Analyses of the dynamic processes of liquid metal filtration
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INTRODUCTION
THE filtration of liquid metals is now becoming a necessary refining step in the production of cleaner metals. This is especially true in the aluminum industry, where increasing demands for superior quality constantly lower the tolerance for nonmetallic inclusions. By filtering the liquid metal with some type of ceramic filter, the content of the nonmetallic inclusions can be substantially reduced. This article presents a fundamental analysis of the dynamic processes of liquid metal filtration using reticulated ceramic filters. Filtration processes associated with reticulated filters are bound to be transient; i.e., the filtration processes, as well as the parameters pertaining to them, change with time. This is because the particles captured inside the filter alter the filter’s properties, a process that has been referred to as “filter aging”. By definition, a filter starts to age as soon as particles start to accumulate inside it. During this period, the filter’s internal structure changes as a result of the continuous accumulation of captured particles as filtration proceeds. These changes will inevitably cause some of the independent variables, upon which the filtration efficiency depends,[1] to change. The most apparent is a filter’s porosity, which will decrease with time, provided there is no release of inclusions. The specific surface area, or equivalent filter web diameter (the two are related, so either can be chosen), also changes. Depending on how the particles are captured (or deposit morphologically), the effect can be different; e.g., if the particles deposit in a uniform fashion onto the filter webs, causing an increase in web diameter (dw), this would have an adverse effect on removal efficiency. Alternatively, if the particles are captured in a dendritic or clustering fashion a CHENGUO TIAN, formerly Doctoral Candidate, Department of Mining and Metallurgical Engineering, McGill University, is Scientist, Division of Manufacturing Technology, CSIRO, Preston, Victoria, 3072, Australia. D. MAZUMDAR, formerly Visiting Professor, McGill Metals Processing Centre, McGill University, is Professor, Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur, 208016, India. R.I.L. GUTHRIE, McDonald Professor of Metallurgical Engineering, and Director, McGill Metals Processing Centre, is with McGill University, Montreal, Canada H3A2B2. Manuscript submitted February 23, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
favorable filtering effect would result since the diameter of the dendritic clusters, acting as new filter webs capturing oncoming particles, would usually be much smaller than dw. The change in interfacial characteristics can also be favorable or unfavorable. As particles accumulate (or porosity decreases), flow resistance increases, causing pressure drops to rise together with interstitial velocities, if the flow rate through the filter is maintained constant. As a result, the drag exerted on either the contacting particles or the particles alread
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