# How to assay sweeps



## MarcoP (Aug 19, 2015)

Hi all, my primary intent is to end up dealing with jewelers but a question I always have to answer "no" is "do you assay". That seems to put an end to my proposals. Where do I find C. M. Hoke's style instructions? How and where do you suggest me to start learning?

Thanks in advance
Marco


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## Palladium (Aug 19, 2015)

This is the thread. http://goldrefiningforum.com/phpBB3/viewtopic.php?f=41&t=4789&hilit=assay+lab
That thread should be placed in the library section..


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## MarcoP (Aug 19, 2015)

I feel an idiot, I stopped reading the same topic just before posting this one. I thought it was only about the setup of a lab and not the procedures ... I should have known better.

Thank you!
Marco

Edit: I was eager of the how to procedure and put that topic on hold...


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## Lou (Aug 19, 2015)

Assaying precious metals is a whole, large, topic by itself. Really depends upon the material.

For karat gold, proof corrected fire assay. For PGMs, usually the liquids from the gold parting are worked up for Pt/Pd. If Rh/Ir/Ru is present, it contaminates the gold in the parting process.


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## kurtak (Aug 19, 2015)

You have already got some good advice in the link provided by Palladium

You might also want to check out the book "Fire Assay Home Study Course" put out by action mining - they have other good books on mining, smelting, refining, assay, etc. as well 

http://www.actionmining.com/

Kurt


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## g_axelsson (Aug 19, 2015)

Another book, don't know if it's good or bad but at least it's free.
The Sampling and Assay of the Precious Metals by Ernest Alfred Smith, 1913
https://archive.org/details/samplingandassa00smitgoog

Göran


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## 4metals (Aug 19, 2015)

But for a small guy, just setting up to buy jewelry related karat scrap and sweeps classic fire assay is the way to go and we can use this thread to give you your own private course if you so desire. There are gravimetric methods for the 4 primary metals which can serve your needs well. 

Why don't you describe exactly what you are looking to assay and what metals you need to pay on, then we can hash out all of the equipment you need and why as well as suggest the methodology. Maybe it won't be C. M. Hoke style it will be GRF style, and represent multiple techniques from different members.

We're due for another meat and potatoes type thread!


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## MarcoP (Aug 20, 2015)

To begin with it seems all jewelers asking for assays are willing to let me work with their floor sweeps so I'm sure I'll have to learn this before anything else. Later on I'll ask more info about Lou's post 

In the last few days, this is why the late reply, I've read about assays, downloaded and looked (not read) them but they all mainly talk about karat and ores. I wanted to read just to get an idea and get familiar with the new terminology.

An analytical scale 0.0001gr was a cold shower but for when I'll be ready I'm sure I'll find a used one.

4metals, in the main assay thread you mention sweeps and if anyone was interested you would post the related process, years later you find someone interested 
On my readings I've also noticed an important factor, honoring the assay. How does that work.

Marco


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## 4metals (Aug 20, 2015)

> honoring the assay


I am not sure what you mean here? Maybe a typo?


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## MarcoP (Aug 20, 2015)

On my readings I've also learned that refiners rarely returns all PMs contained and to me, specially now, will be very hard to return PGMs. I would like to be able to assay and return gold and silver and this will put me head of others returning only gold on karat and silver on sterling. With time I would like to return PGMs too.

Honoring, if the assay turn to be 1% gold but when processing the main batch I get less. How is this handled.

Marco


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## 4metals (Aug 20, 2015)

The process for jewelers sweeps assays is usually done on larger lots, although the lot size doesn't matter, small lots which will be processed in Aqua Regia are generally settled on out-turn. So a fire assay isn't even done. Larger lots are generally fire assayed and then accumulated and shipped to a smelter because their rates are quite appealing if your quantities justify. 

When you acid refine sweeps, two things will happen. One, you will not recover the silver, it remains in with the undissolved as silver chloride. And two, you will leave behind some of the gold. I do work for a good number of refiners who process "residues" from smaller refiners who do process in acid. Their assays are usually high in silver and average 1/4 of 1% gold. So it seems that the small guys acid refining are not getting all of the gold either. I am quite sure that some of the more careful acid sweeps refiners get more of the gold (like the guys who learned here!) and there are some who leave more behind in the residues. But on average there is 1/4 of 1% remaining after refining. The customer does not get paid for this because the acid refiner didn't recover it but the acid refiner gets paid for it when he accumulates enough residues to send out.

