Assay for Platinum Metals in Ore Concentrates J

JOHN SEATH AND F. E. BE,I.IIISH. ITniversity of Toronto, Toronto, Ontario, Canada

S I D E from occurrences in dunites, pyroxenites, and A placer deposits, the platinum metals are generally as- sociated with high proportions of nickel, copper, and iron sul- fides, the mechanical concentration of which results in the accumulation of these base metals.

During an investigation involving such coiicentrates ob- tained from certain nickel deposits of Northern Ontario t'he authors met with considerable difficulty in the assay for platinum metals. This m-as a result of:

Inability of the lead completely to collect the precious metals on a single fusion.

Persistent failure of the flux to slag off sufficient base metals to produce a clean button. Despite repeated scorification of the lead buttons with a great variety of fluxes, cupellation always re- sulted in the deposition of nickel oxide with which much of the platinum met,als was mechanically mixed in the form of the silyer alloy.

3. The presence of much base metal in the lead button pre- vented the use of the all-dry assay in which the precious metals could be weighed directly. This was a great disadvantage in cases where the relative proportions of platinum metals and gold in each sample were fairly constant and it was necessary only to obtain total values.

The authors could find no textbook which dealt with these problems, although a search of the literature revealed that similar difficulties had been encountered and attempt's made toward their solution.

Savelsberg and Fischer (5) discussed the failure of the platinuni fire assay in iron crucibles, and attributed it to the specific affinity of iron for the platinum metals. They also reported that difficulty was experienced in the complete removal of the plati- num metals from high-nickel material. Because of failure to secure a clean button they abandoned the all-dry assay and recommended collection with lead and silver. They also gare an account of their attempt to collect precious metals by means of galena, but again, partly because of the presence of nickel in the matte, the method was unsatisfactory.

Bdam (1) stated that, in the fire assay, precautions are neces- sary if the sample contains quantities of copper and nickel, but details were omitted. Rusden and Henderson (4) dealt with the fire assay of speiss; in order to remove base metals they recoin- mended tn-o fusions of the slag and three pure lead scorificat,ions of the three buttons, but stated that this procedure is not an ar- curate method nf estimating total platinum metals in speiss.

It appeared desirable to investigate (1) the specific effect of copper, nickel, and iron oxides on the fire collection of platinum metals; and (2) the efficiency of standard types of fluxes subjected to varying temperabures.

1.

2.

Assay of South African Sulfide Concentrate A large sample of South African concentrate was obtained

and analyzed for base metals. On the basis of the roasted concentrate the results were 18.6 per cent nickel oxide, 50.2 per cent ferric oxide, 11.6 per cent copper oxide, and 16.1 per cent silicon dioxide.

The following bisilicate flux was used per assay ton of con- centrate:

Litharge, 100 grams Soda ash, 60 grams Borax, 40 grams Silica, 41 grains Silver 200 ma. Flour,' sufficient to pioduce a 30-rram lead regulus

The concentrate was first roasted to remove sulfui witli in- termittent additions of charcoal to remove arsenic. The charge was then placed in the furnace at 9.30" C. The temperature was raised continuously for 45 minutes to about 1200" C. and main- tained for 20 to 30 minutes. The resulting button was cleaned by two washings with a bisilicate flux composed of 60 grams of

soda ash, 76 grams of borax, 19 grams of silica, and 79 grams of litharge. As platinum metals were usually found in the two slags, they were fused by means of 234 grams of the above flux with addition of flour and further lithar e to produce button 2. The heat treatment was that described a%ove.

The same quantity of the latter flux and the same heat treat- ment were also used for two fusions of the original slag, which was invariably found to cont,ain appreciable amounts of platinum metals, producing buttons 3 and 4.

The four buttons, each weighing about 30 grams, were then scorified to form 30 to 40 grams and cupeled at 900" C. The silver bead was treated according to the wet procedure outlined below.

The average of closely checking results was 15.0 mg. of total platinum metals per assay ton of concentrate. In this concentrate the relative proportions of platinum metals were 69.0 per cent platinum, 27.2 per cent palladium, 3.4 per cent gold, and 0.41 per cent insoluble platinum metals. (The result reported for this concentrate by a reputable com- mercial assayer who used a standard wet treatment of the silver bead was l 4 . i nig. of total precious metals per assay ton of concentrate.)

The weights of platinum metals in the first slag (button 3) were found to range from 2.0 to 0.2 mg., in ~vliich a very con- siderable proportion of platinum a,nd palladium was found as well as much of the insoluble precious metals. The second slag (butt'on 4) yielded from 0.3 to 0.05 mg. of platinum metals. Here again there was appreciable platinum and palladium, although most of the residue consided of insoluble platinum metals.

