The magnetic concentration of iron-ores has been so often and so widely studied and discussed among the members of the Institute that any remarks concerning its general importance, from an economic standpoint, would be superfluous; but it is thought that a description of the Ball and Norton ore-separator, designed for cobbing and separating magnetic iron-ores, indicating its essential differences from other machines and the advantages claimed for it, together with some account of recent results obtained in its application to the concentration of such ores, will prove of interest.
This machine is the joint invention of Mr. Sheldon Norton, of Hokendauqua, Pa., and the writer, and has been patented in the United States, Canada, and other countries.
Of the accompanying illustrations, Fig. 1 represents a longitudinal vertical section of the perfected ore-separator, adapted for separating fine ore, and Fig. 2 the same, adapted for cobbing ore of the size of stove- or chestnut-coal. The two designs are identical in principle, and vary from each other in minor details only.
The apparatus consists of a partially-closed chest, having an opening at f, from the feed-hopper, h, through which the ore is delivered to the machine from an ore-pocket or storage-bin provided with means for regulating the flow of ore, so that, when the machine is in operation, the hopper is kept always full. Other openings are provided for the discharge, at t, of tailings; at m, of middlings ; and at c, of concentrates ; also, at e, for allowing free ingress of air to the chest at that point, and at s, where a powerful exhaust-fan is connected. The openings at t and m are kept sealed against ingress of air at those points by means of the hinged and weighted valves, v, v, which discharge the products from the hoppers, p and k, continuously, and in the same proportion as received from above, when a sufficient weight has accumulated upon the inside to cause the contents of the hoppers to leak by the valves. The machine is also provided, as shown, with two drums, Nos. 1 and 2, turning upon the shafts, i and j. These shafts, together with the magnets, a and b, which they also serve to support, stand still, while the drums may be rapidly revolved around the magnets and out of contact therewith.
It will be noticed that the magnet occupies a sector of the drum, the proportions being such that, approximately, one-third of the periphery of the drum is within the influence of the magnetic field, while the upper two-thirds is outside of the field and removed from the magnetic influence. The magnet is so constructed as to present a series of poles of alternately opposite polarity near the inner surface of the drum. In accordance with the well-known phenomena of magnetic attraction, which in the case of powerful magnets is exerted at a considerable distance from the magnetic poles, any magnetizable matter brought near the outer surface of the drum, within the arc covered by the magnet, will be powerfully attracted and drawn into firm contact with the outer surface of the drum.
These drums are composed of a non-metallic and neutral material, such as wood, paper, etc., and they turn in the direction indicated by the arrows.
Just below the feed-hopper an apron of neutral metal, No. 3, is arranged, curving downward and forward in the direction of the rotation of the drum, its lower portion describing a short arc concentric to the surface of the drum. This serves as a chute to direct the stream of ore falling from the feed-hopper within the influence of the first two or three poles of the magnet. A similar but somewhat shorter apron, No. 4, is arranged in like relation to the second drum and magnet, b.
Attention is called to the provisions for applying and directing the air-current, which fulfils an important function in the process of separating ore as developed in this apparatus. The air may enter freely at the openings c and e, and is drawn through and out of the chest by the action of a powerful exhaust-fan connected at s. The air which passes through the chest containing the drums and magnets must, of necessity, follow the course indicated by the arrows in the space below the drums.
When the machine is put in operation the magnets are excited, the drums are revolved in the direction before indicated, the air-current is established through the machine in a direction opposite to that of the rotation of the drums, and ore is supplied through the feed-hopper which is kept always full. The ore passes down the chute under the first drum, and, as soon as it comes within the influence of the magnet, the magnetizable portions are drawn into contact with the drum and through friction upon its surface take on the forward movement of the drum. At this stage, a curious and important result takes place. In accordance with the well-known laws of magnetic induction, a particle of iron brought near a magnet itself becomes a magnet by induction, and the magnetic force tends to bring the longer axis of this induced magnet into a position as nearly as possible parallel to the direction of the magnetic force. So a single particle, or many particles, brought near one pole of a magnet stand on end, as it were; and, in the case of many particles simultaneously influenced by the same pole, they form tufts standing out from the pole, their outer ends repelling each other, but all pointing in the direction of the lines of force towards some focus of opposite magnetic polarity. In conformity with this law, the particles of ore in contact with the drum opposite one of the poles of the magnet stand on end, forming tufts, spreading away from each other at their outer ends. As they are drawn along, however, by friction against the moving drum, when they get to a point midway between two poles they lie down flat against the surface of the drum, and, as they are drawn still further along, they again stand on end—but this time the other end out. So, in passing through the magnetic field, they are tumbled end over end as many times as there are poles in the field. The result is, that every time they are reversed in position opportunity is afforded for any non-magnetic particles of gangue, which may have been entangled with the ore, to fall away from the tufts of magnetite; and this result is still further facilitated by the centrifugal tendency and by the countercurrent of air.
