How to extract gold from the soil ?
Posted by MydeaMedia
By J.C.F. Johnson, F.G.S.
We now come to a highly important part of our subject, the practical treatment of ores and matrixes for the extraction of the metals contained. The methods employed are multitudinous, but may be divided into four classes, namely, washing, amalgamating with mercury, chlorinating, cyaniding and other leaching processes, and smelting. The first is used in alluvial gold and tin workings and in preparing some silver, copper, and other ores for smelting, and consists merely in separating the heavier metals and minerals from their gangues by their greater specific gravity in water. The second includes the trituration of the gangue and the extraction of its gold or silver by means of mercury. Chlorinating and leaching generally is a process whereby metals are first changed by chemical action into their mineral salts, as chloride of gold, nitrate of silver, sulphate of copper, and being dissolved in water are afterwards redeposited in the metallic form by means of well-known re-agents.
In really successful mining it is in the last degree important that the mode of extraction of metals in the most scientific manner should be thoroughly understood, but as a general rule the science of metallurgy is but very superficially grasped even by those whose special business it is to treat ore bodies in order to extract their metalliferous contents, and whether in quartz crushing mill, lixiviating, or smelting works there is much left to be desired in the method of treating our ores.
My attention was recently attracted to an article written by Mr. F. A. H. Rauft, M.E., from which I make the following extract:
He says, speaking of the German treatment of ores and the mode of procedure in Australia, "It is high time that Government stepped in and endeavoured by prompt and decisive action to bring the mining industry upon a sound and legitimate basis. Though our ranges abound in all kinds of minerals that might give employment to hundreds of thousands of people, mining is carried on in a desultory, haphazard fashion. There is no system, and the treatment of ores is of necessity handed over to the tender mercies of men who have not even an idea of what an intricate science metallurgy has become in older countries. During many years of practical experience I have never known a single instance where a lode, on being worked, gave a return according to assay, and I have never known any mine where some of the precious metals could not be found in the tailings or slag. The Germans employ hundreds of men in working for zinc which produces some two or three per cent to the ton; here the same percentage of tin could hardly be made payable, and this, mark you, is owing not to cheaper labour alone, but chiefly to the labour-saving appliances and the results of the researches of such gigantic intellects as Professor Kerl and many others, of whom we in this country never even hear. Go into any of the great mining works of central Germany, and you may see acres covered by machinery ingeniously constructed to clean, break, and sort, and ultimately deliver the ores into trucks or direct into the furnace, and the whole under the supervision of a youngster or two. When a parcel of ore arrives at any of the works, say Freiberg or Clausthal, it is carefully assayed by three or four different persons and then handed over to practical experts, who are expected to produce the full amount of previous metal according to assay; and if by any chance they do not, a fixed percentage of the loss is deducted from their salary; or, if the result is in excess of this assay which is more frequently the case, a small bonus is added to their pay. Compare this system with our own wasteful, reckless method of dealing with our precious metals, and we may hide our heads in very shame."
All really practical men will, I think, endorse Mr. Rauft's opinion. Well organised and conducted schools of mines will gradually ameliorate this unsatisfactory state of things, and I hope before long that we shall have none but qualified certificated men in our mines. In the meantime a few practical hints, particularly on that very difficult branch of the subject, the saving of gold, will, it is hoped, be found of service.
The extraction of gold from the soil is an industry so old that its first introduction is lost in the mist of ages. As before stated, gold is one of the most widely disseminated of the metals, and man, so soon as he had risen from the lowest forms of savagery, began to be attracted by the kingly metal, which he found to be easily fashioned into articles of ornament and use, and to be practically non-corrodable.
What we now term the dish or pan, then, doubtless generally a wooden bowl, was the appliance first used; but they had also an arrangement, somewhat like our modern blanket tables, over which the auriferous sand was passed by means of a stream of water. The sands of some of the rivers from which portions of the gold supply of the old world was derived are still washed over year after year in exactly the same manner as was employed, probably, thousands of years ago, the labour, very arduous, being often carried on by women, who, standing knee deep in water, pan off the sand in wooden bowls much as the digger in modern alluvial fields does with his tin dish. The resulting gold often consists of but a grain or two of fine dust-gold, which is carefully collected in quills, and so exported or traded for goods.
