the semisintering action binds the small particles into larger aggregates, which should not form so much flue dust on roasting in the zinc smelter. 6. If the material contains silver, usually a recovery of about 30 per cent can be expected, depending on the length of time that the leaching solution is in contact with the calcine. The calcine of zinc sulphide tends to reprecipitate the silver from solution, the speed of precipitation being dependent on the acidity of the solution; hence highly acid solutions permit better extraction of the silver than weakly acid solutions. 7. If copper is present, only 10 to 15 per cent of that metal can be recovered. The zinc sulphide in the residue reprecipitates the copper from solution on leaching. Hence this process is not advisable for the treatment of zinc concentrates containing copper minerals. 8. The costs of treating leady zinc-sulphide materials by this method would probably be about the same as those calculated for oxidized ores. The lead content in the average zinc concentrate should be extracted at a complete metallurgical cost of $2 to $2.50 per ton of material. With lead at 4 cents, the normal market price, the lower limit of profitable treatment would be material containing about 4 to 5 per cent lead. Unless material containing less than 4 per cent lead is subject to a penalty at the zinc smelter (as it is in the Joplin district), the bonus paid for the higher zinc content of the concentrate would more than balance the loss of weight and of zinc incurred in the process. It is not probable that any lower grade of material could be worked by this process. LEAD-IRON MIDDLINGS AND COMPLEX SULPHIDES.a As mentioned earlier in this report, some mixtures of galena and pyrite are occasionally regarded as being too complex for.separation and not desirable for shipment to the lead smelters. Usually the iron in the concentrates, after roasting, enters the slag in the lead blast furnace by combining with the silica in the charge. Where only high-grade galena or other lead products are desired, as in the smelting of ores for lead only, or where the smelter is receiving a preponderance of base ores over siliceous ores, or where the cost of shipping the lead-iron mixture to the smelter is too high, it would be desirable to remove this lead by some cheap, simple process. Hence the attempt to apply chloridizing roasting and leaching to this type of product. a Experimenter: M. J. Udy. CHARACTER OF MIDDLINGS. In the disseminated lead district of southeastern Missouri about 50 to 100 tons daily of a lead-iron middling is made in a number of mills, which average 10 to 15 tons each of this product. Part of the lead can be recovered by tabling, but much of it is bound up in a true middling, the treatment of which has presented a difficult problem. Occasionally such middling will contain important amounts of zinc and copper, as indicated by the analyses shown in Table 59 of five different samples received from three mills at different times. In one mill, not represented in the table, the middling is receiving a combined magnetic and table treatment in the attempt to make a zinc concentrate and also a lead concentrate containing less iron. However, it has never yielded as good results as might be desired. TABLE 59.-Analyses of lead-iron middlings from mills in southeastern Missouri. Also some ores may be difficult to treat by ordinary ore-dressing methods, owing to partial oxidation of the ore or to intercrystallization of the various sulphides. Analyses of five materials of this type are given in Table 60. The Bingham Mines Co. ore and the material from the dump of North Star mine are examples of partly oxidized materials that do not give good separation in gravity concentration. The sample from the Bullion Coalition mine is representative of a type of ore so high in iron sulphide that it is difficult to obtain clean lead concentrate and still more difficult to obtain clean zinc concentrate. The ore samples from the Constitution and the Douglas mines represent truly complex microcrystalline ores that yield to no existing ore-dressing method, except that the gangue can be partly removed by fine grinding and flotation. The problems presented by the materials whose analyses are contained in this table are of exceptional magnitude. For 60 years advances have been made in the treatment of complex sulphide ores, but a satisfactory method of treatment is yet to be found. The operators of the electrolytic zinc plants that are supposed to treat such types of ores have their troubles, and installation and operating costs are very high. Material from Bullion Coal- Perct. Perct. Peret. Perct. Perct. Perct. Perct. Perct. Perct. Perct. Per ct. Perct. 18.0 1.1 22.2 14.6 13.1 10.56 0.10 2.65 0.02 30.32 2.42 7.3 8.6 4.9 16.0 5. 12 None. 5.98 .12 18.6 None. 5. 26 Trace 16. 65 None. 23. 