Working principle, process flow, and advantages of a roasting furnace

　　The roasting furnace body is a steel shell lined with refractory, acid-resistant bricks. Three burners are arranged tangentially on the furnace body to provide reheating and to dry the pre-concentrated acid droplets discharged from the nozzles. Within the furnace’s thermal zone (500–800°C), FeCl₂ decomposes according to the following equation: 2FeCl₂ + 2H₂O + 1/2O₂ = Fe₂O₃ + 4HCl.

　　I. Working Principle of the Calcination Furnace:

　　Solid particles (Fe₂O₃) fall in powder form into the conical hopper at the bottom of the roasting furnace and are discharged via a rotary valve, which isolates the furnace’s internal gases from the external atmosphere. A oxide‑block crusher is installed above the rotary valve to break up agglomerates that have fallen from the furnace walls.

　　The gas from the roasting furnace consists of combustion exhaust, water vapor, and hydrogen chloride. It exits the furnace at the top and passes through a cyclone separator, which removes most of the Fe₂O₃ particulates it contains. The separated oxide is discharged via a rotary valve and returned to the roasting furnace. Subsequently, the roasting gas enters the pre-concentrator, where the high‑temperature gas comes into direct contact with the circulating acid, while only trace amounts of oxide remain in the cooled and scrubbed gas before it proceeds to the absorption tower.

　　II. Process Flow of the Calcination Furnace:

　　Industrial-grade hydrated aluminum hydroxide typically contains 6% to 8% adsorbed water. During calcination, as the material is heated above 100°C, this adsorbed water is evaporated and removed. At around 225°C, the aluminum hydroxide first loses two molecules of water of crystallization, transforming into monohydrate boehmite; further heating to 500–560°C causes the monohydrate to lose an additional molecule of water, yielding anhydrous γ‑Al₂O₃. The γ‑Al₂O₃ produced at 500–560°C is a finely dispersed, crystalline alumina that requires further elevation of the calcination temperature to promote crystallization and growth into coarse particles. When γ‑Al₂O₃ is heated to 900°C, it begins to transform into α‑Al₂O₃; at this stage, the transformation rate is very slow, but increasing the temperature accelerates it. Only by maintaining the material at 1050–1200°C for a sufficient duration can γ‑Al₂O₃ be fully converted into α‑Al₂O₃.

　　III. Advantages of the Roasting Furnace:

　　1. Excellent sealing performance and a high degree of automation; 2. Automatic control of roasting temperature with segmented settings; 3. Safe and reliable heating system; 4. Furnace chamber surface temperature below 65°C, resulting in minimal heat loss; 5. Compact equipment design, small footprint, and lightweight; 6. High efficiency, energy-saving, and environmentally friendly; 7. Low operational intensity and a favorable working environment.