Round Tables Recovery of Minerals from Fine & Slimes

During the last half century a great amount of ingenuity and energy has been devoted to the invention of appliances for the recovery of valuable minerals from very fine sands and slimes. The reason for this is that in almost every dressing plant the greatest losses of values, considered relatively as well as absolutely, occur in the treatment of slimes. The natural aim of mill operators to minimize these losses has in recent years received another impetus, from the fact that the gradually diminishing occurrence of high-grade ores makes a more intense recovery of values from existing resources absolutely necessary for profitable operation.

The results of such attempts are found in the appearance of a great number of machines and appliances which make it profitable to-day to rework old dumps, containing the tailings of older processes. A constantly growing difficulty of obtaining skilled labor and the increase in wages made it prohibitory to employ those types of machines which required much attention, and in the middle of the nineteenth century the various concentrating tables of the percussion type, which automatically discharge their products, began to take the place of the intermittent-tables, on which, when the bed had reached a certain thickness, operation had to be stopped in order to allow the skimming by hand of the separate layers formed on the table.

In the course of years, two general types of machines evolved for the treatment of fine sands and slimes: The so-called concentrators and vanners, on the one hand, and the buddies and round tables on the other. For the profitable treatment of the very finest slimes, round tables have as a last resort proved more satisfactory and economical than other machines.

In recent years they have again become the subject of close investigation on the part of the mill operators, with a view towards improving the mechanical features of these machines and thus making them surer of success in the competition with shaking tables and vanners, which during the last decade have to a large extent usurped the function of slime-treatment machines.

It is the intention of this paper to outline the evolution of the so- called round tables from the form in which they first appeared in the dressing plants of Germany and Austria to the various forms in which they are found to-day in the dressing plants the world over.

General Principles

The ultimate success of a machine depends in the first instance on its strict compliance with the laws and principles controlling the work to be performed. No amount of mechanical perfection can assure success when these laws and principles are violated.

To enable the reader to draw correct conclusions as to how far the tables described in this paper have complied with fundamental principles, and in order to point out in which direction future improvements must be looked for, the writer deemed it helpful to explain briefly the principles and fundamental laws which underlie a separation of minerals on so-called round tables as well as on other machines. They are generally known under the name of film sizing.

Film Sizing

If we spread out, in a very thin film, on a slightly inclined plane, water-sized (classified) pulp containing equal-falling particles of mineral, the water running down the incline will not attack all the points of each particle with the same force as in a deeper stream, but the larger particles of lesser density will be exposed at their higher points to a greater force than the smaller, dense particles, because close to the deck of the table the water, due to adhesion, has a lesser velocity than in the upper part of the film.

The result is, that, given a mean velocity of the water, which depends on the inclination of the table, the coarser particles of less density will be carried away by its force, while the denser—that is, the finer—particles will resist the action of the water and remain clinging to the tables. In this manner a separation is made, in which the specific gravity (density) of the particles seems to play the chief role.

An important requirement for a separation of the equal-falling particles on an inclined plane is a thin film of pulp and water, because only in a thin film can a difference in the force of attack on the individual points of the particles of different size take place. If the water flows over the incline in a deep stream all points of each particle are attacked by the water with nearly the same force, and a sorting action will take place according to the law of equal falling.

Therefore, to subject a classified pulp to the action of a deep stream of water is practically the same as subjecting it to another process of classification, which may turn out to be more perfect than the preceding one; but will not effect a separation or concentration of the values.

A further important condition for a successful separation is the proper velocity of the flow of pulp and water over the table; which depends likewise on the inclination of the table. Up to a certain inclination the stream of water flowing over the layer of pulp will have no effect on the particles, because the velocity of the water is too small to move them. If, on the other hand, the inclination is too steep, the velocity will be so great that the denser particles will be carried away with the less dense ones. Only a mean inclination of the table, and therefore a mean velocity of the pulp and water, will permit a separation according to specific gravity.

