The construction of the ordinary reverberatory furnace is too well known to need detailed description here. It consists of a vaulted chamber, containing the ore; through this chamber, the flames and products of combustion from a reverberatory furnace and a current of air are made to pass in a horizontal direction above the ore, which is thus heated. The ore is also stirred by hand with iron rakes, which are passed through small working doors. The hearth of the vault (also called the “ laboratory ” of the furnace) is formed of bricks placed on edge, as close together as possible. No mortar is used, but a little clay is plastered between the bricks. The height of the furnace hearth is about 3½ feet above the floor of the building, on which the labourers stand, and the space underneath the hearth is either occupied by vaults or filled with well tamped rubble. The arch is usually one course of bricks (8 inches) thick; the height between it and the hearth is, in long furnaces, about 24 inches near the bridge, and gradually diminishes towards the other end. This height is less in short furnaces. The best firebricks are used for the fire-box and bridge, and for the hearth and arch of the first few feet of the “laboratory.” The remainder is made of common brick. It is necessary to have a damper in the flue to regulate the draught; the aperture of the flue should not be on a level with the hearth, as in that case the loss by dusting is increased. The brickwork of the reverberatory furnace is supported by longitudinal and transverse iron braces. The working doors have cast-iron frames, and are about 15 inches wide and 9 inches high. The fuel used must of course be a long-flame coal, or wood ; short-flame coal and coke are inadmissible.
For the particular purpose of roasting pyritic ores before chlorination, the temperature on the working floor of the furnace is kept low when the ore is first charged in, and high in the later stages. If a single small floor is used the fire must be alternately checked and urged to secure these conditions. Moreover, when the roasting is nearly complete, the high temperature required renders the gases passing into the flue very hot, and so a corresponding waste of fuel results. To prevent this waste, it is customary for roasting furnaces to be built with a very long hearth, or to have several successive hearths, so as to utilise the waste heat from the portion of the working floor next the fire. Reverberatory furnaces with three floors at slightly different levels are much favoured ; in these a charge remains for a few hours on each of the floors in succession. It is first placed on the floor farthest removed from the fire, and, after a time, is raked down on to the middle hearth, and thence to that nearest the fire, fresh charges being put on the spaces just cleared, so that there are always three charges in the reverberatory furnace in various stages of oxidation.
The most usual form in the Western States of America is the “4-hearth,” in which the length of the hearth is four times its width, so that the dimensions are, say, width 15 feet, length 60 feet. In this case there should be eight working doors on each side. Instead of three floors at different levels, a single continuous floor, gently sloping from the flue towards the fire, is in use at many works in Australia, Mexico, and the United States. At Suter Creek a continuous-hearth furnace was 12 feet wide and 80 feet long, and the mineral was worked in three distinct parts, as though there were three floors. The angle of slope is made large in some Australian furnaces, so that in the course of the “rabbling” or stirring, the ore continually travels towards the fire-box. Furnaces with two or three superposed floors are also used to a limited extent; the lowest floor is next the fire-box, and communicates by a vertical flue with the floor above, and so on. The ore is charged-in on the top floor, and after a time is raked down through the vertical flue on to the next floor. In this case the floors are heated by the gases passing below them as well as above them, and fuel is economised, but the reverberatory furnaces are costly to build and to keep in repair. Merton’s furnace, subsequently described, is one of this kind with automatic rabbling.
It was proposed many years ago to insert a drop of 10 feet between the finishing floor and the floor next to it. The charge, when already red hot, would thus fall vertically downwards in a thin shower against a current of hot air. Some furnaces are said to have been built in this way, but it seems that none are now in existence. The principle is excellent, and is utilised in the Stetefeldt furnace.
When the pyrites to be roasted are rich, it may be an advantage to build dust chambers to ordinary reverberatory furnaces. The amount of gold contained in the dust thus recovered is usually only 1 or 2 per cent, of that contained in the ore, so that in some cases it may be a long while before the dust chamber pays for itself, even if that point is ever reached.
