Nitric Acid Parting of Silver Assay Beads Containing the Platinum Metals and Gold J. SEATH
AND
F. E. BEARZISH, University of Toronto, Toronto, Ontario, Canada
THIS
precipitate, which would consist of the dioxides of palladium, rhodium, etc., as well as compounds of base metals such as lead and iron, was filtered out and then dissolved in 4 cc. of hot hydrochloric acid (1 to 1). To remove traces of platinum this solution was diluted, adjusted in acidity to pH 4,boiled with sodium bromate, and adjusted to pH 6. The precipitate was filtered out and, if necessary, subsequently treated for palladium. The filtrates from the two dioxide precipitations were combined and taken almost to dryness on the steam bath, excess bromate being destroyed by additions of concentrated hydrochloric acid. The residue was dissolved in about 150 cc. of water and the acidity adjusted to about pH 8. Boiling a t this pH caused more complete precipitat’ionof any lead and iron present which would contaminate the platinum. The solution was then filtered, adjusted to pH 6, and boiled for an hour with sodium formate t o precipitate the platinum. The metal was filtered through a Whatman’s No. 42, 7-cm. filter paper, washed with 0.01 N hydrochloric acid to dissolve base metal precipitates, and finally with 1 per cent ammonium chloride solution. After burning in a 5cc. porcelain crucible, the residue was cooled and weighed as platinum. Palladium, when present in t,he parting acid, was precipitated at pH 6. After reprecipitation to remove platinum contamination, the dioxide n’as dissolved in 4 cc. of hot hydrochloric acid (1 to l ) ,and diluted to about 100 cc. n’ith water, and palladium was precipitated with dimethylglyoxime. The precipitate was filtered through a Whatman’s No. 42, 11-cm. filter paper, washed, and burned wet in a 15-cc. porcelain crucible. The residue, after reduction in hydrogen and cooling in carbon dioxide, was weighed as palladium.
investigation was made to provide facts concerning the influence of the platinum metals and gold on certain phases of the nitric acid parting of the silver-precious metals beads. The authors have consistently stressed the fact that the assay bead is usually a polycomponent system and very few data are available from which one can even roughly predict the amount of precious metal dissolved by the acid. Davis (1) reports a method for the separation of the precious metals in silver assay beads by the use of nitric acid, but gives no information regarding the effect of the presence of gold and the other platinum metals on the proportions of platinum and palladium dissolved by the parting acid. In investigating this phase of the parting, the authors have used the nitric acid concentrations and the silver ratio recommended by Davis.
Preparation and Analysis of the Beads The samples of spectrographically pure precious metal sponges and silver foil were wrapped in 25 grams of lead assay foil, compressed in a mold under a pressure of abouj 100 kg. per sq. cm , and cupeled at a cupel temperature of 900 * 25’ C. To assist In the removal of lead, the beads were left in the muffle for 2 minutes after “the blick.” Owing to losses, the ratio of silver to the precious metals was usually betaeen 14 t o 1 and 15 to 1 after cupelation. TABLEI.
PLATINUV AND PALLADIUV I S
Sam- Silver ple Added
SOLCTION Plat]num Added
Mu.
Me.
675.0 255.0 255.0 255.0 255.0 255.0 450.0 675.0 450,O 255.0 225.0 675.0 450.0 450.0 450.0 275.0
15.0 15.0 15.0 15.0 15.0 15.0 15.0 15.0 15.0 15.0 15.0 15.0 l5,O 15.0 15.0 15.0
FIRSTNITRIC i l C I D
Other Precious Silver Loss of Metal ReAdded covered Silver Mu. Mg. Mg. 3 0 . 0 Au 633.9 41.1 12.3 2 . 0 R u 242.7 2 . 0 R h 245.4 9.6 246,s 2 . 0 Ir 8.2 240.6 14.4 2.0 Pd 241.8 13.2 13.1 15: O’Pd 436.9 656.0 19.0 30.0 Pd 12.8 l 5 , O *ill 437.2 241.5 13.5 2 . 0 Au 213.0 12.0 23.8 30: O’Au 651.2 425,5 24.5 1 5 . 0 Ir 16.4 1 5 . 0 R h 433.6 20.0 1 5 . 0 R u 430.0 263.1 1 5 . 0 Au 11.9
The results obtained on parting a series of silver assay beads according to the above directions are reported in Table I. The residues were inquartated, cupeled, and parted until the amount of platinum and palladium extracted by the parting acid was very small. Results for the second and third partings are recorded in Tables I1 and 111.
