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GOLD [symbol Au, atomic weight 195.7(H = 1), 197.2(O = 16)],a metallic chemical element, valued from the earliest ages onaccount of the permanency of its colour and lustre. Goldornaments of great variety and elaborate workmanship havebeen discovered on sites belonging to the earliest known civilizations,Minoan, Egyptian, Assyrian, Etruscan (see Jewelry,Plate, Egypt, Crete, Aegean Civilization, Numismatics),and in ancient literature gold is the universal symbol of thehighest purity and value (cf. passages in the Old Testament,e.g. Ps. xix. 10 “More to be desired are they than gold, yea, thanmuch fine gold”). With regard to the history of the metallurgyof gold, it may be mentioned that, according to Pliny, mercurywas employed in his time both as a means of separating theprecious metals and for the purposes of gilding. Vitruvius alsogives a detailed account of the means of recovering gold, byamalgamation, from cloth into which it had been woven.
Physical Properties.—Gold has a characteristic yellow colour,which is, however, notably affected by small quantities of othermetals; thus the tint is sensibly lowered by small quantitiesof silver, and heightened by copper. When the gold is finely divided, as in “purple of Cassius,” or when it is precipitatedfrom solutions, the colour is ruby-red, while in very thin leavesit transmits a greenish light. It is nearly as soft as lead andsofter than silver. When pure, it is the most malleable of allmetals (see Goldbeating). It is also extremely ductile; asingle grain may be drawn into a wire 500 ft. in length, and anounce of gold covering a silver wire is capable of being extendedmore than 1300 m. The presence of minute quantities ofcadmium, lead, bismuth, antimony, arsenic, tin, tellurium andzinc renders gold brittle, 12000th part of one of the three metalsfirst named being sufficient to produce that quality. Gold canbe readily welded cold; the finely divided metal, in the statein which it is precipitated from solution, may be compressedbetween dies into disks or medals. The specific gravity of goldobtained by precipitation from solution by ferrous sulphateis from 19·55 to 20·72. The specific gravity of cast gold variesfrom 18·29 to 19·37, and by compression between dies thespecific gravity may be raised from 19·37 to 19·41; by annealing,however, the previous density is to some extent recovered, asit is then found to be 19·40. The melting-point has beenvariously given, the early values ranging from 1425° C. to 1035° C.Using improved methods, C. T. Heycock and F. H. Nevilledetermined it to be 1061·7° C.; Daniel Berthelot gives 1064° C.,while Jaquerod and Perrot give 1066·1–1067·4° C. At stillhigher temperatures it volatilizes, forming a reddish vapour.Macquer and Lavoisier showed that when gold is strongly heated,fumes arise which gild a piece of silver held in them. Its volatilityhas also been studied by L. Eisner, and, in the presence ofother metals, by Napier and others. The volatility is barelyappreciable at 1075°; at 1250° it is four times as much as at1100°. Copper and zinc increase the volatility far more thanlead, while the greatest volatility is induced, according to T.Kirke Rose, by tellurium. It has also been shown that goldvolatilizes when a gold-amalgam is distilled. Gold is dissipatedby sending a powerful charge of electricity through it when in theform of leaf or thin wire. The electric conductivity is given byA. Matthiessen as 73 at 0° C., pure silver being 100; the valueof this coefficient depends greatly on the purity of the metal,the presence of a few thousandths of silver lowering it by 10%.Its conductivity for heat has been variously given as 103 (C. M.Despretz), 98 (F. Crace-Calvert and R. Johnson), and 60 (G. H.Wiedemann and R. Franz), pure silver being 100. Its specificheat is between 0·0298 (Dulong and Petit) and 0·03244 (Regnault).Its coefficient of expansion for each degree between0° and 100° C. is 0·000014661, or for gold which has beenannealed 0·000015136 (Laplace and Lavoisier). The sparkspectrum of gold has been mapped by A. Kirchhoff, R. Thalén,Sir William Huggins and H. Krüss; the brightest lines are 6277,5960, 5955 and 5836 in the orange and yellow, and 5230 and4792 in the green and blue.
Chemical Properties.—Gold is permanent in both dry andmoist air at ordinary or high temperatures. It is insoluble inhydrochloric, nitric and sulphuric acids, but dissolves in aquaregia—a mixture of hydrochloric and nitric acids—and whenvery finely divided in a heated mixture of strong sulphuricacid and a little nitric acid; dilution with water, however,precipitates the metal as a violet or brown powder from thissolution. The metal is soluble in solutions of chlorine, bromine,thiosulphates and cyanides; and also in solutions whichgenerate chlorine, such as mixtures of hydrochloric acid withnitric acid, chromic acid, antimonious acid, peroxides andnitrates, and of nitric acid with a chloride. Gold is also attackedwhen strong sulphuric acid is submitted to electrolysis with agold positive pole. W. Skey showed that in substances whichcontain small quantities of gold the precious metal may beremoved by the solvent action of iodine or bromine in water.Filter paper soaked with the clear, solution is burnt, and thepresence of gold is indicated by the purple colour of the ash. Insolution minute quantities of gold may be detected by theformation of “purple of Cassius,” a bluish-purple precipitatethrown down by a mixture of ferric and stannous chlorides.
The atomic weight of gold was first determined with accuracyby Berzelius, who deduced the value 195·7 (H＝1) from theamount of mercury necessary to precipitate it from the chloride,and 195·2 from the ratio between gold and potassium chloridein potassium aurichloride, KAuCl4. Later determinationswere made by Sir T. E. Thorpe and A. P. Laurie, Krüss andJ. W. Mallet. Thorpe and Laurie converted potassium auribromideinto a mixture of metallic gold and potassium bromideby careful heating. The relation of the gold to the potassiumbromide, as well as the amounts of silver and silver bromidewhich are equivalent to the potassium bromide, were determined.The mean value thus adduced was 195·86. Krüss worked withthe same salt, and obtained the value 195·65; while Mallet,by analyses of gold chloride and bromide, and potassium auribromide,obtained the value 195·77.
Occlusion of Gas by Gold.—T. Graham showed that gold iscapable of occluding by volume 0·48% of hydrogen, 0·20%of nitrogen, 0·29% of carbon monoxide, and 0·16% of carbondioxide. Varrentrapp pointed out that “cornets” from theassay of gold may retain gas if they are not strongly heated.
Occurrence and Distribution.—Gold is found in nature chieflyin the metallic state, i.e. as “native gold,” and less frequentlyin combination with tellurium, lead and silver. These are theonly certain examples of natural combinations of the metal,the minute, though economically valuable, quantity oftenfound in pyrites and other sulphides being probably only presentin mechanical suspension. The native metal crystallizes in thecubic system, the octahedron being the commonest form, butother and complex combinations have been observed. Owingto the softness of the metal, large crystals are rarely well defined,the points being commonly rounded. In the irregular crystallineaggregates branching and moss-like forms are most common,and in Transylvania thin plates or sheets with diagonal structuresare found. More characteristic, however, than the crystallizedare the irregular forms, which, when large, are known as “nuggets”or “pepites,” and when in pieces below 14 to 12 oz. weight as golddust, the larger sizes being distinguished as coarse or nuggetygold, and the smaller as gold dust proper. Except in the largernuggets, which may be more or less angular, or at times evenmasses of crystals, with or without associated quartz or otherrock, gold is generally found bean-shaped or in some otherflattened form, the smallest particles being scales of scarcelyappreciable thickness, which, from their small bulk as comparedwith their surface, subside very slowly when suspended in water,and are therefore readily carried away by a rapid current. Theseform the “float gold” of the miner. The physical properties ofnative gold are generally similar to that of the melted metal.
Of the minerals containing gold the most important are sylvanite orgraphic tellurium (Ag, Au) Te2, with 24 to 26%; calaverite, AuTe2,with 42%; nagyagite or foliate tellurium (Pb, Au)16 Sb3(S, Te)24,with 5 to 9% of gold; petzite, (Ag, Au)2Te, and white tellurium.These are confined to a few localities, the oldest and best knownbeing those of Nagyag and Offenbanya in Transylvania; they havealso been found at Red Cloud, Colorado, in Calaveras county, California,and at Perth and Boulder, West Australia. The mineralsof the second class, usually spoken of as “auriferous,” are comparativelynumerous. Prominent among these are galena and iron pyrites,the former being almost invariably gold-bearing. Iron pyrites,however, is of greater practical importance, being in some districtsexceedingly rich, and, next to the native metal, is the most prolificsource of gold. Magnetic pyrites, copper pyrites, zinc blende andarsenical pyrites are other and less important examples, the lastconstituting the gold ore formerly worked in Silesia. A native goldamalgam is found as a rarity in California, and bismuth fromSouth America is sometimes rich in gold. Native arsenic andantimony are also very frequently found to contain gold and silver.
The association and distribution of gold may be considered undertwo different heads, namely, as it occurs in mineral veins—“reefgold,” and in alluvial or other superficial deposits which are derivedfrom the waste of the former—“alluvial gold.” Four distincttypes of reef gold deposits may be distinguished: (1) Gold mayoccur disseminated through metalliferous veins, generally withsulphides and more particularly with pyrites. These deposits seemto be the primary sources of native gold. (2) More common are theauriferous quartz-reefs—veins or masses of quartz containing goldin flakes visible to the naked eye, or so finely divided as to be invisible.(3) The “banket” formation, which characterizes the goldfields ofSouth Africa, consists of a quartzite conglomerate throughoutwhich gold is very finely disseminated. (4) The siliceous sinter at Mount Morgan, Queensland, which is obviously associated withhydrothermal action, is also gold-bearing. The genesis of the lastthree types of deposit is generally assigned to the simultaneouspercolation of solutions of gold and silica, the auriferous solutionbeing formed during the disintegration of the gold-bearing metalliferousveins. But there is much uncertainty as to the mechanismof the process; some authors hold that the soluble chloride is firstformed, while others postulate the intervention of a soluble aurate.
