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INDUSTRIAL AND ENGINEERING CHEMISTRY
motion, it is supported by a 3-mm. wire handle extending through the lower stopper and bent so that the wire and upper portion of the glass tube have the same axis. The tube is held to the wire by a piece of thin metal sheeting soldered to the wire and bent around the glass at two points. In the lower stopper are also placed four slightly bent &mm. glass tubes, which are flanged at the upper end and placed so that the centers are equidistant from the wire. When in operation, four receivers are attached to these delivery tubes by means of rubber stoppers, and then as a drop of distillate leaves the condenser it can be directed by means of the rotating adapter to any one of the delivery tubes and thus to the receiver. Adapters of the type described are free from the usual susceptibility to damage caused by thermal and mechanical strains. The rubber stoppers, do not lower the efficiency of operation, since pressures as low as 1mm. have been obtained, nor do they affect the purity of product, since at no time is the
Vol. 14, NO. 6.
distillate in contact with any material other than glass. Moreover, the individual adapters offer certain advantages. The first model is designed for small fractions, but the capacity can easily be varied by changing the length of the whole system. The use of fragile, preweighed vials as direct receivers is a n important convenience in working with small quantities where losses due to transfer are appreciable. The second suggested design is of such a nature that it is not necessary to change the position of the receivers when changing fractions. Thus, receivers can be held firmly and may even be large and heavy, since changing fractions merely requires twisting the wire handle.
Literature Cited (1) Bruhl, Ber., 21, 3339 (1888).
Ferrocyanide Method for Separation of Hafnium from Zirconium W,4LTER C. SCHUMB AND FRANK K. PITThIAN Massachusetts Institute of Technology, Cambridge, Mass.
M
OST of the methods for the separation of hafnium from zirconium depend upon the repeated fractional crystallization or precipitation of such compounds as the oxychloride, phosphate, oxalate, fluohafnate, etc., and the number of fractionations required in general is large. I n 1932 Prandtl ( 2 ) described a procedure by n-hich the separation of these elements may be effected in comparatively few operations by precipitation of the ferrocyanides, yielding a product containing 90 per cent hafnium oxide and 10 per cent zirconium oxide starting with a material containing about 1 per cent hafnium oxide. Of all the methods reported thus far, this would appear to be the fastest and most efficient. I n certain details the procedure described by Prandtl is somewhat indefinite, such as the actual concentration of sulfuric acid which should be present in the solution prior to precipitation with sodium ferrocyanide and the proper quantity of sodium ferrocyanide to be used in the precipitations. Since the authors’ preliminary experiments with the method gave results somewhat a t variance with those reported by Prandtl, it was decided to examine more closely the conditions under which separation by this procedure may be accomplished. The method described by Prandtl depends upon the fractional precipitation of the ferrocyanides of zirconium and hafnium from a solution containing oxalate and sulfate ion, t h e purpose of which is to form complex ions of different stability. The hafnium is enriched in the precipitate. PROCEDURE. Zirconium (and hafnium) hydroxide is precipitated from a solution of the oxychloride by means of ammonium hydroxide. The precipitate is filtered and treated with just enough dilute sulfuric acid to cause it to dissolve. This takes several hours, because the reaction must be carried out at room temperature. An amount of ammonium sulfate at least equal to the weight of oxide in the solution is added to the clear solution, which is warmed for several hours. If no precipitate forms, a solution of oxalic acid saturated in the cold is added in the ratio of 200 cc. of oxalic acid for each 100 grams of oxide prestnt. To this solution, still warm, is now added with stirring a solution of sodium ferrocyanide containing a weight of SaaFe(CS)s.10H20equal t o that of the oxide in solution.