A fusion of the second slag was regularly inade, but only in rare cases could even traces of platinum metals be detected. The platinum metals from the two washing$! of the button generally weighed from 0.5 to 0.1 mg. and consisted almost entirely of platinum and palladium. In many cases the fusions of the slags and buttons were made in one original pot. As would be expected, the acid charges only slo~vly attacked the clay pot's.

F:ffiicienr?- of Standard Fluxes Fusions were made with fluxes containing acid oxide-basic

oxide ratios of 1 to 1, 2 to 1, and 3 to 1, in each of which the proportions of borax, soda ash, silica, and litharge were widely varied. These charges were subjected to furnace tempera- tures increasing slowly to 1200" and 1300" C. and maintained for 0.5, 1, 1.5, 2 , and 2.5 hours; t'o t,eniperat#ures increasing rapidly to 1200" and 1300" C. and maintained for 1 and 2 hours; and to initial t'emperatures of 1200' and 1300" C. maintained for I , 2, and 3 hours. In all t'liese cases and many others appreciable weights of platinum metals were found in the first and second fusions of the original slag.

The fluxes for roast'ed sulfides recorded by Davis (3) and other recommended charges were tried and the results were comparable to those recorded above.

The proportion of flux to concentrate was increased con- siderably and these charges were subject to the conditions of temperature described above. In these teases the base metal content of the first buttons w i s decreased but appreci- able platinum metal values still remained in the first slag. I n spite of the fact that the first slags carried the base metals in the reduced condition and were almost inwriably homo- geneous in appearance, platinuni metals were consistently found to be present,. High litharge, high soda, and high niter charges also failed t,o extract, all of the platinum metals in the first fusion.

169

170 ISDUSTRIAL . iSD ENGISEERING CHE3IISTRY '\;OL. 12, NO. 3

Specific Effects of Copper, Iron, and Nickel Oxides A synthetic concentrate was made up to contain the same pro-

portions of iron, nickel, silicon, and copper oxides as the roasted South African concentrate.

A weight of the mixture equivalent to 1 assay ton of the sulfide concentrate was salted with 10.46 mg. of platinum, 4.00 mg. of palladium, 0.50 mg. of gold, and 0.05 mg. of iridium and rhodium in equal proportions. The bisilicate flux and the procedure adopted were identical with those described for the natural concentrate.

An average of closely checking results was 14.9 mg. of total precious metals recovered. The original slag yielded about 0.4 mg. of platinum metals and the button washings yielded about 0.3 mg. When the ferric oxide and nickel oxide were omitted from the above charge, the weight of platinum metals recovered from the washed regulus was 14.9 mg. The slag yielded only a trace of platinum metals and the button wash- ings none at all.

When the copper oxide and nickel oxide were omitted from the charge, the weight of platinum metals recovered from the washed button was 14.6 mg. The slag yielded 0.3 mg. of platinum metals and the button washings 0.1 mg. When the iron oxide and copper oxide were omitted, the weight of the platinum metals recovered from the washed button was 13.4 mg. The platinum metals recovered from the slag weighed 1.2 mg. and the button washings yielded 0.3 mg.

If the copper oxide, nickel oxide, and iron oxide were omitted from the above charge, there was a complete recovery of platinum metals in the first regulus. These experiments were duplicated with a wide variety of fluxes and under vari- able temperature conditions. In each case similar results were obtained.

As with the South African concentrate, the unwashed regulus from the iron, copper, and nickel synthetic sample yielded on cupellation the greenish black deposit which was found t o contain mechanically mixed platinum metals-silver alloy. The deposit proved on analysis to be mainly nickel oxide with only a trace of copper and iron.

Wet Analysis of Bead The silver bead was parted at low temperatures according to

the method described by Beamish and Scott (I). The silver was removed as chloride from the parting acid and the palladium re- covered as dirnethylglyoxime. The residue from the sulfuric acid treatment was washed first by decantation with ammonium acetate, then with ammonium hydroxide and water. The filter paper and small residue were ignited and the solids returned to the residue remaining in the beaker. Aqua regia (3 t o 1) was added and the diluted extract filtered off. The nitric acid was re- moved by three evaporations with hydrochloric acid, any silver present was filtered off, and the precious metals were precipitated with sodium formate. The formate precipitate was redissolved and again precipitated as above, in order to eliminate traces of lead which do not precipitate with formate in a pH 6 medium. The aqua regia-insoluble was washed with ammonium acetate, ammonia, and water and added to the formate precipitate and the palladium obtained from the parting acid. The total metals were heated in air, reduced in hydrogen, and cooled in carbon dioxide.

All-Dry Assay for Platinum Metals As suggested above, the all-dry method under certain

limited circumstances offers considerable advantages over any wet method. Adam (1) states that in South Africa it was necessary only to estimate the relative proportions of the platinum metals in particular zones of a mine by periodic examination of the prills from numerous assays, and in the Potgietersrust district where the platinum-palladium ratio is 1 to 1 the total platinoids are reported by simply weighing the bead cupeled without inquartation x i t h silver. He gives

some information concerning the retention of lead by some of the platinum metals.