In the alluvial deposits the associated minerals are chiefly thoseof great density and hardness, such as platinum, osmiridium andother metals of the platinum group, tinstone, chromic, magneticand brown iron ores, diamond, ruby and sapphire, zircon, topaz,garnet, &c. which represent the more durable original constituents ofthe rocks whose distintegration has furnished the detritus.
Statistics of Gold Production.—The supply of gold, and alsoits relation to the supply of silver, has, among civilized nations,always been of paramount importance in the economic questionsconcerning money (see Money and Bimetallism); in thisarticle a summary of the modern gold-producing areas will begiven, and for further details reference should be made to thearticles on the localities named. The chief sources of theEuropean supply during the middle ages were the mines ofSaxony and Austria, while Spain also contributed. The suppliesfrom Mexico and Brazil were important during the 16th and 17thcenturies. Russia became prominent in 1823, and for fourteenyears contributed the bulk of the supply. The United States(California) after 1848, and Australia after 1851, were responsiblefor enormous increases in the total production, which has beensubsequently enhanced by discoveries in Canada, South Africa,India, China and other countries.
| Period. | Oz. | Period. | Oz. |
| 1801–1810 | 590,750 | 1856–1860 | 6,350,180 |
| 1811–1820 | 380,300 | 1861–1865 | 5,951,770 |
| 1821–1830 | 472,400 | 1866–1870 | 6,169,660 |
| 1831–1840 | 674,200 | 1871–1875 | 5,487,400 |
| 1841–1850 | 1,819,600 | 1876–1880 | 5,729,300 |
| 1851–1855 | 6,350,180 | — | — |
The average annual world’s production for certain periodsfrom 1801 to 1880 in ounces is given in Table I. The averageproduction of the five years 1881–1885 was the smallest since theAustralian and Californian mines began to be worked in 1848–1849;the minimum 4,614,588 oz., occurred in 1882. It wasnot until after 1885 that the annual output of the world beganto expand. Of the total production in 1876, 5,016,488 oz.,almost the whole was derived from the United States, Australasiaand Russia. Since then the proportion furnished by thesecountries has been greatly lowered by the supplies from SouthAfrica, Canada, India and China. The increase of productionhas not been uniform, the greater part having occurred mostnotably since 1895. Among the regions not previously importantas gold-producers which now contribute to the annual output,the most remarkable are the goldfields of South Africa (Transvaaland Rhodesia, the former of which were discovered in 1885).India likewise has been added to the list, its active productionhaving begun at about the same time as that of South Africa.The average annual product of India for the period 1886 to 1899inclusive was £698,208, and its present annual product averagesabout 550,000 oz., or about £2,200,000, obtained almost whollyfrom the free-milling quartz veins of the Colar goldfields inMysore, southern India. In 1900 the output was valued at£1,891,804, in 1905 at £2,450,536, and in 1908 at £2,270,000.Canada, too, assumed an important rank, having contributedin 1900 £5,583,300; but the output has since steadily declinedto £1,973,000 in 1908. The great increase during the few yearspreceding 1899 was due to the development of the goldfieldsof the North-Western Territory, especially British Columbia.From the district of Yukon (Klondike, &c.) £2,800,000 wasobtained in 1899, wholly from alluvial workings, but the progressmade since has been slower than was expected by sanguinepeople. It is, however, probable that the North-WesternTerritory will continue to yield gold in important quantitiesfor some time to come.
The output of the United States increased from £7,050,000in 1881 to £16,085,567 in 1900, £17,916,000 in 1905, and to£20,065,000 in 1908. This increase was chiefly due to theexploitation of new goldfields. The fall in the price of silverstimulated the discovery and development of gold deposits,and many states formerly regarded as characteristically silverdistricts have become important as gold producers. Colorado isa case in point, its output having increased from about £600,000in 1880 to £6,065,000 in 1900; it was £5,139,800 in 1905. Somewhatmore than one-half of the Colorado gold is obtained fromthe Cripple Creek district. Other states also showed a largelyaugmented product. On the other hand, the output of California,which was producing over £3,000,000 per annum in 1876, hasfallen off, the average annual output from 1876 to 1900being £2,800,000; in 1905 the yield was £3,839,000. Thisdecrease was largely caused by the practical suspension formany years of the hydraulic mining operations, in preparationfor which millions of dollars had been expended in deep tunnels,flumes, &c., and the active continuance of which might have beenexpected to yield some £2,000,000 of gold annually. This interruption,due to the practical prohibition of the industry by theUnited States courts, on the ground that it was injuring, throughthe deposit of tailings, agricultural lands and navigable streams,was lessened, though not entirely removed, by compromises andregulations which permit, under certain restrictions, the renewedexploitation of the ancient river-beds by the hydraulic method.On the other hand, the progressive reduction of mining andmetallurgical costs effected by improved transportation andmachinery, and the use of high explosives, compressed air,electric-power transmission, &c., resulted in California (aselsewhere) in a notable revival of deep mining. This wasespecially the case on the “Mother Lode,” where highly promisingresults were obtained. Not only is vein-material formerlyregarded as unremunerative now extracted at a profit, but inmany instances increased gold-values have been encounteredbelow zones of relative barrenness, and operators have beenencouraged to make costly preparations for really deep mining—morethan 3000 ft. below the surface. The gold product ofCalifornia, therefore, may be fairly expected to maintain itself,and, indeed, to show an advance. Alaska appeared in the listof gold-producing countries in 1886, and gradually increased itsannual output until 1897, when the country attracted much attentionwith a production valued at over £500,000; the opening upof new workings has increased this figure immensely, from about£1,400,000 in 1901 to £3,006,500 in 1905. The Alaska goldwas derived almost wholly from the large low-grade quartz minesof Douglas Island prior to 1899, but in that year an importantdistrict was discovered at Cape Nome, on the north-westerncoast. The result of a few months’ working during that yearwas more than £500,000 of gold, and a very much larger annualoutput may reasonably be anticipated in the future; in 1905 itwas about £900,000. The gold occurs in alluvial depositsdesignated as gulch-, bar-, beach-, tundra- and bench-placers.The tundra is a coastal plain, swampy and covered with undergrowthand underlaid by gravel. The most interesting and, thusfar, the most productive are the beach deposits, similar to thoseon the coast of Northern California. These occur in a strip ofcomparatively fine gravel and sand, 150 yds. wide, extendingalong the shore. The gold is found in stratified layers, with“ruby” and black sand. The “ruby” sand consists chiefly offine garnets and magnetites, with a few rose-quartz grains.Further exploration of the interior will probably result in thediscovery of additional gold districts.
Mexico, from a gold production of £200,000 in 1891, advancedto about £1,881,800 in 1900 and to about £3,221,000 in 1905. Ofthis increase, a considerable part was derived from gold-quartzmining, though much was also obtained as a by-product in theworking of the ores of other metals. The product of Colombia,Venezuela, the Guianas, Brazil, Uruguay, Argentina, Chile,Bolivia, Peru and Ecuador amounted in 1900 to £2,481,000 andto £2,046,000 in 1905.
In 1876 Australasia produced £7,364,000, of which Victoriacontributed £3,084,000. The annual output of Victoria declined until the year 1892, when it began to increase rapidly, but not toits former level, the values for 1900 and 1905 being £3,142,000and £3,138,000. There has been an important increase inQueensland, which advanced from £1,696,000 in 1876 to£2,843,000 in 1900, and subsequently declined to £2,489,000in 1905. There has been no increase, and, indeed, no largefluctuation until quite recently in the output of New Zealand,which averaged £1,054,000 perannum from 1876 to 1898, butthe production of the two years1900 and 1905 rose to £1,425,459and £2,070,407 respectively. Byfar the most important additionto the Australasian product hascome from West Australia, whichbegan its production in 1887—aboutthe time of the inceptionof mining at Witwatersrand(“the Rand”) in SouthAfrica—and by continuous increase,which assumed largeproportions towards the close ofthe 19th century, was £6,426,000in 1899, £6,179,000 in 1900, and£8,212,000 in 1905. The totalAustralasian production in 1908was valued at £14,708,000.
