Table of Contents
In order to keep the plates in proper condition so that successful amalgamation may be maintained, they must be prepared carefully, and the closest watch kept over them. The setting of the plates has already been described on p. 121. The silver-plated copper table is preferred in California from the ease with which it is kept clean, but is not used in the Transvaal. It is not considered desirable to put on it as much mercury as it will hold, since, if the amalgam is too fluid, losses are sustained by scouring, but, on the other hand, if the amalgam becomes too hard and dry from absorption of gold and silver, further amalgamation is checked and fresh mercury must be added.
The condition of the inside plates is regulated by the amount of mercury supplied to the mortar. In Colorado there is an opening at the front of the battery and above the screen frame, ordinarily covered by canvas, which can be lifted up by the millman, who introduces his arm, and determines by passing his hand over the front plate, whether the right amount of mercury is being added by the feeder. The regulation of the addition of mercury is thus effected without removing the screen frame. Mercury is sometimes added direct to the outside plates, and sometimes their condition is regulated by the additions of mercury in the mortar box. The amount added varies with the conditions of crushing and the richness of the ore, but in general from 1 to 2 ounces of mercury are fed in for every ounce of gold contained in the ore. The finer the state of division of the gold and the more sulphides there are contained in the ore, the more mercury is required. It is fed into the battery at stated intervals of from half an hour to an hour. In some mills amalgamation in the battery is not attempted, no inside copper plates being provided, and under these circumstances it is not usual to feed mercury into the battery.
The practice of feeding mercury into the battery, although still frequently pursued, meets with opposition from certain experienced millmen. The objections urged against it are mainly that the mercury so introduced, and the amalgam formed through its agency, tend to become so excessively subdivided that a high percentage is lost through flouring; moreover the mercury is liable to sicken when the ores contain sulphides. These evils, no doubt, exist, and tend to increase with the percentage of sulphides present, while arsenic and antimony in particular cause heavy losses of both mercury and amalgam if battery amalgamation is attempted, but with ordinary free-milling ores such losses do not appear to be serious. When coarse free gold is present, inside amalgamation is probably advantageous, but otherwise outside amalgamation is now generally preferred.
The amalgamated plates are dressed as frequently as is necessary, the length of time allowed to elapse between two operations depending partly on the richness of the ore. To dress the plates, the battery is stopped, and the “ black sands ” are swept off and kept separately for grinding with mercury. The plates are then rubbed with a hard brush so as to soften and distribute the amalgam, and, if necessary, fresh mercury is added. The amount of mercury put on the plates should be enough to keep their surfaces in a pasty condition, but not enough to gather into liquid drops or to run off. Sometimes cyanide is added in dressing the plates, to assist in the removal of stains, but other chemicals are now seldom used. The plates are dressed once or twice a day, or as often as every two hours when rich ores are being treated, or when stains tend to appear owing to the nature of the ore. Usually in dressing the plates no amalgam is removed, but sometimes a partial clean-up is effected at the same time, the surplus amalgam being wiped off with a piece of rubber.
On the Rand mercury is sprayed on about every eight hours to the upper 3 feet of the plates, the part that is scraped daily. The mercury oozing downwards supplies the lower part. Then a solution of 0.08 per cent, cyanide is sprinkled over the plate, and the mercury is rubbed in with a hard brush. Afterwards the plate is brushed smooth with a soft brush, and then washed down.
Whisk brooms are often used for brushing the plates; these brooms are cut down to a short length so as to be stiff enough. The plate is brushed all over and the amalgam thus thoroughly loosened from it, after which, commencing at the top where the amalgam is thickest, it is subjected to a systematic stiff brushing, each stroke being directed longitudinally down the table, and not towards the centre. The surplus amalgam is thus brushed to the lower end of the plate, whence it is removed, and a thin coating of amalgam is left over the whole surface of the plate, excluding the air and preventing the formation of “verdigris.”
