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.