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Fertilizer Plant Waste

 

Introduction
Manufacturing Process
Treatment of Fertilizer Waste Water
Waste Water from Fertilizer Plant
Effects of Waste on the receiving streams

 

 

Introduction :

Fertilizer industry can be divided into three main categories depending upon  

            1.Fertilizer raw materials

            2. Fertilizer intermediates

            3. Fertilizer products

Fertilizer Intermediates: 

1.      Sulphuric acid

2.      Phosphoric acid

3.      Nitric acid

Fertilizer products:

SOLID

          N - Fertilizers

            1.      Ammonium  nitrate

            2.      Urea

            3.      Ammonium sulfate

            P - Fertilizers     

            1. Super phosphates   NPK fertilizers

LIQUID:

Ammonia fertilizers produced in India can be classified broadly into two groups viz., nitrogens fertilizers, and phosphatic fertilizer.  Plants may be producing only nitrogenous fertilizers like urea, Ammonium sulphate, Ammonium Nitrate, Ammonium chloride or only phosphatic fertilizers like super phosphates;  there are plants where complex fertilizers containing both nitrogen and phosphates like Ammonium phosphate and Ammonium sulphate phosphate are produced

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Manufacturing Process :

Ammonia is the principal intermediate in the manufacture of all nitrogenous fertilizers.  So, except when the

byproduct ammonia will be available from a coke oven, raw materials for nitrogenous fertilizer production

 is the carbonaceous materials, which are required for making ammonia.  So all the nitrogenous fertilizer

plants will have essentially an Ammonia production unit and a reactor where the synthetic ammonia will be

 reacted with other chemicals to produce the final product.  The plant may have auxiliary units to produce

the reacting chemicals also.  Basic process steps in the manufacture of urea, from carbonaceous raw materials

like naphtha are as follows:

 

(i)                  reaction of the carbonaceous materials with steam and air to form a mixture of hydrogen and carbon

monoxide, known as synthesis gas.

(ii)                Reaction  of  the  carbon monoxide with steam over a catalyst to form more hydrogen and carbon

dioxide.

(iii)               Separation and purification of carbon dioxide.

(iv)              Removal of residual carbon monoxide from gas mixture

(v)                Synthesis of ammonia by reacting hydrogen and nitrogen over a catalyst (Nitrogen is supplied as air

 in an earlier step) and

(vi)       Synthesis of urea by treating ammonia with carbon dioxide in a reactor at higher temperature and pressure

 

  • plants using by product Ammonia from other manufacturing plants (coke oven) have to produce carbon dioxide separately for the production of urea
  • Ammonia sulphate may be produced by reacting Anhydrous Ammonia  with Sulphuric Acid,  usually obtained as by-product sulphuric acid from other manufacturing plants
  • Ammonia sulphate may also be manufactured from Gypsum or from Calcium sulphate sludge  obtained from the phosphatic fertiliser plant using Ammonia and Co2  obtained from Ammonia plant.  In this process calcium sulphate is reacted with Ammonia carbonate solution to produce ammonium sulphate.
  • Ammonium nitrate is produced when ammonia is reacted with nitric acid.  Normally the required quantity of nitric acid is produced in the same plant, by oxidizing the ammonia.
  • Super phosphate is produced by merely  mixing the phosphate ore (commonly known as phosphatic rock0 with sulfuric acid to convert the phosphate to 'Monocalcium' phosphate.The by-product calcium sulphate of this process may be used in the manufacture of Ammonium sulphate.
  • Ammonium phosphate is made by treating phosphoric acid with Ammonia, the phosphoric acid production process involves the following steps

    1.      Dissolving phosphate rock in enough sulphuric acid

    2.      Holding the mixture until the calcium sulphate crystals grow to adequate size.

    3.      Separating the phosphoric acid and calcium sulfate by filtration and Concentration of acid to the desired level.

 

 

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Treatment of Fertilizer Waste Water :

Major pollutants in the fertilizers waste water for which the treatment is necessary include oil, arsenic, ammonia, urea, phosphate and fluoride.

The effluent streams can be characterized as either a phosphoric effluent or an ammonia effluent.

