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Iron Vitriol,* FeO,SO, + 7HO, or Chloride of Iron, FeCl + 4HO.-The solutions of gold and iron vitriol are heated separately to 70° C., and the iron vitriol is gradually poured into the solution of gold. When iron vitriol ceases to react upon the solution, the liquid is decanted and the gold, separated in compact lumps, is washed.

Au2Cl2+6FeOSO,

=

2Au + FeCl3 + 2(Fe2O3,3SO3). According to Roset precipitated gold consists of very minute cubes; Morint states that iron vitriol precipitates completely o'0022 part of gold; but Elsner denies this. Levol§ considers this method of extraction to be imperfect, as chloride of silver remains dissolved together with the gold, even after boiling the solution, and the silver also is reduced by the iron salt.

This method, somewhat modified by Newton, is employed. in the American mints.¶

Oxalic Acid, HO,C,O,.-According to Levol this agent precipitates very pure gold, but the process is tedious, especially if the acid is in excess; the liquid is liable to spirt away, owing to the development of carbonic acid, wherefore large vessels must be employed

Au2Cl,+3HOC2O, = 2Au + зCIH + 6CO2.

Jackson states that under certain circumstances gold may be obtained in a spongy form.

Formic Acid, C2H2O, + HO, and Formiate of Potash, KO,C2H2O2, according to Morin,** precipitate gold imperfectly.

Chloride of Antimony, SbCl ̧, mixed with muriatic acid, according to Levol, reduces a somewhat warmed solution of gold after some hours. The gold is placed upon a filter, washed with considerably diluted muriatic acid, and melted in an earthen crucible, with an addition of borax and

* Bgwkfd., xiii., 187.

† Pogg., lxxiii., 8.

DINGL., Bd. 76, p. 38.

|| Ibid., 96, p. 490.

§ Ibid., Bd. 91, p. 232. Bgwkfd., vii, 460.

B.u. h. Ztg., 1852, p 880.

** Dingl., Bd. 110., p. 375. Erdm., J. f. pr. Ch., xlix., 118. B. u. h. Ztg, 1849, p. 125.

[blocks in formation]

saltpetre. 100 parts of gold require 200 parts of chloride of antimony.

3SbCl,+2Au,Cl2 = 4Au + 3SbCl,.

Chloride of Arsenic, AsCl,, produced by dissolving arsenious acid in muriatic acid. According to Levol and Duflos, it quickly and perfectly reduces gold from neutral or faintly alkaline solutions.

[blocks in formation]

Chloride of silver may be reduced to the state of metallic silver by melting it with carbonate of potash, or betteraccording to Gay Lussac-with freshly-burned lime. Levol and Casaseca recommend a solution of caustic alkali and sugar, or melting with saltpetre and carbon. Hornung employs copper and ammonia; Zimmermann, iron, potash, and common salt; Wittstein heats the chloride of silver with moistened coal dust; Kessler uses acetate of iron; Bolley and Brunner the galvanic current; Böttger decomposes chloride of silver by heating it with a solution of crystallised soda, I part dissolved in 3 parts of water, whilst adding some grape sugar, about equal in quantity to the chloride of silver under treatment; Müller employs caustic soda and glycerin.

III. SEPARATION OF GOLD FROM COPPER.

Alloys of gold and copper occur in mints, or they are produced in the form of auriferous and argentiferous black copper when working copper ores containing gold and silver. Gold cannot be perfectly separated from copper; for this purpose the following methods are chiefly employed :

I. Fusion with Lead.-The alloy is either fused with lead and liquated, or it is added direct to the cupelling operation.

If one part of auriferous copper is combined and cupelled with 16 parts of lead, the copper oxidises and enters the litharge, whilst the gold combines with the lead, and finally with the brightened silver. Although very simple,

*DINGL., Bd. 85, p. 77; Bd. 96, p. 175; Bd. 102, p. 320; Bd. 104, p. 42; Bd. 109, p. 373. BERZELIUS, Jahresber., 1845, p. 186. Polyt. Centr., 1849, p. 1196. Pogg., lxxxv., 462. Oesterr.. Ztschr., 1855, p. 216; 1859, p. 68.

this method is only advisable when treating copper rich in gold, by adding it in small quantities (not above 6 per cent) to the cupelling operation of rich lead. Copper poor in gold cannot be treated advantageously by this method, as it causes considerable loss of lead and copper.

The liquation process also cannot be advantageously applied to copper poor in gold, as it not only causes great loss of copper and lead, but also of gold; copper almost always retaining a larger proportion of gold.

