Mechanism Analysis of Ring Formation on Aluminum Phosphate Refractory Bricks During Treatment of Iron-Bearing Waste in a Coal-Fired Rotary Kiln

Technical Summary: Mechanism Analysis of Ring Formation on Aluminum Phosphate Refractory Bricks During Treatment of Iron-Bearing Waste in a Coal-Fired Rotary Kiln

Problem Statement
The steel industry faces significant environmental pressure due to the generation of metallurgical dust and sludge, which contain hazardous substances like alkali metals and chlorides, as well as valuable metals such as iron and zinc. While pyrometallurgical processes, specifically rotary kilns, offer a viable method for recycling zinc-containing waste, their efficient operation is frequently hindered by "ringing" (the formation of accretions on the kiln lining). Ringing leads to declining pellet quality, reduced production efficiency, and increased costs. Although previous studies have examined ring formation using iron ore or oily sludge, the complex and fluctuating composition of metallurgical dust and sludge presents unique challenges. There is a lack of detailed research on the specific corrosion mechanisms of refractory bricks and the subsequent ring formation when treating these specific iron-bearing wastes.

Methodology
This study investigates the corrosion behavior of aluminum phosphate refractory bricks and the mechanisms of ring formation during the roasting of zinc-iron-bearing dust and sludge.

• Materials: Iron-bearing waste samples, including Blast Furnace Dust (BFD) and Oxygen-Converter Sludge (OGs), were sourced from Maanshan Iron and Steel Company. The materials were mixed in a 1:1 mass ratio.
• Experimental Setup: Laboratory simulations were conducted using a muffle furnace. Pressed pellets and loose powder mixtures were placed on rectangular aluminum phosphate refractory bricks (cut from field-retrieved bricks) and heated from 1000 °C to 1250 °C in 50 °C increments for 80 minutes each.
• Analysis: The study utilized X-ray fluorescence (XRF) for chemical composition, X-ray diffraction (XRD) for phase identification, and Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS) to observe microstructures and elemental distribution.
• Thermodynamic Modeling: FactSage 8.4 software was employed to calculate phase diagrams for the CaO-SiO2-FeO-Al2O3 and CaO-SiO2-FeO-Al2O3-MgO systems to elucidate the melting behavior and reaction mechanisms.
• Field Validation: Ring samples were collected from specific zones (8 m, 22 m, and 27 m from the kiln head) of an industrial rotary kiln to correlate laboratory findings with actual production conditions.

Key Results

1. Temperature and Agglomeration: Agglomeration on the refractory bricks was negligible below 1100 °C. Between 1100 °C and 1200 °C, adherents began to form, with powder mixtures showing more severe agglomeration than pelletized mixtures. At 1250 °C, the mixture completely adhered to the bricks, forming a solid corrosion layer that could not be mechanically removed.
2. Corrosion Mechanism:
   • Initial Stage: During reduction, Fe2O3 is reduced to FeO. FeO accumulates on the refractory surface and reacts with Al2O3 to form low-melting-point liquid phases. Thermodynamic calculations indicate that the FeO-Al2O3 system has the lowest melting point among the binary oxides involved.
   • Secondary Stage: As the liquid phase forms, CaO from the waste reacts with Al2O3, and SiO2 reacts with FeO (forming Fe2SiO4). These reactions further lower the melting point of the slag, facilitating the penetration of the liquid into the refractory brick.
   • Solidification and Ring Formation: MgO, initially present in the waste, diffuses into the eroded layer at a later stage. The reaction of MgO with SiO2 and CaO forms high-melting-point compounds, specifically Ca3MgSi2O8. This reaction raises the liquidus temperature of the slag. As the FeO content decreases (due to reduction to metallic iron) and MgO accumulates, the liquid slag solidifies, forming a hard corrosion layer and ring-like accretions.
3. Ring Composition: Industrial ring samples contained metallic iron (MFe), cementite (Fe3C), and slag phases. The presence of Fe3C, formed by the carburization of MFe, significantly lowered the melting point of the iron phase, contributing to the bonding strength and difficulty in removing the rings.
4. Zinc Removal: The zinc removal rate increased with temperature, exceeding 75% at 1100 °C and surpassing 90% at 1200 °C. However, higher temperatures also promote ring formation.

Significance and Claims
The paper claims to elucidate the specific corrosion and agglomeration mechanisms of aluminum phosphate refractory bricks in the context of treating iron-bearing waste. The study identifies that the formation of rings is a multi-stage process initiated by FeO-induced corrosion, followed by the incorporation of CaO and SiO2, and finalized by the solidification of the slag due to MgO participation and FeO reduction.

The authors state that these findings provide practical references for optimizing operating conditions, specifically suggesting that controlling the basicity (CaO/SiO2 ratio) and the composition of the mixture (particularly MgO and CaO content) can reduce liquid slag formation and mitigate ring formation. The research aims to facilitate the application of rotary kilns for solid waste recycling by addressing the key operational challenge of ringing, thereby supporting the industry's goal of reducing environmental pressure through effective waste treatment.