Typical hazardous waste generation nodes and types in the coating industry and environmental management suggestions!

0 Preface

Coating is a general term for a type of liquid or solid material that can form a solid film with protection, decoration or special properties after being applied to the surface of the material.

According to data from the National Bureau of Statistics, China’s paint output in 2019 was 24.

1 Typical hazardous waste generation nodes in the coating industry

1.

In the GB/T4754-2017 "Classification of National Economic Industries", both pigments and coatings belong to the category of "264 coatings, inks, pigments and similar products manufacturing", and film-forming substances, solvents and additives are classified as "266 special chemical product manufacturing" "In the category

Pigments are divided into organic pigments and inorganic pigments

1.

According to the different solvents used in coatings, it can be divided into solvent-based coatings, water-based coatings and powder coatings [1]

The typical hazardous waste types and generation nodes involved in the production of solvent-based, water-based and powder coatings are shown in Table 2

1.

The typical hazardous waste generated during the application of paint mainly includes waste paint and spray paint residue, which belong to the 900-299-12 and 900-252-12 categories in the 2021 edition of the "National Directory of Hazardous Wastes"

2 Types of typical hazardous waste in the coating industry

2.

The types and sources of hazardous waste involved in the pigment production process are shown in Table 3
.
The organic pigment wastewater treatment sludge mainly contains aniline, quinoline, ether, phenol and heavy metal pollutants
.
Aniline substances are one of the raw materials for the diazotization reaction and are used to provide primary amines for subsequent reactions; quinolines, ethers, and phenols are one of the raw materials for the condensation reaction.
Color; heavy metals are mainly used as complex metals for azo and phthalocyanine pigments, but elements such as manganese and zinc can also be used as catalysts
.
The toxic substances contained in inorganic pigment wastewater treatment sludge are mainly heavy metal raw materials, such as chromium, molybdenum, lead, and iron cyanide
.
In addition, impurity metals in the raw materials may also be transferred to the sludge along with the synthesis of inorganic pigments [11]
.

2.
2 Paint manufacturing

Solvent-based coatings are widely used in industrial fields due to their better leveling, anti-corrosion and coloring properties, especially as wood coatings and marine coatings
.
In 2014, the proportion of solvent-based coatings used in China's woodware industry was close to 89%[12]
.
The types and sources of hazardous waste involved in the production of solvent-based coatings are shown in Table 4
.
The film-forming materials of solvent-based coatings are mainly organic resins, of which polyurethane resins and nitrocellulose resins are the most used
.
Yang Yuande et al.
[1] investigated the product structure of wood coatings in China in 2009 and found that 75% to 80% of the wood coating film-forming material is polyurethane, and 15% to 20% of the film-forming material is nitrocellulose
.
In addition, incompletely used pigments, organic solvents, and additives in heavy metals, anilines, and alcohol ether components may also be present in the sludge [13]
.

Water-based coatings are mainly used in the construction field
.
According to incomplete estimates, the proportion of water-based coatings used in China's construction industry is currently close to 90% [6]
.
Taking the construction water-based paint as an example, the typical hazardous waste components that may be involved in the wastewater treatment sludge are shown in Table 5
.
Tang Hongxia [14] analyzed the hazardous characteristics of wastewater treatment sludge from 8 water-based architectural coating plants in Shanghai, and the results showed that the sludge was not flammable, corrosive, reactive and toxic; Huang Jun [15] used Nippon Paint China The waste water treatment sludge of 4 water-based paint production plants under the company was analyzed for hazard characteristics.
The results also showed that the waste was not flammable, corrosive, reactive and toxic, but copper, barium, toluene and ethylbenzene were leaching toxicity Higher
.
Inferred from the source of pollutants, copper and barium should be introduced from the pigment, while toluene and ethylbenzene were introduced from the co-solvent
.

 Table 5 Hazardous waste components that may be contained in water-based paint wastewater treatment sludge

The production process of powder coatings is the melt extrusion and cooling reaction of solid powders.
No liquid is used as a diffusion medium during the reaction.
The adsorbate in the waste activated carbon may contain heavy metals and anilines in the film-forming materials, pigments and additives used.
Pollutants
.
Hao Yaqiong et al.
[16] once identified the ingredients of a certain powder coating material imported into China and found that the main ingredients were polyester, epoxy resin, TiO2, BaSO4 and CaCO3
.
Polyester and epoxy resin are organic resin film-forming materials, and TiO2 is the main component of titanium dioxide
.
BaSO4 is a relatively inexpensive white inorganic pigment.
Because of its wide particle size distribution and low purity, it can be used to replace 10% to 25% of titanium dioxide to reduce costs
.
CaCO3 is a kind of filler and is often used to enhance the deposition and permeability of film-forming resins
.
 

2.
3 Coating and use

After the overspray mist generated during the painting process is absorbed and captured by the water, it is transferred into the spray booth sink under the action of air pumping and condenses under the action of the coagulant to form spray paint residue
.
The types and sources of hazardous waste involved in spraying paint residue are shown in Table 6
.
It can be seen from Table 6 that the composition of the paint residue is basically the same as that of the paint, but due to the addition of the coagulant, it may also contain inorganic metal salts or organic polymer substances
.