Discoloration of the Copper Plates
The plates often become stained by the formation on them of oxides, carbonates, or other compounds of copper through the corrosive action of the water and pulp. Ores containing decomposing sulphides acidify the water and thus cause the corrosion of the plates, a yellow film being formed on the surface of the metal. The presence of carbonic acid in the water is equally harmful, but Aaron points out that the addition of slaked lime to the water neutralises the acid substances and diminishes the tarnishing. The yellow, brownish, or greenish discoloration, the so-called “ verdigris,” appears in spots and spreads quickly, especially on new plates, those which have been silver-plated being less liable to become dirty than the others; whilst when a plate has become covered with a thick layer of amalgam it is not readily discoloured. When these stains appear the plate must be at once cleaned, as the stained part catches little or no gold. The chemicals used for the purpose are generally sal-ammoniac and potassium cyanide, the operation being conducted as follows: The battery is stopped, the plates rinsed with clean water, and a solution of sal-ammoniac applied to the stained parts with a scrubbing , brush, and left covering them for a few minutes in order to dissolve the oxides. It is then washed off, a solution of potassium cyanide rubbed on to brighten the plate, and almost instantly washed off, fresh mercury being then added if necessary. Long brushing with potassium cyanide is necessary, as otherwise the spots reappear when the water is turned on.
What Chemicals to Used for Better Amalgamation
The use of chemicals to aid amalgamation was formerly much more general than at present, although battery-men were less afflicted by the rage for nostrums than pan-amalgamators. Potassium cyanide is used to clean the plates and to promote amalgamation, its action consisting in the removal of oil, grease, and base metallic oxides. 1 or 2 lbs. of potassium cyanide are said by J. H. Hammond to be enough to supply a 40-stamp battery for twelve months when treating free-milling ores, by which the mercury is not much affected. The difference between such a mill and one running on base ores may be judged from the fact that at the Hidden Treasure Mill, Colorado, where there are seventy-five stamps,- 260 lbs. of cyanide were used in a year. Here the plates are dressed every twelve hours with a weak solution containing 2 ozs. of cyanide in 3 gallons of water, the operation being necessitated by the acidity of the water which comes from the mine, and is further contaminated by sulphates formed in the ore. Roskelley used ½ oz. of cyanide dissolved in 4 gallons of water—that is, a solution containing 0.08 per cent, of cyanide—for dressing plates at the Robinson Deep Mill, Transvaal, and , states that that amount is enough to dress ten tables. The tables were dressed every four hours. He found that it saves time to use cyanide in dressing and setting the plates, and keeps the plates in better condition by preventing the appearance of discoloured patches. D. J. Peplar and others consider that discoloration is best prevented by prolonged rubbing with or without cyanide, and that when cyanide is used less rubbing is done, so that, although time may be saved, it is at the expense of efficiency. Potassium cyanide is sometimes added to the water used in the mill. (See below under “Crushing with Cyanide Solution.”) It is said to make the amalgam on the plates hard and dry.
Sodium amalgam is used to revivify sickened mercury, or to maintain it in good condition. It is prepared by heating a basin or iron flask of mercury to about 300° F., and dropping in little pieces of sodium not larger than a pea, one by one. Each addition causes a slight explosion and a bright flash of flame. The sodium may be added with less loss and less danger to the operator if the mercury is kept at a somewhat lower temperature and the sodium stirred into the mercury with an iron pestle or pressed below its surface with a spatula. When about 3 per cent, of sodium has been added to the mercury, the reaction becomes less active and the amalgam is then poured Out upon a slab or shallow dish, allowed to cool and solidify, and then broken up and kept in stoppered bottles under naphtha. When it is necessary to revivify a quantity of mercury, a few small pieces of the amalgam are added to it and stirred in, or are previously dissolved in clean mercury before being added to the impure stuff. The strong affinity of sodium for oxygen, chlorine, &c., enables it to reduce the oxides and other compounds of the base metals which are coating the mercury globules. The sodium hydrate passes into solution, neutralising part of the acidity of the water at the same time. The base metals are redissolved by the mercury which is then in good condition to take up the precious metals or to be caught on amalgamated surfaces or in riffles, but the mercury is not really purified and the base metals in it are soon oxidised again. Sodium amalgam is not much used except in amalgamating-pans or in mercury wells or riffles, or in cleaning retorted mercury—i.e., wherever large bodies of mercury can be directly acted on by it. It is of comparatively small value when added to the mortar of a stamp battery, although this use of it is not unknown. The use of electric currents, galvanic couples, &c., already referred to, has an effect similar to that of sodium amalgam.
