The exothermic mixture for lining high-temperature metallurgical units was developed using plant-derived waste. The composition of the mixture includes refractory clay, aluminum powder, magnesium sulfate (MgSO₄), and a silicon–carbohydrate ingredient as a source of amorphous silica and a burnout additive (PW) [1], as well as an organic binder OC-PW. The purpose of this study was to investigate the phase transformations of this mixture when heated up to 1200 ^оC using the thermal analysis (DTA/TG) method.

The studies were carried out on a synchronous thermal analyzer STA 449 F3 Jupiter (NETZSCH, Selb, Germany).  The results obtained using the STA 449 F3 Jupiter were processed using NETZSCH Proteus software.

For interpretation of thermal effects, thermal analysis of the novel (experimental) exothermic mixture (sample 1) was performed in comparison with a control mixture (without PW and OC-PW additives) (sample 2), and with samples based on the experimental and control compositions but without Al and MgSO₄ (samples 3 and 4, respectively). On the DTA curves of all four samples, a sharp endothermic effect with an extremum at 560 ^oC and a rather intense exothermic effect in the region (depending on sample composition) of 910–950 ^oC were observed. The combination of these effects can be interpreted as the manifestation of aluminosilicate. In the region near 560 ^oC, dehydration associated with hydroxyl groups and amorphization of the substance occur, while the peak at 910–950 ^oC corresponds to crystallization of amorphous phases.

In the DTA curve of sample 3, an endothermic effect with an extremum at 120 ^oC and an exothermic effect at 397 ^oC were observed. The first is related to the loss of free water, while the exothermic effect is due to the formation of new compounds and condensation processes in the carbonaceous material formed. Both effects are accompanied by mass loss on the TG curve.

A series of weaker thermal effects was recorded on the dDTA curves of all studied samples. Among these, two exothermic effects were notable, appearing only in samples 1 (at 649 and 932 ^oC) and 2 (at 649 and 802 ^oC), i.e., those containing Al and MgSO₄. These effects should apparently be considered together with a small endothermic effect on the DTA curve at 648 ^oC, also characteristic only of these samples. Although this temperature is close to the melting point of aluminum (660 ^oC), the effect most likely indicates a prereaction rearrangement between Al and MgSO₄. Local chemical interaction at the interface of these components requires energy absorption prior to the onset of the main exothermic reaction (effect at 649 ^oC). The endothermic effect at 648 ^oC may result from the formation of a transient complex phase (Al–Mg–O–S) or the melting/softening of MgSO₄ in the presence of Al. The second exothermic effect (at 802/932 ^oC) is associated with the formation of a mineral with an olivine structure (forsterite) [2]. Upon further heating, spinel (MgAl₂O₄) is formed [2]. Formation of spinel was also observed by the authors [3–4] during the production of refractories using rice husk silica at firing temperatures of 1050–1170 ^oC. This indicates that the products of the previous reactions continue to interact chemically at high temperatures. However, in samples 1 and 2, this process occurs at different temperatures (932 ^oC and 802 ^оC, respectively), likely due to the presence of organic components in the experimental mixture (sample 1). The combustion of organic components leads to the formation of a porous structure (the residual mass of sample 1 at 1174 ^oC was 80.4%, almost 11.5% lower than that of sample 2 at the same temperature). Since, as shown in [5], plant additives do not affect the phase composition during the production of thermal insulation materials based on clay, the presence of pores in the experimental sample 1 can be considered the reason for the shift of the spinel formation reaction to higher temperatures.

Thus, according to thermal analysis data, the developed composite mixture is exothermic. The shift of the main exothermic reaction to a higher temperature range during heating is caused by the retardation of heat transfer and reactivity due to microstructural changes. In other words, a weakly expressed (retarded) self-propagating high-temperature synthesis (SHS) occurs, making the process more controllable and reducing the risk of explosive behavior. Due to the high porosity, thermal stresses and cracking risks are reduced. As confirmed by corresponding experiments, the thermal shock resistance of the refractory material increases as a result.

This research was funded by the Science Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan (grant number AP 19677767).