Management Of Spent Catalysts In Petroleum Refineries

Management of Spent Catalysts in Petroleum Refineries

MANAGEMENT OF SPENT CATALYSTS IN PETROLEUM REFINERIES David T. Liang Deputy Director Institute of Environmental Science and Engineering Singapore

Abstract Catalysts are indispensable in the petroleum refining and petrochemical industry for routine production of gasoline, diesel fuels, jet fuels, heavy oil hydrocarbons, petrochemicals and plastics. Hydrocarbons (HT and HDS) and residue hydrodesulfurization (RDS) are the major processes for converting crude oil into these petroleum products. During processing, catalysts will become contaminated with impurities in the crude oil feed and become deactivated. When that happens, they are usually sent for regeneration where contaminates are removed. Ultimately, they will be contaminated with coke, sulfur, vanadium and nickel in a manner and at a level that makes regeneration impractical. At this stage, catalysts are considered “spent” and they may pose significant environmental problems, as landfill disposal is no longer accepted as best practice. Hydro-desulfurization (HDS/RDS) of heavy oil produces spent catalysts that contain molybdenum (Mo), vanadium (V), nickel (Ni) or cobalt (Co) at concentration levels that has been found to be economical for recovery. Due to its complex nature, metal recovery from HDS/RDS spent catalysts involves a combination of pyro- and hydro- metallurgical processes. At present, only a handful of companies are capable to do so on a commercial scale and in an environmentally acceptable manner. The energy savings and environment benefits associated with these recycling activities are also quite significant. It has been estimated that recycling of various metal scraps consumes approximately 33% less energy and generates 60% less pollutants than the production of virgin material from ore. However with increasing demand of ever more complex metallic composite and alloy materials in modern manufacturing processes, it becomes imperative to develop appropriate methods for the recovery of these valuable metals. The present paper will provide a brief overview of the management practices and recovery of metals from spent catalysts, with the focus on the technologies, issues and opportunities associated with the recycling of valuable metals. Potential impacts of issues such as the Basel Convention and environmental legislation are also highlighted.

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Management of Spent Catalysts in Petroleum Refineries

Introduction On-going environmental concerns have had major impacts on the refinery industry in general. First there was the response to phase out the lead in gasoline by most developed countries, following the discovery of health hazard it poses on urban, young populations through lead accumulation in their blood. Then there was the discussion in some countries of a possible ban of an additive called MTBE (methyl tertiary butyl ether) which was found to contaminate groundwater through leaky underground storage tanks. Current regulations on the emission of sulfur oxides (SOx) from vehicles have pushed fuel sulfur contents to very low levels (~10 ppm in some jurisdictions). Refineries are now facing the formidable challenge of lowering the sulfur content in their products at a time when the good quality low-sulfur crude is becoming scarce. Technically, removing sulfur from the products during the refining stage is possible, however, the economic impact could be substantial in terms of major process modifications needed. Another major impact on the refinery will be the expected increase in the need for catalyst replacement and disposal of the spent catalysts. This is because proportionally, more sulfur will report to the catalysts that will hasten their service life through sulfur deposition. Safe disposal of these spent catalysts is a significant environmental problem as landfill disposal is no longer generally accepted as the best practice. In many cases, the spent catalysts have been classified as hazardous waste material and are subject to stringent disposal guidelines. Most major refinery companies have set up special disposal practices and only allow authorised waste collectors and processors to dispose the catalyst waste. Metals in the Crude Oil and Catalysts Crude oils are complex mixtures, ranging in consistency from water to tar-like solids, and in color from clear to black. An "average" crude oil contains about 84% carbon, 14% hydrogen, 1-3% sulfur, and less than 1% each of nitrogen, oxygen, metals, and salts. Crude oils can generally be classified as paraffinic, naphthenic, or aromatic, based on the predominant proportion of similar hydrocarbon molecules. Refinery crude base stocks may consist of mixtures of two or more different crude oils. Metals including nickel, iron, and vanadium are often found in crude oils in small quantities and are removed during the refining process. Trace amounts of arsenic, vanadium, and nickel can accumulate in the pore structure of catalysts and poison these processing catalysts. Fluid catalytic cracking (FCC) and hydrotreating are the major processes for converting crude oil into petroleum products in Singapore. FCC catalysts are ultimately contaminated with coke, vanadium and nickel in a manner and at a level that makes regeneration impossible. Hydrotreating heavy oil also produces spent catalysts containing coke, nickel, and vanadium. In this instance, regeneration may be possible by selective removal of nickel, vanadium and iron, but irreversible deactivation ultimately occurs. Catalytic cracking