(Some ambiguity exists in the original description at this point, which is as follows: “ich . . . . eine Losung von soviel , . . Xatriumferrocyanid zuset,zte, als dem Gewicht des in Losung vorhandenen Zirkonium-Hafniumoxydes entsprach.” That this implies an amount of ferrocyanide equal to the weight of the mixed oxides is assumed, since if it were to mean equivalent to, the precipitation of both zirconium and hafnium would be complete and no separation effected. Furthermore, in referring to the addition of ammonium sulfate, in the same paragraph, an almost identical construction is used which leaves no doubt as to its meaning: “Zu dieser Losung setzt man mindestens so vie1 Ammoniumsulfat hinzu, als das Gewicht des in der Losung vorhandenen Zirkonium-Hafniumoxydes betrfigt. ”) The mixture is allowed to stand with stirring at room temperature for several hours. The resulting yellow precipitate is then filtered and converted to the hydroxide by digestion with excess concentrated sodium hydroxide. (The precipitate will be greenish-yellow if iron is present, but this does not affect the efficiency of the separation.) The process may then be repeated with this mixture of zirconium and hafnium hydroxides. I n illustrative data quoted by Prandtl, the addition of 500 grams of sodium ferrocyanide to 1 kg. of zirconium oxide was said to give 40 grams of enriched zirconium oxide. The concentration of the starting material was given as about 1.3 per cent hafnium oxide and that of the product as 20 per cent hafnium oxide. There appears to be a discrepancy between the body of the report, which calls for a 1 to 1 ratio of ferrocyanide to oxide, and these data which call for a 1 to 2 ratio. I n repeating the experiment just referred to, using the 1 to 2 ratio, the authors obtained the following results (calculated to the same basis as Prandtl’s) : The addition of 500 grams of sodium ferrocyanide to 1 kg. of zirconium oxide (12 per cent hafnium oxide) yielded 200 grams of zirconium oxide (18 per cent hafnium oxide). These results differed from those of Prandtl in the amount of material precipitated, the increase in the ratio of hafnium to zirconium, and the efficiency of the enrichment of the hafnium oxide. It was felt that the cause of these discrepancies could be found in the ambiguities in the original paper above referred to. This led to the undertaking of a detailed study of the factors affecting the separation.
ANALYTICAL EDITION
lune 15, 1942
Starting Material The source of the zirconyl chloride containing hafnium used throughout this work was the mineral cyrtolite, a n altered zircon found in fairly large quantities near Bedford, S . Y. The extraction of the oxychloride from the mineral was accomplished by two methods, sodium carbonate fusion and sulfuric acid extraction. In the case of the fusion, the hafnium and zirconium in the mineral were converted to water-insoluble sodium hafnate and sodium zirconate, which were dissolved in sulfuric acid. The hydroxides of zirconium and hafnium were then precipitated from this acid solution, washed and dissolved in concentrated hydrochloric acid, and the oxychlorides were crystallized by evaporation. In the case of the sulfuric acid extraction, the mineral was treated directly with concentrated sulfuric acid and the oxychloride prepared from the acid solution by way of the hydroxide as stated above. The second method was found to be much simpler and much more efficient on large batches than the fusion. By a combination of these two methods, a total of about 3600 grams of zirconyl chloride was obtained from 18 pounds of the mineral. This oxychloride was freed from all other substances by several recrystallizations from concentrated hydrochloric acid solutions, after each of which the crystals were washed with a 1 to 1 mixture of concentrated hydrochloric acid and reagent alcohol. The material thus obtained could be converted to a n oxide which contained about 12 per cent hafnium oxide and 88 per cent zirconium oxide.
