This granulation process uses rotary steel belt granulator to produce particle shaped high concentration compound fertilizer including urea ammonium phosphate (NP) and urea ammonium potassium phosphate (NPK), and please see figure 1 for the production flow.
Fig.1 Production flow of NP and NPK granules by rotary steel belt granulator
1.Raw materials for compound fertilizer (urea, ammonium phosphate, potassium chloride, trace element, etc.)
2. Melting kettle
4. Rotary steel belt granulator
5. Steel belt moved at constant speed
7. Automatic packaging unit
8. Heating jacket pump
9. Cooling water
10. Finished products
Stoichiometric amount of urea, ammonium phosphate or urea, ammonium phosphate, potassium chloride are fed into melting kettle which is equipped with steam heating coil, then the materials are rapidly melted, and eutectic mixture of urea based compound fertilizer is generated. After trace element and additives are added, this compound fertilizer is transferred into the distributor set on the head of rotary steel belt granulator by heating jacket pump, then uniformly dropped down onto the steel belt which moves at a constant speed under the distributor. Cooling water is sprayed on the underside of steel belt for heat exchange, so materials dropped on the steel belt are cooled and solidified into pellets with incomplete spherical shape. Finally, the finished high concentration NP or NPK particles are calculated and packaged by the automatic packaging unit.
The distributor consists of feeding stator inside and barrel outside, and the inner feeding stator is heated by steam to keep the materials in molten state, then the materials are continually delivered to the outer barrel. On the other hand, the barrel rotates at a constant speed, and its linear speed is the same as the moving speed of steel belt. Stepless speed regulation is available, and there are thousands of small holes on the wall of barrel. The hole diameter is determined by the size of finished product, and can be adjusted within 2-20mm. After the melted materials are transferred onto the steel belt from these small holes, the process of condensation, cooling and shape forming starts.
Compared with conventional granulation process of NP or NPK, this granulation process shows some excellent technical innovations as listed below.
1. Water content is not enlarged. During traditional granulation process, in order to increase material temperature and keep material wet, ammonia and sulfuric acid, or steam should be added into the machine. The heat of steam or the reaction heat of ammonia and sulfuric acid is used to raise the material temperature to 70℃, and to increase the water content to 3%-4%, so drying process is a must. On the other hand, due to the indirect heating used by this granulation process, water content of materials is not increased.
2. The production flow is very short.
a. Drier is not required. The low temperature eutectic melting phenomenon of urea based NPK makes the drying process very difficult, and the drying temperature should be much lower than the value of other phosphate compound fertilizers. Thus, the drying parameters are strictly controlled. Two stage drying is preferred, and the equipment is quite large. In order to ensure the normal flow of drying, the temperature of drying medium (hot air or flue gas) should be controlled at 120-130°C for the first stage, and 105°C for the second stage. Under this working condition, the drying intensity of drier is only 1.5-2.0 kg/(m·h). However, the drying intensity for SSP, TSP and GTSP equals to 40-50 kg/(m·h), and the Ay range for MAP and DAP is 25-35kg/(in·h). Due to the low drying temperature, the drier should be very large, and the off gas treatment is also a big project. This granulation process uses rotary steel belt granulator, and the water content of material meets the specifications. So, drying process is not required.
b. Cooling machine is unnecessary. In conventional granulation process, the materials that come from drier are about 70°C, so rotary drum cooler is needed. The critical hygroscopic point of urea based NPK with different constituents is quite low, so the cooling water must be processed with moisture removal treatment in order to avoid air-slake problem of dried fertilizer particles which is caused by the absorption of water in air. If our granulation process is adopted, the temperature of fertilizer granules discharged from the end of steel belt is lower than 45°C, so drier is not needed.
c. Screening, crushing and material returning systems are all eliminated. In the hot wet production system for urea based NPK, the physical properties of thermoplastic materials are deteriorated because of moisture absorption during transportation, screening as well as crushing, and blocking or scaling problem occurs. Single-deck screen is preferred for screening, but it will lead to the heightening of factory and enlarging of area. Our granulation process can cool and solidify the molten materials into solid particles in uniform shapes, and there is no dust. So, screening, crushing and material returning systems are not needed. Usually, the off gas emission is about 6000 in when one ton of urea based NPK is produced, 1000 for granulator, 2000 for cooling machine, and the rest 3000 is for drier. The most commonly seen problem of conventional off gas processing system is blocking. Drier and cooling machine are not required by our granulation process, so off gas is avoided, and off gas processing system is removed.
d. Off gas washing system is not needed. Rotary steel belt granulator won't generate dust and off gas, and the operating condition is very clean. As a result, off gas washing system is not needed, and the cooling water is recyclable.
3. This method requires low energy and maintenance. It uses rotary steel belt granulation method, and the power consumption is only 10kW·h while the steam consumption is only 65kg for producing one ton of NP or NPK. However, these values are 35-40 kW·h and 100kg for agglomeration process, and the fuel oil used is 2.5kg. For conventional slurry granulation process, the power used for every one ton of product is 50-60 kW·h, and the power required is about 20 kW·h for tower granulation. Based on these data, we can see that our product has excellent energy saving performance. In addition, the maintenance is little, and the operation cost is also very low.
4. The finished granules have uniform shape and high strength. This not only means that the finished particles have same size, but also indicates that these particles have same active constituents inside. Meanwhile, the particle strength is 60-80N/grain while the strength is only 20N/grain for agglomeration granulation.
5. This process asks for little investment. Tower granulation equipment draws the attention of many technical staff in this field, and it is a machine suitable for the granulation process of high concentration compound fertilizer including NP and NPK. However, the investment of our rotary steel belt granulator is less than 35% of tower granulation equipment, and our product is also suitable for the production of nitro potassium phosphate. In addition, if tower granulation process is adopted, the investment is quite high.