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raised to make the slag very liquid. The scorifier is taken from the tannin in 300 of water; drops of this solution are placed on a muffle in a pair of tongs and the contents poured into a mould, the white porcelain plate, and as the precipitant is added to the lead lead forming a button in the bottom while the slag floats on top. solution a drop of the latter is removed from time to time on a When cold, the contents of the mould are taken out and the lead glass stirring-rod and added to one of the drops on the porcelain button hammered into the form of a cube, the slag, which is glassy plate. The appearance of a yellow colour shows that all the lead and brittle, separating readily from the metal, which is then ready has been precipitated and that the solution contains an excess of for cupellation. In the crucible method the ore is mixed with from molybdate. From the reading of the burette the lead is calculated. once to twice its weight of flux, which varies in composition, but of The molybdate solution should be of such a strength that 1 cc. will precipitate 0'01 gramme of lead. It is standardized by dissolving which the following may he taken as a type :— a weighed amount of lead sulphate in ammonium acetate and proSodium bicarbonate . . . . 8 parts. ceeding as described above. Potassium carbonate . . . . 3 ,, Zinc.—Chemically the ores of zinc consist of the silicates, carPowdered borax . . . . . 4 ,, bonates, oxides, and sulphides of zinc associated with other metals, Flour 1 ,, some of which complicate the methods of assay. The most modern Litharge . . . . . . 9 ,, and the most generally accepted method is volumetric, and is based The mixture is charged into a round clay crucible from 100 mm. to on the reaction between zinc chloride and potassium ferrocyanide, 125 mm. high, and heated either in a muffle or in a crucible furnace by which insoluble zinc ferrocyanide and soluble potassium chloride at a gradually increasing heat for forty or fifty minutes. At the are formed ; the presence of the slightest excess of potassium ferroexpiration of this time, when the charge should be perfectly liquid cyanide is shown by a brownish tint being imparted, by the soluand in a tranquil state of fusion, the crucible is removed from the tion to a drop of uranium nitrate. The ore (0’5 gramme) is digested furnace and the contents are poured into a mould. The resulting with a mixture of potassium nitrate and nitric acid. A saturated lead button hammered into shape and carefully cleansed from slag is solution of potassium chlorate in strong nitric acid is added, and ready for the cupel. If the button is too large for cupellation, or the mass evaporated to dryness. It is then heated with a mixture if it is hard, it may be scorified either alone or mixed with test of ammonium chloride and ammonia, filtered and washed with a lead before cupellation. The character and amount of the flux hot dilute solution of the same mixture. The filtrate diluted to necessarily depend upon the character of the ore, the object being 200 cc. is carefully neutralized with hydrochloric acid, an excess of to concentrate in the lead button all the gold and silver while dis- 6 cc. of the strong acid is added, and the solution saturated with solving and carrying off' in the slag the other constituents of the ore. hydrogen sulphide, which precipitates the copper and cadmium, Under the most favourable conditions there is a slight loss of gold metals which would otherwise interfere. Without filtering, the and silver in the fusion, the scorification, and the cupellation, both standard solution is added from a burette, and from time to time a by absorption in the slag and by actual volatilization and absorp- drop of the solution is removed on the glass stirring-rod and added tion in the cupel. In ores containing much copper, this metal is to a drop or two of a strong solution of uranium nitrate, previously largely concentrated in the lead button, making it hard and necessi- placed on a white porcelain plate. The appearance of a brown tating repeated scorifications, and in some cases a preliminary tint in one of these tests shows the end of the reaction. When removal of the copper by solution of the ore in nitric acid. This cadmium is not present the copper may be precipitated by boiling leaves the gold in the insoluble residue, which is filtered off', and the the acidulated ammoniacal solution with test lead and titrating, silver in the solution is thrown down by hydrochloric acid. The as before described, without removing the lead and copper from the resulting precipitate of silver chloride is filtered, and the residue solution. The ferrocyanide solution is standardized by dissolving and the precipitate are scorified together. Ores containing much 1 gramme of pure zinc in 6 cc. of hydrochloric acid, adding ammonium arsenic or sulphur are generally roasted at a low heat and the assay chloride and titrating as before. This method is modified in practice by the character of the ores, carbonates and silicates free from is made on the roasted material. Lead.—The “dry” or fire assay for lead is largely used for the sulphides being decomposed by hydrochloric acid, with the addivaluation of lead ores, although it is being gradually replaced tion of a little nitric acid. Copper.—The fire assay for copper ores was abandoned years ago by volumetric methods. One part of the ore is mixed with from and the electrolytic method took its place ; this in turn is now three to five parts of a flux of the following composition