Method of Analysis Since there is no available simple chemical process for the separation of hafnium from zirconium, any analytical method for determining the percentage of hafnium must depend either upon physical measurements or upon the conversion of one definite compound into another. By determining the density of the oxide mixture, one is able to calculate the percentage of hafnium oxide present, because the molecular volumes of the two oxides are very nearly the same. However, the determination of the density of the finely powdered oxide is attended with a certain amount of difficulty, due t o adsorption of gases by the finely divided material, and the accuracy of the results is always open t o question, because of uncertainty concerning the density of pure hafnium oxide. For this reason, density determinations were discarded as a method of analysis after extended trials. The method of analysis used was based, with slight modifications, on the procedure worked out in detail by Claassen ( I ) , which depends upon the fact t h a t zirconium and hafnium form selenites of constant and definite composition which can be converted quantitatively to the oxides (4). Approximately 1 gram of air-dried oxychloride was dissolved in about 200 cc. of distilled water, and to this,solution were added 50 cc. of a 10 per cent solution of selenious acid. A white, flocculent precipitate of the basic selenites of zirconium and hafnium was formed. The mixture was placed on a steam bath and allowed to digest until the flocculent basic selenite was converted to the crystalline normal selenite. This process took between 12 and 15 hours, and rrcautions were taken to prevent evaporation of the solution. &he finely poivdered crystalline selenite was then washed by decantation with about 2 liters of boiling distilled water to remove all excess selenious acid. The washed powder was transferred to an evaporating dish and placed in a drying oven at a temperature of 125" to 140" C. for 10 to 12 hours. Approximately 0.5 gram of this dried powder was accurately wighed into a tared porcelain crucible. The crucible was then hcated very carefully with a Bunsen burner until no further evolution of selenium oxide could be seen, aftrr which the burner was replaced by an air-blttst Rl6ker burner, and the crucible and oxide were ignited to constant weight. The weight of the empty crucible was checked after the ignition. From the weight of the selenite and the weight of the oxide, the percentage of hafnium oxide could be readily calculated by means of the formula - 0.35702 (wt. of selenites)] G y HfOi = 374.86 [ a t . of oxides wt. of oxides
513
The method is accurate to within 0.5 per cent when the amount of hafnium oxide is about 25 per cent.
Factors Influencing Separation EFFECTOF HYDROLYSIS. Because zirconium hydroxide is a somewhat weaker base than hafnium hydroxide, the hydrolysis of salts of these two elements would tend to result in a precipitate richer in zirconium than the original solution. This tendency, if not overcome, would cause a decrease in the amount of separation brought about by the ferrocyanide precipitation, because the precipitate in this case is enriched in hafnium. The extent of hydrolysis is affected by changes in the concentrations of both the hydrolyzable substance and the hydrogen ion. Preliminary experiments indicated that these factors had negligible influence upon the degree of separation achieved. However, until a detailed study of the effect of dilution had been made, the sodium ferrocyanide was added in powdered form in order t o obviate any possible harmful influence of increased dilution. The results of these preliminary experiments also suggested that it might he unnecessary first to convert the oxychloride to the hydroxide, and several comparative series of esperiments showed this to be the case. TABLE I. EFFECT OF VARIATION OF AMOUNT OF A M M O N I ~ ~ SULFATE ON SEPARATION (NHI)&OI Added per 100 Grams of Oxychloride Grams 0 30 40 60 80 95 120
oxychloride irom Ferrocyanide Preoipitate Grams 15 22 14 14 13 15 14
HfOz in Oxide from First Precipitate
% 12.2 15.8 14.7 13.8 12.5 12.0 12.0
Oxychloride irom Secqnd Precipitate Grams 0 47 17 0 0 0 0
Hi02 in Oxide from Second Precipitate
0
ii:o 10.4 .. .. ..
