Calcining colored PGM powders.

[Quirkzunc]

I would like to calcine my colored PGM powders but I don't know how to properly deal with the copious amount of white smoke that is created by subliming ammonium salts in the process. Can anyone who has done this give me some help with a good way to prevent this from being a problem.

 

[Harold_V]

My process was to do the calcining in the fume hood. As you alluded, there's considerable white smoke that is liberated. It's important that the smoke not contain traces of color, otherwise you're losing some of your values in the smoke. Heat the salts slowly, keeping close watch by placing a cold object in the smoke. It should condense smoke without any color. If you find color present, reduce the heat.

As a side note, my hood was filtered to recover, at least in part, lost values. The effort to filter a fume hood for value recovery is time well spent, and can be rewarded with the recovery of several ounces of gold, depending on the nature or the work you do. My main source of refining was bench wastes from jewelry manufacturing----including polishing wastes. I incinerated such wastes in my fume hood, which likely accounts for the majority of lost values due to dusting.

As the salts calcine, you'll notice that there's a considerable amount of white residue that accumulates on top of the lot being calcined. Not being schooled in what may be proper, I simply continued heating until all the salts were calcined, then I'd raise the temperature of the entire lot until it was, for lack of better description, burned off. The reduction in volume is severe, and it often leaves the top portion suspended. My procedure was to push the suspended portion down until it was in intimate contact with the sponge. For this I used a porcelain spatula/spoon. The end result, assuming your solutions are not heavily contaminated, are fairly good quality platinum.

Hope this helps.

Harold

 

[Quirkzunc]

Harold,

Yes! Thank you, that was a very detailed and helpful answer. I will try this with a small quantity and see how it goes.
I usually handle around 1kg of platinum at a time. I can imagine quite a large quantity of smoke will be expelled from this. How much did you perform this procedure on in one go comfortably?
I am thinking along the lines of using a kiln with a special trap for the condensing powder. Do you think so? Or do you think that the method that you have outlined would still be suitable for handling this much.

Regards
QZ

 

[lazersteve]

Harold,

I know you mentioned fused quartz dishes for the melting operation with PGMs, but what about the calcining process? Is a fused silica dish acceptable for both processes? Here's a photo of the dish I purchased for calcining and melting my powders:

[img:503:295]http://www.goldrecovery.us/images/silica_dish.jpg[/img]

My main concern is that the dish is not glazed like porcelin, but rough like a melting dish.

Any advice would be greatly appreciated.

Steve

 

[Harold_V]

How much did you perform this procedure on in one go comfortably?

I found my limitations to be controlled by the vessel in which I did the calcining. Fused quartz dishes are very expensive, and are not readily available in many sizes, at least not from the sources I had at my disposal at the time. I also found that a Fisher burner was fairly well suited (size-wise) to the dish I used. While I don't recall the volume of the dish I used at this point in time, it was roughly 2½" in diameter. Small lots were calcined of necessity-----so it took considerable time to do large volumes of the salt. For me, that was no problem, my other work went on as it ran in the background.

I am thinking along the lines of using a kiln with a special trap for the condensing powder. Do you think so? Or do you think that the method that you have outlined would still be suitable for handling this much.

The first consideration would be the heat source. If you're talking about an electric fired kiln, my immediate reaction is to suggest that you haven't thought the process through. I'm of the opinion that you'd destroy the kiln. You can't pretend the gasses are harmless, nor that they can be eliminated with a trap.

As I recall, Hoke recommended playing a hydrogen torch on the surface as the calcining progressed. I didn't have the luxury of a hydrogen torch, so that is a process I didn't explore.

I'm a creature of habit. If I once found a process that bore fruit, and it wasn't inconvenient, I closed the door on further research. My objective wasn't to build a new mousetrap, it was to refine metals, so I went with known and proven methods, as long as the process was acceptable in my mind. I was more than happy to do so-----and encourage those that are not research oriented to do the same. You can spend your entire life screwing around with a project that could be easily handled by known methods, achieving your goals instead of spinning your wheels. The choice is yours. There is no right or wrong in this issue, not to me. It's all in your objective.