So some would ask, if I have to send it out anyway to get paid for everything why don't I fire assay it and accumulate it and get paid for it all without bothering with the acid refining? The answer, at least in the USA where the jewelry manufacturing has diminished significantly, is it will take so long to accumulate enough sweeps to ship out that you may lose all of your profits before you ship. (Of course you may make a lot of extra money too, depending on how the prices are moving but in recent years the bears have been in charge and the price has been falling. Where are the bulls when we need them?)

So refining for small lots, say incoming 20 pounds or less, acid refining is the method most used. After incineration, that 20 pounds will be about half the weight, or 10 pounds to put up in acid. 

So the question becomes how many pounds will you be getting for processing? If it is a small quantity, you will pay on out-turn and will not be running a fire assay. But if your answer to the jeweler requires you say YES! with a big smile, we can go over the procedures here and you can learn and practice the techniques, and you will be capable of assaying the material. It just isn't needed to process in acid.



> Honoring, if the assay turn to be 1% gold but when processing the main batch I get less. How is this handled.



Wow, I actually answered the question before I posted this response. The answer comes down to what we have been saying about refiners all along. To get and keep your customers your clients need to trust you. You will be picking up material that is un-incinerated from a jeweler The material cannot in any way be sampled in a way that guarantees it is homogeneous. So you have to incinerate it, crush and sift it and sample it before the customer even gets the assay. Generally a refiner has your material a few days before you get the results from the assay. 

So if you have the material for a few days what are you supposed to do? Process and sample it, report it to the customer, and have them tell you it is too low give it back. (I'm from New York, that happens!) 

If you incinerated, screened and processed in acid in the same few days, you will know the out turn, then you can do the math and tell them the percentage yield per pound, based on what you recovered, and you have honored your assay and don't have to wait for the gold.


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## 4metals (Aug 20, 2015)

> I've also learned that refiners rarely returns all PMs contained



Typically a refiner has a minimum payable assay figure for PGM's and a minimum deduction. So if your assay on Platinum is below their minimum payable figure, they get to keep it. Also if your minimum assay is above the minimum payable but doesn't meet the minimum deduction, they still get to keep it. 

PGM's are more difficult to recover in small lots so the minimums actually make sense from a business perspective. When the PGM's are low enough a refiner knows it's not payable and recovers the metals in bulk by cementation or resin after the gold is out, so there is little additional labor. If that labor and handling had to be applied to individual jobs the labor increases significantly. The only people who don't understand this happen to be the people who are losing their metal!


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## Westerngs (Aug 20, 2015)

Agree with everything 4metals states.

What I would do differently.

I would receive material on a 30 day payout contract, that gives me plenty of time to assay and process the material (yes, I did say assay). After incineration, use a magnet to remove magnetics. Smelt balance with some added copper as a collector. Assay for gold, don't bother with silver and PGM's as they will be too low for payout (unless customer asks for Ag, sometimes they do).

To avoid losing money if customer requests material returned after assay, your contract will have fees for smelting and assay.

It is necessary to be extremely honest and fair with all your customers so that they always trust you. Thirty days may seem a long time for settlement, but it is necessary to properly process sweeps for best recovery.


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## MarcoP (Aug 21, 2015)

Westerngs said:


> Smelt balance with some added copper as a collector ... To avoid losing money if customer requests material returned after assay, your contract will have fees for smelting and assay.


Smelt as with cupellation or flux in a silicon carbide crucibles? Good to know about that.

As a general rule, how much floor sweeps should I ask for assay, 50gr? Should I split those 50gr in two batches to assay independently?

If sweeps can be acid assayed that means I only need a 0,1mg analytical scale?

Marco


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## MarcoP (Aug 21, 2015)

4metals said:


> So the question becomes how many pounds will you be getting for processing? If it is a small quantity, you will pay on out-turn and will not be running a fire assay. But if your answer to the jeweler requires you say YES! with a big smile, we can go over the procedures here and you can learn and practice the techniques, and you will be capable of assaying the material. It just isn't needed to process in acid.


