New Fire Assay Method for Rhodium in Ores and Concentrates G.
H. FAYE
and W.
R. INMAN
Mineral Sciences Division, Department o f Mines and Technical Surveys, Ottawa, Canada
b In the proposed fire assay method, rhodium is quantitatively collected in tin. Unlike gold, platinum, and palladium, some rhodium dissolves during the hydrochloric acid parting of tin assay buttons containing copper and nickel. However, the soluble rhodium is recovered easily from the partingacid solution b y precipitation with tin powder. The procedure used for the isolation and subsequent determination of rhodium is described. The method has been applied to the analysis of ores and concentrates, and the results agree favorably with those obtained b y an alternative method.
T
HE COLLECTIOK of precious metals in molten tin during the crucible fusion step of fire assaying is the basis of new methods recently reported for the determination of platinum and palladium ( 6 ) and gold ( 7 ) in ores, rocks, and concentrates. The present investigation was undertaken to determine the efficiency of tin as a collector for rhodium, and also to devise techniques for the isolation of rhodium from the tin assay button prior to its estimation by wet chemical methods. Allen and Beamish ( I ) have sholvn that the classical lead collection method for rhodium possesses certain inherent weaknesses. As an alternative, they developed a method for rhodium ( I S ) , as well as methods for the other metals in the platinum group, in which an iron-copper-nickel alloy is used as the collector (8, 11, 12, 14). This paper describes the successful use of the tin collection technique in the fire assay determination of rhodium in ores, concentrates, and copper-nickel matte. The rhodium values obtained by this method are compared with those obtained by an independent laboratory using a combination of the classical fire assay and a spectrographic finish.
EXPERIMENTAL
Apparatus and Reagents. T h e apparatus and flux used for crucible fusion have been described previously
(6,
7).
Tin powder (-200
972
0
mesh) was ob-
ANALYTICAL CHEMISTRY
t'ained from hnachemia Ltd., Montreal, Ca,nada. .AN105 EXCHAKGE COLUMNS. The anion exchange method used for isolating rhodium from the sample solutions and for subsequently separating any platinum and palladium present was that of Berman and XcBryde ( 3 ) . By experiment it was found that the batch of Amberlite IRA-400 resin (20 to 50 mesh) taken for this work had to be crushed to 100 to 150 mesh to obtain satisfactory separation of platinum and palladium on 0.3 X 2.5 c m . columns. It is suggested that,, for each batch of resin used, experiments be conducted t'o determine t,he optimum resin part'icle size and column dimensions for the separations being considered. STANDARD RHODIUM SOLUTIOS. This was prepared by dissolving Johnson and Natt'hey Specpure ammonium chlororhodite in 100 ml. of LV hydrochloric acid. The rhodium cont'ent of this solut'ion was determined gravimetrically xt-it'h thiobarbituric acid ( 6 ) and was 1.10 mg. of rhodium per milliliter. More dilute solutions were prepared from the stock solution, by appropriate dilutions \\-ith L?' hydrochloric acid. Procedure.
PREPARATIOK OF TIS
~ S S A YBUTTOSS.
