There has been a worldwide shift from prilling
of Urea to granulation because the more desirable products made
in granulator. This shift has taken place to supply growing fertilizer
demand for larger, harder and denser particles, in spite of traditionally
higher costs associated with granulation.
Urea finishing Processes
Urea can be prilled, granulated, flaked and crystallized.
Presently only prilling and granulation is considered important.
Most new plants that plan to ship internationally utilize granulation
because of its far superior handling and storage qualities
Comparative product characteristics are and a brief on two types
of finishing process are given below:
Using a spinning bucket, showerhead or acoustic
vibration makes Prilled urea. The most common method is spinning
bucket. Stamicarbon uses its own design for the buckets,
Snamprogetti and Toyo both use the Tuttle bucket, that is also
being used in our Kakinada plant.
In prilling, the urea melt is concentrated via
vacuum evaporation to 99,8% and fed as quickly as possible into
bucket to minimize buiret formation. The liquid form drops that
then fall down a cylindrical concentrate tower that has either
induced, forced or natural draft airflow. The prills solidify
and are removed at bottom by belt conveying to storage. Some plants
have a fluid-bed cooler in prill tower bottom and others use an
in-line cooler before storage. If low biuret product is to be
produced, the urea melt from the last decomposition stage of the
synthesis plant is first crystallized and the crystals are then
melted before prilling.
The granulation involves spring molten urea through
series of the fine nozzles in a fluidized bed granulator. Cooling
the liquid urea slowly while rolling it in layers, creating a
harder more evenly sized granule makes granules of urea. There
has been a world wide shift from prilling of urea to granulation
because of the more desirable products made in granulators. This
shift has taken place to supply growing fertilizer demand for
larger, harder and denser particles, in spite of traditionally
higher costs associated with granulation.
There have been many granulation processes developed
and operated through the years. These include TVA pan granulation,
C&I Girdler spherodizer spray drum, Norsk Hydro pan granulation
Rotary drum garnulators produce granules by spaying
a concentrated melt (99.0 to 99.8%) onto small seed particles
of urea in a long rotating cylindrical drum. As the seed particles
rotate in the drum, successive layers of Urea are added to the
particles, forming granules. Granules are removed from the granulator
and screened. Off size granules are crushed and recycled to the
granulator to supply additional seed particles or are dissolved
and returned to solution process.
Pan ganulators operate on the same principle as
drum granulators, except the solids are formed in a large, rotating
circular pan. Pan granulators produce a solid product with physical
characteristics similar to those of drum granules.
Since the solids are produced in a vide variety
of sizes, they must be screened for consistently sized prills
or granules. Cooled prills are screened and off size prills are
dissolved and recycled to the solution concentration process.
Urea: Comparative Product Characteristics
* Used, for example for forest application
(3-5 mm size)
The properties in of different finished products
Here some of the results of trials conducted on
granular urea along with other urea products show the properties
of various forms and their comparisons:
Nitrogen losses and fertilizer N recovery from modified urea under
wetland rice on a sandy loam soil in Mandya, Karnataka:
Experiment 2: Paddy
yield comparison according to Urea type and granular urea sizes
at MADA, Alor Setar, Malasiya, showed the following yields:
Experiment 3: Point
placement of Granular Urea:
Due to losses in leaching in Nitrogen as reported
above case particularly paddy and other crops a new concept of
use of Super Granule Urea (SGU) as point placement has been suggested.
An experiment was conducted in BAU, Mymensingh- Bangladesh to
study the yield patterns in different types of urea in a RBD (Randomized
Block Design) in potato crop. The results were as below.
*Plant Nutrition Other than N was common for all
Hence based on results
of above experiments it is clear that:
- Higher the granular size of Urea lesser would
be leaching losses and hence better yields.
- Volatilization losses in granular urea are almost
half of Prilled as well as neem coated urea.
- Lesser quantity of granular urea gives better
yields than granular urea due to saving in leaching and Volatilization
- Point placement of granular urea gives
much better yields to point placement, although prill urea also
responds favorably to point placement.
With most granular NPK Products (excluding the
slurry based nitrophosphate-type processes), agglomeration is
the principal mechanism responsible for the initial granule formation
and subsequent growth. In most agglomeration-type NPK formulations,
50-75% of raw materials is fed as dry solids. These
solid particles are assembled and joined into agglomerates (granules)
by a combination of mechanical interlocking and cementing. The
cementing medium for fertilizer granules are derived from salt
solutions, for example, pre-neutralized ammonium phosphate slurry
and or the dissolution of salts on moist surface of the soluble
solid particles. The size, surface texture, strength and solubility
of solid particles vary widely and have a profound influence on
the granulation characteristics of the mixture.
Accretion refers to the process in which Layer
upon layer of a fluid material (material example, ammonium phosphate
slurry) is applied to a solid particle causing it to grow in size.
