99.5%? Is it possible?

[HAuCl4]

Here are 3 photos taken this afternoon by a friend. Quality is not the best.

One is of a new/untreated crucible with the injection tube in position. Missing (don't have them them), are the slotted crucible cover, the furnace, and the vacuum cleaner "fume processing" device. Maybe I'll make a diagram later this week.

Then you have the tube with characteristic purple stain (this tube bottom part was broken). And another of same tube with the latest new all time high price of last Friday for the August delivery of COMEX gold.

That ceramic tube is about the only "evidence" I still have that I once processed gold using Miller chlorination. An expert connoisseur will recognize it inmediately as authentic.

 

[HAuCl4]

I'm shocked that there was so little fume after been led to believe that was the main concern

Oh no. There are enough fumes to be dangerous, but no fumes problem if you control them at the source before they get mixed with more air.

This was easily controlled with the vacuum cleaner during the process. When pouring the slags, the chlorine injection was stopped and the tube removed, so no more chlorine, and a fume hood was connected to the vacuum cleaner and brought close to the pouring spout, and we wore masks at this point. This was the point of "maximum amount of fumes".

Also this pouring of chloride slags (into another cold crucible) had to be done carefully, because silver chloride at that temperature looked like chicken soup, only more fluid, and it was fuming. Gold stayed in the crucible, and we added more borax and glass at that point after most of the slags were out. Fumes after that were a lot less.

My guess is that with a comparable acid system for the same volume of material, the volume of fumes to control would have been bigger, although of a different nature, and not necessarily easier to process.

 

[4metals]

The corrosivity of the chlorine gas is an issue, the piping to the furnace and the storage of the bottles of gas were a major concern to local firemen. I never had much issue with the gas coming out of the reaction either. I never used an expensive Cottrell electrostatic exhaust either. I exhausted into a 6" quartz glass tube which switched over to CPVC when it was cooler at a distance of 10 feet from the furnace. The chloride fume is water soluble so it actually scrubbed easier than a NO_x fume. The base metals were easily dropped from the scrubber water for waste treatment using caustic.

Not pushing the reaction to get three nines is also smart. If you stop when the clay tube turns purple all of the gold is in the pot. I melted 700 ounce lots of karat (fine gold content) and used French Clay parting crucibles which were used for 1 melt. When the clay tube indicated the reaction was complete I pulled the tube and removed the crucible to let it cool. I never bailed chlorides off the top. In the morning the cold crucible was smacked with a hammer and the 99% fine gold was on the bottom and the silver chlorides had solidified to look like the top of a lemon meringue pie. These chlorides contain between 1/2 and 1 percent by weight fine gold beads which were trapped in the thick foamy chloride. They were crushed and reduced to metal and processed in a silver cell to recover the gold as a slime.

There have been many authors who have detailed the process used at large refineries, these reports combined with a knowledge of chemistry and the ability to experiment helped detail the process.

I still wouldn't take the chlorine gas lightly, it will kill you if there is an accident and it will be a painful choking death. For this reason I advise people to stay away, it requires meticulous maintenance of the delivery system and safety systems for the one time when it goes bad. Continued success makes us complacent, then it happens. Look at what happened in the Gulf.

Considering the non chemical backgrounds of some members I would never recommend the process.

 

[HAuCl4]

Nice idea of using the crucible just once and crushing it. I bet you had hardly any "fumes problem". We recovered most of the gold inside the slags by sprinkling soda ash in the molten slags (in another crucible).

The chlorine cylinder (we had one at a time only) was kept inside a large water tank outside the room (in case of leaks), and a plastic hose inside a hard plastic tube was ran into the furnace room. We never had any incidents, so probably I was taking more risks than I knew at the time.

