Does anyone make a profit refining gold?

[Harold_V]

I designed it, then had Salt Lake Stamp make it (them). While a good stamp can be made using a CNC mill, or a pantograph, a plunge EDM is the best possible choice. The stamp can be heat treated prior to being made, then the impression burned. That eliminates the risk of cracking in heat treat, after all the work is done. While it's not a common problem, the threat exists.

I let the smallest one, the one used on those ingots, go with the business when I sold it. I still have the two larger ones. They require a serious amount of tonnage in order to make a decent impression, so they were of no use to the buyer of my refining business.

The large one measures 2", top to bottom, and the smaller one measures 1-1/8", top to bottom. It's borderline too big to be used with an 8 ton arbor press. The large one doesn't work well at all. I feel it would require 50 tons to imprint well.

Trusting to memory, I believe the one used on those ingots measured 3/4", top to bottom.

Harold

 

[Oz]

Harold,

Those are some very nice looking, professional bars. If you care to elaborate a bit more about the molds you used and your cooling procedures I would be very interested. It sounds like you removed the bars from the mold and then did an annealing operation, but I’m unsure of your description.

Oz

 

[NobleMetalsRecovery]

When selling them on eBay I sold them as flakes, they look somewhat like nuggets.

When selling direct, my buyer likes beads, from a few grams up to an ounce or so in size. The buyer wants to see the "dimple" in the bead, to show the purity.

 

[Harold_V]

If you care to elaborate a bit more about the molds you used

All my molds were made of cast iron. Being a machinist, and operating a commercial shop at the time, I made my own. I've posted on them previously, and may have even posted a picture. I'll include one with this post if you didn't see it. The picture will include the asbestos box I speak of, later. The molds, from left to right, are for 10, 5, 2 (or 3), 1, & ½ oz. ingots.

and your cooling procedures I would be very interested.

I did all my casting with a gloved hand (asbestos glove), handling the melting dish without the aid of holding tools. It was the safest possible way to pour the gold without loss. I also poured in an asbestos box, to contain any spills. So then, the mold was preheated to the point where it would not ignite the carbon that was deposited with an acetylene torch, sans oxygen. This, of course, is very dirty to do----but I did that in my fume hood, where all the free soot was captured.

The mold was preheated, then generously covered with soot. The gold was then (torch) melted and poured immediately. The moment the gold is poured, the torch is played on the surface of the gold, preventing it from solidifying rapidly. A soft flame, leaning towards reducing, is desired, with low velocity, so the flame doesn't disrupt the cooling pattern. The idea is to keep the surface molten and allow the gold to cool from the bottom to the top. That eliminates the pipe, and, done properly, will yield a flat surface on the top face. It's VERY difficult to do by hand, so my results were uneven. It is for that reason I marked the bottoms instead of the tops.

For the record, while a pipe is very important in determining the quality of gold you pour, it is not a desirable feature to include in an ingot. Professionals never allow the pipe---nor should you.

You may recall that Noxx used to use a picture of an ingot as his avatar. That ingot was poured properly. You can see the slightest hint of a center pip, but no pipe. A perfect ingot, in my opinion.

Soot coating of the mold plays a very important roll in pouring a clean ingot. If you don't smoke the mold well, and if you don't move the stream about as you pour, it often will burn through the soot covering and fuse the gold to the mold. It's not a big issue with small ingots, but anything larger than a couple ounces it can be because of the volume of gold being poured.

It sounds like you removed the bars from the mold and then did an annealing operation, but I’m unsure of your description.

No----no processing after pouring the mold. The only exception to that is after marking, I always boiled the ingots in dilute sulfuric, to insure that no surface impurities were included. The gold was so pure and clean that if ingots made contact, they had a tendency to stick to one another.

If the ingot comes out of the mold looking lousy, your only real options are to re-melt and recast, or die strike the resulting ingot. That's usually well beyond the ability of the home refiner. A huge press would be required, as well as a proper die. DO NOT try to melt gold in an ingot mold. It should be poured from either a crucible, or a melting dish.

Harold

 

[Noxx]

It's the first time I see a picture of your molds...

 

[Oz]

Thank you for that Harold. I was aware of the sooting of molds but I do not think I would have done it so thoroughly had I not seen yours first. Your desription of your cooling procedures makes perfect sense now as well.