Harold

 

[Harold_V]

I know you mentioned fused quartz dishes for the melting operation with PGMs, but what about the calcining process?

I'm not sure if I said it that way, or you interpreted it that way. In any case, my intention was to suggest fused quartz dishes, those found at chemical supply houses, for calcining, not melting. I highly recommend you purchase a melting dish that is made, specifically, for melting platinum (and palladium).

My first purchase was a pair of dishes that I recall to be made of a product referenced as Vicor. I may have that wrong. Unlike fused quartz, they were clear in color, and very smooth. The small dishes worked well for palladium, but not for platinum. That, of course, was likely due to my process of playing my (natural gas and oxygen Hoke) torch on the sponge after being calcined. I raised the temperature to the point that all off gassing was eliminated. I found that the platinum sponge fused to the dish, and would release only by taking a small portion of the dish surface with the sponge. I then purchased a fused quartz dish, which didn't present the same problems. My observation was that the temperature that the dish could withstand was well below the melting point of the platinum, so the dish was softened, fusing the sponge to the dish. I have no explanation for the same results with palladium----but it was not a problem. It could be that the surface oxides (palladium) played a role----dunno.

Is a fused silica dish acceptable for both processes?

I would not recommend a dish of that nature be used for melting, not at any time. Melting dishes are readily available for platinum. They resemble fused quart in that they are not clear in color, and are far less expensive. When you melt platinum, you can't avoid melting a small portion of your dish, for the melting point of platinum and quartz (silica) are near one another. You'd severely impact the useful life of your dish were you to use it for melting, possibly destroy it with one usage. A melting dish will be useful for a prolonged period of time. Check with a jewelry supply house for melting dishes made specifically for platinum group metals.

You'll quickly come to realize that you are well limited on the amount of platinum you can torch melt. The largest button I ever melted was slightly over a troy ounce. One larger could have been melted, but I question how much larger. It's very difficult to maintain a uniform heat with platinum because of the extreme temperatures involved. A torch, even a large Hoke torch with a rosebud, is limited in ability. Assuming you could afford the luxury, an induction furnace would be the best choice. They are not really the realm of the hobby refiner.

My main concern is that the dish is not glazed like porcelin, but rough like a melting dish

That would concern me as well, but if you find the sponge will release, I'd go with it. You'll find that you can't remove the sponge totally, as there will be miniscule amounts that form and won't release. My experience proved that the dish, for lack of better terms, became "silvered" with platinum. Don't use your dish for anything but the intended purpose. In other words, have one for platinum and one for palladium. That way you'll minimize the cross contamination. You'll likely figure out soon enough that it isn't easy to make clean separations of one metal from the other.

Harold

 

[lazersteve]

Harold,

Thanks for the insight. I guess I'll hunt down a few more dishes for the Platinum group metals Pt and Pd.

As a side note the dish pictured is sold as a Silminite Roasting Dish (?) and is described as fused silica. It's softening point is listed as 1665C.

Steve

 

[Quirkzunc]

Harold,

Did you ever try using any of the chemical methods for reducing the colored powders, Such as hydrazine hydrate or the formic acid and ammonia solution mixture?
I have used both of these methods. The hydrazine I found to be very effective at reducing all of the colored powder, but I did not like working with a substance with such a high toxicity. For this reason I went back to the formic acid and ammonia mixture that is outlined in Hoke. Although this method is a bit more time consuming and I cannot seem to get a complete recovery I still use this because the formic acid and ammonia are both relatively harmless compared to the hydrazine.
I do remember dabbling with calcining a number of years ago but abandoned it quickly because I could not see a way that I could easily cope with the large quantity of white plumes that are given off. I have been reluctant to have another look at this method again without knowing that I can contain these emmisions. The company that I work for has grown cosiderably and we have many more people around to potentially be affected by any emmisions should they get out of hand.
I have taken in what you explained about having a cold surface upon which to have the white powder condense and we do have a filtered exhaust fan, however this fan is constructed with galvanised iron ductwork and I dont want to introduce any acid vaours to it and start destroying it.

