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

Extraction Of Aluminium Metal From Aluminium Ore (Bauxite)- Done by Jo

Assignment 1: Extraction Of Aluminium Metal From Aluminium Ore (Bauxite)

• Information about aluminium ore (Bauxite)

Bauxite, also known as aluminium ore, is one of the metals of greatest abundance in the world, with ample supply suffice to provide for the world-wide demand for aluminium for centuries. It is mainly composed of one or more aluminum hydroxide minerals, plus various mixtures of silica, iron oxide, titania, aluminosilicate, and other impurities in minor quantities.

The Aluminium metal is extracted from bauxite from two main processes; The Bayer process that involves conversion of iron into pure aluminium oxide, which then go through electrolysis for the aluminium to be seperated

Bauxite
(http://geology.com/minerals/photos/bauxite-14.jpg)

• Extraction process (1) : The Bayer Process

The Bayer process simply refers to the reaction of the bauxite with hydroxide solution. The crushed aluminium ore is treated with sodium hydroxide solution of moderate concentration.Temperatures are typically from 140°C to 240°C; pressures can be up to about 35 atmospheres. High pressure is essential to the solution in order to keep the water in liquid state despite the high temperature of the solution. 

Through the reaction, aluminium oxide reacts with aluminium ore to give a solution of sodium tetrahydroxoaluminate. The equation is stated below.

Through this primary reaction, The impurities, which DO NOT react with sodium hydroxide, can now be separated from sodium tetrahydroxaluminate through the process of filtration. The impurities will be stored in huge lagoons after this process. This red mud have been a significant environmental issue due to its toxic nature.

Impact of the "red mud" on the water bodies
(data:image/jpeg;base64,/9j/4AAQSkZJRgABAQAAAQABAAD/)

The sodium tetrahydroxaluminate is then cooled for precipitation of aluminium hydroxide; The sodium tetrahydroxaluminate is "seeded" with some previously produced aluminium hydroxide, hence providing something for the new aluminium hydroxide to precipitate around.

The equation for this process goes like...

Now as aluminium hydroxide has been yielded, it is heated to get aluminium oxide. The heating takes places at around 1100 - 1200°C due to the strong resistance of aluminium against heat. Through the heating, the aluminium hydroxide is segregated into aluminium oxide and water.

And these processes are the main components of The Bayer Process. The aluminium oxide yielded would then go through electrolysis for complete extraction of the aluminium metal.

• Extraction process (2) : Electrolysis

The extraction is done by electrolysis. But first the aluminium oxide must be made molten so that electricity can pass through it. As Aluminium oxide has a very high melting point (over 2,000°C), melting the metal would not be an economic solution. Hence, aluminium oxide is dissolved in molten cryolite, another aluminium ore, which used to be costly, but now widely available due to availability of lab production.

Cryolite (Na₃AlF₆₎(http://upload.wikimedia.org/wikipedia/commons/thumb/7/7e/
816-_Ivigtut_-_cryolite.jpg/240px-816-_Ivigtut_-_cryolite.jpg)

Along with the molten crytolite, the aluminium oxide goes through electrolysis in an electrolysis cell

Simplified Electrolysis Cell
(http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_35.gif)

From the diagram, you can see an aluminium oxide electrolysis tank. Both the negative electrode (cathode) and positive electrode (anode) made of graphite are present in the tank, for the separation of aluminium metal from aluminium oxide.

 Aluminium is released at the cathode (negative electrode) and sinks to the bottom of the tank, where it is tapped off.. Aluminium ions are reduced by gaining 3 electrons.

Oxygen forms at the positive electrodes. This oxygen reacts with the carbon of the positive electrodes, forming carbon dioxide, and they gradually burn away. 

Through this process of electrolysis, pure aluminium metal could be extracted from bauxite.

Aluminium Metal
(http://upload.wikimedia.org/wikipedia/commons/5/5d/Aluminium-4.jpg)

• Environmental Issues With Extraction Method.

• Industrial waste products

During the extraction metal, many harmful by-products are created such as the "red mud"; It is is a mixture of various chemical compounds,such as a watery slurry of fine rock particles and salts, containing toxic heavy metals. It can even be slightly radioactive if the original bauxite contained radioactive minerals. Furthermore, The mud also has a high pH because of the sodium hydroxide solution used in the refining process, hence, destroying the plants and animal habitats if disposed into the environment. This industrial waste has been a significant subject of concern for the environmentalists over the years.

The flooding of the toxic red mud have affected the
residents in Devescer, Hungary
(http://www.cbc.ca/gfx/images/news/topstories/2010/10/06/tp-hungary-sludge-cp-953461.jpg)

• Waste of lands and other resources

A large space of land is required to carry out different processes for the extraction of the aluminium metal as it is usually produced in large amounts due to the heavy demand for the metal. Also, since vast amount of electricity is needed for electrolysis, (to produce 1 mole of aluminium which only weighs 27 g you need 3 moles of electrons), there will be wastage of fossil fuels which is needed for production of electricity. Hence, not only this extraction process involve unnecessary usage of space, it also contributes to the rapid depletion of the finite natural resources.

• Solutions

The solution for this problem is definitely recycling of the metal; not only does it help saving of raw materials, particularly electrical energy by not having to extract the aluminium from the bauxite, (Recycling aluminium uses only about 5% of the energy used to extract it from bauxite.), the environmental issues regarding the disposal of the industrial wastes can be greatly reduced as the amount of disposed waste materials would significantly drop. 

Hence, recycling eventually seem to be the most efficacious method for solving the environmental issue regarding extraction of metals. 

-THE END-  :D

Reference:

• http://en.wikipedia.org/wiki/Bauxite
• http://www.chemguide.co.uk/inorganic/extraction/aluminium.html
• http://www.bbc.co.uk/schools/gcsebitesize/science/add_ocr_pre_2011/chemicals/extractionmetalsrev3.shtml
• http://www.cbc.ca/news/technology/story/2010/10/07/f-red-mud-bauxite-aluminum.html
• http://en.wikipedia.org/wiki/Aluminium
• http://en.wikipedia.org/wiki/Sodium_aluminium_hydride

*If you are unable to read the equations given above, you can open the images in another tab to see it.

Instructions

Topic: The Extraction of metals and its Impact on the Environment.  

Instructions

1.    In your group of 3, you are to use the blog that you have created to write a report on one the following topic: 

·         A general overview on the method of extraction of metals for:

            

(1)  Extraction of aluminium from bauxite – “odd number” groups

                  

(2)  Extraction of zinc from zinc blende – “even number” groups

·         Explain how the metal is extracted from its ore and how the extraction method has an impact on the environment.

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


Hmm....