The merest trace of any kind of grease or oil is very prejudicial to successful amalgamation, forming a film over the plates and over the little globules of mercury, and thus preventing contact between them and the particles of gold. Grease may consist of the tallow dropped from the miner’s candles, or the oil from the loose steam which is sometimes used to warm the battery feed-water, or from the bearings of the machinery. Acidity of the water and the effects of grease are often corrected by the addition of solid lime to the ore. This has now become a common practice, but its efficacy is denied by Bevington, who states that it does not remove discoloration from plates, and, moreover, tends to make the amalgam strip off.
Amalgamation Plate Angle & Design
The grade or inclination of the plates varies with the nature of the ore to be treated, heavy pyritic ores requiring a higher grade than light quartz, while the coarser the crushing the steeper must be the grade. In California the copper tables have an inclination of from 1½ to 2 inches per foot, the apron plates from ½ to 1¾ inches per foot, with an average of 1½ inches, and the sluice plates from 1¼ to 1½ inches. The narrower plates have a lower slope in order to avoid increased scouring action. With heavily sulphuretted ores a grade of from 2 to 2½ inches per foot is used. On the Rand a slope of 1¼ to 1½ inches per foot is generally used. The steepness of the grade is of great importance, as on it, and on the amount of water supplied, the attainment of the necessary contact between the ore and the plate depends. When the pulp is flowing properly, it travels down in a series of little waves and ripples, and, in consequence of the friction between the plate and the film of water in contact with it, the upper portions of these little waves travel faster than the lower parts, so that the motion becomes one of tumbling over and over. As a result of this, if the plate is long enough, every particle of pulp comes in contact with the amalgamated surface, and the perfect extraction of amalgamable gold, mercury and amalgam is obtained.
Muntz Metal Plates
The use of Muntz metal (which consists of copper 60 per cent., zinc 40 per cent.) for amalgamated plates is of great interest. It differs from copper in catching gold well as soon as the plate is amalgamated, not requiring to be covered with gold- or silver-amalgam before it begins to do good work. Moreover, the amalgamated surface is very superficial, since the mercury does not sink in so far as it does into a plate composed of pure copper, so that only a small quantity of mercury is required to cover it. The result is that cleaning up is easy and rapid, no iron instrument being necessary, but rubber being always sufficient. These properties make it particularly valuable for custom mills, where it is desirable to catch as much as possible without mixing the amalgam obtained from two parcels of ore crushed in succession. On the other hand, as it holds little mercury, it cannot absorb much gold, and must be cleaned-up at frequent intervals.
The mercury on Muntz metal plates does not suffer so easily from “sickening” as that on copper plates; it has been suggested that this is due to the electrolytic action of the copper-zinc couple, which sets free nascent hydrogen, and so reduces the compounds of mercury and other metals which have been formed.
It follows that Muntz metal plates are preferable for ores containing large amounts of heavy sulphides or arsenides. The greenish-yellow stains (called “verdigris” by millmen) which are formed on copper plates when grease and other impurities are present in the battery water, do not appear when Muntz metal is used, and such discolorations as occur on these plates can be better removed by dilute sulphuric acid than by potassium cyanide. At the Saxon Mill, New Zealand, the copper plates formerly required 7 lbs. of cyanide, costing 23s., per month to keep them in order, while the Muntz metal plates, by which they were replaced, could be kept clean by 5 lbs. of sulphuric acid per month, the cost being 3s. 4d. It is stated, however, that, in the treatment of highly acid ores, which have been weathered for some time so that they contain large quantities of soluble sulphates, or in cases where the battery water contains acids, copper plates are less affected than Muntz metal, over which a scum is rapidly formed. In the Thames Valley, N.Z., Muntz metal is preferred in spite of the extremely acid nature of the water and ore.
