Crushing and Screening Circuits Explained

Crushing and screening is the first size-reduction stage in a mineral processing plant. You'll size run-of-mine ore down to a product that grinding circuits can handle economically. A crushing circuit normally has primary, secondary and tertiary duties, and screens close the circuit to control product top size. The closed-side setting (CSS) on each crusher is the main lever you have on that product size. Before you specify any equipment, you need to understand how open and closed circuits, reduction ratio, feed size distribution, moisture and clay all interact.

That's why a crushing and screening circuit isn't just a list of machines. It's a system. The choices you make here ripple through every mineral processing flowsheet downstream. Get the top size wrong and the grinding mill pays for it every hour.

Primary crushing takes the largest feed. You'll see a jaw or gyratory crusher here, often fed directly from the mine or a coarse ore stockpile. Its duty is to reduce blast rock to a size that secondary crushers can accept without bridging. A jaw crusher's moving plate compresses rock against a fixed plate, while a gyratory crushes between a rotating mantle and a stationary concave. The choice between them depends on your feed top size, capacity, and whether the ore is slabby or sticky.

Secondary crushing follows. It reduces the primary product to a finer feed for screening and tertiary crushing. A cone crusher is the common choice. The secondary stage usually runs in open circuit if the product is already fine enough, or in closed circuit if a screen is placed ahead of or behind it. The secondary crusher's CSS is usually tighter than the primary's, and its reduction ratio is lower because the feed is already smaller.

Tertiary crushing is the final stage before grinding. It receives screened feed and delivers a product with a tightly controlled top size. In many flowsheets, the tertiary crusher operates in closed circuit with a vibrating screen. The screen removes product at the desired cut and returns oversize to the crusher. This is the workhorse stage that sets the P80 for downstream ball mills. You'll often set the tertiary CSS to a value slightly finer than the screen aperture, because the screen cannot perfectly separate near-size particles.

Reduction ratio refers to the ratio of the largest feed particle size to the largest product particle size. It is a dimensionless number. For a primary jaw crusher, the reduction ratio is in a range you'll confirm with the manufacturer for your ore. Secondary and tertiary crushers operate at different ratios because each stage handles progressively smaller feed.

The closed-side setting, or CSS, is the minimum distance between the mantle and concave in a cone crusher at the discharge opening. When you tighten the CSS, the crusher produces a finer product. When you loosen it, the product becomes coarser. CSS is not the same as the open-side setting (OSS), which is the maximum distance during the eccentric cycle. The difference between OSS and CSS is the crusher's throw, and it affects capacity as well as product shape.

You can think of CSS as your product size dial. If the screen cut point stays fixed, changing the crusher CSS changes how much recirculating load the circuit carries. Too tight a CSS can overload the crusher and produce excessive fines. Too loose a CSS can push oversized material through, degrading the screen's ability to maintain its cut. That's why you'll adjust CSS during commissioning, not just on paper.

The eccentric speed and chamber profile also matter. A longer parallel zone in the chamber gives more reduction per pass but can reduce capacity. A shorter parallel zone favors throughput over product fineness. The right combination depends on the ore's abrasiveness and the target P80.

Feed size distribution tells you how much coarse material the crusher must handle and how much fine material will pass through without further reduction. A feed with a high proportion of fines can load the screen heavily. A feed with a very wide distribution may require a grizzly or scalper before primary crushing.

Moisture and clay change everything. Wet, sticky ore can blind screen decks, reduce screening efficiency, and cause crusher bridging. If you're dealing with clay-heavy feed, you might choose larger screen apertures, heated decks, or a different screen type altogether. Sometimes the answer is to wash the feed before screening. Other times you'll accept lower screening efficiency and compensate with a larger screen area.

You don't guess at these parameters. You measure them during metallurgical testwork. A few tonnes of representative ore through a pilot plant gives you the data you need to set CSS, choose screen panels, and size the recirculating conveyor. The test program should cover crusher work index, clay content, moisture adsorption, and screening rate under load.

A crushing and screening circuit is one part of an ore dressing process. Its product becomes the feed to grinding, classification, and concentration. If the crushing circuit produces a coarse top size, the grinding mill must do more work. If it produces excessive fines, you lose energy and may create handling problems. The balance between crushing and grinding is a classical trade-off in plant design.

That's why integrated plant delivery matters. The EPC turnkey approach ties crushing, screening, grinding and downstream recovery into one accountable package. According to the company's published figures, Xinhai reports more than 600 EPC+M+O projects across 100+ countries and regions, with a team of 700+ technical experts. Those numbers aren't a guarantee of any single circuit's performance, but they indicate the breadth of ore types the design team has seen.

For further background on ore characteristics and commodity variability, the USGS Mineral Commodity Summaries provides deposit and production data for gold and other metals. The Society for Mining, Metallurgy & Exploration (SME) also publishes comminution design references. Both are useful when you need to cross-check your assumptions.