All of the precious metals we buy and refine are recycled and placed back into the worlds precious metal markets. This helps reduce the demand on newly mined metals. Which in turn, helps reduce the amount of energy required to mine virgin metal, and the amount of acid rock drainage from mine waste rock dumps and tailings ponds.
KMG Gold Precious Metal Recycling recycles all solid precious metal items. Gold, silver, platinum, palladium, and rhodium. In addition to solid precious metals, KMG Gold can also recover precious metal from non-magnetic gold filled items.
We do recycle gold-filled items. All items must be gold-filled or gold rolled (rolled gold) and non-magnetic or lightly magnetic (gold over nickel). All items must be marked "1/10", "1/20", "1/40", "GF", "gold-filled", "rolled gold", or "GP".
We can not recycle gold plated iron (magnetic).
We do recycle nickel and copper, EPNS, silver plate, and brass.
We can not recycle electro-plated gold items marked "clad", "HGE" or "GE".
We do not recycle (at this time), aluminum, iron, or plastics.
We are looking at new innovative ways to recycle electronic components, or ("e-waste"), computer circuit boards and CPU's. This requires a "wet" recycling technique that uses acids or strong salts to either dissolve the precious metals, or dissolve the other metals and plastics, leaving the precious metals behind. Either way, our goal is to recover and recycle all precious metals.
In any strategy to recover precious metals such as gold, it is important to consider the cost of recovery. There is no sense in spending more in processing costs than the value of metal recovered. In particular, the processing of wastes to reduce bulk and concentrate the precious metal is essential to economic recovery. Refining costs are usually based on gross weight shipped in, if for example, waste is sent to a toll refiner. Whether directly recycling high grade scrap or sending materials for refining, there are some broad guidelines that should be followed if efficient, costeffective recovery is to be achieved:
Recycling of contaminated scraps and wastes
High-grade (>20% gold) scraps and wastes such as bench sweepings (or lemel), casting spills and sprues, pot sludge from electrostripping, etc., should be remelted (melted) in a graphite crucible with a suitable flux to take up any dirt, oxides, refractory abrasives and inclusions and other impurities. Typically, a soda-ash flux is used. This has a low viscosity, i.e., is very fluid. The melt should be kept hot and fluid for 30 minutes and stirred occasionally with a graphite rod before being cast into an oiled iron mould and allowed to cool. The slag can be knocked off the cast bar when cold, leaving a clean bar. This can be sampled and assayed prior to re-use, preferably as grain. If there is any doubt about cleanliness of the alloy or if it contains platinum metals, it should be sent for refining.
These include floor sweepings, polishing dusts, spent plating solutions, rags and tissues and plated base metals. Treatment of these is tailored to the type of waste. Fines, floor sweepings, rags and tissues contain low gold contents and need to be incinerate slowly for long times, say 24 hours, under controlled conditions so that fine particles are not blown up the flue. Such "air-tight" furnaces are commercially available. The resulting ashes should be milled, screened, sampled and assayed prior to mixing (with flux in a 5:7 ratio of ash), and fled and melted for 2-3 hours before pouring and casting, as described previously for high-grade material. This material should be refined to pure gold.
Spent cyanide solutions from plating or bombing can be treated with fine zinc or aluminum powder to precipitate the precious metal. An addition of 50g/l of caustic soda will stimulate the reaction. The precipitated metals are then filtered off, washed and dried, and added to the high-grade waste, whose treatment was described above. On a safety note, I will remind you of the danger of putting acids on cyanide solutions; they can generate lethal hydrogen cyanide gas. Investment casting and "buds" resulting from quenching of asks and cleaning the cast trees are also low grade wastes. These can be screened and the bulk of the precious metals will be found in the coarse fraction, which can be sent for refining. Likewise, old crucibles can be crushed, milled and sent for refining.
