gold

I STATE I OF IDAHO C. Ben RO~s, Governor I i • First Editfol'1, May 19a2 Seeond Edition, july 1~2 IPAHO liUREAU 0, MINES AND GEOLOGY John w. finch, Secretary 'nfE RECOVERY 0:E1 GOLD FROM ITS ORES By A. W. iFahrenwald 'Uni ver~ i ty ,.1" I.daho Moslcow, Idaho :j J I Occurrence of Gol d ... ... ... fPrQPorties of Gold - ... ... - ... i- Uses of Gold ... ... - - ... - ... iPl-G»dl1ction of Gold... ... ... ... - 1- ... Production of Gold by sourcels Methods of Detecting and Assayin~ Gold Oro Classif'1 ca tion of Gold Ores ...... ~ - Methods ot Gold Recovery ... - - ... i- - Amalgamation Process ...... - ... ... ... i_ - - ... History ... ... ... ... ... ... - ... - - ,- ... ... ... - - ... Principle of the Process ... 1- - ... - The Process - ... - ... ... ~ - .... - ... ... ... ... Forms ot Gold and Lossos in fAmalgama tion Process Cyanide Process - - ... ... ... ... ... ... - ... - ... - ... Discovery ... - - ... - ... ... ... ... ,- ... ... Principle of the Process The Process - ... ... ... Gravity Concentration - ...... ... I'lota.tion Process ......... - ... ... Principle of the Process The Procoss ... - ... - ... - - ... :- ...... Flow Sheets of Modern Gold Recovqry Processes ... Closed Circuit Grinding (Fiaure 2) .................. - ... Stamp Mill Amalgamation Pro~esses~Figuro 3) Amalgamation Process Employ~ng Ballor Rod Mills (Figure' 4) Cyanide Process (Figure 5) ~ ...... - ...... ... ......... - ...... - ...... Combination Amalgamation, F~otation, and Cyanide ProceSB (Figure 6)- - ... - ... - ... - ..; ......... - - - ... - ... - ...... Plant and Operating Costs in Mod~rn Gold Milling Practice Cost of Erecting Gold Treatmont Elants ...... - - ... - ... - ... - - ... Metal1t.'1lg1cal Testing of Gold Or~s ... ... - - ...... Marketint Gold Bulllon"a.ndRich Gold Products Flotation ,.~ .'JCf~ Gold ... - - - - Introduction ... -_ ......... - ...... Limi ta tiona of Flotatt.on Prqcess Crushing and Grinding - - - i_ - Use of Amalgamation Ahead o~ Flotation - - Ratio of Concentration and qrade of Product ... - - - - - - Machinery Needed - - - ... - 1 - - - - - Examples of Gold Flotation ~ - - - - - - ...... Reagents - - - - - - - - - ~ - - - - - - - ... ... Treatment of the Concentrat~ ... - - - - - - - - ... The Well EquiPPQd Plant - - 1- - - - ... - - - - - - - Eeterences Not Mentioned in the ~ext ... - - - - Common 'Reagents Used in Gold Reco1very Pro cesses - - - - - - - -

 

 

Page 1 2 2 4 6 6 7 7 8 8 8 9 9 9 9 10 10 11 11 11 12 12 12 12 13 14 15 17 17 18 18 20 20 20 tt) 21 21 21 21 24 24 24 25 26 . I Gold o.cure .in nature in the me silver and eheDdcally combined with in veiDel associated With quartz and galena ... ! qhalcopyri te, arsenopyrite, zinc bleJide; With carbonates, espec disseminated in the body ot the sult particles, in the torm of thin tilms '~pbQtOmicrOgraph* ot a polished stlver, and bismuth minerals are pre p1eoes ~e about .001 inch in diame ively iron pyrite and sphalerite, I lic state alloyed Wi th small perQentages of element tellurium. Native gold is found

ous sulfides, notably pyrite and pyrrhotite, ckel and cobalt minerals, and less commonly 11y ankerite. Generally the metal is finely s, or deposited on the surface ot mineral easily visible grains. Figure I is a of a complex gold ore •. Lead, iron, copper, t. The light spots are gold. The smallest The patches marked "pH and "s" are respectsulfide of zinc. In the oxidized parts of veins ~he gold is often associated' with limonIte, and gold-bearing quartz may .ontain ~mall amounts of copper carbonates and manga nese oxides. When gold is apparent~y disseminated in igneous or metamorphic rockS', minute veinlets of quartz or qarbonates usually accompany it. Gold is widely distributed. It is found in ~ea water (less than 1/40 grain per ton), and in insignificant amounts in a great ~ariety of places. The r~ason tDr the occurrence o~ gold in greater quantities in lodes than in neighboring rocks is not known With 4ertainty, but it is considered that it has risen from below with other minerals in solution and has been preclpitated cbemi oa11y where it is found. • ... " • .. t Tellurides or gold are aontaine4 in rich ores in western Australia, in Colorado, and elsewhere. The minera~ calaverite, AuTe2, a bronze-yellow gold telluride,. contains 40 per .oen.~ ot S~ld and the gold-silver telluride, sylvan ite, AuAgTe4,a steel.gray mineral " contains up to 28 per cent of gold. When the gold . tellurides OXidize, the resulting na ive gold is often extremely pure and fine ly divided'" ... and may be covered with tel,urous OXide, called "tellurous" or "mu stard gold .. " It ftaembles yellow clay, bu~ turns bright on heating. AmOl)g the minerals s<5lletimes mi$taken for gold may be mentioned pyri te, marc asi te, and especially chalco~te, i the iron-copper sulfide. These minerals, . however, are very brittle, they OXidize when beated and are qrrtte solub. J# ... ... , .. ", while gold remains bright on heat-ing ito high tempfTatures. it is\ sott I hd '~~leabl : .- I • In panning or concentrating, ma~y people have been mislead by heavy lead minerals of a yello:- ,color, such as he molybdate, chromate, tungstate, and eve n the phosphate, but th.'ese are distin . lshabl.e under a lens or low-power micro scope by their brittleness and transpareno. Nearly all substances which may b-e mistak en for gold are soluble in hot ac ds-. Cold is not. i When gold-bearing v~inshave ~e4ome disintegrated and swept away into alluvial deposits, the particles ot gold, whe*e released, are found in the sand and grave l ot the beds. Gold occurring in this Iway is 'called "allUVial" gold and is reeov ereq by methods of hydraulic mining and ~edging. For methods of recovering allUVial gold, t.he reader is. referred to "Ele entary Methods ot PIa.cer Mining.," by W. W. Staley, Pamphlet No. 35, Idaho Burea of Mines and Geology; "Placer Mining Method s From a mineralogical report on ore of the Mayflower Gold Mines, Inc. By courtes~

 

 

 

of Thos .• H. Hi te, Geologist. IDAHO BUREAU OF MINES AND GEOLOGY PAMPHLET No. 37 Figure 1 Photomicrograph of a polished surface of a complex gold ore. and Costs in Alaska," by N. L. Wimmle~, Bulletin No. 259, U. S. Bureau 0"[ Mines (1927); and Peele's "Mining Engineer' ~ Handbook," published by McGraw-Hill Book Company, New York. PROPER1IFS OF GOLD Gold is the only metal that has a yellow oolor when massive and pure. The color is greatly modified by impurtties, silver lowering the tint, While copper heightens it. Gold exceeds all other metals in malleability and 'ductility. It can be beater, when pure, into leaves one three hundred thousandth of an inch in thickness. The presence of small percenta@es of bismuth, lead, or tellurium, renders it very bri ttle. The specific gravity of gold is 19.27, i.e., it is 19.27 tUnes as heavy'as water. Its electric conductivity is not so great as that or either copper or silver. If tae electric conductivi~ of silver is rated at 100, gold is 76.7. The most effective solvents of gold are potassium ~r sodium cyanides, a mixture of hydrochloric and nitric acids (called aqua regia), and chlorine, or any chemical mixture that liberates free chlorine gas. Gold is readily precipitated from cyanide solutions by metallic zinc (shavings or dust); some zinc is dissolved and the gold is preCipitated as a black sltme o n the zinc. It is preCipitated from its chloride solution by treatmont with ferrou s sulfate, sulfuretted hydrogen or sulfur dioxide. GOld alloys with most metals, but only a tew are of practical use in the arts. The most important are those which gold forms wi th mercury, silver, palladium, platinum, copper, and nickel. Metallurgically , its alloys ot zinc and lead are important in recovering gold from molten lead bullion. USES OF GOLD It is possible here to indicate enly a few of the ways in which gold performs its varied important, and, in some cases, vital functions in modern life. Gold has had a vi tal inf'luence in the world's historY;-". for the lure ot gold has led men to wage war and subjugate whole peoples, and to colonize the most forbidding was tes .• The essential main uses of gold are: (1) Gold held in reserve by the federal government, (2) gold used in dentistry as pure gold and alloys, and (3) pure gol d used in tho arts,or alloyed with other metals. Gold is held in stock by the federal government to provide the gold basis for the currency and credi t requirements of the world's trade. There is practically no gold money in oirculation. The ci:rculating medium is currency, the actual va lue of which is determined primarily by the supply of gold within a country. The sol e importance of the gold reserve is that it guara~ees the convertibility of the currency into gold at par. Such convertibility, however, is possible only When, by proper adjustment of the supply of gold, the currency is maintained at an approximate parity With gold. Tho following table gives the go+d production in millions of dollars for each year from 1919 to 1928 mclusi ve, the! amount consumed each year in arts am.d by the orient, and the balance available for; money: i I I 1- 2 .. ~_:.:..:....",,:~.o I. ~I

