5
Sun et al. [51] demonstrated that in a high-dosage HNO3
solution, nearly 99% of the Cu was successfully leached,
whereas only 3% of the Te was extracted. Subsequently, the
leaching efficiency of Te was significantly improved, reach-
ing 99%, when treated with HCl.
Given that Te can be dissolved in both acidic and alka-
line environments whereas Cu is only soluble in an acidic
media, the use of alkaline leaching represents a potentially
promising approach for separating copper and tellurium.
Moreover, the low corrosiveness of alkaline solutions to
reaction equipment presents a distinct advantage, as it
helps reduce the costs associated with process maintenance.
This aspect holds significant economic value and practical
importance for industrial applications. The selective leach-
ing of Te and Cu is achieved through an aerated alkaline
leaching method in which NaOH is frequently employed
as the leaching agent. In this process, Te is transformed
into a soluble Te(IV) compound, while Cu remains in solid
form within the residue, although it undergoes oxidation to
CuO. The reactions involved are as follows [52]:
Te(s) +2NaOH(aq) +O2(g) → Na2TeO3(aq)
+H2O(l )(3)
2Cu(s) +O2(g) → 2CuO(s) (4)
Oxygen can be introduced by blowing air into the solu-
tion, although the quantity of oxygen must be kept in the
optimum value, as extreme oxygen levels can result in the
oxidation of soluble Te (IV) into its insoluble form (VI)
within the alkaline environments. The separation between
Te and Se can be accomplished through the high levels of
oxidation and transformation of Te and Se into hexava-
lent valence forms. In this process, Se(VI) takes the form
of sodium selenate, which is soluble in alkaline solutions,
while Te(VI) forms sodium tellurate, which remains insol-
uble in the residue, allowing for the efficient separation of
Se and Te [53].
Based on the Cu–Te–Se–H2O system’s Pourbaix dia-
gram, Fan et al. [38] proposed a method for the selective
alkaline pressure leaching of Cu, Te, and Se. They noted
that approximately 90% of the Te can be oxidized to a
soluble form, sodium tellurite (IV), while the Cu remains
in the residue. Within an alkaline environment, both Te
and Se were dissolved in the forms of TeO32– and SeO42–.
Consequently, the alkaline pressure oxidation leaching pro-
cess with NaOH could effectively separate Te and Se from
Cu and other impurities, provided the conditions were
carefully controlled. The challenge of separating Te from
Se was addressed by neutralization of the alkaline solution,
in which TeO3–2 ions were hydrolyzed to TeO2, while Se
remained in the solution. Multiple tests conducted under
various conditions revealed that the leaching efficiencies of
Te and Se were significantly influenced by reaction’s pres-
sure and temperature. Optimal conditions for the process
included an L/S ratio of 6:1, 30- 40 g/L NaOH, total pres-
sure of compressed air of 1 MPa, 120 °C, agitation rate of
400 rpm for 6 hours.
In various industrial facilities like Luilu Metallurgical
Plant in Congo [54], Freeport’s Refinery in the USA [55],
Naoshima’s Smelter and Refinery in Japan [56], and sev-
eral major copper smelters in China such as Zhongyuan
Gold Smelter and Tongling Nonferrous Metals Smelter,
the elimination of Te from the leaching solution is typically
achieved through the reduction of Te (IV/VI) forms using
metallic copper, leading to the production of insoluble cop-
per telluride (Cu2Te) [57,58]. This method allows for the
enrichment of Te by more than tenfold from anode slime
to Cu2Te. However, many companies opt to store or sell the
Cu2Te at low costs instead of pursuing the individual recov-
ery of Te and Cu. This decision is largely taken because of
the intricate physicochemical characteristics of Te, which
pose significant technical challenges in the process of Cu-Te
separation. Cu2Te remains stable over a broad pH range.
The development of an affordable and environmentally
friendly process for Te and Cu extraction from Cu2Te is of
paramount importance for the copper smelters.
Xu et al. [59] introduced both atmospheric and high
pressure oxidizing alkaline leaching techniques with O2
serving as the oxidant for the recovery of Te and Cu from
the Cu2Te. The process mechanisms for the leaching of
Cu2Te at atmospheric pressure are depicted in Figure 3
(a). Initially, the Cu2Te reacts with NaOH and O2, lead-
ing to a selective dissolution of Te into the solution, while
Cu remains in the solid phase as Cu2O. As the reaction
progresses, the unreacted Cu2Te core diminishes, and the
Cu2O layer gradually thickens. Because of the limited
kinetic conditions at atmospheric pressure, the diaffusion of
NaOH and O2 toward the surface of the unreacted Cu2Te
becomes increasingly challenging, resulting in the forma-
tion of a dense outer Cu2O surface. Meanwhile, the Cu2O
on the surface undergoes more oxidation by O2, forming
a fresh outer layer of Cu(OH)2, which subsequently reacts
with the dissolved Na2TeO3 at the liquid-solid interface,
leading to the production of insoluble CuTeO3 on the
particle’s surface. This prompts the reverse transfer of the
dissolved Te from the solution to the solid phase, thereby
contributing to the low Te leaching rate of the alkaline
leaching operation at atmospheric pressure. The mecha-
nisms of the pressure oxidizing alkaline leaching operation
of Cu2Te at 0.7 MPa are depicted in Figure 3. At first, Te
Sun et al. [51] demonstrated that in a high-dosage HNO3
solution, nearly 99% of the Cu was successfully leached,
whereas only 3% of the Te was extracted. Subsequently, the
leaching efficiency of Te was significantly improved, reach-
ing 99%, when treated with HCl.