EFFECT OF VARIATION OF AMMONIUM SULFATE Of the two complex-forming constituents, oxalate and sulfate ions, the former is the more powerful, and i t is reasonable to expect that in its presence the ammonium sulfate would have relatively little effect. The following series of experiments was carried out, in order to determine definitely what effect, if any, the sulfate did have in the absence of the oxalic acid. A series of solutions was made up containing known ratios of oxychloride to ammonium sulfate. To each of these solutions were added 15 grams of solid sodium ferrocyanide per 100 grams of oxychloride, as preliminary experiments had indicated this to be the optimum amount, and the precipitate thus formed was converted to the oxychloride and analyzed. In some cases, a second precipitate formed in the solutions from which the ferrocyanide precipitate had been removed. These precipitates were also converted to the oxychloride and analyzed. The results of this series of runs are shown in Table I. The table shows t h a t there is a certain dependency of separation on the amount of ammonium sulfate present, b u t the magnitude of this effect is small. It also indicates t h e optimum amount of ammonium sulfate to be 30 grams for each 100 grams of oxychloride; and this would be true, were it not for a n additional factor-namely, the formation of t h e second precipitate. The nature of this second precipitate can be understood when one remembers that a permanent, precipitate will be formed when zirconyl and sulfate ions a r e mixed together in a molar ratio of 1 to 1 ( 3 ) . Referring t o Table I, when between 30 and 40 grams of ammonium sulfate have been added, the molar ratio becomes 1 to 1, and the precipitate forms. This precipitate is b u t slightly soluble, and its presence in the solution would interfere with the separation brought about by the ferrocyanide precipitation. Thus 30 grams of ammonium sulfate per 100 grams of oxy-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
2 151// a W
I4V
13
ley 0
1
50
I 150
I 100
I1 200
C C O F SAT OXALIC ACID PER 100 GRAMS OF OXYCHLORIDE
FIQURE 1. EFFECTO F OXALATE-ION CONCENTRATION
chloride cannot be recommended as the optimum amount. It was found that the best amount to use was between 80 and 100 grams. The exact amount added is not of critical importance, but a ferrocyanide precipitate which filters somewhat more easily is formed in the presence of about 100 grams of ammonium sulfate. TABLE 11. EFFECTO F SODlUhl FERROCY.4NIDE NaAFe(CS)o. 10HzO Added per 100 Grams of Oxychloride
HfOz in Oxide Prepared from Precipitate
Grams
%
5 10
18.4 19.7 20.0 18.8 15.3
15 20 30 45 60
13.3
12.0
EFFECTOF OXALATE-ION VARIATIOK.I n an attempt to study the effect of the oxalate ion concentration on the separation, 10 and 20 grams, respectively, of ammonium oxalate mere added to solutions of 100 grams of the oxychloride in 300 cc. of water, and the resulting solutions were treated with 15 grams of sodium ferrocyanide. The ferrocyanide precipitates thus formed were very slimy, and could not be filtered even with the use of filter aids. It was thought that the colloidal nature of this precipitate could be overcome by the presence of more electrolyte in the solution from which the precipitate was formed, and hence ammonium sulfate was added to the solution before the ferrocyanide was precipitated. However, when 10 grams of ammonium oxalate and 80 grams of ammonium sulfate were added to the oxychloride solution, a white precipitate formed which would not dissolve on dilution, but which increased in size as water was added. I n order to prevent the formation of this precipitate, it was found necessary to have the solution acid, either with oxalic or with sulfuric acid. The solutions finally used to determine the dependency of the increase in hafnium content on the amount of oxalate ion were made up to contain 100 grams of oxychloride, 30 cc. of concentrated sulfuric acid, 100 grams of ammonium sulfate, and definite known amounts of a solution of oxalic acid, saturated a t room temperature, in 300 cc. of water. To each of these solutions were added 15 grams of solid sodium ferrocyanide. The precipitates thus formed were filtered, converted to the oxychloride. and analyzed. The results of this
Vol. 14, No. 6
