1. Solid urea feedstock
2. Urea solution or melt
3. Additional solid raw materials
4. Pre-neutralizer
5. Granulator (rotary drum)
6. Dryer (co-current)
7. Cooler (second-stage dryer, counter-current)
8. Screening
9. Oversize crushers
10. Product cooling
11. Product conditioning
12. Bulk storage and process plant dehumidification
13. Bagging

The wide availability of urea, at competitive prices, and its simple handling characteristics makes urea an attractive source of nitrogen for the producer of compound fertilizers. If the particle (granule or prill) size of urea is compatible with other fertilizer materials and precautions are taken to avoid chemical incompatibility, quality physical mixtures (blends) can be prepared and marketed.

However, when urea is used to produce compound fertilizers using steam/water, chemical, or compaction granulation processes, precautions must be taken to avoid process and product quality problems caused by the presence of urea in the formulation. As little as 2%-5% urea in a compound fertilizer formulation will often be quite noticeable with respect to increased hygroscopicity and plasticity (especially during production and at higher than ambient temperatures) of the product. The adverse impact of urea in a compound fertilizer is significantly increased if potassium chloride (muriate of potash, MOP) and/or magnesium sulphate (kieserite) are also present in the formula. The hygroscopicity and plasticity characteristics of urea-containing compound fertilizers may seriously interfere with the operation of the drying, screening, and crushing equipment unless this equipment is specifically designed and operated. The manufacturing and handling aspects of urea-based compound fertilizers has been the topic of publications and workshops.

Furthermore, the storage properties of urea-containing compound fertilizers are often inferior to those of compounds that do not contain urea. The tendency to absorb moisture from the atmosphere in the case of bulk product and the caking of bagged product are the two most undesirable characteristics of compound fertilizers containing urea.

Urea and urea-based fertilizers are in most cases not compatible with ammonium nitrate and ammonium nitrate-containing fertilizers for similar reasons. In storage, a strict separation of such fertilizers should apply in order to avoid both product quality and corrosion issues.

Despite these inherent characteristics of urea-based compound fertilizers, high-quality compound products containing more than 30% urea can be successfully produced and marketed provided certain precautions are taken during the production, storage, and bagging steps. The following is a summary of the critical design and operating criteria that should be considered when producing urea-based compound fertilizers. A process flow diagram of an NPK plant specifically designed to accommodate the use of significant amounts of urea in the formulations is shown in Figure 1. For the manufacturing, storage, and handling of urea and urea-based compound fertilizers, various aspects of the manufacturing and handling operations need to be considered. A discussion of the most important features that should be incorporated into a urea-based NPK granulation process follows is given below.

Solid urea feedstock

Prilled urea is preferred because its small particle size helps to facilitate its relatively homogeneous incorporation into the NPK granule structure. Very small and broken prills are the most desirable.

Urea solution or melt

If the urea content of the formula is more than about 20%, it is preferable to dissolve a portion of the urea to produce a hot (105°C) 75-80% urea solution. The availability of urea solution adds considerable flexibility to the process. The solution is sprayed on top of the bed of material in the granulator. In most instances, a solution of urea is preferred over an almost anhydrous urea melt because it is easier to handle and a solution more effectively promotes agglomeration of the other solid materials used in the formulation.

Additional solid raw materials

Standard-grade materials are preferred. Nongranular run-of-pile (powdered) monoammonium phosphate (MAP) and superphosphate are recommended. If superphosphate is used in combination with urea, it should be first fully ammoniated to minimize unwanted urea-superphosphate reactions that result in the release of water of crystallization contained in the superphosphate. However, as a general rule, the use of superphosphate in combination with urea should be avoided, even if the superphosphate is ammoniated. Standard or granular ammonium sulphate can be used (Ivell, 2019). Standard-grade muriate of potash is preferred because of its usually good flow characteristics and more optimum particle-size distribution. Good flow characteristics of all solid raw materials are highly desired to facilitate handling and accurate metering.

