Roasting

77. Roasting. When the sulphur begins to burn on the first hearth, the fire is kept quite low. The burning of the sulphur can be seen, even in the day time, if the furnace house is dark, as it should be, with no cracks or windows through which the sun can shine, a point of some importance, which is not sufficiently attended to. It is difficult to properly regulate the heat in full day light, but it is even worse if the sun shines in through chinks or windows, so that an open shed is better than an imperfectly inclosed room.

78. Continuous stirring of the ore is useless, for there is a certain quantity of sulphur to be burned, which requires a certain quantity of air, and there is no use in exposing a fresh surface until that exposed is burned, as far as can be done with the moderate heat allowed at this stage of roasting. The guide is, that the ore must be stirred as soon as it is seen to become dark on the surface.

79. The draft is regulated so as to cause as much air to enter as is consistent with the required heat; hence the doors on the working side are left open during all the earlier part of the roasting, though it may be nec-cessary to close them during the intervals of stirring, at least on the first hearth, for the finishing at a high heat. Even the back doors may be left ajar at the rear ends, at times, with great advantage, because the material now under consideration, being nearly half sulphur, requires an enormous quantity of air, without which it cannot possibly burn, and the more rapidly this is supplied the sooner will the oxidation be completed, bearing in mind, of course, that the proper degree of heat must be maintained. At the same time it must be remembered that a high heat at first is an actual disadvantage; the ore roasts faster and better with a low heat, until oxidation is almost complete.

80. In thus starting a furnace, the roasting of the charge on the second hearth will not make much progress until that on the first is so far advanced that a higher heat may be used, when it also will begin to burn, and must be stirred regularly, so that by the time the first is finished, probably in twenty hours, it will be half roasted, or more, and, when moved to the first hearth, will bear a good heat. The roasting is continued, under a gentle heat, as long as the sulphur burns actively, and when a blue flame, or a glow, is no longer perceived on stirring the ore, the heat is gradually increased until it reaches a light red, approaching yellow, if the ore will bear it without melting into lumps, as it will if it consists chiefly of iron or arsenical pyrites.

81. From what has been said about the burning of sulphur, it is evident that the stirring should be done at progressively shorter intervals until oxidation is completed. Care must be taken to stir quite to the bottom, and in every corner; and it is advantageous to clear the hearth entirely, a part at a time, and leave it so for a few minutes to allow of the oxidation of the thin stratum of ore which is not moved by the hoe. The stirring is mainly done from the front of the furnace, with the long hoe; but occasionally the roaster goes to the back, and with a small hoe explores the sides and corners within his reach, raking the ore out, and pushing it toward the middle of the hearth. In ceasing for a time to stir the ore, it is better to leave the surface ridged across by drawing the corner of the hoe over it, because the wavy outline thus given exposes a larger surface for. oxidation, or for heating. This, as remarked by Kustel in his valuable work on this subject, is important enough to be attended to.

82. As the heat, in this kind of furnace, is unavoidably greater near the fire than at other parts, the ore must be changed from end to end, so that every particle may, at one time or another, be brought near to the firewall, or bridge. This is done by drawing it with the hoe into a ridge along the middle of the hearth, or better, diagonally across it; then, with the spade resting on the roller bar as a fulcrum, moving that ore which is farthest from the fire, near to it, placing it on the empty space on the hearth, and the reverse. As long as sparks can be seen on flirting the ore with the hoe, or pouring it off the spade, the roasting is not perfect, nor does the ceasing of this appearance prove that it is so.

83. The roasting, under a strong heat, is continued until all the iron sulphate is decomposed, which may be known by taking a little of the ore on a small sampling shovel, and throwing it into a cup containing a little water. In a minute the water will be clear, and if, on adding a few drops of solution of potassium ferridcy-anide, or red prussiate of potash, a blue or green coloration is seen, the roasting is not perfect. After a little practice, this test may be dispensed with, as the iron sulphate is very easily decomposed. The presence of a little of it is not fatal to the chlorination, but it causes a larger consumption of chlorine.

84. Copper sulphate, if present, remains after iron sulphate disappears, requiring a long time and a high heat for its complete decomposition, which, however, is not absolutely necessary. It gives, with the ferrid-cyanide, a brown or yellow precipitate, which, in the presence of iron sulphate makes a green coloration by mixing with the blue of the iron.