Precious Metals in Parting Acid
MQ 13.9 Pt 6.3 Pt 7.9 Pt 8.6 Pt 9.5 Pt, 1.5 Pd 7.0 Pt 10.7 P t , 1 3 . 9 P d 7 . 1 P t , 28 6 P d 12.2 Pt 9.4 Pt 6.8Pt 12.2 P t 5.6 Pt 3.8 Pt 5.1 P t 12.8P t
TABLE11. PLATISUM Precious Metals Left Ram- after First de Parting
-If Q .
1
2 3
!
The beads were cleaned with dilute acetic acid and parted, first with 25 cc. of nitric acid (1 t o 4), then with 25 cc. of nitric acid (1to l ) ,and finally Il-ith 25 cc. of nitric acid (2 to 1). The three combined parting acids were evaporated on a steam bath to a volume of about 10 cc. to remove most of the nitric acid, diluted with water to 100 cc., and filtered, and the silver was precipitated with a few drops of hydrochloric acid. After standing overnight the coagulated silver chloride was filtered through a Whatman’s No. 42, 7-cm. filter paper. The precipitate and filter paper were heated with 20 cc. of concentrated sulfuric acid to which three 10-cc. portions of concentrated nitric acid were added. This treatment destroyed organic matter and dissolved the silver chloride. When the solution was fumed almost t o dryness, the residual silver sulfate was dissolved in 100 cc. of hot water, and the silver chloride was reprecipitated. The chloride mas then filtered through a 15-cc. S o . A1 filtering crucible, dried at 140 C., and weighed. The filtrates from the two silver precipitations mere combined and taken almost to dryness on the steam bath. The moist residue was evaporated three times with concentrated hydrochloric acid and then dissolved in 50 cc. of n-ater. The acidity of this solution was adjusted to pH 4 by means of a filtered 10 per cent sodium bicarbonate solution using bromophenol blue as the indicator. TKOcubic centimeters of a filtered 10 per cent sodium bromate solution were added and, aft’er boiling for 15 minutes to oxidize the platinum, the solution was adjusted to an acidity of pH 6, using bromocresol purple as indicator. Any
6 7 8 9 10 11 12 16
31.1 10.7 9.1 8.4 6.0 8.0 5.4 9.3 17.8 7.6 8.2 32.8 17.2
T4BLE 111.
Silver hdded
8
639
SOLUTIOX Silver Recovered
Loss of Silrer
Mg.
Mg.
MQ.
455.0 170.0 140.0 140.0 95.0 120.0 95.0 155.0 270.0 130.0 125.0 495.0 85.0
431.5 164.4 129.1 132.6 88.4 107,i 91.8 137.5 258.0 123.8 118.7 483.9 82 2
3.5 5.6 10.9 7.4 6.6
12.3
Silver Loss of Recovered Silver
IT
Precious Metals in Parting Acid 1 0 Pt 5 4 Pt 4 3 Pt 4 0 Pt 3 2 Pt, 0 5 Pd 4 5 Pt 3 6 Pt, 1 0 Pd 4 7 Pt, 1 2 Pd 2 6 Pt 3 7 Pt 5 6 Pt 2 8Pt 2 1Pt
THIRDNITRIC ACID
Precious Metals in Parting ricid
Total P t and P d Recovered after 3 Partings
3 4 Pt 0 1 Pt
15.1Pt 12.3 P t 12.6 P t 14.9 Pt 2.0Pd 14.9 Pt 15.0 Pt
5.3 4.8 4.4 2.3 3.5 0.8
85.0 72.0 66.0 71.0 55.0 12.0
82.8 65.7 65.0 70.0 50.8 9.6
1.0 1.0 4.2 2.4
2 2 Pt 0 0 Pd 3 4 Pt 0 7 Pt
3.4 3.9 2.6
80.0 25.0 40.0
76.6 24.2 38.2
3.4 0.8 1.8
0 0 Pd 1 OPt 2 0 Pt
2.2 6.3
ACID
Mg.
3.2 17.5 12.0 6.2 6.3 11.1 2 8
P.iLLIDIU\I SOLUTION
P L ~ T I N U M4 S D
Precious .\I et ais Left after Sam- Second Silver Parting hdded ple
.O .1
SECoSD NITRIC
AND P.4LLADIT;M IN
Sll
{
29.8 Pd 1 4 . 1 Pt 1 4 . 4 Pt
INDUSTRIAL A S D ENGINEERING CHELMISTRY
640
Any final residues after the last parting were treated with aqua regia, and the platinum and palladium, if any, were determined as above.
