Extraction of zinc from zinc blende

REQUIREMENTS

Explain how the metal is extracted from its ore.

How the extraction method has an impact on its environment

Suggest solutions that are in place to address the problems that you have mentioned.

ZINC BLENDE

http://wikis.lib.ncsu.edu/images/f/fc/ZnS.png

A mineral form of Zinc Sulfide.
The primary ore of zinc, occurring in usually yellow-brown or brownish-black crystals or cleavage masses, essentially ZnS with some cadmium, iron, and manganese.
An ore is a type of rock that contains minerals with important elements including metals.

http://www.thefreedictionary.com/Zincblende
http://en.wikipedia.org/wiki/Ore

EXTRACTION OF ZINC FROM ZINC BLENDE

BLAST FURNACES

(1) The zinc sulphide ore is roasted in air to give impure zinc oxide.

2ZnS_(s) + 3O_2(g) ==> 2ZnO_(s) + 2SO_2(g)
ZnCO_3(s) ==> ZnO_(s) + CO_2(g)

(2) The impure zinc oxide can be treated to extract the zinc:

(a) It is roasted in a smelting furnace with carbon (reducing agent) and limestone (to remove the acidic impurities).

C_(s) + O_2(g) ==> CO_2(g)
(very exothermic oxidation, raises temperature considerably)

C_(s) + CO_2(g) ==> 2CO_(g)

(C oxidised, CO₂ reduced)

ZnO_(s) + CO_(g) ==> Zn_(l) + CO_2(g)

(zinc oxide reduced by CO, Zn undergoes O loss)

or

direct reduction by carbon:

 ZnO_(s) + C_(s) ==> Zn_(l) + CO_(g)

 (ZnO reduced, C oxidised)  

The carbon monoxide acts as the reducing agent i.e. it removes the oxygen from the oxide.

(3) The impure zinc is then fractionally distilled from the mixture of slag and other metals like lead and cadmium out of the top of the furnace in an atmosphere rich in carbon monoxide which stops any zinc from being oxidised back to zinc oxide.

(4) The zinc can be further purified by a 2nd fractional distillation or more likely by dissolving it in dilute sulphuric acid

http://www.docbrown.info/page04/Mextractd.htm#Zinc


IMPACT ON THE ENVIRONMENT

The products of this process of extracting the zinc from zinc blende includes Sulphur Dioxide, Carbon Dioxide and Carbon Monoxide.

(i) Sulphur Dioxide
Sulfur dioxide is one of the major cause of acid rain, which has acidified soils, lakes and streams, accelerated corrosion of buildings and monuments, and reduced visibility.
www.cleanairtrust.org/sulfurdioxide.html

(ii) Carbon Dioxide

Carbon dioxide is a greenhouse gas which will result in global warming.
The above map shows the percentage of predicted warming due to the direct effect of carbon dioxide  
CO₂ can contribute to acid rain and smog in cities.
http://www.sciencedaily.com/releases/2010/05/100503161435.htm

(iii) Carbon monoxide
Carbon monoxide (CO) is only a very weak direct greenhouse gas, but has important indirect effects on global warming. Carbon monoxide reacts with hydroxyl (OH) radicals in the atmosphere, reducing their abundance. As OH radicals help to reduce the lifetimes of strong greenhouse gases, like methane, carbon monoxide indirectly increases the global warming potential of these gases.

http://www.ghgonline.org/otherco.htm

SUGGESTIONS

venturi scrubber



As explained above, alkaline sorbents are used for scrubbing to remove SO₂. Depending on the application, the two most important are lime and sodium hydroxide (also known as caustic soda).
http://en.wikipedia.org/wiki/Flue-gas_desulfurization#Scrubbing_reagent


 Wellman–Lord process

The Wellman-Lord process is a regenerable process to remove sulfur dioxide  without creating a throwaway sludge product.
It is possible to scrub sulfur dioxide by using a cold solution of sodium sulfite, this forms a sodium hydrogen sulfite solution. By heating this solution it is possible to reverse the reaction to form sulfur dioxide and the sodium sulfite solution.
http://en.wikipedia.org/wiki/Wellman%E2%80%93Lord_process.