roller crusher product size distribution

Roller Crusher Product Size Distribution: An Overview

The product size distribution from a roller crusher is a critical performance metric that directly impacts downstream processing efficiency, product quality, and overall operational economics. This article delves into the key factors influencing this distribution, including crusher type, operational parameters, and feed characteristics. It will analyze typical output gradations, provide comparative data, and discuss optimization strategies to achieve desired sizing outcomes. A real-world case study from the aggregate industry illustrates practical application.

Factors Influencing Size Distribution

The fineness and shape of the output material are not arbitrary but are controlled by several interconnected factors:

1. Crusher Type and Design:

   • Smooth/Roll Crushers: Primarily produce a uniform size product with minimal fines through compression crushing. The product size is largely determined by the gap setting between the rolls.
   • Toothed/Dentated Roll Crushers: Employ a combination of compression, shear, and tearing action. This results in a broader size distribution, often with more intermediate-sized particles and controlled fines generation, suitable for breaking friable materials.

2. Operational Parameters:

   • Roll Gap: The single most important parameter. A smaller gap yields a finer product.
   • Roll Speed Differential: In dual-roll crushers, running rolls at different speeds enhances shearing action, altering the particle shape and distribution.
   • Feed Rate: An inconsistent or excessive feed rate can lead to improper nipping of material, causing uneven sizing and increased production of oversize particles.

3. Material Characteristics:

   • Feed Size Distribution: A well-graded feed is more likely to produce a well-graded product.
   • Moisture Content: High moisture can lead to material packing and clogging, disrupting the crushing process.
   • Abrasiveness & Hardness: These affect roll wear, which gradually increases the effective gap and coarsens the product over time if not compensated for.

Typical Product Size Distributions: A Comparative Analysis

While exact distributions are material-specific, general trends can be illustrated. The table below contrasts idealized output from smooth roll crushers with different settings and toothed roll crushers processing similar feed.

Table: Comparative Product Size Distributions for Different Roller Crusher Setups
| Crusher Type / Setting | Feed Size (mm) | Product PSD (Cumulative % Passing) |
| :--- | :--- | :--- |
| Smooth Roll Crusher (Coarse Gap) | 50-0 | ~100% passing 30mm, ~40% passing 10mm |
| Smooth Roll Crusher (Fine Gap) | 50-0 | ~100% passing 15mm, ~65% passing 5mm |
| Toothed Roll Crusher | 50-0 | ~100% passing 35mm*, ~85% passing 10mm |

Note: Toothed rolls can produce a slightly larger top size due to their grabbing action but generate more mid-range material. Data is illustrative based on industry handbook references (e.g., "Mineral Processing Plant Design, Practice, and Control").

Optimization and Control Strategies

Achieving target specifications requires active management:

• Regular Gap Checks & Adjustment: Manual or hydraulic systems must be used to maintain the set gap as wear occurs.
• Pre-Screening (Scalping): Removing fines from the feed before crushing increases capacity and reduces roll wear.
• Post-Crushing Screening (Closed Circuit): Returning oversize material from a screen to the crusher is the most effective method for tight control over top size and distribution.

Real-World Case Study: Limestone Aggregate Production

A quarry in the Midwest USA producing road base material faced inconsistency in its final product gradation from its tertiary crushing stage using an old hammer mill. The goal was to produce a consistent 0-19mm (0-3/4") product with reduced fines generation.

• Solution: Installation of a twin smooth roll crusher in closed circuit with a vibrating screen.
• Implementation: The roll gap was set at 22mm. All crusher discharge was fed onto a screen with a 19mm aperture. Oversize (+19mm) material was conveyed back to the crusher feed (closed circuit).
• Result: The product size distribution became highly consistent and predictable, meeting state DOT specifications reliably. Fines generation (-4.75mm) was reduced by approximately 15% compared to the previous system due to the compression-dominated breakage of the roller crusher versus impact breakage of hammer mills.

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Frequently Asked Questions (FAQs)

Q1: Can a roller crusher produce a one-size product?
No commercial crusher produces a true "one-size" product. Roller crushers—especially smooth roll types—can produce a relatively narrow size distribution band centered on the set gap dimension when fed with appropriately sized material in an open-circuit configuration.

Q2: Why does my roller crusher start producing more fines over time even with constant settings?
This is most commonly caused by progressive wear on the roll shells or teeth. As they wear down microscopically over time without adjustment of mechanical gap compensation systems like shims or hydraulic cylinders; effectively increases allowing slightly larger pieces through which may then be re-crushed creating additional fines within your process stream if you have recirculating loads present downstream via screening operations etcetera

Q3: What is better for minimizing fines: smooth or toothed rolls?
Generally speaking; Smooth rolls operating at correct speeds differentials generate fewer ultra-fines (<1mm) because they employ primarily compressive breakage which tends toward creating cracks along natural grain boundaries rather than pulverizing particles excessively as might occur from aggressive tearing/shearing actions associated heavily serrated tooth profiles found on some designs meant for sticky materials where grabbing action needed first before fracture occurs

Q4 How does feed size affect output gradation?
If fed excessively large pieces relative designed maximum input dimension; not only risks damaging equipment but also leads poor nip angle causing slippage inefficient crushing resulting wider unpredictable output ranges including unwanted oversized fragments Conversely consistently sized smaller feeds yield more uniform predictable products closer target specification

Q5 Is possible adjust product shape using roller crusher?
Yes Particle shape influenced significantly by type mechanism employed Smooth rolls under correct conditions tend yield more cubical particles with lower flakiness index compared jaw cone impactors Toothed rolls however may produce slightly elongated fragments due initial gripping tearing action though still typically superior many other primary secondary stages terms achieving desired angularity for applications like asphalt concrete where interlock important