Authored by Cui Changhao, Liu Meijia, Meng Bangbang, Li Lili, and Yan Dahai
(1. Institute of Solid Waste Pollution Control Technology, Chinese Academy of Environmental Science2. Key Laboratory of Hazardous Waste Identification and Risk Control, Chinese Academy of Environmental Science)
Summary:With the rapid development of the coatings industry, the types of hazardous waste involved are constantly increasing, and the management of hazardous waste has become a key factor affecting the sustainable development of the coatings industry. By analyzing the coating industry's process flow and typical hazardous waste generation points from the three stages of pigment production, coating manufacturing, and coating application, this document outlines the typical types of hazardous waste and their disposal methods, providing a reference for the regulation of hazardous waste in the Chinese coatings industry. Furthermore, it proposes environmental management recommendations, including simplifying the identification procedures for solid waste properties of water-based coatings, conducting research on the management of hazardous waste in powder coatings, and strengthening environmental supervision of downstream coating users.
Keywords:- Pigment production; Paint manufacturing; Paint application; Hazardous waste; Recommendations for handling
Typical hazardous waste generation nodes and types in the coating industry, and corresponding environmental management countermeasures.
CUI Changhao, LIU Meijia, MENG Bangbang, LI Li, YAN Dahai
(1. Research Institute of Solid Waste Management, Chinese Research Academy of Environmental Sciences; 2. State Environmental Protection Key Laboratory of Hazardous Waste Identification and Risk Control, Chinese Research Academy of Environmental Sciences)
Abstract: The rapid development of the coating industry results in a significant increase in the types of hazardous waste involved. The management of hazardous waste has become a key factor affecting the sustainable development of the coating industry. This paper describes typical hazardous waste generation nodes, types of typical hazardous wastes, and treatment and disposal methods for pigment production, paint manufacturing, and coating application, in order to provide reference for the supervision of hazardous wastes in China's coating industry. Additionally, this paper proposes environmental management countermeasures for the coating industry from three aspects: simplifying the identification procedures for the attributes of water-based paint solid waste, conducting research on the environmental management of powder coatings, and strengthening the supervision of hazardous waste during the coating process.
Key words: pigment production; coating manufacturing; paint; hazardous waste; recommendations
0 Introduction
Coatings are a general term for liquid or solid materials that, after being applied to a material surface, form a solid film with protective, decorative, or special properties. They consist of four components: the film-forming material, solvent, pigments, and additives.[1]。The film-forming materials are typically oils or polymers, used to bind all components together. The solvents primarily consist of organic solvents and water, in which the film-forming materials are dispersed to form a viscous system. The pigments are divided into organic pigments and inorganic pigments, which enhance the durability of the coating while imparting different colors. Additives are auxiliary materials used in the formulation of coatings, which improve the properties of the coating and promote film formation.[2].
According to data from the National Bureau of Statistics, China's paint production reached 24.39 million tons in 2019.[3], accounting for approximately 31% of the global paint production.[4]。Specifically, the production of architectural coatings reached approximately 6.94 million tons, accounting for 28% of China's total coating production; and the production of industrial coatings reached approximately 10.95 million tons, accounting for 45% of China's total coating production.[5-6]"With the rapid development of coating technology, the types and quantities of hazardous waste involved in the industry are constantly increasing, and the management of coating hazardous waste has become a key factor affecting the sustainable development of the entire industry chain. The author will analyze the generation nodes and types of typical hazardous waste in the three stages of coating raw material production, coating manufacturing, and coating application, and propose corresponding environmental management suggestions, in order to provide a reference for the regulation of hazardous waste in the coating industry."
1. Typical nodes for the generation of hazardous waste in the coatings industry
1.1 Coating Raw Materials
In GB/T 4754-2017, paints and coatings are classified under "264 Manufacture of Paints, Inks, Pigments and Related Products," while film-forming materials, solvents, and additives are classified under "266 Manufacture of Special Chemical Products." The hazardous waste types associated with film-forming materials, solvents, and additives primarily consist of organic resins, waste acids, waste alkalis, and other organic compounds, which do not represent the typical hazardous waste characteristics of the paints and coatings industry. Therefore, the term "paints and coatings raw materials" in the text specifically refers to pigments.
