The Determination of Rhenium Estimation in Pyrolusite LOREN C. HURD'
AND
CLARENCE F. H I S K E I , University of Wisconsin, Madison, Wis.
A n analytical method for the determination of rhenium in the presence of large amounts of manganese dioxide has been developed and demonstrated on synthetic mixtures. Analysis of a representative group of pyrolusi te samples from many parts of the world indicated that the naturally occurring mineral contains inappreciable amounts of rhenium. The highest concentration observed was 0.2 part per million parts of mineral.
T
HE presence or absence of rhenium in manganese min-
erals in general and pyrolusite in particular has been a controversial question since the time of the discovery of the element. Although i t has been shoRn (4) that some earlx methods used for isolating rhenium from manganese concentrates were unreliable and that manganese compounds in general ordinarily contain inappreciable amounts of the element, evidence indicated t h a t pyrolusite from selected sources contained small but definite amounts of rhenium. During a n earlier spectrographic examination ( 7 ) of the acid-insoluble sulfide concentrates obtained from a few samples of western American pyrolusite, evidence was obtained which indicated the presence of rhenium. Routine examination of about fifty random samples of widespread geographic origin revealed its presence in twenty-two specimens. Identification was in most cases based upon one or two lines and i t was knoRn that concentrations were on the border line of the sensitivity of the method. It was not fully realized a t that time that any method involving idgntification based up2n the appearance of one line of the 3460 A. triplet or the 4880 A. line was hazardous because of the virtual coincidence of iron, manganese, and molybdenum lines, some of which n ere not recorded in the conventional atlases. All the samples available for the study had been previously analyzed for elements known to be detrimental in dry cells of the Leclanch6 type and with one or two exceptions were found to have normal compositions. Because it is axiomatic in the dry-cell industry that a chemical analysis of a given pyrolusite does not necessarily prognose its beha\ ior in a cell, all samples had been tested as dry-cell depolarizers. About half of the samples were of such a nature that the cells in which they were incorporated had subnormal shelf lives and were deficient on drain tests. X h e n the results of the spectrographic examination R ere compared t o the electrical data, with but three exceptions the samples thought to contain rhenium were unsatisfactory as dry-cell constituents. Because of this striking correlation, samples of high-grade Montana pyrolusite were compounded with rhenium dioxide to yield products containing 0.075, 0.0075, and 0.00075 per cent of rhenium. The materials along with uncontaminated ore were incorporated in dry cells which were subjected to intermittent and continuous drain tests. It was found t h a t the cells containing rhenium were inferior t o t h e controls in 1 Present
every respect and that the deficiencies were in proportion to the rhenium concentrations. Figure 1illustrates the condition of the inner surfaces of the zinc cans a t the end of 72 hours of intermittent drain through 80 ohms. It was evident that the presence of rhenium in pyrolusite was objectionable if the mineral mas to be used in the construction of Leclanch6 cells. The need for an adequate method of quantitative analysis was likewise apparent. An added stimulus to the development of a method for the analysis of rhenium in the presence of large amounts of manganese lies in the possibility that, if a naturally occurring manganese ore containing rhenium, element 75, can be found, element 43 will likewise be present. The repeated failure of investigators to confirm the isolation and identification (11) of the middle homolog of the manganese-rhenium series lends support to the I-ieIv that its properties are as yet unknown.
Historical
Two general methods have been proposed for the isolation of rhenium from manganese concentrates. Those procedures involving manipulations based upon an assumed similarity
FIGURE 1. Upper Upper Lower Lower
address, 228 Rosemore 4 v e . , Glenside. Pa.
623
R H E N K M I S PYROLUSITE Blank right. 0.00076 per cent rh,enium left. 0.0075 per cent rheni,um right. 0.075 per cent rhenlum
left.