series of runs are shown in Figure 1. An examination of this curve shows that, although the optimum amount of oxalic acid solution is approximately 100 cc. of an oxalic acid solution, saturated a t room temperature, for each 100 grams of oxychloride, i t is not necessary to regulate this amount too closely. Any volume between 90 and 110 cc. of theoxalic acid solution i.: satisfactory. EFFECT O F VARYIKG AMOUST O F SODIUM FERROCYANIDE ADDED. A series of solutions was made up containing 100 grams of oxychloride, 30 cc. of concentrated sulfuric acid, 100 grams of ammonium sulfate, and 100 cc. of saturated oxalic acid solution, in 300 cc. of water. To each of these solutions was added a different known amount of solid sodium ferrocyanide, and the precipitate thus formed was filtered, converted to the oxychloride, and analyzed. The results of this series of experiments, shown in Table 11, confirmed the previously drawn conclusion that the optimum amount of sodium ferrocyanide is 15 grams for every 100 grams of oxychloride in solution. A detailed study of the effects of acid concentration and dilution was next carried out, using the optimum amounts of oxalic acid and sodium ferrocyanide and ammonium sulfate, as previously determined. EFFECTOF ACID CONCENTRATION. A series of solutions was made up containing 100 grams of oxychloride, 100 grams of ammonium sulfate, 100 cc. of oxalic acid solution saturated a t room temperature, and different known volumes of concentrated sulfuric acid in 300 cc. of water. Each solution was then treated with 15 grams of solid sodium ferrocyanide, and the precipitate thus formed was filtered, converted to the oxychloride, and analyzed. The results are shown in Table
111. These results clearly indicate, as was expected from preliminary experiments, that the concentration of free acid in the solution may vary from 0 to about 5 N with no noticeable effect on the separation, and that large quantities of sulfate ion in the presence of oxalate ion have no effect, which confirms the work previously described on the effect of ammonium sulfate. It would appear that the addition of sulfuric acid is unnecessary, but, if it is not added, the ferrocyanide precipitate is difficult to filter, until about 30 cc. of acid has been added, after Rhich no further difficulty is encountered. For this reason, the recommended amount of acid is about 30 cc. of concentrated sulfuric acid for each 100 grams of oxychloride TABLE111. EFFECTOF ACIDCOXCENTRATION HzSOh Added Oxychloride per 100 Grams Recovered from Ferroof Oxycyanide P p t chloride
cc.
Grams
Sulfate PresHfOi in Oxide e n t per 100 N(Approx ) from Ferro- Grams of Oxy- with Respect cyanide Ppt. chloride to HzSO, 70 Molrs
EFFECT OF DILUTION. I n order to determine in more detail the effect of dilution, a series of precipitations was made from solutions, the total volumes of which were 300, 400, 500, 800, 1000, and 1500 cc. I n no case was there any difference in the amount of separation realized, thus confirming the conclusions drawn from preliminary experiments. However, in the more dilute solutions, the ferrocyanide precipitate was much more difficult to filter, and for this reason it was found advisable always to precipitate from a solution having a total volume of between 400 and 600 cc. per 100 grams of oxychloride. EFFECT O F TIME INTERV.4L BETWEEN PRECIPITATION AND FILTRATION. Since there was a possibility that the composition of the ferrocyanide precipitate might change on standing in contact with the solution from which it was formed, a
ANALYTICAL EDITION
June 15, 1942
515
50 cc. of water should be added, and the solution left standing series of experiments was made in which the time before until it is clear. To this is added, for each 100 grams of oxyfiltration was varied. The shortest time interval between chloride, a solution containing 15 grams of sodium ferrocyanide precipitation and filtration was 1 hour, the longest was 2 in about 50 cc. of water-a nearly saturated solution. The ferrocyanide solution is added dropwise from a buret over a period weeks. I n some cases stirring was provided, in others t h e of several hours, while the zirconium-hafnium solution is being mixture was not stirred. In no case was there any difference stirred vigorously. in the amount of separation, but the precipitates which had The yellow precipitate formed by the addition of the sodium stood for some time were much easier to filter. The best ferrocyanide to the zirconium-hafnium solution should be allowed to stand for a t least 1 hour, and preferably for 10 or 12 hours results were obtained when the precipitate was allowed to before filtration. Suction is used for the filtration, and the prestand overnight before filtration. Longer standing tended cipitate is drained as dry as possible but is not washed. The preto cause a certain amount of decomposition of the ferrocyacipitate is then trannferred from the funnel to a large beaker, nide, and since this introduced iron into the solution, i t was where it is suspended in water. To this suspension is added ammonium hydroxide, which converts the zirconium and hafobjectionable. EFFECTOF SUCCESSIVEFERROCYANIDE PRECIPITATIONS.nium ferrocyanides to the hydroxides. The hydroxide precipitate, after being