Revolutionary New Technologies for Stripping Precious Metals
PM-Strip™ is a revolutionary new technology for recycling PM from scrap materials. PM-STRIP represents a technology step change for the PM refining industry and replaces smelting, which is more expensive, slower, and less sustainable.
PM-STRIP recovers PM from any type of scrap consisting of a PM plating on a base metal substrate. The key advantage of PM-STRIP is that it works by dissolving the PM plating without dissolving the base metal. The plating is thereby stripped from the substrate, and the plated PM are recovered in metallic form with a high level of purity, while the base metal substrate is recovered undamaged and can be immediately sold or recycled separately. PM-STRIP successfully recovers PM even from impure coatings, e.g., alloys of gold or silver with any other metals, and, unlike conventional methods, is effective irrespective of the thickness of the coatings.
Stripping and Recovering Pure Silver Plated on Copper
Stripping Ag from Cu in a Basket
Stripping Gold Plating
- Is capable of recovering gold and silver from the most difficult types of scrap that cannot be economically recycled by traditional smelting or other chemical methods due to low concentrations of gold or silver or the presence of other metals that are dangerous (e.g., cadmium) or have high melting temperatures (e.g., tungsten).
- Can treat all types of plated, filled, layered, or karat gold filled scrap, such as karat gold filled jewelry, silver plated ware, silver-cadmium oxide coating on copper plates, gold and silver on brass, and many others.
- Is completely effective regardless of the thickness of the gold or silver coating.
- Also works on scrap that contains silver and a base metal or base metal alloy in its mass, provided only that the scrap is ground to a powder before processing.
We have successfully used PM-STRIP to recover the following, but this technology also has application to a wide range of other filled/plated PM scrap:
- Gold and silver from gold and silver filled scrap.
- Gold and silver from karat gold filled scrap, e.g., jewelry.
- Gold from gold plated copper wires.
- Gold from plated brass pins.
- Silver from silver plated scrap ware.
- Silver from silver-cadmium oxide coated on a copper base material of the type normally used in relay contacts, switches, profiles, contact tips, etc.
- Silver from silver-tungsten material, such as electrical contacts.
- Silver from silver plated copper wires.
- Silver from silver filled copper-zinc perforated tapes.
- Palladium from palladium lead frames.
Drawbacks/Limitations of Current Methods
The most widely used method for recycling plated materials with a low PM concentration is smelting, which subjects the materials to high temperature in an enclosed furnace. However, smelting is not as economically efficient as PM-STRIP, and, when the materials contain volatile dangerous metals, such as cadmium, lead, or beryllium, smelting results in their volatilization, creating poisonous gas emissions. Also, when the materials contain metals with very high melting temperatures, such as tungsten or tungsten carbide, recovering targeted PM becomes even more difficult and expensive.
There are other known chemical processes to recycle plated PM. But chemical solutions traditionally used to oxidize and dissolve PM are normally even more aggressive with respect to base metals. As a result, these solutions dissolve the base metals of the substrate faster than they dissolve the plated PM. Consequently, the use of such solutions as aqua regia to strip gold from a base metal substrate or nitric acid to strip silver from a base metal substrate, leads to the complete dissolution of the scrap item, including both the plating and the substrate. When the substrate's weight is much greater than the weight of the plating, as is typically the case, this is undesirable, because it generates large quantities of leaching solutions loaded mostly with base metals. Such solutions containing dissolved metals require special treatment. In addition, the dissolution processes are time consuming and generate large amounts of toxic gases.
The most widely used chemical process as an alternative to the above for recovering PM is cyanidation, which applies less aggressive chemical solutions that still oxidize and dissolve PM but are not extremely corrosive for the substrate. However, well-known disadvantages of cyanidation include (i) very slow processing cycles, particularly with respect to silver, which reduces the commercial value of the process, (ii) high toxicity of the leaching solutions, (iii) reduced selectivity towards copper, resulting in the leaching of copper with the plated PM, and (iv) the generation of large amounts of toxic wastes.
There are also other known chemical processes for recovering PM plated on a base metal substrate by undercutting the plating. But this is possible only when the PM plating is thin enough that a chemical solution can be used to begin dissolving the underlying base metal. Thus, the PM is liberated, before the entire base metal substrate is dissolved, and can be recovered in a non-oxidized state. The problem with this approach is that it is difficult to control the strength of the leaching solution and the residence time of the items so that the entire PM plating is undercut and can be recovered while only a small portion of the underlying base metal is dissolved. Also, a large volume of items with PM plating can contain plating or filling of different thickness with different metal compositions. In such case, the base metal substrates can also be different, so the same leaching solution may act faster on some base metals and slower on others. Consequently, the plating/filling can remain on some items, causing losses of unstripped PM, which can make this undercutting approach problematic on a commercial scale.
Advantages of PM-STRIP
PM-STRIP is technically and commercially superior to conventional refining and smelting:
- Fast, with cycle times measured in minutes. PM can be monetized in days vs. waiting months for payment when plated materials are sent to a smelter.
- 100% effective.
- Stripped PM are fully recovered in metallic form.
- Base metal substrates are left intact without the need for further processing.
- Simple, easy to operate, and cost-effective.
- Chemical solutions can be highly loaded with PM only..
- Processing costs are low. PM-STRIP is uniquely suited for scrap with low concentrations of PM that could not otherwise be processed at commercial scale with conventional refining methods.
- Gold plated and gold filled materials can be mixed for processing, which is not feasible with conventional processes.
- Uses modular systems that can be built to meet the needs and budgets of small-medium refiners or can be easily scaled to meet the requirements of the largest refiners.
- Is sustainable and replaces the use of cyanide and aqua regia.
We have patents pending for various aspects of PM-STRIP. An IP Overview comparing PM-STRIP in detail with "prior art" is available as appropriate.
We have bench scale units in our Laval laboratory that are available to process refiners' samples.