Saint Petersburg Mining University, Saint Petersburg, Russia:

V. N. Brichkin, Head of Department, Doctor of Engineering Sciences
V. V. Vasiliev, Associate Professor, Candidate of Engineering Sciences, Vasilev_VV@pers.spmi.ru
I. S. Bormotov, Post-Graduate Student
R. I. Maksimova, Student

The use of hydrochemical, biochemical, hydrometallurgical and thermochemical methods is an integral part of modern technological approaches to processing of rebellious natural and manmade raw materials. Within the framework of the hydrochemical conversion method, it becomes possible to rationalize the use of lime in the composition of middlings and waste in production of chemically precipitated calcium carbonate, which has a multipurpose purpose and is of interest for repeated and multiple use, including reduction of technology-related greenhouse gas emissions. This work presents the experimental results on soda conversion of tricalcium hydroaluminate (TCGA), which creates favorable prospects for enhancing recovery of aluminum from alkaline solutions, raising production completeness and increasing efficiency of caustic alkali turnover. The laboratory studies used highprecision process and analysis equipment, including: LabSys Evo derivatograph from Setaram, carbon analyzer of TOC-L analyzer (Shimadzu) with an attachment for measurements in solid samples SSM-5000A, single- and multi-reactor systems of parallel synthesis Auto-LAB and Auto-MATE II by HEL and others. It is found that soda conversion of TCGA proceeds at the maximum completeness at a temperature of 95 °C, total alkali concentration in the solution of 70 g/l with respect to Na₂O and at the molar ratio of (Ca₃Al₂(OH)₁₂)/(Na₂CO₃) = 1/3. These conditions ensure the alumina recovery at the level of 88–89% and the corresponding degree of TCGA conversion. The fundamental possibility of obtaining aluminate solutions with reduced caustic modulus, satisfying the conditions of their decomposition and providing the possibility of partial replenishment of caustic alkali losses in the technological cycle of alumina production, has been substantiated and experimentally proved.
The research was carried out using the laboratory facilities of the Center for Collective Use and the Scientific Center for Problems of Processing Minerals and Industrial Waste of the Mining University.
The study was supported by the Russian Science Foundation, R&D Grant Agreement No. 18-19-00577 date 26 April 2018.

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