Fig. 1 shows the experimental setup used for the Li₂CO₃ precipitation in this study, wherein a batch-type reactor was used, and the experiments were performed under open-system conditions. The Li₂CO₃ precipitation experiment based on CO₂ bubbling was conducted as follows: The volume of the reaction solution used was 20 L, and the initial concentration of lithium and pH value were adjusted using lithium hydroxide (Sigma, USA) and hydrochloric acid (Sigma, USA), respectively. The initial lithium concentration and pH values were approximately 7,400–7,900 mg Li/L and 11–11.5, respectively, unless stated otherwise. The lithium concentration and pH condition of the solution was identical to that of the wastewater generated after the waste battery treatment provided by the ER Corporation. The temperature of the solution was maintained at 35°C throughout the experiment using a temperature-control unit (JeioTech, Korea). The reason than the temperature condition of 35°C was chosen, because, during preliminary tests of the lithium carbonate precipitation, the solution reached a temperature of approximately 35°C. During these experiments, the reaction solution was stirred vigorously using a mechanical stirrer. The experiment started with the addition of CO₂ to the solution. The CO₂ gas flow rate was controlled using a mass-flow controller to provide a constant supply of 3–9 L/min (Line-tech, Korea). In a conventional CO₂ bubble supply, CO₂ gas was directly injected into the reaction solution via a commercial bubble diffuser provided by the ER Corporation. However, in the CO₂ microbubble supply, CO₂ gas was injected into the reaction solution via a rotating-nozzle-based microbubble generation system, as reported in our previous studies. [29, 30]. Fig. 2 shows the conventional and microbubble bubbling methods used in this study. The water flow rate, water pressure, and gas pressure were approximately 6 L/min, 2.5 bar, and 2 bar, respectively.

The changes in the lithium concentration and pH of the solution were monitored during the experiment. To analyze the change in the lithium concentration, a small quantity of the solution was sampled at regular time intervals and was measured using ion chromatography Dionex ICS-1000, Thermo Fisher Scientific, USA). The quantity of Li₂CO₃ precipitate, the maximum Li₂CO₃ reaction ratio, and reaction rate were calculated using Eq. (1–3), respectively:

where C_Li,0 is the initial lithium-ion concentration, C_Li,t is the lithium-ion concentration at time t, C_Li,min is the lowest lithium-ion concentration (mg Li/L), and V is the solution volume (20 L).

pH was measured using a pH-measuring electrode. The reason for measuring pH is that the concentration of H⁺, expressed as pH, is crucial for the Li₂CO₃ precipitation, expressed by the Eq. (4 – 8). The reaction equations for the dissolution of CO₂ gas and its reaction with Li⁺ ions are given in Eq. (4 – 8) as follows:

Eq. (4–8) exhibit that the Li₂CO₃ precipitation progresses with increasing CO₂ supply, thus increasing the H⁺ concentration in the solution and decreasing the pH. Hence, a higher pH favors the lithium carbonate reaction.

The precipitated lithium carbonate powder was collected at the end of the reaction, and its diameter was measured after drying. The diameters were measured using scanning electron microscopy (SEM) (JSM 7800F, Jeol, Japan).

Fig. 3 shows the findings of the Li₂CO₃ precipitation reaction based on the CO₂ supply and the bubbling systems, i.e., microbubbling and conventional, inferring two interesting observations. First, in the Li₂CO₃ precipitation reaction, the CO₂ microbubbling system shows an increase in the performance compared to that of the conventional CO₂ bubbling system. Fig. 3(a) shows the change in the Li⁺ concentration in solution with the bubbling type. The initial Li⁺ concentration decreases from approximately 7,400 to 3,500 mg Li/L in 300 min using the conventional CO₂ bubbling system, in which the maximum Li₂CO₃ reaction ratio is approximately 53%, whereas in the CO₂ microbubbling system, the initial Li⁺ concentration decreases from approximately 7,600 to 2,500 mg Li/L in 220 min and the maximum Li₂CO₃ reaction ratio is approximately 67%. Hence, the CO₂ microbubbling system gives approximately 14% higher maximum Li₂CO₃ reaction ratio than that of the conventional CO₂ bubbling system. It is noted that significant lithium concentration shows changes after 40–60 min of reaction initiation. This appears to be the minimum time required for the Li₂CO₃ precipitation to begin. A similar trend is observed in previous studies [26]. Fig. 3(b) shows the quantity of Li₂CO₃ precipitates, calculated using Eq. (1) and the results in Fig. 3(a), and the effect of CO₂ bubbling types on Li₂CO₃ precipitation. The quantity of Li₂CO₃ precipitates increases to 590 g over 220 min in the CO₂ microbubbling system, which is approximately 2.7 g/min on average. In conventional CO₂ bubbling, the quantity of Li₂CO₃ precipitates increases to 420 g over 300 min, which is approximately 1.4 g/min on average. Therefore, the precipitation reaction rate of the CO₂ microbubbling system is approximately 1.9 times higher than that of conventional CO₂ bubbling. Over the same time interval (220 min), the CO₂ microbubbling system (2.68 g/min) shows a 1.6 times higher average precipitation reaction rate than that of the conventional CO₂ bubbling system (1.63 g/min). Similar trends are observed when the CO₂ flow rate is doubled (6 L CO₂/min) (Fig. S1), and this improvement could be attributed to greater gas-liquid contact area and longer residence time in the solution for the microbubbling system compared with that of the conventional bubbling. The experiment in Fig. 3 was carried out under the following conditions. The initial pH and the temperature of the feed was 11 and 35°C, respectively, and CO2 flow rate was 3 L/min.