washed by decantation until all the ferrocyanide All the previous discussions have dealt with a single ferrohas been removed, is dissolved in concentrated hydrochloric acid, cyanide precipitation; in each case the solution remaining from which solution the oxychlorides may be obtained by evaporation. still contained a major portion of the zirconium and hafnium The filtrate from the ferrocyanide precipitation, which still originally present. It seemed reasonable t o expect that contains a major portion of the zirconium and hafnium, may bfurther precipitations could be made from this same solution, treated with further portions of sodium ferrocyanide and thus and that the subsequent precipitates should be of varying separated into several fractions of varying hafnium-zirconium content. I n each case the ferrocyanide precipitate is treated as hafnium content. A series of experiments was made in which outlined above. The final filtrate after the desired number of the solution from the first precipitation was treated with ferrocyanide precipitations have been carried out will, in most successive portions of sodium ferrocyanide, until five precases, still contain some hafnium and zirconium which must be cipitations in all .had been made from the initial solution. recovered. This is done by precipitating the hydroxides, washing them, dissolvin them in concentrated hydrochloric acid, Each of the precipitates was analyzed (Figure 2). and evaporating to o%tain the oxychlorides. This curve shows that, by starting with a material conEach of the fractions thus obtained can be further fractionated taining about 12 per cent hafnium oxide, five fractions could in the same manner, and by combining fractions having the same be obtained in which the composition varied from 19 per hafnium-zirconium ratio, and continuin the precipitations, the zirconium and hafnium can gradually f e separated from each cent to 3.8 per cent hafnium oxide. The amounts of each of other in any desired state of purity. these fractions were approximately the same, about 15 grams per 100 grams of oxychloride initially used. Using the procedure outlined above, in four successive ferrocyanide precipitations the hafnium oxide content of an Recommended Procedure oxide mixture was enriched as follows: 12 to 20 per cent; 20 to 36 per cent ; 36 to 62 per cent; and 62 to 80 per cent As a result of the previous detailed studies of the conditions for the separation of hafnium from zirconium by the ferroSummary cyanide method, the following procedure is recommended as giving the maximum separation: A systematic study of the factors involved in the separation of hafnium from zirconium by the ferrocyanide method, proThe oxychloride is dissolved in 300 cc. of water for each 100 posed by Prandtl, has resulted in modifications of the procegrams of the oxychloride. To this solution are then added 30 cc. of concentrated sulfuric acid, 100 grams of ammonium sulfate, dure to give the maximum degree of separation. and 100 cc. of oxalic acid solution, saturated at room temperature, The factors studied include the dilution and acid concenfor each 100 grams of oxychloride. The solution a t this point tration, each of which affect the extent of hydrolysis of the should be perfectly clear. If there is a white precipitate, an extra zirconium and hafnium ferrocyanides precipitated, the amounts of ammonium sulfate, oxalic acid, and sodium ferrocyanide added, the time interval allowed between precipitation and filtration, and the effect of successive ferrocyanide precipitations carried out upon the same initial solution. The optimum conditions for each of these factors have been determined. By the procedure described an oxide mixture containing initially 12 per cent hafnium oxide and 88 per cent Zirconium oxide mas converted in four operations into an oxide mixture containing 80 per cent hafnium oxide and 20 per cent zirconium oxide. The Claassen method for the analysis of zirconium-hafnium mixtures has been found entirely satisfactory as to accuracy and reproducibility of results. Certain minor modifications have been made in the technique to adapt it to the present problem. Large quantities of hafnium-high zirconyl chloride can be easily extracted from Bedford (S. Y.) cyrtolite by means of concentrated sulfuric acid.
i
0 I
2
3
4
5
N U M B E R OF 15-GRAM PORTIONS OF S O D I U M FERROCYANIDE
FIGURE 2. EFFECT OF SUCCESSIVE FERROCYAXIDE ADDITIOKS
Literature Cited
(1) Claassen, 2. anal. Chem., 117, 252 (1939). (2) Prandtl, 2. anorg. allllern. Chem., 208, 420 (1932). (3) Ruer and Levin, Z b i d . , 46, 449 (1905). (4) Schumb and Simpson, J . A m . Chem. Soc., 53, 921 (1931). CONTRIBUTION KO. 85 from t h e Research Laboratory Chemistry, Massachusetts Institute of Technology.
of Inorganic