Undoubtedly the greatest ofthe gold discoveries made in thelatter half of the 19th centurywas that of the Witwatersranddistrict in the Transvaal. Byreason of its unusual geologicalcharacter and great economicimportance this district deservesa more extended description. The gold occurs in conglomeratebeds, locally known as “banket.” There are several series ofparallel beds, interstratified with quartzite and schist, the mostimportant being the “main reef” series. The gold in this conglomeratereef is partly of detrital origin and partly of the geneticcharacter of ordinary vein-gold. The formation is noted for itsregularity as regards both the thickness and the gold-tenor ofthe ore-bearing reefs, in which respect it is unparalleled in thegeology of the auriferous formations. The gold carries, on anaverage, £2 per ton, and is worked by ordinary methods of gold-mining,stamp-milling and cyaniding. In 1899, 5762 stampswere in operation, crushing 7,331,446 tons of ore, and yielding£15,134,000, equivalent to 25.5% of the world’s production.Of this, 80% came from within 12 m. of Johannesburg. AfterSeptember 1899 operations were suspended, almost entirelyowing to the Boer War, but on the 2nd of May 1901 they werestarted again. In 1905 the yield was valued at £20,802,074,and in 1909 at £30,925,788. So certain is the ore-bearingformation that engineers in estimating its auriferous contentsfeel justified in assuming, as a factor in their calculations, avertical extension limited only by the lowest depths at whichmining is feasible. On such a basis they arrived at more than£600,000,000 as the available gold contained in the Witwatersrandconglomerates. This was a conservative estimate, and wasmade before the full extent of the reefs was known; in 1904Lionel Phillips stated that the main reef series had beenproved for 61 m., and he estimated the gold remaining to bemined to be worth £2,500,000,000. Deposits similar to theWitwatersrand banket occur in Zululand, and also on theGold Coast of Africa. In Rhodesia, the country lying northof the Transvaal, where gold occurs in well-defined quartz-veins,there is unquestionable evidence of extensive ancientworkings. The economic importance of the region generallyhas been fully proved. Rhodesia produced £386,148 in 1900and £722,656 in 1901, in spite of the South African War; theproduct for 1905 was valued at £1,480,449, and for 1908 at£2,526,000.
The gold production of Russia has been remarkably constant,averaging £4,899,262 per annum; the gold is derived chieflyfrom placer workings in Siberia.
The gold production of China was estimated for 1899 at£1,328,238 and for 1900 at £860,000; it increased in 1901 toabout £1,700,000, to fall to £340,000 in 1905; in 1906 and 1907it recovered to about £1,000,000.
| Year. | Australasia. | Africa. | Canada. | India. | Mexico. | Russia. | United States. | Totals. |
| 1881 | 1,475,161 | .. | 52,483 | .. | 41,545 | 1,181,853 | 1,678,612 | 4,976,980 |
| 1882 | 1,438,067 | .. | 52,000 | .. | 45,289 | 1,154,613 | 1,572,187 | 4,825,794 |
| 1883 | 1,333,849 | .. | 46,150 | .. | 46,229 | 1,132,219 | 1,451,250 | 4,614,588 |
| 1884 | 1,352,761 | .. | 46,000 | .. | 57,227 | 1,055,642 | 1,489,950 | 4,902,889 |
| 1885 | 1,309,804 | .. | 53,987 | .. | 46,941 | 1,225,738 | 1,538,325 | 5,002,584 |
| 1886 | 1,257,670 | .. | 66,061 | .. | 29,702 | 922,226 | 1,693,125 | 5,044,363 |
| 1887 | 1,290,202 | 28,754 | 59,884 | 15,403 | 39,861 | 971,656 | 1,596,375 | 5,061,490 |
| 1888 | 1,344,002 | 240,266 | 53,150 | 35,034 | 47,117 | 1,030,151 | 1,604,841 | 5,175,623 |
| 1889 | 1,540,607 | 366,023 | 62,658 | 78,649 | 33,862 | 1,154,076 | 1,587,000 | 5,611,245 |
| 1890 | 1,453,172 | 497,817 | 55,625 | 107,273 | 37,104 | 1,134,590 | 1,588,880 | 5,726,966 |
| 1891 | 1,518,690 | 729,268 | 45,022 | 131,776 | 48,375 | 1,168,764 | 1,604,840 | 6,287,591 |
| 1892 | 1,638,238 | 1,210,869 | 43,905 | 164,141 | 54,625 | 1,199,809 | 1,597,098 | 7,102,172 |
| 1893 | 1,711,892 | 1,478,477 | 44,853 | 207,152 | 63,144 | 1,345,224 | 1,739,323 | 7,772,585 |
| 1894 | 2,020,180 | 2,024,164 | 50,411 | 210,412 | 217,688 | 1,167,455 | 1,910,813 | 8,813,848 |
| 1895 | 2,170,505 | 2,277,640 | 92,440 | 257,830 | 290,250 | 1,397,767 | 2,254,760 | 9,814,505 |
| 1896 | 2,185,872 | 2,280,892 | 136,274 | 323,501 | 314,437 | 1,041,794 | 2,568,132 | 9,950,861 |
| 1897 | 2,547,704 | 2,832,776 | 294,582 | 350,585 | 362,812 | 1,124,511 | 2,774,935 | 11,420,068 |
| 1898 | 3,137,644 | 3,876,216 | 669,445 | 376,431 | 411,187 | 1,231,791 | 3,118,398 | 13,877,806 |
| 1899 | 3,837,181 | 3,532,488 | 1,031,563 | 418,869 | 411,187 | 1,072,333 | 3,437,210 | 14,837,775 |
| 1900 | 3,555,506 | 419,503 | 1,348,720 | 456,444 | 435,375 | 974,537 | 3,829,897 | 12,315,135 |
| 1901 | 3,719,080 | 439,704 | 1,167,216 | 454,527 | 497,527 | 1,105,412 | 3,805,500 | 12,698,089 |
| 1902 | 3,946,374 | 1,887,773 | 1,003,355 | 463,824 | 491,156 | 1,090,053 | 3,870,000 | 14,313,660 |
| 1903 | 4,315,538 | 3,289,409 | 911,118 | 552,873 | 516,524 | 1,191,582 | 3,560,000 | 15,852,620 |
| 1904 | 4,245,744 | 4,156,084 | 793,350 | 556,097 | 609,781 | 1,199,857 | 3,892,480 | 16,790,351 |
| 1905 | 4,159,220 | 5,477,841 | 700,863 | 576,889 | 779,181 | 1,063,883 | 4,265,742 | 18,360,945 |
| 1906 | 3,984,538 | 6,449,749 | 581,709 | 525,527 | 896,615 | 1,087,056 | 4,565,333 | 19,620,272 |
| 1907 | 3,659,693 | 7,270,464 | 399,844 | 495,965 | 903,672 | 1,282,635 | 4,374,827 | 19,988,144 |
| 1908 | 3,557,705 | 7,983,348 | 462,467 | 504,309 | 1,182,445 | 1,497,076 | 4,659,360 | 21,529,300 |
Alloys.—Gold forms alloys with most metals, and of these manyare of great importance in the arts. The alloy with mercury—goldamalgam—is so readily formed that mercury is one of the mostpowerful agents for extracting the precious metal. With 10% ofgold present the amalgam is fluid, and with 12.5% pasty, while with13% it consists of yellowish-white crystals. Gold readily alloys withsilver and copper to form substances in use from remote times formoney, jewelry and plate. Other metals which find application inthe metallurgy of gold by virtue of their property of extracting thegold as an alloy are lead, which combines very readily when molten,and which can afterwards be separated by cupellation, and copper,which is separated from the gold by solution in acids or by electrolysis;molten lead also extracts gold from the copper-gold alloys.The relative amount of gold in an alloy is expressed in two ways:(1) as “fineness,” i.e. the amount of gold in 1000 parts of alloy;(2) as “carats,” i.e. the amount of gold in 24 parts of alloy. Thus,pure gold is 1000 “fine” or 24 carat. In England the followingstandards are used for plate and jewelry: 375, 500, 625, 750 and916.6, corresponding to 9, 12, 15, 18 and 22 carats, the alloyingmetals being silver and copper in varying proportions. In Francethree alloys of the following standards are used for jewelry, 920,840 and 750. A greenish alloy used by goldsmiths contains 70% ofsilver and 30% of gold. “Blue gold” is stated to contain 75%of gold and 25% of iron. The Japanese use for ornament an alloyof gold and silver, the standard of which varies from 350 to 500,the colour of the precious metal being developed by “pickling” ina mixture of plum-juice, vinegar and copper sulphate. They maybe said to possess a series of bronzes, in which gold and silver replacetin and zinc, all these alloys being characterized by patina havinga wonderful range of tint. The common alloy, Shi-ya-ku-Do, contains70% of copper and 30% of gold; when exposed to air itbecomes coated with a fine black patina, and is much used in Japanfor sword ornaments. Gold wire may be drawn of any quality, but itis usual to add 5 to 9 dwts. of copper to the pound. The “solders”used for red gold contain 1 part of copper and 5 of gold; for lightgold, 1 part of copper, 1 of silver and 4 of gold.
Gold and Silver.—Electrum is a natural alloy of gold and silver.Matthiessen observed that the density of alloys, the composition ofwhich varies from AuAg6 to Au6Ag, is greater than that calculatedfrom the densities of the constituent metals. These alloys areharder, more fusible and more sonorous than pure gold. The alloysof the formulae AuAg, AuAg2, AuAg4 and AuAg20 are perfectlyhomogeneous, and have been studied by Levol. Molten alloys containingmore than 80% of silver deposit on cooling the alloy AuAg9,little gold remaining in the mother liquor.
Gold and Zinc.—When present in small quantities zinc renders gold brittle, but it may be added to gold in larger quantities withoutdestroying the ductility of the precious metal; Péligot proved that atriple alloy of gold, copper and zinc, which contains 5.8% of the last-named,is perfectly ductile. The alloy of 11 parts gold and 1 part ofzinc is, however, stated to be brittle.
Gold and Tin.—Alchorne showed that gold alloyed with 137th partof tin is sufficiently ductile to be rolled and stamped into coin, providedthe metal is not annealed at a high temperature. The alloysof tin and gold are hard and brittle, and the combination of the metalsis attended with contraction; thus the alloy SnAu has a density14.243, instead of 14.828 indicated by calculation. Matthiessen andBose obtained large crystals of the alloy Au2Sn5, having the colourof tin, which changed to a bronze tint by oxidation.
