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Summary The results of investigation of MgO participation in the binding of SO2 with lime-containing materials as sorbents are presented. Experiments of SO2 binding into solid phase using model samples of reactive grade MgO and CaO varying the mole ratio of MgO/CaO from 9:1 to 1:9 were carried out. Besides, dolomite and limestone samples with different MgO/CaO mole ratio (from 1.24 to 0.13) and samples of ashes formed at combustion of Estonian oil shale (containing 35-40% of carbonates) and its semicoke were studied Initial samples, intermediate and final products were subjected to chemical, IR-spectroscopy, X-ray and BET specific surface area analyses. The results of the present study confirmed the active participation of MgO in the binding of SO2 into the solid phase. In addition to CaSO4 the formation of Ca,Mg-double sulphate CaMg3(SO4)4 and ß-MgSO4 was observed. The presence of CaMg3(SO4)4 was fixed in a large temperature range 400-900°C and that of ß-MgSO4 in between 500-700°C. The optimum temperature range for formation and durability of CaMg3(SO4)4 was 700-800°C.

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Journal of Thermal Analysis and Calorimetry
I. Klimova
T. Kaljuvee
L. Türn
V. Bender
A. Trikkel
, and
R. Kuusik


In order to elucidate the influence of Ca and Mg carbonates with or without the presence of boron, manganese and copper compounds on the thermal stability of ammonium nitrate (AN), thermodynamic analysis of different reactions between AN and additives was carried out. Temperature dependency of Gibbs free energy changes ΔG T and equilibrium composition of reaction products were calculated for a set of reactions using the HSC software. Main solid compounds that can form in the systems of AN and carbonates, were Ca(NO3)2 and Mg(NO3)2, Ca(OH)2 and Mg(OH)2, CaO2 and MgO2, CaO and MgO, and N-containing gaseous compounds NO, N2O and NO2. As a result of H3BO3, MnO2 and CuSO4 addition, the content of CuO, Cu2O and MnO as solids and SO2, SO3 and HBO as gaseous reaction products reached the same level. Thereby, their equilibrium concentrations did not depend on the carbonate origin of CaCO3, MgCO3 or CaMg(CO3)2. Small amount of CuSO4, H3BO3 or MnO2 additive (0.01–0.05 mol) in the system, practically, did not influence the temperature dependencies of ΔG T of the reactions between AN and CaCO3 or CaMg(CO3)2. The influence of additives taken in the larger amount (0.5 mol) was evident and, depending on the additive and reaction, shifted their proceeding temperatures in either direction by more than 300–400 K.

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