copper. The leading consumers of copper are wire mills and brass mills,
which use the copper to produce copper wire and copper alloys,
respectively. End uses of copper include construction materials,
electronic products, and transportation equipment. Once refined, copper
can be used as a powder in automotive, aerospace, electrical and
electronics equipment, in anti-fouling compounds, various chemicals and
medical processes. Compounds of copper include fungicides, wood
preservatives, copper plating, pigments, electronic applications and
specialized chemicals.
Copper can be produced as either a primary product or as a co-product of
gold, lead, zinc or silver. It is mined in both the Northern and
Southern Hemisphere and primarily consumed in the Northern Hemisphere
with the U.S. as a primary producer and consumer.
Primary Production of Copper
Copper is mined in open pits and below ground. The ore usually contains
less than 1% copper and is often associated with sulfide minerals. The
ore is ground, concentrated, and slurried with water and chemical
reagents. Air blown through the mixture attaches to the copper, causing
it to float to the top of the slurry. The copper is then removed with a
skimmer. The tailings remain and are dewatered and disposed of in
tailing ponds. The water is recovered and recycled.
One of two processing methods are used to refine concentrated copper.
Pyrometallurgy, or smelting, is used on ore with copper sulfide and iron
sulfide minerals. The concentrate is dried and fed into a furnace. The
minerals are partially oxidized and melted, resulting in segregated
layers. The matte layer refers to the iron-copper sulfide mixture which
sinks to the bottom. The slag, which refers to the remaining impurities,
floats on top of the matte. The slag is discarded on site or sold as
railroad ballast and sand blasting grit. Sulfur dioxide gases are also
collected and made into sulfuric acid for use in hydrometallurgical
leaching (discussed below) or sold off-site.
The matte is recovered and moved to the converter, a cylindrical vessel
into which the copper is poured. Air, lime and silica are added to react
with the metal oxide. Scrap copper may also be added. Iron slag is
removed and often recycled back into the furnace. Sulfur dioxide is
captured and converted into sulfuric acid. The converted copper, known
as "blister copper," is recovered.
The blister copper then undergoes "fire refining." Air and natural gas
are blown through the copper to remove any remaining sulfur and oxygen.
The copper is cast into copper anodes and placed in an electrolytic
cell. Once charged, the pure copper collects on the cathode and is
removed as 99% pure. The copper can be sold to wire-rod mills or further
processed into rods. Anode slime refers to impurities that sink to the
bottom of the electrolytic cell.
The second method for refining copper is called the hydrometallurgical
process. This process begins with oxidized copper ores or oxidized
copper wastes. The oxidized material is leached with sulfuric acid from
the smelting process. The sulfuric acid is percolated through piles of
oxidized metal and collected with acid resistant liners.
Further refining may be performed using one of two processes. In
cementation, the acidic solution of copper is deposited on to scrap iron
in an oxidation-reduction reaction. After sufficient amounts of copper
have been plated, the copper is further refined using the
pyrometallurgical process. However, this process is rarely used.
Solvent extraction is more commonly used to refine copper. An organic
solvent in which copper is soluble is introduced. As the copper is more
soluble in the organic layer than the aqueous, it enters an
organic-copper solution and is separated. Sulfuric acid is added to
strip the copper from the organic solvent into an electrolytic solution.
In the electrolytic process, called electrowinning, the copper plates
out onto the cathode. The cathodes are sold as-is or made into rods
on-site or made into starting sheets for other electrolytic cells.
All remaining organics and acids are reused. Further, sulfur is fixed
throughout the process to meet Clean Air Act Standards. If the sulfur
content of the gas is over 4%, the sulfur compounds are made into
sulfuric acid for use in the process or for sale to fertilizer
manufacturers. Slurries with less than 4% sulfur are classified as RCRA
hazardous wastes because of sulfur, cadmium, lead and other metals.
Secondary Copper Processing
Secondary copper processing involves two steps: metal pretreatment and
smelting. Pretreatment includes cleaning and concentrating the copper.
Concentrating is done manually or mechanically and includes sorting,
stripping, shredding and magnetic separation. The metal can be further
refined using pyrometallurgical methods — including sweating, insulation
burning, or drying — or hydrometallurgical methods — including flotation
and leaching. The concentrated metal is then smelted. Generally, copper
is fire refined, similar to primary copper smelting operations although
the exact procedure depends on the quality of copper scrap.
Pollution Output and Prevention in Copper Processing
Primary and secondary copper processing produce similar pollutants with
similar pollution prevention opportunities. Air emissions include
particulates and sulfur dioxide. Particulate air emissions usually
include iron and copper oxides, but many contain other metal oxides,
sulfates or sulfuric acid. Particulates are usually captured using
emissions control equipment. Depending on the composition of the
emissions some recovery of heavy metals may be possible.
In addition, secondary copper processing produces air emissions from the
removal of excess oils and cutting fluids. The air emissions are usually
captured using baghouses. After-burners may also be used to fully
combust products.
Sulfur dioxide is usually captured using single stage electrostatic
precipitation. Once captured, the sulfur dioxide is converted into
sulfuric acid and sold or reused in process.
Liquid wastes from the copper processing plant include large quantities
of water. Most of the water can be reused with minimal refinement. The
leaching process creates some sulfuric acid liquid waste. The sulfuric
acid is almost always directly reused. Electrolytic refining procedures
also produce some liquid waste. This waste is usually sent to waste
water treatment facilities and discharged.
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