Preneutralizer

A standard, atmospheric tank-type pre-neutralizer is preferred. An NH₃:H₃PO₄ mole ratio of about 0.5 to 0.6 is recommended in the preneutralizer to achieve a low free water content while still maintaining a fluid and pumpable slurry. This is especially important if a significant amount of sulphuric acid is also neutralized in the unit. The pre-neutralized slurry at a mole ratio of about 0.5 .to 0.6 should be about 127°C and contain no more than 15% free water. The preneutralizer and its auxiliary equipment (agitator, pumps, and piping) should be constructed of corrosion-resistant materials to facilitate the use of a mixture of phosphoric and sulphuric acid, thus adding considerable flexibility to the process. In general, if a higher mole ratio (for example 1.5) is used, especially if sulfuric acid ls also present, the free water content of the slurry will have to be higher to facilitate pumping because the ammonium sulphate crystals tend to thicken the slurry.

The length-to-diameter ratio should be at least 3. The greater length, compared with granulators used in most ammonium phosphate and many North American NPK plants, facilitates granule formation and gives the operator more flexibility in controlling the agglomeration process. The NH₃:H₃PO₄ mole ratio of the material discharged from the granulator may be quite variable depending upon the properties of the raw materials. However, a mole ratio between 1.0 and about 1.8 is expected to be optimum. The lower mole ratio (1.0) will minimize the evolution of ammonia from the granulator and thus simplify operation of the scrubbing system and tend to improve control of the process. The free moisture of the product discharged from the granulator will usually be in the range of about 2% to 3%. The recycle-to-product ratio is expected to vary from about·3 to 6. A ratio of 6 is recommended for design. If the process is based on a solid ammonium phosphate source (for example, powdered MAP) and a preneutralized slurry is not used, then a design recycle-to-product ratio of 3 is usually sufficient.

Granulator (rotary drum)

In high mole ratio NP(K) formulations, for example 28-28-0 and 19-19-19, the urea share in the formulation is in the order of around 30-35 percent. This tends to cause a number of difficulties, including a very high solubility, a very low melting point, and a low critical relative humidity (CRH). Such grades are much more soluble than DAP because of the high urea levels. The moisture required for granulation is therefore significantly lower and the required recycle ratio higher. To some extent, the impacts of these characteristics can be mitigated by partial cooling of the recycle, as decreasing the granulation temperature reduces solubility. Cooling both product and oversize material has an added benefit: carrying out crushing at a lower temperature, well below the melting point, helps reduce crusher build-up (Ivell, 2019).

Co-current dryer

The length-to-diameter ratio of this rotary unit should not exceed about 6. The superficial velocity of the air (at outlet conditions) should not exceed 2.4 m/sec maximum (2.0 m/sec preferred]. The maximum temperature of the outlet air should not exceed 80°C, and the relative humidity (RH) of the air at this temperature should not exceed 15%. An outlet air temperature of 75°C at 15% RH is recommended as the basis for design. The moisture of the material discharged from the dryer should not exceed 1.0%; a value of 0.8% is recommended for design provided a second stage dryer (process cooler) is used. The temperature and humidity profile in the dryer is extremely important, and the optimum values will vary from product to product primarily depending on the CRH of the fertilizer material, as shown in Figure 2. It is important to note that if the material becomes overheated, the surface of the granules will become liquescent and kneadable and material will stick to the interior walls and to the flights of the dryer near the discharge end, as such a length-to-diameter ratio of no more than 6 is recommended to avoid ‘over-drying’ and possible melting of the material near the discharge end of the dryer.

This unit is essential because it performs two important functions in a urea-based NPK granulation plant. First, it acts as a low-temperature second-stage dryer to ensure that the product moisture is 0.6% or less; and second, the retention time in the unit allows the relatively soft and plastic material to harden, thus improving the operation of the screening and oversize crushing systems.

Cooler (second-stage dryer, counter-current)

The length-to-diameter ratio of the process cooler should not be less than 6; 7 to 8 is preferred. The inlet air should be dehumidified and/or heated to ensure that the RH is 60% or less as it enters the process cooler. The superficial air velocity should not exceed 2.0 m/ sec. The low velocity is recommended because of the relatively large amount of minus 1.0 mm particles in the material. It is usually preferred that these small particles be removed (separated) by the screens rather than by the airflow/dust collection system. The unit should be designed to achieve a material (discharge) temperature of not more than about 54°C at a free moisture content of no more than 0 6%. Further cooling of the product fraction (after screening) is performed in a separate operation.