Another requirement for successful separation is the proper consistency of the pulp. If too thick, that is, if the percentage of solids in the diluting water is too great, the water cannot act undisturbed upon the individual particles contained in the pulp because of interference of the particles among themselves. If the pulp is too thin, larger table area is required to do the same amount of work; in other words, the capacity is unnecessarily diminished.

What the consistency of the pulp should be in any individual case is a problem that can only be satisfactorily solved by careful experiments and trials.

Film Sizing Appliances in General

Appliances that depend partly on separation by falling through a horizontal current of water and partly upon the differential rate of travel along the bottom of the stream have been in use from immemorial periods. They constitute the most primitive type of a dressing plant and illustrations of them are found in Agricola’s De Re Metallica, published in 1556 (see Figs. 1 and 2). They are found in almost identical, but doubtless independently invented, forms among semi-civilized races all over the world.

Buddles

As usually constructed, these consist of a rectangular inclosed wooden trough, at the head of which the pulp to be treated is fed in, an additional stream of water being sometimes run in near the head of the trough. The inclination of the trough, the quantity of water, and the resulting velocity of the current are so proportioned to the size and weight of the particles of mineral that the heavier mineral remains at rest in the trough, while the lighter is carried off. In order to prevent particles of the lighter material from being entangled in and held back by the heavier, it is necessary to work the material lying on the floor of the trough up against the current, which is usually done by means of a broom, rake, or hoe. The clean

heads thus obtained are usually shoveled or raked out from time to dime, while the water runs off continually. Fig. 3 shows a buddle as described above.

Round Tables in General

From the rectangular buddle described above, developed in the natural course of evolution the round table.

A large number of very narrow rectangular buddles arranged around a common center, with their longitudinal dividing walls removed and the vacant sectors between the buddles filled out, form the so-called round table, either of the form of a cone (convex) or that of a funnel (concave), according to whether the inclination of the surface is away from or towards the center.

The principle of action of the round table is identical with that of the rectangular buddle, except for the fact that the velocity of the pulp as it flows down the table is not uniform, because on a convex table the pulp spreads out over a larger surface, while on a concave surface it is crowded into a smaller area. In the first case the thickness of the film of pulp and therefore its velocity gradually decrease,

while the reverse takes place in the concave or funnel-shaped table. The less dense particles rushing towards the discharge end of the table will therefore accumulate at this portion of the table in a thicker mass in the case of a convex table and in a thinner mass in the case of a funnel-shaped table. Inasmuch as these particles are destined to go to the tailings dump or to another machine, this feature does not seriously affect the concentration of values, which takes place in the upper portion of the table.

Round Tables of the Intermittent Type

The first round tables, like the rectangular buddles, were intermittent machines; that is, after having been fed with pulp and water for a certain period, the operation had to be stopped to allow the skimming of the various layers of material formed.

Convex Round Tables—Intermittent Action

Figs. 4 and 5 illustrate one of the old constructions of a convex round table. The table A forms a ring with an outside diameter of 20 ft. and an inside diameter of 6 ft., which gives a radial length of the table of 7 ft. The deck consists of boards, nailed on the sills B, which are arranged star-shaped. The surface is carefully trued with a plane. The lower end of the deck is surrounded by a board wall,

A1, from 9 to 12 in. high. The openings O in this wall permit the discharge of the tailings into the tailings launder B. The inner concentric wall A2, which is 12 in. high, is joined by the conical distributing apron H, which receives the feed through the funnel C. The center post K carries the step bearing for the spindle S, which at its upper end is supported by the bearing L1 and is revolved by means of the gearing T and the pulleys P.

The pulp is brought to the table in the launder R1, which delivers it into the funnel C, whence it flows on the table via the apron H. Two sockets C1 cast integral with the funnel, carry the arms D on which are fastened the windlasses N and N1, by which the brushes F and F1 can be adjusted. These arms with their brushes make from 10 to 12 rev. per min. The object of the brushes is to keep the layer of ore smooth. In proportion as it increases in thickness the brushes are raised by means of the windlasses.

