Deposits in Gas-fired Rotary Kiln for Limonite Magnetization-Reduction Roasting: Characteristics and Formation Mechanism

Abstract

The formation mechanism of deposits in commercial gas-fired magnetization-reduction roasting rotary kiln was studied. The deposits ring adhered on the kiln wall based on the bonding of low melting point eutectic liquid phase, and the deposit adhered on the air duct head by impact deposition. The chemical composition and microstructure of the deposits sampled at different locations varied slightly. Besides a small amount of quartz and limonite, main phases in the deposits are fayalite, glass phase and magnetite. The formation of the deposits can be attributed to the derivation of low melting point eutectic of fine limonite and coal ash, and the solid state reaction between them. Coal ash, originated from the reduction coal, combining together with fine limonite particles, results in the accumulation of deposits on the kiln wall and air duct. Fayalite, the binder phase, was a key factor for deposit formation. The residual carbon in limonite may cause an over-reduction of limonite and produce FeO. Amid the roasting process, SiO2, originated from limonite and coal ash, may combine with FeO and reduce the liquefaction temperature, therewith liquid phase generates at high temperature zone, which can significantly promote the growth of deposits.

The Deposit Formation Mechanism in Coal-Fired Rotary Kiln for Iron Ore Pellet Production: A Review

“…Furthermore, because of the pressure exerted by the rolling pellet layer, the volume of the primary deposit is continuously compressed and eventually forms a tight structure. The newly formed deposits continue to grow on the primary deposits, thickening layer by layer, further impeding the advance of the rolling pellets layer [30]. [27].…”

Effects of Pellet Basicity on the Simulated Deposit Formation in Coal-Fired Rotary Kilns for Iron Ore Pellet Production

“…In the rotary kilns fueled by pulverized coal, preheated iron ore pellet powder and coal ash are the material basis for deposit formation. The characterization of the deposits showed that the main mineralogical phases include hematite, silicate, and some minor glass phases. − Some studies have found differences in the mechanism of deposit formation in different parts of the rotary kiln. , The primary manner to form the deposits near the burner zone in the rotary kiln was the hematite crystallization and diffusion, while the liquid phases played a secondary role in the deposit formation. Furthermore, bentonite and coal ash contain high SiO 2 , Al 2 O 3 , Na 2 O, and K 2 O, which will promote the generation of liquid phases and intensify the deposition phenomenon. ,, In addition, insufficient combustion of pulverized coal caused partial zones of the reduction atmosphere in the rotary kiln, resulting in the generation of Fe 2+ , and formed low-melting-temperature compounds with veinstone minerals such as SiO 2 and CaO Al 2 O 3 . − It has been found that the deposition phenomenon became more severe in rotary kilns for producing fluxed iron ore pellets. , Unfortunately, the effects of the basicity of iron ore pellets on the deposition formation mechanism are still not fully understood.…”