THE EXTRACTION OF GOLD.

West Coast Times, Issue 324, 6 October 1866, Page 1 (Supplement)

THE EXTRACTION OF GOLD.

At the last monthly meeting (Sept. 10) of llio Ko\«l (Society the following paper on the a1)a 1 ) ye subject was road by Mr 11. A. Thompson :— The paper I have the hoaov to lay before the Royal Society has be en compiled from my notes of experiments extending over the last sixyOE-isp.ven years, and entered on with* a. .Yww °f diminishing the heavy loss fflj4£s# ( now sustained in reducing quartz,., 'Mfc^ al ' jP '' portion of these expei'imenjjJliHaj'e been carried out at the Port P«HBffl|SflSipany's works at Clunes by the jHHsj!&f the conipanyyor in conjunction <i}P!Jppp!in, and. are the more important, as onThat large establishment there ib every facility for conducting the trials upon a working scale, while an assay office attached to the works allows of every step beuig tested with the accuracy which alone can make the results obtained reliable. It has long been known that a greater loss occursjn the treatment of -jpldsores than is the case with any other metal ; and although this subject has attracted the notice of scientific practical men for jnany years, the advance hitherto made has hardly been commensurate with the attention bestowed on it. In the old gold-mining works of Europe and Sonth America, the loss runs from tweuty r nve per cent upwards, not- - withstanding the accumulated experience of s' veral generations of minerS ; and, in California, 'Professor Sillimaty reports that his examinations of tailings from the different works in the Grass/ Valley showed a loas of eighty dollars- pay, four ounces — oi gold per ton ; and he ud.is, on the authority of one of the moat experienced and cautious, of metallurgists of Cali- | fornia. that the saving in a large number of cases wa,i barely thirty per cent, of thegross contents of the oiv, as determined *by his own careful a=say3, both of the ore and the waste. In this colony assays of tailings from' many different gold-fields) have led me to the conclusion that the average loss ' sustained in" crushiiig is not less- than thirty- five per cent, of the total gold coulents, ojp, at the lowest calculation, L 1,000,000, 000,000 per "annum. I doubt if any woik^ are sustaining less than twenty-five per cent, loss, and several instances have come under my notice where there was a large per centage cf sulphides in the quartz, and the loss of gold was iit consequence from fifty to seventy per c,ent. of the tota' contents. It has been commonly asserted that this colony was' far behind other mining countries in the adaptation of improved methods bf extracting the gold, but judging from tho most reliable, returns we can obtain, this certainly is notthe case, and I think that either as regards the character of our machinery, or the attention given to improved ' methods of treating the ore, wo have no reason to be ashamed of the position we hold. Tt should not be matter for surprise tha*- it has taken us fifteen years to solve the problem which has baffled other gold miners for 'four times that period, with all the resources of an old count'-y, cheap labor, and cheap" material, at their command. For convenience tho gold found in mineral veins may be divided into two classes : first, the free gold, meaning that deposited in the quartz or slate maa form rendering it capable of liberation by the ordinary process of crushing ; aud, second, the pyritous gold, meaning that deposited with aud enveloped by the sulphides of