Cutting Fines in Aggregate and Mineral Crushing Operations
Operators from the Pilbara to the Hunter Valley know that fines are more than a nuisance in the crushing process. They cut into product yield, drive up dust loads on site, and erode the margin on every tonne that leaves the plant gate. In Australia, where haul distances are long, labour rates are high, and compliance with state-level dust and noise regulations is strict, controlling fines generation is not optional. It is the difference between a plant that turns a profit and one that simply grinds through fuel and wear metal.
The conversation around fines often starts with the screen and ends with the stockpile. The truth sits in the middle, in the crushing chamber itself, where particle-on-particle action and reduction ratios combine to either preserve product shape or shatter it into unusable dust. Adjusting feed preparation, closed side setting, chamber geometry, and the way material is recirculated can pull the fines curve back into a profitable shape without sacrificing the throughput that production planners rely on.
Why Fines Drain Margin in Australian Operations
In iron ore circuits around Port Hedland and gold operations near Kalgoorlie, fines below about 6 mm often have to be sold at a discount or processed through a separate washing plant. In the Bowen Basin's coal preparation chain, excessive fines push raw coal towards the tailings stream, reducing yield on the coking product. The economic pain is not theoretical. For a mid-sized hard-rock quarry in regional New South Wales, dropping fines generation by just two percentage points can recover six figures in a year once wash water, screen media, and dust suppression are added back into the calculation.
Australian sites also carry operating costs that magnify the problem. Diesel on a remote Western Australian run is rarely cheaper than what a coastal plant pays, and the cost of running a water cart across a dusty footprint around Mt Isa adds up fast. The dust fallout in dry inland conditions can also breach site licence limits, which means a quiet tightening of the closed side setting is usually safer than running the crusher wide open and hoping the baghouse copes.
Fines generation is a function of three things: the energy applied per unit of mass, the number of compression events a rock experiences, and the competence of the feed. None of these are fixed by the machine alone. They are set by how the plant is configured upstream and downstream of the crusher. That is where operators have the most leverage.
Tighten the Feed Before It Reaches the Chamber
The cheapest fines are the ones never made. Anything done to remove already-fine material before it enters the crushing chamber pays back twice, because that material no longer consumes energy, no longer wears the manganese, and no longer ends up in the product pile at a discount. A scalping screen ahead of the primary jaw, set at a generous cut, pulls off the natural fines from the run-of-mine feed and feeds them straight to the product conveyor or a dedicated bypass.
In many Australian quarries, the scalping screen is the single most underused piece of kit. Plant managers often inherit a feed arrangement built when throughput was the only KPI, and a vibrating grizzly is treated as a glorified dirt catcher. Reversing that mindset, and giving the screen the same maintenance attention as the crusher itself, is a low-capital way to make the rest of the circuit behave. The wider the screen aperture relative to the crusher CSS, the less chance that a near-size particle is crushed twice.
Pre-crushing sizing also matters. Blasted material in the Pilbara typically tops out at 800 to 1000 mm; in a hard-rock quarry outside Brisbane, the ROM may be limited to 400 mm by the loader bucket. The same crusher will produce very different fines curves at these two feed sizes. Where the loader can be fitted with a bucket that limits top size, or where a mobile jaw is used to pre-break oversize, the downstream cone or impactor runs more gently. Less work, fewer fines, and a quieter plant that the local crew does not dread walking past on a forty-degree arvo.
Adjust CSS and Chamber Geometry with Intent
Closed side setting is the most direct handle an operator has on fines generation. Every extra millimetre of CSS is a millimetre where material can exit without being nipped, which sounds like it should mean more fines, but the relationship is more subtle. A CSS that is too tight forces the crusher to work harder on near-size material, creating flake and sliver. A CSS that is too wide allows the chamber to compact, with the bed of material acting as its own breaker plate and producing excessive fines in the bed layer.
The art is to match CSS to the feed grading curve, the desired product top size, and the crusher's eccentric speed. For a secondary cone in a basalt or granite operation, a CSS 4 to 6 mm tighter than the nominal product top size, with the right CSS through the liners as they wear, usually keeps the product inside spec without overworking the chamber. When operators explore trapezium mill options, they sometimes configure these units downstream to take the remaining fines load off the cone, but the upstream discipline still matters.
