Empress Catherine II St Petersburg Mining University, Saint Petersburg, Russia

A. B. Lebedev, Research Fellow at the Research Centre for Resources Processing, Candidate of Technical Sciences, e-mail: 2799957@mail.ru
V. Yu. Bazhin, Professor, Head of Metallurgy Department, Doctor of Technical Sciences, e-mail: bazhin-alfoil@mail.ru
I. T. Zhadovskiy, Associate Professor at the Department of General and Physical Chemistry, Candidate of Chemical Sciences, e-mail: sergaur@mail.ru 

Polymorphism typical of calcium orthosilicate is a phenomenon that is being studied more and more widely replacing the energy consuming crushing process. Polymorphous β and γ transitions of Ca₂SiO₄ take place as sinters, which contain the corresponding modifications of orthosilicate or pyroaluminate of calcium, are cooling down and then self-disintegrate. The latter process is used in alumina industry. The process duration should be limited to an optimum indicator. To solve the above problem, the authors examined the following modifications of calcium orthosilicate: α, α', β, γ. Differential thermal analysis revealed the existence of 5 phase modifications: α, α', α'', β, γ. It was confirmed that the difference between β and γ phases is due to the arrangement of tetrahedrons [SiO₄]⁴⁺ relative to the Ca²⁺ cations in the calcium orthosilicate lattice. The two structures have different lattice symmetry and parameters and correspondingly, different physical and chemical properties. The stable modification takes place at the temperatures of 539–550 ^oC and has the lowest free energy. The paper describes a relationship between the free energies of β and γ modifications and the temperature. It was found that a new phase is completely formed in 30 min at 350 oC. The selfdisintegration temperature of pure calcium orthosilicate was established (i.e. 525 ^oC). A dropping temperature model was built. It was found that the less the principal quantum number W_n of the foreign cation, its valency and electronegativity are and the more the atomic radius is, the weaker the stabilizing properties are. The total thermal effect was determined with belite formation – 100,273.86 J/mol. The thermal effect of the polymorphous β → γ-Ca₂SiO₄ transition with selfdisintegration is 5,568.44 J/mol higher. The practical value of this factor is that it determines the specific area of the self-disintegrated powder and the microstructure of its grains. Total savings may amount to 8 bln roubles per year. Rapid cooling of the sinter entails supercooling of the β phase. As a result, the self-disintegration process is associated with 60% belite and 30% aluminate.

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