The digger of to-day having discovered payable alluvial dirt at such a depth as to permit of its being profitably worked by small parties of men with limited or no capital, procures first a half hogshead for a puddling tub, a "cradle," or "long tom," and tin dish. The "wash dirt," as the auriferous drift is usually termed, contains a considerable admixture of clay of a more or less tenacious character, and the bulk of this has to be puddled and so disintegrated before the actual separation of the gold is attempted in the cradle or dish. This is done in the tub by constantly stirring with a shovel, and changing the water as it becomes charged with the floating argillaceous, or clayey, particles. The gravel is then placed in the hopper of the cradle which separates the larger stones and pebbles, the remainder passing down over inclined ledges as the cradle is slowly rocked and supplied with water. At the bottom of each ledge is a riffle to arrest the particles of gold. Sometimes, when the gold is very fine, amalgamated copper plates are introduced and the lower ledges are covered with green baize to act as blanket tables and catch gold which might otherwise be lost.
A long tom is a trough some 12 feet in length by 20 inches in width at the upper end, widening to 30 inches at the lower end; it is about 9 inches deep and has a fall of 1 inch to a foot. An iron screen is placed at the lower end where large stones are caught, and below this screen is the riffle box, 12 feet long, 3 feet wide, and having the same inclination as the upper trough. It is fitted with several riffles in which mercury is sometimes placed.
Much more work can be done with this appliance than with the cradle, which it superseded. Of course, the gold must be coarse and water plentiful.
When, however, the claim is paying, and the diggings show signs of some permanency, a puddling machine is constructed. This is described in the chapter called "Rules of Thumb."
Hydraulicing and ground sluicing is a very cheap and effective method of treating large quantities of auriferous drift, and, given favourable circumstances, such as a plentiful supply of water with good fall and extensive loose auriferous deposits, a very few grains to the ton or load can be made to give payable returns. The water is conveyed in flumes, or pipes to a point near where it is required, thence in wrought iron pipes gradually reduced in size and ending in a great nozzle somewhat like that of a fireman's hose. The "Monitor," as it is sometimes called, is generally fixed on a movable stand, so arranged that the strong jet of water can be directed to any point by a simple adjustment. A "face" is formed in the drift, and the water played against the lower portion of the ledge, which is quickly undermined, and falls only to be washed away in the stream of water, which is conducted through sluices with riffles, and sometimes over considerable lengths of amalgamated copper plates. This class of mining has been most extensively carried out in California and New Zealand, and some districts of Victoria, but the truly enormous drifts of the Shoalhaven district in New South Wales must in the near future add largely to the world's gold supply. These drifts which are auriferous from grass roots to bed rock extend for nearly fifty miles, and are in places over 200 feet deep. Want of capital and want of knowledge has hitherto prevented their being profitably worked on a large scale.
The extraction of reef gold from its matrix is a much more complicated process, and the problem how most effectively to obtain that great desideratum—a complete separating and saving operation—is one which taxes the skill and evokes the ingenuity of scientific men all over the world. The difficulty is that as scarcely any two gangues, or matrixes, are exactly alike, the treatment which is found most effective on one mine will often not answer in another. Much also depends on the proportion of gold to the ton of rock under treatment, as the most scientific and perfect processes of lixiviation hitherto adopted will not pay, even when all other conditions are favourable, if the amount of gold is much under half an ounce to the ton and even then will leave but a very small profit. If, however, the gold is "free," and the lode large, a very few pennyweights (or "dollars," as the Americans say) to the ton will pay handsomely. The mode of extraction longest in vogue, and after all the cheapest and most effective, for free milling ores where the gold is not too fine, is amalgamation with mercury, which metal has a strong affinity for gold, silver, and copper.
As to crushing appliances, I shall not say much. "Their name is legion for they are many," and the same may be said of concentrators. It may be old-fashioned, but I admit my predilection is still in favour of the stamper-battery, for the reason that though it may be slower in proportion to the power employed, it is simple and not liable to get out of order, a great advantage when one has so often to depend on men who bring to their work a supply principally of main strength and stupidity. For the same reason I prefer the old draw and lift, and plunger pumps to newer but more complicated water-lifters.
