“Daily Share 4.29” – Methods for Handling Spent Catalysts

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Petrochemical waste catalysts often contain various toxic components, mainly heavy metals and volatile organic compounds, posing significant environmental risks. Therefore, their harmless treatment and disposal are of great importance. Furthermore, petrochemical waste catalysts contain high levels of precious metals or other valuable metals; in some cases, these levels are even much higher than those in certain low-grade ores. Due to their high metal content, they can be recycled as secondary resources. The comprehensive utilization of waste catalysts from the petrochemical industry not only improves resource utilization but also helps to avoid the environmental problems caused by these waste catalysts, thereby enabling sustainable development. 1. How much waste catalyst is there? It is reported that 500,000 to 700,000 tons of waste catalysts are generated globally each year, of which waste refining catalysts account for a large proportion. As the sales volume of refining catalysts in our country increases year by year, the amount of waste refining catalysts generated also increases accordingly. If waste refining catalysts are not managed scientifically, their toxic and harmful components will pollute the environment and harm human health, and some of the precious metal resources contained within them will also be lost. Therefore, the effective treatment and utilization of spent refinery catalysts has become a very important issue. Currently, FCC catalysts account for a large share of the market, comprising approximately 68.9% of the total amount of catalysts used in oil refining ； The proportions of catalysts used for hydrorefining, hydrocracking, and catalytic reforming are 9.4%, 6.2%, and 3.3% respectively ； Other types of refining catalysts account for about 12.2%. In 2015, China’s oil consumption reached 585 million tons (estimated value), and the amount of waste refinery catalysts generated amounted to 207,000 tons (estimated value). 2. Major Components and Their Contents The major components and their contents of several fresh catalysts used for catalytic cracking, hydrorefining, hydrocracking, and catalytic reforming are shown in Table 2. Due to the requirements for catalytic reactivity, some fresh catalysts inherently contain toxic and harmful components. If NiO is present in hydrorefining and hydrocracking catalysts, it is a carcinogenic substance. During the refining process, some toxic and harmful components in crude oil end up in the catalysts. The main components and their contents in used refining catalysts are shown in Tables 3–4. As can be seen from Table 3, heavy metals such as Ni, V, and Fe may be deposited on the surface of the spent FCC catalyst, and small amounts of elements such as Na, Mg, P, Ca, As, and Cu can also deposit on the spent catalyst. Furthermore, in order to suppress the activity of heavy metals deposited on the catalyst, a certain amount of passivator is usually added to the system, and passivators containing Sb are also toxic substances. Metals such as Ni and V deposit on the spent hydrorefining catalysts, and depending on the feedstock, impurities such as As, Fe, Ca, Na, and clay can also deposit on the catalyst, reducing its activity or even rendering it inactive. Due to the strict requirements of the catalytic reforming process regarding feedstock, there are very few toxic and harmful components in its spent catalyst. The surface of the spent catalyst is mostly covered with carbon deposits. Given the long operating time of the unit, sulfur, nitrogen, metals, and other elements from crude oil also accumulate on the catalyst surface. 2. Hazards of used refining catalysts: Used refining catalysts may contain many toxic and harmful components, such as NiO; when its mass fraction exceeds 0.1%, such waste catalysts are considered hazardous solid waste ； Similarly, for elements such as V, Sb, and Ti, when their mass fraction exceeds 3%, the waste catalyst also falls under the category of hazardous solid waste. If used waste refining catalysts are left piled up outdoors for an extended period, they not only occupy large amounts of land but also their toxic and harmful components can enter water bodies and soil as a result of rainwater runoff, causing damage to water bodies, soil, vegetation, and living organisms. These substances can further threaten human health through the food chain. Furthermore, the particle size of spent FCC catalysts is very small, making them easily inhaled and thus posing a threat to human health. 3. Treatment of spent catalysts: Generally, various methods are employed to regenerate spent petroleum refining catalysts. If the regenerated catalysts fail to achieve the required reactivity for reactions, different treatment and utilization methods are applied depending on their composition. 3.1 Recycling: Catalyst regeneration is a major method for this purpose; it enables the reuse of hydrogenation catalysts. Moreover, the cost of catalyst regeneration is low, making it an economical approach for dealing with waste catalysts. Catalyst regeneration is divided into two methods: in-reactor regeneration and out-of-reactor regeneration. In-vessel regeneration mainly involves coking regeneration using water vapor and air, or nitrogen and air, within the reaction apparatus. This can prevent damage to humans caused by some toxic substances generated during catalyst removal. However, in-situ regeneration presents problems such as device corrosion, localized overheating, long burning time, and low removal efficiency. Ex-situ regeneration involves removing the deactivated catalyst from the reactor and sending it to a regeneration facility for restoration. Off-line regeneration can meet requirements such as precise control of regeneration conditions, short downtime of the unit, and a high decoking rate. Today, 90%–95% of regenerated catalysts in the United States and Europe are processed using off-site regeneration methods. About 90% of the regenerated catalysts in the world are hydrogenation catalysts. Most hydroprocessing units built abroad after 1996 no longer include facilities for regeneration. CRI, Eurecat, and Tticat are the major overseas companies for catalyst ex-situ regeneration. In China, extracorporeal regeneration is mainly carried out by Zibo Hengji Chemical Co., Ltd. and Sinopec Hunan Changwang Chemical Co., Ltd. The methods for catalyst regeneration cannot restore the catalyst to its technical specifications as those of a fresh catalyst, and whether it is worth regenerating depends on the type of impurities deposited on the catalyst. 