2NaCl + 2H2O → Cl2 + H2 + 2NaOH
A membrane cell is used to prevent the reaction between the chlorine and hydroxide ions. If this reaction were to occur the chlorine would be unbalanced to form chlorine and hypochlorite ions:
Cl2 + 2OH– → Cl– + ClO– + H2O
Because of the corrosive nature of the chlorine produced, the anode has to be made from a un reactive metal such as titanium.
Mercury is used as a negative electrode or cathode that works with the titanium anode to keep the highly reactive products involved apart when electricity is passed through brine.
Because mercury is highly volatile, mercury contamination occurs throughout the process, commonly leading to both the product (caustic soda) and the wastewater stream containing small amounts of mercury.
As mercury is enormously toxic, all of it needs to be recycled to ensure there are no dangers to the environment.
However the Chlor-alkali process has caused a large amount of environmental problems. Chlor-alkali plants have allowed for large amounts of mercury to be disposed in a harmful way as they released mercury into rivers and into the atmosphere.
“Mercury in the air may settle into water bodies and affect water quality. This airborne mercury can fall to the ground in raindrops, in dust, or simply due to gravity (known as “air deposition”). After the mercury falls, it can end up in streams, lakes, or estuaries, where it can be transferred to methylmercury through microbial activity. Methylmercury accumulates in fish at levels that may harm the fish and the other animals that eat them. Mercury deposition in a given area depends on mercury emitted from local, regional, national, and international sources. The amount of methylmercury in fish in different water bodies is a function of a number of factors, including the amount of mercury deposited from the atmosphere, local non-air releases of mercury, naturally occurring mercury in soils, the physical, biological, and chemical properties of different water bodies and the age, size and types of food the fish eats. This explains why fish from lakes with similar local sources of methyl mercury can have significantly different methylmercury concentrations
Birds and mammals that eat fish are more exposed to methylmercury than any other animals in water ecosystems. Similarly, predators that eat fish-eating animals are at risk. Methylmercury has been found in eagles and otters. Analyses conducted for the Mercury Study Report to Congress suggest that some highly-exposed wildlife species are being harmed by methylmercury. Effects of methylmercury exposure on wildlife can include mortality (death), reduced fertility, slower growth and development and abnormal behavior that affect survival, depending on the level of exposure. In addition, research indicates that the endocrine system of fish, which plays an important role in fish development and reproduction, may be altered by the levels of methylmercury found in the environment.”2
An example of a company that has used the chlor alkali process is Canadian-Oxy Ltd., Squamish, British Columbia. This company produces NaOH, Cl2 and HCl, which are used by the pulp and paper industry. During the years 1986-1989 it has released excess amounts of mercury into the atmosphere and has caused numerous problems.
“A distinguishing feature of this plant is that the mercury cells are not contained within a building but are open to the atmosphere. Consequently, mercury emissions from the cells are released directly to the atmosphere and are difficult to measure using the methodology specified in the standard reference for mercury emissions. Cell emissions front this plant, therefore, are calculated from quarterly monitoring of ambient mercury concentration (μg/m3) and air flow rate (m3/min) at six locations in the cell area to give a mass/unit time of mercury released.
For 1987, total mercury emissions from the cell area were 466.3 kg, above the allowable limit of 335.8 kg/year based on the rated capacity. This excess, emitted during the third quarter of 1987, was caused by a combination of several factors:
- the plant was operating at its highest recorded production levels due to increased electrical capacity and good market conditions
- high overall cell temperatures (caused by high production levels)
- high ambient temperatures (caused by hot summer weather)
- seawater cooling deficiencies
-
equipment malfunctions“3
Apart from the loss of mercury to streams and as plant emissions, the two remaining sources of mercury loss from chlor-alkali plants are to products (produced from chlorine, sodium hydroxide, and hydrogen) and as solid wastes generated during plant operations.
“Because of the presence of small amounts of mercury in hydrogen and caustic soda (traces of which remain after product purification), when these chemicals are used to manufacture other products, such as sodium hypochlorite and hydrochloric acid, the new products contain traces of mercury. Although this may be regarded as product contamination, mercury concentrations remain within product specifications.
