Converting Minerals to Metals - Mining and the Environment: From Ore to Metal - page 260

ACID FLOW

Stripped Organic
Solution

Acid
Make up

Leach Solution Spent Electrolyte

Ore

Pure
Copper
Cathodes

Leaching Extracting Stripping Electrowinning

FIGURE 6.8
Stages Involved in Solvent Extraction and Electro-
winning of Copper from Low-grade Oxide Ores

Spent
Ore

Copper Loaded
(pregnant) Leach Solution

Copper Loaded
Organic Solution

Copper Loaded
Aqueous Solution

About 20 percent of all copper produced is pro-
duced by the SX/EW process. In Latin America,
the total is closer to 40 percent whereas in the
USA the total is approaching 30 percent.

IONIC COPPER FLOW

Recovering Copper from the Pregnant Leach Solution

Each copper leaching method results in a pregnant leach solution (PLS). The method used
for recovering copper from the leach solution is the solvent extraction-electrowinning process
( Figure 6.8 ; U.S. Congress, Office of Technology Assessment 1988). The widely applied
leach-solvent extraction-electrowinning process (or in short SX/EW) process has its roots
in analytical chemistry where it is used to separate one metallic ion from another. It was
first used as a large-scale process during World War II for the recovery of uranium from
its ores. The key to the process is the development of organic extractants that are specific
to the metal to be extracted. The first extractant that was specific for copper and used at a
commercial scale was developed by General Mills Corporation and sold under the name
LIX 64® (LIX for Liquid Ion Exchange and Roman for 1959 - the year of the first patent).
Ranchers Exploration and Development Corporation at its Bluebird Mine in Arizona in
1968 first demonstrated it on a large scale. Today, worldwide, about 20 percent of all cop-
per is produced by the SX/EW process. In Latin America, the total is closer to 40 percent,
whereas in the USA the total is approaching 30 percent (Tilton and Lansberg 1997).
Solvent extraction stage - The solvent extraction (SX) part of the process occurs in
two steps and each step includes a process of ion exchange between two solutions. In the
first step the pregnant leach solution is mixed vigorously with a kerosene-based solvent
containing an organic chemical specifically designed to extract copper. Here the cop-
per is extracted away from the aqueous phase leaving behind most of the impurities that
were in the pregnant leach solution. Since in the solvent extraction stage, copper ions are
exchanged for hydrogen ions, the aqueous phase is returned to its original acidity and
recycled to the leaching step of the process. During the ion exchange, the lighter organic
copper bearing solution floats on top of the heavier leach solution, much like oil floats on
water, and can be easily separated and directed to the next processing step.
In the second step the copper exchange is reversed in another tank that strips the copper from
the organic solution. The copper-laden organic solution is mixed with a strong sulphuric acid
solution known as the electrolyte or aqueous solution. In the mixing and settling stage, the cop-
per is transferred from the organic to the aqueous solution, filtered and then pumped to the
electrowinning (EW) tank house. During the process, the organic phase is reconstituted in its
hydrogen form and, in its barren organic phase, is returned to the extraction stage of the process.

Today, worldwide, about 20
percent of all copper is produced
by the SX/EW process.