Heat of Dissolution of Gibbsite and Boehmite

Navigation: Models ➔ Alumina Models ➔ Heat of Dissolution of Gibbsite and Boehmite

Related Links: Precipitation3, Alumina 3 Bayer Species Model

---

Introduction

The dissolution of Gibbsite (trihydrate alumina, Al2O3.3H2O or Al[OH]3) and Boehmite (monohydrate alumina, Al2O3.H2O or AlO.OH) in Bayer liquor are endothermic (heat absorbing) reactions. The energy absorbed by these reactions has a large effect on the predicted steam requirements in Digestion and the optimisation of energy use. Conversely, in Precipitation, Gibbsite (or "hydrate") coming out of solution is an exothermic (heat releasing) reaction and produces enough energy to have a significant effect on the process (e.g. requirement for interstage cooling).

The enthalpy of phase change of Gibbsite and Boehmite are dependent on liquor composition and temperature, with temperature being the most important parameter.

Heat of Dissolution of Gibbsite

Depending on the species in the project, the dissolution of Gibbsite may be written as:

Al[OH]3(s) + NaOH(aq) → NaAl[OH]4(aq)

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

Al2O3.3H2O(s) → Al2O3(aq) + 3H2O(l)

Heat of Dissolution:

• 252.3 kJ/kg of Gibbsite (Al2O3.3H2O or Al[OH]3) at 0°C reference temperature
  • This is equivalent to 386.0 kJ/kg Alumina (Al2O3) at 0°C

The Precipitation reaction or Gibbsite reversion are simply the reverse, and the Heat of Reaction is the negative of the Digestion dissolution reaction.

Heat of Dissolution of Boehmite

Depending on the species in the project, the dissolution of Boehmite may be written as:

AlO.OH(s) + NaOH(aq) + H2O(l) → NaAl[OH]4(aq)

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

Al2O3.H2O(s) → Al2O3(aq) + H2O(l)

Heat of Dissolution:

• 170.3 kJ/kg of Boehmite (Al2O3.H2O or AlO.OH) at 0°C reference temperature
  • This is equivalent to 200.4 kJ/kg Alumina (Al2O3) at 0°C

The Boehmite reversion reaction is simply the reverse, and the Heat of Reaction is the negative of the Digestion dissolution reaction.

Notes

• The default value in the Precipitator model -252.3 kJ/kg Gibbsite (Al[OH]3) at 0°C.
• It is recommended to use a reference temperature of 0°C for Heat of Reaction (HOR) override. If HOR override data is at a different temperature, this can be specified in the Reaction Editor. SysCAD will then 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.

Discussion Page New Reaction Heat Features in Alumina Precipitation Models

• 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.

References

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