In dressing new Muntz metal plates the following method is adopted in New Zealand:—The surface of the plate is scoured with fine, clean sand ; then it is rinsed with water, and washed with a dilute (1 to 6) solution of sulphuric acid. Mercury is then applied and rubbed in with a flannel mop until it wets the surface of the plate (i.e., amalgamates with it) in one or more places, after which the mop is given a circular movement, passing through these spots, until the amalgamation of the surface spreads from them over the whole plate.
The discoloration of the Muntz metal plates is prevented by the weak electric current produced, as has been already stated. The same effect can, according to Aaron, be obtained when ordinary copper plates are in use, by placing them in contact with iron or some other metal which is positive to copper. Strips of iron bolted to the top and sides of the plate are said to be sufficient for the purpose, the copper being in that case unaffected by the acidity of the water, which causes oxidation and dissolution of the iron only. Janin’s experience does not support these views.
Shaking Copper Amalgamation Plates
A shaking copper plate has been recommended to be used either below or in place of the ordinary amalgamating tables, especially in cases where these do not appear to give good results. An ordinary fixed copper plate requires an inclination of from 1 to 2 inches per foot, in order to keep it clear of sand, when the plate is of the same width as the battery. If, however, the plate is subjected to a short rapid shake, the sand is kept from packing, and amalgamation is well performed with a grade of only ¼ to ½ inch per foot, or the amount of water needed with the pulp may be greatly reduced and better contact thus obtained. For these plates, silver-plated copper is the material employed. They are affixed to a light wooden frame which is moved by a crank-shaft, revolving 180 to 200 times per minute, placed on one side, with a throw of 1 inch at right angles to the direction of the flow of the pulp. In some mills, a longitudinal shake is given to the plate instead of this side shake. The frame may be hung on rods from above, but is more conveniently supported on four short iron springs, forming rocking legs. The width of the tables is made as great as possible, while the length is of less importance, as, the thinner the current of pulp flowing over them, the better the chance of the gold particles coming in contact with the plates and being retained. These shaking plates were first used in Montana in 1878, and have since been employed at several Californian mills. It is advantageous to add to them an amalgam- and mercury- saver. A simple device for this purpose is to nail a strip of wood, half an inch thick, across the copper plate near the top, thus forming a shallow riffle, the angle of which is soon filled with sulphides and coarse sand, which are kept in agitation by the movement of the table. This is stated by W. M ‘Dermott and P. W. Duffield to be the most effective contrivance yet devised for catching quicksilver and hard amalgam. If the inside copper plate should become hard by accident or neglect, chips of amalgam escaping through the screens are retained in this riffle, and, becoming spherical by rolling up and down under the effect of the shaking motion, increase in size just as a snowball does when rolled in snow.
In 1904, P. Carter described the shaking plates used below the ordinary plates at the Ferreira Gold Mine. Each plate caught an average of 15.05 ozs. of fine gold per month. Each ordinary plate at the same time was catching nearly 400 ozs. per month. Two-thirds of the values caught on the shaking plates came from black sands. The plates were 11 feet long by 4 feet 6 inches wide. They were fixed on old Frue vanners, and had a fall of 4 inches in 12 feet.
Mercury Traps
Another method of saving mercury and amalgam, which would otherwise be lost in the tailings, consists in the application of mercury wells or riffles. A mercury well consists of a shallow gutter filled with mercury, over the surface of which the pulp flows or through which it is forced to pass by suitable machinery. Attwood’s amalgamator, formerly much used in California, was a machine of the latter class. Such wells or traps are usually placed between the successive plates, the pulp dropping from the end of a plate on to the surface of the mercury in the well, and then passing on to the next plate. The practice of placing a well between the screens and the amalgamating tables has been condemned, as it prevents proper supervision being kept over the feeding of mercury into the battery, overfeeding being difficult to detect under these circumstances.
