Treatment of FCC spent catalysts

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Fluidized catalytic cracking (FCC) is an important component of modern petroleum refining processes. In the FCC process, heavy feedstocks are cracked at certain temperatures in the presence of a catalyst, and it is the main method for lightening heavy oils. In Europe and the United States, FCC gasoline accounts for about 36% of the total gasoline production ； About 71% of refineries in Japan are FCC-type refineries, and FCC gasoline accounts for 25% to 30% of the total gasoline production. In recent years, catalytic cracking technology in our country has developed rapidly, and the processing capacity of FCC units continues to increase. Currently, gasoline produced by FCC units accounts for about 70% of the total gasoline production, with the overall processing capacity reaching 180 Mt/a. It is evident that, in the domestic oil refining process, the catalytic cracking process plays a crucial role. On August 1, 2016, the newly issued **List of Hazardous Wastes** classified FCC waste catalysts as hazardous waste of category HW50. The rationale for this classification is that FCC waste catalysts contain nickel, and after calcination at 700°C, this nickel exists in the form of nickel oxide in a concentration of over 0.1%. Since nickel oxide is considered a carcinogen of category 1, FCC waste catalysts are included in the list of hazardous wastes. There are doubts among many experts in the industry regarding whether FCC spent catalysts will produce nickel oxide after calcination at 700°C, with a mass fraction exceeding 0.1%; some research institutions are also conducting laboratory evaluations to verify this. However, until a definite conclusion is reached, the industry is more focused on the reuse or comprehensive utilization of FCC spent catalysts in order to alleviate the cost pressures associated with waste disposal. According to statistics, in 2015, the output of waste refinery catalysts in China exceeded 200 kt, with waste catalysts generated by FCC units accounting for the vast majority. In 2015, Sinopec’s FCC units had a total processing capacity of 72.79 Mt, with actual processing volume of 69.41 Mt; this resulted in the generation of approximately 50 kt of FCC waste catalysts. At a disposal cost of 3,000 yuan per ton of such waste, this would add at least 0.6 yuan per ton to the refining costs. At the same time, if the FCC spent catalysts are not properly disposed of in the future, the harmful components contained in them pose risks of contaminating the environment and harming human health; moreover, some of the precious metal resources present in these spent catalysts will be lost, resulting in a waste of resources. Therefore, the reduction and resource utilization of spent FCC catalysts have attracted increasing attention from researchers. 1. Sources and properties of FCC spent catalysts. FCC spent catalysts have three sources: the main source is that during the operation of FCC units, high temperatures and the toxic effect of heavy metals cause a decrease in the catalyst’s activity, rendering it unable to meet the requirements of catalytic cracking reactions. To maintain a certain level of activity in the catalysts in FCC units, it is necessary to regularly add fresh catalysts and remove some of the catalysts that have become deactivated from the regenerator (balance catalysts) ； Secondly, there are fine particles with a particle size of less than 20 μm resulting from factors such as collisions between the catalyst in the device and high-speed fluid streams (main air and feed oil), container walls, as well as catalyst particles, or thermal decomposition; these particles cannot be captured by the primary and secondary cyclone separators, but can be collected by the subsequent tertiary cyclone separator. These waste catalysts are thus the fine powder produced by the tertiary cyclone ； The third source is the ultrafine catalyst powder collected from the desulfurization and denitrification sludge of flue gas, which results from the application of flue gas purification technology in catalytic cracking. The typical properties of spent catalysts are shown in Table 1. During the FCC reaction process, toxic and harmful substances in the feed oil (such as Ni, V, etc.) enter the catalyst as the reaction progresses and accumulate there, resulting in a high concentration of heavy metals like Ni and V in the FCC spent catalyst. In addition, to reduce the toxic effect of harmful heavy metals on catalysts, some refineries add a certain amount of vanadium passivator to the equipment; such vanadium passivators typically contain metals such as Sb and Bi, which can also end up in the catalysts. The large quantity and variety of heavy metals mentioned above also require proper disposal in order to reduce potential harm to water bodies, soil, animals, plants, and human health. 