Treating base metal ores

The following is an excerpt from Mining Explained, published by The Northern Miner.

Base metal ores go through a more complex treatment process than gold to reach their ultimate commercial form. Because most base metal ores contain metallic sulphide minerals, the greatest challenge is separating the sulphur from the metal and then ensuring the sulphur is contained in some manner so that it cannot damage the surrounding environment.

Unlike gold, base metals are not usually produced in a near-pure form at the mine site. That’s because most mines are not sufficiently large nor well-situated to warrant the construction at the mine site of all of the plants (mills, smelters, refineries, etc.) needed to convert the metal ore into pure metal.

Instead, each base metal mine attempts to remove locally as much of the waste rock as possible from its ore and ship the enriched product, or concentrate, to a strategically situated smelter.

Crushing and grinding in a concentrator are practised in the same manner as in a cyanide mill, and for the same purpose — to liberate the valuable minerals from the surrounding worthless rock.

q Flotation — Grinding of base metal ores is done in water to which certain oils and synthetic chemicals are added. Then the resulting pulp is swirled around in rectangular tanks arranged in series. These tanks are known as flotation cells. Controlled air and further chemicals, called flotation reagents, are added. The air forms bubbles in the pulp, the flotation reagents coat the metal sulphides (but not the waste particles) and cause them to stick to the bubbles, which, in turn, carry the sulphides to the top of the tanks.

The sulphide-carrying bubbles are scraped from the top of the flotation machines, while the worthless material left behind sinks to the bottom and is discharged.

A recent innovative approach, designed to improve flotation recoveries, is to use column flotation cells. These are 10-metre-tall (33-ft.) tanks in which a tall column of froth can develop. Water sprayers at the top wash unwanted minerals from the froth so that only minerals that are chemically attracted to the bubbles are retained.

– Dewatering — Later, the metal-bearing bubbles, now known as a concentrate, have the water removed from them before shipment. This is typically done by pressure filters. This dewatering step is carried out to reduce the weight of the concentrate if the shipping distance is great, and to prevent freezing (in rail cars, for example) during winter shipment.

By employing various flotation reagents, different kinds of metal sulphides can be floated or separated, one at a time. Thus, if an ore contains copper, zinc and iron sulphides, it is possible to make separate concentrates for each metal.

Flotation is occasionally applied to gold ores, to make a gold flotation concentrate that is then treated with cyanide, with or without roasting. This method is especially applicable when the metal is very fine and intimately associated with minerals containing sulphur or arsenic. In this manner, it is possible to make, say, 10 tonnes (11 tons) of gold concentrate from 300 tonnes (330 tons) of mill feed. This concentrate can be economically treated much more intensely (finer grinding and longer agitation) than the original ore.

– Heavy-media separation — Another method of concentrating ores and minerals by floating (but actually quite different from the flotation method described above) is known as the heavy-media separation or sink-float process. Here, the differences in specific gravity of the various minerals in the rock mined are used to achieve the desired separation of ore from gangue.

In the sink-float plant, coarsely crushed ore is fed into a suspension of finely ground ferro-silicon (or magnetite or another heavy mineral) in water. This suspension is carefully maintained at a specific gravity between that of the desired feed and the unwanted minerals, or gangue. As a result, the heavier mineral components sink and are retained, while the lighter constituents float and are discarded.

Another way of using the density of minerals to help concentrate them is gravity. In most gravity processes, a slurry of ore in liquid is fed into a centrifuge or other chamber that agitates the slurry, causing lighter particles to spin away and heavier ones to sink. Similarly, gold is often recovered on a vibrating table, where lighter materials are washed away and heavier grains remain.

Simple mechanical separation of this kind is often the cheapest method of recovery, and consequently it is most often used at the start of the process. Tailings from the mechanical separators are then sent to other processing circuits to recover more of the valuable minerals.

Heavy-media separation is also a basic step in the process of recovering diamonds. Diamonds and other heavy minerals are separated from the crushed ore; then other recovery processes separate the diamonds from the other heavies. Because diamonds tend to adhere to organic compounds, “grease tables” often recover diamonds successfully. Also, diamonds are fluorescent when subjected to X-rays, and separation plants have been designed to produce diamond concentrates using this method.

– Magnetic separation — Some low-grade iron ore can be treated using magnetic separation because all iron minerals are magnetic to some degree. In this process, the crude ore, which may grade less than 30% iron, is mixed with water and ground to a suitable fineness. The pulp is then passed over a revolving magnetic drum, to which the magnetic iron minerals adhere. They are scraped off and retained, while the gangue particles are discarded with the water.

The magnetic portion is dewatered and filtered, but the concentrate produced is too fine to be used in a blast furnace at a steel plant (the destination of most iron), so it is “pelletized,” that is, mixed with a suitable bonding agent and rolled on a pan or in a drum until marble-sized, iron-rich balls are formed. These pellets are dried and baked, then shipped out to steelmakers.

Other methods of metal recovery include “photometric sorting” — where the distinct light-reflection properties of metallic minerals are used to separate them from gangue minerals — and various simple gravity-based processes.

Heavy-mineral concentration and magnetic separation are two approaches that have been useful in recovering other metals, such as vanadium and titanium, whose principal ore minerals are both heavy and magnetic. Magnetic separation can also be used to extract low-value magnetic minerals from heavy-mineral concentrates, leaving behind more valuable minerals, such as zircon and rutile (titanium oxide), for recovery and sale.