Recovering precious metals from polishing sweeps

[Ageo308]

Hi guys, I'm after the simplest method of trying to extract the precious metals from polishing sweeps. So all you masters out there let me have it

P.S I'm not after refining and separating the metals from each other, just separating all the gunk from the precious metals then form it into a bar.

cheers

 

[Harold_V]

I recommend Hoke's book as well----regardless of the direction you may take.

Incineration is mandatory.

Unless you investigate chemical refining, your best chance is incineration and then furnace reduction. You'd need a good flux, and perhaps even a collector to recover the finely divided particles. It's not exactly something you can do with success without some serious equipment.

I'm of the opinion that you won't enjoy much success by any other means.

Harold

NOTE; this is the condensed version of this thread, it was originally posted when the GRF was relatively new. Notice Hoke was not available as a download on the forum back then.

 

[Ageo308]

Hi guys. I'll definately be getting the book so thanks.

Would the correct term to be used here is smelting?

 

[Harold_V]

Not in my opinion, but I can see where it might be a gray area.

Smelting, as I understand it, is the reduction of metals from chemical compounds (ores). What you'd be working with isn't remotely the same in that it won't contain sulfides or other troublesome elements. Could be I'm tripping over semantics, however.

I never referenced my melting operation as smelting when I processed my wastes, although in many ways the process was identical. I reserve the term smelting to working with ores------anything else is melting---with or without flux.

Incidentally, if you choose to do this with a furnace, one of the constituents of your flux should be fluorspar. It, better than anything else I ever tried, thins the flux exceedingly well, which is a factor in not losing traces of values to prills. The negative aspect is its aggressiveness towards crucibles and furnace lining. The flux will pour water thin, but it takes along with it a generous portion of the components you hope to retain (read that as the crucible and furnace lining). It's the price you'll pay for good extraction, and, in my case, was a negligible loss, although not cheap.

Harold

 

[Ageo308]

Hi guys, while I'm waiting for the book to be sent out I came across these instructions to prepare polishing sweeps ready to refine.

Let me know what you think

Standard methods of sweeps burnout are ineffective and produce a product similar in appearance to lava rock, and which is more, rather than less, difficult to refine. The following method requires about the same amount of work as ineffective methods but reduces to sweeps to a material that can be melted to obtain a bar of metal

Polishing sweeps are composed of grease (the binder in polishing compound), sandy grit (the active ingredient in polishing compound), lint (from the buffs) and very tiny balls of karat gold (from the gold that was polished). Sometimes it will also include paper and buffs.

The first step is to remove all organic material. Take the dust and put it in a series of casserole dishes. The dust must not be more than 1" high and it must not be packed into place. This is very important. If sufficient air does not reach the sweeps, they will not burn completely. If there are buffs and large amounts of paper, these must be put aside for a separate burning.

Place the casserole dishes in your burnout oven. You can stack them, but make sure that there is sufficient space for air to freely circulate through them. If using an electric burnout oven, the door must be left slightly ajar so that sufficient air enters the oven.

Burnout at 1350° F until it has stopped smoking and an additional hour has passed. When it has cooled and you examine the sweeps, you should observe no blackness and no lumps. Typically, it is a uniform, light gray, fine sand. If examined a loupe, you will find tiny balls of gold scattered in the sand.

If you attempt to melt at this point, the sand will turn to glass and its sheer volume will overwhelm the metal and microencapsulate the gold (you’ll get a lump of black glass will little beads of gold throughout it). To avoid this, you must first remove most of the sand. This is done with lye (also called sodium hydroxide or caustic soda).

Lye is very corrosive, so take the normal precautions that you would take with any corrosive material (wear rubber gloves, eye protection etc.) Make a saturated lye solution by adding just enough water to the lye to cause it all to dissolve. Lye gets hot when you add water, so use cold water. Now, for every cup of polishing sweeps, add 10 cups of lye/water solution. Heat the solution to about 200° F for about 1-2 hours in a stainless pot (do not use aluminum or any other metal- the lye will rapidly corrode aluminum).

Allow to cool to about room temperature. Then carefully pour off the lye/water, being careful not to pour off the gold particles in the bottom. Rinse by filling the pot with fresh water, allowing the gold to settle and then pouring off the water. There will still be a lot of sand but it will not longer be overwhelming. The gold will be visible. Dry by placing the pot back on the hot plate at low temperature.