K i t h the exception of certain tests involving leaching of the copper-nickel matt'e, all samples of ore and matte analyzed by the proposed method rvere roasted a t 750' to 800' C. for approximately one hour Iiefore being mixed with the flux and taken through the crucible fusion process which has been described previously ( 7 ) . AK.~LYSIS OF ASSAY BCTTONS. Each assay button was granulated and treated with concentrated hydrochloric acid as described earlier (6, 7 ) . After the alloy had been decomposed, the contents of the beaker n-ere diluted with an equal volume of distilled water. The suspension was then st'irred wit'h a motor - driven, polyethylene - coated stirrer, and approximately 7 grams of powdered tin was added to precipitate copper and any rhodium present in solution. When it was evident that the amount of copper precipitated was less than 0.3 to 0.4 gram, then approximat'ely 0.5 gram of copper (as the chloride) was added to ensure that sufficient copper was present' t o act as a n efficient carrier for bhe rhodium. After the suspension had been stirred for 20 minutes, the supernatant solution was decanted through a pad of paper pulp (from Whatman Yo. 31 paper) sup-
ported on a filter disk. The solids tvere washed several times by decantat'ion with 2 5 hydrochloric acid, and the washings were passed through t'he filt'er pad. hpproximat'elj- 50 nil. of 1 2 S hydrochloric acid was added to the solids in the beaker, and then 30Tc hydrogen peroxide was added cautiously in small portions unt'il the solids appeared t o be dissolved and excess peroxide was present. After the beaker h i d been heated for a fen. minutes t o ensure complete dissolution of the residue, i t was placed under the filter pad and the pad was washed wit,h 20 nil. of a 3 to 1 mixture of 8.Y hydrochloric arid and 30% hydrogen peroxide to dissolve any fines deposited during rlecnntation. The solution !vas evaporated until salts began to cryst'allize; then, with the aid of an infrared heat lamp, tin was volatilized from the sample nit11 mixed hydrochloric and hydrolironiic. acids (6). The salts reniaiiiing after the t,in volatilization step were c>autiously treated ivit'h several 5- to 10-nil. port,ions of a 3 to 1 mixture of 3 2 s hydrochloric acid and 3 0 7 , hydrogcn peroxide (bromine may be e v o h d 1-igorously when large quantities of h bromides are presriit 1, and the solution was then evaporated to inci1)icSnt dry-
ness.
\Then necessary, gold can bc removed from the sample at this etnge by the procedure of Fsye and Inman ( 7 ) . K h e n base metals Ivere present: t'he salts were taken up in either 0.25 or 0.50 ml. of 12K hydrochloric acid and diluted tto either 50 or 100 nil. n-ith water (depending on the amount' of base metals present), to give a solution with a pH of approximately 1.5. Iron, copper, and nickel were then renio1-ed by passing the solution through cation exchange columns (Dowes 50 \T7-X8) of appropriate size (4, 10). The effluent solution was evaporated just to dryness in t,he presence of a few milligrams of sodium chloride, and bhe salts were taken up in 1.5 ml. of 12N hydrochloric acid and diluted t o 25 nil. ivith water. Rhodium was then separated from the other platinum metals by anion exchange chromatography according to the method of Berman and McBryde (S), and determined spectrophotometrically with stannous bromide ( 2 ) . I n several test's involving milligram quantit,ies, rhodium was determined gravimetrically with thiobarhituric acid (6)*
RESULTS
Recovery of Rhodium from Salted Charges. To determine t h e efficiency of tin as a collector for rhodium during the crucible fusion process, 115-gram portions of t h e flux were each salted IT i t h known quantities of rhodium i n the manner described previously (6). These charges were fused to produce tin buttons weighing approximately 20 grams each. The buttons were treated by the procedure given above, except that, after parting the granulated alloys in hydrochloric acid and decanting off the parting solutions, only the parting residues were analyzed for rhodium. The results of these experiments are given in Table I.
Table I.
Recovery of Rhodium from Salted Charges
Test Suniber
____Rhodium
Taken ~-
1 2 3
11 110 '"'0 --
Recovered
P!z.
11 10i 210
irg. __-__ 11 .O"
ll.B 15.2 Iktermined graviiiietricaily with thio-
1 5
151"
1)arhituric acid.
