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.

Sources: http://en.wikipedia.org/wiki/Flue-gas_desulfurization
 http://www.bbc.co.uk/schools/gcsebitesize/science/aqa_pre_2011/rocks/fuelsrev5.shtml
http://www.eoearth.org/article/Flue_gas_desulfurization                           http://www.ehow.com/way_5855225_sulfur-dioxide-removal.html 

      http://www.osha.gov/SLTC/cadmium/index.html

      http://en.wikipedia.org/wiki/Venturi_scrubber (Image)