Rock Hardness and Abrasivity Based Equipment Selection

Rock hardness and abrasivity based equipment selection is the first decision that fixes the lifetime cost of a crushing plant. The two properties are not interchangeable: a rock can resist breakage strongly and still be gentle on steel, or break easily and consume wear parts at an alarming rate. A mismatch rarely shows up as lost tonnage; it shows up as a wear-part invoice and as downtime.

Two properties that are constantly confused

Strength is the stress a rock withstands before it fails, measured in the laboratory as unconfined compressive strength. Abrasivity is the rate at which the rock wears the metal it touches, and it depends largely on mineral composition, above all on quartz content. Quartz sits at 7 on the Mohs scale and is harder than tool steel, which is why even a nominally soft sandstone can be severely abrasive when it is quartz-rich.

On site the distinction reads like this: strength tells you which crushing principle will finish the job, abrasivity tells you which principle will do it at an acceptable parts consumption. No equipment list should be written before both questions are answered.

Strength classes

Rock mechanics uses an established classification for unconfined compressive strength. Boundaries vary slightly between sources, but the following bands are widely accepted:

Field behaviour cannot be predicted from intact strength alone. Joint spacing, weathering and bedding govern the fragment size delivered by blasting and therefore the character of the feed the primary stage will see.

How abrasivity is measured

Two indices dominate. The Cerchar index is derived from the width of the wear flat left on a hardened steel stylus scratched across the rock surface under a defined load, and it is usually read through these bands:

The second index is the Bond abrasion index, reported in grams of metal lost by a standard steel paddle in a tumbling test. In practice a value below 0.1 is read as low, 0.1 to 0.4 as moderate, and 0.4 and above as high abrasivity. Limestone typically sits in the lowest band, granite and basalt between moderate and high, quartzite at the top. Both indices are sample-specific: the same rock name can return different values from different quarries.

Compression or impact

Crushing principles fall into two families. Jaw and cone crushers break material by squeezing it between two surfaces, in slow and controlled contact. Impact crushers break material by striking it at high speed, which yields cubical product and a high reduction ratio in one stage but raises the relative velocity between metal and rock. On abrasive feed, that high relative velocity drives blow bar and liner consumption up quickly.

This is exactly why a cone crusher is preferred in the secondary and tertiary stages when abrasivity is high: compressive contact loses less metal for the same tonnage.

Rock hardness and abrasivity based equipment selection: the data set

Before an equipment list is drafted, the file should contain compressive strength results from representative samples, an abrasivity index, quartz or equivalent quartz content, the expected fragmentation after blasting, moisture and clay content, and the target product fractions with any shape requirement. Clay and moisture raise the risk of packing in compression stages and change how the feed must be prepared. Assembling that data set is a mining engineering task and forms the core of the enquiry sent to suppliers.

Marking the sampling points on the quarry plan pays off later: when the rock changes, the source of the deviation can be traced. Adding blast pattern notes and observed fragment sizes to the same file lets the primary feed opening be chosen from records rather than from assumption, and reduces retesting when a second face or a second quarry is opened.

Frequently asked questions

Is one sample enough

No. Strength and quartz content vary both laterally and with depth. The average of samples from several levels is assessed together with the least favourable value.

Is hard rock always abrasive

No. Strength and abrasivity are separate properties; a hard carbonate rock free of quartz can be far less abrasive than a soft, quartz-rich sandstone.

How are wear costs compared during selection

On a per-tonne basis. For each plant option the expected change interval and the downtime per change are placed side by side, and the difference in initial price is judged together with those two figures.