If melting by torch, first wrap it tissue paper and soak in alcohol. Use a partially covered crucible such as a Burno crucible or a casting crucible. These steps will help prevent the gold dust from being blow about by the torch.

 

[Harold_V]

I'm not sure you want to hear what I think. :wink:

I don't agree with anything posted. Total nonsense-----not to say that it doesn't work. It probably does.

I processed polishing wastes for over 20 years and never experienced hardening. To suggest that a product akin to lave rock is formed is absurd. That is a sure sign of a process that isn't proper for the purpose.

If you incinerate as suggested, and do it indoors, you'll quickly come to understand how stupid it is. The described process is a coking operation, totally unsuited for the desired result. By roasting the material, you can expect the production of smoke that would fill a huge room, cover everything with a waxy deposit, and you'd likely never get rid of the smell, which would rival that of cooking your dog. Incineration should involve burning, not roasting. You'll understand that the first time you attempt to process polishing wastes.

Sorting paper products, as suggested, makes absolutely no sense. They should be burned, along with the other materials. That's what the process is for.

You can effect a perfectly good incineration by simply burning these materials in a shallow container, one that permits oxygen to access the material for complete combustion. It must be heated to the point where carbon burns, or about 1,100° F. The ash that results is fine and easily screened.

There is no need for the use of lye, which is far more dangerous than the use of acids. A drop of lye in your eye is pretty much a guarantee of blindness. A wash with HCl before digesting the values will suffice. An alternate would be a digest with nitric acid, assuming one expected a silver recovery. You must tailor the process to the source, in other words. Wastes from the silver bench would be handled differently from wastes from the gold bench.

All too many times we are told that there's more than one way to do most things. While it's true, there is generally one way that is best suited----and that's the one you, or anyone, should seek. The choice is yours.

The most efficient way to process these wastes is to incinerate, then melt in a furnace, using litharge as an oxidizer and collector. That involves cupellation to eliminate the lead, and is likely beyond the ability of the average home refiner. Your other option is to incinerate and then chemically refine the residues.

Follow Hoke's advice and you'll be well served, although you should avoid using gasoline for incineration.

Harold

 

[Ageo308]

Thanks Harold, although I must admit when my jeweller friend tried this process, he did burn in the kiln as stated then melted straight away and the piece actually turned into a real hard piece of glass. So he's trying this method today to actually see if any of this rings true.

I'm assuming the cheapest method is the chemical way yes? Once I get Hoke's book I will study it to death thanks for the advice Harold.

 

[Harold_V]

Thanks Harold, although I must admit when my jeweller friend tried this process, he did burn in the kiln as stated then melted straight away and the piece actually turned into a real hard piece of glass.

Of course it did. For all practical purposes, he melted with the idea that the values would magically appear as a solid piece. It doesn't work like that. Melting that material without a collector, and a thinning agent for the silica, which should be left behind but dissolved with soda ash, is a mistake. Much of the material won't dissolve, so there must be a thin flux to absorb the non-metallics and allow the metallics to collect and settle. Your friend is now stuck with something that will require either processing by ball mill, or it must be broken very fine, properly fluxed, and run with a collector. Had he stayed away from the furnace, that would not be the case.

Mind you, I have no quarrel with anyone that chooses to process polishing wastes by furnace----I'm simply stating that the process presented isn't a good one. It works, but the losses are likely to exceed what should be considered acceptable. That, of course, depends on what was used as a flux, but the indication is this person didn't have a clue.

So he's trying this method today to actually see if any of this rings true.

This method? Not sure of your meaning, but if you're implying the one I recommended, it never failed me. I processed polishing wastes on an ongoing basis, for more than 20 years.

Im assuming the cheapest method is the chemical way yes?

I'd prefer to have you think of it as a functional way for those that don't have at their disposal a great reducing furnace to process such wastes, yet a need to achieve a decent recovery. The problem with melting, even after the removal of considerable sand, is that it's very hard to collect the miniscule bits of values. Unless the flux is very fluid, and a collector of sorts is used to agglomerate the often colloidal sized particles, much of it is lost in the slag, some identifiable as prills, with the rest so fine it isn't recognizable. Have your friend pay for a fire assay of his piece of glass so he'll understand how poor was the advice he followed.

