for larger image
Copper Electrorefining - Last step
- The end result of of the copper smelter operations is anode
copper which is 99% pure (see graphic on the left). An example
where copper is further purified is at the Asarco
electrolytic plant in Amarillo, Texas.
- The 99% pure copper from the anode furnace is cast into 750-pound,
3-inch thick, rectangular slabs called anodes.
- The final step is a process called electrorefining. This
is done by electrolysis where the copper anodes from the smelter
become positive electrodes and thin, three-foot square sheets
of pure copper are used as negative electrodes. Forty-five anodes
and 46 cathodes are interleaved and immersed in an electrolysis
tank filled with a solution of sulfuric acid and copper(II) sulfate.
- Oxidation occurs at the anode, where copper metal is converted
to copper(II) ions with the release of two electrons. At the
cathode the opposite reaction occurs: copper(II) ions are joined
with two electrons to form copper metal. During the migration
of metal ions from anode to cathode through the aqueous solution,
the impurities drop to the bottom of the electrolysis cell. Some
of these impurities are gold, silver, nickel, selenium, tellurium,
and platinum which are themselves recovered to be used.
- After two weeks, the cathodes are pulled from the tank- --
each one about 350 pounds of 99.99% pure copper. These are washed,
bundled, and sold. A new set of cathode starter sheets is inserted
and the process continues. After two more weeks, the second set
of cathodes is pulled and the remainder of the anodes are melted
and cast into new full-size anodes.
- Anode - oxidation: Cu metal (impure)---> Cu+2
+ 2 electrons
- Cathode - reduction: Cu+2 + 2 electrons --->
Cu metal (pure)
In-situ Copper Mining:
- Another type of mining being tried by BHP at Florence, AZ
is in-situ copper mining. A weak acid solution is injected in
wells to dissolve the copper in the ore without removing it.
The concentrated copper solution is then pumped out of recovery
- Hydrometallurgical routes have become more popular to research
because: energy costs are lower; more pollution-conscious communities
require "zero discharge" type conditions; whether for
air or water, as smelting processes are made "leak proof"
the impurities become a problem by concentrating in the copper;
hydrometallurgical processes can separate impurities better;
the operating temperatures are lower making pilot plants easier
to operate; sulfur dioxide is a gas that has to be made into
acid. (Elemental sulfur is formed in hydrometallurgical processes.)
- Hydrometallurgical methods of concentrating copper from
low grade ores:
- The leaching of the "oxide" materials is relatively
easy and relies on dump, vat and agitation leaching using dilute
sulfuric acid solutions.
Both the Asarco Ray Mine and the Silverbell Mine are leaching
low-grade copper with weakly acidic solutions. Silverbell produces
50 tons of SX/EW copper per day (design capacity).
Copper Ore - Arizona Mining Association
- Leaching sulfide materials, whether in ore or concentrate,
requires a chemical oxidizing agent ferric ions (Fe+3).
These special ferric ions can be generated by reactions with
air. The oxidation can be assisted by either pressure (as in
an autoclave) or more commonly with bacteria. The importance
of the bacteria and our understanding of their potential only
introduces an extra variable into the process. But different
copper minerals require different conditions to extract the copper.
- However, sulfuric acid is not the only reagent that can dissolve
copper from concentrate. For example, BHP has already patented
a process using ammonia to dissolve part of the copper concentrate.
(This is the basis of the Coloso cathode production plant in
- The Mission Mine has an an ore body with lots of limestone,
which precludes using acid leaching. Leaching with NaOH was tried
here in the 60's with costly success. It worked, but it cost
more than the copper recovered was worth.
Copper Applications in
Mining & Extraction How Do They Do That?
In Situ Mining
In-situ mining is designed to co-exist with other land uses,
such as agriculture. The in-situ mine will typically consist
of a series of injection wells (A) and recovery wells (B). The
wells are built with acid-resistant concrete and a PVC casing.
At the level where the drill hold penetrates the copper-bearing
rock (C), perforations in the casing allow a weak acid solution
to be pumped into the ore to dissolve the copper. Then the copper-rich
solution is drawn up through the recovery wells for processing
at a nearby SX-EW (Solvent Extraction, Electrowinning) facility.
|This article is reprinted with permission from
the publication, On CU, January - March, 1997, Vol. 1,
|On CU is the quarterly publication
BHP Copper, a business group of
The Broken Hill Proprietary Corp., Ltd.
© 1997 BHP Copper