Database with SIT (Specific ion Interaction Theory) aqueous activity model for the H₂O-CO₂-LiCl-NaCl-Li₂CO₃ system up to 70 °C

Introduction

Like lithium hydroxide (LiOH), Li carbonate (Li₂CO₃) is a salt used as a precursor to compounds used in Li-ion batteries. It is produced by carbonation at \(T >\) 70 °C (to favor its precipitation as it presents retrograde/inverse solubility) of a Li brine either by sodium carbonate (Na₂CO₃) adding :
$$\ce{2 Li+ + Na2CO3_{(s)} <=>> Li2CO3_{(s)} + 2 Na+}$$ , or by CO_2(g) bubbling (needing the brine to be alkalinized by e.g. NaOH), the so-called ‘direct carbonation’ :
$$\ce{2 Li+ + CO2_{(aq)} + OH- <=>> Li2CO3_{(s)} + H+}$$
Ramirez-Velazquez et al. (2024) studied the direct carbonation of a LiCl brine, with a [LiCl]=4.4 M, making this a ‘high’ ionic strength electrolyte. A e.g. PHREEQC, model simulating this process has thus to include an aqueous activity model suitable for high ionic strength aqueous solutions, such as the Pitzer one, or simplier i.e. with less number of equations and less parameters, the SIT one (Specific ion Interaction Theory ; e.g. Grenthe et al., 1997) :
$$ \ln \gamma_i = -\frac{z_i^2 \cdot A(T) \cdot \sqrt{I}}{1+1.5\sqrt{I}} + \sum_{k}\epsilon_{(i,k)} \cdot m_k $$ , where \(\gamma_i\) , \(z_i\), \(A(T)\), \(I\), \(\epsilon_{(i,k)}\), and \(m_k\), are respectively the activity coefficient of an \(i\) ion, its charge, the Debye-Hückel \(A\) term at a given \(T\), the aqueous solution ionic strength, the cation-anion binary/doublet interaction parameter (\(i\) and \(k\) have opposite charges), and the \(k\) ion molality.

Brief description of the database

The reactions' thermodynamic constant value as function of temperature is calculated using the polynomial coded in PHREEQC for which the A₁-A₅ coefficients were taken from Thermoddem ; except for the Li₂CO₃ dissolution reaction, for which the van’t Hoff equation (see this page) is prefered (better fit to the measured data).

The espilon's (\(\epsilon_{(i,k)}\)) have been obtained by fitting / optimization on mean activity coefficient measured values of salts as function of their concentration given by the 2014 CRC handbook of chemistry and physics and by Mamontov and Gorbachev (2020).

Downloading and testing the database

With the above button, you downloaded 5 files :

Place them in a same folder and run the .phr file. You should obtain the 3 following plots. Note that the model outcomes with the present database fit better to the measured data rather than with the 'as-it' sit.dat database provided with the PHREEQC package (see the next 3 following plots).

Results with the present database (lines=model outcomes, symbols=measured data) :

Results with the 'as-it' sit.dat database provided with the PHREEQC package (appended with the Li₂CO₃ dissolution reaction and its \(K(T)\) calculated with the van't Hoff equation) (lines=model outcomes, symbols=measured data) :