2026/11/11
The core principle of a coating drum in preventing caking is that it uses a rolling bed to evenly coat the granules with an anti-caking agent or coating oil, forming a hydrophobic barrier that blocks moisture migration between the granules and the external environment. The correct timing for the coating process is after cooling and before packaging. Deviating from this logic will significantly reduce the coating effect.
Fertilizer caking is essentially caused by the granule surface absorbing moisture, dissolving, and recrystallizing, forming crystal bridges at the granule contact points, binding loose granules into hard lumps. The function of the coating drum is to "block" the moisture channels before these crystal bridges form.
The main body of the coating drum is an inclined rotating cylinder, similar in structure to a rotary drum granulator, but with a lower rotation speed and no lifting plates on the inner wall, only a few guide plates. Cooled granules enter the cylinder from the upper end and spiral along the inner wall under slow rotation, forming a continuously churning bed of material. The atomizing nozzle evenly sprays liquid anti-caking agent onto the granule surface. As the granules tumble, they repeatedly contact the atomized droplets, causing the coating liquid to spread and adhere to the surface, forming an extremely thin, continuous protective film. This film increases the contact angle of water on the granule surface, slowing down moisture absorption; it also forms a physical barrier between granules, preventing direct crystal bridge connections. Experiments show that the caking rate of compound fertilizer treated with this coating can be reduced by more than 80%.
The coating drum is located after the cooling machine and before the packaging machine; this is not an arbitrary arrangement in the process. If the granules are too hot when entering the coating stage, the coating liquid will evaporate or decompose rapidly upon contact with the hot granules, resulting in a discontinuous film and leaving weak points. Simultaneously, when the hot granules cool inside the packaging bag, internal moisture migrates outward and recondenses under the film, negating the coating effect. Studies also indicate that excessively high fertilizer packaging temperatures significantly increase the tendency to caking. Therefore, reducing the granule temperature to a safe level during the cooling process is a prerequisite for the coating drum to function effectively.
The process parameters for the coating drum need to be set around achieving "uniform coverage." The rotation speed is typically 8-12 r/min, much lower than the granulator's speed, to allow the material bed to tumble slowly rather than be violently scattered, giving the coating solution sufficient time to spread on the particle surface. The inclination angle is generally 3°-5°, controlling the particle's forward speed and residence time within the drum. The residence time is typically 1-3 minutes; too short a time results in incomplete coverage, while too long a time leads to excessive particle friction and damage to the coating layer.
Regarding particle temperature, multiple process data indicate that the particle temperature entering the coating process should ideally be controlled below 35-40℃, and the finished product temperature should not exceed 40℃. This is a practical window balancing the fluidity of the coating solution and the stability of the coating layer. The dosage of coating agent depends on the formulation and target: the typical dosage for liquid anti-caking agents is approximately 1.5 kg/ton of fertilizer, and for solid anti-caking agents, approximately 3 kg/ton of fertilizer. Other anti-caking agents have a recommended dosage of 2.0–3.5 kg/ton of fertilizer, which can form a double-layer film structure of "oil inside, powder outside" on the particle surface. The specific dosage should be determined through small-scale tests based on the particle specific surface area, storage humidity, and target shelf life.
The coating drum should not be selected as a standalone machine; its effect depends on the upstream line and material state. On a rotary drum granulation production line, the npk fertilizer granulator determines granule shape, surface roughness, and size distribution, all of which affect coating coverage; upstream, a fertilizer crusher machine controls fine powder content and particle uniformity. In npk fertilizer formula processing, different salt systems and hygroscopicity require different coating dosages and film types. For blending routes, an npk bulk blending machine must also ensure that particle size and surface condition are uniform before coating, otherwise segregation and uneven film coverage may occur. The fertilizer coating drum should therefore be positioned after cooling and before packaging, with its rotation speed, inclination, and dosage matched to the granulator, crusher, and blender upstream. In this way, the coating drum becomes part of a line-level anti-caking design rather than a single post-treatment device.
Q: Coated drums have been prepared, but the finished product still clumps. What should be checked?
Coating is not a "cure-all" for clumping. First, check the particle condition before coating: if the particles themselves have insufficient strength, poor particle size distribution, rough surface, or high residual moisture after cooling, coating can only cover the surface and cannot repair the defects of the particles themselves; clumping will still occur. Secondly, check the coating parameters: Insufficient atomization pressure will form large droplets, preventing the coating solution from spreading evenly and causing localized adhesion or clumping; insufficient rotation speed or residence time will result in low coating coverage. Thirdly, check the packaging temperature: If packaging is done when the finished product temperature exceeds 40℃, moisture migration during the cooling process inside the bag will re-initiate crystal bridging. Finally, check the storage conditions: Even if the coating is qualified, long-term exposure to a high-humidity environment may still cause gradual penetration of the film layer. The check sequence should proceed from particle state to coating parameters, then to packaging and storage, addressing each stage step by step, rather than simply increasing the amount of coating agent.
The anti-caking ability of the coating drum is based on four prerequisites: "qualified particle state, correct temperature window, matching parameters, and correct sequence." By placing the coating drum after cooling and before packaging, using a low-speed rolling bed to achieve uniform coating, and then locking the process window with four parameters—rotation speed, tilt angle, temperature, and dosage—the anti-caking effect can be reproducibly guaranteed.