Pigments are divided into two categories: organic pigments and inorganic pigments. Organic pigments are pigments synthesized using organic compounds. They can be further divided into categories such as azo pigments, phthalocyanine pigments, and quinone pigments. Inorganic pigments are colored materials consisting of fine particles of inorganic compounds, and are mainly divided into titanium-based pigments, lead-chrome pigments, and iron-based pigments according to the type of coordination metal.[7]The typical hazardous waste types and generation points in the production of organic and inorganic pigments are shown in Table 1.
1.2 Paint Manufacturing
based on the different solvents used in coatings, they can be categorized into solvent-based coatings, water-based coatings, and powder coatings.[1]. Solvents in solvent-based paints typically consist of organic solvents such as aliphatic hydrocarbons, aromatic hydrocarbons, alcohols, and ketones. Water-based paints only use water as a solvent. The manufacturing of powder coatings involves melting and cooling solid materials, typically without the use of solvents. The production processes for solvent-based, water-based, and powder coatings are illustrated in Figures 1, 2, and 3. The manufacturing process for paints is a physical dispersion and mixing process, and does not involve any chemical reactions.
The typical hazardous waste types and generation points involved in the production of solvent-based, water-based, and powder coatings are shown in Table 2. Hazardous waste primarily generated from solvent-based coatings includes sludge and waste organic solvents from wastewater generated during production and cleaning, as well as water-based coatings generate only sludge from wastewater treatment due to the absence of organic solvents. Typical hazardous waste for powder coatings is activated carbon. Taking a specific example of a company in Shandong, Zhejiang, and Hunan with an annual production capacity of 72,000 tons of solvent-based coatings, 3,000 tons of water-based coatings, and 1,000 tons of powder coatings, the total amount of sludge generated from wastewater treatment is approximately 140 tons per year, with a generation coefficient of 0.002 tons/ton; the amount of waste organic solvents generated is approximately 300 tons per year, with a generation coefficient of 0.004 tons/ton; and the total amount of waste activated carbon generated is approximately 0.9 tons per year, with a generation coefficient of 0.0009 tons/ton.
1.3 Coating Application
Typical hazardous waste generated during coating application mainly includes waste coatings and paint residue. According to the 2021 edition of the "National List of Hazardous Wastes," these are classified as 900-299-12 and 900-252-12 respectively. Waste coatings refer to coatings that have become unusable, deteriorated, substandard, or obsolete. Paint residue primarily forms through the settling of overspray and mist.[8]。Manual spraying results in at least 40%~60% of overspray, while electrostatic spraying also results in over 10% of overspray.[9]. Taking the automotive coating industry as an example, according to data from the China Automotive Industry Association, in 2019, China's automobile production reached 25.721 million vehicles. If we assume that each vehicle uses 35 kg of coating, the total amount of automotive coating required is 90,000 tons. Assuming a utilization rate of 60%, at least 540,000 tons of coating waste will be generated.[10].
2. Typical types of hazardous waste in the coatings industry
2.1 Pigment Production
The types and sources of hazardous waste involved in the pigment production process are shown in Table 3. The wastewater sludge from the treatment of organic pigments mainly contains pollutants such as aniline, quinoline, ethers, phenols, and heavy metals. Aniline compounds are one of the raw materials for the diazotization reaction, used to provide a primary amine for subsequent reactions; quinoline, ether, and phenol compounds are one of the raw materials for the condensation reaction, which combine with the primary amine to produce color; heavy metals are mainly used as complexing metals in azo and phthalocyanine pigments, but elements such as manganese and zinc can also be used as catalysts. The toxic substances mainly contained in the wastewater sludge from the treatment of inorganic pigments are heavy metal raw materials such as chromium, molybdenum, lead, and iron cyanide. In addition, impurities in the raw materials may also be transferred to the sludge during the synthesis of inorganic pigments.[11].