2. Treatment methods for FCC spent catalysts 2.1 Reuse of balance agents Many studies have been conducted, both domestically and internationally, on the utilization of FCC spent catalysts. RFCC units for processing residue oil (with high metal content) abroad account for a small proportion of the total FCC units; usually, the waste catalysts discharged from FCC units can be used as balance catalysts and supplied to RFCC units for reuse. In most FCC units in our country, they are RFCC units; the balance metal discharged from these units has a high content and low activity, and only a small portion of it can be used as a catalyst for starting up operations. Another scenario is when the catalyst in the unit is severely poisoned and contains high levels of heavy metals; refineries typically add balancing agents to \"replace\" the severely poisoned catalyst, followed by the addition of new catalysts. Typically, the balanced doses of these recycled materials are very small, and a large amount of FCC spent catalyst still needs to be dealt with; therefore, other disposal methods must be found. 2.2 Landfilling Landfilling FCC spent catalysts is a relatively simple method that is still in use to this day. Before being landfilled, foreign waste catalysts must have their toxic and harmful substances converted into non-toxic and harmless ones before they can be disposed of in this way. In the United States, even if landfilling is permitted, high fees must be paid, and legal responsibilities are imposed on both those who approve the landfilling and its owners. In China, approval from the relevant authorities is required; it can only be carried out after it is confirmed that dumping waste catalysts does not cause environmental pollution, and the costs involved are very high. Although the landfilling method is simple, it merely transfers the waste catalysts to another location; it provides only a temporary solution rather than a permanent one. As land resources become increasingly scarce, this single method of disposal faces more and more restrictions. 2.3 Magnetic separation: Waste FCC catalysts contain relatively high amounts of metals (Ni, V, and Fe). The magnetic separation technique utilizes the magnetic properties of these metals to separate the waste catalysts with a high metal content. The separated catalysts, which retain relatively good performance, can then be reused in FCC units. As early as the 1970s, magnetic separation technology was put into use abroad. With continuous improvements to the equipment, it evolved through high-gradient magnetic separators, the replacement of electromagnets with rare earth permanent magnets, and later, the successful development of permanent magnet technology, which led to significant advancements in this field. Typically, the magnetic separation unit of this technology is combined with the catalytic cracking unit. Patent US7431826B2 proposes a magnetic separation method that separates the magnetic separation unit from the catalytic cracking unit; this invention allows for cost reduction and enables the reuse of more balancing agents. It is estimated that this method can result in a 20% to 40% reuse rate of balancing agents. Magnetic separation technology was introduced in our country in 1998 and put into industrial use at the Luoyang Refinery in the same year, but it falls under the category of electromagnetic technology. Permanent magnet technology was put into industrial use at the North China Petroleum Branch in 2002. Subsequent to further development, Sinopec Engineering Construction Co., Ltd. collaborated with other companies to develop the NBMS magnetic separation process. This process makes use of permanent magnet technology to create rollers with high magnetic induction strength, enabling two-stage or multi-stage separation. It does not require the use of chemicals and generates no wastewater, making it a clean processing method. At present, this process is being used industrially in the heavy oil catalytic cracking unit of the North China Petrochemical Branch. The trial results are shown in Table 2: after replacing part of the fresh catalyst with the recoverer in proportion, various parameters of the unit remained basically stable. The yield of light products was roughly the same as before the addition of the recoverer, and the product distribution stayed essentially unchanged. Magnetic separation methods can extend the service life of catalysts, but their purpose is essentially to reduce the amount of waste catalysts emitted, without carrying out harmless treatment of such waste catalysts. 2.4 Synthesis of molecular sieves from FCC spent catalysts: Although some of the primary structural elements in FCC spent catalysts have been damaged, they still contain a small amount of molecular sieve structure. If this structure can be utilized to process the spent catalysts into high-value products, it will not only generate economic benefits but also help protect the environment. In this regard, many scholars abroad have conducted exploratory research; Basaldella et al. synthesized octahedral zeolite molecular sieves and Type A molecular sieves by hydrothermal and alkali fusion activation of waste catalysts, and the catalysts prepared through their modification exhibited excellent catalytic performance. Al-Sheeha et al. recovered alumina from spent catalysts and converted it into pseudoboehmite; the resulting