SPECIAL DIVISION. I. LEAD. 24. LEAD ORES. Galena (lead monosulphide) PbS, with 86.6 Pb; cerusite (lead carbonate) PbCO,, with 76.6 Pb; anglesite (lead sulphate) PbSO4, with 68.3 Pb; pyromorphite 3Pb,P2Os+ PbCl, with 76.3 Pb; crocoisite (lead chromate) PbCrO4, with 63.2 Pb; wulfenite (molybdate of lead) PbMoО, with 57 Pb. 25. ASSAYS OF LEAD IN THE DRY WAY. The results of these assays are inaccurate, as the lead is liable to slag off and to volatilize, which is promoted by the presence of other volatile substances (arsenic, zinc, antimony), and also by reason of a possible contamination of the lead by other metals (copper, arsenic, antimony, bismuth). The highest yield which can be obtained from pure galena is 85.25 per cent., but in poorer ores the loss of lead may be 5 per cent. greater. The results are sometimes calculated to whole per cents., but more frequently from 5 to 5 per cent. The condition of the ore, whether the lead is sulphurized or oxidized, and whether the substance is pure or contains more or less of earths and foreign metallic sulphides, will decide the choice of the assay method. I. Sulphurized Substances. A. Gelena, etc., without foreign metallic sulphides (ZnS, FCS, Cu,S, Sb2S, ASS).-Precipitation assay: the assay sample is either decomposed by alkalies alone (Upper Harz assay with potassium carbonate, assay with potassium cyanide), or together with metallic iron (in an iron pot or clay crucible), with the following reactions : The lead sulphide is decomposed by the alkalies at a comparatively low temperature (7PbS + 4K,CO ̧ = 4Pb+ 3(K ̧PbS) + K2SO, + 4CO). The sulpho-salt (K,PbS,), which otherwise would pass into the slag, is either decomposed by iron at a high temperature, 3(K,PbS,) +3Fe=3Pb+3(K,FeS2), or, as is the case in the Upper Harz assay with potash, by a suitable admission of air at a lower temperature. By this process the K,S of the sulpho-salt is completely converted into K,SO,, but the PbS only partly into PbSO,, so that, by increasing the heat, the still remaining PbS is decomposed by the PbSo, as follows: (PbS+ PbSO, = 2Pb+2SO,). In the first case, the presence of carbon (black flux, flour, tartaric acid, etc.) promotes desulphurization. 1. Rich galena (with little earths). a. Assay in an iron pot (Belgian assay).—This is the best method of assaying lead, as pure galena with 86.6 Pb yields 84.25 to 85.25 per cent. of lead, therefore, with a loss of but 1 to 2 per cent. and sometimes only 0.5 per cent. of lead. It also permits the use of a larger charge, and, the iron pot being a good conductor of heat, the operation can be more quickly executed and at a comparatively low temperature. When large quantities of earth are present, more slag will be formed, and consequently the resulting loss of lead will increase, according to Percy, at the following rate: 1.80 to 7.90 per cent. when the ore contains from 10 to 90 per cent. of calcium carbonate, and 1.18 to 35 per cent. when 10 to 90 per cent. of silicic acid is present. The sample is less frequently fused without any fluxes (Flintshire) than with alkaline fluxes. An addition of carbon (black flux, flour, argol) checks the oxidation of the lead, promotes the reduction of the lead carbonate and lead sulphate which may be present, and prevents the oxidation of the iron sulphide. The latter (oxide of iron) vigorously attacks the walls of the pot and retains particles of lead when the contents of the pot are poured out. Fluor-spar is a good flux for heavy spar. Silver and gold pass entirely into the lead, but only traces of zinc and iron. Copper divides itself between lead and slag. A large part of the antimony passes into the lead, and while a part of the arsenic volatilizes, very little of it passes into the lead and the largest part forms spiess with iron. 50 grammes (771.61 grains) of ore are placed in an iron pot previously heated to dark redness, in a coke fire in the wind furnace, or in a gas furnace. From 50 to 100 grammes (771.61 to 1543.23 grains) of black flux or potassium carbonate with 15 to 20 per cent. of flour are added, then 2 to 3 grammes (30.86 to 46.29 grains) of borax, and finally a covering of common salt 5 millimeters (0.197 inch) thick. The charged crucible is then placed between the darkly glowing coal in the furnace. The latter is closed, and the temperature gradually raised in about five minutes to complete redness, and this is kept up until the contents of the crucible are in quiet fusion without foaming (three to five minutes). The granules of lead floating on the top should be submerged 1 B. u. h. Ztg. 1871, p. 61. by means of an iron spatula or wooden rod. The furnace is then closed for a few minutes, after which the pot is taken out and allowed to cool off somewhat. Its contents are poured into a mould, which should have been previously coated with graphite and heated. If the contents are poured out while too hot, a film of lead may remain adhering to the iron, and, if too cold, the lead will partly spread over the slag. The mould, after having been allowed to cool off, is turned over, and the hard, black slag is quickly broken off from the lead button to prevent it from becoming moist, as it would then not separate quite as well. The lead button is then washed with hot water or diluted sulphuric acid, dried, and weighed. The slag is again smelted with some potassium carbonate and flour or black flux for about 10 to 12 minutes, and then poured The time required for the fusion, counted from placing the ore into the pot to the first pouring out, is from 10 to 15 minutes. The iron pots will bear from 40 to 50 operations. In many works lead matt free from copper, poorer ores, and slag are assayed according to this method. out. England (Flintshire): 500 grains (32.4 grammes) of rich galena, 500 grains sodium carbonate, and 50 grains argol; for poorer ores: 350 grains of sodium carbonate, 150 grains borax, and 50 grains argol. The ore is mixed in a mixing scoop with a long spout (Fig. 6a, p. 32), with three-fourths to four-fifths of the quantity of the flux. The mixture is pushed to the front of the scoop, next to this the remainder of the flux is placed, and behind this the borax. The whole is then carefully poured into the dark glowing pot and subjected to the fusing operation mentioned above. In pouring out the contents the slag is kept back in the pot by means of a wooden rod, and is again fused with 20 to 30 grains of sodium carbonate and 5 to 10 grains of argol. The yield of lead from pure galena is generally from 84.25 to 85.25 per The difference in the results of the assay is nearly the same for cent. I B. u. h. Ztg. 1862, p. 242; 1871, p. 61. |