SECOND SERIES OF PRELIMINARY TESTS. As these tests showed that it was possible to leach most of the lead from all three ores tested, and with encouraging consumptions of acid for two of them, a number of other ores from different localities were tested to determine whether they would respond in like manner. The results are shown in Table 9. TABLE 9.-Results of preliminary tests of leaching lead carbonate ores with acidified brines. [Tests by M. J. Udy. 1,000 c. c. of brine; 100 grams of ore, crushed to 10 mesh.] a 1 equivalent of H2SO4=0.474 pound per pound of lead. In each test a pulp consisting of 1,000 c. c. of brine to 100 grams of ore was prepared. The leaches were agitated all night and on the next day were filtered for analysis. The temperature at which these experiments were run was that of the laboratory, about 20° C. The extractions of lead from the Daly Judge carbonate ore were not as high as was expected from comparison with the figures in Table 2. Later tests showed that it was difficult to get more than 1 per cent of lead into solution in a brine at the laboratory tempera tures, hence the use of 10 volumes of solution to 1 weight of ore was slightly below the requirements for the proportion of lead in this ore. Only one type of ore, Iron Blossom No. 10, gave discouragingly low extractions. This ore was really a silver ore containing a small amount of lead. In all of the materials tested the silver content was disregarded for the time being, as the object was to study the lead in as many different ores as possible. As regards acid efficiency the ore from the Scranton mine and the tailings from the Wilbert and the Horn Silver dumps seemed to use less acid per unit of lead SULPHURIC ACID CONSUMED, POUNDS PER TON 600 700 600 500 400 200 ૩ 3 5 7 The amount of acid used in each test is expressed as an equivalent to the lead. That is, 1 pound of lead is equivalent to 0.474 pound of sulphuric acid (anhydrous), or, roughly, onehalf pound, so that in general 1 pound of sulphuric acid should convert 2 pounds of lead to a soluble form, provided some of the acid is not used up by other constituents of the ore. The data in Table 9 were used in preparing the curves in figure 3, except that the results have been converted to the number of pounds of acid that gave an extraction of a definite number of pounds of lead from a ton of ore. This is the information that is needed in any calculations as to the financial possibilities 100 0 20 40 60 80 100 120 140 160 180 200 220 FIGURE 3.-Curves showing relation of lead extraction to con- ore; 3, Chief Consolidated ore; 4, American Flag ore; 5, tailing from Wilbert dump; 6, tailing from Horn Silver dump; 7, Scranton ore; 8, Daly Judge ore. of such a process. From these curves it is evident that with some of the ores the amount of lead extracted after adding more than two equivalents of acid is actually smaller than with less acid. This result is probably due to the excess of sulphate radical present that tends to reprecipitate lead sulphate from the solution. The materials that acted in this way are the ones that did not contain noteworthy amounts of acid-consuming constituents, and hence gave high acid efficiencies. Down to the limit of the amount of acid necessary to convert the lead carbonate to a soluble form it is best not to use any more acid than such material seems to require. With some of the other materials the addition of more acid always resulted in 9001 the extraction of a little more lead. THIRD SERIES OF PRELIMI- Another set of argentiferous lead carbonate ores was treated in the same way, with the results presented in Table 21 (p. 53), which is incorporated with the section on leaching such ores. The data on the lead extraction from these ores have been calculated and plotted in figure 4, to make the results comparable with those in figure 3. Although the various ores tested vary widely in lead SULPHURIC ACID 200 அ 100 0 20 40 60 80 100 120 140 160 180 200 220 LEAD EXTRACTED, POUNDS PER TON content, it is a strange FIGURE 4.-Curves showing relation of lead extraction to con coincidence that in only a few ores will the use of more than 150 pounds of sulphuric acid per ton of ore give a higher extrac sumption of acid in second series of leaching tests with acidified brine. 