I reckon and hope about 20 pounds, 5 more, 5 less.


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## 4metals (Aug 21, 2015)

> To avoid losing money if customer requests material returned after assay, your contract will have fees for smelting and assay.



This is usually done on larger lots, there are per pound in fee's and assay fee's. Generally the customer producing 20-25 pounds of unburnt sweeps for processing is small enough that he wants to see his rate expressed as a single number, say 10% or whatever you deem reasonable. When you start adding on extra fee's, that is where some clients get turned off. The bigger guys expect it but the smaller guys like a simple number. 

If you do as Westerngs say's, you are doing what larger refiners do and you then do have to address the issue of honoring the assay. In that case processing in acid can be an issue. (Remember you don't get the silver or about 1/4% of the gold.) With most sweeps, the assay will be very accurate and what is in there is what can be recovered. 



> Smelt balance with some added copper as a collector.



I am a little confused about what Westerngs means by this. Generally sweeps are incinerated, crushed and sifted. The oversize from the sifting process is processed by removing the magnetics and melting the balance with copper. I have generally referred to this as the oversize fraction of the sweeps. 

Some refiners do smelt the entire lot with copper to produce a bar with all of the precious metals in it, which can be assayed. Then there is no need to put the powdered sweeps in acid but the gold must be recovered from the copper based bullion. The issue with melting sweeps, and I have done it, is fluxing. Different polishing compounds and buffs included in the sweeps require different flux combinations which can be difficult to figure out. If this is what you are going to do, first assay the powder after incineration and then assay the copper based bar to assure you have recovered all of the values. 

I believe Westerngs was referring to melting the oversize and preparing the powder for shipping to a large smelter, if this is the case, 30 days would be a good thing. But you need enough business to collect enough powder to ship in economical processing quantities, which at a minimum is 1 full drum of prepared sweeps. I drum usually weighs 400 - 500 pounds after burn weight, so it started with about 1000 pounds incoming. I do not think this is the scale MarcoP is looking to process as you need to see this quantity every 2 weeks or so so you are not holding the metal too long, unless you are financially capable of hedging.


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## 4metals (Aug 21, 2015)

> As a general rule, how much floor sweeps should I ask for assay, 50gr? Should I split those 50gr in two batches to assay independently?



You are making a very dangerous assumption if you think you can grab a small sample of unburnt sweeps and determine the value of the entire lot from that sample. Unburnt, uncrushed and unsifted sweeps are not homogeneous, your sample will not match the entire lot. The only way this is done is to take the entire lot, incinerate it, crush it, and sift it. Then you can blend the powders and take a representative sample. 

The best sweeps assayer in the world will not get good numbers from a bad sample. 

If you do as you suggest, take a small unprocessed sample for assay, you *will* run into issues honoring your assay. 

As I write this I realize that Harold ran his refinery by processing sweeps in acid and he never fire assayed the material. I would value his insight here.


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## 4metals (Aug 21, 2015)

So judging from where this thread has gone, we have to decide how you intend to process the material before we even get to whether or not you will be doing sweeps assays.


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## MarcoP (Aug 21, 2015)

4metals, you say that no assay is done for small lots and those are paid on out-turn. In my case I'll have to deal with small lots of ~20 pounds of unburned sweeps but they all require an assay before I get the main batch. 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? There are few reasons why I would do that.

So the assay seems to be a requirement around here.
Assay of incinerated, crushed and sifted material with magnetic removed from oversize. How can I assay for silver and gold?
If I had to acid-assay should I flux out base metals, melt into shots, use a nitric bath to go after silver and palladium, precipitate silver chloride and convert with diluted sulfuric and steel nails in a plastic tumbler, cement any palladium with copper bar. Then do aqua regia for gold and platinum? Or ... ?

For the main batch, if there is enough silver to make it worth while, should I flux out base metals, melt into shots with silver as collector, silver nitrate cell and then go after the gold from the anode sludge?
Or ... ? You mentioned that for small lots acids could be avoided.

Edit: I'd love to hear Harold voice on this too.

Marco


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## g_axelsson (Aug 21, 2015)

MarcoP said:


> As a general rule, how much floor sweeps should I ask for assay, 50gr? Should I split those 50gr in two batches to assay independently?


