The following is a dissertation on methods of recovery & refining for the jewelry industry. It is based upon my experiences of over 20 years in the industry. It is designed to help jewelry factories implement procedures & systems that will improve the recovery of precious metals removed during the finishing process & increase returns when refining.
There are in reality four different classes of material in a jewelry factory that materials ought to be segregated into: – bullion, stone removal, sweeps & solutions, as these 4 types of materials get processed & handled with different methods during refining. Each of these areas may contain sub-categories that may warrant additional segregation based upon a factories needs & lot size. There are different fees & charges associated with processing different materials & a customer-focused refiner ought to guide a client to maximize the client’s bottom-line returns by not adding unnecessary charges & fees.
Bullion is anything that can be melted – scrap, sprues, clean sprue grinding, stripping, etc.
Stone removal is commonly called break-out & is a method of recovering diamonds & other gemstones from finished jewelry.
Sweeps refers to anything that gets burned, crushed & sifted to homogenize for sampling & assay – polishings, buffs, brushes, floor sweeps, rubber wheel grindings, sludges, miscellaneous shop debris, etc.
Solutions would be primarily ultrasonic, but may also be stripping or bombing solutions & even perhaps, if an analysis warrants it, wastewater at the end of process stream before release into a sewer line.
With any materials being accumulated for refining, there are several steps a factory ought to take prior to shipping to a refiner:
Examine your materials. Know what you are shipping.
Photograph your materials both before & after packaging. Provide a copy of the photos to the refiner prior to shipping so there can be no disagreement about the nature of what you have shipped.
Know the net weight of your material. Weigh your materials on your own scale, noting the gross (materials + whatever packaging is used), tare (the packaging), & through deduction, the net weight of the materials.
Seal your packaged materials in a manner that cannot be duplicated or compromised. For example, refiners usually provide numbered plastic padlock type seals to lock barrels containing sweeps. You may substitute your own padlocks or use evidence tape, wax seals, etc. to decrease the likelihood of tampering.
Ideally, it is always good practice to segregate materials by karat & generally (but not always, depending on the size of the lot) keeping platinum group metals separate. One can determine with simple math what the expected yield, before refiner’s fees, ought to be. Example: if one has 100 troy ounces (t.o.) of scrap, 50 t.o. of which is 10 K & 50 t.o. of which is 14K, the yield ought to be 49.95 t.o. of .9999 fine gold, 20.8 from the 10K @ 41.6% & 29.15 t.o. from the 14K @ 58.3%. On materials such as clean sprue grindings & stripping, there will be some melt loss due to abrasives that end up in the sprue grindings & moisture from the stripping, but if the client has a history, he can determine his average melt losses for these types of materials & take that into account on expected yield. In some cases where a client has a small amount of material, particularly any type of filings or grindings, it is just as well to melt gold & platinum group metals together & simply assay for each element. A good refiner will inform their client as to what would be most cost effective for them.
Experience has shown that rubber wheel grindings are best prepared as sweeps & not as meltable bullion. The carborundum in the rubber wheel makes for a very messy, difficult & incomplete melt, leaving a lot of metal incorporated in the fluxes used to melt. This leaves a precious metal bearing by-product (slag) that will have to eventually be refined. By preparing rubber wheel grindings as sweeps, all the precious metal is recovered in one shot.
With stone removal, it is always best to segregate the material by karat & whenever possible, have an inventory of the stones. The reason for doing this is to be able to determine what the expected yield ought to be.
To determine yields for stone removal: If one has 100 t.o. of 10K scrap with that contains 1 t.o. of stones, there ought to be 99 t.o. of metal @ 41.6%, yielding 41.18 t.o. of .9999 fine gold.
Sweeps are a much harder material to estimate expected yield & often a client can only base their assumption on history. The best course a client can take is ‘rep’ or witness his sweeps being weighed, processed & sampled. No refiner ought to discourage a client from doing this & in fact really should encourage a client to be there to see for themselves the procedure (this applies to bullion & stone removal lots as well).
If a client has the wherewithal, they can weigh their goods before & after every step of the finishing process to determine what metal is being removed in which areas of their shop. They can then know, for example, how much metal was removed during sprue removal, during stripping or bombing, during lapping, during polishing, during setting, etc.
This practice is really the only way to determine what metals can reasonably be expected to be recovered from refining & does not take into account things like employee theft (this occurs in every shop), unscrupulous accountability from refiners (dishonest refiners? Who would believe that…) & inefficient recovery methods (more on that to follow).
We also try to persuade our clients to weigh their consumables – polishing compounds, buffs, brushes, emery paper, etc. on a scheduled basis. For example, if one starts a month with 100 lbs of polishing compounds, buffs, brushes, etc. & ends the month with 25 lbs un-used, there ought to be approximately 75 lbs of material accumulated for refining – polishing sweeps, used buffs & brushes, spent emery paper, etc.
