Thursday, 6 September 2012

Extraction of zinc from zinc blende

Zinc blende is ground and concentrated by Froth floatation, a process for selectively separating hydrophobic materials from hydrophilic. This is where a powerful air current is blown through the suspension, creating dense foam on its surface, and then the foam is skimmed away, settling the gangue of the zinc ore, concentrating it. Roasting it in air, impure zinc oxide is produced ( 2ZnS(s) + 3O2 (g)à 2ZnO(s) + 2SO2 (g) ) by combustion. The zinc ore can calcined to produce zinc oxide, added with coke/carbon as reducing agent and limestone, and heated at 1673 K in fire clay retorts by Belgian Process (ZnCO3(s) à  ZnO (s) + CO2 (g)), an endothermic thermal decomposition. At this high temperature, the metal is volatile so it distils over and is condensed, leaving behind impurities (ZnO(s) + C(g) = Zn(s) +CO(g)). The crude metal is spelter. The metal may be refined either by electrolysis or by fractional distillation. The zinc is fractionally distilled from the mixture of slag (mud like material insoluble in molten metal) and other metals like lead, iron and cadmium out of the top of the furnace with carbon monoxide atmosphere, preventing zinc from being oxidised back to zinc oxide. The resulting layers can be tapped off. It is refined more by dissolving it in sulfuric acid and purified electrolytically. After reacting ZnO (s)+ H2SO4 (aq) à ZnSO4 (aq) + H2O (l), the zinc sulfate is electrolysed to get zinc.

http://www.transtutors.com/chemistry-homework-help/d-and-f-block-elements/zinc-and-its-compounds.aspx ; http://www.newton.dep.anl.gov/askasci/chem03/chem03435.htm ;
http://www.docbrown.info/page04/Mextractd.htm


> http://cst-www.nrl.navy.mil/lattice/struk/b3.html*). [image: Picture of> lattice; Click for Big Picture]
> http://cst-www.nrl.navy.mil/lattice/struk/b3.html(Watch> video on structure of Zinc Blende:*
> http://www.youtube.com/watch?v=m_7z-7dp8BQ&feature=player_detailpage*)

The extraction of Zinc from Zinc Blende: Reduction has to be carried out in closed vessels or furnaces in which where the zinc vapor can be condensed in the absence of air. For thermal reduction the concentrate is roasted a treatment that may be carried out in two stages multiple-hearth furnace followed by sintering on a traveling grate or in a single-stage operation in which a proportion of the roaste material is fed back to the sintering machine. Sintering is necessary for transforming the material into suitable lumps to allow air to flow through it during the subsequent reduction process.  Reduction of the zinc oxide can be done by various methods. In the so-called standard process, reduction is effected in horizontal retorts in a retort furnace or distilling furnace, which comprises a lower part containing regenerative chambers for preheating of  the gas and combustion air and an upper part in which retorts, arranged in tiers one above the other, are heated by the hot-flame gases. A retort of this kind is a rectangular distilling vessel, about 6or 7ft. long and about 1 ft. square in cross section. It is made of fireclay and has only a short service life, having to be renewed every four to six weeks. The zinc vapor escapes from the retorts and is collected in the condensers as liquid metal. At the end of about 20 hours the process has been completed; the liquid zinc and zinc dust are then removed from the condensers, and the residual matter is removed from the retorts by special machines. In the New Jersey process  reduction is effected in large vertical retorts about 14m (45 ft) high and of rectangular cross section, lined with silicon-carbide bricks in the hottest parts. The advantage over the horizontal retort method is that reduction can be performed as a continuous operation. The charge consist of briquettes made of a mixture of zinc oxide material (roasted blende) and bituminous coal. The briquettes are fed automatically to the resort, the residual matter being extracted by a screw conveyor at the bottom. The mixture of carbon monoxide and zinc vapor is discharged from the top of the retort into a condenser, where the metal is precipitated in liquid form.
Chemical Formula: (Zn,Fe 2+)S
Mineral Name: Spharelite
Color:Yellowish brown, orange, red, black, greenish yellow.
Hardness: 3.5 - 4
Specific Gravity: 3.9 - 4.1

Impact on the environment and its soloution

-cadmium vapour - severe lung irritation, pulmonary edema, and in some cases, death; long term inhalation of it can cause emphysema, a disease affecting the ability of the lung to absorb oxygen) and can damage the kidneys.

-Carbon dioxide emitted by machines (e.g., dieselpowered
heavy vehicles) involved in extracting
and transporting ore: cause global warming.  Reduce by planting more trees and maintaining forest & nature … 


Zinc in rivers – Decrease amount of oxygen carrying capacities of blood in fish
 -Main cause is the sulfur dioxide produced which is poisonous; at a level of 0.5%, it will kill. It reacts with rain water to cause acid rain, which can affect animals and plants & corrode buildings. The immediate effects of inhaling sulfur dioxide on humans are coughing, chest pains and breathlessness. Sulfur dioxide is thought to contribute to bronchitis and lung diseases. Sulfur dioxide react with water vapour to form acid rain. Acid rain can damage lakes and fish, trees and building materials such as stone, concrete, metal and cement. This can affect the environment by just changing the pH. It can be reduced by changing it into other useful industrial materials.
 
 
http://www.mechanicalebook.com/topics/topicz/zinc.htm)

3 comments:

  1. Good job on explaining the steps of the extraction of zinc, it's very detailed. However, i feel there are more possible impacts on the environment. I've also realised that you did not state the solutions to the impacts that you have reasoned out.

    ReplyDelete
  2. I'm sorry, i have just realised that you did state the solutions. However, i feel that there could be more solutions to overcome the impacts.

    ReplyDelete
  3. Hi there. I think your blog post was rather good. The explanation for the extraction was very detailed , but I think that it can be further simplified as some words are ... too complex for our level to comprehend ? But GOODJOB still ! (:

    Impacts and solution can be further described. Like. Other than just stating you can elaborate. Like for example " it can be reduced by changing it into other useful industrial materials " you can be specific and state a few examples. (: goodjob nonetheless.

    - HUIFANG. ^^

    ReplyDelete