4metals touched at this but I would like to emphasize it. As a general rule, never let a customer pick what to assay. That is something that you should do yourself so you are certain that the sample is a representative sample of the whole lot.
As a customer you should always be there to see how the sample is made to make sure the refiner doesn't try to pick a barren sample to lower the buying price.

If we stick to this rule it means that the customer is always watching when the refiner is taking their sample.

Of course there are exceptions like when a large batch of electronics with the same card or components so every card or component is virtually identical, but that's outside of this discussion.

Göran


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## 4metals (Aug 21, 2015)

> 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?



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?



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.


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## Harold_V (Aug 22, 2015)

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


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## justinhcase (Aug 22, 2015)

> 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.



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.


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## 4metals (Aug 22, 2015)

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.


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## 4metals (Aug 22, 2015)

Here is a PDF I wrote a few years back when teaching a group of assay students, ironically it is just what you need Marco.

View attachment Assay of Jewelers Sweeps.pdf


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## Westerngs (Aug 23, 2015)

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.


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## MarcoP (Aug 23, 2015)

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


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## g_axelsson (Aug 23, 2015)

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


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## MarcoP (Aug 23, 2015)

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


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## Lou (Aug 23, 2015)

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.


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## 4metals (Aug 23, 2015)

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.


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## 4metals (Aug 23, 2015)

> 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.



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.


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## 4metals (Aug 23, 2015)

Scorification is another assay technique which was not covered in the previous PDF I posted, so here is another.

View attachment Assaying Scorifications.pdf


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## Harold_V (Aug 23, 2015)

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)


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


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## 4metals (Aug 23, 2015)

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.


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## Harold_V (Aug 24, 2015)

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


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## MarcoP (Aug 26, 2015)

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


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## 4metals (Aug 26, 2015)

> I think it is wiser to begin to assay with acids.



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.


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## MarcoP (Aug 27, 2015)

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


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## 4metals (Aug 28, 2015)

> produce a cleaner melt in a, yet, unused 3" fused silica,



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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## Westerngs (Aug 28, 2015)

4Metals,

I stopped doing scorifications many years ago. I should point out that I only did scorifications on gold filled materials. Anyway, I switched to straight cupellations using 2" cupels and 20 grams lead. to a 0.5 gram sample I added 0.5 gram silver inquart. 

Many side-by-side assays as well as ICP analysis proved to me that the scorification losses were excessive and the modified straight cupellation was better.


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## MarcoP (Aug 28, 2015)

Idea is good, I see the point ... I can melt already (need only fluxes) with a Hoke torch (the old owner used oxy-methane). Unfortunately I don't have any furnace yet, just an incinerator that needs to be modified to pyrolize instead (originally was your idea in an old thread of mine) or make my own to pyrolize and use the incinerator as is for other uses.
As far I can see a cupellation furnace is totally different from others so I'll get either one or the other one when needed.

I'm ok with melting small lots already but I still need a way to incinerate/pyrolize, till now on hold but I need to come up with something shortly. Tomorrow I'll try go to the jeweler to see the volume of the sweeps to get an idea on the size it needs to be. Anyhow I'm still undecided if I'll be using gas or coke (both with an air blower), will see what work best for me.


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## Lou (Aug 28, 2015)

Why don't you just built a small rotary furnace yourself?

They sure are nice! They can't be that hard to build. We've built most of our furnaces here.

Lou


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## MarcoP (Aug 28, 2015)

I already have a rotary depopulating machine in my head and a 3D drawing already half done, I stopped drawing when I understood I could use a wash machine, fire bricks and a grill motor. If the project goes well I could build a rotary one for pyrolysis. As I understand those can be used to smelt also, any other uses?


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## 4metals (Aug 28, 2015)

> Unfortunately I don't have any furnace yet, just an incinerator that needs to be modified to pyrolize



If you have an incinerator, you don't necessarily need to pyrolyze for processing jewelers sweeps. What is wrong with the incinerator? Maybe we can help you get it up and running. Pyrolysis was mentioned because it can be done in a gas melter cheaper than investing in an incinerator, but if you have an incinerator already all it needs is an afterburner with a retention time of 1 second at 1600 degrees F.

