Mining Rule of Thumb

The primary usage of Rules of Thumb should be in the development of conceptual designs and feasibility studies or, when a quick decision is required in the solution of an operating problem. Although an approximated answer, derived from a Rule of Thumb may solve an immediate problem, Rules of Thumb are not a substitute for the application of sound engineering and design methodologies. Although we firmly believe that the presented Rules of Thumb provide great continuing value to our industry, McIntosh Engineering does not guarantee their validity, nor do we (or the referenced individual sources) accept responsibility for application of the Rules of Thumb by others. Where possible, direct quotes have been provided from individual references; however, it is possible that referenced sources may not have directly stated the Rule of Thumb for which they are assigned credit. Although we have endeavored to accurately quote all individual references contained in the Rules of Thumb compilation, we apologize in advance for any misquotes that may be attributed to individual sources. We will provide updates to the Rules of Thumb compilation, as we become aware of corrections that may be necessary.

Field: Crushers and Rockbreakers

Area: Crusher Design

• The approximate capacity of a jaw crusher for hard rock application at a typical setting may be obtained by multiplying the width by 10 to get tonnes per hour. For example, a 48 by 60 crusher will have a capacity in the order of 600 tph when crushing ore in a hard rock mine. Jack de la Vergne
• The capacity of a jaw crusher selected for underground service should be sufficient to crush the daily requirement in 12 hours. Dejan Polak
• For most applications, 7:1 is the maximum practical reduction factor (ratio) for a jaw crusher, but 6:1 represents better design practice. Jack de la Vergne
• For most applications, 6:1 is the maximum practical reduction factor (ratio) for a cone crusher, but 5:1 represents better design practice. Jack de la Vergne
• Corrugated liner plates designed for jaw crushers (to avoid a slabby product) result in shortening liner life by up to two-thirds and they are more prone to plugging than smooth jaws. Ron Doyle

Area: Crusher Installation

• The crushed ore surge pocket beneath a gyratory crusher should have a live load capacity equal to 20 minutes of crusher capacity or the capacity of two pit trucks. Various
• It will take six months to excavate, install, and commission an underground crusher station for a typical jaw crusher. For a very large jaw crusher or a gyratory crusher, it can take nine months. Jim Redpath
• The desired grizzly opening for an underground jaw crusher is equal to 80% of the gape of the crusher. Jack de la Vergne
• The combination of a jaw crusher and a scalping grizzly will have 15% more capacity than a stand-alone jaw crusher. Ron Casson
• As a rule, scalping grizzlies are rarely used anymore for (large) primary crushers. The exception is when ore contains wet fines that can cause acute packing in a gyratory crusher. McQuiston and Shoemaker
• The product from a jaw crusher will tend to be less slabby and more even-dimensioned without a scalping grizzly, since slabs do not pass through so readily under this circumstance. A. L. Engels
• Removal of the scalping grizzly for a primary jaw crusher can cut the liner life by 50%. It also makes it more difficult to clear a jam when the jaws are filled with fines. Ron Doyle

Area: Crusher Selection

• For a hard rock mine application below 600 tonnes/hour, select a jaw as the primary crusher. Over 1,000 tph, select a gyratory crusher. Between these capacities, you have a choice. Chris Ottergren
• For a hard rock mine application below 540 tonnes/hour, a jaw crusher is more economical. Above 725 tonnes/hour, jaw crushers cannot compete with gyratory crushers at normal settings (6 -10 inches). Lewis, Cobourn and Bhappu
• For an underground hard rock mine, a gyratory crusher may be more economical in the case where its required daily production exceeds 8,000 tonnes of ore. Jack de la Vergne
• If the hourly tonnage to be crushed divided by the square of the required gape in inches is less than 0.115, use a jaw crusher; otherwise use a gyratory. (If the required capacity in metric tph is less than 162 times the square of the gape in metres, use a jaw crusher.) Arthur Taggart
• Nearly all crushers produce a product that is 40% finer than one-half the crusher setting. Babu and Cook
• The product of a jaw crusher will have a size distribution such that the -80% fraction size (d80) is slightly less than the open-side setting of the crusher. For example, if the open-side setting is 6 inches, the d80 product size will be 5¾ inches. Unknown
• In a hard rock mine, the product from a jaw crusher will tend to be slabby, while the product from a gyratory crusher may tend to be blocky, the latter being easier to convey through transfer points on a conveyor system. Heinz Schober
• Impact crushers (rotary or hammer mills) have the capacity for high reduction ratios (up to 40:1), but are rarely applied to hard rock mines. Since they depend on high velocities for crushing, wear is greater than for jaw or gyratory crushers. Hence, they should not be used in hard rock mines that normally have ores containing more than 15% silica (or any ores that are abrasive). Barry Wills

Area: Rockbreakers

• The capacity of a hydraulic rockbreaker is higher (and the operating cost lower) than a pneumatic rockbreaker. For these reasons, most new installations are hydraulic, despite the higher capital cost. John Kelly
• For underground production rates less than 2,000 tpd, it may be economical to size the ore underground with rockbreakers only, otherwise, an underground crusher is usually necessary when skip hoisting is employed. John Gilbert
• The operating cost for a stand-alone rockbreaker will be approximately 30% higher than it is for a crusher handling the same daily tonnage. John Gilbert
• The capacity of one rockbreaker on a grizzly with the standard opening (± 16 by 18 inches) is in the order of 1,500-2,000 tpd. John Gilbert.
• For skips that fit into a standard 6 by 6 shaft compartment, the maximum particle size that is normally desired for skip hoisting is obtained when run-of-mine muck has been passed through a grizzly with a 16-18 inch opening. Skips hoisted in narrow shaft compartments may require a 12-14 inch spacing, while oversize skips may handle muck that has passed a 24-30 inch spacing. Jack de la Vergne
• A pedestal mounted rockbreaker installed should be equipped with a boom that enables a reach of 20 feet (6m). Peter van Schaayk