Assaying Zinc Box Residues in Cyanide Leach Process

Several methods, both wet and dry, for the assay of zinc-box residues from the cyanide process, have been described in recent years, and each of them has been claimed to be superior to all others. A paper, entitled “Assay of Zinc Precipitates,” was published in the School of Mines Quarterly to the purport that the scorification method for the assay of zinc-box residues was absolutely unreliable.

In order to shed light on this matter, the following experiments were undertaken by Messrs. C.B. Hollis and F. D. Kehew, undergraduate students at the Massachusetts Institute of Technology.

The zinc-box residues used were obtained through the courtesy of Mr. H. H. Batcheller. The samples were very rich and varied greatly in the fineness of their condition.

Scorification-Assays

In the preliminary tests, the charge, which was weighed on a chemical balance, consisted of 0.1 Assay Ton of residues mixed with from 30 to 35 grammes of test-lead and placed in a 3-in. scorifier, over this an additional quantity of test-lead (from 30 to 35 grammes) was placed for a cover, and borax-glass, varying in quantity from 3 to 15 grammes, was sprinkled over the top of each charge of the various assays for a cover. The charges were scorified in a muffle-furnace heated to the ordinary temperature which is used in scorification; in some cases the door of the muffle was left open, while in others it remained closed. Generally, the charges spit badly, especially in the assays that were made with door of the muffle left open, or in those in which the door was opened too quickly. The results of the preliminary assays showed: 1. That in order to obtain approximately uniform results, the material submitted to the assay must be in sufficiently fine condition to pass through a 200- mesh sieve. 2. That the ordinary chemical balances are not sufficiently delicate to afford accurate results in handling these residues which are so rich in gold and silver. 3. That a large quantity of borax-glass is absolutely necessary (from 3 to 10 grammes for 0.1 Assay Ton of residues); and 4. That spitting can be avoided, provided the muffle be heated to a high temperature before the introduction of the charge, and provided the door of the muffle be kept closed until the content of the scorifier has become thoroughly liquefied; after this the temperature may be lowered.

The quantity of zinc-box residues received amounted to 458 grammes and, upon sizing, it was found that 146 grammes, or 31.8 per cent., remained upon a 125-mesh screen; 64 grammes, or 13.9 per cent., passed through a 125-mesh screen and was caught on a 160-mesh screen; and 248 grammes, or 54.1 per cent., passed through a 160-mesh screen. The entire quantity of residues was then put on a 160-mesh screen and the material that sifted through was treated on a 200-mesh bolting cloth, yielding 290 grammes of very fine material, less than 200-mesh in size, on which the tests were made.

In order to mix the sample thoroughly, the entire quantity of fine material was placed in a 38-oz. bottle closed with a glass stopper and steadily shaken for 20 minutes, the bottle and its content being alternately shaken and rotated. The mixed material was then poured out upon a glazed paper, on which it was rolled 100 times, finally being spread out in a thin layer covering an area 18 in. square. Spatula-samples to the number of 450 were then taken, which constituted a new sample, weighing 102 grammes. A chemical analysis of the new sample showed that it contained 9.09 per cent, of copper and 14.3 per cent, of zinc.

The assays were made in a muffle-furnace heated with coke, and the cupels used were of the ordinary bone-ash variety made at the Institute of Technology. Ninety per cent, of the material forming the cupels was of sufficient fineness to pass through an 80-mesh screen.

Scorification Assay

Four charges, Nos. 1, 2, 3 and 4, each of 0.05 Assay Ton in weight, were weighed on an assay balance sensitive to 0.02 of a milligramme and treated as follows:

No. 1. 0.05 Assay Ton of the residues was mixed with 35 grammes of test-lead in a 3-in. scorifier; 30 grammes of test-lead were then added to the top of the charge, followed by a cover of 10 grammes of borax-glass.

No. 2. The same as No. 1.

No. 3. 0.05 Assay Ton of the residues was mixed with 6 grammes of litharge in a 3-in. scorifier. Additional test-lead was added, amounting to 40 grammes, followed by a final cover of 10 grammes of borax-glass.

No. 4. 0.05 Assay Ton of the residues was mixed with 1 gramme of fine charcoal and 35 grammes of test-lead; the mixture was then covered with 30 grammes of test-lead and a final cover of 10 grammes of borax-glass. The scorifiers were placed in the muffle as shown in Fig. 1. The muffle was very hot and the door was kept closed for about 5 minutes, after which it was opened.

