of reasonably pure zinc oxide, because so few impurities of the zinc ore are soluble in ammonia. Copper compounds can be precipitated from these solutions before boiling, by the use of metallic zinc. As hinted at in the discussion of leaching (p. 86), the chief difficulties are due to deterioration of the solution and to losses of ammonia in the tailing and by vaporization. Machinery capable of leaching ores in confined tanks, and of absorbing all the ammonia evaporated from the solutions at any part of such a plant would have to be invented. Further, the cost of heating and boiling solutions to recover ammonia is somewhat higher than any of the unit costs usually allowed in plants treating ores of this kind. Ammonia sulphates and chlorides and other compounds formed in the leaching of the ore would have to be decomposed, in order to liberate the ammonia from the solution, by the use of lime after the zinc had been precipitated, and this would add a complication. The method deserves further study, but at the present time it is probable that methods can be developed which will not call for such complicated plants. CAUSTIC ALKALI SOLUTIONS. Caustic alkali solutions of zinc (sodium zincate) can be decomposed by dilution, zinc oxide being precipitated, but evaporation of the solution after precipitation, in order to obtain a strong solution of caustic for further work, is rather expensive. Such solutions can also be electrolyzed, but usually give a spongy deposit, and the market for zinc dust is limited. Consequently there seems to be nothing to favor processes involving the use of caustic alkali solutions. SULPHATE SOLUTIONS. Sulphate solutions of zinc might be precipitated by such reagents as ammonia or caustic soda, giving a hydrate of zinc and a sulphate of the ammonia or the alkali metal which could be recovered or discarded. As regards ammonia, the solution of ammonium sulphate would later have to be boiled with lime added, in order to recover the ammonia for reuse. Many of the same objections that apply to the direct leaching of ore with ammonia solutions also apply to this latter process. Furthermore, the amount of ammonia absorbed in the precipitate would be very liable to make the losses of ammonia too high. Caustic soda, as a precipitant, is too expensive. Even crude sodium carbonate can not be delivered to many points in the intermountain region or elsewhere for less than 1 cent per pound. Beside, the sulphuric acid used in leaching the ore is not regenerated as in electrolytic precipitation. This is not a serious drawback as sulphuric acid can be made very cheaply as soon as a market for it is assured. If lime could be used as a precipitant it would be much cheaper than either ammoniacal or alkaline precipitants; also most precipitates of metal hydrates made with lime are well flocculated and easy to filter. On that account rather thorough tests were made of a method of using lime as a precipitant of zinc sulphate solutions, which involved converting the zinc sulphate solution to zinc chloride solution by the use of a solution of calcium chloride, followed by. precipitation of the zinc as zinc hydrate by the use of lime. The proposed method involved running a solution of calcium chloride onto the ore, adding sulphuric acid to bring the zinc into solution, leaving calcium sulphate in the ore and yielding a solution of zinc chloride containing the excess of calcium chloride. In precipitation of this solution with lime the calcium chloride would be regenerated. The reactions involved are: CaCl2+H2SO,+ZnO (in ore)=CaSO,+Residue+H2O+ZnCl2 ZnCl2+Ca(OH)2=Zn(OH)2+CaCl2 This process utilizes two very cheap chemicals, sulphuric acid and lime. Enough calcium chloride would have to be added to make up for mechanical losses of that chemical. Consequently, investigation of the two reactions outlined above was necessary. PRECIPITATION OF ZINC SULPHATE SOLUTIONS WITH CALCIUM CHLORIDE. The first step was to see whether zinc sulphate solutions could be converted to chloride of zinc by the use of calcium chloride. To do this, a series of fractional precipitations of the sulphates were made by adding to known amounts of zinc sulphate solutions 0.5, 0.6, 0.7, 0.8, 0.9, 1, and 1.1 equivalents of calcium chloride. The results are contained in Table 28. In each test the calcium chloride solution acted on the zinc sulphate solution for about 60 minutes before filtration. No filtration difficulties in the handling of the calcium sulphate precipitate were experienced. The solution of zinc used contained 4.24 grams of zinc per 100 c. c. TABLE 28.-Precipitation of sulphates from zinc sulphate solutions with calcium chloride. From these tests it can be seen that while an 0.5 equivalent of calcium chloride will precipitate the calculated amount of calcium sulphate from a zinc sulphate solution containing 5 per cent Zn, the theoretical equivalent to all the sulphates will precipitate only 87 per cent of the sulphates. The remaining 13 per cent of the sulphates is probably dissolved calcium sulphate. There is no published figure on the solubility of calcium sulphate in zinc chloride solutions, but the solubility of CaSO, in hydrochloric acid solution of 6 per cent strength is 6.12 parts per 100 parts of water, whereas the solutions used contained 2.36 parts at 25° C. As the solutions contained nearly 5 per cent Zn, or more than 10 per cent ZnCl2, the conclusion drawn. is probably justified. One encouraging feature was that the precipitated calcium sulphate contained almost no zinc. When the theoretical or slightly more than the theoretical amount of calcium chloride was used no zinc was adsorbed in the precipitate and lost. PRECIPITATION OF ZINC CHLORIDE SOLUTION WITH LIME. As the tests had shown that fairly efficient precipitation of the sulphates from solution could be obtained by use of calcium chloride, tests on the precipitation of zinc chloride solutions with lime wero next in order. TESTS WITH PURE ZINC CHLORIDE. At first pure zinc chloride solutions were prepared, in order that the analysis of the precipitate might be studied without having any complicating effect due to the presence of dissolved calcium sulphate. Two solutions of zinc chloride were prepared, one containing 10 per cent Zn and one 5 per cent. Each of these solutions was tested with a series of additions of lime in amounts varying from 0.5 to 1.0 equivalent to the zinc. The mixtures were allowed to stand for 24 hours to insure complete reaction. The lime used contained 47.4 per cent CaO and 29.9 per cent MgO. The data on these tests is given in Table 29. TABLE 29.