The principle of this ancient, valued procedure is described here because of its historical interest, and with the thought that young people coming into the field should be aware of the basic procedures of the trade, even though they may not pursue them.
In the case of precious metal refining, it is generally more efficient to send scrap to a reputable refiner where sophisticated equipment and years of experience will insure accuracy. In addition, the procedures described below use dangerous acids that mandate industrial quality ventilation and safety precautions, which makes the decision to send out refining chores even more compelling.
The method described below is appropriate for refining batches of clean scrap metal such as cut off pieces, sprues and filings. Polishing residue and floor sweeps require a radically different approach because such scraps are likely to contain a wide assortment of miscellaneous materials.
Preparation of the initial Materials
Buffing off (Removing flammable materials)
Magnet Use (Remove Iron Particles)
Boiling in hydrocloric acid (Sn, Pb, Zn, Fe, Al are dissolved to become chlorides)
Alloy down to Au 250 at the highest, roll it out thinly.
Dissolve in Nitric Acid
(Ag, Cu, Zn are dissolved to form nitrates)
Transform the precipitate into Gold
dissolve in Aqua Regia (Au to H[AuCl4]);
reduce to Au with FeSO4;
Melt the Gold.
Transform the solution into Silver
Precitipate it with NaCl
(AgNO3 Becomes AgCl);
reduce to Ag with Zn and dilute H2SO4;
Melt the silver.
The higher the proportion of precious metals in a container of material sent for refining, the simpler the treatment at the refinery. This affects the length of treatment time and therefore the cost of refining. For this reason it is best to remove from scraps any foreign materials such as pitch, plaster, wood, etc. The pile of scraps is stirred with a magnet to remove iron and steel particles, then it is burned to remove organic materials such as paper, wax and sawdust. It is not necessary to melt the remaining sweeps together.
Because they are handled differently by a refiner, it is best to package large clean pieces of scrap separate from filings, which in turn are kept separate from sweepings. The more clearly these categories can be defined, the easier it will be for a refiner to process. The job is made easier still if you can be clear about what metals a given batch of scrap is likely to contain; this is especially true where platinum metals are involved.
This category includes sheet and wire scrap that result from fabrication, trimmings from castings such as sprues, and unwanted objects such as might be received from a customer or bought for weight value. As far as possible the scraps should be sorted according to fineness or purity of each alloy. The scrap should be weighed and placed in a rigid container such as a cardboard box or steel can. Plastic bags are not recommended because they can be torn by the sharp edges of scrap pieces. The container should be labeled with the total weight and, if possible, with the name or alloy of the scrap, such as “46 ounces, Ag 925 (sterling)” or “320 grams scrap, primarily Au 333.”
The term filings includes not only the powder that is the result of using a file, but the chips, curls and particles that result from drilling, turning and grinding. Residue from abrasive papers (retrieved by burning) are also included in this category.
Filings are weighed and collected into a plastic bag that is then packed into a rigid box that can be well sealed with tape. It is labeled, for instance, as “216 grams of Au 585 “lings (free from platinum metals)” or “326 grams Ag 925 filings.”
All dust and residue swept from the work bench or retrieved from the sweeps drawer or bench skin are called sweeps or lemel. These are kept separate from floor sweepings or residue collected from scratchbrushing and polishing, which have considerably less valuable. Bench sweeps are cleaned with a magnet then packed and labeled in the same manner as filings.
The recovery of platinum and platinum group metals is exceptionally difficult and expensive, particularly if the individual platinum group metals have to be separated from each other. Because of this, scrap that contains platinum should always be carefully separated from other scraps and labeled accordingly, such as “11.8 grams scrap with about 90% platinum content.” Even in the case of sweeps the approximate platinum content should be stated, for instance, “19.4g mixed filings (Au, Ag and about 20% Pt and Pd).”
When the electrolytic baths used in plating become unusable, the metal contents are precipitated out chemically and sent to a refiner for recovery. Rhodium plating baths are concentrated by evaporation and the concentrated solution is sent to a refiner to recover the metal.
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