142. D. Concentrations rich in Gold and Silver, and containing much Lead, etc. It appears that certain rich ores, containing much lead, and other obstructive metals, do not yield the gold well when treated in the ordinary way. To meet this case Ottokar Hofman has devised and patented the following modification of the process.

The ore is subjected to a thorough chloridizing roasting, then washed with water, as described in the case of silver-bearing concentrations, leached for silver, and again washed to remove all hypo. It is then removed from the vat, dried sufficiently for chlorination, returned to the vat and chlorinated. The gold is then leached out, and if there still remains a considerable quantity of silver, the ore is again leached with hypo for its extraction. The results are said to be very satisfactory.


143. E. Unconcentrated Ore. This material, containing gold or silver, or both gold and silver, is treated in the same way as described for concentrations of similar character, except that, if it is only moderately charged with sulphides, the furnace is made hotter before being charged, and the roasting is effected more speedily, and with less draft in the furnace, the rule being only to allow as much as suffices to prevent the escape of fumes at the doors.

If, as often happens, the ore contains too little sulphur to effect the chloridation of silver, it is necessary, if that metal is present, to add a certain quantity, say four or five per cent of crushed pyritous ore, one per cent of sulphur, or from two to three per cent of copperas. The pyrites or sulphur must be mixed with the ore before roasting, but copperas may be put in later, as described in certain cases occurring in the treatment of concentrations.

144. Unconcentrated ores are not often treated for gold by lixiviation. They are crushed as coarsely as is compatible with good roasting, but nevertheless are often troublesome to leach, on account of clay, talc, etc., so that, while concentrations can be leached in a bed of from two to four feet in thickness, without difficulty, or special appliances, unconcentrated ores will, in many cases, not admit of more than ten inches, and, in extreme cases, they cannot be worked by leaching.

This difficulty is sometimes overcome by crushing the ore in a wet battery. The water removes the slimy matters which impede filtration, while the heavier portion of the ore is retained in "catchpits." This is in fact a species of concentration, although a very imperfect one. As the slimes usually contain a considerable quantity of silver, they must also be preserved, and, since they cannot be leached, must be treated in some other way. Amalgamation is usually adopted. On the whole, the plan is scarcely to be recommended. In cases of difficult leaching the suction pipe, (54) may be used.

145. When about to begin leaching with the aid of a suction pipe, it must be filled with liquid. When operating on ore which is not chlorinated with gas, this is done by connecting the pipe with the water supply, and passing the water upward through it, and the ore mass, as directed in beginning the washing of silver ore (139). If a vent pipe (55) is in use, it must be plugged as soon as the space below the filter is full of water. If the ore has been chlorinated for gold, it is not desirable to introduce the water in this way, because the solution of gold would be too much diluted. It is then better to let the pipe down into the trough, and stop the end with a tight plug. On introducing water into the vat, as described in the gold leaching, it passes through the ore, and, displacing the air, fills the pipe. When the water stands permanently above the ore, and air bubbles have ceased to rise, the plug is removed, the vent pipe stopped, and the leaching is allowed to proceed, aided by the weight of the column of liquid in the suction pipe, or, more correctly, by the pressure of a corresponding column of air. The suction pipe may be filled before charging the vat with ore, if so desired, and in the case of silver ore the water may even rise to the top of the filter, thus avoiding all trouble from air; but when chlorine gas is to be used, the water must not cover the aperture through which the chlorine is admitted.

To reduce friction, and give the best effect, the the suction pipe should be large; say, for a ten foot pipe, one and a half or two inches in diameter. It is impossible for it to take air at the lower end, with either of the arrangments mentioned, nor can any enter at the top as long as the vent pipe is closed and there is liquid in the vat. I have observed that a fall of six feet more than doubles the flow of liquid through a bed of ore fifteen inches deep.