One drawback of these tables is, that, due to the variable current of the pulp, the bed forming upon the deck changes its inclination and that it remains too loose, permitting the formation of furrows,

which prevent a perfect separation of the minerals. It is therefore not possible to make a clean product in one operation, but the several products must be re-treated on another set of tables.

The work is intermittent, due to the necessary stops for skimming. It takes from 2 to 3 hr. for the table to form a layer ready for skimming.

Convex round tables of the intermittent type similar in design and operation to the one shown in Figs. 4 and 5, but making use of masonry instead of wood construction, are also built. (See Figs. 6 and 7.)

Concave Round Table—Intermittent Type

The substructure of the concave or funnel-shaped table is very similar to that of the convex table, as will be seen from the illustrations, Figs. 8 and 9. The deck A, which rests on the sills B, is surrounded by a wooden wall A1 from 12 to 15 in. high, supporting the; conical feed apron H which receives the pulp for distribution on the table.

In the center the table is cut off by a wooden barrel A2, 6 ft. in diameter and extending 12 in. above and 18 in. below the deck surface. The upper part of the cylinder is perforated to allow the passage of the tailings, which are carried off by the tailings launder R.

The distribution of the pulp, which is brought to the table by the launder R1 is effected via the funnel C, from which radiate the four

launders D which spread the pulp on the feed apron H. The launders D also carry the windlasses N by means of which the position of the brushes is regulated.

The spindle revolves 10 times per minute. Owing to the fact that the four revolving launders distribute the pulp more uniformly over the table, the surface of the bed of minerals remains smoother on a concave than on a convex table, which results in better work.

The principles of operation of the concave table are practically the same as those of the convex tables. It is likewise intermittent.

The great drawbacks of intermittent action were soon realized, and efforts were directed to the construction of tables which discharged the various products automatically, thereby making the operation a continous one.

Round Tables of the Continuous Type

The principle of continuous action was first applied to the round table by making the table itself rotate, and allowing the pulp as it travels down the incline to come under the influence of stationary sprays of water, which wash the various products from those portions of the table on which they are formed, and as soon as they are formed, into suitable collecting launders.

In this manner a clean surface is ready to receive another layer of pulp when the table has made a complete revolution; in other words, the action is continuous.

Having thus achieved continuous action, capacity was the next item of importance. So, rotating tables were made of larger and larger diameter. In this process of evolution two obstacles were encountered. As the tables grew larger, they required more extensive and costly foundations and substructures, occupying excessive space in the mill. Then they became so heavy as to require an excessive amount of power, and, being unwieldly the motion became less steady, and as a consequence the separation less perfect.

These obstacles were overcome: the first, by placing several decks on the same central shaft one above the other; the second, by making the table stationary and allowing the fixtures, consisting of the feed launder, the spray pipes, and the collecting launders, to revolve.

All modern round tables belong to one or the other of these types as they evolved from the first rotary round table, installed in the mills at Clausthal in the Harz mountains in 1853.

In principle, continuous-acting round tables have not undergone material changes since their introduction. But numerous mechanical improvements have been made, chiefly in the choice of material for the deck covering. Wood, which was used in the construction of the first tables, was subject to the undesirable property of warping. It was first replaced by cast iron, which for a great number of years was used for deck material as well as for other parts of the machinery, although the turning of the castings to a true surface was exceedingly expensive then.

In the beginning of the twentieth century a covering of rubber was tried on a wooden substructure. The first experiments with this material were carried on in the Harz mountains at the Hulfe Gottes mine. The technical results were altogether satisfactory, but the cost of this material proved too high in the long run. Next a cement cover was tried with a thickness of 3 cm. (1.25 in.). It was found that this cement cover would crack through the cold of the winter. To prevent this a series of nails was driven into the deck before the cement cover was applied, these nails being set 10 cm. (4 in.) apart and protruding slightly above the deck.

