Question

Give the raw materials,process description,flow sheet,advantages and disadvantages,engineering problems faced in the production of NITRO PHOSPHATE(please give flow sheet)

Answer 1

Designation Nitrophosphate process is limited to where nitric acid is used primarily to acidulation of phosphate rock.

First process developed was well know as "Odda Process" which was developed by Erling johnson. We will discuss this Odda process used and further developed by "Norsk Hydro".

1) Raw Material for nitrophosphate production

Main raw materials used in production of Nitro Phosphate are phosphate rock and nitric acid.

2) Flow Sheet for Nitro Phosphate Production and Process Description

Process Description with engineering problems.

The process above can be summarized as

1.Digestion of phosphate rock.

In this process phosphate rock is reacted with nitric acid (around 60 %). To Produce calcium nitrate and phosphoric acid.

Along with calcium phosphate other minerals also reacts with nitric acid as follows.

  

The reactions are exothermic and normally takes place between 50 to 65 Degree Celcius in two consecutive SS vessels with agitators. The main part of nitric acid fed directly to the digestion tank. A part of nitric fed is first used to wash crystals and then returned to the digestion tanks. The degree of dissolution depends upon excess nitric acid, degree of agitation, rock surface. Particle size less than 8 mesh and residence time of 1 hour is generally sufficient for dissolution of 99 % phosphorous. Dissolution of phosphate rock liberates small amount of HF and SiF4 with small amount of nitrogen oxides.

If too much oxides are formed, this is can suppressed by addition of urea. Foaming due to presence of CO2 and organic compounds in rock can be reduced by rigourous agitation. Acidulation is followed by removal of undissolved solids like silica. As silica can erode equipments.

2. Crystallization of calcium nitrate tetrahydrate.

Norsk Hydro process uses batch crystallizers which are operated sequentially for overall continuous operation and giving large and uniformly sized crystal. The crystallizers are equipped with cooling coils with a 20% aqueous ammonia solution as coolant. When coldest crystallizer reaches the end temp material is discharged to buffer tank and then fed to calcium nitrate filters at steady state.

3. Removal of the calcium nitrate tetrahydrate crystals by filtration.

In this step each filter has two rotary drums. Mother liquor from first drum goes to mother liquor tank. and calcium nitrate cake goes to second filter where it is resuspended in wash acid and filtered. And then washed with nitric acid and water. Resulting acid is collected in wash tanks partially recycled to filters and remainder is utilized in acidulation process. Calcium nitrate crystals are flushed from filters with molten calcium nitrate and sent to conversion section.

4. Neutralization of the mother liquid (with the addition of ammonium nitrate if required).

Mother liquor or filtrate from filters is nuetrilized with gaseous ammonia which comes from evaporators in crystalizers. In this stem calcium is precipitated as calcium fluoride and dicalcium phosphate. The balance of phosphorous stays in solution as ammonium phosphate.

5. Evaporation of water from the neutralized liquid.

In evaporation step the water content of the neutralized liquid is reduced to 0.5 % at about 180 C by low pressure evaporation.

6. Mixing of the evaporated liquid with potassium salts

The potassium source can either regular commercial-grade KCl / K2SO4. Specially designed mixers give very efficient distribution of potassium in melt.

7. Prilling of the mixture in an air-cooled prill tower.

The NPK mixture is now ready to get prilled. Prilling is done in air cooled prill tower where NPK mixture is droped from top with help of prill bucket. Prill bucket is specially designed for anti clogging. When first bucket is removed for cleaning is replaced by another clean bucket. formed prills are removed from bottom with help of rotating bottom and stationary scrapper. The amount of off size product is normally below 10%.

8. Screening, cooling, and coating of the final product.

Prilled granules are then screened, cooled before coating. Cooling is done by fluid bed or rotaing type drum coolers. Oversized particles are crushed and rescreened before sending back to prilling tower top with undersized particles.

Advantages of nitrophosphate  process.

Main advantage of this process is whole process can be automated to considerable errors. Other main and cost effective advantage is dual benifit of nitric acid used in process. Firstly as in digestion of phosphate rock and in secondly in nitrogen nutrient in final product.

Dissdvantages of nitrophosphate  process.

Man disadvantage is raw material cost as raw materials acquire around 80% of the production cost of fertilizer.