The assay / refining lab..

[4metals]

Layout of a 10 by 20 garage for assay / refining


If I were to make a small refinery with a capacity for fire assay and acid refining the following working plan would be my starting point.

Divided down the long wall into 2 rooms 10 by 10 each. Access to the rear room through a 30” door.

The acid hood is on the end with the garage door.

The acid room contains a 4 foot long 30 “ deep hood and a scrubber. This hood will do double duty both as a hood for acid refining and a hood for assay parting. (not at the same time)
A cabinet to store all chemicals. (Heated with a thermostat and light bulbs for cold winters)
A sturdy work table. Windows behind the work table can provide make up air in warmer weather and can be equipped with an exhaust fan to remove dust if you are performing disassembly of e-scrap. Storage beneath, for drums and buckets.

The rear room has a section divided off to house an analytical balance and a balance room work table with cabinets above. The main part of the rear room has a metal hood to house an assay furnace which can do double duty as a melting furnace to melt your fine gold. You could also do limited burnouts in a metal hood. One wall has a sink for lab use and to be shared with the acid room, and a lab table with cabinets above.

If you were a one man show you could push out some serious refining out of a small place like this. All you need is the scrap.

A few years after this thread was written I had a client set up a "container refinery" in a 20 foot shipping container. Very similar to this. He was a gold buyer and needed to do stone removal and assays, he kept the furnaces outside the container which was inside a 750 sq. ft. building with a garage door in the back. He could only get a 1 year lease so we built the refinery so he could move and reset up quickly as his leases expired.

 

[Harold_V]

If you were a one man show you could push out some serious refining out of a small place like this. All you need is the scrap.

I was a one man show, and turned out one hell of a lot of gold and silver, and with far less space than you have provided.

Do keep in mind I had a lot more space for other appliances and devices, but my fume hood and counter area, which included a laundry type sink, was all contained within a space that was no more than 8' x 14'. That was by design, so I could perform all of the required functions without walking far.

Nice layout, 4 metals!

Harold

 

[semi-lucid]

If you were going to describe the capacity of a refinery by the size of the reaction vessels, how would you characterize the following vessel sizes;

5 Liter
12 Liter
22 Liter
50 Liter
72 Liter

The number of vessels in use would also be relevant, but at what point does it become practical to use a larger vessel?

And with cylindrical and spherical vessels, do you generally work at about half full, or maybe a little more than half full? Need to leave room for foaming up?

The reason I ask is this affects the fume hood design. I do recall 4metals writing about using a 72 liter vessel outside of the hood, but I would rather have the vessel inside the hood, even if it is connected to a scrubber.

I'm curious to know what size vessel Harold generally used in his operation.

John

 

[4metals]

Semi

Based on the sizes you are listing you are talking about stock glass vessels with 4" conical necks. The nice thing about them is you can install a head on the reactor and direct the fume into a condenser which allows you to use less nitric. Plus the vessels are interchangeable because they all have 4" necks.

The standard loading capacity for karat gold is as follows
5 liter 20 T.O.
12 liter 50 T.O.
22 liter 100 T.O.
50 liter 200 T.O.
72 liter 300 T.O.

These quantities can be processed by filling the reactor a bit more than half way. The actual in practice numbers I use are 7.5 T.O. of total metal per liter of acid.

The following pictures are of a setup I did for a small refiner. The reactors are 5 and 22 liters. Notice the condensers are permanently mounted on the wall and connected by a corrugated teflon hose. This way the exhaust setup after the condenser is always lined up because the condenser never moves. Plus they're fragile and expensive so once they're mounted it is pretty safe and secure.

 

[semi-lucid]

Once again, thanks for the information. If I ever get this going, I'm going to have to send you a consultants fee.

Those look like Allihn condensers, maybe 500mm or so. I need one of them. I could go for those flasks too.

No fume hood at all in the picture. With the exhaust system and a little room ventilation, I guess you can get by without a hood. But I would still rather have one.