INDUSTRIAL AKD EKGIXEERING CHEMISTRY
624
VOL. 10, KO. 11
of rhenium to iiianganese have been previously discussed (4) The results ieported by the discoverers of the elenient (IO) indicate that manganese minerals contain no rhenium. Unfortunately the aiialytical procedure followed in the examination of the eighteen hundred odd iniscellaiieous minerals reported n a s not clearly disclosed. TT7hether or not the manganese-containing samples were analyzed by alternate oxidation and reduction of the sainple, follov-ed by x-ray examination of the suhlimates, is not definitely known. Since this procedure was followed in the majority of the cases, it is probable that the same teclinique 1 \ 9 8 applied to the hausmaniiitP, rhodonite, psilomelane, and liauerite Ppecimens. If so, the values reported are without significance. I n the presence of a large excess of basic material only a fraction of the rhenium present in a saniDle may berernoved by this treaiment: When the sainple is reduced in a hydrogen atmosphere, rnetallic rhenium is formed. This is nonvolatile under ordinary conditions. Upon introducing oxygen into the system, the rhenium is oxidized to rhenium heptoxide, which in the absence of bases is volatile. I n the presence of the hundred thousand fold excess of basic oxide encountered in actual samples, the heptoxide reacts to form essentially nonvolatile perrhenates. It is not to he expected that substantial percentages of the rhenium present will escape during the alternate reduction and oxidation and the prediction may be readily suhstantiated by experiment.
extract having a color somewhat similar to that of rhenium. Any procedure involving final estimation of the rhenium by the standard colorimetric method m u s t make adequate provision for the separation of the two interfering elements. Separation of rhenium from large amounts of molybdenum may be accomplished by a steam distillation from a sulfuric acid solution. The original method of Geilniann (1) involving distillation in a stream of moist hydrogen chloride has been modified by Kronniann ( 8 ) and Soddack ( 9 ) . Direct distillation of small amounts of rhenium from concentrated acid solutions of manganous sulfate iq not practical because of the mechanical difficulties e n c o u n t e r e d when l a r g e amounts of anhydrous manganous sulfate separate out during the distillation. The determination as finallv developed consisted of a conibination of the colorimetric and distillation methods. Because the \-arious steps leading up to the final estimation have never been investigated under the conditions peculiar t o the type of sample under consideration, it was necessary to check each step individually. The principal operations involved are solution of the pyrolusite, extraction of rhenium and molybdenum, solution of the extract, separation of the rhenium from the bulk of the molybdenum b y distillation, and colorimetric estimation of the rhenium in the distillate.
In one series of experiments conducted by the senior author, manganese dioxide was moistened with sufficient potassium perrhenat,e solution to yield samples which when dry contained 0.1, 0.01, and 0.001 per cent of rhenium. Fifty-gram samples of the mixtures were heated alternately in oxygen and hydrogen at 600" C. The sublimates and condensates were collected in a trap cooled with acetone and carbon dioxide snow. Between oxidation and reduction the system was swept out' with nitrogen. Each cycle required 2 hours and three complete cycles extending over 6-hour periods were carried out in duplicate on each sample. Condensates in the tube and trap were washed out with 5 per cent sodium hydroxide to which hydrogen peroxide was added. Analysis of t,he extracts indicated the presence of between 70 and 10 micrograms of rhenium in the two richest samples and none in the most dilute sample. Similar analysis of a series of pyrolusite, columbite, wulfenite, rhodochrosite, hausmannite, and keilhauite specimens by Noelck (12) had previously failed to reveal the presence of rhenium. It vias evident that but a minute fraction of the rhenium present was recovered by the treatment and that the possibility existed that manganese minerals might contain rhenium which would not be detected when the minerals were analyzed according to the Xoddack method.
SOLUTION OF SANPLE. When manganese dioxide or potassium permanganate is digested with hydrochloric acid in the presence of potassium perrhenate there is no appreciable loss of rhenium ( 3 ) . Although the results reported were on samples of such size that the gravimetric sulfide-nitron method could be used, a similar set, of data indicated that when smaller amounts were dissolved under similar conditions the variations in apparent recoyeries were of the same order as the precision of the colorimet,ric method of analysis. The presence of a ten thousand fold excess of manganese did not interfere with the extraction. As a niatter of precaution, all samples of pyrolusite involved in t,he preliminary experiments and in later analyses were dissolved and digested at teniperatures belo\y 80" C. in large Erlenmeyer flasks.