Of course, some of this material ends up in the ultrasonic baths after pre-polish & lapping, but this method will provide a very good indication that all the consumables & refinable material is accounted for. It is best to segregate & weigh these consumables by type. It is very difficult to get a manufacturer to follow these practices & see the value of doing so, particularly in the U.S. where everyone is labor-cost conscious trying to compete with cheap overseas labor.
However, those who follow these practices, who have tightened up their internal security & recovery methods & who rep their materials during refining, readily see the benefit of doing so. If refining yields fall below expectations, they can readily determine where the problem lies.
In regards to solutions & waste water, this area of recovery is probably paid attention to the least. Typically, shops will have settling tanks to collect various process solutions such as ultrasonic solution, tumbling water & polisher’s hand wash water. Most shops have the notion that the settling tanks are going to collect whatever is thrown into them. We have run dozens of waste water analysis in different factories & have found that without exception, factories are losing significant, recoverable precious metals in their waste water. One large manufacturer in Long Island City was losing more than ½ ounce of fine gold per day in their waste water.
This is almost always due to the practice of dumping spent ultrasonic solutions into the settling tanks. One of the properties of ultrasonic detergents is that it is designed to hold particulate matter in suspension. When ultrasonic solution is introduced into a settling tank, the water in the settling tank adopts this property of particulate suspension, depending on dilution from other sources such as polisher’s hand wash. We set up a number of factories, both in NY & in Merida, Mexico, to collect & treat their ultrasonic solutions separately from the rest of their waste water stream, greatly increasing their recovery.
Analyses have shown that in spent ultrasonic solutions there is on average anywhere from 0.75 t.o. to 1.5 t.o. of .9999 fine gold contained per 100 gallons of which roughly 60% will settle out with time. The other approximately 40% stays in suspension & will work its way through any settling tank system.
This suspended gold is of such a small particulate size that labs we use to perform solution analyses refer to it as “dissolved” in solution. Filtering becomes an exercise in futility as ultrasonic detergents tends to clog any filters small enough to capture this gold & the cost of time, supplies & maintenance spent on trying to filter waste water is at best a break-even effort. In treating ultrasonic solutions, we use a chemical called flocculant, which is basically powdered/granulated clay, similar to what is used to manufacture clumping cat litter. When PH conditions are right, the flocculant blooms & encapsulates everything in the solution being treated. A sludge forms & can easily filtered through a course media to decant the clarified water. This decanted water can then be safely sent to a drain.
It should be noted that we first started using flocculant to address environmental issues that shops in NYC were having with the Dept. of Environmental Protection for excessive silver, copper & other restricted non-precious metals. We found as a bonus that flocculant also removed 90%+ of the gold in waste water & that is when we came to the realization that the real problem was the introduction of the ultrasonic solution into settling tanks, both from an environmental & precious metal recovery standpoint.
Very simply, if you do not put ultrasonic solution in the settling tanks, you lose the issues with the D.E.P. & you recover more gold. Smaller shops that do not have the room to set up a system to treat their ultrasonic solution with flocculant can get a large stock pot from a kitchen supply & evaporate their ultrasonic solutions down to a paste which can then be refined.
In regards to bombing & stripping solutions, my hopes are that someday shops will abandon these methods of pre-finishing because of health & environmental factors. Unfortunately, there is no new technology that works as efficiently as these cyanide-based methods for removing casting scale. Stripping solution ought not to contain any gold as it is a reverse-plating method, meaning the cyanide contained in the solution dissolves the outer layer of gold & the gold is electroplated onto a strip pot.
The solution is poured off & the gold is scraped off the pot to be melted & refined. The gold contained in bombing solution can be easily plated out. There are commercially available units designed for specifically this purpose. The real problem with either of these methods is proper treatment of contained cyanide & disposal.
This is a whole other issue & I would love to share my experience in this area as there are a lot of misconceptions & bad practices related to cyanide & proper handling, treatment & disposal. In either case, a small sample of these solutions can be taken, after mixing, & sent to a lab for analysis of precious metal contained. With either of these gold-removal methods, it is very simple to weigh product before & after the process to determine what precious metal has been removed & ought to be recovered.
A common practice I have seen in factories is the addition of granulated lime to settling tanks. I have heard different reasons for this & none of them are valid. Lime will not decrease odors emanating from a settling tank. Lime will not help precious metals settle to the bottom. The one thing the addition of lime to a settling tank is guaranteed to do is increase the costs of recovery & refining. The addition of lime increases the weight of material that must be handled & decreases the % by weight of precious metals contained. Home Depot sells a product called Simple Green that is a terrific deodorizer/disinfectant. One cup of this once a week in your settling tanks will keep down unpleasant odors.