Table of Contents
Any amalgamation test is found to possess most merits and fewest limitations when reduced to the smallest possible dimensions permitting uniform action and certain treatment. A limit is always reached at some point in this reduction in size, where some condition no longer can be fulfilled. In many continuous mechanical tests the limit is set by irregularity, resulting from complexity and intricacy. In tests of a complicated chemical nature, the instability of small masses and fluctuations in physical surroundings make the work often difficult to practice.
The amalgamation test, however, meets no actual barrier to reduction of scale, until the difficulty of procuring a sample is encountered. Down to this point the obstacles are eliminated in the method described. A further decrease in the size of the portion would necessitate a finer pulverization of the sample; and this is prohibited in the purpose of the test. By giving heed to large pieces of gold when they occur, this necessary portion is reduced to a minimum size; and under these conditions the test is found often valuable.
The small amalgamation-test of the laboratory is not always reliable as a basis for important decisions as to the character and commercial treatment of ores. The conditions of continuous practice on a large scale are not always reproduced in the brief laboratory-test. Sometimes such a reproduction would be very difficult, or impracticable; sometimes it is not even attempted, and the test is performed in a rough way, without regard to the effects of small variations in the methods pursued.
The aim of a laboratory amalgamation-test is to learn how much of the gold or silver in an ore can be economically recovered by amalgamation on a commercial scale. The yield can be increased, perhaps, by additional chemical treatment or by extreme comminution of the ore; but this might cost more than the value of such increase. Moreover, in practice, other operations often follow amalgamation; and these operations may remedy a defective extraction by amalgamation, more economically than could be done by directly increasing that extraction.
The test should be performed, therefore, upon a uniformly-sized material, and under conditions that are precisely known and capable of being exactly duplicated, or purposely modified.
The crucial feature of the test is the effectiveness of the contact made between the gold or silver of the ore and the amalgamating-surface. This may depend upon the temperature; the duration of contact; its force, due to gravity or impact; the hardness, plasticity or liquidity of the amalgam; the roughness or other quality of the amalgam-surface; the scouring action of the ore upon the amalgam, or the suspension and removal of metal once amalgamated; the supporting power of the currents; the fineness of the ore; the liberation of the metal from the gangue by comminution; the cleanness and freshness of the mineral; the molecular stress in the metal after grinding or rubbing; the oxidation and chemical changes of the ore during the test, and before the test; the kind of gangue; the kind of metal; the kind of amalgam; and the cleanness of everything. The test should be conducted with a realization of the significance of these conditions; and if so conducted, it often may give results capable of interpretation, and helpful as suggestions, though its general verdict be unfavorable.
Methods of laboratory-amalgamation are classified as follows, in accordance with similar commercial methods.
Amalgamation During Grinding
This consists in wet crushing or grinding in contact with mercury or amalgam, usually in small arrastras, amalgamating-pans or other laboratory grinding-apparatus; followed, generally, by further addition of water, stirring, settling, and final collection of the mercury or amalgam containing the precious metal. Suitable chemical treatment often is applied to aid the process. Unquestionably this method amalgamates more of the gold and silver than any other. The material may be a crude ore, such as a “ free-milling ” gold-ore, or it may have been previously oxidized or chloridized by roasting. And the amalgamation may be either simple, or assisted by added chemicals, such as copper sulphate, sodium chloride, etc., often used with silver-ores—in which case the operation may be a complex chemical one. However conducted commercially (e.g., in barrel, patio or arrastra), the chemical reactions generally require the aid of some accompanying positive mechanical or grinding-action when practised in the laboratory.
Grinding during amalgamation has been recommended also for hand-work with a compact field-outfit, and has long been used by prospectors and assayers. For operations on a large scale, it has its economic counterparts in the arrastra-treatment of gold-ores, in stamp-milling with battery-plates, and in the numerous patented mills in which amalgamated plates are exposed to the pulp during crushing. In it, the freshly fractured mineral presents its surface of clean gold for amalgamation before the metal has been distorted by pressure, or coated by reaction with gangue, etc.
In a small test of this class, the amount of precious metal amalgamated generally is estimated from the difference between the assay-value of the ore, at the start, and that of the tailings at the end. The result, showing the percentage of free-milling gold in the ore, usually is higher than can be expected in any treatment on the large scale other than that of the arrastra. In such a laboratory-test, moreover, there is usually some difficulty in separating all the mercury from all the pulp. The quantity of pure mercury required is large, compared with the small amount of ore and gold present; the result when calculated upon differences in assay-value is less satisfactory than a positive determination (such as might be reached by retorting the mercury—which is an inconvenient and burdensome affair, preventing rapid, frequent and easy tests). Moreover, power is needed in the laboratory, or else much arduous and cautious hand-work must be performed; and, finally, there is uncertainty as to the exact degree and the uniformity of pulverization incidental to the treatment.
Agitation with Mercury
This method consists in circulating, stirring or otherwise agitating a wet ore, already pulverized, mercury being added at the beginning of the operation, and, at the end, removed with the gold and silver it has collected. The results may conform to a large degree with those of ordinary stamp-milling and plate-amalgamation; but here again, figures are obtained which, uncorrected, are likely to indicate possibilities not to be realized in practice.
It has been customary, always, to use a comparatively large quantity of mercury in these tests; and the retorting of so much mercury containing so little silver and gold is troublesome in accurate work. Therefore, in this, as in the grinding- test above described, the result generally is estimated from the difference between the assay-value of the original ore and that of the tailings. There is often difficulty, moreover, in causing all the mercury to settle. Mercury, which has floured, may remain persistently with the metallic sulphides, or, when removed, may be mixed with sulphides carrying gold and silver that should not be reckoned as free-milling.