I
The most sensitive method yet developed for the estimation of rhenium is that of Geilinann ( 2 ) . Inasmuch as the variables of this method had been established over the ranges encountered in ordinary analytical work ( 6 ) , i t mas used as a basis for the present procedure. I n addition to molybdenum, the interference of which was to be expected when dealing with pyrolusite samples, chloroplatinic acid responds to t'he stannous chloride-thiocyanate treatment to yield an ether
ll
Experimental
EXTRACTION. Rhenium and molybdenum were extracted by adding stannous chloride to the hydrochloric acid solutions of manganese chlorides in amounts sufficient to reduce all ferric and manganic salts present. Stannic chloride has been shown to be n-ithout significant influence upon the intensity of the color produced (6). Khen reduction of the salts to the divalent state was evidenced by the change from a yellow-orange to the clear pink of manganous chloride, enough 20 per cent potassium thiocyanate solution was added to give a concentration of 0.5 gram per 100 ml. of solution. This \ m s followed by 20 per cent stannous chloride sufficient to provide 0.6 gram per 100 ml. The solution was shaken in a separatory funnel and after 7 minutei was extracted with successive portions of ether until a practically colorless ether layer was obtained, Four 60-ml. portions were generally sufficient.
NOVElIBER 15, 1938
ANALYTICAL EDITION
Oxidation of the rhenium oxythiocyanate extract was one of the most' critical steps in the procedure. If the combined extracts were evaporated to dryness, decomposition of the complex compound usually folloived and an objectionable residue remained. The best results were obtained if the bulk of the ether was removed by evaporation over a water bath. FT-hen but 5 to 10 nil. remained, 15 ml. of hydrochloric acid (1 to 1) were added and the remainder of the ether was removed by blon-ing a jet of air across the surface of the liquor. Hydrogen peroxide (30 per cent) was then added to oxidize the rhenium to perrhenic acid. Although alkaline hydrogen peroxide yielded a colorless solution, its w e was objectionable because of the tendency of such solutions to precipitate sulfur when acidified for distillation. Potassium chlorate in acid solut,ion and hydrogen peroxide in neutral solution ivere not as satisfactory. Unless the oxidation n-as carried out in such a manner as to yield a colorless solution free from sulfur. low results were obtained.
DISTILL~TION. The distillation of rhenium from sulfuric acid solutions of the oxidized extract was studied in the presence and absence of hydrogen chloride. It n a s observed that quantitative recoT eries of 2OO-microgran~amounts of perrhenic acid could not be made using the conventional distillation method. Accordingly, a large number of determinations were made in which the rate of distillation, the volume of hydrochloric acid, the volume of nater, and the temperature of distillation were varied. Finally carbon dioxide mas substituted for the hydrogen chloride and the method modified in the fo1loTr ing manner: The rhenium as perrhenate was placed in the diqtilling flask along 111th 150 to 250 ml. of 98 per cent sulfuric acid The contents of the flask )!ere heated to betneen 270" and 290" C. and maintained at this temperature 1% hile steam and carbon dioxide were pa-ed through at such a rate as to ensure the distillation of about 250 ml. in 2 hours. The carbon dioxide floiv na? regulated so that the volume vas from one-third to one-half that of the steam. In Table I is t o be found a summary of the results obtained on the analysis of fifty-one consecutive samples. T a n m I. DISTILLATIOS OF RHESI~U No. of Samples
Rhenium Taken /
Rhenuim Found (Av.) Y
Maximum Deviation
Average Deviation 7
Determinations carried out on pure molybdic oxide under the same conditions indicated t h a t ivhen as much as 10 grams was added t o the distilling flask the distillate never contained over 0.5 mg. of molybdenum. Inasmuch as the preliminary extraction of digested pyrolusite samples had a t no time indicated the presence of such large quantities of molybdenum, i t was assumed t h a t the possibilities of interference mere slight. ESTIMATION. Because the previous investigations on the effect of sulfuric and hydrochloric acids upon the development of the rhenium (6) and molybdenum (6)colors were not extended into the high concentration ranges encountered in distillates