C.—3

206

The average extraction from acid ore, say, from sdwt. or 6dwt. stuff, may be put clown at 70 per cent. Concentrates of 3oz. value will give an extraction of from 90 to 98 per cent, in three or four weeks' treatment. With regard to the treatment in the presence of metals having an affinity for cyanogen, the principle usually adopted is to apply the weakest solution which has been approved by experience. Having obtained the gold in solution, precipitation in the zinc-boxes is, as a rule, a simple matter. The only points requiring attention are to keep a sufficient stock of zinc-shavings in the compartment, and to regulate the flow of solutions so as not to incur danger of the fine precipitate being carried away. The zinc-shavings are prepared usually by turning zinc dies on a lathe. Precipitation of the gold varies somewhat with the different classes of ore. Its completeness appears to depend, in a measure, in a slight excess of cyanide of potassium being present in the solutions. Eoughly speaking, it may be said that if the solutions leaving the boxes assay more than 2dwt. per ton the precipitation is not what it should be. This may be caused by the solution flowing too quickly, or to the paucity of zinc in the boxes, or it might be to insufficient cyanide being in the solution. The zinc-shavings in the boxes may require replenishing every day to replace the amount consumed, or may run a week at a time without requiring attention. The construction of zinc-boxes is shown on Kg. 8. The box consists practically of a wooden launder fitted with baffle-boards, which divide the launder in such a way that solutions are forced upwards through the zinc-shavings with which the large compartments are filled. The top division is advantageously used as a settling-tank to collect any sand which may come through the filter, or a separate settling-tank may be used, particularly in plants where solutions are pumped direct from the filter-vats. The zinc-box compartments are fitted with removeable trays, consisting of wooden frames supporting wire gauze of about -Jin. mesh. This, while it carries the zinc, allows most of the fine gold precipitate to fall through into the bottom of the box. The filter —launder—shown on the side of the zinc-box need not necessarily be attached to same, but may be connected by a small wooden V launder with the plugholes in the zinc-box compartments. The zinc-box sketched is large enough for each 1,000 or 1,500 tons of monthly-plant capacity. But, although it is generally found best to have at least two zinc-boxes to a plant, it is, of course, not necessary to increase the number of them in proportion to the tonnage to be treated. The increase in precipitating-capacity can just as well be attained by increasing the width of the boxes. Having reconverted the gold into metallic form by passage of the solution through the zincshavings, we have it as a sludgy-looking precipitate commonly known as " slimes," and the next step is to get it into marketable shape. This is done by separating from the zinc, drying, roasting, and smelting. The clean-up, which takes place once or twice a month, is conducted as follows : — A small amount of clean water is run through the boxes to remove the cyanide solution which might otherwise be injurious to the workman. The zinc-shavings are taken out, being twisted and scrubbed in the water to remove, as far as possible, anything adhering to them. In some cases there is quite a thick plating on the zinc which cannot well be removed by scrubbing, but this is usually ignored, as the shavings are returned to the boxes, and this plating will go into the precipitate of the next clean-up. Having removed nearly all the coarse zinc, the precipitate contained in the water is allowed to settle; an addition of a little alum will considerably assist this process. Most of the clear water is then siphoned or pumped off, and the precipitate, together with the remaining small quantity of water, is drawn off into calico, or linen, or filter press. After drying sufficiently to handle with a scoop the precipitate may be further dried in iron pots, and is then ready for roasting and smelting. The object of roasting is so to oxidize the greater portion of the zinc, which has, in the form of small chips and shavings, fallen through the zinc-box trays, so as to cause it to combine in the subsequent smelting with fluxes, and leave the bullion fairly fine. Oxidation by the aid of atmospheric air is sufficient, but a certain amount of the zinc-oxide subsequently becomes reduced by the carbon of the plumbago melting-pots and re-enters the bullion. A good method of roasting has been found by adding a little nitre, say, from 3to 10 per cent, to the precipitate. It is best applied as a strong solution before drying the precipitate so that it gets equally mixed with the whole mass. In the subsequent roasting the nitre not only assists by yielding up oxygen to the zinc, but to some extent also appears to flux the zinc-oxide, forming zincate of potash, which is not so readily reduced as zinc-oxide. In case the precipitate is very sandy, owing to tailings coming through the filters, nitre roasting is not successful, as it tends to cake. By the addition of nitre the tendency of the precipitate to dust on stirring up in the roasting furnace is minimised. The amount of flux required in smelting is reduced, and the resulting bullion is better. In roasting the precipitate care should be taken not to raise the temperature much above a dull red heat so as to avoid partially fusing it into a pasty mass, and not to stir too violently, especially at the commencement of the roast, or dusting and consequent loss will be the result. The furnace sketched in Fig. 12 will conveniently take 501b. of precipitate at a time. The precipitate having been sufficiently roasted, the next step is to mix it with suitable quantities of flux, and smelt in plumbago-pots. The fluxes commonly used are bicarbonate of soda, borax, and sand. In the case of sandy precipitate, of course, further addition of sand is omitted, and it may be found advisable to add a small quantity of fluor spar. The proportions of fluxes and precipitate vary within very wide limits. For general guidance, it may be stated that where much sand is present —which would give a glossy but thick-flowing slag—the best corrective is more soda, with the addition, if necessary, of a very little powdered fiuor spar. For too basic slag—a dull lustrous one —which is too stiff, more borax will generally do good. Precipitate and fluxes are well mixed, and charged into the plumbago crucibles. The smeltingfurnaces may be constructed to take three pots at a time ; but the furnaces require to be well built, as the heat required for this first fusion is rather in excess of the ordinary gold-melting temperature*