2. Treatment with Sulphuric Acid.-Of late this method has been frequently applied to argentiferous and auriferous copper, as its yield of metal is better. The copper is dissolved in either boiling concentrated sulphuric acid or hot dilute sulphuric acid.

Rich and pure alloys of gold, silver, and copper, are dissolved in concentrated acid, thus yielding gold and silver in a pure state, and copper as copper vitriol.

Impure black copper, containing little gold and silver, but more lead, antimony, and arsenic, is best treated with hot dilute acid, thus yielding copper vitriol, and leaving gold and silver along with the impurities in the residue, from which they are then extracted.

3. Treatment with Chlorine according to Plattner's Method.-At Mr. Vivian's works, near Swansea, this method is employed for extracting the small amount of gold contained in Russian copper coins. Upon re-melting the coins the gold concentrates in the lower part of the liquid metal; this part is then granulated, and the gold extracted from it by means of chlorine.

Treatment of Auriferous and Argentiferous Black Copper by means of Boiling Concentrated Sulphuric Acid, as performed at Septèmes.*

Silver produced in smelting works, foreign coins, ornaments, &c., if containing more than o'6 per cent of gold, are treated direct with sulphuric acid. Substances poorer in gold are previously melted with copper, if there is great demand for copper vitriol, in order to produce greater

quantities of such vitriol as a by-product; otherwise the poorer substances are also dissolved direct in sulphuric acid.

a. Substances containing from 6 to 750 parts of gold in 1000 are melted for three hours in plumbago crucibles in quantities of 60 lbs., and granulated in a copper vessel. The granulated copper is then boiled in cast-iron pans in quantities of 30 or 34 lbs. with 2 or 3 times its weight of sulphuric acid of 50° B. It is stirred with an iron rod, and the firing is interrupted when the distillation of sulphuric acid ceases; this takes place in about two hours. The distilled sulphuric acid of 27° B. is collected in a condenser, and is from time to time concentrated to 50° B. in a leaden pan. The sulphurous acid formed is conducted into a chimney. The pans used for dissolving last, on an average, three months; the platinum pans formerly employed were more expensive, required purer acid, and only admitted of lower charges.

The contents of the pans, consisting of silver and copper vitriol, and very finely divided gold, are cooled in a leaden pan to 30° C., and then transferred to vessels filled with boiling water, where they are stirred till completely dissolved. The firing is now interrupted, the stirring continued for a short time, and the liquid (being of 17° B.) is allowed to clarify. The gold, some silver vitriol, and sulphate of lead having deposited in about an hour, the clear liquid is drawn off by means of a syphon into a vessel containing plates of argentiferous copper, to precipitate all the silver; this will take about seven hours. The remaining lixivium of copper vitriol, of 20°, is again used for dissolving silver vitriol, and thus becomes concentrated to 24° B. The copper vitriol it contains is crystallised out, and the remaining acid lixivium again used for dissolving. The precipitated silver is washed with hot water and pressed into the form of bricks by a hydraulic press; these bricks still contain 8 or 10 per cent of water, and, when fused, have a fineness of 997.

When 8 or 10 lbs. of gold have collected in the clarifying vessel, they are removed, dried, and rich auriferous substances of a fineness above 750 being added, they are melted with silver from one to three times the weight of the gold, granulated and boiled in sulphuric acid of 50° B. The silver

is dissolved, and the fine gold left behind is washed, dried, and melted.

b. Substances containing less than six parts of gold in 1000 are fused with so much copper that 1000 parts of the alloy produced will contain 700 parts of auriferous silver, this being the best proportion for separating gold and silver, and also yielding less impure deposits of gold in the clarifying vessels. If a larger production of copper vitriol is required, an alloy containing 600 parts of auriferous silver is produced; the resulting gold deposits are then more impure and retain more silver. 60 lbs. of the alloy are melted in three hours and granulated. Charges of about 42 lbs. are dissolved in iron pans in sulphuric acid of 66° B. The resulting impure deposits of gold are melted with borax and saltpetre, yielding ingots, which are cupelled and again treated with sulphuric acid.

The poorer the substances under treatment, the impurer and the richer in silver are the resulting gold deposits, and the more expensive is the refining of these deposits.

Sometimes if the production of copper vitriol is not specially intended, the poor substances are treated direct with sulphuric acid of 55° B., and the gold deposits are either cupelled or treated again with acid when they have accumulated in sufficient quantity; the gold is then separated by the quartation method.

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