Galvanic Action in Amalgamation
In amalgamation in the mortar, on plates, or in pans, not only are free metals absorbed, but the dissolved salts, and, to a less extent, the insoluble compounds of the heavy metals, are reduced and amalgamated, chiefly by galvanic action. The copper of the plates, or the iron of the mortar or pan, constitutes the positive element, and all metals less oxidisable than this reacting metal are reduced by it, and are then amalgamated by the mercury. In this way iron reduces both lead and copper, although, if these are present in the form of undecomposed sulphides, this action will be very slight. Now, if lead is introduced into the amalgam, the latter becomes pasty, and is subjected to considerable losses, and copper has an equally harmful effect. It is for this reason that the arrastra is found to be better than the stamp battery or even the pan for certain plumbiferous ores. This action of iron is of course enormously increased if the ores are subjected to a chloridising roast before being amalgamated, as in the Reese River process for the treatment of auriferous silver ores.
In some mills, this galvanic action has been increased by the passage of a weak electric current through the charge by means of a dynamo. The amalgamated plates or the walls of the pan are connected with the negative pole, while the positive pole is formed of a plate of lead or iron dipping into the pulp. Under these conditions the mercury is still further protected from attack, and remains bright and lively, but the deposition of base metals in it is favoured, and the stronger the current the more this action is induced. Consequently, such methods are attended with the best results when dealing with ores containing little or no copper, lead, &c., since in these cases the strength of current can be increased, and the mercury kept clean, without any ill effects. The principle is made use of in Bazin’s centrifugal amalgamator, Molloy’s hydrogen amalgamator, and other similar machines.
Designolle Process of Amalgamation
In this process a solution of mercuric chloride is used. It was tried at the Haile Mine, South Carolina, the method being as follows:—Charges of 600 lbs. of roasted ore were placed in cast-iron barrels with 1,000 lbs. of cast-iron balls, or in pans. The barrel was partly filled with water, and 1 gallon was added of a solution containing 1.7 per cent, of mercuric chloride, the same amount of hydrochloric acid, and twice as much salt, if the ore contained; less than 15 dwts. of gold per ton. This would be equivalent to about 10 parts of mercury to 1 part of gold. The barrel was rotated for twenty minutes and then discharged into a settler, and the suspended amalgam caught on copper plates. The mercury was supposed to be reduced by the iron thus:
HgCl2 + Fe = FeCl2 + Hg,
and the metallic mercury thus freed amalgamated with the gold. If no common salt was present, some mercurous chloride was formed according to the equation:
2HgCl2 + Fe = Hg2Cl2 + FeCl2,
and the subsequent reduction of the insoluble calomel by iron was not complete. Hydrochloric acid was supposed to hasten the amalgamation by setting up some electrolytic action.
The total cost of the process at the Haile Mine was said to be only 35 cents, or 1s 7½d., per ton, but it was abandoned when the percentage of iron in the material under treatment increased, owing to improved methods of concentration. Large quantities of oxide of iron were then amalgamated, and rendered the resulting mass harder to treat than the ore itself. By repeated washing, settling, and regrinding with fresh mercury, it could be partially purified, but not without a loss of gold. It is stated that 87 per cent, of the gold in the ore was extracted.
How to The Clean Amalgamation Plates
The amalgam, both on the inside and outside plates, does not accumulate evenly, but in ridges and knots which serve as nuclei for the collection of more. It is not advisable to allow the coating of amalgam to become very thick, since, although the plates catch better as the amalgam accumulates, losses may be experienced by scouring. The scraping of the outside plates usually takes from ten to fifteen minutes for each battery. The amalgam so obtained is ground with more mercury in a clean-up pan in order to soften it, the skimmings from the mercury wells, &c., being added to the charge. The inside plates are not scraped until the amalgam stands up in ridges on it; the operation may be necessary as often as twice a week, but it usually takes place twice a month, when a general clean-up is made. All amalgam, however obtained, unless it is already hard and dry, is usually at once separated from the excess of mercury contained in it by being squeezed in filter-bags, and the pasty residue alone passed to the clean-up pan or the retort.