1, tailing from Ontario dump; 2, tailing from Bullionville dump; slimes from Eureka Hill dump; 4 to 7, Michigan-Utah ores; 8, Chief Consolidated ore; 9, slimes from Copper Queen dump; 10, Shattuck ore; 11, tailing from Dry Valley dump; 12, tailing from Daly West dump; 13, slimes from Bullion Beck dump; 14, Nevada United ore. tion of lead than is obtainable with that amount of acid. In those experiments, however, the main consideration governing the amount of acid used was the possible extraction of the silver in the ore. Therefore the data on the lead extractions are only of technical interest. TESTS OF TAILING FROM WILBERT DUMP. Two of the samples that seemed to give the best acid efficiencies were the Wilbert tailing and the Scranton ore. These were chosen for further experimentation. It was necessary to know the minimum time required to extract the lead and the minimum amount of brine that would hold it in solution. It also seemed possible that the application of stronger acid solutions would dissolve the lead faster than weaker acid solutions containing the same weight of acid in a greater volume of brine. All of these points were tested on a sample of the Wilbert tailing, of the screen analysis given in Table 5 (p. 19). The charges consisted of 100 grams of ore, usually with 1,000 c. c. of saturated brine unless otherwise stated. The material was placed in ordinary 2.5-liter acid bottles, which were placed on their sides. on a cyanide agitator and rolled over about once every two seconds. After agitation the contents were filtered on suction filters consisting of a pump, flask, and a Büchner porcelain funnel. PER CENT 90 80 70 50 2 These data are presented in Table 10. Some of the results are plotted in figure 5. The curves showing the effect of time on the acid efficiency represent two series of tests, one of solutions containing two equivalents of acid, and one of solutions containing four equivalents of acid. The curve showing the percentage extraction of the lead represents the series of tests of solutions containing two equivalents of acid. The corresponding curve for the results with solutions containing four equivalents of acid would practically coincide with this curve, and was not plotted. The percentage of efficiency of the acid when four equivalents are used is considerably lower both efficiency curves were no advantage to be gained 30 20 10 TIME, HOURS 3 20 FIGURE 5.—Curve showing effects of time on lead extraction and acid efficiency in leaching Wilbert tailing with acidified brine. than when two equivalents are used, so plotted. The curves show that there is by prolonging a leach for more than two hours, either on the score of lead extraction or of acid efficiency. In fact, after two hours both the lead extraction and the acid efficiency fall off to some extent. One would expect that any excess of acid, on long standing, would gradually waste itself on other constituents of the ore; hence, the fall in efficiency of the acid is easy to understand. It may be that on long standing certain other constituents of the ore would also precipitate some lead, as lead is a metal that might be easily precipitated from solution by finely divided iron or particles of other minerals such as zinc oxide or zinc sulphide. As these tests showed that two hours of leaching was sufficient, it was thought that a series of leaches of varying acidity might reveal the most advantageous concentration of acid to use. However, the percentage of extraction of lead and the acid efficiency seemed to be unaffected, or at least were not affected in 2-hour leaches. The strength of the acid solution may possibly have some effect on the extraction or on the acid efficiency in leaches of less than two hours' time. This point was not investigated, as the results of the former series of tests, with two of the extremes of acid concentration used, showed that about two hours of contact between the solution and the ore was needed to get the most satisfactory extraction. TABLE 10.-Results of leaching Wilbert tailing with acidified brine. The solubility curves for lead chloride in brines (fig. 2, p. 22) indicate that at ordinary room temperatures (about 20° C.), solutions containing as much as 1.5 per cent lead when saturated with respect to lead can be obtained. Such a concentration could not be expected in practice, but it might be possible to get solutions containing considerably more than 1 per cent of lead. Hence a series of tests were run to determine the minimum amount of brine, of a given strength, necessary to leach the lead from the Wilbert tailing, as in the previous tests leaches consisting of 1,000 c. c. of brine to 100 grams of ore that contained only 5.5 per cent of lead had been used. |