JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II PRINCIPLES OF EXTRACTIVE METALLURGY Previous Question Paper (November/December - 2020 - R18)

A fluidized bed roaster is a reactor designed to facilitate efficient gas-solid reactions by suspending fine ore particles in an upward-flowing stream of process gas (typically air or oxygen-enriched air). The particles behave like a boiling liquid, ensuring high heat and mass transfer rates.

Sketch Description: The reactor consists of a cylindrical steel shell lined with refractory bricks. At the base, a distributor plate (or tuyeres) ensures uniform gas distribution. The feed system introduces fine ore at the side or top, while the product (calcine) is discharged through an overflow pipe. Off-gases exit from the top through a cyclone separator to recover entrained dust.

• Functioning: The roasting gas is blown upward through the distributor plate at a velocity sufficient to overcome the weight of the particles (the minimum fluidization velocity). This causes the bed to expand, creating intense turbulence. The high surface area contact between the gas and solid allows for rapid oxidation reactions (e.g., 2ZnS + 3O₂ → 2ZnO + 2SO₂). The process is highly exothermic, and temperature is controlled by injecting water or cooling coils directly into the bed.
• Advantages:
  • High Reaction Rates: Excellent gas-solid contact leads to rapid and uniform roasting.
  • Thermal Efficiency: The high turbulence ensures uniform temperature distribution, preventing local hot spots or sintering.
  • Process Control: The system responds quickly to changes in feed rate or gas composition.
  • Heat Recovery: Large amounts of heat can be recovered from the off-gases for steam generation.
• Limitations:
  • Feed Size Sensitivity: Requires finely crushed ore; coarse particles may not fluidize, while extremely fine particles may be carried away (elutriation).
  • Dust Handling: High dust carry-over necessitates the use of large-scale cyclone separators and electrostatic precipitators.
  • Capital Cost: The auxiliary equipment (blowers, dust collection systems, and control instrumentation) makes the initial investment relatively high.
  • Operational Complexity: Maintaining a steady state requires precise control of gas velocity and particle size distribution to avoid bed collapse or excessive entrainment.

A Multiple Hearth Roaster (MHR) is a vertical, cylindrical furnace designed for the continuous roasting of sulfide ores. It consists of a series of circular hearths arranged one above the other, enclosed within a refractory-lined steel shell. A central rotating shaft extends through the entire height of the furnace, equipped with horizontal arms and rabble blades on each hearth.

• Mechanism: The ore is fed at the top and moves downward through the hearths by the action of the rabble blades, which plow the material toward alternating drop holes (center and periphery).
• Gas Flow: Air or roasting gases are introduced at the bottom and flow counter-currently to the ore, facilitating efficient heat exchange and oxidation.
• Temperature Control: Each hearth can have independent air supply and fuel burners, allowing for precise temperature zoning to manage exothermic roasting reactions.

The Fluidized Bed Roaster (FBR) differs fundamentally in its operational principle and efficiency:

Key distinctions include:

• Fluidization: In FBR, air is blown upward through a perforated hearth (distributor plate) at a velocity sufficient to suspend the ore particles, creating a boiling-like state that ensures near-isothermal conditions.
• Throughput: FBR offers significantly higher throughput per unit volume compared to MHR.
• Off-gas: FBR produces a more concentrated sulfur dioxide (SO₂) stream, which is advantageous for sulfuric acid production, whereas MHR off-gases are often more dilute.
• Maintenance: MHR is prone to mechanical wear of the central shaft and rabble arms, whereas FBR maintenance is primarily focused on the air distribution system and dust collection (cyclones).