Second, when CO₂ is oversupplied, it seems that the re-dissolution of precipitated Li₂CO₃ occurs (Fig. 3(a)). The Li⁺ concentration slightly rebounds after reaching the lowest point for the CO₂ microbubbling and conventional CO₂ bubbling systems. For instance, the Li⁺ concentrations increase again after reaching the lowest values of 2,500 and 3,500 mg Li/L in CO₂ microbubbling and conventional CO₂ bubbling systems, respectively, which could be attributed to the re-dissolution of the precipitated Li₂CO₃. Fig. 3(c) shows the variation in the pH of the solution as the Li₂CO₃ reaction proceeds. In Fig. 3(c), the CO₂ microbubbling system shows a remarkable pH decrease to 9.4 at 240 min. For the conventional CO₂ bubbling, the pH decrease starts at 200 min, giving pH values of 9.7, 9.6, and 9.5 at 300, 320, and 340 min, respectively. For both CO₂ microbubbling and conventional CO₂ bubbling, Li₂CO₃ re-dissolution starts at a pH of approximately 9.4–9.7. We will discuss the re-dissolution phenomena in the next section 3.2.

Fig. 4(a) and (b) show SEM images of the Li₂CO₃ precipitates based on the CO₂ microbubbling and conventional CO₂ bubbling systems, respectively. The diameters of Li₂CO₃ precipitates are approximately 45 and 500–750 μm in CO₂ micro-bubbling and conventional CO₂ bubbling systems, respectively. Considering the cathode material synthesis for battery, Li₂CO₃ with a smaller diameter is commercially more viable; hence, the Li₂CO₃ precipitates obtained using the CO₂ microbubbling system are expected to have excellent utilization value.

In this section, the primary factors affecting the Li₂CO₃ precipitation in a CO₂ microbubbling system are investigated, which include pH, CO₂ flow rate, and temperature. Fig. 5 shows the effect of the initial pH of the solution on the Li₂CO₃ precipitation in the CO₂ microbubbling system, expressed as the Li⁺ concentration and pH changes. The experiment in Fig. 5 was carried out under the following conditions. The initial pH and the temperature of the feed was 10.7 or 11.5 and 35°C, respectively, and CO₂ flow rate was 6 L/min.

A high initial pH results in the increase in the maximum Li₂CO₃ reaction ratio. For instance, when the initial pH is 11.5, the Li⁺ concentration in the solution decreases from 7,400 to 2,500 mg Li/L, and the maximum Li₂CO₃ reaction ratio is approximately 66%. However, at initial pH 10.7, the Li⁺ concentration in the solution decreases from 7,400 to 4,300 mg Li/L, with a maximum Li₂CO₃ reaction ratio of 39%. An interesting observation was made when Li⁺ concentration and pH are examined together, as shown in Fig. 5: the pH at which the Li⁺ concentration in the solution reaches its lowest value is similar, at approximately 9.4–9.7, regardless of the initial pH. The Li₂CO₃ precipitation terminates when Li⁺ concentration reaches the lowest value. For instance, at an initial pH of 11.5, the lowest Li⁺ concentration is 2,500 mg Li/L at 110 min, and the solution pH is 9.7, whereas for an initial pH of 10.7, the lowest Li⁺ concentration is 4,300 mg Li/L at 70 min with a solution pH of 9.4. Irrespective of the initial pH, it appears that the shift of the dominant carbonate species from carbonate to bicarbonate ions leads to the termination of Li₂CO₃ precipitation reaction and the re-dissolution of precipitated Li₂CO₃. Considering that the bicarbonate-carbonate pKa₂ at 35°C is 10.2 [31], the dominant carbonate species at pH 9.4 – 9.7 is bicarbonate ion (approximately 76–86%) [31] (Eq. (9)). As the pH decreases, the carbonate ions corresponding to the Li₂CO₃ reactants become increasingly scarce and the bicarbonate ions become increasingly the dominant species. Therefore, re-dissolution of the precipitated Li₂CO₃ occurs as the concentration of bicarbonate increases relative to that of carbonate. It is noted that the solubility of lithium bicarbonate is approximately 7 times higher than that of Li₂CO₃ [32].

Therefore, the Li₂CO₃ precipitation stops at pH 9.4–9.7, and the precipitated Li₂CO₃ is redissolved by bicarbonate ions, which is presumed to increase the Li⁺ concentration in the solution. The results in Fig. 5 demonstrate the effect of the initial solution pH on the Li₂CO₃ reaction.

