hydroxide to a dirty brown color when the reaction was nearly completed. Manganese sulphate is not hydrolized in the presence of crushed lime rock in cold solutions and hence is not completely removed with the iron and the alumina. 6. The precipitate of calcium sulphate formed in the first reaction of the process carries very little zinc and can be discarded. Not more than 1 per cent of the total zinc is lost in this way. REMOVAL OF MANGANESE. USE OF CHLORIDE OF LIME IN MODERATE AMOUNTS. On account of the discoloration of the zinc hydroxide precipitate with manganous hydroxide and its oxidation products, further tests were necessary in which removal of the manganese by oxidizing it with some form of chlorine was sought. In the first set of tests made, chloride of lime was used to supply the chlorine. Twenty pounds of CaOCl, per ton of solution were used in each test, and a series of tests were made in which various amounts of sulphuric acid were added to liberate more or less of the chlorine from the chloride of lime. These amounts of sulphuric acid are recorded in the columns of "acid equivalents." The calcium chloride and chloride of lime were added together, forming the first precipitate, which was removed by filtration, then crushed-lime rock was added to remove iron and alumina, followed by precipitation of the zinc. with a paste of slaked lime. The results are contained in Table 31. TABLE 31.-Results of treating zinc sulphate solutions with lime compounds for removal of manganese. [In each test: 20 pounds of CaOCI, per ton of solution; 500 c. c. of solution; 30.50 grams Zn per liter; time, 4 hours.] Grams. P. ct. 0.0 28.6 Trace. .125 46.4 Trace. .25 39.2 Trace. .5 50.3 Trace. 1.0 20.3 1.5 2.0 Grams. P. ct. P. ct. P. ct. Grams. P. ct. P. ct. P. ct. 0.09 P. ct. The table shows that the manganese content of the final precipitate was low; also it was observed that the precipitate was of a satisfactory white color. The lime content of the precipitate was too high, as insufficient time had been allowed for the reaction of the lime on the zinc chloride solution. On that account further treat ment of these precipitates with zinc chloride solutions, to see whether the lime would react and go into solution as calcium chloride, was thought best. This was done with the precipitates from the next set of tests about to be described. USE OF LARGER AMOUNTS OF CHLORIDE OF LIME, As the improved color of the precipitate was encouraging, another series of tests was made with greater amounts of chloride of lime, in the hope that most of the manganese could be precipitated as the dioxide, in a marketable product. The results of the tests are reported in Table 32. The use of the greater amounts of chloride of lime seemed to give erratic results, and the precipitates were colored with manganese, especially those in the first and third tests, which were very brown. However, the lower percentage of lime left in the precipitates, as compared with the previous set of tests, was a favorable feature. TABLE 32.-Results of treating zinc sulphate solutions with excess CaOCl to remove manganese. [500 c. c. of solution, with 3.05 per cent Zn content, used in each test; solution agitated 4 hours at 20° C.] The content of lime left in the precipitates was still too large, so the precipitates of this last series of tests were further treated with a 5 per cent ZnCl, solution, to correspond with the conditions for precipitation of zinc chloride solutions with lime in a series of countercurrent agitators and thickeners. The data on these tests are contained in Table 33. Test 4 was made with a mixture of precipitates, from one of the former sets of tests, having a high lime content. TABLE 33.-Results of treating zinc hydroxide precipitate with zinc chloride solution to remove lime. 2. 3. 4. Grams. P. ct. P. ct. P. ct. P. ct. P. ct. Grams. P. ct. P. ct. P. ct. P. ct. 8.18 P. ct. 0.13 .39 .73 .08 This series of tests shows that 4 hours' agitation of the precipitate with fresh zinc chloride solution removes practically all of the lime, leaving less than 2 per cent CaO in the precipitate, although not much of the magnesia was removed. The percentage of zinc was not raised much and the percentage of chlorine was increased. It was noticed that the precipitates tended to combine with the zinc chloride of the solution to form hard cakes, supposedly oxychloride of zine, which will set somewhat like magnesium oxychloride cement. When the zinc hydroxide is later calcined to form zinc oxide, all of the zinc chloride would be volatilized and could be caught from the calciner gases and returned to the zinc chloride solutions. For some reason the manganese content of the residual zinc hydroxide was lowered markedly, although the precipitate still had a dark color. TESTS WITH CHLORINE WATER. As no advantage was to be gained from using large amounts of chloride of lime for preventing manganese from entering the precipitate, it was next determined to try chlorine gas for the oxidation of the manganese, as well as to try the effect of heating the solution while using chloride of time. The conditions of the five tests made were as follows: No. 1. Hot, CaOCl2, 20 pounds per ton of solution. No. 2. 