Thermochemical Decomposition of Calcium Sulfate in Presence of Carbon and Hydrogen Mix

Results of research of thermodynamic modeling of thermochemical decomposition of CaSO4 by a mix of carbon and hydrogen are given in the article. Influence of temperature on degree of carbon interchange by hydrogen on a degree of formation in systems CaO, 3, CaS was determined. Interchange of C by 2 reduces formation of 3, CaS, and formation of has an extreme character. With the help of method of research planning adequate equation of regression has been defined and optimum technological parameters ( = 1660 and 40 % degree of carbon interchange by 2), providing full decomposition of CaSO4 to were determined. The composition of a gas phase of CaSO4 restoration, containing 16 components was established. By reduction of pressure to 0,001 P it is possible to lower temperature of process to 1400 . The chemical equation of CaSO4 interaction with carbon and hydrogen was found.


Introduction
At heat treatment of phosphogypsum in presence of natural coals basic restoration CaSO 4 up to CaO occurs according to reaction [1]: 2CaSO 4 + C = 2CaO + 2SO 2 + CO 2 (1) The structure of flying coals includes hydrogen [2,3] which also restores CaSO 4 : CaSO 4 + H 2 = CaO + SO 2 + H 2 O The purpose of the present research is definition of regularities of restoration CaSO 4 by mixtures containing carbon and hydrogen. Research was carried out by a method of thermodynamic modeling employing program complex Astra [4] based on the fundamental principle of entropy maximum [5,6]. The basis of the information on a database consists of thermodynamic properties of the individual substances systematized at the Institute of high temperatures of the Russian Academy of Sciences and the USA National bureau of standards [7,8].

Experimental
The research was carried out at temperature interval of 500 -1800 K at pressure (P) 0,1 MPa. Proceeding from [9] initial quantity of carbon moles in system comprised 1,5. Influence of the degree of carbon moles interchange by hydrogen (Ȗ) on the degree of formation CaO was determined in the work.
In figure 1 the information about influence of Ȗ and temperature on the process, from which it follows, that the basic compounds in systems are CaCO 3 , CaSO 4 , CaS and CaO. CaCO 3 is formed at low temperatures. So, at Ɍ = 500 K the degree of CaCO 3 formation changes as follows:

Research results
To define the optimum parameters of restorating CaSO 4 to CaO in CaSO 4 -C-H 2 system we carried out the research using rototable plan of the second order [9]. As independent variables Ȗ and Ɍ have been used. Parameters of the plan and the matrix of research planning are given in tables 1 and 2.
The composition of the gas phase of the system at Ȗ = 40 % is given in figure 5 from which it follows, that SO 2 , H 2 S, CO 2 , H 2 O and CO are the principal components of the gas phase. At Ɍ = 1600 K the gas phase, for example, contains 16 substances: 24,02 % ɋɈ 2 ; 54,15 % SO 2 ; 16,73 %