82 06 21.14 3.23 TESTS OF MIDDLINGS FROM A MISSOURI MILL. Eight chloridizing roasts of lead-iron middling from the Bonne Terre mill, in Missouri, were made on the thin-bed down-draft roaster of the Knight-Christensen type. The material, being relatively coarse table middling, packed easily. The first three roasts settled onto the grate so tightly during roasting that getting air through the roasting mass was difficult. On that account enough clay (5 per cent) was added to the fourth roast to ball up the charge when moistened, and thus leave it in fluffy condition for roasting. The extraction of lead was 94 per cent, as compared to 75 per cent without clay. In roast 5 a high percentage of salt and a high blast pressure, and no clay, were used with the purpose of volatilizing all of the lead so that leaching would not be necessary. Due to the packing of the charge, the results were no better than on the first three roasts. Roasts 6, 7, and 8 were made with material balled by adding the proper proportion of slime. This slime was from oxidized lead ore from the La Motte mine, in southeastern Missouri. The slime, which contained 2.5 per cent lead in the form of carbonate and was high in iron and alumina, was of claylike consistence. The mixture balled on the roaster in good condition for easy roasting. Extractions of 95 to 100 per cent of the lead were obtained. RESULTS OF TESTS. The detailed data on all the tests are not given herein, but a summary of the results is shown in Table 61, following. Each charge was roasted and the percentage of lead, zinc, and copper that volatilized was determined from the weights and analyses of the heading and the calcine. Two samples of the calcine were then leached, one with neutral saturated brine and the other with acidified brine, in order to recover any lead that did not volatilize and to catch any copper that would dissolve. The results show that as much as 80 per cent of the total lead content can be volatilized, hence the quantity of brine necessary for leaching the remaining lead is small. Also, 25 39094°-18-Bull. 157-11 TABLE 61.-Results of tests of middlings from Bonne Terre mill in southeastern Missouri. a Determined from weights and analyses of heading and calcine. c Determined by analysis of tailing. 8. b Determined by analysis of solution. d Determined by adding recovery by volatilization and solution. to 40 per cent of the zinc was extracted. As this zinc is in the solution containing the lead, the problem of proper treatment of the solution is a serious one. Precipitation of lead from such solutions with lime would cause the lead to be accompanied by the zinc. Electrolytic precipitation of the lead from lead-zinc solutions has been tried, as described elsewhere in this report, with the result that the lead was deposited as sponge metal free from zinc. The sponge metal was melted into bars of lead without much difficulty. As much as 60 per cent of the copper was extracted in test 7, as shown by the analyses of the tailing. The acid leaches always removed more copper than the neutral leaches, in which it was occasionally possible to observe the formation of a white insoluble compound of copper, probably basic cuprous chloride. No work has been done on the recovery of a zinc product from the middlings tested, although the experience of one company producing them indicates that it can be done. As this company is finding it possible to make a zinc concentrate from similar material, it is not improbable that the roasted, leached tailing could be passed through a magnetic separator, or ground and treated by flotation in order to float the zinc sulphide from the roasted iron mineral. The excellent results in extracting the lead when the material was balled with a small amount of clay or slimes, and the facts that the process is cheap and yields metallic lead indicate that it should receive further attention by the companies operating in the southeastern Missouri district. In roasting this material it was noticed that a large amount of elemental sulphur distilled off ahead of the roasting zone and contaminated the lead-chloride fume from the down-draft roaster. Possibly the sulphur could be distilled or otherwise recovered from this fume and sold, thus lessening the cost of treatment by the process. However, no attempt was made to recover the sulphur in the tests of this material, because they were made simultaneously with tests of many other materials. The treatment of a mixed fume of elemental sulphur and lead chloride has not yet received attention at the Salt Lake City station. Hence the methods mentioned in connection with the oxidized ores may require changes. TESTS OF MATERIAL FROM BULLION COALITION MINE IN UTAH. As regards mixed-sulphide ores such as are typified by the Bullion Coalition sample, its analysis (Table 60) is much like that of the leadiron middlings except that the percentage of gangue materials is somewhat higher and the most valuable metal content outside of the lead and zinc was silver. The material being in lumps as mined, could be crushed to any desired mesh. Extensive tests were carried out with this ore, both in the Christensen down-draft roaster and in the Holt-Dern shaft roaster. |