How to Crush Stone: Primary, Secondary, and Tertiary Stages Explained
How stone crushing works stage by stage: primary jaw, secondary cone or impactor, tertiary finishing, compression vs. impact, and open vs. closed circuit.
How to Crush Stone: Stages, Machines, and Circuits
Crushing stone is a staged process. You reduce big rock in steps — primary, secondary, and sometimes tertiary — because no single machine takes a 30-inch boulder down to a 3/8-inch spec in one pass without choking or wearing out fast. Each stage has a job, a machine suited to it, and a handoff to the next. Understand the stages and you can size a circuit that actually makes the product you are trying to sell.
The three crushing stages
Primary crushing takes run-of-quarry rock and knocks it down to a manageable size — typically getting a 24- to 36-inch feed down to something in the 4- to 8-inch range. This is heavy, blunt work. The jaw crusher owns this stage: two plates, one fixed and one moving, squeezing rock by compression. Jaws are simple, tolerant of hard and abrasive stone, and hard to jam with the occasional tramp piece. A grizzly feeder ahead of the jaw scalps out the dirt and undersize before it ever hits the chamber, which protects the machine and boosts throughput.
Secondary crushing takes that 4- to 8-inch primary product and reduces it further, toward the 1- to 3-inch range. This is where you choose your tool by material:
- Cone crushers compress rock between a spinning mantle and a fixed concave. They are the standard secondary for hard, abrasive stone — granite, trap rock, basalt — and they make well-shaped, consistent product.
- Horizontal shaft impactors (HSI) throw rock against blow bars and aprons instead of squeezing it. They hit a higher reduction ratio in one pass and make a cubical product, which is why they are popular in softer stone, limestone, concrete, and asphalt recycling. The catch is wear — blow bars are consumables.
Tertiary crushing is the finishing stage when you need a lot of small, tightly spec'd product — think manufactured sand or a clean 3/8-inch aggregate. Cones or vertical shaft impactors (VSI) do this work, tightening the product to a narrow size band.
Not every job needs all three. A pit making road base might run jaw-to-screen and stop. A producer selling concrete aggregate and manufactured sand runs all three plus screening.
Compression vs. impact
The core choice at every stage comes down to how the machine breaks rock:
- Compression (jaw, cone) squeezes rock until it fractures. Lower wear, better on hard and abrasive material, more slabby product shape.
- Impact (HSI, VSI) shatters rock with velocity. Higher reduction per pass, better cubical shape, but faster wear and more fines and dust. Best on softer, less abrasive stone and recycled material.
Match the method to the rock. Running an impactor on hard, abrasive granite will eat blow bars and drive your cost per ton through the roof. Running a cone on wet, sticky, low-abrasion material can pack and slow you down. This is the same jaw-vs-impact-vs-cone decision that trips up a lot of first-time buyers.
Open circuit vs. closed circuit
How you handle the oversize coming off a crusher defines the circuit:
- Open circuit: Material passes through the crusher once and moves on. Simpler, cheaper, faster — but you accept a wider size range and some oversize in the product. Fine when the next stage or a screen will catch it.
- Closed circuit: A screen sits after the crusher. Anything larger than spec gets recirculated back through the crusher until it passes. This gives you a tight, on-spec product and is standard for finished aggregate. It costs throughput and adds a conveyor loop, but it is how you hit a hard size guarantee.
Most producers run their final stage in closed circuit — the recirculating load is what turns "roughly the right size" into a salable, spec'd grade.
Building the circuit for the product you sell
Start from the product, not the machine. Decide what grades your market buys — clean #57, dense-grade crusher run, a decorative 3/8-inch spread — and work backward to the stages and screens that make them. That usually means a primary jaw, a secondary cone or impactor, and a screen closing the loop, with conveyors moving material between them.
Sizing that spread — matching crusher capacity, screen area, and stockpile conveyors so no stage starves or backs up the others — is where a lot of money gets spent well or poorly. If you are piecing together a crushing and screening train, it is worth walking current crusher, screen, and conveyor listings against your target tonnage before you buy. RPG Equipment brokers this equipment nationwide and can help you match machines to the product you are trying to move.
Have questions about how to crush stone or spec a crushing circuit? Contact RPG Equipment — we work with buyers and sellers across the US and Canada.
RPG Equipment is a heavy equipment brokerage based in Worcester, MA specializing in grinders, crushers, screeners, and material processing equipment. Contact us to buy or sell equipment, or browse more articles.