Much of the effort to develop the Na/S battery was aimed at its use in electric vehicles. Current applications of this advanced battery system are now mainly in the stationary battery area, but feasibility studies were done on the recycling of this system before the EV development efforts were suspended. Sodium/sulfur batteries contain reactive and corrosive materials, but not toxic ones. By treatment of the battery waste, the reactivity problems can be removed.
The major difficulty in recycling this chemistry is that most of the constituents have low value or are difficult to recover in a form that could be used in a high-value application (e.g., the beta" alumina electrolyte). A patented proposed recycling scheme has been evaluated on a pilot scale and found to be acceptable from a cost and technical standpoint . This process replaced incineration, which was used earlier in the development program, but judged too expensive for large numbers of batteries. In the recycling process, the batteries are shredded and the soluble constituents extracted with water. The resulting sodium polysulfide solution is acidified to generate hydrogen sulfide, which can be converted to sulfur in a small-scale Claus process reactor. The remaining Na2S04 solution can be converted to sulfuric acid and sodium hydroxide, which are sold or used in the process. Insoluble ceramic, graphite and metal cell case materials are also recovered. Sulfur recovered in this way was recycled into new Na/S cells that showed identical performance to cells built with virgin sulfur.
Estimated processing costs were deemed acceptable at $6 to $10/kWh of batteries based on a 5,000 (metric) ton per year plant size. This compares favorably to the $40 to $60/kWh incineration cost. The relatively low value of the recovered materials prevents this recycling process from being completely self-supporting. Other more valuable forms of sulfur could be recovered with a modified process, but markets for them may be limited because of the Exclusion Principle. A detailed examination of the cost benefits in this area has not been done.
A specification was drawn up for a pilot recycling plant (250 tons/year) that would meet German safety and environmental standards, but the plant was not constructed because the quantity of returned batteries was insufficient to support it. Analyses of solution from laboratory-scale recycling were carried out for chromium, which is regulated for toxicity, and levels were found to be below EPA limits. TCLP tests on cells also show amounts of leachable chromium that are within EPA standards.
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