Performance Evaluation of Two Different Industrial Foam Filters with LiMCA II Data
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CERAMIC foam filters (CFFs), as shown in Figure 1, appeared in the late 1970s and early 1980s. Now, they are one of the most commonly used filter types for filtration of molten aluminum. Filtration removes solid inclusions and greatly improves the mechanical properties of aluminum.[1] When contaminated metal flows into a filter, inclusions are collected on the inner surfaces of the filter by interception and gravity mechanisms.[2] CFFs operate in a mode of deep-bed filtration where inclusions smaller than the pore openings are captured. The random structure of the CFF interface forces the molten metal to flow in a tortuous path, which allows inclusions to come in contact with, and be collected by the filter. The liquid metal cleanliness analyzer (LiMCA) was patented by Don Doutre and Roderick Guthrie[3,4] in 1985. Since the early 1990s, the LiMCA system has been successfully applied in aluminum industries monitoring the number of inclusions.[5,6] The technique is based on the electric sensing zone (ESZ) principle,[7] in which a constant electric current flows between two electrodes separated physically by an electrically insulating sampling tube with a small orifice. The orifice allows both the electric current and molten aluminum to pass in and out of the tube in a cyclic manner. When a nonconducting particle passes through the orifice, the electric resistance within the ESZ rises, causing a voltage pulse. The amplitude of this voltage pulse is roughly proportional to the volume of (spherical) particles and the number of pulses equals the number of particles. A disadvantage with the LiMCA system is that it does not analyze the shape or type of inclusions but rather the number and size. One measurement cycle lasts for MARTIN SYVERTSEN and SARINA BAO, Research Scientists, are with the SINTEF Materials and Chemistry, 7465 Trondheim, Norway. Contact e-mail: [email protected] Manuscript submitted July 9, 2013. Article published online December 10, 2014. 1058—VOLUME 46B, APRIL 2015
typically 70 to 90 seconds, compared with a typical cast time of 1 hour. The LiMCA continuously counts the inclusions. LiMCA II is used to study various melt treatment processes. In addition to inclusion removal and inclusion release from filters,[6] also stirring,[8] degassing, gas fluxing,[9,10] and grain-refining behaviors[8] can be monitored by LiMCA. When the filtration efficiency of inclusions in a filter is discussed, a typical way of using LiMCA II data is to take averages for the whole cast.[11] Little work has been done with a focus on how the changes in inlet inclusions concentration affect the filtration efficiency. Guthrie and Li[6] calculated the inclusion removal efficiency in a filter as a function of time. However, the scattering in the calculated removal efficiencies is large and no uncertainty is estimated. Use of the LiMCA II data to study the dynamic behavior of filter is the main focus in the current work. Four experiments using two different types of industrial CFF filters have been carried out and are presented. LiMCA II data from these fou
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