Cordero Gallium Project
north central Nevada, USA
(U.S. GEOLOGICAL SURVEY MINERALS YEARBOOK-1999 62.4)
Because of the low yield in processing gallium for optoelectronic devices or integrated circuits, substantial quantities of new scrap are generated during the various processing stages. These wastes vary in purity with respect to gallium content depending upon the processing steps. Gallium arsenide (Ga-As)-based scrap, rather than metallic gallium, represents the bulk of the scrap that is recycled.
During the processing of gallium metal to make the Ga-As device, waste is generated in several stages. If the ingot formed does not exhibit single crystal structure or if it contains excessive impurities, the ingot is considered to be scrap. Typically, there is some Ga-As that remains in the reactor after the ingot is produced and this may be recycled as well. During the wafer preparation and polishing stages, significant quantities of wastes are generated. Before wafers are sliced from the ingot, both ends of the ingot are cut off and discarded due to impurities concentrated at the tail end of the ingot and crystal imperfections at the seed end. These ends represent as much as 25% of the weight of the ingot. As the crystal is sliced into wafers, two types of wastes are generated-saw kerf, which is essentially Ga-As sawdust, and broken wafers. When the wafers are polished with an abrasive lapping compound, a low-grade waste is generated.
During the epitaxial growth process, various wastes are produced, depending on the growth method used. Because Ga-As is a brittle material, wafers may break during the fabrication of electrical circuitry on their surfaces. These broken wafers may also be recycled. Gallium content of these waste materials varies from less than 1% to as much as 99.99%. In addition to metallic impurities, the scrap may be contaminated with other materials introduced during processing such as water, silicone oils, waxes, plastics, and glass (Kramer, 1998, p. 15).
In processing Ga-As scrap, the material is crushed and then dissolved in a hot acidic solution. This acid solution is neutralized with a caustic solution to precipitate the gallium as gallium hydroxide, which in turn, is filtered from the solution and washed. The gallium hydroxide filter cake is re-dissolved in a caustic solution and electrolyzed to recover 99.9% to 99.99% gallium metal (Kramer, 1998, p. 15). Most Ga-As manufacturers recycle their scrap or sell it to metal traders, recyclers, or suppliers, who can recover the gallium and return it to the customer. Generally the prices commanded by Ga-As scrap parallel the price fluctuations of 99.99%-pure gallium metal. Also, prices are dependent on the type of scrap and its gallium content. Ga-As scrap that is recycled is considered new scrap, meaning that it has not reached the consumer as an end product, and it is present only in the closed-loop operations between the companies that recover gallium from Ga-As scrap and the wafer and device manufacturers (Kramer, 1998, p. 15). 8 Prepared by Deborah A. Kramer [Telephone: (703) 648-7719; Fax: (703) 648-7722].