Chamber geometry plays an equally large role. Concave profiles, eccentric throw, and the angle of the chamber all change how particles break. A short-head cone in a tertiary role will produce fewer fines than a standard cone running at the same CSS, simply because the parallel zone is longer and particles are broken once rather than repeatedly. Closed-circuit product gradation depends on this geometry being right, because the screen can only reject what the crusher has already produced.
Screens, Returns and Closed-Circuit Logic
A crushing circuit is a system, not a chain of machines. Fines are generated in the crusher, but the screen decides what becomes product and what goes back around. If the return conveyor is overloaded, the crusher gets a mixed feed with material that was already in spec, and that material is broken again unnecessarily. The result is a fines curve that climbs sharply below the screen aperture.
A well-tuned closed circuit in Australia typically runs with a circulating load of 25 to 40 percent of new feed. Below that, the screen is not doing its job and oversize is sneaking into the product. Above that, the crusher is overworking near-size material and the fines line is climbing. Operators in the Hunter Valley often use load cells on the return belt as the primary control signal, trimming feeder speed to keep circulating load in band rather than chasing CSS adjustments after the fact.
Screen media choice is the quiet lever. A screen with a high open area, properly tensioned and not blinded, sends more near-size material back as oversize than a worn cloth that allows fine material to leak into the product stream. That leakage inflates the apparent fines percentage even when the crusher is behaving. A common audit on a Queensland hard-rock site is to compare the screen undersize gradation with the actual product gradation; if they differ by more than five percent at any size, the screen is the problem, not the crusher.
Lubrication and thermal stability of the crusher itself is part of the circuit logic as well. A cone running hot will drift CSS as the head lifts, and that drift will quietly push fines generation upward over a shift. How planetary gear thermals interact with oil flow shows the same principle in a closely related way, and the takeaway is the same: stable oil temperature means stable CSS, and stable CSS means a predictable fines curve.
Matching Crusher Type to Material Behaviour
No single crusher is the lowest-fines option for every material. The right answer depends on the work index, the feed grading, the moisture content, and the product spec. The comparison below outlines the main crusher types in common use across Australian quarrying and mining operations, with a focus on fines behaviour.
| Crusher type | Typical role in circuit | Fines tendency | Best-fit material | Key operating note |
|---|---|---|---|---|
| Single-toggle jaw | Primary, open or closed circuit | Moderate to high when CSS tight | Hard rock, abrasive ore | Keep CSS at 1.5 to 2 times product top size to avoid slab |
| Cone (standard) | Secondary | Moderate | Granite, basalt, iron ore | Run with correct feed distribution; avoid segregated feed |
| Cone (short-head) | Tertiary, in closed circuit | Low to moderate | Hard rock, manufactured sand feed | Parallel zone length controls flake; use correct liner profile |
| Horizontal shaft impactor | Secondary or tertiary on soft rock | High unless feed controlled | Limestone, low-AI ores | Add a curtain gap; reduce rotor speed to drop fines |
| Vertical shaft impactor (VSI) | Sand making, tertiary shaping | Tunable, can be very low with rock-on-rock | Manufactured sand, shaping | Rock-on-rock is lowest fines; cascade mode increases them |
| Hammer mill or hammer crusher | Tertiary on soft to medium rock | Very high | Coal, soft limestone, gypsum | Limit to applications where product spec tolerates fines |
For most Australian hard-rock operations chasing a low-fines product, the combination is a primary jaw, a secondary standard cone, a tertiary short-head cone, and a final VSI in rock-on-rock mode for shaping. The VSI gives the operator a final tool to pull down the fines percentage by tuning the rotor tip speed and the cascade feed ratio, which is harder to do on a cone.
The trap is treating the table as a rigid answer. Material competence changes with every blast, and operators in regions such as the Yilgarn or the Tanami have learned to read the rock before trusting the setting sheet. A competent ore wants a different chamber profile than a weathered one, and the fines curve will tell the truth within a few hours either way. The best Australian sites are the ones that adjust CSS and feed distribution daily, rather than waiting for a liner change to fix what the operators could have managed with the controls they already have.