This precaution, however, did not altogether prevent cracking, and new experiments with covering material were made, with the result that a concrete foundation with a thin cement surfacing proved in the end to be the cheapest and most satisfactory deck covering. A 3-cm. (1.25-in.) layer of concrete was placed on the deck, made of rough boards, and finished up with cement. This also permitted the surface to be made more or less rough, depending on the character of the slimes to be treated.

A concrete cover was first employed on the so-called Harz two-deck round table (Fig. 10), the upper deck being funnel shaped, the lower cone shaped, both decks being fastened to and revolved by the same main shaft.

This arrangement of decks had the great advantage of making possible the finishing of the product on the lower deck without taking up much mill space, although at the expense of much water. On the other hand, the increased weight of the table due to the concrete and cement covering made it impracticable to lubricate properly the toe and step bearing of the main shaft. In the end it became necessary to separate the decks, and at first the upper deck was made revolving while the lower one remained stationary.

This new method of covering the table with concrete and cement had the other great advantage that it permitted giving the table in loco an inclination best adapted to the nature and the consistency of the slimes as found by trial. It also permitted a ready change of this inclination should a change in the nature of the feed demand it.

One disadvantage of these tables is, that a small unevenness of the surface, which even with the greatest skill in turning will occur, is apt to cause an accumulation of the heavier material in concentric rings, particularly when the table is overfed.

In recent years iron decks have come into use again, for various reasons. First, the technical improvements in the manufacture of iron appliances have made the manufacture of iron decks much cheaper than in previous years. An iron deck is also much lighter than a wooden table with a concrete cover, and hence requires less power. In round tables of the percussion type, like the Bartsch table, wooden decks would not offer the necessary resistance, so that in these machines iron decks are used.

Examples of Round Tables of the Continuous Type

Revolving Tables with Stationary Fixtures

Figs. 11 and 12 show a revolving round table with stationary distributing and receiving launders, and stationary wash-water pipes. The deck A has the shape of a slightly inclined cone and, if made of wood, it rests on a cast-iron plate which is supported in the center by a spider B. The latter is keyed to the vertical spindle D, which at the bottom revolves in a step bearing E, being driven at the top by a worm wheel F, through a worm G. The latter forms the end of a horizontal shaft provided with a driving pulley H.

The distributing launder J is a ring-shaped vessel divided into two compartments. The smaller one receives the pulp through the feed launder R. Into the larger one flows the wash water from the circular tube C, which also provides the spray pipes C1. From the distributing launder J pulp and water reach the tables through small short pipes in the bottom of the launder.

As the table revolves, each part of it passes under the pulp compartment and immediately after under the wash-water compartment of the distributing launder J, remaining under the influence of the latter until it again passes under the pulp compartment.

The operation is continuous. The coarsest, barren mineral particles are immediately washed off the table into their respective section of the receiving launder L. A middlings product and the concentrates which settle on the upper portion of the deck are washed off by the spray water into other sections of the receiving launder.

The diameter of the table varies with the fineness of the slimes to be treated. A common dimension is from 16 to 24 ft. The inclination varies from 1 to 10, to 1 to 12. The table makes from 15 to 20 rev. per hour, corresponding to one revolution in 3 to 4 min.

The average capacity is 10 gal. per minute of slimes containing from 8 to 10 per cent, of solids. The quantity of wash water is 25 gal. per minute. The power required is 0.25 h-p.

In a more modern construction of revolving table, Fig. 13, the substructure consists of iron beams and corrugated iron, which is followed by a cement cover, forming the deck.

The Evans Slime Table.—In the early seventies there appeared in the copper mills of the Lake Superior district revolving round tables patented by Mr. Evans, which differed from the tables theretofore in use by having a stationary conical feed apron or head, which extended halfway down the incline and was supposed to protect the headings formed on the revolving portion of the deck until they were ready to be washed off.

