Tungsten in Low-Grade Ores Quant i tut i ve Spectrographic Deter m i nu t io n DAVID KAUFMAN, Massachusetts Institute of Technology, Cambridge, Mass., Metal Hydrides Incorporated, Beverly, Mass.
AND
S . K. DERDERIAN,
A method is described for the determination of tungsten in ores and products from ore treatment. This method is applicable from 0.001 to 0.3% tungstic oxide and as much as 50% ferric oxide can be tolerated. Direct burning of ore samples in a direct current arc in graphite electrodes does not provide good results. By diluting samples with silica so that the concentration of tungstic oxide was less than 0.05% and then mixing samples 1 to 1 or 1 to 2 with silver chloride, satisfactory results could be obtained. The precision is good and can be improved. In routine operation a probable error of 5% is realized.
T
DEVELOPMENT OF ANALYTICAL METHOD
HE problem of determining tungsten spectrographically in
ores and ore products has been studied by a number of workers (1,4-7,lO) and for specific problems satisfactory methods have been developed. Some of these methods are direct, and some require chemical concentration. The problem a t hand concerned the determination of tungsten in siliceous ores and ore products which contained from 0.001 to 0.3% tungstic oxide and from 0.01 to 50% ferric oxide. No direct spectrographic method reported in the literature would give quantitative results a t a concentration of tungsten as low as O.OOl%, and all the direct methods list iron as a serious interference. Wilson and Fieldes (IO) and Scobie ( 7 ) developed combined chemical and spectrographic methods applicable to the problem, but the chemical separations are too involved for rapid control analyses on a large scale. Ahrens ( 1 ) has suggested a relatively simple procedure for the direct determination of tungsten in siliceous ores, but the limit of sensitivity is about 0.0270 tungstic oxide. In Ahrens’ procedure finely ground samples are placed in a graphite cup, and volatilized in a direct current arc, and the spectrum is photographed. The tungsten concentration is then determined by densitometry of the photographic plates. Silicon is used as an internal standard. Apparently the ores studied by Ahrens contained little iron.
Preparation of Standards. Three samples of gravity concentrates from the ores under investigation had been analyzed chemically in the laboratories of the Climax Molybdenum Company. The gravity tailing was primarily silica; therefore, dilution of these concentrates with pure silica reconstituted the original ore and provided a rapid method for the preparation of standards. The three gravity concentrates contained 0.396, 0.227, and 0.09370 tungstic oxide, respectively. A series of standards ranging from 0.396 to O.OOl70 tungstic oxide was obtained by using the three chemically analyzed concentrates for the highest points and diluting the 0.093% concentrate with quarts crystals to obtain the others. All grinding was done by hand in an agate mortar. An investigation of the spectrum of the standards showed that the only tungsten line that did not have eitber an iron or titanium line coincident w j t h i t was 4924.61 &. (8). (There is a very strong iron line only 0.5 A. away, 4294.13 A ) The spectrograph employed (located a t Metal Hydrides Incorporated, Beverly, Mass.) was a Paschen mounted grating instrument designed by the senior author. It has a dispersion of 4.0 A. per mm. in the first order and uses a 10-cm. (4inch), 4.1meter focal length, 15,000 line per inch original grating. By adjusting the exposure conditions and using a 10-micron slit, the tungsten 4294.61 and the iron 4294.13 A. lines were not only resolved but sufficiently separated to allow densitometry of the tungsten line if the iron concentration was not too high. All plates were densitometered ‘on an Adam Hilger densitometer equipped with a Tinsley galvanometer and galvanoscope. A variable opening disk was rotated in front of the slit to obtain the correct exposure. The spectra were photographed on Eastman 40 plates and after development were densitometered without calibration.
