Abstract: This article investigates the sources of odors in the investment casting process, analyzes the composition and concentration of these odors, and proposes practical treatment measures, providing a scientific basis for environmental management.
Introduction
Investment casting, also known as lost-wax casting, is widely regarded by casting experts due to its high dimensional accuracy, smooth surface finish, low pollution, flexible casting structure design, absence of sand cores, low investment and production costs, and low technical proficiency requirements for workers. It is called “green casting” and has undergone rapid development in recent years with an expanding application range.
However, being “green” only refers to its comparison with traditional casting processes (such as green sand mold casting). It does not mean that investment casting is devoid of environmental pollution issues. While issues with smoke and dust in casting factories have been well-addressed with the increasing awareness of environmental protection and advanced dust removal technologies in China, complaints about odors have become frequent in recent years. To address these issues, it is necessary to comprehensively understand and analyze the pollution sources and pollution levels generated during the investment casting process and adopt corresponding measures to control these sources.
1. Production Process and Pollution Emission Standards
1.1 Production Process
Based on the type of coating binder, domestic investment casting is mainly divided into two types: water glass mold shell and silica sol mold shell. The main production process is as follows:
Wax → Wax Pattern Making → Sand Shell Making → Dewaxing of Sand Shell → Firing of Sand Shell → Metal Melting and Pouring → Sand Removal and Shell Stripping → Gating Removal → Shot Peening → Grinding and Repair Welding → Semi-finished Product Sales
1.2 Emission Standards
According to the Emission Standard of Air Pollutants for Foundry Industry (GB39726-2020), the exhaust pollution indicators generated during the casting process must meet the relevant standards outlined.
The current domestic emission standards for the casting industry do not set specific limits for odors (except for surface coating processes). Due to the lack of standards, the issue of odors in the investment casting industry has not received attention from environmental departments and workers. However, based on investigations at relevant enterprises, odors do exist in workshops.
2. Odor Sources and Intensity
2.1 Production Process and Odor Sources
Based on an investigation of investment casting enterprises in a certain industrial park in Zhejiang (with over 30 casting enterprises and strong reactions from surrounding residents regarding odors), the waste gas pollution factors generated by these enterprises.
Odors in the investment casting process are mainly related to casting wax, including processes such as wax pattern making, dewaxing of sand shells, and firing of sand shells. The main components of casting wax are paraffin wax and auxiliary materials (such as resins), which have high viscosity and molecular weight, large internal cohesive energy, and can improve the dropping and melting points of wax patterns, refine the grain structure, and enhance thermal stability and internal cohesion. During high-temperature processing, incomplete combustion of melted casting wax generates smoke and odors, including waxy smells, burnt smells, and other organic waste gas odors.
2.2 Odor Intensity
The detection data for dewaxing steam and firing exhaust gas from a certain enterprise, respectively.
3. Odor Treatment Processes
3.1 Dewaxing Odor Treatment Process
A water scrubber is installed at the end of the dewaxing exhaust gas to wash and remove odors. However, a single water scrubber has limited removal efficiency for dewaxing exhaust gas. Based on analysis, two reasons contribute to this: short residence time of steam in the water scrubber due to high steam pressure, leading to insufficient water-steam contact; and secondary volatilization of organic substances absorbed and washed down in the spray water due to high gas temperature, resulting in odors.
To address these issues, a shell-and-tube condenser is added in front of the water scrubber. The outer periphery of the shell-and-tube condenser passes indirect cooling water to cool and decompress the gas, and then it passes through water spraying for more effective odor removal. The detection data after treatment.
3.2 Firing Odor Treatment Process
Firing exhaust gas has obvious dust and heavy odors. Although a water scrubber is used for treatment, dust is still visible at the end, and odors remain strong. To improve removal efficiency, a wet electrostatic precipitator (WESP) is added at the end of the water scrubber. The detection data after treatment.
4. Analysis of Treatment Process Improvement
To further reduce the impact of firing exhaust gas in the entire industrial park, further purification of the exhaust gas is necessary. For an enterprise with exhaust gas temperature below 40°C at the outlet of its existing facilities, a wire mesh demister and activated carbon adsorption fixed bed are added at the end of the water scrubber + WESP system for further odor reduction. The detection data after further purification.
5. Conclusion
(1) Based on the investigation and analysis, the odors in the investment casting process mainly originate from dewaxing steam and firing exhaust gas.
(2) Dewaxing steam can be effectively deodorized by passing through a condenser for cooling and decompression, followed by absorption treatment in a water scrubber.
(3) Firing exhaust gas can significantly reduce odor concentration through water scrubbing + WESP. However, the concentration of non-methane hydrocarbons remains high. Further pretreatment of the exhaust gas (such as cooling and demisting) followed by activated carbon adsorption purification can achieve remarkable treatment effects.
Through the above analysis, we have identified the sources of odors and appropriate treatment methods, providing a practical scientific basis for pollution control and environmental management in this industry.