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Preparation of Ores

Metalliferous ores straight from the mine are seldom directly suited for metal smelting. Quite often their metal content is too low e.g., only 0.8% copper or 5% lead for processing in the furnace, or they may be composed of minerals containing different metals requiring different kinds of metallurgical treatment.

For these reasons, must ores have to undergo variety of preparatory processes, referred to as dressing for the removal or separation of waste matter or other minerals, so that the concentration of the desired mineral is increased. Some of the processes used more particularly in ore dressing are sorting; comminution (crushing and grinding); sizing (by screening); classifying (e.g., the grading of finely divided material by rates of settling); separation (e.g., by magnetism, electrical conductivity, specific gravity, etc.).

Preparation Ores

The preparation plants are basically situated at the mines, so that only the processed ore, free of waste matter, has to be transported to the smelting works. Preparation starts with crushing and grinding (Fig.1). The degree of comminution (size reduction) to be applied will depend on the size of the ore lumps and on the requirements of the subsequent treatment to be applied. Sizing and classifying i.e., grading the comminuted material according to particle size are important operations in ore dressing. To relieve the crushing and grinding machines of unnecessary load, particles that have been sufficiently reduced in size are removed by screening (Fig.5).

Sizing of relatively coarse particles can most efficiently be performed by screening, and screens of many kinds are used for the purpose. Small particles below about 1mm in size can usually be more suitably sized by classification based on different rates of settling of different particles sizes in water. For e.g. the Hardinge countercurrent classifier (Fig.6) is slowly rotating drum on the inner surface of which are located spiral flanges. As the classifier rotates, the coarser particles are settled out, moved forward by the spiral flanges, and repeatedly turned over in a forward motion, releasing any finely divided material mixed with them.

It is not possible, within the scope of this article, to describe all the many processes and types of equipment employed in the preparation of metalliferous ores. However, three important methods of treatment will be dealt with.

In wet-mill concentration the differences in specific gravity of different minerals are utilized for separating them. The metalliferous ores – sulphides and oxides – are as a rule specifically heavier than the waste material. Separation of the ore from the waste may be effected in a settling classifier with the aid of water in motion, the underlying principle being that the differences in specific gravity are associated with different rates of settlement of the particles (Figs. 2 and 7).

Coarser particles, ranging in size from about 0.5 to 30 mm, can be settled out in a machine known as a jig in which a horizontal stream of water is subjected to a rhythmical up-and-down motion. Finer-grained materials, approx. 0.3 to 0.5 mm in size may be treated on a table concentrator a slightly inclined plate on which the lighter material is separated from the heavier by a thin, shallow stream of water.

This hydromechanical separating action may be augmented by the action of gravity developed by oscillating or jolting motions applied to the table (shaking and bumping tables Fig.4) these concentration processes are referred to as tabling.

Flotation is an important and commonly used separation process which is based on the fact that some of the components in the comminuted minerals are wettable, whereas others are water-repellent (hydrophobic Figs 3 and 8). The hydrophobic particles have an ability to hold air bubbles by surface action, the nature of the film on the outside of the particles being the controlling factor.

Finely divided air which is introduced into the pulp the mixture of solids and water in which flotation is performed, adheres in the form of bubbles to these particles, more particularly the metalliferous components of the pulp, and causes them to rise to the surface. Here they collect in a mass of froth and are removed by a skimmer device. The hydrophilic components remain behind in the pulp. As a rule, these are the worthless minerals (gangue), which are removed as tailings from the flotation machine.

The sulphides of heavy metals are readily floatable, and flotation is therefore an important method for the concentrating of copper, lead and zinc ores. A further development has been the selective flotation of two or more useful minerals, particularly the ores of different metals, which can thus be collected as separated concentrates This principle is, for example, applied to the preparation of sulphidic lead-zinc ores.

The floatability of minerals can be controlled by certain chemical additives called flotation agents. These are of various kinds: Frothers, whose function is to produce froth by combining the air bubbles into a stable froth which will buoy up the ore particles. Oils and allied substances are used as frothers.

Collectors are substances that increase the water repellency and make the ore particularly receptive to the attachment of air bubbles. Collectors usually consist of synthetic organic compounds.

Other flotation agents help to regulate the process. Other depressors can make hydrophobic minerals temporarily hydrophilic and can in this way help in the selective separation of one mineral from another by depressing one, thereby inhibiting its flotation. The depressed mineral can subsequently be made hydrophobic again by an activating agent. The various agents for regulating the flotation process in this manner are inorganic compounds, mostly salts.

Magnetic separation: If a comminuted and classified ore is brought into a magnetic field, the magnetic components generally the useful metalliferous ore can be extracted and thus separated from the nonmagnetic residual material. The treatment is carried out with the aid of magnetic separators, of which there are many kinds.

A drum separator is explained in Fig.9. This method of separation plays an important part, for example, in the concentrating of certain iron and manganese ores. Those substances which are attracted by a magnetic field are called paramagnetic. These are subdivided into strongly magnetic and weakly magnetic substances.

Various techniques have been applied for the separation of both categories of material. In general, magnetic fields of greater intensity high intensity magnetic separators have to be employed for dealing with weakly magnetic ores. In both categories wet and dry processes are employed, depending on whether or not water is used as an aid in the process.

The magnetic properties of certain ferrous minerals can be enhanced by suitable preliminary heat treatment. For instance, in the case of siderite a particular kind of iron ore, which is a carbonate-FeCO3 the carbon dioxide can be expelled by heating the ore in a kiln. As a result, the carbonate is converted into the strongly magnetic compound named ferroso-ferric oxide (Fe3O4), which can readily be separated by a magnetic action.