furic acid. Iron(I.1) solution was added dropwse from a weight buret until the meter showed a large potential change. Then the excess iron(I1) was titrated automatically with standard cerium(1V) solution from the Gilmont buret. Cerium(1V) solution was always used for the automatic titrations to prevent changing solutions in the titrator and to maintain constant conditions in the end point determination. The average value for the weight of ceriuni(1V) solution equivalent to 1 gram of iron(I1) solution obtained for 10 titrations is 12.3089,with a standard deviation of 0.0008 or 0.006 relative yo (Table I). This standard deviation is about twice the weighing error for the iron(I1) solution. The average value for 10 determinations of the weight ratio of chromium(V1) solution t o iron(I1) solution is 2.4620, with a standard deviation of 0.0002 or 0.01 relative yo (Table I). These data show the over-all precision for the determinations using large samples with the back-titration technique, and that the per cent standard deviations are lorn because of the large sample sizes. To determine the precision of only the automatic part of the titration, tht. iron(I1) solution was diluted to a concentration of
about 0.03 meq. per gram, and direct titrations of small weighed portions were made with the titrator. The results of these titrations of 0.01 meq. or less (Table 11) show a precision of 0.5 relative yo. Assuming 0.01 meq. or less of cerium(1V) would be used in all titrations of plutonium, over 5 meq. of iron(I1) should be used in the backtitration to keep the standard deviation of the determination below 0.00005 meq. or 0.001 relative yo for the automatic titration part of the determination. By using large samples with the back-titration technique, highly precise results are obtained rapidly. ACKNOWLEDGMENT
The assistance and suggestions of Gilbert Apprill and James Deal of the instrument group of Los Alamos Scientific Laboratory were very helpful in modifying the titrator for remote operation. LITERATURE CITED
(1) Barredo, J. M. G., Taylor, J. K., Trans. Electrochem. SOC.92, 437 (1947).
P., “New Instrumental Methods in Electrochemistry,” pp. 382-90,Interecience, New York, 1954.
(2) Delahay,
Table
II.
Direct Titration of Iron(ll) with Cerium(1V)
Net Fe Soln., Buret Ce Added, Gram Reading Meq. 0.1994 0.2155 0.1327 0.3295 0.1953 0.2736
133.4 143.1 89.0 219.2 129.8 184.1
Fe, Meq.1 Gram
0.006270 0.006726 0.004183 0.010302 0.006101 0.008653
0.03144 0.03121 0.03152 0.03127 0.03124 0.03163 Av. 0.03139 Std. dev. O.OOOl7 Std. dev., 700.5
(3) Kordatski, ITT., Wulff, P., 2. anal. Chem. 89, 241 (1932). (4) Lingane, J. J., ANAL.CHEM.20, i97 (1948). (5) Lingane, J. J., “Electroanalytical Chemistry,” pp. 128-35, Interscience, New York, 1953. (6) Malmstadt, H. V.,Fett, E. R., ANAL. CHEM.26, 1348 (1954). (7) Muller, R. H., Lingane, J. J., Zbid., 20, i95 (1948). (8) Robinson, H. A., Trans. Electrochem. SOC.92, 445 (1947). (9) Shenk, W.E., Fenaick, F., IKD. Em. CHEY.,ASAL.ED.7, 194 (1935). (10) Vogels, Henry, Bull. sci. acad. roy. Belg. (5),19, 452 (1933). RECEIVED for review November 3, 1958. Accepted March 5, 1959.
