Electrical Resistivity Measurement of Petroleum Coke Powder by Means of Four-Probe Method
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INTRODUCTION
EFFICIENCY of aluminum smelting through Hall– He´roult process is directly influenced by the quality of the carbon anodes used in the electrolysis cells. Low electrical resistivity, high mechanical strength and homogeneity, and also low reactivity towards air and carbon dioxide are very important quality indices of carbon anodes.[1] Carbon anodes for aluminum industry are made of coke particles mixed with coal tar pitch. This mixture, which is called anode paste, is then compacted and baked to form a solid block. Usually, a wide size range of coke is used in the anode recipe: the coarse aggregates, ranging between 0.104 and 4.750 mm, and the fine coke particles smaller than 0.104 mm. The electrical resistivity of a baked anode is affected by both
G. ROUGET, B. MAJIDI, and H. ALAMDARI are with the Department of Mining, Metallurgical and Materials Engineering, Universite´ Laval, 1065 avenue de la Me´decine, Que´bec, QC, G1V 0A6, Canada, and also with NSERC/Alcoa Industrial Research Chair MACE3 and Aluminum Research Centre, REGAL Universite´ Laval, 1065 avenue de la Me´decine, Que´bec, QC, G1V 0A6, Canada. Contact e-mail: [email protected] D. PICARD and G. GAUVIN are with the NSERC/Alcoa Industrial Research Chair MACE3 and Aluminum Research Centre, REGAL Universite´ Laval. D. ZIEGLER is with Alcoa Primary Metals, Alcoa Technical Center, 859 White Cloud Road, New Kensington, PA, 15068. J. MASHREGHI is with the Department of Mathematics, Universite´ Laval, 1065 avenue de la Me´decine, Que´bec, QC, G1V 0A6, Canada. Manuscript submitted September 26, 2016. Article published online July 6, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS B
anode processing parameters and the resistivity of the coke aggregates.[2,3] The electrical resistivity of beds of coke is also measured in some calciners to follow the coke calcination level in their process.[4] As the electrical resistivity of baked anodes is influenced by the electrical resistivity of coke aggregates, an accurate characterization of those aggregates is of prime importance. This characterization is performed following a well-established standard test method in the aluminum industry. Electrical characterization of calcined coke is typically performed with a two-probe setting,[5–8] according to the ISO 10143 standard method. According to this method, a pressure of 3 MPa is applied on the bed of particles to ensure a mechanical contact between the particles and minimizing the interface electrical resistance. Stainless steel plungers, which also act as electrodes, are used to apply the pressure and supply the current. The ISO 10143 standard method is however not an ideal method for electrical characterization of coke particles. The main reason is that two-probe measurement is used for low conducting materials, with a resistance exceeding 100 X. For better conductive materials such as coke (~500 lX m), a four-probe setting is privileged.[8–10] This is explained by the fact that in a two-probe setting, the internal resistance of the voltmeter as well a
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