Ring formation in a rotary kiln is a nodulation phenomenon resulting from the combined effects of raw materials, thermal regulation and equipment operation. The enrichment of harmful elements such as sulfur, alkalis, chlorine and phosphorus in raw materials tends to produce low-melting-point molten phases including silicates and sulfates at high temperatures, which significantly lowers the sticking temperature of the materials. Excessive fines, insufficient material strength, and uneven particle size distribution and moisture content make powders more prone to adhere to the kiln lining and form an initial deposition layer.

 Unstable thermal conditions are the primary inducing factor. Excessively high local temperature in the burning zone, poor flame profile and concentrated high-temperature zones promote massive liquid-phase formation. Improper control of oxidation-reduction atmosphere facilitates the formation of low-valence iron minerals, further intensifying material adhesion. Meanwhile, unreasonable kiln rotational speed and filling ratio, poor burner performance, and fluctuations in ventilation and negative pressure lead to uneven material rolling and local overheating. The materials adhered to the kiln wall are continuously compacted, sintered and solidified.

 Through repeated melting and solidification, the deposition layer keeps thickening and eventually forms hard and dense annular rings, which impair the internal ventilation of the kiln, material calcination and normal operation of the equipment.