-Precipitation of zinc chloride solutions with lime. The conclusions drawn from these tests were as follows: 1. The magnesian lime used precipitated the zinc in slightly more than the calculated proportion, especially with 10 per cent Zn solutions. 2. The excess of zinc precipitated beyond the theoretical proportion is doubtless a basic salt, as an excess of zinc oxide is known to form such basic compounds with zinc chloride solutions. 3. The purest precipitate was obtained with half of the theoretical amount of lime necessary, and as the theoretical amount was more nearly approached, the precipitate contained more and more lime and magnesia which had not reacted with the zinc chloride solution to form calcium or magnesium chlorides. 4. The precipitates prepared from 10 per cent Zn solutions were rather difficult to filter, but the precipitates from the 5 per cent Zn solutions were easily filtered. On drying the precipitates and preparing them for analysis, those from the 10 per cent Zn solutions were also sticky and difficult to pulverize. 5. It is possible to precipitate all the zinc from a zinc chloride solution without using an excess of lime. 6. Fine grinding of the lime, or the preparation of a lime paste by slaking, before addition to the zinc solutions, gives more complete reaction; also less washing of the precipitate is necessary to remove calcium chloride. A lime paste was found to be the best and most reliable form in which to add the precipitant. EXPERIMENT WITH OXIDIZED ZINC Ore. Although the grade of precipitate obtained in the test with zinc chloride was not satisfactory, on account of the large amount of lime and magnesium compounds left in the precipitate, an attempt was made to precipitate the solution obtained by treating oxidized zinc ore from the Wolftone mine of the Western Mining Co., of Leadville, Colo. The ore was mixed with strong sulphuric acid and calcined at 600° C. in order to break up colloidal silicic acid, iron sulphates, and other compounds. The ore was then leached with water and the resulting zinc sulphate solution was used for a series of precipitation tests. The usual procedure would involve leaching the calcine with calcium chloride solution, followed by purification of the solution with crushed limestone before filtering from the residue, but these steps were conducted separately in order that the precipitates of calcium sulphate, etc., might be examined separately. On that account calcium chloride solution was added to the water solution of zinc sulphate (containing small amounts of other metal sulphates), and the resulting precipitate of calcium sulphate was filtered from the solution. After filtration, ground lime rock was agitated with the solution for about one hour for the purpose of removing iron and aluminum from the zinc chloride solution by hydrolysis. Then this precipitate was filtered and the purified zinc chloride solution was treated with the theoretical proportion of a paste of freshly slaked lime to precipitate zinc hydroxide and regenerate the calcium chloride solution. The zinc hydroxide precipitate was then filtered and dried. In normal operation the reaction of the calcium chloride on zinc sulphate would take place in the presence of the ore, and likewise the purification with lime rock. The results of this test are contained in Table 30. TABLE 30.-Results of treating zinc-sulphate leaching solutions with lime compounds. [500 c. c. of solution used in each test.] 23.5 Grams. P. ct. P. ct. Grams. P. ct. P. ct. P. ct. P. ct. P. ct. P. ct. Hr. Min. 5 44.6 26.9 47 22. 4 0.5 7.81 1248 66.7 80.2 85.5 93.4 13.5 .94 1.30 41.0 2.18 .70 13.5 5.7 6.66 20.3 51.7 2.16 .42 6.6 3.5 6.04 From these results the following conclusions can be drawn: 1. The reaction of the lime paste on the zinc chloride solution requires more than one hour before it is approaching commercially satisfactory completion. Four or eight hours' time is better, and the indications are that 24 hours would be still better. 2. The lime content of the zinc hydroxide precipitates is high unless the time of reaction is more than four hours. It is probable that the coating of zinc hydroxide over lumps of calcium hydroxide slows down the reaction of zinc chloride on the lime, making necessary long agitation in order to obtain a complete double decomposition. 3. The preparation of a lime paste of sufficient strength that 1 c. c. is equivalent to 0.2 gram of zinc was found to be easily attainable. 4. The chlorine content of the air-dried precipitate of zinc hydroxide was variable but seemed to tend toward an average of 6 per cent. This chlorine is probably present as basic chloride of zinc. When the hydrate of zinc is ignited to form zinc oxide most of the chloride of zinc would be volatilized and would have to be caught again in water for reprecipitation. 5. The ore that was treated in this series of tests contained manganese, which was precipitated with the zinc, discoloring the zinc |