In cleaning-up, the stamps are hung up, two batteries at a time, the screens, inside plates, and dies are all taken out and washed in tubs, and the “ headings,” or contents of the mortar, consisting of the pulp, mercury, sulphides, and pieces of iron and steel, amounting in all to a quantity sufficient to fill two or three buckets, are carefully scraped out and panned or fed into the mortar of one of the other batteries, which has not yet been cleaned up. In California, the headings from the last batteries are panned, the iron removed by a magnet, and the remainder ground with mercury in the clean-up pan. G. O. Smith states that mortar-box sands, as well as the sands from mercury traps and from washing the screens, are very rich, and must be ground with mercury in a clean-up pan. Roskelley considers that mortar-box sands from 10-dwt. rock would contain about 10 ozs. of gold to the ton after a three months’ run. Amalgam is found adhering to the inside of the mortar and to the dies, and is carefully detached and added to the clean-up pan, and all the plates are well scraped with a sharp-edged piece of hard rubber or an iron tool, care being taken not to scratch them. Hard amalgam is removed by a chisel, care being taken not to lay bare the copper surface. Fresh mercury is then added, and brushed over the plates, which are finally smoothed with a soft brush. After the plates have been redressed, the batteries are restarted, and the next ones stopped and cleaned up. Three men can clean up forty stamps in from five to seven hours, ten stamps being thus idle for the whole of this time.
The amalgam obtained from the batteries, outside plates, mercury wells, or sluices, is rarely clean enough for immediate retorting; it is usually found to contain mixed with it grains of sand, pyrites, magnetite, and other minerals, together with fragments of iron and other foreign substances. The skimmings from mercury wells are still more impure. These materials must be purified by grinding with fresh mercury and washing before they can be passed to the retort. The scraps of iron consist of fragments of shoes, dies, shovels, picks, hammers, and drills : they are knocked about in the mortar until a quantity of gold and amalgam has been driven into their interstices. At the Jefferson Mill, Yuba County, California, about ½ ton of such scrap, picked out by hand or by a magnet, had accumulated in 1885. It was attacked by warm dilute sulphuric acid until the surface had all been dissolved off, and the residue was then well washed, and gold to the value of $3,000 thus recovered. The shoes, dies, &c., which were too large for this treatment, were boiled in water for half an hour, and then struck by a hammer, when the gold dropped out. In small mills the dirty amalgam is ground in a mortar by hand with fresh mercury and hot water, until it is reduced to an even thin consistency, when the dirty water is poured off, and the mercury poured backwards and forwards from one clean porcelain basin to another until the pyrites, dirt, &c., have risen to the surface, when they are skimmed off. The skimmings obtained are put back into the mortar, and re-ground by themselves with fresh mercury. The clean mercury is then squeezed through canvas or wash-leather, when the greater part of the gold and silver contained in it, together with about one and a-half times its weight in mercury, remains in the bag, the rest of the mercury, with a small quantity of the precious metals dissolved in it, passing through. The mercury was formerly always squeezed by hand, but it is now sometimes squeezed by machinery, a ram working in a cylinder pressing out all surplus mercury from amalgam contained in a canvas bag. The ram is worked by compressed air or water.
In large mills a clean-up revolving barrel is often employed to mix the amalgam. This is made of iron, and is similar in construction to chlorination barrels, but without the lead lining. At the Plumas Eureka Mill, the barrel is 3 feet in diameter and 4 feet long, and revolves twenty times a minute; the charge is 700 lbs. of amalgam and 20 lbs. of mercury, or more if the amalgam is very rich. A dozen or more iron balls or pieces of iron, such as worn-out battery shoe shanks, are put into the barrel, which is filled nearly up to the top with water, and then revolved for from six to twelve hours. The use of the iron is to help to mix the amalgam and mercury, but it causes some loss by flouring, and is omitted in Australian mills. The barrel is then opened and washed out with water, the tailings being run over amalgamated plates and through a mercury well or some other form of amalgam-saver, after which the amalgam is scooped out of the barrel and squeezed in wash-leather.
The Clean-up Pan is even more extensively used than the barrel. One of the oldest in use in the United States, the Knox Pan, is still a great favourite, especially for treating battery sands, skimmings, &c. It consists (Fig. 38) of an iron pan, 5 feet in diameter and 14 inches deep. Wooden or iron shoes are attached to the arms, g, which make from twelve to fourteen revolutions per minute. Iron shoes are considered better for brightening or polishing the particles of gold contained in the pyrites, and so rendering them fit for amalgamation. The charge for this pan is about 300 lbs. of impure amalgam, mercury, concentrates, skimmings, &c. The charge is made into a pulp with water and ground for three or four hours, after which more mercury is added, and mixing is carried on for a few hours longer, before the pulp is diluted, settled, and discharged. The tailings suspended in the water are usually passed over amalgamated plates, and are then often caught in settling pits, and either sold or subjected to further treatment on the mill, as they are frequently of high value. The mercury is squeezed through canvas, and the amalgam retorted.