I would be interested to know if you ever compared these methods before settling on calcining.

Regards

QZ[/quote]

 

[Harold_V]

It's softening point is listed as 1665C.Steve

You'd likely have problems with it working with platinum, assuming you worked towards the melting point, which is greater than 1665C (3224F, or roughly 1773C) I can't help but think it would work quite well for calcining, however. You can use porcelain for that process, but it's so sensitive to thermal shock that I quickly moved away from its use.

I don't recall what I paid for the small diameter fused quartz dish I purchased, which was larger than the Vicor (?) pieces, perhaps 3" in diameter, but I recall it was no less than $50, and that was many years ago, when a dollar was still worth 45¢ .

Harold

 

[Harold_V]

Harold,

Did you ever try using any of the chemical methods for reducing the colored powders, Such as hydrazine hydrate or the formic acid and ammonia solution mixture?

I did not, nor did I have any desire. As I've said, my objective was to refine metals, not to research various processes. I knew that the volume I'd generate would never be large (platinum group), and understood that the process recommended by Hoke worked to satisfaction, so I followed her instructions and was satisfied with the results.

I could not see a way that I could easily cope with the large quantity of white plumes that are given off. I have been reluctant to have another look at this method again without knowing that I can contain these emmissions. The company that I work for has grown cosiderably and we have many more people around to potentially be affected by any emmisions should they get out of hand.

I can't imagine a method by which workers would be exposed, not unless you don't work in a fume hood. There's not a doubt in my mind that a system could be set up to contain the emissions---perhaps going so far as a wet scrubber. It's a matter of how far you'd like to pursue the issue. There are certain hazards included with working with metals, as we all understand. We make calculated decisions and go accordingly.

There is considerably more knowledge available for the platinum group than is presented in Hoke's book, some of which I possess, as does GSP. I've not researched any of it, nor did I apply any of it. As I said, I was happy with the results I achieved from following Hoke----but would quickly explore the newer information were I still involved in refining, especially if I found myself facing large volumes of platinum. In all my years of refining, the total amount I processed was about 75 ounces. Hardly a large volume.

I have taken in what you explained about having a cold surface upon which to have the white powder condense and we do have a filtered exhaust fan, however this fan is constructed with galvanised iron ductwork and I dont want to introduce any acid vaours to it and start destroying it.

My recommendation to check the discharge by that method was not intended as a procedure to eliminate the emission. The suggestion I mentioned is also in Hoke's book, and is intended to insure the operator that he/she is not losing values, nothing more. To condense the fumes on a galvanized surface could prove a mistake, as you allude, and would not be very effective in eliminating all of the fumes. My entire hood/exhaust system was made of asbestos, which bordered on indestrutable from exposure to acid fumes. The degeneration was exceedingly slow, with all components still very serviceable after more than ten years of almost constant use. It was not uncommon for my hood to run for weeks without being turned off.

I never looked at the discharge from my hood, but I am of the opinion that the vast majority of the fumes were condensed on the filter, which was nothing more than a fiber glass furnace filter. They are key to easy recovery of values after the fact. The filters were incinerated as they were replaced, with the ash placed in storage for future furnace reduction. Considering silica is very much a part of the flux recipe I developed, the small amount of glass added from the filter was actually to advantage.

Harold

 

[Quirkzunc]

Harold,

Thankyou for you time. It has been good to read about the travells of someone who has been down similar paths as I have. Your clarity of thought on the the subject is enviable.
I will post the details when I have some news about my trials calcining trials.

