Coating Machine

The coating machine is a core piece of equipment in the post-processing stage of fertilizer production lines, enabling the functional upgrading of granules. By uniformly applying functional coating materials to the surface of fertilizer granules, the machine imparts diverse properties—such as anti-caking capabilities, slow or controlled nutrient release, aesthetic coloring, and biological enhancement—thereby serving as key processing equipment that elevates fertilizer from a "basic nutrient product" to a "functional product."

The coating machine plays a pivotal role in shaping the final quality of the fertilizer. Although granules emerging from the granulation, drying, and cooling stages possess their basic form, issues such as rough surfaces, susceptibility to moisture absorption and caking, and excessively rapid nutrient release limit their commercial value and application effectiveness. By creating a functional coating layer on the granule surface, the machine effectively blocks direct contact with external moisture and slows the dissolution and release of nutrients, while simultaneously improving the granules' appearance and flowability. In the production of slow- and controlled-release fertilizers, the coating machine is essential for achieving "precision controlled release." The application of various coating materials transforms fertilizers from products with a singular nutrient-supply function into multifunctional products that integrate anti-caking, slow/controlled release, biological enhancement, and physical protection.

Types and Characteristics of Coating Materials

The choice of coating material directly affects coating performance and fertilizer characteristics.

Anti-caking agents: These include mineral oil, paraffin wax, and anti-caking powders (such as talc, bentonite, and diatomaceous earth). They are low-cost and easy to apply, making them suitable for preventing caking in conventional fertilizers.

Slow-release polymer materials: These include synthetic high-molecular-weight materials such as polyvinyl alcohol, polyurethane, epoxy resin, and acrylic resin. They provide dense coatings, precise release control, and thin coating layers, though they come at a higher cost.

Natural polymer materials: These include natural substances such as tung oil, rosin, lignin, and starch. They are biodegradable and environmentally friendly, though their release control precision is relatively lower.

Sulfur materials: Used in the production of sulfur-coated urea; they are cost-effective and simultaneously provide sulfur nutrients to crops.

Composite coating materials: These involve the combined use of multiple materials to achieve multiple functions, such as anti-caking, slow release, and biological enhancement.

Control of coating process parameters

Controlling the process parameters of the coating machine is crucial for ensuring coating quality.

Drum rotation speed: Speed ​​directly affects the tumbling frequency of the granules and the uniformity of the coating. Low-speed models (8–12 rpm) are suitable for high-viscosity coating agents, while high-speed models (15–20 rpm) offer higher processing capacity. The speed is typically maintained between 6 and 15 rpm.

Coating agent spray rate: Generally 0.5%–2% of the granule weight, precisely regulated by adjusting spray duration and flow rate. Excessive spraying leads to granule agglomeration, while insufficient spraying results in incomplete coating.

Hot air temperature: Hot air facilitates drying and accelerates the curing of the coating layer. Traditional heating systems exhibit temperature fluctuations of ±5°C, whereas thermal oil circulation systems can limit fluctuations to ±2°C.

Granule size adaptation: For a particle size of 1–2 mm, the drum inclination angle should be set to 1°–1.5° with a rotation speed of 6–10 rpm; for a particle size of 2–4.75 mm, the inclination angle can be increased to 2°–3° with a rotation speed of 10–15 rpm.

Application of Coating Machines in the Production of Slow- and Controlled-Release Fertilizers

Coating machines play an indispensable, core role in the production of slow- and controlled-release fertilizers. Manufacturing these fertilizers typically requires multiple coating steps: first applying an inner slow-release layer, followed by an outer protective or functional layer.

Polyurethane coating process: A dense polyurethane film is formed by spraying a mixture of polyurethane prepolymer and a curing agent onto granular fertilizer within a drum coater. This process enables the precise release of nutrients at specific time points.

Sulfur coating process: Molten sulfur is sprayed onto the granule surface to form a sulfur coating. This layer not only provides slow-release functionality but also supplies sulfur as a nutrient for crops.

Multilayer composite coating: Coating layers with distinct functions are applied sequentially to the granule surface—an inner slow-release layer, a middle controlled-release layer, and an outer anti-caking protective layer.

Positioning of the film coating machine in the production line

The coating machine represents the final step in the post-processing stage of a fertilizer production line, situated between the cooling and screening units and the automatic packaging system. It integrates seamlessly with upstream and downstream equipment: it receives qualified granules from the cooling and screening process and transfers the coated finished product to the automatic packaging line. A buffer hopper links the coated granules to the automatic packaging line, preventing the coating machine from having to stop and wait.

Development Trends in Coating Technology

Coating technology is evolving towards greater precision, functionality, and environmental sustainability.

Precision controlled release: Achieving a precise match between nutrient release and crop demand curves through multi-layer composite coatings and precise process control.

Biodegradable coatings: Developing biodegradable coating materials based on natural polymers to reduce microplastic pollution.

Smart-response coatings: Developing smart coating materials that release nutrients in response to environmental signals such as soil temperature, moisture, and pH levels.

In-line inspection and feedback control: Integrating near-infrared or thickness sensors to monitor coating quality in real-time and automatically adjust process parameters.