Determination of Platinum and Palladium in Ores and Concentrates New Fire Assay Method M. E. V. PLUMMER and F. E. BEAMISH
,
University o f Toronto, Toronto, Ont. Canada
b A method for determining microgram amounts of platinum and palladium in ores and concentrates involves a collection by an iron-copper-nickel alloy in a clay crucible from a fused mixture of the oxidized ore, sodium carbonate, borax, and graphite. The fusion is accomplished b y a standard air-gas furnace. The accuracy is comparable to that of the single available assay method applied under optimum conditions and nickel does not interfere. While the efficiency of recovery of the remaining platinum metals has not yet been definitely ascertained, these metals do not interfere with the recovery of platinum and palladium,
A
NEW approach to the analytical extraction of platinum and palladium from ores (7) involved a reduction
by the walls of a carbon crucible of iron, nickel, copper, platinum, and palladium in a sodium carbonate-borax glass medium. The metal alloy mas removed from the slag and dissolved in appropriate acids. The solution was passed through a cation exchange resin to remove base metals. It is of interest to compare the data in Table IV with those recorded by the senior author in publications on the efficiency of recovery of platinum and palladium by the classical fire assay (3, 4). Under optimum conditions, the initial lead buttons obtained from salted samples yielded a recovery of 98.8% of the platinum and 98.1% of the palladium originally added t o the synthetic samples. Under the recommended coiiditions the new method yielded a recovery of 99.3y0 platinum and 98.2% of pallad ium.
Because the principles of this method suggested useful metallurgical applications, an effort was made to overcome such objectionable characteristics as the large sample of ore required, the specially constructed and expensive carbon crucibles, the need for a high frequency furnace, the excessively large button, and certain procedural complications. The result has been the successful collection by iron-copper-nickel of platinum and palladium from relatively small samples of ore by fusion in a standard gas furnace. Some evidence indicates that copper is itself a good collector of platinum and palladium, and by increasing its proportions in the ore, a rclatively low temperature can be used for assay fusions. APPARATUS A N D REAGENTS
The apparatus and reagents have VOL. 31, NO. 7, JULY 1959
1141
been described ( 7 ) . A gas-fired &{onarch Engineering furnace, Curtis Bay, Md., was also used. Ore concentrate was made from ores of the Sudbury district of Ontario. The percentages of the main constituents were: iron 40.70, copper 5.20, nickel 6.14, sulfur 34.00, silica 5.10, h i e 2.0, alumina 4.5, and magnesia 2.0. The concentrate was analyzed for platinum and palladium by the classical fire assay using the following composition: one assay ton of ore concentratc was mixed with 35 grams of soda ash, 20 grams of silica, 50 grams of litharge, and 5 mg. of silver powder. The lead buttons from two such assays were scorified together and then cupelled to give a silver bead weighing about 10 nig. This bead was analyzed by a chromatographic ( 7 ) and a spectrographic method (6) (Table I). Artificial Sulfide Concentrate. The following mixture resembling the natural concentrate mas used to ascertain the ability of an alloy of copper, nickel, and iron to collect platinum and palladium: iron(1T) sulfide 64.6 grams; copper(I1) sulfide 7.5 grams; nickel(I1) sulfide 9.3 grams; silica 5.1 grams; lime 2.0 grams; alumina 4.5 grams; and magnesia 2.0 grams. Xnety-five grams were taken for each assay.
the feasibility of producing metal buttons. Different forms of carbon-eg., flour-could be used, but graphite provided the most predictable weight of metal alloy. Flour as about 25% as effective as graphite. The amount of graphite required t o produce a 20-gram button was determined experimentally. T o avoid the formation of a sulfide matte, the ore or concentrite must be completely roasted t o metal oxides. The melting point of the extracting alloy may be controlled to some degree by an external addition of copper oxide. The detailed characteristics of each of the three base metals as collectors of each of the platinum metals must await further research. The new assay procedure significantly affects the reduction of slag losses of platinum and palladium. Nickel is a selective and persistent carrier of platinum and palladium and its presence in the slag from the classical fire assay increases the risk of some loss of platinum metals (8). With high proportions of nickel this loss may be irrecoverable by fire assay with lead as the collector (3, 4). Because nickel and copper are preferentially reduced, the new method has the advantage of a practically complete tranEference of the nickel t o the button. The slags resulting from the new fire assay on this natural ore concentrate revealed the complete removal of platinum and palladium. Fire assays made on synthetic ores salted with platinum and palladium and containing high proportions of nickel resulted in slags containing both nickel and pla t'inum metals. These slagb may be retreated to remove completely the nickcl and remaining traces of platinum and palladium. With synthetic ores containing a high proportion of copper but no nickel or iron, the slag losses were small. The significance of ore and button composition complicates the ascertaining of the optimum button weight. Accumulated data indicate the combined influence of the per cent nickel or copper recovery and the total button weight of the extraction of platinum and palladium from the natural concentrate (Table 11).