Total Gold Produced .nd Peroentage Available for Arts anQ.IOrient and Money (Joseph'Kitchin, Harvard Rer. of Economic Statistics (1928) Year Total Produotion: c~nsumed1n Arts: Balance available 1919 1920 1921 1922 1923 1924 1925 1926 192? 1928 365 335 331 319 36? 394 394 399 401 409 and Orient for Money 304 110 66 218 188 334 216 154 148 163 61 226 265 101 1?9 60 1?8 245 253 246 It is seen that the amount of new gold likely to be available for monetary uses is difficult to calculate because of the uncertainty {)f purchases of gold by oriental countries. In the past 10 ye~rs the average annual addition to the monetary gold stock has been about 1.8 per cent of the total stock, and in the l ast three years about 2.3 per cent. Economists have estimated that an annual increas e of 3 per cent in the gold stook is required to provide the gold base for the inc reasing currency and credit requirements of the world's trade. Such an estimate, however, is seriously effected by price levels, etc. Gold alloys of various degrees of fineness (parts of gold in 1000) are available in a variety of colors. Red, yellow, green, and white gold can be produced by a suitable choice of the added silv~r and base metal alloying constituents.

White gold alloys, which were originally introduced as platinum substitutes, are widely used in ihe less expensive grades of jewelry and are produced in both the solid and filled grades ranging from 10 to 20 carats* fine. These alloys are usually of the g¢)ld-nickel-copper-zinc type and the whi test grades are difficult to fabricate on account ef hardness. Where better working properties are required, the gold-pall~dium and gold-palladium-nickel alloys are used. The color of the softer white galas is not very pleasing and the low-carat gold alloys tarnish to some extent. Gold-platinum and gold-palladium ~loys find an important application in making spinnerets used in the production ~f rayon, the substitute for silk. Gold is used in limited quantity in the manufacture of scientific apparatus where it fills important needs. A large amount of gold is used in. dentistry. Gold is the base of a large variety of alloys used in dental work~ Platinum and palladium raise the melting paint of gold, increase its hardness, ~d lighten its color. aertain of these alloys possess unusually high tensile strength, in excess or 150,ooe pounds per * A carat is one part in 24 parts •. Ftlt~ example, a 22 carat gold is 22 parts go ld and two parts of other alloying metal. square inch •.. Reoently oonsiderable interest ihas developed in white casting alloys consisting essentially of palladium-gold-silven and copper with the palladium content rangi ng tram 14 to 25 per cent. The appear~nce ot these alloys in the mou~h is far less conspicuous than the yellow gold henetofore used. PRODUCTION OF GOLD For some concise and pertinent statements of gold production since the discovelr of Amerioa, the reader is referred to a long press bulletin dated November 5, 19 29, issued by the Director of the U. S. Bureau of Mines. This notice states that sin ce the discovery ot America world production ot gold has been approxtmately 1,P03,500,OOO ounces; that this gold would make a cube 38.5 feet square. Also a great volume of statist~cal information regarding the world production ot gold since 1492 is contained in ~conomic Paper No. 6 by Robert Ridgway and th e staft ot the Common Metals Di.vision iot the U., S. Bure'au ot Mines, which may be obtained from the Su.perintendent of Documents,Goyermnent Printing Office, Washington, D. C., price 20 cents.' , The tollowing table gives the Jold production ot the world, by countries: 'IlABLE II Gold Production o,r th,e World, by Countries, since 1920 (Mineral Industrr) United states(inc. Central America South America - - - Transvaal - - - Rhodesia - - - - - - West Africa - - Congo, Madagascar, etc.- - - Total Africa - - Russia '(inc. Siberia) Other Europe - - - - Total Europe British India -East Indies -Japan and Chosen China and others - - -

Total Asia - - Australia and New Zealand , ;! :' . '.' , . ~~~~~~~~~~~~~~~~~~~~~~~ . . . '.. Total for World - - - - -,:$398,55?,OOO:$40~,15a"OOO:$406,338,OOO:$403,366,OOO .(a) Preliminary estimates *McGraw-Hill Book Co." New York .1- 4In Table III is given the gold pro4uction o~ the United States, by states. TA$E III Production or Gold in the tunited states, by states(a) Alaska. Alabama . . . • • • Arizona • • • California • • Colorado • • • Georgia •• Idahll. •• Montana •• Nevada •• New Mexico •• North Carolina Oregon • • • • Pennsylvania • South Carolina South Dakota Tennessee Texas • • Utah ••• Virginia •• Washington • Wyoming •• 1926 1927 1929 Fine oz. Fine ez. Fine oz. Fine oz, • • • • 232,200 581,700 346,297 140 12,840 57,707 : 170,8~ 20,105 121 13,303 106 15 :. 286,961 416 164 181,832 10 .. . .8 ,.8 3.3 286,298 • • • • • 203,088

564,981 259,111 ,15 15,2()9 56,076 149,445 26,098 34 14,425 126 I; . . . . .... 322,681 426 324 199,518 • • • • • 19,398 • • • 330,604 . . . . . 189,519 513,249 258,564 34 20,351 59,661 177,730 34,961 131 11,865 987 10 318,095 537 556 211,418 : 379,669 10 211,108 409,020 220,285 58 19,59'7 55,854 158,991 33,02~ 1'14, : 17,65'7 . . . 7.4 5. 312,328 653 1,263 237,221 . . . . " .: . . . . . 16,414 34 3,972 48 1930 (b) Fine oz. 399,779

10 1~1,428 438,912 214,195 203 20,748 45,724 134,410 29,576 184 13,975 639 • • • • • 402,422 1,030 1,122 200,103 . .. . . . 3,72(: 450 Total 2,238,616 2,117,253 t 2,144,720 2,056,629 2,053,629 Philippine Islands • 96,426 B8 r531 178,934 'rotal • • 2,335,042 2,197,125; 2,233,251 2,208,386 2,252,593 (a) Figures of Bureau ot the Mint and U. S. Bureau of Mine s. {b} Prel~inary figures. Of the countries, Africa leads by i8. large margin. Of the states, California ranks No .. 1 in gold produotion; south Dakota a close second; Utah, third, and Idaho ranks ninth. The gold produced in Idaho COIQ,eS ~rom placer and dredging operations, from the mining and milling of gold ores, as a b~-product in the milling and smelting of lead alld zinc ores, and to a small extent fro~ copper ores. The principal districts fram Which ,gold is now produced in the United states are the WhiteWOOd district, South Dakot~; the Marysville, OrOVille, Folsom, Gras s Valley, and Ja,ckson districts, Ca,lif'ornl~; the Cripple Creek, Eureka, San Meg uel, (Telluride), a.nd California. districts, Cblorado; the West Mountain (Bingham Ca nyon), Tintic, and Park City districts, Utah; the Verde, Warren, and Ajo districts, Ari zona; ,~1,5- . the Robinson, Jarbridge, Tonopah, and Comstock districts, Nevada; the Southeaste rn district, Seward Peninsula, an.d Yukon Basin, Alaska; and the Summit Valley (But te). Montana. Production br Gold by Sources Table IV gives data of gold prodtlct1on in the United states, by sources. 'TABLE IV Gold produced in the United States*, by Sources (Mineral Industry) :Copper-: :l'ry and Copper Lead Zinc : lead and. Lead:si11ceous ore ore ore :Copper-: zinc Year Placers ores lead- ore Total zinc I • ores 1919 704,377 1,7BO,567 186,701 41,945: 86 1,~43 38,563 2,753,282 1920 66>3,002 1,523,331 171,055 46,776: 3,657 3,731 31,435 2,382,987

