Saturday 1 September 2012

Solutions to reduce impact of zinc extraction on the environment

In order to reduce the impact of zinc extraction from zinc blend on the environment, release of sulfur dioxide must be reduced. A set of technologies, called flue-gas desulfurization (FGD), can be used to remove sulfur dioxide.
A large flue gas desulfurization unit in a power plant
Here is a list of common methods categorised under flue-gas desulfurization:

1)Wet scrubbing using a slurry of alkaline sorbent, usually limestone or lime, or seawater to scrub gases;
2)Spray-dry scrubbing using similar sorbent slurries;
3)Wet sulfuric acid process recovering sulfur in the form of commercial quality sulfuric acid;
4)SNOX Flue gas desulfurization removes sulfur dioxide, nitrogen oxides and particulates from flue gases;
5)Dry sorbent injection systems.
Scrubbing with a basic solid or solution


SO2 is an acid gas, and, materials used to remove the SO2 from the flue gases are alkaline. The reaction taking place in wet scrubbing using CaCO3 (limestone) produces CaSO3 (calcium sulfite) and can be expressed as:

CaCO3 (solid) + SO2 (gas) → CaSO3 (solid) + CO2 (gas)
When wet scrubbing with  Ca(OH)2 (lime) , the reaction also produces CaSO3 (calcium sulfite) and can be expressed as:

Ca(OH)2 (solid) + SO2 (gas) → CaSO3 (solid) + H2O (liquid)

To partially offset the cost of the FGD installation, in some designs, the CaSO3 (calcium sulfite) is further oxidized to produce marketable CaSO4 · 2H2O (gypsum). This technique is also known as forced oxidation:

CaSO3 (solid) + H2O (liquid) + ½O2 (gas) → CaSO4 (solid) + H2O

A natural alkaline usable to absorb SO2 is seawater. The SO2 is absorbed in the water, and when oxygen is added reacts to form sulfate ions SO4- and free H+. The surplus of H+ is offset by the carbonates in seawater pushing the carbonate equilibrium to release CO2 gas:

SO2 (gas) + H2O + ½O2 (gas)→ SO42- (solid) + 2H+

HCO3- + H+ → H2O + CO2 (gas)


There are two other methods to remove sulfur dioxide:

     Wellman-Lord Process


Firstly, Sulfur dioxide (SO2) is absorbed in a solution of sodium sulfite (Na2SO3) in water which forms sodium bisulfite (NaHSO3). 

Water temperature is steadily lowered in the tank which converts the sodium bisulfite to sodium pyrosulfite (Na2S2O5), a compound with low solubility and precipitates. 


It was said that "The Wellman-Lord process uses a wet generative process to reduce stack gas sulfur dioxide concentration to less than 250 ppmv and can achieve approximately 99.9 percent sulfur recovery."


  1. Venturi scrubber
Other Wet Scrubbing Systems
This method uses an equipment, the Venturi-rod scrubber which is a  converging and diverging section of duct, as shown in the figure to
 the right. 
      
The converging section of duct first accelerates the gas steam 
containing sulfur dioxide to a high velocity. The high speed atomizes 
steam at the duct's choke point into small water droplets. 
      This creates the surface area needed to give the temperature drop (similar to Wellman-Lord) whereby sulfur dioxide is removed from 
the emission as the droplets are reheated into a gas and expelled. 


                 Controlling exposure of cadmium to the atmosphere can be done through engineering controls. Engineering controls include such things as isolating the source and using ventilation systems.

      Ventilation is one of the most important engineering controls available to the industrial hygienist for improving or maintaining the quality of the air in the occupational work environment. 

      Broadly defined, ventilation is a method of controlling the environment with air flow. Industrial ventilation generally involves the use of supply and exhaust ventilation to control airborne contaminants, in the workplace, to acceptable levels. Other major applications of industrial ventilation are to prevent fire and explosions, and to control temperature, humidity, and odors. 


                 Excess Zinc also poses a threat to the environment. The way to prevent exposure of excess Zinc in the environment is to hold the extraction reaction in an enclosed area and to make sure that all Zinc extracted are kept in places of correct conditions, i.e. with no water and heat.

Friday 31 August 2012

Environment Impact of Extraction of Zinc from Sphalerite


Environment Impact

The  extraction of zinc from sphalerite will produce large amount of sulfur dioxide as well as cadmium vapour.
Sulfur dioxide is poisonous.  Residues from the process such as smelter slag also contain a significant amount of heavy metals, which is damaging to the environment. This affects the atmosphere greatly.
Furthermore, the soils contaminated with excess zinc due to the extraction process will also interfere will the soil’s ability to be able to absorb essential nutrients such as manganese and iron. In order words it will affect the plant ecosystem whereby the plants are not able to grow well and greatly influence the whole food web.
Sources: (Both this post on environment impact and the Process of Extraction of Zinc-Last Post)

Extraction of zinc from zinc blende

Question: 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.
1)What is Zinc Blende/Sphalerite/Black-Jack

Sphalerite
It is a mineral that is a chief ore of zinc. It has a chemical formula of (Zn,Fe)S. It consists mainly of Zinc Sulfide in crystalline form but always almost contain variable amounts of Iron. The Iron content affects it's coulou and it is usually found in association with galena, pyrite, and other sulfides with calcite, dolomite and fluorite.

Zinc Blende has a crystal structure. In the structure, zinc and sulfur atoms are tetrahedrally coordinated. It's structure is closely related to diamond.

2)How zinc is extracted from zinc blende

Firstly, after the grinding of the ore,
zinc sulfide undergoes froth floatation, a process to separate materials based on whether it is hydrolilic or hydrophobic, to obtain an ore concentration. Note that at this point of time, only 50% of the ore concentration is zinc.

Next,
Roasting of the ore concentration will then convert the zinc sulfide into zinc oxide and sulfur dioxide. Based on the equation below:

2 ZnS + 3 O2 → 2 ZnO + 2 SO2

The sulphur dioxide is needed for the leaching process later on.
After this process, prometallurgy is by reducing the zinc oxide with carbon or carbon monoxide at a high temperature of 950 degrees Celcius.

From this reducing process, zinc vapour is distilled.

The equations for the reaction are as follows:

2 ZnO + C → 2 Zn + CO2

ZnO + CO → Zn + CO2                      

OR

After the  roasting of the ore, an electrowinning process will leach off zinc from its ore

ZnO + H2SO4ZnSO4 + H2O

And thereafter, electrolysis is used to produce zinc metal via this equation:

2 ZnSO4 + 2 H2O → 2 Zn + 2 H2SO4 + O2

Note that the sulfuric acid is then recycled to be used in the leaching stage.

en.wikipedia.org/wiki/Sphalerite

Friday 24 August 2012

Chemistry-Metals

In this blog, we will be discussing on specific chemical properties and issues on metals and we hope to receive your comments as well. If you have any comments or queries, please  feel free to leave us a message.

Group Members:
Chen XuanLin - Research and Post
Chua Tian Tian - Research and Post
Michelle Ten - Pictures and Design