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Study on the separation process of aluminum electrolysis waste cathode carbon and electrolytic float

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Tutor: XieGang
School: Kunming University of Science and Technology
Course: Non-ferrous metallurgy
Keywords: spent potlining of aluminum electrolysis,floatation,recycle
CLC: X758
Type: Master's thesis
Year:  2013
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When piled up in the open air, spent potlining created during overhaul of aluminum electrolysis cell pollutes environment because toxicants in it such as cyanide and soluble fluoride permeate into the earth with rainwater. However, both of carbon and fluorides, which are chief components of spent potlining, are industrial valuable materials worth recovering. If valuable ingredients in spent potlining could be recycled, it will be undoubtedly turning "waste" into wealth.In this paper, carbon and electrolyte, which were main components of spent potlining, were separated efficiently by flotation so that they could be recycled separately. Besides, after being detoxified, waste water produced in the flotation was innocuous to the environment. Floatation, which had a short technological progress, a low cost and a prominent economical benefit, was easy to be industrialized. In order to improve the floatation results, the chemical and physical characters of spent potlining provided by an aluminum manufacturer in Yunnan province were studied. And technological conditions together with floatation reagent system of floatation were optimized.X-ray diffraction figures showed that the main components of spent potlining were C、Al2O3、Na3AlF6、NaF and a small quality of SiO2、Fe2SiO4、NaAlSiO4(nepheline) and NaAl7O11(β—alumina) etc., most of which could be divided into two categories i.e. carbon and electrolyte. Elemental analysis, microstructure, burning characteristics and crushing and grinding characteristics were studied on spent potlinings with different service life, which were particularly SPL1(1016days), SPL2(1560days) and SPL3(1936days). The results showed that as the service life increased, the carbon content decreased and the electrolyte content increased, moreover, dissemination particle size of electrolyte in the carbon substrate became smaller and more complex and crushing and grinding of spent potlining became more difficult. Carbon and electrolyte in spent potlining with a short service life could be separated effectively, but spent potlining with a long service life was hard to be recycled by floatation. In order to improve floatation results, SPL3with a carbon content36.1%was chose as the researching target and the floatation progress of SPL3 was studied in detail.The influence on floatation results of raw-feed particle size, rotating speed of floatation machine, slurry density and the stirring time after collecting agent were added were studied so that the optimum conditions could be obtained. The results showed, the best particle size of raw feed was-200mesh particles taking90percents, the best rotating speed of floatation machine was from1600to1800rpm, the best slurry density was from25%to33%and the stirring time after collecting agent was added should be less than10minutes.Characteristics of three different collecting agents, adding collecting agent by several times, emulsification of collecting agent and mixing of different collecting agents were studied. Results revealed that among the three collecting agents, gasoline showed the best selectivity and the worst collecting ability. And light diesel oil showed the worst selectivity and the best collecting ability, while selectivity and collecting ability of kerosene was in the middle. Adding collecting agent by several times or using emulsified kerosene as collector could improve the floatation results. Moreover, the emulgators could not only promote the emulsification of kerosene but also improve the carbon content and recovery of concentrate. The mix of gasoline and kerosene performed better than the mix of gasoline and light diesel oil, and when the mass fraction of gasoline in the mix was75percent, both of the mixes performed the best.The influences on floatation of foaming agents, dispersing agents and pH regulators were also studied. The results showed that, by adding foaming agents into the slurry, carbon content of rough concentrate could be increased and floatation rate could be enhanced, while at the same time carbon recovery could be kept. Terpenic oil could be used as foaming agent and the suggested dosage was20g/t. Adding sodium silicate into the rod mill while grinding could increase carbon content of rough concentrate, and the recommended dosage was500g/t. Dispersing agents could improve carbon content in rough concentrate slightly and reduce carbon recovery of rough concentrate remarkably, therefore, the adding of dispersing agents was not necessary during production. The function of pH regulators was the same to that of dispersing agents; the adding of pH regulators was also not preferable in production. The optimum condition was fixed after a large number of experiments, and the closed floatation flow sheet of single-stage roughing, single-stage scavenging and two-stage cleaning was adopted. The floatation results were as follow:carbon content in concentrate was82.3%, carbon recovery of concentrate was88.5%, carbon content in tailings was6.8%and electrolyte recovery of tailings was89.3%. Compared to the exploratory-experiment result, the optimum-condition-experiment results were obviously improved. Particularly, carbon content in concentrate was increased by9.2%, carbon recovery of concentrate was increased by3.3%, carbon content in tailings was decreased by18.8%and electrolyte recovery of tailings was increased by21.9%. Through researching on floatation progress of SPL3with a carbon content of36.1%, the paper proved the feasibility of recycling spent potlining with low carbon content by floatation and improved the flotation results. Besides, the researching results could also be used for reference while researching floatation progress of coal and graphite.
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