iron, copper, antimony, and lead, but principally with iron,- in tin's colony. Free" Gold — The greater, part of the free gold is deposited in particles large enough to be liberated and retained fry the ordinary, reducing process ; hut in all guld- ' bearing quartz, a certain amount of gold exists which cannot be retained by the usnnl mode of treatment ; and. in some few exceptional cases, the proportion of this five »\,ld is so considerable as to /form nearly 'one half of the gold Jost. This fine gold, when seen in the quartz, presents the nppearance of minute patches .of gilding, and must be detached in thin flat pieces, readily floated away in running w ate/. A part may be pyritous gold, liberated in the breaking np of the iron ores. In practical working, some of this fine gold must always be lost, even where the greatest care is used j but a large proporti n of that riow carried away would be retained with the pyrites, by any efficient mode of concentrating the latter. The pyritous gold is so incorporated with the iron and other ores, that it cannot \>e separated by the means used for extracting the free gold. A question his arisen whether this • pyritons gold exists in combination with the sulphides, or in a metallic state. Experi ments made at Clunes, u.»ing hyposulphite of soda as the dissolving a^cnt, showed a ,trace of other than metallic gold in rich pyrites, but-jione in auriferous antimony ore. In the first case, the quantity was evidently so small as to be of no practical importance, nearly all the \ gold In ing mechanically deposited with the pyrites. Ih a sample washed from the blanket strakes, in which the particles of pyrites , average about 1-lOOOth of an inch in diavineter, gold can be seen on the broken faces in. still minuter particles, and I believe the great bulk of the pyritous gold is in this finely divided state. Pyrites in this co'ony have been found to contain 200 ounces of gold per ton, and in New South Wales o\er 2000 ounces of gold per ton ; but these were probably picked specimens, and would not represent the average yield of the sulphides in-the vein. ■ I have, _ however, half a'tpii of pyrites leady for treatment at the Good Hope mine, holding nearty 100 ozs of gold to the ton, and whenever quartz 'veins contain "ft paying amount of free g"old, and carry from two to five per cent, lit pyrites, the latter are nearly certain to be rich in gold; i have as yet met with no instance where tlieyield of the pyrites was not in proportion to -their per centage in the quartz aqd the amount of free gold that could be obtained from the ore. But it is not the sulphides that exist in the quartz veins only that are auriferous. Many of the blue slaty beds, at a distance of several' fathoms from tne mineral veins, contain pyrites in scattered crystals, studding the rock. "These crystals have been collected and, on assay, gave from 5 dwts to la dwts of gold per ton. This fact may thrdw some light on the cause of quartz veins b ing frequently productive above, and poor .below, .the water line— a circumstance usually ascribed <o the pyrites being undocoi.iposed below the water. This hardly sufficient to account for the sudden failure of the gold in depth, in some cajses ; and it is possible that the existence of large quantities of undecompo'sed pyrites in the adjoining slate-beds may have An impoverishing effect, -by, holding, the gold, and preventing its aggregation ia the quartz veins. Hitherto, I have only referred to the sulphides found in the quartz or slate, .but in

