Thermochemical Processes Episode 13

The refining of metals 353 show that CS for FeO-containing slags is somewhat higher than that of CaO-rich slags reflecting the Gibbs exchange energies FeO C .1/2/S2(g) D FeS C .1/2/O2(g); logK D 2.59 CaO C .1/2/S2(g) D CaS C .1/2/O2(g); logK D 2.30 G1850 D 91500kJmol 1 G1850 D 81500kJmol 1 Entering into the magnitude of the sulphide capacities is the fact that the FeO–SiO2 system is Raoultian, while the CaO–SiO2 system shows strong negative departures from Raoult’s law. Finally, the activity coefficient of sulphides in solution in slags is inversely related to the oxide activity, and hence in the equilibrium constant for the sulphide forming reaction MSfMSgpO1/2 MOfMOgpS1/2 the activity coefficient ratio sulphide/oxide is very much higher in the case of the calcium ion than for the ferrous ion, to an extent which more than balances the equilibrium constant which appears to favour the calcium ion. Using the method described above, the sulphide capacity of a multicom-ponent slag may be calculated with the exchange oxide/sulphide equilibria weighted with the metal cation fractions, thus logCS.Fe2C,Mn2C,Ca2C,Mg2C/ D X.Fe2C,Mn2C,Ca2C,Mg2C/ logCS.Fe2C,Mn2C,Ca2C,Mg2C/ Similarly the removal of phosphorus from liquid iron in a silicate slag may be represented by the equations 2[P] C 5[O] C 3fO2 g ! 2fPO4g; logK D ixi logKi (i D Ca2C, Fe2C etc. which are the cationic species in the slag phase). Fellner and Krohn (1969) have shown that the removal of phosphorus from iron–calcium silicate slags is accurately described by the Flood–Grjotheim equation with logK.Ca2C/ D 21 and logK.Fe2C/ D 11 and concluded that the term in x.Ca2C/logK.Ca2C/ is the only term of impor-tance in the dephosphorizing of iron. 354 Thermochemical Processes: Principles and Models The thermodynamics of dilute solutions Many reactions encountered in extractive metallurgy involve dilute solutions of one or a number of impurities in the metal, and sometimes the slag phase. Dilute solutions of less than a few atomic per cent content of the impurity usually conform to Henry’s law, according to which the activity coefficient of the solute can be taken as constant. However in the complex solutions which usually occur in these reactions, the interactions of the solutes with one another and with the solvent metal change the values of the solute activity coefficients. There are some approximate procedures to make the interaction coefficients in multicomponent liquids calculable using data drawn from binary data. The simplest form of this procedure is the use of the equation deduced by Darken (1950), as a solution of the ternary Gibbs–Duhem equation for a regular ternary solution, A–B–S, where A–B is the binary solvent lnS.ACB/ D XA lnS.A/ C XB lnS.B/ G.ACB//RT Here, the solute S is in dilute solution, and the equation can be used across the entire composition range of the A–B binary solvent, when XA C XB is close to one. When the concentration of the dilute solute is increased, the more concentrated solution can be calculated from Toop’s equation (1965) in the form lnS.ACB/ D XB/.1 XS/lnS.B/ C XA/.1 XS/lnS.A/ .1 XS/2G.ACB//RT This model is appropriate for random mixtures of elements in which the pair-wise bonding energies remain constant. In most solutions it is found that these are dependent on composition, leading to departures from regular solu-tion behaviour, and therefore the above equations must be confined in use to solute concentrations up to about 10 mole per cent. When there is a large difference between S.A/ and S.B/ in the equation above, there must be significant departures from the assumption of random mixing of the solvent atoms around the solute. In this case the quasi-chemical approach may be used as a next level of approximation. This assumes that the co-ordination shell of the solute atoms is filled following a weighting factor for each of the solute species, such that nS A/nS B D XA/XB exp[ .Gexchange//RT] where nS A and nS B are the number of S–A contacts and S–B contacts respectively, and the Gibbs energy of the exchange reaction is for B S C A ! A S C B The refining of metals 355 The equation corresponding to the Darken equation quoted above is then [1/S.ACB/]1/Z D XA[.A.ACB///.S.A//]1/Z C XB[.B.ACB///.S.B//]1/Z In liquid metal solutions Z is normally of the order of 10, and so this equation gives values of S.ACB/ which are close to that predicted by the random solu-tion equation. But if it is assumed that the solute atom, for example oxygen, has a significantly lower co-ordination number of metallic atoms than is found in the bulk of the alloy, then Z in the ratio of the activity coefficients of the solutes in the quasi-chemical equation above must be correspondingly decreased to the appropriate value. For example, Jacobs and Alcock (1972) showed that much of the experimental data for oxygen solutions in binary liquid metal alloys could be accounted for by the assumption that the oxygen atom is four co-ordinated in these solutions. The most important interactive effect in ironmaking is the raising of the activity coefficient of sulphur in iron by carbon. The result of this is that the partition of sulphur between slag and metal increases significantly as the carbon content of iron increases, thus considerably enhancing the elimination of sulphur from the metal. Other effects, such as the raising of the activity coefficient of carbon in solution in iron by silicon, due to the strong Fe–Si interaction, have less effect on the usefulness of operations at low oxygen potentials