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By Ma Junxin, Zhengzhou Zhongji Machinery
For a small or medium hard-rock project, equipment selection often begins with a production target. A plant may be designed for 50 tonnes per hour, 100 tonnes per hour or 200 tonnes per hour. That figure is important but it does not by itself define the duty of a crushing and grinding circuit.
A jaw crusher and ball mill can both appear adequately sized on paper while the plant struggles once material starts moving through the full circuit. In many cases, the problem is not the capacity of an individual machine but how the stages interact and the assumptions made between them.
The starting point is the feed. Maximum lump size establishes the basic opening requirement for the primary crusher, but average size and feed distribution are equally important. A feed consisting mainly of coarse rock behaves differently from one containing a significant proportion of fines. Moisture and clay can alter the duty further, particularly where material becomes difficult to feed, screen or transfer when wet.
A useful feed description should therefore cover normal size distribution, fine content, moisture and expected variation across the orebody. These factors influence the feeder, crusher, screen and conveyors before material reaches the grinding stage. The next question is what the crushing circuit must deliver to the mill. This is where equipment selection can become too fragmented. The crusher is selected first and the mill considered later, even though the two stages should be evaluated as part of the same circuit.
A ball mill does not receive “crusher capacity”. It receives a defined tonnage with a particular size distribution. If the crushed product is consistently too coarse, the mill must use more energy to achieve the required grind. Wide variations in feed size can also make mill operation less stable. The objective, therefore, is not simply to determine whether the primary crusher can produce the required tonnes per hour. The crushing and screening circuit must provide a suitable and reasonably consistent mill feed.
Depending on the ore and target grind, this may require secondary crushing, screening and recirculation. In other applications, a simpler arrangement may be sufficient. Screening is particularly important at this interface. A screen determines which material moves forward and which returns for further size reduction. Once recirculation is introduced, the internal tonnage through parts of the plant can exceed the fresh-feed rate.
For example, a circuit receiving 100 tonnes per hour of fresh ore may require the secondary crusher, screen and associated conveyors to handle substantially more than 100 tonnes per hour because oversize material is circulating within the circuit. Ignoring this circulating load can leave a crusher apparently well sized while the screen or conveyor becomes the actual bottleneck. The same principle applies to grinding. A ball mill should not be selected on tonnes per day alone. Feed size, target grind size, ore hardness, effective operating hours and the classification system all contribute to the actual duty.
A plant producing a fine product for downstream processing places a different demand on the mill from one requiring a relatively coarse grind. This is why catalogue capacity and performance on previous projects should be treated as useful references rather than guarantees of performance on a new ore. Testwork becomes important where key parameters cannot be reliably established from project data. Ore hardness and grindability are two obvious examples. Two ores that appear similar can require significantly different amounts of grinding energy.
Recovery should also remain distinct from mechanical equipment selection. Crushing and grinding prepare material for the downstream process, but they do not determine whether copper, gold or another mineral will achieve a particular recovery. That outcome depends on mineralogy and the selected metallurgical process. Mechanical duty can be defined at an early project stage while metallurgical performance is confirmed through appropriate testing.
Operating hours are another consideration that can materially change equipment sizing. A project targeting 1,000 tonnes per day does not necessarily require the same hourly capacity in every operating scenario. A plant achieving 20 effective operating hours per day has a different hourly duty from one realistically operating for 12 or 16 hours after maintenance, stoppages, feeding interruptions and shift changes are considered.
Daily production targets should therefore be translated into realistic hourly requirements before equipment is selected. For a small hard-rock project, the more useful early design question is not which crusher and ball mill to buy. It is what each stage must deliver to the next. Once feed conditions, crusher discharge, screening duty, circulating loads, mill-feed size, operating hours and grinding targets are clearly defined, equipment selection becomes more reliable.
The machines matter but it is how they are matched as a system that ultimately determines whether the plant performs as intended.
Author Bio
Ma Junxin represents Zhengzhou Zhongji Machinery, a China-based manufacturer of crushing, screening, grinding and conveying equipment for mining and quarry applications.
Website: https://zhengzhouzhongji.com/
