A PHREEQC model of Li2CO3 solubilization in NaCl and LiCl aqueous solutions up to 70 ℃

Context & Goal

Lithium carbonate (Li2CO3) is a precursor to compounds used in lithium-ion batteries. It is produced from Li-bearing natural raw brines through 2 steps :

• (1) Treatment(s) to concentrate Li, giving a Na-K-Li/Cl-SO4-B brine (see Figure)
• (2) Precipitation by brine carbonation - i.e. sodium carbonate (Na2CO3) adding - at 80-90 ℃ (e.g. An et al., 2012 and Cheng et al., 2013) (see Reaction)

In order to simulate the carbonation/precipitation step, it is necessary to have a model of solubility of both solids (Li2CO3 and Na2CO3) in Na-K-Li/Cl-SO4-B aqueous solutions.

As a first stage, here is presented a model of Li2CO3 solubilization in NaCl and LiCl aqueous solutions as compared with experimental measurements

Model

The model was built using the PHREEQC software (Parkhurst and Appelo, 2013), with the following features :

• Aqueous system made of the following species : H2O, OH-, H+ ; CO32-, HCO3-, CO2 ; Cl- ; Li+ ; Na+, and the following equilibria with their thermodynamic constants as function of temperature :
  • Solid-liquid equilibrium : Li₂CO₃ = 2 Li⁺ + CO₃^2-
  • In-water equilibria : H₂O = OH^- + H⁺ ,AND CO₃^2- + H⁺ = HCO₃^- ,AND HCO₃^- + H⁺ = CO₂ + H₂O

• The Pitzer model (e.g. Plummer et al., 1988) for calculating ions' activity

Results

The model simulates the retrograde/inverse solubility of Li2CO3 in pure water and NaCl and LiCl aqueous solutions.

The model reproduces quite satisfactory the Li2CO3 solubility measurements in pure water and NaCl and LiCl aqueous solutions, but needs to be improved to minimize the deviation to the measurements.

Modeled (line) versus measured (symbols) Li2CO3 solubility in pure water (measurements from Lide, 2005)

Modeled (lines) versus measured (symbols) Li2CO3 solubility in NaCl aqueous solutions (m : molality ; measurements from Cheng et al., 2013))

Modeled (lines) versus measured (symbols) Li2CO3 solubility in LiCl aqueous solutions (m : molality ; measurements from Cheng et al., 2013))

Outlooks

The model can be improved by - e.g. optimization of the Pitzer model parameters -.

Beside Li+, Na+ and Cl-, the carbonated brine presents significant concentrations in K+ and Cl- and SO42- and Br- (see above) ; those ions should be included in the system.

To fully simulate the carbonation reaction (see above), solubility of Na2CO3 should be also studied.

References

• Lide (2005) CRC Handbook of Chemistry and Physics
• An et al. (2012) Recovery of lithium from Uyuni salar brine. Hydrometallurgy
• Cheng et al. (2013) Solubility of Li2CO3 in Na-K-Li-Cl brines from 20 to 90 ℃. Journal of chemical thermodynamics
• Parkhurst and Appelo (2013) Description of input and examples for PHREEQC version 3 - A computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations. USGS report
• Plummer et al. (1988) A computer program incorporating Pitzer's equations for calculation of geochemical reactions in brines. USGS report