The authors' conclusions concerning the efficiency of the all-dry method are derived from considerable experience with its applications and may be summarized as follows:

Over a wide range of compositions and extended cupella- tion treatment a t 850" to 900" C. the lead was never quan- titatively removed. -4s much as 40 to 50 mg. of lead may remain with an 80-mg. bead.

TABLE I. RECOVERY OF PL.4TIXCM h1ETALS BY ALL-DRY &SAY

wt. of

S O

Wt. of

Insolu- Total after 30 ble Rh Precious Min. at

Pt Pd Au and Ir Metala 1300' C. Mg. M g . Mg. 1MQ. M g . M g .

-- Metal Added- Bead

Precious &letah by Wet Analysis of Same

Bead .Mg.

1 1 5 . 0 5 . 0 5 . 0 ... 2 5 . 0 2 5 . 1 25.1 2 20.0 20 .0 2 . 0 ... 4 2 . 0 4 2 . 5 4 1 . 8 3 1 5 . 0 1 5 . 0 . .. 3 0 . 0 29 .9 29 .9 4 2 . 0 2:o 2 . 0 ... 6 . 0 G.1 6 . 1 J 5 . 0 1 . 0 5 0 . 0 ... 5 6 . 0 5 5 . 9 5 5 . 7

1 5 . 0 5 . 0 5 . 0 5 . 0 30 .0 2 6 . 9 2 6 . 2 b 1 5 . 0 5 . 0 5 . 0 1 . 0 2 6 . 0 2 6 . 2 25 .9 9 15.0 5 . 0 5 . 0 0 . 5 25 .5 2 5 . 6 2 5 . 5

5 10.0 2 . 0 100.0 . . . 112.0 112.0 111.9

Contrary to the findings of Adam ( I ) , the authors observed that continued treatment of the bead a t about 1300" C. often results in increasing losses of platinum metals. Beads con- taining 15 mg. of platinum, 10 mg. of palladium, 5.0 mg. of gold, and 0.5 mg. of insolubles lost about 2 mg. of platinum metals during the second hour's heating at 1300" C. and about 2 mg. in the third hour. On some occasions even with low insolubles the authors found tha t cupellation at 1300' C. in air over a period of an hour did not quantitatively remove the lead. I n such cases the use of oxygen instead of air hastened the removal of lead and consequently reduced the danger of precious metal losses.

Like Adam ( I ) , the authors found tha t a bead containing a high proportion of palladium retained lead tenaciously while platinum reduced this tendency.

The insolubles, when present in the proportions usually associated with lode deposits, did not adversely affect the dry assay. If the bead contained more than about 4 per cent of insolubles there was a marked retention of lead even after a n hour's heating a t 1300" C. in the presence of an oxygen atmosphere. There were also considerable mechanical losses of precious metals due to the incoherence of the bead. These effects were accentuated in the case of large beads.

Final treatment of the bead by the well-known blowpipe method is not recommended by the authors. Only an opera- tor with considerable experience can avoid serious loss of precious metals.

Cupels of 3X bone ash were found the most satisfactory for the final high-temperature treatment. In the case of morganite the beads were cleaned only with difficulty. Cupels made of bone ash mixed with cement were found unsatis- factory.

PROCEDURE FOR DRY TREATMEXT. The lead collection without silver is conducted as described above. The beads after cupella- tion at about 900" C. are transferred t o small depressions in 3X bone ash vessels and placed for 30 minutes in the muffle a t 1300" C. through Fhich a stream of oxygen is conducted. The beads are cleanedwith concentrated acetic acid, dried, and n-eighed. Applying this procedure, the South African concentrate described above yielded 15.1 mg. as compared with 15.0 mg. obtained by wet analysis. Direct examination of the beads yielded no lead as chromate.

Table I illustrates the degree of accuracy obtained by ap- plying this dry method to synthetic concentrates containing high proportions of base metals and using the fluxing pro-

MARCH 15, 1940 ANALYTICAL EDITION 171

cedure described above for the natural concentrate. The quantities of platinum metals recorded roughly represent the various compositions encountered by the authors.

No. 7 illustrates the fact that with high percentages of insolubles the dry assay is unsatisfactory. This is due to both retention of lead and loss of precious metals. Direct examination by means of chromate always revealed consid- erable lead where the composition and weight were similar to KO. 7 . I n many cases beads similar to S o . 2 yielded small amounts of lead. This was found to be caused by the high pro- portion of palladium in a bead weighing about 40 mg. or more.