The results given in Table I show that tin is an efficient collector for rhodium during the crucible fusion process; also, that the intermetallic compounds containing rhodium are essentially insoluble during t h e hydrochloric acid parting of the pure tin buttons. The behavior of rhodium in this respect is similar to that of p1:itinuni and palladium ( G ) , and gold ( 7 ) . Slag Loss The slags from teqts 4 and 5 , and the slag from a n unreported test n i t h 14 mg. of rhodium, were each crushed, mixed with 30 grams of Sn02 and 5 granis of pondered coke, and fused to produce tin buttons neighing approximately 20 grams each. These buttons 17 ere analyzed for rhodium by the procedure given :above and contained 0.2, 0.05 and 0.0S7c, respectirely, of the rhodium originally taken. Recovery of Rhodium from Synthetic Samples Containing Copper and Nickel. Because rhodium (and palladium) is encountered most frequently in ores and rocks containing iulfides of iron, copper, and nickel, it v a s desirable to determine t h e behavior of rhodium during the crucible fusion and button-parting operations n hen appreciable quantities of these base metals were present. I n previous work ( i ) ,except in the case of copper-nickrl matte, a preliminary acid 1eac.h to remove the bulk
of the base metals from the sample was less convenient than the direct fusion of the roasted sample material. Thus, after a n oxidizing roast the sample to be fused consists primarily of a mixture of copper, nickel, and iron oxides and various silicates; the proportion of each constituent in the mixture can vary over wide limits. During the fusion process, practically all the copper and nickel in the sample form an alloy with tin, while most of the iron remains in the silicate slag. Therefore, in preparing each of the synthetic samples, various quantities of copper and nickel oxides were mixed n i t h the flux, and the mixture was salted with known quantities of rhodium. b s before, these samples n-ere fused to produce assay buttons n-hich TI ere then analyzed for rhodium. I n certain preliminary experiments the buttons were parted in hgdrochloric acid and the parting-acid solutions n-ere rejected as tests 1 to 5 indicated that rhodium should remain insoluble during the parting operation, However, when the parting residues were analyzed, Ion rhodium values were obtained. I n subsequent tests in which the parting-acid solutions 17-ere analyzed. 10 t o 35Yc of the rhodium originally present had been dissolved during the parting operation and ivhen the individual rhodium values were added to those from the corresponding parting residues, a material balance was obtained. Platinum and palladium (6) and gold (7') remain quantitatively in the parting residue when conditions are siniilar to those used in the above euperinients. T o overcome the problem of losing rhodium during parting. the entire assay button should be dlssolred, the tin distilled from the sample, the base metals removed by cation euchange, and the rhodium isolated by anion exchange. Accordingly, a serier of assay buttons JTas prepared from synthetic samples containing various known quantities of copper, nickel, and rhodium. Each of the granulated buttons was first treated with hydrochloric acid, and the insoluble residue was then separated from the solution b y decantation. The residue was dissolved as usual in a mixture of hydrochloric acid and hydrogen peroxide. The cooled button solution was oxidized by the cautious addition of hydrogen peroxide, then combined with the solution obtained from the residue. The combined solutions were analyzed for rhodium. The results obtained in these tests are given in Table 11, -1. Recovery of Rhodium from PartingAcid Solutions with Tin Powder. Because of t h e comparatively large quantities of tin (20 t o 35 grams) present in the sample solutions used in the above tests, considerable time
Table II. Recovery of Rhodium from Synthetic Samples Containing Copper and Nickel
Base Metals Taken Copper Sickel
~~
Rho diu in Taken Found
A-ANALISIS OF ESTIREBCTTOS SOLCTIONS
Grams
P!z. -__
1.0 1.0 1.0
1.0 1.0 1.0
11 33 66
11 32
1.0 1. 0 1. 0
1 .o 1. 0 1.0
1.10 1.10 1.10
1.12 1.10 1 13
1. 0 1. 0 I .0 0.5 0.5
1 0 1.0 1.0 1. 0 1.0
11 22 33 110 110
14 21 34 108 108
68
was required t o volatilize this metal. Also, in certain of these t'ests some difficult,y was encountered when a slimy precipitate containing tin and base metals separated out during the preparation of the solutions for removal of the base metals by cation exchange. Because of these problems i t was decided to use a metallic reducing agent to recover rhodium from the parting acid. The resultant rliodiurn-bearing precipitate could later be combined Jrith the parting residw. If this approach were surcessful, essentially all the nickel and most of the tin in the original button noultl be eliniinatecl. leaving only the precipitated copper. the excess reducing agent. and vsrioiis other easily reduced minor c,oiietituents to be handled along with the parting residue. Po\\-clered tin v a s c.1iosc.n as the escess could later be removed by volatilization. I n acidition, tin n-ould not react vigorously with the diluted parting acid and thus ~voulcl be dispersed in a uniform suspensio:n to give conditions suitable for efficsient reduction. By experiments irith a ,series of synthetic parting-acid solutions containing rhodium, tin, coplm, and nickel, the rhodium n-as quantitatively precipitated along with copper n-hen the solutions m-ere stirred for 10 to 20 minutes with excess pondered tin. As 1 gram of tin theoretically reduces approximately 1 gram of copper, 7 grams of powdered tin should provide an adequate excess, even when the VOL. 34;
NO. a, J U L Y 1962
973
Table 111.