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Management of Spent Catalysts in Petroleum Refineries

breaks complex hydrocarbons into simpler molecules in order to increase the quality and quantity of lighter, more desirable products and decrease the amount of residuals. Catalytic cracking is similar to thermal cracking except that catalysts facilitate the conversion of the heavier molecules into lighter products. Use of a catalyst in the cracking reaction increases the yield of improved-quality products under much less severe operating conditions than in thermal cracking. Typical temperatures are from 850-950 degrees F at much lower pressures of 10-20 psi. The catalysts used in refinery cracking units are typically solid materials (zeolite, aluminum hydrosilicate, treated bentonite clay, fuller's earth, bauxite, and silica-alumina) that come in the form of powders, beads, pellets or shaped materials called extrudites. F luid Catalytic Cracking The most common process is FCC, in which the oil is cracked in the presence of a finely divided catalyst which is maintained in an aerated or fluidized state by the oil vapors. The fluid cracker consists of a catalyst section and a fractionating section that operate together as an integrated processing unit. The catalyst section contains the reactor and regenerator, which with the standpipe and riser forms the catalyst circulation unit. The fluid catalyst is continuously circulated between the reactor and the regenerator using air, oil vapors, and steam as the conveying media. A typical FCC process involves mixing a preheated hydrocarbon charge with hot, regenerated catalyst as it enters the riser leading to the reactor. The charge is combined with a recycle stream within the riser, vaporized, and raised to reactor temperature (9001,000 degrees F) by the hot catalyst. As the mixture travels up the riser, the charge is cracked at 10-30 psi. Spent catalyst is regenerated to get rid of coke that collects on the catalyst during the process. Spent catalyst flows through the catalyst stripper to the regenerator, where most of the coke deposits burn off at the bottom where preheated air and spent catalyst are mixed. Fresh catalyst is added and worn-out catalyst removed to optimize the cracking process. Treatment Processes Throughout the history of refining, various treatment methods have been used to remove non-hydrocarbons, impurities, and other constituents that adversely affect the properties of finished products or reduce the efficiency of the conversion processes. Treating can involve chemical reaction and/or physical separation. Typical examples of treating are chemical sweetening, acid treating, clay contacting, caustic washing, hydrotreating, drying, solvent extraction, and solvent dewaxing. Sweetening compounds and acids desulfurize crude oil before processing and treat products during and after processing. Following the Second World War, various reforming processes improved gasoline quality and yield and produced higher-quality products. Most of these involved the use of catalysts and/or hydrogen to change molecules and remove sulfur. A number of the more commonly used treating and reforming processes are described in this chapter of the manual.

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Management of Spent Catalysts in Petroleum Refineries