1921 677,435 1,548,251 52,765 61,924: 78 159 4,400 2,345,CnO 1922 537,910 1,560,789 130,928 35,415: 2,8~'7 2,46'7 22,935 2,293,251 1923 551,929 1,510,098 2'71,690 .38,109: 3,321 3,801 25,969 2,404,913 1924 450,720 1,602,469 310,413 39,947: 22'7 1,995 38,560 2,444,331 1925 436,251 1,414,446 348,025 50,3'78: 375 5,515 52,384 2,307,3'74 1926 457,717 1,295,570 365,223 45,724: 1,194 3,335 63,763 2,232,526 192'7 451,215 1,162,404 367,639 41,504: 1,563 2,631 80,076 2,10'7,032 1928 416,832 1,195,8~4 414,771 35,959: 130 6,973 77,595 2,148,064 Per cent, 1919 25.6 64.7 6.8 1.5 •• • II ••• 1.4 . • • II ••••• Per cent, * 1928 19.4 55.7 19.3 1.'7 .. . • 0.3 3.6 .......... Philippine Islands and Porto Rico exeluded. Here we see that the gold recovered from placers has declined from 25.6 per cent in 1919 to 19.4 per cent in 1926, from dry and siliceous ores from 64.'7 pe r cent to 55.'7, from copper ore increased from 6.8 per cent to 19.3 per cent, fro m lead ore increased from 1.5 per cent to 1.'7 per cent, and from copper-lead and copper-zinc ore increased from 1,4 per cent in 1919 to 3.6 per cent in 1928~ The se changes probably are due more to new and improved metallurgical methods (flotati on in particular) than to trends in mining or to discoveries of new ore. MElliOns OF DETEGT~G AND ASSAYING GOLD ORE Gold occurring free in quartz can be easily identified by the man in the field by simply crushing a few pounds of the rock in an iron mortar or on an anvil, an d carefully panning with a miner's "gold pan." Particles of gold will appear in th e pan after the bulk of the quartz Band i,has been washed off. Gold associated wit h various sulfides and as telluride is ~ch less easy to detect by simple methods. Ores of this type require a fire assay and this should be done by a competent an d reputable assayer. J I There 1s a certain type ot aSayer who is willing to, and aotually does, give tavorable assay reports .nen t re is absolutely no gold in the sample subm1 tted to him for assay. This pe son olaims he has a new and secret method or prooess that gets the gold and t t the old :fire assay is not capable of recovering the gold, allot which is: nonsense. The motive baok of such olaims 1s dishonest. Methods of assaying at given in a number of books on the subject. A Manual of Fire Assaying by lton and Sherwood, published by the McGraw-Hill Book Company,Inc., 370-7 h Avenue, New York, is one of the best. There are many reputable public assayers, and their assaying is dependable. state schools ot mines sometimes do trivate assaying. The Idaho School of Mines, however, although it does mak assays for Idaho people, does not cater to this kind of work and preters to ave tbe prospector, or .whoever is intereste d in gold assays, send his sampl.s to the public a~sayers. Their labor~ atories are as well equipped to mal<:e rapid and reliable assays as are the schools of mines. A short list of c9mpetent assayers is published under Professi onal Classification in the Engi*eering and Mining Journal. A list also may be obtained from the nearest school of mines or your state mine inspector. The fire assay is the only reli,ble method of deter.mining gold in ores, It is comparable to commercial smelt~ng methods and it gives the true gold

content of t~e ore, and should be ac~epted as final. The assayer reports the gold co.tent of an ore in terms of ounces per ton and in dollars. The mint value of g~ld is $20,57813 per troy ounce, 1000 fine. The number of ounces present times ~e value $20.67 is the assay value of the sample in gol d. CLASSIFICATION OF GOLD ORES From the viewpoint of gold recoVery methods, and for convenience to the reader, gold ores may be classified as follows: 1) Quartz ores in which the gold is free, i.e., in the form of metallic gold particles ranging in s~ze from particles easily visible to the eye to particles as fine as dust and detectable only with a highpower microscope. 2) Ores in which the gold is free but intimately associated physically with such minerals as pyrit~ or galena, either in the crystal or as a painted film on the crystal surface. 3) Ores in which the gold content, as a compound with tellurium or partly as free gold, is ass~ciated almost wholly with one or several minerals in the ore such as galena, pyrite, arsenopyrite, chalcopyrite, etc. "The Ores of Copper, Lead, Goldl, and Silver," by Charles H. Fulton, U. S. Bureau of Mines Technical Paper 143, contains a great deal of information bearin g on this subject. METHODS OF GCLD RECOVERY Gold is recovered from its ores by one, or a combination, of the following methods: I. II. III. IV. Amalgamation Cyani da t ion Concentration ~T:g All gold and/or silver ores y~eld a high percentage of their gold content when smelted with lead or copper ores ,and an almost complete saving of gold is affected ia the process of ret1~ing thG lead and copper. Molten copper or lead are strong collectors of gold and s 11 ver. Gold is separated from copper bullion electrolytically and from ~ead bullion generally by agitation With a small percentage of metallic zinc for which it has a strong affinity and with wh ich 1 t forms a chemi cal compound" Table V gives th0 percentage of gold produced by the various methods of recovery. TABLE V Percentage of Gold Produced by Various Methods of Recovery* (Mineral Industry) Year Amalgamation Cyanidation Placers Gold Silver Gold ': Silver Gold SilviJr ! 1919 31.5 C.5 28.5 18.8 1920 28.5 .4 24.3 13.4 1921 32.6 .45 23.9 16.0 1922 35.4 .3 23.6 14.4 1923 28.4 .3 25.3 14.3 1924 30.6 .4 23.0! 13.9 1925 32.6 .3 20.7 9.5 1926 30.6 .3 20.4 8.3 1927 30.8 .3 17.9 7.~ 1928 31.9! .4 17.0 7.3 1929 : 29.6: .35 15.8 5.3

 

*Philipplne Islands and Porto Rico excluded. **Both crude ores and concentrates. 25.6 25.3 28.9 23,5 22.9 18.4 18.9 20.5 21.4 19.4 19.8 0.15 .13 .18 .1 .1 .1 .1 .1 .1 .1 .1 Smeltlng** Gold Silvur 14.4 21.9 14.6 17.5 23.4 28.0 27.8 28.5 29.9 31.7 34.8 ... 85,5 86.1 83.4 85.2 85,3 85.6 90.1 91.3 92.1 92.3 94.3 Much of the gold produced by smolting comes as a by-product in the smelting of copper and lead ores. Due to the flotation process there has been an increase in the tonnage of gold concentrates smelted. Comparatively little true gold ore is diroctly sold for smolting. Smelting charges and deductions on the full weight of ore, with the cost of freight and loading in accessibility and bad roads, often make it advisable to adopt some method whi ch may be metallurgi cally inferior, as regards percentage recovery, but which puts the precious metal in a form such as bullion, r:ich precipi tate, or high grade concentrate, in which it is more profitably salable. AMALGAMATION PROCESS Histor.l

The amalgamation process is one of the oldest known methods of gold recovery. It is mentioned by Pliny iin his "Natural History" and descriptions of amalgamation processes for both go~d and silver are to be found in various 16th Century treatises. The histo~y of the process is given in detail by Pearcy (John Pearcy, Silver and Go~d, 1880). Principle of tho Proces~ The process is based on the fact that mercury dissolves gold. A saturatod solution of gold in mercury cantai~s 13,5 per cent of mercury. PhYSically the alloy 1s a paste a.nd is cal1t;}d tf~lgatn." t 8 The amalg&matton prooess compri see (1) crusl;l.ing the ore in water to pass, say, about a 14 or 20 (somatimes fi'er) mesh sieve, (2) passing the crushed ore in tho torm of a thin pulp over a m~rcury-oovered copper, or silver-ooated, plate~ ",3) removing the gold-lOOrcurt amalgam at regular intervals and re-dress ing tho plates wi th new mercury, and. (4) distilling the mercury from the gold amalgam, thereby producing nearly p~re gold, and recovering the mercury for ~~se on the plates. In the early practice, the cru~hing was done by gravity stamps. In a number of placos this practioe is still retained. Tho modern trend, however, is to use sGcondary crushers of the jau or gytatory typo, and a ball or rod mill to reduce to whatever mosh is desired. The techni~e ot the steps 2, 3, and 4 of the process bas changod little in the many years of the uso of tho amalgamation process. Forms of Gold and Lossos in Amalg~on Pro<!..~~ Orcs that can be troated successfully by amalgamation are called "froemilling" orcs. It is practically nG,vG1' possible to obtain high extraction of gold by amalgamation alone. This is due principally to two reasons, namely, (1) fino (flour) gold that fails to sottle upon,and make contact with,thc plat(' E~ and (2) a typo of gold occurronco in which the gold particles arc brown and lusterless. This may be due to coatings of' oxide of iron, manganese, or telluri um. Free-milling ores soldom yield more than a 70 per cent rocovo~y. This 1s the oXplanation of tho existence (mostly in the past) of tailing p~lesabout the country assaying several dollars a tan in gold. Many or most of them have since been profitably retreated by cyanida.t1on or flotation. Present operators, even the small ones, are not satisfied ~th the gold recovery possible by plate , amalgamation only, but follow the plates .with cyanidation or flotation, or tabl e concentration. Ama~gamation as used today recovers only relatively coarse gold and for this purpose plays an important role in gold metallurgy. Cyanide acts too slowly in dissolving coarse gold, but readily dissolves fine gold. Flotation concentration is not effective on Goarse gold. Tbe small operator who wishes only to treat a small tonnage of high-grade ore in remote places, and looks for quick returns rather than for 100 per cent recoveries, probably still will find the amalgamati or. process the best available method. A small stemp or preferably a ball mill and a few square feet of mercury-covered oopper plate usually meet his needs. Howeve r, to the larger operator amalgamation is now only an adjunct to a more elaborate plant of high over-all efficiency. THE CYANIDE PROCESS Discovery: The cyanide process was inventeld in 1887 just at a time When there was pressing ~eed tor improvement in the treaJtment of gold ores. It was about then that the industry, atter languishing for many years, was receiving a fillip from the discovery of the Rand gold field. Ait that time the available gold extraction processes ;were cheap but in general icould o nly extract some 60 per cent of the

