Thermochemical Processes Episode 12

Chapter 13 Extraction metallurgy The important industry for the production of metals from naturally occurring minerals is carried out at high temperatures in pyrometallurgical processing, or in aqueous solutions in hydrometallurgical extraction. We are concerned only with the former in this book, and a discussion of hydrometallurgy will not be included. The pyro-processes are, however, also mainly concerned with liquids, in this case liquid metals and molten salt or silicate phases. The latter are frequently termed ‘slags’, since very little profitable use has been found for them, except as road fill or insulating wool. The processes are invariably designed to obtain relatively pure liquid metals from the natural resource, sometimes in one stage, but mainly in two stages, the second of which is the refining stage. The movement of atoms in diffusive flow between the liquid metallic phase and the molten salt or slag, is there-fore the most important rate-controlling elementary process, and the chemical reactions involved in high-temperature processing may usually be assumed to reach thermodynamic equilibrium. The diffusion coefficients in the liquid metal phase are of the order of 10 6 cm2 s 1, but the coefficients in the molten salt and slag phases vary considerably, and are structure-sensitive, as discussed earlier. As in the case of the diffusion properties, the viscous properties of the molten salts and slags, which play an important role in the movement of bulk phases, are also very structure-sensitive, and will be referred to in specific examples. For example, the viscosity of liquid silicates are in the range 1–100 poise. The viscosities of molten metals are very similar from one metal to another, but the numerical value is usually in the range 1–10 centipoise. This range should be compared with the familiar case of water at room temperature, which has a viscosity of one centipoise. An empirical relationship which has been proposed for the temperature dependence of the viscosity of liquids as an Arrhenius expression is D 0 exp.E/RT/ where the activation energy is 20–50kJmol 1 for liquid metals, and 150–300kJmol 1 for liquid slags. This expression shows that the viscosity decreases as the temperature is increased, and reflects the increasing ease with 324 Thermochemical Processes: Principles and Models which structural elements respond to an applied shear force with increasing temperature. Another significant property in metal extraction is the density of the phases which are involved in the separation of metal from the slag or molten salt phases. Whereas the densities of liquid metals vary from 2.35gcm 3 for aluminium, to 10.56 for liquid lead, the salt phases vary from 1.5 for boric oxide, to 3.5gcm 3 for typical silicate slags. In all but a few cases, this difference in density leads to metal–slag separations which result from liquid droplets of metal descending through the liquid salt or slag phase. The velocity of descent can be calculated using Stokes’ law for the terminal velocity, Vt, of a droplet falling through a viscous medium in the form d2g.p / t 18 where d is the particle diameter, and p are the densities of the slag and the particle material respectively, and is the slag viscosity. Densities of liquid chlorides vary according to the size of the cation, from 1.4 for LiCl to 4.7 for PbCl2, and so there are processes in which the metal floats on the liquid salt, as in the production of mangesium by molten chloride electrolysis. The principles of metal extraction Metal–slag transfer of impurities The diffusive properties play the rate-determining role in determining the transfer of impurities from metal to slag, or salt phase, but obviously the thickness of the boundary layer is determined by the transport properties of the liquid non-metallic phase. A metal droplet will carry a boundary layer for the diffusion transport from the bulk of the metal droplet to the metal–slag inter-face. The flux of atoms from the metal to the slag can therefore be described in terms of the transport of atoms across two contiguous boundary layers, one in the metal and the other in the slag. In the steady state when both liquids are stationary the flux of impurity atoms out of the metal will equal the flux of atoms away from the interface and into the slag. Using the simple boundary layer approximation where J D .D/υ/.Cbulk Cinterface/ and applying this to the two phases, M S S.I/ S.B/ Jn D Js and DSυM D .CM.B/ CM.I// Extraction metallurgy 325 where D and υ are the diffusion coefficient of the atoms being transferred from metal to slag, and the boundary layer thickness respectively. The concentra-tions, CM.B/, CS.B/ are in the bulk, and CM.I/, CS.I/, are at the metal–slag interface. Since thermodynamic equilibrium is assumed to exist at the inter-face, the equilibrium constant for the partition of the impurity between metal and slag KM S would be related to the interface concentrations. KM S D CM.I/ CS.I/ and writing ji for D/υ, then when CS.B/ − CS.I/ jS/jM D KM S.CM.B/ CM.I///CM.I/ This condition applies when the equilibrium content of