Other Effects of Extraction Of Zinc From Zinc Blende

While the extraction of zinc brings about numerous economic benefits to communities and countries, it can also have serious ramifications on the enivonment. These consequences can be categorized into several aspects- water, soil and air.

Environmental Impact on Water

Sources of groundwater pollution due to the extraction of zinc can come from potential sources such as include runoff, leachate, and seepage from tailings impoundments as well as waste rock piles. Zinc is extracted from sulphur bearing ores, such as zinc blende ore, which is actually zinc sulphide. This in turns encourages the generation of acid.  Acid in ground and surface water causes it to become acidified, which causes greater potential for the leaching of heavy metals and facilities the transport into bodies like rivers or lakes. The existence of bacteria like thiobacillus ferroxidans are catalyse the oxidation of sulphur-bearing ores which further encourage the generation of acid. Zinc extraction also increases the instances for zinc wastes produced during zinc extraction to enter water bodies. The problem is worsened by abandoned mines as existing waste and contaminants are not cleared and continue to leak into water bodies unnoticed. For example, in Wales, Cwm Rheidol mine, near Aberystwyth, which practiced zinc extraction over the centuries, has historically discharged large amounts of zinc into the River Rheidol.

Since water bodies are polluted with zinc and other contaminants due to zinc extraction, the pH of the water decreases while the acidity levels of the water increases, which leads to the pollution of water. When the water becomes polluted with zinc and other waste products produced during zinc extraction, it deteriorates the quality of the water. This will affect the marine life and local communities. Marine life such as fish can die due to the overconsumption of lead, and pollution of water bodies means it cannot support marine life. It also has devastating effects on the ecosystems there, affecting the food chains and biodiversity. Communities who depend on these water bodies for drinking water and other uses are also unable to continue using the water there and if they consume the water, they are undesirable health effects like nausea, respiratory disorders and anaemia.

Environmental Impact on Soil

Discharge, runoff, leachate, and seepage from tailings impoundments,

pits and underground workings, as well as waste rock piles, which are results on zinc extraction.  In addition, deposition of wind-blown particulates from dry tailings impoundments is also a potential cause of zinc contminatoon in soil. Spills of fuel, flotation reagents, cleaning solutions, as well as other chemicals such as zinc sulphide used during the extraction process can also result in zinc contamination of soil. Zinc one of the most soluble metal elements in soils, and can have toxic impacts on the soil as it can increase the acidity of the soil, thereby affecting the fertility of the soil and causing mineral deficiency. This affects the growth of plants and other organisms in the soil. It has repercussions to soil quality as well as the organisms in the soil thereby affecting the ecosystem in the soil. Contaminated soil can have disastrous effects since leakages can affect the quality and contaminate groundwater and water bodies. 

Environmental Impact on Air

dust from the dry surfaces of tailings impoundments, as well as waste rock piles, resulting from zinc extraction is the main cause of air pollution due to zinc mining. Tailings impoundments are not completely covered by water, therefore being dry enough for wind transport. This gives rise to pollution of water and soil, whereby due to winds, the contaminants and zinc metals contaminate the water and soil. Humans can also suffer from health problems like respiratory problems due to the dust blown in from the dry tailing impoundments.

Example

The Red Dog extracts zinc from heDeLong Mountains which contains large amounts of zinc deposists.  Leaching of metals from the ore-rich DeLong Mountains into Red Dog Creek has caused widespread pollution in water bodies. Red Dog Creek cannot support any fish, partly because of high metal concentrations.  The contaminated water empties into the Ikalukrok Creek, which in turn empties into the Wulik River.  It is one of the richest fisheries in northwestern Alaska and hence this results in .  Ikalukrok Creek is a wintering ground for Dolly Varden (Arcticchar), and Wulik River is a major habitat for Dolly Varden and spawning salmon, and the contaminated water affected the spawning of salmon and growth of salmon.  The Wulik River also flows to the small Inupiat Eskimo community of Kivalina, about 60 miles downstream from the mine. This affected communities relying on the water for livelihood for fishing and as a resource for drinking water from the river. As a result, they lost their source of water and livelihood.

In the early 1980s, before mining began, the Alaska Department of Environmental Conservation (DEC)records showed natural concentrations of zinc in Red Dog Creek between 7 and 52 parts per million (ppm),

with one reading of 273 ppm. Summer 1990, where zinc extraction was in full force, samples from the creek showed consistent zinc measurements

above 100 ppm, with an early July measurement of 970 ppm, which showed that due to zinc extraction, the amounts of zinc has increased rapidly, over legal levels, meaning that there was pollution. Alaska Department of Fish and Game (ADF&G) biologists have identified discoloration in the form of a milky orange plume of precipitate beginning at Red Dog Mine and flowing

downstream for about 20 miles.  The source of the water is seepage from the saturated zone associated with the ore body exposed by extraction, which contains high concentrations of zinc.

Images:

The town of Picher, Okla., is nestled among huge lead-laced piles of rock. Its population has dwindled to a fraction of its former size as people gradually move away from the Tar Creek Superfund site left from years of lead and zinc mining.

http://www.msnbc.msn.com/id/24555711/ns/us_news-environment/t/pollution-brings-end-oklahoma-mining-town/#.UEH8JNYgfSY

Reference:

http://www.epa.gov/osw/nonhaz/industrial/special/mining/techdocs/leadzinc.pdf

EXTRACTION AND BENEFICIATION OFORES AND MINERALS: LEAD & ZINC (USA Environmental Protection Agency)

http://www.lenntech.com/periodic/elements/zn.htm

Extraction of Zinc

Method of Extraction of Zinc from Its Ore, Zinc Blende

Concentration:

The zinc blende ore is concentrated using the froth floatation process, in which the ore is mixed with water and pine oil and some sodium ethyl xanthate in a tank. Air blows on this mixture. This results in particles “wetted by oil” (Sharma) rising to the surface in the form of froth, which can be skimmed off and collected. The impurities, on the other hand, are “wetted by water” (Sharma) and thus sink to the base of the tank. 

