7
precipitated as TeO2. According to Mokmeli et al. [58], Te
(IV) has the lowest solubility in a solution in pH of 4.5 at
25 °aC. To neutralize the alkaline leaching solution, H2SO4
is commonly employed. Consequently, Te can be recovered
as TeO2 precipitate by changing the pH to 4.5, and the
reaction is as follows:
Na2TeO3(aq) +H2SO4(aq) → TeO2(s)
+Na2SO4(aq) +H2O(l )(5)
Xu et al. [59] employed an oxidizing alkaline leaching
method to achieve the separation of Cu and Te, followed
by the neutralization of the alkaline leaching solution for
the precipitation of TeO2. Through the neutralization pro-
cess using H2SO4 to adjust the pH to 4.5, they successfully
recovered tellurium with a TeO2 precipitation efficiency
exceeding 95%, closely aligning with the anticipated theo-
retical value of approximately 97%.
Shao et al., [49] investigated the impact of several fac-
tors like reducing agent concentration, reaction time, tem-
perature, and agitation speed, on the selective precipitation
of Te from the leaching solution. Na2SO3 excess coefficient
(NEC) was employed to quantify Na2SO3 concentration
based on the research of Xu et al., [66]. They found that
under suitable conditions, including 16 of NEC, 10 min-
utes of reduction time, and 80 °C of temperature, approxi-
mately 99.83% of Te in the leaching solution was reduced
to crude Te. According to the microscopic morphology
analysis presented in Figure 5 at 90 °C for 60 minutes,
when NEC was less than 13, there were no correspond-
ing SEM images due to the low precipitation rate of Te.
However, as the Na2SO3 dosage increased, the number of
crude Te particles gradually raised and reached a peak at 16.
Alongside, needle-like crystals composed primarily of Ca,
S, and O appeared on the crude Te surface. The needle-like
or rod-shaped crystals were mainly CaSO4, which was a
result of the saturation of Ca2+ and SO42− in the leaching
solution. Although their saturation state could be disrupted
due to the formation of some SO42− after the reduction of
Na2SO3, it led to the precipitation of CaSO4. Increased
Na2SO3 content generated more SO42−, further which
facilitated the growth and coarsening of CaSO4. Hence, a
minimal Na2SO3 dosage could be beneficial to inhibit the
generation of CaSO4when Te(IV) in the leaching solution
is reduced into crude Te.
In the fields of PVC and semiconductors, achieving a
higher purity of Te is essential, as even minor contaminations
can negatively impact the operation of these equipment.
The electrowinning process is broadly used in industry for
this purpose [67]. Additionally, the electrodeposition-redox
Figure 4. The proposed flowsheet for extraction of Te and Cu from Cu2Te [60]
Figure 5. SEM images of Te after selective reduction of
leaching solution with different NEC values: (a) 13 (b) 16
(c) 18 (d) 20 (e) 24 (f) 32 [49]
precipitated as TeO2. According to Mokmeli et al. [58], Te
(IV) has the lowest solubility in a solution in pH of 4.5 at
25 °aC. To neutralize the alkaline leaching solution, H2SO4
is commonly employed. Consequently, Te can be recovered
as TeO2 precipitate by changing the pH to 4.5, and the
reaction is as follows:
Na2TeO3(aq) +H2SO4(aq) → TeO2(s)
+Na2SO4(aq) +H2O(l )(5)
Xu et al. [59] employed an oxidizing alkaline leaching
method to achieve the separation of Cu and Te, followed
by the neutralization of the alkaline leaching solution for
the precipitation of TeO2. Through the neutralization pro-
cess using H2SO4 to adjust the pH to 4.5, they successfully
recovered tellurium with a TeO2 precipitation efficiency
exceeding 95%, closely aligning with the anticipated theo-
retical value of approximately 97%.
Shao et al., [49] investigated the impact of several fac-
tors like reducing agent concentration, reaction time, tem-
perature, and agitation speed, on the selective precipitation
of Te from the leaching solution. Na2SO3 excess coefficient
(NEC) was employed to quantify Na2SO3 concentration
based on the research of Xu et al., [66]. They found that
under suitable conditions, including 16 of NEC, 10 min-
utes of reduction time, and 80 °C of temperature, approxi-
mately 99.83% of Te in the leaching solution was reduced
to crude Te. According to the microscopic morphology
analysis presented in Figure 5 at 90 °C for 60 minutes,
when NEC was less than 13, there were no correspond-
ing SEM images due to the low precipitation rate of Te.
However, as the Na2SO3 dosage increased, the number of
crude Te particles gradually raised and reached a peak at 16.
Alongside, needle-like crystals composed primarily of Ca,
S, and O appeared on the crude Te surface. The needle-like
or rod-shaped crystals were mainly CaSO4, which was a
result of the saturation of Ca2+ and SO42− in the leaching
solution. Although their saturation state could be disrupted
due to the formation of some SO42− after the reduction of
Na2SO3, it led to the precipitation of CaSO4. Increased
Na2SO3 content generated more SO42−, further which
facilitated the growth and coarsening of CaSO4. Hence, a
minimal Na2SO3 dosage could be beneficial to inhibit the
generation of CaSO4when Te(IV) in the leaching solution
is reduced into crude Te.
In the fields of PVC and semiconductors, achieving a
higher purity of Te is essential, as even minor contaminations
can negatively impact the operation of these equipment.
The electrowinning process is broadly used in industry for
this purpose [67]. Additionally, the electrodeposition-redox
Figure 4. The proposed flowsheet for extraction of Te and Cu from Cu2Te [60]
Figure 5. SEM images of Te after selective reduction of
leaching solution with different NEC values: (a) 13 (b) 16
(c) 18 (d) 20 (e) 24 (f) 32 [49]