On both these points, however, I am constrained to admit that my opinion has recently been somewhat shaken.
I have lately seen two appliances which appear to mark a new era in the scientific progress of mining. One is the "Griffin Mill," the other the "Lemichel Siphon Elevateur."
The first is in some respects on the principle of the Huntingdon Mill. The latter, if the inventor may be believed and the results seem to show he can be, will be a wonderful factor in developing not only mining properties where a preponderance of water is the trouble, but also in providing an automatic, and therefore extremely cheap, mode of water-raising and supply, which in simplicity is thus far unexampled. Atmospheric pressure alone is relied on. The well-known process of the syphon is the basis, but with this essential difference, that a large proportion of the water drawn up to the apex of the syphon is super-elevated to heights regulated by the fall obtained in the outlet leg. This elevation can be repeated almost indefinitely by returning the waste water to the reservoirs.
The Lemichel Syphon is a wonderful, yet most simple application of natural force. The inlet leg of the syphon is larger in diameter than the outlet leg, and is provided at the bottom with a valve or "clack." The outlet leg has a tap at its base. At the apex are two chambers, with an intermediary valve, regulated by a counterpoise weighted lever. The first chamber has also a vertical valve and pipe.
When the tap of the outlet leg is turned, the water flows as in an ordinary syphon, but owing to the rapid automatic opening and shutting of the valve in the first chamber about 45 per cent of the water is diverted, and may be raised to a height of many feet above the top of the syphon.
It need not be impressed on practical men that if this invention will perform anything like what is claimed for it, its value can hardly be calculated. After a careful inspection of the appliance in operation, I believe it will do all that is stated.
Another invention is combined with this which, by a very small expenditure of fuel, will enable the first point of atmospheric pressure to be attained. In this way the unwatering of mines may be very inexpensively effected, or water for irrigation purposes may be raised from an almost level stream.
The Griffin Mill is a centrifugal motion crusher with one roller only, which, by an ingenious application of motive force, revolves in an opposite direction to its initial momentum, and which evolves a force of 6000 lb. against the tire, which is only 30 inches in diameter. For hard quartz the size should be increased by at least 6 inches. It is claimed for this mill that it will pulverise to a gauge of 900 holes to the square inch from 1 1/2 to 2 1/2 tons per hour, or, say roughly, 150 tons per week.
The Huntingdon mill is a good crusher and amalgamator where the material to be operated on is comparatively soft, but does not do such good work when the stone is of a hard flinty nature.
A No. 4 Dodge stone-breaker working about 8 hours will keep a five-foot Huntingdon mill going 24 hours, and an automatic feeder is essential. For that matter both are almost essential for an ordinary stamper battery, and will certainly increase the crushing capacity and do better work from the greater regularity of the feed.
A 10 h.-p. (nominal) engine of good type is sufficient for Huntingdon mill, rock breaker, self-feeder and steam pump. A five-foot mill under favourable circumstances will crush about as much as eight head of medium weight stamps.
The Grusonwek Ball Mills, made by Krupp of Germany, also that made by the Austral Otis Company, Melbourne, are fast and excellent crushing triturating appliances for either wet or dry working, but are specially suited only for ores when the gold is fine and evenly distributed in the stone. The trituration is effected by revolving the stone in a large cylinder together with a number of steel balls of various sizes, the attrition of which with the rock quickly grinds it to powder of any required degree of fineness.
More mines have been ruined by bad mill management probably than by bad mining, though every experienced man must have seen in his time many most flagrant instances of bungling in the latter respect. Shafts are often sunk on the wrong side of the lode or too near or too far away therefrom, while instances have not been wanting where the (mis) manager has, after sinking his shaft, driven in the opposite direction to that where the lode should be found.
A common error is that of erecting machinery before there is sufficient ore in sight to make it certain that enough can be provided to keep the plant going. In mines at a distance from the centre of direction it is almost impossible to check mistakes of this description, caused by the ignorance or over sanguineness of the mine superintendent, and they are often as disastrous as they are indefensible. Another fertile source of failure is the craze for experimenting with untried inventions, alleged to be improvements on well-known methods.
