Thursday, 2 March 2017

Crushing Strength

Crushing Strength
Crushing strength is the minimum pressure needed to crush individual particles. Determining the crushing strength, or hardness, will help determine handling and storage requirements of a chosen granular product.1
Measure crushing strength by applying pressure to individual granules. A simple finger test can be used in the field.
  • Granule crushed between thumb and forefinger is “soft”.
  • Granule crushed between forefinger and a hard surface is “medium hard”.
  • Granule not crushed between forefinger and hard surface is “hard”.
Crushing Strengths of Typical Fertilizers
Fertilizer Type2
Crush Strength
Prilled Urea
Granule Urea
Granule Ammonium Sulfate
Prilled Ammonium Nitrate
Granule Diammonium Phosphate
Granule Monoammonium Phosphate
Granule Triple Superphosphate
Prilled Potassium Nitrate
Granule Potassium Chloride
Granule Potassium Sulfate
Any granule with a less than three (<3) crush strength should not be broadcast with spinner speeds over 700 rpm. Verify granules are not pulverized before increasing speed.
1 T.P. Hignett. Fertilizer Manual. (Netherlands: Kluwer Academic Publications and Springer Science+Business, 1998), p. 478-479 with kind permission from Springe Science+Business Media B.V.
2 Hignett, Fertilizer Manual, p.481, table18.6.

Crushing strength of urea prills DISCUSSION MATTER

Crushing strength depends upon Vacuum condition, free ammonia present in molten urea prill size distribution.The crushing strength of fertilizer particles differs greatly depending on the chemical composition. Mechanical resistance is the ability of the fertilizer to resist the stresses imposed upon them in the handling chain. The mechanical resistance depends on surface structure and particle strength. Dust and fines normally arise during handling from
Water absorption
• Poor surface structure and particle strength
• Low mechanical resistance
• Mechanical stresses in the handling chain
• Wear and tear from equipment (scrapers, screw feeders, grain trimmers etc) See also how to prevent dust formation.
Crushing strength is the minimum pressure needed to crush individual particles. Determining the crushing strength, or hardness, will help determine handling and storage requirements of a chosen granular product.
generally the crushing strength in our unit about 700 gm to 1.2 Kg per prill. The crushing strength of Granule Urea is about 1.3 to 3.2 kg per granual.
The crushing strength widely depends upon moisture & Prills Temp
• MAXIMUM STRENGTH 930 Gram/prill at 0.1777% Moisture.
• Below 0.12% Moisture strength reduced 698 gram/prill.
• Above 0.3% moisture strength=640 g/prill.
• At 40 deg C Maximum strength 850 g/prill
• Below 34 deg C the strength reduced 640 g/p.
• Above 66 deg C the strength reduced 640 g/p.
Plant. Product results with in plant battery limits remained in prescribed range but after piling in bulk storage amount of under size increases significantly. We took following actions and their results remained fruitful.
- Our Bulk storage is equipped with "Heating & Ventilation system" which was not in service since commissioning. We make it operative.
- We make operative the ventilation system installed at "Conveyor Belt Circuit" .

In the given case,bulk storage is not equipped with ventilation & heating system. In my opinion following steps will be helpful
1- Speed of Bucket:
Check the design operating speed of bucket against the load of 75 t/hr. from the reference performance curve of bucket. Operate bucket slightly above the prescribed speed keeping in view the available margin for under size in final product. This will help in improving the crushing strength of prills. This practice is also good in case of old buckets.
2- Operating Conditions Of Tower:
Operate tower under slightly pressurized conditions with out going beyond the limit where attrition of prills starts due to inter collusion. This will provide extra air cushion to prills which will facilitate the cooling phenomena and will help in improving the crushing strength of prills.
3-Conveyor Belt Circuit:
- Cover open areas (Hoods) on conveyor belt circuit in order to avoid direct contact of product with atmosphere.
- Carry out product analysis of samples being collected from different belts. This will also help in sorting out the problems in conveyor belt system affecting the product quality.
4- Piling of Product in Bulk Storage:
Make heaps/piles of final product in bulk storage after calculation of appropriate heap/pile height based on
- Crushing strength
- Angle of Repose which is generally 27 degrees for urea prills.
5- Recycling of Sweep Urea from Bulk Storage:
In case of recycling, sweep urea from bulk storage
- Make sure strainer for bucket is installed.
- Increase cleaning frequency of sweep urea tank.
Sequential implementation with appropriate optimization can give good results.

 If your plant is producing prills, please give the following info.
1- what is the operating speed of your prilling bucket. Low speed generally produces product
A) with low crushing strength due to insufficient heat transfer from the core of grain to its surface. Surface due to large area, cools down more rapidly while the inner core remains hot and soft which latterly causes disintegration of product.
B) initially with less amount of under size.
2- How old is the prilling bucket ?
3- Pattern of prilling bucket holes and their condition?
4- Under what conditions you are operating Prilling Tower i.e Vacuum or Pressurized?
5- What are the "ambient conditions" in the vicinity of plant?
6- How far is the bulk storage from plant?
7- Are the conveyor belts being provided with ventilation system & covered in order to avoid direct contact with open atmosphere?
8- Is your bulk storage equipped with central heating and ventilation system?
9- In what pattern, the product is stacked in bulk storage?
10- Recycling of sweep/powdered urea

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