Disc Granulator (Pan Granulator)

Equipment Introduction

A disc granulator, also known as a pan granulator or disc pelletizer, is a wet granulation device that utilizes the centrifugal force, friction, gravity, and adhesion between materials generated by a rotating disc to cause powdery materials to continuously roll, agglomerate, and grow within the disc, ultimately forming granules. It is widely used in compound fertilizers, organic fertilizers, mineral powders, metallurgical powders, building material powders, and solid waste resource utilization, and is a very common type of equipment in powder pelletizing and granulation processes.

The disc granulator is characterized by its relatively simple structure, intuitive operation, highly adjustable parameters, and good visualization of the granulation process. It is suitable for small- to medium-scale continuous production and also for applications requiring flexible adjustment of particle size and pelletizing state. Compared to some forced extrusion granulation equipment, the disc granulator is more suitable for materials with a certain degree of cohesion that can be wet-granulated; compared to a drum granulator, its operation is more intuitive and control is more flexible, making it suitable for scenarios with high requirements for particle appearance and size uniformity.
Equipment Structure Composition
The disc granulator mainly consists of the following core components:

Equipment Working Principle and Granulation Process

Working Principle
The disc granulator is a wet tumbling granulation device that uses an inclined rotating disc as its working chamber. Powdered material forms a dynamic tumbling bed under the combined force field generated by the disc's rotation. Atomized liquid binder wets the particle surface and forms liquid bridges under surface tension, causing the particles to adhere to each other. During continuous tumbling, the binders continuously adsorb surrounding powder and undergo compaction and rounding, ultimately forming dense spherical particles. The equipment utilizes the centrifugal force gradient caused by particle mass differences to achieve self-classification; qualified particles automatically overflow and are discharged, while unqualified particles remain in the disc to continue growing.

The core forces include three: centrifugal force, directed radially outward, pushing the material to the disc edge and achieving particle classification; gravity, directed vertically downward, causing the material to roll down the inclined disc surface, creating tumbling motion; and liquid surface tension, acting on the wetted particle interface, forming liquid bridge forces that cause particles to adhere to each other. The dynamic balance of these three forces determines the granulation effect.

Granulation Process
The granulation process consists of four stages:

Stage 1: Nucleation. Powdered material is continuously fed from the top of the equipment, while a liquid binder is sprayed through a spray system. A thin water film forms on the surface of the powder particles. Due to the surface tension of water, the water and powder combine to form tiny nuclei with a diameter of 0.5–1 mm. The key at this stage is uniform wetting, avoiding localized over-wetting or under-wetting.

The second stage is stratification. The nuclei move upwards on the rotating disk and then roll down the inclined surface under gravity. During this rolling process, the nuclei continuously adsorb surrounding unaggregated powder, gradually enveloping and enlarging, increasing the particle diameter from 1 mm to 2–3 mm. This process is called the snowball effect.

The third stage is compaction and rounding. The particles are constantly subjected to collisions and compression during continuous tumbling, expelling internal air, increasing particle density, and gradually smoothing and rounding the surface. At this stage, the particle diameter remains relatively stable, but the density and strength are significantly improved.

The fourth stage is self-classification and discharge. When the particles reach the target size (typically 2–5 mm), their increased mass leads to a greater centrifugal force, causing the particles to migrate towards the edge of the disc and eventually be discharged naturally. Smaller particles remain in the disc due to insufficient centrifugal force and continue to grow. This self-grading effect is the core advantage of disc granulators, with a return ratio controllable at 15%–25%.

Key Process Parameters:

Disc diameter: 1.0–6.0 meters, directly determining the upper limit of production capacity.

Disc inclination angle: 40°–55°; a larger angle results in shorter residence time and smaller particles, while a smaller angle results in longer residence time and larger particles.

Rotation speed: 8–28 RPM; too fast a speed causes the material to stick to the wall without tumbling, while too slow a speed prevents pellet formation.

Disc edge height: 200–450 mm, determining the bed depth.

Material moisture content: 10%–15% for chemical fertilizers, 15%–30% for organic fertilizers; too low moisture results in loose, dusty particles, while too high moisture leads to excessive agglomeration and clumping.

Raw material particle size should be less than 2 mm (80–120 mesh). The material bed thickness should be maintained at 10–25 cm. The scraper gap should be set at 2–3 mm.

Common Problems and Solutions:
If particles are too small and dusty, the cause is insufficient moisture or excessive rotation speed. Increase the water spray volume, decrease the rotation speed, or decrease the tilt angle. If particles are too large and clumping, the cause is excessive moisture or excessive residence time. Reduce the water spray volume, increase the tilt angle, or increase the rotation speed. If material sticks to the disc, the cause is high material viscosity or insufficient disc surface smoothness. Check the scraper gap and consider using an anti-stick coating. If the pelleting rate is low, the cause is uneven raw material particle size or improper binder. Strengthen pretreatment screening and adjust the type and amount of binder. If particle strength is insufficient, the cause is insufficient moisture or insufficient residence time. Appropriately increase the moisture content and decrease the tilt angle to extend the residence time.

