Preliminary studies have been conducted on alternatives to the pyrometallurgical processing of Ni/MH batteries. Hydrometallurgical treatment provides metal salts as products, which may offer market stability benefits in certain circumstances compared to the primary metals produced by smelting. Another advantage is that the separation and recovery of other valuable constituents such as titanium, vanadium, zirconium, and rare earths may become possible. The U.S. Bureau of Mines (USBM) conducted exploratory research on hydrometallurgical processing options for several years [32,33], concluding that these options are indeed feasible for battery scrap containing either AB2 or AB5 hydride alloys. AB2 metal hydride electrodes typically contain about 54% Ni + Co, 42% Ti + V + Zr, and 4% other elements (Al, Cr) by weight. The AB5 electrode consists of a LaNi5 type alloy on a nickel substrate. The alloy contains about 33% rare earths, 10% Co, 50% Ni, 0.12% Fe, and 6% other metals (Mn, Al). USBM evaluated several different leaching protocols and acid solutions for extraction efficiency on whole batteries, cracked batteries, and components. A two-stage leaching process was found to be particularly effective for concentrating the titanium (Ti), vanadium (V), zirconium (Zr), and chromium (Cr) species in solution. Preliminary precipitation tests to recover partially separated metals from solution were run using pH adjustment, carbonate precipitation or oxalate precipitation, although the optimum methods for producing the highest purity products were not determined. Nickel and cobalt could be recovered by electrowinning or solvent extraction as well as by precipitation techniques.
Operating revenue that could be generated from chemical separation or physical/chemical separation processes for recycling Ni/MH batteries was compared to a pyrometallurgical process in a report prepared for the National Renewable Energy Laboratory (NREL), a DOE facility located in Golden, Colorado , The pyrometallurgical process has similarities to the process operated by INMETCO. Revenues (or costs) were estimated for both AB2 and AB5 hydride alloy battery designs. Other general assumptions in the cost calculations were that the plant was sited in California and was processing 30,000 metric tons of EV batteries annually. The chemical process is based on an acid leach of the battery materials, followed by precipitation of all but the nickel and cobalt, which are recovered by electrowinning. The major products recovered are nickel-iron scrap, steel scrap, polypropylene and nickel metal. In the physical/chemical separation process, the battery electrodes are physically separated prior to chemical processing and the metal hydride alloy powder is recovered and returned to hydride alloy producers. The rest of the procedure is very similar to the chemical process.
For the pyrometallurgical process, all of the battery electrodes and powders are smelted to form a ferronickel product and a slag that is enriched in hydride alloy constituents. Slag from batteries containing AB2 alloy could be smelted further to produce ferrovanadium while rare earth producers may be interested in the enriched rare earth content of the slag from AB5 batteries. The only other products are steel scrap and a very low-grade furnace slag remaining after the smelting of the ferrovanadium product.
In the most favorable case (physical separation/chemical process), the revenue from the recovered products obtained by the recycling process was predicted to be between $16.70/kWh of batteries processed for the AB5 alloy and $18.50/kWh for the AB2 alloy. This is largely because of the value of the credit assumed for the physically separated hydride alloy scrap, although the process is still predicted to generate a small amount of revenue without it. The revenue of $9.50/kWh from the chemical process is second best for the AB5 alloy, and the pyrometallurgical process comes in third at $4.15/kWh. For the AB2 alloy, the pyrometallurgical process looks better at $7.50/kWh, but the chemical process does not generate revenue at -$.50/kWh. The better cost performance of the chemical process in the case of the AB5 alloy is a result of both somewhat lower processing costs and a significantly higher product credit because of the cobalt content.
Little follow-on evaluation has occurred for Ni/MH battery recycling processes since the earlier studies described above were completed in about 1994 in spite of the potential benefits that were shown . Mitsui Mining and Smelting Company Limited reported one additional study in 1995 , Nickel, cobalt and rare earth elements were the major materials that were targeted for recovery from the battery. Following mechanical processing, a sulfuric acid leach was applied as the first step in a hydrometallurgical process. Rare earth elements can be separated by the double salt method and then other impurities (copper, zinc) removed by solvent extraction or sulfide precipitation. The final solution contains nickel and cobalt, which are recovered in high purity by electrowinning. A conceptual flow diagram of the process was presented, but continuous testing and other evaluations had not yet been done.
Optimization of the hydrometallurgical-type processes is far from complete, and feasibility and pilot scale-up experiments must still be performed. In order to capitalize on the value of cobalt in the AB2 battery system (worth approximately $18 per pound), and the value of rare earth materials for industrial applications (worth approximately $8 to $10 per pound) , a more detailed examination of the economics associated with hydrometallurgical processes is needed. Two factors that seem to be contributing to the low level of interest are the current low prices in the primary metals market and the slow increase in the number of fielded EV batteries. Operators of existing commercial smelting operations are unwilling to accept small, infrequent shipments of battery manufacturing scrap, and returns of end-of-life batteries from the field are expected to be miniscule for some time yet. Even waste processors that are willing to accept small waste shipments, such as INMETCO, will not pay for the scrap at the current level of volume and price. Research on improved, more comprehensive recycling processes is not occurring because profits in the near term will be too low to justify the investment. However, the relatively high cost of the Ni/MH battery system makes it important to maximize the recycling credit that can be obtained, and this will only happen with the development of a more comprehensive recycling process.
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