3 Disposal methods of paint hazardous waste

The waste slag and wastewater treatment sludge produced in the process of pigment manufacturing and paint production are relatively complex and contain a large amount of heavy metals and organic pollutants
.
The common treatment and disposal methods of waste residue and wastewater treatment sludge are landfill or incineration, but in recent years, there have been researches on the reuse of paint waste residue
.
Yilmaz et al.
[17-18] used thermosetting coating waste as a filler and reinforcing agent for low-density polyethylene and methanol copolymers
.
The results show that the addition of 10%, 20% and 30% of thermosetting coating waste can increase the polymer strength of low-density polyethylene and copolymerized methanol
.

The conventional treatment and disposal methods of spray paint residue are similar to waste residue and wastewater treatment sludge, mainly landfill or incineration
.
However, in recent years, there have been many studies on the recycling and reuse of paint residue.
Zhang Yufei [19] started from the formation stage of paint residue and regenerated the paint residue into part of the raw material of solvent-based coatings by selecting a suitable coagulant.
Its physical and chemical properties can reach agricultural use.
The standard for the use of low- and middle-grade coatings for cars; Gerace et al.
[20] used calcium oxide to chemically dehydrate the sprayed paint residue and then vacuum dehydrated.
The powder obtained has a solid content of more than 98%, which can be used as a car sealant; Dalmazzo et al.
[21- 22] The spray paint residue is added to asphalt or cement to be used as a lightweight construction material
.
In addition, research on the treatment of spray paint residue by hydrothermal method and Fenton method has been gradually carried out [23-24], using hydrothermal technology to convert spray paint residue into bio-oil and gas fuel, or oxidizing paint residue by Fenton reagent.
Improve biodegradability
.

4 Suggestions for the environmental management of typical hazardous wastes in the coating industry

The coating industry involves a wide variety of typical hazardous wastes, and its management is a long-term process
.
Accurately identifying the types and pollution characteristics of paint hazardous waste is of great significance to the establishment and improvement of hazardous waste management methods and risk control methods in the paint industry in China [25]
.
In view of the characteristics of China's coating industry waste and current management methods, the following three suggestions are put forward
.

(1) Simplify the identification procedures for the properties of water-based paint solid waste
.
The waste generated during the production and use of water-based coatings needs to be identified in accordance with the methods specified in the GB5085 "Hazardous Waste Identification Standards" series to confirm that it is not corrosive, toxic (including leaching toxicity and toxic substance content), flammability and reaction It can be managed as general industrial solid waste only after it has hazardous characteristics such as sex
.
Our country’s management thinking for water-based paint solid waste is correct, but the identification of hazardous waste generally suffers from time-consuming and high-cost problems.
Some small and micro enterprises are unwilling to waste time and cost in identifying them.
Undoubtedly, the ecological environment management department will increase the amount of paint waste.
The difficulty of supervision
.
As a kind of environmentally friendly coatings, the development of water-based coatings is bound to be the focus of the development of the coatings industry.
It is recommended to set content control values ​​for the characteristic pollutants in coating waste, such as chromium, zinc, ethylbenzene, xylene, etc.
[26], simplify Identification index for solid waste of water-based paint
.

(2) Carry out research work on powder coating hazardous waste management
.
The production process of powder coatings is quite different from solvent-based and water-based coatings
.
Typical hazardous wastes such as wastewater treatment sludge and waste organic solvents are not involved in the melting and cooling of solid materials .
However, solid residues formed by heavy metal and aniline pollutant pigments and organic resin film-forming substances used in the production of powder coatings still pose environmental risks and may cause the powder coatings themselves to become hazardous wastes
.
It is recommended to carry out research work on the environmental management of powder coating hazardous waste, and to promote the dynamic revision of the "National Hazardous Waste List" in time with the newly revised Solid Waste Law
.
Strengthen the research on the categories and pollution characteristics of powder coating hazardous wastes, clarify the generation nodes and types of characteristic hazardous wastes in the production process, and classify the "List" "HW12 dyes and coating wastes" according to the types of coatings, aiming at the hazards generated by powder coatings Create entries for waste
.
At the same time, scientifically demonstrate the hazardous properties of commonly used film-forming materials for powder coatings, such as epoxy resins and acrylic resins, and conduct research on whether waste activated carbon with a small amount of production can be included in the exemption management
.

(3) Strengthen environmental supervision of companies that use coatings downstream
.
Coating companies are mostly small and micro companies such as automobile spraying and paint painting.
They have problems such as large numbers, wide distribution, and low scale.
They have low awareness and management awareness of hazardous waste generated during the coating process, and spray paint slag, etc.
Hazardous waste may be disposed of randomly
.
It is recommended to strengthen the management of hazardous waste in the use of paint, pinch "one end and one end", record and register the types and quantities of paint used from the source, and verify the amount of paint slag produced by spraying and the disposal method from the end, and supervise The enterprise establishes two ledgers of the “source and end” and ensures the supervision coverage of the full cycle of spray paint slag and the traceability of the problem
.