The main sources of solid wastes produced at mercury cell plants are:
- brine saturator sludges
- recycled brine sludges
- caustic soda product
- wastewater treatment plant sludge
- waste solids produced as a result of cell maintenance and retort use
For those plants employing some form of solid waste treatment for mercury recovery, the final product is an inert mercury-free solid (residue) that is landfilled. Monitoring wells are installed and sampled to determine if there is any migration of mercury to the groundwater. “4
Over the years people have developed a more efficient way of using mercury as a way of doing the Chlor-alkali process and have cut down emission to a safer number.
“Following discussions with federal and provincial regulatory authorities, the company immediately improved its housekeeping practices and modified process equipment, pumps, and tanks to improve sealing and prevent mercury emissions from the cell area.
Modifications included the following:
- new mercury pump tank covers were made of fibre-reinforced plastic (FRP)
- mercury pump tanks were upgraded to improve the seal between the cover and the tank
- a new head end-box wash-water system was installed (to improve cooking and reduce emissions)
- high-performance shaft seals were installed on the mercury pumps and solid mountings were used to prevent mercury entrapment
- mercury level indicators were installed on the mercury pump tanks to minimize mercury leaks
- end-box venting was improved by resloping the vent lines
- the liquid drain seal of the heat exchanger was deepened to allow operating at a higher vacuum without re-entrainment of condensed liquids (re-entrainment caused pressure fluctuations that produced puffing of mercury from the mercury pump tanks)
- decomposer lids were replaced with an improved design that reduced the number of leak points
These modifications were started during the fourth quarter of 1987 and were completed in 1988. As a result, mercury emissions during the fourth quarter of 1987 were lower, reflecting the process modifications and equipment improvements. For 1988 and 1989, mercury emissions were well below allowable levels, again reflecting the process modifications and equipment improvements. “5
From this I conclude that the chlor alkali process has been greatly improved over time as the EPA established national effluent limitation guidelines for mercury discharges from the chlor alkali industry under sections 301 and 304 of the Clean Water Act By June 29, 1985. Plenty of companies have set goals to achieve maximum increase in chlor-alkali cell energy efficiency, lower production costs and exceed industrial purity standards. Also some industries have developed a way in which they can remove mercury from industrial wastewater as the cellulose of Acetobacter xylinum can be employed as an environmentally friendly adsorbent for the removal. Adsorption of mercury by cellulose of Acetobacter xylinum is reasonably fast.
2.
a)
- Methyl mercury can enter the food chain, or it can be released back to the atmosphere by volatilisation.
- Sunlight can break down methyl mercury to Hg (II) or Hg (0), which can leave the aquatic environment and re-enter the atmosphere as a gas.
- Inorganic mercury compounds usually attack the liver and kidney. Low-level exposures to these compounds do not normally cause problems, as they are not retained in the body long enough to cause damage
- Methyl mercury can easily pass the blood-brain barrier and affect the brain.
- It affects the immune system, alters genetic and enzyme systems, and damages the nervous system, including coordination and the senses of touch, taste, and sight.
- Methylmercury compounds are much more dangerous, as they cause irreversible damage to the central nervous system, leading to numbness in the extremities, lips and tongue, followed by deafness, lack of balance, blurring and restriction of vision.
- Methylmercury also has a longer half-life than inorganic mercury, so it has greater potential to accumulate in the body.
b)
Cmax / Ci = e-k [1/(1-e-k)]
Cmax= max accumulated concentration Ci= Ingestion rate
For methylmercury (CH3Hg) the half life is 70 days
k = 0.693 / t½
0.693 / 70 days k = 0.0099 days-1
Cmax = 2 (mg/day) x e-0.0099 x 1 / (1 - e-0.0099)
Cmax = 201mg
For mercury (Hg) the half life is 6 days
k = 0.693 / t½
k = 0.1155
Cmax / Ci = e-0.1155 [1/(1-e-0.1155)]
Cmax = 16.34 mg
REFERENCES
1 http://www.chemicals-technology.com/projects/chlor_alkali/
2 http://www.epa.gov/hg/eco.htm
3 http://www.ec.gc.ca/CEPARegistry/documents/pubs/eps-1-ha-2/chap4.cfm#anchor4.2
4http://www.ec.gc.ca/CEPARegistry/documents/pubs/eps-1-ha-2/chap4.cfm#anchor4.2
5http://www.ec.gc.ca/CEPARegistry/documents/pubs/eps-1-ha-2/chap4.cfm#anchor4.2