A rule in the most scientific of card games, whist, is "when in doubt lead trumps." It might be paraphrased for mining thus: "When in doubt about machinery use that which has been proved." Let some one else do the experimenting.
The success of a quartz mine depends as much on favourable working conditions as on its richness in gold. Thus it may be that a mine carrying 5 or 6 oz. of gold to the ton but badly circumstanced as to distance, mountainous roads, lack of wood and water, in some cases a plethora of the latter, or irregularly faulted country, may be less profitable than another showing only 5 or 6 dwt., but favourably situated.
It is usually desirable to choose for the battery site, when possible, the slope of a hill which consists of rock that will give a good foundation for your battery.
The economical working depends greatly on the situation, which is generally fixed more or less, in the proximity of the water. The advantages of having ample water for battery purposes, or of using water as a motive power, are so great that it is very often desirable to construct a tramway of considerable length, when, by so doing, that power can be utilised; hence most quartz mills are placed near streams, or in valleys where catchment dams can be effectively constructed, except, of course, in districts where much water has to be pumped from the mine.
If water-power can be used, the water-motor will necessarily be placed as low as possible, in order to obtain the fullest available power. One point is essential. Special care must be taken to keep the appliances above the flood-level. If the water in the stream is not sufficient to carry off the tailings, the battery should be placed at such a height as to leave sufficient slope for tailings' dumps. This is more important when treating ore of such value that the tailings are worth saving for secondary treatment. In this case provision should be made for tailings, dams, or slime pits.
Whether the battery is worked by water, steam, or gas power, an ample supply of water is absolutely necessary, at least until some thoroughly effective mode of dry treatment is established. If it can be possibly arranged the water should be brought in by gravitation, and first cost is often least cost; but where this is impossible, pumps of sufficient capacity, not only to provide the absolute quantity used, but to meet any emergency, should be erected.
The purer the water the better it will be for amalgamating purposes, and in cold climates it is desirable to make provision for heating the water supplied to the battery. This can be done by means of steam from the boiler led through the feed tanks; but where the boiler power is not more than required, waste steam from the engine may be employed, but care must be taken that no greasy matter comes in contact with the plates. The exhaust steam from the engine may be utilised by carrying it through tubes fitted in an ordinary 400 gallon tank.
Reducing appliances have often to be placed in districts where the water supply is insufficient for the battery. When this is so every available means must be adopted for saving the precious liquid, such as condensing the exhaust steam from the engine. This may be done by conducting it through a considerable length of ordinary zinc piping, such as is used for carrying the water from house roofs. Also tailings pits should be made, in which the tailings and slimes are allowed to settle, and the cleared water is pumped back to be again used. These pits should, where practicable, be cemented. It is usual, also, to have one or two tailings dams at different levels; the tailings are run into the upper dam, and are allowed to settle; the slimes overflow from it into the lower dam, and are there deposited, while the cleared water is pumped back to the battery. Arrangements are made by which all these reservoirs can be sluiced out when they are filled with accumulated tailings. It is well not to leave the sluicing for too long a period, as when the slimes and tailings are set hard they are difficult to remove.
Where a permanent reducing plant is to be erected, whatever form of mill may be adopted, it is better for many reasons to use automatic ore feeders. Of these the best two I have met are the "Tulloch" and "Challenge" either of which can be adapted to any mill and both do good work.
By their use the reducing capacity of the mill is increased, and the feeding being regular the wear and tear is decreased, while by the regulated feeding of the "pulp" in the battery box or mortar can be maintained at any degree of consistency which may be found desirable, and thus the process of amalgamation will be greatly facilitated. The only objection which can be urged against the automatic feeder is that the steel points of picks, gads, drills, and other tools may be allowed to pass into the mortar or mill, and thus cause considerable wear and tear. This, I think, can be avoided by the adoption of the magnet device, described in "Rules of Thumb."