 

values and in some cases gave no extract! on whatever. Sma 1 ting, although effe ctive, was costly and required the supply of large quantities of rich ore, together wi th lead ore and cheap fuel at no g!rea t di stance from the gold mines. The cyanide prooess was put forWard by J. S. MacArthur and R. W. and W. Forrest, and was reooived at first ~th deriSion, partly because the chemical I _i 9 ,.. j was rare and had been little Btudi~d. Oyanide was best known at the t~e as a deadly poison. . The process was first used ma~nly on the tailings from amalgamation in the Transvaal, but was applied later t~ both gold and silver ores without previous amalgamation, and this practice beqame the general rule by 1925. For a thorough review of the ~arly history of the cyanide process, the reader should refer to a 'serial ar1icle by A. W. Allen in the September 3rd, and October 1st and 8th, 1927, numbers of the Engineering and Mining journal. Principle of the Process The process is based on two scientific fa~ts, namely, (l) that gold is soluble in dilute solutions of pot 4ssium or sodi urn cyanide, and (2) that the dissolved gold is precipitated from its cyanide solution by metallic zinc. The Process The process comprises the following steps: (1) Crushing the ore to a suitable fineness in cyanide solution,or in water and cyanide, then added to the ore-water pulp. (2) Agitation of the pulp to effect dissolution of the gold content of the pulverized ore~ (3) Separation of the gold-contain~ng cyanide solution from the pulverized ore. (4) Precipitation nf the gold from the cyanide solution by passing it over zinc shavings, or agitating it with zinc dust. (5) Refining the blac~ gpld-zinc precipitate which contains an excess of zinc, and small amounts df other elements, producing gold bullion • • If silver is present, the gold and silver are separated by dissolving the fO silver wi th sulfuric or ni tric acid. (6) Melting and casting the gold bulli on into bars for shipment to the mint. In modern practice the gold solution prior to precipitation is subjected to a vacuum treatment to remove dissolvod air. This treatment is known as the Crowe process. This treatment gives a more rapid and thorough preaipitation of the gold by the zinc with less consumption of zinc . For gold extraction the strength of cyanide solution ranges from .01 per cent to .05 per cent. Lime also is used in the process in just sufficient amount to maintain the solution in an alkaline condition; two or three pounds per ton usually are sufficient. For roasted concentrates more lime may be needed and a cyanide solution as high as 15 or 20 pounds of cyanide per ton of solution. The cyanide process is highly technical and for its successful operation the attention of an experienced me~allurgist is required. Not all ores yield to high rooovery by the cyanide process. Arsenical and antimonial ores always have given "trouble. They require a preliminary low temperature roast. l Gold ores con~aining constituents, such as copper2 , readil y soluble in cyanide solution, consume so much cyanide as to make the process inap plicable. It is on these ores that the flotation process has found wide application. 1 Leaver, E. S. and Woolf, ~. Wi th Arsenic and Antimonial 2 Leaver, E. S. and Woolf, J. Sulfide-Acid Precipitation: (l929). A., C~anidc Extraction of Gold and Silver Associntec

 

Ores: U. S. Bureau of Mines Tech. Paper 423 (1928). A., E~toct of Copper and Zinc in Cyanidotion with .Am. Iinst. of Min. & Met. Engrs. Tech. Pub. No. 250 -; 10 I , ~ ... ORA VITY cQNCENTRATICN Woolen blankets have long been u~~d for catching gold at Brazilian mines, where mercury is sickened by bismuth ~nd tellurium minerals. They were introduoe d into early Californian and Australian mills, and have been used in mills t~eating the rich Goldfield, Nevada, bre. They are genorally laid overlapping on inclined tables, the pulp flowing bver them for an hour, or several hours, when they are folded, replaced by trelsh blankets, and the accumulation washed otf in a tank. On the Rand, amalgamation on copper plates had been generally discarded by 1925, owing to the difficulty of pre~enting theft and the danger from mercury poisoning. Instead of passing the crushed ore from the tube mills over amalgamat ed plates, it is concentrated on a surface of corduroy, Which retains the heavy particles including all coarso gold. The corduroy also catches osmiridium and other valuable minerals of th~ platinum group which had previously been lost . Wool blankets ware found equally effecti va, but corduroy gives a less bulky concentrate, and the lock-up of gold is very small. Canvas gave a still smaller bulk, but caught only 75 per cent as much gold; riffles were only 57 per cent as effective. Canvas concentration as practiced in California is described by W. H. $torms, Engineering and Mining Journal, Vol. 60, (July 13, Nov. 9 and 16, 1895), and by E. B. Preston, Bulletin 6, California, State Mineralogist. Gravi ty tables and vanners often serve a useful purpose in gold recovery flow sheots. Flotation, however, is rapidly replacing these and other methods of gravity concentration. 'IHE FLOTATION PROCESS Flotation probably is the most important gold recovery process today. Practically the only kind of gold the flotation process will not recover is coarse gold, i.e., coarser than about 35 to 48 mesh. It recovers fino, free gold, gold associated in any form wi~ sulfides, and gold in oxidized lead and copper ores. A combination of amalgamation, cyanidation, and flotation almost insures high gold recovery from any gold ore. ~nen gold is recovered from its ore by flotation, a high grade conc0ntrato contains the gold. This concentrate may be ground and~ nith_or Hithout rousting, treated with cyanide solution, with the ultimate production of gold bullion, or it may be, and most often is. shipped to a lead or copper smelter Where it is paid for on the basis of its precious motal and base metal contents. It is a process easily available to the small operator who may use it wi th good results wi th a minimum of me'tallurgic al advice. Principle of tho Process The prinCiple of the process is briofly tha t most metals and metallic minerals crushed in water and treated with a small amount of certain reagents (xanthates and aerofloat for example)' collect this reagent on their surfaces, giving them a "dry"tondcncy (like pafr"affin) toward water. In this condition bubbles introduced into tho water in which tho particles aro stirred and suspend ed attach to these particles. A frotlhing or foaming agent, such as pine oil, is added to produce an intense froth which collects on the top of tho pulp. This froth contains the gold and sulf1des or other floatable substances. (For a statement of the history and invention of the flotation process, see the July 15, '1916, issue of Chemical and Metallurgical Engineering, by Georgo E. Collins

, - ~l IDAHO BURBAU OJ' IIINJJ8 AND GEOLOGY Mill feed bin (1) + ® Fe e. der WQfdr~ BQ"II or Fd mi"® Wafer CIt1,ssifier@ OverFlow ~..2t:fl~r~ r---;:-, ~ ,--: ''f. ® I 'C~Qn;de or I Btfwl clOSt1tief I flolation I l Sand l process L - - _J 'f - _.J ® Boll o~ tube mill : -Sinsle ~tage grinding L ____ ~ ---Double" .' FLOW-SHEET or CLOSED-CIRCUIT GRINDING UNIT Figure 2 Mill feed bin0 Mercur s-F WQfer Stam hotter ® from batter S on e old ToiJin9c Melting furnace ® + ® t ® Gold bullion Slat} F"LOW-SHEET Ot STAMP MILL AMALGAMATION PROCESS Jlgure 3 I. PAMPBLBT No. 37 ~ I 4) Traps are poekets Iprovlded in launders at end of the plates to catch e~eess mercury flowing off the plates. This mercury may ~e used tor dressing plates or'added to stamp mortar, Qr, if it contains enough gold, it is squeezed in canva$ or ch~ols and retorted. 5) Retorts and conde~ser my be purchased from The Mine and Smelter Supply Company, Denver. Heating is done in an oil-fired ~r.naoe. T~perature of 3600 C. is required. Care slilould be taken not to inhale mercury fumes. 6) The oil-fired cru¢ible furnace 1s suitable for small plants. Larger plants u:ee an oil-fired tilting furnace. 7) Slag is produced by adding soda and borax. It is broken up and strunped or shipped to a smelter. 8) Gold bullion is s¢ld to the United States Mint. Figure 4 is a flow sheet of the amalgamation process, in which grinding is accomplished in a ball or rod mill. Plates are shown both before and after the classifier. The bulk of the gold, if it is free, will be caught on the plate immediately following the ball mill. The use of a plate between the ball mill and the classifier.may cause dilution troubles, since water in liberal volume must be added to the pulp to cause it to flow freely over the plate. This plate may be omitted and only the plate follo~ng the classifier used. In this case, however, considerable coarse gold will build up in the classifier which, of course, is ~eoovered periodically by cleaning' it out. A small amount of mercury may be ted at intervals into the ball mill, if a plate

