Dry roller pressingGranulationworkmanshipIt was first applied in the field of fertilizers for granulation of potassium chloride. Due to the special physical and chemical properties of potassium chloride, it is used for the granular (1) required for blending fertilizers (BB fertilizers)-4mm）Potassium chloride is mostly usedroll-inLegal production. in recent years,roll-inGranulation has achieved rapid development in the production of compound fertilizers.

roll-inThe working principle of granulation is:

Dry materials aggregate into dense and hard materials under high pressurePlate, called rollerCompression process;sectorThe process of crushing, granulating, and sieving into particles is called granulation.roll-inThe function is to squeeze out the air between particles and make the particle spacing close enough to generate attractive forces such as van der Waals forces, adsorption forces, crystal bridges, and embedded connections.roll-inThe particle strength of granulation is mainly formed by the intermolecular forces between molecules.

This process has a short creation time, easy operation, convenient product variety replacement, low investment, and fast results. Especially as a secondary processing of basic fertilizers, combined with soil testing and fertilization, it reduces agricultural field costs and increases agricultural benefits, which is widely welcomed.

Process characteristics:

1No adhesive required

The commonly used method for producing compound fertilizers in China is steam/slurry granulation. After condensation and crystallization between material molecules, water vapor acts as a binder during the granulation process. But during the drying process, it is necessary to remove the water inside and on the surface of the particles, otherwise the fertilizer is prone to clumping during storage. Dry granulation does not require additional additives and only utilizes the intermolecular binding force of the material itself, simplifying the process and reducing energy consumption.

2Diverse sources of raw materials

The raw material routes for the production of compound fertilizers by roll granulation are quite extensive, and currently more than 20 types of materials have been successfully used for extrusion granulation: ammonium nitrate, urea, ammonium chloride in nitrogen fertilizersammonium sulphatePhosphorus fertilizers include ammonium phosphate, tricalcium phosphate, heavy calcium phosphate, phosphate powder, etc. Potassium fertilizers include potassium chloride, potassium sulfate, etc. Trace elements such as boron, iron, copper, zinc, manganese, etc. can also be added.

In addition, the use of roll granulation technology can flexibly change product formulas in a very short period of time, and one set of equipment can produce more than 30 types of products with different formulas.

In mixed fertilizers, the properties of a certain component may have a positive or negative impact on the squeezing of the mixture, such as the low melting point of urea and the tendency of superphosphate to soften under pressure. Both of these components can act as adhesives at low concentrationsThe function. However, when the content of these two components in the fertilizer formula is high, it will cause the plate to become soft. At this point, it is necessary toSqueezingmakegrainofDuring continuous operation,sectorThere are 30 on the conveyor belt or in the aging warehouseThe hardening time of minutes makes the plate harder and more brittle.

The hardness of fertilizer components determines the energy consumption required for extrusion, and the average hardness of mixed fertilizers is determined by the formula. For example, when a component with a low percentage and high hardness is added, the impact of the high hardness component on energy consumption is minimal. On the contrary, when the percentage of components with high hardness in the mixture is high (such as magnesium vanadium water, ammonium sulfate, or potassium sulfate), the extrusion force required for producing the plate is high, and the power consumption is also high.

Another important characteristic parameter is the wear resistance index of the components. Wear resistance and extrusion force are the main factors causing surface wear of the roller shaft. In order to ensure the service life of the worn surface, for highly wear-resistant fertilizer raw materials such as Thomas steel slag, the shaft sleeve of the roller shaft should be made of highly wear-resistant special steel.

The actual density of fertilizer raw materials determines the apparent density of the plate and the production capacity of the roller press. In the production of mixed fertilizers, the apparent density of the plates is determined byThe density of each component and its content in the mixture are determined, so as the fertilizer formula changes, the rollerThe ability of the press is also changing.

3. Raw materialsThe impact on product strength：

roll-inGranulationworkmanshipThere are no special requirements for the particle size of the raw materials, and the moisture requirement is less than3%,Both granular and fine powder materials are available, but the powder size is not up to standardSometimes raw materialsBetter to use.

During the process of particle formation,powderThe shape (spherical, cubic, plate-shaped, needle shaped) andpowderThe size distribution range is affectedGreen formingImportant factors.

In the field of fertilizersroll-inThere is a universal rule in the process that for particles with relatively rough structures, onlya large number ofPowder can fill larger pores. At this point, the binding force between molecules is more likely to form. In general, only van der Waals force cannot generate enoughstrength. There are some natural adhesive like substances in fertilizer raw materials that have a bonding effectThey are: crystal bridges formed between particles, chemical reactions between mixed components, and bridges between particlesTake it.