John

 

[4metals]

The condensers are vigereaux (sp?) type. 24" long for up to a 22 Liter flask with a 29/42 tapered fitting. The bigger reactors need a 36" condenser I believe with 45/50 fittings.

The reason I don't use fume hoods with the reactors is the exhaust is up so high due to the condenser it's hard to squeeze it all in. Besides, the reaction is contained, I never disassemble the reactor when it's hot, or even warm for that matter. Always the next day, so there are no fumes. All of the acid filtering and transferring and precipitation is done under a hood.

Another thing you probably can't see in the picture is the heads for the reactor are custom made. The leg connected to the condenser has alot of condensed acid dribbling back into the reactor. In a normal head it dribbles down the side. This always seems to find its way across the ground glass seal and drip out. Never fast, but the clamps are metal, could never find them made of anything else, so first the paint goes and then the die cast and you're buying more clamps. I have my glassblower add a small extension inside the head so the acid drips down and falls back into the center of the reactor, away from the seal.

If you look in the photo of the 5 Liter reactor working there is a spare head on the shelf to the left. You can see the modification I'm talking about there.

A good scientific glassblower is a good guy to know.

Noxx edit: Vigreux condenser

 

[semi-lucid]

The condensers are vigereaux (sp?)

Ahh "Vigreux" condensers. That's not what I'm looking for. I just want a simple condenser for a distillation rig.

All of the acid filtering and transferring and precipitation is done under a hood.

That makes sense. Handling those 22 liter flasks would require care and caution though, and a fifty liter would be a bit much to pick up and pour. I've been thinking about a tilting holder. I like to build computer models of anything I'm thinking of building, and that's the stage I'm in right now.

I have my glassblower add a small extension inside the head so the acid drips down and falls back into the center of the reactor, away from the seal.

That's a good tip.

John

 

[Lou]

A good scientific glassblower is a good guy to know.

Truer words!!! They are a rare breed, but most any of them that are in the business have to be good.


My questions:

How do you stir these reactors? The usual overhead stirrer deal with a teflon stir blade? Gas stirring? Pump stirring?



Comment:
I don't have any photos handy to show you what I'm talking about, but you might want to consider this arrangement instead of the usual flange for a reactor head--the clampless design:

It's more fragile and more expensive, but you can avoid using a clamp as the fumes won't leak out easily. A little bit of Kel-F (you should use this now!!) put into the groove will make for a great seal.

Some of the problem might be that you're gasket/o-ring is the wrong elastomer and doesn't have enough give. You can always put a sacrificial copper gasket in lieu of PTFE or Viton.

 

[Harold_V]

I'm curious to know what size vessel Harold generally used in his operation.
John

Aside from one handled Pyrex cylinder, size of which I do not recall, but it held about five gallons, I used nothing larger than 4 liter beakers and cylinders. Cylinders were used exclusively for precipitation of re-refined gold, with a target volume of roughly 18 ounces. Any more and great difficulty was experienced in precipitation. That was in spite of adding three liters of ice to the solution prior to precipitation of the gold.

You'll come to discover that large vessels get expensive real fast---so unless you have a real need, I recommend you stick with beakers 4000 ml and smaller, and use them in multiples when necessary. Unless you have an exceptional supply of material to process, it isn't likely you'll have need for large vessels.

Do keep in mind, my method of operation was to process individual batches, to avoid having to assay. Worked for me, and very well, for the entire duration of my business.

Harold

 

[4metals]

Lou,

I started with reactors in the mid '70's and at the time the joints were all ground glass. To replace a reactor to accommodate a new head seems excessive so I've always stuck with what I had because a collection of vessels which matched and were interchangeable with a bunch of heads. If I were re-tooling today I would look into the heads you describe. I use stopcock grease on the ground glass surfaces and clamp it. As I said to Semi, if you route the return from the condenser so it doesn't flow across the seal it's fine.