the slag of the impurity being transferred would be high, but the bulk of the slag is large compared to the volume of the descending metal particle. When CS.B/ is not much less than CS.I/ jS/jM.CS.I/ CS.B// D .CM.B/ CM.I// D .jS/jM/..CM.I//KM S/ CS.B// and hence for the flux out of the metal, JM the equation JM D jMjS.CM.B/ CS.B/KM S//KM SjM C jS is deduced, which on re-arrangement takes the form .CM.B/ CS.B/KM S/ D JM..1/jM/ C .KM S/jS// which is analogous to Ohm’s law where .CM.B/ CS.B/KM S/ is the potential drop, JM is the current, and .1/jM/ C .KM S/jS/ represents two resistances, RM and RS, in series. The comparison of the magnitude of the two resistances clearly indicates whether the metal or the slag mass transfer is rate-determining. A value for the ratio of the boundary layer thicknesses can be obtained from the Sherwood number, which is related to the Reynolds number and the Schmidt number, defined by NSc D /D by the equation NSh D 0.332N1/2N1/3 326 Thermochemical Processes: Principles and Models for each of the two phases. Using the braces ( ) for the slag phase, and [ ] for the metal phase, the ratio between the Sherwood numbers of the two phases is .NSh/ kSDM .NRe/1/2.NSc/1/3 .ubulk/[1/6D1/2] [NSh] kMDS [NRe]1/2[NSc]1/3 [ubulk].1/6D1/2/ where is the kinematic viscosity, equal to /, and k is the mass transfer coefficient, and kS/kM D j /jM D DMυS/DSυM it follows that S 1/2 1/6 1/3 F.u,,D/ D υM D .ubulk/1/2[1/6D1/3] and hence RS/RM D KM SF.u,,D/DM/DS ³ KM S.DM/DS/2/3 Usually DS < DM, and hence RS > KM SRM. The transfer in the slag phase is therefore rate-determining in the transfer of a solute from the metal to the slag phase. When the two liquid phases are in relative motion, the mass transfer coef-ficients in either phase must be related to the dynamical properties of the liquids. The boundary layer thicknesses are related to the Reynolds number, and the diffusive transfer to the Schmidt number. Another complication is that such a boundary cannot in many circumstances be regarded as a simple planar interface, but eddies of material are transported to the interface from the bulk of each liquid which change the concentration profile normal to the interface. In the simple isothermal model there is no need to take account of this fact, but in most industrial circumstances the two liquids are not in an isothermal system, but in one in which there is a temperature gradient. The simple stationary mass transfer model must therefore be replaced by an eddy mass transfer which takes account of this surface replenishment. When only one phase is forming eddy currents, as when a gas is blown across the surface of a liquid, material is transported from the bulk of the metal phase to the interface and this may reside there for a short period of time before being submerged again in the bulk. During this residence time tr, a quantity of matter, qr will be transported across the interface according to the equation 1/2 qr D 2cbulk tr which depends on the value of the diffusion coefficient in the liquid, D. If the container has a radius r, then the liquid is blown across the container Extraction metallurgy 327 by a tangential stream of gas, and begins to submerge at the wall of the container. The mass transfer coefficient is given under these circumstances by an equation due to Davenport et al. (1967) 161/2 D1/2 3 r where u is the surface velocity of the liquid. For the converse situation where an inductively heated melt is in contact with a gas, a typical value in a labo-ratory study involving up to about one kilogram of liquid metal, the mass transfer coefficient is approximately given by k D 0.05/r1/2 (Machlin, 1960). When both phases are producing eddies a more complicated equation due to Mayers (1962) gives the value of the mass transfer coefficient in terms of the Reynolds and Schmidt numbers which shows that the coefficient is proportional to D0.17. 1.9 2.4 k1 D 0.0036.D1/.Re1Re2/0.5 0.6 C Sc0.83 2 1 In many studies of interphase transport, results are obtained which show a dependence on the diffusion coefficient somewhere between these two values, and therefore reflect the differing states of motion of the interface between studies. The electron balance in slag–metal transfer The transfer of an element from the metal to the slag phase is one in which the species goes from the charge-neutralized metallic phase to an essentially ionic medium in the slag. It follows that there must be some electron redistribution accompanying the transfer in order that electro-neutrality is maintained. A metallic atom which is transferred must be accompanied by an oxygen atom which will absorb the electrons released in the formation of the metal ion, thus [Mn] C 1/2O2 D fMn2Cg C fO2 g where [Mn] indicates a manganese atom in a metallic phase and fMn2Cg the ion in the slag phase. In another example electro-neutrality is maintained by the exchange of particles across the metal–slag interface fO2 g C [S] D fS2 g C [O] For any chemical species there are probably many ways in which the transfer across the metal–slag interface can be effected under the constraint of the ... - tailieumienphi.vn