Summary With ores or concentrates containing high proportions of

nickel oxide, one or two fusions of the slag are necessary for complete collection of platinum metals in a lead regulus.

High proportions of copper and iron oxides have very little deleterious effect on the lead collection of platinum metals.

Various monosilicates, bisilicates, trisilicates, and excess litharge and niter charges subjected to wide temperature treatments did not materially improve the first fusion re- covery of the platinum metals contained in concentrates of high nickel content.

In the absence of high proportions of rhodium, iridium, and ruthenium proper washing of the lead regulus permits the a p plication of the all-dry assay. If the proportion of insoluble to soluble platinum metals is very much greater than that usually present in lode deposits, t,he assay bead is no longer coherent and lead is retained even after I to 3 hours’ treat- ment a t 1300’ C. in oxygen. If the proportion of palladium to platinum is about 1 to 1 or greater, a weight of concentrate should be chosen to produce a bead weighing about 10 or 15 mg. Even where a small amount of lead is retained, the ab- sence of silver facilitates wet analysis of the bead.

The all-dry assay has a limited application. It should not ordinarily be used for the original discovery of platinum metals.

Literature Cited ( l j Adam, H. R., J . Chem. M e t . Mining Sac. S. Africa, 29,106-15

(3) Beamish, F. E., and Scott, M., IND. ENG. CHEM., Anal. Ed., 9,

(3) Davis, C. W., U. S. Bur. Mines, Tech. Paper 342 (1924). (4) Rusden, H., and Henderson, J., .I. Chem. )Vet. Mining Sac. S.

(5) Savelsberg, W., and Fischer, A., Mctall u. Erz , 31, 451 (1934).

(1928).

460 (1937).

Africa, 28, 181-98 (1926).

Dropping Electrode with a Constant Head of Mercury E. F. RIUELLER, National Bureau of Standards, Washington, D. C.

OST of the dropping electrodes used in polarographic measurements have no provision for automatically

maintaining a constant head of mercury, al- though it is obvious that such a constant head can be obtained very simply by using the principle of the Mariotte flask. To one unfamiliar with polarographic litera- ture, it seemed prob- able that the device had already been used but on discussing the m a t t e r w i t h E. R. Smith of the Physical Chemistry Section of t h i s b u r e a u , i t a p - peared that this was not the case. He ac- cordingly made and tried the device, and has kindly supplied the following comment and description.

Measurements made with the polarograph generally require com- p a r i s o n s of wave heights which are re- c o r d e d a t d i f f e r e n t times. These wave heights depend on a number of variables, among which is the rate of dropping of the mer- FIGURE 1

cury electrode. For special purposes it is adtantageous to ar- range the electrode so that its dropping rate can be varied arbitrarily, u-ithin limits, by changing the height o,f the column of mercury or by changing the external pressure on the column, but for most analyses it is necessary and sufficient to have substantially a constant pressure acting on the droplets a t the tip of the capillary both during successive determina- tions and during an initial calibration.

For this purpose, a constant head of mercury may be very simply obtained by usin the principle of the Mariotte flask, as shown in Figure 1 (not firawn to scale). The reservoir is filled with mercury through tube A , which is open to the atmosphere and drawn to a bore of about 1 mm. at D. Electrical connection to the dropping electrode, C, is made through E. By gentle suc- tion at B, the level of mercury in A is brought down to tip D and then stopcock B is closed. As long as the level of the mercury in the reservoir is above D, a constant head, H , is maintained with- out attention to the height or amount of mercury in the reservoir, Instead of sealing the capillary, C, to the reservoir, it may be at- tached by a short piece of rubber tubing, as indicated by the dot- ted lines at F , or by means of an interchangeable ground joint. With a suitable set of interchangeable capillaries different d r o p ping rates may be obtained.

Stopcock B should be lubricated, but the stopcock above F and the ground joint need not be. They might be partially lubricated in such a way as to avoid contact between the lubricant and the mercury. If used dry, some kind of spring would be needed to keep the parts in position. Even without lubricant, there should be no leakage, since the mercury would not enter the narrow spaces, and air should not leak in because the pressure is above atmospheric. A dust cap for tube A adds a toutrh of elegance.

The same device may also be used where it is preferable to use a single capillary sealed to the mercury reservoir and an adjust- able head. In this case t,he mercury reservoir is made in the form of a tube, perhaps 8 or 10 mm. in inside diameter and somewhat less then 76 cm. long, fitted with a perforated stopper at the top. A long glass tube, corresponding to A in the figure, passes through the stopper. If this tube is of small bore and if the reservoir of mercury is nearly filled at the start, the level in the tube will fall to the point corresponding to D after a. small quantity of mercury has been allowed to escape through the capillary. Capacity for a supply of mercury can be provided in a bulb at the top of the long reservoir. By using a two-hole stopper, a stopcock corresponding to B may be included.