I never use a stirrer on these reactors for processing karat gold, never have, never needed it. For one I only mix karat so the silver is under 7 1/2 % so it doesn't crust over. Sometimes some small chunks get buried in silver chloride but I just leave it in the chlorides when I filter and reduce them with sugar and assay the silver bar for gold. As long as I know where the metals are I don't go after them on a lot by lot basis, that's too costly. I believe in assaying for accountability.

These vessels also are quite handy for chemical stone removal. For this process you do not want to use a stirrer of any sort, the stirrer moves the loose diamonds in a circular pattern and scores the glass. You only have to lift one of these reactors after this happens, from stirring with diamonds, one time and have the bottom fall out to teach you the lesson. Don't ask me how I know this!

For simplicity sake Harold's setup of multiple 4 liter beakers can process a-lot of gold too! It comes down to lot size. For the small lab we are describing in this thread, Harold's way may do just fine. I just hope everyone knows to use heavy walled beakers for less breakage.

 

[qst42know]

My how I envy you big budget guys.

4metals. There is only one thing I would change about your room layout. The sink (with an eyewash fitting) should be in the acid room.

 

[4metals]

42know

Maybe it's because I work with the big budget guys I think this way. The EPA loves to drop in on the bigger guys and they don't like drains in the acid room, the room next to it is OK for some reason I don't get but in the acid room they think you're dumping everything so they make you monitor the discharge point. That gets expensive because a certified lab has to do it.


Your point about the eyewash is well taken and if you notice the sink is right in front of you when you open the door, I put the hinges that way for quick and easy access from the acid room. I'm kind of partial to assayers, and we like to keep our glassware clean so a lab needs a sink too!

 

[Lou]

Dow grease (stopcock grease) is great for vacuum work and keeping stopcocks nice. Kel-F is a whole different animal--it's a fluorinated grease akin to other fluorocarbons with regards to inertness and it has a very low vapor pressure. It is not reactive at all--you can boil it in aqua regia and it would have no effect.


Beakers work great for evaporation and removing nitric acid, but I do not like them for dissolution--too much potential for spatter even with a good watch glass. If it's something really small, then erlenmeyer flasks are the way to go because most of your refluxing nitric and hydrochloric acids will condense before they make it out of the top neck. For doing really quick removal of nitric acid with zero losses, I suggest a rotary evaporator. They cut down time drastically.


Lou

 

[qst42know]

If you could lay it out anyway you wanted it perhaps a conjoined wall with a waist high eyewash bowl in the acid room? Something on the wall not very practical for dumping? OSHA not the EPA likes a clear path. Not from experience mind you, but just a thought.

 

[semi-lucid]

Getting back to assaying for a minute. What would be the weight of a typical assay sample? Is there a range? I know I need to read a book or two to find the answers to questions such as this, but I haven't taken the time to read about assaying yet.

 

[goldsilverpro]

The sample size depends on the type material you are assaying. It also is determined by the size cupels and assay crucibles you are using. If you are assaying karat gold or other high value material, most people use 0.25 gm or 0.5 gm samples. It is always best to run at least two samples (duplicates).

 

[4metals]

Fire Assay of Karat scrap

Equipment for fire assay of Gold and Silver

Analytical balance capable of weighing to 0.0001 grams
Assay furnace (Vncella kilns) 8" x 8" x 8" nice small heats up quickly easy rebuild
Sheet metal hood and exhaust blower
Acid resistant exhaust hood and corrosion resistant exhaust blower
Hot plate (electric kitchen griddles are great and inexpensive)
Coors ceramic annealing cups
50 ml beakers
Wire brush
Cupel tongs
Heat resistant gloves
High temp crayon
Multi depression muffin tin (so you don't mix up samples)
tweezers
pliers
squeeze bottle

Supplies for fire assaying

Reagent Nitric Acid
Bone ash cupels 1 1/4" diameter
Sheet lead (foil) 3" X 3"
Fine silver shot
Bone ash
Distilled water

