Fig. 7. about 50 to 52 millimeters (1.96 to 2.04 inches) wide in the clear, and 8 to 10 millimeters (0.31 to 0.39 inch) deep, made of not too refractory clay, and has rather thin sides. It is lined with reddle, chalk, or oxide of iron, and, if necessary, in order to increase the surface, the lined sides are marked with a spatula in such a manner that radial furrows, running from the centre towards the edges, are formed. It is then placed in the muffle of a muffle-furnace (Fig. 25) and heated at a gradually increasing temperature until it glows; the heating must be the more gradual the more fusible the sample. (Antimonial and arsenical metals are more easily fused than metallic sulphides, antimony glance, lead sulphate, and “fahlerz" containing mercury). The mouth of the muffle is left open, with the exception of a low layer of pieces of wood charcoal touching each other, and continued in a forward direction. These pieces in a glowing state heat the oxidizing air current. The roasting dish must occasionally be turned around during the operation. It is taken from the muffle when the mass has ceased to burn, and oxidation is complete. This is indicated by the heated mass ceasing to fume and no longer emitting odors of sulphurous or arsenious acid, and by the metallic lustre having been replaced by an earthy appearance. If this should not be the case, the roasting dish must be placed back into the muffle until these signs make their appearance. The now roasted sample may be somewhat sintered together. It is then rubbed with the iron knob, b, of a wooden-handled spatula, after being loosened from the edges of the roasting dish with the knife's edge, a, of the rod. This tool (Fig. 8) is about 195 millimeters (7.67 inches) long, and consists of the iron head b, about 16 millimeters (0.63 inch) diam., and the steel knife-blade a, set in a wooden handle. The roasting is repeated once or several times, but the mass must be rubbed up previous to each roasting. Fig. 8. It is then mixed with 1 or 2 volumes of powdered wood charcoal, or 20 to 25 per cent. of graphite, and the roasting dish with its contents is again placed in the muffle and brought to a glow. By this process the sulphates, antimoniates, and arseniates formed during the oxidizing period are reduced to metallic sulphides, antimonides, and arsenides, while the volatile products of oxidation escape (reducing roasting). These compounds when all the carbon has been consumed (which may be readily recognized by the manner of glowing) will be again converted into oxides; sulphurous, arsenious, and antimonious acids being evolved in the operation. But new sulphates, antimoniates, and arseniates will constantly be formed, and these, if the sample is to be roasted as completely as possible (for instance, copper ores, but lead ores in a less degree), can only be removed by repeating the rubbing up of the assay sample twice or three times, mixing it with charcoal powder, and glowing until the coal is completely consumed, although even after this small quantities of sulphates will nevertheless remain. When the roasted sample has become sufficiently cold, it is placed in an iron mortar and mixed with 20 to 50 per cent. of ammonium carbonate. A small conical heap of the mixture is formed in the roasting dish, this is covered with an empty roasting dish and quickly ignited until the odor of ammonia can no longer be detected. When this is the case, the last traces of sulphuric acid in the roasted sample will have been volatilized in the form of ammonium sulphate. (Lead and bismuth sulphates are only incompletely decomposed by ammonium carbonate.) The roasting dish is now taken from the muffle and allowed to cool off. The sample is then placed in a mortar and rubbed up. Modifications. When the ores are refractory (for instance, copper pyrites), powdered charcoal or graphite is added to the sample before roasting, in order to shorten the time required for the operation. Very fusible substances which evolve vapors (such as "fahlerz” containing mercury) must be heated very gradually. To diminish the loss of metal (for instance, of silver and gold) the temperature must not be raised higher than is absolutely necessary. The loss from this cause is greatest with ores containing antimony, arsenic, zinc, etc. 3. Fusion. The sample is brought into a liquid state, either by itself, or with fluxes. During this process the resulting products (metal button or regulus, speiss, matt, slags) arrange themselves in layers according to their specific gravities, and are separated from each other, either by breaking to pieces the clay assay-vessels in which they have been fused, after they have become cold, or they are poured out while still in a fluid state, into iron or bronze moulds, where the separation then takes place. Sometimes the fluid, oxidized substances are absorbed by the porous sides of the assay-vessel, leaving the metal button behind (cupellation of lead, and refining copper on the cupel). The following distinctions are made according to the object of the fusion: a. Oxidizing fusion.