Copper Bullion Assay

Copper Bullion Assay

Table of Contents

Someone some time ago remarked that some chemists still insist on telling us how to determine copper by the electrolytic method. The writer must confess that he believes that everything is not known definitely as yet as to how the exact amount of copper is determined in such material as purest commercial electrolytic copper. Some of us are not yet convinced that the pure copper atom is at all times deposited from an acid solution and that no oxygen or hydrogen will, under certain conditions, accompany it. Difficulties with impurer material are frequent, but erroneous results are not always apparent, since these do not speak so readily for themselves as those in the first case. By this is meant that if a chemist finds 100 per cent, of copper in electrolytic copper it is pretty certain that he perceives the result to be wrong, while if he finds 0.1 per cent, of copper too much in a very impure material he is far more likely to be unconscious of his error. These, in a wide experience, are frequent happenings and the writer has always looked with interest to publications on this subject. In turn, he feels justified in giving to others a few of his own observations.

Up to a few years ago there were two methods of electrolytic copper assay in technical use, which had in common, that the amount of copper deposited on the cathode did not exceed 2 g., and they differed in that, for the one a large sample (20 to 80 g.) was taken from the general sample by a splitting device, dissolved, and from the solution a small portion (to contain 1 or 2 g. of copper) measured out for the electrolytic deposition; in the other method the copper was determined, generally in 1-g. portions of the separated coarse and fine portions of the sample, all of which passed a 16 or 20-mesh screen, being accomplished by a 40-mesh screen, and the average assay was figured according to the weight-ratio of the parts. To use larger quantities (5 g.) for electro-deposition on such copper as Lake or commercial electrolytic had been recommended some years ago, but this had never been attempted in a systematic way with crude coppers.

John T. Stoddard published a paper in which he pointed out the feasibility of rapid electrodeposition of metals, with analytical accuracy, on stationary gauze cathodes and without special stirring device for the electrolyte, it occurred to the writer that this idea, in properly modified form, could be taken up in the laboratories of the copper smelteries and refineries for the purpose of more accurate copper determinations in the metallic materials; however, not by reducing the time usually required for deposition, but by multiplying the quantity of sample taken for the assay. It was reasoned that gauze cathodes, for much work, would be too flimsy and changeable in weight through abrasion and that perforated platinum sheet would answer the purpose better. For certain work, cathodes with larger perforations had been recommended and used before by G. A. Heberlein of Great Falls, Mont. It developed after a few trials that with the new perforated cathodes 5 g. of copper, from crude-copper solution, could be as quickly and cohesively deposited as 1 g. on the smooth cathode, with equal gross surface, within the customary time of 18 to 20 hr. With the perforated cathode a current of 0.5 ampere was applied, while with the smooth cathode 0.15 ampere had been found to be the upper limit for good work. It was found impossible to deposit, in good form, 5 g. of copper on a smooth cathode within 24 hr. Most important with the 5-g. method is the fact, that the general limit of agreement between duplicate determinations was moved from the second percentage decimal to the third, whatever the absolute accuracy might be. This in itself is sufficient proof of the fairness of each of the 5-g, charges taken from the general sample. Besides, it is the writer’s opinion that there is far more simplicity in this method than in the one in which a much larger sample is dissolved and an aliquot portion of the solution, with a small quantity of copper (1 to 2 g.), measured out.

The operation of this method is now carried out in the following manner:

Stock solutions

Nitric acid (sp. gr. 1.42) one part to one part of water; sulphuric acid (sp. gr. 1.84) one part to two and one-half parts of water; sodium chloride, 5.5 g. to the liter. The proper ratio of the fine portion of the sample, 5/C/F + 1 grams (C=weight of coarse; F= weight of fines in grams), is weighed and the remaining part of the 5 g. made up with the coarse portion. This is dropped into a 300-cc. lipless Jena beaker (tall form) which seems to be the smallest size that can safely be used for the solution of 5 g. of copper. Tests made by placing a filter paper over cover glass and beaker and snugly folded down over the edge of the latter


revealed no loss of copper mechanically carried away. Ten cubic centimeters of stock sulphuric acid is first added to the copper, followed by 25 cc. of stock nitric acid; the purpose of the initial sulphuric acid is to retard the action of the nitric acid but afterwards to aid in the solution of the oxides present. The beaker, covered with a watch glass, is placed in a cool part of the hood until the copper is dissolved and is then placed on a steam plate, carefully avoiding- boiling, until complete disappearance of nitrous fumes; 2 cc. of the sodium chloride solution is now added and the hot solution gently shaken until the silver chloride is well coagulated. Filtration is then performed through a 7-cm. filter into a beaker of the same size and shape as the original. To make up the full acidity of the electrolyte 30 cc. more of the stock sulphuric acid and 3 cc. of the nitric is necessary, all of which is run through the filter to insure complete recovery of the copper. The whole is diluted to 225 cc. with distilled water.