Regards
QZ

 

[skeeter629]

Does anyone have a suggestion as to where one can purchase the fused quartz dishes for calcining? I would like to find the ones like Steve has in his PGM DVD. I have studied Hoke and Steves DVD. I have approximately 10 pounds of catatlyc converter beads, and want to have all the other necessary supplies before starting. Some warmer weather would be nice as well.

 

[lazersteve]

I bought mine from Technical Glass Products.

These guys are great, I have had them build all of my quartz lab ware and they are excellent at it. I wish I had an unlimited budget to get them to fab me an entire lab worth of quartz goodies.

Steve

 

[rusty]

I purchased these from flebay.

 

[Harold_V]

I purchased these from flebay.

I paid over $100 for one of those things more than 25 years ago.

Harold

 

[rusty]

I purchased these from flebay.

I paid over $100 for one of those things more than 25 years ago.

Harold

These were listed by a seller located in Ontario at $35.00 each, at first only purchased 2 giving lasersteve a heads up on the auction. Then after a couple of days the last remaining fused quartz dishes were still on auction bought these.

Steve thought that the rounded bottoms would pose a problem for calcining, I have not used the dishes yet so I'm not able to make a comment on how well they work or do not.

Because of Steve's concern of the rounded bottoms performance had been keeping an eye open for flat bottom dishes to appear on ebay, in the past year nothing has been listed.

So what is the verdict Harold will these rounded bottom dishes work for calcining my precious metals salts. To make myself clear the bottoms are not totally round the dish will sit perfectly with out falling over.

Also I though of using them on a sand bath to keep the temperature down to avoid losing values in the smoke.

Regards
Gill

 

[skeeter629]

I would use it in a sand bath as well, to help control the heat. You also wouldn't have to worry about it tipping over. Gill - If you would like to sell one, PM me.

 

[lazersteve]

Gill,

I never got around to buying any of the dishes although another member did turn up with quite a few.

The round bottom ones would work for calcining, but the inner layer of the salt pile would tend to be protected from the heat unless you drive it up pretty high, or stir the salts during the process.

Now days I use zinc in very dilute HCl to convert my PGM salts, a lot less smokey and much quicker to boot.

Steve

 

[Harold_V]

So what is the verdict Harold will these rounded bottom dishes work for calcining my precious metals salts. To make myself clear the bottoms are not totally round the dish will sit perfectly with out falling over.

Mine was round, and it worked perfectly well.

Also I though of using them on a sand bath to keep the temperature down to avoid losing values in the smoke.

I'm not convinced that's necessary. Unlike Vicor, fused quartz is very stable where thermal shock is concerned. In theory, you should be able to plunge one that is red hot in water and suffer no consequences. Maybe if the were free I'd try that, but suffice it to say they are very rugged so long as you don't drop or hit them with a hard object.

I think you'll find in calcining that you can get by perfectly well with a Fisher burner directly on the dish. Start with a low heat, but as the calcining progresses, a full burner can be applied with no worries. I did that with mine, and I had one pound delivery on my gas pressure (not the commonly available four ounces) and had no problems with controlling heat, nor with over heating.

To be truthful, if you can convert by chemical means (as Steve suggested), I think I'd endorse that process, especially with palladium, which oxidizes readily. It's really pretty to see, but not desirable.

In any case, you sure didn't go wrong buying those fused quarts dishes. Prior to buying mine, I had used a Vicor dish for platinum and found I had fused some platinum to the dish, thanks to it melting beneath the converted platinum. You won't have that problem with quartz.

Harold

 

[Macgiver]

Gill,

I never got around to buying any of the dishes although another member did turn up with quite a few.

The round bottom ones would work for calcining, but the inner layer of the salt pile would tend to be protected from the heat unless you drive it up pretty high, or stir the salts during the process.

Now days I use zinc in very dilute HCl to convert my PGM salts, a lot less smokey and much quicker to boot.

Steve

Hey LSteve,

Can you direct me to some good instructions on using zink and HCl to convert my 3 pounds of Ir salts? Not looking forward to all the smoke and time involved calcining.

Thank you mucho.

CW