Crusher Choke Feed Oversize Throughput Loss Case Study

A crushing circuit can lose net throughput while every major item appears to be running. The feeder moves ore, the crusher draws power, the screen vibrates, and the downstream conveyor carries material. The production loss appears only when engineers separate gross internal circulation from useful product leaving the circuit.

This case study follows a secondary crusher and screen circuit that cannot supply the grinding plant at the required rate. The first explanation is that the mine is not delivering enough ore. The evidence shows a different constraint: the crusher is often not choke fed, the closed-side setting has drifted open, the screen is returning excessive oversize, and recirculating load is consuming capacity that should have become net product.

The purpose is to show how a mining or mineral processing engineer connects crusher level, screen oversize, recirculating load, closed-side setting, power draw, particle-size evidence, and validation before deciding whether to change feed rate, adjust the crusher, clean the screen, or modify upstream fragmentation.

Case Context

The plant uses a secondary cone crusher in closed circuit with a vibrating screen. Screen undersize feeds the grinding plant. Screen oversize returns to the cone crusher. The circuit target is:

as dry screen undersize to the grinding circuit. During the problem shift, the circuit sustains only:

The crusher is not tripping. The crusher motor is loaded. The screen is operating. However, the recirculating oversize conveyor runs heavily, crusher level is unstable, and product-size samples show too much material near and above the screen aperture.

Item	Normal or target value	Current event value
Net screen undersize to grinding	650\ \text{t/h}	520\ \text{t/h}
Screen oversize recycle	180\ \text{t/h}	420\ \text{t/h}
Crusher motor power	560\ \text{kW}	610\ \text{kW}
Cone crusher closed-side setting	18\ \text{mm}	24\ \text{mm} measured
Time with stable choke level	more than 85\%	about 62\%
Screen aperture	22\ \text{mm}	22\ \text{mm}
Screen deck condition	clean	partial blinding and pegging
Product P80	16\ \text{mm}	24\ \text{mm}

The values are simplified for education. A real crusher review must use the equipment manual, liner profile, crusher speed, chamber design, feed gradation, ore hardness, moisture, tramp protection, screen media condition, structural and guarding requirements, and site-specific safety controls.

Event Evidence

Operations report that the crusher feed bin is not empty. That rules out simple mine starvation as the only explanation. The relevant observations are:

1. crusher level alternates between high surges and low-level operation;
2. the closed-side setting measured during shutdown is larger than the operating target;
3. the screen oversize conveyor carries much more material than usual;
4. product P80 is coarser than the grinding plant requires;
5. crusher power is high relative to useful net product;
6. screen panels show near-size pegging and clay-rich blinding.

The question is not whether rock passes through the circuit. It does. The question is whether enough rock leaves the circuit as acceptable product.

Net Throughput Shortfall

The immediate production shortfall is:

Relative to target:

Over a 20\ \text{h} operating day, the lost grinding feed is:

This is large enough that treating the issue as ordinary shift variability would hide a real circuit constraint.

Recirculating Load Screen

For a closed crusher-screen circuit, a useful operating metric is recirculating load ratio:

where:

• R is screen oversize returning to the crusher;
• P is screen undersize leaving as net product.

Normal operation:

Current operation:

The screen feed, ignoring minor losses and inventory changes, is:

Normal screen feed:

Current screen feed:

The circuit is therefore pushing:

more material across the screen while producing:

less useful product. That pattern is typical of a closed circuit trapped in internal recirculation.

Oversize Fraction

The screen oversize fraction is:

Normal:

Current:

Nearly half the screen feed is being returned as oversize. That is not a small measurement fluctuation. It is consistent with a crusher product that is too coarse, a screen that is not passing near-size material efficiently, or both.

Choke-Feed Effect

Cone crushers are normally more stable when the chamber remains properly filled. A stable choke feed helps maintain inter-particle breakage, consistent residence time, and predictable product size. Poor feed control can create segregation, low bed depth, uneven liner loading, and a coarser product.