When any difficulty in depositing the copper is experienced it may be attributed to insufficient dilution of the electrolyte; addition of more water will overcome it.

The perforated platinum sheet constituting the closed cathode cylinder is 6 x 11 cm. and has seven perforations of about 0.5 mm. to the linear inch. The total height of the cylinder and stem is 23 cm.; they are somewhat unnecessarily heavy, weighing about 23 g. Those cylinders that have been in daily use for four years on Anaconda material lost on the average 1.7 milligrams per year. Others, which are used for copper high


in gold, gain very appreciably in weight after every deposition; this being due to the deposition, with the copper, of that portion of the gold which is soluble in nitric acid. In course of time these cylinders become gold-plated. With the heavy current used, it is important that the beakers are well covered, so that there can be no loss of solution by spraying. Fig. 1 illustrates the scheme to accomplish this with the aid of several sets of split watch glasses. Fig. 2 shows the construction of an electrolytic stand with series connections. The ammeter is not visible. It is here shown chiefly to call attention to the knife switches on the top of the bar which enable the operator to remove the cathode and simultaneously to close the circuit, which is not possible with the generally used plug. It is of the utmost importance to transfer the cathode; with its copper deposit as quickly as possible from the electrolyte into pure water, so as to lose a minimum of copper by resolution. It is done as follows: The covers are first removed from the beaker, then the base block from under the latter with the right hand, holding the beaker with the left. Up to this moment the current passes uninterruptedly. Now the forefinger of the right hand closes the switch and simultaneously the thumb presses the spring releasing the cathode; the left hand at the same time draws the beaker and cathode down and away from the anode (the usual corkscrew spiral). By this time the right hand is free to transfer the cathode to several clean waters and to absolute alcohol, after which it is dried over a Bunsen burner.

As already stated, the current employed for each assay is 0.5 ampere; the time (overnight) for complete deposition about 18 hours. The latter point is arrived at when polarization (bubbles) at the cathode has taken place for about one-half hour. The beaker is then filled with distilled water; so that the electrolyte touches the cover-glasses and the current permitted to continue for another half-hour. Each electrolyte is tested for the presence of copper by saturation with hydrogen-sulphide gas.

The question of the accuracy of results should in all cases be subjected to analysis. It has already been hinted that after all the copper has been deposited on the cathode, there is a possibility of re-solution when removing it from the acid electrolyte. Tests made on numerous individual determinations showed that the electrolytes contained 0.0092 per cent, of the original copper. Table I shows, on the other hand, that the copper which is deposited is not entirely pure, that in this case it contained 0.0093 per cent, of its weight of impurities deposited from the electrolyte. These two figures balance each other very closely and in commercial work usually no corrections are made.assay-impurities-contained-in-copper-bullion

In many laboratories it has been customary to precipitate the silver electrolytically with the copper on the cathode and then to subtract from the gross percentage that of the silver as determined by separate assay. This works well up to a certain silver content; 0.3 per cent, or 100 oz. per ton probably being the limit. When under this method the first difficulties with a good copper deposit are experienced, it is well to eliminate the silver from the electrolyte. On a number of occasions this was the only precaution necessary to bring about an excellent deposit of electrolytic copper.

When certain impurities in crude copper, or copper-bullion, exceed a certain quantity, it becomes impossible to electro-deposit from its solution a copper of sufficient purity for commercial analytical requirement. We have already seen that bismuth is all deposited with the copper, but this metal is rarely met in disturbing quantity in converter copper. When met in larger quantity in black coppers it must be eliminated before the electrodeposition of the copper; a method for which will be described further on.

Selenium is an element which is more readily deposited from solution than copper and we meet it in appreciable quantity in the converter coppers from our Southwest. When the copper on the cathode is dissolved in dilute nitric acid the selenium remains, at least in part, as a red coating, and some chemists weigh this and use the weight as a minus correction in their copper assay. This, no doubt, is but an approximation. The following is a very simple method to remove quantitatively the selenium from any, quantity of copper, the silver being removed at the same time.