The phosphoric acid effluent is high in fluoride concentration, low in pH, High in phosphate and high in suspended solids.  Standard practice has been contain the water for reuse, allowing enough time for solids sedimentation.

n      Oil  is removed in a gravity separator

n      Arsenic containing waste is segregated and after its concentration the solid waste is disposed off in a safe place.

n      Phosphate and fluoride bearing wastes are also segregated and chemically coagulated by lime; clarified  effluent which still contains some amount of phosphate and fluoride is diluted by mixing with other wastes.

The other effluent type is characteristic of ammonia production and ammonia containing products.  Most of the contamination comes from ammonia production itself.  It is characteristically high in ammonia from effluent gas scrubbing and gas cleaning operations and high in sodium hydroxide or carbonate from gas cleaning process.

Several alternatives are there for the treatment of Ammonia bearing wastes

1.      Steam stripping

2.      Air stripping in towers

3.      Lagooning after pH adjustment

4.      Biological nitrification and denitrification

For  all practical purposes, steam stripping for the ammonia removal from fertilizer wastes have been found to be uneconomical.

Removal of Ammonia gas from the solution in an air stripping tower, packed with red wood stakes, is found to be very  efficient method.  Very encouraging results are obtained from some laboratory and pilot plant studies conducted by NEERI in the removal of Ammonia by simply lagooning the waste.  It was found that considerable reduction in the Ammonia content can be accomplished  just by retaining the Ammoniacal waste in an Earthen tank about 1m deep, for a day or two after a pretreatment of the waste by lime increase the pH to 11.0.  Of course with urea containing waste, no reduction in urea content is observed within this period' Thus waste containing both urea and Ammonia required  to be retained in the Lagoon for a longer period, to allow urea to decompose to ammonia first.

Biological  nitrification involves oxidation of Ammonia to nitrate via nitrite under aerobic condition;  this followed by the denitrification of the nitrified effluent under anaerobic condition; in which gaseous N2 and N2O is the end product and is released to the atmosphere.  The denitrification  requires addition of some quantity of carbonaceous matter in the reactor.

In all the Ammonia removal method described above, urea remain as untouched.  If urea removal is required the urea containing wastes must be retained for  a sufficiently long time in an earthen lagoon to allow it to decompose first to ammonia.

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Waste Water from Fertilizer Plant:

A variety of wastes are discharged from the Fertilizer plant as water pollutants in the form of

1.      Processing chemicals like Sulphuric acid

2.      Process intermediate like Ammonium, Phosphoric and etc.

3.      Final products like urea, Ammonium sulphate, Ammonium phosphate etc.

In addition to the above, oil bearing wastes from compressor houses of ammonia and urea plants, some portion of the cooling water and the wash water from the scrubbing towers, for the purification of gases, also come as waste.

Wash water from the scrubbing towers may contain toxic substances like Arsenic, Monoethanolamine, Potassium carbonate etc. in a Nitrogenous fertilizer plant, while that in a phosphatic fertilizer plant may contain a mixture of carbonic acid, hydrofluoric acid.  Both alkaline and acidic wastes are also expected from the boiler feed water treatment plant, the wastes being generated during the regeneration of anion and cat ion exchanger units.

Additional pollutants like phenol and cyanide  will be introduced in the list of pollutants in the fertilizer plant where ammonia is derived from the waste ammonia cal liquor of the coke ovens.

Average characteristics of the waste water from a typical Indian fertilizer plant producing both nitrogenous and phosphatic fertilizer  is given below.

Parameter Value
pH   7.5 to 9.5    
Total solids, mg/l 5400 mg/l
Ammonia Nitrogen 700 mg/l
Urea Nitrogen 600 mg/l

Phosphate 

75 mg/l
Arsenic  1.5 mg/l

Fluoride 

15 mg/l  

 

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Effects of Wastes on Receiving streams:

All the components of the waste from the fertilizer plants induced adverse effects in the stream. Acids and Alkalis can destroy the normal  aquatic life in the stream.  Arsenic, Fluorides and Ammonium salts are found to be toxic to the fishes.  Presence of different types of salts renders the stream unfit for use as a source of drinking water in the down stream side.  Nitrogen and other nutrient content of the waste encourages growth of aquatic plants in the stream.

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