This material may be taken as containing 96 to 98% copper, a few ounces of gold per ton (varies considerably with different ores), some hundreds of ounces of silver per ton, and small quantities of impurities.
Though the estimation of silver has so far not been considered, the student will find it convenient to estimate both silver and gold when conducting the assay.
Method adopted.– Many methods have been tried and are in use. Briefly, they come under one or other of the four following classes:—
1. ‘All-fire’ (or dry).
2. ‘All-fire’ (or dry) corrected.
3. Combination (wet and dry).
4. Combination (wet and dry) corrected.
1. The all-fire method is somewhat similar to the fire method given for mattes. Ten portions of 3 gms. each are scorified at a high temperature, and the resulting buttons are combined by rescorification till suitable for cupellation.
2. The all-fire corrected method is simply the all-fire method plus an estimation of the slag loss and cupel absorption loss.
3. The combination method will be described.
4. The corrected combination method is simply the last method, corrected as before described.
On reference to American literature, the student will find numerous methods given, more especially combination methods. For the present he may estimate the gold and silver by the combination method given, and under the head ‘Accuracy’ he will find a summary of the research work of some of the most celebrated American chemists.
In the method described the blister copper is dissolved in nitric acid, leaving most of the gold as a residue. NaCl or HCl is added to precipitate the silver. The gold and silver are separated from the copper by filtration, and then are estimated by the Fire method.
Apparatus.—Pulp scales and assay balances and weights, beakers and covers, etc., the muffle furnace and accessories.
Reagents.—The usual assay reagents, together with nitric acid and salt.
Details of the Operation.—Weigh out in duplicate 30 gms. of borings of blister copper, taking care that any fine particles are evenly distributed in the two lots (the finer portions generally differ in value from the long drillings).
Transfer each lot to a 750 c.cs. beaker. Add 100 c.cs. distilled water and 50 c.cs. HNO3 (S.G. 1.42), and cover. When the violent action ceases add 50 c.cs. more of HNO3 and warm till the copper is all dissolved (takes about one hour). Boil off part of the HNO3 (taking from 20 to 30 minutes). Remove the beakers and dilute with distilled water to 400 c.cs. Run in sufficient normal NaCl solution to precipitate all the silver. Avoid large excess (test). Either stir vigorously for 20 or 30 minutes (or use air jet), or allow the precipitate to settle all night. Filter through double filter papers 15 cm. diameter, of good quality. Wash the pp. well to the point of the filter. On top of the AgCl add 6 gms. test grain lead Transfer the moist papers and pp. with lead to 2½ inch scorifiers, on the bottoms of which is spread 2 gms. of test lead, Dry and burn the papers and contents in the muffle below a dull red (takes about 20 minutes). Add a few gms. litharge and 2 gms. borax glass, fuse for a few minutes and pour.
Cupel the buttons at a low temperature (feathers). Part and weigh as usual.
Note.—The gold may be separately estimated by filtration before adding NaCl.
Calculations.—Principles and method as before.
Accuracy of Methods.—Authorities give the following:—
“Ricketts and Miller.”—Fire method, good results for gold; low for silver. Combination method, low results for gold; about the same for silver.
“ Furman ”—referring to mattes—gives the loss of silver by scorification as 2% to 4%. Seems to recommend the Combination method for silver, but does not state its accuracy for gold.
“Ledoux” (1900).—Combination method gives low results for gold, and leads one to infer that the results for silver are good. With blister copper containing 96% to 98 % copper and 1 to 5 oz. gold per ton, the gold results are .1 to .3 oz. higher by the All-fire than by the Combination method. He considers Dr Godshall’s combination method, using H2S to precipitate silver, good for silver but low for gold.
“ Van Liew.”—Summarises “Ledoux’s” earlier work, in which carefully sampled blister copper was submitted for assay to many leading American chemists with the following results:—
Silver by Scorification.—9 chemists reported, with a variation of 6.06% on an average of 159.36 oz. per ton.
Silver by Combination method.—15 chemists reported 10 results, with a variation of 8.24% on an average of 154.9 oz. per ton.
Briefly, 19 chemists give 25 determinations, using these two methods, with a variation of 8.25%, or an average of 156.3 oz. per ton.
Silver by Corrected Scorification.—5 chemists reported results; average before correction 151.95 oz., after correction 158.96 oz. That is, uncorrected there is at least a loss of 4.43% silver.
Silver by Corrected Combination.—4 chemists reported results; average before correction 156.26 oz., after correction 160.30 oz. That is, uncorrected there is an average loss of at least 2.52%.
Gold.—20 chemists reported 26 determinations by the various methods, with results varying from .501 to .205 oz., averaging .307 oz., with an extreme variation of 96.4% of the average.
10 chemists reported by scorification, averaging .362.oz.
16 chemists reported by the Combination method, averaging .283 oz.
That is a loss of 21 .8% gold by the Combination method, assuming .362 oz. to represent 100%.
Briefly, the uncorrected Scorification method gives a loss of over 4.43% silver. The Combination method gives a loss of 3.0% silver, and 21.8% gold. Van Liew then gives a method (combination) giving an increase of 6.75% gold on the older Combination method (N. Y. Engineering and Mining Journal, April 1900). Yet, from Ledoux’s figures, quoted by Van Liew, there must still be a loss of about 17% as compared with the All-fire method.
The above results have been given in some detail, to show the student the necessity of ascertaining the degree of accuracy attained in such work, and with the hope that they may lead him to take part in the excellent work done in this direction by American chemists. Such research must eventually result in accuracy combined with speed.