EXPERIMENTAL
The most significant improvement in the new procedure is the method of reduction, which permits control of the proportion of base metals reduced and, thus, the size of the button. Preliminary experiments with a roasted sulfide concentrate niixed in a clay crucible with carbon, sodiuni carbonate, and borax, and fused in an air-gas furnace indicated Table I. Determination of Platinum and Palladium b y Classical Fire Assay
(Ounce per ton) Chromatographic Spectrographic 1% PdPt Pd 0.0313 0.0195 0.0355 0.0195 0.0325 0.0198 0.0285 0.0190 0.0324 0.0192 0.0340 0.0185 0.0335 0.0180 0.0290 0.0195 0.0332 0 0195 0.0330 0.0185 0 0320 0.0200 .iv. 0,0326 0.0192 0 0320 0 0192
Table II.
Smiple, NO.
-4 I3 C 1) E I;
G
11
1142
Samples il through D are natural and E through H synthetic concentrates. Nearly 3 grams of nickel in the slag of sample A did not affect the recovery, by classical fire assay, of the microgram amounts of platinum and palladium but 21 grams of nickel in the slag of sample E prevented the quantitative collection of platinum and particularly of palladium. Sample F with a high iron content failed to yield the missing 1 mg. of palladium. This phenomenon may be due to various conditions whose effects are being investigated. Samples G and H contained high copper; H was subjected to a relatively more efficient roasting to remove sulfur, with the result that the button appeared free from adhering matte. The lower values for G were due to a small amount of matte from which significant amounts of platinum and palladium were recovered; the pertinent values recorded in Table I1 were obtained from a mixture of this matte and thc corrrsponding slag. PROCEDURE
Preparation of Alloy Button. One hundred grams (3.43 assay tons) of orc concentrate were roasted, with frequent stirring, in a 6-inch porcelain dish in t h e assay furnace at 980' C. for 2 hours. The cooled calcine, weighing 81.6 grams, was crushed to pass a 45mrsh screen, mived with 42.4 grams of soda ash, 27 grams of borax glass, 9.5 grams of powdered graphite, placed in a 30-gram fife assay crucible, and then put into the gas furnace at 1050' C. During 30 minutes the temperature was raised to 14%' C. The gas was turned off and the crucible removed from t h e furnace to cool. Crucibles must not be swirled because this causes the metal to become intermixed with the slag. Pouring the melt into an iron mold caused some metal to become intermixed in the slap. When cold, the crucibles were broken open. Only metal and slag phases were prcsent. The small amount of slag adhering t o the button was removed by tapping with a hammer. Dissolution of Button. The 20gram button was placed in a 600-ml. beaker u-ith 1.50 ml. of concentrated hydrochloric acid and treated as described ( 7 ) . The combined solutions were reduced in volume to 150 ml. and
Effect of Composition and Weight of Button on Extraction of Platinum and Palladium from Roasted Sulfide Concentrates
Carbon Added, Grams 5
7 9 11
10 10 10
10
Biitton Wt., Grams 6.3 11.0 17.8 28.5 31 10
21 21
ANALYTICAL CHEMISTRY
Present in Sample,
Grams
Cu 5.20 5.20 5.20 5.20 . . . .
52 52
Xi
6.14 6.14 6.14 6.14 52 , .
. .
..
Fe 40.7 40.7 40.7 40.7 ..
52 , .
..
Found in Button,
Grams Cu Xi Fe 2.93 3.22 0.07 4.45 5.32 1.10 4.85 5.84 7.00 5 . 0 2 5 . 8 i 17.3 .. 31 ,
, .