TYPICAL WORKING CURVE FOR SAMPLES
10050.01 0.02
0.05 0.1 0.2
At first an attempt was made to analyze for tungsten by simply burning a weighed sample directly in a direct current graphite arc. When 20-mg. samples were burned completely (2 minutes) a t 10 amperes (400 volts applied), consistent results were not obtained, although the working curve ( yigure 1)was reproducible. The blackening of tungsten 4294.61 A. was measured and referred to standards exposed on the same plate. Normally four standards were burned in addition to the samples and a working curve was constructed for each plate. All observations were made in triplicate. Some improvement might have been obtained by using an internal standard. However, the technique of Ahrens (silicon as an internal standard) could not be applied because of the wide variation in sample composition. Fortunately, the necessity for an internal standard was eliminated by use of a new technique which is described below. Silver Chloride Volatilization. Some experiments were conducted to determine whether or not tungsten could be volatilized selectively. The senior author ( 3 )had found that silver chloride
0.5
%WO3 513
ANALYTICAL CHEMISTRY
614
Table I. Effect of Dilution w i t h Silioa in Assay of Typical Concentrate b y Silver Chloride Volatilization Asshy
of DiluMixture, tion % WOI Factor 0.10 1
Mix-
turea. MS. I . Nodilution 2 . 200 shmple
inn
zoo sbmB~e
3.
200 &iiio&
4 . 200
300
5 . 20nsamp1e 400 silica 6 . 200 No. 5
zoo
i . 200 N ~ 6.
zoo
0
b
Aasay of
Sample. 70 WOa 0.10(
0.055
1.5
0.083
0.038
2
o.oi6
0.031
2.5
0.078,
0.027
3
0.081
0.014
6
0.084,
o.ow
12
n.io8c
Av. % WO,
Probhblek Error,
% WOa
...
...
n.mo
n.002
P.E.
%
the actual analysis 2O-ma. portions of chhe silver chloride mixture
...
...
100 mg. of cadi mixture ground with 200 mg. a i AgCI. Prohnhle error of individual determination (9). P,E, =
o,6i,/dd+i?: +
.
dP . . . . n-1
. d"
d = deviation iron1 mean of individual value n = number oi lues
' Values
omitted in oslouhtiona.
-
assisted the volatilization of a large number of otherwise refractory elements. The principle of the method is similar to that of the pyroelectric concentration procedure described by Scribner and Mullin (8). One concentrate sud one tailing were selected for test work. Each was ground with silver ohloride (Mallinckrodt analytical reaeent) in the followine ratios: 2 Darts of samde to 1 D a r t of silver ohloride, 1 part of sample i o 1 part ofbilver cfiloride, and 1 p a t of sample to 2 parts of silver chloride. Twenty
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I
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spectrographic resultv showed reasonably good agreement with chemical determinations (Table 11). Detailed Description of Analytical Procedure. Original samples must be dry and preferably finer than 200-mesh. If there is m y question of sample uniformity, a representative portion of approximately 5 grams should be ground in an agate mortar until no grittiness is apparent,.
out, in triplicate. For tailing samples no initial dilution with'quartz was required, and a 100-mg. portion of the original sample was ground directly with 100 mg. af silver chloride. From this point on the procedure was the same as far the caneent.rates, except that the standards contained 0.002. 0.006, 0.012, and 0.023% tungstic oxide, respeotively. All concentrates were burned for 40 seconds a t 10 amperes. Tailings were burned for only 30 seconds. A rotating disk with a 180" opening decreased the background sufficiently for both tailings and concentrates, so that no background corrections had t,o be made. Normally 8 samples and 4 standards (36 hurnings) were taken on one plate. Some time could have been saved by calibrating plates and burning fewerstandards, hut B calibration procedure was not available above 4000 A. on the spectrograph used. Without calibration i t was necessary t,o draw a new working curve for each plate. Reproducibility of Analysis. In order to obtain some idea of the reproducibility of the spcctrographio method, eight typical samples were chosen and analyzed repeatedly on seven different plates. For each analysis a separate portion of the original sample was weighed out and ground with silica. On two of the plates, all the samples were treated as tailings and biter dilut,ion were ground with an equal weight of silver chloride. On the other five plates, the samples were treat,ed as eoncentrnt,es and
~~~
from a n 0.02% tungstic oxide sample was volatilized f r o g the 1part of sample t o 1part of silver chloride mixture in 30 seconds. The iron was volatilized as hell, but nevertheless, because of the shorter burning time and lower arc temperature, the background a n the d a t e s was ereatlv decreased. It was found with CT 20-ma. ..
