Glossary

Aggregates — hard, inert materials used for mixing in various-sized fragments with a cementing material to form concrete or mortar.

Air separators — a machine where air is the carrying medium instead of water, and par­ticles are separated by centrifugal force. Static separators are similar in operation to hydrocyclones, but dynamic separators have internal rotating mechanisms that can be controlled to ensure that the separation is as sharp as possible and that a minimum amount of material passes through the separator unclassified.

ANFO — ammonium nitrate-fuel oil. The most widely used explosive for mining in the late 20th century, because it is inexpensive, safe, powerful, and easy to handle, it will continue to dominate explosive rock breakage operations for many decades.

Autogenous mills — ginding media for rocks of various sizes and generated by shatter­ing and abrasion of the ore being ground.

Ball mills — machines that use separate grinding media-steel balls, cast iron balls, or, sometimes, ceramic balls. The ball charge normally contains a mixture of sizes related to both the size of the feed and the required product size.

Black powder — an explosive used for fireworks for many centuries. The first explosive used for breaking rocks in mines, this deflagrating explosive is made from potassium nitrate, composed of saltpeter, carbon, and sulfur. Its speed of reaction is 400 m/sec.

Bradford breaker — a tumbling mill to break coarse coal that is brittle and shatters eas­ily when dropped. The coal enters the feed end, is lifted by the rotating mill, and shatters by impact when it falls. The mill has holes in the cylindrical section through which small particles of the shattered coal leave the mill. Larger particles are repeatedly dropped until they shatter while the tougher shale does not shatter and leaves the mill at the dis­charge end.

Classifiers — machines in which broken particles are separated into fine and coarse (or light and heavy) streams.

Colloid mills — high-speed mills in which rotating pins pass each other with little clear­ance between the pins, creating intense turbulence and shear forces in the slurry close to the pins. See also pin mills.

Compartment mills — ball mills divided into compartments by vertical diaphragms. The first compartment has large balls to grind coarse particles; the second one has finer balls to grind smaller particles. The apertures in the division heads retain balls and larger par­ticles in the compartment. Occasionally, a mill has three compartments. Used mainly as dry mills for grinding cement clinker, they are also used for grinding refractory gold ores prior to roasting. The grinding media in cement clinker mills are usually alloy steel balls. In wet-grinding compartment mills, alloyed steel rods are occasionally used as the grind­ing media in the first compartment.

Cone crusher — high-speed gyratory crusher with the crushing chamber designed to accept a finer feed than a conventional gyratory crusher and produce a finer discharge.

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Crushers — machines that reduce the size of boulders and pebbles by compression and impact.

Deflagrating explosives — low-energy explosives that burn rapidly with intense heat. Large volumes of gas are generated by the chemical reactions.

Detonating explosives — high-energy explosives that decompose so rapidly that they explode. The copious volumes of gases produced by the chemical reaction have very high pressures and are efficient and dangerous explosives.

Disk mills — machines where stirring is carried out by disks attached to a shaft rotating at high speed and used for very fine grinding.

Dynamite — a solid explosive that is much safer than nitroglycerin by itself, which is more difficult to confine and handle. Dynamite contains 75% nitroglycerin and 25% kieselguhr.

Edge mills — machines that have a stone or concrete disk rotated on a stone base. Cereal grains are ground between the moving rim of the disk and the fixed stone base. The edge mill was the first type of roller mill. Horse-powered edge mills are still used in many small villages where they are used to grind small amounts of grain each day.

Explosives — a mixture of chemicals that produces heat and a large volume of gas when they react. The gas exerts rapid pressure on walls that confine it. Explosive rock break­age occurs when the explosive is placed in rock and detonated and the gas moves or shatters the confining rock.

Fire setting — building fires against rock walls and keeping them burning for hours until the rock heats enough to crack so that it can be removed easily.

Grain roller mills — counter-rotating rolls used in grain mills. The break rolls cut most of the bran from the endosperm, the scratch rolls break composite bran-endosperm par­ticles, and the reducing rolls grind purified semolina to finished flour. The roll surfaces may be corrugated or smooth depending on their use.