In the laboratory this method is used mostly for testing unroasted gold-ores, to compare with plate-amalgamation in milling. Such a use of liquid mercury has been found, by observing certain precautions, to give results corresponding to those of commercial practice with solid amalgam.
A roasted ore may require a positive grinding-action to break apart clots remaining from roasting. Such material, if tested by this method, must be thus mechanically prepared before the simple agitation with water and mercury begins.
In economic practice this test has its counterparts in the various patented devices for effecting an intimate contact between ores and liquid mercury. In the action of these devices, ore is made to ascend through liquid mercury, to flow across mercury or to fall upon mercury; it is impelled by gravity or centrifugal force; or it is moved by various agitators, aided by currents of water, and sometimes supplemented by heat, chemicals and electrolytic action. Much ingenuity has been devoted to such inventions; but they have not found general favor. As a rule, they do only the work of plate-amalgamation at a somewhat higher cost.
For a simple agitation-test, in which no crushing is to take place during the amalgamation, close attention must be given to the manner of preparatory crushing. By dry crushing, the gold contained in ore is rubbed, distorted and exposed to the mechanical action of every kind of impurity and gangue-substance present. Some of these substances may be harmful in the test, while they may or may not be equally so under the commercial conditions that the test is expected to imitate. The behavior of grease, graphite or talc is well known. Oxides, sulphides and other natural substances are also known to affect, chemically or physically, the refractory qualities of ore.
With wet crushing and immediate amalgamation, as contrasted here with dry crushing and subsequent wetting, there is partial elimination or segregation of some of these substances at the outset. Amalgamation can begin in practical, wet-crushing appliances often before all the ore is reduced to the size to which it is destined to be crushed. Moreover, in wet crushing on a commercial scale, air does not reach the fresh fractures of ore, and can have no action in surface-tension ; whereas, in dry crushing, films of air, or of dry solid material can remain upon the gold surfaces, and seriously interfere with amalgamation, especially if uncleanness has been permitted, or if the fines are not removed as fast as they have been produced.
In a simple treatment of this type, with any ordinary apparatus, an attempt to keep the mercury unbroken often gives too low a result. Treating the ore with water and only a moderate quantity of mercury, in cylindrical bottles, for example, by rotation around the axis, fails to draw the sulphides to the liquid mercury surface and thus expose all the material to amalgamation. Rotation in alternating directions gives the ore opportunity to settle during the change of direction, and causes the mercury to flow over the accumulated ore until finally the continuity of the liquid metal is broken. Attempts to economize to any great extent, in the proportion of mercury used, reduce the area of the amalgamating-surface until, in an extreme case, the mercury may be in the form of a globule, remaining at one end of the bottle, for a long time failing to come into contact with certain minerals of the ore, or with the gold which it is desired to amalgamate. A rotating spherical flask, or a specially constructed receptacle made by joining the ground tops of two glass funnels to retain the mercury in a V-shaped groove, is an improvement over the cylindrical flask; but even this permits the passage of certain constituents around the mercury; and complete amalgamation may fail. A cylindrical bottle, rotating on an inclined axis is subject to the same objection.
A grooved channel, more easily cleaned and managed than a closed receptacle, has been used in a few tests for amalgamating small portions of ore, before concentrating upon the wooden batea, the aim being to avoid excessive flouring of the mercury. This appliance was made by turning a deep annular V-shaped groove in the upper face of a wooden disk, 10 in. in diameter, of turned sugar-pine, shellacked and rubbed down to a dead finish. The disk is tilted at a constant angle by a pin projecting downward 1 in. from the bottom at the center. When resting upon a level surface, this pin gives the disk two points of bearing; the one being the lower end of the pin, the other, any point on the lower edge of the periphery. The ore is placed in the groove with mercury and water, and a slow, gyratory motion is given to the whole, by rolling upon a level surface. A small depression or shallow cavity is made at one point in the bottom of the groove to retain at least a part of the mercury, and to arrest particles of gold that travel more slowly around the groove than the flowing ore, water and mercury. A small quantity of mercury suffices for the test, and this can be recovered easily, with the gold that it contains, at the conclusion of the treatment. The test is subject to the objection that float-material, from certain ores, always forms when the surfaces upon which they lie are alternately flooded and drained. The device can be used sometimes, however, with the batea, and is convenient in its limited way.
Tumbling the ore with mercury and water, or rotating end-wise in small bottles or tubes at a moderate speed, while the ore is in the form of a thin pulp, flours the mercury and fails to recover gold more expeditiously obtained by other means.
In all these agitation-tests, where the speed is moderate, the ore of a certain specific gravity, or gold of a certain degree of fineness, is liable not to make the necessary contact with the liquid metal. The mercury seeks the lower level, where it quietly flows, never making contact with certain parts of the ore.
Agitation in Contact with Amalgamated Surfaces
By this method, the wet pulp, already crushed, is passed over amalgamated plates or agitated in contact with amalgamated surfaces, usually of silver or copper. This test, practiced with care, may yield results strictly comparable with outside-plate amalgamation. The surfaces employed may be in the form of amalgam-plates or of agitators of special construction, or they may be the amalgamated interior surfaces of retaining receptacles such as the pan, spoon or batea; but in any case they must be carefully prepared and kept in good condition. A copper surface coated with soft silver-amalgam is found highly satisfactory in such tests.
On a larger scale, the test can be made with a small stamp-battery and plates, such as some metallurgical laboratories possess, thus approaching still nearer, the exact conditions of practice. Nevertheless, there are limitations to the practicability and scope of such tests; and it may often happen that the quicker and easier small test can be employed with advantage to settle some special questions. Many features of the large- scale laboratory-test have been brought out by Prof. R. H. Richards.
