Sizing a primary crusher: feed size, throughput, reduction

A badly sized primary crusher shows itself quickly: either it blocks because the lumps do not fit, or it runs half empty because it was bought oversized "to be safe," or it becomes the site bottleneck as soon as trucks arrive in bunches. All three mistakes come from the same place: sizing on averages, when a primary is sized on extremes — the largest lump and the peak throughput.

This article explains which inputs drive the choice, in what order, and what to send the manufacturer to get a proposal whose assumptions hold.

1. The largest lump sets the opening

The first input is not throughput: it is the size of the largest lump that will reach the feed. It depends on blasting (fragmentation), on the shovel or loader, on the truck, and on what a grizzly or scalper retains upstream. It is not derived from an average: it is measured on the actual rock, or set as an operating rule the site commits to holding.

Manufacturers commonly cite a rule that the largest lump must stay well below the feed opening — often expressed as a fraction of that opening — so the lump engages instead of bridging. The exact figure varies by crusher type and manufacturer; they state it for their model, and it is that figure you check against your maximum lump, not your typical one.

That rule has a direct consequence: reducing the largest lump — through blasting or an upstream grizzly — sometimes lets you drop one crusher size, with the foundation, building and budget that go with it. That trade-off is made before choosing the model, not after.

2. The feed curve and the target product set the reduction

Two size distribution curves bracket the crusher: the feed (with its F80, the size that 80% of the mass passes) and the expected product (P80). The ratio between the two is the reduction asked of the primary.

A primary — jaw or gyratory — delivers a limited reduction in a single pass. Asking for more means either setting it too tight (capacity drops, wear rises) or accepting a coarser product than planned and pushing the work onto the secondary. So "what product does the downstream circuit expect?" is asked at the same time as "which crusher?", and the answer sets the setting (CSS or OSS depending on the type) around which the manufacturer tabulates capacity.

An often-forgotten point: the share of fines already in the feed. Material with a lot of fines passes partly uncrushed; upstream scalping that bypasses them around the crusher frees capacity and reduces wear. Conversely, wet, clayey material packs — and no capacity table accounts for that without correction.

3. Peak throughput, not average throughput

The site's nominal throughput (tonnes per hour of production) is not what the primary sees. Three factors distort it:

• Utilization: the hours during which the crusher actually produces, compared with calendar hours — planned stops, blockages, waiting for trucks, maintenance. Annual tonnage divided by the hours actually available gives an hourly rate well above the "average."
• Feed peaks: on a truck-fed primary, arrivals are not continuous. Two trucks dumping back to back create a peak the dump hopper and the crusher have to absorb. Sizing is done on that peak, or on a hopper and feeder that smooth it.
• Planned growth: if the mine plan calls for a ramp-up in three years, that rate is what sizes the crusher — or you accept replacing it later, which is planned inside a shutdown window, not in an emergency.

Manufacturers' capacity tables are established for a reference rock (given density, size distribution, moisture). For your rock they are adjusted with correction factors: bulk density, hardness, moisture, share of fines. A tabulated capacity read without those corrections is an optimistic capacity.

4. The rock sets the type and the wear

Compressive strength, abrasiveness (a laboratory-measured index, not an estimate), moisture, clay content: these four inputs steer the crusher type — compression for hard, abrasive rock, impact for soft rock — and the cost of wear parts over the service life. The Bond work index (Wi) adds an indication of the fragmentation energy required, useful to check installed power.

That data usually exists in the project's geological study or metallurgical testwork. When it is missing, a laboratory test on a representative sample costs little compared with the decision it informs.

5. The crusher is not sized alone

A primary is the centre of a system, and each element of that system has its own sizing:

Forgetting one of these at selection time means discovering it at installation time — when it costs the most.

The mistakes that cost a crusher

• Sizing on average throughput and finding the bottleneck at the first peak.
• Choosing the opening "just big enough" to save one size, and living with daily blockages.
• Reading a tabulated capacity without correcting for density, moisture or fines.
• Not planning access for wear-part changeouts, or lifting capacity.
• Dealing with the structure and foundation after signing the purchase order.

What to send to get a serious proposal

1. The largest lump size (measured or set as a rule) and the feed size distribution curve.
2. The target product (P80 or desired setting) and what the downstream circuit expects.
3. Annual tonnage, actual operating hours, feeding mode (trucks, loader, conveyor) and peaks.
4. The rock: strength, abrasiveness, moisture, clay, Bond index if available.
5. The site: space, height, existing structure, available power, climate, shutdown window for installation.

With those inputs, the selection — type, model, setting, associated equipment — is made with written assumptions, and the proposal can be compared with another on identical grounds. That is the work our equipment sales and supply service covers and, where the project warrants it, a feasibility study that places the crusher in its complete circuit. To get started, tell us about your feed and your throughput.

Frequently asked questions

Should I allow a capacity margin? Yes, but a reasoned one: the margin that covers feed peaks and the planned ramp-up, not an arbitrary factor. An oversized crusher runs at a low fill ratio, which degrades product shape and cost per tonne.

Can the peak be absorbed by the hopper rather than the crusher? Partly. A properly sized dump hopper and feeder smooth truck dumps; the crusher is then sized on a steadier rate. The trade-off is between hopper size and crusher size.

What if the rock data is missing? Have a representative sample tested (strength, abrasiveness, Bond) before choosing. A proposal made without that data should say so explicitly and state the assumptions used.