Develop new technology to encourage domestic processing of automobile catalytic converters for recovering platinum-group metals.
Automobile catalytic converters are used to convert pollutants in automobile exhaust into water vapor, carbon dioxide, and nitrogen. About 1 million troy ounces of platinum-group metals arc used each year in the United States to manufacture catalytic converter. It has been estimated that about 60 pct of the used catalytic converters are being collected. Most of these used catalytic converters were not processed in the United States but were exported to Japan or Europe for processing. In 1991, about 250,000 troy ounces of platinum- group metals were recovered from scrap catalytic converters collected in the United States. Each short ton of decanned catalyst may contain 30 troy ounces of platinum-group metals valued at over $ 15,000. Development of an economic process for the recovery of platinum-group metals that can be operated on a small or large scale may increase the percentage of catalytic converters collected and encourage domestic processing to recover a platinum-group metal concentrate. This would significantly decrease the dependence of the United States on imports for these critical and strategic metals.
Description of Process
The process developed by the U.S. Bureau of Mines takes advantage of the unique properties of cyanide to selectively dissolve platinum-group metals from automobile catalytic converters and recover a metal concentrate. Cyanide is a traditional industrial chemical used in the recovery of gold and silver. Automobile catalytic converters are first collected and decanned from the type 409 stainless steel canisters. The monolith catalyst “biscuits” consist of a ceramic support with an alumina wash coat and are crushed to minus 8 mesh (-2 mm) for leaching. The pellet catalyst consists of-3 mm alumina solids and is leached as decanncd. Leaching is conducted in an autoclave with no agitation using a 1-pct sodium cyanide solution at a pH of 12.5, heated to 160° C for 1 h. Filtering separate, the valuable pregnant leaching solution that contains primarily platinum-group metals from the waste residue. Subsequent heating of the solution to 275° C for 1 h decomposes the metal- cyanide complexes, destroys all residual cyanide, and precipitates the platinum-group metals.
Best results were obtained from leaching used pellet-type catalysts where greater than 9, pct of the platinum and palladium and 80 pct of the rhodium were dissolved. Leaching used monolith catalysts dissolved almost 90 pct of the platinum and palladium and 75 to 80 pct of the rhodium. The leached residues from processing both types of catalysts passed the toxicity characteristics leaching procedure (TCLP) and are considered nonhazardous wastes. Recovery of the dissolved platinum- group metals from the pregnant leach solution as metallic concentrate by thermal decomposition of the metal-cyanide complexes was greater than 99.8 pct. The concentrate contained 60 to 70 pct platinum-group metals and is an excellent feed for a refinery. Thermal treatment also destroyed all of the free cyanide in the solution, ultimately leaving a barren solution that analyzes less than 0.2 ppm total cyanide and meets drinking water standards for cyanide. Evaporation of the barren solution yielded a sodium carbonate-bicarbonate residue, which may be marketable as a fluxing agent in melting operations.
Research has progressed from 20-g laboratory tests through a 2,000-g (4.4-lb) process development unit to the current 45-kg (100-lb) pilot plant. The pilot plant was built in cooperation with two process cooperators and a refiner cooperator. Each process cooperator supplied a short ton of catalyst to test in the pilot plant and $10,000 to help defray processing costs. Few problems were encountered in operation of the pilot plant, and preliminary results indicate that extractions of platinum- group metals are comparable to earlier results.
The U.S. Department of the Interior has applied for a patent on the cyanide leaching method for recovering platinum-group metals. Additional patent information may be obtained by contacting the Office of Technology Transfer, 801 Seventh St. NW, Washington, DC 20241 (telephone: 202-501-9323).