SOME THERMODYNAMIC ASPECTS OF WO3 RECOVERY BY SILICON

Practical interest, with the goal of resource conservation, is the technology of arc welding with powder wire in which the fillers are used, the tungsten oxide is WO₃ , and the material (ferrosilicon) with a reducing agent is silicon. In the work, a thermodynamic estimate of the probability of nine standard reactions under tabular thermodynamic data of reagents was carried out in the temperature range from 1500 to 3500 K. Among the reactions, the reaction of direct reduction of WO₃ oxide by silicon and the reaction of a tungsten-silicon compound with the formation of tungsten silicides are considered. As possible products of the reactions were considered W, WSi₂ , W₅Si₃ . The reduction reaction of the oxide was recorded on 1 mole of O₂ , and the reaction of the compounds of tungsten with silicon – on 1 mole of W. The probability of reactions was estimated by their standard Gibbs energy. As standard for reagent substances in the range 1500 – 3500 K, the following states were selected: W_(solid) , WO_{3 (solid, liquid)}  with a phase transition at 1745 K, WSi_{2(s., l.)}  with a phase transition at 2433 K, W₅Si_{3(s., l.)} with a phase transition at 2623 K, Si_{(s., l.)} with a phase transition at 1690 K, SiO_(l.) , SiO_{2(s., l.)} with a phase transition at 1996 K. In order to assess the degree of effect on the thermodynamic properties of the possible evaporation reactions in the tungsten oxide WO3 arc, the thermodynamic characteristics of one of the reactions were calculated in which the WO_3(l.)  state was selected as the standard state in the same temperature range. Thermodynamic analysis shows that at the reduction of WO₃ the formation of silicides WSi₂ and W₅Si₃ is most likely, then tungsten. The thermodynamic probability of formation of these silicides due to the reactions of the tungsten-silicon compound in standard states turns out to be substantially lower. The reducing ability of silicon in reactions with the formation of SiO₂ decreases with increasing temperature, while in the reactions with formation of SiO, on the contrary, it increases. Consequently, in the system under consideration at high melt temperatures (more than 2500 K), a change in the composition of the gas phase due to the formation of SiO is more likely. At temperatures below 1750 K, the slag phase can become more acidic due to the resulting silicon oxide SiO₂ . The evaporation of WO₃ in the arc increases the thermodynamic probability of the reduction reactions occurrence, but more at a low temperature. 

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