Charge No. 1 spit badly, doubtless due to its position in the muffle; charge No. 3 spit to a slight extent; while charges Nos. 2 and 4 did not spit. Charge No. 4 became covered over very quickly, owing to the charcoal in the mixture, but the resultant button of lead was so large that it was necessary to rescorify it. Although zinc-ores require a high temperature for the fusion, the heat was lowered as soon as possible after the muffle was opened, in order to slag off the copper and avoid a second scorification. The fused material poured well and the color of the scorifier indicated that the buttons could be cupelled with safety. Both the slags and the cupels were assayed by the crucible method, the results being given in Table I. The silver and gold-beads from the slag of charges Nos. 1 and 2, and those from the cupels used in tests Nos. 2 and 3, sank into the cupels which were reassayed. This additional assay may account for the low results in silver and gold that were obtained in tests Nos. 2 and 3.

The silver-gold beads were weighed, but as they did not contain sufficient silver to part them they were recupelled, with the addition of chemically pure silver; the cupels of this latter cupellation were not reassayed. The parting was done with nitric acid of 1.16, 1.2 and 1.27 specific gravities. The results obtained are given in Table I.

Charges Nos. 5, 6 and 7, which were similar in all respects to charges Nos. 1 and 2, were next weighed and placed in the muffle as indicated in Fig. 2. The temperature of the muffle was that used in the ordinary assay, and the door of the muffle was kept closed for 10 minutes. It was then opened and charge No. 6 was seen to spit twice; the door was then closed and, through an opening in the muffle, charge No. 6 was seen to spit a third time. The door of the muffle was then opened and the scorification completed. The slags and the cupels from these charges were assayed as in the former tests, the results being given in Table I.

In order to ascertain whether a scorifier of larger size would be beneficial or not, charge No. 8 was assayed in a 3-in. scorifier, and charge No. 9 in a 4-in. scorifier. Charge No. 8 consisted of 0.05 Assay Ton of residues placed in the bottom of a 3-in. scorifier and covered with 65 grammes of test-lead, followed with a final cover of 10 grammes of borax-glass. Charge No. 9 consisted of 0.05 Assay Ton of residues, prepared as in charges Nos. 1, 2, 5, 6 and 7, with the exception that a 4-in. scorifier was used in place of a 3-in. one. The tests were placed in the muffle as shown in Fig. 3. The muffle was closed and both charges were seen to spit badly. They were then allowed to become covered and were later poured and treated in a manner similar to the earlier tests. From the results of these tests, which are given in Table I., charges Nos. 8 and 9 were rather peculiar. The gold in No. 9 was very low, the silver was very high and an exceedingly large quantity of silver was recovered from the slag. Charge No. 8, also, showed a high percentage of silver in the slag. The cause of these odd results was not apparent.

Charges Nos. 10, 11 and 12 were then made as follows:

Charge No. 10 consisted of 0.05 Assay Ton of residues, 60 grammes of test-lead, 10 grammes of litharge and 12 grammes of borax-glass, thoroughly mixed together in a 4-in. scorifier.

Charge No. 11 consisted of 0.05 Assay Ton of residues mixed with 30 grammes of test-lead; on this were placed an additional 30 grammes of test-lead, followed by a cover of 10 grammes of litharge and a final cover of 12 grammes of borax-glass.

Charge No. 12 consisted of 0.05 Assay Ton of residues mixed with 30 grammes of test-lead in a 4-in. scorifier, over which were placed 30 grammes of test-lead and a final cover of 14 grammes of borax-glass. Charges Nos. 10, 11 and 12 were placed in the muffle as shown in Fig. 4, and the door of the muffle was kept closed for 10 minutes. Charge No. 10 fused very quietly and did not even tend to jump. Charge No. 11 was less quiet, but did not spit, and charge No. 12 was quiet. The charges were allowed to become covered and were then poured, the resultant buttons, slags and cupels being assayed as in the former tests. (See Table I.)

The results from charges Nos. 11 and 12 were low, and the gold obtained from charge No. 10 was especially so. The silver-gold button obtained from the assay of the cupel used for charge No. 10 sank into the cupel, which had to be reassayed.

