How to assay sweeps

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As far I can understand there is only one way around: let the customer understand that I need the entire lot for pretreatments if he wants a proper assay. If he doesn't like the assay he will receive the burned material, good for him. In the case he doesn't like the assay, a small guy, should just let him walk away?
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Exactly!

One scenario is to tell the customer you need the entire lot. You will incinerate it and crush it and sift it to sample the powders, then you will melt the oversize with copper and assay that as well. That is a considerable bit of work, so to protect yourself, you can say I will charge X percent. If you do not like my assay, I will still have the material you can have it back but it will cost you X dollars per pound that I processed, and X dollars per assay. Then at least you recoup what you put into the material in terms of your time. If they like the assay, then you process it and make sure the X percent you charge covers everything. So your rate will be given as a percentage with a per pound and assay fee only entering the picture if they do not like the assay.

On larger lots, customers often witness the incineration, crushing sifting and sampling and take a sample to have assayed on their own. I have never seen this on small lots.

Basically if you get to take the entire 20 pounds, process and sample it as a sweep powder, and report an assay, the guy either has to be there for that process or trust you. There is that trust word again. You have to build up trust to get repeat business.

As far as assaying is concerned, you will need to learn to do a bullion assay because the melted oversize fraction is essentially a bullion assay. Then you will need to learn to do a sweeps assay.

Sweeps assays are of 2 parts, the first part is a fusion. That is where all of the values are collected in the lead contained in the flux. The second part is the cupellation, which is essentially what is done for a bullion assay.

So for learning purposes, you need to learn a bullion assay first, and a fusion second. I have listed the equipment needed for the small assay lab before but I can repeat it in this thread.

I will wait for input from others who have done this so we can get the insight of different refiners who have done this before, both the processing and the assaying.

For the main batch, if there is enough silver to make it worth while?
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Let's assume the jeweler is polishing karat jewelry, Back in the day a good polishing sweep had an after burn assay of 1 ounce per pound. That is 6.8% gold assay. So after burn a 20 pound lot will be around 10 pounds. At 6.8% assay it will contain 10 ounces of gold. If we assume an alloy mixture of 50% gold karat then there were 20 ounces of karat ground into the pound of burnt sweeps. Since the gold alloy the jeweler started with had around 8% silver in it to start, 8% of 20 ounces = 1.6 ounces of silver. Not enough to justify the acid to go after it or the cost of the assay. If he mixes sweeps and he polishes silver as well, that is a different story.
 
I've read the majority of posts on this subject. I have an opinion. Several, really, and not necessarily in any particular order. I'll just toss out my thoughts.

To start, unless one is serving a large producer, the amount of polishing/floor sweeps one will receive is typically way too small to even consider performing an assay. Yes, there are exceptions, but, for me, they were few and far between (although I still didn't assay), and those were a pleasant surprise for the owner, as the yield was far greater than their expectations, anyway.

It is true that one does not recover 100% of the values by acid recovery. For that reason, my wastes were either later processed by furnace, or sold to a refiner who further processed the low grade residues by furnace, using litharge to collect the values.

Having said the above, if one's fee structure is reasonable, the owner(s) of the waste material are often more than pleased to have their material processed without an assay, especially when they are provided a settlement in a week, often less. That was my common practice, so unless the volume was huge, where incineration delayed the eventual recovery, settlement rarely exceeded a week.

Trust is key to success. If a customer does not trust you as a refiner, you are better served to explain to them that it would be in their best interest to seek another refiner. What they won't understand is that it really is in YOUR best interest, as, failing to heed this advice is asking for one's reputation to get thoroughly trashed, as once a customer assumes you have not been honest with them, no amount of fence building will repair their opinion of you---and they will speak poorly of you to their peers. The very kind of experience you MUST avoid if you hope to be successful as a refiner.

I processed for more than 20 years, and never assayed, aside from the few times I extracted values from ore----and enjoyed complete success. It can be done. You just have to build trust with your customers. You do that by being honest with them.

I would be inclined to suggest to a potential customer that if an assay is required, they would most likely be better served sending their wastes to a major refiner, where they will most likely not be treated fairly, and will "enjoy" paying all manner of fees, in the end, costing them far more than they'd pay if they trusted you as an individual. That worked for me.

