galena, and the slag will also be covered with a lustrous film. The yield of lead from pure galena is from 82 to 83 per cent. of Pb.-New York: 10 grammes (154.32 grains) of ore, 25 grammes (385.80 grains) of black flux, three loops of iron wire, which are taken out after the fusion is complete, and a covering of common salt. The yield from pure galena is 78.4 to 78.6 per cent., with a difference of 1 to 2 per cent. in the various assays.-Upper Harz: The assay was formerly conducted in the same manner as in Freiberg, but now iron pots are used. b. Upper Harz, assay with potassium carbonate.-A muffle-furnace is required for this method of assaying. Low crucibles (Fig. 39) may be used, as the charge contains no carbon, and several crucibles can be placed in the muffle at one time. The result of this assay is not as accurate, the yield being somewhat less than with the methods described above, as the success of the operation depends on the proper "cooling of the assay," for which there is no guide but experience. This method is therefore chiefly available for uniform ores only, the approximate yield of which is known. It has been almost abandoned at the present time. Charge: 12.5 to 15 grammes (192.90 to 231.48 grains) of potassium carbonate are placed in a small crucible (Fig. 39, p. 66). To this is added 5 grammes (77.16 grains) of galena, and both are thoroughly stirred together with the mixing spatula. In case basic earths are present, 1 assay spoonful of borax is placed upon the mixture, and upon this a covering of common salt 5 millimeters (0.196 inch) thick. The charge is then placed in the thoroughly heated muffle-furnace, where it remains, with the mouth of the muffle closed, until it has come into perfect fusion (that is, when no more deposits are perceptible on the edges of the crucible). To decompose the sulpho-salt by oxidation, the mouth of the muffle is then opened for about 10 to 15 minutes, until the crucible ap pears dark and the vapors above it have greatly diminished or entirely disappeared (this is called cooling the assay). Thereupon the furnace is brought back to its first temperature, completely closing the muffle, in order to decompose the still remaining sulphurized lead by the sulphate which has been formed. The crucible is then taken out and allowed to cool off, and the lead buttons are freed from adhering slag. If the assay has been successful, the slag is completely fused, and the lead button has a pure lead color, but not much metallic lustre, as, if this is the case, the heat has been too strong. For ore containing antimony: 10 grammes (154.32 grains) of ore, 35 grammes (540.13 grains) of potassium carbonate, 1 gramme (15.43 grains) of saltpetre, and a covering of common salt. 30 minutes are required for fusing, 10 minutes for cooling, and 10 minutes for the final heating of the assay. 3. Galena containing large quantities of earths.-The English method (p. 87) is employed with a strip of sheetiron in the form of a horse-shoe, but stronger fluxes (caustic alkalies) are used, which, to be sure, attack the crucibles more energetically, and larger charges, as for instance: 100 grammes (1543.23 grains) of assay sample, 100 to 150 grammes (1543.23 to 2314.8 grains) of caustic soda, 150 to 250 grammes (2314.8 to 3858 grains) of potassium carbonate or sodium carbonate, strip of iron in the form of a horse-shoe 25 millimeters (0.098 inch) wide, and 4 millimeters (0.157 inch) thick. From 1 to 1 hours are required for perfect fusion, and until the iron is free from lead globules. B. Lead monosulphide with foreign metallic sulphides (galena with zinc blende, iron pyrites, etc.; lead matt, etc.). 1. Roasting and reducing assay.-The result of this assay is inaccurate, as the lead oxide is liable to slag off and foreign metallic oxides to be reduced, the metal of which contaminates the lead. For this reason the assay with sulphuric acid is frequently used instead (Rammelsberg smelting works in the Lower Harz). 