Procedure
Cupellation
1. Cut the lead foil into 3" squares and fold into "boats"
(a boat is made by folding the foil in half over your index finger crimp both ends fold one towards you and one away so the boat will stand upfor you to put in the sample and the silver inquarts. Always wear gloves when handling lead.)
2. Weigh a sample of approx 0.5 grams of karat scrap and place in foil boat
3. Weigh out 1.5 grams of fine silver (to inquart the gold) and place in the foil boat
4. Roll the foil tightly around the sample and silver shot. Compact tightly by squeezing with pliers
(be careful not to squuze too hard to force the sample to split out the side of the lead foil)
5. Prepare 2 samples for a gold assay in the same manner, if you need a silver assay prepare a foil boat
the same as above but do not add silver.
6. You now have 3 rolled up foil boats with the sample weights recorded ready for cupellation.
7. Preheat the furnace to 1850 degrees
8. Number the cupels with the heat resistent crayon and place in the oven to pre-heat
9. When the empty cupels and furnace are up to temperature place the foil boats in the depressions
of the proper cupels.
10. Close the door of the furnace and wait about 10 minutes for the lead to melt, this is classically known
as the "opening" of the cupels. If you peek into the oven you will see all the cupels with a bright glow
of molten lead in the center of each cupel.
11. Be careful not to leave the oven open too long because you don't want to freeze the cupellations, a quick
peek through a slightly opened door should suffice.
12. It is now time to "drive the lead", this is accomplished by leaving the door open just a crack to allow
air to oxidize the lead which will be absorbed into the cupel along with all of the base metals.
13. After 30 minutes the door should be closed and the cupellation continued for another 15 minutes.
14. The beads will now appear to be bright shiny round balls of glowing silver. The furnace can be turned off now
and the cupels allowed to slowly cool. If you need to use the oven for more assays, you can remove the cupels
and place an old cupel on top of each hot cupel so it cools slowly and avoids spitting.
15. When the cupels have cooled, the bead can be pried (it should come easily) out of the cupel and the bottom
is buffed with a wire brush to remove any adherant bone ash.
Parting
16. It is now time to part the beads in the acid hood. The only beads that are parted are the samples you added silver to.
The beads that you didn't add silver to are usually gold in color, they are used to calculate the silver
content of the sample by subtracting the average weight of gold from the 2 samples you are assaying
for gold. The calculations will be at the end of the parting section.
17. You will be transferring the beads and gold from cupels to beakers to annealing cups so make sure you
number beakers and cups so there is no mixup.
18. Add the beads to numbered beakers and add 40ml of a mixture of 1 part ntric acid and 2 parts distilled water.
19. When mixing the acids, always add acid to water, never add water to acid!!
20. Heat the beakers for ab
out 15 minutes in the first acid. The silver will dissolve and leave behind gold particles
which look like coffee grounds. When the reaction stops you can change the acid. The particles are heavy and
behave well. (meaning they rinse well and with reasonable care will not pour off when rinsing) Gently pour off the
acid leaving the gold at the bottom, squirt with a stream of distilled water from a squeeze bottle. After a few
seconds the gold settles and can be decanted as well.
21. All of the used acids and rinses contain silver, collect them in a glass bottle for future recovery, you can recycle
this silver and re use it over and over.
22. Now add a mixture of 1 part Nirtic acid and 1 part distilled water to the beakers. Again about 40ml per beaker,
continue heating on the hot plate for 15 to 30 minutes.
23. Again decant and rinse the gold, saving the rinses as you did the first time. rinse well, at least twice.
24. Now transfer the gold into the annealing cups by flowing the granules with a stream of distilled water from the
squeeze bottle. Allow it to settle for a few seconds and carefully decant the water, again be careful not to loose
any gold particles by pouring too fast. Drain as much water as possible.
25. Place the annealing cups on the warm hot plate and allow them to dry, if you rush this step and the gold isn't dry
it will spit when annealed in the furnace and you will lose gold and have to start again!
26. Place the dry annealing cups in the furnace at about 1000 degrees for 5 minutes or until they glow from the heat.
27. Remove the annealing cups and allow them to cool.
Weighing & Calculations
28. The annealed gold will have a nice gold color and if it doesn't there are other precious metals in with the gold.
If the gold isn't a nice uniform yellow color after annealing the following calculations will not be correct. (because
your gold is contaminated with other metals)
29. Transfer the gold to the balance and weigh and record the weights.
30. Divide the ending weight in grams by the starting weight. Multiply by 100 to get % gold in sample.
31. To get a silver number weigh the small bead you did not part. Divide the weight of the bead by the starting
weight and multiply by 100 to get % gold and silver in the sample. Now subtract the average gold percentage
from step 30 from the gold and silver percentage to get the silver percentage. Reported as silver by difference.