the old auriferous drifts cf Jj.illarat, trunk' of old trees are found embedded in die ) gravel, and on tbis'old' timber pyrites l-avt [• frequently formed. One beautiful speci- . men of crystallised white iron pyrites de- » jiO3itqd on a piece of wood, taken from a s drift immcrl iately below the trap rqfk, gave • by ( ssay 40 ozs of gold per ton. In anoi ther case, where the old trunks were burst ! open, nnd only the sulphides formed in the heart of the tree were saved, the yield was over thirty pennyweights of gold per too. Some of the fine dust obtained in. washing out the gold at the 'Royal Saxon claim, Ballarat, yielded by assay . over fifteen ounces of gold per top. When placed under the microscope, this cSust was seen to be composed of minute crystals of pyrites aggregated into round pellets, from I -300th to l-100th of an inch in diameter, the suiface being roughened by the projecting angles of' the crystals, and unwaterworn. These results show that the deposition of goldalong'with pyrites has. b.eu in operation at a comparatively recent date, and is probably still going on. To enter into all the details of the researches made would extend the paper to too great, a length ; and it will be sufficient to state that careful trials, including ass'aj'S of total contents, different modes of amalgamation to ascertain the amonnt of free gold, dud microscopic examinations, indicated that the bulk of the gold lost was enveloped in pj'rites, except, in rare instances not more than oae-foui th of the loss being free gold, and this was usually small flaky piece.-, floitcd off with the water. TJie proportion of .free gold left iv the waste -tailings will depend an the fine ox coarse particles in which it may have * hecn deposited in the quartz, and also on the mare or lets perfect character oi tho. means used for retaining it. A small per-cantago albo exists in the form of gold still attached to-particles of quartz. The plan first adopted by our miners was to roast tho quartz in stacks, i-i the; opon air or 'in kilns, to G\idise the siriphidu-s and to liberate the gold, whilo the qua.:t2 was rendered more friable and easy' to crush. After several years' trial, this system was given up, as it was found to be rather injuriQus than otherwise. At a low heat the pyrites in the interior of the quartz >\vere little changed, while the free gold was coated with a film of some material — probably bulphur, — which impeoefl the ,action of the mercury on it. When the roasting was carried on with a higher ■ degree of heat, the- oxide of iron formed on the exposed faces- of the quartz acted as a flux, and a glazed surface of clay was formed, in which "numerous minute globules of gold could be be discerned under the microscope, similar to those found in the waste -tailings when crushing roasted quartz. In the inteiior of the quartz black, veins were formed by the melted mono-sulphide of iron ; and other experiments led to the conclusion that a portion of the melted gold was diffused through these blaclc veins in a form which rendered it more difficult to separate than when in its natural state. A careful experiment made on quartz roustedMn a cupola furnace, with a superabundance of heated air, showed that the loss of gold sustained in crushing was at least ten per cent, more than it would have been if the.same qunrtz were crushed raw. Even if the pyritous gold could have been liberated previous to crushing the quartz, much of it would have been loit iv *lre treatment"- found to be the most ' economical for extracting the free gold. These experiments indicated that the attempt to liberate the pyritous gold by roasting the quartz before it was crushed only increased the loss, and that attention should be directed to the separation of this pyrilous gold from the sand after the -latter had passed the blanket strakea used for retaining the fitre gold. This entailed the necessity of operating on the waste tailings in bulk ; and as this is the principle on which the, improvements are based in the Californian aud uiost other gold mines, it may be advisable to htate the reasons which induced a different line of experiment. The fi.st attempts to secure this gold were by means of tine grinding and amalgamating, with the following results. By re- crushing the tailings with mercury in a Chilian mill, about twentyfive per cent, of the gold was obtajned', with very careful working. In the arastra, with grooves in the bottom of the basin for mercury, the return was increased to thirty-three per' cent, of the" assayed contents. Several other plans based on the same principle of re. grinding the sand in mercury gave similar results ; and even with careful hand amalgamation, when the material tried was cleau pyrites, we could not make much improvement on the above return. Dr Percy, to whom this question Jjad been referred, states that fiv> n pyrites containing 25$ ounces of gold per ton he only obtained 8^- ounces, when ground with mercury ; but b}' roasting the pyrites before operating he obtained nearly 25 ounces p^r ton. "A sample of rich pyritous quartz tried by me a few months ago, was, ground ,to a fine powder, and then rubbed up with mercury and hot water, to extract all the free gdld. The pyrites were then se'pa*rated by hand-washing, roasted, and amalgamated, when tht-'y yielded 140 ounces of gold per ton. In the two last mentioned cases the process, was conducted with a care and .perfection such as could not be carried out on a large scale by any machine at present known, and are onby quoted as a sample of numerous trials all tending to the same result, and indicating that rlecomposion of the pyrites was a necessary preliminary to aay plan for ex trading the gold to profit. The cost of this routing, in addition to that of grindins, so increased the expense of operating on the waste tailings that it^was only in rare cases of exceptional richness where they could have bee,n so treated profitably-. But as more than seventyfive per cent, of the gold lost was in the sulphides, another course was open, viz.— to separate the sulphides from the comparatively worthless sand, thus re ducing the bulk cf the mateiial to be acted on, ami then extract the gold. This is'the system we have been endeavoring to bring to nerfecUou for several years, and appears to be the only course at present known by means of which we can hope to reduce the loss of gold within reasonable grounds. As regards the larger grains of pyrites, this is partially effected on the blanket strakes, and it is the sulphides obtained from them, together with, a portion separated from the waste tailings, which have been operated on at the Clunes works, pi a cost of about Ll per ounce of goldxextraeted, leaving L 8 per ounce for profit. The common reverberatory furnace was first used for roasting, but it was found tc require such a large expenditure of laboi

