153 154
Hammet and Loren and Frumkin have showed that the rate of hydrogen evolution on a sodium amalgam surface is essentially the same as on a mercury surface polarised electrically to a potential equal to that established by the sodium-sodium ion equilibrium. The rate dependence on the amalgam concentration raised to the power 0,5 then follows directly from the Tafel equation using polarisation data for mercury surfaces at low current densities. This approach also predicts that the reaction . velocity should vary inversely as the same fractional power of sodium
149
ion activity. Bronsted and Kane did not carry out any quantitative work but did report that the addition of sodium chloride to neutral and alkaline solutions decreased the reaction velocity.
In the previous experimental work mentioned, it was normal practice 158 T 60
to use a glass reaction vessel. Frumkin ’ “ noted that the decom position rates in alkaline solutions when carried out in a polystyrene cell were markedly lower than those carried out in a glass vessel. The relationship observed by Bronsted end Kane and the conclusions drawn from it are therefore only applicable in alkaline solutions where the reaction is aided by catalytic effects.
The mechanism of hydrogen evolution at mercury and amalgam cathodes 159
in alkaline solutions has been examined by Bockris and Watson . Mercury, in an alkaline solution, was polarised with a definite current
density until the rate of decomposition of the amalgam formed became equal to the rate of metal cation discharge, and hence the amalgam po tential reached its limiting negative value. The current density measured under such conditions is a measure of the hydrogen evolution rate. It was concluded that the slowest step in amalgam decomposition under these
conditions was the chemical interaction of sodium with water. ^"Na-^Hg + H2° (Na )eo0 + (0E )ho0 + HAds,
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Frumkin has carried out a similar electro chemical study of amalgam decomposition using polystyrene cells. He concluded that hydro gen evolution from alkaline solutions on mercury, at pH less than 10 was due to hydrogen ion discharge and at pH greater than 10 to a chemical interaction between sodium and water molecules. Hie rate of direct
water molecule discharge was small compared with the other two mechanisms. Experiments in the pH range 2 to 10 showed that the rate of hydrogen gas evolution was directly proportional to the hydroxonium ion concentration.
The decomposition of sodium amalgam in aqueous media has been used 82
as a model for slag-metal reactions by Shanahan . A model involving
y H
the reaction of 0.2 Ha amalgam with ^ sulphuric acid solution was employed to assess the efficiency of mixing produced by various agitation techniques.
3*1 Choice of model system
It was noted in the previous chapter that a number of workers have
QQ Qrt. 9 already attempted to develop room temperature models of the L.D. process * * All of these models involved a reaction between the gas jet and a solute
in the bath, rather than a reaction between the slag phase and the bath. This is a serious criticism in view of the work of Meyer and Trentini 11, 16, 21, 49 > 50> 51 wk0 }lave shown that a slag foam exists within the converter during much of the blow and that a significant proportion of the metal in the converter is suspended as droplets in the foam.
It appears that reactions between the droplets and the foam play an important role in the overall refining process. If this is so, it is essential that these reactions are replicated in any room temperature model that is used to study the basic oxygen steelmaking process.
Of the various reactions that occur between the metal and the slag, it is the oxidation of carbon that has the greatest influence on the form ation of the slag foam since it is this reaction that produces the carbon monoxide gas in the foam. Hence, an essential requirement for any model of the process is that it should reproduce the production of gas during a reaction between the metal and the slag. The choice of suitable model systems is however very limited. The reaction between sodium, dissolved in mercury, and an acidified aqueous pha.se provides a room temperature system which does meet the above requirements,
It is this system that has been used as a basis for the model developed in this work.
Mercury provides a suitable solvent for sodium, whilst also being effectively inert and immiscible with the aqueous phase. Just as slag floats on steel, the aqueous phase will float on mercury, although the phase density ratio is somewhat larger in the model than in steelmaking.
The important physical properties of mercury and water are compared with those of a typical L.D. slag and iron in Table 1. Additions of sodium, do however tend to lower both the density and the surface tension of
mercury The maximum solubility of sodium in mercury at room temperature is O.65 wt.^(Figure 7)«
The viscosity of water can be increased significantly by additions of glycerol, a factor made use of by a number of workers studying model systems involving an aqueous phase *^9. The viscosity ratio for the slag and metal in steelmaking is about 10:1. A 60 vol.$ glycerol-water mixture gives a similar ratio, for the phases in the model system. Figures 8 (a), (b), (c) and (d) show the variation of viscosity, density, surface tension and interfacial tension with mercury, for various glycerol-water mixtures. The interfacial tension between mercury and water-glycerol solution is relatively high, just as it is for slag-metal interfaces. This has been shown to be an important consideration when modelling slag-metal systems Glycerol is chemically inert, with respect to the components in the present model system, it is completely
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