Upon the smaller scale, this method has often been combined with a concentration-test performed at the same time, an amalgamated batea or other vanner being used for concentration. The cleanup and quantitative determination of the gold and silver from the comparatively large amalgamated surface necessary in such a vanning-test is, however, difficult; and generally the values amalgamated are estimated from a comparison of the assay-values of the pulp at the start and at the end. This difference-method for ascertaining the “free-milling” gold-value of an ore is objectionable. Moreover, in a small test with pan or batea, it is difficult to avoid some accidental loss of material; there is difficulty in preparing a satisfactory amalgamating-surface, and preserving it in good condition, while exposed to the air repeatedly during the test; and the small amount of ore that can be handled, and the irregularity of the treatment, make the results somewhat uncertain.
During a concentration-test on a wooden batea or miner’s horn, one can recover much of the “ free-milling ” gold upon a fragment of soft, silver-amalgam or in a small globule of liquid mercury. Imperfect contact, however, between the gold of the ore and the amalgamating surface makes the result uncertain; and when mercury is used, and not amalgam, the breaking up and flouring of the mercury sometimes makes the method inconvenient. Nevertheless, it is possible to perform a kind of approximate, small-scale test in this way; and the direct determination for the “ free-milling ” metal is a satisfaction.
If the actual conditions of large-scale tests, with stamp-mill and plates, or other apparatus, are not present, there is little reason for operating large and inconvenient quantities of material. Between the large test on a commercial scale, and the small one of the assay-laboratory, the intermediate sizes have little to commend them. Large mechanical devices, circulating the pulp automatically over amalgamated surfaces are sometimes useful; but, in general, large size is an inconvenience, and the heavier machinery involved is not so closely held under inspection. Usually, a large plate-surface or a large quantity of amalgam or mercury is required; and this is more difficult to prepare and to recover with quantitative accuracy. To obtain the advantages of a systematic and positive small-scale laboratory-test for amalgamating a prepared pulp, the quantity taken should be as small as will permit good work in the original preparation of the sample.
The Size of the Test-Portion
The size of a sample for amalgamation affects the details of manipulation and the significance of the test. There is a minimum quantity smaller than which a true sample cannot be; and this limit is controlled by the principles of sampling. Larger portions can be employed to an indefinite extent, but with progressively-increasing inconvenience and expense—and this without limit. It is the minimum limit only which demands consideration.
While the quantity of material used in a test generally should be as large as is demanded of an accurate sample, yet, with some ores and under some circumstances, even such a quantity may be unnecessarily and inconveniently large. Sometimes it is justifiable to work with smaller portions, provided the limitations of the sample are understood. The test is intended to show, accurately and positively, for the sample taken, the amalgamating-quality of the gold; but the average amalgamating-quality of a gold-bearing material may be represented truly by a sample so small as to be influenced in its assay-value by the possible occurrence, for example, of a nugget. Such an influence might vitiate a much larger portion, and must be guarded against by other means than by mere increase of the quantity tested.
For assay-purposes, visible particles of native gold are removed from the fine screens and are assayed separately, and these separate values are assigned to the larger portions from which they were taken. In sampling for amalgamation, on the other hand, it may be desired to let this gold remain, in order to observe its action in the test. But such particles may vary from an infinitesimal size up to that of appreciable value; and thus may introduce an uncertain factor. It is known at the outset, that the sample may vary from a true average, by reason of the presence or absence of such particles; but if the total value of all the products of the test is known at the end, the original assay of a different sample is only of secondary importance; for the object of the test is to determine, not the average value of the ore from a given mine, but the proportion of gold that can be extracted by amalgamation, as represented in a given sample.
The possible size of these particles, that are allowed to remain, would ordinarily govern the necessary size of the sample. If we made unqualified provision for the possible occurrence of large gold particles, we should need to take a test-portion of almost hopeless magnitude. But when such particles are found in amalgamation they must receive special attention, and this, together with the method and manipulation employed, determines the necessary size of the actual test-portion employed.
To determine the relation between the coarseness of crushed ore and the necessary weight of the sample taken for the sole purpose of ascertaining average value, we can carry the relation between the weight and necessary fineness of an assay-ton sample to larger portions of larger mesh.
If one assay-ton be suitable for the assay when the fineness is 100-mesh, we may calculate how large a portion from coarser screens is required, on the supposition that the gold particles may be as large as the screen-opening, while all other conditions remain the same. In such a case, the weight of the sample ought to vary as the cube of the diameter of the closely- sized ore-particles which constitute it.
This principle may be expressed in the equation:
W = zD³
in which D is the diameter of the ore-particles (gold, mineral or gangue); W, the weight of the sample needed; and z, a factor for each different kind of ore. If W= 1 A-T., and 0.0055 inches be taken as the size of the opening of the 100-mesh screen, a numerical value for z is obtained, which, substituted in equations with other values of D, will give the values for W tabulated below.
The table shows merely a ratio between the coarseness of the ore and the corresponding weight of the necessary sample. It is recognized that this sample is of minimum size. Much more than this quantity would be desirable, were the duplication of assays impossible. With coarse ore, it is seen that the calculation leads to portions of enormous size.
The influence of a cube of gold of the full size of the screen-mesh indicated in the foregoing table would cause a variation of $ 1.05 in the reputed assay-value of the ore. The irregular shape of pieces of native gold might make this variation considerably greater. Two or three of these large gold-particles, or a gold-value that corresponds to them, will be found in excess or deficit, however thorough the mixing, or careful and uniform the sampling. A variation of several dollars per ton in the assay-value of the ore may be expected, therefore, from work with samples of native gold-ores of these sizes.