~
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~
the 0.046%standard could be volatilized ih 40 seconds: Recause the majority of concentrates analyzed about 0.1%
burned. ' I f ~ t h edilution was ins;ffioieht to bring the conc&tratian below 0.0570, the results were generally high rsthrr than low, although a low result was expected.
A series of different dilutions was made on one concentrate to determine just how critical the dilution factor was. The datn are presented in Table I. It is apparent that consistent results can he obtained as long as the dilution factor is large enough t o hring the tungstic oxide concentration of the mixture below 0.05%. The poor result obtained with mixture 7 probahly can be attributed to bad sampling because of the high dilution. Most of the tailing samples contained less than 0.02% t,ungstic oxide so that dilution with silica was unnecessary. A ratio of 1 part of srtmple to 1 part of silver chloride gave complete volatiliastian of all the tungsten in the tailings and gave better sensitivity than the 1 part of sample to 2 parts of silver chloride employed for concentrates. When concentrates and tailings were both analyzed by bhe partial burning technique, satisfactory c h e c h were obtained between calculated feed values and actual assay of feed, and the
Figure 2
V O L U M E 21, NO. 5, M A Y 1 9 4 9 -
~-
~-
*
-
.
-
615
__
determined. T h e Drobable error determined in t h s manner was compared with the probable error for this same sample -~ . .Per ~ Cent ~ Tungstic Oxide . given in Table 11. In Table I1 Plate Plate Plate Piate Piate Plate Plate ProbProhthe probable error was obtained dalllpl? Chein. T-4, T-5, T-6. T-9, T-10. T-11, T-12, able able from seven analyses comprisvaluesu 2:16 2:1 2:l 1:l 2:l 2:l 1:l .it-. ErrorC Error, 7c NO. ing three samples each, but oic 0.077 0.056 0.066 0.068 0.065 0.080 0.066 0.064 0.067 0.004 6.0 each group of three samples 13341 0.08 0,070 0.0713 0.092 0.086 0.088 0.080 0.092 0,083 0.006 7.2 133422 0.17 0.172 0.176 0.160 0.164 0 . 1 4 4 0.176 0.165 0.008 4.8 was takrn froni a separate mix148422 0.19 0.180 o:iSs 0 . 1 7 2 0 . 1 7 2 0.180 0 . 1 7 2 0.164 0.175 0.005 2.8 ture of sample, silica, and silver 165C 0.12 0.096 0122 0.12fi 0 . 1 1 4 0.100 0.098 0.128 0.111 0.007 6.3 0.09 0.064 0 072 0 076 0.062 0.076 0.064 0.062 0.068 0.005 7.3 169-CT chloride rather than from the 0.0090 0 0078 0.0094 0.0094 0.0093 0.0092 0.0082 0.0089 0.0004 4.5 0 01 lR9-TI mixture described above and 0 . 0 0 7 0.0057 0 0038 0 0033 0.0052 0 0058 . . . . 0.0050 0.005j 0.0002 3.6 1107 each of the seven analyses was Cheirii ea1 analywh niade ti? h n a l d (;iiernsey, Departmrnt of hletallurgy, llassarhusetts Institute of Techmade on seven different plates noloyy. b 2:l--2 yarts AgCI, 1 part yarnple. 1:l-1 part .lgCI. 1 Imrt sample. with seven different sets of C Prohal>If, r r r o r formilla, rrfrr to Tahlr I. Mean i)robaLle error, 3.376. standards. . . __ -~ A similar determination was carried out, with sample 1107, but no dilution with silica was required, and 200 mg. of the original sample were mixed directly with 200 mg. of silver chloride. Fifteen 20-mg. portions were burned, compared with standards, and treated as above. Tlic data are presented in Table 111. Table 11.
