Precious Metal Refining
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 Principle of “Quarter Refining”
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.
Preparing the Metal
- Large pieces of foreign materials are picked out and iron or steel particles are removed with a magnet.
- Any combustible materials such as wood, paper, pitch, wax, grease, etc. are burned away through a gentle annealing.
- Any non-precious metal impurities such as lead or tin (from soft solders), zinc, iron, aluminum, or other metals are dissolved in hot concentrated hydrochloric acid. The scraps are set into a safe vessel and covered with a generous amount of acid and remain there until bubbles no longer rise from the scraps. This step is especially important if tin is present, because even a minute amount will contaminate precious metal.
- The solution is poured off and the metal is washed.
- The scraps are then melted together. This process relies on the ability of acids to attack base metals and in the process isolate gold. Because metals become resistant to acid when combined with gold, it is vital that the scrap contain no more than 25% gold (Au 250). If the scrap is of a higher gold content it is first necessary to add base metals so as to “dilute” the alloy down to a point where it will be susceptible to acids. This is known as “quartering” the scrap, and explains the name of the process.
- The scrap is melted together, stirred well with a quartz or carbon rod and poured into water so as to create small spherical pieces called shot. In the case of smaller amounts of scrap, the material is melted to a blob on a charcoal block, quenched and rolled out to become as thin as possible. This thin sheet is then cut into small pieces. In both cases the intention is to create finely divided material with a maximum surface area for attack by the purifying acid of the next step.
Dissolution in Nitric Acid
- Use extreme safety precautions
- The prepared metal is placed in an appropriate container and covered with slightly diluted nitric acid. In this bath, non-precious metals and silver are converted to nitrates, while gold falls out as a sediment.
- The nitrate-containing solution is poured off and the residue is treated again with nitric acid, this time slightly heated to accelerate its corrosive action. This repeats the action of the first step. When no further dissolving appears to be taking place, this solution can be poured off and added to the previous acid for subsequent reclaiming of the silver present.
- The residue is washed with distilled water and caught in a paper filter.
Dissolution in Aqua Regia
- The residue is next dissolved in Aqua Regia, which dissolves impurities and of course the gold itself. It is important that the previous step has removed any silver present in the scrap. If more than 5% silver were remaining at this step, the metal would become cloaked with silver chloride, which is impervious to acid.
- Excess Aqua Regia is removed from the solution through evaporation in the draft of a fume hood until it is as thick as syrup.
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.
Reduction of the Gold
- Metallic gold is precipitated from the solution of salts that was formed by the Aqua Regia by adding iron (II) sulfate solution. When the addition of further iron (II) sulfate solution produces no more precipitation the reduction is complete.
- The solution is allowed to sit for a minimum of two hours, after which time it should have settled. The liquid is decanted to leave behind a residue that contains fine gold. This is thoroughly rinsed, then dried.
- The fine gold is melted into an ingot, which can be tested for purity as described earlier.
Reduction of the Silver
- To reclaim silver from the solution that was set aside after the first step in the gold reclamation process, table salt is added to provoke the formation of silver chloride. This is added and stirred until all precipitation of silver chloride has stopped.
- After allowing the curd-like precipitate to settle out, the solution is stirred, which will cause the precipitate to ball up. The remaining solution can be discarded (safely!) because the recovery of the copper it contains does not pay for itself.
- The residue is rinsed until the runoff washing water no longer registers acidic with litmus paper.
- The sediment is next mixed with dilute sulfuric acid and zinc is added to a point of saturation. This will precipitate silver according to the following equation.
- The liquid is poured off and the residue is melted to create an ingot. It may be tested for purity as described earlier
Treatment at the Refinery
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.”
Sweeps or Lemel
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.
Scrap Containing Platinum
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.