Fig. 14 gives the main outlines of the table. A is a launder to conduct the slimes from the catch pit or slime box to the distributor B, which has a partition B1 to separate the clear water from the puddled water or slime water. The clear water is supplied by pipe P to the distributor, and runs over one-half of the table, while the slime water runs over the other half, being controlled by the division piece L. The sand and water being on one side of distributor B run through its perforated bottom, and are distributed equally over one-half of the stationary head C, and run on the rotating table D into the circular launder N, then through the waste pipes OO.

The headings remain on the upper part of table D, and after concentration are shielded from the action of clear water by passing under the spiral-shaped stationary head C, until they reappear at the end of the revolution from under the widest part of the apron. Through the action of clear water the proper grades of ore are washed about half-way down the rotating table D. They then come in contact with the diagonal perforated pipe E, and are rewashed by a succession of small jets from perforations of small pipes. The ore passing between the jets is carried around the rotating table E until it comes in contact with a jet of water from pipe F and conducting board G. The jet F conducts the ore into hutch H through pipe I.

The middle or second-grade ore is washed off table D by the perforated pipe E, and is deposited in hutch J through pipe K to be re-washed.

The speed of the machine is one revolution in 80 sec. The pitch or incline of the table is 1.25 in. to 1 ft. The pitch of head is 1.75 in. to 1 ft. The capacity of the machine is from 25 to 35 tons per day of 24 hr.
The same type of table was afterwards used at the Washoe works in Anaconda, Mont., for treating copper slimes. To save mill room

the tables were built with two decks on the same shaft, as shown in Fig. 15.

At these works the construction and operation in the near future of a 20-deck revolving round table is under contemplation. This subject, I am informed, is treated in a separate paper to be presented at this meeting.

Stationary Tables with Revolving Fixtures

The Linkenbach Round Table: In 1878 Linkenbach designed for the Ems Lead and Silver Works, in Germany, a round table which is the prototype of all tables belonging in this class.

Figs. 16 and 17 illustrate an early construction of this table. It consists of a conical deck A with a cement surface on a rough masonry foundation. Before hardening, the deck is turned true and smoothed. Where mill space is abundant this deck can easily be made of larger diameter, this being one of the chief advantages of this type compared with the revolving table.

The vertical spindle A1 carrying the fixtures revolves at the bottom in a step bearing M, while to the upper end is keyed the worm wheel N, which is set in motion by the worm E. The latter forms the end of the horizontal shaft F provided with a driving pulley H.

The feed is delivered through pipe I into the revolving distributing launder J, which is suspended from the pipes K. Below it, likewise suspended from K, is the wash-water launder L. The water is distributed throughout the fixtures by way of the hollow spindle A1.

Suspended from the revolving beams B and revolving with them is the collecting launder G, the individual sections of which deliver their respective products into the stationary sump launders O, O1 O2 O3 by means of pipes of different lengths.

The separation and automatic discharge of the minerals take place in the same manner as in the rotating tables, with this difference : That the change of the place of delivery where the pulp is spread on the deck is caused by the revolution of the fixtures in the first case and by the revolution of the table itself in the second case.

The wash-water pipes and spray pipes are so arranged that their positions can be changed within certain limits. This permits the pointing of the streams of water in the direction which they must assume in order to wash the various products formed on the deck into the respective sections of the receiving launder.

To save mill space, Linkenbach tables were built with three decks, one above the other. Fig. 18 shows an installation of tables of the Linkenbach type built by Fried. Krupp in Germany.

They are built with diameters of from 19.5 to 32.75 ft. The capacity of the largest size is from 2,000 to 2,400 lb. of material per hour. The number of revolutions is from 0.25 to 0.43 per min. The power required is 0.75 h-p. The clear water consumed per minute is from 48 to 55 gal.

Principle of Operation of the Linkenbach Table

The action of this table is continuous; that is, the pulp is spread on the table and is allowed to separate into various layers, which are washed off successively into respective sections of the receiving launder, which revolves with the fixtures.

