Established Materials Technology: Front Page
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CR3 Update: Recycling of Strategic Metals
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By B. Mishra, C.D. Anderson, P.R. Taylor, C.G. Anderson, D. Apelian, and B. Blanpain
Posted on:
4/20/2012 12:00:00 AM...
Editor’s Note: TMS has forged cooperative agreements with several carefully selected organizations that actively work to benefit the materials science community. As part of this relationship, TMS provides updates on the activities of these organizations. This installment, by the Center for Resource Recovery & Recycling (CR3), focuses on the recycling of strategic metals. The Center for Resource Recovery & Recycling is a research center established by Worcester Polytechnic Institute, Colorado School of Mines, and K.U. Leuven.
NEW SUPPLY OPPORTUNITIES
The supply of metals and materials for use in manufacturing commercial commodities will come under significant pressure in the near future. Resources from nature will diminish, as recovery from earth becomes progressively difficult. The sources are likely to get leaner and their exploitation ever less viable. This change in the supply scenario, of course, will depend on the specific type of metal and material and its demand. In addition, from the U.S. perspective, excessive dependence on foreign imports for some metals will place constraints on availability due to socio-political and economic reasons. For some of the “exotic” metals, such as rare earths, molybdenum, rhenium, ruthenium, tungsten, and platinum group metals (PGMs), recycling and recovery from spent secondary resources will be the only viable options for sustainable growth, as demand is predicted to remain high and the indigenous resources low for these materials.
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Figure 1. The consumption of primary metals in the United States indicates a steady trend from 1960–1995, but the recycled metals consumption shows a growth of more than 200 percent.1 (Click on images to enlarge.)
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Figure 2. Criticality matrix for selected imported metals.2
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Table I. Yttrium, Terbium, and Dysprosium Show a Shortfall in Supply Compared with Other Rare-earth Metals
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Recycling of metals in the United States has shown a growing trend over the past five decades (Figure 1). The reuse, reprocessing, recovery, and recycling of metals and materials are steps toward the achievement of sustainability. Recycling provides energy conservation, better environmental control, and improved economic process viability for most metals and materials. The significant growth of the recycling industry for such materials as iron and steel, aluminum, copper, zinc, and lead has been supported by these provisions, enabling the supplies of these materials to keep up with demand. However, some other industrially significant metals indicate higher demand than supply. As examples, terbium, dysprosium, and yttrium are used in fluorescent light fixtures, while neodymium and samarium are used in permanent magnet production. Figure 2 also shows critical metals that will experience a higher impact of supply restriction due to high risks in supply. Ways to recover these from spent sources must be developed, as shown in Table I. These are the metals that require special attention from the recycling and recovery perspectives.
RECOVERY AND RECYCLING APPROACHES
Driven by the superalloy sector, which accounts for 80 percent use of rhenium metal, an annual growth in demand of an average of five percent for rhenium is predicted over the next five years. Rhenium metal is also used as an additive to tungsten and molybdenum-based alloys, filaments for mass spectrographs and ion gauges, and rhenium-molybdenum alloys in superconductors at 10K. Due to its high wear and arc-corrosion resistance, it is also an important electrical contact material due and is also used in Re-W thermocouples for high temperatures.
The current global production of rhenium is estimated at 50 mT, with growth supported by an expansion in primary production capacity, greater recycling of rhenium-bearing superalloy scrap and increased use of superalloy “revert.” However, the future supply in the United States will always be strained due to net import reliance of 85 percent. In addition to being produced as a byproduct of molybdenum, it is possible to recycle rhenium during processing and after its use as a means of fulfilling growing demand.
Recovery and refining of rhenium, tungsten, and molybdenum from W-Re, Mo-Re, and other superalloy scraps have been carried out via an oxidative pyrometallurgical roast technique. Initially, the scrap is roasted at 1,000°C under an oxidizing atmosphere to convert the contained rhenium to water-soluble rhenium pentoxide (Re2O7). The volatile rhenium pentoxide is condensed in the cooler part of the tube furnace. This condensed material is then sent for digestion in water. The aqueous rhenium (ReO4–) is subsequently precipitated as potassium perrhenate, upon the addition of potassium chloride via the following reaction:
KCl + ReO4"– = KReO4 + Cl– (1)
The potassium perrhenate is filtered and further purified via continued dissolution and recrystallization. After purification, the salt is dried and sent for reduction under a hydrogen atmosphere at approximately 350°C via the following reaction.
