Primary, Secondary, and Tertiary Crushing: Explained

What is primary crushing?

Primary crushing reduces large rock to manageable size. A jaw or gyratory crusher is used. The feed size can be 600-1200 mm. The product after primary crushing is 80-300 mm. Why: this makes handling and transport easier.

Key numbers.

• Reduction ratio: 4:1 to 8:1. – why: sets expected product size.
• Feed size: up to 1020 mm. – why: matches jaw inlet capacity.
• Throughput: 50-1500 t/h depending on model. – why: capacity must match mine.

Machine basics. A jaw crusher uses compressive force. The movable jaw presses against the fixed jaw. The rock breaks. CSS (closed side setting) and OSS (open side) define product size. CSS on a jaw is the distance between liners at the bottom. Why: CSS controls product gradation.

What is secondary crushing?

Secondary crushing reduces primary product. Cone crushers, impact crushers are common. The target size is 10-80 mm. Why: this prepares material for tertiary stage or sale.

Key parameters.

• Reduction ratio: 2:1 to 4:1. – why: ensures controlled fines.
• CSS range: typical cone adjusts 6-40 mm. – why: controls final gradation.
• Motor power: 75-400 kW typical. – why: matches torque requirement.

Cone crusher principle. The mantle revolves eccentrically. Rock is crushed in the chamber. OSS and CSS vary during cycle. CSS strongly affects capacity and gradation.

What is tertiary crushing?

Tertiary crushing polishes product. It gives final size and shape. Machines include high-speed cone, vertical shaft impact (VSI), and roll crushers. The output is 0-10 mm for sand. Why: it meets concrete and asphalt specs.

Key numbers.

• Target product: 0-5 mm for manufactured sand. – why: required for fine grading.
• Throughput: 20-800 t/h. – why: depends on prior stages.
• Energy: often higher per ton than primary. – why: finer comminution needs more energy.

VSI action uses high speed and rock-on-rock crushing. This gives cubical particle shape. Why: improves concrete workability and strength.

Core technical terms and how they matter

CSS. Closed side setting. It is minimum gap in crusher. It controls top product size. OSS. Open side setting. It is maximum gap. Reduction ratio. Feed size divided by product size, ratio shows crushing depth. Throughput. Tons per hour passing machine. Power match. Motor horsepower and gearbox adequacy. Why: wrong match reduces life and raises costs.

Typical reduction rules. Jaw 4-8:1. Gyratory 6-10:1. Cone 3-6:1. Impact depends on rotor speed. Why: guides stage planning.

Equipment list and parameter table

Example plant for hard rock basalt. It shows typical machine sizes and performance.

Why these choices. Jaw for big feed. Cone for mid-size, stable gradation. VSI for shape and fines control.

Performance data: energy, downtime, maintenance

Measured energy for crushing stage ranges 0.5-3.5 kWh/t. Hard ores push the number higher. Why: hardness and feed size matter.

Typical wear and uptime.

• Wear parts life: 500-2000 hours for liners. – why: depends on abrasiveness.
• Planned major service: 1000-3000 hours. – why: bearing checks, liners, seals.
• Unplanned failure rate: 1-5% annually in well run plants. – why: caused by tramp metal or poor feed distribution.

Maintenance cycles. Daily checks for oil and belts. Weekly for lubrication points. Monthly for full inspection. Why: reduces unplanned downtime.

Two verified project cases

Case A: Limestone plant, east China. Client needed 450 t/h for road base. Plant design: C6X jaw primary, CI5X impact secondary, VU sand making tertiary. In trial run product met grade. Customer note: installation finished in 30 days. Operator report: stable output and low vibration. Why: correct machine match and feed control solved bottleneck.

Project details.

• Feed: 350 mm limestone. – why: suits jaw inlet.
• Output: 0-31.5 mm, 31.5-63 mm blends. – why: met highway spec.
• Downtime first year: 2.4%. – why: proper spare parts plan.

Case B: Basalt road aggregate, SE Asia. Client required 500 t/h. Plant: HPT cone plus VSI6X sand system. Result: round-the-clock production for three months trial. Client comment: better particle shape, lower cement usage in concrete mixes. Why: VSI improved shape.

Selection guide and decision tree

Step 1. Define feed size and hardness. Why: feeds size limits crusher choice.

Step 2. Set target product sizes. Why: determines stages needed.

Step 3. Choose primary machine by intake. Why: prevents choke.

Step 4. Match motor power to torque needs. Why: avoids motor stall.

Decision points.

• If feed > 800 mm use gyratory or heavy jaw. – why: needed capacity.
• If final product < 5 mm use VSI or fine cone. - why: to get cubical sand.
• If abrasiveness high increase liner change frequency. – why: to control wear cost.

Installation and maintenance practical steps

Installation. Lay foundation to spec. Align shaft and motor carefully. Why: prevents vibration and premature wear.

Start-up. Check lubrication. Run empty for 30 minutes. Then feed gradually. Why: avoids liner shock.

Maintenance. Keep spare liners and belts on site. Record hours daily. Why: speeds repairs.

Cost and ROI notes

Capital cost varies with capacity. Operating cost depends on energy and wear. Why: both determine cost per ton.

Improve ROI by:

• Optimizing CSS and feed gradation. – why: reduces overcrushing.
• Scheduled lubrication. – why: extends bearing life.
• Using proper liners. – why: lowers spare part spend.

Common user questions

Q1: How to choose CSS for cone crushers?

Start with vendor table. Test with sample feed. Reduce CSS stepwise. Why: prevents sudden overload. Check product gradation each change. Use closed loop control if possible.

Q2: What is realistic energy per ton?

Expect 0.5-3.5 kWh/t across stages. Soft rock lower. Hard ore higher. Why: comminution energy rises with hardness and feed size.

Q3: How often replace liners?

Replace when wear reduces capacity or changes product. Typical life 500-2000 hours. Why: worn liners lower throughput and change shape.

Staged crushing is predictable. Choose machines by feed, hardness, and product. Monitor CSS, power draw, and wear. Why: these control cost and output. For tailored plant design contact SBM technical team. They supply matched machines and field support.