Harold
 
Trust is key to success. If a customer does not trust you as a refiner, you are better served to explain to them that it would be in their best interest to seek another refiner. What they won't understand is that it really is in YOUR best interest, as, failing to heed this advice is asking for one's reputation to get thoroughly trashed, as once a customer assumes you have not been honest with them, no amount of fence building will repair their opinion of you---and they will speak poorly of you to their peers. The very kind of experience you MUST avoid if you hope to be successful as a refiner.
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Very good advice.
If some one seems preoccupied with your honesty the chances are that they are the dishonest one's.
He that lieth down with dogs shall rise up with fleas.
 
OK Marco, so from the refiner's perspective on the forum, you don't gain much from doing an assay. From your customers perspective, they won't ship much without an assay, and from your competition who are already getting the work, of course you need to assay and do everything they do. So what do you need to do to break into the business? In my opinion, you need to do whatever it takes to get the work and price your work accordingly.

That means you have to be set up to burn (incinerate) the material in front of your customer if he is willing to come watch. Then you can crush, sift, and sample and give the customer a sample. The customer will have it assayed, you will have it assayed, and you pay on assay (honoring the assay) the difference is you build the 1/4% into your charges. With an in house capability to assay, you assay the material before you put it in acid, and assay the residues. You will then know if you are better than the 1/4% or worse. You keep the residues so you can be paid on them eventually, think of it as money in the bank. If you price your work so that you will be able to afford to do business even while you have to wait on the last bit of gold from the residues, you can make it work. For small batches you will likely develop a trust relationship and the customer will stop coming to watch, then you can process without even assaying if you want.

The way I see this, in your situation the ability to produce a sample which can accurately represent the lot, and the ability to assay that sample, is the key.

If a jeweler is doing the quantity of sweeps to require your services, he likely is also generating karat scrap. That is an easy melt and assay, much easier than sweeps. So by learning to assay, you will be able to fully satisfy what your customers needs to have before they give you the work.

But refiners have a terrible reputation, as Harold eluded to in his post. If a refiner that your potential client is currently dealing with is not being honest with his customer, the customer probably does not realize it. The refiner is quoting rates that you cannot match because he is making it up elsewhere. (at the customers expense) So your business model must enable you to give the customer the ability to witness his material and see for himself that even though your rate may appear higher, your returns are better and the customer wins. It may be a tough sell, but if you understand the entire process and all of the pitfalls you can do it.

Also realize that Harold did not have to worry about employee theft. He did all of the processing himself. If your business were to grow to the point where you need to hire workers, then you have a need to assay to protect yourself from inside theft as well.

As you may have guessed, I am in favor of the analytical end of the business. Never hurts to know exactly how much gold you are dealing with, and nothing tells you that better than a fire assay.
 
Hello 4 Metals.

What I meant to say when I stated I would smelt balance with added copper is this:

1. I would incinerate entire sweeps lot (customer witnessed, as you suggested)
2. Run powerful magnet through ashes to remove magnetics (those won't have any precious metals on them and can be returned to customer)
3. Smelt ashes (sans magnetics) with copper added as a collector to produce a bar
4. Assay bar for gold only, pay on assay

AS I also stated, make sure customer is aware of smelt and assay fees if they want to have material returned at this point.

We always follow this procedure, we always assay, even on lots as small as 1 t.o. total gold content. It helps to keep customer trust, it helps with accounting, and it helps with employee theft issues. We run so many gold assays that the assay price really drops due to volume.

Yes, we have had customers ask for their material to be returned, we gladly hand over bar AND slag. Almost always they end up returning to us once they realize their replacement refiner paid them less than we would have.

As everyone has stated, trust is key.
 
I can't thank you all enough, I do have lot of study ahead and on the mean time I'll have to buy what I need and practice with it.

Marco
 
Thanks for the interesting thread and the assay pdf, very easy to follow.

I've been thinking about setting up an assay lab for a while now. I went so far as bidding on an analytical scale a month ago but when the price tag went past half of new price I let it go and it sold for closer to 2/3 of the price for a new scale.

My assaying days will have to wait for a while longer. 8)

Göran
 
Göran, just yesterday I spoke with a big jeweler over the phone and we setup a meeting for December, he was clear about the fact that an used analytical scale could easily put me into troubles, he highly recommended to get a new, and so under warranty, scale.

Marco
 
Wow! Just get your scale calibrated or else buy a scale with internal calibration (such as FACT by Mettler Toledo) and it's fine so long as you check periodically.

