Extraction of Zinc from Zinc Blende

l   Explanation for Extraction of zinc from zinc blende

Extracting metal from ores

 Zinc ores are dug from underground mines using blasting, drilling, and hauling techniques.

http://www.madehow.com/Volume-2/Zinc.html



the zinc blende, aka sphalerite



According to reactivity series of metals, zinc is not so reactive; it can be extracted by reducing its oxides with carbon.

Extracting zinc from zinc blende

Ø  Zinc blende is a mineral that is the chief ore of zinc. It consists largely of zinc sulfide in crystalline form but almost always contains variable iron.

Structure: http://www.youtube.com/watch?v=6lCQ6LT3Esg

Ø  Grind the zinc ore to a fine powder and mix it with water, pine oil, and flotation chemicals. Stir the mixture, and make the zinc ore “float” to the surface. A variety of chemicals are used to coat the zinc ore so that zinc ore will not be wet, and will not become too denser to float. Then air is injected producing bubbles in the mixture. Use an impeller to stir the mixture. The coated minerals will follow the bubbles to the bottom of the container, while the zinc ore is still on the surface (since zinc ore cannot adhere to the bubbles). Use the scraper to remove the froth on the surface which contains the zinc.

Ø  The froth is filtered to remove the water and oils. Then it is sent to a blast furnace. This reaction will happen.

2ZnS(s) + 3O₂ (g) → 2ZnO(s) + 2SO₂ (g) 

After this reaction, the material has been completely converted to zinc oxide, ZnO.

Ø  Mix ZnO with coke, and send to the blast furnace, which can generate 1204°. The reaction will happen, ZnO + C → Zn +CO. However, when zinc cools down, it may re-form zinc oxide due to the presence of air. To reduce this reformation, molten lead is added while zinc is hot. The lead will dissolve zinc at 550°. When it is cooled to 440°, the lighter zinc will separate from lead and is cut out from the top.

Ø  Keep the zinc molten for several hours; the other metal (like lead) will sink to the bottom. The almost pure zinc which forms on the top can be carefully removed.

Next let the ore mixed with half its weight of powered coal and put it into fireclay retort furnaces. This reaction will occur: ZnO + C = Zn +CO. The reduction begins at 800 degrees and zinc will be produced. This reaction will increase rapidly with the rise in temperature. 

l  The problems caused by extraction of zinc

1.Air pollution

The production for sulfidic zinc ores produces large amounts of sulfur dioxide and cadmium vapor.  

      what are the consequences?

      The sulfur dioxide gas will diffuse in the air and react with the water vapour in the air.

       SO₂+H₂O→HOSO₃ 

 Which is followed by

         HOSO₂· + O₂ → HO₂· + SO₃ 

In the presence of water, sulfur trioxide (SO₃) is converted rapidly to sulfuric acid: 

          SO₃ (g) + H₂O (l) → H₂SO₄ (l)

      The dillute sulfate acid in the air acidifies the rain so that it becomes acid rain and can corrode the limestone sculpture or metal in the building.

       Example

Since 1998, Harvard University wraps some of the bronze and marble statues on its campus, such as this "Chinese stele", with waterproof covers every winter, in order to protect them from erosion caused by acid rain (or, actually, acid snow)



2.Water pollution

Smelter slag and other residues of process also contain significant amounts of metals(most are Zn) which will make water pollution.        

       what are the consequence?       

 

       Zinc pollutes the water because of large quantities of zinc in wastewater of industrial factories. The purification of these wastewater is not fine, leading to zinc-sludge being deposited on river banks. It may also increase acidity of the waters.

       There are fishes able to build up zinc in their bodies while living in these contaminated water bodies. It is able to bio magnify up the food chain within the fish bodies.

        Zinc presence in soil is a threat to animals as well as plants. The animals take in high concentration of zinc into their bodies which will damage their health. Plants have a zinc uptake to which the system cannot handle and breaks down. On top of that, zinc can interrupt activities of micro-organisms and earthworms in the soil, causing break down of organic matter to slow down greatly. Soluble zinc can also contaminate groundwater leading to well waters not fit for consumption. 

         If a human eat much food or drink lots of water with high proportion of zinc, he will accumulate lots of zinc in his body. Although humans can handle proportionally large concentrations of zinc, too much zinc can still cause eminent health problems, such as stomach cramps, skin irritations, vomiting, nausea and anaemia. Very high levels of zinc can damage the pancreas and disturb the protein metabolism, and cause arteriosclerosis. Extensive exposure to zinc chloride can cause respiratory disorders.

Read more: http://www.lenntech.com/periodic/elements/zn.htm#ixzz25NkcDMme

Read more: http://www.lenntech.com/periodic/elements/zn.htm#ixzz25NkUxiiy

l   

Solution to the problems.

1.        Air pollution

Use CaO to react with SO₂  in order to avoid air pollution.

Reaction: CaO + SO₂ → CaSO₃ 

 After the reaction, CaSO₃ can be used to make CaSO₄ which is a common laboratory and industrial chemical. It can be used as desiccant. It is a good way to recycle and reuse the SO_2.

2.        Water pollution

Use active carbon to adsorb the metal ions(the most metal in the water is ion form). The used carbon can also be regenerate by different ways.

 For more information:

http://www.lenntech.com/library/adsorption/regeneration/activecarbon-regeneration.htm

 

Role of team members:

Zhang Zequn: Explanation for Extraction of zinc from zinc blende

Gan Kai Ming: Water pollution of the extraction of zinc

Tao Tianhao: Air pollution of the extraction & Solutions & General design

 PS:  for the technical problem, not all pictures are available in the blog yet.