If you need it I can post an Excel spreadsheet for the calculations.

This description is meant to get you started in fire assays of karat gold, there are many nuances which you will learn from experience. A good textbook to get you further is Fire Assaying by Shepard & Dietrich.

 

[goldsilverpro]

4metals,

The assay setup (cupellation only) you've outlined will only work for high grade gold such as karat gold, dental gold, or gold bullion. Since about 80 - 90% of the 10,000 plus fire assays I've done have been on electronic materials, I think that should be discussed also. Besides all the equipment, etc., that you've mentioned, you will also need:

Assay crucibles. For general work, I prefer the 30 gram size.
Cone mold - available in 2, 3, or 6 depressions.
Crucible tongs

Litharge (lead oxide). I prefer the yellow rather than the red.
Borax - anhydrous or borax glass
Soda ash
Sugar or flour. I prefer sugar.
Silica sand.
Other occasionally used chemicals such as fluorspar and yellow sulfur.

The furnace should be heavy duty with thick insulation. Otherwise, the temperature will drop severely when putting the cold crucibles into the furnace. I prefer Cress furnaces with good temperature controllers.

 

[4metals]

GSP

The list you gave for e-scrap is almost identical for the sweeps assays on jewelery materials as well. I have methods for that process as well, and will post them if anyone shows interest.

The only way I have assayed e-scrap is after it has been processed into either copper based bullion or digested in aqua regia. If you have any processing techniques to assay e-scrap I'd love to enter a discussion on the methods. I'm quite sure they would fit nicely into the general flavor of this forum.

Another addition to the list would be scorifiers and granulated lead for scorifications.

I have never seen or used yellow litharge, what is the difference and why do you prefer it?

 

[goldsilverpro]

4metals,

Excellent writeup. I have a few comments on the particular way I did things. I'm not criticizing. Both ways will work.

Equipment and supplies:
Hot plate - I found that an old electric skillet is excellent for parting and drying the gold. I assume that's what you meant. They are very controllable. If I used an actual hot plate, I would set the parting crucibles in a white, flat-bottomed CorningWare dish.
Pliers - See 15 below
Cupels - For silver assays, bone ash is best since you can see the feathers. For electronic assays, I prefer the composite cupels (bone ash + portland cement), since they are much more durable (and cheaper). Although about any size of 1 inch, or bigger, is good, I prefer the 1-3/4" size for electronic materials.
Coors ceramic annealing cups - Actually, annealing cups are clay and are made by such companies as DFC. I think what you mean are Coors porcelain crucibles. I prefer the high form ones that hold about 30 ml. I use them for parting, drying, and annealing.

