Scories

Slag Demystified: What Mills Don’t Tell You

Slag isn’t ‘waste.’ It’s the natural result of cleaning metals. When raw ore meets super-high heat (over 1,500°C in furnaces), unwanted stuff melts together into a hot liquid mix of silicates, calcium oxides, et l'aluminium. This lava-like flow gets removed from pure metal – like skimming fat from soup. But here’s what factories won’t tell you: slag’s makeup is unique. German slag differs from Indian slag because of:

• Where the ore comes from
• Materials added during melting
• How it’s cooled (water spray vs. air cooling)

I’ve tested slag with 40% silica against types full of calcium compounds. This difference isn’t bad – it’s useful. Dutch road builders specifically order electric furnace slag for seaside roads because its extra iron fights salt damage better than regular stone.

Slag Types: A Field Guide for Engineers

Blast Furnace Slag (BFS): The Cement Saviour

Formation: Molten slag water-quenched at 1,000°C → glassy granules (GBFS).
Pour la pointe: GBFS reactivity depends onglass content. Aim for >90% vitreous material – check mill certificates. Lower grades cause slow strength gain.

Steel Slag: The Road Warrior

The Expansion Myth: Oui, fresh BOF slags expands like baking dough. Why? Free lime (CaO) hydration. But here’s the fix:

1. Hot-stage processing: Inject CO2 during cooling → forms stable calcite
2. Weathering: Minimum 6-month outdoor exposure (monitor pH < 10.5)
3. Accelerated aging: Steam curing at 80°C for 48 heures

Field Observation: Japan’s Tohoku Expressway uses 100% aged steel slag aggregate. After the 2011 quake, sections with slag base showed 70% less cracking than gravel-based stretches.

The Underdogs

• Copper Slag: Blasting abrasive so aggressive it strips hull paint in half the time of garnet sand. Requires dust suppression systems.
• Ferronickel Slag: Indonesia ships this to Dubai as road base – its high MgO content prevents bitumen stripping in heat.

Processing Breakdown: Where Profit is Made

Crushing Circuits – The Heartbeat

Crusher Selection Truths:

• Concasseurs à mâchoires: Best for >800mm lumpy slag. Wear cost: $0.18-0.30/tonne
• Rupture de cône: Ideal for 50-250mm feed. Produce 60% cubical aggregate
• Concasseurs VSI: Essential for road aggregates. Enhances particle shape but costs 40% more in wear parts

Operator Hack: Run slags through electromagnetavant secondary crushing. Recovered metal scrap offsets 15-30% of processing costs.

The Weathering Yard Dilemma

Steel slag stockpiles aren’t just sitting – they’re chemically evolving. Smart plants:

• Install drainage layers under piles
• Monitor pile temperatures (target <70° C)
• Test expansion monthly (EN 1744-1 test)
  Coût: Weathering adds $1.50-4.00/ton but prevents $20+/ton in liability claims.

Slag Cement: The Carbon Killer

Why GGBFS Beats Fly Ash

The Grinding Reality

Producing high-reactivity GGBFS requires:

• Vertical roller mills (not ball mills)
• 4,500-5,000 Blaine fineness
• Moisture control <0.5%
  Energy Penalty: Grinding consumes 35-50 kWh/ton – still 60% less than OPC production.

Aggregates That Outperform Nature

Roads That Bite Back

Steel slag’s PSV (Polished Stone Value) de 68-72 beats granit (50-55). UK’s Highways Agency mandates slags on M25 curves after wet-season skid crashes dropped 45%.

Concrete’s Secret Weapon

Air-cooled BFS aggregate:

• Low thermal expansion (9 μm/m°C vs limestone’s 12)
• Alkali-silica reaction neutralization
  Étude de cas: Dubai’s Burj Khalifa used 340,000 m³ of BFS concrete to combat 50°C setting temperatures.

Beyond Construction: Unusual Profit Streams

Farming’s pH Fixer

Steel slag’s 8.5-11.5 pH makes it perfect for acidic soils. Malaysia’s palm oil plantations apply 2 tons/hectare:

• Raises soil pH from 4.2 → 6.0 in 6 mois
• Adds soluble silicon for stalk strength
  Caution: Test for heavy metals – Cd < 2 mg/kg, Cr < 80 mg/kg.

Mineral Wool’s Dirty Secret

70% of stone wool is actually slag wool. Production energy:

• basalt: 2,800 Kwh / tonne
• Slag-based: 1,100 Kwh / tonne
  Tip: Use slags with 38-42% CaO for optimal fiberization.

Equipment Deep Dive: Making Crushers Last

Slag’s Wear Mechanisms

• Abrasion: SiO2 content >18% accelerates wear
• Impact: Uncrushed metal “tramp” damages rotors
• Corrosion: Sulfides attack manganese steel

SBM China’s Edge (https://www.mill-sbm.com)

Le concasseurs à percussion handle slags better because:

• Hybrid breaker plates (ceramic inserts + manganese)
• Rotor impact tips welded with Hardox 500
• Vibration sensors trigger metal detection shutdown
  Maintenance Hack: Schedule hammer rotation every 250 hours – extends life by 3x.

The Real Economics of Slag

Pricing Power

Hidden Revenue Streams

1. Carbon credits: 1 ton GGBFS = 0.65 carbon credits
2. Récupération des métaux: $15-40/ton from ferrous scrap
3. Landfill avoidance fees: $7-20/tonne

The Future Is Pre-Activated

Leading research (Fraunhofer Institute, 2023) showsmechanochemical activation could revolutionize slag:

• High-pressure grinding to nano-particles
• Chemical activators (Na2SO4 or KOH)
• Result: 7-day strength equal to OPC
  This eliminates grinding costs – the final barrier to 100% slag cements.

Waste is Just Resource in Disguise

Last year, I visited a Tokyo data center made with 92% slag concrete – cool inside, no cracks, standing strong through earthquakes. Nearby, rice fields fed with slag glowed bright green. This isn’t just for show; it’s practical resourcefulness. Le 2.5 billion tons of old slag piles worldwide aren’t problems – they’re unused resource banks. With modern crushers and processing, we’re not just recycling slags; we’re reusing our own industrial leftovers. Le “waste” era is finished. Welcome to the age of valuable materials.