An approach to evaluation of the slag melt in the gas-slag-metal system report 2.
The chemical composition of the solidified solid slay does not always correspond to the composition of the molten slag. Therefore, the variation of the basicity of the slags is based on the evaluation of the static state of the solidified melt. However, the slag in the molten state is a mixture of active components which are in constant interaction. Consequently, using this procedure, we cannot evaluate the basicity (as the static property of the melt) and instead we can evaluate the properties of the components of the slag melt existing in the conditions of constant energy exchange not only with each other but also with the components of other phases which contact with the slag-gas and/or metallic phase.
Evaluation of the basicity of the slag melt is essential for determining the possibilities of interaction of the component of the slag with each other and, most importantly, with the components of the contacting phases (metallic and gas). However, this interaction process takes place at the temperatures of the metallurgical melts (approximately 1500- 2000[degrees]C). In these conditions, the compounds which the investigators can find in the solidified melt do not exist in the melts.
The determination of the actual composition of a fluoride-oxide melt can be carried out (with specific conditions) by the methods of high-temperature x-ray diffraction investigations and analysis of the local structure of the atoms of the melt (1). The method is suitable for investigating the artificially developed conditions, simulating the actual conditions of existence of the liquid slag melt but cannot be used for investigating the actual metallurgical processes.
The chemists, concerned with low - temperature interactions, do not use the term basicity. Instead, they use the concept of the activities of the elements of the solution, for example, pH level which determine the capacity of a solution for the formation in the solution of a free hydrogen ion. The level of pH is used to determine the acid or alkaline solution, i.e., not as much the content of the components of the solution as the activity of the free ions which may form as a result of the exchange reactions in the solution and take part in the interactions.
The slag melts as high-temperature solutions according to the chemistry laws should be characterised not by the composition of the initial components formed as a result of the metallurgical process in the solidifying slack, but should be characterised by the activity (the capacity to enter interaction) of the free elements of the melt. For example, one of the reasons for corrosion of metals in ionic melts may be the active (free) oxygen or halogen ions (2), (3).
In the case of pyrometallurgical processes in which the slag is used, we should evidently talk about the activity of free oxygen because metallurgical slags are the oxide or fluoride-oxide systems.
The free oxygen ion may form in metallurgical slags which are an integral part of the gas-slag-metal ternary system, as a result of the exchange reactions or in the slag, between the slag components with the metal and gas that are in contact with these component.
The exchange reactions may take place between the basic and amphoteric oxides since the acid oxides can form with the basic oxides the same complex compounds which are difficult to disrupt (4), (5).
It is necessary to determine the ratio of the amount of the basic and other oxides for ensuring specific exchange reactions. In the case of the slag-gas system there is interaction inside the slag and between the slag and the gas; in the gas-slag-metal system--inside the slag and between the slag, the gas, and the metal. When using a complex, the determination of the reasons is more complicated and, most importantly, the oxidation potential depends not only on the partial pressure of oxygen in the gas atmosphere but also on the possibilities of formation of free oxygen in the slag melt (6), (7).
The formation of the free oxygen ions in the slag melt can take place only in the presence of the necessary ratio in the slag of the oxides of calcium, aluminium and silicon (in normal slag systems). This ratio determines the amount of the basic and amphoteric oxides in relation to the acid oxides and enables the formation of free oxygen ions or prevents this formation.
(1.) Shpak A.P. et al., Structural special features of the melts of oxide systems, Akademperiodika, Kiev, 2003
(2.) Delimarskii Yu.K., Electrochemistry of ionic melts, Metallurgiya, Moscow, 1978.
(3.) Novokhatskii I.A., Gases in oxide melts, Metallurgiya, Moscow, 1975.
(4.) Burdonov B.A., Distribution of oxygen between the metal and slag with different compositions, in: Physical-chemical fundamentals of the processes of melting of steels and alloys, Metallurgiya, Moscow, 1985, 73-77.
(5.) Konishchev B.P., Evaluation of the basicity of oxides taking into account differences in the affinity of acid and basic oxides, in: Advanced methods of welding, new materials and constructions in welding production, 1979, No. 3, 38-42.
(6.) Novikov V.K., et al., Izv. AN SSSR, Metally, 1979, No. 3, 38-42.
(7.) Arsent'ev P.P. and Koledov L.A., Metallic melts and their properties, Metallurgiya, Moscow, 1976.
V.V. Lakomskii, G.M. Grigorenko and L.N. Chubov
E.O. Paton Electric Welding Institute, Kiev