The operation of roasting pyrites in an ordinary furnace with three floors may be described as follows: The furnace being hot, and the flame from the fire-box reaching completely across the first flour, the ore is charged-in on the third floor and spread out by the rabbling tool. The weight of the charge may be taken as from 12 to 18 pounds per square foot of floor space, varying according to the nature of the ore, a high percentage of sulphur necessitating small charges. The layer of ore is 2 or 3 inches deep. It is not spread quite evenly, but made to form a series of parallel ridges by means of the rabbling tool, so as to increase the surface exposed to the air. The working doors may be closed at first to heat the ore quickly. Moisture is at once given off in great quantities, and the sulphur soon begins to burn with a blue flame. When this is seen to take place, all the working doors are opened, and the charge is energetically rabbled, with little intermission, until the sulphur flame disappears. If this is not done, clots are formed which are afterwards difficult to break up. The air for the combustion of the sulphur is supplied by holes in the fire-bridge and from the working doors. The flames and heated products of combustion from the fire tend to rise above the colder air, and move along next to the arch of the reverberatory furnace, while the air forms a, sheet between these gases and the ore. In practice, although the air is introduced below them, nevertheless the reducing gases from the fire partially mix with the air, and greatly reduce its oxidising power. Moreover, the combustion of the sulphur in the ore on the first two floors further reduces the amount of free oxygen present in the current of air, and roasting on the third floor is, therefore, largely dependent on air derived from the working doors.
When the sulphur flames have abated, and the charge has been heated almost to redness, it is transferred to the middle floor, where it is raised to a dull red heat and most of the sulphating takes place. During this stage the ore swells considerably, so as to occupy much more than its original bulk. All the lumps previously formed should be broken up on this floor. Dabbling is continued until the ore is uniformly dull throughout, so that, on turning it over, the fresh surfaces appear but little brighter than that which has been exposed for some time. The charge is then transferred to the floor next the fire. There is now little risk of the formation of lumps, and the charge may be allowed to reach a bright red heat. Rabbling is of less importance than before, as little oxidation takes place, the chief reaction which occurs being the decomposition of the sulphates already formed. As long as this is still going on, the ore emits the odour of sulphur dioxide. When no further odour can be detected, and the ore can be piled up so as to maintain a vertical face, shows no bright specks on its glowing surface, emits no sparks if some of it is tossed up by the working tool, and is inclined to become black very readily from cooling, the charge is said to be “dead” or “sweet” and is ready to be withdrawn. It should be observed that, when the ore contains much sulphur, its particles at a low red heat, appear less coherent than when cold, and flow almost like water so that the charge cannot be made to form a heap with steep sides. Care must therefore be taken when the ore is on the middle floor to prevent any part of the charge from flowing out of the working doors, which it is very liable to do when being rabbled.
Kustel states that the best means of rapidly ascertaining whether a charge is completely roasted is to throw a little of the ore into some water, and then to plunge a bright iron rod into the liquid. If the rod remains bright the ore is ready for withdrawal, but if sulphates still remain undecomposed, the surface of the iron will be darkened. This is not a safe test with all classes of ore, as the presence of sulphate of iron in the water would not be detected in this way. A more trustworthy and equally simple test is to add a few drops of chloride of barium to the water. A white cloud, which consists of BaSO4, indicates the presence of soluble sulphates. In most cases the water need not be filtered before it is tested, but even when filtration is necessary the whole operation can be performed in two or three minutes.
Certain insoluble sulphides may be detected by boiling the roasted ore with caustic potash, filtering, and adding a solution of lead carbonate. A brown colour or black precipitate denotes the presence of sulphides. This test is used at the Great Boulder, according to D. Clark.
Although roasting is carried on until the ore is said to be “dead,” it is not practicable to eliminate the whole of the sulphur, and a small percentage both of insoluble and of soluble sulphur can be found in roasted ores. The insoluble sulphur is, of course, more detrimental to gold extraction by chlorine than is soluble sulphur. It is not usual in roasting for chlorination to leave more than about 0.1 to 0.15 per cent, of insoluble sulphur in the ore, but much larger percentages of soluble sulphates are often left undecomposed in roasted ore.
The charge is withdrawn by a scraper, and falls by gravity through a hole in the floor of the reverberatory furnace near the working door into a pit below. This hole is covered by a plate while roasting is being performed.
The time of roasting depends chiefly on the ore, but may be shortened by more continuous rabbling than most workmen can perform, the work being somewhat exhausting. In a three-floor reverberatory furnace, concentrates with 15 per cent, of sulphur usually remain eight hours on each of the three floors. The fuel used is either wood or flaming-coal. If the flame from the coal is not long enough to reach across the first floor, this will not be heated uniformly; in that case the part of the charge next the fire-bridge is finished first and must be moved away, while that from the other end of the floor is brought up nearer to the fire. This causes a great increase in the labour, besides occasionally leading to the withdrawal of a charge of which a part is not quite “ dead.” The draught is regulated by the damper in the flue leading to the dust chamber, and by opening and closing the working doors.
The consumption of coal for fuel is usually from 10 to 20 per cent, of the weight of the ore.