【FAQ】How should granulation binders be properly selected?

Binders are key additives in wet granulation processes (such as drum or pan granulation) that facilitate the agglomeration of fine powders; their type and dosage directly influence the granulation rate, granule strength, and drying energy consumption. Proper selection depends on material characteristics and the requirements for the finished product. Binders generally fall into three categories: Water-based: Water is the most inexpensive binder, suitable for organic fertilizers containing inherent adhesive substances (e.g., humic acid, molasses); however, it is less effective when used alone for inorganic powders. Organic: Examples include sodium carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), and gelatinized starch solutions. These binders offer strong adhesion; adding just 0.5%–2% can significantly improve the granulation rate, making them particularly suitable for NPK products requiring high granule strength. However, they are costly, and some varieties are not easily biodegradable, so caution is advised when using them in organic fertilizers. Inorganic: Examples include bentonite, attapulgite, and diatomaceous earth. These serve both binding and adsorption functions. Added at levels of 3%–8%, they improve material flowability and reduce granule hygroscopicity, making them suitable for producing slow- or controlled-release fertilizers. Selection principles: For organic materials with high moisture content, a combination of bentonite and water is recommended, as it adjusts moisture levels while enhancing binding. For NPK fertilizers based primarily on inorganic salts, an aqueous CMC solution (1%–2% concentration) is suggested; this should be continuously sprayed into the granulator via a metering pump, with the atomized droplet size controlled at 100–200 μm to ensure uniform coverage. The total binder dosage is generally kept at 2%–5% of the dry material weight; excessive amounts result in overly dense granules that are difficult to dry, while insufficient amounts lead to a low granulation rate. In practice, the process can be monitored by observing the "snowflake-like" appearance of the material inside the granulator: if fine powder is flying about, the binder dosage is insufficient; if large clumps form, the dosage is excessive. Binders should be fully dissolved (or gelatinized) in water and passed through a filter to remove insoluble matter and prevent nozzle clogging. Prior to each production run, small-scale trials must be conducted to determine the optimal dosage, and detailed records must be maintained to correlate binder batches and usage rates with the strength data of the finished product, thereby enabling continuous formula optimization. Ultimately, the finished product must meet the following standards: compressive strength of ≥15 N (for organic fertilizer) or ≥30 N (for compound fertilizer), and a dry pulverization rate of <5%.