 

is used i~~ediately following the mill. Gold also accumulates in the ball mill and can be recovered only when the mill is down for relining. Notes on Figure 4 flo! shee!: i) See (1) Figure ~. 2) For the small ~erator the ball mill is preferable. At the Homestake a small amount of mercury is added to the ball mill. 3} The addi tion ot some new mercury is required at this point. The mercury consumption is determined by the amount of gold in the feed. The 10s8 of mercury in well regulated plants is about 0.1 oz. per ton of are milled. Plate area reqUired varies; the average is about two square feet pe~ ton milled. 'In stamp milling, six to eight tons of water are required for each ton of ore. With use of ball mills, this large volume of water causes classifier problems and necessitates introduction into the system of a thiekener. Oscillating Circular plates obvia te these problems. ' 4) See (4) Figure 3. 5) The classifier overflow should not contain probably more than three tOO$ of water to each ton of are. The overflow can, however, be diluted as desired for proper plate operation. 6} Plates used he~e catch only relatively fine gold. The coarse gold remains in the ball mill and classifier circuit, from whe~e it is removed by periodic clean-ups. 7) See (5) Figure 3. 8) See (6) Figure 3. 9) The clean-up m~rcury from the various places where it is caught contain~ a considerable excess of mercury. This is removed by ~'luee~ing in. a c.anvas or chamois bag. The exoess is retu~ned for. ~e~se on the plates. I '"' 13 .. IDAHO BUREAU 01' MINES AND GEOLOGY PAMPHLET No. 8'1 BaJl or rodmi"'® C/ean~u Mercur . Wat~r 1# new mercury Mer cur Class I fr er ® Mercury Sand Gold tJmot 'Qtn Plafes .. @ Squeeze in ® TQiling canvas or Ghomois £X'cess mercl.lr Gold amalgam RflTort(J) old' FLOW-SHEET Of AMALGAMATION PROCESS EMPLOY'NG SAL.L OR ROD MILL "AN.D CLOSED-CJRCUIT GRINDING Figure 4 10) In cleaning out the ball mill, the classifier and the mercury traps, i~ is necessary to include in the cleanup sand and conc~ntrate as well as the mercury and the gold it contains~ This matorial is placed in a small mill along with ~ small charge of balls. The mill is rotated for several hours, discharged, and the mercury separated from the pulp. The mercury is strained through canvas to remove excess mercury. The pulp is returned to the ball mill. Figure 5 represents a flow sheet of the cyanide process with its variati0P~ The solid lines give tho standard all-sliming process including agitation of the pulp in two or more agitators (Pachuca tanks or mechanical-air agitators),

 

 

thickening and filtration of tho pulp. The thickener over-flow is the strong gold solution and goes, as shown, to the preCipitation department. In some plants the counter-current decantation process is used, and in this case the agitators are omitted. The counter~current decantation process includes an agitator, and a series of thickeners. The pulp from the agitator is pumped to a thickener that makes an over-flow (which is the strong gold solution) and a thickened pulp. The thlck pulp is pumped to a second thickener that also makes a thickened pu~p and a clear over-flow. The system comprises a series of tanks and the jPulp advances from tank to tank through the system. The final tank makes a thickened pulp quite free from gold solution and may be discarded, or may, as shown, be filtered for recovery of the last trace of gold solution. Fresh walter is admi tted to the final thickener of the system and advances, by pumping from tank to tank, to the second thickener of the system, or counter to the flow of the thickened pulp. The over-flow from the second thickener conta mls some free cyani de and gold in solution, and is returned to the ball mill or classifier after it has been made up to the required strength by addition of new cyanide. The counter current decantation process is shown in the flow sheet by the dotted lines. If the combination process of sand-leaching and slime-agitation is to be employed, the flow sheet is modif'ied as shown by the waved lines. Some ores give up their gold value so readily that the extraction may be accanplished by sand percolation or leaching in porous-bottomed tanks. From one to several days is required for the di ssolu tion of the gold •. Notes on FiSRre 5 flow sheet: I) See. (I) Figure 2. 2) See (2) Figure 2. 3) New cyanide and lime are added, usually at this point, in amount equal to consumption of these reagents. Lime may be added to ore bin. 4) See (3) Figure 2. 5) See (4) Figure 2. 5) The Pachuca tank, a tall, small diamoter tank, provided cen trally wi th an air lift, is lTRlch used. Tho Dorr mechanical-air agitator is also widely used. The object is to have large capacity tanks in which pulp can be thoroughly agitated and aerated. Gold is dissolved in this operation. A number of tanks are made available and the procosis is either intermittent or continuous .• .. 14 - J IDAHO BUREAU 01' MINES AND GEOLOGY PAMPHLET No. :n .r-I.n ! ore bI n (]) { ® Feeder ® 1- . Acid lime and new cyanide t ® Ball or rod or tube mill !r-----------t Cla.ssifier ® Parf discarded Mi,y wifh wafer and pump fa toil r(Jct! FLOW~SHEET OF CYANIDE PROCESS SHOWING ALT£RNATIVE S(HEMES Figure 5 i 7) The Dorr thieDne~, a large diameter, relatively shallow tank, containing ~low-mov1ng pulp raking mechanism, is most"; Widely used. Clear solu tion (containing the gold) overflows the rim! of the tank and thick pulp discharges through a spigot ~t the bottom of the tank. 8) The thickened pulp ot (7) is further dewatered and waterwashed on filter. The filter is continuous and of the vacuum type. The dried cake contains 8 to 10 per cent

 

 

water. 9} & 10) It is neoessary to remove oompletely gold solution from the solids. This is best done by repulping the dried cake with water and refiltration. Solutions from both filters are weak :in cyanide and gold, and are used in the ball mill circuit, along with enough new cyanide to make up to proper strength. 11) The overflow from (7) is the strong gold solution and is ready for treatment with zinc for precipitation of gold val ues. It is fitr"st sprayed into. a vacuum tank which rC!lX)ves dissolved oxygen. This is the Crowe process. 12) The solution is then agitated with zinc dust. Some zinc goes into solution and the gold comes doWn as a black precipi tate. The solution called "barren solution" is removed from the precipitate by filtration in pressuretype plate and frame filters. The ' filter medium is canvas and filter paper. Part of this solution is discarded and part is kept in tlhe system for re-:use, because it contains a small percenta~e of gold solvent. 13) The gold precipitate contains considerable zinc which, after a roast, is fluxeid off in a furnace, or it is removed by treatment with acid. 14) A system of treatring the ore and water pulp in a cyanide solution to effec.t gold dissolution known as the "counter current deoantation system" is often used in place of the agitation system. It already has been briefly described. 15) Sand-leaching is an old and well established practice applicable to some ores. It is still practiced at the Homestake mine. It saves grinding, but requires large tank volume and building area. Figure 0 is a combination amalgamation, flotation, and cyanidation flow sheet. Straight flotation is shown by the solid lines. When amalgamation is employed to catch roarse gold, its introduction into the flow sheet is indicated by the dotted lines. Amalgamation mayor may not be necessary and this can be learned only through metallurgical studies in the laboratory or by careful studies in the plant. If a combination of ftlotatioin and cyanidation is required to give the desired extracti 011, and if' calculations indicato the combination to be more economical than oither process alone ,tho cyanide process is introduced as indic ated by the waving lines. The flotation concentrate m~y go direct to the smelter or it may be cyanided either arter, or without, a preliminary roasting. Roasting may be, and most often is, necessary for. good extraction of the gold from the conccntra te. Direct disposal of the concentrate to the smelter is the simplest, most conv~ient, and ~~ally the most economical, procedure to follow. ... 15 "" mAHO BtJUAU a.1dNI:s AND GEOLOGY C-' • CD I Inc ore bin +® ----1 ~ rwater 8 in .some cases mercury 801lorro ill® or here t I r- _ Water a. mercury- __ I «:, f @) I Amolgqmatif(n plate l i ' L- - - Rec.:;;~; ~~!d_ --7- --, Sand Cyonldlzation @

by ogifafion 8 filtration or ceo. 8. filtration c +.~ r® . I I Overflow, : I I I , @) t &ruI1gqmftionlP-l!J.1s I I . ~ LAmolS'om __ ?_-I PAMPHLET No. 37 Afer",C)~ ~g~ @ I I I L "yer~tY-+-f I @) • @ Retort Set! Figure .s ThlcfIener I I " - -~ L..J Reagenf '"8" t ® + ® MelFI ng. furqqce Co ditioner rlof. tion r ug,her cells (!) Concentrate Ucil® .. Waste t • Slog Gold bullion Flofofion cleaner cells ® Toil r d/r~cf to FLOW-SHEET OF COMBINATION AMALGAMATION, FLOTATION. AND CYANIDE PROCESSES Figure 6 Flotation tails may contain silf'ficient value~ to consider tbelir treatment by cyanidation. This alternative ~s indicated in the flow sheet. Combination processes are mor~ expensive in first cost and to operate, and are justified only after thoro'\.lgh metallurgical studies and calculations. The above flow sheets do not ~nclude all operating plant details. They are here included simply to give a general picture of gold recovery processes. Each ore requires individual consideration for best metallurgical and commercial results. Notes on Figure 6 flow seeet: 1) See (1) Figure 2. 2) See (2) Figure 2. 3) See (3) Figure 2. 4} See (3) Figure 3. In this flow sheet excessive dilution not only in the cl~ssifier, but also in the flotation system, must be avpided. The pulp should contain not much in excess of 30. pelr cent mter. It large volume of water is added at this pOint, or at (14), a thickener in the flow sheet will be reqU!ired. 5) See (4) Figure 2. Certain flotation reagents may be added at this pOint. 6) In the flotation process it is often necessary to give the pulp a prelimin8?Y agitation with certain reagents in advance of the flotation cells. This device is called a condi tioner. It is a small tank in which is installed a pulp-stirring mechanism. 7) & ll} The first set of oel1s racei vi'ng the pulp make a concen tra te containing the go~d values and a tailing. The tailing generally is discarded. Should it contain economic values, it may, as is being done in a number of plants, be treated by the cyanide process.