Refining Techniques for Gold Scraps and Wastes
When we consider refining of contaminated scraps and wastes from jewellery manufacture, there is a tendency to think of refining them back to pure gold (and recovering platinum metals, if present). This will be re-alloyed with silver and copper, etc., to produce a carat gold once again for new jewellery manufacture. However, often all that is really needed is to ensure the removal of nonmetallic and other base metal impurities to leave a clean gold-copper-silver alloy for re-allaying to the desired composition. This approach is possible and can be considered if scrap is being recovered and refined in-house. A pyrometallurgical refining process for such "upgrading" of scrap has been developed and used successfully by a major European alloy producer, with economic benefit compared to refining back to pure gold. There are a number of refining techniques available for recovering gold, but not all are suitable for small-scale refining in a jewellery production environment. It is worth noting that:
This is the technique that forms the first part of the fire assay process. It involves the addition of lead to the unrefined gold material. This is heated in air to around 1000-1100 degrees Celcius (1830-2010 degrees Farenheit), when the gold-containing metal will dissolve in the lead. All base metals, including the lead, are oxidised to form a lead oxide slag, leaving behind a gold-silver bullion which will also contain any platinum group metals (PGMs) present. If pure gold is required, further refining steps are necessary to separate out the gold. If parting is used, then additional silver will need to be added to the original material (as in the fire assay procedure) to result in the bullion containing greater than 25% gold.
Aqua regia process
This process is probably the most widely used at the small-medium scale by jewellers and refineries alike and can produce gold of up to 99.99% purity. It is based on the fact that aqua regia (a mixture of hydrochloric and nitric acids in a 4.5:1 ratio) can dissolve gold. Thus, the scrap gold is dissolved in the acid mixture to form soluble gold chloride. Silver chloride is precipitated and filtered off. The gold is selectively precipitated from solution by a reducing agent and filtered off, washed, dried and the resultant powder melted to a solid button or ingot. In practice, the scrap is grained to increase surface area and treated with a series of aqua regia acid additions, aimed at using only a small excess of acid without leaving any undissolved gold.
Gentle heating will speed up dissolution. Copious brown fumes of nitrogen oxide are emitted during this stage, and fume abatement systems must be utilised to stop emission of these toxic fumes and to comply with pollution laws. It is also worth noting that suitable storage and safety procedures are required with the use of these strong acids. Filtering of the yellow-green solution removes insoluble silver chloride, the insoluble PGMs and any non-metallics such as abrasives and inclusions. Gold can be selectively precipitated using a number of reducing agents such as ferrous sulphate (known as Copperas), sodium bisulphite and sulphur dioxide gas. Others include hydrazine, formaldehyde, oxalic acid, hydroquinone and some sugars. Some emit copious quantities of gas and some are carcinogenic. After precipitation is complete, the solution should be allowed to stand overnight to allow the fine gold particles to settle as a sludge on the bottom, otherwise there is a danger of washing some of it away.
The major part of the liquor can be decanted off and the remaining part with the gold can be filtered. This filtrate is washed with acid and then water, dried, and placed in a crucible for melting and graining. Full details of this process are given in the excellent book by Roland Loewen, and I commend it to anyone wishing to use this process. I do occasionally come across jewellers trying this process who complain that they have lost a considerable amount of the gold. This suggests that either they are not fully dissolving all the gold in the first stage or, more probably, not precipitating all the gold in the reducing step. Never throw the liquor away before analysing it for gold content!
Besides aqua regia, it is possible to dissolve gold in other aggressive acids such as a mixture of hydrochloric acid and 50% hydrogen chloride or chlorine gas in hydrochloric acid. Both are somewhat hazardous and the latter is slower than aqua regia. The jeweller using this process must be aware of the risks and ensure he has trained chemists and proper safe facilities.
The simplest and cleanest kinds of precious metal waste are old jewelry, the metal parts of old dentures, and the clean scrap metal that is produced in the manufacture of jewelry or dentures. The refining of this material is easy because there is little or no dirt to be removed, and the job is profitable for the same reason. Old jewelry may be made of Sterling silver, or of base metal plated with silver; or of art gold in which pure gold is alloyed with more or less of other metals; or of base metal plated with gold; or of platinum, which also is alloyed with other metals.
Dental restorations such as bridge work, plates, fillings, etc., are often made of high quality gold; sometimes of platinum; sometimes of alloys in which gold, platinum, silver, copper, palladium and other metals are combined; sometimes of metals as inexpensive as stainless steel. Non-metallic substances are often found in conjunction with these precious materials: the crystal of the watch-case; the porcelain tooth in the denture; the sapphire in the ring. In the jewelry factory and the dental laboratory we find metalbearing wastes generated in the processes of manufacture, and it is with these that we shall concern ourselves most, partly because they are the stuff on which the refiner spends most of his time, and partly because the processes involved are more complex.
These wastes are sometimes of quite high intrinsic value, as in the case of the sprues chipped from castings, or the little shavings produced by the engraver's tools, or the clippings that fall on the jeweler's bench. The neater the worker, the higher the value per ounce; that is, the less trash, the fewer match ends, bits of paper, cigarette ashes, etc., that have to be removed.