Water Soluble MAP Process

 

Highlights of the technology

Irrigation fertigation is practiced extensively in modern agriculture, which needs fully water soluble fertilizers in order to avoid the plugging of pumps, spraying system and connecting pipes.

Mono-Ammonium Phosphate (MAP) provides two essential nutrients, Nitrogen and Phosphorus, it is one of most required fully water soluble fertilizers on the market.

MAP is often produced by synthesis of phosphoric acid with ammonia. When the crude phosphoric acid by wet-process is used, the MAP obtained contains too much insoluble materials to meet the requirement by irrigation fertigation.

This technology concerns the production of fully water soluble MAP from crude phosphoric acid by wet-process, including the pre-treatment of crude phosphoric acid, the neutralisation of MAP, the purification and concentration of MAP solution, the crystallization and conditioning of MAP crystals.

Phosphoric Acid Pre-treatment

From the wet-process, the filtered phosphoric acid still contains some calcium sulfate, other soluble sulfate salts, fluorine.

This step consists to reduce the sulfate salt and fluorine as much as possible by defining the right additives and their amount.

Phosphoric acid Neutralization

The pipe reactor technology is proposed to synthesize MAP from phosphoric acid and ammonia.

The pipe reactor consists of a tubular reactor under pressure with a mixing chamber in which NH3 gas and phosphoric acid are mixed homogeneously and instantaneously synthesized into MAP, and its main advantages are:

  • Obtaining a homogeneous MAP solution within a very short time,
  • Easiness of operating,
  • Easiness of controlling the pH value of MAP solution.

Separation of insoluble content

Filter press is opted to separate the insoluble content in the MAP solution after the neutralization step. By combination of the crude phosphoric acid pre-treatment and the separation of insoluble after the neutralization, most of the impurities are removed from the MAP solution.

MAP Crystallization

Indirect Forced Circulation (IFC) crystalliser type was selected to produce the crystalline form of Mono-Ammonium Phosphate.

In the IFC crystalliser, contrary to Forced Circulation (FC) and Draft Tube Baffle (DTB) crystallisers, large crystals are not in contact with the circulation pump neither with the agitator. Consequently, the average size of crystals is bigger and the size distribution is narrower than those with FC and DTB technologies.

In summary, the adequate operating conditions for the crude phosphoric acid pre-treatment, the separation of insoluble and crystallisation allows to get a fully water soluble and highly pure grade of MAP 12:61.

Process description

Section 100 - Neutralization

The crude phosphoric acid by wet process contains some insoluble, sulfate salts and fluorine. Some chemical compounds are added into the crude phosphoric acid in a sedimentation tank (T101), and the insoluble is precipitated out and separated from the phosphoric acid.

After this pre-treatment, the phosphoric acid is neutralized with gaseous ammonia in a pipe reactor (R100). In the pipe reactor flash tank (T103), the vapour is separated from the MAP solution and is sucked into the scrubber (S600) for its washing.

The synthesized MAP solution flows down to the tank (T104), which receives also the wash solution from the scrubbing system. The mixed solution is then pumped to the purification section.

Section 200 - Purification

After the neutralisation, some insoluble materials are formed and will precipitate in the settling tanks (T200A/B). The clear part flows down to the clear solution tank (T201), and the turbid part is pumped and filtered in the filter press (F200). The purified solution is collected in the tank (T201).

A second filter press could be necessary to furtherly remove the insoluble and to ensure a highly pure grade of MAP crystals.

The deeply purified MAP solution is collected in the tank (T202), which receives also the treated the mother liquor of the crystallization.

Section 300 - Concentration

The purified MAP solution is concentrated in a forced circulation evaporator (S300) operating under vacuum, and the vapour generated is condensed in the condenser (E301).

A steam ejector (J300) keeps the evaporator (S300) and condenser (E301) under vacuum condition and rejects non-condensable gases to air scrubbing system.

The concentrated MAP solution is collected in the tank (T300), which is fitted with a steam coil to keep the solution temperature.

Section 400 - Crystallization

The concentrated MAP solution is mixed with the recycling mother liquor and introduced in the upper section of the crystallizer (S400) where a part of the sensible heat of the liquor is converted in latent heat of vaporisation of the water. The loss of water in this zone supersaturates the slurry which recirculates to the lower section and comes into contact with small crystals allowing further crystals growth.

Crystallizer (S400) works under vacuum which is maintained by extracting the process vapour via condenser (E400) and steam ejector (J400).

The separation of the crystal slurry and the mother liquor is carried out by sedimentation. Mother liquor leaves the crystallizers via the external circulation loop where flow rate is assured by the propeller pump (P400), while the MAP crystals are recovered in the elutriation leg.

The MAP crystals from the crystallizer contains some mother liquor, which is mostly removed by the Centrifuge separator (S401). Then the MAP crystals are conveyed to the conditioning section through screw conveyor (W400).

SECTION 500 - Conditioning

MAP crystals are dried and cooled in a horizontal fluidized bed cooler (FBC500) having two separated compartments.

For the first compartment, atmospheric air is filtered in air filter (S500), heated in the air heater (E500) and supplied to by the drying blowing fan (M500) for the drying of MAP crystals.

For the second compartment, the filtered atmospheric air is supplied by the cooling blowing fan (M501) for the cooling of MAP crystals.

An exhaust fan (M502) sucks and sends the dusty air from FBC500 to the air scrubber.

The dry and cold MAP crystals could be mixed with anticaking powder before bagging.

Section 600 – Air treatment

Mother liquor treatment

Mother liquor from the crystallization section is treated with additive. The insoluble precipitates and is sent out of the battery limit, while the cleared MAP solution is recycled to the evaporation section.

Air Scrubbing

All process condensates are collected in the condensate tank (T600) and distributed in the process by pump (P600 A/B). The excess of condensate is sent out of the battery limit.

The dusty air from fluidized bed cooler (FBC500) and the vapors from the Ejectors (J300&J400) are sucked into the wet scrubber (S600) by fan (M600) for scrubbing.

The scrubber is constituted of two parts, a venturi section and a cyclonic separator.

The dusty air is introduced through the venturi where it is sprayed with washing solution. The fluid dynamics imposed by the venturi enhances the contact between the air and the washing solution. Thus enhanced coalescence of the particles is obtained.

The saturated air is then relieved of water and agglomerated dust in the cyclonic separator. To reduce the quantity of water carried away with the exhaust gas, a two-step droplets separator is installed at the outlet of the cyclonic separator.

The washing solution from the venturi and the separator is collected in tank (T601) equipped with an agitator (A601). The washing solution is continuously circulated by scrubber pump (G601 A/B) into the venturi.

To control the concentration of washing solution in the tank (T601), part of the solution is recycled to the neutralisation section. To compensate for the recycling and the water lost in the exhaust gas, process condensate is added in tank (T601) so that the concentration and level in the tank stay constant.

Cleaned air is discharged to the atmosphere.

 

Expected figures
Crude phosphoric acid  
State: Liquid
Composition:

P2O5: 20 - 26%

CaO: 1 - 2%

Fe2O3: 0.1 - 0.5%

Al2O3: 0.1 - 0.5%

MgO: 0.1 - 0.5%

F: 1 - 2%

SO3: 0.3 - 1%

Product quality  
Water soluble MAP crystals  
Total Nitrogen (N): 12%
Water soluble P2O5: 61%
Insoluble: 0.2%
Water content: 0.3%
pH value: 4.5 (1% solution)
Temperature: 40°C