Pyrolysis excels for circuit boards because a typical incinerator cannot handle the smoke and chemicals that come off circuit boards.


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## 4metals (Aug 28, 2015)

> I switched to straight cupellations using 2" cupels and 20 grams lead. to a 0.5 gram sample I added 0.5 gram silver inquart.



That sounds like a good thing to try all in one step. Essentially a scorification in a cupel.


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## MarcoP (Aug 28, 2015)

4metals said:


> Pyrolysis excels for circuit boards because a typical incinerator cannot handle the smoke and chemicals that come off circuit boards.


 That's great to know and so my incinerator would probably work (http://goldrefiningforum.com/phpBB3/viewtopic.php?f=56&t=21613) modified or as is?

Marco


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## 4metals (Aug 29, 2015)

What is the inside diameter of the incinerator? And the inside height?


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## MarcoP (Aug 31, 2015)

4metals said:


> What is the inside diameter of the incinerator? And the inside height?


ID is 35cm (14")
Height from the base of the stainless steel container is 50cm (20") to the center of the exhaust, 60cm (54") to the bottom of the lid.

I've tried to create an afterburner in the exhaust while testing with some ICs but with poor results, will have to try again by directing the fumes into a little coke furnace.


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## 4metals (Aug 31, 2015)

Ok, a few things I have questions about. First, from this photo you provided, does the flame pass under a basket, up around the sides of a basket, or through the contents of the basket? And, the bottom looks like the floor seen outside the unit, I hope there is a refractory bottom.




Unfortunately the burner comes in perpendicular to the circumference, I would like it better if it were tangential as the swirling of the flame path makes any incompletely combusted fume to remain in the heat zone a bit longer. 

Incinerators use afterburners because it is hard to get enough air into the incinerator for complete combustion in the time the smoke from the burning takes place before it passes out of the unit as smoke. Generally the type burner you have gives you enough air for the gas you are using in the burner and does not take into consideration the air required to completely combust what you have in there. But with the dimensions you gave, it would be relatively easy to put a closed chamber in the unit and force the smoke from the closed chamber to exit the chamber at the bottom where the fire will ignite it. 

I would first try to fabricate a round crucible rest that would be located in front of the flame. The flame would then bounce off the rest and begin to circulate in the unit. Then I would get a crucible that will allow at least 1 inch (2.54 cm) clearance all around for the flame and burning smoke to exit from under the crucible. Then I would cast a heavy lid out of refractory cement to sit on top of the crucible with a hole in the center into which you cement a 1" pipe that is flush with the bottom of the lid and comes up enough for you to thread on a 90 degree elbow. Then, thread in a nipple long enough to direct the fumes to the side of the crucible and into a 90 degree elbow facing down and continue the piping all the way down to the height where the flame comes out of the burner. 

Now you place your unburnt sweeps in the crucible and fit the lid so the combustible fumes will be directed back to the flame zone. Now fire it up. The flame should provide heat to cause the sweeps to give off their volatiles and the smoke should be burned in the process of coming back up the side of the crucible after being directed back down into the heat zone by the pipe. A spiraling flame will help the smoke stay in there a little longer. Actually afterburners only need a 1 second dwell time in a properly sized and correct temperature afterburner. 

In theory, the sweeps will get hot enough in the crucible to pyrolyze because there is no air in the crucible, or very little. The sweeps will give off all of their volatile gasses and essentially pyrolyze. Then, after the burn is complete, you will have to spread the pyrolyzed sweeps in a shallow tray and light them. They should burn like charcoal and not smoke just like a barbecue. Once the carbon is gone crush, sift and assay.


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## nickvc (Aug 31, 2015)

You want the truth about jewelers floor sweeps you cannot ever recover all the values using a wet method, the volume of material to the amount of values will never balance, if you get 100 grams first time you may get 10 next if you do it right, if you get more you didn't extract the values properly the first time. I did hundreds of them and in honesty I can't say I was perfect but my customers got treated right, I did my best which was better than many so called professional or commercial refiners.
You cannot ever extract it all using acids the same as cats but give your customer a fair shake and you have them for life and make a few bucks 8)


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## 4metals (Aug 31, 2015)

I think that most refiners who refine jewelers sweeps in acid just put it straight up in aqua regia. Back in the day, I did that on small lots requiring fast turnaround but I was always paying on assay. I knew I left gold behind but I also knew I would get paid for it eventually. 