 

[4metals]

I chose CPVC because of the temperature. Something about exhausting over a furnace with plastic made me nervous, that's why I fed the fume directly over the furnace into a quartz glass pipe for 10 feet before transitioning to CPVC. (Did you ever see a PVC fueled fire? I did once in a plating shop it was like someone threw kerosene on it.) I suppose the physical scrubber could have been PVC but the deal I got at the time had it all in schedule 80 CPVC. The scrubber could have also been fiberglass, I like the vinyl ester resin as it withstands the corrosivity well.

Using a single cylinder and having it in a water tank is a good approach but we were doing 6 to 10 melts per day so we needed a bank of 8 tanks. Because of the way they were delivered and the distance, we purchased chlorine 4 tanks at a time. They were outside, 100 feet away from any building and enclosed by fire department rules. All of the delivery piping was double walled and we even ran it inside a dedicated 6" exhaust duct constantly pulling air out of the shop. That duct ended about 5 feet from the point of use. A pressure gauge told us the line pressure and in case of a leak in the feed the air was already going out. Fortunately the exhaust ducting on feed piping was never put to the test. When I design something I always make it like I have to be the guy over the furnace, sometimes I was, so I'm kind of anal about the safety I build into a process.

We never processed the crucibles separately from the regular melting room crucibles and stir rods, so I don't know how much metal was in the crucibles but I do know that our accountability of the gold between the melting and the slimes from the silver cell ran inside the margin of error for fire assays so it was negligible.

My point again about Chlorine gas is you can make it safe, even super safe, but I don't believe you can make it backyard safe!

 

[HAuCl4]

Maybe a little paranoid about the chlorine? :lol: Although 8 tanks in a fire could easily get out of hand...

It looks very safe and simple and not too costly. I think you did good.

Can you describe further your furnace/crucible setup?. Electric, induction or gas-fired?. Tilting furnaces?. Crucible covers?. You already said that you didn't slag the melts.

Did you further flux the pure gold afterwards with nitre/other?. What was your average fineness stopping at just the "purple stain" point without pushing further?.

 

[4metals]

The melting was done in an induction furnace, I prefer the push up type. I never covered the crucible because the ability to see the melt and control the chlorine feed rate was important to me.

The purity averaged 99.2% with silver being the remaining impurity. For that reason toughening with niter wouldn't help much.

Our goal for purity was .9995 back in the day so upgrade was necessary. For reasons of speed, and so no metal was ever out of the vault overnight, the 99 fine gold was shotted and digested in aqua regia. The acid stayed a beautiful red color as there were no base metals and due to careful nitric additions we were gassing by lunchtime. This process produced .9995 metal or better every time and acid use was half that of putting up karat gold directly. An added benefit was waste treatment was easy due to the lack of significant base metals.

If I had to do it over, I would consider solvent extraction to get to .9999 but in the early '80s I was happy with removing the bulk of the impurities quickly with chlorine followed by an aqua regia cleanup.

 

[HAuCl4]

Wow. 1 day refining with AR!. What vessels did you use to digest those big amounts in AR?. Glass or something else?. I bet you had a "fumes problem" bigger than with Miller.

How/to whom did you sell your refined products?. Mint or something?. Did they pay full price?.

I have avoided AR like the plague, except for a few experiments and batches containing palladium and platinum. Call me paranoid but never liked the idea of a broken vessel full of gold in solution, even with safety measures, it could get expensive very fast.

I still cringe when I see videos of people with a glass flask on top of an electric heater, nothing underneath, and dissolving gold.

After the failed Miller refinery "experience", all I did was inquarting 23-77 with silver and nitric digesting, whenever I had accumulated enough from small batches and running low on fine. It has been a hobby more than a business (or never a business, but didn't cost me money, but could never live off it??. Maybe that's better put.). I routinely got 997-998 with inquarting, and that was good enough to exchange with the jewelers. In the very rare cases where my gold didn't roll like butter, fluxing with nitre and/or ammonium chloride fixed the problem.

I always kept the best pieces that I bought directly (not from jewelers) and cleaned them and attempted to resell them at a premium instead of melting them and refine them.

