Steel / Metal / Sponge Iron Plants

Extreme heat: Reheating furnaces, annealing ovens, and EAFs generate gases at 600-1200°C requiring specialized high-temp fans with refractory protection and water cooling. Metal fume toxicity: Zinc (galvanizing), manganese (ferromanganese), lead (brass), and hexavalent chromium (stainless steel) fumes require 99.5%+ capture efficiency due to severe health hazards. Explosive atmospheres: Sponge iron off-gas contains 20-30% CO creating explosion risk—fans must be non-sparking with proper grounding. Abrasive scale: Mill operations generate iron oxide scale dust that erodes fan components 5-10mm/year without AR steel protection. High capital cost: Large EAF fume systems cost ₹2-5 crore requiring careful economic justification despite regulatory mandates. Downtime sensitivity: Steel production operates continuously—fan failures causing production stops cost ₹50 lakh to ₹2 crore per day necessitating redundancy and rapid maintenance.

Capture method: Direct evacuation (DEG - Direct Evacuation Gun) extracting fumes directly from furnace through water-cooled duct during melting, plus canopy hood capturing fumes during charging and tapping. System components: (1) Water-cooled ductwork: Initial section handles 800-1200°C post-combustion gases, water jackets maintain duct wall <100°C. (2) Evaporative cooler: Water spray cooling reduces gas from 600°C to 200°C before bag filter. (3) Bag filter: High-temp bags (aramid or fiberglass) capture metal oxide fumes to <20mg/Nm³. (4) ID fan: 200,000-400,000 m³/hr at 350-500 mmWC. Control strategy: Dampers modulate extraction rate—high during melt (maximum fume), low during idle, closed during tapping (prevents CO explosion). Capture efficiency: Properly designed DEG systems achieve 90-95% capture vs 70-80% canopy-only systems. Payoff: Recovered metal oxide dust (20-50 kg/ton steel) worth ₹5-15/kg can offset operating costs.

Sponge iron (DRI - Direct Reduced Iron) rotary kilns produce iron from ore using coal/natural gas, generating large volumes of off-gas (8,000-15,000 Nm³/ton DRI) containing: CO (20-30%), CO2 (15-20%), H2 (1-3%), N2 (balance), plus dust (iron/carbon particles). GCP functions: (1) Dust removal: Cyclones + bag filters capturing 50-200 g/Nm³ dust load to <50mg meeting regulations. (2) Cooling: Gas exits kiln at 200-400°C, cooled to 80-120°C for fabric filter compatibility. (3) CO recovery (optional): Clean CO-rich gas can fuel process heating (drying, preheating) saving coal. Equipment: Multi-cyclone pre-cleaner (70-85% efficiency removing coarse dust), evaporative cooler or heat exchanger, pulse-jet bag filter (25,000-80,000 m³/hr), ID fan (300-500 mmWC), stack. Critical design: All equipment must be non-sparking (aluminum/bronze vs steel) preventing CO ignition. Cost: GCP for 200 TPD plant = ₹3-5 crore capital but mandatory for environmental compliance.

Pickling (acid cleaning) of steel generates HCl or H2SO4 acid mist requiring corrosion-resistant systems: Capture: Enclosed pickling tanks with side-slot extraction pulling acid fumes from tank surface before escaping to atmosphere. Extraction rate 500-2,000 m³/hr per meter tank length. Scrubber system: Cannot use bag filters (acid destroys fabric)—require wet scrubber neutralizing acid. Packed tower scrubber: Acid fumes contact counter-flowing caustic solution (NaOH 2-5%) neutralizing HCl→NaCl+H2O. Packing media (polypropylene balls/saddles) provides contact area. Efficiency 95-99% reducing HCl from 500-2000 mg/Nm³ to <50mg. Materials: Ductwork: PVC or FRP (fiberglass reinforced plastic). Scrubber: PVC or PP (polypropylene). Fan: FRP construction with PP impeller. Maintenance: Weekly: Check caustic pH (maintain 8-10), inspect nozzle spray pattern. Monthly: Clean packing media, verify mist eliminator. Quarterly: Inspect fan for corrosion pitting. Cost: Complete system for 2-tank pickling line (40,000 m³/hr) = ₹25-40 lakh.

EAF principle: Melt steel scrap using electrical arc (3,000-4,000°C) between graphite electrodes and metal charge. Process: (1) Charging: Lower electrodes, open roof, charge scrap basket (60-150 tons) into furnace using overhead crane. Close roof. (2) Melting: Raise electrodes, strike arc (300-600V, 50,000-150,000 amperes!). Arc melts scrap pool forming at bottom, additional scrap continuously added as melt progresses. Oxygen injection (500-2,000 Nm³/heat) burns carbon, silicon, manganese (exothermic oxidation accelerating melting). Power-on time 35-55 minutes depending on scrap quality and transformer power (50-150 MVA typical). (3) Refining: After complete melt, sample chemistry, adjust composition with alloy additions (ferrosilicon, ferromanganese, lime for slag conditioning). (4) Tapping: Tilt furnace, pour molten steel (1,600-1,650°C) into ladle. Tap time 3-8 minutes. (5) Slag-off: After steel tapped, tilt further dumping slag into separate slag pot. Electrical power: Transformer 50-150 MVA (megavolt-amperes) stepping down grid voltage (33-110 kV) to furnace secondary (300-1,000V). Power consumption 350-550 kWh per ton liquid steel. At ₹6/kWh = ₹2,100-3,300/ton energy cost (dominant operating expense). Air pollution control: Primary fume capture: Roof-mounted duct (4th hole) extracting fume directly from furnace during melting. Volume 80,000-250,000 m³/hr, temperature 150-400°C, dust loading 5-30 g/Nm³ (iron oxide, lime, carbon). Secondary fume capture: Building exhaust fans capturing fugitive emissions escaping primary system. Treatment: Cooling + bag filter reducing to <50 mg/Nm³. Advantages EAF vs blast furnace: (1) Uses scrap (cheaper than iron ore), (2) Flexible (on/off as needed vs continuous blast furnace), (3) Lower capital (₹5-15 crore per ton annual capacity vs ₹20-40 crore blast furnace), (4) Suitable small-scale (10,000-500,000 ton/year vs BF minimum 1 million+). Disadvantage: High electricity cost—only viable with cheap power or valuable scrap/alloy steel.

