Heat of Dissolution of Gibbsite and Boehmite

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Related Links: Precipitation3, Alumina 3 Bayer Species Model

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Introduction

The dissolution of Gibbsite and Boehmite in Bayer liquor are endothermic (heat absorbing) reactions. The energy absorbed by these reactions in Digestion accounts for a significant fraction of the energy requirements in the Bayer process. Conversely in Precipitation, the crystallisation of hydrate (Gibbsite) releases a significant amount of energy that has an effect on the process.

Discussion Page New Reaction Heat Features in Alumina Precipitation Models

NOTE: The approach of using a fixed HOR is a "shortcut" only for approximate calculations, but can lead to inaccuracies depending on the Bayer liquor heat capacity correlation and thermodynamic data used for other involves species. A more thorough methodology is to fully define the thermodynamic properties of the involved species and allow SysCAD to calculate the HOR. See the discussion page to the right for more information.

Heat of Dissolution of Gibbsite Correlations

The dissolution of Gibbsite (trihydrate alumina, Al2O3•3H2O) in Bayer liquor is an endothermic (heat absorbing) reaction. The energy absorbed by this reaction accounts for a significant fraction of the energy requirement in digestion in the Bayer process and has a large effect on predicted steam requirements and the optimisation of energy use. Conversely in precipitation, Gibbsite coming out of solution is an exothermic reaction and releases enough energy to have a significant effect on the process.

The enthalpy of dissolution of Gibbsite is dependent on liquor composition and temperature with temperature being the most important parameter.

The Heat of dissolution at 0°C reference temperature is:

• Gibbsite Dissolution in kJ/kg Gibbsite (Al2O3.3H2O or Al[OH]3) at 0°C = 252.3 for the reaction

Al2O3.3H2O(s) + NaOH(aq) -> Al2O3(aq) + NaOH(aq) + 3H2O(l) or

Al[OH]3(s) + NaOH(aq) -> NaAl[OH]4(aq)

• Gibbsite Dissolution in kJ/kg Alumina (Al2O3) at 0°C = 386 for the reaction

Al2O3.3H2O(s) -> Al2O3(aq) + 3H2O(l)

The Heat of reaction for the precipitation reaction is simply the negative of the dissolution reaction and is an exothermic reaction meaning that it releases heat.

Example of How to Apply in Reaction Block

The dissolution reaction in Digestion:

Al2O3.3H2O(s) + NaOH(aq) = Al2O3(aq) + NaOH(aq) + 3H2O(l)

Extent: Fraction Al2O3.3H2O(s) = 1.0

HeatOfReaction: msFixed = 252.3 / Al2O3.3H2O(s) At 0

The alumina precipitation reaction:

Al2O3(aq) + 3H2O(l) = Al2O3.3H2O(s)

Extent: Fraction Al2O3(aq) = 0.01

HeatOfReaction: msFixed = -386 / Al2O3(aq) At 0

NOTES:

1. The Precipitator and Precipitator3 models may have the Heat of Precipitation reaction built in when using certain methods. The default value used for Gibbsite Dissolution in kJ/kg Gibbsite (Al2O3•3H2O) at 0°C is 252.3.
2. If the user does have the Heat of Reaction (HOR) value at 0°C then this is the recommended temperature to use when specifying the HOR override.
3. However, if you have a HOR value at a different temperature, then you can specify this in the reaction file. SysCAD will use this value, together with the implemented Alumina species model to calculate the HOR at 0°C, which is used in the enthalpy balance. Please see Heat of Reaction for more information.

Heat of Dissolution of Boehmite Correlations

The dissolution of Boehmite (monohydrate alumina, AlO[OH]) in Bayer liquor is an endothermic (heat absorbing) reaction. The energy absorbed by this reaction accounts for a significant fraction of the energy requirement in the Bayer process for refining alumina and has a large effect on predicted steam requirements and the optimisation of energy use. Conversely in precipitation, the reaction releases a significant amount of energy that has a significant effect on the process.

The Enthalpy of phase change of Boehmite is dependent on liquor composition and temperature with temperature being the most important parameter.

The Boehmite Heat of dissolution at 0°C is:

• Boehmite Dissolution in kJ/kg Boehmite(Al2O3.H2O or AlO.OH) at 0°C = 170.3 for the reaction

NaAl[OH]4(aq) -> AlO.OH(s) + NaOH(aq) + H2O(l) or

2 NaAl[OH]4(aq) -> Al2O3.H2O(s) + 2 NaOH(aq) + 2 H2O(l)

• Boehmite Dissolution in kJ/kg Alumina (Al2O3) at 0°C = 200.4 for the reaction

Al2O3.H2O(s) -> Al2O3(aq) + H2O(l)

Example of How to Apply in Reaction Block

While writing the dissolution reaction in Digestion:

Al2O3.H2O(s) + NaOH(aq) = Al2O3(l) + NaOH(aq) + H2O(l)

Extent: Fraction Al2O3.H2O(s) = 0.8

HeatOfReaction: msFixed = 170.3 / Al2O3.H2O(s) At 0

NOTES:

1. If the user does have the Heat of Reaction (HOR) value at 0°C then this is the recommended temperature to use when specifying the HOR override.
2. However, if you have a HOR value at a different temperature, then you can specify this in the reaction file. SysCAD will use this value, together with the implemented Alumina species model to calculate the HOR at 0°C, which is used in the enthalpy balance. Please see Heat of Reaction for more information.

References

1. "The Heat of Dissolution of Gibbsite at Bayer Digestion Temperatures", Langa, Light Metals 1985, 197-208