Haven't refined anything in years, but maybe soon. Will have to borrow a torch here, and a crucible there...and steal some cookware from my wife. :roll:

Most of all what I liked best was to learn about human nature, if you get my drift...

 

[4metals]

250 gallon steam heated Pfaudlers will handle 6000 ounces per digestion, considering the gold was in at 99% that's 5900 plus ounces out per batch, it went up quickly and scrubbing with caustic in one scrubber and peroxide in the other there was good fume control.

The gold was shipped overseas and Brinks was there every night for bars fine going out and karat coming in. They paid a few cents under per ounce, like $.20, if I remember.

Made a lot of money but worked a lot of hours, I like the consulting business much more and the hours are better.

 

[HAuCl4]

Very nice. Excuse my ignorance: What is a "Pfaudler"?.

I once helped "improve" an AR digesting setup made of double walled PVC, but I was never happy with the original design, and how they handled the filtering, etc afterwards, etc. I collected my small consulting fees, left them a list of recommendations, and moved on. They didn't/wouldn't want to change much, just to take care of the fumes, and some safety aspects. It looked very unprofessional setup and handling. Afterwards, I heard they broke a 5 gallon pyrex full of dissolved gold and had to "mop it", etc, etc.

I stopped giving free/unsolicited advice to people not long after, and started charging more for my services!.

Edit: Found in google: http://www.pfaudler.com/

 

[HAuCl4]

Do those glass lined reactors/vessels wear inside?. How did you go about that?.

How did you filter afterwards, before precipitation?. Dropping with SO₂ I presume or other?.

I'm fascinated!. You are/were the largest commercial refiner-person I have ever met, asides of Mr. Trepanier from the Canadian Mint!.

Thanks for the answers!.

 

[goldsilverpro]

Pfaudler is probably the primary manufacturer of glass-lined kettles/reaction vessels. The type I always used was made of steel and had a thick (I would guess about 1/4"-1/2") blue glass lining on the inside which was impervious to acids. There was a steam jacket around it for heating the solution inside. I usually used a 50 gallon one but they are available in any size, starting with 5 - 10 gallons. They cost many thousands of dollars new but there are tons of used ones available. For dissolving large quantities of metal, like karat gold, they can't be beat. Never buy a used one if any part of the glass lining has been damaged or patched. They have patch kits but they're not worth a damn. About the only way to properly repair the glass is to send it back to Pfaudler for re-lining - terribly expensive. When putting material into the kettle, don't drop heavy parts into it. I always placed the material in it carefully before adding the solution, to avoid chipping the glass. The glass is actually quite tough and durable but, you have to still be very careful.

I tried to find a photo of just the kettle but couldn't find one. Most have stirrer and pressure assemblies on top but all you need is the kettle (the vessel), itself. In this photo, for most dissolving, all you need is the bottom part, the kettle. Most people cover the kettle with a sheet of heavy inert plastic or fiberglass. About 2/3 to 3/4 of the cover is hinged and is raised in order to add or remove material. Usually, an inert exhaust pipe (usually high temp plastic) is mounted vertically to the back stationary part of the cover, in order to suck out the fumes and transport them to a scrubber. The shape of the kettle in the photo is fairly typical, but they come in various shapes. At one place I worked, we had several new dish-shaped kettles, 5 or 6 feet in diameter and a foot or two deep.
http://www.alibaba.com/product-gs/213307266/Glass_lined_reactor/showimage.html

I just found a photo of the inside (and, outside) of a small 10 gallon one. The ones I used were solid on the bottom, without the drain hole as in this photo. Click on View 2 more Photos and then click on Next or the small photo on the left.
http://www.aaronequipment.com/UsedEquipment/Tanks/Glass+Lined/77183.html

 

[HAuCl4]

Thanks GSP. Hanging a basket made of fiberglass inside the PVC reactor was the way the folks did it in the setup I inspected (max 200 Oz at a time, so small reactor). That way the metal never touched or wore the lining of the reactor. Still this Pfaudler looks like A LOT better piece of equipment.