Extraction of Platinum from Binary and Ternary Systems SILVER-PLATINUM. The results reported in the tables for samples 6 and 11, each containing 15.0 mg. of platinum, indicate that a t least three successive partings with nitric acid are necessary to dissolve most of the platinum. The weights ofiplatinum recovered from the aqua regia solutions of the residues from the third partings were nil and 0.4 mg., respectively; hence, the total recovery for sample 6 was 14.9 mg. and for sample 11, 14.8 mg. SILVER-PLATINUM-PALLADIUM. Sample 5, containing 15.0 mg. of platinum and 2.0 mg. of palladium, required two nitric acid partings to dissolve all the palladium. A total of 14.9 mg. of platinum was recovered after three partings and the aqua regia solution of the residue yielded no platinum. Bead 7, containing 15.0 mg. each of platinum and palladium, required two nitric acid partings to dissolve 14.9 mg. of the palladium and three partings to dissolve 15.0 mg. of platinum. Sample 8, containing 15.0 mg. of platinum and 30.0 mg. of palladium, yielded 29.8 mg. of palladium after two partings and 13.6 mg. of platinum after three partings. The residue of sample 8 was dissolved in aqua regia and yielded 1.5 mg. of platinum, a total platinum recovery of 15.1 mg. IN FIRST XITRICACIDSOLUTION TABLE IV. PALLADIUM
Samde 1
2 3 4
5 6
Palladium Added Mo. l5,O l5,O 15.0 15.0 15.0 15.0
Other Precious Metal Added
Mo. 15.0Rh 15.0 Ir l5,ORu 1 5 . 0 Au 3 0 . 0 Au
...
Silver Added Mg. 460.0 460.0 460.0 280.0 325.0 230.0
Silver Recovered
'MQ. 448.2 446.1 445.9 270.4 315.4 219.8
Loss of Palladium Silver Recovered Mg. Mg. 11.8 12.8 13.5 12.6 14.1 13.5 5.6 15.0 9.6 15.1 10.2 15.0
SILVER-PLATINCM-GOLD. The presence of gold in the assay bead seems to assist the dissolving of platinum rather than hinder it. This has been discussed by Smith ( 2 ) . Bead 1, containing 30.0 mg. of gold and 15.0 mg. of platinum, required only two nitric acid partings to dissolve 14.9 mg. of platinum. Sample 9, containing 15.0 mg. each of platinum and gold, required two partings to dissolve 14.8 mg. of platinum. With sample 10, containing 15.0 mg. of platinum and 2.0 mg. of gold, three partings yielded 14.1 mg. of platinum and subsequent aqua regia treatment 1.0 mg. more. When the silver ratio was reduced as in the case of bead 16, containing 15.0 mg. each of platinum and gold, two partings dissolved 14.9 mg. of platinum. S I L v E R - P L A T I N u h ~ - I R I D I ~ ~ The f. presence of iridium in the assay bead seems to lessen the dissolving of platinum in the parting acid. Sample 4, containing 15.0 mg. of platinum and 2.0 mg. of iridium, yielded 12.6 mg. of platinum after three partings. The aqua regia solution of the final residue yielded 2.7 mg. more, making a. total recovery of 15.3 mg. of platinum. Bead 13, containing 15.0 mg. each of platinum and iridium, produced 5.6 mg. of platinum from the first parting acid. SILVER-PLATINUM-RHODIU~I. Rhodium aibo seems to retain platinum in the parting of assay beads. Sample 3, containing 15.0 mg. of platinum and 2.0 mg. of rhodium, yielded a total of 12.3 mg. of platinum after three partings and a further 2.5 mg. by the aqua regia treatment, making a total recovery of 14.8 mg. From the first parting acid of bead 14, containing 15.0 mg. of both platinum and rhodium, 3.8 mg. of platinum were recovered.