Catalytic Hydrotreating Catalytic hydrotreating is a hydrogenation process used to remove about 90% of contaminants such as nitrogen, sulfur, oxygen, and metals from liquid petroleum fractions. These contaminants, if not removed from the petroleum fractions as they travel through the refinery processing units, can have detrimental effects on the equipment, the catalysts, and the quality of the finished product. Typically, hydrotreating is done prior to processes such as catalytic reforming so that the catalyst is not contaminated by untreated feedstock. Hydrotreating is also used prior to catalytic cracking to reduce sulfur and improve product yields, and to upgrade middle-distillate petroleum fractions into finished kerosene, diesel fuel, and heating fuel oils. In addition, hydrotreating converts olefins and aromatics to saturated compounds. Catalytic Hydrodesulfurization Process Hydrotreating for sulfur removal is called hydrodesulfurization. In a typical catalytic hydro-desulfurization unit, the feedstock is deaerated and mixed with hydrogen, preheated in a fired heater (600-800 degrees F) and then charged under pressure (up to 1,000 psi) through a fixed-bed catalytic reactor. In the reactor, the sulfur and nitrogen compounds in the feedstock are converted into H2S and NH3. The reaction products leave the reactor and after cooling to a low temperature enter a liquid/gas separator. The hydrogen-rich gas from the high-pressure separation is recycled to combine with the feedstock, and the lowpressure gas stream rich in H2S is sent to a gas treating unit where H2S is removed. The clean gas is then suitable as fuel for the refinery furnaces. The liquid stream is the product from hydrotreating and is normally sent to a stripping column for removal of H2S and other undesirable components. In cases where steam is used for stripping, the product is sent to a vacuum drier for removal of water. Hydrodesulfurized products are blended or used as catalyticreforming feedstock. Other Hydrotreating Processes Hydrotreating processes differ depending upon the feedstocks available and catalysts used, it can be used to improve the burning characteristics of distillates such as kerosene. Hydrotreatment of a kerosene fraction can convert aromatics into naphthenes, which are cleaner-burning compounds. Hydrotreating also can be employed to improve the quality of pyrolysis gasoline (pygas), a by-product from the manufacture of ethylene. Traditionally, the outlet for pygas has been motor gasoline blending, a suitable route in view of its high octane number. However, only small portions can be blended untreated owing to the unacceptable odor, color, and gum-forming tendencies of this material. F CC Catalysts According to one estimate (Avidan 1992), that the capacity of the worldwide FCC catalyst production in 1990 was about 1,100 ton/day or 400,00 ton/year. Assuming that 90% of the production capacity is needed to replace spent catalyst from the FCC units, the total amount of spent FCC generated in the world is then about 360,000 t/y. Rao (1993) estimated that there were about 336 FCC units operating around the world, each processing 3,300 t/d of feed and requiring 2-3/t/d of fresh catalyst make up. This gives a worldwide 2nd Asian Petroleum Technology Symposium Program

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Management of Spent Catalysts in Petroleum Refineries

total spent FCC catalyst between 250,000 to 368,000 t/y. Following table gives the geographical distribution of FCC units worldwide and the approximate generation rate of spent FCC catalysts : Region

No of units

Est. Spent catalyst t/y 1

% of total

North America Asia Pacific Latin America Europe Middle East/Africa Eastern Europe Other 2

176 48 37 34 17 13 11

161,000 43,800 33,800 31,000 15,500 12,000 10,000

52 15 11 10 5 4 3

Total

336

306,600

100

Note: 1. The generation rate is estimated from the average fresh make up required (2.5 t/d) for each unit and the number of units given in Rao (1993). 2. There 11 FCC units unaccounted by Rao (1993). Hydrotreating Catalysts The world generation of the spent metal-bearing, hydrotreating (HT) catalyst was estimated (Dakota Catalyst, 1996) to be about 87,500 t/y and spent desulphurization catalyst at 297,500 t/y from the petroleum refinery industry. Metal Bearing

North America

Other

Wor ldwide

Single Metal (Ni, Co) Desulphurization

25,000 110,000

62,500 187,500

87,500 297,500

Total

135,000

250,000

385,000

Noting that the estimate for the FCC catalyst in the previous section is for the year 1990. In order to get a more current estimate of FCC consumption, one can use the rule of thumb which suggests that the ratio between the amount of FCC and HT catalyst consumption should be about 5:1. This means that if we take the more current estimate of the HT (single metal) catalysts, 87,500 ton shown above as being correct, then the worldwide FCC consumption for 1996 should be 437,500 t. Given that there is elutriation loss associated with the FCC system which is typically about 10%, the recoverable spent FCC catalysts would then be about 393,000 t in 1996.

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Management of Spent Catalysts in Petroleum Refineries