There are many mines where 3 to 4 dwt. of gold cover all the cost, the excess being clear profit. In fact there are mines which with a yield of 1 1/2 to 2 dwt. a ton, and crushing with water power, have actually yielded large profits. On the other hand, mines which have given extraordinary trial crushings have not paid working expenses. Everything depends on favourable local conditions and proper management.
Having decided what class of crushing machinery you will adopt, the first point is to fix on the best possible site for its erection. This requires much judgment, as success or failure may largely depend on the position of your machinery. One good rule is to get your crusher as reasonably high as possible, as it is cheaper to pump your feed water a few feet higher so as to get a good clear run for your tailings, and also to give you room to erect secondary treatment appliances, such as concentrators and amalgamators below your copper plates and blanket strakes.
Next, and this is most important, see that your foundations are solid and strong. A very large number of the failures of quartz milling plants is due to neglect of this rule.
I once knew a genius who erected a 10-Lead mill in a new district, and who adopted the novel idea of placing a "bed log" laterally beneath his stampers. The log was laid in a little cement bed which, when the battery started, was not quite dry. The effect was comical to every one but the unfortunate owners. It was certainly the liveliest, but at the same time one of the most ineffective batteries I have seen.
In a stamp mill the foundations are usually made of hard wood logs about 5 to 6 feet long, set on end, the bottom end resting on rock and set round with cement concrete. These are bolted together, and the "box" or mortar is bolted to them. The horizontal logs to carry the "horses" or supports for the battery frame should also be of good size, and solidly and securely bolted. The same applies to your engine-bed, but whether it be of timber, or mason work, above all things provide that the whole of your work is set out square and true to save after-wear and friction.
Considerable difference of opinion exists as to the most effective weight for stamps. My experience has been that this largely depends on the nature of your rock, as does also the height for the drop. I have usually found that with medium stamps, say 7 to 7 1/2 cwt. with fair drop and lively action, about 80 falls per minute, the best results were obtained, but the tendency of modern mill men is towards the heavier stamps, 9 cwt. and even heavier.
To find the horse-power required to drive a battery, multiply the weight of one stamp by the number of stamps in the battery; the height of lift in feet by the number of lifts per minute; add one-third of the product for friction, and the result will be the number of feet-lbs. per minute; divide this by 33,000 which is the number of feet-lbs. per minute equal to 1 h.-p. and the result will be the h.-p. required. Thus if a stamp weighs 800 lb. and you have five in the box, and each stamp has a lift of 9 in. = 0.75 ft. and strikes 80 blows per minute, then 800 x 5 x 0.75 x 80 = 240,000; one-third of 240,000 = 80,000 which added to 240,000 = 320,000; and 320,000 divided by 33,000 = 9.7 h.-p. or 1.9 h.-p. each stamp.
The total weight of a battery, including stamper box, stampers, etc., may be roughly estimated at about 1 ton per stamp. Medium weight stampers, including shank cam, disc, head, and shoe, weigh from 600 to 700 lb., and need about 3/4 h.-p. to work them.
The quantity of water required for the effective treatment of gold-bearing rock in a stamper battery varies according to the composition of the material to be operated upon, but generally it is more than the inexperienced believe. For instance, "mullocky" lode stuff, containing much clayey matter or material carrying a large percentage of heavy metal, such as titanic iron or metallic sulphides, will need a larger quantity of water per stamp than clean quartz. A fair average quantity would be 750 to 1000 gallons per hour for each box of five stamps. In general practice I have seldom found 1000 gallons per hour more than sufficient.
As to the most effective mesh for the screen or grating no definite rule can be given, as that depends so largely on the size of the gold particles contained in the gangue. The finer the particles the closer must be the mesh, and nothing but careful experiment will enable the battery manager to decide this most important point. The American slotted screens are best; they wear better than the punched gratings and can be used of finer gauge. Woven steel wire gauze is employed with good effect in some mills where especially fine trituration is required. This class of screen requires special care as it is somewhat fragile, but with intelligent treatment does good work.
The fall or inclination of the tables, both copper and blanket strakes, is also regulated by the class of ore. If it should be heavy then the fall must be steeper. A fair average drop is 3/4 inch to the foot. Be careful that your copper tables are thoroughly water-tight, for remember you are dealing with a very volatile metal, quicksilver; and where water will percolate mercury will penetrate.