Gold and Iron.—Hatchett found that the alloy of 11 parts goldand 1 part of iron is easily rolled without annealing. In these proportionsthe density of the alloy is less than the mean of its constituentmetals.
Gold and Palladium.—These metals are stated to alloy in all proportions.According to Chenevix, the alloy composed of equal partsof the two metals is grey, is less ductile than its constituent metalsand has the specific gravity 11.08. The alloy of 4 parts of gold and 1part of palladium is white, hard and ductile. Graham showed that awire of palladium alloyed with from 24 to 25 parts of gold does notexhibit the remarkable retraction which, in pure palladium, attendsits loss of occluded hydrogen.
Gold and Platinum.—Clarke states that the alloy of equal partsof the two metals is ductile, and has almost the colour of gold.
Gold and Rhodium.—Gold alloyed with 14th or 15th of rhodium is,according to Wollaston, very ductile, infusible and of the colour of gold.
Gold and Iridium.—Small quantities of iridium do not destroy theductility of gold, but this is probably because the metal is only disseminatedthrough the mass, and not alloyed, as it falls to the bottomof the crucible in which the gold is fused.
Gold and Nickel.—Eleven parts of gold and 1 of nickel yield analloy resembling brass.
Gold and Cobalt.—Eleven parts of gold and 1 of cobalt form abrittle alloy of a dull yellow colour.
Compounds.—Aurous oxide, Au2O, is obtained by cautiouslyadding potash to a solution of aurous bromide, or by boilingmixed solutions of auric chloride and mercurous nitrate. It formsa dark-violet precipitate which dries to a greyish-violet powder.When freshly prepared it dissolves in cold water to form an indigo-colouredsolution with a brownish fluorescence of colloidal aurousoxide; it is insoluble in hot water. This oxide is slightly basic.Auric oxide, Au2O3, is a brown powder, decomposed into its elementswhen heated to about 250° or on exposure to light. When a concentratedsolution of auric chloride is treated with caustic potash,a brown precipitate of auric hydrate, Au(OH)3, is obtained, which,on heating, loses water to form auryl hydrate, AuO(OH), andauric oxide, Au2O3. It functions chiefly as an acidic oxide, beingless basic than aluminium oxide, and forming no stable oxy-salts.It dissolves in alkalis to form well-defined crystalline salts; potassiumaurate, KAuO2·3H2O, is very soluble in water, and is used in electro-gilding.With concentrated ammonia auric oxide forms a black,highly explosive compound of the composition AuN2H3·3H2O,named “fulminating gold”; this substance is generally consideredto be Au(NH2)NH·3H2O, but it may be an ammine of the formula[Au(NH3)2(OH)2]OH. Other oxides, e.g. Au2O2, have been described.
Aurous chloride, AuCl, is obtained as a lemon-yellow, amorphouspowder, insoluble in water, by heating auric chloride to 185°. Itbegins to decompose into gold and chlorine at 185°, the decompositionbeing complete at 230°; water decomposes it into gold and auricchloride. Auric chloride, or gold trichloride, AuCl3, is a dark ruby-redor reddish-brown, crystalline, deliquescent powder obtained bydissolving the metal in aqua regia. It is also obtained by carefullyevaporating a solution of the metal in chlorine water. The goldchloride of commerce, which is used in photography, is really ahydrochloride, chlorauric or aurichloric acid, HAuCl4·3H2O, andis obtained in long yellow needles by crystallizing the acid solution.Corresponding to this acid, a series of salts, named chloraurates oraurichlorides, are known. The potassium salt is obtained by crystallizingequivalent quantities of potassium and auric chlorides.Light-yellow monoclinic needles of 2KAuCl4·H2O are deposited fromwarm, strongly acid solutions, and transparent rhombic tables ofKAuCl4·2H2O from neutral solutions. By crystallizing an aqueoussolution, red crystals of AuCl3·2H2O are obtained. Auric chloridecombines with the hydrochlorides of many organic bases—amines,alkaloids, &c.—to form characteristic compounds. Gold dichloride,probably Au2Cl4, = Au·AuCl4, aurous chloraurate, is said to beobtained as a dark-red mass by heating finely divided gold to 140°-170°in chlorine. Water decomposes it into gold and auric chloride.The bromides and iodides resemble the chlorides. Aurous bromide,AuBr, is a yellowish-green powder obtained by heating the tribromideto 140°; auric bromide, AuBr3, forms reddish-black orscarlet-red leafy crystals, which dissolve in water to form a reddish-brownsolution, and combines with bromides to form bromaurates correspondingto the chloraurates. Aurous iodide, AuI, is a light-yellow,sparingly soluble powder obtained, together with free iodine, byadding potassium iodide to auric chloride; auric iodide, AuI3,is formed as a dark-green powder at the same time, but it readilydecomposes to aurous iodide and iodine. Aurous iodide is alsoobtained as a green solid by acting upon gold with iodine. Theiodaurates correspond to the chlor- and bromaurates; the potassiumsalt, KAuI4, forms highly lustrous, intensely black, four-sided prisms.
Aurous cyanide, AuCN, forms yellow, microscopic, hexagonaltables, insoluble in water, and is obtained by the addition of hydrochloricacid to a solution of potassium aurocyanide, KAu(CN)2.This salt is prepared by precipitating a solution of gold in aqua regiaby ammonia, and then introducing the well-washed precipitate intoa boiling solution of potassium cyanide. The solution is filteredand allowed to cool, when colourless rhombic pyramids of theaurocyanide separate. It is also obtained in the action of potassiumcyanide on gold in the presence of air, a reaction utilized in theMacArthur-Forrest process of gold extraction (see below). Auriccyanide, Au(CN)3, is not certainly known; its double salts, however,have been frequently described. Potassium auricyanide,2KAu(CN)4·3H2O, is obtained as large, colourless, efflorescenttablets by crystallizing concentrated solutions of auric chlorideand potassium cyanide. The acid, auricyanic acid, 2HAu(CN)4·3H2O,is obtained by treating the silver salt (obtained by precipitatingthe potassium salt with silver nitrate) with hydrochloric acid; itforms tabular crystals, readily soluble in water, alcohol and ether.
Gold forms three sulphides corresponding to the oxides; theyreadily decompose on heating. Aurous sulphide, Au2S, is a brownish-blackpowder formed by passing sulphuretted hydrogen into asolution of potassium aurocyanide and then acidifying. Sodiumaurosulphide, NaAuS·4H2O, is prepared by fusing gold with sodiumsulphide and sulphur, the melt being extracted with water, filteredin an atmosphere of nitrogen, and evaporated in a vacuum oversulphuric acid. It forms colourless, monoclinic prisms, which turnbrown on exposure to air. This method of bringing gold intosolution is mentioned by Stahl in his Observationes Chymico-Physico-Medicae;he there remarks that Moses probably destroyedthe golden calf by burning it with sulphur and alkali (Ex. xxxii. 20).Auric sulphide, Au2S3, is an amorphous powder formed when lithiumaurichloride is treated with dry sulphuretted hydrogen at −10°.It is very unstable, decomposing into gold and sulphur at 200°.
Oxy-salts of gold are almost unknown, but the sulphite and thiosulphateform double salts. Thus by adding acid sodium sulphiteto, or by passing sulphur dioxide at 50° into, a solution of sodiumaurate, the salt, 3Na2SO3·Au2SO3·3H2O is obtained, which, whenprecipitated from its aqueous solution by alcohol, forms a purplepowder, appearing yellow or green by reflected light. Sodiumaurothiosulphate, 3Na2S2O3·Au2S2O3·4H2O, forms colourless needles;it is obtained in the direct action of sodium thiosulphate on gold in thepresence of an oxidizing agent, or by the addition of a dilute solutionof auric chloride to a sodium thiosulphate solution.
Mining and Metallurgy.
The various deposits of gold may be divided into two classes—“veins”and “placers.” The vein mining of gold does notgreatly differ from that of similar deposits of metals (see Mineral Deposits).In the placer or alluvial deposits, the precious metalis found usually in a water-worn condition imbedded in earthymatter, and the method of working all such deposits is based onthe disintegration of the earthy matter by the action of a streamof water, which washes away the lighter portions and leaves thedenser gold. In alluvial deposits the richest ground is usuallyfound in contact with the “bed rock”; and, when the overlyingcover of gravel is very thick, or, as sometimes happens, when theolder gravel is covered with a flow of basalt, regular mining byshafts and levels, as in what are known as tunnel-claims, may berequired to reach the auriferous ground.
The extraction of gold may be effected by several methods;we may distinguish the following leading types:
1. By simple washing, i.e. dressing auriferous sands, gravels, &c.;
2. By amalgamation, i.e. forming a gold amalgam, afterwardsremoving the mercury by distillation;
3. By chlorination, i.e. forming the soluble gold chloride andthen precipitating the metal;
4. By the cyanide process, i.e. dissolving the gold in potassiumcyanide solution, and then precipitating the metal;
5. Electrolytically, generally applied to the solutions obtainedin processes (3) and (4).
1. Extraction of Gold by Washing.—In the early days of gold-washingin California and Australia, when rich alluvial depositswere common at the surface, the most simple appliances sufficed.The most characteristic is the “pan,” a circular dish of sheet-ironor “tin,” with sloping sides about 13 or 14 in. in diameter.The pan, about two-thirds filled with the “pay dirt” to be washed,is held in the stream or in a hole filled with water. The largerstones having been removed by hand, gyratory motion is givento the pan by a combination of shaking and twisting movements so as to keep its contents suspended in the stream of water, whichcarries away the bulk of the lighter material, leaving the heavyminerals, together with any gold which may have been present. Thewashing is repeated until enough of the enriched sand is collected,when the gold is finally recovered by careful washing or “panningout” in a smaller pan. In Mexico and South America, instead of thepan, a wooden dish or trough, known as “batea,” is used.