Selenium and copper cannot be completely separated by the alkaline sulphide method, as there is always some insoluble selenide formed with the heavy metal. Selenium is also precipitated with the copper when the latter is separated as sulphocyanate from a number of other elements. In fact, however, any method that, would necessitate the precipitation of 5 g of copper as a chemical compound for the purpose of its purification would be unsatisfactory. The precipitation of the selenium from the electrolyte by means of sulphur dioxide naturally suggested itself, but it was soon found that here also the two elements combined to form copper selenide.

Upon further investigation it was learned that in the presence of copper and silver, in sulphate solution, the selenium, when liberated by sulphur dioxide, had a strong preferential affinity for silver and if the latter was present, in sufficient excess, the selenium would be precipitated quantitatively as silver selenide, Ag2Se, without a trace of copper. The excess of silver is then precipitated as chloride and with the silver selenide is filtered and washed with dilute sulphuric acid and water. In case too much of an excess of salt solution has been used, it is best now to evaporate to sulphuric acid fumes, in order to drive off the chlorhydric acid and thus avoid its deleterious effect on the character of the copper deposit.

The presence of an appreciable quantity of tellurium will also interfere with the accuracy of the electrolytic copper determination and it must, accordingly, be eliminated before the electrodeposition of the copper. However, it is not often present in disturbing amount. Tellurium is not very readily precipitated from the copper sulphate solution by sulphur dioxide, but whatever portion of it is reduced combines with copper to form copper telluride. It is no rival of selenium in the latter’s affinity for silver. A complete elimination method will be described with that of the bismuth.

Antimony and arsenic are not very readily electro-deposited from nitric-acid solution and only small percentages of the original quantities are found in the electrolytic copper; yet they become troublesome at times, because they are generally the elements found in largest quantity associated with copper. It has been claimed that certain compounds, “dopes,” added to the electrolyte will entirely prevent the deposition of these elements. In tests made in the Anaconda laboratory with commercial electrolytic copper it was found with the addition of such dopes the results were very noticeably high.

To eliminate antimony and arsenic when they become disturbing factors, the writer prefers the double-deposition method, which has proved to be very satisfactory and in proof of which the data of Table II are illustrative. To execute this method the original deposit on the cathode is treated with the usual precautions and is then placed, together with the anode, in a regular depositing beaker, the complete electrolyte added and the whole covered as per scheme of Fig. 1. Only gentle heat is applied to insure the re-solution of the deposit and the solution is digested long enough to expel the reduction products of the nitric acid. When, in these cases, the copper deposit has a slight coating of arsenic or antimony, there is no re-solution of the copper during the time of removal of the cathode-cylinder from the electrolyte, since the former elements are first subject to attack.

Lead is one more element which is usually present in copper in small quantity, but which, when present even in larger quantity, may be, readily eliminated in the course of the regular routine of preparing the electro-analysis. In the first place a major part of it may be separated as sulphate from sulphate solution and the small remaining part can be electrodeposited from nitric acid solution as peroxide on the anode; it being only necessary that the latter presents sufficient surface for the adhesion of the peroxide.

There is yet to be described a method by which bismuth and tellurium may be eliminated and which will, at the same time, include the elements for which individual methods have been given.


For this the writer believes the simplest process to be the precipitation with ferric hydroxide in slightly ammoniacal solution with an addition of ammonium carbonate to precipitate the bismuth. To carry out this method the copper is dissolved and the silver eliminated as usual, 0.1 g. of ferrous sulphate having been added to the 5-g. copper charge. The solution is made slightly ammoniacal and a little ammonium carbonate added; it should then be kept near the boiling point for about 15 minutes. The ferric hydroxide carries down with it all of the selenium, tellurium, antimony and arsenic, but unfortunately, and this is the drawback of the method, it also carries with it an appreciable quantity of copper, so that after filtering and washing the precipitate with ammoniacal water it must be re-dissolved, re-precipitated, filtered and washed, and the whole procedure repeated four or five times to insure the recovery of all of the copper. In each re-precipitation the same conditions are to be observed as in the first one. All the filtrates are united and the regular quantity of sulphuric acid is added. Should the volume of solution be too great it must be evaporated to the desired mark.

