Requirements of Crushing Plant Systems for High-Grade Aggregate Production

1 Introduction
2 Related articles on Designing Buildings

[edit] Introduction

High-grade aggregates are used in applications where material properties such as particle size distribution, particle shape, strength, durability and cleanliness are important. These include concrete, asphalt, railway ballast and other forms of civil engineering and infrastructure construction.

Producing aggregate to a required specification generally involves an integrated system of feeding, crushing, screening, conveying, stockpiling and, where necessary, washing or other processing. The configuration of a crushing plant depends on the geological characteristics of the feed material, the required products, product ion capacity, site constraints and applicable environmental requirements.

Both fixed and mobile crushing plants can be used for aggregate production. Fixed installations are generally suited to long-term, high-capacity operations, while mobile or modular systems may provide greater flexibility where equipment needs to be relocated within a quarry or between projects.

[edit] Crushing and material processing

High-grade aggregate production commonly requires more than one stage of size reduction. A typical process begins with primary crushing of run-of-quarry material, followed by secondary crushing and, where required, tertiary or quaternary crushing. Screening between and after crushing stages controls the particle size distribution and allows oversize material to be returned for further processing.

The type and arrangement of crushers should be selected according to the feed material and required products. Jaw crushers are commonly used for primary crushing because they can accept relatively large feed sizes. Cone crushers are widely used for secondary and tertiary crushing of hard and abrasive rock, while impact crushers and vertical shaft impact crushers can be used where particular particle shapes or crushing characteristics are required.

Cone crushers generally crush material through compression. Depending on the crusher design and operating conditions, inter-particle crushing can occur when a suitable volume of material is maintained within the crushing chamber. This can contribute to particle shaping, but a cone crusher is not necessarily the most appropriate equipment for every aggregate specification.

A well-designed crushing circuit should maintain a consistent feed to each stage. Excessively variable feed rates can reduce throughput, increase wear and affect product consistency. Feed hoppers, vibrating feeders, scalping screens and surge capacity may therefore be incorporated to regulate material flow.

Pre-screening can also remove naturally occurring fines, soil or unsuitable material before primary or secondary crushing. This can reduce unnecessary crushing and wear while helping to maintain the required quality of the final aggregate.

[edit] Particle shape, grading and screening

Aggregate specifications may place limits on particle size distribution, flakiness, elongation, fines content and other characteristics. The required properties depend on the intended use and the relevant material standard or project specification.

Particle shape is influenced by the geological properties of the source rock and by the crushing process. A multi-stage circuit may be used to produce a more cubical product and reduce the proportion of flaky or elongated particles. However, the selection of crushing equipment should consider the complete process rather than assuming that a particular crusher type will always produce a specified particle shape.

The closed side setting of a crusher is one of several operating parameters that influence product size. Other factors include feed size, feed distribution, crusher speed, chamber configuration, material characteristics and the use of open- or closed-circuit operation.

Screening is essential for separating crushed material into specified size fractions. Multi-deck vibrating screens are commonly used to classify material and may be arranged so that oversize material is returned to a crusher for further processing. This closed-circuit arrangement can improve control over the final product size distribution.

Screening systems should be designed for the required throughput and material characteristics. Factors affecting performance include screen area, aperture size, vibration characteristics, feed distribution, moisture content and the tendency of material to block or blind the screen.

Typical aggregate products may be supplied in size ranges such as 0–5 mm, 5–10 mm and 10–20 mm, although the required fractions vary according to the applicable specification and intended use.

[edit] Fixed, mobile and modular crushing plants

A fixed crushing plant may be appropriate for a quarry or other operation with a long-term resource and relatively stable production requirements. Fixed installations can accommodate larger processing equipment, permanent material handling systems and substantial stockpiling capacity.

Mobile crushing plants can be relocated more readily and may be used close to the quarry face, reducing the distance that unprocessed material must be transported. They can also be used for temporary projects, construction materials recycling and operations where the processing location changes over time.

Mobile plants may incorporate primary or secondary crushers, screens, conveyors and other equipment. Their capacity and configuration are constrained by transport, weight and space requirements, and mobility should be considered alongside production capacity, maintenance access and site infrastructure.

