Responses of Lithium-Modified Bath to a Shift in Heat Input/Output Balance and Observation of Freeze-Lining Formation Du
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IN the aluminum electrolysis process, a stable heat balance of the reduction cell is critical to achieve high current efficiency. However, global electricity and aluminum markets are driving dynamically varying economics for aluminum production. To meet these new industrial demands, a new cell technology, to extend the heat balance and amperage operating window of smelters, has been developed.[1] This new technology aims to shift each cell in the series circuit (termed potline) from one steady state to another with minimum disturbance to cell operation, by adjusting both the heat loss (output controlled by the shell heat exchanger) and power input (the input amperage) to the cell. During the heat balance shift, the change in heat loss, especially through side walls, may disrupt the self-adjusting JINGJING LIU is with the Department of Chemical and Materials Engineering, The University of Auckland, 1023 Auckland, New Zealand. Contact e-mail: [email protected] MARK TAYLOR is with the Department of Chemical and Materials Engineering, The University of Auckland and also with the NZ Product Accelerator, The University of Auckland, 1023 Auckland, New Zealand. MARK DORREEN is with the Light Metals Research Centre, The University of Auckland, 1023 Auckland, New Zealand. Manuscript submitted May 18, 2017. Article published online November 10, 2017. 238—VOLUME 49B, FEBRUARY 2018
melting/freezing process of the ledge,[2,3] giving rise to a large variation in the ledge thickness.[4] In some smelters, the electrolysis bath contains LiF, introduced by the impurity in the raw material alumina[5,6] or as a bath modifier.[7] The LiF may be as high as 4 pct in the industrial cell, accumulated in a long term.[8] An industrial trial with the addition of lithium salts had been carried out over 10 years in a smelter by adding Li2CO3 instead of LiF to reduce the cost.[7] It was observed that the power consumption and fluorine waste was reduced, and higher current efficiency was achieved even with serious bottom sludge problem. It is commonly accepted that LiF causes bath properties change, such as a decrease in bath liquidus temperature and alumina solubility, but increase in the bath conductivity.[9–11] The composition of side ledge may also change according to the alteration of bath chemistry. Therefore, the freezing/melting process of the ledge needs to be understood very well when the bath is modified by LiF, especially during the heat balance shift between different steady states. In the previous work,[12] the heat balance shift measurements in a controlled laboratory analogue have been conducted in the cryolite-AlF3-CaF2-Al2O3 bath system, where both the heat loss (output) and the furnace power (input) can be altered independently and measured. In this paper, heat balance shifts in the same laboratory experiment will be investigated when the METALLURGICAL AND MATERIALS TRANSACTIONS B
cryolite-AlF3-CaF2-Al2O3 bath system was modified by Li2CO3. When the heat balance is shifted between different steady states, responses of the mol
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