Superphosphate is made by acidification of rock phosphate with sulphuric acid. Similarly, TSP is made by acidification of rock phosphate with phosphoric acid. In both these processes, fluorine is evolved which till recently was creating a lot of pollution problems similar to those in the phosphoric acid industry.
Various processes have been developed for the recovery of this fluorine as in the phosphate industry, and hydro-fluosilicic acid is recovered. It is possible to recover 30 to 40 percent of the total fluorine in the rock as hydro-fluosilicic acid and this can, in turn, be converted into sodium silico fluorine or cryolite.
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In India, there are two plants operating on this process and a few others are in the developing stage. Gas from the superphosphate and TSP is piped to the adjacent fluorine based industries and such units work as ancillary units and, thus, gainfully utilize the fluorine. Indian know-how is available for proper conversion of these gases to AIF and cryolite.
Emissions:
With proper controls and recovery systems, it is possible to bring down the fluorine-based gas as follows:
The existing super phosphate factories which don not recover fluorides have emissions as high as 0.6-0.7 kg/F/tonne of super phosphate.
Image Source : environmentmagazine.org
Hydrocarbons:
In ammonia synthesis, methane is the main hydrocarbon that is formed by the reaction:
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In order to reduce the inerts from the synthesis loop, this has to be purged out and normally the methane is burnt in the flare header converted to CO Hydrocarbon pollution, therefore, as such does not pose any significant problem. Even if ventors unburned, the amount of ammonia produced would not be more than 5 kgs per tonne (standard 4 kg/tonne).
The main source of hydrocarbon is the effluent gas from oil refining. This problem could be solved by either reducing emissions, using developed technologies or burning the hydrocarbon efficiently as is generally done. The latter solution, however, is not being heavily criticized in view of the world shortage of energy.
Particulate Matter:
These are emitted from various industries but mainly from:
1. Boilers, especially powerhouse boilers based on coal
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2. Boilers from sugar factories based on coal/bagasse
3. Dust from cement kilns
4. Dust from mineral grinding mills
In the fertilizer industry, the dust could emanate from:
1. Granulation plants
2. Urea prilling towers and
3. Grinding mills for coating agent/rock phosphate.
Solution (i) Dust emission could be reduced by installation of good efficient cyclones or filters. In case of dust, which is hydroscopic such as that containing ammonium nitrate, cyclones followed by wet scrubbers have been successfully used.
Now cyclonic scrubbers have been installed at Trombay for washing the dust from complex fertilizer granulation plants, using a venturi scrubber. The efficiency of such scrubbers, at inlet dust level of 2 mg/litre, is 97 percent.
In other industries, particulate matter can be reduced by installing efficient cyclones, electrostatic precipitators, bag filters and wet scrubbers.
Emission Standard:
The BIS is considering the framing of emission standard to guide the Central and State Governments and other local bodies in establishing rules regarding air pollution.
In arriving at a decision on the tolerance limits, the local authorities should give due consideration to local conditions and, in special cases, they may relax the limits or make them more rigid.