# Heat of Dissolution of Gibbsite and Boehmite

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## Heat of Dissolution Papers

The dissolution of Gibbsite (and Boehmite) in Bayer liquor is an endothermic (heat absorbing) reaction. The energy absorbed by this reaction accounts for a significant fraction of the energy requirements in the Bayer process. Conversely in precipitation, the reaction releases a significant amount of energy that has an effect on the process.

For a detailed discussion of Heat of Dissolution of either of these species, SysCAD users with a current maintenance agreement, can download detailed papers on these two topics at the link below. The linked paper gives details for the derivation of the heat of reaction and also equations for hand calculation estimates as a function of temperature.

## 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. For a detailed paper on this topic see [Gibbsite Dissolution Reference Paper]

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 on Heats of Reaction.

## 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. For a detailed paper on this topic see [Boehmite Dissolution Reference Paper]

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 on Heats of Reaction.