and nine in turning over the sand— so 'asto Maw of every portion being exposed*. to the action of the heated air for a sufficient length of time, to ensure perfect 'oxidation of the sulphides — as to render it a very c<>&tly process. To remedy this defect, a new oxidating furnace was designed, which has now been in uso at the Clunea works for thiee years. This is a reverberatory furnace with an inclined bed,- from thirty to fifty feet long, and from live ttf six feet wide. The .bed i*< inclined at an angle that will allow the undisturbed sand to remain at rest on it, but stitt muke it easy to- rake down through' don-ways at tho side. The sand to be roasted, is fed in ,at the upper end of the bed, and .is gradually raked down — its place being supplied by fresh charges — until it readies the lower end of the Led, completely debulphurised, and is then discharged through a narrow opening between the bed and the fii c bridge. This furnace, iray be supplied \\ith heiiteduir — by tubes over the fire, hollow fire-burs communicating with a hollow bridge, and, if neces-sary,-\vith a coil of air pipes in the ashpits ; the object being' to supply a Jarge amount of oxygen in the heated air, for the purpose of combining with the sulphur and arsenic,, forming sulphurous and aryt-nious acids, ■ which puss off iv a g:iseous stat'-- ; and also for converting the sulphides into oxide.?, having no deteriorating action on the meicur}', and capable of ready disintegration, to al'tofy of tho liberation of the gold. The * ' sand is spread over the bed of the furnace in a th.iv layer, and \\ quires about, two hours' tiuje to be roasted peifectly at a dull red heat, Soon as it comes out of the furnace, the heated sand -is quenched with water ; and. wheij cool, it is ground and amalgamated in a -dam") state in Chilian- * innis— a very good 3ystem, first introduced into this colony by Mr tliaclc. About two Imndredweight of roantud sand is placed iv i\\Q mill for a'charge, with half its weight „! mercqrj'. This is ground for half an hour, the mercury breaking up, sfiid being ii&ti'ibuted through the sand in small globujes. When it is supposed. t!ie mercury lias- had time to absorb the <, r old, water is idmitted, and the globules collect together again ; tbe sand is then flushed out, and another charge placed in tliCj mill. Some of the jrcken mercury escapes with tho taud, ar.d )'rovisi"ii must be m\do for its separation Vom the waste before the^ latter finally usses away,- The following return gives he results obtained at the Chines works ! or the firat six months of the current. I'ear, operating on the pyritous sand in ;he way ascribed before : — Number of ons, concentrated pr treated, 183; amount . )f gold obtained, 5390z 17dwt; cost of:oncentrating and reducing, LSGO ; profit >n the ( six months' work, L 1422 5s 8d ; proportion of the^ total gold contents )btained, 87 per -cent; loss of mercury, ! p Slb per tow (if mm id treated. Ibis loss/ vas heaviest at the beginning of the year ; n the last parcel treated it was reduced to •6lb- of mercury person. Changes, are row in contemplation, intended to decrease . liis Joss still more, and ,at the sarue time ncrease the per' eentage of gold obtained. But etfen in its prvsent state tliis is ajood practical system of treating the ulphides giving fsir returns both as re:ards the protit and the proportion of - ;old extracted ; and it should be noted hat this is not a mere laboratory experincnt, but the results obtained