By adopting technology based on the slurry granulation process, the NPK A plant can manufacture complex fertilisers (NPK) such as 28:28:0 (Uramphos), 18:46:0 (Samrat), 19:19:19 (Sampurna), and 14:35:14 (Sampatti). Zuari’s NPK plant incorporates the latest in pipe reactor technology, which ensures that the fertilisers produced in this plant are some of the highest fertiliser grades in the country. The NPK plant has reduced the effluent it generates by adopting numerous processes. The particulate emissions of the NPK plant is considerably lower than requisite industry standards owing to the utilisation of a high efficiency scrubber. Water that is used for washing in the manufacturing process is collected in a recovery tank, recycled, and used once again in the plant reducing the effluent discharge.
NPK granulation Plant consists of
- Size separation by sieving
- Product cooling
- Oversize crushing and conveying along with fines
- Scrubbing of various air streams from granulator dryer and cooler
Recycle solids consisting of crushed oversize as well as the fines separated in the process screens are routed back to the Ammoniator Granulator (AG) at the back end through a chute. All the fresh solid feeds like Urea, Potash, filler are also added to AG through the same chute. AG rotates at approx. 9 rpm causing a rolling/cascading bed which mixes the incoming solids uniformly. Hot slurry consisting of Ammonium Phosphates with a mole ratio of 1.4 is prayed on the rolling/cascading bed through slurry spray nozzles. Liquid ammonia is introduced below the rolling bed for further ammoniation and also supplies heat of reaction. The rolling/cascading action of the solids on which the slurry is sprayed causes the growth of granules through the process of agglomeration and accretion as well as rounding off of the granules to spherical shape. The granules having assorted size then flow to the dryer through a chute.
Solids flowing into the dryer has a moisture content of 1.8~2.2%. The co-current dryer is a rotary equipment having specially designed flights to effect a good cascading of the solids across the maximum cross section so that the air is intimately contacted with the cascading solids and the solids are thus dried. Depending on the operating temperature at the exit of AG, the firing in the furnace generating hot air will vary. In the limiting case there will be no fuel combusted in the furnace and the internal heat of the solids will be adequate to dry the solids to a moisture content of 1.0 ~1.5% at the exit of dryer. Depending on the grade produced (e.g. 19:19:19), air needs to be dehumidified before being used in the dryer.
Dried solids having a moisture content of 1.0 ~ 1.5% moisture are then routed to 3 nos process screens and distributed on the entire width of the screen through a Screen Drag Feeder. The screens are double deck, vibrating type screens. The upper deck mesh has 4mm opening and separates the oversize. The lower deck mesh has an opening of 2mm and separates the undersize. The product is taken out from the top of the bottom deck mesh from where it flows to the product cooler through a conveyer belt.
Product is cooled in a rotary drum cooler using air in a counter-current flow and thereby the temperature of the product is reduced to 45 deg.C before sending the product to the storage bins or for bagging.
Cyclones and Scrubbing
For effectively taking care of the dust and unreacted ammonia leaving with the air stream in different operations in the plant, high efficiency cyclones and venturi scrubbers as well as spray scrubber are employed. Dust and/or ammonia ingress is generated in the equipment like the Preneutraliser , Ammoniator Granulator, Dryer, Product Cooler, Crushing mills, screens, conveyers etc. Cyclones separate most of the particulates from the air stream which are recycled back. To take care of ammonia and smaller particulate emissions following systems are installed.
Fumes Scrubbing System
The fumes scrubber scrubs the moist fumes coming out of Ammoniator Granulator as well as the Pre-neutraliser. In the venturi scrubber, a two stage scrubbing is employed using two venturis in series. Some Phosphoric acid is added to the scrubber. The acidic media will effectively recover ammonia. Spray nozzles are provided in the cyclonic separator of the venturi as a tertiary scrubbing step.
Dryer scrubbing System
Dryer Scrubber scrubs the air coming out from the Dryer Cyclones. Originally the plant had tumbler type of scrubber to scrub the fine particulate matter coming from the dryer cyclones. As a part of on-going improvement in controlling emissions, these tumbler type scrubbers have been replaced with venturi type scrubbers which afford a much more intimate contact between the scrubber liquor and the dust bearing gas. As a result, the efficiency of the venturi scrubber is improved substantially.
Dedusting Scrubber System
Dedusting scrubber caters to scrubbing the dust laden gases from the Dedusting system as well as the product cooler. This system incorporates a venturi type scrubber to remove particulates from the incoming gases so that the gas exiting the stack is well within statutory norms.
Liquid Effluent Control
Complex fertilizer plants like our NPK Plant ‘A’ and NPK Plant ‘B’ are net consumers of waste water. This is because the principal reaction involved in the production of complex fertilizers is the neutralisation reaction between Phos. Acid and ammonia. This reaction is a highly exothermic reaction and is carried out in the pre-neutraliser vessel. During the reaction a lot of heat is evolved and the temperature of the process shoots up. A lot of water is required to be added in the pre-neutraliser to control its temperature at approximately 125 deg.C. This is done by consuming the scrubber liquor from the various scrubbing systems in the plant. Scrubbing system typically contains approximately 30% solids formed by reaction between Phos acid and ammonia recovered from the Granulator/pre-neutraliser/dryer air stream as well as the fertilizer dust recovered in the scrubbers. Balance 70% is water. Thus, depending on the P205content of the grade, phos. acid consumption varies and thereby the process water consumption also varies. However, all the grades are net consumers of waste (process) water.
Solid Waste Control
There is no generation of solid waste in the complex fertilizer plants like our NPK ‘A’ which uses imported concentrated phos. acid (P205 = 54%).