, ,
21 21
..
..
10 , . ..
entrate PJ.
,-
0.112 0.112 0.112 0.112 9.93 9.93 9.53 9.53
In Button I n Slag ___ Pt Pcl Pt Pd 0.066 0.050 0.030 0.061 0.035 O.OG6 0.110 0.065 Nil Nil 0.066 0.110 0.065 Nil Nil 0,066 0,110 O.OG5 Kil Nil
10.17 10.17 10.17 10.17
9.83 9.76 9.20 9.42
8.95 9.16 9.72 0 97
0.018 0.060
Si1 Nil 0.211 0.280 0.069 0 147
Slag Wt.,
Grams 150 148 128 114 72 158 95 100
concentrated nitric acid was added slowly until reaction ceased. The nitrates were then converted to chloride. The volume was reduced to about 150 ml., and the solution was tested for acidity ( 7 ) , diluted to about 300 ml., filtered through a 9-em. Whatman filter Ixqxr, and washed well with water. The small amount of siliceous residue contained nrither platinum nor palladium. Tlie filtrate was diluted to 1500 mi. and the pII adjusted to 1.5 with hydrochloric acid. The solution was passed through a column of Dowex 50X8 resin, treated to remove carbon, boron, and selenium, and then passed through the small Dowex 50-XS column as descrihcti (7'). The effluent from the small coliimn was transfrrred to a 10nil. volurnctric flask froin which a 4.00nil. aliquot was transferred t o a crucible prior to bciiig applied to the chromatographic paper (6). The colorimetric nwthods for the determination of platinum and palladium have been described (1, 7, 9). The results of the button andysis are given in Table 111.
Table 111.
Sample KO.
Analysis of Metal Button and Slag for Platinum and Palladium
Metal Button Button Ounce per Ton wt., grams Pt Pd
1Q 2a 3a 4a
5a 6a 7= 8C
9 1& lld
Av a
*
c
d
OF SLAG
Tlie classical fire assay with lead as a collector may result in losses of platinum :md palladium to the slag (3, 4). Experiments showed that an assay ton of ore containing 3 grains of nickel yielded on1y 88% of platinum and 847, of palladium. Although normally this method was unaffected by the presence of nickel in the ore, it was necessary to analyze many of the slags to determine the platinum and palladium losses directly. The slag from 3.43 assay tons of the above concentrate contained only 300 mg. of nickel. I3ecause this amount is below the quantity IThich affects the efficiency of platinum-palladium recovery by classical assay, and because this recovery is not significantly reduced by the presence of iron or copper, the slags from this method could be analyzed by lead collection. Each lot of slag was mixed with 200 grams of litharge, 4 grams of flour, and 10 mg. of silver powder, and fire asBayed. The lead buttons weighing about 55 grams were scorified and cupelled. The silver beads were analyzed as described (7') (Table 111). T o determine directly the ability of lead t o collect platinum and palladium from the slag, two slags were salted with palladium and platinum as follom: Two 30-gram crucibles were lined with 8 X 8 inch sheets of cellophane, weighing 2.3 grams. The slags were poured into the cellophane and 5 ml. of multicomponent precious metal solution were added. The crucibles were left overnight in a steam cabinet. The contents of each of the salted samples were crushed and fire assayed as described above (Table 111).
To evaluate further the efficiency of a copper, nickel, and iron alloy as a colLector for platinum and palladium, a n
0.0316 0.0319 0.0329 0.0326 0.0319 0.0300 0.0318 0.0322 0.0327 0.0346 0.0315 0 0326 0 0310 0 0331 0 0336 0 0305 0 0321
0.0184 0.0195 0.0201 0.0204 0.0185 0.0375 0.0192 0.0189 0.0205 0.0205 0.0186 0 0179 0 0192 0 0196 0 0191 0 0188 0 0192
119 124 12i 132 133.5 119 123 125 123 121 ...
0.0006 49.5by 0.0011 0.0005 43.gb5 0.0001 < O . 0006