 

8} The grade of the concentrate may be markedly increased by a second frothing treatment. The tailing from this second flotation treatment is returned to the initial flotation cells, or it may be returnod to the ball mill and ground 9) & wi th the oro. 10) The gold concentrate is most conveniently sold to a smeltor. In some instances, howevor, it may be more economical, be cause of the isolated location of the mine, to cyanide the concentrate produ~ing gold bullion. The concentrate mayor may not require a preliminary roasting before cyanidation. From some concentrates the gold is readily extracted by cyanide while others are highly refractory. 12) See Flow sheet Figure 5, 14),15),16), and 17) See Figure 3~ PLANT AND OPJRATlNG c~ IN MODmN GOLD MILLmG PRACTICE i Information Ciroular No. 5433,j U. S. Bureau of Mines, March, 1931, entitled "Amalgamation Practice at Porcupinei United Gold Minas, Ltd., Timmons, Ontario," written by Ronald A. Vary, gives'co~plete details of operation, refining, and costs ot a modern 30-ton gold amal~tion plant employing also blankets and tables. The operating cost per ton of are treated is $1.851. Infbr.mation Qircular No. 5408, U. S. Bureau of Mines, February, 1931, entitled "Milling Methods and Costs at tlhe Homestake Mine, Lead, South Dakota," by that famous gold metallurgist Allan J. Clarkt is a veritable treatise on the subject of modern gold milling. The are assays about $5.00 a tpn, 52.8 per cent is recovered by amalgamation, and 30.2 per cent by oyanidation. The total gold recovery is 93.0 per oent. The total milling cost for the year 19219 was $~503 per ton of ore milled. Detailed costs of every part of the operatiQn are given. Information Circular No. 6236, U. '8. Bureau of Mines, February, 1930, enti t1ed "Milling Praotice at the Alaiska Juneau Concentrator, tt by P. R. Bradley, is another excellent contribution to the subject of milling gold ores. The outstanding features of the operations upon tho Alaska Juneau are are an are body of such size and character as to permit a caving system of mining whereby are is mined and delivered to the mill at a cost of $0.25 to $0.29 per ton; rejection by screening and ha~d-sorting at approximately 50 per cent of the material mined, at a cost of $0.13 to $0.15 per ton rejected; milling of the sorted ore at a cost of $0.31 to $0.33 per ton milled, giving a total operating cost of $0.51 to $0.5? per ton trammed from the mine. Information Circular No. 64?6, U. S. Bureau o~ Minos, July, 1931, entitled "Milling Methods and Costs at the .l\rgonaut Mill, Jackson, California," is an article giving complete metallurgioal and operating data on a present day gold milling plant employing amalgamatitin and gravity concentration. Tho mill heads in 1929 assayod $6.0? in gold; 69.0 per cent is recovered by amalg~ation and 18 per cent by concentration. Theora is crushed in stamps through a 24 mesh screen, and amalgamation is affected both in the mortar and on shaking tablos. The total milling cost in 1929 was $0.95 per ton ,of ore treated. Information Circular No. 6411" U. S. Bureau of Mines, March, 1931, entitled "Milling Methods and Costs at the Spring Hill Concentrator of the Montana Mines Corporation, Helena, Montana," by L. N. Grant, is an example of a plant that rec ently has been converted from cyanidation to all-flotation recovery of gold. The gold is associated with pyrite, arsenopyrite, and bismuth. Modern eqUipment is used throughout the flow-sheet. Tho total milling cost per ton of ore treated is $1.00? "Gold Milling in Canada,t' Bulletin of tho Canadian Institute of Mining and Metallurgy, January, 1930, by J.J.Denny, is another excellent roforence for those interested in modern gold reoovery methods.

   

COST OFERECTma GOLD TREA'IMENT PLANI'S ~ '~ The cost of a metallurgical plant for gold recovery depends upon a nwnbor of factors. Large tonnage plants Qost loss por ton of ore to be treated than small ones. Simplo flow shoets are less costly than those for complox orcs; henco, tho difficulty of extracting the gold is a primo factor in determining the initial plant cost. Aocessibility and locat1on,which detormine cost of materials and transportin.g and installing oquiprnont, arc lar!Q variable factor s. '\"!' 17 .... The cost of building a straightl amalgamation mill will run from $450 to $700 per ton of ore treated per 24 hours., All.slime cyanide plants cost from $aoo to $1,400 per ton per 24 hours; a1l-floltation plants, from $600 to $800, depending upon the size of plant (tons of ore Itreated per 24 hour.s). For metallurgical cost and ope~ating data, the reader is referred to the following manufaoturer's bulletins: Bulletin 2901 - Resultsl in Modem Flotation: Denver Equipment Company, l419-l7th street, Denver, Colorado. Metallurgical Bulletin - The General Engineering Company, 159 Pielrpont Avenue , Salt Lake City, Utah. Notes on the Flotation ~rocess - Southwestern Engineering Corp., 606 South Hill Street, Los Angeles, California. "The Trend of Flotat iOlli, tt - Colorado School of Mines Quarterly, Vol. XXIV, No.4, October 1929. The above publica~ions, the first three of which may be had on request, and the last at a small charge, contain data sut"fi cient to answer practl cally any question pertaining to ore dressing and milling. METALLURGIC.AL TESTING OF GOLD ORES Gold ore having been found in What appears to be sufficient quantity to be of commercial interest, the problem jarises as to 1 ts proper metallurgical trea tment. The large company knows the proper course to take, namely, that a thorough metallurgical study by a competent IIfetallurgist should first be made to determ ine the most economic extraction, i.e., the type of metallurgical treatment that wil l net the greatest return per ton of ore treated. The inexperienced, however, may not get off to a good start. In short, the proper procedure is to obtain the help and advice of a reputable and experienced metallurgist. MARKETllJG OOLD BULLION.AND RICH GOLD PRODUCTS Gold is the easiest of all metals to sell, The only procedure necessary is to take or send by express the gold bullion, pure or impure, to the nearest United states Mint or government assay office, where the metal will be assayed and settlements made on the basis ot $20.67183 per troy ounce, 1000 fine. Silver in gold bullion is paid for at the Gurrent market price. Base metal content is not paid for. The Treasury Department maintains an assay office in New York, Helena, Montana, Deadwood, South Dakota, Sal t Lake, Utah, and Seattlo, Washington. At 'New Orleans, Louisiana, and Carson ¢ity, Nevada, there arc mints that are conduct ed as assay offices. All the insti tutions mentioned are bullion-purchasing offices for the largor institutions in Philadelphia, San FranCisco, ~nd Denver, where coining operations aro centered. The assay officos also assay gold and silver ores for prospectors and any others interested. The charge for gold or silver is $1,.00. Gold-bearing matorial, such as gold ore or gold concentrate, will be bought by any copper or lead smelter. The minimum amount of gold sottled for, say, in the case of a lead or copper concentrate, is ,01 to .05 ounces par ton. Often the entire assay content is paid for, but most smelters pay only for 95 per cent of the assay content, and the price paid varies fram $19 to $20 per ounce.

.. 18 I, Gold concentrate invariably contamls silver. The smol tor pnys for 90 to 95 per cent ot tho silver content as dete~nod by fire assay, at the New York quotation, if more than one ounce is present. The value of a ton of ore or concentrate at the smelter is determined not only on the gold content, but also Ion (1) the valuable recoverable base metals, such as lead, copper, and bismuth, (2) the undesirable constituents from the purely smelting point of View, sucb as, for example, zinc, sulphur, arseniC, and antimony, and (3) tho desirable co~sti tuents from a fluxing point of view, such as, for example, iron or limo, or slilica~ Lead in concentrate 1-s usually paid for at 90 per cent of the lead contant (wet assay less Ii per cent) at the New York quotation, loss 1.65 cents pe~ pound. If less than 5 per cent of lead is present, it is not paid for. Paymen t for copper in con contra te is determined on the bas is of the wet assay less than 1 to 1.5 per cent aind therefore, naturally, when the o.opper con tent of an are or concentrate is lCiSS than 1.5 or 1 per cent, it is not paid for. A second doduction of 2i to 3~ cents per pound of coppor is made to cover fro igb. t , .se 11 in g, and del ivory charges. Zinc is an undesirable constituent in mnolting gold orcs or concentrates. Tho freo maximum per cent allowable, at nearly all smelters (excepting those employing the Waolz process) is 5 tb lQ per cont~ The penalty is from 25 to 50 conts per unit (one unit equals 20 pounds per ton) for each unit in excess of tho allowable amounts. Sulphur is penalized at tho ra;te of about 25 oents per uni t above tho allowablo free maximum per cent which ranges from one to four per cent. Tho maximum penalty is $2.50 a ton. Silica may be penalized, or it may command a premium, If a smelter is S0 located that it rocoives an excess of basic ore or concentrates, i,o., matorjal containing high per centages of iron, it will pay a premium for the silica. FOl example, tho Consolidated Mining anid Smelting Company of Canada, Trail, B. C., pays a premium for silica at tho ra~e of ? cents a unit and penalizes iron at tho rate of 5 cents a unit. It thus pays tho seller to "shop around" an.d learn the condi tions prevailing at the various smelters. He should write to severnl of the smelters within possible shipping distanoe of his ptroperty, giving a complete chemical analysis of the matorial for sale, and obtain from thE) smelter a quotation of the net value of a ton of arc at that smeltpr. These quotations, in conjunction with freight rates, which the smoltor al~o is in u position to give, provides, of coursc, the necessary data upon which a decision is reached as to tho most profitable place to do business. The smelter also makes a base charge for smelting the ore. This basocharge varies \nth tho grade of the ore from $4 to $10 per ton. Freight rates also are based on the value of the are. The higher the value of the material, tho higher the freight rate. Good referencos on the subject of buying and selling ores and metallurgical products are as follows: .. c .• H. Fulton - "Tho Buying and Selling of Metallurgical Products:" U. S. Bureau of Minos Technical P~'or 83, (1915). Arthur J. Wcinig and Irving A. Palmer - "The Trend of Flotation;" Colorado School of Mines Quarterley, Vol, XXIV, No.4, october 1929. ... 19 Spurr and Wormser .. '''Mar. 'ke~11ng ot Metals and Miner. als;" MeOrs -Hill Book Compuy,370 Seventh Al'8nue New Y rk City. FLOTATlCE qF METALLIC GOID Int~oduction Gold has a constant and invari~ble value of $20.6? an ounce fixed by law,