The “cradle” is a simple appliance for treating somewhat largerquantities, and consists essentially of a box, mounted on rockers,and provided with a perforated bottom of sheet iron in which the“pay dirt” is placed. Water is poured on the dirt, and the rockingmotion imparted to the cradle causes the finer particles to pass throughthe perforated bottom on to a canvas screen, and thence to the baseof the cradle, where the auriferous particles accumulate on transversebars of wood, called “riffles.”
The “tom” is a sort of cradle with an extended sluice placed onan incline of about 1 in 12. The upper end contains a perforatedriddle plate which is placed directly over the riffle box, and undercertain circumstances mercury may be placed behind the riffles.Copper plates amalgamated with mercury are also used when thegold is very fine, and in some instances amalgamated silver coins havebeen used for the same purpose. Sometimes the stuff is disintegratedwith water in a “puddling machine,” which was used, especially inAustralia, when the earthy matters are tenacious and water scarce.The machine frequently resembles a brickmaker’s wash-mill, and isworked by horse or steam power.
In workings on a larger scale, where the supply of water is abundant,as in California, sluices were generally employed. They are shallowtroughs about 12 ft. long, about 16 to 20 in. wide and 1 ft. in depth.The troughs taper slightly so that they can be joined in series, thetotal length often reaching several hundred feet. The incline of thesluice varies with the conformation of the ground and the tenacity ofthe stuff to be washed, from 1 in 16 to 1 in 8. A rectangular troughof boards, whose dimensions depend chiefly on the size of the planksavailable, is set up on the higher part of the ground at one side of theclaim to be worked, upon trestles or piers of rough stone-work, at suchan inclination that the stream may carry off all but the largest stones,which are kept back by a grating of boards about 2 in. apart. Thegravel is dug by hand and thrown in at the upper end, the stoneskept back being removed at intervals by two men with four-prongedsteel forks. The floor of the sluice is laid with riffles made of stripsof wood 2 in. square laid parallel to the direction of the current, andat other points with boards having transverse notches filled withmercury. These were known originally as Hungarian riffles.
In larger plant the upper ends of the sluices are often cut in rockor lined with stone blocks, the grating stopping the larger stonesbeing known as a “grizzly.” In order to save very fine and especiallyrusty particles of gold, so-called “under-current sluices” are used;these are shallow wooden tanks, 50 sq. yds. and upwards in area,which are placed somewhat below the main sluice, and communicatewith it above and below, the entry being protected by a grating sothat only the finer material is admitted. These are paved with stoneblocks or lined with mercury riffles, so that from the greatly reducedvelocity of flow, due to the sudden increase of surface, the finerparticles of gold may collect. In order to save finely divided gold,amalgamated copper plates are sometimes placed in a nearly levelposition, at a considerable distance from the head of the sluice, thegold which is retained in it being removed from time to time. Sluicesare often made double, and they are usually cleaned up—that is,the deposit rich in gold is removed from them—once a week.
The “pan” is now only used by prospectors, while the “cradle”and “tom” are practically confined to the Chinese; the sluice isconsidered to be the best contrivance for washing gold gravels.
2. The Amalgamation Process.—This method is employed toextract gold from both alluvial and reef deposits: in the firstcase it is combined with “hydraulic mining,” i.e. disintegratingauriferous gravels by powerful jets of water, and the sluicesystem described above; in the second case the vein stuff isprepared by crushing and the amalgamation is carried out inmills.
Hydraulic mining has for the most part been confined to the countryof its invention, California, and the western territories of America,where the conditions favourable for its use are more fully developedthan elsewhere—notably the presence of thick banks of gravel thatcannot be utilized by other methods, and abundance of water, eventhough considerable work may be required at times to make it available.The general conditions to be observed in such workingsmay be briefly stated as follows: (1) The whole of the auriferousgravel, down to the “bed rock,” must be removed,—that is, noselection of rich or poor parts is possible; (2) this must be accomplishedby the aid of water alone, or at times by water supplementedby blasting; (3) the conglomerate must be mechanically disintegratedwithout interrupting the whole system; (4) the gold must be savedwithout interrupting the continuous flow of water; and (5) arrangementsmust be made for disposing of the vast masses of impoverishedgravel.
The water is brought from a ditch on the high ground, and througha line of pipes to the distributing box, whence the branch pipessupplying the jets diverge. The stream issues through a nozzle,termed a “monitor” or “giant,” which is fitted with a ball andsocket joint, so that the direction of the jet may be varied throughconsiderable angles by simply moving a handle. The material ofthe bank being loosened by blasting and the cutting action of thewater, crumbles into holes, and the superincumbent mass, oftenwith large trees and stones, falls into the lower ground. Thestream, laden with stones and gravel, passes into the sluices, wherethe gold is recovered in the manner already described. Under themost advantageous conditions the loss of gold may be estimated at15 or 20%, the amount recovered representing a value of abouttwo shillings per ton of gravel treated. The loss of mercury isabout the same, from 5 to 6 cwt. being in constant use per mile ofsluice.
In working auriferous river-beds, dredges have been used withconsiderable success in certain parts of New Zealand and on thePacific slope in America. The dredges used in California are almostexclusively of the endless-chain bucket or steam-shovel pattern.Some dredges have a capacity under favourable conditions of over2000 cub. yds. of gravel daily. The gravel is excavated as in theordinary form of endless-chain bucket dredge and dumped on to thedeck of the dredge. It then passes through screens and grizzliesto retain the coarse gravel, the finer material passing on to sluiceboxes provided with riffles, supplied with mercury. There arebelt conveyers for discharging the gravel and tailings at the end of thevessel remote from the buckets. The water necessary to the processis pumped from the river; as much as 2000 gallons per minute isused on the larger dredges.
The dressing or mechanical preparation of vein stuff containing goldis generally similar to that of other ores (see Ore-dressing), exceptthat the precious metal should be removed from the waste substancesas quickly as possible, even although other minerals of value that aresubsequently recovered may be present. In all cases the quartzor other vein stuff must be reduced to a very fine powder as a preliminaryto further operations. This may be done in several ways,e.g. either (1) by the Mexican crusher or arrastra, in which the grindingis effected upon a bed of stone, over which heavy blocks of stoneattached to cross arms are dragged by the rotation of the arms abouta central spindle, or (2) by the Chilean mill or trapiche, also knownas the edge-runner, where the grinding stones roll upon the floor,at the same time turning about a central upright—contrivanceswhich are mainly used for the preparation of silver ores; butby far the largest proportion of the gold quartz of California,Australia and Africa is reduced by (3) the stamp mill, which is similarin principle to that used in Europe for the preparation of tin and otherores.
The stamp mill was first used in California, and its use has sincespread over the whole world. In the mills of the Californian type thestamp is a cylindrical iron pestle faced with a chilled cast iron shoe,removable so that it can be renewed when necessary, attached toa round iron rod or lifter, the whole weighing from 600 to 900 ℔;stamps weighing 1320 ℔ are in use in the Transvaal. The lift iseffected by cams acting on the under surface of tappets, and formedby cylindrical boxes keyed on to the stems of the lifter about one-fourthof their length from the top. As, however, the cams, unlikethose of European stamp mills, are placed to one side of the stamp, thelatter is not only lifted but turned partly round on its own axis, wherebythe shoes are worn down uniformly. The height of lift may bebetween 4 and 18 in., and the number of blows from 30 to over 100per minute. The stamps are usually arranged in batteries of five;the order of working is usually 1, 4, 2, 5, 3, but other arrangements,e.g. 1, 3, 5, 2, 4, and 1, 5, 2, 4, 3, are common. The stuff, previouslybroken to about 2-in. lumps in a rock-breaker, is fed in through anaperture at the back of the “battery box,” a constant supply ofwater is admitted from above, and mercury in a finely divided stateis added at frequent intervals. The discharge of the comminutedmaterial takes place through an aperture, which is covered by athin steel plate perforated with numerous slits about 150th in. broadand 12 in. long, a certain volume being discharged at every blowand carried forward by the flushing water over an apron or tablein front, covered by copper plates filled with mercury. Similarplates are often used to catch any particles of gold that may be thrownback, while the main operation is so conducted that the bulk of thegold may be reduced to the state of amalgam by bringing the twometals into intimate contact under the stamp head, and remain in thebattery. The tables in front are laid at an incline of about 8° and areabout 13 ft. long; they collect from 10 to 15% of the whole gold;a further quantity is recovered by leading the sands through a gutterabout 16 in. broad and 120 ft. long, also lined with amalgamatedcopper plates, after the pyritic and other heavy minerals have beenseparated by depositing in catch pits and other similar contrivances.
When the ore does not contain any considerable amount of free goldmercury is not, as a rule, used during the crushing, but the amalgamationis carried out in a separate plant. Contrivances of the mostdiverse constructions have been employed. The most primitive isthe rubbing together of the concentrated crushings with mercury iniron mortars. Barrel amalgamation, i.e. mixing the crushingswith mercury in rotating barrels, is rarely used, the process beingwasteful, since the mercury is specially apt to be “floured” (seebelow).
