Fig. 3. The possible reaction mechanisms
In acidic condition, TMPD complexed with H3BO3 by hydroxyl condensation and dehydration forming monocyclic boric acid ester, (Equilibrium equation 1), and the pH value had no change in this process. Excess TMPD reacted with monocyclic boric acid ester forming spiro boric acid ester, (Equilibrium equation 2), and H+ released in this process. Formation of spiro boric acid ester and release H+ made the pH value dicrease in the extraction process. The high extraction efficiency indicated that reaction 1 and 2 could easily perform. In alkaline condition, TMPD might complex with [B(OH)4]- by hydroxyl condensation and dehydration forming boric acid esters, (Equilibrium equation 3 and 4), and the pH value had no change in these processes. The low extraction efficiency indicated that performation of reaction 3 and 4 was more difficult than the former.

3.1.2 Effect of TMPD concentration

The concentration of TMPD is an important factor affecting the equilibrium of boron extraction, and the stoichiometry of the complex of H3BO3 and TMPD can be calculated according to the effect of the TMPD concentration on the boron distribution ratio (D ) by using the slope ratio method[30]. The experiments were carried out at the concentration of TMPD varied in range of 0.02~0.6 mol/L. It can be clearly seen from Fig. 4 that extraction efficiency and distribution ratio of H3BO3 both increase with the increasing of TMPD concentration.
As mentioned in section 3.1, boric acid can react with TMPD to form different boric acid esters and the formation of boric acid esters relate to the mole ratio of TMPD/H3BO3. The extraction equilibrium equation can be expressed as following formula:
\({H_{3}\text{BO}}_{3(aq)}+{n\text{TMPD}}_{(org)}\leftrightarrow{{H_{3}\text{BO}}_{3}n\text{TMPD}}_{(org)}+{aH}_{2}O_{(aq)}+{bH}_{(aq)}^{+}\)Eq. (4)
Fig. 4. Effect of TMPD concentration on distribution ratio and extraction efficiency. pHini  = 4.6; O/A = 1; [H3BO3]ini = 0.20 mol/L.
According to Eq. (4), the two-phase extraction equilibrium constant (K ) could be given as follows:
\(K=\frac{\left[{H_{3}\text{BO}}_{3}n\text{TMPD}\right]_{(org)}{[H_{2}O]}_{(aq)}^{a}{[H^{+}]}_{(aq)}^{b}}{{[H_{3}BO_{3}]}_{(aq)}{[TMPD]}_{(org)}^{n}}\)Eq. (5)
The \(D_{H_{3}\text{BO}_{3}}\) value, which represents the distribution ratio of H3BO3 can be determined as follows:
\(D_{H_{3}\text{BO}_{3}}=\frac{[{H_{3}\text{BO}}_{3}n\text{TMPD}]_{(org)}}{[{H_{3}\text{BO}}_{3}]_{(aq)}}\)Eq. (6)
Then, Eq. (5) can be transformed as follows:
\(K=\frac{D_{H_{3}\text{BO}_{3}}{[H_{2}O]}_{(aq)}^{a}{[H^{+}]}_{(aq)}^{b}}{{[TMPD]}_{(org)}^{n}}\)Eq. (7)
By taking logarithms and rearranging, Eq. (7) convers to:
\(\log D_{H_{3}\text{BO}_{3}}=logK+n{log[TMPD]}_{(org)}-alog{[H}_{2}O]_{\left(\text{aq}\right)}-b{log[H}^{+}]_{\left(\text{aq}\right)}\)Eq. (8)
Keeping the initial pH of the aqueous phase unchanged, Eq. (8) can be represented as:
\(\log D_{H_{3}\text{BO}_{3}}=n{log[TMPD]}_{(org)}+C\)Eq. (9)
The value of n in Eq. (9) refers to the complex ratio between TMPD and H3BO3. The plots of\(\log D_{H_{3}\text{BO}_{3}}\) versus\({log[TMPD]}_{(org)}\) was shown in Fig. 5. When the mole ratio of TMPD and H3BO3 in initial two phases was less than 1:1, the slope was 1.17, which indicated that the complex ratio between TMPD and H3BO3 was 1. When the mole ratio of TMPD and H3BO3in initial two phases was more than 1:1, the slope was 2.17, which indicated that the complex ratio between TMPD and H3BO3 was 2. This result indicated that the formation of the complex was connected with the concentration ratio of TMPD/H3BO3. It doesn’t mean that below a certain ratio all esters are one form and above that they are all in another. Different complexes may co-exist when the extraction reaches equilibrium, and when TMPD is excessive, TMPD is more likely to form a 2:1 complex with boric acid.
Fig. 5. Plot of log D B vs log [C TMPD]. pHini  = 4.6; O/A = 1; [H3BO3]ini = 0.20 mol/L.

