Research Related to Liquid Metal Quality (LiMCA) Studies
Thanks to break-through technology at McGill in the early 1980’s, it is now possible to monitor the quality of liquid metals in terms of the numbers of extraneous inclusions. Thus, LiMCA (Liquid Metal Cleanliness Analyzer) sensors make use of the Electric Sensing Zone (ESZ) principle to detect inclusions within a sample of liquid metal drawn into an electrically insulated sampling tube. First conceived and developed at McGill University, rugged commercial versions of the instrument were later designed and assembled by Bomem Inc. (now ABB), and successfully used in Alcan's smelters around the world, prior to general commercialisation.
MCA principle (left), and the LiMCA system in operation (right).
A second version, LiMCA II, has since been constructed. This was made available in 1994 for the Aluminum Industry. Bomem and Alcan received a prestigious R&D 100 Award that year from R&D magazine, LiMCA II being voted one of the most technologically significant products of 1994. In the meantime, the study of liquid metal quality at McGill, using our own equipment continued, aimed at extending the technique’s application to other metals, such as alloys of copper, magnesium and most importantly, liquid steel. Similarly, we have been researching at the MMPC means to enhance the analytical power of the resistive pulse principle, to allow us to discriminate between different types of inclusions within a melt. In aluminum, for example, microbubbles are relatively harmless compared with carbide or silicate inclusions and need to be distinguished from a metal quality control point of view. To do this, we pioneered the application of digital signal processing technology (DSP) to LiMCA, which, combined with pattern recognition techniques, provided the necessary processing capability in analyzing the voltage pulses generated by the passage of particles through the electric sensing zone.
Aqueous Particle Sensor Systems (APS III) for full scale water model studies of ladle – tundish –mold operations, have been a continuing item for our McGill – based studies. Recently, we have developed a new ground-up aqueous system for our on-going tundish modeling work in which we measure the flotation of hollow glass microspheres (40 – 200 um) of S.G. 0.4 in a full-scale model tundish of RTIT – QIT’s 12 tonne, 4 strand, billet operations. This tool for monitoring and measuring the size distribution of inclusions and microbubbles is the valuable water analog equipment for advancing research on cleanliness of melts. Its advantage over an equivalent Heraeus APS system is that it is “in situ” and continuous, vs. offline with the Heraeus Electro-Nite equipment. The latest experimental work using this analytical device was performed in a water analog set-up by our graduate and Undergraduate students performing their co-op term with McGill Metals Processing Centre (Ajay Panicker, Giacomo di Silvestro, Karim Selim, and Xiuyuan Zhu). Below the Figure presents the newly developed bubble generator following our findings of bubble dispenser used in the ladle-shroud, experiments.
APS III (Aqueous Particle Sensor Systems) set-up. New Microbubble Generator for Water Analog systems
LiMCA for Low Temperature Melts
Our latest research accomplishments are related to designing and fabrication a new LiMCA equipment for low temperature melts. Among the MMPC researchers contributing to the realization of this first to be produced unit of this kind are Dr. Usman, Dr. Luis Calzado, under the supervision of Dr. Mihaiela Isac, and Professor Roderick Guthrie. Below is presented a picture of LiMCA for low melting point alloys.
LIMCA setup for Low Melting Point Allows, e.g. Cerrolow, including: the LiMCA sensor tube with the two electrodes immersed in molten Cerrolow, a battery, and a battery charger, a vacuum pump, an oscilloscope, a voltage amplifier, a power supply, a data acquisition system, and an analog to digital convertor.
The new LiMCA system for use with low melting point alloys (Cerrolow, Sn, Al, Zn, etc.), consists of a glass tube containing a small orifice located near the bottom of the closed-end tube. The tube is placed into a larger container of liquid metal, in this case, liquid Cerrolow. The liquid metal, containing the inclusions or micro-bubbles to be detected, is drawn through the small orifice (300 – 3,000 μm. dia.), into the tube, by creating a vacuum inside the tube, in the order of 380 torr (760 torr=1 Atm). The tube is made of an electrically insulating material (e.g. alumino-silicate, boro-silicate, or quartz). A minimum of two electrodes are required; the one located inside the tube acts as the anode, the other electrode, outside the tube, acts as the cathode, as shown in the figure below presenting the set-up of two operational electrodes.
A heavy DIRECT electrical current (6A-30A) is applied between the electrodes and electrons flow from positive to negative (outside electrode) through the liquid metal being drawn through the small orifice, into the tube. The passage of an inclusion or entrained microbubble into the orifice, within the metal flowing through the orifice, will temporarily increase the electrical resistance measured within the orifice. This magnitude of this voltage pulse is detected, and can be measured, from which one can estimate the size of the particle that has passed through.
Thus, when an inclusion enters the orifice, it displaces its volume of conducting fluid, causing a temporary rise in electrical resistance. In the presence of the current, the increase in resistance generates a voltage pulse. The magnitude of the voltage pulse is a function of the volume of the particle. The duration of the pulse is related to the transit time of the inclusion through the orifice (ESZ). The micro-voltage pulses associated with liquid metals, need to be amplified by a thousand (X1,000), using a custom-made amplifier, shown in the LiMCA set-up above. These magnified pulses are now in the millivolt range. As such, they can then be measured digitally, using the data acquisition system, shown in the figure above. The size distribution and total concentration of foreign particles, or microbubbles, within the metal sampled, can then be displayed in real time, on the computer screen, as shown above.
Microbubble Generation in Water and Low Temperature Melts Systems
Another significant milestone in the MMPC’s research on improving the cleanliness of steel and low temperature melts represented the designed and fabrication of a new Microbubble Generation system presented in the figures below, as experimental set-ups for Cerrolw melts, as well as for water analog system.
Microbubble Generator in low temperature melts (left) & Microbubble Generator in water (right)
The Microbubble Generator is used by our current graduate students, PhD Rohit Kumar Tiwari and M.Sc. student Ajay Panicker, PhD Giacomo Daniel Di Silvestro, together with our current Co-op students Daniel Bonneau, Ruohao Li, and Shutian Get to perform their experiments using a water analog system, and to create the assumptions and experimental conditions and settings for our research on creating microbubble in a low melting point alloy, i.e. Cerrolow 136. However, creating and monitoring the number and distribution of microbubbles in low melting points alloys represent an intermediate stop, in our research, before transfer of this new knowledge to steel melts, to improve their cleanliness using microbubbles to remove all oxide particles from them. Presently, this is not possible with current technologies.