Test Sumber
Determination of Rhodium in Ores and Concentrates
Sature of Sample Copper-nickel matte Copper-nickel matte Copper-nickel matte Copper-nickel matte Copper-nickel matte Copper-nickel matte Copper nickel matte Copper-nickel matte Copper-nickel matte Copper-nickel matte
Sample J5-t.
Proposed Independent Method Laboratory Troy oz./ton 0 052 0 0.51 0 053 0 050 0 058 0 047 0 050 0 053 0 051 0 053 0 055 ( 0 007 0 007
assay ton '/s assay ton 3a I/s assay ton 4' I/* assay ton 5. assay ton 6* 4 grams ib 6 grams 8b 8 grams 9b 6 grams lob 6 grams Average 11 Flotation concentrate 1 assay ton 12 Flotation concentrate 1 assay ton 13 Flotation concentrate 1 assay ton 0 008 14 Flotation concentrate 1 assay ton 0 009 1.5 Flotation concentrate 1 assay ton 0.008 Average 0 008 0 008 ( 9 ) C 16 Sickel ore 1 assay ton 0 030 17 Sickel ore 1 assay ton 0 031 18 Sickel ore 1 assay ton 0 031 19 Xichcl ore 0 031 1 assay ton 20 Sickel ore 1 assay ton 0 030 21 Sickel ore 0 033 1 assay ton Average 0 031 0 035 (6," In tests 1 to 5, the samples were leached in 12-Y hydrochloric acid before fusion. In tests 6 to 10, the entire button solution was analyzed; in all other tests the partingacid solution m s treated with tin powder t o recover rhodium. Figures in parentheses indicate number of replicate determinations. la
2a
m
-
-
copper-nickel matte may be given a preliminary acid leach to remove the bulk of the base metals without losing rhodium to the leach liquor. Platinum and palladium (6) and gold ( 7 ) are also insoluble during the leaching of the copper-nickel matte. DISCUSSION
At the present time, a satisfactory explanation cannot be given for the difference between the solubility of rhodium and that of platinum and palladium (6) and gold ( 7 ) during the parting of assay buttons containing appreciable quantities of copper and nickel. I n a few experiments n i t h synthetic samples containing copper and nickel and with copper-nickel matte, little or no rhodium dissolved during the buttonparting operation, even though the experimental conditions were similar to those described in this paper. Attempts to find conditions under which the behavior of rhodium n-ould be predictable during the parting operation have, thus far, been unsuccessful. ACKNOWLEDGMENT
sample solutions contain as much as 4 grams of copper. T o determine the efficiency of this treatment, a series of assay buttons containing k n o m quantities of copper, nickel, and rhodium was prepared. Each button was parted, the suspension of intermetallics in the parting acid was treated with evcess tin powder, and the resultant precipitate, together with the parting residue, was analyzed for rhodium according to the experimental procedure given above. The results of these experiments are given in Table 11,
