Table of Contents
Concentration of grains from 10 to 30 mm. is effected by hydraulic jigs with two compartments, and in the case of the smaller grains down to 2 mm. by jigs with five compartments. The construction of the jigs is the same in both cases. Fig. 3 gives the details of a jig with two compartments; it is formed of three cast-iron plates which support the bearings of the eccentric shaft, joined by a wooden casing or wooden walls so as to form two communicating chambers for pistons and screens. This construction has no special advantage beyond facilitating the transportation and mounting of the jigs. But in some details the Monteponi jig differs greatly from those in general use.
The eccentrics have a variable stroke. A first eccentric fixed to the shaft is surrounded by a moving eccentric; the first has a flange which partly covers the second at the side, and both have holes through which the bolt is passed to hold them together. The holes being at a different distance in the two eccentrics, the combination forms a kind of vernier caliper, which allows variation in the eccentricity.
Eccentrics of three sizes are used: one for strokes up to 20 mm.; a second for strokes between 10 and 40 mm., and a third for strokes between 30 and 80 mm.
With five holes in each partial eccentric, 25 combinations of different strokes can be obtained between the two extremes. The superiority of the system consists in the facility with which the eccentricity can be regulated, and in the assurance that this eccentricity cannot vary during the work of the jig. This eccentric is shown in Fig. 4.
The discharge of the concentrated material is made by pipe for the coarser grains; by pipe and suction through the sieve into the hutch beneath for the sands. The pipe varies in diameter from 13 to 51 mm., according to the classes treated. It is placed, slightly inclined towards the outside, and transversely to the screen, at about half the height of the layer of grains. On the bottom of the pipe, in the middle of the screen, a hole is bored, through which the grains with the water rise through the pipe and flow away.
The jig separates the grains in layers of different density. The pipe gives an outlet to the layer of valuable mineral as fast as it rises on the screen. The discharge is made at intervals, especially for the small grains, and is stopped when waste is found mixed with the ores.
In jigs treating grains larger than 10 mm., the ore falls on sorting-tables of perforated iron sheets. The jigs have two discharge-pipes, one for each compartment, and the division between the compartments is raised only as high as the pipe, to allow free movement to the upper layer. The first pipe discharges principally a mixture of galena, barite, and cerussite; the second discharges smithsonite and calamine. Sorting on the outside tables gives finished products.
The jigs with five compartments, for sands between 2 and 10 mm., discharge at the same time by pipe-discharge and hutch. A bed of iron disks—the waste from punching-machines— spread on the screen gives the resistance necessary for the separation of the sands and secures the continuous production, above the bed, of a layer of ore, which is forced out through the pipe-discharge as it is formed. To close the spigot, a stopper of some sort is employed, or else a bend, which can be turned upwards when it is desirable to stop the outflow. The screens have perforations larger in diameter than the maximum diameter of the sands, and the products from the pipe and from the hutch of the same compartment have nearly the same composition. Table II. shows the principal features of the jigs in use at Monteponi.
All these jigs are directly fed by the vibrating-screens, and perform continuous work. The mixed products from the jigs for sand—for instance, the mixtures of galena, barite, cerussite, and smithsonite—are separated by closed jigs, with one compartment of 0.45 by 1.20 m. free surface of screen, giving beds of different ores, which can be removed by hand at intervals.
For sands below 2 mm. to 0.05 mm. the Shaking table has been in use since 1898. This apparatus is well known also in other countries, since the Fried. Krupp Grusonwerk bought the patent and introduced it into almost all mining regions.
The Shaking-table is built in two types: one for fine sands below 2 to 0.5 mm., the other for sands of 0.5 down to 0.05 mm. They are identical in principle. The former is shown in Fig. 5. A rectangular table is placed horizontally in the direction of the movement, and slightly inclined in the other direction. It rests on six inclined springs, and receives an Shaking motion from an eccentric, exactly like the vibrating- screens; the table is covered with linoleum. Its inclination may be regulated during the progress of the work by wedges placed between the table and the frame, which rests on the springs. The mixture of water and sand from the hydraulic
classifier is distributed by a short longitudinal hopper to the upper angle at the side of the eccentric, while the water flows away transversely. The grains are discharged on the table, running in parabolic lines, according as their specific gravity is greater and their diameter smaller. The spray-pipe placed at the upper side of the table pours out a slender stream of water which holds the grains suspended. Lengthwise grooves depressed in the linoleum prevent a too rapid fall of the heavy grains (without stopping the fall of the waste), and force them under the short spray-pipes placed at the end opposite to the hopper, where they are divided into groups of different character and specific gravity, and pushed towards the outlet.