I do not recommend the home refiner attempt to melt such material, and if the decision to do so is made, the last thing that should be done is to remove the silica as recommended. If you'll study a good fluxing recipe, you'll come to understand that silica is a very important component. The one I used included the addition of silica, so I see no reason to eliminate what is a necessity.

By the way, my flux was assayed after the recovery of values and was so low in content that it was not acceptable at a refinery that specialized in recovering values from low grade wastes. The formula I developed worked very well, but it was very aggressive in that it would dissolve crucibles and furnace lining in short order. That was the chief reason I got such good extraction. In my case, the damage created was more than offset by a virtually complete recovery. I relied on silver chloride to act as my collector-----which turned out to work exceedingly well, at least from all indications.

Harold

 

[Ageo308]

I thought I would update you guys, my friend didnt take any photos but he said the result was better than expected. This time when he melted it (doing the process, that's listed above) it didn't turn to glass but just stayed more liquidy and the gold is much more visible and it's coming together. He said the only thing that's left is to remove the tripoli (polishing compound) and rouge, so I was wondering since we have come this far what's the best way to remove these 2 things. Is there any acids that will dissolve them?

cheers

 

[Harold_V]

All of it?

Harold

 

[Ageo308]

Did he melt all of it? Is that what you're asking Harold?

 

[Harold_V]

No------I'm asking if he recovered all of the gold. You have to understand that there is no reason for minute particles to gather. The density of the flux makes it difficult for them to move about, especially by gravity. Polishing waste, unlike filings and larger pieces, is made up of mostly invisible particles. So fine that the dust, once well incinerated, is purple in color (colloidal). It shifts to a yellow/green/beige color instantly when AR is introduced, a sure sign that you are dissolving the miniscule particles that I speak of. Those are unlikely to be recovered well by melting unless the flux is very thin, you have a collector of sorts, and the heat is sustained for a period of time.

Best thing for you to do is pursue this as you are, then have your flux assayed. If I'm wrong (and I certainly can be), your flux will assay low, fractions of an ounce/ton. If it assays high, you're on your way to understanding what I've been trying to have you understand.

Harold

 

[Ageo308]

Ok Harold, we hit a road block (as I expected). We are still stuck with the rouge and tripoli left on the metals. If we apply AR will that dissolve the gunk (rouge/tripoli)? If not what chemical will do the job?

cheers

 

[Harold_V]

You are likely stuck with a good amount of your values in the flux as well.

If you're hell bent on running this material as you suggest, you may discover that a boil in HCl & water prior to melting will serve to reduce the amount of rouge & tripoli, which is, if memory serves, just iron in a given state of oxidation.

I'm still at a loss to understand why you don't use the chemical process. It does an adequate job, at least as good as melting, assuming you don't use a collector.

If you get involved in chemical extraction, do not discard the material when you're finished. It will still contain substantial value, and can be sold to the proper refinery (assuming you have enough). Heat it to dryness and store it for the future.

Harold

 

[Ageo308]

Well Harold I'm just waiting for the Hoke's book to arrive so I can perform the chemical extraction.

In the mean time (while I wait) if you don't mind could you break down the process from start to finish (including the chemicals used) I'm after extraction of just gold at this present time.
Your information is tops and I thank you for your patience

 

[Harold_V]

My approach was very simple.

1} Incinerate in a shallow pan, heating from below. Ignite the material to eliminate the majority of smoke and stink. It will readily burn once hot enough. Incinerate until all carbonaceous material has been eliminated. There should be no glowing embers. Allow them to burn completely. Insure the pan is hot enough by heating with a torch from the bottom. Don't burn through the pan. An old stainless fry pan works very well for this operation. Find them at Goodwill or other second hand stores. DO NOT USE ALUMINUM.

2} Screen after cooling, to remove any sizeable pieces that would be difficult to dissolve without inquartation.

3} Run a magnet through the material to remove free iron particles. They should be placed in the stock pot, not discarded. They generally will transfer miniscule traces of values.

4} Place screened wastes in a beaker, or a container that will tolerate being heated.

5} With an approximate mixture of 50/50 tap water and HCl, cover the contents with solution, then place on hot plate. Heat slowly until contents are brought to a boil. Stir regularly to prevent the container from overheating and breaking from thermal shock.

6} After a reasonable boil period, (contents will change color), remove from heat and fill container with tap water. Stir well.