Charge No. 13 was similar to charge No. 10, and consisted of 0.05 Assay Ton of residues, 60 grammes of test-lead, 10 grammes of litharge and 12 grammes of borax-glass, all thoroughly mixed together in a 4-in. scorifier.

Charge No. 14 consisted of 0.05 Assay Ton of residues and 30 grammes of litharge, mixed together in a 4-in. scorifier, and covered with 30 grammes of test-lead, with a final cover of 12 grammes of borax-glass.

Charges Nos. 13 and 14 were placed in a hot muffle in the position shown in Fig. 5. The muffle was then closed for 10 minutes. Charge No. 13 fused quietly and had no tendency to spit, while charge No. 14 spit several times after the door of the muffle was opened. These charges were treated in a manner similar to the previous tests, except that, in the assay of the cupel of test No. 14, trouble was encountered with the silver-gold button, which accounts for lack of results given under this heading in Table I. The results for gold in tests Nos. 13 and 14 were low, an effect which seems to be true of all assay-charges containing litharge.

A complete summary of the data obtained in tests Nos. 1 to 14, inclusive, are given in Table I.; and in Table II. are given the weights of the lead buttons and other data relative to the assays of the slags and cupels of these tests.

A study of the results shows that the addition of charcoal to the charge seems to aid the scorification. Also, that all charges in which litharge was used (Nos. 8, 10, 11, 13 and 14) gave low results for gold. The addition of the litharge, however, seemed to prevent the spitting of the charge during fusion.

The value of the residues in gold lies evidently between 4,690 and 4,698.4 oz. per ton, as the results from 6 of the 14 charges are within these limits, and 3 of these 6 are practically identical, i.e., 4,694 oz. The results for silver in 4 of the tests were between 4,175.2 and 4,179.6 oz. per ton; 4,178.5 oz. per ton being taken for the quantity present.

Estimating the value of gold at $20.67 per oz. and silver at $0.50 per oz., the value per ton of the residues was:—for gold, $97,025, and for silver, $2,089. On the basis of these values, a comparison of the highest and lowest content of gold as determined by the scorification method and the percentage of variation from the correct value is:

Ounces above, 4.4, value $91, or 0.09 per cent.
Ounces below, 96.8, value $2,000.85, or 2.06 per cent.

The extremely low results of test No. 10 (54.8 oz.,  1.16 per cent.) are omitted from this calculation.

Summarizing the results of the determinations of silver in a similar manner, there were obtained the following figures :—

Ounces above, 495.5,  or 11.85 per cent.
Ounces below, 30.1, or 0.72 per cent. The high results obtained in test No. 9 (87.9 oz., or 2.1 per cent.) were not included in this calculation.

The results for the silver determinations were less uniform than were those for the gold, but the value is apparently between 4,175 and 4,220 oz. per ton, 9 results out of the 14 being within this range. The charges containing litharge gave low results for silver in 2 out of 5 cases.

The quantity of gold found in the slags was generally less than that found in the cupels—a result which is unusual.

From the results obtained in tests Nos. 1 to 14, the best method for the treatment of these zinc-box residues is as follows :—The charge should consist of 0.05 Assay Ton of residues mixed with 35 grammes of test-lead in a 3- or a 4-in. scorifier and covered with a layer of 30 grammes of test-lead, followed by a final cover of from 10 to 12 grammes of borax-glass. The filled scorifier should be placed in a hot muffle (in order that the fusion shall occur rapidly) and the door should be closed for fully 5 minutes after the charge has been fused. During the time that the door is closed, no air whatever should enter the muffle. When the charge has become thoroughly fused, the door of the muffle should be opened and the remainder of the assay conducted in the usual manner.

Confirmatory Wet Assays

In order to confirm the results obtained in the scorification-assays, Mr. Hollis made a duplicate determination of gold and silver in the zinc-box residues by the wet method of Mr. C. Whitehead. These results (charges Nos. 15 and 16) were, respectively, for gold, 4,698.8 and 4,694.8 oz. per ton; and for silver, 3,841.6 and 3,555.6 oz. per ton. The results for gold are practically the same as those obtained in the scorification-assay. The results for the silver, however, are very much lower and are doubtless due to the incomplete precipitation of the silver bromide which is soluble to a certain extent in too strong a solution of potassium bromide. This effect is analogous to the action of silver chloride, for, if a solution of silver nitrate be precipitated by salt in a solution that is not sufficiently diluted, the silver chloride will not be all thrown down, some of it being dissolved in the strong brine.