The blanket tables are simply a continuation of the mercury tables, but covered with strips of coarse blanket, green baize, or other flocculent material, intended to arrest the heavier metallic particles which, owing to their refractory nature, have not been amalgamated.
The blanket table is, however, a very unsatisfactory concentrator at best, and is giving place to mechanical concentrators of various descriptions.
An ancient Egyptian gold washing table was used by the Egyptians in treating the gold ores of Lower Egypt. The ore was first ground, it is likely by means of some description of stone arrasts and then passed over the sloping table with water, the gold being retained in the riffles. In these the material would probably be mechanically agitated. Although for its era ingenious it will be plain to practical men that if the gold were fine the process would be very ineffective. Possibly, but of this I have no evidence, mercury was used to retain the gold on the riffles, as previously stated. This method of saving the precious metal was known to the ancients.
At a mine of which I was managing director the lode was almost entirely composed of sulphide of iron, carbonate of lime or calcspar, with a little silica. In this case it has been found best to crush without mercury, then run the pulp into pans, where it is concentrated. The concentrates are calcined in a common reverberatory furnace, and afterwards amalgamated with mercury in a special pan, the results as to the proportion of gold extracted being very satisfactory; but it does not therefore follow that this process would be the most suitable in another mine where the lode stuff, though in some respects similar, yet had points of difference.
I was lately consulted with respect to the treatment of a pyritic ore in a very promising mine, but could not recommend the above treatment, because though the pyrites in the gangue was similar, the bulk of the lode consisted of silica, consequently there would be a great waste of power in triturating the whole of the stuff to what, with regard to much of it, would be an unnecessary degree of fineness. I am of opinion that in cases such as this, where it is not intended to adopt the chlorination or cyanogen process, it will be found most economical to crush to a coarse gauge, concentrate, calcine the concentrates, and finally amalgamate in some suitable amalgamator.
Probably for this mode of treatment Krom rolls would be found more effective reducing agents than stampers, as with them the bulk of the ore can be broken to any required gauge and there would consequently be less loss in "slimes."
The great art in effective battery work is to crush your stuff to the required fineness only, and then to provide that each particle is brought into contact with the mercury either in box, trough, plate, or pan. To do this the flow of water must be carefully regulated; neither so much must be used as to carry the stuff off too quickly nor so little as to cause the troughs and plates to choke. In cold weather the water may be warmed by passing the feed-pipe through a tank into which the steam from the engine exhausts, and this will be found to keep the mercury bright and lively. But be careful no engine oil or grease mingles with the water, as grease on the copper tables will absolutely prevent amalgamation.
The first point, then, is to crush the gangue effectively, the degree of fineness being regulated by the fineness of the gold itself. This being done, then comes the question of saving the gold. If the quartz be clean, and the gold unmixed with base metal, the difficulty is small. All that is required is to ensure that each particle of the Royal metal shall be brought into contact with the mercury. The main object is to arrest the gold at the earliest possible stage; therefore, if you are treating clean stone containing free gold, either coarse or fine, I advise the use of mercury in the boxes, for the reason that a considerable proportion of the gold will be caught thereby, and settling to the bottom, or adhering to amalgamated plates in the boxes, where such are used, will not be afterwards affected by the crushing action, which might otherwise break up, or "flour," the mercury. On the whole, I rather favour the use of mercury in the box at any time, unless the ore is very refractory—that is, contains too great a proportion of base metals, particularly sulphides of iron, arsenic, etc., when the result will not be satisfactory, but may entail great loss by the escape of floured mercury carrying with it particles of gold. Here only educated intelligence, with experience, will assist the battery manager to adopt the right system.
The crushed stuff—generally termed the "pulp"—passes from the boxes through the "screens" or "gratings," and so on to the "tables"—i.e., sheets of copper amalgamated on the upper surface with mercury, and sometimes electroplated with silver and afterwards treated with mercury. Unless the quartz is very clean, and, consequently light, I am opposed to the form of stamper box with mercury troughs cast in the "lip," nor do I think that a trough under the lip is a good arrangement, as it usually gets so choked and covered with the heavy clinging base metals as to make it almost impossible for the gold to come in contact with the mercury. It will be found better where the gold is fine, or the gangue contains much base metal, to run the pulp from the lip of the battery into a "distributor."
