2026/07/06
Compound fertilizer products are highly susceptible to caking during storage and transportation due to moisture absorption, pressure, and temperature changes, severely impacting their effectiveness and commercial value. Cold spray coating technology is a key technology developed to address this need. After the granules are cooled to a suitable temperature, an anti-caking agent is sprayed on them, followed by dusting, forming a functional protective film on the granule surface. This film provides both anti-caking and slow-release enhancement. This article analyzes the process from four aspects: granule pretreatment after cooling, coating oil spraying, dusting device, and the anti-caking effect.
I. Cooled Granules: The Temperature Barrier Before Film Formation
The first and crucial step in the cold spray coating process is ensuring the granule temperature drops below 45℃. Traditional anti-caking coating treatments often suffer from high product temperatures, leading to damage to the anti-caking film structure during coating and storage, severely affecting the anti-caking effect.
When the particle temperature is too high, the sprayed coating oil will become too fluid due to the high temperature, failing to solidify evenly on the particle surface, and may even result in an excessively thin film or loss of coating. Simultaneously, high temperatures will accelerate the volatilization and decomposition of the active ingredients in the anti-caking agent. Therefore, before entering the coating roller, the particles must be cooled to room temperature or slightly above room temperature by a cooler. Some advanced devices also employ water-cooled coating technology, simultaneously cooling the particles during the coating process to further ensure the stability of the film formation.
II. Coating Oil Spraying: Uniform Atomization is Key After cooling, the particles are weighed via a metering belt and enter the coating roller. The coating oil is pumped to the atomizing nozzles inside the roller, where it is evenly sprayed onto the surface of the fertilizer particles in a mist form.
The coating oil formulation is typically a compound system of mineral oil, surfactants, paraffin wax, and other additives. In actual production, some coating oils are solid at room temperature and need to be heated to 80-90℃ to melt before being pumped in. To prevent temperature drops during transport from causing reduced flowability and nozzle clogging, advanced nozzle devices utilize compressed air-assisted atomization to ensure even application of the coating oil. The spraying amount is generally controlled at approximately 1.5-2.0 kg per ton of fertilizer.
III. Powdering Device: A Mechanical Isolation Layer After the Oil Film
After the coating oil is sprayed, a powdering process follows. The powdering device uses a loss-in-weight screw feeder to add powdered anti-caking agents (such as diatomaceous earth, talc, kaolin, etc.) into the coating rollers. These powders adhere evenly to the surface of the not-yet-fully-cured oil film as the particles roll, forming a mechanical isolation layer.
The oil film provides a chemical barrier—reducing particle surface tension and hygroscopicity; the powdering provides a physical barrier—preventing direct particle contact and the formation of crystal bridges. The synergistic effect of both significantly improves the anti-caking effect. The finished fertilizer, after coating and powdering, is conveyed by an elevator to the finished product silo for packaging.
IV. Anti-caking and Slow-release Effects A mature cold-spray coating process can achieve a product looseness of over 95%, maintaining good flowability even after 6 months of storage. Simultaneously, the film layer possesses a certain degree of hydrophobicity and low permeability, slowing the rate at which moisture enters the granules and nutrients diffuse outward, thus achieving a certain slow-release effect. Some advanced formulations can also incorporate functional ingredients such as nitrification inhibitors and urease inhibitors into the coating oil, further improving fertilizer utilization.
Summary: The compound fertilizer cold-spray coating process, based on the technical line of "cooling pretreatment—atomized oil spraying—powder coating isolation," constructs a protective film on the granule surface that combines moisture protection, barrier properties, and slow-release functions. This process not only solves the long-standing problem of fertilizer caking but also provides a low-cost, high-efficiency technical path for the functional upgrading of fertilizers.
The cold spray coating process, with its three‑step sequence of cooling pretreatment, atomized oil spraying, and powder isolation, is the final quality‑enhancing step in modern compound fertilizer manufacturing. This coating technology, when applied via a fertilizer coating machine, not only ensures over 95% product looseness even after six months of storage but also imparts a slow‑release function that improves nutrient efficiency. However, its full benefit is realized only when integrated with the upstream production chain—from precise npk fertilizer formula processing that defines the nutrient ratio, to the selection of a suitable npk fertilizer granulator machine (such as a double roller press granulator for dry extrusion or a drum granulator for wet agglomeration), and finally to the blending stage where a npk blending machine or BB fertilizer blender ensures homogeneity before coating. For producers operating a complete npk blending fertilizer production line, the coating unit is the critical final module that differentiates premium products—preventing caking, reducing dust, and enabling custom release profiles. Moreover, when the coated granules are later used in a npk bulk blending machine to create custom BB blends, the uniform film ensures that each particle retains its anti‑caking and slow‑release properties, preserving blend integrity during transport and application. In essence, cold spray coating is not an isolated add‑on but a strategic integration that elevates the entire npk blending fertilizer production line, delivering fertilizers that are easier to handle, more efficient in the field, and more valuable to the end‑user.