The residues were assayed also by the wet method suggested by Messrs. Charles H. Fulton and C. H. Crawford, which is called the “ combination wet and dry method of assay.”

The method was used exactly as described, with the exception that the filter and content were not scorified, but were assayed in a glazed crucible after having been separately burned. The data obtained by the combination wet and dry method are given in Table III.

The data given in Table III. show that the results for gold were somewhat lower than those obtained by the scorification-assay, while those for silver were very much lower.

Crucible Assays

Three portions of residues were taken, of 0.05 Assay Ton weight, and to each was added 15 grammes of soda, 10 grammes of borax-glass, 90 grammes of litharge and 2 grammes of argols; an excessive quantity of litharge was used, in order to slag the copper and the zinc. The fusion was made in a “ G ” crucible, which had previously been glazed with borax- glass, and each charge was fused for 35 minutes. One charge ate through the crucible, one would not pour, the third only seeming satisfactory. The results of the good test are given in Table IV., charge No. 22. Four additional charges were made, Nos. 23, 24, 25 and 26, all similar to No. 22, with the exception that the quantity of borax-glass was increased to 15 grammes in each charge. These charges worked satisfactorily in the furnace, but the results, which are given in Table IV., were not all that was hoped for.

The data given in Table IV. show that the results for gold and silver averaged much lower than the quantities obtained in the scorification-assays. The quantity of silver obtained was very much lower, a result which may be due to the large quantity of litharge used in the charge, or to the quantity of copper present in the sample. The slag also from the crucible assay is richer in both silver and gold, and the second slags and second cupels should have been assayed,—an omission which is to be regretted.

In order to verify the results obtained in the scorification method by Mr. Hollis, a duplicate set of experiments were made by Mr. Kehew on residues from the same lot of samples. Mr. Kehew conducted the assays in a muffle that was fired by gas, and measured the temperature of the experiment tests by a Le Chatelier pyrometer.

The same care was observed in taking the samples, and the same button-balance was used to weigh the samples, although a different set of assay weights was used.

Three charges were made, as follows:—0.05 Assay Ton of residues was mixed with 35 grammes of test-lead in a 3-in. scorifier; on this were placed 30 grammes of test-lead and a final cover of 10 grammes of borax-glass. The charges were placed in a very hot muffle, which was of the dimensions 12 in. by 6.25 in. by 4 in., and the door was closed for 5 minutes; the door was then opened and the heat lowered. No spitting took place. The charges were run so that they were just driving, but upon pouring it was found that the temperature had not been sufficiently high to decompose all of the charge. The results were therefore rejected. Three similar charges were then made, Nos. 27, 28 and 29, and placed in the muffle in the position shown in Fig. 6. The test was conducted as before, but at a higher temperature (780° C. by pyrometric measurement). The resultant lead buttons were too large for cupellation and they were rescorified, with the addition of 2 grammes of silica. The second lead button was cupelled, weighed, recupelled, with the addition of C.P. silver, and parted with three strengths of nitric acid, having specific gravities, respectively, of 1.16, 1.20 and 1.28. The results obtained are given in Table V.

By noting the position of these charges in the muffle, it is seen that the quantity of gold found in the slag increased with the increased temperature, i. e., it was greatest in the back of the muffle and least in the front.

Owing to the fact that these assays as conducted in a 3 or 3.5-in. scorifier yielded a button of lead which was too large to cupel, the subsequent assays were allowed to become covered over with the slag, which was then poured as much as possible; the scorifier was then replaced in the muffle and the scorification continued until the “lead-eye” was of a diameter of 0.5 in.; the content of the scorifier was then poured.

Charges Nos. 30, 31, 32 and 33 were of the same composition as Nos. 27, 28 and 29, and were placed in the muffle in the position shown in Fig. 7. The slags from these assays were ground, passed through a 40-mesh sieve and assayed. The buttons from two of the assays passed into the cupel and were lost; the other two were weighed and parted. The data given in Table V. show that more gold was recovered from the slag in tests Nos. 31 and 33, which were in the back of the muffle, than in Nos. 30 and 32, which were in the front. The temperature in the back of the muffle was 780° C., while in the front it was 720° C.