I tried magnifying Noxx’s avatar pic to no avail, perhaps he will humor me and post a higher resolution picture of it.

I am impressed with the accuracy in mass you achieved in your finished product. I imagine careful pre-weighing and little or none left in the melting dish. I was curious though that you marked and then boiled in sulfuric to remove any impurities. Would that not have changed the mass considering the fine weight tolerances you stamped? I’m not trying to be critical, just curious.

Lastly, is there anything you would have done differently from the above procedure if you were pouring Pd or Ag?

Thanks again for your time!
Oz

 

[Harold_V]

I am impressed with the accuracy in mass you achieved in your finished product. I imagine careful pre-weighing and little or none left in the melting dish.

I used an analytical (beam type) balance to weigh the gold. It was purchased surplus, from the U of U. I lived in Salt Lake City when I refined. There was a local company operated by the Ruefenacht family that made balances, bullion, assay and analytical. I had them make me a set of weights. They were, at that time, certified to calibrate. When the father died (old age), the business closed. They no longer exist.

Casting exact weights is not easy. For one, when you melt the gold, unless it has been previously melted, a miniscule amount goes off as colloidal particles. You can see evidence of that by observing the color of the asbestos melting tray. It is purple because of the huge amount of gold that was melted.

I cast shot, then weighed the proper amount, usually allowing a grain or less over weight. Not all of it will pour every time---even when you have applied fresh borax to the dish. Ten ounces was the target weight.

I was curious though that you marked and then boiled in sulfuric to remove any impurities. Would that not have changed the mass considering the fine weight tolerances you stamped? I’m not trying to be critical, just curious.

You actually asked a good question, so no offense taken.

Being pure, gold is not affected. The cleaning process was to insure that any potential material that may have been imbedded in the ingot in the marking process was eliminated. If anything, the ingot would return to the marked weight. I was very clean with my processing and handling, so I never noticed any changes----but I liked to insure that what I provided was realistic.

Lastly, is there anything you would have done differently from the above procedure if you were pouring Pd or Ag?

I did pour several silver ingots---many 100 ounce, and a few smaller ones. You must use preventive measures when you pour silver because of its affinity for oxygen. It doesn't oxidize easily, but it will absorb nine times its volume in oxygen, expelling it as it solidifies. If you've read Hoke, you likely remember reading about the surface often being disfigured by spires of silver, the result of oxygen leaving the surface rapidly as the silver solidifies.

Silver can be melted in a graphite crucible to minimize the intake of oxygen, or even covered with a layer of carbon (charcoal). My procedure was to pour while holding a smoking torch on the mold. It would then be played over the surface while the silver cooled to minimize uneven shrinkage.

As far as palladium goes-----I never tried to cast any. I melted a small amount, but left virtually all of my palladium as sponge, and sold it that way.

There are monumental problems with casting palladium---due to its high melting point. It is not advised to use a carbon barrier, for the platinum metals form carbides. That would eliminate the use of graphite or iron molds. I have no idea how palladium is cast. What little I melted was simply allowed to freeze in the melting dish, then removed before it stuck to the dish. Sorry----I'm not any help with the platinum metals.

Harold

edit: corrected two spelling errors

 

[viacin]

Harold, I think you answered every question I've ever had about the molding process. This should be moved to the tutorial section, seriously. There is a lot of wisdom in your words.

btw, you were a machinist? Me too. I'm going on 5 years this Oct ( I know, still a noob). I learned the old scool way of doing things hands on from the shop floor from one of the best old timers in town. I learned how to program CNC's last year though, so most days I end up programming and setting up the machines now. I miss the manual lathes and mills some days, but the promotion to head programmer was worth it . lol, if I keep typing I'll have to move this to the about me section so I'm shutting up now.

 

[Lou]

Very good pictures Harold, that's about how I do it. My only comment is that I just put my molds over the vent hole in the furnace, bottom to the furnace lid and let them heat that way. Then when it comes time to cast, I turn down the burner from 12 psi propane to 4 psi and close the choke, richening it up in the furnace. Pull the molds off with tongs, soot them (usually I start the torch while the melt is coming to casting heat) and set them in perlite. The perlite keeps the molds very hot for a long time. As for the flame--I have a fan tip that gives a 3" fan (its original purpose was for glassblowing--heating a section of tubing evenly), I just play this slightly lean across the surface. Any spills immediately stick to the perlite, which is white and especially good for noticing gold. Perlite's so light that any spill can be recovered just by shaking the perlite for a minute or two and then removing all but what's left on the bottom, where all the metal-stuck-to-perlite accumulates.

