Comminution is the size reduction stage of a mineral processing plant, where run of mine ore is broken down until the valuable mineral is exposed. The equipment gets most of the attention, and the circuit around it decides what that equipment can deliver. A mill rated for a given tonnage will not reach it when the feed arriving at the inlet is coarser than the design assumed. Circuit design and equipment sizing are the same problem approached from two directions.
A Circuit Is a System, Not a List of Machines
Every size reduction stage exists to hand the next stage a feed it can handle. Crushers reduce blasted rock to a size a mill can accept, and mills grind that product down to the particle size at which the mineral separates from the waste rock. Classification sits between and after those stages, returning oversize material and passing acceptable material forward. Break one of those handovers and the whole line runs below its rating.
Two numbers govern the conversation. F80 is the size at which 80 percent of the feed passes and P80 is the same measure applied to the product, and Bond related the specific energy a grinding circuit needs to the inverse square root of those two sizes. Every machine in the line has an F80 it is designed to receive and a P80 it is expected to produce. A design either honours those figures or it caps the plant.
Open and Closed Comminution Circuits
An open circuit passes material through a machine once and sends everything out the other side. A closed circuit screens or classifies the discharge and returns the oversize to the same machine, releasing only material that meets the target size. The recycled fraction is the circulating load, and it is a design input rather than a fault to be engineered out.
| Factor | Open circuit | Closed circuit |
|---|---|---|
| Product size control | Wide distribution, some oversize passes forward | Tight control against a target P80 |
| Overgrinding | Material stays in the machine after reaching size | Material leaves as soon as it is at size |
| Auxiliary equipment | No recycle screening or pumping needed | Classification and recycle handling required |
| Capital and layout | Fewer units, simpler plant footprint | More units, more instrumentation |
| Where it suits | Coarse duty and pre-crushing stages | Final grinding ahead of concentration |
MPT builds for the closed arrangement. Titan mills are offered with a mill sump and discharge trommel fitted with slotted wedge wire for scats removal, and the modular comminution circuit carries cyclone support steel with underflow piping routed back to mill feed. Those two details are what make the recycle path physical rather than theoretical.
What Are the Stages of Comminution?
Blasting starts the work and the plant takes over from there. Primary crushing handles run of mine rock and reduces it to a size the next stage can accept. Secondary and tertiary crushing carry that further where the flowsheet calls for it, and grinding takes the product down to liberation size. Not every plant runs all of them, and a single stage crushing circuit feeding an autogenous or semi autogenous mill is common where the ore allows it.
Primary Crushing Sets the Standard for Everything Downstream
Primary reduction decides the feed size every later stage inherits. Undersize that stage and the mill receives rock it was never designed to accept, which shows up as reduced throughput rather than as an obvious failure. Oversize it and capital sits idle while the plant pays to move material the circuit did not need reduced that far. Ore competency does the most damage here, because hard or abrasive feed drives liner and media wear before any other stage sees it.
Reduction ratio is the practical constraint, since a single machine can only take feed down so far in one pass. MPT builds rolls crushers with twin manganese steel lined rollers and adjustable gap settings, which is where the product size of that stage is actually set. Autogenous and Titan GDS scrubbers handle intensive washing and impurity removal for material that arrives bound up rather than simply oversized. The four lines in the comminution range cover crushing and scrubbing alongside two mill families.
What Are the Main Types of Comminution?
Two mechanisms do the work. Crushing applies compression to coarse rock held between two surfaces, and grinding uses impact and attrition between a tumbling charge and the ore. Hukki showed that the energy and size relationships behind them hold only over narrow size ranges, which is why crushers are not simply pushed finer and finer. Scrubbing is preparation of a different kind, separating agglomerated material without fracturing the mineral itself.
Mill Selection Follows the Ore, Not the Catalogue
Mill choice comes down to ore hardness and two numbers the rest of the plant hands it: the feed size the crushing stages will deliver and the P80 the concentration circuit needs. Harder ore demands more specific energy per tonne to reach the same product size, which is what a work index measures. That single figure moves mill diameter and installed power at the same time, and media consumption follows. Getting it from a test rather than an assumption is the difference between a circuit that meets its nameplate and one that never does.
The Titan gear driven range is manufactured in house in Benoni and comes in overflow, grated, peripheral discharge and rod mill configurations. The design is rated to carry a full process load with a 40 percent ball charge at 80 percent of critical speed, which is where measured mill power draw peaks in laboratory testing. Bi-directional operation is stated to extend liner life by up to 70 percent without losing performance, and a single modular skid base carries up to 1.3 MW. Grinding media selection belongs to the same decision, since media size and density set how energy is transferred to each particle.
What Sizing Does to Throughput and Energy
Energy demand rises steeply as the target particle size falls, so every micron of unnecessary grinding is paid for continuously over the life of the plant. Comminution accounts for close to 40 percent of total mine site energy use, and the grinding mill is typically the largest single consumer on the site. An oversized mill invites overgrinding, producing fines the downstream circuit cannot use and sometimes cannot tolerate. An undersized mill caps tonnage and pushes the circulating load up until classification becomes the bottleneck instead.
Power draw is the signal that tells you where the mill actually is. The Titan automatic ball charger runs on that principle: a PLC monitors mill power draw, and a deviation from setpoint releases weighed media into the feed hopper to restore it. Variable speed control on the drive gives the same kind of latitude on the other side of the equation, letting mill speed move as ore competency changes across zones.
Classification capacity is the constraint most often missed. A correctly sized mill in a closed circuit still fails when the screens handling its discharge cannot pass the recycle volume the design created. Screen area and aperture have to be sized against the circulating load rather than against nominal plant feed, and deck configuration follows from that. The same applies to the pumps and piping carrying recycle.
Testing a Circuit Before It Is Built
Ore changes over the life of a mine, and a circuit sized on one composite sample will meet its numbers only while the feed resembles that sample. Running representative ore through a pilot plant tests the flowsheet against real reduction ratios and real recycle behaviour before the capital is committed. Wear rates come out of the same exercise. It also exposes the handover points, where a stage that performs well in isolation starves or floods the one after it.
Modular plants built to this purpose have run from five tonnes an hour on trommel feed screening duty up to a 60 tonnes an hour scrubbing and screening plant on manganese in Gabon. Assembly and dry commissioning happen in the workshop before shipment, so the circuit is proven as a circuit rather than as a set of separately specified machines. A staged build also keeps the sizing decision open until more of the orebody has been tested.
Sizing the Circuit for the Ore You Actually Have
The questions that decide a comminution circuit are answerable before anything is fabricated: what the ore’s competency is and what P80 the concentration step requires. The cost of that grind, in energy per tonne for every year the plant runs, follows from those two answers. MPT designs and builds circuits against those answers rather than fitting equipment to a generic flowsheet, under a quality system certified to ISO 9001:2015. Send your ore characterisation and target throughput to the team through the contact page and the sizing conversation can start from data.