C—3

164

The leaching-vats, as described, may be of any size or shape—round, square, or oblong—made of wood, and the inside, bottom of course included, brushed over with melted paraffin (2) ; a convenient depth would be 5-J-ft. or 6 ft. On the bottom is placed a filter-bed of (commencing at the bottom) coarse clean quartz pebbles, -Jin. to fin. size, and free from pyrites or other sulphides, this layer occupying, say, 6 in. or 7 in. in depth ; resting on this another layer of smaller quartz pebbles or gravel ; and on this again still smaller quartz gravel, or coarse sand, the whole being topped with fine clean sand. The whole depth of such filter-bed would be 12 in. to 14 in. There must not be any sulphides or any other reducing agents, or sea-shells or limestone, or black oxide of iron in these filter-beds, nor, indeed, anything that reduces the colour of the permanganate solution when steeped in it for a day or two. Quartz pebbles and quartz sand, when free from pyrites or other sulphides, answer the purpose very well, but should be tested by steeping samples in a little permanganate solution for, say, two days : if the solution by the end of that time will be found to retain its true colour, the materials will be suitable for making the filter-bed. The bluestone basalt road-metal at Dunedin, broken up, is found very suitable ; but the scoria about Auckland and the road-metal there is utterly unsuitable, as it contains a good deal of the magnetic or black oxide of iron, which has a very deleterious effect on the permanganate solution, and was a source of much trouble in experiments conducted in Auckland in March last. For small laboratory experiments pounded glass makes a good filter-bed ; but if it be pounded in an iron mortar it is liable to contain small particles of metallic iron, which is also very injurious to the solution. The ore, crushed and passed through a 30-wire sieve (900 holes to the square inch), or the concentrates, or concentrated tailings, as the case may be, after thorough dead- or sweet-roasting, and tested for deadness as described above, are placed in the leaching-vat on the filter-bed to the depth of 3 ft. to 4 ft. or 4-Jft., and the permanganate solution run on gently and evenly till it covers the ore and stands a few inches above the level of it. The solvent action is so rapid that the short wooden pipe leading to the precipitating-vat or charcoal filter-bed may be opened at once. The permanganate solution is then allowed to trickle slowly from the solution-vat (1) into the leachingvat (2) at the same rate as the outflow from the leaching-vat into the precipitating-vat or filter-bed. The percolation proceeds much faster with the acid permanganate than with alkaline cyanide. Gold begins to make its appearance in the outflow usually within a quarter of an hour to half an hour after this overflow begins. The presence of gold in the outflow is indicated by the greenish-blue or greenish-violet colour produced when some ferrous sulphate solution is added to a portion of it in a test-tube or a wine-glass. The richness in gold of this outflow increases for some time till it reaches a maximum, then it goes level (continues of the same quality) for some time, varying in different ores (according to the coarseness of the particles of gold) from an hour up to perhaps five or six hours, or still longer in some cases. The richness then diminishes very gradually ; and when this diminution is thoroughly pronounced it may be taken to indicate that all the recoverable gold is in solution, and only requires washing out with water instead of the permanganate. The perman-ganate-solution tap is therefore closed, and leaching with water continued for a few hours till very little or no more gold is coming through. The man in charge of the leaching operations tests a sample of the outflow from time to time in a glass test-tube or wine-glass by adding to it a small quantity of the protosulphate-of-iron solution, and he judges the degree of richness by the depth of greenish-blue or brown colour produced. At Mount Morgan Mine, in Queensland, the process-manager recognises eight degrees of richness by the degree and shade of colour so produced by the sulphate of iron. The degrees are— (1) light trace, (2) trace, (3) strong trace, (4) light black, (5) black, (6) strong black, (7) rich, (8) very rich. If the outflow should not show the gold indication nor the pink colour for any considerable time—say, an hour or so—it is something in the charge that is robbing it, owing to imperfect roasting. A sample of it should then be tested as follows: A small quantity of the permanganate solution—say, five drops—should be put in a wine-glass or test-tube, and the outflow should then be run into this ; when, if it rapidly destroys the colour of the five drops of permanganate, it is an indication that, from insufficient roasting, there is in the charge in the leaching-vat either protosulphate of iron or some metallic sulphide or arsenious acid, or the sulphide of some metal. Of course, if the test for effective dead-roasting or calcining mentioned near the beginning of this article were properly made, and with a satisfactory result, this will not happen, and the gold will come early. As the leaching proceeds, the purple-red-violet colour will be deepening in the outflow, and could, when very deep, be economically returned into the vat with all its load of gold in it; for so long as the rich red colour is there the solution is capable of dissolving more gold, and so enriching itself still further. The gold is recovered from the leach in the precipitating-vats by the addition of a strong solution of freshly prepared green crystals of ferrous sulphate (protosulphate of iron, or "green vitriol "). This solution of sulphate of iron is made by dissolving the green crystals in water. The addition of iron nails and some sulphuric acid keeps the solution fresh for a long time—many days ■ —and strengthens it by the formation of more sulphate by the action of the acid on the iron. Instead of dissolving the green crystals of ferrous sulphate, the iron precipitating solution may be made by steeping scrap-iron, pieces of iron (not galvanised), fencing-wire, or iron nails, or indeed any pieces of iron, in dilute sulphuric acid, one part of strong acid to eight or ten parts of water, or even weaker still. The first, effect of the sulphate-of-iron solution is to quite decolourise or bleach the permanganate colour in the precipitating-vats ; and enough must be added to do this. The next and almost immediate effect is to throw the gold out of solution, or, in other words, to precipitate the gold in a very fins state of division—so fine, indeed, that the precipitate has not the ordinary yellow colour of gold at all, but appears dark-brown when there is much present, or bluish-green or violet when there is only a little present. The sulphate of iron solution must be stirred into the liquid very thoroughly, so as to come into full contact with every part, and thus precipitate all the gold. The fine gold will settle to the bottom in the form of a brown-coloured gold slime, which is really pure gold, in the course of twenty-four