3.1.3 Effect of H3BO3concentration

To understand the effect of H3BO3concentration on the extraction efficiency of H3BO3 in the simulated brine, batch experiments were performed at pH=6.0 with varying H3BO3 concentration (0.08~0.6 mol/L). The results are shown in Figures 6 and 7. The extraction efficiency of H3BO3decreased with the increasing of H3BO3concentration in aqueous phase, as can be seen from Fig. 6. The extracted H3BO3 in organic phase increased rapidly when the initial H3BO3 concentration below 0.4 mol/L. Continuous increasing initial H3BO3 concentration, the extracted H3BO3 did not increase, keeping a constant concentration of 0.25 mol/L.
Fig. 6. Variation in concentration of H3BO3 in organic phase and efficiency with the initial H3BO3concentration. [TMPD] = 0.4 mol/L; pHini =6.0; O/A = 1.
The mole ratio of TMPD and extracted H3BO3 decreased with the decreasing of the mole ratio of TMPD and H3BO3 in the initial two phases while the equilibrium pH of the raffinate changed inversely, as shown in Fig. 7. When the mole ratio of TMPD and H3BO3 in the initial two phases was greater than 2.12, the mole ratio of TMPD and extracted H3BO3 was greater than 2.38, which indicated that two TMPD molecular complexed with single H3BO3 molecular following Equilibrium 2, and amounts of TMPD molecular which had no effect on H3BO3 existed in organic phase. The released H+ made the pH of aqueous decreased. With the mole ratio of TMPD and H3BO3 in the initial two phases decreasing from 2.12 to 0.87, the mole ratio of TMPD and extracted H3BO3 decreased from 2.38 to 1.59. Bimolecular complex and monomolecular complex occurred in this process following Equilibrium 1 and 2. The H+ released in Equilibrium Equation 2 made the pH of aqueous decreased. The pH of aqueous showed a slight increase when the mole ratio of TMPD and H3BO3 in the initial two phases decreased from 5.10 to 0.87, which indicated that the proportion of Equilibrium 2 decreaced in this extraction process. Continuously decreasing the mole ratio of TMPD and H3BO3 in the initial two phases, the pH of raffinate had no significant difference from the initial simulated brine, and the mole ratio of TMPD and extracted H3BO3 kept a constant of 1.59. This indicated that Equilibrium 1 played the leading role, and the TMPD molecular complexed with H3BO3 molecular one by one. Similary to the higher mole ratio of TMPD and H3BO3 in initial two phases, amounts of TMPD molecular which had no effect on H3BO3 existed in organic phase.
Fig. 7. Variation in mole ratio of TMPD and extracted H3BO3 and equilibrium pH value of raffinate with respect to the mole ratio of TMPD and H3BO3 in initial two phases. pHini =6.0; O/A = 1; [TMPD] = 0.4 mol/L.