Then I read Harold's method for sweeps with a pre-treatment with Hydrochloric. I did pre treatments on sweeps with high silver using nitric and still left gold behind. I never, pre GRF, used an HCl pre-treat. It almost sounds counter intuitive but I tried it and it makes a difference. Having thought about it I think the real benefit is in the ability of the material to filter better having leached out the tin and eliminated the colloidal gold it forms. The gold comes out much cleaner and the yields are better than straight 
aqua regia. If you consider the concentration of gold in the acid, then factor in the fact you cannot rinse it well if it drips all day, and it is easy to see why it pays to prevent the tin from doing any harm. Rinse well and get more of the gold where you want it and not in the residues. 

The examples I made earlier in this thread about gold assays for acid refined sweeps are from small refiners who ship out their residues, I do not know who they are but it is likely they are not adding the additional HCl steps. A lot of those little guys have the customer witness so the more they leave behind the more they make. 

But I agree with Nick 100%, take into account you will never get it all (but get as much as you can using an HCl leach) and pay the customer based on good returns, give them a fair shake and they will stick with you.


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## MarcoP (Sep 1, 2015)

True that I want to earn as much as I can, but it is also true that I'm extremely honest and this always gave me high returns, at work and in life. I will leave behind less I can, hurting the retirement plan but giving the customer most I can. I need to prove my self.

Nick, thanks for the insight! The wet method is just to get me started, I will invest back all the refinement earnings into fire assay and into improving, or moving to a new, lab.

4metals, you did hit two nails in the head. The burner seems to be the main problem. It was designed to work with city gas but at the moment I have none in my lab. I've modified the burner to work with GPL but still not enough luck, even if little better. The flame is pretty short, too much oxygen also. I can see that from the redness of the basket, it's all at the center and doesn't wrap around the basket. With this flame even your afterburner won't work properly, I think.
The second nail is the bottom part, you are right, you see the floor and it needs a refractory base!

This weekend I was able to get a GPL burner which is also bigger in diameter and, the week before, a custom made stainless steel round coke grill, about 25cm Ø (10"). Before I do as you suggest I want to try to send the fumes thru the small furnace, I will post a picture while testing the new flame and the afterburner.

What I like from your idea is the lid, stopping light particles from flying around... I thought this incinerator would have worked out the box but it's not the case.

Marco


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## Lou (Sep 1, 2015)

They definitely must be roasted.

In the past, I would do these first by gentle incineration, then milling with borax:carbonate:fluorospar (3:2:0.5) and adding an equal weight of silver crystal as the sweeps weighed.
Smelt at 1100 C, pour a cone, check the slag under a loupe and with XRF. Cone would then get remelted, thief sampled, put into an anode and sent to the cell. 
Gold by parting of three 1 g samples; silver was not paid, Pd by DMG, platinum by AA.

Wet chemically, the post incineration HCl wash is a definite requirement. You run into real issues with some sweeps that have big chunks and shanks of Pt and Pt solders. Take a while to dissolve. 


Lou


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## MarcoP (Sep 1, 2015)

Lou, that sounds like a plan and one day I'll be able to choose which method suits me best.

Rotary furnace to incinerate or to smelt? I'm having troubles with the interior refractory lining, I believe it should have a smooth surface so the slag won't stuck but then, if I use it for smelting I can't use it for incineration. Am I far?

Soon I'll be making, as said before, a rotary depopulating tool and next a rotary incinerator/milling machine (if slightly inclined should concentrate the material) following the same principle. This to show that "rotary" and all its benefits was in my head already.

Marco


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## Lou (Sep 1, 2015)

Rotary furnace for smelting not incinerating. Typically you put in a layer of insulating brick in the shell, then a hard face brick to stand the flux. Grout it all in with SuperHighMull 3200 F high alumina mortar. Plan on re-bricking it periodically. 

Incineration you want indirect gas fired, low turbulence. You can also do electric with afterburner.


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## MarcoP (Sep 1, 2015)

Savage Bond Air Setting 70% Alumina Mullite Mortar?


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## Lou (Sep 2, 2015)

Sounds right.


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