I've used quite a few variety of scales (beam, load cell, tortion) in my time and still have a few of the old ones that could read hundredths of a mg!

Nowadays, with continuing calibration verification being easier and simple to conduct, buying a new scale should not be a requirement. Find a good used balance that has its calibration logs kept in good order. We keep records of weekly checks and I compare amongst my three lab scales daily for consistency. Once a year we have a third party calibrate and seal.

This is a good policy to have. Invest in a traceable calibration set and keep records. Not only is it the right thing by the customer and yourself (they can weigh heavy and they can weigh light), it lets you know when the scale is acting off. As a refiner, you never want to take that risk on the scale.
 
Westerngs,

Very interesting approach. I have used the direct smelt approach but with some slight variations. First I always crushed and sifted the material and assayed the sweeps before melting. Then I mixed assayed batches together based on their assays. I always mixed the flux and the powdered sweeps together well in a large double cone blender before putting them in the rotary melt furnace, the copper was added last as copper shot.

We had to assay first because it would be impossible to melt a small lot like MarcoP is discussing, 10 to 15 pounds after incineration in the large #2 US Rotary Melter we were running to process our sweeps. Our average smelt quantity was 1000 pounds before fluxing on average.

Invariably we ran across jobs which had to be processed alone. We did this in a gas fired crucible furnace. In 1995, Roland Lowen published a book called "Small scale refining of Jewelers waste", in that book he detailed the process we used in the crucible furnace very well. (In the library, information to purchase this excellent book is provided) Unfortunately it was 15 years after we did it but it is essentially the same. Initially we did it as he detailed but eventually we too added copper to the melt to act as a collector. The problem with a crucible vs. a rotary furnace is the charge is not rolling and picking up the values as effectively in a crucible furnace and the slags always left behind values. Since we had fire assays for the material from the powders, we were able to pay out based on the sweeps assay and the slags were re-melted on Fridays, which was always slag re-melting day. Re-melting the slags always pays off, any slags!

Of course we had a copper cell room where we refined the copper based bars and recovered the values in the slimes from the cells. I also mentioned we mixed large smelt lots together based on assay. The benefit of this was we were able to keep the platinum group metals in separate melts so those all went into cells where we were looking for PGM's. The benefit of that was we were able to effect higher concentrations of PGM's for recovery so we got the PGM's out quicker.

We also assayed the copper based bullion bars and found assaying after scorification necessary due to the high copper.

I do not believe it can be cost effective for a small refiner to go this route, even just the crucible melt version. The hang up of metals in the cells (time wise) is still substantial when you consider you can get 99.75% of the gold out by aqua regia refining essentially in 2 days. This is my belief, however I am always open for discussion and feel we all benefit from a good, well informed debate.
 
Just get your scale calibrated or else buy a scale with internal calibration (such as FACT by Mettler Toledo) and it's fine so long as you check periodically.
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Spoken by a man who is both practical and does refining for a living.

I always believe in keeping a traceable standard weight, (I like 250 mg) near the scale. At least once a day, I weigh the standard and note the weight in my permanent lab notebook. Then you know when the scale is acting up. The good standard weights are expensive and come in a felt lined hinged top box with a special pair of tweezers to pick it up and place it on the pan so no finger oils affect the weight. It stays accurate and you can have it recertified periodically when you have your balance recalibrated.

With the price of metals today, you can never have too much documentation certifying the accuracy of your analytical balance.
 
Westerngs said:
2. Run powerful magnet through ashes to remove magnetics (those won't have any precious metals on them and can be returned to customer)
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I'm not sure I agree. Having processed this material for more than 20 years, it was my experience that in spite of careful incineration (to the point of fully excluding carbon), magnetic bits still carried traces of values. I dealt with them by sending the magnetics to my stock pot, where I recovered even copper, although not in great quantity. That way, even traces of vales were recovered, and became part of my retirement plan.

It should be understood that my long term plan was to use the traces of base metals (copper, in this case), as well as silver chloride, to become the collector of greater values when the collected material was processed by furnace. That proved to be quite effective in my operation, as an assay of the slag (by a third party) after recovery of values indicated that further treatment would not be justified. Those who intend to process the stock pot by chemical processing may wish to exclude the recovery of copper, so they may not want to send the magnetics to their stock pot.