Given that Te can be dissolved in both acidic and alka-
line environments whereas Cu is only soluble in an acidic
media, the use of alkaline leaching represents a potentially
promising approach for separating copper and tellurium.
Moreover, the low corrosiveness of alkaline solutions to
reaction equipment presents a distinct advantage, as it
helps reduce the costs associated with process maintenance.
This aspect holds significant economic value and practical
importance for industrial applications. The selective leach-
ing of Te and Cu is achieved through an aerated alkaline
leaching method in which NaOH is frequently employed
as the leaching agent. In this process, Te is transformed
into a soluble Te(IV) compound, while Cu remains in solid
form within the residue, although it undergoes oxidation to
CuO. The reactions involved are as follows [52]:
Te(s) +2NaOH(aq) +O2(g) → Na2TeO3(aq)
+H2O(l )(3)
2Cu(s) +O2(g) → 2CuO(s) (4)
Oxygen can be introduced by blowing air into the solu-
tion, although the quantity of oxygen must be kept in the
optimum value, as extreme oxygen levels can result in the
oxidation of soluble Te (IV) into its insoluble form (VI)
within the alkaline environments. The separation between
Te and Se can be accomplished through the high levels of
oxidation and transformation of Te and Se into hexava-
lent valence forms. In this process, Se(VI) takes the form
of sodium selenate, which is soluble in alkaline solutions,
while Te(VI) forms sodium tellurate, which remains insol-
uble in the residue, allowing for the efficient separation of
Se and Te [53].
Based on the Cu–Te–Se–H2O system’s Pourbaix dia-
gram, Fan et al. [38] proposed a method for the selective
alkaline pressure leaching of Cu, Te, and Se. They noted
that approximately 90% of the Te can be oxidized to a
soluble form, sodium tellurite (IV), while the Cu remains
in the residue. Within an alkaline environment, both Te
and Se were dissolved in the forms of TeO32– and SeO42–.
Consequently, the alkaline pressure oxidation leaching pro-
cess with NaOH could effectively separate Te and Se from
Cu and other impurities, provided the conditions were
carefully controlled. The challenge of separating Te from
Se was addressed by neutralization of the alkaline solution,
in which TeO3–2 ions were hydrolyzed to TeO2, while Se
remained in the solution. Multiple tests conducted under
various conditions revealed that the leaching efficiencies of
Te and Se were significantly influenced by reaction’s pres-
sure and temperature. Optimal conditions for the process
included an L/S ratio of 6:1, 30- 40 g/L NaOH, total pres-
sure of compressed air of 1 MPa, 120 °C, agitation rate of
400 rpm for 6 hours.
In various industrial facilities like Luilu Metallurgical
Plant in Congo [54], Freeport’s Refinery in the USA [55],
Naoshima’s Smelter and Refinery in Japan [56], and sev-
eral major copper smelters in China such as Zhongyuan
Gold Smelter and Tongling Nonferrous Metals Smelter,
the elimination of Te from the leaching solution is typically
achieved through the reduction of Te (IV/VI) forms using
metallic copper, leading to the production of insoluble cop-
per telluride (Cu2Te) [57,58]. This method allows for the
enrichment of Te by more than tenfold from anode slime
to Cu2Te. However, many companies opt to store or sell the
Cu2Te at low costs instead of pursuing the individual recov-
ery of Te and Cu. This decision is largely taken because of
the intricate physicochemical characteristics of Te, which
pose significant technical challenges in the process of Cu-Te
separation. Cu2Te remains stable over a broad pH range.
The development of an affordable and environmentally
friendly process for Te and Cu extraction from Cu2Te is of
paramount importance for the copper smelters.
Xu et al. [59] introduced both atmospheric and high
pressure oxidizing alkaline leaching techniques with O2
serving as the oxidant for the recovery of Te and Cu from
the Cu2Te. The process mechanisms for the leaching of
Cu2Te at atmospheric pressure are depicted in Figure 3
(a). Initially, the Cu2Te reacts with NaOH and O2, lead-
ing to a selective dissolution of Te into the solution, while
Cu remains in the solid phase as Cu2O. As the reaction
progresses, the unreacted Cu2Te core diminishes, and the
Cu2O layer gradually thickens. Because of the limited
kinetic conditions at atmospheric pressure, the diaffusion of
NaOH and O2 toward the surface of the unreacted Cu2Te
becomes increasingly challenging, resulting in the forma-
tion of a dense outer Cu2O surface. Meanwhile, the Cu2O
on the surface undergoes more oxidation by O2, forming
a fresh outer layer of Cu(OH)2, which subsequently reacts
with the dissolved Na2TeO3 at the liquid-solid interface,
leading to the production of insoluble CuTeO3 on the
particle’s surface. This prompts the reverse transfer of the
dissolved Te from the solution to the solid phase, thereby
contributing to the low Te leaching rate of the alkaline
leaching operation at atmospheric pressure. The mecha-
nisms of the pressure oxidizing alkaline leaching operation
of Cu2Te at 0.7 MPa are depicted in Figure 3. At first, Te