Modular systems provide an intermediate approach, allowing individual process units to be transported and assembled in different configurations. They may be suitable where production capacity needs to be expanded or where a plant is expected to be relocated after a project has been completed.

The selection between fixed, mobile and modular systems should take account of:

Expected production capacity.
Project duration.
Characteristics and location of the material resource.
Required aggregate specifications.
Site access and available space.
Transport and relocation requirements.
Power and fuel availability.
Maintenance requirements.
Environmental constraints.

[edit] Material cleanliness and environmental control

Aggregate quality can be affected by excessive fines, clay, silt and other contaminants. The processing system should therefore be designed to achieve the required level of cleanliness.

Where the feed material contains significant quantities of fine or adherent material, washing, scrubbing, dewatering or other processes may be required. The suitability of water-based processing depends on the material, required product and availability of water, as well as arrangements for water treatment and management.

Dust is generated during crushing, screening, conveying and material transfer. Effective dust control is important for worker health, equipment reliability, environmental compliance and, in some circumstances, product quality.

Dust control measures may include:

Water spray systems.
Enclosures around crushers and transfer points.
Local extraction and dust collection systems.
Covered conveyors.
Appropriate transfer chute design.
Regular cleaning and maintenance.

The most appropriate method depends on the material, climate, process design and applicable environmental requirements.

[edit] Automation, monitoring and maintenance

Automation can improve the consistency and efficiency of aggregate production by monitoring and controlling the interaction between plant components. Control systems may regulate feed rates, crusher settings, conveyor operation and other process variables.

Monitoring systems can provide information about production rate, material flow, power consumption, vibration, temperature, hydraulic pressure and other operating conditions. This information can be used to identify abnormal operating conditions and support maintenance planning.

Wear of crusher liners, screen media and other components can affect production capacity, energy consumption and product characteristics. Maintenance should therefore include regular inspection and replacement of components before wear significantly affects plant performance.

Important maintenance considerations include:

Crusher liners and other wear parts.
Bearings and lubrication systems.
Screens and screen media.
Feeders and hoppers.
Conveyors and belt tracking.
Hydraulic systems.
Electrical and control systems.
Dust suppression and extraction equipment.

Predictive and condition-based maintenance techniques may be used alongside planned inspections and routine servicing. The objective is to maintain safe and consistent production while reducing unplanned downtime.

[edit] Energy efficiency and plant optimisation

Energy consumption is an important consideration in aggregate production. Crushing hard rock requires significant energy, and inefficient plant design can increase operating costs unnecessarily.

Plant efficiency depends on the interaction between all stages of the process. Oversized equipment, poor material flow, excessive recirculating loads, unsuitable crusher settings and unnecessary crushing can all reduce efficiency.

Measures that may improve overall performance include:

Matching equipment capacity between process stages.
Maintaining consistent feed conditions.
Removing unsuitable material before unnecessary crushing.
Using closed circuits where appropriate.
Controlling recirculating loads.
Maintaining crushers and screens in good condition.
Reducing conveyor spillage and unnecessary material handling.
Monitoring energy consumption and production rates.

The energy source may also influence plant selection. Fixed installations may use grid electricity where available, while mobile plants may use diesel engines, electric drives, hybrid systems or other arrangements depending on site conditions.

[edit] Planning for consistent aggregate quality

The production of high-grade aggregate requires consideration of the complete processing system rather than the performance of individual items of equipment. The characteristics of the source material should be established through appropriate geological investigation and testing before the crushing process is designed.

The required aggregate properties should then be defined according to the intended application and relevant standards or project specifications. Crushing stages, screening arrangements and any washing or additional processing can be selected to achieve these requirements.

A well-designed aggregate production system should provide:

Suitable feed preparation.
Appropriate crushing stages.
Controlled material flow.
Effective particle size classification.
Consistent particle shape where required.
Control of contaminants and excessive fines.
Adequate stockpiling and product separation.
Effective dust and environmental management.
Reliable maintenance access and procedures.
Monitoring and control of production quality.

The configuration may need to be reviewed as the geological characteristics of the source material change. Regular testing of aggregate products and monitoring of plant performance can help maintain compliance with the required specification while identifying opportunities to improve production efficiency.