At the Simmer and Jack Proprietary Mines, where the mill contains 280 stamps, there are five clean-up pans 36½ inches in diameter by 13½ inches deep.
The position from which the greater part of the gold is obtained in a clean-up varies according to the ore and the method of treatment. In California from 50 to 80 per cent, of the gold saved is caught on the single plate inside the battery, the remainder being caught on the outside plates and the sluice- plates, or being contained in the concentrates. In the Grass Valley, at the Original Empire and the North Star Mills, from 70 to 85 per cent, was caught inside the battery. The amalgam from the battery plates is usually richer than that from the outside plates, especially if the gold is coarse. The reason for this is that coarse gold, being easily amalgamable, is almost all caught on the inside plates, while fine gold, even if amalgamated; in the battery, forms a more fluid amalgam which passes through the screens and is caught outside. Coarse gold forms a richer and stiffer amalgam than fine gold, for the reason already given on p. 15. At the two mills last named the value of the plate amalgam was $4.50 per oz., and that of the battery amalgam $8.50 per oz. According to Peplar the amalgam caught on inside plates is of very low grade, on account of the large percentage of iron that amalgamates on them. Roskelley found at the Robinson Deep Mine, where inside plates are not used, that 95 per cent, of the gold saved by amalgamation was caught within 3 feet of the top of the table.
About every three or six months the plates are scraped with a sharp iron chisel or palette knife, or scaled by chipping with a hammer and chisel. At the North Star Mill, California, the plates are immersed in boiling water so as to soften the amalgam before they are scraped. On the Rand the plates are generally steamed and scraped every two or three months, according to Denny. They are sometimes “sweated” in California by heating them over a wood fire before scraping them. At the Empire Mill, Grass Valley, California, the sweating of the outside and apron plates of four batteries produced bullion to the value of $19,000. These plates had been down for eighteen months, and the ore which had been run over them averaged 18 dwts. of free gold per ton. After scaling and sweating, the plates may require replating. In course of time they are worn out, the copper becoming brittle and worn into holes, but they usually contain gold enough when discarded to pay for a new set.
Several methods are in use for recovering the gold from old plates. For example, they may be dissolved in nitric acid, when the gold is left nearly pure. A more economical method of detaching the gold, much used in Australia, is described by W. M‘Cutcheon as follows :—The plates are placed on the hearth of a reverberatory furnace, or on a fire made with logs in the open air, and the mercury expelled at a gentle heat. If the temperature is too high, the gold sinks into the copper at once, and the copper must then be dissolved. After the mercury has been driven off, the plate appears to be more or less coated with gold on one side. This surface is treated with hydrochloric acid for eight or ten hours, and the plate is then replaced on the hearth and exposed to a dull red heat until well blackened. On plunging it into cold water, the gold now scales off, and is collected and freed from copper by boiling in nitric acid.
The method in use on the Rand for scaling plates before casting them aside is described by I. Roskelley as follows :—A mixture of ½ lb. sal-ammoniac, ½ lb. nitre, ½ lb. hydrochloric acid, and a pint of water is applied to the plate with a soft brush, and allowed to stand for about fifteen minutes. Afterwards the plate is heated over a good fire. When it has become quite black, which will be in about half an hour, it is dipped into a bath of water, when the scaling can be washed off.
Some millmen drive off the mercury before applying the above mixture; others do so afterwards. Should all the scaling not come off, the parts needing it are treated over again. The scalings are afterwards collected and mixed as follows :—1 part scalings, 1 part sulphur, 1 part borax, and 1 part sand. This is melted in a crucible and the gold extracted.
Old copper plates are often melted down and sold to refineries, where they are useful for mixing with gold and silver bullion in making-up the alloy for parting.