sampled ore has now reached the ore bin k and is drawn off through the gate u and chute into the car w, which removes it to mill or furnace, as the case may be. If 1 ton only were sampled, the first sample would be 400 pounds, the second 80 pounds, the third 16 pounds, and the fourth 3.2 pounds from 1 ton. In this case the ore could be finally divided by the Bridgman apparatus.

65. Advantage of Height in a Mill.-It will be noted that the Taylor and Brunton sampling mill is higher than that shown in Fig. 23. This permits the ore to be raised high enough to descend by gravity from one machine to another, thus economizing in power, as the second and third elevators of Fig. 23 are not required. In this plant, little bin capacity is needed, but by using a scraper line under the spout as the ore comes from the discard a series of bins can be filled.

66. Use of the Drier.-The drier is used only for ores that are quite moist, such ores forming lumps when ground fine. Soft damp ores when passing through rolls simply flatten out in sheets, thus making it necessary to dry such ores before attempting to sample them by hand or machinery.

ROASTING AND CALCINING ORES

INTRODUCTION

DEFINITION

1. Ore roasting is the process by which certain chemical changes are produced by the aid of heat, but at a temperature so comparatively low that the ore does not fuse. It is one of the most important operations in metallurgy, because the quality of the product from this process controls the results in the various processes that follow.

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2. The most common example is the oxidizing roast, by means of which sulphides are converted into oxides by the oxygen of the air, the sulphur passing off principally as sulphur dioxide SO,. Arsenic and antimony are also oxidized, the object being to form as much arsenic trioxide As,O, or antimony trioxide SbO, as possible, because these compounds pass off by volatilization. Other compounds of sulphur, arsenic, and antimony which are not volatile are also produced by roasting; these, however, can generally be changed to a condition in which they can be volatilized by the reducing roast, which consists in the admixture of fine coal to take away oxygen, atmospheric oxygen being excluded during this operation. When the sulphur, arsenic, and antimony have practically all been removed by a series of alternate oxidations and reductions, the ore is said to be dead-roasted or sweet-roasted. For the sulphatizing roast the operation is conducted at an especially low

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temperature with a small supply of air and with the ore bed somewhat deeper than usual; by this means considerable sulphur is converted into sulphur trioxide SO,, which combines with the metals to form sulphates.

3. In the chloridizing roast, certain metals, especially silver, are changed to chlorides by mixing their ores with common salt NaCl after practically all the sulphur, arsenic, antimony, etc. have been expelled.

PURPOSES OF ROASTING

4. The object of roasting the sulphides of lead and zinc is to convert them into oxides, which are afterwards reduced to metal in smelting furnaces. In the case of copper sulphide, the purpose generally is to burn off the excess of sulphur, leaving enough so that when smelted it will combine. with all the copper to form an artificial sulphide called matte, which always contains iron. Copper-sulphide ores with 4 per cent. or less of copper are often roasted to sulphates, or sometimes to chlorides, the copper being subsequently dissolved by some liquid.

5. Silver ores are often chloridized so as to prepare them for amalgamation. Gold ores that are to be treated by the chlorination process generally contain sulphides or arsenides, and as these minerals are detrimental to chlorination, such ores are dead-roasted. If oxides of copper, calcium, or magnesium remain after this roasting, common salt is added to the ore towards the end of the roast to convert them into chlorides; otherwise they will absorb chlorine gas during the process of chlorination. It often happens that with finely pulverized ores and slimes which are to be treated by the cyanide process, the solutions will not easily pass through the ore. Again, some ores, for instance tellurides, are so compact that the solutions cannot enter and attack the gold. In such cases roasting will often remedy the trouble by producing physical changes in the ore.

6. Iron ores are sometimes roasted to remove carbon dioxide CO, and water and also to remove sulphur. Zinc carbonate is also roasted to remove carbon dioxide. This kind of roasting is termed calcining. Lump ores are sometimes heated in order to eliminate moisture and allow them to crush more readily. The heat in drying sometimes expands the rock and produces small cracks. Ores that are to be crushed fine and screened generally have the moisture driven from them by heat to render them more friable.

7. Roasting is usually preliminary to some metallurgical process, but in some cases it is preliminary to mechanical concentrating. Mixtures of blende and pyrite are roasted for the purpose of concentration by magnetic concentrators.

Experiments have been made with hematite and limonite iron to convert the ferric oxide Fe,O, into magnetic oxide F,O,, but they have not proved commercially successful. On the other hand, magnetic iron ore containing sulphur has been successfully converted into hematite by roasting to eliminate the sulphur.

METHODS OF ROASTING

8. The methods adapted for roasting fine ore are not suitable for coarse ore, and the methods used for coarse ore are still less suitable for fine. Fine ores are generally treated in beds only a few inches deep, which are turned over and over to expose all parts to the air. With coarse ores this would be troublesome in various ways; they are, therefore, roasted in masses from 6 to 30 feet deep, the necessary air passing up through the mass between the lumps. Fine ore cannot be treated in this way, because it would pack so solidly that the air could not pass through it. A diameter of inch is probably the limit or boundary line between coarse and fine ore for roasting purposes, and in most cases the largest particles in a lot of fine ore are much