2.2 Paint Manufacturing
Solvent-based paints are widely used in industrial applications due to their excellent leveling, corrosion resistance, and coloring properties, particularly as wood coatings and marine coatings. In 2014, the proportion of solvent-based paints used in China's wood coating industry reached approximately 89%.[12]. The types and sources of hazardous waste generated during the production of solvent-based coatings are shown in Table 4. The main film-forming components of solvent-based coatings are organic resins, with polyurethane resins and nitrocellulose resins being the most commonly used. Yang Yuan De et al.[1]In 2009, a survey of wood coating products in China revealed that 75% to 80% of wood coatings used polyurethane as the film-forming material, while 15% to 20% used nitrocellulose. Furthermore, incomplete use of pigments, organic solvents, and additives containing heavy metals, anilines, and ether-based components may also be present in the sludge.[13].
Water-based paints are primarily used in the construction industry. Estimates suggest that the proportion of water-based paints used in the Chinese construction industry is approximately 90%.[6]。Taking water-based architectural coatings as an example, typical hazardous waste components that may be present in the wastewater sludge generated from their treatment process are shown in Table 5. Tang Hongxia[14]Regarding the hazardous property analysis of wastewater sludge from 8 water-based architectural coating factories in Shanghai, the results indicate that the sludge does not exhibit flammability, corrosivity, reactivity, or toxicity; Huang Jun[15]A hazardous property analysis was conducted on the sludge from the wastewater treatment of four water-based paint production plants under the China Paint Co., Ltd., using the sludge as a sample. The results indicated that the waste does not exhibit flammability, corrosivity, reactivity, or toxicity. However, the leaching toxicity of copper, barium, toluene, and ethylbenzene was found to be high. based on the source of pollutants, it is inferred that copper and barium are introduced from pigments, while toluene and ethylbenzene are introduced from solvents.
Table 5: Potentially hazardous waste components that may be present in wastewater sludge from the treatment of water-based paints.
The production process for powder coatings involves the melting and cooling of solid powders, and the reaction does not use liquids as a dispersing medium. Waste activated carbon may contain heavy metals and aniline-based pollutants from useful film-forming materials, pigments, and additives. Hao Yaqiong et al.[16]During the identification of imported powder coating materials, it was found that the main components were polyester, epoxy resin, and TiO.2、BaSO4and CaCO3. Polyester and epoxy resins are organic resin-based film-forming materials, and TiO2 is the main component of titanium dioxide. BaSO4As a relatively inexpensive white inorganic pigment, it can be used as a substitute for titanium white powder at a concentration of 10%~25% to reduce costs due to its wide particle size distribution and lower purity.3It is a type of filler, commonly used to enhance the depositability and permeability of the coating resin.
2.3 Coating Application
The overspray generated during painting is absorbed and collected by water. Under the action of suction, it is transferred to the paint room water tank and, due to the action of a coagulant, it precipitates and forms paint sludge. The types and sources of hazardous waste contained in the paint sludge are shown in Table 6. As shown in Table 6, the composition of the sludge is essentially the same as that of the paint, but due to the addition of a coagulant, it may also contain inorganic metal salts or organic polymer substances.
3. Methods for disposing of coating hazardous waste
The composition of sludge and wastewater generated during paint and coating production is complex and contains large amounts of heavy metals and organic pollutants. Common disposal methods for paint waste and wastewater sludge include landfilling and incineration, but recent studies have explored the reuse of paint waste. Yilmaz et al.[17-18]The solid-state coating waste can be used as a filler and reinforcing agent for low-density polyethylene and copolymer methanol. The results show that adding 10%, 20%, and 30% of solid-state coating waste can improve the polymer strength of low-density polyethylene and copolymer methanol.