such as those at carbon-saturation in the blast furnace. The effect of one solute on the activity coefficient of another is referred to as the ‘interaction coefficient’, defined by dlnA D εA; B lnA D lnA C εBXB where °A is the activity coefficient of component A at infinite dilution in the binary M–A system, M being the solvent. The self-interaction coefficient of solute A, εAA, represents the change of the activity coefficient of solute A with increasing concentration of the solute A. In industrial practice, the logarithm to the base 10, together with the weight per cent of the components is used rather than the more formal expressions quoted above, and so the interaction coefficient, e, is given by the corresponding equation logA D log° C eA[%B]...etc. Table 14.1 shows some experimental data for the interaction coefficients in iron as solvent. The linear effect of the addition of the solute B only applies over a limited range of composition, probably up to 10wt% of the solute B, because this is the limit of the composition range beyond which the solute will begin to show departures from Henry’s law. 356 Thermochemical Processes: Principles and Models Table 14.1 Interaction coefficients of solutes (ð102) in liquid iron at 1850K Solute A C Added element B O S N Si Mn C 22 10 9 O 13 20 9 S 24 18 3 Si 24 25 5.5 11 10 – 5.5 14 0 3 6.5 2.5 9 32 0 Numbers rounded to the nearest 0.5. The refining of lead and zinc The metals that are produced either separately or together, as in the lead–zinc blast furnace, contain some valuable impurities. In lead, there is a signifi-cant amount of arsenic and antimony, as well as a small but economically important quantity of silver. The non-metals cause the metal to be hard, and therefore the refining stage which removes them is referred to as lead ‘soft-ening’. This is achieved by an oxidation process in which PbO is formed to absorb the oxides of arsenic and antimony, or, alternatively, these oxides are recovered in a sodium oxide/chloride slag, thus avoiding the need to oxidize lead unnecessarily. The thermodynamics of the reactions involved in either of these processes can be analysed by the use of data for the following reactions .4/3/As C 2PbO D .2/3/As2O3 C 2Pb; and .4/3/Sb C 2PbO D .2/3/Sb2O3 C 2Pb; G° D 25580 82.6TJ G° D 27113 41.7TJ In the case of the direct oxidation, the oxygen partial pressure must be greater than that at the Pb/PbO equilibrium, while in the process involving sodium-based salts, the oxygen pressure is less than this. The two equilibrium constants for the refining reactions KAs D a2/3fAs2O3g/a4/3[As] and KSb D a2/3fSb2O3g/a4/3[Sb] (since aPb D aPbO D 1/ show that the relative success of these alternative processes depends on the activity coefficients of the As and Sb oxides in the slag phase. The lower these, the more non-metal is removed from the metal. There is no quantitative information at the present time, but the fact The refining of metals 357 that the sodium salts of oxy-acids are usually more stable than those of lead, would suggest that the refining is better carried out with the sodium salts than with PbO as the separate phase. Another metal which accompanies silver in blast furnace lead is copper, which must also be removed during refining. This is accomplished by stir-ring elementary sulphur into the liquid, when copper is eliminated as copper sulphide(s). The mechanism of this reaction is difficult to understand on ther-modynamic grounds alone, since Cu2S and PbS have about the same stability. However, a suggestion which has been advanced, based on the fact that the addition of a small amount of silver (0.094wt%) reduced substantially the amount of lead sulphide which was formed under a sulphur vapour-containing atmosphere, is that silver is adsorbed on the surface of lead, allowing for the preferential sulphidation of copper. The removal of silver from lead is accomplished by the addition of zinc to the molten lead, and slowly cooling to a temperature just above the melting point of lead (600K). A crust of zinc containing the silver can be separated from the liquid, and the zinc can be removed from this product by distillation. The residual zinc in the lead can be removed either by distillation of the zinc, or by pumping chlorine through the metal to form a zinc–lead chloride slag. The separation of zinc and cadmium by distillation An important element that must be recovered from zinc is cadmium, which is separated by distillation. The alloys of zinc with cadmium are regular solutions with a heat of mixing of 8300 XCdXZn Jgram-atom 1, and the vapour pressures of the elements close to the boiling point of zinc (1180K) are pZn D 0.92 and pCd D 3.90atmos The metals can be separated by simple evaporation until the partial pressure of cadmium equals that of pure zinc, i.e. p° dCdXCd D p°ZnZnXZn and using these data the zinc mole fraction would be 0.89 at 1180K. It follows that the separation of cadmium must be carried out in a distil-lation column, where zinc can be condensed at the lower temperature of each stage, and cadmium is preferentially evaporated. Because of the fact that cadmium–zinc alloys show a positive departure from Raoult’s law, the activity coefficient of cadmium increases in dilute solution as the temperature decreases in the upper levels of the still. The separation is thus more complete as the temperature decreases. A distillation column is composed of two types of stages. Those above the inlet of fresh material terminate in a condenser, and are called the ‘rectifying’ ... - tailieumienphi.vn