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: Mineral Processing

Area: Concentrate

• The typical moisture content of concentrates shipped from the mine is often near 5%. If the moisture content is less than 4%, the potential for dust losses becomes significant. Ken Kolthammer
• The moisture content of concentrate measured by a custom smelter will invariably be 1% higher than was correctly measured by the mine when it was shipped. Edouardo Escala
• If the moisture content of the concentrate is above 8%, problems with sintering and combustion are usually avoided. Unfortunately, concentrates stored in a cold climate generally require maximum moisture content of 5% to avoid handling problems when frozen. Concentrate subject to both spontaneous combustion and a cold climate are usually dried to less than 4% and sometimes as dry as ½%. Ken Kolthammer

Area: Filtration

• When designing the filters required for a proposed mill, the scale-up factor from bench tests is approximately 0.8. Donald Dahlstrom
• When determining vacuum pumps for filter installations required for a proposed mill, the scale-up factor from bench tests is approximately 1.1. Donald Dahlstrom

Area: Flotation

• When designing the flotation circuit for a proposed mill, the scale-up factor for flotation retention times obtained from bench tests is approximately two. Mining Chemicals Handbook (Cyanamid)
• To determine a preliminary water balance for a proposed flotation circuit, the pulp density may be assumed to be 30% solids (by weight). Rex Bull
• As a rule, water-soluble collectors may be added anywhere in the circuit, but oily, insoluble promoters should always be added to the grind. Keith Suttill
• For roasting to be exothermic to the extent that no fuel is required to sustain reaction, the flotation product must contain at least 17% sulfur. Therefore, the target is 18%. Dickson and Reid

Area: General

• A concentrator (mill) requires up to 3 tons of water for each ton of ore processed. It is therefore important to operate with the with the maximum practical pulp density and minimum practical upward or horizontal movement. The basic philosophy requires movement over the shortest possible distances between processing units and makes use of gravity to save on power consumption. Wayne Gould
• In the arid climates, mills operate with less than one ton of new water for each ton of ore processed. The balance of the process water required is recovered from de-watering concentrate, thickening the tails, and re-circulation from tailing ponds. Norman Weiss
• A mill at the mine (and related facilities) accounts for approximately 85% of the total electrical power consumption for an open pit operation, but only about 45% for a typical underground mine. Alan O'Hara
• For a typical underground mine, the cost for electrical power for the mill (concentrator) will be approximately 35% of the total electrical power cost for the mine. Fred Nabb
• A mill built entirely of second-hand equipment and controls may be constructed for half the cost of one built "all new" with state-of-the-art automated monitoring and controls. Bruce Cunningham-Dunlop

Area: Gravity Separation

• For simple methods of gravity separation with water as the medium, a specific gravity differential of at least 1.5:1 between the minerals to be separated is desirable. Fuerstenau, Peterson and Miller
• Common methods of gravity separation (jigs, tables and spirals) require close particle sizing and a difference in specific gravity differential of at least 2:1 between the minerals to be separated. V. P. Kenyen
• For gravity separation to be possible, the ratio of the difference in density of the heavy mineral and the medium and the difference between the light mineral and the medium must be greater than 1.25. Arthur Taggart
• Most all wet gravity separation equipment is sensitive to the presence of slimes (minus 400 mesh). Slimes in excess of 5% should be avoided. More than 10% causes serious separation problems. Chris Mills

Area: Grinding

• Fine ore bins that provide feed to the grinding circuit should have a capacity equal to 30 hours of processing. Northern Miner Press
• Grinding is a low-efficiency, power-intensive process and typically can account for up to 40% of the direct operating cost of the mineral processing plant. Callow and Kenyen
• For purposes of design, it may be assumed that a ball mill will carry a 40% charge of steel balls; however, the drive should be designed for a charge of 45%. Denver Equipment Company
• A grate (diaphragm) discharge ball mill will consume 15% more power than an overflow (open) discharge ball mill even though the grinding efficiencies are the same. Lewis, Coburn, and Bhappu
• Other things being equal, the larger diameter the drum, the more efficient the grinding. However, this phenomenon stops when the diameter reaches 12.5 feet (3.8m). Thereafter, the efficiency bears no relation to diameter. Callow and Kenyen
• The ball mill diameter should not exceed 100 times the diameter of the grinding media to avoid sloughing. Bond and Myers
• In pebble mills, the individual pieces of media should be the same weight, not the same volume, as the optimum size of steel ball. Bunting Crocker
• The power draft (draw) in a pebble mill can easily, quickly, and automatically be controlled to an extent that cannot be done on a ball mill. Bunting Crocker
• The ratio of length to diameter of a rod mill should not exceed 1.4:1 and the maximum length of a rod (to avoid bending) is 20 feet. As a result, the largest rod mill manufactured measures fifteen feet diameter and is 21 feet in length. Lewis, Coburn, and Bhappu
• For most applications, 70:1 is the maximum practical reduction factor (ratio) for a ball mill, but 60:1 represents typical design practice. Jack de la Vergne
• Rubber liners in ball mills may have a service life of 2-3 times that of steel liners. W. N. Wallinger
• The capacity of a mill with synthetic rubber liners is approximately 90% that of the same unit with steel liners. Yanko Tirado
• The capacity of a grinding mill for a given product operating in open circuit is only 80% that of the same unit operating in closed circuit. Lewis, Coburn and Bhappu
• A dual drive (i.e. twin motors and pinions driving a single ring gear) may be more economical than a single drive when the grinding mill is designed to draw more than 6,000 HP (4.5 Mw). Rowland and Kjos
• Geared drives are currently available up to 9,500HP. Barrat and Pfiefer
• A direct drive ring motor (gearless drive) is the only option for an autogenous mill rated over 20,000 HP. Mac Brodie