Winning the metals

The following is an excerpt from the ninth edition of Mining Explained, published by The Northern Miner.

Digging ore from the earth is only half the battle. Often just as challenging and costly is the processing of ore, which takes place in mills, smelters and refineries.

At first glance, the interior of a mill is a baffling maze of tanks, pipes, pumps, conveyors, motors, chemicals, pulps and solutions, yet this seeming confusion is actually a carefully designed system constructed for one objective: to recover the valuable minerals locked up in the ore.

The end-product from a mill is called a concentrate, or, in the case of gold and silver, a dore bar of the metal itself.

All milling and concentrating processes begin with a crushing and grinding stage, which usually represents most of the total cost of processing.

Ore minerals are usually found in and among grains of other ore minerals or (relatively) worthless gangue minerals. As a result, some ores are more difficult to process than others.

For instance, a complex sulphide ore containing microscopic particles of sphalerite in small blebs of galena or other sulphides presents a special challenge to the metallurgical engineer, that is, to design a milling process that will liberate these various constituents from each other as cleanly and economically as possible so that each may be recovered.

— Primary crushing — The milling process begins with the primary crusher, which is normally situated below the mine workings so that broken ore can be dropped down an ore pass to be crushed and then hauled to the surface in a skip. This is so because loading skips with small, 15-cm-wide pieces of ore is more efficient than loading it with larger chunks.

The primary crusher is usually a jaw crusher. Ore falls into the opening between a pair of metal jaws at the top and is crushed by the short, rapid, motion of the one movable jaw — a process not unlike an animal using its jaws to chew food.

A few large-tonnage mills use a gyratory crusher as the primary crusher. It consists of a heavy, gyrating head which works inside a crushing bowl fixed to the main frame. Rock falling into the bowl is caught and broken up by the gyrating head.

— Secondary crushing — A secondary crusher is frequently needed when the product from the primary crusher is too large for efficient grinding. In North America, the main type used is the cone crusher. This is a close cousin of the gyratory crusher, though the speed of the cone crusher is greater and it is designed to handle smaller pieces of rock.

Vibrating screens are used to control the size of the final product from the crusher building. Ore that falls through the openings in a screen is called “undersize,” and it will find its way into the mill on a conveyor belt. Ore that is too large to fall through the openings is called “oversize”, and it returns to the crusher along with a separate conveyor belt.

Crusher buildings are typically equipped with dust control and ventilation systems designed to eliminate the buildup of harmful dust.

Most mills operate seven days a week, 24 hours a day, compared with the mine, which may operate only five days a week. If there is a difference in work schedules, crushed ore must be stored on surface in sufficient quantities to keep the mill going around the clock. The customary place to store the ore is in a fine ore bin next to the mill. These bins generally contain enough ore to keep the mill operating for at least 48 hours.

— Grinding circuit — Ore from the storage bins is fed, together with water, to the first circuit in the mill building. This is known as the grinding circuit and consists of one or more ball mills or rod mills. As each mill revolves, the ore within rolls over itself, causing it to be crushed and ground. The steel balls (or steel rods) in the mill assist this process.

A particularly economic form of grinding, used today wherever possible, is “autogenous.” This process involves the use of rock-against-rock contact to crush and grind ore to the required size, eliminating the cost of periodically replacing steel balls.

“Semi-autogenous” grinding refers to the addition of some steel balls to the grinding stage to supplement the rock-on-rock breakage.

There is normally considerable variation in the size of particles discharged from a grinding mill. Some will be too coarse or too fine for the chemical separation of their constituents to work effectively.

If the crushing and grinding process is not carefully controlled, some ore particles get reduced to sub-micron sizes. These are called slimes and may interfere with subsequent treatment processes. That is why many crushing and grinding circuits are now controlled by computers.

Particles that are too coarse are separated from the balance of the material in a classifier and then returned to the grinding mills. Traditional classifiers consist of a box set on a slope and a mechanism for moving material up the incline. Newer plants may use a hydrocyclone, which separates various sizes of particles by spinning, as in a centrifuge.

When a ball mill and classifier work together as a unit, the process is known as a closed circuit. The ground ore produced will have a certain maximum size, and the amount of “fines” is limited. Usually a ball mill grinds in closed circuit for maximum control and efficiency.

A rod mill may be used to prepare feed for a ball mill, in which case it may operate without a classifier or in open circuit.

The goal of the mill circuit is to crush and grind the ore down to what is termed its “size of liberation,” that is, the maximum size to which the rock must be ground to ensure that individual mineral grains are separated from one another.