B. The results in Table I1 show that both microgram and milligram quantities of rhodium can be recovered quantitatively from synthetic samples containing as niuch as 3.5 grams of combined copper and nickel. Satisfactory results can be obtained, either by analyzing the solution obtained by the direct decoinposition of the rrhole assay button. or hy recovering the soluble rhodium from the pnrting acid solution with pon-dered tin and thus eliminating the nickel nrid most of the tin from the sample solution. The latter approach is the more rapid and convenient, especially rvhen comparatively large quantities of nickel are prese n t in the sample. Application to O r e s and CopperNickel Matte. To determine its practicability, the proposed method was applied to the determination of rhodium in a sample of copper-nickel matte, in a flotation concentrate, and
974
ANALYTICAL CHEMISTRY
in a specimen of massive sulfide nickel ore. These materials were also analyzed for rhodium a t the metallurgical laboratories of Falconbridge Kickel Mines Limited, Richvale, Ontario, by a method in which fire assay beads obtained by conventional techniques n'ere analyzed spectrographically (9). The origin and compositions of the copper-nickel matte and the flotation concentrate have been described previously (6, 7 ) . The sample of nickel ore was obtained from the Geological Survey of Canada. On analysis a t the hlines Branch, i t contained 5.470 nickel, 0.3% copper, 36.3% iron, 32.0% sulfur, 0.19 troy 02. per ton of palladium, and 0.022 troy oz. per ton of platinum. Table I11 gives both the rhodium values obtained b y the proposed method and those obtained by an alternative method on the materials mentioned above. Analytical Results. Table 111 shows t h a t the results obtained by the proposed method agree favorably n i t h those obtained by a combination of the classical fire assay and a spectrographic finish. The results for the copper-nickel matte indicate that concordant rhodium values are obtained either by treating the entire button solution, or by recovering soluble rhodium from the parting acid n ith tin powder and thus eliminating nickel and a large proportion of tin from the sample. Table I11 also shon-s that for convenience the
The authors acknowledge the assistance of P. E. 11oloughney in performing much of the evperimental work. They are also grateful to Falconbridge Nickel Mines Ltd. for supplying samples of flotation concentrate and copper-nickel matte, and also for the analyses used for comparison in this paDer. LITERATURE CITED
(1) Allen, W.F., Beamish, F. E., ASIL. CHEV.22, 451 (1950). ( 2 ) Berman, S. S., Ironside, R., Can. J . Chem. 36, 1151 (1958).
(3) Berman, P. S., McBryde, JT, A, E., Ibzd., 36, 835 (1958). (4) Coburn, H. G., Beamish, F. E., Lenis, C. L , - 1 s ~CHEM. ~ . 28. 1297 (1956). (5) Currah, J. E., McBryde, W. Ai.E., Cruikshank, A. J., Beamish, F. E., Ibid., 18, 120 (1946). (6) Faye. G . H.:Innian, U-.It.: Ibiti., 33, 278 (1961j. (7) I b i d . , p. 1914. (8) Iinvanaugh, J. AI., Beainish, F. E., I b i d . , 32,490 (1060). (9) Lewis. C. L.. Can. Mininu X e t . Bull.. ' March 19.57. ' (10) Marks, X. G., Beaniieh, F. E., A s'bL. CHEV.30, 1464 (1938). (11) Plummer, 11.E. V.,Beamish, F. E., Ibid., 31, 1141 (1959). (12) Plumnier, 11. E. I-., Leir-is, C. L., Beamish, F. E., Ibid., p. 254. (13) Sant, B. It.,Beamish, F. E., I b i d . , 33, 304 (1961). (14) Tertipis, G. G., Beamish, F. E., Ibid., 34, 108 (1962). RECEIVED for review April 2, 1962. Accepted May 10, 1962. Published by permission of the Director, Mines Branch, Department of Mines and Technical Surveys, Ottawa, Canada.
