1
25-092
The Role of Oxidizing Agents in the Recovery of Precious Metals
and PGMs During the Alkaline Leaching of Selenium-Rich
Metallurgical Wastes
L. Sanchez-Calderon
Missouri Univ. of Science and Tech., Rolla, MO
W. Kirk
Missouri Univ. of Science and Tech., Rolla, MO
M. Silwamba
Missouri Univ. of Science and Tech., Rolla, MO
L. Alagha
Missouri Univ. of Science and Tech., Rolla, MO
ABSTRACT
Selenium-rich metallurgical waste, also known commer-
cially as “crude selenium,” is a byproduct of the copper (Cu)
refining process. This by-product, contains ~90% selenium
(Se), precious metals, platinum group metals (PGMs), and
other critical elements. This study investigated the leach-
ing efficiency of Se from the sludge under alkaline condi-
tions using different oxidizing agents to enrich PGMs and
precious metals in the leach solid residue. The effectiveness
of each oxidizing agent during leaching was assessed and
compared. Additionally, Eh-pH diagram for Se was estab-
lished under specific experimental conditions and utilized
to inform process optimization. Key variables, including
leaching time, temperature, and oxidant concentration,
were evaluated for their impact on the efficiency and the
selectivity of the oxidative alkaline leaching process.
INTRODUCTION
The U.S. Department of Energy has published a list of criti-
cal minerals based on their significance to green energy and
national security, and the risk associated with their supply
chains (Figure 1). Relevant to this manuscript, tellurium
(Te), platinum (Pt), and palladium (Pd) are included in the
list. On the other hand, some elements are not classified
as critical minerals but often contribute to the economic
feasibility of critical minerals processing. These elements
include gold (Au) and silver (Ag), which have market prices
of approximately $38,000 and $462 per kilogram, respec-
tively (Sim et al., 2024).
Copper anode slimes are byproducts of Cu production
that originate from the electrorefining step. These slimes
are always enriched with valuable elements, including pre-
cious metals, critical minerals, and residual Cu (Xing &
Lee, 2017). Most refineries leverage these byproducts to
recover additional elements and diversify income streams.
Typically, the recovery process begins with extracting the
remaining Cu using a selective lixiviant, such as sulfuric
acid (Chen et al., 2015). After the removal of Cu from the
slimes, the subsequent steps are determined by the concen-
tration of each remaining element. Refineries may target
Ag and lead (Pb) using wet chlorination process. The wet
chlorination process leverages the ability of Ag and Pb to
form insoluble chlorides (Ding et al., 2017). This is usually
achieved by leaching the material with hydrochloric acid,
leading to the formation of silver chloride and lead chloride
precipitates (Hoffmann, 1990).
Other elements such as Au and PGMs can be recovered
by various approaches. Au and PGMs can be selectively
separated from the solution using a solvent extraction pro-
cess or selective precipitation. Reagents for solvent extrac-
tion include dibutyl carbinol or tributyl phosphate, while
precipitation can be achieved using reagents such as zinc
(Zn) for Au or ammonium chloride for PGMs, resulting in
the formation of ammonim hexachloropalladate or hexa-
chloroplatinate (Raiguel et al., 2020).
Removing precious metals is extremely challenging
due to their noble characteristics. Attempting to selectively
leach them could potentially result in the unintended leach-
ing other materials as well. In this study, a selective leach-
ing process was implemented specifically for Se removal.
Previous studies have successfully demonstrated the effec-
tiveness of alkaline leaching for Se extraction. However,
25-092
The Role of Oxidizing Agents in the Recovery of Precious Metals
and PGMs During the Alkaline Leaching of Selenium-Rich
Metallurgical Wastes
L. Sanchez-Calderon
Missouri Univ. of Science and Tech., Rolla, MO
W. Kirk
Missouri Univ. of Science and Tech., Rolla, MO
M. Silwamba
Missouri Univ. of Science and Tech., Rolla, MO
L. Alagha
Missouri Univ. of Science and Tech., Rolla, MO
ABSTRACT
Selenium-rich metallurgical waste, also known commer-
cially as “crude selenium,” is a byproduct of the copper (Cu)
refining process. This by-product, contains ~90% selenium
(Se), precious metals, platinum group metals (PGMs), and
other critical elements. This study investigated the leach-
ing efficiency of Se from the sludge under alkaline condi-
tions using different oxidizing agents to enrich PGMs and
precious metals in the leach solid residue. The effectiveness
of each oxidizing agent during leaching was assessed and
compared. Additionally, Eh-pH diagram for Se was estab-
lished under specific experimental conditions and utilized
to inform process optimization. Key variables, including
leaching time, temperature, and oxidant concentration,
were evaluated for their impact on the efficiency and the
selectivity of the oxidative alkaline leaching process.
INTRODUCTION
The U.S. Department of Energy has published a list of criti-
cal minerals based on their significance to green energy and
national security, and the risk associated with their supply
chains (Figure 1). Relevant to this manuscript, tellurium
(Te), platinum (Pt), and palladium (Pd) are included in the
list. On the other hand, some elements are not classified
as critical minerals but often contribute to the economic
feasibility of critical minerals processing. These elements
include gold (Au) and silver (Ag), which have market prices
of approximately $38,000 and $462 per kilogram, respec-
tively (Sim et al., 2024).
Copper anode slimes are byproducts of Cu production
that originate from the electrorefining step. These slimes
are always enriched with valuable elements, including pre-
cious metals, critical minerals, and residual Cu (Xing &
Lee, 2017). Most refineries leverage these byproducts to
recover additional elements and diversify income streams.
Typically, the recovery process begins with extracting the
remaining Cu using a selective lixiviant, such as sulfuric
acid (Chen et al., 2015). After the removal of Cu from the
slimes, the subsequent steps are determined by the concen-
tration of each remaining element. Refineries may target
Ag and lead (Pb) using wet chlorination process. The wet
chlorination process leverages the ability of Ag and Pb to
form insoluble chlorides (Ding et al., 2017). This is usually
achieved by leaching the material with hydrochloric acid,
leading to the formation of silver chloride and lead chloride
precipitates (Hoffmann, 1990).
Other elements such as Au and PGMs can be recovered
by various approaches. Au and PGMs can be selectively
separated from the solution using a solvent extraction pro-
cess or selective precipitation. Reagents for solvent extrac-
tion include dibutyl carbinol or tributyl phosphate, while
precipitation can be achieved using reagents such as zinc
(Zn) for Au or ammonium chloride for PGMs, resulting in
the formation of ammonim hexachloropalladate or hexa-
chloroplatinate (Raiguel et al., 2020).
Removing precious metals is extremely challenging
due to their noble characteristics. Attempting to selectively
leach them could potentially result in the unintended leach-
ing other materials as well. In this study, a selective leach-
ing process was implemented specifically for Se removal.
Previous studies have successfully demonstrated the effec-
tiveness of alkaline leaching for Se extraction. However,