Saint-Petersburg Mining University, Saint-Petersburg, Russia¹ ; Mining and petroleum department, Faculty of Engineering, Al-Azhar University in Cairo, Egypt²:

A. B. ElDeeb, Post – Graduate student, Metallurgy Department^1,2, e-mail: s175000@stud.spmi.ru

Saint-Petersburg Mining University, Saint-Petersburg, Russia:
V. N. Brichkin, Head of Metallurgy Department, Doctor of Technical Sciences, e-mail: Brichkin_VN@pers.spmi.ru
V. G. Povarov, Leading Researcher of the Center for Collective Use, Doctor of Chemistry Sciences, e-mail: povarov_vg@pers.spmi.ru
R. V. Kurtenkov, assistent, Metallurgy Department, Candidate of Technical Sciences, e-mail: Kurtenkov_RV@pers.spmi.ru

The results of studying the activating effect of carbon on the sintering performance of two-component limestone-kaolin mixture and subsequent hydrometallurgical processing of sinter are presented. Samples of charcoal and used anodes from aluminum electrolysis plants were added to the kaolin-limestone mixture in the range of 1–4% carbon of the charge mass. Briquetted mixtures were sintered in the established technological mode, with a constant heating and cooling rate of materials at an isothermal holding temperature in the range of 1250–1360 °С. The temperature of phase transformations and the values of thermal effects were estimated by using thermo gravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis. The phase composition of the sinter was studied by X-ray diffraction analysis (XRD) and the particle size distribution of the sinter samples formed as a result of the self-disintegration process was studied by laser microanalysis. Sintered samples of the obtained particle size and without additional grinding were subjected to soda leaching under the same technological conditions in terms of temperature and process duration, initial concentration of solid in the pulp, composition and concentration of the solution. The sludges obtained from leaching the sinters were filtered and washed with distilled water then examined by (XRD) analysis to determine the phase composition and by X-ray fluorescence spectrometry (XRF) analysis to determine the chemical composition. Extraction of alumina into the solution was evaluated by the Al₂O₃ content in sinter and sludge. The results of experimental studies showed that the largest increase in the extraction of alumina from kaolin ore is more than 7% with a carbon content in the charge from 1.5 to 3.0%, depending on the nature of the carbon material. Further increase of the corresponding carbon additive in the charge causes a decrease in Al₂O₃ extraction, which is associated with the formation of hard-to-open mullite. At the same time, the activating role of the carbon additive has a thermodynamic energy and kinetic nature, which explains a more complete assimilation of lime with the formation of end products and a decrease in the metastable stability of the  β-form C₂S. The latter causes its accelerated transition to a thermodynamically stable γ-modification of C₂S, increased self-disintegration of sinter, and noticeable amorphization of sintering products.
Studies were conducted with the involvement of the laboratory base of the Center for Collective Use of the Mining University.
The work was carried out with the financial support of the Russian Science Foundation under the Agreement No. 18-19-00577 of April 26 2018 of grant for fundamental scientific research and exploratory scientific research.

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