Although the gold more frequently occurs in a finely-divided condition, yet large pieces of gold may occur in many ores, and are sure to be found in certain ores. Hence, unless we use very large test-portions, we must decide what size of gold particles can be classified as part of the ore, and what size must be removed or specially considered. Having done this, we can decide how large an ore-portion one must use in the test.
By making special allowance for small particles of gold in the sample, it is possible to perform rapid and sufficiently decisive preliminary tests with comparatively small portions of ore. The following statements are offered in justification of this opinion:
Amalgamation-tests are often made upon a series of sizes of the same material, to determine the degree of comminution most suitable for economically liberating the gold from the gangue. This series may include finely-crushed material, of which one or two assay-tons could be regarded as a fair sample. The large pieces of malleable gold, flattened and not crushed in obtaining such a finely-divided sample, must be removed from the ore to make the remaining small quantity uniform, and to protect the gold particles from further objectionable distortion in dry grinding. In the ordinary course of ore-dressing, and certainly in preparing an ore for amalgamation, this malleable gold, collected on the screens, is kept separate from the ore to be still further crushed. The value of the gold thus removed is assigned to the larger portion of ore from which it was taken; but its removal diminishes the assay-value of the finer ore passing the screens, so that the fine ore no longer represents the original value of the whole. The return of this coarse gold to the fine ore would make the sample necessary for assay or amalgamation as heavy as if the ore had not been thus crushed; and, consequently, this gold, once removed, is never again added; but a small sample of the finely-crushed and screened ore is assayed, and to the result the proportionate value of the coarse gold is added. But the laboratory-test should ascertain the behavior, in contact with the mercury, of this coarse gold, as well as the finely-divided gold which passed the screens. Hence, instead of putting this gold back into the finely-divided ore from which it was taken, and thus making a sample that cannot be fairly assayed or divided, it is better to test by itself the behavior of the coarse gold, not unnecessarily obscured by the presence of inactive ore. In this way, the gold of a very large portion of ore may be represented in a small sample: The large pieces, by screening or picking them out from the sample; the smaller particles, by treating them in the finely-divided ore. All the information obtainable from an assay after exact sampling can be thus gained from a series of tests on a smaller scale.
Since all free-milling gold and its products can be recovered and assayed at the end of the small test, particles of gold, if large enough to require separate attention, influence the results in a way readily understood and interpreted. If they do not amalgamate, they are found unamalgamated in contact with the mercury. If they are absent from a given sample, but present in others, the fact is indicated in the final results, and can be explained by further tests. When large pieces of gold are absent in the whole ore-body from which the sample was taken, the small ore-portion is an accurate sample. Hence variation in the value of the samples need not obscure results relating to the possibilities of amalgamation.
The small portion permits a test more delicate and exact than can be conveniently made after increasing the scale. Sampling the several products becomes unnecessary; they are smelted in crucibles without subdivision, or, if small, are treated by scorification. The amalgamated gold and silver can be weighed and quantitatively determined, without rough handling or retorting.
A lot of ore received at the mill is never a true sample of the whole mine; and no single sample, however large, represents all the conditions that may hinder amalgamation. To obtain complete information it is often necessary to make tests upon ores and vein-material from different places, representing degrees of superficial oxidation, etc. As a rule, the various pertinent questions involved cannot be answered by a single test.
These considerations may often justify tests upon ore-samples as small as 100 grams; and such tests are frequently of great value. A sample of this size yields no more tailings than can be treated by a single crucible-assay, and contains sufficient value to permit the assay for gold and silver in any of the products obtained. These products are scorified and cupelled if they are small, or fluxed in crucibles if they are large. A portion of this size can be regarded as a satisfactory sample of moderately fine ore. When the ore is coarse, it is always possible to make the tests in duplicate; but in that case, the gold itself, if also coarse, should receive special attention, as it would occur in few particles, materially altering the average value.
In tests upon 100-gram portions, each milligram of gold recovered, represents $6,029, and each milligram of silver, 0.2916 oz. per ton. The 100-gram portion is also convenient in showing directly the percentage-weight of all the products obtained, and the values of these products per ton of original material are easily calculated.
The Laboratory Test
The chief purpose of the present paper is to describe a method by which the amalgamated gold is to be recovered, weighed and thus determined directly; the mercury or amalgam used is to be small in amount; the work is to be done automatically, as far as possible; and, at the end of the test, the pulp is to be left in a condition suitable for whatever concentration-test remains to be made upon the small portion of ore that has been used.
To secure an invariable certainty of the essential contact, with the smallest practicable quantity of mercury and an amount of ore relatively so large, requires the employment of a positive and uniform system. If liquid mercury be used, it must not “ flour.” Every particle of the ore must be brought into contact with an amalgam-surface that is small. The process must do more than effect, simply, an uncertain movement of a little mercury through a large quantity of ore; and no ordinary form of ore-conveyor can be reduced in dimensions and successfully applied to such small-scale work, by reason of the difficulty of cleaning, and of complications in small dimensions. The clean-up must be expeditious; the apparatus must be accessible and adjustable, and the action must be continuously open to the inspection of the operator.