The slurry type accretion granulation processes are used to produce
TSP and some nitrophosphate compounds. The accretion
process is quite different from the agglomeration process parameters
for optimum operation of these slurry type accretion granulation
processes is often quite different from those used in agglomeration
processes. With a slurry type granulation applied, dried and hardened
to a relatively firm substrate consisting of granules that are
often product size or nearly product size. In this process, layer
upon layer of new material is applied to a particle, giving the
final granule a onion-skin like stricture. In the
process, of course, some agglomeration of particles also occurs,
but this is not a predominant granule formation mechanism.
The recycle - to -product ratio for accretion
type granulation is normally higher than that of the required
for the agglomeration type processors. Accordingly, for a given
production rate, the material handling capacity of the process
equipment has been larger for the accretion type plants.
Soil Reaction of Urea (irrespective of type of
At the field, when the urea contacts the soil,
a quick hydration and dissolution of granule is produced, and
it disappears from the sight once dissolved. Then urea is subjected
to the soil and climates own factors, and it breaks down
until it reaches the proper form to be absorbed by the plants.
Plants take up nitrogen from soil in the mineral
forms of nitrogen, both ammonium and nitrate before converting
it to plant protein nitrogen. Plants vary in their preference
to utilize either form of nitrogen. Nitrate is considered the
main source because it is more mobile in the soil. On addition
to soil, urea dissolves into soil solution and is converted to
ammonium and then to nitrate.
Conversion is favoured by
Unfavorable conditions of ammonium ions:
- Persistent drought
- High temperatures ( > 25° C) and
- Soil which has a low capacity for adsorbing
- Soil which has a high pH
- Dry cloddy soils that limit the physical
mixing of urea with soil can increase volatilization.
- Do not mix with Superphosphates unless
applied shortly after mixing. Urea will react with Superphosphates,
releasing water and resulting water molecules and resulting in
a damp material which is difficult to store and apply.
- Do not apply urea either forms of prilled
or granular along with seeds or close to root zone resulting in
death of seed /seedlings.
Critical Humidity of urea at 30° C is 75.2%.
(Critical humidity is level of humidity for a given temperature,
at which a product begins to take up moisture from the atmosphere.
Urea (both granular and prilled) is completely
soluble in water. Its maximum solubility is 30 kg of urea per
100 litres of water. Be aware that with water it gives a cold
reaction, so take care on cold and frosty mornings.
Questions related to Fertilizer
in Urea is in form of:
(Nitrogen After dissolution in soil it is converted to Ammonium
2. Immediate after
dissolution in soil urea is in what form?
Ans: Ammonium Ions
3. Most favorable
temperature for conversion of Urea from Ammoniac form to Nitrate
Ans: -20 to 370C
5. What is the Average
size of prilled Urea?
Ans: 1.7 mm
6. What is Volatilization?
Ans: It is evaporation
of Urea as ammonia into atmosphere when left exposed to sunlight
7. What is Critical
Humidity of Urea at 300C?
Ans: 75.20C (CH is
the level of humidity for a given temperature at which a product
begins to take up moisture from atmosphere.
8. What is maximum
solubility of Urea in kg per 100 liters of water.
Ans: 30 kg in 100
liters of water.
9. What is a chemical
formula of Urea? CO2(NH2)2/ CO(NH2)3/ CO(NH2)2 / CO(NH3)2
10. Why plant is
killed when Urea is applied to root zone Plant?
Ans: Due to toxicity
caused by high pH and ammonia concentration around moist root
11. Urea should not
be mixed with what fertilizer and why?
due to release of water molecules and creating damp material.
12. Urea should not
be stored with what fertilizer and why?
Ans: Ammonium Nitrate
CRH Urea -75.2%, AN -59.4% but Urea +AN -18.1%
13. Percentage of
Sulphur in ammonium Sulphate
Ans: S- 23%
14. Percentage of
Nitrogen in Calcium Ammonium Nitrate & form of N
Ans: Ammoniacal N
12.5% & Nitrate N 12.5%
15. Chemical Formula
for 21% zinc sulphate and called as
Ans: Zinc Sulphate
Heptahydrate (ZNSO4. 7H2O)
16. Percentage of
Boron in Borax
17. Forms of Nitrogen
& their percentage in 19:19:19
Ans: Nitrate 4.0%
Anominiacal Nitrogen 4.5% & Urea Nitrogen 10.5%
18. Form of Nitrogen
in Multi K
Ans: Nitrate 13%
19. Weight of sample
drawn from complex and straight fertilizers( other than micro
nutrients) by Inspector
Ans: 400 gm
20. Who authorizes
a Fertilizer Inspector to draw Fertilizer sample
Ans: A fertilizer
Inspector is appointed by State or Central Government through