iur actual working on a large scale, ihider conditions srhere each "step of the process is ac•urately tested. The next question re[n'iring attention the best method of eparating the sulphides from the waste nilmgs, and ihis has been a difficult pro)!em to solve. There is so little difference >etween the respective specific gravities if the quartz and iron pyrites, bat the separation of one ffpm he other, in any- known machine, even vith p-n'ticles of nearly the same size, vould be imperfect ; but this ■ diflicylty is •-istiy increased through the pyrites being uore friable than tho qu u\z, ami therefore , noSc^n under the stamps into . much miallcr particles. This difference in size jounterbalances the difference iv the spe:ific gravities, where water concentration mly is used, thus nullifying the principle >n which air the systems -of ore-dressing n general use.are based. An attempt was , nade to classify the sand, liuCit \^as found hat more than half the gold in the waste ailings was enclosed in particles of sand md pyrites in such a initiate state of diviiion that they could be passed through me wire gauze, hiving ;360(T meshes ta* he square inch ; ami as it was evidently < mpracticable to pass "100 tons of sand per l,ay through sieves oi: 1 this kind, the idea ' jf direct classification \\as given up. "A rial was -mude of the classifying boxes n'troduced by .Air Ulrich, Vhere the ioaiaer sand and heavier sulphides pass )at with the water flowing from a lower, ;he lighter particles from an upper, ;-,cape. By this method the sand can be livided into several different qualities-; but :be classification is nttt according to size )nly, and is therefore imperfect. The joarsA- particles of pyrites were retained m the blanket strakes, but the finer pieces floated away, and no dressing-machine liitherto tried would retain more than a small proportion of these fine pyrites. The bi-st result was <> >tamed from the round, concave buddie. «'itli the improi-e-ments pafei: 1 ? I hy Mr .slunday; and this machine •"-• 'io>v being wo; Iced to advantage at Cln-r*. and other ••[■hices. but it falls ' far short of the requirements of the case, and the endeavor to discover another system was, in consequence, nat relaxed. After . trying moit of 'the old *dres^ ing machines, and m.i:iy modifications ' of old plans, whicb/ it was hoped might overcome the difficuliies in the way, a trial waa made of tho percussion table, a' dressing machine rnucji used in Germany , and South America. Ynis is a table from ten to fourteen feet lo: I.1 '.- and from four to six feet wide, slung" i-y moans of four * chains leading bftck, and with iv head, resting agains^a block of timber. It is pushed forward by of a lever, .md when released swings back against the block with a' smart blow', jnaki-.ig frorn^twenty to fifty blows pet-'miniue. The sand and water U ru'i on 1 1 tlie bend of the table, and' flows down it, carrying off ihe lightermaterial, the heavier b.^ing retained on the table, and gradually brought up to the head' by the force of the percus>-ion blow. In dressing ' ordinary (»res a table. of ihe size meiuioned will put through from, one to one and a half tousMn twelve hours, and the material retained on it is still mixed with suck a propofition of tbe poor wa'ite as to require a second, and sometime^ a third dre-s-iusr. These known c. 'cts evidently rendered the peicussion tab*.e,< inapn'.icablo to the concentration of th 6 sulphides in this colony,, where ma-

