53. It will be noticed that the size of the ore chambers j enlarges somewhat from the top to the bottom. This is to provide for the expansion that takes place in the ore while roasting.

For calcining carbonate ores to remove carbon dioxide CO, or roasting such iron ores as contain small percentages of sulphur that must be expelled, the radial partitions a are not used. Also, for such ores the combustion chamber f is extended downwards to include the chamber s, because no reducing effect is needed.

CHEMISTRY OF ROASTING

54. In roasting sulphide ores, the sulphur and the metals are both oxidized, most of the former passing off in the gaseous state as sulphur dioxide SO,, while the metallic oxides remain in a more or less porous condition.

PbS+30 PbO + SO2
ZnS+30= ZnO + SO,

Cu,S+30= Cu ̧O + SO2

Cu,S+40=2CuO + SO2

55. A certain amount of the sulphur is burned to sulphur trioxide SO,. Some of this passes off in the draft, but part of it unites with the metallic oxides to form sulphates, which are not volatile, for example:

and

=

PbOSO, PbSO,
CuO + SO, = CuSO

The sulphur trioxide SO, also acts as a very efficient. oxidizing agent, both oxidizing sulphides and changing the lower to the higher metallic oxides; for instance,

and

Cu2S + 3SO1 = Cu2O+ 4SO,

Cu2O +SO ̧= 2CuO +SO,

56. In roasting pyrite FeS, there are several distinct stages. One of the two atoms of sulphur in this mineral is quite readily volatilized (FeS, + heat = FeS +S). The first stage in the roasting consists in the oxidation of this one

atom of sulphur, which burns with the characteristic blue flame of sulphur. In proof that this atom of sulphur is actually volatilized, it is only necessary to point to the fact that by scooping out small holes in the top of an open heap of roasting pyrite and lining these holes with fine ore, a certain amount of sulphur can be collected in them. Indeed, this sulphur sometimes condenses in the upper part of the heap during the early stages of roasting to such an extent that it closes up the air passages and so hinders the roasting. Any such crust must be broken up. This same crust forms on the surface of the ore when roasting fines in hearth furnaces. Consequently the rabbling should be frequent during this period; but after the first atom of sulphur has been burned off, the rabbling need not be done so often.

57. During the second stage of roasting, the iron and the remaining atom of sulphur are oxidized, according to the following equation:

FeS+30= FeO + SO2

Much of the ferrous oxide FeO formed by this reaction unavoidably becomes further oxidized to ferric oxide Fe,O,. The ferric oxide is undesirable in smelting, because it has to be reduced again in the smelting furnace.

The

58. As stated in Art. 2, the quantity of sulphates produced is greatest when the temperature is comparatively low, when there is not too much draft, and when the bed of ore is rather deep. It is because these conditions are so well fulfilled in heap and stall roasting that unusually large amounts of sulphate are formed by those methods. temperature at which sulphate forms most readily differs for different metals. Iron is sulphatized at a lower temperature than the other metals, yielding ferrous sulphate FeSO,. As the temperature increases, this is decomposed into ferrous oxide FeO and sulphur trioxide SO,, or sulphur dioxide and oxygen SO, +0. The sulphatizing and decomposition of the other common metals proceed in about the following order: copper, silver, zinc, and lead. It is to be understood,

however, that one metal is not completely sulphatized before others begin to be; and the same may be said in regard to the decomposition of sulphates. Zinc and lead sulphates are decomposed only at very high temperatures. In the slagging of lead ores, described in Art. 14, some sulphur is eliminated by the following reaction:

2PbSO, + SiO, = Pb,SiO, +2SO, +20

59. Arsenic and antimony behave in roasting much the same as sulphur, but a larger proportion of these are oxidized to the non-volatile condition (arsenates and antimonates) than in the case of sulphur. On the other hand, some arsenic and antimony may be volatilized as sulphide before oxidation takes place.

60. In the reducing roasts, oxygen is taken away from the higher oxides and from sulphates, arsenates, and antimonates by carbon, carbon monoxide, and sometimes by sulphur, where iron disulphide is used as a reducing agent. The reactions differ with different conditions of temperature, etc. The following examples are given of this reduction:

Fe„As„O ̧+2C = Fe„O, + As„0, + 2CO

8

3

61. In the chloridizing roast of silver ores, the principal reaction is

Ag2SO, +2NaCl = 2AgCl + Na2SO ̧

It often (and with the Stetefeldt furnace probably always) happens that the silver is not completely chloridized in the furnace; but if the hot ore is left in a heap for a number of hours, certain chemical reactions take place, which in many cases greatly improve the chloridization.

62. The roasting of carbonate ores consists simply in calcining the ore; that is, driving off carbon dioxide by heat.

In the case of iron carbonate, it is practically impossible, commercially, to prevent the FeO being oxidized to Fe,O,, though this is not done purposely.

In the various kinds of roasting there are, in addition to the reactions given above, various others of a more or less complicated character, which it is not necessary to discuss here.

63. Fig. 19 shows the Argall multitubular roaster. It consists of four tubes, or cylinders, a arranged side by side. and held together by two heavy tires b. Each of these tires rests upon a pair of grooved friction pulleys c set so that the roaster has a little slope from the feed end d to the discharge end e.

Each of the

tubes a is 29 feet long and 25 inches in diameter inside the

brick lining. At the feed end, the tubes are set into the hood d, into which the ore is fed from the hopper f. As the furnace is revolved by the friction pulleys c, the ore enters the tubes a. The revolution, combined with the slope of the furnace, causes the ore to gradually travel to the lower hood e, from which it discharges through the holes g into the receiving hopper h. The furnace is heated from the firebox i, which is arranged to burn oil, but can, of course, be modified to burn coal. The oil, mixed with air, is introduced through the holes k as a spray, and the hot gases pass into the hood e and thence through the tubes a of the roaster and pass off to the dust chamber m and a chimney beyond.

The object of using four cylinders instead of one is to provide a larger amount of brickwork to absorb the heat from the firebox, this heat being again given out to the ore.

The furnace has been used to roast some of the gold ores of Cripple Creek, Colorado. When treating 48 tons of ore in 24 hours in one furnace, the 2 per cent. of sulphur originally in the ore was reduced to 0.1 per cent., according to statements made. If such is the case, the roaster is entitled

to take rank among the best.

64. The Zellweger roaster has been recently introduced at Gas City, Kansas. This furnace, while a straightline reverberatory 135 feet long by 15 feet wide, has a rolling stirrer that moves slowly from the feed to the discharge. The stirrer consists of a heavy shaft carried by wheels, 6 feet in diameter, rolling on tracks in the depressed wheel pits on either side of the hearth. On this shaft are a number of collars carrying blades for rabbling the ore. These collars lock on the shaft when traveling from the feed to the discharge end of the furnace, and as they revolve with the shaft, the blades scoop up ore during one half a revolution and discharge it during the other half, thus gradually moving the ore forwards. During the return trip, the stirrers revolve around the shaft with the collar, but do not displace the ore more than to rake it. This furnace has external