S'OL. 10, so. 11
SILVER-PLATINUM-RGTHEKIUM. Ruthenium also seems to lessen the dissolving of platinum in the parting acid. Sample 2, containing 15.0 mg. of platinum and 2.0 mg. of ruthenium, yielded 15.1 mg. of platinum after three partings. Bead 15, which contained 15.0mg. of both platinum and ruthenium, produced 5.1 mg. of platinum from the first parting acid. Extraction of Palladium from Binary and Ternary Systems SILVER-PALLADIUM. The result for sample 6 (Table IV) indicates that one nitric acid parting is sufficient to dissolve completely the palladium in a bead containing the reported amounts of silver and palladium. SILVER-PALLADIUM-PLATINUM. The effect of the presence of platinum in the assay bead on palladium extraction has been discussed in the preceding section. SILVER-PALLADIUM-GOLD. The results for samples 4 and 5 (Table IV) indicate that gold does not prevent the dissolving of palladium in the parting acid. A single parting was sufficient in each case. SILVER-PALLADIUM-IRIDIUM. Iridium causes retention of palladium in the residue from the first parting. Bead 2 (Table ITr),containing 15.0 mg. each of palladium and iridium, required two partings to recover 14.9 mg. of palladium. SILI-ER-P.~LLADIUM-RHODIU~~. Rhodium, like iridium, lessens the dissolving of palladium in nitric acid. Bead 1 (Table IV), containing 15.0 mg. of both palladium and rhodium, yielded 12.8 mg. of palladium from the first parting acid. The second parting acid contained 259.0 mg. of silver and 2.1 mg. of palladium, making a total palladium recovery of 14.9 mg. SILVER-PALLADIUM-RUTHEXIL-X. Ruthenium also causes the retention of palladium. The first parting acid of bead 3 (Table IV) produced 13.5 mg. of palladium. The second parting acid yielded 250.9 mg. of silver and 1.6 mg. of palladium; the total palladium recovery was 15.1 mg.
Extraction of Platinum and Palladium from Polycomponent Systems An assay bead was prepared from 15.0 mg. of gold, 15.0 mg. of platinum, 15.0 mg. of palladium, 1.0 mg. each of iridium; rhodium, and ruthenium, and 550.0 mg. of silver. The silver recovered after cupelation weighed 532.5 mg. The platinum recovered from the first parting acid weighed 8.6 mg. and the palladium 14.8 mg. A second assay bead was prepared from 30.0 mg. of gold, 15.0 mg. of platinum, 5.0 mg. of palladium, 0.5 mg. each of iridium, rhodium, and ruthenium, and 430.0 mg. of silver. The first parting acid contained 424.4 mg. of silver, 7.6 mg. of platinum, and 5.0 mg. of palladium. These beads roughly represent proportions of the platinum metals and gold often found in platinum ores.
Observations The color of the nitric acid extract is no indication of the proportion of platinum present. Solutions containing very appreciable amounts of platinum were water-white. Under the conditions described a considerable amount of platinum is often precipitated a t p H 6 despite sodium bromate treatment; a reprecipitation is therefore necessary. Base metals such as iron, which may be introduced through the reagents, and residual lead from the assay bead precipitate a t p H 6 and contaminate the platinum. Preliminary adjustment to about pH 8 favors the complete precipitation of the base metals. The solution, after filtering, is adjusted t o p H 6 to separate the palladium.
UOT’EMBER 15, 1938
ANhLYTIC-kL EDITION
Summary When a bead with a silver-platinum ratio of about 15 to 1 is parted with nitric acid, e\*en three successive treatments will not alwavs dissolve all the nlatinum. Bead.. containing only sih-er and palladium with the above silver ratio require only one parting with nitric acid to dissolve the palladium completely. The presence of gold in the assay bead seems to assist the dissolving of platinum and palladium in the nitric acid
6-21
Iridium, rhodium, and ruthenium definitelv interfered with the dissolving of platinum and palladium in the parting acid. in the bead decreased the action of The presence of the first parting acid on palladium.
Literature Cited D ~c J ~v, ~ B ~ L t r .Mines , Paper 270 (1921) (2) Smith, E. .I.,“The Sampling and Assay of the Precious Metals,” Charles Griffin and Co , Ltd , 1913. RECEIVED July i , 1938.
A Rapid Method for Gold in Cyanide Plating Solutions JOSEPH B. KUSHNER, 301 Echo Place, S e w York. N. Y.
I
S DETERlIISII1-G the gold content of cyanide elec-
troplating solutions for control or evaluation purposes, most of the commonly known methods are too long drawn out or involved to be of routine use to the practical analytical chemist who must make many such determinations each day. The evaporation method ( 5 ) , the copper sulfate method ( d ) , the zinc-lead acetate method ( I ) , and the hydrochloric-ferrous sulfate method (,2) n-ere found by the author to be unsuitable because of the excessive amount of time or the tedious operational procedure required. A method suggested by IF7eisberg (7) proved to be sufficiently speedy (an analysis can be made in 2 hours) and simple but lacked accuracy, as some of the gold precipitated in colloidal form and could not be retained on a filter paper or matting. However, further experimentation resulted in the improved method described in this paper.
