THE ACCUMULATION OF AMALGAM ON COPPER PLATES.

Otago Witness, Issue 2209, 2 July 1896, Page 18

THE ACCUMULATION OF AMALGAM ON COPPER PLATES.

Although every millman of even limited experience in the amalgamation of gold ores is probably aware that copper plates will in time become coated by the accumulation of gold amalgam, it may be that many do not know to what extent this accumulation occurs in the treatment of ores dissimilar to those with which they are familiar and under conditions foreign to those with which they have to contend in any given locality. The facts hereinafter stated wore observed in the 50-atamp " combination " mill operating the Drum Lummon mine, situated at Marysville, Montana, U.S. The material treated in this mill consists of quartz and ores of somewhat variable character, .coDtiining gold and silver in native form, a« well as in chemical union with the t ulphides of iron and.oopper. There is also present a little lead, some arsenic and antimony, an,d a little zmo. The . last four minerals, however, &re of irregular occurrence, and but sparingly distributed throughout the ore, and yield only a ' trace ia the analysis q{ tbo &&fira£a miterial is

thfs mill. The free gold, which is In a condition of fine division and rarely visible to the naked eye, carries silver as an alloy ; and the native silver, which is only occasionally observed, is present in the richer ores in wire and leaf form. Of the two precious metals, the qre worked during the pant three years and ten months, the period under review, contained aboub £oz of gold aud from 7oz to 12oz of silver per ton of 20001b. The ore after being crashed in the stamp battery and passed through a 30-meßh steel- wire' screen, runs over amalgamated copper plates, and is then concentrated on Frne vanners, to the distributor of which is affixed a small ooppsr plate for saving any amalgam escaping from the battery plate; the tailings from the vanners being subsequently amalgamated in pant. Amalgamation upon copper plates placed inside the mortar is riot practised in this mil], better results having been obtained by amalgamation on outside or apron plates only. It is not the purpose of this paper to enter the wide field of dioousßioa as to the relative merits of inside and outside amalgamation ; bat it may be observed in pasting that an extended experiment upon the two methods proved that a higher per--centage of saving was gained by amalgamation outside the battery than by the use of inside plates in addition to the apron plates: The' apron plates in use in this mill are 54in wide and Bft long ; made of £in rolled copper plate,' electro-plated with.lqz of, silver to tho square foot. They are set with a" fall of 2£ia to the foot ; are cleaned up once in 24 hours, and dressed once every two, three, or four hours, j as may be deemed necessary by reason of their snrf ace condition, which varies according to the mineral contents v of the ore submitted to them. In dressing, a weak solution of cyanide of potas3fciurai»usedwben.necesiary. The copper plates upon the, Frue vanners ate 46in wide and 18in deep, made or £in rolled copper plate, j electro-plated with loz of silver to the square foot, and are subjected to the same treatment aB the apron plates. At the daily clean up the surface of the apron plates is first washed ; then the amalgam whioh has collected during tho preceding 24 hours is slightly softened and loosened by sprinkling a small quantity of mercury over the plate, and rubbing the surfaca with a cloth or whiskbrush, after which the amalgam is removed with a stiff rubber scraper, 4-in wide, made out of rubber belting. In this process the daily accumulation of amalgam is removed, as completely as possible without the application of a steel Kcraper,* the use of which is not permitted, except for the removal of blisters, or any fixed impurity which may occasionally be found upon the plate. The foregoing brief summary of the method employed in this mill- will be sufficient to explain the general condition* sur/ounding the amalgamation of the free gold contained in the ore, and the treatment to which the copper plates are exposed. To return to the special subject indicated by the title of this paper. One of the copper plates employed in this mill was recently removed, after having been in continual service for three years and ten months. During this period the battery of five stamps which it served ciushed 14,942 toui of ore, yielding from amalgamation on this plate 64260z of bullion, .-541*5 fine in gold and 443 9 flue in tilverPgiving a standard assay value of £2 10a .per ounce,' ajjd showing the recovevy of £1 per ton- of ore crashed. In fixing .the total tonnage of ore passed over this plate, «nd the yield in bullion therefrom, the total tonnage and production of the mill during this period has been divided by 10. As all the stamps have been operated continuously, this proportion should correctly represent the duty performed by the particular plate under consideration. ' ' ' The agreeable task of removing the amalgam known to have accumulated upon, this plate | (whioh was new when put in) was then undertaken, and' the accumulation or • scale Was removed by striking the back aud front of the plate with a light hammer, n. small block ofwood being used to deaden and distribute the blow. By this means the plate is slightly buckled, causing the amalgam to scale off, and leaving upon the original electro-plated surface but an insignificant film or" layer, ' which is subsequently removed with a chisel or soraper. Other methods, such as sweating with hot water and immersing the plates in chemical solution?, have been tried, but have proved unsatisfactory; aud the process of buckling the plate, although a somewhat drastic and unscientific method, has been found to effect the recovery, of the highestpercentage of the accumulated amalgam. After this .treatment the plate is usually bo damaged as to ba unfit; for further service, but, notwithstanding that every visible portion of amalgam has been removed; the copper plate- is - still «found to carry a considerable quantity of gold and mercury ; and upon being cut up and melted into a bar the value of the gold and copper contents amounts to more than twice the value of & new plat 9. "Hence the substitution of new plates for old is an expenditure which 1 can be viewed without concern, The results obtained from the foregoing.treatment of the particular copper plate here tpeciued were both interesting and of substantial value, the scale or accumulation of amalgam at the head of the plate being no less 'than o'l6in in thickness, and gradu&lly decreasing to & bare l-16in at the lower end. \ The total weight of the amalgam so recovered was 4601b avoirdupois, which upon being retorted yielded 6051b, or 38 per cent, of crude bullion, and produced after melting a gold bar weighing 863 lcz troy, which upon assay proved to have a total fineness of 993 9, being 431/4 fine in gold and.s62&fiaeid silver, : and having a standard value of £1 18s per ounce, thus making the total value of bullion recovered from the accumulated amalgam upon | this one plate no less than £1668. j It may be remarked that the results recorded ; in this instance are not by any means excep- 1 tion&l. Other plates in this mill, placed in • service at the same time «s the one forming the subjeot of this paper, appear tp be coated with an accumulation of amalgam of equal weight ' and value ; and, moreover, within the writer's ' experience of other pla'.cs removed from this mill in past years' is an instance in which one , plate yielded accumulated amalgam in excess of £2200. An examination of the facts observed in the course of cleaning this plate and converting ' the amalgam into bullion reveals one or two .

