Abstract
Background, Aim and Scope. Cleaning processes are inevitable for product manufacture and maintenance of product functions. In ordinary metal processing, metal parts are greased to avoid possible friction and confrontation by pressing or cutting. To deal with metal parts with various materials and shapes, suitable chemicals and devices have been selected on a site to site basis. Almost all enterprises with cleaning process are small and medium-sized enterprises(SMEs) in Japan. Each process has plant-specific functions and constraints depended on its cleaning requirement. Chloride compounds have widely been utilized as a cleansing agent because of their inexpensiveness, nonflammability and high capability for precision cleaning. Because the discharge of such chemicals has become an issue, many alternative agents and improved processes have been developed. Available technologies for environmentally benign processes are strongly depended on plant-specific conditions. For the reduction in risks caused by cleaning process, the environmental aspects of cleaning should be assessed appropriately considering plant-specific functions and constraints. This paper aimed to analyze the difference of environmental impacts among metal cleaning processes, which are utilizing chlorinated carbon and hydro carbon, by life cycle assessment(LCA). This analysis enabled discussions of the way how to evaluate and interpret environmental impacts originated in metal cleaning considering plant-specific conditions. Human health impacts were quantified on the basis of the life cycle of cleansing agents.
Methods. The inventory data of cleaning processes were acquired through field investigations of plants. The cleaning functions of investigated plants were different from each other. Functional units in LCA should be carefully defined to take into account cleaning requirements appropriately for comparisons of processes. In this paper, "Daily operation" and "Cleaning of unit weight of products" were adopted and compared.
Results. The impacts attributable to cleaning process were the largest among the processes in the life cycle of a cleansing agent. Especially, photochemical oxidant creation due to emitted cleansing agent was a dominant contribution to human health impacts. Global warming, air pollution in urban area and toxic chemical emission had the same order of contribution. According to the evaluation results for a specific plant, several actions toward the reduction in emission volume could reduce the total human health in the life cycle of cleansing agents. Although hydro carbon has been regarded as an alternative agent reducing environmental impacts, its result was quite similar to a process using chlorinated agent with well-suitable machine under the plantspecific conditions. Changing the functional unit from "Daily operation" to "Cleaning of unit weight of products" revealed that some plants with small impacts during daily operation have larger human health impacts per unit weight of products than plants with large daily impacts.
Discussion. Because the mass of each metal part is considerably small comparing the total throughputs on site, in ordinary product LCA, the inventories associated with metal cleaning might be regarded as trivial loads and can be usually cut off. The obtained results in this paper can be regarded as the hidden impacts in such assessments. Comparing the obtained results with an existing LCA result of product including metal parts, it was revealed that metal cleaning cannot be neglected to evaluate the impacts in product LCA appropriately. If one includes cleaning processes in product LCA, the impacts due to the use of cleansing agent would be recognized and effectively reduced through the supply chain of products. For the sake to achieve such reductions, the cleaning requirements on a process should be defined comprehensively and quantitatively.
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