85. If it is desired to roast to complete decomposition of copper sulphate, a test is made by washing a small quantity of the ore on a filter, in a glass funnel, with hot water. To the water which passes through the filter is added ammonia, and if even a very little copper is present, a blue color is produced. It will not do, in this test, to add the ammonia without removing the ore by filtration, because the blue color would then be produced by copper oxide, which remains after the sulphate is decomposed.

86. When the roasting is finished, a careful panning or horning of a little of the ore will, in general, show the gold plainly; if not, then grinding finely in a mortar, and panning, should make it visible, at least with a lens. But it happens with some ore that very little of the gold can be seen even in this way, perhaps because of its extreme fineness, causing it to be lost in the most careful washing. No sulphides should be visible, or only a few minute particles which the grinding may have brought out.

The roasting of the first charge being completed, it is drawn from the furnace and spread on the cooling floor. The second charge is moved down to the first hearth, and a third is put on the second hearth. The charge now on the first hearth, having been all the time roasting on the second, will, if it has been properly attended to, be almost done, and, with a good heat, will be finished in a few hours, while the third, having consequently but a short time to roast on the second hearth, will take longer on the first, when moved down, but this irregularity will gradually disappear, and after a few days the charges, if equal in quantity and properly attended to, will remain an equal length of time on each hearth, and the discharges will take place regularly.

87. A furnace with two hearths, or even with three, does not allow of a continuous strong fire. The heat is reduced each time that a charge is moved down, and not until a strong smell of burning sulphur is no longer evolved by the roasting ore, is the heat raised gradually to the finishing point. The great fault of most beginners is in using too much heat, especially in the early stages of roasting. A waste of fuel and of time is caused by using a higher heat than is necessary, for the oxidation proceeds faster under a low heat at first, and the ore assumes a better condition for the action of chlorine, being more porous, and lighter. Especially is this the case when lead is present. Even at the finish, too high a heat may melt the gold particles together, or to a spherical form, which is disadvantageous, because of exposing a smaller surface than any other for the chlorine to act upon. The workman must use judgment, and acquire experience, before he can roast well, and with the least consumption of fuel.

88. The roasting of concentrated sulphides, consisting chiefly of iron or arsenical pyrites, requires from 20 to 30 hours, in a reveberatory furnace with two or three hearths.

89. The roasted and partially cooled ore is moistened, by spraying with water from a hose, the finger being placed partly over the nozzle to spread the stream, or a sprinkler may be employed. To equalize the moisture, the ore is mixed by means of a hoe or shovel. The degree of moisture required varies. If the gold is in very fine particles, the ore is made only so damp as not to dust, and to cohere slightly on being compressed in the hand. If the gold is coarser, more moisture is required, sometimes as much as can be used without making the ore so wet as to settle into a compact mass, for it must be loose and porous in the chlorinating vats. The use of more water than is necessary is disadvantageous, because it causes a greater consumption of chlorine.

It is beneficial to allow the ore to remain a number of hours, even a day or two, in a moist heap before proceeding further, as this enables the moisture to penetrate and soften any lumps which may have been formed in the roasting. If this is done, the surface of the heap should be sprayed from time to time, to counteract drying, and before charging a vat the ore must be re-examined, to insure its being in the right condition as to moisture. It is next to be placed in the vat.

90. Charging the Vat. If the filter in the vat is made of sand, on the gravel, and is wet from previous use, it is advisable to dry it somewhat by laying on it a few hundred pounds of dry roasted ore, which soon becomes moist by drawing the water out of the sand, whence it will be seen that, if the moistened ore were put in it at once, it would become too wet, and, by packing, would obstruct the passage of the chlorine, causing a back pressure which would force the gas through the water joints of the generator. After the dry ore has been an hour or so in the vat, and has become damp it is removed, and the charging is proceeded with. If only gravel and burlap are used on the perforated false bottom, for a filter, this precaution is not needed, and time is saved.

91. When charging a vat for chlorination, the discharge pipe is stopped with an oiled plug, and hung up, and the moist ore is sifted into the vat. The lumps which are retained by the sieve, are removed from time to time, to be afterwards recrushed, with rollers or otherwise, and returned to the roasting furnace. Any scraps of iron which may have found their way into the ore, and which would be very detrimental to the chlorination, are removed by the sifting. As the ore cannot be spread evenly over the filter by the sifting, and as it packs a little where it falls, it is distributed and loosened with an iron rake. The vat is filled to within four inches of the top, and the ore is very slightly packed around the side, by pressing it with the hand, so that the chlorine cannot make its way between it and the staves, but is forced to permeate the entire mass.