The second type, or small Shaking table for sands finer than 0.5 mm., is trapezoidal in form, and has no spray-pipe at the outlet; and the hopper at the entrance is replaced by a screen placed a few centimeters above the table, with which it oscillates. The purpose of this screen is to remove the excessively large grains, and to deliver the material evenly. This delivery is made first upon a raised section (A., Fig. 6), less inclined than the rest of the table, B, so as to hold the grains, while the accompanying water flows away transversely. The two sections, A and B, carry semicircular grooves, which diminish in depth towards the side of the outlet. The grooved area is limited by a parabolic line, as shown in Fig. 6.
This table serves also to treat the mixed products from the larger table of the first type, and all the other intermediary fine products. For this work, a screen is used with perfora-
tions of 1 mm., corresponding to the maximum diameter of the grains which the table can treat successfully. The details of the two types of Shaking-tables are given in Table III.
Difference Between Hydraulic Jigs & Shaking Tables
In all old mills, sands of 1 to 2 mm., and even below 1 mm., are treated by hydraulic jigs with suction. The defects of this system are numerous. In the first place, sizing on screens, and still more by trommels, of grains smaller than 2 mm., is difficult and far from exact; and the work of the machines for classification is costly and delicate. Suction-jigs for fine sands never give well-finished products; for below 2 mm. the pipe-outlet which serves to regulate the thickness of the bed, while maintaining on the screen of the jig a constant layer of ores of the same composition as that which sifts through the screen, cannot be used. The metallic value, or average specific gravity, of the ore which sifts gradually diminishes from one end of the jig to the other, without any sharp separation between the ores of different quality. The shaking-table has the additional advantage of using less power in order to obtain better products, as can be seen by comparing the results of the two systems:
When we consider that half of the products of the suction- jig are submitted to an extra concentration or separation, we see that the advantages of the oscillating-table are increased to about 50 per cent., and, apart from the best results in work, there is also considerable economy in installation.
Belt for Treating the Slimes
When argillaceous ores are treated, there are found in the last products from the hydraulic classifiers very fine ores, which run over the tables without sinking into the grooves. These fines have a diameter below 0.02 mm., and would go to enrich the slimes in the settling-ponds if there were no way to separate them. The method employed for this purpose serves, also, to recover the useful ores which might be drawn away by the water accompanying the products of the tables and jigs. It consists of a rubber belt, slightly inclined transversely, 0.60 m. wide, stretched over two drums, of which one serves to give the motion and the other the necessary tension. Every 60 cm. it is supported by rollers with regulated inclination, so as to have the belt almost horizontal at the side of the entrance of the slimes, while the inclination is progressively increased at the side of the outflow of the products. Fig. 7 shows such a belt.
The pulp, reaching the belt through a rubber pipe, with almost no velocity, flows out on the belt, on which it deposits the solid particles, leaving the clear water to flow away. The motion of the belt carries the deposits to the water-sprays, which force them to the edge of the belt, making the lighter portion flow out with the water. The results are different products, some finished and some middlings, which can be treated on a second belt.
The force necessary to operate the slime-belt is merely that required to turn the belt on the pulleys without a load. A belt 4 m. in length treats 40 liters of slime, and requires 60 liters of clear water per minute.
According to the degree of concentration of the slime, more or less material can be treated on the belt, up to a maximum of 240 kg. of dry material per hour. The average is 100 kg., for, generally, concentration of the slime is avoided, so as to prevent losses by being carried away.
There are belts made two or three times the length, with two or three sections of introduction for the slime to be treated.
The belt does the same work as the Linkenbach revolving buddle; but it has the advantage over the latter of occupying less space. The life of a rubber belt, carefully managed, is about two years.
Separation of the Middlings
Middlings from the process of concentration may be divided into two classes:
- Mixtures of ores of too close a specific gravity to be easily separated at the first operation; for instance, cerussite and barite, blende and pyrite, calamine and limonite; or certain mixtures of sufficiently different specific gravity, but produced in the work of concentration, which are further treated either to remove a mineral which is found in the raw material in too small a quantity to be directly concentrated, or to take away from the waste all trace of useful mineral. Such are mixtures of galena with cerussite and barite, zinkiferous limonite and dolomite, as well as the ferruginous calamine and dolomite at the calamine-mill of Monteponi.
- Mixed minerals which require a previous breaking to separate them.
