britagem wikipedia
Crushing (Mineral Processing): An Overview of the Britannica Definition and Industrial Practice
The term "britagem" is the Portuguese word for "crushing," a fundamental unit operation in mineral processing and construction aggregates. As defined in the Encyclopædia Britannica and standard engineering references, crushing is the process of reducing large solid materials—such as run-of-mine ore or quarry rock—into smaller, manageable fragments using mechanical force. This article synthesizes the Britannica definition with modern industrial practice, detailing the stages of crushing, equipment types, key comparisons between primary and secondary crushers, and real-world applications. It also addresses common operational questions regarding energy consumption, feed control, and safety.
1. The Core Definition and Purpose of Crushing
According to the Encyclopædia Britannica (entry: "crushing"), the process involves applying compressive force to break a material's internal cohesion. Unlike grinding, which produces fine particles (typically <1 mm), crushing generally yields coarse to medium particles (from 150 mm down to 5 mm). The primary purpose is threefold: (a) to liberate valuable minerals from the gangue (waste rock) for subsequent concentration, (b) to produce construction aggregates of specified size and shape, and (c) to increase the surface area for chemical reactions (e.g., in cement production or heap leaching). Crushing is always the first mechanical stage in a comminution circuit, followed by grinding if finer particle sizes are required.
2. Stages of Crushing: Primary, Secondary, and Tertiary
Crushing is rarely a single-step operation. Industrial circuits typically employ three stages, each with distinct equipment and reduction ratios (the ratio of feed size to product size).
| Stage | Feed Size (Typical) | Product Size (Typical) | Reduction Ratio | Main Equipment |
|---|---|---|---|---|
| Primary | 500–1500 mm | 100–300 mm | 3:1 to 6:1 | Jaw crushers, Gyratory crushers |
| Secondary | 100–300 mm | 20–100 mm | 4:1 to 8:1 | Cone crushers, Impact crushers (horizontal shaft) |
| Tertiary/Quaternary | 20–100 mm | 5–20 mm | 3:1 to 5:1 | Short-head cone crushers, Vertical shaft impactors (VSI) |
Key Differences Between Primary and Secondary Crushers:
| Parameter | Primary Crusher | Secondary Crusher |
|---|---|---|
| Location | At the mine face or quarry pit (fixed or semi-mobile) | Inside the processing plant (fixed) |
| Feed Size | Very large (up to 1.5 m) | Smaller (already reduced by primary) |
| Mechanism | Compression only (jaw or gyratory) | Compression (cone) or impact (HSI) |
| Capacity | Very high (up to 10,000 t/h) | Moderate (up to 2,000 t/h) |
| Wear Cost | Lower per ton (due to massive liners) | Higher per ton (due to finer work) |
| Product Shape | Generally poor (elongated) | Better (more cubical, especially with impact) |
3. Equipment Selection: Compression vs. Impact
The choice of crusher depends on the material's abrasiveness, hardness, and moisture content.
- Jaw Crushers (compression): Best for hard, abrasive rocks (granite, basalt). They operate by a fixed and a moving jaw. Britannica notes that jaw crushers are the "workhorse" of primary crushing due to their simplicity and reliability.
- Gyratory Crushers (compression): Similar to jaw crushers but with a circular gap. They are used for very high-capacity primary crushing (e.g., in large copper mines) because they can handle continuous feed.
- Cone Crushers (compression): Used for secondary and tertiary crushing. They produce a more uniform particle size than jaw crushers. The mantle and concave are the wear parts.
- Impact Crushers (impact): Use high-speed rotors with hammers or blow bars. They are ideal for softer, less abrasive materials (limestone, recycled concrete). They produce excellent cubical shape but have higher wear costs on abrasive feed.
4. Real-World Case Study: Carajás Iron Ore Mine (Brazil)
To illustrate the practical application of crushing, consider the S11D mine (Vale S.A.) in Carajás, Pará, Brazil—the world's largest iron ore project. The term "britagem" is standard in Brazilian mining operations..jpg)
The Challenge: The mine extracts ore with a top size of 1,200 mm. The product must be reduced to 50 mm for the 900 km slurry pipeline to the port.
The Solution (Primary Crushing): Vale installed four semi-mobile gyratory crushers (from FLSmidth) at the mine pit. Each crusher handles 10,000 t/h. The gyratory crushers were chosen over jaw crushers because:
- They accept a larger feed size (1,500 mm) without bridging.
- They have a continuous throughput, which matches the conveyor system feeding the beneficiation plant.
The Solution (Secondary/Tertiary): After primary crushing, the ore (now at 300 mm) is fed to HPGR (High-Pressure Grinding Rolls) and cone crushers in the plant. The cone crushers reduce the ore to 50 mm. Notably, the plant uses dry magnetic separation after crushing, eliminating the need for water in the initial stages—a significant environmental advantage in the Amazon region.
Result: The crushing circuit achieves a reduction ratio of 24:1 across three stages, with an energy consumption of 0.8–1.2 kWh/t (within the industry standard for hard iron ore). The project reduced operational costs by 20% compared to older Carajás plants due to the use of semi-mobile crushers that move closer to the mining face, reducing truck haulage distance.
5. Frequently Asked Questions (FAQs)
Q1: What is the difference between crushing and grinding?
A: Crushing produces particles from 150 mm down to 5 mm using compressive or impact forces. Grinding (using ball mills, SAG mills) reduces particles below 1 mm, often to 50–200 microns, using attrition and impact. Crushing is cheaper (lower energy per ton) and is always done first.
Q2: Why is a "closed circuit" used in secondary crushing?
A: In a closed circuit, the crusher product passes through a screen. Oversized material is returned to the crusher, while undersized material moves forward. This ensures the final product has a maximum size limit and reduces over-crushing (production of unwanted fines). It improves energy efficiency by 10–15% compared to an open circuit.
Q3: How do I choose between a cone crusher and an impact crusher for secondary duty?
A: Use a cone crusher for hard, abrasive rock (e.g., granite, quartzite) where wear cost is the main concern. Use an impact crusher for soft to medium rock (e.g., limestone, dolomite) or for recycling concrete, where you need a high percentage of cubical particles and can tolerate higher wear. As a rule: if the material's silica content is >15%, choose a cone.
Q4: What is the typical energy consumption of a crushing circuit?
A: For a primary jaw crusher, energy use is 0.2–0.5 kWh/t. For secondary and tertiary cone crushing, it is 0.5–1.5 kWh/t. Total crushing circuit energy is typically 1–2 kWh/t, which is only 10–20% of the total comminution energy (the rest is consumed by grinding mills)..jpg)
Q5: What safety measures are critical in a crushing plant?
A: The three critical measures are: (1) Isolation – lockout/tagout (LOTO) on all motors and hydraulic systems before maintenance; (2) Dust control – water sprays or baghouse filters to suppress silica dust (respirable crystalline silica is a known carcinogen); (3) Rock box / chute design – to prevent flying rock fragments from the discharge point. Additionally, modern plants use metal detectors before the crusher to prevent tramp iron damage.
6. Conclusion
The Britannica definition of crushing as a "mechanical reduction of material" is accurate but only scratches the surface. In industrial practice, "britagem" is a highly engineered, multi-stage process that balances particle size, energy efficiency, and wear costs. The choice of equipment—from massive gyratory crushers in Brazilian iron ore mines to compact impact crushers in urban recycling plants—depends entirely on the material's properties and the required end product. Understanding the stages and the trade-offs between compression and impact is the foundation of any successful mineral processing operation.