Fig. 1 shows a simple appliance, made to test the practicability of some of these conditions, and found useful, repeatedly, in the study of many details arising in connection with laboratory-amalgamation. The gold is collected upon a small amalgamated plate, placed in the carrier shown in the figure; and at the end of the operation this plate is removed, melted and finally parted. In this apparatus, other qualities are sacrificed to extreme simplicity; but its efficiency as a test-amalgamator must not be judged by its small plate-area. The possibility of observing the behavior of all parts of the ore with so small an amalgam-surface makes such an apparatus often most useful. In using it, however, strictly to determine the total proportion of free-milling gold contained in an ore, the ore must be passed over the plate more than once; and, for this purpose, modifications of the apparatus are more satisfactory; but in the small apparatus shown, a very high percentage of all the free-milling gold is collected in the first contact. Moreover, the operation can be repeated any number of times upon the same ore, without special attention, loss of material, or harm to product.
The apparatus consists of a flask, A, provided with a rubber stopper and funnel, B and C, and a hard rubber attachment, D, for holding a small amalgamated plate. The operation requires that a uniform stream of ore, so small that at least half an hour shall be necessary for the passage of a 50-g. portion, should be delivered upon the amalgamated plate. Success depends upon skill in manipulation, the perfect condition of the amalgamated surface and the suitable size of the orifice, at which the flow of the ore-particles is restricted to a uniform stream, falling through water, which is ascending by displacement. Under the proper conditions, this flow of the pulp is uninterrupted, so long as ore remains in the funnel.
For the amalgamated plate, a suitable size has been found to be ¼ by 5/8 inches. It should be as thin as practicable without danger of amalgamating through. Soft copper foil 1/64 in. thick is sufficiently heavy. A silver-plated copper foil, or better, an amalgamated silver plate, is used when the assay has shown silver to be insignificant, and only gold is to be determined.
It is, of course, necessary that the whole surface of so small a plate be in perfect condition. Much of the gold amalgamates at the first point of contact, where the small stream falls upon the plate. A longer plate does not correspondingly increase the amalgamation. This first impact, at the point where the current changes its direction and spreads over the surface, should be carefully observed, as indicating much concerning the amalgamating-properties of the ore.
When silver foil is employed, it is annealed and cut into suitable pieces, which are amalgamated a short time before use. The surface may be roughened, if desired, by striking a sharp blow upon a fine-cut file placed upon it. It can be cleaned with dilute hydrochloric acid, and then, after washing in water, flooded with dilute nitric acid for a moment. After the removal of the mercury, the plate may be kept a short time, for the amalgamated surface to become somewhat hardened.
The carriage, D, is provided with a groove, F, 1/16- in. deep, in which the plate fits nicely, and is held from sliding downward by the raised rim, G, at the foot. This rim is also important in guiding the pulp over the plate. Like the rim at the sides, it is 1/16- in. high. Meeting this obstruction, the pulp parts into two streams flowing to the sides. Thus any fragment of coarse gold, that might be swept along in a straight line, reaches a turning-point near the foot of the plate where its high specific gravity takes it below the current of moving ore, and permits it to remain a longer time in contact with the mercury, and thus receive a better opportunity to amalgamate.
The carriage for holding the plate is attached to the support at the side by a small hard-rubber pin. (All metal should be avoided in the construction on account of its possible corrosion or amalgamation.) The plate thus can be inclined as desired, the angle depending to some extent upon the fineness of the ore, the shape of the crushed particles, the roughness of the amalgam-surface, and the specific conditions desired. But the proper angle is higher than if water, flowing over the plate, helped to carry the pulp. There is here almost no current. The apparatus is filled with water at rest, the ore falls by gravity through it, each particle striking and traversing the plate. Hence an inclination of from 30° to 45,° according to the conditions of the test, is necessary.
Under the circumstances, this high angle is apparently not detrimental to amalgamation. On particles of a given size and specific gravity, the force of gravity alone may produce, at the higher angle, the same normal pressure of particle upon amalgam, as would the force of gravity combined with that of flowing water upon plates less highly inclined. On the other hand, in the treatment of particles of different specific gravity and size, the higher angle favors a more thorough amalgamating action upon such as would be easily suspended and carried away in flowing water.
The support, H, is held firmly to the stem of the funnel by the rubber band, I, indicated in the figure. Since funnels of different sizes may be used for different purposes, the connecting-pin is made sufficiently long to permit moving the carriage towards, or away from, the support, so that the amalgamating- plate may be always directly beneath the orifice E.
For most tests a 3-in. funnel is best. A portion of ore exceeding 50 grams would be desirable for numerous reasons; but the long time during which the ore remains stationary in the funnel makes it liable to become compact. With a 50-g. portion, however, this need not occur, if the orifice be of correct size and shape, and the ore fairly uniform in fineness. Towards the orifice, the stem of the funnel is drawn down abruptly to a smaller diameter, and the end is ground square, leaving a circular opening 1/12 in. in diameter. An orifice as small as 1/16 in. diameter may be used for fine ore, but the tendency to pack is correspondingly increased; and the slow passage of fine ore through the smaller aperture would make it necessary to reduce the size of the portion to something less than 50 grams.
Larger portions of ore, or portions that, for any reason, quickly become compact, can be held in suspension by directing a small blast of air through a capillary tube, from above to the top of the stem of the funnel. Ore once thoroughly wetted will not retain air-bubbles in its downward course, so to interfere with the uniform flow through the orifice; but since the air-pump introduces a complication, and the whole of the ore is exposed to its oxidizing action for a large part of the time, and since, moreover, it causes a separation of the ore into heavy and light, and coarse and fine, which pass the plate in different times, the use of the air-blast is not generally desirable.
To make the test, the flask is filled with water, the amalgamated plate placed in its carriage, and the rubber stopper holding the funnel, support and plate is inserted. All air is thus excluded from the flask; and hence, when the stopper is inserted, a small excess of water is forced up into the funnel. This should be allowed to remain. The portion of ore to be tested is now added. This should have been carefully reduced to some one of the uniform sizes of ore usually adopted in such work, and have been screened to remove any foreign substance. It may be placed first in a small dish, moistened with water, and stirred about with the water to displace interstitial air, which otherwise might collect in a bubble at the orifice, and obstruct the stream by capillary action.
