163

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while the sulphur will be partly removed in fumes, and partly converted into the harmless sulphate of soda. The difficulty with antimony-sulphide may also be met fairly, provided the ore does not contain more than, say, 5 or 6 per cent, of that mineral, by roasting without salt, but keeping the temperature down so as not to fuse the sulphide, and keeping the draft strong, and very slowly raising the temperature, but so as not to fuse any part of the charge, and finishing up with a brightred heat, continued for several hours, or until fumes cease to come off, and till the charge answers the test given above. This treatment will send most of the antimony off in fumes, and convert what remains of it into the tetroxide of the metal, which has very little injurious action on the permanganate solution. Ores containing much carbonate of lime, carbonate of magnesia, or other carbonates, are not amenable to the process, as there would be an expenditure of acid incurred in the proportion of about one part of sulphuric acid for one part of carbonate of lime in the ore. This means for every ton of ore an expenditure of 22f lb. of sulphuric acid (costing, say, 25.) for every per cent, of carbonate of lime in the ore. If, for example, the ore contained 5 per cent, of carbonate of lime, 1 cwt. of sulphuric acid (costing, say, 10s.) would have to be added to each ton of ore before the permanganate solution would dissolve any of the gold. The presence of much lime would be objectionable also (even with the addition of the acid) from the fact that the sulphate, not being very soluble, would tend to clog and retard the leaching. The plant required for the permanganate process will be—(l) a vat in which to make up the solution ; (2) leaching-vats; (3) either precipitating-vats or else charcoal filters, and in this last case a neat, clean, small furnace in which to burn the charcoal, from the ash of which the gold is recovered by fusion with borax. The solution- and leaching-vats, (1) and (2), should be made of wood (the harder the better), just similar in every way to the wooden vats now used for the same purposes in the cyanide process. But as wood, especially green new wood with its sap in it, has a reducing or decolourising and therefore destructive effect on the permanganate solution, these vats should be painted inside with a coating of paraffin, which has no reducing or injurious action on the solution. The paraffin coating or lining should be put on in the following way : The solid paraffin, or paraffin wax as it is called (costing about Bd. per pound), is melted in an iron pot over the fire, and heated till fumes begin to rise from it. It is then brushed in the melted state by means of a brush on the inner surface of the vat, in patches of 1 or 2 square feet at a time, till the whole inside surface is thoroughly covered with a skin of the paraffin. A hot flame, as from a painter's scarifying lamp (costing £1), to heat the wood strongly in front of the brush, secures a more durable coating of the paraffin, as it sinks more into the hot wpod and takes a better hold of it. Any metal surface, such as the heads of nails inside the tank, should also be carefully paraffined, as all metals have a very destructive action on the permanganate solution. A coating of paraffin would do no harm to the precipitating-tanks, but it is not necessary in that case, and may be cheaply substituted by an application of a mixture of pitch and tar to prevent absorption; or, as recommended by Eissler and others, the precipitating-vats may be lined with sheet lead. Neither lead nor any other metal, nor the mixture of pitch and tar, can, however, be used for lining or coating the solution- or leaching-vats —(1) and (2). Indeed, an application of paraffin as above described is the only efficient lining for these vats. Glass would do, or tiles, or slate; but wooden vats lined with paraffin seem preferable from an economic point of view to any other materials for these (1) and (2) vats. The vats may be of any size and any shape; square or oblong would be just as good as round, and probably cheaper. The great thing is to have them well paraffined to begin with. One thorough dressing of paraffin as described should not require renewing for months, but if at the discharging of a vat a naked place is noticed it will be an easy matter to rub it dry and apply the paraffin. The permanganate solvent solution contains 12 lb. of common salt, about 14 lb. of strong sulphuric acid, and about 6 oz. or 7 oz. of permanganate-of-potash crystals, all dissolved in every 100 gallons of water. The solution is made up as follows : The salt and permanganate in the proportion named above are weighed out and thrown into the paraffined solution-tank ; the water is then run in, and the contents of the vats stirred, so as to dissolve the salt and permanganate. The sulphuric acid is then mixed with about six or seven times its bulk of water in a separate vessel —say, a stoneware jar, glass jar, paraffined wooden bucket, or vessel of any kind lined with lead (the acid being poured into the cold water and not the water into the acid). This acid mixture is then poured into the vat, which already contains the salt and permanganate, and the whole is stirred with a wooden rod or stake, to insure a uniform mixture. The wooden rod should be withdrawn as soon as the stirring (which need not occupy more than half a minute) is finished, as bare wood weakens the solution. The solution thus prepared has a fine deep violet-red colour and an acid salt taste. It is quite harmless in small quantities; indeed, when mixed with eight or ten times its bulk of water it would make a very wholesome and agreeable summer drink. So long as the solution retains a violet or reddish or pink colour it has the property of dissolving gold, and may be used over and over again for this purpose, becoming richer in gold and paler in colour at each time. It cannot, however, be used again after the gold has been precipitated from it, as the same agents that throw down the gold will bleach or decolourise the solution, and thereby utterly destroy its power of dissolving gold. So soon as the reddish or violet colour is destroyed, from whatever cause, the gold-dissolving power is quite gone. If the colour is weakened,or made paler it may, however, be restored by the addition of more permanganate of potash, either with or without the addition of salt or sulphuric acid. In some cases, when the colour is nearly gone but the acid taste still perceptible, it is economical to revive the colour by a small addition of permanganate dissolved in water. One soon becomes acquainted with the colour of the most effective solution, and it is an easy matter to keep the colour up by such additions.

22—C. 3.