As observed in connection with the hydraulic jigs for coarse grains, the mixtures of the first class are separated by stratification on the closed hydraulic jigs with one compartment, removing the products by hand, and layer by layer, as soon as stratified.
The fine-grained mixtures which contain waste are usually concentrated in a special section of the washery, provided with suitable classifying- and separating-apparatus.
Separation of the mixtures of the second class begins with crushing, more or less extreme, according to the nature of the material. The machines for crushing used in Sardinia are the stone-breaker, the rolls and the ball-mill. Of the first two types in Sardinia there is nothing special to be said.
The ball-mills used are chiefly the Krupp and the Ferraris.
Ferraris Mill.—The Ferraris wet ball-mill possesses the advantages of great simplicity of mounting and small requirements of space and power for the same capacity. The steel plates which form the lining do not need to be adjusted, being held in place by the lateral steel walls and the sand formed by the crushing of the ores. There being no central shaft, large lumps of ore can be introduced into the mill, and workmen can easily enter for repairing and cleaning.
The mill is made in two forms: one for coarse grinding (from 5 to 15 mm.), the other for fine grinding (from 0.5 to 5 mm.). The following description of the first form may serve for both, except as to the differences mentioned below.
The mill consists of a drum supported on four carrier-wheels and driven by a spur-gear securely fixed to the drum, which engages with a spur-pinion keyed to the counter-shaft. The drum is divided by an annular perforated partition into two compartments. The larger or crushing-compartment is 61 3/8 in. in diameter by 30 in. long. It is lined with manganese-steel plates with projecting ribs, and contains about 1,000 lb. of forged steel balls 4 in. and 6 in. in diameter. The smaller or screening-compartment, about 10 in. in length, is divided into a series of pockets by means of a cone projecting into the crushing-compartment, and a series of radial partitions extending therefrom. The periphery of this compartment is open, and is surrounded by a screen of the desired mesh. The material passing through the screen falls into a housing surrounding the lower half of the screening-compartment.
The ore to be crushed is fed into the crushing-compartment with the water, and, when reduced to pieces smaller than the holes in the annular partition, passes through into the screening- compartment, where the material which is fine enough passes out through the screen, and the oversize is elevated by the radial partitions until it slides back on the surface of the cone into the crushing-compartment, where it undergoes further crushing.
The drum is rotated at 20 rev. per min., and requires from 5 to 6 h.p. at its full load. The capacity at this speed on quartzose ore, broken by crusher to pass through a 2-in. ring, is approximately:
The weight of the mill, including balls, is approximately 7.5 tons.
In this type, the peripheral plates are detached from the inner walls of the drum, leaving between them and the projecting bars a space of 12 mm., through which the water carries into the sizing-compartment the grains below 12 mm. In the second or fine-grinding form (Fig. 8), the peripheral steel plates are close
to the inner walls of the drum, and the water with grains below 10 mm. runs out through holes in the walls which divide the ball-chamber from the sizing-chamber. In both forms the screen is placed at the periphery of the sizing-chamber, and the material rejected by the screen is raised by the radial partitions to the point where it can slip over the exterior surface of the cone and return to the crushing-chamber.
A Ferraris ball-mill requires 7 h.p., with 20 rev. per min., and 80 liters of water per min. The quantity crushed per hr. depends on the quality of the ore and the size. In general, the product is greater from brittle ores like quartz than from tough minerals like diabase. A quartzite mineral in large pieces is crushed to 3 mm. at the rate of 4 tons in 3 hr., or 1.33 tons per hr. If the ore has been broken beforehand to 50 mm., 1.5 tons per hr. can be crushed to an average size of 1.5 millimeters.
The broken ore is sent to the separating-machines after having been sized, if a screen of more than 2 mm. in size is used. In this case the sizing is accomplished by the vibrating-screen. If the crushing is pressed below 2 mm., hydraulic classifiers are applied to the pipe which carries the water and sand, as described above.
In the mill at Monteponi for the fine crushing of mixed ores, the first hydraulic classifier feeds a jig of five compartments; the others feed the Ferraris oscillating/shaking-tables.
At the Rosas mine, there are five ball-mills forming five sections. The ball-mills receive the material which has been broken by the stone-breaker to 2 in. and crush it to 2 mm., at the rate of 1.5 tons per hr. per mill. But diabase impregnated with blende and galena is found to be very difficult to crush.
Each section is composed of one ball-mill, two jigs and three shaking-tables. There is one special section, composed of a distributing-trunk, a classifying-pipe, and eight shaking-tables, to treat the middlings from the five crushing-sections.