Gyratory crushers — machines that are similar in principle to jaw crushers but the fixed plate is the inner surface of an inverted cone. The moving plate is a cone that fits into the fixed cone. The moving surface is mounted on a large shaft called the main shaft, which is suspended from the top. The surfaces of the cones converge. The center cone rotates and gyrates, as its surface moves toward and away from the fixed cone during each revo­lution. During the gyration, rocks are caught in the wedge between the plates, and crushed and broken fragments are released.

Hammer and gad — a heavy stone used to drive a lighter, wedge-shaped stone into a crack in a rock to detach a boulder from the rock wall.

Hammer mill — high-speed rotating hammers that break rocks by impact. Broken parti­cles are flung against a screen, the fine particles pass through the screen and are dis­charged, and the coarse particles are retained for further breakage. In an out-of-hole hammer drill, or drifter drill, each blow is struck at the end of the drill string out of the hole, and energy is transmitted through the drill rods and couplings to the bit. In a down-the-hole hammer drill, the hammer piston is at the bottom of the hole and directly contacts the bit.

Hand stones — moving stones that almost touch fixed stones and are moved by muscle power. Cereal grains and ore particles are ground between them.

High-pressure grinding rolls (HPGRs) — Spring-loaded counter-rotating rolls that do not touch but which break pebbles and particles by exerting immense pressure on a bed of particles. The product emerges as a sheet of agglomerated particles that must be deagglomerated prior to further processing.

Hydrocyclones — cylinders into which slurries are introduced tangentially at high speed and rotate around the inner wall. The coarser or heavier particles are held by centrifugal force at the wall and spiral toward one end of the cylinder where they are diverted by a cone to an apex from which they are collected. The finer or lighter particles form a spiral in the center of the cyclone, which rotates in the same direction but moves in the oppo­site direction, and are collected through a vortex finder. Centrifugal force is the separat­ing mechanism.

Jaw crushers — machines that have two converging steel plates, one of which is fixed and the other which moves a short distance back and forth. Rocks fall into the wedge between the plates as the moving plate recedes, then are crushed when it moves for­ward, and broken fragments are released when it recedes again.

Jet mills — machines with fixed cylinders into which very high speed jets of particles are introduced so that they collide and cause the particles to break autogenously by impact and abrasion.

Mortar and pestle — the size-reduction device that has remained unchanged in princi­ple since the Stone Age. The pestle (rod with a flat-surfaced hammer) is moved up and down in the mortar (bowl-shaped vessel), striking the base of the mortar during each cycle and breaking particles caught between the mortar and pestle.

Nitroglycerin — the first high-energy explosive. An oily liquid made by reacting glycerin with nitric or sulfuric acid, it is a detonating explosive, with speed of reaction up to 9,000 m/sec.

Pebble mills — machines for grinding media for pebbles from stream beds or natural deposits or for pebbles formed by breakage of coarse rocks.

Pin mills — high-speed machines in which rotating pins pass each other with little clear­ance between the pins, creating intense turbulence and shear forces in the slurry close to the pins. See also colloid mills.

Portland cement — made by burning a mixture of lime and clay.

Querns — a moving stone traveling in a rotary motion. Solid particles are added through a hole in the center of the moving stone and are broken as they travel to the periphery; the ground product is then discharged from the periphery.

Rake or spiral classifiers — tanks into which ore-water slurries are fed, with the coarser particles settling and being removed by a rake or spiral and the fine particles overflowing the top of the tank and being removed in a launder. Gravitational force is the separating mechanism.

Rittinger classifier — the original hydraulic classifier. The slurry flowed horizontally across settling boxes of increasing size, with the coarsest particles settling in the first box and finer products in later boxes.

Rod mills — machines that have grinding media for alloyed steel rods that usually are the same length of the mill shell.

Roll crushers — spring-loaded, counter-rotating rolls in which rock pebbles are broken by compression individually and no bed of agglomerated particles is formed.