Scrap types: Heavy melt scrap (thick plates, beams, rails—minimal dust), shredded scrap (automobiles, appliances shredded to <12-inch pieces—moderate dust), turnings/borings (machine shop waste—very dusty), foundry returns (high sand content). Dust generation: Scrap handling (loading, conveyor transfer, magnetic crane lifting/dropping) creates impact releasing adherent dirt, rust, coatings (paint, zinc from galvanized), sand from casting scrap. Uncontrolled scrap yard: 50-500 mg/m³ dust in immediate area. Health hazards: Lead (from painted scrap, battery scrap), cadmium (electroplating), hexavalent chromium (stainless steel), manganese (alloy steel), silica (sand in foundry returns). Fire risk: Oil-soaked turnings ignited by cutting torch fires, spreads rapidly. EAF charging fume: Dropping scrap basket into hot furnace (1,200-1,500°C residual heat from previous heat) vaporizes surface contaminants creating massive fume plume (10,000-50,000 mg/Nm³!) exhausting from furnace openings (door, roof gap). Without capture, fills building with dense smoke. Dust control strategies: (1) Water spray suppression: Spray scrap pile, conveyor transfer points with mist (10-50 micron droplets) knocking down dust 60-85%. Simple, low capital (₹2-8 lakh), but contaminated runoff requires treatment. (2) Enclosed transfer + extraction: Cover scrap conveyor/chute, extract with fan (20,000-80,000 CFM), bag filter. Expensive (₹40 lakh-₹2 crore) but 95%+ capture. (3) Building exhaust: High bay exhaust fans (total 200,000-500,000 CFM) diluting fugitive dust keeping <5 mg/m³ worker exposure. (4) Good housekeeping: Minimize drop heights (reduces impact energy and dust generation), regular vacuum cleaning (industrial HEPA vacs), paved surfaces vs dirt yards. EAF primary fume hood: Capture at source immediately above charging point extracting smoke plume before dispersing. Critical for compliance and worker health.

Ladle function: Refractory-lined steel vessel (15-200 ton capacity) receiving molten steel from furnace (EAF, BOF, LRF), transporting to caster or secondary metallurgy (LRF, vacuum degassing). Problem with cold ladle: Steel tapped at 1,580-1,650°C into cold ladle (ambient 25°C, refractory thermal mass ~5-30 tons) causes: (1) Severe temperature drop: 30-80°C drop (varies by ladle size, tapping time, steel mass). 1,620°C steel drops to 1,550°C after tap. (2) Skull formation: Steel freezing on cold refractory forming skull (solidified layer) reducing ladle capacity, contaminating next heat. (3) Refractory thermal shock: Rapid heating causing cracking, spalling, premature lining failure (refractory life reduced 60-80 heats vs 120-180 with preheat). (4) Casting problems: If steel temperature drops below liquidus (1,510-1,530°C typical), premature solidification in tundish/mold causing breakouts (molten steel bursting through solidified shell—catastrophic). Preheating solution: Heat ladle to 900-1,200°C before tapping using ladle preheater (burner firing into ladle). Burner types: (1) Direct-fired burner: Natural gas/LPG burner (1-5 million kcal/hr) inserted into ladle opening firing against refractory. Preheat time 4-10 hours (cold ladle) or 1-2 hours (warm ladle from previous heat). (2) Oxy-fuel burner: Higher flame temperature (2,800°C vs 1,900°C air-fuel) reducing preheat time 50%. But requires oxygen supply. Combustion air: Burner requires 8-15 Nm³ air per Nm³ natural gas. 2 million kcal/hr burner = 200 Nm³/hr gas × 12 = 2,400 Nm³/hr air = 2,900 m³/hr at ambient. Blower delivering 3,000-4,000 m³/hr at 50-150 mmWC pressure. Exhaust: Hot combustion products (1,400-1,600°C) exhausting from ladle top. Requires overhead hood (5,000-15,000 m³/hr extraction) capturing for worker safety. Benefits of preheating: (1) Minimize temperature loss (<15°C drop with 1,100°C preheat vs 60°C cold), (2) No skull formation, (3) Refractory life 2-3× longer (saves ₹5-20 lakh per reline), (4) Casting temperature control ensuring quality, (5) Productivity (prevents heat delays from temperature issues). Cost: Ladle preheater burner + blower + hood = ₹8-25 lakh. Fuel cost ₹500-2,000 per preheat. But refractory savings + quality benefits justify easily.