terial and labor are so costly, however useful it may- be under more favorable conditions in this respect. The object in i trying' the table was, therefore, to see if

it" defects could not be remedied, or the 'percussion principle applied to more advantage. Careful observation of the

•worlcing of a percussion-table soon led to the conclusion that the cause of its imperfect action was the heavy bank formed by the sand, which prevented the blow from producing its full effect on the heavier particles ; and it was. evident that the action" would be much improved if the sand on the table could be kept' loose in a semi-fluid state, so as to allow the blow to produce a maximum effect. When fi.iely ground, ore is suspended in disturbed water. A blow given to the side of the vessel containing the mixture will check the current, and tend to settle the materials in suspension in the order of their specific gravity, the heavier particles falling first ; and even where gold or any of the sulphides are in such a fine state of division as to float on the surface of the water, a similar blow will at once cause them to sink, and at the same time draw thagn towards the point where the blow is applied. This is the action of the percussion table ; and when the sand on the table is kept loose, the pyrites, however finely crushed, are thrown down by the sudden check given to the current of .water by the percussion blow, and drawn bejow the surface of the sand on the table,'where they are protected from the action of the water, and gradually accumulated towards the head, the point where the blow is given. To apply this principle with success, several details require to be attended to. If the sand is allowed to form a hard bank on the bed of the table, the pyrites cannot settle into it readily; and if the sand is kept too loose, the motion of the table forms a wave, -which tends to throw the pyrites to the surface, and again exposes them to the risk of being carried off by the current of water. Numerous experiments were made, to ascertain*the form of stir.er best calculated to meet these requirements. That finally adopted is not unlike the prong of a sluice-fork, and is marine of quarter-inch nailrod iron, each stirrer being eighteen inches in length, with the end slightly curved. They are set about one and a half inches apart in rows, each row being set into an .axle working on gudgeons, nine inches above the' bottom of th'eT table, and th~e curved ends of the stirrers re°t on the bottom of the table, at eveiy pr t of the stro'ie, the axles allowing each row of stirrers to rise or fall with the table. The bed of the table is covered with light boiler plate to reduce the wear, as grooves are soon formed in wooden bottom, w jich impede the action of stirrers. The sand and water are passed over a distributing board, .which deliver them in an even sheet over'the sloping head, clear of the sand in the table. The suspending chains have regulating screws on each for the purpose of adjusting the levels ; the upper chains are fixed, but the lower ones pass over and are attached to a roller, by means of which the inclination of the table can be altered at pleasure without disturbing the cross levels. When put to work, the table is set with a slight inclination to the head, arid is gradually lowered whenever the sand at the head collects to

over twjo inches and a half in depth. After ■working a longer or shorter time, accord-

ing as the sand operated on may be poor in sulphides or the contrary, the table will become loaded with pyrites. The tail-

ings should,, then be diverted to a spare machine, and clean water only be allowed to run over the table, In a few minutes the bulk of the pyrites will have

collected at the head, when the^table must be stopped, the -pyrites shovelled out, and then the work resumed as before. Hitherto

the machine has only been worked at the Good Hope mines, the table used there being a small one, 2ft 9in wide, with abed seven feet long. Through this was passed the waste tailings from four head of • stamps (i.e., from thirty-five to forty tons per week), and carefully sampled at short intervals before going on to the table and after leaving it, the samples being all filtered through close woven calico. The assay of these samples, made at the office of the Port Phillip Company, gave the following results : — Gold contents of waste tailings before going on to the table. 17 dwts 22 grs per ton ; gold contents of waste tailings after 1< aving the table, 3 dwts 4 grs pei» ton ; amount of gold retained in sand on

the table, 14 dwts 18 grs per ton ; equal to 82-3 per cent, of total gold contents. The gold-bearing material saved on the table consisted principally of decomposed pyrites converted into oxide of iron. It is probable that each of these particles of

oxide contains a nucleus of undecomposed sulphides, yet the oxidation is sufficient so to .redHce the specific gravity as to render it much more difficult to effect separation from the quartz sand, and the saving of such a large proportion of this material is equivalent to a saving of

" from ninety to ninety-five per cent, of /the undecomposed sulphides. Three of these tables are now in course of erection at the Good Hope mine, and one at the Chines, ' where the effective working of the tables on different kinds of materials will be carefully observed, and the results laid bex fore the society in another paper. Hitherto, the close concentration of tne sulphides in the waste tailings could not be effected without such* an increase in the loss as as more than balanced the gain, through having to treat a smaller^ quantity of pyritous sand. With the table a much higher degree of ' concentration can ho effected without risk than by any other means, and there will be a decrease in the' cost per ounce of gold ( - '-acted. No doubt experience will lea.: i.> improvements in the working and construction of tins table, but even in its present form, it surmounts the difficulty which has so long stopped the way ; is simple,- inexpensive, and easily erected, and* when worked in conjunction with the system now; in use at Cluries for extracting the gold, will retain from seventy to seventy-five per cent, of tbe gold at present lost in the ■waste, tailings. • "*