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By Academician Mikhail SCHULTZ, Vladimir KONAKOV, Dr. Sc. (Chem.), RAS Institute of Chemistry of Silicates named after Grebenshchikov
One key factor in the development of chemical production is continuous control of parameters which reflect the complexity and nature of the on-going reactions. As compared with a century ago when these parameters were the time, temperature and pressure, in our day and age still greater importance is being attached to parameters reflecting the composition and chemical structure of the initial, intermediate and final products.
Being constantly monitored in the course of technological processes is a range of physico-chemical properties of the substances involved. For water solutions the most common factor is the pH index as reflecting the activity of hydrogen ions. In neutral solutions pH=7, in acidous it is lower, and in alkaline ones it is greater. This parameter is being used in areas like research and technological chemistry, biology, medicine, crime detection and food industry. Thus it has become part and parcel of our daily life.
As for high-temperature chemical production technologies, of primary importance is the determination of acidic-alkaline properties of oxide alloys for the assessment of the physico-chemical processes involved, their prognostication and for obtaining materials with preset properties. This factor is important, among other areas, for glass production (sintering, caking of batch, founding and casting of glass, interaction of the glass melt with the inner lining of the furnace), metallurgy (a wide range of equilibriums between metal, slag and reactive atmosphere-decarboxilation, desulphurization, dephosphorization, etc., slag interaction with refractory materials of equipment), production of cement (monitoring gas atmosphere, laboratory control of alkalinity).
But as different from aqueous solutions, for oxide melts there is still no common concept which would make it possible to interpret the chemical interactions taking place within them from the point of view of acidic-alkaline theory. Incidentally, a similar situation with respect to aqueous solutions existed at the start of the 20th century. At that time the progress of industry set the task of elaborating methods of continuous monitoring of chemical processes within the temperature range of the existence of these materials. The solution of this problem resulted in the acceptance on an international scale of the range of pH values and the development of the appropriate control devices.
But nevertheless direct application of the results obtained for water solutions with respect to oxide melts is impossible because the latter have many basic distinctions (low dielectric permeability, high viscosity, broad bank of temperature "existence", etc.). At the same time the experience accumulated in the process of determination and norm fixing of pH values-the development of the principles of scale building, methods of standards introduction and the identification of the corresponding states, development of the acidic-alkaline concept, etc.-make it possible to look forward to a successful solution of the similar problem for oxide melts.
As the "bearer" of their acidic-alkaline properties one can use the pO parameters, the so-called oxygen factor, in as much as it is connected with
Scale of base value of oxides and structural-chemical groups for 950 o C: a-borates; b-germanates; c-silicates. lgK 4 -dissociation constant with break-off of oxygen ion.
the activity of oxygen ions. In ferrous and non-ferrous metallurgy it determines the equilibrium between metal and slag, and in the physical chemistry of glass it influences the polymerization and depolymerization of the structures of glass-forming materials, the formation in the glass lattice of element-oxide groups, oxidation- reduction equilibrium, etc. What is more, the effectiveness of production processes of metals, glass and cement depends to a large extent on the resistance of refractories used in the industrial equipment to the impact of oxide melts. Of no little importance in this respect are the differences in the acidic-alkaline properties of both.
Thus, one of the central problems of high-temperature physical chemistry consists in the formulation and development of the general theoretical notions about the properties of oxide melts in terms of acids and bases. And in practical terms, it is important to achieve quantitative determinations of their acidic-alkaline characteristics.
There exist different approaches to the description of the interactions involved. Their analysis has made it possible for us to suggest a generalized theory based on the principle of dissociation. In other words, it is assumed that any substance in a liquid state can undergo processes of dissociation - the splitting-off of positive or negative ions. And that means that oxide melts are regarded as liquids which add on, or split off oxygen ions with two negative charges in the course of acid-alkaline transformations. And since oxygen is present in all oxide melts, that means that the proposed approach is applicable to the description of the acidic-base properties of them all.
For building the pO scale it was necessary to choose the standard state, or condition. With this aim in view we suggested the one corresponding to pure silicon dioxide since its structure and physico-chemical properties have been studied in the greatest details and since most of the industrial glasses, metallurgical slags, natural minerals and oxide coatings have a silicate base.
The next step was an experimental assessment of pO values of other oxide materials and the modelling of experimental results. It turned out that of the greatest practical value are binary silicate glasses and melts containing oxides of lithium, sodium and potassium, since they are basic elements for many industrial and special glasses. Also of interest are germanate and borate melts in view of their increased acidity (as compared with silicon) in such systems of oxide-vitriforms.
A special place in the studies of melts belongs to systems Na 2 O- B 2 O 3 -SiO 2 . Their distinct feature consists in the fact that added to the interacting mixture of the two typical vitriforming oxides is the base of sodium oxide. Because of that there starts in the system a "competition" between them for the priority interaction with the latter. And its nature depends on the ratio of acidic-base properties of the vitriforms (glass producers).
The result of the latest investigations has been an experimental alkalinity (basicity) scale of the structural-chemical groupings and oxides. The physico-chemical aspects of it have been elaborated to a sufficient extent which makes it possible to move on to the next stage-development of state (federal) standards of measurements of the pO factor. The introduction of this type of monitoring in areas like ferrous and non- ferrous metallurgy, glass manufacture and, to a certain extent, in the production of cement, will pave the way to a technological breakthrough, comparable with the effect of the introduction of the pH scale for water solutions.
Illustrations supplied by the authors.
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