interesting features- whioh appear to be worthy 1 or record. * A« stated above, the percentage of bullion til the .amalgam was 38 per cent. \ whereas the amalgam from the daily olean-np never contains more ihan fcO per cent, or bullioD, and frequently nob mora ttftn 10 pet cent. This is no doubt due to the oircomstanoi that the amalgam remaining upon the surface of the plate is subjected to gre»tet_pompieiiiori th»n that whioh is cleaned off and merely strained through canvas sacks. The accumulated amalgam obtained 'from the plate in the shape oJ"ioale does not appear, to suffer any visible alteration m form or Az& during the procons of retorting. A piece, one* eighth ioch thick and one inch fiquaro.-wiU pag^f through the ordeal of retorting and emerge »S crude bullion, having preserved, without ap« parent loss, its original dlmoniione, and Still re? taining on its sumoe any ripple mark or lm.-> perfections which it bore during ita existence at &m»lgara, A comparison of the fineness of bullion,obtained ' from the accumulation of ' amalgaui upon this plate with the average flnoneM o£ bullion produced from the daily clean-up of -the' plate, during iti service of three years and' ten months, of whioh one would suppose the accumu* hted amalgam to be a fair sample, shows that, to a striking degree, this is nob the case. The averagejiueness of bullion obtained from daily clenu-ups daring tho period mentioned was Aa, 5U f 5; Ag,443°8 ;—tofa»!, 985 4, whereas the .fineness of bullion- obtained ■ from* the accumulation upon this plate dririog the iame \ period was An, 431*; Ag, 582 s;— total, 9939,; from whioh it will- be observed that although the total fineness of the latter, 993*6, is greater than .the fineness of the former, 935 4,- the gold finenens is 1101 -less, and the silver fineness 118*6 more, • ' That the total fineness of bullion from the accumuUtsd amalgam should be higher than that obtained, from the daily oleau-ups is not a matter for surprise, sines the anaalg*m gathered from the plates from day to- day would naturally contain a higher percentage of impurity and base metal thau' the amalgam which adhered to the surface of the plate ; bafc it does not clearly appear why the gold finencis of the latter should be to muoh lower .than that of the former, the difference being represented by a corresponding increase in -the silver fine* ness. ■. In explanation of this inconsistent feature, it has been suggeited that the native* silver con* tamed in -the ore has a -greater tendency to accumulate upon the plates than. the free gold, owing to its stronger affinity with the amalgamated surface. If theory furnishes any authority for.auoh a statement, it is rudely disproved by aotnal experience in this particular instance ; for test samples .of the accumulated amalgam from the head and tail of thia plate pro*ve the former to be '020 finer in gold th»n the latter, with a corresponding increase in the silver* fineness of the amalgam from tho loner end of the plate. Furthermore, .the amalgam saved upon the, copper plates forming the distributor "of the Frue vanners invariably shons, upon assxy, a lower gold and a higher silver.fiueness than the bullion recovered from the treatment of the BRms ore upon the battery' 'plite, .«. sample ,of this yanntir amalgam yielding bullion assaying Au, 380 5 ; Ag, 602:0— total, 982 5. In other words, th'c silver, instead ' of showing ,a strong »ffipifcy with' the .amalgamated surface, gives' evidence of, a 'persistent tendenoy to escape amalgamation,' as prbved> by the foregoing assays, which show that the gold .fineness of bullion- i$ highest nearest to thejbattery, and gives place to a atendily-in-oreasing silver fineness as the amalgam ii deposited upon the copper plates at grester distance from the battery discharge. '; ' The natural conclusion .to ba drawn from these fact* and figures would justify the expec- < tation that the bullion derived, from the accumulation of amalgam on copper plates would be of equal (if not of higher) gold fineness to that recovered from the daily clean-ups upon' tha plates during the period in whioh the accumulation was in progress. The foregoing faefctf, however, incontestably prove that such is not the case ; and the purpose of this paper will be served if the facts herein presented, which have been prepared with due regard 'to acouraoy, are the means of suggesting an explauatiou of this interesting incou«l<it< ncy, which the writer most frankly oon ''eJKO-i himself unable bo supply, —Australian Mining Sfcitndacd.