Singapore and the Regional Market The amount of spent FCC and hydrotreating catalysts produced by petroleum refineries in the Southeast Asia is estimated to be around 20,000 tones and 5,000 tones per annum, respectively, and the demand for petroleum refining catalysts is expected to have a steady growth in foreseeable future. Thus, the existing and future environmental problems posed by these materials is considerable. The current practice of Singapore petroleum refineries to ship these catalysts overseas is both expensive, and likely to be made illegal as international protocols such the Basel convention come into force. As the petroleum industry is one of the largest export earners for Singapore a domestic solution to the recycling and disposal of catalysts requires urgent investigation. Although the total quantity of spent catalyst generated in Singapore and the region is small, with changing legislative environment worldwide it is expected to have to deal with its own waste. Thus, the existing and future environmental problems posed by these materials is considerable. As the petroleum industry is one of the largest export earners for Singapore a domestic solution to the recycling and disposal of catalysts requires urgent investigation. US EPARegulations The U.S. Environmental Protection Agency (EPA) is proposing to amend the regulations for hazardous waste management under the Resource Conservation and Recovery Act (RCRA) by listing, as hazardous wastes, three residuals from petroleum refining processes because certain disposal practices may present a risk to human health or the environment. EPA is also proposing not to list as hazardous eleven process residuals. This action proposes to add the toxic constituents found in the wastes to the list of constituents that serves as the bases for classifying wastes as hazardous. This action is proposed pursuant to RCRA section 3001(b) and section 3001(e)(2), which direct EPA to make a hazardous waste listing determination for ``refining wastes.'' The effect of this proposed regulation would be to subject these wastes to regulation as hazardous wastes under Subtitle C of RCRA. Additionally, this action proposes to designate the wastes proposed for listing as hazardous substances subject to the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), and to adjust the one-pound statutory reportable quantities (RQs) for these substances. In support of the Agency's regulatory reinvention efforts, this action also proposes changes to the RCRA regulations to promote the environmentally sound recycling of oil-bearing residuals. Specifically, the Agency is proposing to broaden the existing exemption for certain wastes from the definition of solid waste. These include oil-bearing residuals from specified petroleum refining sources inserted into the petroleum refining process, and spent caustic from liquid treating operations when used as a feedstock. Today's proposal also would exempt from the definition of hazardous waste mixtures of clarified slurry oil (CSO) storage tank sediment and/or in-line filter/separation solids with tank wastewaters, provided that the waste is discharged to the oil recovery sewer before primary oil/water/solids separation, and ceramic support media separated from spent hydrotreating/hydrorefining catalysts. Finally, EPA is proposing to apply universal treatment standards (UTS) under the Land Disposal Restrictions program to the Petroleum Refining Wastes proposed for listing in this rulemaking. 2nd Asian Petroleum Technology Symposium Program

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Management of Spent Catalysts in Petroleum Refineries

The Basel Convention In 1989, 105 countries and the European Union signed the Basel Convention on the Control of Transbondary Movement of Hazardous Waste and Their Disposal. It was a response by the international community to incidents of indiscriminate final disposal or dumping in developing nations by others. The emphasis is now focused on regulating shipments for recycling, which has raised numerous interpretations and questions whether it is a justifiable interference in world trade (Alter, 1997). Earlier definitions on hazardous waste also put many inert, harmless scrap such as stainless steel in question due to its content of nickel. Clearly, the definition of waste under Basel Convention has a broad reach and is complicated and controversial. Its implications on trade of metal scraps will have impact of the amount of metals that is recycled worldwide. For this reason, its application and enforcement has not yet been fully accepted. However, the underlying goals principles of the Basel Convention, which is to assure sound environmental management has not been lost. Incidents of indiscriminate dumping will no longer be acceptable internationally. Each country will have to learn to deal with its own waste and manage it in the most effective manner. Competing Technologies for Processing Spent Catalyst There are three main methods for the treatment of spent catalysts, they include: (1) encapsulation and stabilization of heavy metals in spent FCC catalysts (2) metal recovery and separation from spent hydrotreating catalyst, and (3) glassification and vitrification for stabilization together with other industrial solid wastes which may contain heavy metals such as As, Zn, Pb etc in a form suitable to use as aggregate for road paving and concrete applications. Current paper only reviews the metal recovery technologies as they are considered the most advanced state for metal recovery from waste materials and will prevent the dispersion of toxic elements in the environment while completing the life cycle of these valuable resources. Spent catalysts are truly a valuable source of metals as they contain up to 10% molybdenum and/or vanadium, 3% nickel or cobalt and 50% alumina. The F CC Catalysts Recycling Fluid catalytic cracking (FCC) and hydrotreating are the major processes for converting crude oil into petroleum products. FCC catalysts are ultimately contaminated with coke, vanadium and nickel in a manner and at a level that makes regeneration impossible. Hydrotreating heavy oil also produces spent catalysts containing coke, nickel, and vanadium. In this instance, regeneration may be possible by selective removal of nickel, vanadium and iron, but irreversible deactivation ultimately occurs. These spent catalysts poses a significant environmental problem as landfill disposal is no longer generally accepted as the best practice. When the price of metals are high, it is economical to recover these from hydrotreating catalyst. On the other hand, metal concentrations on residual FCC catalysts are always to low to justify similar treatment. One technology developed by Dakota Catalyst Products uses a three stage process to treat alumina and mullite based catalysts in a manner that:

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Management of Spent Catalysts in Petroleum Refineries

1. separates alumina, mullite and base metals from hydrocarbons and sulphur using a cement kiln, 2. releases the alumina from metals by selective reduction and gravity separation in an arc furnace to yield a purified raw material, and 3. size the alumina and mullite for sale. In principle, the approach is straightforward and does not involve any exotic processing or engineering. There is a large market for the treatment of spent catalysts and the economics grow steadily more favourable as landfill charges escalate. Potential limitations of the process include an inability to cope with waste streams of variable composition, incomplete volatilization of carbon and sulphur during calcination, and lack of disposal or reuse scheme for waste gypsum. Management of the Spent Catalysts There are only a handful of companies that are capable of recycling the metals from spent HDS and RDS catalysts. The catalysts processed originate mainly from various catalytic operations in the petroleum industry like hydrodesulfurization, hydro-demetalization, hydrotreating, hydrorefining, and hydrocracking. The catalyst consist of an alumina base with molybdenum oxide and nickel or cobalt oxide as the active ingredient. During use in the oil refineries, the catalyst becomes "spent" due to adsorption of sulfur, carbon, vanadium, nickel, iron and other elements which inhibit the catalytic process. Spent catalyst is received in bulk or packaged in flowbins and contains 5 to 25% oil. The most frequently received materials are hydrodesulfurization (HDS) and residual desulfurizing (RDS) catalysts. In the United States, the Federal and state government environmental regulations must be followed in transporting, receiving and in particular, storage of the spent catalyst and other feed materials. A bar code system is applied to assure that the allowed storage time is not exceeded. Representative pre-shipment samples are required for evaluation. The samples are analyzed and evaluated to determine compliance with regulations and compatibility with economical and technical requirements of the process. The customer is then notified as to whether the tested material can be successfully recycled. Upon arrival of the feed shipment, the material, is compared with the pre-shipment sample, before it can be unloaded. The Ni/V Recovery Technology Overview They invariably involve a combination of hydro and pyrometallurgical operations in order to fully recover the various metal values. Typically, all valuable components of the catalysts are converted into four major marketable products: vanadium oxide, molybdenum trioxide, alumina trihydrate and a nickel-cobalt concentrate. Other metals typically found in these wastes may be present in lesser concentrations. The principal products recovered from one of the process developed by CRIMET are a nickel-copper-cobalt concentrate, alumina trihydrate, and chrome oxide. A complete list is shown below: Alumina Trihydrate Calcium Tungstate (Scheelite) 2nd Asian Petroleum Technology Symposium Program

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Management of Spent Catalysts in Petroleum Refineries

Chrome Oxide Molybdenum Trisulfide Molybdic Oxide Nickel-Cobalt Concentrate Vanadium Hydroxide Wet Cake Vanadium Oxide Other flow sheet such as Kingtech’s involves different unit operations and recovers the following salable products: Alumina-silica aggregate Ammonium Molybdate Ammonium Vanadate Ferro Molybdenum Ferro Vanadium Molybdic Oxide Nickel-Cobalt Concentrate Sodium Vanadate Sodium Molybdate Vanadium Pentoxide It is clear that both processes are designed to recover nearly almost all the available metal values from the spent catalyst and they are economically viable and can be regarded as an integral part of the best practices in the management of spent catalysts. Molybdenum and Vanadium Separation, Recovery and Products The high purity sodium molybdate and vanadate solution is acidified with sulfuric acid and treated with hydrogen sulfide and/or sodium sulfide in a vented reactor tank. Molybdenum is selectively precipitated as molybdenum trisulfide: Na2MoO4 + 3 H2S MoS3 + 2 NaOH + 2 H2O The yield of this reaction exceeds 99.9%; typically less than 10 ppm Mo remains in solution. Vanadium is reduced to the tetravalent state and remains in solution as vanadyl sulfate. This is reflected by following equations. 2 NaVO3 + H2S + 3 H2SO4 Na4V4O12 + 2 H2S + 6 H2SO4