Procedure:
8 - I usually numbered the crucibles, but not the cupels. It was too easy to keep track of them. At one place, we ran 36 cupels at a time in a beautiful DFC gas assay furnace. To keep track of the beads, I put them in a spot plate, on which I had numbered the depressions with a magic marker.
9 - I always preheated the cupels, at full temp., for about 20 minutes or more, before adding the lead. If you don't drive out all the moisture, the moisture will spit molten lead out of the cupel and you will have to start over. I also preferred to set the front row of cupels back about an inch from the front of the floor of the furnace. Otherwise, they can freeze up (turn black), since they can get too much air.
10 - 10 minutes seems like a lot of time, normally. Once the lead changes from black to a bright glow, they are ready to start driving. I would guess this normally takes 2 or 3 minutes. Rarely, some will stay black and won't open, no matter how long you leave them. You can try to open them by raising the temp and/or putting them in the back of the furnace. If this doesn't work, the last resort is to hold a splinter of wood near the top of the lead with tongs. The carbon will sometimes reduce the black lead oxide to lead metal. If this doesn't work, start over.
12 - Sometimes, if you have a lot of cupels, the ones in the back will take forever because they are not getting enough oxygen from simply cracking open the door. For this reason, I always drilled about a 3/8"-1/2" hole through the back of the furnace, centered and about 2" or 3" above the furnace floor. This allowed airflow over the top of the cupels.
14 - I don't like timing the cupellation. Some are done sooner than others, due to such things as their position in the furnace. Also, when cupelling the lead from the fusion of electronic parts, the weight of lead will vary. I like to remove each cupel from the furnace when it is finished. When the bead changes from a bright, rotating, glowing orb to a gray metallic bead, all the lead is out and it is finished. Silver sprouting can occur with a high percentage of silver or too high a temperature. I cupelled at about 1750-1800F. Gold losses can occur with sprouting. If I had trouble with sprouting, I covered the cupel carefully with an equally hot upside down cupel and then removed both together from the furnace. This doesn't work as well with bone ash, since it can easily crumble when handling it with tongs, especially when the cover cupel is used multiple times.
15 - I found that 6" electrician pliers work well for removing stuck beads from cupels. Use them vertically. Squeeze and twist. The slight squeeze will loosen any bone ash that is stuck to the bottom of the bead. It can then be easily brushed off.
16 - If you're only interested in gold, add silver to to the original sample. If you are interested in both gold and silver, the procedure is - wrap in lead and cupel - weigh and record the weight - wrap in lead with added silver and cupel - part - dry - anneal - weigh - calculate.
17-23 - I always flatten the bead using a very clean hammer and anvil before parting. I added about 15 ml of acid (Note: I only used .25 gm samples for karat gold and less acid was needed) to the porcelain crucibles and heated it to slight steam before adding the beads. I first used a 1/7, HNO3/distilled H2O mix. When I got no more fizzing, I decanted and rinsed 3 times with hot distilled water, allowing each rinse to heat for awhile on the hot plate. I also made sure the entire inside of the crucible was well rinsed down with water from a squirt bottle - otherwise, the dried silver nitrate can contaminate to gold. This was repeated with a 3/2, HNO3/distilled H2O mix. It is best to use reagent grade HNO3. However, if the technical grade contains no chlorides, it will work fine.
24-25 - I parted, dried, and annealed in the same porcelain crucible. I decanted so that the gold ended up all together in a pile on one side of the bottom of the crucible. Annealing was done on a triangle over a bunsen burner. The gold must be thoroughly dry before annealing or it will spit out of the crucible. The gold will quickly change from brown to golden color when annealing. The hot crucible was set on a firebrick, or other good insulator, to cool before weighing. If you set it on metal, the crucible will crack, with a loud pop.

I have never seen or used yellow litharge, what is the difference and why do you prefer it?

The yellow litharge is PbO and the red oxide is Pb3O4. Actually, the yellow is called litharge and the red is called red lead oxide. The percentage of lead only varies by a couple of percent. I don't really remember why I preferred the yellow, other than that's what all the books recommend and that's the way I was taught. I have used both and they both will work. I remember that there were slight differences in their usage, but I can't recall what they were.