If we observed any particular section of the table during a complete cycle of operation we would see, if conditions were ideal, approximately the following picture: In flowing down the table the individual particles of the pulp will settle on the deck in concentric rings according to their specific gravity, the densest nearest to the top and the less dense nearest the circumference. If we have a pulp containing, for instance, galena and blende as the valuable constituents with, say, quartz as a gangue, that section of the table over which the pulp distributor has just passed will immediately be exposed to the action of the wash water, which washes the gangue off the table into its respective section of the collecting launder. There remains then on that section of the table near its circumference a ring

of the next dense particles, consisting of middlings containing blende, some galena, and some quartz. This layer of material is now attacked by the first set of spray pipes that come along in the course of revolution and is washed into its section of the receiving launder. Next the layer of blende higher up is attacked by a following spray, then a layer of blende mixed with some galena still higher up, and at last the pure galena particles nearest to the top of the table, all being washed into separate compartments of the receiving launder. After the last spray has passed that section of the table, it is completely cleared to receive a new layer of pulp from the pulp distributing launder, which follows in the wake of the last water spray.

By the proper distribution and direction of the sprays in relation to the pulp distributor a more or less perfect separation can be made, as well as a number of finished products, middlings, and clear tailings.

Percussion Pound Tables

The success of percussion tables of the rectangular type for separating valuable minerals from their gangue, particularly when several metals are to be recovered, as, for instance, from an ore containing galena and blende, has led to the application of the percussion principle to round tables.

A well-known table belonging to this class is the Bartsch table, which is operating successfully in various dressing plants in Germany. It is manufactured by the Humboldt Engineering Works Co. at Kalk, Germany.

The Bartsch Round Table

As illustrated in Figs. 19 to 23, this table consists of a cone-shaped deck A made either of two cast-iron plates, turned true and covered with a coat of durable paint, or of a rubber sheet stretched over a wooden substructure. The spindle X, supported by a step bearing A1, moves freely within the spider R, the latter forming the central support of the table, which on its circumference rests on the rollers G. To the central part of the spindle are

fastened the four hollow arms Y on which are suspended the distributing launder M, the curved spray pipe K, and the rods N1 which carry the collecting launder N.

As will be seen, the curved spray pipe K with its auxiliary sprays L starts from the end of the distributing launder M. All sprays receive the necessary wash water under constant pressure through the hollow arms Y from the water tank G. The latter is provided with a float regulating the inlet valve. The spindle X also carries the funnel S which receives the pulp through the launder R1. From the funnel it flows through the pipes T into the distributing launder M, which feeds the table.

The mechanical operation of the table is performed by the spindle X, which not only rotates the fixtures, consisting of the distributing launder M, the spray pipes K and L, the funnel S, and the collecting launder N, but also imparts to the table the tangential shocks in the following manner:

On the shaft E is keyed a cam F, which strikes a roller, carried on a bearing I underneath the table, at the moment when the table, due to the tension of the spring P, is in its artificial state of rest against the stop Q.

During the forward thrust the spring is further compressed, and at the end of the motion the table is jerked back against the stop Q, the direction of the jerk being opposite to that of the rotation of the fittings. By means of sprocket wheels, and chains E1 E2 the motion of shaft E is transmitted to shaft H, at the end of which is a worm H1 which rotates the worm wheel H2 at the bottom of the spindle X. This arrangement permits of a change in the speed of revolution whenever desired.

Fig. 21 shows a somewhat different arrangement for regulating the speed of revolution by the use of step pulleys in place of sprocket wheels, and the shaft F which carries the cam is set in motion by a spur wheel on shaft E.

The pulp is spread on the table through the perforations of the distributing launder M, and if the table has the proper inclination the gangue will leave the table, while the valuable (denser) minerals will cling to it until they come under the influence of the spray water.

The individual products are washed into the respective sections of the collecting launder N, which rotates with the other fixtures, and they are finally discharged through the pipes U, of different length, into the stationary circular launders Z1, Z2, Z3, Z4.