of the Geilmann-Veibke type, i t was necessary to determine the effect of these variables before attempting to evaluate the efficiencies of various modifications of the conventional distillation. A study of the effect of hydrochloric acid upon the intensity and stability of the colors produced by optimum amounts of stannous chloride and potassium thiocyanate in acid concentrations between 0.2 N and 9.15 ;V indicated t h a t whereas the rhenium complex was remarkably stable in 4 *V hydrochloric acid, the molybdenum color was almost completely bleached in 12 minutes. I n 4.5 S hydrochloric acid the color produced by molybdenum was negligible a t the end of 6 minutes, whereas the rhenium color was unaffected. Sulfuric acid in concentrations between 1 AT and 12 IV ivas
62.5
found to have much less effect upon the intensity of color produced with 400 micrograms of rhenium and 400 micrograms of molybdenum than corresponding concentrations of hydrochloric acid. The rhenium-containing solutions reached their maximum intensity in 10 ' V sulfuric acid. It therefore seemed possible that in mixed hydrochloric-sulfuric acid solutions of proper concentration the development of color clue to the molybdenum thiocyanate complex could be inhibited without greatly altering the extent of the rhenium reaction. Optimum concentrations appeared to be 9.3 N sulfuric acid and 4.9 iY hydrochloric acid. Accordingly, colorimetric comparisons nere made to ascertain vhether or not rhenium could be determined in the presence of molybdenum and if so what the approximate concentration limit of molybdenum was. Two hundred micrograms of rhenium as perrhenate and varying amounts of molybdenum as molybdate were added to 75 ml. of mixed acid of the above concentrations. Stannous chloride and potassium thiocyanate were added in the proper amounts, and a t the end of 6 minutes comparison was made with similar solutions containing no molybdenum. I n Table I1 arc to be found the results obtained.
TABLE 11. EFFECT OF Molybdenum Added
Rhenium Added
Y
Y
200 200 200 400 400 400
200 200 200 200 200
200
hlOLYDBESUM
Estimated 198 196
202 201 201 200
Molybdenum Added
Rhenium Estimated
/
Y
1000 1000 1000 2000 2000 2000
200 207 204 225 232 240
I n acid concentrations as high as those used, the various organic solvents such as ether, butyl acetate, or cyclohexanol which are ordinarily employed as extractors in the determinations cannot be used. Xot only is their solubility greatly increased by the presence of so much acid, but when extraction is made the molybdenum reaction is reversed. Colorless solutions containing molybdenum b u t no rhenium will upon extraction yield colored nonaqueous solutions, the concentration of which varies directly with the number of extractions. It was concluded that, since the amount of molybdenuni distilling over with the rhenium did not exceed 2 mg., a direct colorimetric estimation could be made, provided the acidities were properly adjusted.
Procedure The composite procedure as finally developed was as follolvs : One hundred grams of finely pulverized pyrolusite TTere placed in a 1-liter Erlenmeyer flask and moistened with 50 ml. of water and 200 ml. of hydrochloric acid (sp. gr. 1.2) were added as rapidly as the frothing and the energetic reaction would permit. After the initial reaction had subsided, the flask TTas placed on a hot plate and warmed to GO" to 80" C. Solution required 8 o r more hours and it ivas usually necessary to make small additions of acid from time t o time. When the reaction was complete, the silica and insoluble matter settled out as a light yellow sand and the solution was free from suspended matter. After cooling, the sample was diluted to about 300 ml. and filtered on a Biichner funnel. The precipitate and the filter paper n-ere returned to the flask and 10 to 15 ml. of water were added, followed by 25 ml. of hydrochloric acid (sp. gr. 1.2). The liquid was heated to boiling and filtered. The two filtrates were united and placed i n a separatory funnel. A 20 per cent stannous chloride solution was then added in small portions until all ferric and manganic compounds vere reduced to the divalent state. To the resulting clear pink solution 20 per cent potassium thiocj-anate was added in amounts to yield a solution containing 0.6 gram per 100 ml. This was follomd by reducing agent in sufficient quantity to yield a solution containing 0.5 gram of stannous chloride per 100 ml. -4fter 7 minute