Charges Nos. 34 and 35 consisted of 0.05 Assay Ton of residues, mixed with 6 grammes of litharge in a 3-in. scorifier, on which were placed 40 grammes of test-lead, followed by a cover of 10 grammes of borax-glass. The charges were placed in a very hot muffle and the door was closed for 10 minutes, after which it was opened and the temperature allowed to fall to 780° C. As soon as the buttons had become covered, the slag was poured from them, but the buttons finally obtained were too large for cupellation and had to be rescorified. All slags and cupels were assayed as usual. Although charge No. 34 was fairly satisfactory, for some unaccountable reason the results for charge No. 35 were too low.

Charges Nos. 36 and 37 consisted of 0.05 Assay Ton of residues, mixed with 65 grammes of test-lead with a cover of 10 grammes of borax-glass, and charges Nos. 38 and 39 consisted of 0.05 Assay Ton of residues, with 10 grammes of litharge and 30 grammes of test-lead, having an additional quantity of 30 grammes of test-lead placed on top with a final cover of 10 grammes of borax-glass. Charges Nos. 36 and 38 were placed at the back of the muffle and Nos. 37 and 39 at the front. The temperature of the muffle was maintained as nearly as possible at 790° C. Owing to the door of the muffle having been opened too soon, charges Nos. 37 and 39 spit, the former quite badly. The slag from No. 38, which was at the back of the muffle, contained more gold than that from No. 39, which was in front, but the slag from charge No. 37, in the front, carried more gold than that of No. 36, which was at the back.

Considerable difficulty was encountered in parting the buttons when the ratio of silver to gold was 2.5 to 1, even after they were annealed and rolled thin. No difficulty resulted, however, when the ratio was 3.5 to 1. The data pertaining to these assays are given in Tables V. and VI.

Taking the best results, i.e., charges Nos. 27, 28, 29, 30, 31, 32, 33, 36 and 37, an average of 4,699.2 oz. gold per ton is obtained; the difference between the highest and the lowest results being 18.4 oz., which corresponds to 0.39 per cent. Averaging the results for silver from these assays, the figure of 4,209.7 oz. per ton is obtained; the difference between the highest and the lowest results being 29.6 oz. per ton, or 0.7 per cent.

In those charges in which litharge was used, Nos. 34, 35, 38 and 39 (omitting No. 39), the average determination of the gold was 4,697.3 oz. per ton, and of silver, 4,202.4 oz. per ton.

A comparison of the final average results obtained from the scorification-assays of the zinc-box residues, obtained by Mr. Hollis and Mr. Kehew, is given in Table VII.

Confirmatory Wet Assay

Mr. Kehew confirmed his results of the scorification-assay by the wet method of Mr. C. Whitehead, as follows:—0.05 Assay Ton of the residues was placed in a 250-c.c. casserole and 50 c.c. of water was poured over it, followed by 25 c.c. of strong nitric acid (sp. gr. 1.42). The casserole was then placed on a hot plate and allowed to stand for 2 hours. The residue after filtration should have consisted of gold and other insoluble material, but the button obtained by cupelling this residue contained silver, which necessitated a second cupellation, with the addition of C. P. silver. The slags and cupels were assayed, the final results being given in Table VIII.

The results for gold as shown in Table VIII. are not as high as those of many of the scorification-assays; the results for silver are quite a little lower, and confirm the results obtained by Mr. Hollis.

From the foregoing experiments, the following conclusions may be drawn.

1. That the zinc-box residues must be in a sufficiently fine state of division to pass at least through a 200-mesh screen.
2. That assay balances, or balances of equal delicacy, must be used for weighing the residues.
3. That the results obtained by the scorification-assay, when properly made, are as accurate for the determination of the gold as those of any other method tried, and more accurate for silver than the Whitehead wet method or the combination wet and dry method.
4. That the most satisfactory charge is 0.05 Assay Ton of residues, mixed with 35 grammes of test-lead in a 3- or 4-in. scorifier, with 30 grammes of test-lead placed on top and a final cover of 10 grammes of borax-glass.
5. That a large quantity of borax-glass is absolutely necessary.
6. That the spitting of the charge can be avoided by placing the scorifiers in a very hot muffle, keeping the muffle door closed for at least 5 minutes after the charge had become fused, and then opening the door and reducing the temperature to from 780° to 800° C.
7. That the addition of a small quantity of litharge to a charge seems to lessen the danger of spitting, but when so added the results for silver will probably be low.