2 VO3SO4 + S + Na2SO4 + 4 H2 4 VO3SO4 + 2 S + 2 Na2SO4+ 8 H2O

From the environmental point of view this is the "best available technology", for vanadium separation. The yield of vanadium exceeds 99.9%; only 10-20 ppm V remain in solution. After separation of this element,according to conventional methods, applied by other vanadium producers (SX, ammonium meta vanadate or "vanadium red") substantially higher residual concentrations of over 100 ppm V are observed in tail liquors. The vanadyl hydroxide solids are separated by centrifugation and subsequently dried and partly sintered on a belt furnace operated under oxidizing conditions to obtain a granulated vanadium 2nd Asian Petroleum Technology Symposium Program

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Management of Spent Catalysts in Petroleum Refineries

oxide product. The granulated vanadium oxide product is sold mainly to ferro vanadium producers. Wastewater Treatment Water from the vanadium hydroxide countercurrent thickener washing operation and rain water collected within the plant boundaries are directed to the water treatment plant. Traces of heavy metals, if present, are removed as hydroxides. Sodium hydroxide and flocculants are applied as reagents. The precipitated solids, removed in a clarifying operation followed by filtration, are recycled to the first stage leach. The discharge water consistently meets current EPA BAT "Best Available Technology" effluent standards. The wastewater treatment facility has, in fact, operated now 10 years without a single metals related permit excursion. Spent Caustic Make up caustic must be added to the sodium aluminate loop and spent caustic can in many cases beneficially replace fresh caustic requirements. Some caustic solutions from oil refining facilities contain sulfides and hydrosulfides. This caustic is also recyclable in this process, but is only partially used because sulfides are oxidized to sulfates before being utilized. Modifications to the process are considered to utilize sulfidic values of such materials in the molybdenum precipitation step. Spent Sulfuric Acid Spent sulfuric acid, if of sufficient purity and strength (20%), is acceptable for recycle/reuse and applied for acidification in the precipitation of molybdenum trisulfide and consumed and for neutralization of caustic. Some contaminants in sulfuric acid, such as Hg, cannot be tolerated and as in the case of spent caustic, a thorough analysis and pre-evaluation procedure is required before accepting spent acids at the facility to identify those which can be recycled. Silica Based Catalysts Silica based catalysts cannot be fed to the first leaching step due to mineral formation and consequently caustic and alumina losses. Some silica based catalysts can, however, be coprocessed. Vanadium can be extracted in a separate atmospheric leaching step, using sodium aluminate from the main loop as lixiviant. The leach solution is combined with the first stage leach raffinate and directed to the molybdenum vanadium separation step. The nickel containing leach residue is added to the solids from the second stage of the main loop and sold as nickel-cobalt concentrate. Spent Tungsten Catalyst Tungsten catalyst containing usually about 10% W and/or up to 3% Ni or Co cannot be directly coprocessed with the Ni-Mo or Co-Mo types of spent catalyst due to potential contamination of the molybdenum and vanadium products with tungsten. A 2nd Asian Petroleum Technology Symposium Program

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Management of Spent Catalysts in Petroleum Refineries

complementary, but separate, processing method has also been developed for this type of catalyst. The nickel contained in the atalyst is recovered in the nickel-cobalt concentrate within the present process. Conclusions Present paper describes a new process for the complete resource recovery that is simple, effective, economical and environmentally benign. The metal leaching process for spent catalysts, has a selectivity that is >95% for the metals. The undissolved residue contains mainly Al2O3 and SiO2, can be used in the production of masonry bricks. The pH of the rich liquor is then adjusted by adding dilute sulfuric acid to co-precipitate Mo and V, with a recovery rate of >99%. The lean liquor contains mostly dissolved Ni or Co which can be recovered through precipitation by caustic additions. The final liquor is then sent to an ion exchange step for final recovery and cleanup of residual metals. The final effluent is then discharged after a simple wastewater treatment process, which was found to meet all environmental requirements. The entire process has been found to be simple, costeffective with minimal environmental impacts while achieving the goal of complete resource recovery from spent catalysts commercially. The technologies represent significant advancement in the recovery of metals from spent catalysts and other solid wastes. This is the leading edge of an environmentally sound technology and sets the standards for handling various metal bearing solid wastes in the future. References 1. T. LaRue, et al., "AMAX Port Nickel - A New Dimension in Reclaiming spent Catalysts", Paper presented at the 1988 Spring AICHE National Meeting, March 6-10, 1988. 2.

E. Wiewiorowski, "Selective Extraction of Molybdenum and Vanadium from Spent

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