Roller mills — machines that have steel cylinders attached to horizontal rotating shafts. There are two types of mills: (1) those in which particles are caught between counter­rotating rolls and broken as they pass between the rolls (used to grind cereal grains and small rock pebbles); and (2) those in which particles are caught between a rotating roll and the bowl with which it is in contact. This type of mill usually has an air separator built into the structure with the rollers. The separator classifies the product from the roll­ers and returns the reject for further grinding. It is used to grind soft rocks such as coal, limestone, and phosphate rock.

Rotary rock drills — rotating a drill bit against the rock, penetration is caused by apply­ing pressure to the top of the bit. Soft rock requires lower feed pressure and faster rotation speed than hard rock.

Saddlestones — horizontal fixed stones, over which a moving stone travels in a back — and-forth motion.

SAG mills (semiautogenous mills) — machines where steel balls occupy up to 12% of the mill volume as supplemental grinding media. The steel balls are needed to break critical-size particles—those that are too small to be broken effectively by abrasion and too large to be broken effectively by impact. The name SAG mill is currently used for all primary mills whether fully autogenous or semiautogenous.

Shorthead cone crushers — machines designed to produce a very fine discharge.

Size reduction — the breaking of materials such as grains, pebbles, rocks, and ore to a smaller size using available tools or machines driven by available sources of energy.

Stamp mills — mechanized mortars and pestles driven by waterpower from about 1500-1850, then by steam power. The main machines used for fine crushing or coarse grinding of ores for 400 years were replaced in the 20th century by fine crushers and ball mills.

Stirred mills — machines where the material being ground is kept in motion by stirring rather than tumbling. Grinding media and particles acquire kinetic energy by stirring with an impellor or by agitation. Particles are broken by abrasion and impact when they are caught between grinding media, colliding with themselves or with the wall. Stirred mills are used to make very fine particles. The general rule is that higher-speed mills use smaller grinding media and produce finer particles. Vertical high-speed stirred ball mills are called peg mills, and horizontal high-speed stirred ball mills are called disc mills.

Tower mills — vertical cylindrical machines in which the stirring is carried out by screw — type impellors operating at relatively low speeds.

Tumbling mills — machines with horizontal rotating cylinders containing grinding media and particles to be broken. The mass moves up the wall of the cylinder as it rotates and falls back into the toe of the mill when the force of gravity exceeds friction and cen­trifugal forces. Particles are broken in the toe of the mill when caught in the collisions between the grinding media themselves and the grinding media and the mill wall. Mills may be batch or continuous. In continuous mills, feed enters one end and broken pro­duct leaves the other. In batch mills, the material to be ground is charged to the mill, which is then closed and run until the material is ground. Then the mill is stopped and the load in the mill is dumped out of the mill and another load is put in. Usually, mills are identified by the type of grinding media. In tumbling mills, the grinding media and particles acquire potential energy that becomes kinetic energy as the mass falls from the rotating shell.

Vertical roller mills — today’s version of the age-old edge mill. They use up to four very large rollers which rotate in a spinning bowl. Feed enters the center of the bowl, is ground as it flows between the rollers and the bowl, and is discharged from the periph­ery of the bowl. It is normally entrained in an upward-flowing air stream and classified in a separator. The reject returns to the center of the bowl where it is ground again.

Vertical-shaft impact crushers — machines with a feeder rotating at high speed inside a fixed cylinder. The feeder is designed to accept rocks at its center and propel them from its outer rim at high speed so that they are flung against the fixed cylinder and broken by impact. The wall of the fixed cylinder is designed to hold compacted broken rock to min­imize wear.

Vibrating and nutating mills — machines with very small balls kept in rapid motion in a confined volume by moving the mill shell at very high speed.

Water mills — mechanized querns used for grinding cereal grains. They were replaced by roller mills in large-production flour mills starting about 1890 but are still used in many villages located on rivers and streams and in small mills that produce stone — ground flour.

Work index — a parameter developed by Fred Bond that defines the resistance of a material to size reduction in a machine. The kilowatt-hour per ton required to reduce the material from theoretically infinite feed size to 80% passing 100 pm is calculated from plant data but can also be predicted from laboratory tests that measure the grindability of the material.

Copyright © 2005 by the Society for Mining, Metallurgy, and Exploration.
All rights reserved. Electronic edition published 2009.