5 grammes (77.16 grains) of ore are roasted in a roasting dish. This is mixed in the assay vessel with 7.5 to 15 grammes (115.7 to 231.48 grains) of potassium carbonate and flour, or black flux. Upon this is placed 1.25 to 1.5 grammes (19.29 to 23.15 grains) of glass, then 0.25 to 0.5 gramme (3.85 to 7.71 grains) of thick iron wire, upon this 1.25 to 1.5 grammes (19.29 to 23.15 grains) of borax, then a covering of common salt, and on top of all a piece of coal. By fusing the roasted charge in the muffle- or wind-furnace at not too high a temperature, the lead oxide and lead sulphate are reduced (if the temperature is too high, many other metallic oxides are also reduced), and the foreign oxides and earths contained in the sample are slagged off by the aid of the potassium carbonate in the black flux, as well as of the borax and glass. The heating should be done with the greatest care, as the contents swell up very much. 20 to 30 minutes are required for smelting in the muffle-furnace after the "flaming" in the muffle has ceased, and from 15 to 20 minutes in the wind-furnace after the flames get under way. Hungary: 10 grammes (154.32 grains) of roasted ore are charged with 11.5 grammes (177.44 grains) of black flux, and this is covered with a layer of 15 to 20 grammes (231.48 to 308.65 grains) of common salt. It is fused by keeping up a strong fire under the muffle for The yield is from 10 to 12 per cent. Pb less than from an assay with metallic iron. It will be larger if charcoal dust is added in roasting, but the button will be less pure. half an hour. 2. Assay with sulphuric acid (combined dry and wet method). This gives more accurate results than the fore - going processes, as the foreign metals are removed before fusion, but there will be always a loss of lead in the lastnamed operation. 5 to 10 grammes (77.16 to 154.32 grains) of the ore are ground as fine as possible. It is decomposed by digesting it with nitro-muriatic acid (aqua regia) in a glass flask with straight walls (Fig. 15, p. 42). A few drops of sulphuric acid are added, and it is then evaporated to dryness. The dry mass is digested with diluted sulphuric acid, filtered, and washed. The filter is freed from water between blotting-paper, and, with the residuum (lead sulphate and insoluble earths, clay, etc.), is dried in a roasting dish under the muffle, and finally incinerated at as low a temperature as possible. The mass is then ground up, and charged in a high crucible with 15 grammes (231.48 grains) of black flux (1 part saltpetre and 3 parts argol), and then with 1 to 1.5 grammes (15.43 to 23.15 grains) of iron. The charge is slowly heated, and then strongly heated after the "flaming" in the muffle has cased (15 to 20 minutes).—If the assay sample should contain antimony, which partly remains as lead antimoniate with the lead sulphate, the process is as follows: The assay sample is decomposed by nitric acid, to which some tartaric acid has been added. It is neutralized with sodium carbonate, and the antimony is extracted by digesting the mass for about half an hour in a solution of sodium sulphide containing sulphur. It is then filtered and washed, and the residuum is treated in the same manner as in the sulphuric acid assay. II. Oxidized Substances. A. Lead oxides free from earths (litharge, minium, skimmings (Abstrich) etc.).—5 grammes (77.16 grains) of the sample are fused at not too high a temperature with 12.5 to 15 grammes (192.90 to 231.48 grains) of potassium carbonate with 30 to 35 per cent. of flour, or black flux, and covering of common salt, with small pieces of coal on the top, in the same manner as in the roasting and reducing assay. If the sample should contain any sulphur, 0.25 to 0.5 gramme (3.85 to 7.71 grains) of iron wire are added. 20 to 25 minutes are required for fusion in the muffle-furnace after the "flaming" in the muffle, and 13 to 15 minutes in the wind-furnace after the flame is under way. B. Lead oxides with earths.-The charge is the same as in II. A, with the exception that from 25 to 30 per cent. of borax are added, and 5 to 10 minutes more are required for fusion. C. Salts of lead oxide, namely: 1. Lead carbonate (cerussite), lead chromate (crocoisite), lead phosphate (pyromorphite), mimetene (lead arsenate), and yellow lead ore (wulfenite).-The charge is the same as in II. A, with an addition of from 20 to 30 per cent. of borax, according to the presence of more or less earthy substances. With pyromorphite containing arsenic from 5 to 10 per cent. of iron is added to separate the arsenide of iron. Time for fusion the same as in II. A. Charges for oxidized ores according to Percy: 500 grains of ore, 350 grains of sodium carbonate, 150 grains, or less, of borax, and 50 grains of argol. The charge is fused in an iron pot (p. 87).-Lead carbonate (cerussite): 500 grains of ore, 500 grains of sodium carbonate, 100 grains of argol, and 30 grains of borax. The charge is fused for about 20 minutes in a clay crucible in the wind-furnace, and poured out (p. 91).—Lead phosphate (pyromorphite): 300 grains of ore, 400 grains of sodium carbonate, 20 grains of powdered charcoal, and 30 grains of borax; or, 350 grains of sodium carbonate, 100 grains of argol, 30 grains of borax, and some metallic iron. Fusing time: 25 to 30 minutes, counting from introducing the charge until it is poured out. |