product had a specific surface area of over 200 m2/g and a pore volume of more than 0.3 mL/g, making it suitable for reuse as an aluminum source in the synthesis of molecular sieves. Escardina et al. hydrothermally synthesized 4A-type zeolite molecular sieves using spent catalysts as raw materials; the products obtained had a high degree of crystallinity and good quality. Many researchers in China have also studied the synthesis of molecular sieves from FCC spent catalysts. Yang Haining and others used catalytic cracking spent catalysts as raw materials; they first activated these catalysts and then, in the presence of an external silicon source, synthesized Y-type molecular sieves using hydrothermal in-situ crystallization techniques. The results showed that it is possible to produce Y-type molecular sieves with high crystallinity through the in-situ synthesis using spent catalysts. Li Liang also used FCC spent catalyst fine powder as a raw material; under the presence of an external silicon source, he controlled the synthesis of Y-type molecular sieves by adding inorganic salts and adjusting factors such as the silicon-to-aluminum ratio, alkalinity, crystallization temperature, and crystallization time in the synthesis system. The results show that under suitable synthesis conditions, fine FCC spent catalyst powder can be used to synthesize ultra-fine Y-type molecular sieves with a large specific surface area and high crystallinity, and the resulting products exhibit good hydrothermal stability. Synthesizing molecular sieves from spent catalysts provides a new approach for their reuse, offering good prospects for application. 2.5 FCC Spent Catalyst Refined Products: Petroleum refining products such as lubricants and paraffins contain unstable components that can reduce the stability of these products and cause their color to darken over time; therefore, it is necessary to refine and purify these oils. Generally, those unstable components are mostly highly polar substances. The pore structure and specific surface area of FCC spent catalysts are comparable to those of ordinary clay, and their performance even exceeds that of clay; therefore, their structural advantages can be fully utilized to replace clay in the purification of oils. Several domestic researchers have used FCC spent catalysts as adsorption purifiers for oils such as lubricants and diesel. Zhu Jun and others used three types of spent catalysts to refine lubricants, and the results showed that all three catalysts had a certain degree of refining effect. Furthermore, they refined lubricating oil by mixing waste catalysts with clay in a certain ratio. The results showed that the pore structure of the waste catalysts facilitates adsorption, and their spherical particle shape reduces the frictional resistance between the oil and the particles, resulting in an effective removal of impurities; thus, it can replace part of the clay in the process of refining lubricating oil. Yang Shicheng and others used waste catalysts to replace part of the clay in the purification of paraffin. Their research found that this method is easy to operate, feasible from a technical standpoint, and that the heavy metals present in the waste catalysts do not contaminate the paraffin product. The industrial application of FCC spent catalysts to replace part of the clay-processed oil products essentially only reduces the amount of clay used and its purchase cost, without fundamentally addressing the issues of reducing the volume of FCC spent catalysts and reusing them as resources. 2.6 Revitalization of spent FCC catalysts The revitalization of spent FCC catalysts is generally achieved through chemical treatment: the spent catalyst is mixed with specific chemical reagents and soaked to remove heavy metals such as Ni and V from it, thereby restoring part of the catalyst’s specific surface area and pore volume, allowing it to be reused in catalytic cracking units. As early as the 1960s, ARCO Oil Company developed chemical demetallization technology, which uses hydrogen sulfide and chlorine to treat FCC spent catalysts. After washing the samples, metal sulfides and chlorides are separated, thereby restoring the pore structure and catalytic activity. At the end of the 20th century, Cosatal Company improved the demetallization process, resulting in high yields of regenerated catalyst products; however, the improved demetallization process still caused severe equipment corrosion, which limited its industrial application. Domestically, scholars such as Wu Yu have developed a new organic coordination method for reviving FCC spent catalysts. Through experimental design and analysis, they identified the key factors and their order of influence on the revival effect, and by scaling up the experiments, they obtained revived FCC spent catalyst samples with good catalytic activity. A company in Qingdao has used an inorganic-organic coupling method to reactivate FCC waste catalysts, and industrial production has been established. The regenerated catalysts have been put into industrial use in more than 10 FCC units at various refineries such as Qilu, Yanshan, Qingdao