EXTRACTION
Zinc Blende, also known as Sphalerite
(zinc sulfate) , is a major ore of zinc, a mineral. It can be found in many
locations worldwide. Zinc is often extracted from sphalerite as it
contains 40-50% of zinc.
Zinc metal is extracted from sphalerite using extractive metallurgy. Froth flotation is a process for separating minerals from gangue by taking advantage of differences in their hydrophobicity. Hydrophobicity differences between sphalerite and waste gangue (worthless material that surrounds, or is closely mixed with it) are increased through the use of surfactants and wetting agents. The selective separation of the minerals makes processing complex (that is, mixed) ores economically feasible. The flotation process is used for the separation of a large range of sulfides, carbonate and oxides (in this case, sphalerite) prior to further refinement.
Diagram: froth
flotation equipment
Afterwhich,
the sphalerite is roasted using pyrometallurgy,
where it is heated to a temperature of 600-740 degrees celcius, to allow oxygen
in the air to react with it. This is known as an oxidation reaction, where the
zinc sulfate is oxidized to zinc oxide.
2ZnS + 3O2 à 2ZnO + 2SO2
There are 2 ways
to obtain zinc from zinc oxide. The first way is to add coke (carbon) to ZnO in
a fireclay retort. The carbon will react with ZnO to form Zn (solid) and CO2
(gas).
ZnO + CO à Zn + CO2
Another type of extraction
is electrolytic. Dilute sulfuric acid (H2SO4) is added to ZnO,
forming an aqueous zinc sulfate (ZnSO4) solution and hydrogen gas; which is
removed as effervescence. As cadmium, am impurity, is found in the solution, it
has to be removed. Zinc dust is added to the solution, forming a precipitate of
cadmium sulfate and zinc solids.
IMPACT ON ENVIRONMENT &
SOLUTIONS
Before metals can be extracted
from their ores, the land must be mined for the ores. These mined lands are
usually unable to support any plant or animal life. During extraction, an
enormous amount of waste material is also generated. About 90% of the
extraction generates into waste and needs to be disposed of. If not carefully
disposed, it may leak into streams or soil in the nearby water bodies. In order
to dispose of the waste properly, huge landfill sites are needed to be dug and
take up a lot of space.
The smelting of ores can cause
air pollution. Gases such as sulphur dioxide are constantly released into the
air. Sulfur dioxide is the major precursor of acid rain, which acidified soils,
lakes and streams, accelerated corrosion of buildings and even monuments which
reduced their visibility. Sulfur dioxide causes fine particulate soot, which is
a significant health threat.
In addition, these processes
require a lot of energy and energy is usually generated from burning fossil
fuels, which happens to be in scarce supply.
Plus, after all the zinc has been
mined, the mine is abandoned and they are ‘robbed of their pillars’, which
involves mining the pillars that supported the mine roof, causing it to
collapse and eventually causing subsidence at the surface.
In the 1980's,
the U.S. Bureau of Mines, together with state geological surveys, conducted
detailed studies of the physical hazards associated with the old mining areas.
The studies discovered more than 1,500 open shafts and nearly 500 subsidence
collapsed mines in the Tri-State. A total of 599 mine hazards were found around
Galena, concentrated in an area that they call "Hell's Half Acre." In
1994 and 1995, the U.S. Environmental Protection Agency (EPA) together with the
local people filled in all the mine collapses and shafts in the town of Galena,
Kansas. Top soil was used to cover the chat and boulders in the area.
However the environmental problems were not resolved yet. Since the
mining has stopped, pumping has also stopped, causing the abandoned mines to
fill with water. These water become contaminated by minerals such as iron sulfide
(both pyrite and marcasite), which had remained in the mine walls or was left
behind by the miners, as well as by other metallic sulfides in the mines. This contaminated
water, in turn, contaminates the local ground
water, springs, as well as the surface water.
The EPA has been helping in the cleanup since early 1980's, and
work is ongoing. Early cleanup efforts were positioned at the area around
Galena as they had some of the worst contamination. One such effort was the
provision of safe water supply for rural residents whose wells had been contaminated.
Two new wells have been constructed in the deep aquifer, and a new rural water
district was formed, currently providing over 500 households with a long-term
source of clean drinking water.
references
from
