Mintek

C H A P T E R   4


ADDITIONAL CONSIDERATIONS

  • Pyrosim's Energy Balance
  • Factors That Affect the Mass Balance
  • The Temperature Controlled Model
  • The Energy Controlled Model
  • Ideal Mixing of Complex Components Model


  • Some additional considerations have evolved that have enabled more reliance to be placed in the predicted values obtained from Pyrosim.

    Pyrosim's Energy Balance
    The energy requirements for a process are automatically calculated by calculating the difference between the enthalpy of formation of the products and reactants in the system.

    Incorrect phase data may be used for the final process enthalpy balance if care is not taken to specify the correct phase data and force Pyrosim to use this data. This is best explained by using an example.

    For example: Consider the problem of heating a typical slag from 25 ºC to 1700 ºC.

    The slag's composition is SiO2=25 %, Al2O3=25 %, CaO=25 %, MgO=25 % (all mass %). According to Osborne et al. the slag liquidus temperature will be about 1650 ºC. However at 1650 ºC all the pure compounds are solids.

    Pyrosim's routine searches for the compound of interest and then selects the data applicable to the specified temperature. Thus, Pyrosim would use solid phase data, for each pure compound, to calculate the energy requirements needed to heat a liquid system from 1650 ºC to 1700 ºC.

    An error is therefore occurring because the enthalpy of transition from solid to liquid is erroneously being omitted.

    The scarcity of data for complex systems means that all Pyrosim energy balances conducted in this way will always contain some error.

    ¤ You can eliminate part of this problem by doing the following:
    Specify a separate species (by editing the thermodynamic data file, if necessary) with the correct phase data (i.e. the liquid phase data). This will be extrapolated to the lower temperature of interest. For example, create a compound CaO(l) that consists of only the data for the liquid phase of CaO but is valid across the entire temperature range from 298 ºC to 3500 ºC.

    As enthalpy is a state function, the enthalpy function for each phase will account for the total enthalpy of each compound up to the specified temperature. By specifying the new compounds CaO(l), SiO2(l), Al2O3(l) and MgO(l) as the only species allowed to be present at equilibrium in the slag phase, the energy balance will be as correct as possible (given the form of the thermodynamic data).

    Figure30
    Figure 30: A correctly specified system for heating a slag with individual pure component data given for a different phase at the specified temperature

    In reality this discrepancy in the energy balance may not be of any significance and would need to be checked. When producing ferrochromium, the total energy required to heat the feed added to that required for the chemical reactions taking place is far greater than that energy associated with heating a relatively small amount of slag using incorrect phase data.

    However, if the process in question contained a large recycle stream consisting of hot slag (the enthalpy of which was incorrectly calculated) the potential error could be very significant.

    Factors That Affect the Mass Balance
    An attempt has been made to describe the influence that some factors are known to have on the prediction of the mass balance using the equilibrium model.

    The Temperature Controlled Model
    For a temperature controlled model the predicted mass balance is affected by four factors:

    The mass balance is not affected by the temperature of the feed streams, or the mineralogical phase description of a feed material.

    The Energy Controlled Model

    Ideal Mixing of Complex Components Model
    Pyrometallurgical slags, in particular, are usually non-dilute systems which render the procedure of extending activities for binary dilute systems to higher order systems inaccurate. In order to ensure that Pyrosim could deal with a large variety of non-dilute systems, both simple and complex, the Ideal Mixing of Complex Compounds approach has been adopted in preference to other models based on extensive experimental investigation.

    Jones and Botes state that: "Many oxide systems are characterised by strong acid-base chemical interactions, leading to negative deviations in thermodynamic activities that are much larger than those predicted by the ideal solution or regular solution models" Silicate systems are cited as an example wherein the activity of silica varies greatly within a narrow compositional range.

    The Ideal Mixing of Complex Components (IMCC) model is based on the identification of complex components, e.g. 2MgO.SiO2 (in a slag phase) and Fe3Si (in a metallic solution). Thermodynamic data for these components is recorded in the Pyrosim database and the specification of a complex component like 2MgO.SiO2 in the equilibrium specification of a slag phase will contribute in part to accounting for the activities of MgO and SiO2 of the system (by accounting for some chemical interaction between MgO and SiO2). The specification of other complex compounds will also contribute in part to the individual activities and thus, by specifying a seemingly complicated system a realistic model is obtained.

    The species present in the equilibrium specification do not relate directly to the mixture speciation. They should be regarded as intermediate compounds, present to theoretically account for non-ideal chemical interaction between reference compounds, en route to a final product composition.

    This model thus allows non-ideality to be accounted for without relying on an analytical model of the activity function, which could contain large experimental uncertainty, and in most cases is limited to simple, well documented, systems.

    An important assumption made is that chemical interactions dominate in the formation of species and physical interactions are assumed to be negligible.

    Thus applying the model to situations where no available activity information is present is a trivial procedure. Simply select all species that contain combinations of the reference compounds present.

    If a specie or phase has incomplete data, Pyrosim will extrapolate the available data using the functional form of DGfº. The associated errors are expected to be small when compared to the typical error associated with DGfº.

    A limitation of such a model is its reliance on the accuracy of tabulated thermodynamic data.


    Contact Information
    Pyrometallurgy Division, Mintek,
    200 Hans Strijdom Drive, Randburg, 2125, South Africa
    Private Bag X3015, Randburg, 2125, South Africa.

    Phone: +27 (11) 709-4642
    Fax: +27 (11) 793-6241


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    Copyright © 1996-97 George la Grange, Mintek, glg@pyro.mintek.ac.za
    19 June 2001