The distributor is a wooden box the full width of the "mortar," having a perforated iron bottom set some three to four inches above the first copper plate, which should come up under the lip. The effect of this arrangement is that the pulp is dashed on the plate by the falling water, and the gold at once coming in contact with the mercury begins to accumulate and attract that which follows, till the amalgam becomes piled in little crater-shaped mounds, and thus 75 per cent of the gold is saved on the top plate.
I have tried a further adaptation of this process when treating ores containing a large percentage of iron oxide, where the bulk of the gold is impalpably fine, and contained in the "gossan." At the end of the blanket table, or at any point where the crushed stuff last passes before going to the "tailings heap," or "sludge pit," a "saver" is placed. The saver is a strong box about 15 in. square by 3 ft. high, one side of which is removable, but must fit tight. Nine slots are cut inside at 4 in. apart, and into these are fitted nine square perforated copper plates, having about eighty to a hundred 1/4 in. holes in each; the perforations should not come opposite each other. These plates are to be amalgamated on both sides with mercury, in which a very little sodium has been placed (if acid ores are being treated, zinc should be employed in place of sodium, and to prevent the plates becoming bare, if the stuff is very poor, thick zinc amalgam may be used with good effect; but in that case discontinue the sodium, and occasionally, if required, say once or twice in the day, mix an ounce of sulphuric acid in a quart of water and slowly pour it into the launder above the saver). Underneath the "saver" you require a few riffles, or troughs, to catch any waste mercury, but if not overfed there should be no waste. This simple appliance, which is automatic and requires little attention, will sometimes arrest a considerable quantity of gold.
We now come to the subsidiary processes of battery work, the "cleaning" of plates, and "scaling" same when it is desired to get all the gold off them, the cleaning and retorting of amalgam, and of the mercury, smelting gold, etc.
Plates should be tenderly treated, kept as smooth as possible, and when cleaning up after crushing, in your own battery, the amalgam—except, say, at half-yearly intervals—should be removed with a rubber only; the rubber is simply a square of black indiarubber or soft pine wood.
When crushing rich ore, and you want to get nearly all the gold off your plates, the scraper may be resorted to. This is usually made by the mine blacksmith from an old flat file which is cut in half, the top turned over, beaten out to a sharp blade, and kept sharp by touching it up on the grinding-stone. This, if carefully used, will remove the bulk of the amalgam without injury to the plate.
Various methods of "scaling" plates will be found among "Rules of Thumb."
Where base metals are present in the lode stuff frequent retortings of the mercury, say not less than once a month, will be found to have a good effect in keeping it pure and active. For this purpose, and in order to prevent stoppage of the machinery, a double quantity is necessary, so that half may be used alternately. Less care is required in retorting the mercury than in treating the amalgam, as the object in the one case is more to cleanse the metal of impurities than to save gold, which will for the most part have been extracted by squeezing through the chamois leather or calico. A good strong heat may therefore at once be applied to the retort and continued, the effect being to oxidise the arsenic, antimony, lead, etc., which, in the form of oxides, will not again amalgamate with the mercury, but will either lie on its surface under the water, into which the nozzle of the retort is inserted, or will float away on the surface of the water. I have also found that covering the top of the mercury with a few inches of broken charcoal when retorting has an excellent purifying effect.
In retorting amalgam, much care and attention is required.
First, never fill the retort too full, give plenty of room for expansion; for, when the heat is applied, the amalgam will rise like dough in an oven, and may be forced into the discharge pipe, the consequence being a loss of amalgam or the possible bursting of the retort. Next, be careful in applying the heat, which should be done gradually, commencing at the top. This is essential to prevent waste and to turn out a good-looking cake of gold, which all battery managers like to do, even if they purpose smelting into bars.