3.1.4 Effect of O/A

The influence of volume ratio of organic phase to aqueous phase (O/A) on boron extraction was investigated in the range of 0.2~2.5. It can be seen from Fig. 8 that the extraction efficiency of H3BO3 increased with the increasing of O/A while the concentration of H3BO3 in the organic changed inversely. The rate of improvement in extraction efficiency slowed down when O/A was greater than 1.0. When O/A was equal to 1, the single-stage extraction efficiency reached 86%, and the boron concentration of organic was 0.17 mol/L. Therefore, O/A of 1.0 is a more appropriate phase ratio condition. We then experimentally determined the maximum boron loading capacity of organic phase at 0.4 mol/L TMPD concentration was 0.40 mol/L (in H3BO3). When the phase ratio is 2.5, the extraction efficiency can reach 97%, but the boron content loaded in organic is only 0.078 mol/L, which is far less than the maximum boron loading capacity. Therefore, in order to improve the utilization rate of extractant and reduce the amount of extractant, multi-stage extraction must be adopted.
Fig. 8. Effect of O/A on the extraction efficiency of H3BO3. [TMPD] = 0.4 mol/L; pHini = 4.6.

3.1.5 Effect of Salting-out effect

Since SL brines in the Qaidam Basin usually contains high concentration of magnesium chloride[31,32], the salting-out effect of magnesium chloride on boron extraction process was investigated. Four groups of magnesium chloride solutions (0~4.5 mol/L) containing 0.20 mol/L H3BO3 were prepared as the aqueous phase, and the pH of the four groups was adjusted to 1.3, 3.0, 4.6, and 6.0, respectively. As shown in Fig. 9, with the continuous increase of MgCl2, the difference of salting-out effect is obvious under different initial pH conditions: when the pHini is 1.3, the extraction efficiency of H3BO3 increases with the increase of the concentration of MgCl2, which is a positive salting-out effect; when the pHini ≥3.0, the changes of the extraction efficiency of H3BO3 show a similar trend, first increasing, then decreasing and then increasing. Since the addition of MgCl2 will promote the extraction of boric acid, and at the same time, it promotes the conversion of H3BO3 to [B(OH)4]- and polyboronic oxide anions, resulting in the reduction of the extraction efficiency of H3BO3. Therefore, the obtained data is the experimental result that the two interactions reach a balance. It can be concluded that the addition of MgCl2 does not always show positive salting-out effect for the dibasic alcohol extraction system, which is significantly different from the unary alcohol extraction system[25], and the lower the pHini , the more promoting effect of MgCl2 on boric acid extraction obviously. Additionally, the SL brine (pH=4.6) contains 4.27 mol/L Mg ions (shown in table 2) and in this concentration, the extraction efficiency of single-stage exceeds 80%, which also indicates that the extraction process has a high extraction efficiency without acidification.
Fig. 9. Changes of extraction efficiency with the MgCl2concentration at different pHini , [TMPD] = 0.4 mol/L; O/A=1.

3.1.6 Effect of Temperature

The effect of temperature on boron extraction by TMPD was performed in the temperature range of 293~333 K. Fig. 10 shows the linear relationship between log D against 1/T . Based on the slope of the straight line in Fig. 10, the enthalpy change (ΔH) of the extraction process can be calculated using the Van’t Hoff equation:
\(\log{D=\ -\frac{\text{ΔH}}{2.303R}\ \frac{1}{T}}\ +C\)Eq. (10)
where R is the universal gas constant (8.314 J·mol−1·K−1). The enthalpy changeΔH can be evaluated to be 17.24 kJ·mol−1, indicating the extraction reaction is endothermic. Therefore, appropriately increasing the temperature is beneficial for the extraction of boron. The Gibbs free energy ΔG were calculated by:
\(\text{ΔG}=\ -2.303\text{RT\ }\text{log\ }K\)Eq. (11)
Two equilibrium constant K values under different conditions were obtained from the fitting curve in the Fig. 5. Consequently, the changes in free energy at 293 K are calculated as follows: ΔG = −5.20 kJ·mol-1 when the mole ratio of TMPD and H3BO3 in initial two phases is less than 1:1, and ΔG = −9.42 kJ·mol-1 when the mole ratio of TMPD and H3BO3 in initial two phases is more than 1:1. Both are negative values, indicating that the extraction of boric acid by TMPD/CCl4 extraction system can proceed spontaneously.