The diameter of the table is 13 ft. It is capable of treating about 0.5 ton of slime per hour. The forward thrust of the table producing the shock is from 10 to 20 mm. During one-half revolution from 100 to 120 shocks are imparted to the table. A complete revolution of the fixtures takes place in about 2 min.

From 0.25 to 0.5 h-p. is required for operation. The consumption of spray water per hour is from 12 to 18 gal. for light material, and up to 35 gal. for heavy material.

Principles of Separation of the Bartsch Percussion Round Table

While gliding down the incline, the tangential shocks imparted to the table compel the particles of mineral to follow a path which is the resultant of the straight path down the incline and the path at right angles to it, due to the shock.

The distance traveled in the direction of the incline is greater, the less dense and the larger the particles, while the distances traveled in the direction of the tangential force causing the shock are practically the same for all particles. This is due to the fact that the weight and therefore the momentum of these particles is approximately the same.

As a consequence, the resultant paths traveled by the individual particles will form different angles with that generatrix of the cone on which the said particles started on their journey down the table, the magnitude of the angle being a function of density, the largest angles corresponding to the densest, the smallest angles to the least dense particles.

This process is repeated during each time interval between two successive shocks, and each individual particle is advanced not only into another generatrix of the cone, but also into a larger parallel circle of the table; that is, towards the lower circumference of the same.

The distances between two parallel circles measured in the direction of the generatrix, due to acceleration, become greater and greater, while the angle formed between the resultant path of the particles and the generatrix in which this particle happened to be at the beginning of the last time interval, becomes smaller and smaller.

If we connect the positions occupied in succession by one and the same particle during its travel over the whole table surface, we obtain a curve, which in its horizontal projection corresponds to a spiral.

Since the individual particles of a classified feed have different densities, we shall get different curves which will be more or less steep. The least steep curve which intersects the circumference of the table at a point farthest from the generatrix on which the particle started its journey, corresponds to the densest particle, and vice versa.

If we treat, for instance, a pulp containing as valuable minerals galena and blende, with a gangue of, say, quartz and graywacke, the table, after being set in operation, will present at any individual moment the picture shown in Fig. 24.

The pulp is spread upon the surface 1-2-3-4, and the gangue is washed into the section E1—E2 of the collecting launder N. On the surface 2-3-5-6 the separation of a second-grade (less dense) blende product takes place, which is washed into the section E2-E3 of the collecting launder; on surface 5-6-7—8 is made a first-class blende product and washed into section E3-E4 of the launder; finally a second-class galena product is separated out on surface 7-8-9, and a first-class galena product on surface 8-9-11-10-8, and washed respectively in sections E4—E5 and E5—E1 of the collecting launder.

Revolving Round Tables

with Adjustable Inclination of the Deck (System Demuth)

Various attempts to construct tables with adjustable slope have been made, with more or less success.

Such a table is built by the Firma Groeppel in Bochum, Germany, and is illustrated in Fig 25. At this moment the writer has at his disposal no drawing showing the details of construction of this table.

It appears, however, that the sections which compose the table overlap each other slightly, and are fastened at their upper ends to a sleeve which can be moved up and down the main vertical shaft, thereby changing the slope of the deck within certain limits. The deck covering, which is either rubber or linoleum, is in one piece.

Round Tables with Conoidal-Shaped Decks.—The generatrix of the deck surface of all tables described in the preceding pages is a straight line. At the Reduction Works of the Boston & Montana Mining Co. at Great Falls, Mont., tests are being made at present with round

tables, having a deck the generatrix of which is the arc of a circle, whose chord has an inclination from the horizon varying with the fineness of the pulp to be treated.

The description of these tables and the results obtained, I am told, form the subject of a separate paper to be presented at this meeting.

It is the writer’s belief that by referring to these tables he has arrived at the latest stage in the evolution of round tables, and therefore at the end of the task which he started out to accomplish in presenting this paper.