When raw materialsSize<<span=''>0.5mmAt this time, crystal bridges play an important role. They are usually formed into granules by adding a small amount of water to the raw material, dissolving a small amount of the raw material, and during the drying process of the raw material, the solid recrystallizes,We can utilize the formed crystal bridge，Obtain high strength and low porosityPressed products.However, the amount of water added should generally not exceed 2%. As shown in the table above, for potassium sulfate, the moisture content in the feed is generally greater than 1%. However, when potassium chloride is extruded and granulated, it needs to be done in a dry state. Some fertilizers (such as urea) have particularly strong water absorption properties and can easily absorb water from the air. When urea is present in the mixed fertilizer, the critical humidity of the air is lower than that of each component in the mixed fertilizer, resulting in high viscosity and difficulty in granulation. So, it is necessary to process raw materials containing urea in a relatively dry environment, such as adding a dryer in areas with high humidity.

When raw materialssize<1< span=''>When mm,Utilizing the plasticity of raw materials is also a feasible method.When the temperature of the feed is increased, the plasticity of the raw material can be increased, resulting in tight compressionPreform.The feeding temperature of fertilizer raw materials determines the plasticity of fertilizers. Hot materials can obtain the same particle strength and density with lower specific pressure than cold materials, while with the same energy input, hot materials can obtain products with higher strength and density than cold materials. For example, potassium fertilizer requires a specific pressure of 70KN/cm at room temperature, while at 120-140℃Below, only a specific pressure of 50KN/cm is needed. Potassium sulfate can only be used between 70-100℃Only then can it be tightened, with a specific pressure of 70KN/cm. Increasing the temperature is also beneficial for crystal bridges, as more grains are produced during the recrystallization process.

For some mixed fertilizers, chemical reactions between the components also play a bonding role.When single component fertilizer raw materials cannot form enough rollersWhen it comes to pressure, they cantake shapeSuch as a mixture of phosphate rock and urea.

4energy consumptionRegarding investment:

Roll granulation is the process of granulating materials at room temperatureCompared to other methods, it does not require drying measures such as fuel or coal. Production only requires electricity consumption, per ton of productThe power consumption is about 40kWh.

Due to the elimination of the drying process, the process flow is simple and the investment is relatively low. According to IFDC (International Fertilizer Development Center) in 1987A detailed study was conducted in the year, which proved that in developing countries, for a newly built compound fertilizer plant, the rollerPressure granulation is economically superior to steam granulation or slurry granulation. For an NPK compound fertilizer plant with an annual output of 100000 tons, the investment in steam granulation is 20% higher than that in extrusion granulation, while the investment in chemical granulation or slurry granulation is 88.8% higher. In terms of production cost (for formula 15-15-15), steam granulation is 3% higher, while chemical or slurry granulation is 12% higher.

For the roll granulation device, the economic scale effect is not significant, and the device capacity ranges from 1 ton/hour Platform -50Ton/hour Taiwan can carry out more economical designs, so it has a wide range of applicable capabilities.

5Flexible formula

A set of extrusion granulation equipment can produce up to dozens of products, and small-scale production is also feasible. According to the requirement of clearing the total amount of raw materials during switching and the degree of impact of existing materials on the new formula, a maximum of 3 batches of products can be produced per shift (8 hours). Therefore, the compound fertilizer produced by roll granulation can meet the needs of small batches and multiple varieties of compound fertilizers.

The most obvious example is urea. In the case of relatively low urea content, i.e. 10-15%, the plasticity of urea makes it act as a binder in the material. This way, during the extrusion process, only low pressure and energy consumption are required. In addition, it can reduce the wear of the extruder, extend its life, and the strength of the product is also high. However, as the percentage of urea continues to increase, the extruded thin sheet material will become soft and sticky. So the highest percentage content of urea in the formula is generally between 30-40%, depending on the properties of other components in the formula. To overcome the above problems, we suggest using other nitrogen fertilizers (such as ammonium sulfate) instead of urea content greater than 25%, and adding ammonium sulfate has another advantage of adding sulfur to the formula.

The main advantage of the fertilizer extrusion device is its ability to produce more nitrogen, phosphorus, and potassiumAll formulas are applicable, and the choice of formula mainly depends on market conditions (i.e. price and supply). But the rollerPressure also has certain limitations, mainly because certain raw materials (such as urea) cannot coexist with other raw materials under harsh weather conditions (such as high humidity). Therefore, it is recommended to control the percentage content of urea below 30% and further reduce it in seasons with high humidity. Otherwise, the raw material will become viscous due to water absorption and clog the hopper.