The conventional disposal methods for paint flakes, such as those used for handling waste and wastewater sludge, typically involve landfilling or incineration. However, in recent years, there has been increasing research into the recovery and reuse of paint flakes. – Zhang Yufei[19]Starting from the stage of paint flake formation, by selecting appropriate binders, paint flakes can be regenerated into part of the raw materials for solvent-based coatings, achieving the performance standards for low-grade coatings used in agricultural vehicles. Gerace et al.[20]Utilizing calcium oxide to chemically dehydrate spray paint flakes followed by vacuum dehydration, the resulting powder has a solid content exceeding 98% and can be used as a sealant for automotive applications. (Source: Dalmazzo et al.)[21-22]"Alternatively, the paint flakes can be added to asphalt or cement to create lightweight building materials. Furthermore, research into the hydrothermal and Fenton process treatment of paint flakes is also gradually being developed."[23-24]Utilizing hydrothermal technology to convert paint sludge into bio-oil and gaseous fuel, or oxidizing paint sludge with Fenton's reagent to enhance biodegradability.
4. Typical Hazardous Waste Environmental Management Recommendations in the Coatings Industry
The types of hazardous waste involved in the coatings industry are numerous, and their management is an ongoing process. Accurate identification of the types and characteristics of hazardous waste generated in the coatings industry is crucial for establishing comprehensive hazardous waste management methods and risk control measures for the Chinese coatings industry [25]. based on the characteristics of waste generated in the Chinese coatings industry and the current management practices, the following 3 recommendations are proposed.
(1) Simplify the procedure for identifying the properties of solid waste from water-based paints. Solid waste generated during the production and use of water-based paints must be identified according to the methods specified in GB5085 "Standards for Identification of Hazardous Wastes." After confirming that the waste does not exhibit hazardous characteristics such as corrosivity, toxicity (including leaching toxicity and content of toxic substances), flammability, and reactivity, it can be managed as general industrial solid waste. The approach of China regarding the management of solid waste from water-based paints is correct, but the widespread identification of hazardous waste as time-consuming and costly is a problem. Some small and micro-enterprises are reluctant to spend time and cost on identification, which undoubtedly increases the difficulty of environmental management departments in supervising paint waste. As an environmentally friendly paint, the development of water-based paints will undoubtedly be a key focus of the paint industry. It is recommended to set content control values for characteristic pollutants in paint waste, such as chromium, zinc, ethylbenzene, and dimethylbenzene [26], to simplify the identification of solid waste from water-based paints.
(2) Conduct research on the management of hazardous waste from powder coatings. The production process of powder coatings differs significantly from solvent-based and water-based coatings. It does not involve typical hazardous waste such as wastewater treatment sludge and waste organic solvents during the melting and cooling of solid materials. However, pigments and solid residues formed from heavy metal and aniline-based pollutants, as well as solid film-forming materials used in powder coating production, still pose environmental risks and may render the powder coating itself a hazardous waste. It is recommended to conduct research on the environmental management of hazardous waste from powder coatings, and to promptly update the "National List of Hazardous Waste" in conjunction with the revised solid waste regulations. Strengthen research on the categories and pollution characteristics of hazardous waste from powder coatings, clarify the nodes and types of characteristic hazardous waste generated during production, and classify the "HW12 Dye and Coating Waste" in the "List" according to the type of coating. At the same time, scientifically verify the hazardous properties of commonly used film-forming materials in powder coatings, such as epoxy resins and acrylic resins, and conduct research on whether small amounts of spent activated carbon can be included in the exemption management.
(3) Strengthen environmental oversight of coating manufacturers and downstream users. Coating manufacturers and users are typically small and micro-enterprises involved in automotive spraying, painting, and other applications. They often lack awareness and management capabilities regarding hazardous waste generated during coating processes, and there may be instances of improper disposal of waste such as paint residue. It is recommended to strengthen the management of hazardous waste generated during coating processes, focusing on both the "beginning and end" phases. This involves registering the type and quantity of coatings used at the source, and verifying the amount and disposal methods of paint residue generated during spraying at the end. This will encourage companies to maintain two records, one for the entire coating process from source to end, and to ensure comprehensive oversight and traceability throughout the entire process.
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