Shift historian data show:

Crusher level state	Fraction of operating time	Estimated net product mode
stable choke feed	62\%	650\ \text{t/h} equivalent
low or surging feed	38\%	430\ \text{t/h} equivalent

A simple weighted throughput screen is:

This does not exactly equal the observed 520\ \text{t/h} because screen blinding, CSS drift, feed gradation, and measurement uncertainty also contribute. It is close enough to show that unstable chamber level can account for a large part of the shortfall before any mechanical failure is assumed.

Closed-Side Setting Drift

The closed-side setting controls the minimum crusher discharge opening. A drift from 18\ \text{mm} to 24\ \text{mm} is:

Relative to the target:

An opening that is one-third larger than target can send more near-size and oversize material back to the screen. The measured product P80 moved from:

to:

The product P80 is now larger than the 22\ \text{mm} screen aperture. That is physically consistent with elevated recycle and grinding feed that is too coarse.

Power and Specific Energy

Crusher motor power alone can be misleading. A heavily loaded crusher is not necessarily producing useful product. A simple net-product specific energy is:

where P_{motor} is in kW and \dot{m}_{product} is in t/h. The units reduce to kWh/t.

Normal condition:

Current condition:

The increase is:

The motor is doing more work per tonne of useful final product. That supports the recirculation diagnosis: energy is being spent on material that returns to the crusher instead of leaving as acceptable product.

Corrective Action

The correction should not be “push more feed” until the internal circuit constraint is removed. Extra feed can overload the screen, increase recirculating load, raise crusher stress, and reduce product quality.

The corrective package is:

1. Reset and verify closed-side setting to the approved value after checking liner profile and hydraulic setting calibration.
2. Restore stable choke feed by tuning feeder control, removing surge triggers, and maintaining a minimum crusher level band.
3. Clean or replace blinded and pegged screen panels; confirm aperture, deck tension, vibration, and spray or dust-control condition.
4. Temporarily cap fresh feed until recirculating load falls below the release threshold.
5. Check feed gradation and oversize boulders from blasting or primary crushing; update the upstream fragmentation trigger if large rocks are driving surge operation.
6. Add alarms for high recirculating load, crusher low-level operation, CSS drift, high net-product specific energy, and screen oversize fraction.
7. Review crusher bearing temperature, vibration, tramp events, liner wear, and drive torque before returning to full-rate operation.

These actions separate symptoms from causes. A truck-dispatch change or conveyor speed increase will not fix a crusher chamber that is not producing the right size distribution.

Validation Package

The release test should prove that net product, size, energy, and mechanical condition all returned to the operating envelope.

Throughput and Recycle

Set acceptance targets:

and:

At target product rate, the maximum permitted recycle is:

This is higher than the historical 180\ \text{t/h} but far below the event value of 420\ \text{t/h}. It gives room for ore variability without accepting a trapped recirculating-load condition.

Screen Feed Check

At release threshold:

Compared with normal:

This is a plausible screen load increase if the screen is clean and product size is controlled. It is not the 13.3\% overload observed during the event.

Specific Energy Check

If corrected operation draws:

at:

then:

That is close to the normal value of 0.862\ \text{kWh/t}. The release should still compare energy against ore hardness, feed size, moisture, and liner condition before treating it as a universal target.

Size and Mechanical Evidence

The circuit should also demonstrate:

1. product P80 below the grinding feed limit for the ore campaign;
2. measured CSS within tolerance before and after the trial;
3. stable crusher level above the minimum choke-feed band for the required fraction of time;
4. screen panel inspection with no unacceptable pegging or blinding;
5. belt-scale reconciliation around feed, recycle, and product streams;
6. crusher bearing temperature, vibration, and torque within alarm limits;
7. operator handover notes explaining the temporary feed cap and release criteria.

Engineering Decision

Do not declare the circuit recovered because the crusher motor is loaded or because fresh feed briefly reaches target. The release decision should be based on useful product leaving the circuit.

For this case, the correct decision is:

Hold the circuit below full fresh-feed target, restore CSS and choke feed, clear screen blinding, validate recirculating load below threshold, then release full-rate operation only after net product, P80, specific energy and mechanical condition are stable.

The main lesson is that closed crushing circuits can hide poor performance inside internal recycle. Engineers should distinguish gross tonnage moved through equipment from net compliant product delivered to the next process.