-In this process the following may serve as oxidizing agents: the oxygen of the air, demanding open vessels for the operation (cupels, calcining and roasting dishes), which must be heated in the mufflefurnace (for instance, cupellation of lead, refining copper, assay of cobalt and nickel); or fluxes yielding oxygen, and then open or covered assay-vessels (pots, crucibles), and muffle, wind, and blast furnaces may be used (saltpetre in the Cornish assay of copper and in the assay of chromium, lead oxide in the assays of fuel, silver and gold); or both at the same time (refining of black copper). The resulting oxides are more frequently slagged off by themselves or by solvent agents added as a flux (borax, glass, etc.), than absorbed by the porous vessel used for fusing (cupels). b. Reducing fusion.-This operation is seldom executed by itself with reducing agents (coal, flour, colophony, potassium cyanide), but generally in connection with fluxes (potassium or sodium carbonate), in order to allow of a better collection of the particles of metal (as from litharge, white-lead ore); or in connection with reducing, fluxing, and solvent agents (borax, glass, phosphorus salt). A definite low temperature must then be used to reduce one metallic oxide, while the metallic oxides more difficultly reducible, are slagged off with the earths which may be present (assays of lead, copper, and tin ores). Muffle, wind, and blast furnaces are used. The vessels used for this process (crucibles, pots) should be roomy, as the mass puffs up. This is caused by the formation of carbonic oxide which ignites above the vessels. This phenomenon is called "flaming," the end of the operation being generally indicated by its cessation. c. Purifying fusion.-This is more frequently used in connection with oxidizing fusion (p. 36) and reducing fusion (p. 37) than by itself (assay of smalt, assay of thin matt). d. Precipitating fusion.-By this process metallic sulphides (in assays of lead, bismuth, and antimony) or arsenical metals (in the assay of lead ores and nickel and cobalt ores containing bismuth) are decomposed by iron. The desulphuration of the metals is promoted by suitable fluxes (potassium or sodium carbonate, black flux), or the slagging off of earthy and other admixtures is effected (borax, glass, alkalies). e. Mixing fusion, to prepare alloys by fusing different metals together (gold and silver in quartation). f. Remelting, in order to produce the sample in another form (as, for instance, by granulation, p. 24). g. Liquating fusion (liquation).-Liquation of easily fusible substances from more refractory substances (assay of antimony glance). 4. Sublimation and distillation.-The sample is placed, either by itself or with fluxes, in crucibles, tubes, or retorts, and heated until the substances volatilize, and the vapors are then condensed as sublimates (flaky arsenic, flowers of sulphur, realgar), or as distillates (mercury, zinc) in suitable condensers. 8. OPERATIONS BY THE WET METHOD. These may be 1. Assays by gravimetric analysis.1 Fig. 9. a. The sample is dissolved in acids, in a porcelain dish covered with a watch-crystal. Or a bellied flask is used for the purpose (Fig. 9). This either stands upright and is provided with a funnel, or is placed in a slanting position to prevent the liquid, in case it effervesces, from being thrown out of the mouth of the flask. The vessel may be heated on 1 Rammelsberg, quant, Analyse, Berlin, 1863. Wöhler, Mineralanalyse, Göttingen, 1861. Sonnenschein, quant. Analyse, Berlin, 1864. RoseFinkener, Mineralchemie, Leipzig, 1865. Fresenius' quant. Analyse, 6 Aufl. 1871. Classen, quant. Analyse, Stuttgart, 1857. Menschutkin, analyt. Chemie, Leipzig, 1878. Bolley, techn.-chem. Unters., Leipzig, 1879. Muspratt's techn. Chemie, 3 Aufl. |