Sometimes special difficulties crop up in the process of separating the gold from the amalgam. At the first "cleaning up" on the Frasers Mine at Southern Cross, West Australia, great consternation was excited by the appearance of the retorted gold, which, as an old miner graphically put it, was "as black as the hind leg of a crow," and utterly unfit for smelting, owing to the presence of base metals. Some time after this I was largely interested in the Blackborne mine in the same district when a similar trouble arose. This I succeeded in surmounting, but a still more serious one was too much for me—i.e., the absence of payable gold in the stone. I give here an extract from the Australian Mining Standard, of December 9th, 1893, with reference to the mode of cleaning the amalgam which I adopted.
NEW METHOD OF SEPARATING GOLD FROM IMPURE AMALGAM.
I had submitted to me lately a sample of amalgam from a mine in West Australia which amalgam had proved a complete puzzle to the manager and amalgamator. The Mint returns showed a very large proportion of impurity, even in the smelted gold. When retorted only, the Mint authorities refused to take it after they had treated two cakes, one of 119 oz., which yielded only 35 oz. 5 dwt. standard gold, and one of 140 oz., which gave 41 oz. 10 dwt. The gold smelted on the mine was nearly as bad proportionately. Thus, 128 oz. smelted down at the Mint to 87 oz. 8 dwt. and 109 oz. to 55 oz. 10 dwt. The impurity was principally iron, a most unusual thing in my experience, and was due to two causes revealed by assay of the ore and analysis of the mine water, viz., an excess of arsenate of iron in the stone, and the presence in large proportions of mineral salts, principally chloride of Calcium CaCl., sodium NaCl, and magnesium MgCl2, in the mine water used in the battery. The exact analysis of the water was as follows:—
Carbonate of Iron FeCO3 2.76 grains per gallon Carbonate of Calcium CaCO3 7.61 grains per gallon Sulphate of Calcium CaSO4 81.71 grains per gallon Chloride of Calcium CaCl2 2797.84 grains per gallon Chloride of Magnesium MgCl2 610.13 grains per gallon Chloride of Sodium or Common Salt NaCl 5072.65 grains per gallon Total solid matter 8572.70 = 19.5 oz. to the gallon.
It will be seen, then, that this water is nearly four times more salt that that of the sea. The effect of using a water of this character, as I have previously found, is to cause the amalgamation of considerable quantities of iron with the gold as in this case.
I received 10 oz. of amalgam, and having found what constituted its impurities proceeded to experiment as to its treatment. When retorted on the mine it was turned out in a black cake so impure as almost to make it impossible to smelt properly. I found the same result on first retorting, and after a number of experiments which need not be recapitulated though some were fairly effective, I hit on the following method, which was found to be most successful and will probably be so found in other localities where similarly unfavourable conditions prevail.
I took a small ball of amalgam, placed it in a double fold of new fine grained calico, and after soaking in hot water put it under a powerful press. The weight of the ball before pressing was 1583 gr. From this 383 gr. of mercury was expressed and five-eighths of a grain of gold was retorted from this expressed mercury. The residue, in the form of a dark, grey, and very friable cake, was powdered up between the fingers and retorted, when it became a brown powder; it was afterwards calcined on a flat sheet in the open air; result, 510 gr. of russet-coloured powder. Smelted with borax, the iron oxide readily separated with the slag; result, 311 gr. gold 871-1000 fine; a second smelting brought this up to 914-1000 fine. Proportion of smelted gold to amalgam, one-fifth.
The principal point about this mode of treatment is the squeezing out of the mercury, whereby the amalgam goes into the retort in the form of powder, thus preventing the slagging of the iron and enclosure of the gold. The second point of importance is thorough calcining before smelting.
Of course it would be practicable, if desired, to treat the powder with hydrochloric acid, and thus remove all the iron, but in a large way this would be too expensive, and my laboratory treatment, though necessarily on a small scale, was intended to be on a practical basis.
The amalgam at this mine was in this way afterwards treated with great success.
For the information of readers who do not understand the chemical symbols it may be said that
FeCO3 is carbonate of iron; CaCO3 is carbonate of calcium; CaSO4 is sulphate of calcium; CaCl2 is chloride of calcium; MgCl2 is chloride of magnesium; NaCl is chloride of sodium, or common salt. -----------------------------------------------
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