At Schemnitz, Kerpenyes, Kreuzberg and other localities inHungary, quartz vein stuff containing a little gold, partly free andpartly associated with pyrites and galena, is, after stamping in mills,similar to those described above, but without rotating stamps,passed through the so-called “Hungarian gold mill” or “quick-mill.”This consists of a cast-iron pan having a shallow cylindrical bottomholding mercury, in which a wooden muller, nearly of the sameshape as the inside of the pan, and armed below with several projectingblades, is made to revolve by gearing wheels. The stufffrom the stamps is conveyed to the middle of the muller, and isdistributed over the mercury, when the gold subsides, while thequartz and lighter materials are guided by the blades to the circumferenceand are discharged, usually into a second similar mill,and subsequently pass over blanket tables, i.e. boards coveredwith canvas or sacking, the gold and heavier particles becoming entangledin the fibres. The action of this mill is really more nearlyanalogous to that of a centrifugal pump, as no grinding action takesplace in it. The amalgam is cleaned out periodically—fortnightly ormonthly—and after filtering through linen bags to remove the excessof mercury, it is transferred to retorts for distillation (see below).
Many other forms of pan-amalgamators have been devised. TheLaszlo is an improved Hungarian mill, while the Piccard is of thesame type. In the Knox and Boss mills, which are also employedfor the amalgamation of silver ores, the grinding is effected betweenflat horizontal surfaces instead of conical or curved surfaces as in thepreviously described forms.
One of the greatest difficulties in the treatment of gold by amalgamation,and more particularly in the treatment of pyrites, arises fromthe so-called “sickening” or “flouring” of the mercury; that is, theparticles, losing their bright metallic surfaces, are no longer capableof coalescing with or taking up other metals. Of the numerousremedies proposed the most efficacious is perhaps sodium amalgam.It appears that amalgamation is often impeded by the tarnishfound on the surface of the gold when it is associated with sulphur,arsenic, bismuth, antimony or tellurium. Henry Wurtz in America(1864) and Sir William Crookes in England (1865) made independentlythe discovery that, by the addition of a small quantity of sodium tothe mercury, the operation is much facilitated. It is also stated thatsodium prevents both the “sickening” and the “flouring” of themercury which is produced by certain associated minerals. Theaddition of potassium cyanide has been suggested to assist theamalgamation and to prevent “flouring,” but Skey has shown thatits use is attended with loss of gold.
Separation of Gold from the Amalgam.—The amalgam is firstpressed in wetted canvas or buckskin in order to remove excess ofmercury. Lumps of the solid amalgam, about 2 in. in diameter,are introduced into an iron vessel provided with an iron tube thatleads into a condenser containing water. The distillation is theneffected by heating to dull redness. The amalgam yields about30 to 40% of gold. Horizontal cylindrical retorts, holding from200 to 1200 ℔ of amalgam, are used in the larger Californian mills,pot retorts being used in the smaller mills. The bullion left in theretorts is then melted in black-lead crucibles, with the addition ofsmall quantities of suitable fluxes, e.g. nitre, sodium carbonate, &c.
The extraction of gold from auriferous minerals by fusion, except asan incident in their treatment for other metals, is very rarely practised.It was at one time proposed to treat the concentrated black ironobtained in the Ural gold washings, which consists chiefly of magnetite,as an iron ore, by smelting it with charcoal for auriferous pig-iron,the latter metal possessing the property of dissolving gold inconsiderable quantity. By subsequent treatment with sulphuric acidthe gold could be recovered. Experiments on this point were madeby Anossow in 1835, but they have never been followed in practice.
Gold in galena or other lead ores is invariably recovered in therefining or treatment of the lead and silver obtained. Pyritic orescontaining copper are treated by methods analogous to those ofthe copper smelter. In Colorado the pyritic ores containing goldand silver in association with copper are smelted in reverberatoryfurnaces for regulus, which, when desilverized by Ziervogel’s method,leaves a residue containing 20 or 30 oz. of gold per ton. This issmelted with rich gold ores, notably those containing tellurium, forwhite metal or regulus; and by a following process of partial reductionanalogous to that of selecting in copper smelting, “bottoms”of impure copper are obtained in which practically all the gold isconcentrated. By continuing the treatment of these in the ordinaryway of refining, poling and granulating, all the foreign mattersother than gold, copper and silver are removed, and, by exposing thegranulated metal to a high oxidizing heat for a considerable time thecopper may be completely oxidized while the precious metals areunaltered. Subsequent treatment with sulphuric acid renders thecopper soluble in water as sulphate, and the final residue containsonly gold and silver, which is parted or refined in the ordinary way.This method of separating gold from copper, by converting the latterinto oxide and sulphate, is also used at Oker in the Harz.
Extraction by Means of Aqueous Solutions.—Many processeshave been suggested in which the gold of auriferous deposits isconverted into products soluble in water, from which solutionsthe gold may be precipitated. Of these processes, two only areof special importance, viz. the chlorination or Plattner process, inwhich the metal is converted into the chloride, and the cyanide orMacArthur-Forrest process, in which it is converted into potassiumaurocyanide.
(3) Chlorination or Plattner Process.—In this process moistened goldores are treated with chlorine gas, the resulting gold chloride dissolvedout with water, and the gold precipitated with ferrous sulphate,charcoal, sulphuretted hydrogen or otherwise. The process originatedin 1848 with C. F. Plattner, who suggested that the residues fromcertain mines at Reichenstein, in Silesia, should be treated withchlorine after the arsenical products had been extracted by roasting.It must be noticed, however, that Percy independently made thesame discovery, and stated his results at the meeting of the BritishAssociation (at Swansea) in 1849, but the Report was not publisheduntil 1852. The process was introduced in 1858 by Deetken at GrassValley, California, where the waste minerals, principally pyrites fromtailings, had been worked for a considerable time by amalgamation.The process is rarely applied to ores direct; free-milling ores aregenerally amalgamated, and the tailings and slimes, after concentration,operated upon. Three stages in the process are to be distinguished:(i) calcination, to convert all the metals, except goldand silver, into oxides, which are unacted upon by chlorine; (ii.)chlorinating the gold and lixiviating the product; (iii.) precipitatingthe gold.
The calcination, or roasting, is conducted at a low temperature insome form of reverberatory furnace. Salt is added in the roastingto convert any lime, magnesia or lead which may be present, intothe corresponding chlorides. The auric chloride is, however, decomposedat the elevated temperature into finely divided metallicgold, which is then readily attacked by the chlorine gas. The highvolatility of gold in the presence of certain metals must also beconsidered. According to Egleston the loss may be from 40 to 90%of the total gold present in cupriferous ores according to the temperatureand duration of calcination. The roasted mineral, slightlymoistened, is introduced into a vat made of stoneware or pitchedplanks, and furnished with a double bottom. Chlorine, generallyprepared by the interaction of pyrolusite, salt and sulphuric acid,is led from a suitable generator beneath the false bottom, and risesthrough the moistened ore, which rests on a bed of broken quartz;the gold is thus converted into a soluble chloride, which is afterwardsremoved by washing with water. Both fixed and rotating vats areemployed, the chlorination proceeding more rapidly in the lattercase; rotating barrels are sometimes used. There have also beenintroduced processes in which the chlorine is generated in thechloridizing vat, the reagents used being dilute solutions of bleachingpowder and an acid. Munktell’s process is of this type. In theThies process, used in many districts in the United States, the vatsare rotating barrels made, in the later forms, of iron lined with lead,and provided with a filter formed of a finely perforated leadengrating running from one end of the barrel to the other, and rigidlyheld in place by wooden frames. Chlorine is generated within thebarrel from sulphuric acid and chloride of lime. After charging,the barrel is rotated, and when the chlorination is complete thecontents are emptied on a filter of quartz or some similar material,and the filtrate led to settling tanks.
After settling the solution is run into the precipitating tanks. Theprecipitants in use are: ferrous sulphate, charcoal and sulphurettedhydrogen, either alone or mixed with sulphur dioxide; the use ofcopper and iron sulphides has been suggested, but apparently thesesubstances have achieved no success.
In the case of ferrous sulphate, prepared by dissolving iron indilute sulphuric acid, the reaction follows the equation AuCl3 + 3FeSO4= FeCl3 + Fe2(SO4)3 + Au. At the same time any lead, calcium,barium and strontium present are precipitated as sulphates; it istherefore advantageous to remove these metals by the preliminaryaddition of sulphuric acid, which also serves to keep any basic ironsalts in solution. The precipitation is carried out in tanks or vatsmade with wooden sides and a cement bottom. The solutions arewell mixed by stirring with wooden poles, and the gold allowed tosettle, the time allowed varying from 12 to 72 hours. The supernatantliquid is led into settling tanks, where a further amountof gold is deposited, and is then filtered through sawdust orsand, the sawdust being afterwards burnt and the gold separatedfrom the ashes and the sand treated in the chloridizing vat. Theprecipitated gold is washed, treated with salt and sulphuric acidto remove iron salts, roughly dried by pressing in cloths or on filterpaper, and then melted with salt, borax and nitre in graphitecrucibles. Thus prepared it has a fineness of 800-960, the chiefimpurities usually being iron and lead.