How did you ladle or siphon the stuff out, solid and liquid, for filtration, etc.?

 

[HAuCl4]

I made a quick draw (sorry for the quality) for the PVC design that I mentioned before. I added another PVC valve at the bottom to the original design that I encountered and bettered somewhat.

The removable basket sits inside the reactor with the gold shot, it has a bottom with several large holes (like a colander), and a fiberglass filter.

The cover has a tube to plug the fume scrubber sucking air.

Basket was loaded with the shot and put in place, then hot HCl added, then nitric. After reaction complete, the basket was removed with most of the AgCl. The rest of AgCl settled on the bottom and was "brushed" towards the drain hole, with first valve open and bottom valve closed. Then closed the top valve and drained into a bucket with a little gold chloride/HCL by opening the bottom valve. Most of the chloride was out at this point. Then the main solution was filtered with a Buchner funnel made also of PVC. The Chloride washings, etc, also filtered.

Then precipitation with SMB in the funnel vessel.

It seems efficient in writing, but it was quite messy in practice, in my opinion. Too much man-handling of liquids, and rolling in, rolling out vessels, funnels, etc.

 

[4metals]

I found that digestion was both quicker and more complete under a little pressure, our reactor was an open topped, and no outlet on the bottom of the reactor. To create pressure I had a very heavy lid which was also coated with the blue glass, the top had a tab connected to a chain hoist to lift the reactor top off. The seal was made with a heavy flat viton gasket which sealed well under the full weight of the lid.

The top also had a 2" hole which I connected to a 24 inch diameter by 48" deep cylindrical tank. There was also a 1" hole for chemical feed. The 2" hose went in the bottom of the tank. On top of the tank was an exhaust to a scrubber. I had sight glasses so I could see how much water was in the tank. If I kept the tank empty, the reactor operated at atmospheric pressure, but if I added water to the tank the pressure inside increased to displace the weight of the water.

It was an easy cheap way to pressurize the reactor.

The top was lifted off after disconnecting the hoses and the steam lines (all quick disconnect) and the solution was siphoned into a filter and by using a vacuum filtration setup drawn directly into a gassing tank which was fiberglass reinforced to withstand the vacuum so the solution was pulled directly into the tank where it was to be gassed. Less handling less losses. The final bit of solution and any chlorides were removed by dumping the reactor into buckets. The reactor was mounted to pivot about its center of gravity when empty. So emptying was easy.

I've seen the wok shaped glass lined reactors GSP mentioned, they were used to concentrate gold potassium cyanide to make salts at a Lea Ronal refinery in Freeport. I went to an auction last winter at that plant, which had changed hands to Rhom & Haas and the reactor was there for bidding. The glass was in rough shape though so I passed. GSP is right to have them re-lined is very pricey.

 

[goldsilverpro]

4metals,

Excellent suggestions for creating some pressure. A little pressure always helps.

We siphoned what we could and dipped the rest out. I always fashioned dippers by cutting a round plastic gallon jug in a certain manner.

 

[4metals]

GSP Wrote;

We siphoned what we could and dipped the rest out. I always fashioned dippers by cutting a round plastic gallon jug in a certain manner

So you were the master of purple fingers in your day! 8)

 

[goldsilverpro]

So you were the master of purple fingers in your day!

True. The purple always came off with cyanide, though.

 

[Harold_V]

So you were the master of purple fingers in your day!

True. The purple always came off with cyanide, though.

That worked for me. (Sure did leave your fingers feeling slimy, though!)

Harold

 

[HAuCl4]

Thanks for the detailed description 4metals. A few questions:

Did you get rid of fumes/vapors at the end before lifting the top off?.

Was there an HCl stink at all during the operation?.

How did you load the shot inside the tank at the beginning?. 180 Kg of material to load is not trivial.

What about the acid pumps to load the reactor?.

Also: Were there any bits of undigested metal trapped in the chloride slimes?.

Thanks also GSP.