6deficiencies:

The only drawback of producing compound fertilizers through roll granulation is that the shape of the product is irregular particles, but the irregular particle shape does not affect the application of fertilizers. Fertilizer produced by rolling granulation has sufficient strength and powderLess dust, no clumping, narrow particle size distribution range, and good fluidity. Fertilizers produced by rolling granulation, especially granular potassium fertilizers, have been widely used in mechanical fertilization abroad, proving that their particle shape will not be a factor affecting fertilizer application.

workmanshiptechnological process:

Roll granulation processIt consists of four parts, described as follows:

1．Pre processing of raw materials:Storage, measurement, and mixing

Fertilizer raw materials such as urea, ammonium phosphate, potassium chloride, etc. are transported to the storage bin by bucket elevators and belts. The number of storage bins, the complexity of measurement and mixing techniques, and the number of components in the product formula are related. To ensure the flexibility of the device, we recommend having more than 4 feeding bins, preferably 6: 2 for nitrogen fertilizer, 2 for phosphorus fertilizer, 1 for potassium fertilizer, and 1 for filler.

When producing compound fertilizers, intermittent or continuous mixing operations are generally used. The raw materials of each component in the formula are weighed in the weighing bin, and after meeting the formula requirements, they are all added to the intermittent or continuous mixer for mixing. A balance bin is installed in the system, and other equipment after the balance bin operates continuously. Trace elements can also be added to the mixer. In order to ensure uniform particle distribution of trace elements, high-strength mixers are selected. The mixed material and return material are lifted by a bucket elevator and mixed with the return material in a dual axis paddle mixer. The return material is the unqualified particles after crushing and screening, and the ratio of circulating material to new material is about 35:65.

2Rollerpress

mixtureThe material is fed into the gap between two rollers rotating in opposite directions through a vertical spiral feederDuring the compression process, the pressure on the material gradually increases. When the gap between the two shafts is at its minimum position, the material is subjected to the maximum pressure, and then gradually decreases until it reaches zero. During the extrusion process, due to the reduction in material volume, the apparent density of the mixture can increase by 1.5-3 times.rollerThe compressed product is a thick piece of 5-20mmofsectorThe structure and size of the roller press are determined by many parameters, which are generally determined based on material properties and experiments.

Applicable raw materialsThere are certain requirements for particle size, shape, moisture content, plasticity, hardness, temperature, etc, enterThe material bitten into the area needs to have good fluidity,at onceRequire the material to have a low moisture content, as the flowability deteriorates when the moisture content of the material reaches a certain level.

The extrusion process of materials includes: a. rearrangement of materials; b、 Fragmentation; cPlastic deformationThree stages. These stages may not necessarily be sequential, they may occur intermittently. The result of extrusion is that the compression ratio of general materials is close to 2.5:1, or the removal rate of gas in the feeding is over 60%. In order to achieve better extrusion effect, the feed particle size should be of different sizes, with a suitable range between 0.1 and 1.0. Materials that are too fine (less than)1.0mm）There is too much gas content, and materials that are too large require more energy consumption.

The hardness or plasticity of the material affects the pressure required for the extrusion process. Some materials, such as potassium chloride, diammonium phosphate, and urea, have good plasticity and can be extruded into granules effectively; Potassium sulfate, ammonium sulfate, magnesium sulfate, and phosphate powder have poor plasticity and extrusion effect, making them difficult to shape. For ingredients that are difficult to shape, an appropriate amount of adhesive can be added to achieve better results. Urea, potassium chloride, and a small amount of water can act as adhesives.

3. Granulation and screening

The plates need to be further broken and graded to obtain the required particle size. There is one under the roller press machinetowerbiaxPlate crusher, pre crushing of platesA small piece of about 50mm.Small pieceRelatively easy to transport and susceptible to subsequent damageGranulatorsFurther fragmentation. The crushed raw materials are divided into large particles, product particles, and small particles on the grading screen. The fine particles on the lowest sieve are recycled back to the mixer and then extruded into shape. The oversized particles that are trapped in the upper layer of the multi-stage sieve need to be further crushed. After crushing, return to screening.

Choosing the right crushing and screening equipment is crucial forRoll granulation processThe economy is crucial. The shape of the particles (usually cubic) and the size range of the particles (1)-5 millimetersOr 2-4 millimeters）The amount of circulation after crushing and screening determines the output and capacity of a set of equipment.

4．Post processing of finished products:Polishing and repairing

Under normal circumstances, the final product needs to be polished and trimmed. Irregular shaped particles are rounded at edges and corners using a grinding drum. The fine powder generated during the polishing process is sieved through a layer of sieve and circulated. When repairing, anti caking agents are usually added.

For environmental protection purposes, the entire device needs to be equipped with a bag filter for dust collection.