Results of Se\en Separate Analyses of Eight Typical Sanlples by Silver Chloride Volatilization Method
~
~~~
~
ANALYZED B Y
2PARTS AGCL 1 PART h C L
0.002
0.06
0.012 0.023 0.04
% WO, after dilution were ground with double their weight of silver
chloridr. From each sample-silver chloride mixture, 20-mg. samples were weighed into the electrodes in triplicate. I n Tahle I1 each figure represents the mean of threz burnings. .In enlargement of the region of the 4294.6 A. tungsten line is shown in Figure 2, which gives a good indication of how the lines appear on the viewing screen of the densitometer. In spite of t h r high iron concentration in sample 133 C-1, the tungsten line car] be adequately resolved. Typical working curves for conct'nt rates and tailings are demonstrated in Figure 3. Sources of Error. The probable error for a single deterinination (mean of three burnings) is approximately 5%. Part of this error is inherent in t.he burning; part may be attributed to sanipling bot,h from the original material and from the mixtures d e r grinding; and part may be the result of variation in the working curves on the individual plates. The error involved in the burning step can be determined by multiple analyses from a aiirgle griiiding made on one plate with one set of standards. To determine this error a concentrate (2TC) and a tailing (1107) a-we treated as follows:
.i 200-nig. portion of sample 27C1 was ground with 000 mg. of
silica. Two hundred milligrams of this mixture were then ground with 400 mg. of silver chloride. Fifteen duplicate samples of 20 nig. each were taken from the silver chloride-sample mixtuw and hurtled on one plate against one set of standards (in tripli(wtcl), and the results were arbitrarily divided into groups of three. Each group was then considered as a separate analysis, arid the meail and probahlr wrms for the five analysw were
Thr probable error for a number of determinations made from a single grinding and compared with a single set of standards is much smaller than that obtained froin different grindings compared with different sets of standards. This indicates that a considerable increase in precision could be obtained by more careful attention t'o sampling and grinding and by better control of development conditions. Effect of Iron Concentration. .1s noted above, most direct methods for the determination of low concentrations of tungsten (below 0.1 yo)are applicable only when the iron concentration is low. However, some of the ore products contained large amounts of pyrite. High iron causes difficult,y chiefly because of the interference of iron with most of t,he high iniensity t'ungsten lines. At high concentrations iron line 4294.13 A. becomes very broad and has a dark background in its immediate neighborhood. If tungsten line 4294.61 is weak, it may be completely lost in the background. In order to determine how much iron can be tolerated at various concentrations of tungsten, a concentrate which previously had been analyzed at 0.066% tungstic oxide was diluted with different amounts of silica and analyzed. These same diluted samples were then further diluted with ferric oxide and analyzed. The results w e presented in Table I\-.
Tahle 111. Comparison of Probable Error for Single and 3Iultiple Grindings Sample Z i C , Single grinding 0.070 0.076 0.064 0.068 0.066 0.069 0.003 4.4
Mean c/o WOa Probable error P.E., 7c -
Table I \ .
Sample 1107. % It Oa RIult11,lr grinding Single irom grinding Tablr I I
c/c \\OaAIllltlnle rrindinng from Tahle I1
....
0 0065 0 0064
0 0052 0 0051 0,005& 0,0052 0 0001 1 9
0 06i 0 004 6 0
.... ,...
....
.... 0.0035 0 , 0002 3.6
--______ - - -~ Effect of Added Iron on Tungsten 4ssa) at Yarions Concentrations of Tungsten
L , 1 r,art sainplc 2 p a r t s silica 0 . 3 parts KO.1 1 part Fez08 3 . 1 part sample 3 parts silica 4 . 3 parts S o , 3 1 part Fez08 5 , 1 part S o . 3 1 part Fe2Oa u All samples mixed with 1 .. -. .
~
..
-
~~
2 ;i
0,022
0.02''
0 , 0 1ii
0.016
0.0165
0 . 0 16 J
25
0.0124
0.0135
30
0,0083
0,0105
p a r t i