200 c. c. of saturated chlorine water per 500 c. c. of solution, cold. No. 3. 400 c. c. of saturated chlorine water per 500 c. c. of solution, cold. No. 4. 200 c. c. of saturated chlorine water per 500 c. c. of solution, boiling. No. 5. 400 c. c. of saturated chlorine water per 500 c. c. of solution, boiling. The data on the precipitates obtained in these tests is contained in Table 34. The chlorine was applied to the solution at the same time as the lime rock, as this provides the conditions under which manganese should theoretically be precipitated. Actually very little manganese was eliminated from the solution; later it was thought that the manganese must be present in the oxidized solutions as manganates or permanganates of zinc or calcium, all of which are very soluble. In these tests insufficient lime for complete precipitation was used, as it was hoped that with the smaller amounts of lime zinc hydrate of reasonable purity could be obtained in four hours' reaction. The results indicate that the precipitate can not be kept free of lime unless the lime and the zinc chloride solution are in contact at least eight hours or more. The precipitates obtained in this series of tests were white and contained sufficiently low percentages of iron and manganese to be acceptable. Only the lime and chlorine are left, according to the analyses recorded. Later it was found that sulphates also were present in serious amounts. TABLE 34.-Results of treating zinc sulphate solutions with chlorine to oxidize manganese. [500 c. c. of 3.05 per cent Zn solution in each test.] The chlorine used in these tests had been applied as chlorine water, which diluted the zinc chloride solution. In the next series of tests chlorine gas was applied directly to cold solutions. TESTS WITH CHLORINE GAS. The designing of this set of tests depended on the following considerations: It had been shown that the treatment with fresh zinc chloride solution for a sufficient length of time would dissolve most of the lime left in the precipitate. Hence a series of cyclic leaches of ore, with purification and precipitation of the liquor during each cycle was designed. The precipitate of each test was partly dewatered and then treated with fresh solution from the succeeding leach, followed by dewatering and treatment with the fresh solution of the next succeeding leach. In this way the zinc chloride solutions were treated by countercurrent agitation and thickening with the lime precipitant. The final precipitate was further washed with water and the wash water was analyzed for zinc. Then the zinc hydroxide precipitate was ignited to form zinc oxide, and to drive off all zinc chloride so that it might be recovered and retreated in solution. The ore used for these tests was from the Bullion Coalition mine, in the Stockton district of Utah. It was mixed with sulphuric acid in amount equivalent to the zinc in the ore, and was calcined at 700° C. in order to break up all colloidal silica, iron sulphate, and aluminum sulphate, leaving only zinc sulphate to be leached with the calcium chloride solution returned from precipitation of the previous cycle. In this way calcium sulphate was precipitated in the pulp. Then chlorine gas was bubbled through the pulp till it 86198-19-Bull. 168—7 smelled strongly, after which ground calcium carbonate was added and the pulp was agitated for two hours to cause hydrolysis and precipitation of iron, aluminum, and manganese. The pulp was then ready for filtration and precipitation. The data on these cyclic leaching and precipitation tests is contained in Table 35. The high percentage of manganese in the second precipitate was thought to be due to the use of lime in excess of the theoretical proportion. In nearly every test the solution was agitated for about four hours after the lime was added, but in the eighth and ninth tests only about two hours' time was allowed, which accounts for the higher proportion of lime in the precipitates from these two tests. The chlorine content in the precipitates varied considerably as they tended to crack and shrink while on the filter, with the result that good washing was difficult; hence various amounts of zinc chloride were left in them. It is possible that repulping of this precipitate with water, followed by a second filtering would be necessary. The precipitates were also analyzed for sulphates, and surprisingly large amounts were found. Later, it was determined that the amount of sulphates present in the final precipitate was very nearly equivalent to the amount of gypsum that can be dissolved in the zinc chloride solution. The remedy would be to use solutions containing higher proportions of zinc and also to use a slight excess of calcium chloride in order to lower the solubility of the calcium sulphate due to the "common ion" effect. After drying, weighing, and analysis, the precipitates were ignited at a low red heat, in order to form zinc oxide and to drive off any zinc chloride present. After burning, most of the precipitates had a yellow tinge. The analysis of the ignited oxide shows that it was still considerably contaminated with calcium sulphate. |