Ind. Eng. Chem. Res. 1993,32, 2485-2489
2485
Catalyzed Electrochemical Dissolution for Spent Catalyst Recovery Laura J. Silva,’ Lane A. Bray, and Dean W. Matson Battelle, Pacific Northwest Laboratories, Richland, Washington 99352
Increasing concern for pollution prevention and waste disposal has created aneed for clean alternatives for spent catalyst recovery or disposal. A new technique uses catalyzed electrochemical dissolution to oxidize and subsequently dissolve typical catalyst metals and metal contaminants so that they may be isolated and reclaimed. Hydrocarbons and carbonaceous materials adhered to the catalyst surface are oxidized to carbon dioxide and water a t moderate temperatures and ambient pressure. Sulfides are converted to aqueous sulfur species. Supercritical fluid extraction is an optional pretreatment step to remove hydrocarbons adhered to the catalyst. The technique described here offers advantages over conventional spent catalyst reclamation processes. In this paper, recovery of a spent hydrodesulfurization catalyst is described.
Introduction Expanded use of catalysts for the production of fuels and chemical feedstocks will continue in response to (1) economic pressures to upgrade heavier crudes and other feeds having high levels of impurities, (2) competitive pressures to achieve higher conversions using less energy, and (3) pressures to increase reaction selectivities to minimize waste production. While the incentives for catalyst processes are great, all catalysts gradually lose activity through coking, poisoning by metals, sulfur, or halides,or loss of surfacearea from sinteringat high process temperatures. Regeneration is possible where the catalyst deactivation can easily be reversed. For example, naphtha reforming catalysts are routinely regenerated by removal of carbon accumulation and can be reused for multiple cycles. However, the economic life of a catalyst is ultimately limited by regeneration costs and extent of irreversible deactivation. The need for clean alternatives for spent catalyst recovery or disposal becomes more apparent as concerns about pollution and waste disposal grow. The U S . petroleum refining industry consumesmore than 50 million pounds per year of fresh hydrocracking, hydrotreating, and naphtha reforming catalysts (Hoffman, 1991). Spent catalysts are typically disposed of in landfills or, if the metal value is high enough, sold to a metals reclaimer. The direct cost of and potential liability associated with landfill disposal has increased. Reclaimingspent catalyst materials typically involves conventional metallurgical processes such as roasting, acid leaching, caustic dissolution, reactions with H2S or C12, and calcination (Elvin, 1989; Ward, 1989; Parkinson, 1987; O’Sullivan, 1992). A new process developed at Battelle uses catalyzed electrochemicaldissolution to destroy the carbon adhered to the catalyst and dissolve catalyst metals in an aqueous solution for further recovery of the raw materials. In this process, shown in Figure 1, spent catalyst is added to a solution containing small (catalytic) amounts of elements that form kinetically active, strongly oxidizing ions such as cerium(IV) or silver(I1). The oxidizing ions are regenerated at the anode; they act in a catalyticmanner carrying electrons from the solid surface to the anode of the electrochemical cell. The couples found applicable are shown below: Ce3++ Ce4++ eEo = -1.61 V (1) Ag+ + Ag2+ + eEo = -1.98 V (2) A cerium oxidizer was used for the technique described in this paper. For this procedure, solution is added to the anode side of an electrochemical cell. A t the anode, 0888-588519312632-2MWM~V0
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2486 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993
the dissociation of water: 2H20
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Such side reactions represent inefficiencies in electric current utilization, and the extent to which they occur depends on the reaction conditions and the kinetics of these reactions relative to the desired reactions. The net anodic reactions must be balanced by corresponding cathodic reactions. Cathodic reactions are determined by the species present in the catholyte, cell potential, and mass transport characteristics in the cell. Once all the spent catalyst material is dissolved, subsequent steps are required to separate the various aqueous metal ions into valuable product streams. Buildup of dissolved reaction products, such as sulfate, would be limited by performing another separation or taking a purge from the recycle stream. The design of these separation steps depends on the chemistry of the specific catalyst to be treated and may be driven by solubility limits. Conventional hydrometallurgical techniques may be used for separations. The complexity of the catalyst chemistry will ultimately determine whether it would be practical to remanufacture the catalyst onsite or process the spent catalyst offsite. Catalyzed electrochemical dissolution for recovery of spent catalyst promises several advantages over conventional approaches. First, the adhered carbonaceous material is completely oxidized to carbon dioxide electrochemically at low temperatures (