I had a theory when I refined. That theory being that ANYTHING that came in contact with values should not be discarded, but properly treated. Thus, even my discarded gloves were processed.

Harold
 
I never did the magnetic separation until it was sifted. The oversized material that didn't go through a 10 mesh screen were separated by the magnet from the other oversized pieces. The magnetics were offered back to the customer, but they were really interested in the other oversize material. That could often be melted with some flux and yield a bar with no copper additions, and the assay was usually pretty high. Although it depended on what the factory was doing, some customers had a lot of oversize meltable and some had none.

All of the magnetics were accumulated and when we had a few 55 gallon drums, it was shipped up to the old Handy & Harmon plant in South Windsor Ct. (Oh boy, now I'm dating myself 8) !) where it was melted with copper. After all of the melt charges and the copper charges it was still payable, but not by much. I would guess that 10 pounds of the material by itself wouldn't yield much more than a trace of gold if processed the same way.

But I can not think of any refining residues that weren't shipped off to a low grade refiner somewhere for recovery, even hydroxides with traces of silver and a lot of copper hydroxide.
 
Interestingly (at least to me), that's pretty much the procedure I followed, although by chance. I had no clue what I should have been doing, but I figured separating the large pieces that would not readily digest in aqua regia was a good idea.

Anything removed by a magnet was sent to a mortar, where it was mashed about a little, to liberate anything that might be hitch-hiking, such as saw blades with traces of filings still in the gullets of the teeth. Once well mashed, I screened, to separate the fine particles from the coarse, then sent the coarse things to the stock pot. I even used a magnet in the sifted material, repeating the above operation. Didn't try to remove all traces of magnetics, just the coarser stuff. It, too, was sent to the stock pot, then the remaining fine material was given an extended boil in HCl, which removed the balance of iron.

Gold from this operation tended to come out very clean, as the bulk of contaminants were removed before the gold was dissolved. This operation also proved to be of serious value, as the resulting gold chloride would filter very easily, which it often did not if the HCl cycle was omitted.

Harold
 
Initially to build customers' trust I'll have to assay at least the first time.

I've explored assay by furnace and thanks to you all I've have fair better picture of the process and the equipment, the last being a quite risky investment so I think it is wiser to begin to assay with acids. I bet I'll have to follow known procedures as I would with the entire lot. With small lots the losses percentage is greater, is there anything I should know, a tip, anything that could give a better return?

* Just remembered a "one beaker style" (or similar) post by Harold and going to read it...

Marco
 
I think it is wiser to begin to assay with acids.
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By this do you mean assay by processing a small sample of the prepared material? If you were to prepare the entire lot, and then take a pound (or 500 grams for you metric guys) and refine it. Just do the leach with HCl and then Aqua Regia refining with good rinsing and preferably vacuum filtration to produce a fine gold button. You can use this result to report the "assay" to the customer. Then he will either request his prepared sweeps back or give you the go ahead to process.

I think the most important thing you have to set up first is a small incinerator, a ball mill, and a sifter. That is what you need to get the sweeps material into a form to sample. You will need a hood for the acid refining portion as well.

If your potential clients are asking you for assays on sweeps, you will likely have to give them assays on bullion as well if you get that far with them. For bullion, a fire assay will be the easiest way to go, just melt the material, sample the bar, and assay the sample. Then if your customer likes the numbers, you proceed.

Again, I am of the opinion that having the skills to assay will never hurt you in the refining business.
 
Yes, that is exactly what I meant. I'm not happy with the incineration I'm doing now and milling will be next (I can crush but, eg. SMD). I already have different sizes sifters, not smaller then 10 mesh I reckon, so I'm half way there. In a week or so I'll have another surgery and if things goes well I'll rent out a place outside the country and be able to run a ball mill without annoying neighbors. Surely I'll have more freedom in general and it won't hurt.

I don't have yet a vacuum filtration setup but I'll be able to properly wash the first drop and, if needed, redissolve and drop again to produce a cleaner melt in a, yet, unused 3" fused silica, yet again to be, glazed with borax and no other fluxes will be used while melting (I think I'm doing my homework...).

Marco
 
produce a cleaner melt in a, yet, unused 3" fused silica,
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Are you melting with a torch? You do not have a melt furnace of any type? A homemade gas melt furnace would be very helpful here, if it is big enough you can use it for pyrolysis of the sweeps which is very handy to incinerate the sweeps without a lot of pollution.
 

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