Determination of Gold WEISBERGPROCEDURE.After thorough stirring, permitting any sediment to settle, pipet a 50-ml. sample into a 500-ml. Erlenmeyer flask. Place the flask under a good draft hood and cautiously add concentrated sulfuric acid until vigorous action ceases. Add 50 ml. more of acid and place the flask over heat, boiling for about 45 minutes until all the gold has precipitated in brown sponge form. Boil 5 to 10 minutes longer to coagulate the gold. Cool the flask and filter into a tared Gooch crucible, carefully washing all particles of gold onto the filter. Wash with hot dilute sulfuric and then with hot water until the washings are no longer acid. Dry and ignite in the Gooch until the brown sponge turns golden yellow in color. Cool to room temperature and weigh. Invariably a small but definite amount of gold precipitated in such a finely divided state as t o pass through the asbestos filter mat of the Gooch, even after continued matting with gold. This was evidenced by the faint bluish brown color of the filtrate and the presence of the Tyndall effect on examination of the filtrate in a beam of light’. T h a t the gold came down in part in a colloidal state could only have been due to the weak ionization of the concentrated sulfuric acid. This was proved by the fact that, if the sulfuric acid containing the gold in finely divided form was diluted with a large volume of water and further boiled, most of this colloidal gold precipitated. However, this change in method was hardly feasible because of the danger of explosion and spattering, the additional time required, and the fact that the colloidal gold was not completely precipitated. The author found that whenever silver was present in the electroplating solution (green gold), the gold almost always
came down perfectly and the supernatant sulfuric acid was crystal clear and showed no Tyndall effect. It was decided that this perfect precipitation was brought about by a mutual suspensoid precipitation-i. e., the type that occurs when a suspensoid solution of ferric hydroxide is mixed with a suspensoid solution of arsenic trisulfide-the assumption being that the particles of silver formed in the early part of the process neutralize the charges on the finely divided gold particles and precipitate with them in a coagulated mass from which the silver dissolves on further boiling with sulfuric acid, leaving t’he gold in sponge form. If so, then adding a measured amount of a soluble silver salt to a sample containing only gold, prior to the addition of the sulfuric acid, would bring about the same results. This was found to be the case. Several experiments showed that a wide latitude in the amount of silver salt was possible without ill effects, but in general the optimum amount of silver nitrate solution that could be added mas just sufficient t o combine with the free cyanide present in the electrogilding solution. With this in view, the method of Weisberg was revised as follows: If the solution to be tested is high in REVISEDPROCEDURE. gold content (0.5 to 20 grams per liter) take a 10-ml. sample; if low in gold content (0.5 gram or less per liter), take a proportionally greater sample. Transfer sample t o a 500-ml. Erlenmeyer flask, dilute with 50 ml. of pure water, and add sufficient 0.1 IV silver nitrate from a buret to combine completely with the free cyanide, using 5 ml. of a 2 per cent solution of potassium iodide as indicator (this is the equivalent of Liebig’s method for the determination of free cyanide, 6 ) . Place the flask under a good draft hood and cautiously add concentrated sulfuric acid until vigorous action ceases. Nom add 50 ml. more of sulfuric and heat the flask to boiling, adjusting the flame so that the ebullition does not become too violent. Discontinue heating the moment the precipitate of gold turns light brown in color and the sulfuric acid is absolutely clear. Decant the supernatant acid and treat the precipitate with 50 ml. more of concentrated sulfuric acid, heating to the boiling point to dissolve any silver sulfate that may be present. Decant this acid, leaving as little as possible in t h e flask. Carefully dilute the remaining acid with 200 ml. of distilled water and filter the contents onto a tared Gooch crucible lined with asbestos. Wash the precipitate with hot dilute sulfuric and then with hot wat’er until the washings are no longer acid. Dry and ignite at red heat until the brown sponge turns golden yellow. Cool to room temperature and weigh.
To compare the accuracy of this method with that of the evaporation method, tests were made on standard samples prepared as follows: c. P. gold weighed accurately to within 0.1 mg. was dissolved in a minimum amount of aqua regia and carefully evaporated
,