Charcoal is used as the precipitant at Mount Morgan, Australia.Its use was proposed as early as 1818 and 1819 by Hare and Henry;Percy advocated it in 1869, and Davis adopted it on the large scaleat a works in Carolina in 1880. The action is not properly understood;it may be due to the reducing gases (hydrogen, hydrocarbons,&c.) which are invariably present in wood charcoal. The processconsists essentially in running the solution over layers of charcoal,the charcoal being afterwards burned. It has been found that thereaction proceeds faster when the solution is heated.
Precipitation with sulphur dioxide and sulphuretted hydrogenproceeds much more rapidly, and has been adopted at many works.Sulphur dioxide, generated by burning sulphur, is forced into thesolution under pressure, where it interacts with any free chlorinepresent to form hydrochloric and sulphuric acids. Sulphurettedhydrogen, obtained by treating iron sulphide or a coarse mattewith dilute sulphuric acid, is forced in similarly. The gold isprecipitated as the sulphide, together with any arsenic, antimony,copper, silver and lead which may be present. The precipitateis collected in a filter-press, and then roasted in muffle furnaceswith nitre, borax and sodium carbonate. The fineness of the gold soobtained is 900 to 950.
4. Cyanide Process.—This process depends upon the solubilityof gold in a dilute solution of potassium cyanide in the presenceof air (or some other oxidizing agent), and the subsequent precipitationof the gold by metallic zinc or by electrolysis. The solubilityof gold in cyanide solutions was known to K. W. Scheele in 1782;and M. Faraday applied it to the preparation of extremely thinfilms of the metal. L. Eisner recognized, in 1846, the part playedby the atmosphere, and in 1879 Dixon showed that bleaching powder,manganese dioxide, and other oxidizing agents, facilitated the solution.S. B. Christy (Trans. A.I.M.E., 1896, vol. 26) has shown that thesolution is hastened by many oxidizing agents, especially sodium andmanganese dioxides and potassium ferricyanide. According toG. Bodländer (Zeit. f. angew. Chem., 1896, vol. 19) the rate of solutionin potassium cyanide depends upon the subdivision of the gold—thefiner the subdivision the quicker the solution,—and on theconcentration of the solution—the rate increasing until the solutioncontains 0.25% of cyanide, and remaining fairly stationary withincreasing concentration. The action proceeds in two stages; inthe first hydrogen peroxide and potassium aurocyanide are formed,and in the second the hydrogen peroxide oxidizes a further quantityof gold and potassium cyanide to aurocyanide, thus (1)2Au + 4KCN + O2 + 2H2O = 2KAu(CN)2 + 4KOH + H2O2; (2) 2Au + 4KCN + 2H2O2 = 2KAu(CN)2 + 4KOH.The end reaction may be written4Au + 8KCN + 2H2O + O2 = 4KAu(CN)2 + 4KOH.
The commercial process was patented in 1890 by MacArthur andForrest, and is now in use all over the world. It is best adapted forfree-milling ores, especially after the bulk of the gold has been removedby amalgamation. It has been especially successful in theTransvaal. In the Witwatersrand the ore, which contains about9 dwts. of gold to the metric ton (2000 ℔), is stamped and amalgamated,and the slimes and tailings, containing about 312 dwts. per ton,are cyanided, about 2 dwts. more being thus extracted. The totalcost per ton of ore treated is about 6s., of which the cyaniding costsfrom 2s. to 4s.
The process embraces three operations: (1) Solution of the gold;(2) precipitation of the gold; (3) treatment of the precipitate.
The ores, having been broken and ground, generally in tube mills,until they pass a 150 to 200-mesh sieve, are transferred to the leachingvats, which are constructed of wood, iron or masonry; steel vats,coated inside and out with pitch, of circular section and holding up to1000 tons, have come into use. The diameter is generally 26 ft., butmay be greater; the best depth is considered to be a quarter of thediameter. The vats are fitted with filters made of coco-nut mattingand jute cloth supported on wooden frames. The leaching is generallycarried out with a strong, medium, and with a weak liquor, in theorder given; sometimes there is a preliminary leaching with a weakliquor. The strengths employed depend also upon the mode ofprecipitation adopted, stronger solutions (up to 0.25% KCN) beingused when zinc is the precipitant. For electrolytic precipitation thesolution may contain up to 0.1% KCN. The liquors are run offfrom the vats to the electrolysing baths or precipitating tanks, and theleached ores are removed by means of doors in the sides of the vatsinto wagons. In the Transvaal the operation occupies 312 to 4 daysfor fine sands, and up to 14 days for coarse sands; the quantity ofcyanide per ton of tailings varies from 0.26 to 0.28 ℔, for electrolyticprecipitation, and 0.5 ℔ for zinc precipitation.
The precipitation is effected by zinc in the form of bright turnings,or coated with lead, or by electrolysis. According to Christy, theprecipitation with zinc follows equations 1 or 2 according as potassiumcyanide is present or not:
(1) 4KAu(CN)2 + 4Zn + 2H2O = 2Zn(CN)2 + K2Zn(CN)4 + Zn(OK)2 + 4H + 4Au;
(2) 2KAu(CN)2 + 3Zn + 4KCN + 2H2O = 2K2Zn(CN)4 + Zn(OK)2 + 4H + 2Au;
one part of zinc precipitating 3.1 parts of gold in the first case, and2.06 in the second. It may be noticed that the potassium zinccyanide is useless in gold extraction, for it neither dissolves gold norcan potassium cyanide be regenerated from it.
The precipitating boxes, generally made of wood but sometimes ofsteel, and set on an incline, are divided by partitions into alternatelywide and narrow compartments, so that the liquor travels upwardsin its passage through the wide divisions and downwards through thenarrow divisions. In the wider compartments are placed sieveshaving sixteen holes to the square inch and bearing zinc turnings.The gold and other metals are precipitated on the under surfaces ofthe turnings and fall to the bottom of the compartment as a blackslime. The slime is cleaned out fortnightly or monthly, the zincturnings being cleaned by rubbing and the supernatant liquorallowed to settle in the precipitating boxes or in separate vessels.The slime so obtained consists of finely divided gold and silver(5-50%), zinc (30-60%), lead (10%), carbon (10%), together withtin, copper, antimony, arsenic and other impurities of the zinc andores. After well washing with water, the slimes are roughly dried inbag-filters or filter-presses, and then treated with dilute sulphuricacid, the solution being heated by steam. This dissolves out thezinc. Lime is added to bring down the gold, and the sediment, afterwashing and drying, is fused in graphite crucibles.
5. Electrolytic Processes.—The electrolytic separation of the goldfrom cyanide solutions was first practised in the Transvaal. Theprocess, as elaborated by Messrs. Siemens and Halske, essentiallyconsists in the electrolysis of weak solutions with iron or steel plateanodes, and lead cathodes, the latter, when coated with gold, beingfused and cupelled. Its advantages over the zinc process are that thedeposited gold is purer and more readily extracted, and that weakersolutions can be employed, thereby effecting an economy in cyanide.
In the process employed at the Worcester Works in the Transvaal,the liquors, containing about 150 grains of gold per ton and from0.08 to 0.01% of cyanide, are treated in rectangular vats in which isplaced a series of iron and leaden plates at intervals of 1 in. Thecathodes, which are sheets of thin lead foil weighing 112 ℔ to thesq. yd., are removed monthly, their gold content being from 0.5 to10%, and after folding are melted in reverberatory furnaces toingots containing 2 to 4% of gold. Cupellation brings up the gold toabout 900 fine. Many variations of the electrolytic process as aboveoutlined have been suggested. S. Cowper Coles has suggestedaluminium cathodes; Andreoli has recommended cathodes of ironand anodes of lead coated with lead peroxide, the gold being removedfrom the iron cathodes by a brief immersion in molten lead; in thePelatan-Cerici process the gold is amalgamated at a mercury cathode(see also below).
Refining or Parting of Gold.—Gold is almost always silver-bearing,and it may be also noticed that silver generally containssome gold. Consequently the separation of these two metals Isone of the most important metallurgical processes. In additionto the separation of the silver the operation extends to theelimination of the last traces of lead, tin, arsenic, &c. whichhave resisted the preceding cupellation.
The “parting” of gold and silver is of considerable antiquity.Thus Strabo states that in his time a process was employed for refiningand purifying gold in large quantities by cementing or burningit with an aluminous earth, which, by destroying the silver, left thegold in a state of purity. Pliny shows that for this purpose the goldwas placed on the fire in an earthen vessel with treble its weight ofsalt, and that it was afterwards again exposed to the fire with twoparts of salt and one of argillaceous rock, which, in the presence ofmoisture, effected the decomposition of the salt; by this means thesilver became converted into chloride.
The methods of parting can be classified into “dry,” “wet” andelectrolytic methods. In the “dry” methods the silver is convertedinto sulphide or chloride, the gold remaining unaltered; in the“wet” methods the silver is dissolved by nitric acid or boilingsulphuric acid; and in the electrolytic processes advantage is takenof the fact that under certain current densities and other circumstancessilver passes from an anode composed of a gold-silver alloyto the cathode more readily than gold. Of the dry methods onlyF. B. Miller’s chlorine process is of any importance, this method, andthe wet process of refining by sulphuric acid, together with theelectrolytic process, being the only ones now practised.