Roasting:

The concentrated zinc blende ore is roasted in the presence of air so that zinc sulphide will be converted to zinc oxide.

2 ZnS + 3 O₂ →2 ZnO + 2 SO₂

Reduction:

Mix the roasted ore with half its mass of powdered coal or coke (carbon) and placed “into a fireclay retort furnace” (Sharma).

ZnO + C →Zn + CO₂

At such a high temperature, zinc is gaseous. Thus, gaseous Zn could mix with the CO₂, so that when they are removed reoxidation of zinc could occur, giving us ZnO again.

To make the reoxidation minimal, smelters heat the ZnO with excess carbon so that the CO₂ formed is converted to CO.

CO₂ + C →2CO

As a result, the reduction of zinc oxide can be illustrated as follows:

ZnO + C →Zn + CO

Purification:

Using electrolysis, the zinc metal compound is broken apart to obtain pure zinc metal.

Effects of Extraction of Zinc from Its Ore, Zinc Blende

During the extraction process of Zinc form its ore, Carbon Monoxide and Sulphur dioxide is released into the atmosphere. This could lead to pollution of water sources or formation of acid rain.

Carbon monoxide is an industrial gas and may be collected from the extraction process to be used in other industrial processes, such as the purification of Nickel through the Mond Process.

Sulphur Dioxide similarly has several uses outside its toxicity. It may also be used as a reducing agent, or as a means of forming sulfite salts.

Instead of being released as waste gases, both products can be collected and reused for other industrial purposes. 

Reference:

Images:

“Froth floatation process” http://www.goiit.com/templates/default/images/chapters/gen_princp/fig.4.gif

Sharma, Neeraj, Prof. "Extraction of Zinc." Industrial Aspects of Inorganic Chemistry. 2007.

http://www.newton.dep.anl.gov/askasci/chem03/chem03435.htm

http://chem-guide.blogspot.sg/2010/04/occurrence-and-extraction-of-zinc-from.html

Short note on contributions:

Method of Extraction – Radhika

Effects of Extraction – Wei Liang

Solution to Problems –Elizabeth

The Extraction of Aluminium from Bauxite

Sources:

1. http://entrancechemistry.blogspot.sg/2011/05/extraction-of-aluminium-from-bauxite.html
2. http://www.chemguide.co.uk/inorganic/extraction/aluminium.html
3. http://sam.davyson.com/as/physics/aluminium/siteus/extraction.html
4. http://en.wikipedia.org/wiki/Bauxite
5. http://en.wikipedia.org/wiki/Bayer_process
6. http://en.wikipedia.org/wiki/Hall%E2%80%93H%C3%A9roult_process
7. http://en.wikipedia.org/wiki/Red_mud

About Bauxite: 

Bauxite is an aluminium ore that contains a mixture of aluminium oxides( Al(OH)₃, γ-AlO(OH), and  α-AlO(OH etc.) as well as iron oxides,silicon dioxide and titanium oxide.

As aluminium is high up in the reactivity series, a pure sample of aluminium cannot be obtained by reduction with carbon. Furthermore, aluminium forms a stable compound with oxygen and cannot be displaced easily.

Therefore Aluminium has to be purified with two different methods, the Bayer Process or Electrolysis.

Bayer Process:

Industrial set-up for the Bayer Process.

During the Bayer Process, crushed bauxite is broken down or digested by a solution of hot concentrated NaOH solution, according to this reaction: Al₂O₃ + 2 NaOH + 3 H₂O → 2 NaAl(OH)₄

_{In the digester Sodium tetryhydroxoaluminate will be dissolved as solution while the other impurities in the ore will not react and remain insoluble.} 

Next All of these solids are filtered out. The residue is known as 'red mud'.

Then sodium tetryhydroxoaluminate is cooled in the precipitator. Crystallisation takes place, producing a precipitate of Aluminium hydroxide, Al(OH)3, or alumina  and sodium hydroxide.

NaAl(OH)₄ → Al(OH)₃ + NaOH

Aluminium Hydroxide is then purified in the Rotary Kiln by heating to temperatures between 1000 to 1200 degrees celsius. Aluminium hydroxide is then decomposed into aluminium oxide and water.

2 Al(OH)₃ → Al₂O₃ + 3 H₂O and is collected for electrolysis.

Electrolysis or the Hall-Heroult process:

simplified Electrolysis cell.

Aluminium Oxide is first dissolved in a mixture of molten cryolite, Na3AlF6, and Aluminium fluoride to lower the melting point of aluminium oxide in the set-up shown above. A negative current will be passed through the carbon lining of the 

 set-up while a positive current will be passed through the carbon anodes. This electrolyses the aluminium oxide, cryolite 

 and the aluminium fluoride,  causing liquid aluminium to be formed in the cathode. Meanwhile oxygen gas reacts with 

 carbon from the anode to form carbon dioxide. 

 Liquid Aluminium is taken out with a siphon and is transported elsewhere to be cast into aluminium ingots.

Environmental impact:

Red Mud expelled in the early part of the experiment is accumulated in small containments or disposed of in the lakes and rivers. As the mixture is a strong alkali, it would adversely affect the pH of the soil, leading to complications in its disposal.

We believe that as red mud is a mixture of the waste products from bauxite, such as haematite and silicon dioxide, it can be taken away for iron to be produced instead of dumped.

Research by: Wei Liang and Radhika

Write up by: Elizabeth