crusher and screen behaviour

Crusher and Screen Behaviour: Key Interactions in Aggregate Processing

The efficient operation of an aggregate production plant hinges on the interdependent behaviour of its primary components, notably the crusher and the screening unit. Their performance is not isolated; rather, it forms a closed-loop system where the output of one directly dictates the efficiency and output of the other. Understanding this interplay is critical for optimizing yield, product quality, and overall plant throughput.

The Crusher’s Role: Creating a Size Distribution

A crusher’s primary function is to reduce the size of incoming feed material. Its “behaviour” is characterized by parameters such as:

• Reduction Ratio: The ratio of feed size to product size. Higher ratios increase throughput but may produce more fines.
• Crusher Type and Settings: Jaw crushers typically produce a broader, more slabby product. Cone crushers, depending on their cavity design and CSS (Closed Side Setting), can be tuned for finer or coarser products. Impact crushers tend to produce a more cubical product but generate a higher proportion of fines.
• Feed Characteristics: The hardness, abrasiveness, moisture content, and initial gradation of the feed material significantly influence crusher wear, power draw, and output gradation.

The crusher does not produce a single-sized product. Instead, it generates a size distribution, often following a rough logarithmic pattern. This output contains both desired product sizes and unwanted oversized material (“scalping”) or excessive fines.

The Screen’s Role: Classification and Feedback

The screening unit acts as a classifier. Its behaviour is defined by:

• Separation Efficiency: No screen is 100% efficient. Some undersize material will report to the oversize fraction (“carryover”), and some oversize may contaminate the finished product (“pegging” or “blinding”). Efficiency depends on screen aperture shape/size, deck inclination, vibration mechanism (e.g., circular vs. linear), feed rate, and moisture content.
• Deck Configuration: Multiple decks allow for simultaneous separation into several specific size fractions (e.g., 0-5mm, 5-20mm, 20-40mm).

The most critical behavioural aspect of the screen is its function within the circuit:

• Pre-Screening (Scalping): Removing fines from the crusher feed increases crusher efficiency by reducing packing and wear.
• Post-Crushing Classification: Directing undersize material to final product stockpiles.
• Closed-Circuit Operation: Routing oversize material back to the crusher for further reduction. This is where the most dynamic interaction occurs.

System Behaviour in a Closed Circuit

In a typical closed-circuit configuration (e.g., a secondary cone crusher with a return feed), the behaviour becomes cyclical:

1. Crusher discharge lands on the screen.
2. The screen separates material at a predetermined cut point (e.g., 40mm).
3. Oversize (+40mm) is returned as “circulating load” to the crusher feed.
4. Undersize (-40mm) is sent to final product.

The key metric here is the Circulating Load (CL)—the ratio of returned material to fresh crusher feed. A moderate CL (typically 100-200%) is generally beneficial; it ensures the crusher operates with a full chamber for effective inter-particle crushing and provides a cushion against wear. However, excessively high CL leads to:

• Crusher overloading and increased power draw.
• Reduced effective throughput as the circuit churns material.
• Accelerated wear on conveyors and the crusher itself.

Conversely, too low a CL may indicate an overly large crusher setting or inefficient screening, leading to oversized material in the final product.

Empirical Observations and Operational Adjustments

Plant operators monitor this behaviour through:

• Gradation Analysis: Regular sieve tests on crusher discharge and screen products provide objective data on size distribution changes.
• Power Monitoring: A rising amp draw on the crusher motor often signals an increasing circulating load or harder feed material.
• Visual Inspection & Sound: Changes in screen sound (e.g., slapping from oversized material) or visible blinding of screen panels indicate immediate issues.

Adjustments are made based on this feedback:

• If final product contains too much oversize: The CSS of the cone/impact crusher may be reduced OR screen media may be checked for damage/pegging.
• If excessive fines are produced: The CSS may be opened slightly OR feed rate/material characteristics should be reviewed.
• To manage circulating load: The feeder rate to the crusher or screen stroke/speed may be adjusted.

Conclusion

The behaviour of the crusher and screen is fundamentally synergistic. The crusher defines the potential size envelope, while the screen controls what fraction of that envelope becomes product versus recycle. Optimal plant performance is achieved not by maximizing individual component output in isolation, but by carefully balancing their interactions through measured adjustments based on gradation data and system feedback loops. Effective management of this relationship minimizes recirculation waste energy while maximizing production of in-specification aggregate—a core objective in mineral processing economics