 

 

as already stated. It is the only metal Whose value is not effected by the law of supply and demand, or by economiQ depressions. Hence, naturally, in times ot depression when the market prices ot other metals are so low as to discourage base metal mining, there developes ~ncreased interest and activity in gold minin g. In such times too, conditions are decidedly favorable to profitable gold mining because of cheap labor, supplies, and transportation. The School of Mines at the Uni'\tersity of Idaho receives almost daily letters, or people call in person. requesting information relative to methods of gold rec overy. It is the purpose of this shor~ paper to give a tew examples of gold flotatiol and to discuss the application of t~otation to the treatment of gold-bearing materials. Metallic spld, like metallic capper and metallic silver, floats readily if proper flotation conditions are pro~ided. SUrprisingly coarse gold can be succes s fully floated even in the absence of froth stabalizing minerals such as the sulf ides. In the case of quartz and oJ¢ldized siliceous ores, the condition necessar:' is veIy similar to that required for flotation of metallic copper in Michigan oopper ores. Suffi cien t frothing aaent is added to produce a -'clean white foa m which runs freely ever the lip of t~e flotation cello. The metallic gold pal:"ti ell are entangled in the watery froth w~ich readily breaks up on leaving the flo~ati (. cell. When the gold is free, but ass~ciated with sulfides, the free (liberat.ed) gold and the gold locked in the sul~ide particles float in a froth that readily forms on the surface o~ the pulp in the flotation cell. This sulfide-laden froth 1s much more stable. The gold in fine placer sand or in black sand resulting from sluicing and table concentration operations also itloats freely. Limitations o~ Flotation~~~ The flotation process of cours~ has its l~itatlons. Very coarse gold cannot be lifted py the bubbles. It is di~ticult to indicate by the use of sieve scale numbers, for example, such as 48 me~ or 85 mesh, the size of particles that can be floated. The shapes of the parti:cles determine this, assuming. of course, th e particle surfaces are clean and lus~rous. Much of the gold in ores is in the for m of thin flakes, and same of it OCCU~B as compact particles. The gold particles in most black sand concentrates have the form of thin. discs. Pieces of this aha pe as large as i millimeter in diameten float readily. "Float" or "flour" gold, whi c!: always has given trouble in amalgamation .and gravity concentration processes, f loa: easily. Crushin~ and Grin din...& The purposes of crushing and gr!ind1ng are (1) to liberate the gold and sulfide from the worthless rock, and (2) to reduce the material to a fineness that can be lvIati •• 1a ttae tlotatiQll mai1Jle. It 18 difficult to a.p coarse sand in ctr011l.a"tOA aa4..,...1011 tnth eells, 8IldperilapalO __ ah sand is about as coarse aa can be suooea.tul.ly had! 4. Gold ores of the compla aul.t1 de olas8 otten require very tine grinding, d in some cases the tinest grinding po.8ible in practioal operation is not suttiF~ent to give satisfactory l1beratlQn. On such ores, it is ot course ~possib~e to get satistactory reooveries by flotation . Fortunately this type of ore is not! often encountered.

 

Use. ot Amal.gamat~on Ahead ot Flotati on In any case, ooarse gold proba~ly should be removed by amalgamation, although this operation often may be ~-voidedi Without sacrifice in total recovery. When grinding is done in a ball mill in ~lose circuit with a olassitier, the ooarse gold accumulates in the system and ~es not escape in the classifier over-tlow. Muoh EP 1d is retained in the ball m~ll in the liner joints and this is reoovere d when the mill 1s shut down for rel~ing. The classifier may be "cleaned up" at regular intervals. The clean-up majt;erial is dewatered and shipped to the smelt er or it may be treated in a clean-up barrel with mercury. There may, however, be certain. economic advantages in catching the coarse tree gold on plates from which the ~ld may be recovered daily. As already stated, the freight rate charged by the railroads is based on .the grade (value) of the conoentrate, the higher the $rade, the higher the freight rate. Also otten the base charge made by the s~lter for smelting the concentrate 1s on a sliding scale, the Charge tnoreasi~ with eoncentrate grade. When amalgamation is employed ~n conjunction wi th flotation, it must precede the flotation treatment, since the tlotation reagents seriously prevent SOld frt t._ attaching to the meroury. Also ro~se of the mill water recovered from the tails is out ot the question, beoause it contains tho harmful chemicals. Ratio of Ooncentra~ion and Grade of Product The flotation process, unlike tho amalgamation or cyanide processes, in most cases, is not a gold reoovery proce~s. The gold value is contained in a concentr ate which is a small and ¥ariable fraction of the original ore. Clean quartz ores, naturally, give a very high-grade concentrate and high ratio of oonoentrat ion. Massive sulfide ores give lo~-grade concentrate and low ratio of concentration. In flotation concentration of gold ore, naturally, the effort is to make as high a ratio of conoentrati~n as possible. This is accomplished by skillful use of flotation equipment and chemical reagents and is a job for the experienced flotation metallUrgist. ~ohiteEl ~eeded The machinery needed in the average small gold flotation plant is a rock crusher, ball mill, classifier, flotation machine, reagent feeders, and a concen trate filter. Examples of Gold Flotation ExamPle No.1 'The gold was partly free and p.rtly intimatoly associated with pyrite. The silver occurs prtnoipally with tetrahedrite. Results: Feed Tails Concentrate Recoveries Gold 0.210 oz. 0.041 oz. 44.00 oz. 80.5% + 21 Silver 15.08 oz. 4.07 oz. 3000.00 oz. 72.8% I MallY years ago tho i.8 ore 1188 tJated by I!Itrail#lt oyan1dation, but the reco

veries were low and the consumpti~n or cyanide high. I .ple No •. ! The ore is a very intimate mix,~ure of arsenopyrite, pyrite, and pyrrhotite, with calcite. The gold and silver ~re associated wi th the sulfides. Results: Heads Concentrates Tails Recovery Ex!!$l~ No.3 !JQlS .2£. 0.35 to 0.40 7.00 to 9.00 0.03 91e;O - 92.5% This is oxidized siliceous or~, the gold being free and not associated with sulfide minerals. Results: Heads Concentrates Tailing Recovery -Gold- oz. .59 4.34 .08 88.00 % The concentrate of this test ~hen recleaned in a second cell assayed 8~25 ounces of gold per ton. ExatP1e No.4 In this ore the gold is assoc1ated with pyrite, galena, sphalerite, bi,smuthinit e, and a few other sulfides. nder a microscope the gold appeared free as filling between the mineral erysta 8 and incased in the sulfides, parti cularly the bismutheni te. Amalgamation un~er favorable conditions gave an extraction of 40 per cent of the gold. Resul ts t Heads Concentrate Tailing Gold oz. 1.53 8.16 .48 Gold oz. Silver oz. Lead % Heads 1.62 17.P. 1.5 Concentrate 23.62 227.+1 19.8 Tail:ing .47 2.8 .15 Silver oz. 17.1 . 90.4 3.8 Recovery ~ Silver ?7% Recovery Bismuth % ~ Silver ~ .04 1.84 72% 85% 90% The sample was grqWld to 68 ~r cent .,.325 mesh. -22-