Fig. 6 shows the effect of the CO₂ flow rate on the Li₂CO₃ precipitation. Fig. 6 (a) and (b) show the variation in the Li⁺ concentration in the solution and the quantity of Li₂CO₃ precipitate with respect to the CO₂ flow rate, respectively, where the initial pH and the temperature of the feed was 11 and 35°C, respectively, and CO₂ flow rate was from 3 to 9 L/min. As the CO₂ flow rate increases, the time required for the lithium concentration in the solution to reach its lowest value decreases. Fig. 6(a) reveals that quantitatively, the Li concentrations decrease from 7,400 mg Li/L to the lowest value of 2,500 mg Li/L in 240 min at 3 L CO₂/min; from 7,800 mg Li/L to the lowest value of 2,300 mg Li/L in 140 min at 6 L CO₂/min, and from 7,700 mg Li/L to the lowest value of 2,800 mg Li/L in 80 min at 9 L CO₂/min. Fig. 6(b) shows the quantity of Li₂CO₃ precipitates calculated using Eq. (1) and the results presented in Fig. 6(a). The effect of the CO₂ flow rate on the Li₂CO₃ precipitation reaction rate was more clearly expressed in Fig. 6(b). At 3 L CO₂/min, 570 g of Li₂CO₃ precipitates in 240 min with 2.4 g/min reaction rate; at 6 L CO₂/min, 630 g of Li₂CO₃ precipitates in 140 min with 4.5 g/min reaction rate, and at 9 L CO₂/min, 550 g of Li₂CO₃ precipitates in 80 min with 6.9 g/min reaction rate. Hence, a proportional relationship exists between the CO₂ flow rate and the average Li₂CO₃ precipitation reaction rate.

Fig. 7 shows the effect of the solution temperature on Li₂CO₃ precipitation, where the initial pH and the temperature of the feed was 11 and 20 or 35°C, respectively, and CO₂ flow rate was from 6 L/min. Fig. 7 (a) and (b) show the changes in the Li⁺ concentration in the solution and the quantity of Li₂CO₃ precipitates, respectively, at different temperatures of the solution. A high solution temperature is advantageous in terms of the maximum Li₂CO₃ reaction ratio and the quantity of precipitates. Quantitatively, Fig. 7(a) shows that the lithium concentration decreases from 7,700 to 2,500 mg Li/L at 20°C (68% of maximum Li₂CO₃ reaction ratio) and from 7,900 to 2,200 mg Li/L at 35°C (72% of maximum Li₂CO₃ reaction ratio). Additionally, Fig. 7(b) reveals that the maximum quantities of lithium carbonate precipitates at 20 and 35°C are approximately 550 g at 100 min, corresponding to an average of 5.5 g/min, and 630 g at 120 min, corresponding to an average of 5.3 g/min, respectively. Although the precipitation reactant generation rates are similar, the temperature of 35°C leads the increase in the maximum lithium carbonate precipitation compared to 20°C condition. However, temperature has complementary positive and negative effects on the lithium carbonate precipitation. For example, higher temperatures are negative in terms of reaction because they reduce the solubility of carbon dioxide but positive in terms of precipitate formation because they reduce the solubility of lithium carbonate. [33]. Therefore, the effect of temperature is further investigated via additional experiments.

Fig. 8 shows the effect of temperature control on lithium carbonate precipitation, expressed as a change in lithium concentration, in which the initial pH and the initial temperature of the feed was 11 and 35°C, respectively, and CO₂ flow rate was from 6 L/min. The CO₂ gas supply is stopped at 110 min (dotted line, Fig. 8), which shows a difference between the two cases wherein the solution temperature is maintained at 35°C and when it is adjusted to 70°C. Increasing the solution temperature shows a positive effect on Li₂CO₃ precipitation. For instance, when the temperature is constant at 35°C, no considerable change in Li⁺ concentration is observed after the CO₂ supply stops. Quantitatively, the Li⁺ concentration reaches the lowest value of 2,500 mg Li/L from an initial concentration of 7,900 mg Li/L at 110 min, then remains at approximately 2,800 mg Li/L. The maximum Li₂CO₃ reaction ratio is approximately 68%. On the other hand, after the CO₂ supply stops, the Li⁺ concentration in the solution gradually decreases with increasing temperature of the solution from 35 to 70°C. For instance, the Li⁺ concentration decreases from an initial 7,900 mg Li/L to 2,500 mg Li/L at 110 min and then gradually decreases to 1,900 mg Li/L at 190 min under temperature control. The maximum Li₂CO₃ reaction ratio is approximately 76% owing to the temperature-dependent solubility change of Li₂CO₃. Unlike several other materials, Li₂CO₃ exhibits decreasing solubility with increasing temperature. Therefore, solubility is reduced by increasing the temperature of the solution, and additional precipitation likely occurs. This approach demonstrates a good strategy to improve the reaction ratio of lithium carbonate precipitation without re-dissolution.