crushing and screening process
Crushing and Screening Process: A Comprehensive Overview
The crushing and screening process is the fundamental stage in mineral processing, aggregate production, and construction waste recycling. It involves reducing run-of-mine (ROM) ore or raw quarry material into smaller, uniform particles, followed by classification into different size fractions for downstream use. This two-stage operation—comminution (crushing) and classification (screening)—determines the efficiency, product quality, and economic viability of any hard-rock or gravel operation. The process is not a single step but a carefully engineered sequence of equipment, each with a specific reduction ratio and energy input, designed to meet exact product specifications while minimizing fines and operating costs.
1. The Core Stages: Crushing and Screening
Crushing is the mechanical process of applying compressive, impact, or shear forces to break rock. It is typically performed in stages to gradually reduce the material from a maximum feed size (e.g., 1,000 mm) to a final product (e.g., 0–25 mm). The primary, secondary, and tertiary crushers each have distinct roles:
- Primary Crushing: Handles the largest feed (up to 1.5 m). Jaw crushers or gyratory crushers are used, producing a coarse product (150–300 mm).
- Secondary Crushing: Reduces the primary product to 50–100 mm. Cone crushers or impact crushers are standard here.
- Tertiary/Quaternary Crushing: Produces fine aggregates (0–40 mm) or mill feed. High-speed cone crushers, VSI (Vertical Shaft Impact) crushers, or HPGR (High-Pressure Grinding Rolls) are employed.
Screening is the separation of crushed material by size using vibrating screens or grizzly feeders. It serves two purposes: (a) removing undersized material before a crusher (pre-screening) to prevent over-crushing, and (b) grading the final product into marketable fractions (e.g., 0–5 mm, 5–10 mm, 10–20 mm). Screening efficiency is critical—poor screening leads to recirculation loads and reduced crusher throughput.
2. Process Flow and Equipment Selection
A typical stationary crushing plant follows a closed-circuit layout:
- Feed Hopper + Vibrating Grizzly Feeder: Removes fines (< 50 mm) and scalps oversized rock.
- Jaw Crusher (Primary): Reduces 600–1,000 mm rock to 150–200 mm.
- Cone Crusher (Secondary): Further reduces to 40–80 mm.
- Double-Deck Vibrating Screen: Separates into three fractions: oversize (> 40 mm) returns to the cone crusher (closed circuit), mid-size (20–40 mm) goes to tertiary crushing, and undersize (< 20 mm) is stockpiled.
- VSI Crusher (Tertiary): Shapes the aggregate (cubical shape) and reduces to 0–20 mm.
- Final Screening: Produces final products (e.g., 0–5 mm sand, 5–10 mm, 10–20 mm aggregates).
Mobile vs. Stationary Plants: The choice depends on project duration, feed source, and mobility needs.
| Feature | Stationary Plant | Mobile Plant (Track/Wheel) |
|---|---|---|
| Setup Time | Weeks to months | Hours to days |
| Capacity | High (500–3,000 t/h) | Low to medium (100–600 t/h) |
| Capital Cost | High (civil works, foundations) | Lower (no concrete works) |
| Best For | Long-term quarries, mines | Construction sites, recycling, short-term projects |
| Power Source | Grid electricity | Diesel generator or hybrid |
| Mobility | Not movable | Self-propelled or trailer-mounted |
3. Key Process Parameters and Optimization
- Reduction Ratio: The ratio of feed size to product size. A jaw crusher has a ratio of 4:1 to 6:1; a cone crusher, 3:1 to 5:1. Exceeding these ratios causes excessive wear and energy consumption.
- Closed-Side Setting (CSS): The smallest gap between crusher liners. Adjusting CSS controls product top size. For a cone crusher, a 10 mm decrease in CSS can increase fines by 15–20% but reduces throughput by 10%.
- Screen Media: Polyurethane or rubber screens reduce blinding (clogging) for wet or sticky materials, while steel wire screens are cheaper for dry, abrasive rock.
- Moisture Content: Above 5% moisture, screening efficiency drops sharply. In such cases, washing screens or dry classification (air classifiers) are required.
Energy Efficiency: Crushing is energy-intensive. According to a study by the U.S. Department of Energy, comminution accounts for 1–2% of global electricity consumption. Using HPGR instead of a conventional cone crusher for tertiary crushing can reduce energy use by 20–30% (Fuerstenau, 1995).
