Extraction Of Zinc Metal From Zinc Blende (Sphalerite) - Done by Valerie

Assignment 2: Extraction of Zinc Metal from Zinc Blende (Sphalerite)

• Information about Zinc Blende (Sphalerite)

Sphalerite, another name for Zince Blende, is the cheif ore of Zinc and is found in thousands of locations worldwide. Zinc is the twenty-third most abundant element in the Earth's crust.It is primarily consisting of zinc sulfide in crystalline form. However, it almost always contains variable iron. Typically, it is found associated with galena, pyrite, and other sulfides along with calcite, dolomite, and flourite.Zinc metal is extracted from sphalerite using extractive metallurgy in froth flotation. Froth flotation is a process which selectively separates minerals using the difference between hydrophilic and hydrophobic materials. 

Sphalerite
( http://crystal-cure.com/pics/sphalerite.jpg )

• Extraction Process

The Zinc ore, Sphalerite, is to be crushed into fine particles and concentrated in concentrators or mills before froth floatation could work. This is then formed into a slurry using water and other surfactants and wetting agents. The hydrophobicity differences between sphalerite and the waste gangue are increased. The slurry, also known as the pulp, of both hydrophilic and hydrophobic particles are then introduced to floatation cells. 

Step 1 :


Froth flotation
( http://www.1.mixing.net/sites/default/files/publications/mineral_17.png )


Step 2 :

Flotation Cell
( http://www.zzywzg.com/en/UploadFiles/2011324153811454.gif )

The sphalerite is then roasted (heated in air), where it is heated to temperatures of 600 to 740 degree celsius. This allows the oxygen in the air to react with it, producing sulfur dioxide from the zinc blende. The zinc sulfate then produced the oxide of zinc after being oxidised. 

  2ZnS + 3O₂ → 2ZnO + 2SO₂

Zinc Oxide 

( http://cn-zinc-oxide.com/images/zinc.jpg )

To obtain Zinc metal, there are two further methods:

Firstly, the Zinc oxide formed is mixed with coke (carbon) in a fireclay retort furnaces. The coke then reacts and the metal can be distilled off and condenses to obtain Zinc.

ZnO + CO → Zn + CO

Secondly, using electrolysis, specifically electrowinning to obtain metals from their ores. Zinc can be extracted from the zinc sulfate solution by passing through electric current through it. Bonds in the compound are broken and thus, pure zinc metal is produced. 

Electrowinning Process

( http://www.tms.org/pubs/journals/JOM/0304/fig3.gif )

Therefore, Zinc metal is obtained.

Zinc Metal
( http://fullmetalzinc.com/wp-content/themes/zinc/images/zinc.png )

• Impact on the Environment 

Zinc mining left behind numerous of physical hazards and environmental issues. The process of extraction of Zinc from sulfuric zinc ores such as Sphalerite produces large quantities of sulfur dioxide and cadmium vapor. These substances are released into the environment and impacted water resources, soils, vegetables, crops and so on. In Belgian towns of La Calamine and Plombiéres between 1806 and 1882, about 1.1 million tonnes of metallic zinc and 130 thousand tonnes of lead were mined and smelted. This resulted in the sediments of the Geul River to contain significant amounts of heavy metals like zinc and cadmium.

Water in a stream contaminated by iron sulfide that flows across the state line near Picher, Oklahoma

During extraction, about 90% of the extraction generated is waste material and requires disposing. If not cautiously disposed of, it could possibly leak into streams or soils in nearby water bodies. Large landfills sites are necessary to dispose of the waste properly and this requires much space. The smelting of these ores also cause air pollution as gases such as sulfur dioxide would constantly be released into the air. Sulfur dioxide is one of the main precursor of acid rain. This acidifies soils, lakes, streams and corrodes building and etc. Furthermore, going through these processes requires large amounts of energy which is typically generated from fossil fuels which are a non-renewable source of energy that is already scare. 

• Solutions

These environmental problems are yet to be fixed. I feel that Zinc should be constantly recycled instead of extracted from mines. This would help to minimise waste produced through the extraction process and moreover, the zinc can be continuously recycled without losing its physical or chemical properties. Zinc can be recycled at all stages of its production and use, from the scraps made during the production of galvanised steel sheets to the end-of-life products.  Thus, I think that the best way to go about this is to continue to recycle Zinc and thus reduce environmental issues that are caused by Zinc extraction.
Zinc recycling circuit

References: 
http://en.wikipedia.org/wiki/Sphalerite#Occurrence
http://chem-guide.blogspot.sg/2010/04/occurrence-and-extraction-of-zinc-from.html
http://chemistryeblogging2012.blogspot.sg/2012/08/how-is-zinc-extracted-from-zincblende.html
http://yfrcyx.blogspot.sg/2012/09/extraction-of-zn-from-zincblende.html
http://en.wikipedia.org/wiki/Electrowinning
http://wiki.answers.com/Q/How_is_zinc_extracted_from_zinc_blende
http://en.wikipedia.org/wiki/Zinc_smelting#Electrolysis
http://wiki.answers.com/Q/What_is_the_impact_of_zinc_extraction_on_the_environment
http://www.zinc.org/info/zinc_is_recyclable
http://www.kgs.ku.edu/Extension/ozark/mining.html
http://www.ncbi.nlm.nih.gov/pubmed/21573711