3.2 Landfilling Landfilling waste catalysts is a relatively simple method. Landfilling deactivated catalysts at designated waste sites has also been a traditional treatment method throughout history. Inactive spent hydrogenation catalysts can also be landfilled after meeting harmless standards. Abroad, spent catalysts must meet certain standards before they can be landfilled. In the United States, environmental laws impose strict restrictions on the landfilling of waste catalysts. Toxic substances must be converted into non-toxic ones before the spent catalyst is landfilled. Therefore, in the United States, waste catalysts cannot be discarded arbitrarily without permission from the relevant authorities; even when such catalysts are landfilled, a considerable amount of taxes and fees must be paid. It also stipulates that owners of approved waste dumps are held legally accountable, as are the owners of the waste being landfilled. There must be lifelong responsibility for the environment of the waste site, and continuous monitoring of the surrounding environment and groundwater is required. Therefore, the cost of landfilling waste catalysts is becoming increasingly high. Even in some cases, waste catalysts with **pollutant levels exceeding a certain threshold are prohibited from being landfilled. Issues related to the domestic environment are also attracting increasing attention. Landfilling can only proceed after obtaining approval from the land resources and environmental management authorities. An assessment by authorized agencies is required to confirm that landfilling of waste catalysts will not cause harm to the land or the environment, prior to approval being given for such landfilling to take place in a proper manner. However, with the development of society, it will become increasingly difficult and more costly to dispose of waste catalysts through landfilling. 3.3 Cement Raw Materials Cement is primarily composed of sand and minerals, mainly containing silicon, aluminum, and calcium oxides. The main components of the cracking catalyst are SiO2 and Al2O3, which are also present in relatively high amounts; the proportions of other components are low. Moreover, after treatment, it is essentially non-toxic and does not cause environmental pollution. In particular, after being calcined at high temperatures, it forms a multi-component inorganic composite that is difficult to decompose and does not release any toxic substances. Therefore, it can be used as a partial substitute for cement, with no safety or environmental concerns. In the United States, cement kilns process approximately 60,000 tons of waste catalysts per year. The cement plant affiliated with Maoming Refining and Chemical Co., Ltd. in China has also tried out this technology. Cement plants are very large and require a large amount of raw materials. There is a certain amount of spent cracking catalyst available. However, the demand from cement plants remains low, and it is not possible to meet the continuous supply of raw materials. Moreover, cement plants operate on a continuous basis and cannot frequently change their formulations, which also limits the use of spent catalysts in such plants. 4. Catalyst Recovery 4.1 Dry Method: Generally, the waste catalyst is heated together with a reducing agent and a flux in a high-temperature furnace to melt it; the active metal components in the waste catalyst are then reduced and melted into metal or alloy form for recovery, to be used as raw materials for alloys or alloy steels. The carrier in the spent catalyst forms slag with the flux and is disposed of accordingly. Dry methods generally include: oxidation roasting, sublimation, and chlorination volatilization. Since this method does not use water, it is generally referred to as the dry method. Catalysts such as CoO-MoO3/Al2O3, NiOMoO3/Al2O3, and W-Ni can all be recovered using this method. 4.2 Wet method: Strong acids or strong bases can be used, or other solvents can be employed to dissolve the main metal components of the waste catalyst ； The solution containing the main metal is filtered to separate the liquid from the solid. Upon separation, water-insoluble sulfide salts or metal hydroxides can be obtained ； After drying, it is further processed as needed to become the final desired product. In the wet method for treating waste catalysts, the carrier exists essentially in the form of insoluble residues. Without proper treatment methods, these large amounts of solid insoluble residues can cause secondary pollution to the environment. If the solid insoluble residue still contains active metal components from the spent catalyst, it can also be reduced using a dry method. Hydrogenation waste catalysts can generally be recovered using wet methods. First, extraction or dry distillation is carried out to remove oils from the deactivated hydrogenation waste catalysts; thereafter, the active metal components in these catalysts are dissolved. The various active metal components present in the solution are then separated and extracted through methods such as extraction and back-extraction, or by using anion and cation exchange resins. 4.3 Combined dry and wet method: When a hydrogenation waste catalyst contains two or more active metal components, the combined dry and wet method is usually employed to achieve the desired outcome. Using dry or wet methods alone for recovery makes it difficult to achieve the desired treatment results, and it also generates large amounts of residue or waste liquid. The wet-dry combination method is widely used in the refining process for the recovery and treatment of hydrogenation waste catalysts. For example, in the recovery of molybdenum from hydrorefining catalysts, most methods involve calcination followed by liquid impregnation to dissolve the molybdenum in a solution, after which separation is carried out. 4.4 Non-separation method: This method does not involve separating the active metal components of the spent catalyst from the carrier, nor does it require separating two or more active metal components from each other. It is a method for the direct utilization and recycling of waste catalysts. Since this method does not separate the active metal components from the carrier, it requires less energy during the recovery of waste catalysts, results in lower recovery costs, generates fewer waste emissions, and helps to minimize the risk of secondary pollution. This method is a commonly used approach in the field of waste catalyst recycling. For example, when recovering Fe-Cr medium-temperature shift catalysts, the iron and chromium components in the leachate are often not separated from each other. It can be directly used to recycle and recreate new catalysts. 4.5 Ion exchange method In hydrogenation catalysts, active metals such as molybdenum, nickel, and cobalt are primarily used in Co-Mo/Al2O3-based hydrogenation desulfurization catalysts and Co-Mo/Al2O3-based hydrogenation denitration catalysts for petroleum refining. The Miyazaki plant of Inu Chemical Co., Ltd. in Japan uses a combination of ion exchange and solvent extraction to separate Al2O3 from waste catalysts, and then recovers cobalt and molybdenum in the form of molybdenum oxide and cobalt chloride. The process of this method is relatively complex, but the purity of the recovered product is high, making it suitable as a raw material for chemical reagents. Conclusion