Petrochemical, and Shenchи Chemical; the results show that these regenerated catalysts can replace some new catalysts in catalytic cracking units. In addition, to address issues such as the high pollution level and fine particle size of the three-spin fine powder, which did not meet the requirements for use in catalytic cracking units, the company, after thoroughly examining the physical properties of this fine powder, pre-treated it and used it as a carrier to develop the UPC-type catalyst as a substitute for fresh catalysts. The UPC-type regenerated catalyst has been industrially applied in the catalytic cracking units of several refineries in the North Sea, Zhongyuan, and Changling areas. The operation data of its catalytic cracking unit at the Zhongyuan Petrochemical Plant are shown in Table 3. The industrial application of this FCC spent catalyst revival technology fundamentally solves the problem of pollution caused by spent catalysts, achieving zero emissions of solid waste from catalytic cracking units. This not only promotes sustainable environmental development but also reduces the need to extract mineral resources. 2.7 Recovery of valuable metals from FCC spent catalysts There are two types of metals present in FCC spent catalysts: one is rare earth elements, which exist in the form of trivalent ions or oxides within these catalysts. This form allows them to react easily with related compounds, enabling the recovery of the rare earth elements. Another type is heavy metals such as Ni and V. During catalytic cracking, Ni, V, and other such elements in crude oil accumulate on the catalyst, thereby turning the catalyst into a carrier for various heavy metals. Therefore, the recovery of metals (rare earths, Ni, V, etc.) from FCC spent catalysts is not only economically beneficial but also one of the methods for treating such catalysts. Regarding the rare earth elements on FCC spent catalysts, scholars have conducted the following research. Studies by He Dafan and others have shown that an extraction system composed of P507(HEH/HHP) and kerosene can effectively extract rare earth elements from the leachate of FCC spent catalysts; subsequent back-extraction removes non-rare earth impurities (such as Al, Fe, etc.), enabling the recovery of La and Ce. Yuan Zhiwei and others chose P507 as the extractant for recovering rare earths; under these process conditions, the recovery rate of rare earths was not less than 87%. The study indicated that an increase in temperature enhances the kinetic energy of the leaching agent, improving its mobility and thus accelerating the entire extraction reaction and increasing the leaching efficiency. While studying the recovery of rare earth elements from FCC spent catalysts, Wu Shufeng and his colleagues also recovered aluminum elements. Hydrochloric acid was used to leach the elements to be recovered, and oxalic acid was then employed to precipitate the rare earth elements. The remaining precipitate, along with wastewater, could be reacted with calcium aluminate to produce the flocculant polyaluminum chloride. For Ni and V, the most commonly used extraction and separation method is acid treatment of the waste to convert Ni and V into ionic forms in solution, followed by separation based on their different precipitation pH values. Lai investigated the extraction process of Ni and V from spent catalysts, and the results showed that when the pH value was greater than 6.0, Ni could hardly be leached; whereas when the pH value was between 4.0 and 8.0, V was difficult to leach. Chao Yang and others, in collaboration with other researchers, conducted experiments on the leaching and recovery of Ni, V, and Mo from spent catalysts, developing new techniques for the recovery of these valuable metals. The recovery rates for V, Mo, and Ni were 90%～95%, 80%～85%, and 95%～98%, respectively. Some researchers first calcine the spent catalyst to separate metal Ni from other substances, converting the non-metallic phases in the catalyst into substances insoluble in acid, and then proceed with the separation of nickel. Since nickel-containing FCC spent catalysts are basically not composed of a single metal, different separation methods need to be tried based on the characteristics of the specific system in order to extract and separate nickel and achieve its recovery. 3. Conclusion Based on the composition and performance characteristics of FCC spent catalysts, scholars at home and abroad have proposed various treatment methods. In the long run, landfilling will surely be banned. The use of balance agents can only enable the reuse of some waste catalysts, while magnetic separation methods merely reduce the amount of FCC waste catalysts produced without enabling the recycling of such catalysts. Using FCC spent catalysts as raw materials to synthesize high-value-added products, reviving such catalysts, or recovering valuable metals from them will be future development trends. This approach not only improves the utilization rate of resources but also helps protect the environment, while yielding economic benefits; it thus holds great research value and broad prospects for development