The Role and Application of Disc Granulators in Fertilizer Production Lines

Disc granulators are core forming equipment in fertilizer production lines, playing a crucial role in converting powdered materials into spherical granules. They serve as a technological hub connecting front-end powder processing with back-end finished product processing. The particle size distribution, strength, and sphericity of the output granules directly determine the quality of the finished product, and the granulation capacity directly determines the maximum capacity limit of the entire production line.

1. Compound Fertilizer (NPK Compound Fertilizer) Production Line:
The primary granulation equipment. The process location is after ingredient mixing and before drying. The raw material is a mixture of various single-element fertilizer powders. Water, dilute phosphoric acid, or a special granulation aid are used as binders. Key control points are moisture content of 10%–15% to prevent urea hydrolysis and temperature control below 60°C. A typical configuration is a single disc granulator with a diameter of 3.0–4.5 meters, with a capacity of 5–15 tons/hour. The return ratio is 15%–25%. Compared to drum granulators, the investment is 30%–50% lower, making it more suitable for small and medium-sized compound fertilizer plants.
2. Organic Fertilizer Production Line:
The main equipment is the granulation or finishing equipment. It operates after fermentation, composting, crushing, and screening. Raw materials include fermented livestock manure, straw powder, and biogas residue, with a moisture content of 30%–40%. It needs to be dried to below 20% or adjusted with dry additives. The binder uses water, molasses, and humic acid solution. Key control points are a moisture content of 15%–30% and a fiber content below 15%. A typical configuration is a 2.0–3.5 meter diameter disc granulator with a capacity of 2–8 tons/hour. Due to the high fiber content and viscosity of organic materials, a robust scraper configuration is required, along with online moisture detection and automatic spray adjustment. The drying temperature must be strictly controlled below 80°C to prevent organic carbonization.

3. Organic-Inorganic Compound Fertilizer Production Line:
The core equipment is the granulation equipment. It operates after the mixing of organic materials and inorganic fertilizer ingredients. The raw material is a mixture of 40%–60% organic powder and 40%–60% inorganic fertilizer powder. The key challenge is poor compatibility at the organic-inorganic interface and difficulty in pelletizing. The solution is to add 2%–5% bentonite or attapulgite as a binder and control the organic particle size to less than 1 mm. A typical configuration is a 3.0–4.0 meter diameter disc granulator with a capacity of 3–10 tons/hour.

The disc granulator's interface with upstream and downstream equipment is as follows: Upstream, it receives uniform powder from the mixer with a moisture content of 10%–15%. Upstream, it receives precisely fed materials according to the formula by an automatic batching system with an accuracy of ±0.2%. Upstream, it receives 15%–25% of the returned material conveyed by a return belt and mixes it with new material for re-granulation. Downstream, it outputs wet pellets to the rotary dryer with a moisture content of 15%–20%, requiring matching with the dryer's feed rate. Downstream, it receives the returned material after screening by a drum screener, forming a closed-loop cycle. Downstream, the dried, high-temperature granules are conveyed to the cooling machine and need to be cooled to below 40°C.

Five key control elements in the granulation process:

Moisture:Chemical fertilizer 10%–15%, organic fertilizer 15%–30%. Control method: online moisture meter with automatic spray adjustment. Excessive moisture leads to a sharp increase in drying energy consumption; insufficient moisture leads to low pelletizing rate and excessive dust.

Particle Size:Adjustable from 2–5 mm. Control method: adjusting tilt angle, rotation speed, and disc edge height. Uneven particle size leads to low screening efficiency and increased return ratio.

Temperature:Below 60°C to prevent urea hydrolysis. Control method: controlling feed temperature and avoiding frictional overheating. Excessive temperature leads to raw material decomposition and nitrogen loss.

Uniformity:Coefficient of variation less than 8%. Control method: stable operating parameters and uniform raw material particle size. Poor uniformity leads to uneven coating and uneven fertilization.

Strength:Greater than 3 N/granule. Control method: controlling moisture and extending residence time. Insufficient strength leads to breakage during transportation and high pulverization rate.

Coupled control with the drying process:
If the granulator output particles are too large (greater than 6 mm), it will lead to uneven drying and high central moisture content. The tilt angle should be reduced and the rotation speed lowered.

If the granulator output particles are too small (less than 1 mm), it will result in high dust levels and a heavy load on the exhaust gas treatment. The tilt angle should be increased and the moisture content increased.

If the granulator output moisture content is too high (greater than 20%), it will increase drying energy consumption by 30%–50%. The water spray volume should be reduced and the spray system checked.

If the granulator output moisture content is too low (less than 10%), it will result in loose particles that pulverize after drying. The water spray volume should be increased and the rotation speed lowered.