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largely replaced by volumetric methods. In the electrolytic method 40-6 per cent. Potassium carbonate from 0'5 to 5 grammes of ore are treated in a flask or beaker, with 31-3 „ Sodium bicarbonate . a mixture of 10 cc. of nitric and 10 cc. of sulphuric acid, until 15-6 Borax thoroughly decomposed. When this liquid is cold it is diluted 12-5 Flour with cold water, heated until all the soluble salts are dissolved, The mixture is charged into a clay crucible and heated for twenty transferred to a tall, narrow beaker, and diluted to about 150 cc. minutes at a good red heat. When the mixture has been in a The electrodes are attached to a frame connected with the battery tranquil state of fusion for a few minutes it is poured into a mould. and the beaker is placed on a stool, which can be raised so that the When cold, the button is hammered, cleaned carefully from slag, electrodes are immersed in the liquid and reach the bottom of the and weighed. The proportion is calculated from the amount of ore beaker. The electrodes consist of two cylinders of platinum (placed used, and the result is expressed in parts in a hundred or percentage one inside the other) about 75 mm. high, the smaller of the two of the ore. Various impurities, such as copper, antimony, and 37 mm., and the larger 50 mm. in diameter, both pierced with 10 sulphur, go into the lead button, so that the result is generally too to 12 holes 5 mm. in diameter, evenly distributed over the surfaces high. The most accurate method for the determination of lead in to facilitate diffusion of the liquids. The surfaces of the cylinders ores is the gravimetric method, in which it is weighed as lead are roughened with a sand blast to increase the areas and make the sulphate after the various impurities have been separated. Nearly deposited metals adhere more firmly. Each cylinder has a platinum all lead ores contain more or less sulphur; and as in the process of wire fused to the upper circumference to connect with a clamp from solution in nitric acid this is oxidized to sulphuric acid which unites which a wire leads to the proper pole of the battery. The smaller cylinder is generally the negative electrode on which the copper is with the lead to form the very insoluble lead_ sulphate, it is simpler deposited. framework carrying the clamps is arranged so that to add sulphuric acid to convert all the lead into sulphate and then a number ofThe determinations may be made at one time, the wires evaporate until the nitric acid is expelled. The salts of iron, copper, &c., are then dissolved in water and filtered from the in- from the clamps running from a rheostat, so arranged that currents soluble silica, lead sulphate, and calcium sulphate, which are washed of any strength may be used simultaneously. The cylinder, having carefully weighed, is placed in position, the beaker containing with dilute sulphuric acid. The insoluble matter is treated with a been solution is adjusted, and the current passed until all the copper hot solution of alkaline ammonium acetate, which dissolves the the precipitated. This generally requires from two to twelve hours. lead sulphate, the other materials being separated by filtration. is cylinders are then removed from the solution and washed with The lead sulphate, re-precipitated in the filtrate by an excess of The water, the one holding the deposited copper being washed sulphuric acid and alcohol, is then filtered on an asbestos felt in a distilled Gooch crucible, washed with dilute sulphuric acid and alcohol, with alcohol, dried, and weighed ; the increase in weight represents copper contents of the ore. The deposited copper should be ignited, and weighed. Lead sulphate contains 68‘30 per cent, the firmly adherent and bright rosy red in colour. Silver, arsenic, and of metallic lead. . if present, are precipitated with the copper and affect the There are several volumetric methods for assaying lead ores, but cadmium, of the results ; they should be removed by special the best known is that based on the precipitation of lead by accuracy ammonium molybdate in an acetic acid solution. The lead sulphate, methods. Yolumetric methods are more expeditious and require less appaobtained as described above and dissolved in ammonium acetate, is acidulated with acetic acid diluted with hot water and heated to ratus. The potassium cyanide method is based on the fact that, boiling-point. A standardized solution of ammonium molybdate when potassium cyanide is added to an ammoniacal solution of a of copper, the insoluble copper cyanide is formed, the end of is then added from a carefully-calibrated burette. As long as the salt solution contains lead, the addition of the molybdate solution the reaction being indicated by the disappearance of the blue colour causes a precipitation of white lead molybdate. An excess of the of the solution. One gramme of the ore is treated in a flask with precipitant is showm by a drop of the solution imparting a yellow a mixture of nitric and sulphuric acids and evaporated until all the colour to a solution of tannin, prepared by dissolving one part of nitric acid is expelled. After cooling a little, water is added, and
