Water Soluble C.N. from ODDA Process

 

Highlights of the technology

The technology concerns the production of Water Soluble Granulated Calcium Nitrate, prepared from CNTH crystals as sub-product in ODDA type processes.

Calcium Nitrate mother liquor treatment

From an ODDA nitrophosphate process, the calcium nitrate contained in the NP mother liquor is crystallized by cooling, and then CNTH crystals are separated from NP mother liquor by a belt or rotary filter. The cake of CNTH crystals is heated and re-dissolved with clean water or process condensates. Besides calcium nitrate, the CN mother liquor obtained contains also some impurities like nitric acid, phosphoric acid and insoluble. This mother liquor needs further treatment to produce ammonium nitrate, calcium ammonium nitrate etc.

This process describes the treatment of CN mother liquor in order to obtain a highly pure CN solution, which meets the requirement to produce a fully water soluble CN granules at the end. The process includes mainly three parts: neutralization of free acids, purification and concentration.

Calcium Nitrate Granulation

The heart of the granulation process is a drum granulator composed of:

  • a rotary drum consisting in a cylindrical horizontal drum turning around its axis and fitted with lifters,
  • a fluidized table installed inside the drum and fed with conditioned air.

In the granulator, two operations take place simultaneously:

  • size increase,
  • crystallization and cooling.

Those operations are obtained as follows: The lifters bring the seed material to the upper part of the drum from where it falls on the surface of the fluidized bed table. On the table the product is cooled. The slope of the table allows the product to fall down along the table into the lower part of the drum. While falling, it is coated with molten calcium nitrate that is sprayed through nozzles disposed in the granulator. The lifters return the freshly coated material towards the upper part of the drum to be further crystallized and cooled on the fluidization table. The same cycle is then repeated as many times as necessary to reach the desired grain size.

The number of cycles is determined by the residence time in the drum, obtained by an adjustable overflow threshold. An external fan ensures the evacuation of the air out of the granulator.

Process description

Section 100 - Neutralization

After the dissolution of CNTH crystals, the CN mother liquor is introduced in the first neutralization tank (B101) together with calcium carbonate. Water is sprayed over the solution level to break the foam formed by the reaction.

Then the acidic CN solution flows down by gravity into the second neutralisation tank (B102), where the ammonia is injected to adjust the right pH value of the CN solution.

The neutralised CN solution is fed to the CN buffer tank (B103).

These tanks B101, B102 and B103 are fitted with a steam coil to heat the CN solution, if necessary.

Section 200 - Purification

Purification

The additive is used in the process to help the precipitation of insoluble content in the calcium nitrate solution.

The additive is introduced proportionally to the flow rate of CN solution in the tank (B202), and the mixed CN solution flows down to settling tank (B203).

The clear solution resulting from the settling tank (B203) feeds the adjustment tank (B204) by gravity. The turbid solution is collected at the bottom of settling tank (B203) flows down to the centrifuges (P202 A/B). The mud resulting from this separation operation is conveyed by screw conveyor (T201) to the pumping hopper (B207) from where is sent back to the existing nitrophosphate plant by an eccentric screw pumps (P203 A/B).

The clear liquid leaving the centrifuge machines is collected in the tank (B206) and can be sent by the pump (P204 A/B) to the adjustment tank (B204).

Final grade adjustment

The clear CN solution in tank (B204) is analysed in order to determine the amount of Ammonium Nitrate to be added to reach the desired grade.

The solution with a desired proportion of CN and AN flows down by gravity to the buffer tank (B205) and then transferred to the evaporation section through pump (P205 A/B).

Section 300 - Concentration

The concentration unit operates partly under vacuum, and the MP Steam provides the heat necessary to evaporate the water of CN solution.

Vapour collected in the vapour head (VH301) is transferred to and condensed in the main condenser (E302). The condensate is collected and distributed in the process by pump (P302 A/B). The excess of condensate is sent out of the battery limit.

A vacuum system (J301) is needed to ensure the evaporator (E301) and condenser (E302) under vacuum condition and to reject non-condensable gases to scrubber (S601).

Section 400 - Granulation

Granulation

The concentrated CN solution from concentration section is introduced in the homogenizer tank (B401) along with the recycled crushed oversized particles from the screening section. Tank (B401) is equipped with steam coils and an agitator (A401) to prevent the solution from crystallizing. The resulting melt is sprayed by pumps (P401 A/B) in the granulator (D401).

The granulator (D401) is fed with recycled material from the screening section (S403) and the seed crusher (Z401). Atmospheric air is filtered (S401) and heated (E401) if necessary and then blown by fan (M401) into the fluidized table of the granulator to cool down the freshly coated granules.

The calcium nitrate melt is sprayed on to the cooled granules during their falling from the fluidized table and crystallized.

Screening and crushing

At the outlet of the granulator the product is conveyed (T401) and lifted (T402) to the screening section.

The double deck screen (S403) separates the oversize product, the onsize product and undersized product. Oversize product is crushed by a hammer crusher (Z402) and sent to the homogenizer (B401) to be re-dissolved. The undersized product is returned to the granulator through conveyor (T406). Part of the onsize product is crushed by a seed crusher (Z402) and recycled to adjust the recycle ratio though diverter (T408).

The remaining onsize product is sent to the next section 500.

Section 500 – Cooling and conditioning

Onsize product is cooled in the fluidized bed cooler (E502) by conditioned air coming from air conditioning unit (S501, E501).

The cooled onsize product is sent to the coating drum (D501) to be coated with anti-caking agent to enhance storage and transportation properties.

Optionally, the fluidized bed cooler could be replaced by a plate/plate vertical exchanger cooled with cooling water.

The coated product is conveyed to the storage unit.

Section 600 – Air treatment

Dedusting

Due to the high hygroscopicity of the product, the dedusting air must be heated to decrease the relative humidity. Some air from the air conditioning unit is heated in the steam heater (E504) before being injected in each suction point of the dedusting network.

Scrubbing

The dusty air from the granulator (D401), the FBC (E503) and the dedusting points are collected and sent to the wet scrubber (S601).

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 (B601) 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 (B601), part of the solution is recycled to the existing evaporation section. To compensate for the recycling and the water lost in the exhaust gas, make-up water (process water or process condensates) is added in tank (B601) so that the concentration and level in the tank stay constant.

Through fan (M601), cleaned air is discharged to the atmosphere through a fume stack.

 

Expected figures
Mother Liquor typical specification  
State: Liquid
Temperature: 60°C
Density: 1.7 g/cm3
Composition:

Ca(NO3)2 : 61%

HNO3: 3%

H3PO4: 0.4%

Insoluble: 3.6%

Water: 32%

Product quality  
Total soluble Ca++: 19%
Total N – NH4: 1.2%
Total N – NO3: 13.8%
Insoluble: 700 ppm (lower content if required)
Temperature: 50°C
Water content: 17%
Granulometry: 2 mm < 90% < 4 mm
Crushing strength: 2.5 kg min. on 3,0 mm granules
Specific raw materials consumptions (kg/t of final product)  
CN Mother Liquor: 1220 kg
CaCO3: 35 kg
NH3: 2 kg
AN solution (100%): 60 kg