I! _l,e No.5 I This is highly siliceous gold 4re containing small percentages of iron sulfide and galena. Amalgamation reootered only 24% of the gold, and cyanidation 'l4/fo. Results: Heatls Con¢entrate Tailing RecQvery ~~. 2.35 32.00 .43 83.00 % The sample was ground through ~ -65 mesh sieve. Extraction of the gold in the -200 mesh part of the tail was $0%. E:x:$le No. 6 This is highly siliceous gold Qre with a small amount of pyrite. Amalgamatic" alone recovered 75 per cent of the gold. Cyanidation recovered 90 per c nt of the gold. Results: Headis Con een tra te Tailing ReCCi)very Ex~ple No.7' Gold oz. .43 6.26 .03 93.00 % This is highly oxidized siliceous gold ore. Results: Heads Concentrate Tailing Recovery, Example ~o. 8 Gold oz. ,54 20.76 .03 94.00 % S.iIvar .2!' 1.09 29.8 .4 63.0 % This was a black sand concentrate resulting fram concentrating by panning Snako River sand assaying 50.22 oun¢es per tan. The gold in the sand was in the form of thin plates from 0.1 to 1.0 millimeters in diameter by about .01 millime ters thick. The black sand was about 35 to 65 mesh in size. A 500-gram charge was floated in a gravity-flow mechanical flotation machine with propor flotation reagents. The following results were obtained; Gold oz. ~r oz. Recovery Heads 50.22 Concontrate 8620.00 1680.0 Tailing 1.67 2.0 97.3 % Tbe value of the concentrate in gold is $172,400.00. These results bring out unmistakably the high potentialities of flotation

as a gold rocovery process. - 23 The gold in river sand is fine~ much of it rete!'red to as "flour Bold," and 1 ts recovery always has present~d a hard, if not insurmountable, problem. Gravi ty concentration gives a black: sand concentra'te which contains much, but not all, of the gold. The loss of sold is high and recovery 01' gold from the black sand concentrate in most inst~nces cannot be efficiently made by working it with mercury, and cyanide acts tbo slowly. In view of these resul ts, the :placer miner may :rind it advantageous to add to his kit 01' apparatus a flotation cell and a few chemical reagents. These results suggest that poseibly in the future river sands may be profitably handled by washing or screenin~and flotation of the fine gold containing sand. The concentrate will be a nearly pure gold ready for sale to the Unit ed States Mint. Rragents The reagents required for gold flotation are few and simple. The following usually are sufficient: Soda ash, 0.5 lb. to 2 lb.; sodium amyl or ethyl xanthate alone or sodium aerofloat;, .005 to 0.05 lb. per ton. If aerofloat (15 per cent strength) is used, pine oil is not necessary, but if xanthate is used, pine oil is needed for frothing. In some cases, a very small addition of copper sulfate, or sodium cyanide, is beneficial. On oxidized ores sodium sulfid e assists materially, Close regulation of reagent feed is essential. FOr this purpose, standard and approved reagent feeders should be used, of which there are several on the market. Treatment qt the Concen tra1!E!. The concentrate, either with or without thickening in a sui'table thickener, is fil tared on a continuous vacuum fil tar. Where the tonnage of concentrate to handle is large, thickeniag of the concentrate as it' comes off the flotation machine, is standard practice. At small plants, often the concentrate is tiltorc ( direct. The filter makes a product, containing 8 to 10 per cent moisture. The We~l ~quip~ed Plant The well equipped flotation plant, of course, includes automatic weighing apparatus and samplers for sampling both mill feed and tails. The plant includes also a suitable assay office and assayer. A complete metallurgical balance sheet and cost data are kept by the metallurgist ruld his assistants. The sma1l scale operator, however, gets along Without these facilities and interests himself mainly in getting ore into the mill and smelter returns. It is remarkable too the good work tha t can be done by flotati on with hay-wire equ~pmont and common sense. t' 24 I Austin, L. S.: Metallurgy of the Common Metals. (1921) John Wiley & Sons, Inc., New York. Chapters XII, XIII, and XIV. Bray, John L.: The Principles ~f Metallurgy. (1929) Ginn & Co" New York. Chapters X and XI. Clennell, J. E.: The Cyanide Handbook. McGraw-Hill Book Co., New York. Gaudin, A. M.: Flotation. (1'J32) McGraw-Hill Book Co., New York. Hayward, Carle R.: An Outline ot Motallurgical Practice. (l929)~. Van Nostrand Company, Inc., 250 Fourth Avenue, New York. Chapter XIV. Julian and Smart: Cyaniding Gold and Silver Ores. Griffin & Co., London. Liddell, Donald M.: Handboolc of Non-Ferrous Metallurgy, Part II. (1926) McGraw-Hill Book Co., New York. Chapters XXVIII and XXVIX. Maclaurin, J. S.: Tho Dissolution of Gold in a Solution of Potassium Cyanide. Vol. 53 (1893), pp. 724-738; Vol. 67 (l895), p. 199. Jour. Chem. Sao., London. MacFarren, H. W.: Cyanide Practice. McGraw-Hill Book Co., New York. Richards, Robt. H.: Text Book Qf Ore Drossing. (1909) Gravity stamp and

 

 

Amalgamation, Chapter V. McGraw-Hill Book Co., Now York. Roso, T. Kirko: The Metallurgy of Gold, Griffin & Co., London. Taggart, A. F.: Handbook o f Ore Dressing. (1927) John Wiley and Sons, Now York. Thomson, F. A.: Stamp Milling and Cyaniding, 1st Edition. (1915) McGrawHill Book Co., Now York. White, H. A. and others: Text Book of Rand Metallurgical Practice, 3rd Edi tion. i- 2f) MERCURY, Hg, a solvent of gold, is used in gold amalgamation processes., Cost per pound - - ~ - - - - $0.72., CYANIDE, KIN, or NaCN, and cyanamide cyanide, has the following uses in gold metalll~gy: Cleantng copper plates for mercury coatings, dissolving gold and silver in the cyanide process, and for depressing iron and zinc, and for activating gold in gold flotation. In cyaniding,the s~lution contains from 2 to 8 lbs. cyanide per ton of solution. tn flotation, from .1 to .01 Ibs. per ton of ore are added to tij.e pulp. Cost per pound"barpel lots,$O.15. LIME, CaO, is made cheaply by calcining limestone. Used in the cyanide process (1 to 10 lb. ~er ton of ore) to neutralize pulp acidity and to promote classiftcation of pulps. Used in the flotation procesf to depress iron minerals and as conditioner. It should not be used in gold flotation, Cost per ton - $R.50. ZINCllJST, Zn, used in the cyanide process (about .1 lb. per tun vi' ore) for preCipitating gold from cyanide solution. Cost per pound - - $0.10. ' SODIUM CABfONATE, Na2C03' has a vari~ty of uses. As a flux in gold refining and fire assaying; in flotation pulps to maintain alkalinity and to promote selectivity of mineral flotation. Cost per 100 Ibs. - - - - $1.15. SODIUM SILICATE, (Soluble glass; sodium meta-silicate; liquid glass; water glass) is a valuable reagent in flotation of some ores. It is a strong depressant of gangue slime, producing a high-grade concentrate. Often b~nefio1al (1 to 4 lb. per ton) in flotation of oxidized ores. Cost per lb. (30%)100-lb.drums,$0.60. SODIUM SULFIDE, Na2s, in carefully controlled amounts (i to 3 lb. per ton) is an effective depressant of iron and zinc. Often highly beneficial in flotation of some ox1dizad gold ores. Cost per lb.(60 to 52%)100-lb, Qrums,$0.035! ZINC SULFATE, ZnSC4, is used occasionally alone but more often in conjunction with cyanide to depress zinc and iron minerals in flotation. Cost per 100 lbs, - - - $3.25. , COPPER SULFATE, CuS04' blue vlttol, is widely used (i to 3 lb. per ton) as a strong activator ot zinc in the flotation process. It also aids gold flotation from some ores when used in very small (.005 lb. per ton) quantities. Cost per 100 lbs. - - - - - $3.25. FERROUS SULFATE, FeS04' is a standard precipi tan t of gold from gold chloride solutions. HYDROGEN SULFIDE, H2S, a gas soluble in water, also of gold from gold chlor.ide solutions. of sulfuric aci d on a sulfi de, usually strong sulfidizing agent of oxide ores purpose in flotation. I - 2e~ an effective precipitant It is made by the action iron sulfide. It is a and is used for this LEAD ACETATE, (sugar of lead), ~bt (C2H302}2' 3H20, is occasionally us~d in aiding tho proci~itation of gold and silver from cyanide

 

solutions in the cyanide process. Lead nitrate may be used instead. Cost per lb. in 100 lb. barrel, $0.11. pmE OIL, steam distilled, is wtdely and principally used as a frothing agent in the flotation process. Cost per gal. - - - - $0.45. CRESYLIC ACID is used for the s$lle purpose as pine oil and is widely ~mployed particularly in lead ore flotation. It is effective on gold ore flotation. Cost per gal. - - - - $0.50. XANTHATES, ethyl, ~yl,. and butyl, are universally used as mineral flotation promoters in the flotation process. They float metallic gold, but must be used in minute amounts ~ from .05 to .001 lb. per ton of oro. Amyl xanthate i.s particularly effective on oxSUIJru:RIC ACID ~ idized ores. Cost per 100 lbs. - - $O.20~ H2S0 , used in assaying in parting gpld-silver beads. Silver is dissolved by it~ At some cyanide plants sulfuric acid is usod to dissolve the excess of z:inc from the zinc-gold preCipitate resulting from the treatnPnt of gold solutions with zinc dust~ Sulfuric acid is also an activator of pyrite that has been previously depressed with lime. Cost per ton tank - ~ $15,00. AEROFLOAT is oresyiic acid with phospho-pentasulfide in different strengths. It is a much used promoter in flotation; Cost per 100 Ibs. - - $20.00. NITRIC ACID, HNO~, is used in a$sayi~g to part gold-silver. beads. When 'mixed WIth hydrochloriO acid in the proportion of 82 parts hydro~ cloric to 18 parts nitric aCid, aqua regia is formed, which is a strong solvent of gold and platinum. Cost per lb. in carboys $0.055~ 1- 27 ,.