The conversion of silver into the sulphide may be effected byheating with antimony sulphide, litharge and sulphur, pyrites, or withsulphur alone. The antimony, or Guss und Fluss, method waspractised up till 1846 at the Dresden mint; it is only applicable toalloys containing more than 50% of gold. The fusion results in theformation of a gold-antimony alloy, from which the antimony isremoved by an oxidizing fusion with nitre. The sulphur andlitharge, or Pfannenschmied, process was used to concentrate thegold in an alloy in order to make it amenable to “quartation,” orparting with nitric acid. Fusion with sulphur was used for the samepurpose as the Pfannenschmied process. It was employed in 1797at the St Petersburg mint.
The conversion of the silver into the chloride may be effected bymeans of salt—the “cementation” process—or other chlorides, orby free chlorine—Miller’s process. The first process consists essentiallyin heating the alloy with salt and brickdust; the latter absorbsthe chloride formed, while the gold is recovered by washing. It is nolonger employed. The second process depends upon the fact that, ifchlorine be led into the molten alloy, the base metals and the silverare converted into chlorides. It was proposed in 1838 by LewisThompson, but it was only applied commercially after Miller’s improvementsin 1867, when it was adopted at the Sydney mint. SirW. C. Roberts-Austen introduced it at the London mint; and it hasalso been used at Pretoria. It is especially suitable to gold containinglittle silver and base metals—a character of Australian gold—but ityields to the sulphuric acid and electrolytic methods in point of
The separation of gold from silver in the wet way may be effectedby nitric acid, sulphuric acid or by a mixture of sulphuric acid andaqua regia.
Parting by nitric acid is of considerable antiquity, being mentionedby Albertus Magnus (13th cent.), Biringuccio (1540) and Agricola(1556). It is now rarely practised, although in some refineries boththe nitric acid and the sulphuric acid processes are combined, thealloy being first treated with nitric acid. It used to be called “quartation”or “inquartation,” from the fact that the alloy best suitedfor the operation of refining contained 3 parts of silver to 1 of gold.The operation may be conducted in vessels of glass or platinum, andeach pound of granulated metal is treated with a pound and a quarterof nitric acid of specific gravity 1.32. The method is sometimesemployed in the assay of gold.
Refining by sulphuric acid, the process usually adopted forseparating gold from silver, was first employed on the large scale byd’Arcet in Paris in 1802, and was introduced into the Mint refinery,London, by Mathison in 1829. It is based upon the facts that concentratedhot sulphuric acid converts silver and copper into solublesulphates without attacking the gold, the silver sulphate beingsubsequently reduced to the metallic state by copper plates with theformation of copper sulphate. It is applicable to any alloy, and isthe best method for parting gold with the exception of the electrolyticmethod.
The process embraces four operations: (1) the preparation of analloy suitable for parting; (2) the treatment with sulphuric acid;(3) the treatment of the residue for gold; (4) the treatment of thesolution for silver.
It is necessary to remove as completely as possible any lead, tin,bismuth, antimony, arsenic and tellurium, impurities which impairthe properties of gold and silver, by an oxidizing fusion, e.g. withnitre. Over 10% of copper makes the parting difficult; consequentlyin such alloys the percentage of copper is diminished by theaddition of silver free from copper, or else the copper is removed by achemical process. Other undesirable impurities are the platinummetals, special treatment being necessary when these substances arepresent. The alloy, after the preliminary refining, is granulated bybeing poured, while molten, in a thin stream into cold water which iskept well agitated.
The acid treatment is generally carried out in cast iron pots;platinum vessels used to be employed, while porcelain vessels are onlyused for small operations, e.g. for charges of 190 to 225 oz. as at Okerin the Harz. The pots, which are usually cylindrical with a hemisphericalbottom, may hold as much as 13,000 to 16,000 oz. of alloy.They are provided with lids, made either of lead or of wood lined withlead, which have openings to serve for the introduction of the alloyand acid, and a vent tube to lead off the vapours evolved during theoperation. The bullion with about twice its weight of sulphuric acidof 66° Bé is placed in the pot, and the whole gradually heated.Since the action is sometimes very violent, especially when thebullion is treated in the granulated form (it is steadier when thinplates are operated upon), it is found expedient to add the acid inseveral portions. The heating is continued for 4 to 12 hours accordingto the amount of silver present; the end of the reaction is knownby the absence of any hissing. Generally the reaction mixture isallowed to cool, and the residue, which settles to the bottom of thepot, consists of gold together with copper, lead and iron sulphates,which are insoluble in strong sulphuric acid; silver sulphate mayalso separate if present in sufficient quantity and the solution besufficiently cooled. The solution is removed by ladles or by siphons,and the residue is leached out with boiling water; this removes thesulphates. A certain amount of silver is still present and, accordingto M. Pettenkofer, it is impossible to remove all the silver by meansof sulphuric acid. Several methods are in use for removing thesilver. Fusion with an alkaline bisulphate converts the silver into thesulphate, which may be extracted by boiling with sulphuric acid andthen with water. Another process consists in treating a mixture ofthe residue with one-quarter of its weight of calcined sodium sulphatewith sulphuric acid, the residue being finally boiled with a largequantity of acid. Or the alloy is dissolved in aqua regia, the solutionfiltered from the insoluble silver chloride, and the gold precipitatedby ferrous chloride.
The silver present in the solution obtained in the sulphuric acidboiling is recovered by a variety of processes. The solution may bedirectly precipitated with copper, the copper passing into solutionas copper sulphate, and the silver separating as a mud, termed“cement silver.” Or the silver sulphate may be separated from thesolution by cooling and dilution, and then mixed with iron clippings,the interaction being accompanied with a considerable evolution ofheat. Or Gutzkow’s method of precipitating the metal with ferroussulphate may be employed.
The electrolytic parting of gold and silver has been shown to bemore economical and free from the objections—such as the poisonousfumes—of the sulphuric acid process. One process depends upon thefact that, with a suitable current density, if a very dilute solution ofsilver nitrate be electrolysed between an auriferous silver anode and asilver cathode, the silver of the anode is dissolved out and depositedat the cathode, the gold remaining at the anode. The silver is quitefree from gold, and the gold after boiling with nitric acid has a finenessof over 999.
Gold is left in the anode slime when copper or silver are refined bythe usual processes, but if the gold preponderate in the anode theseprocesses are inapplicable. A cyanide bath, as used in electroplating,would dissolve the gold, but is not suitable for refining, because othermetals (silver, copper, &c.) passing with gold into the solution woulddeposit with it. Bock, however, in 1880 (Berg- und hüttenmännischeZeitung, 1880, p. 411) described a process used at the North GermanRefinery in Hamburg for the refining of gold containing platinumwith a small proportion of silver, lead or bismuth, and a subsequentpatent specification (1896) and a paper by Wohlwill (Zeits. f. Elektrochem.,1898, pp. 379, 402, 421) have thrown more light uponthe process. The electrolyte is gold chloride (2.5-3 parts of pure goldper 100 of solution) mixed with from 2 to 6% of the strongesthydrochloric acid to render the gold anodes readily soluble, whichthey are not in the neutral chloride solution. The bath is used at65° to 70° C. (150° to 158° F.), and if free chlorine be evolved, whichis known at once by its pungent smell, the temperature is raised, ormore acid is added, to promote the solubility of the gold. The bathis used with a current-density of 100 ampères per sq. ft. at 1 volt(or higher), with electrodes about 1.2 in. apart. In this process allthe anode metals pass into solution except iridium and other refractorymetals of that group, which remain as metals, and silver,which is converted into insoluble chloride; lead and bismuth formchloride and oxychloride respectively, and these dissolve until thebath is saturated with them, and then precipitate with the silver inthe tank. But if the gold-strength of the bath be maintained, onlygold is deposited at the cathode—in a loose powdery condition frompure solutions, but in a smooth detachable deposit from impureliquors. Under good conditions the gold should contain 99.98% ofthe pure metal. The tank is of porcelain or glazed earthenware, theelectrodes for impure solutions are 12 in. apart (or more with puresolutions), and are on the multiple system, and the potential differenceat the terminals of the bath is 1 volt. A high current-densitybeing employed, the turn-over of gold is rapid—an essential factorof success when the costliness of the metal is taken into account.Platinum and palladium dissolved from the anode accumulate in thesolution, and are removed at intervals of, say, a few months bychemical precipitation. It is essential that the bath should notcontain more than 5% of palladium, or some of this metal willdeposit with the gold. The slimes are treated chemically for theseparation of the metals contained in them.
Authorities.—Standard works on the metallurgy of gold are thetreatises of T. Kirke Rose and of M. Eissler. The cyanide processis especially treated by M. Eissler, Cyanide Process for the Extractionof Gold, which pays particular attention to the Witwatersrandmethods; Alfred James, Cyanide Practice; H. Forbes Julian andEdgar Smart, Cyaniding Gold and Silver Ores. Gold milling is treatedby Henry Louis, A Handbook of Gold Milling; C. G. Warnford Lock,Gold Milling; T. A. Rickard, Stamp Milling of Gold Ores. Golddredging is treated by Captain C. C. Longridge in Gold Dredging, andhydraulic mining is discussed by the same author in his HydraulicMining. For operations in special districts see J. M. Maclaren, Gold(1908); J. H. Curle, Gold Mines of the World; Africa: F. H. Hatchand J. A. Chalmers, Gold Mines of the Rand; S. J. Truscott, WitwatersrandGoldfields Banket and Mining Practice; Australasia: D. Clark,Australian Mining and Metallurgy; Karl Schmeisser, Goldfields ofAustralasia; A. G. Charleton, Gold Mining and Milling in WesternAustralia; India: F. H. Hatch, The Kolar Gold-Field.