Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/773

Rh GOLD is led from a suitable generator beneath the false bottom, and rises through the moistened ore, resting on a bed of broken quartz below the false bottom, converting the gold into a soluble chloride, which is afterwards removed by washing with water. The precious metal is then precipitated as metallic gold by sulphate of iron. The process has been greatly improved in America by Kiistel, Deetken, and Hoffmann; with proper care it is a very perfect one, and yields 97 per cent. of the gold originally present in the ore. It is stated not to cost more in California than 50s. a ton. Any silver origin- ally present in the ore is of course converted into chloride of silver and remains with the residue, from which it may be extracted by the solvent action of brine or by amalga- mation. I-.’qfin.in_y or Partin_«7 Gold from other 1l[etaIs.—Strabo states‘ that in his time a process was employed for refining and purifying gold in large quantities by cementing or burning it with an aluminous earth, which, by destroying the silver, left the gold in a state of purity. Pliny shows that for this purpose the gold was placed on the ﬁre in an earthen vessel with treble its weight of salt, and that it was afterwards again exposed to the ﬁre with two parts of salt and one of argillaceous rock, which, in the presence of moisture, effected the decomposition of the salt; by this means the silver became converted into chloride. In a similar process still practised in New Granada the granulated argentiferous gold is mixed with one part of common salt and two parts of brick dust. In the presence of moisture, effected by the passage of aqueous vapour through the porous pots in which the mixture is heated, the salt acts on the brick dust, producing silicate of soda, and the evolution of hydrochloric acid affords a source of chlorine for the silver. The chloride of silver formed fuses readily and drops off, exposing a fresh surface of the alloy to the action of the gas. Various methods for separating gold from silver or other alloys appear to have been in use from ancient times. Among these may be mentioned prolonged oxidation by exposure to air, and treatment with sulphur, sulphide of antimony, and corrosive sublimate. In the Harz, 2 ounces of the granulated alloy of gold and silver were mixed and heated with 1 ounce of sulphur, litharge being added to separate the gold remaining in the sulphide of silver. Purtz'n{/ by Nitric Acid, the old process of reﬁning, is now practised in England by only one ﬁrm, although in some refineries both the nitric acid and the sulphuric acid processes are combined, the alloy being ﬁist treated with nitric acid. It used to be called “ quartation,” froin the fact that 4 parts of the alloy best suited for the opera- tion of reﬁning contain 3 parts of silver and 1 of gold. The operation may be conducted in vessels of glass or platinum, and each pound of granulated metal is treated with a pound and a quarter of nitric acid of speciﬁc gravity 1'32. It is the method employed in the assay of gold (see .lssAY1NG). I343,/iizizzg by Snip/un-ic Acid is the process usually adopted for separating gold fro1n silver on the large scale. It appears to have been proposed in France by Dizé at the beginning of the present century. It was actually in use in France in 1820, and was introduced into the Mint reﬁnery, London, by Mr Mathison in 1829.‘-3 It is based upon the facts that concentrated hot sulphuric acid con- verts silver and copper into soluble sulphates without attacking the gold, the sulphate of silver being subsequently reduced to the metallic state by copper plates with the formation of sulphate of copper. About 80 lb of the granulated alloy are boiled for three 1 Fabbroni, Ann. Chz'm., t. lxxii. p. 25. 2 Report on the Royal Mint, 1837, Appendix, p. 59. 7-19 or four hours in a platinum vessel (ﬁg. 9) with 25 times its weight of sulphuric acid of speciﬁc gravity 1'84. The sulphurous acids which arise are partially condensed before being allowed to pass into the air. When the acid has Z a, T FIG. 9.—Reﬁnery Siphon and Alembic. ceased to act on the metal, a small quantity of sulphuric acid of speciﬁc gravity 1'53 is added, and, after a second boiling, the contents of the vessel are allowed to settle. The supernatant liquid is then withdrawn from the gold, which falls to the bottom of the vessel, and is diluted until its density is 1'21 or 126. The silver is usually precipitated from solution by copper plates, but sometimes iron is used, and the silver is roughly dried and compressed by an hydraulic press before it is melted into ingots. The gold, which is often again treated with sulphuric acid, is then washed and melted into ingots that contain from 997 to 998 parts of gold in 1000. The operation of parting may be conducted in iron or platinum vessels; the use of the former was advocated by M. Tocchi, and they are still extensively employed. Magniﬁcent vessels of platinum have, however, been made in England by Messrs Jolm- son, Matthey, 6: Co. The alloys best suited for the operation contain from 800 to 950 of silver and 50 to 200 of copper and gold, but the proportion of gold must not ex- ceed 200 parts in 1000. Reﬁners obtain alloys in suitable proportions by mixing together auriferous silver and argenti- ferous gold, the proportions of the respective metals having been previously indicated by assay. By such an arrange- ment, silver which contains but the 00004 part of gold, or 2'25 grains in the troy pound, may be proﬁtably treated. Cost of I3e;ﬁm'ng.——Tl1e charge to the public for reﬁning depends in a great measure on -the amount of metal to be operated upon and its richness. In England, however, it may be considered to be about 1d. per ounce for the silver and 4d. per ounce for gold. In France the charge is about 90 cents to 1 franc 25 cents for a kilogramme of silver. The Lower Harz smelting works produce annually from 50 to 55 cwts. of test silver of an average ﬁneness of 950 silver and 50 gold per 1000 ; the proportion of the latter metal is, however, variable, being lowest (3 per 1000) in the silver obtained from clean lead ores, and highest (10 per 1000) in that separated from argentiferous copper ores,—that from the mixed copper and lead ores being of intermediate richness. The silver, in quantities of 25 kilogrammes, is reﬁned upon small tests in a muﬂie, and when sufficiently puriﬁed is granulated by ladling it into water, whereby thin ﬂattened granules suitable for dissolv- ing are obtained. The parting vessels (ﬁg. 10) are of porcelain which, to protect them against fracture by irregular heating. are covered with wire netting and plastered over with a mixture of clay and smithy scales. They are mounted in a frame and set loose in an iron pot with a hemi- spherical bottom, which is heated by a ﬁre from below; the pot also serves to catch the contents of the porcelain vessel if the latter should be accidentally broken. The cover is perforated by a hole in the centre for the passage of a lead pipe to carry off the sulphurous acid fumes,