron will.

Well I tried to help but you seem so stubborn in your ideas about what will and what will not work, I am beginning to feel I am wasting my time trying to help you understand this, and you seem to wish to believe what you want to be true, maybe you will just have to prove it to yourself what will, and what will not work, do not believe me, just try melting your materials and see if you can separate metals, or try melting your gold in a steel dish and see for yourself, which theory here is correct, I just know I do not want to try it with my gold, with my gold, I know that I want to stick with what I know works, from what I have learned from others here on the forum.

 

[nickvc]

Butcher is giving you the truth whether you want to hear it or not.
Think of this, when you braze or solder steel together you need structural strength in most cases, think of aircraft engines and to get that integrity you need a strong join, why do they use gold or silver in these solders when you could use just lead or tin solder, the answer is that the metals form an alloy when using silver and gold at the points of contact forming a strong joint. If you don't believe me melt some gold and stir it with a steel rod then try and get all the gold back off that rod by heat.
There is no easy way to do what you want to do so take the advice given if you want a successful outcome and stop trying to follow a route that will never get you there.

 

[Sodbuster]

JustRandy :

Get your torch out and see if you have any success with trying to remove (all) the solder from a piece of copper wire.

You could even try it with an old sweat joint on a piece of plumbing. You will never get all the solder off of the copper.

This has been explained by a hand full of some of the best and most knowledgeable on the forum.

If you chose not to accept the information given, I challenge you to try this idea with solder and copper so as to not have any loss of value.

Please do show us some pictures of your success.

Ray

 

[JustRandy]

But I guess the "why" isn't important. If you guys are all in agreement that it won't work, then I guess it won't work. It surprises me, but apart from this one project this is more metalurgy than I'll ever need to know lol

Apparently you guys missed me saying that.

I appreciate the insight though. No, I'm not stubborn, I just like learning. Yet, metalurgy is something that takes a lot of effort to learn and at the end, I'm no better off. I just want to get the gold off this metal so I can sell the gold and use the steel for something productive... and not kill myself in the process lol.

Acid and electricity doesn't sound as safe as baking a pie and there are a 1000 tutorials, none of which seem to be complete. Where do I even aquire acid? What strength? Will battery acid from the autoparts store work? Do I use a plastic container? What kind of plastic? Tuperware? ABS? PVC? Do I use a battery? 12 volts with infinite current? What if there is a short, a spark or too much current passing thru the acid? I think sulfuric acid releases hydrogen gas right? I don't want to turn my garage into the Hindenburg. Suppose I survive getting the gold into an acid solution, now what? What do I add to get the gold out? And where do I get it? So I'm faced with the prospect of learning all this, of which I know nothing, except that its dangerous and complicated and you guys want to give me a hard time because I want to be darn sure that a simple process like melting it off won't work lol. Or maybe I could sand the gold off with sandpaper and pan the gold out of the dust in water. Will that work? Then again, I can't sand every odd shape and some is just too small.

 

[jimdoc]

Can you post pictures?

Jim

 

[Marcel]

Just grind the gold down and process the powder. Why all the trouble?

 

[JustRandy]

Can you post pictures?

Jim

 

[JustRandy]

Just grind the gold down and process the powder. Why all the trouble?

Yes I just thought of that last night. So that was my next question... Can I sand it with sandpaper and use my gold pan and some water?