2KReO4(s) + 7H2(g) = 2Reo
+ 2KOH(s) + 6H2O(g) (2)
The metallic rhenium is first washed with distilled water, and then with 95 percent ethanol to remove any residual alkali salts.3
The process of rhenium recovery from spent platinum rhenium catalyst relies on the use of sulfuric acid for the dissolution of alumina, rhenium, and to some extent, platinum. The rhenium rich solution is separated from the platinum-containing residue and separated from the aqueous aluminum using ion exchange. Rhenium is subsequently eluted from the organic amine resin by way of hydrochloric acid addition. After elution, the rhenium rich eluate is neutralized using ammonium hydroxide. This solution is then evaporated to form a super-saturated solution, and cooled to allow for crystallization of ammonium perrhenate. After continued redissolution and recrystallization, a high purity ammonium perrhenate precipitate is obtained.4
A process for the electrolytic decomposition of rhenium superalloys has shown favorable results (U.S. patent 0110767). The developers describe a process where titanium baskets, which act as the electrodes and containing superalloy scrap, are fed to a polypropylene electrolysis cell containing an 18 percent HCl solution. The electrolytic dissolution is carried out for 25 hours at a frequency of 0.5Hz, current of 50 amps, voltage of 3–4V, and a temperature of 70°C. The remaining scrap is then filtered from the pregnant solution and sent for further dissolution in sodium hydroxide/peroxide solution. These processes that have shown technical viability on a laboratory scale have to be further optimized for commercialization.
As to trends for other strategic materials, tantalum has a recycling rate of approximately 20 percent while molybdenum shows less than a 30 percent rate of recycling in the United States. Yet, tantalum and molybdenum contained in scrap imported by the United States exceeds 30mT and 200mT, respectively. Tantalum shows a downward trend due to weak demand for capacitors in consumer electronics. The supply of columbite-tantalite (coltan) from South Africa and China has kept the supply available and the prices low. Tantalum will likely see a higher demand in the future owing to its superior dielectric properties (capacitors with higher capacitance voltage, better heat transfer and frequency characteristics) and its excellent corrosion resistance (chemical, medical, and pharmaceutical industries; fabrication of heating elements, surgical implants, combustion turbines and jet engines.) Tantalum shows good thermal conductivity and is useful as a heat transfer surface in acidic or corrosive environments.
Molybdenum’s high melting point makes it the metal of choice for heat shields, heating elements, electrodes, and other high temperature components. Its primary use is as an alloying agent in steels (SS, Cr-Mo, etc.) and Ni-based superalloys (Hastelloys), which will keep the demand high. Other applications for molybdenum include electrodes for electrically heated glass furnaces, nuclear energy applications, and missile and aircraft parts. It is also used as a catalyst in the refining of petroleum, as filament material in electronic/electrical applications and in support radio and light bulbs, and as material for arc resistant electric contacts and thermocouple sheaths.
Some of the current recovery processes of Mo from molybdenum oxide in spent catalysts employs sodium chloride salt roast and leach:5
MoO3 + 2NaCl + 1/2O2(g)
Na2MoO4 + 2HCl(g) (3)
Na2MoO4 + NH3(aq)
(NH3)6[MoO4] (4)
and from molybdenum sulfide in spent catalysts using the following reactions:6
MoS2 + 3Na2CO3
+ 2H2O + 3.5O2
Na2MoO4 + 2Na2SO4
+ 2H2O + 3CO2 (5)
CONCLUSION
Recycling of critical and strategic metals will become a necessity, as demand will outstrip the supply in the future, particularly in the United States, due to import fluctuations. A controlled and reliable source for spent secondary resources will be required, while technologies need to be developed that are optimized, not only economically, but also from energy and environmental perspectives. Better separation and scrap sortation schemes must be adopted, followed by adequate beneficiation and chemical/metallurgical recovery processes. Just as with steel and aluminum, primary production of these strategic metals will have to be supplemented by secondary recovery for sustainability.
REFERENCES
1. G. Matos and L. Wagner, “Consumption of Materials in the United States, 1900–1995,” USGS Report, pubs.usgs.gov/annrev/ar-23-107/aerdocnew.pdf, 1998.
2. Board on Earth Sciences and Resources, Minerals, Critical Minerals, and the U.S. Economy (Washington, D.C.: National Academies Press, 2008).
3. B. Heshmatpour, J. Less-Common Metals, 86 (1982), pp. 121–128.
4. M.I. ElGuindy, Rhenium and Rhenium Alloys, ed. B.D. Bryskin (Warrendale, PA: TMS, 1997), pp. 89–97.
5. B.B. Kar, B.V.R. Murthy, and V.N. Misra, Int. J. Miner. Process., 76 (3) (2005), pp. 143–147.
6. Kyung Ho Park, B. Ramachandra Reddy, D. Mohapatra, and Chul-Woo Nam, Int. J. Miner. Process., 80 (4) (2006), pp. 261–275.
B. Mishra (Professor and the Center Co-Director of CR3), C.D. Anderson (Ph.D. student); P.R. Taylor (Professor); and C.G. Anderson (Professor) are at the Colorado School of Mines; D. Apelian is a Professor and Center Co-Director of CR3 at the Worcester Polytechnic Institute, Worcester, MA, and B. Blanpain (Professor & Center Co-Director of CR3) is at KU Leuven.
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