Chlorination

92. Chlorination. Time may be saved by starting the chlorine generator as soon as the vat is about half charged, as the filling will be completed before the gas can reach the surface.

The progress of the chlorine is watched, after the charge of ore is in, by making a few little excavation and holding therein a glass rod dipped in ammonia, which instantly shows the presence of chlorine by a dense white fume. The moist stopper of the ammonia bottle may be used instead of a glass rod. As soon as the chlorine has risen to within a few inches of the surface of the ore, the holes are lightly filled, and the cover is let down into its groove, or rabbet, and luted with a paste made of clay and sand, in such proportions as not to crack in drying; or the luting is kept moist by placing wet cloths on it. The plug hole in the cover is left open until the vat is quite filled with chlorine, as shown by the fume when ammonia is held to the hole, which is then tightly plugged.

If at this time the chlorine generator is not required for another operation, and if the evolution of gas has become so slow that it only bubbles through the wash bottle at intervals of many minutes, it is well to leave it connected with the vat; otherwise it is disconnected, the lead nipple in the vat is plugged, or the vent pipe connected, and the chlorine pipe is either applied to another vat, or the end is passed out of doors. The vat is now left undisturbed for from 12 to 40 hours, to allow the chlorine to act on the gold.

When two or more vats are impregnated at one time, the forked lead pipe is used, the flow of chlorine to each vat being controlled by a pinch-cock, for which a clothes pin answers very well, applied on the rubber part of the branch pipe; or a lead pipe may be laid alongside of the vats, with a nipple and lead faucet to connect with each of them by a short piece of rubber tube.

93. Leaks in any part of the apparatus are easily located by the aid of ammonia, as above explained, and the operator's nose should be excused from this duty, or serious consequences may follow. Leaks in pipe-joints can usually be stopped with oil or lard, and those in the vat-cover with a paste of oil and sand, or earth. The charging of a vat accupies from one to two hours, as the sifting is very tedious.

94. Making Chlorine. To produce enough chlorine for three or four tons of roasted ore, the charge for the generator is about:

Manganese........................... 30 pounds

Salt. . .......................... 40

Water........................... 36 "

Sulphuric acid of 66° Beaume........... 70 "

Or the same of salt and manganese,

Water............................... 32 "

Acid of 63° Beaume................... 74 "

The cleaning out pipe of the generator being securely plugged, the salt, manganese, and water may be put in at any time, and stirred with a wooden paddle. The acid is weighed or measured, and kept at hand in a vessel of lead, or enamelled iron, though the enamel generally soon peels off. The naked iron does very well for strong acid. The cover of the generator being let down, the water joints all filled, and the pipes connected, a fire is made in the fire-place, or steam is turned into the heater, to warm the apparatus some time before the chlorine is wanted.

When all is ready for the chlorine to enter the vat, a portion of the acid is poured into the generator through the funnel, and mixed by turning the stirrer. Chlorine soon begins to pass, and can be seen in the wash bottle by its greenish color. It must pass in a lively stream through the water, and when the current slackens, more acid is put in until the whole is used.

From time to time the stirrer is turned carefully. Sometimes it must be lifted a couple of inches before the impacted stuff can be loosened, and this is the reason why the water joint in this part is made two inches deeper than in the others; otherwise the lifting of the stirrer might cause an escape of gas which would be injurious, and might be dangerous, to the workman. During the operation the apparatus is gradually heated, but not so as to make the contents boil.

By the time the charge is exhausted the test by means of ammonia should indicate an abundance of chlorine issuing from the holes in the vat covers. If this is not the case, more manganese and salt must be put into the generator, through the opening provided for that purpose, but, as the workman is liable to suffer serious inconvenience from the escaping gas while doing this, it is better to avoid the necessity by putting in enough at first. If an additional quantity of the other materials is used, more acid will also be required, unless an excess was taken for the charge, but this can be introduced without inconvenience by means of the pipe and funnel. The relative proportions of the materials vary according to their purity, and the quantity of chlorine required depends much on the character, and more or less perfect roasting, of the ore; hence no fixed rule can be given for either.

95. When it is desired to prepare the generator for another operation, the end of the rubber tnbe leading from the wash bottle is passed out of doors, or connected with a charged vat. Water is poured into the water chamber, b, and passes to the interior of the generator through notches cut in the lower edge of the cover for that purpose, and fills it, expelling all gaseous chlorine, which would otherwise escape into the room on raising the cover, or removing the plug at d. The water is allowed to stand for some time in the generator, to dissolve the sodium sulphate formed by the action of sulphuric acid on salt, which sometimes forms a hard incrustation, if too much heat has been applied. The plug at d is then withdrawn, and the generator is emptied through the pipe, which should deliver out of the room.

If there is any impacted residue which cannot be removed by a stream of water from a hose, directed through one of the openings, the main cover is raised and the stuff carefully scraped off the bottom, but if not, the recharging can be done by the opening, h, without raising the cover. If the residue contains much unchanged salt, the proportion used should be reduced. An excess of manganese shows itself by its blackness. When the ingredients have been correctly proportioned, only a dark grey sediment remains with the more or less acid liquid.