7} After solids have settled well, decant solution. Test for values, then discard if barren. It is rare to find values in the solution unless you have allowed nitrates to be included.

8} Repeat the above operation until the rinse water is clear, or nearly so.

9} Apply AR @ 5 ounces per troy ounce of values expected to be recovered. Use only what is necessary, to minimize the amount of nitric that must be evaporated later. Heat to boiling, and stir regularly. Do not allow the material to dry on the bottom of your container, which will lead to thermal shock and cracking, as above. Make your AR @ 4 HCl/1 nitric, not less. If in doubt, move up to 5 HCl/1 nitric. Too much HCl does no harm, unlike too much nitric.

10} When values are in solution (note color change of solids, and the yellow color of the solution), and you see no fumes coming from the container, remove from heat and add a few ounces of water. Stir, then allow contents to settle.

11} Decant, then add more water to solids and stir well. Repeat this operation until you have removed the bulk of values. Note the color of the solution, which will slowly shift towards clear as it is washed.

After the final decantation, place the solids in a filter (Sharkskin works very well for this operation) and extract all solution possible.

Combine all solutions from the AR process and evaporate to eliminate water and nitric. Add a few drops of sulfuric to the evaporation process early on, which will precipitate any traces of lead that may be included in the process. Be very careful about adding sulfuric to hot solutions. It is best added while they are cold. Stir as it is added to prevent local generation of steam.

After evaporation, take up with water, filter, precipitate, and wash appropriately.

I suggest you do none of the extraction processes until you've read Hoke's book---so you have a general understanding of what you're doing, and why. None of this is difficult----it becomes routine after you've done it a few times. Do not allow the outline to discourage you.

Dry the waste solids by heating, then save them. Do not store them wet----they will give off fumes endlessly and destroy anything of value in the immediate vicinity by corrosion. Please take my word on this part (the solid waste material, after acid processing) ---it is a wonderful savings plan. The values remaining are not substantial, but they add up. Remember------you will have left silver behind in this operation, not just traces of gold.

Note that this process will not be suited to processing wastes high in silver. It will remain in the solids. If you process waste from the silver bench, it becomes somewhat more complicated. I can talk about that if and when it is an issue. It is a waste of time to pursue traces of silver with each batch.

Harold

 

[Ageo308]

Hi Harold, last night we started your technique and got till the end of the rinsing process and ready for the AR process. Everything is looking good so far. My partner and I read Hoke's book a few times to understand the evaporation process especially as you mentioned but 1 thing we can't find is when making AR, does it matter if you mix HCl to Nitric or Nitric to HCl? Tomorrow night will be the final stage and I will update you on the progress.

thanks again

 

[Harold_V]

when making AR, does it matter if you mix HCl to Nitric or Nitric to HCl? Tomorrow night will be the final stage and I will update you on the progress.

thanks again

I don't recall ever having any problems with mixing those two acids, regardless of the sequence of combining them. If any heat is generated, it is miniscule, so it's not a problem.

I did my mixing in a graduate, pouring from stoppered bottles, in random sequence. I usually poured my nitric first, then the HCl. It was then poured into the container where it was to be used. The only caution, aside from common ones, is to not mix any that you don't intend to use immediately. It tends to gas a little, so it's not a combination that you should make and store.

I suggest you use an excess of HCl. It does no harm, and guarantees you use all of the nitric. I favored a 4:1 ratio, and often went as high as 5:1.

Harold

 

[Ageo308]

Harold we are basically using a large syringe to transport the acid from the container to the beaker. It has the exact measurements so we can do it nice and slow (for the experimenting stages).

It's much more cleaner and we didn't spill any.

Basically we are going for the 4:1 in the AR mix but if in doubt for whatever reason as you mentioned will move up to 5:1.

Fingers crossed everything will go as planned.

 

[Ageo308]

Ok Harold we did the final process, as we are letting it dry and before we melt which flux will suffice? We tried melting whilst still wet using borax and it seemed like it did the job fine. I think we didn't do the last bit correctly thow as it still has a brown mud on the bottom along with the gold and other metals.

If you don't mind could you go into a bit more detail from the AR process onwards? We are going to try some more tomorrow night.

Also for the filter we used the coffee filters which seemed like it did a very good job (have you ever used that before)??

anywayz I look forward to your valuable input.

thanks