4. Real-World Case Study: Metso’s Lokotrack in Urban Recycling (Munich, Germany)
Background: In 2021, a construction and demolition waste recycling company in Munich needed to process 300,000 tonnes/year of mixed rubble (concrete, brick, asphalt) into a reusable 0–32 mm base course for road construction. The challenge was space constraints and noise limits in a residential-adjacent site.
Solution: The company deployed a Metso Lokotrack LT106 jaw crusher (primary) and a Lokotrack ST2.8 mobile screen in a two-stage closed circuit. The LT106 crushed the feed (max 600 mm) to 0–120 mm. The ST2.8 screen separated at 32 mm; the oversize returned to the LT106 via an integrated conveyor. The 0–32 mm product was directly used as a sub-base, meeting German TL SoB-StB 04 standards.
Results:
- Throughput: 220 t/h (vs. 180 t/h with the previous stationary plant).
- Fuel consumption: 22 L/h (diesel) – 15% lower than the old electric plant due to reduced recirculation.
- Noise level: 78 dB(A) at 10 m, compliant with local night-time limits.
- Payback period: 14 months, driven by avoided landfill fees (€80/tonne) and sale of recycled aggregate (€12/tonne).
This case demonstrates that a mobile crushing and screening process can be economically and environmentally superior to a fixed plant in urban settings.
5. Common Challenges and Mitigation
- Over-crushing (fines generation): Caused by excessive recirculation or wrong CSS. Fix: Use pre-screening to remove fines before the crusher; adjust CSS based on feed hardness.
- Screen Blinding: Wet clay or fibrous material blocks apertures. Fix: Use ball-deck cleaners, heated screens, or switch to a washing screen with water sprays.
- Crusher Blockage (jamming): Non-crushable material (e.g., rebar) enters the chamber. Fix: Install magnetic separators and metal detectors before the crusher.
- Dust Emissions: High silica dust is a health hazard. Fix: Water spray systems, dust extraction hoods, and enclosed conveyor transfer points.
6. Frequently Asked Questions (FAQ)
Q1: What is the difference between a closed-circuit and open-circuit crushing process?.jpg)
- Open circuit: Material passes through the crusher once and goes to the next stage. It is simpler but produces a wider product size distribution. Closed circuit: The crusher discharge is screened; oversize material is returned to the crusher for further reduction. This ensures a consistent top size and higher product yield, but requires additional conveyors and a screen.
Q2: How do I choose between a jaw crusher and an impact crusher for primary crushing?
- Use a jaw crusher for hard, abrasive rock (granite, basalt) where compressive strength exceeds 200 MPa. Use an impact crusher for softer, less abrasive materials (limestone, recycled concrete) where you need a higher reduction ratio (up to 15:1) and a more cubical product shape. Impact crushers have higher wear costs per tonne on hard rock.
Q3: Why is screening efficiency important, and how is it calculated?.jpg)
- Screening efficiency is the ratio of the actual amount of undersize material passing through the screen to the theoretical amount present in the feed. A 90% efficiency means 10% of the desired fine material is incorrectly sent to the oversize stream. Low efficiency increases recirculation load, reduces crusher capacity, and wastes energy. It is calculated as: Efficiency (%) = (Mass of undersize in feed – Mass of undersize in oversize) / Mass of undersize in feed × 100.
Q4: Can the same crushing plant handle both hard rock and recycled concrete?
- Yes, but with limitations. A jaw crusher + cone crusher setup can handle both, but the cone crusher requires a minimum feed size and will wear faster on concrete due to rebar. For mixed waste, a jaw crusher with a hydraulic release system (to pass non-crushable steel) is recommended. Impact crushers are more versatile for recycling but have higher wear on hard rock.
Q5: What is the typical cost breakdown for a crushing and screening operation?
- For a 250 t/h stationary plant, the breakdown is approximately: Wear parts (liners, screens): 30–40% of operating cost; Energy (electricity/diesel): 25–30%; Labor and maintenance: 20%; Blasting and loading (if applicable): 10–15%. Wear costs are highest for impact crushers (up to $0.50/tonne) and lowest for jaw crushers ($0.05–0.10/tonne).
7. Conclusion
The crushing and screening process is a mature, data-driven engineering discipline. Its success hinges on selecting the right equipment for the material's abrasiveness and hardness, optimizing the closed-circuit configuration, and maintaining high screening efficiency. Modern trends—such as mobile plants, hybrid power, and automation (e.g., Metso’s IC process control)—are reducing operating costs and environmental impact. For any project, a thorough test of the feed material (JKTech or Bond work index) and a pilot-scale trial are recommended before full-scale plant design.