One thing I've noticed is that you get a better ingot the hotter the mold is, and the faster you get all of the metal in the mold. Also, the slower you cool, the flatter the ingot will be.


As for palladium, like platinum and rhodium, those are best melted via induction and cast with a positive argon pressure, or more preferably, negative pressure due to vacuum. Palladium is a particular nuisance because it sometimes ''spits'' when you're heating the powder via torch because it oxidizes and flies off. Molds for Pt, Rh, and Pd can either be cooled copper (Rh and Pt freeze almost instantly, Pd less so). Usually though, pressed alumina with a zirconia stucco is used for crucibles and for the molds when casting these three.The mold is preheated to about 2500F/1300C so it doesn't crack when 3000+F metal goes into it. If one even desired, one could melt them in the mold.

Casting temps depend on size of whatever you're casting but it's a more narrow window than other metals with lower m.p.'s. I can carry a crucible of cast iron all around the shop and pour a heavy mold--I can't do that with any PGM. Hell, some of the PGMs can't be torch melted in a reasonable quantity no matter how hard you try.

I've cast Pt and Pd before for rings but never in quantity for ingots. I don't sell ingots of the PGMs, if someone wants them in bulk and won't take sponge (which is odd), I have sintered alumina crucibles that I just pack with sponge and melt in, topping them off til full. A full one holds exactly 100g of platinum. Turn it over when it's still hot and the metal falls out. Generally I rinse with warm water and they're off.


Lou

 

[Oz]

Thank you Harold and Louis for sharing your expertise.

I have made buttons with Ag about an ounce in mass w/o the difficulties referenced in Hoke, perhaps I have just been lucky. I have an iron mold that I need to make some anodes with and set up a silver cell as my Ag is contaminated with PGM. It is then that I will need the advice I am getting here for casting bars.

I actually chose to add Pd to the short list because of its lower melting point compared to Pt and Rh, I also have more of it. With the drop in PGM prices it is more tempting to work/play with them vs. selling.

The perlite tip will be handy. I used to anneal old heavy springs by taking them to a cherry heat for 10 minutes or so and then burying them in wood ash to cool over night. It did a great job of taking out internal stresses and leaving the high carbon soft, ready to take to the anvil to forge into tools.

It would be wonderful to have an induction furnace like Lou but I am set up rather crudely using an acetylene torch for now. I am aware of the problems with acetylene and platinum but have not heard the same with Pd. I should talk to my welding shop to see about getting hydrogen for a Hoke torch down the road.

I would be tempted to make my own hydrogen as it is simple enough to make safely the small amounts I would use. I have spare oxy tanks that i thought of using but am unsure as to the permeability of these tanks with hydrogen. Filling them to pressure would be my greatest concern, as I would want to avoid a 30% stoichiometric ratio with air when filling. Possibly purging the tank with dry ice (CO2), and then drawing a reasonable vacuum before filling. Purely an intellectual pursuit at this time as it would be lot of hassle to be sure, but fun with additional uses once set up. Has anyone here done it? Thoughts?

 

[viacin]

I'm saving all this into a wordpad file for refrence.

 

[goldsilverpro]

Lou,

I've been off the forum for awhile and just read your PM on silver. Give me a day or two to think about your questions.

This one goes out to GSP/Harold, any information on what's needed to (legally/officially) stamp a product as 9999 fine? Even if my analyses and my assayers' agree, I still am hesitant to stamp any ingot.

I agree with Harold. Either stamp it as Fine Gold or as 9995. You can pretty much rely on the pipe and crystal structure at that purity. No matter what other assay methods are available, the gold industry normally uses fire assay. Fire assay is supposedly only accurate to .02%, which makes it impossible to make a claim of 9999.

The only time I was required to make 999.9 Fine gold was when the company I worked for needed that purity for making gold plating salts. Any less purity would show up negatively in the product. We originally had about 30, or more, impurities analyzed, by spectro, on the first couple of batches, and then subtracted the total percentage of impurities from 100%. I don't know if that's the standard method now for high purity gold, but it was then.

I never stamped my gold bars and I retained the pipe. The biggest problem with a pipe is that slag gets trapped in it, if you're not careful.