Screening

Single-deck horizontal gyratory-type screening units are recommended. The screen wire should be stainless steel and of the square-mesh style to obtain ‘precision screening’ and minimize the amount of irregular shaped particles that lead to caking during long-term storage. If inclined, electrically (or motor) vibrated screens are used, they should be for oversize separation only, not for ‘precision screening’ of the product fraction, The hourly loading of the oversize screen should not exceed about 20 t/m2, and the loading of the horizontal gyratory-type product screen should not exceed 50% of this value (25%-30% preferred).

Oversize crushers

Double rotor chain mill-type crushers or double row cage mills are preferred. The discharge assembly of the mill should not be restricted and should be constructed of flexible rubber (conveyor belting) panels that can be flexed from the outside by an operator using a hammer. A flared-type discharge assembly for the crushers is recommended to help avoid the accumulation of solids. The crushed oversize should be recycled to the oversize screen on a closed-loop basis to ensure that only the fine material fraction (undersize) from the screens is returned to the granulator as recycle. With good control of granulation, the production of oversize is minimized. This is important because the production of too much oversize material disturbs the solids surface/liquid-phase ratio in the granulator, which leads to the further production of excessive quantities of oversize material.

Product cooling

Either a rotary-drum or fluidized-bed unit is recommended. A single-pass, counter-current, cascade-type cooler is also acceptable. The cooling unit should be located immediately ahead of the product conditioning unit. The RH of the cooling air at inlet conditions should not exceed 50%. The temperature of the product discharged from the cooler should not exceed about 43°C. In all cases the temperature of the cooled product should be about 5°C above the average ambient temperature to avoid absorption of atmospheric moisture on the surface of the material when it is stored in bulk for a short period prior to bagging. An indirect plate type cooler may be used if care is taken to avoid deposition of product material onto the cooling plates due to product plasticity or remaining moisture.

Product conditioning

A standard rotary drum-type product conditioning unit is recommended. Screw-type mixing units should be avoided because they tend to grind and break the product granules. Kaolin clay applied in the order of about 0.5% (weight bases) is usually sufficient for most urea-based NPKs. An oil- or wax-type liquid binder may also be sprayed into the product prior to the application of the clay conditioner to help adhere the clay to the surface of the granules, thus decreasing the evolution of dust during handling.

Bulk storage and process plant dehumidification

If ambient conditions normally exceed about 50% RH for extended periods (more than about 8 h), provisions should be made to dehumidify the process plant and bulk storage buildings. Cooling and proper ventilation may also be necessary for worker comfort and safety. Adequate ventilation is especially important because all buildings should be tightly constructed and closed to maintain a dry inside environment (RH of 40%-50%).

Bagging

The urea-based compound fertilizer products should be bagged in moisture proof bags shortly after production. Direct bagging from the production unit is not recommended because off-specification product could inadvertently be bagged and flexibility is lost in handling product during grade-change periods. In addition, bag set (caking) is minimized if the fresh product is allowed to ‘pile set’ for a short period of time (10-12 h). Bags may consist of an impermeable polyethylene inner bag and a polypropylene outer bag for mechanical strength.

References

D. Ivell, NPK production routes, Fertilizer International 492, September – October 2019, pages 1 – 5

Links to related IFS Proceedings and recordings

648, (2009), Urea Technology – Past, Present and Future,
J. Visser.
692, (2011), Technology and Factors Affecting Fertiliser Screening Performance,
N.E. Smith.
725, (2013), Urea-based NPK Granulation – Examination of Constraints and Potential Solutions,
S R Doshi.
767, (2015), Update on Fluidised Drum Granulation Technology and its Applicability for Different Fertilisers,
S Valkov.
783, (2016), Granulation of Complex Fertilisers,
H Kiiski, A. Kells.

External resources

Urea-Based NPK Plant Design and Operating Alternatives, Workshop Proceedings, Special Publication SP-15, 1991, International Fertilizer Development Center, Muscle Shoals, AL, U.S.A..
Compatibility of various solid inorganic fertilizers, compatibility table, Fertilizers Europe, 2016.
Schultz, J.J. (1989). Production of Granular NPKs in Ammonium Phosphate Plants-Some Important Differences, Technical Bulletin T-36, International Fertilizer Development Center, Muscle Shoals, AL, U.S.A.
Manufacturing fertilizers with POLY4 (polyhalites) – urea-based NPK blends, handbook, version 04-2020, International Fertilizer Development Center, Muscle Shoals, AL, U.S.A.