Secondary machining techniques are the processes used to modify the output of primary manufacturing processes further to improve the material properties, surface quality, surface integrity, appearance and dimensional tolerance (Patra, Biswajit and Samantray 2011, p. 76). Various secondary machining techniques include heat treatment, coating, etc. Heat treatment is the heating of metals to change their physical and chemical properties, without allowing it change shape. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, normalizing and quenching. During Heat-Treating process, a quench is mostly used as an integral part. The liquid usually quenches the use of mineral oils, water-based solutions or molten salts. Quenching operations usually contribute to various health and safety hazards to workers. These may include; exposure to chemicals, working in high temperatures, and the risk of explosion. This study looks into dangerous gases produced during heat treatment process in metal extraction, their health and safety risks and the impacts they have on the environment.
Gas Quenching
Gas quenching is an environmental friendly and more easily controlled alternative to oil and salt baths. It’s primarily used to speed up cooling. It is majorly applied in vacuum furnaces but is suitable for all suitable furnaces. Nitrogen, argon, hydrogen, and helium are suitable gas. In the gas carburizing process, a low carbon bearing carrier gas is used, which is enriched with a hydrocarbon gas such as propane or methane, to increase and control the carbon availability of the atmosphere. The carrier gas is usually of the endothermic gas type produced from a substoichiometric mixture of a hydrocarbon and air at an elevated temperature in the presence of a catalyst (Dossett, Boyer and Howard 2006, p. 20). Alternatively, a nitrogen-methanol mixture, injected into the furnace can be used to produce a synthetic endothermic gas. During the reactions, carbon monoxide and nitrogen oxide gases are produced to the environment in both cases.
Health effects and safety risks of gases evolved during heat treatment
Ambient NO2 exposure may increase the risk of respiratory tract infections through the pollutant’s interaction with the immune. Sulfur dioxide contributes to respiratory symptoms in both healthy and those with underlying pulmonary diseases (Robert and Peter 1996, p. 10). Controlled human exposure studies have demonstrated that experimental SO2 exposure causes changes in airway physiology, including increased airways resistance. Both acute and chronic exposure to carbon monoxide is associated with increased risk for adverse cardiopulmonary events, including death.
Environmental pollution by the dangerous gases and Protection.
Emission of irritant and corrosive gases like sulfur dioxide, hydrogen sulfide, and hydrogen chloride usually contribute to air pollution and causes corrosion of metals and concrete within the plant and in the surrounding environment (Rajan, Sharma and Ashok 1992). Mostly, vegetation does not tolerate such gases as they block the stomata in the plant leaves preventing proper photosynthesis from occurring. Such gases, when released into the atmosphere, combine with the nuclei upon which the water vapor condense and hence fall back as acidic rain. The acidic rain is dangerous for human consumption and also corrodes metal works. The mitigation process for such hazardous gases to the environment is carried out before reaching the environment. For instance, sulfur dioxide, one of the primary contaminants, is recovered as a sulfuric acid when present in sufficient quantity. Otherwise, to meet emission standards, sulfur dioxide and other hazardous gaseous wastes are controlled by scrubbing. Particulate emissions are commonly controlled by fabric filters and electrostatic precipitators.
Conclusion.
Heat treatment as a secondary machining technique is advantageous to the material but the processes done prove to be hazardous to human health and environment at large, the carbon monoxide, sulfur dioxide, and nitrogen dioxide are all dangerous gases evolved during the heat treatment. The industries are thus supposed to implement techniques of capturing these gases and not releasing them into the air at their toxic levels. These include scrubbing for sulfur dioxide.
References
B. B. Patra; Biswajit Samantray 2011 Engineering Chemistry I. Dorling Kinderslei. Pp. 75-77.
Dossett, Jon L; Boyer, Howard E. (2006). Practical Heat Treating. ASM International. Pp. 17-22.
Rajan, T.V.; Sharma, C. P.; Sharma, Ashok (1992). Heat Treatment: Principles and Techniques. Prentence Hall. P.1
Robert W. Cahn, Peter Haasen, eds (1996). Physical Meturllugy. Volume 2. Elsevier Science. Pp. 10-11.