The moistened ore can be transferred to the funnel by the aid of a small stream of water from a wash-bottle, without loss of material and without the use of an excessive quantity of water. At once the ore begins to flow through the orifice in a uniform stream to the plate. Nothing need interrupt the continuous flow from the beginning to the end of the operation. Moving downward, over the surface of the plate, the ore causes a slight water-current after the start; and the plate, which might not seem sufficiently inclined at the first moment, immediately clears itself. This current can be started by a few grains of barren sand, when the conditions makes this refinement necessary.
After the ore has passed, if the operation is to be repeated, the water in the funnel is decanted into a second flask like the first. The second flask is then completely filled by further adding fresh water, and the funnel, support, plate and complete attachment is transferred from the first flask to this second one. The first flask, now containing ore that has passed over the plate once, and water that has been made turbid by the falling ore, is filled to the top with fresh water, to occupy the space resulting from the removal of the stopper and attachment, and then inverted over the funnel, keeping the neck under water to hold the column.
A flask of the required neck-diameter cannot be inverted and inserted into a funnel of this small diameter without danger of losing some material, unless a cap or suitable cover is made to hold the column of ore and water while inverting it. A useful cap for flasks of this size can be made by turning a vulcanite disk 1 5/8 in. in diameter and 1/8 in. thick, and cutting out the center concentrically to a depth of 3/32 in. and a diameter of 1½ in., leaving thus a raised rim, 3/32-in. high, around the edge of a disk 1/32 in. thick. A bent handle of glass or vulcanite may then be cemented to the disk and so shaped as to lie along the side of the neck of the flask when the loose-fitting cap is in place. The handle is held easily while placing the flask, and no trace of the contents ever is lost. The ore at once falls into the funnel; the slime settles; and the water remains in the flask. After sufficient time has elapsed for the ore and slime to settle from the inverted flask to the funnel, the flask is set aside, with its water unchanged, for a third operation, if such be desired.
The second passage of the ore over the amalgamated plate generally, but not always, yields a small additional quantity of recovered metal. With different ores this second yield averages less than 10 per cent, of the total free-milling value, but sometimes, by reason of defective first treatment, or otherwise, it is considerably more. A third passage over the plate generally shows that all the free-milling gold was collected by the first two. To learn how effective the previous treatments have been, one merely substitutes a fresh plate for the old one in a final test. By repeated treatment, the recovery of value thus quickly approaches a limit; and four repetitions can be regarded sufficient.
At the end of the test, the amalgam-plate is removed, washed with water, ignited gently in a muffle to remove the mercury, melted before the blow-pipe to produce a uniform alloy, and parted in nitric acid to recover the gold. When silver is to be determined, the thin copper plate is dried, after washing with water, scorified and cupelled as in the assay of an ore, or treated in a wet way, as some assayers prefer.
Larger portions of ore can be used only by placing a part of the ore-portion at a time upon the funnel. This increases the time required for the test; but throughout that time it may be seen that the plate is constantly performing its amalgamation-work as well as any corresponding area of surface could do, upon such an ore-portion as is put upon it.
The above method is useful in studying the action of different amalgams upon ore, in observing carefully the readiness with which the gold or silver particles of an ore amalgamate, and in obtaining from an assay-portion a preliminary idea of the free-milling qualities of an ore. The operation is simple; there need be no loss of material; and, at the end, the amalgamated gold and silver are quickly recovered.
Equipment for Prolonged Amalgamation
Fig. 2 shows a more effective automatic apparatus, adapted to systematic tests upon a somewhat larger scale. It causes all the ore to pass repeatedly over the small amalgam-surface, until the action is complete, or the desired conditions have been duplicated.
The apparatus, being made of glass, is fragile, and requires the use of a suction-pump, which introduces a complication. It is, moreover, restricted to certain small dimensions to produce the continuous automatic action desired. Opportunity is afforded, however, to observe the progress of amalgamation; and the simplicity of the process, when the apparatus once is made, the cleanness of all details and the thoroughness of the work, make the test fascinating to conduct and satisfactory in result.
One form of apparatus upon this general principle was made in which the pulp was caused to fall upon and flow across an amalgamated plate like that shown in Fig. 1. The advantage of being able to replace the amalgamated plate with liquid mercury, however, is of much importance in a test of this character; and, as will be seen, means are provided to convey the pulp in actual contact across the level surface of the mercury without disturbing the liquid metal.
In Fig. 2, representing this apparatus, the amalgamating surface is shown as a globule of mercury, a. If liquid mercury is not wanted, a small amount of soft silver-amalgam, or a small amalgamated plate could be used.
The size of the apparatus can be increased to treat portions of ore larger than those recommended here, but only within certain limits. The small tube through which the ore is elevated should have an inside diameter not much greater than 5/32 in., to give the capillary conditions necessary to elevate all the ore with absolute certainty. The large tube, A, has been used with a diameter of 1 in. and a length of 23 in. above the valve, v.
The tube, A, is securely clamped to a suitable support. The funnel tube, B, is left free to slide up or down through the stopper at the top of the tube, A, thus permitting the opening or closing of the ore-passage, v, at the bottom. The main apparatus is thus held rigidly and permanently in position, the only detachable parts being the lower tube, W, containing the amalgam, a, and held in place by the rubber connectors, r, r. The air-inlet should be a substantial capillary tube drawn down to a diameter of 1/75 in. at the orifice at the bottom, and turned at its lower end to form a bend, as shown in the figure, in order to cause the current of air to impinge upon and against the flowing stream of water and ore.