The marked progress in rapid electrodeposition of metals in recent years, by means of high current density and the use of rotating electrodes or the rapid circulation of the electrolyte, is well known. These methods, undoubtedly, have their high merit for special purposes, where time is the chief factor. They do not seem to have appealed to the chemists of copper works where large numbers of accurate determinations are required and in which the work of the electric current during the night requires no attention whatever, while rotating electrodes, motors, and violently stirred electrolytes would be sources of much concern.

Assaying for Silver and Gold


assay of copper bullion 1

In the admirable paper quoted, is mentioned the date when the 5-g. assay was first proposed as a standard for refined metal.

The author omitted, however, to mention a later paper, which contains a proposal for the use of a single “ stock solution ” of mixed acids, which has been adopted by the American Brass Co. and others.

The suggestion is made that such a scheme would save time and manipulation with converter metal. Unless the low current of 0.5 ampere and the large excess of nitric acid, recommended by Mr. Keller, have been proved necessary to hold up antimony and selenium, the use of the mixed solution which we have adopted will permit the use of 1 ampere of current per square decimeter of cathode surface, until the solutions have become colorless. At this point, we wash the covers and reduce to 0.5 ampere to prevent oxidation or contamination. The ordinary split electrodes have been used. With strong current, at least, the use of an excess of nitric acid tends to the neutralization of the solution before the end of the period, from the formation of too much ammonia, and this is detrimental. Antimony deposits more easily than arsenic, and tends to drag the latter down with it. There must be some such explanation for the fact that we have made hundreds of deposits of pure cathodes from Lake material containing no antimony, but as high as 0.5 per cent, of arsenic. By placing an assay beaker in the Frary solenoid, or rotary device, a cathode deposit with less than 0.01 per cent, of impurity is easily obtained in one operation from 3 g. of whitneyite, carrying from 7 to 10 per cent, of arsenic. In such a case, it is necessary to test several times for end point and remove the plate as soon as no brown tint appears (at once) when hydrogen sulphide water is added to the test portion on the spot, plate.

Edward Keller’s proportions of solvent figure out as 14 cc. of strong nitric to 11.4 cc. of sulphuric acid (1.84).

Our stock solution is made up in the following, proportions by volume: 7 cc. of nitric acid (1.42), 10 cc. of sulphuric acid (1.84), and 25 cc. of water. Ordinarily, 40 cc. is taken for a 5-g. sample, unless the arsenic is over 0.5 per cent., in which case the amount of solvent is increased.

It may not be generally known that one chemist, Ferdinand Andrews, of the Raritan Works, is depositing regularly 10 g. of copper from refined metal in 16 hr. with the perforated cathode, described by Mr. Keller, and a double anode, consisting of an inner spiral and outer frame in one piece, similar to the Hollard type advertised by Eimer & Amend, New York. The current strength is 1 ampere, and the first solvent is pure nitric acid, to which is added 6 cc. of sulphuric acid and 10 cc. of ammonia, after the nitrate has been evaporated to the crystallizing point. Here, as in the single-mixture method, ammonium salts are depended upon to assist in holding back the impurities.

If the same quantity of sample can be plated out with half the current required by the solid cathode, or if 10 g. can be taken, the perforated cathode certainly offers advantages in the assay of refined metal as well as crude bullion.

Removal of Selenium and Tellurium

When these elements are the principal impurity, they have for years been successfully removed by a very simple scheme which takes care of Mr. Keller’s objection that the metals, under certain conditions, form compounds with silver, or copper, when reduced with sulphur dioxide.

The nitrate solution is evaporated with excess of sulphuric acid until the dry residue is white. Dissolve in 60 cc. of water and wash into a lipped beaker, placing the tall beaker under a funnel fitted with a 3-cm. filter. Heat the solution to boiling, remove from plate, and charge with SO gas for 10 min. Gas, free from chlorine, is generated by slowly dropping a saturated solution of C. P. sodium sulphite from a separatory funnel into a large round-bottom flask half filled with concentrated C.P. sulphuric acid. After settling a few hours, at least, filter into the original electrolytic beaker, and wash free from acid with hot water. Boil gently to remove most of the sulphur dioxide gas. Ignite the filter in a porcelain crucible at a red heat until the residue is oxidized, to volatilize the harmful elements.

Re-dissolve the residue in 1.5 cc. of nitric acid and add to the main solution, and electrolyze in the usual manner. A little more nitric acid and 10 cc. of ammonia may be added, if necessary, to hold back antimony and bismuth.

assay of copper-bullion