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Industrial & Engineering Chemistry Research pubs.acs.org/IECR Article mass of lithium in solution divided by its initial mass (eq 1). The final solution volume was inferred as the sum of the volumes of the feed Li-rich brine and the precipitant solution (Na2CO3 or NaOH). c×Vc×V recovery = initial initial final final c×V (1) initial initial The mass purity of precipitate in Li+ was calculated according to eq 2 purity = mass of Li2CO3(s)eq mass of precipitate (2) where the equivalent mass of Li2CO3 was determined from the measured Li+ concentration in the collected precipitate samples (approximately 100 mg of the dried precipitate, see Section 2.4). 3. RESULTS AND DISCUSSION 3.1. Lithium Precipitation with Na2CO3. The influence of several operating parameters on lithium precipitation using Na2CO3 was analyzed, addressing in particular (i) the effect of different CO32−/Li+ molar ratios, (ii) the effect of solution temperature and ionic strength (given by NaCl and KCl dissolved salts) and (iii) the effect of the presence of divalent cations (namely, calcium, magnesium, and strontium) and anions (namely, sulfate and bromide ions) in the Li-rich feed brine. 3.1.1. Influence of the [CO32−]/[Li+] Ratio. The influence of the [CO32−]/[Li+] operating ratio on Li+ recovery and purity was investigated. Five precipitation scenarios were carried out within the [CO32−]/[Li+] range from 0.25 to 2 (mol/mol). Note that the [CO32−]/[Li+] value of 0.5 represents the stoichiometric precipitation condition, while lower and upper ratio values refer to under- and over-stoichiometric conditions with respect to the excess or lack CO32− ions, respectively. A constant temperature of 50 °C and a 300 rpm stirring rate were maintained in all experiments. Details of the reacting quantities for each test are reported in Table S2 in the Supporting Information. Li+ recovery, eq 1, and purity, eq 2, observed at the end of all experiments (after 2 h) are shown in Figure 4. Figure 4. Li2CO3 recovery and purity as a function of the [CO32−]/ [Li+] ratio. Li+ recovery significantly increases from ∼30 to ∼60% using a CO32−/Li+ ratio of 0.25 and 1, respectively. On the other hand, only a slight increase is noticed when increasing the CO32−/Li+ ratio from 1 to 2, that is, from ∼60 to ∼65%. Therefore, all the hereinafter reported experiments were carried out using a CO32−/Li+ ratio of 1. Purity ranges between 98 and 90%, slightly decreasing at high CO32−/Li+ ratios. In all these cases, the impurities are attributed mainly to trapped Na2CO3, remaining in the liquor entrained within the particle cakes after filtration. 3.1.2. Influence of Temperature and Ionic Strength. Li2CO3(s) solubility decreases when the temperature is increased (see also the Supporting Information); thus, a beneficial effect of temperature on the precipitation rate is expected. In particular, the influence of temperature on the Li2CO3 precipitation process was studied by performing experiments at 50 °C and at 80 °C with and without the presence of other monovalent ions in solution, namely, Na+ and K+. The presence of dissolved ions (e.g., Na+ and K+) increases solution ionic strength, which can be calculated as i=n I = 0.5 cz2 i=1 13593 where I is the solution ionic strength and ci and zi are the i-th ion concentration and valence, respectively. Four precipitation tests were carried out using a starting (before Na2CO3 solution addition) 0.70 M LiCl solution (i) as apuresalt(I=0.70M)orwith(ii)1.5MKCl(I=2.20M), (iii) 2.0 M NaCl (I = 2.70 M), and (iv) both 2.0 M NaCl and 1.5 M KCl (I = 4.20 M). Such NaCl and KCl concentrations were chosen based on preliminary calculation regarding the actual selectivity properties of the Li-MFCDI against monovalent and divalent ions present in the treated brine, as discussed in the introduction and shown in Figure 1. Details for all the four investigated cases are reported in Table S3 in the Supporting Information. In all experiments, solutions were stirred at 300 rpm and a double excess of a 2.0 M Na2CO3 solution (CO32−/Li+ ratio of 1), fed at a flow rate of 10 mL/ min, was employed. Li+ concentration evolution over time during the precip- itation tests is shown in Figure 5. A final Li+ concentration of ∼15% lower than the ideal solubility value is obtained in pure LiCl solutions at 50 and 80 °C (Figure 5a), thanks to the over-stoichiometric amount of CO32−. Note that, in Figure 5a, the experimental point determined at 3 h was likely affected by some measurements errors, for example, a possible wrong dilution before analysis; therefore, it was excluded from the interpolated Li concen- tration trend. When other ions are present, Li concentration further decreases reaching values ∼25% lower than the ideal solubility value for the case of single K+ or Na+ ions added (Figure 5b,c). This is induced by the ion salting-out effect between Na+, K+, and Li+ ions that leads to a Li2CO3 solubility decrease. The lower Li2CO3 solubility induces a higher precipitated Li2CO3 mass (higher reaction yield) and, in turn, a lower final Li+ concentration in the solutions. The observed results are in accordance with data reported in the literature29,30 and better discussed in the Supporting Information. Finally, the simultaneous presence of Na+ and K+ ions causes a considerable drop in Li+ concentration, in the range of ∼50−60% lower than the ideal solubility at 50 and 80 °C (Figure 5d). It should be also observed that Li2CO3(s) precipitation is more than two times faster at 80 °C (∼20 min) than that at 50 °C (∼1 h), but with high ionic strength solutions, the kinetics of the precipitation at medium temperatures seems to be enhanced and the precipitation occurs at a comparable time. Figure 6 shows the Li recovery and purity as a function of solution ionic strength and temperature. For the tests at 80 °C at 0.70 and 4.20 M ionic strength, also recovery and solid purity after the EtOH washing step are reported. https://doi.org/10.1021/acs.iecr.2c01397 ii Ind. Eng. Chem. Res. 2022, 61, 13589−13602 (3)PDF Image | Recovery of Lithium Carbonate from Dilute Li Rich Brine
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