When the ore reaches the limit of the arc covered by the magnetic field it is no longer attracted, and takes on a tangential movement, which carries it away from the drum. It has now, however, passed the edge of the second apron, and, on leaving the first drum, comes within the influence of the magnet of the second drum, where similar operations are repeated, a portion being finally discharged as concentrate at c. The function of the second drum and magnet is to differentiate the product from the first drum into two portions, which may be conveniently designated as middlings, discharged at m, and concentrate, discharged at c. The middlings consist of particles of ore with adhering portions of gangue, which may require a little finer crushing to effect their mechanical liberation ; or, they may consist in part of iron compounds having a smaller degree of magnetic susceptibility than the pure magnetite. The separation of the middlings from the mass delivered to the second drum may be effected in two ways: If the drums have the same speed of rotation, a weaker magnetism in the second magnet will allow these less magnetic particles to drop away; or, if the magnets have approximately. the same force in the two drums, a higher speed of rotation of the second drum will throw these particles off by reason of the centrifugal force overpowering the centripetal magnetic attraction, the magnet having the smallest influence upon the leaner portions of the mass.
Modifications of this apparatus have been designed, which adapt it for use in conjunction with a stream of water mingled with the ore, so constituting a wet process.
In designing this apparatus, it has been assumed that, in order to separate effectively a mass of commingled ore and gangue, containing a large percentage of fine dust, positive and contending forces must necessarily be brought into active play.
The importance and special efficiency of the means provided for suppressing the movement and discharge, along with the ore, of the finer particles of gangue, and of the means for differentiating the product into several portions graded according to the percentage of iron in each portion, and its magnetic susceptibility, should not be overlooked. It is upon the application of these means that the ability depends to perform the critical operation of converting non¬Bessemer into Bessemer ores by the elimination of phosphorus-, sulphur-, and titanium-bearing compounds.
In the concentration of ores by the dry process the control of the dust developed in the rapid movement of large quantities of ore, is generally difficult. When the ore reaches this machine, the finer portions of the material are as readily and perfectly dealt with and separated as the coarser, the concentrates, however finely pulverized, being delivered from the machine in an absolutely clean condition, and free from non-magnetic dust and the dust carried away by the air-current being also as wholly free from any magnetic material.
The peculiar action of the ore in passing through the complex or multipolar magnetic field has already been described. Upon the mode of arrangement of the poles the efficiency of the apparatus largely depends. In this machine the magnetism of the field of attraction performs a two-fold function, viz., to attract the magnetizable matter of the ore and, as it is moved through the machine by friction upon the revolving drum, to turn it over and over so as to allow the gangue to fall away, and also to permit the air-current to take effect on all sides of every particle of the ore.
If the importance of this operation is once recognized, it will be readily understood that there can be only one possible efficient arrangement of the poles, and that it is not a matter of indifference whether they conform to to this or some other plan.
The positive character of the functions above enumerated renders it possible to make an effective separation under the conditions of a very heavy supply of ore to the machine. The easy working- capacity of a machine having drums of 24 inches diameter and 24 inches working-face is from 15 to 20 tons per hour of ore granulated to pass 16- to 20-mesh screens.
The power required is from 1 to 1½ horse-power in electricity for each drum, and ½ to ¾ horsepower to drive the machine.
These machines have been applied to the treatment of a considerable number of ores with results shown in the appended tables. The most remarkable of these was the conversion of Port Henry Old Bed ore into a Bessemer ore, carrying Fe, 71.10; P, 0.037. This concentration was made from the crude ore, carrying Fe, 58.7; P, 2.25; the Bessemer concentrate representing about 65 per cent, of the original mass.
An earlier type of this machine was in use during nearly the whole of the year 1889, at Benson Mines, St. Lawrence county, N. Y., concentrating the lean magnetite mined there, which in the crude ore carries about 40 per cent. Fe, and is above the Bessemer limit in both phosphorus and sulphur. The concentrated product found a market at Pittsburgh, Pa. The analyses in Table A show the general character of the results; those in Table B, the results obtained there with the improved machines of the later type; and the analyses in Table C, the results of latest concentrations.
The new works at this mine (a detailed description of which it is hoped may be offered at a future meeting) are provided with crushing-machinery having sufficient capacity to crush daily 800 to 1000 tons of ore, sized to 16-mesh and finer, and the separation of this quantity of ore is to be effected at the outset with three separators of the type represented in Fig. 1, having a working-face on the drums of 24 inches.
In the minds of some persons who have been studying the general subject of ore-concentration, and employing the apparatus hitherto available, inability to convert some of the well-known ores high in phosphorus and sulphur, such as Port Henry Old Bed, Little River, Croton, etc., from non-Bessemer into Bessemer ores, has led to the inference that some as yet undiscovered refinement of preparation was essential to their successful treatment. The entirely successful and economical management of these ores with this machine has proved that the essential matter in the problem was to provide a separator that would separate ore that had been properly prepared. The preparation of any particular ore being wholly dependent upon its physical and chemical characteristics, its final separation would depend upon the efficiency of the separator, which would constitute the last and supporting link in the chain of operations. It is believed that this machine offers a practical solution of the problem of concentrating magnetic ore such as has not been hitherto provided.