A small blow-hole, 0.01 in. or thereabouts in diameter, can be made in the side of the elevator tube above the rubber connector, r. This aperture can be kept closed, except when needed, by covering it with a short piece of rubber tubing like the connector, r, fitting around the tube and sliding over the opening to close it. Such an opening insures a lighter column of material in the tube, S, than that in the tube, A, and is a safeguard against the reversal of direction of the circulating material. In ordinary working, however, it is unnecessary; and the aperture should remain usually closed.
At the beginning of the operation, the ore-tube, A, being supported firmly in a vertical position, the tube, W, is removed from the rubber connectors, r and r, and the mercury, or amalgam, a, is placed in the depression at the bottom of the W- tube. A globule of mercury, 0.5 gram in weight, will suffice for the test, or a small, flattened particle of soft silver-amalgam can be used. The capillary air-inlet is inserted, but at the beginning it is drawn upward somewhat from the position in which it is seen in the figure, and is turned in the opposite direction, so that the orifice points in the direction of the moving current, and not against it, as is shown in the figure. The tube, W, is then attached by the rubber connectors, r and r. The funnel-tube, B, is now pushed downward, closing the valve at v. This valve, v, can be made by grinding the stem of the tube, B, into the contraction of the tube, A, until it becomes a closely-fitting glass valve; or better, the lower end of the tube can be covered with a short piece of thin, tightly-fitting rubber tubing, which will conform to the shape of the valve-seat formed by the contraction of the tube, A, if left unground. The severe usage a valve in this position is likely to receive makes the latter course satisfactory, as well as simple in construction. This valve should remain closed while the ore is being added.
In adding the ore, it is preferable to moisten it first, so as to avoid capillary difficulties due to interstitial air. A convenient portion is 100 grams, which will occupy a height of 10 or 12 in. in the tube, A, leaving sufficient space above for water; the ore never rising as far as the orifice, o. The coarseness may range between 20- and 120-mesh, according to the purpose of the test.
The ore is poured into the apparatus through the funnel-tube, B, the stopper in the neck being removed, and the passage of the ore accelerated, if necessary, by attaching the suction-tube to the pump. The last part of the ore is washed in with a fine jet of water from a wash-bottle, thus avoiding an excessive quantity of water. The ore passes downward through the stem, B, until it meets the obstruction, b, and passes into the outer tube, A.
The ore having settled, the suction-pump is running uniformly, and the tube, A, is partly filled with water, above the ore. Air is passing through the capillary air-inlet, up through the tube, S, and out at the “ suction ’’-tube.
Sufficient water is now added to the apparatus to raise the level to c. The tube, S, should dip beneath the surface of this water (as shown at d) in such a way as to cause some agitation of the water from the air entering through S, and thus give the ore entering the tube, A, from the tube, S, opportunity to fall through the column of water, and not to be blown and spattered upon the sides of the tube, A. This water is added, as was the ore, by lifting the stopper in the tube, B.
While the apparatus is being filled with water, there will be a current of water passing through the stem, B, into A, back through the aperture, o, into B, and thence downward through v, where no obstruction to the flowing water is met, while the ore is held back in the outer tube, A, by the continued closure of the valve, v. The water thus passes downward through the orifice, v, into the tube, W, in which the air, entering through the air-inlet, meets the moving current of water and impels it onward, while the orifice is turned in the direction of motion. The water, now mixed with air, passes into the side-tube, S, and rises, on account of the difference in head between the solid column of water in A, and the air- and-water column in S. The water is thus passed repeatedly around the apparatus and over the mercury surface—but thus far no ore has been permitted to pass over the mercury surface, and the exact adjustment has not been made which produces the correct velocity of the current over the mercury.
At this time the air-inlet may be turned so as to direct the jet of air against the stream and to cause a depression of the surface of the water in the tube, W, directly over the mercury. The condition of the water-current at this point is under perfect adjustment. It is desired to decrease the cross-sectional area of the stream directly at the point where the mercury is placed, until the correspondingly increased velocity will keep the mercury surface clear when the ore is passing. This adjustment is made by raising, or lowering, the air-inlet tube—a minor regulation, the extent of which is governed by the size of the capillary orifice and the suction-pressure.
The air itself must not come in contact with the mercury, and too shallow a stream of pulp over the mercury surface would cause the mercury globule to break and be swept along with the pulp, thus injuring the test in one of its most important features. It is possible to thrust the air-inlet tube downward so carelessly and so roughly as to drive all the mercury up through S, and over into the ore; but this never need happen. There is no difficulty in keeping the entire portion of mercury in one mass, unbroken, after once knowing the action of the air- and water-currents in the apparatus.
So far it has been assumed that the apparatus has been working upon water alone, and that ore has not been permitted to pass the valve, v. Fig. 2 shows the apparatus in this state. The ore now can be admitted by raising the tube, B, a short distance, when the ore passes downward through the valve, v, and meets the downflowing water which is also passing downward through v. There must be a free passage of ore on all sides of the stem of the tube, B, at the valve, v, that no part of the ore may be held back from circulation. The ore and water mix directly below v, and the adjustment of the valve regulates the proportion of ore to water in the pulp.
The thin pulp now passes through the tube, W, where its behavior at the amalgamating-surface is under perfect adjustment by the regulation of the capillary air-inlet tube. If the ore-particles tend to lodge around the mercury more than is desired, the inlet-tube can be pushed downward somewhat. Never, however, must the air be directed in such a way as to agitate the surface of the stream above the amalgam and break the current into bubbles directly over the mercury. The orifice of the air-inlet tube is lowered to such a level that the impinging current of air directed against the stream, without breaking into bubbles, glides over the surface of the water against the current. The air-current then turns back upon itself, follows the water back past the orifice whence it issued, and finally breaks into bubbles in the upper part of the limb of the W tube, as shown in the figure.
The pulp passes up through the tube, S, without lodgment of any of the heaviest ore-particles and thence over into the tube, A. The operation is continuous. Every particle is drawn across the amalgamating-surface as many times as is desired, and the behavior of the metal upon the mercury can be easily seen in this tube of small diameter. The conditions are favorable for amalgamation at each passage of the ore, for there is perfect control of the two opposing-tendencies; the one, the tendency to lodge, caused by insufficient velocity of current; the other, the tendency to clear the plate or disturb the mercury, caused by high velocity. This is regulated in either case by the position of the capillary air-inlet tube.
The suction-pump, by means of which the circulation is effected, must be uniform in action. Any pump that produces a constant suction without pulsation can be used. The equipment of almost every laboratory, nowadays, demands a supply of tap-water under a fairly constant head; and when such water-supply is available, the Richards pump is found most serviceable. While the ore, ascending through the tube, S, appears to be exposed to conditions favoring oxidation, this action is but momentary, and, as is usual in amalgamation, the ore is exposed to the action of aerated water.
At the close of the amalgamation-test, when it is desired to stop working and effect a separation of ore from mercury, the tube, B, is simply lowered and the valve, v, thus closed. The apparatus now begins to work upon water alone, ore no longer being admitted, and the mercury surface washes free from adhering ore by the water-current. The capillary air-inlet tube is now drawn upward somewhat, and turned carefully to deliver its air in the direction of the current, in such a way as to avoid disturbing the mercury.
The operation is continued after closing the valve, v, until all the ore-particles have settled to the bottom of A. The W tube next can be drawn downward some distance through the rubber connectors, r and r, which fasten W to S and A; but before removing W entirely, the connector at the right is closed with some form of pinch-cock, to avoid a rush of water through W when the connection is first broken. The suction is then stopped, and W is taken away from the apparatus entirely, leaving the rubber connectors behind—the one at the right closed with the pinch-cock retaining all the water in A. The contents of W can be removed without inconvenience or loss, and the mercury can be evaporated, scorified or parted, as desired, in order to obtain the gold and silver.
The ore, A, is now allowed to flow out into a suitable dish, by opening the valve, v, and removing the pinch-cock attached temporarily to the rubber connector. The material so collected is ready to be treated in any other test desired, or to be dried and assayed.
In any ore crushed and treated without sizing, there is always an appreciable amount of slime. This slime remains persistently suspended in the water under any treatment, and is to be recovered at the end, and its importance noted. Only a small quantity of water being required for the test, the slimes will be obtained in a somewhat concentrated form. All this water, with its suspended material, is saved; the solid material is collected from it, by settling or filtration, and the sediment is dried, weighed and subsequently assayed, or treated otherwise, according to the needs of the case.
At the conclusion of the test with ordinary gold-ores, the value is in three portions : Free-milling gold, gold in the tailings, and gold in the slimes. The last would be lost in certain processes of further treatment; and the extent of the loss can be determined by drying and scorification-assay of the slimes. Often, on the other hand, it is desirable to make only one portion out of slimes and tailings.
Ordinarily, the gold of the tailings, whether including the slimes or not, should be treated by some concentration-process to complete the test. This subsequent test can be made with the batea, horn-spoon, pan, hand-jig, or other suitable device.
Finally, after concentration, the value is in four portions: Free-milling gold, concentrates, tailings and slimes. Except the free-milling gold portion, which is to receive other treatment, these should be dried and weighed, and the percentage of concentrates, tailings and slimes should be calculated from these weights. The whole of each portion then should be used for the assay. From these assays are calculated the values in dollars of gold and ounces of silver upon the double basis:
(1) The value in gold and silver of each ton of each product obtained in this manner.
(2) The value in gold and silver that one ton of the original material would yield in the form of the products,
(a) as free-milling gold,
(b) as concentrates,
(c) as tailings,
(d) as slimes.
The sum of these values shown by this second calculation is the assay-value of the original ore, and should check with the results of the regular assays of the ore, as far as sampling will permit.
The duration of the operation must depend upon the rate of flow and the size of the sample used. The first passage of the ore over the mercury amalgamates a large part of the free-milling gold, the exact proportion varying in different ores. In a treatment of 100 grams for two hours in this apparatus, the ore flows across the mercury about ten times; and this time may be regarded as long enough for results comparable with economic milling. A trifling difference in the rate of flow, which may decrease or increase the number of passages of the ore, is compensated by the amalgamation-efficiency of each passage of the ore, so that a treatment of this duration, with this approximate rate of flow, can be regarded as doing work of a definite character. The rate of flow should be regulated according to the behavior of the pulp as it passes over the mercury surface, to make sure that every particle is drawn over that surface in actual contact.
Nothing prevents the use of larger samples of ore but the longer time required and the need of dividing the resulting products for sampling and assaying. The advantages of the method are brought out more fully by employing portions not too large, and by an apparatus of the moderate size here described. The apparatus, once in place, is found ever ready for use, and the time and attention required to use it are but trifling. The small quantity of amalgam is recovered without loss or inconvenience, and the value of the metal amalgamated is learned in a short time by evaporating the mercury, fusing the metallic residue before the blowpipe, parting and weighing; or, if the case demands it, by scorifying, cupelling and parting, as in ordinary assaying.