The magnetic properties of certain minerals have long been recognized, and their concentration through magnetism can lay no claim to novelty. A patent was awarded in England on a process for separating iron minerals by means of a magnet in 1792, and in this country a separator having a conveyor belt for presenting ore beneath electromagnets excited by cells was employed in separating magnetite from apatite in New York State in 1852. The earlier attempts at magnetic separation naturally were directed toward the separation of the most strongly magnetic substances. The first separators were employed in separating iron from brass filings and turnings, metallic iron from furnace products, and magnetite, the most strongly magnetic of minerals, from gangue. The next step in the process was in roasting, or calcining, certain iron minerals which might by such means be transformed into strongly magnetic compounds and separated from their admixtures. The steady development of improved apparatus and more intense fields has constantly broadened the field of magnetic separation until minerals previously considered nonmagnetic are separated commercially.

Beginning with the crude machines which employed permanent magnets to attract the magnetic particles and brushes to detach the material so collected, a great variety of separators has been devised and patented, and many of them have been placed in commercial operation. The magnetic separator has been developed, in most instances, for the exploitation of individual ore deposits, and the different types and modifications so produced might well form subject matter for a book. In the United States alone over three hundred patents have been granted.

In view of the above facts the broad practice of magnetic separation is incapable of monopoly and its application is not determined by any one machine. The process suitable to the treatment of the ore under consideration having been carefully chosen, it will be found that any one of several machines will perform the functions of the actual separation.

In its own field, which will be hereinafter outlined, magnetic separation is a useful adjunct to the specific-gravity processes, but it is in no sense a competitor with these processes except in the concentration of magnetite iron ores, and in this application is a success backed up by many years of profitable operation.

Magnetism Applied to Ore Dressing

All substances—solid, liquid, and gaseous—are either attracted or repelled by a magnet, though in most cases this influence is too feeble to be apparent except with delicately adjusted apparatus. The atmosphere has a definite magnetic attractability and the magnetic behavior of solids may be said to be controlled by the magnetic qualities of the surrounding medium; if a substance is more permeable to magnetism than air, it is attracted; if less permeable, it is repelled. The permeability of air (air being the most common medium) is taken as 1, and the permeabilities of all other substances are referred to it as unity. The permeabilities of substances more strongly attracted than air are therefore represented by values greater than 1, and are called paramagnetics; substances less permeable than air are represented by values less than 1, and are called diamagnetics. The permeability of the diamagnetics is so nearly unity that the phenomenon of magnetic repulsion is not a familiar one.

The lines of force of a magnetic circuit pass along the path of least resistance; in other words, they pass through the most permeable substance available. Paramagnetic particles, introduced into a magnetic field, tend to aline themselves in the direction of the lines of force in precisely the same manner that a compass needle alines itself with the magnetic meridian. Paramagnetics concentrate the lines of force, while diamagnetics cause the lines of force to go around them. The passage of lines of force through particles induces magnetic polarity in them, and they gather in tufts or chains. North pole to South pole, and are all held by the energizing magnet. The force with which these particles are attracted is a function of their permeability, the intensity of the field, and the time they are subjected to its influence.

The paramagnetic minerals have been divided for convenience into two classes: those which are attracted and held by a common permanent magnet, called ferromagnetic minerals, and those not so attracted, referred to as feebly magnetic minerals. The ferromagnetic minerals are magnetite, pyrrhotite, ilmenite, chromite, and franklinite in typical specimens.

Various investigators have made attempts to determine the specific magnetic permeabilities of minerals, but without uniformly reliable results. This is due to two causes: the variable magnetic permeability of the same mineral from different localities (and even of different specimens from the same locality) and the unreliability of the available methods for determining the permeability of minerals. The rod method, employed in testing the permeability of iron, is not applicable to minerals, as rods of sufficient length of pure minerals are not available. The methods employed are based on a comparison of the permeabilities of crushed and sized minerals with crushed and sized cast iron, or filings. The figures so obtained are affected by the size and shape of the particles tested, the amount tested, whether the charge is packed tight or loose, etc., which makes the results of comparative value only.

In view of the above facts the permeabilities of various minerals, as determined, do not form a reliable guide in the consideration of ores, and a preliminary test of the ore in question must be made to determine the applicability of magnetic separation, unless the ore be magnetite or one capable of transformation into magnetic oxide.

While chemically pure minerals possess magnetic permeability independently of any iron they may carry, in practice it is almost always the effect of a trace or more of iron, either chemically combined or present as an impurity, that is utilized for separation. In minerals which combined iron renders separable the magnetic permeability varies more or less regularly with the amount of iron combined; but minerals which depend for their separation upon the presence of iron as an impurity are subject to wide variations in permeability, and are therefore more unreliable subjects for magnetic separation.

The paramagnetic metals are iron, nickel, cobalt, manganese, chromium, cerium, titanium, palladium, platinum, and osmium. Oxygen is paramagnetic (liquid air is attracted and held by a magnet) and sulphur is diamagnetic; the oxides are therefore more likely to be magnetic than the sulphides of the same metals, and in like manner the oxides are usually more strongly magnetic than the carbonates. That the chemical composition of a substance does not determine its magnetic properties, however, is strikingly shown in the mineral pyrite (FeS2, 46.7 per cent. iron) which is too feebly magnetic to be separated by the most intense field yet produced; and also in the bromide of copper, a compound of two diamagnetic elements, which is paramagnetic. The occurrence of strongly magnetic galena at Gem, Idaho, is another striking instance of the variable magnetic behavior of minerals.

The crystalline form of a compound has an effect on its magnetic properties, as has also water of crystallization. The temperature at which separation takes place also exercises an influence: Langguth (” Elektromagnetische Aufbereitung,” p. 5) separated readily a zinc blende, warm, which was with difficulty effected when cold.

Much has been written concerning the magnetic properties of various salts and alloys, in the investigation of which peculiar manifestations of magnetism have been observed. While throwing light, perhaps, on the magnetic behavior of matter, these results are hardly of importance in the practical subject of magnetic separation.. (For the theories regarding the magnetic properties of matter the reader is referred to the writings of Poisson, Coulomb, Ampere, Becquerel, Weber, Burgman, Kohlrausch, Plucker, Tyndall, Faraday, Delcasse, Dolter, Wiedman and others.)

Field of Magnetic Separation

The applications of magnetism to ore dressing fall naturally under two heads: the concentration of magnetic minerals from their gangues, and the separation of two or more minerals of similar specific gravity in the products of a preliminary water concentration.

Magnetic concentration has been applied principally to the treatment of magnetic iron ores, eliminating the gangue, and at the same time effecting a partial separation of phosphorus and sulphur minerals which are frequent and objectionable contaminations. The concentration of these magnetite ores is the oldest, and to-day one of the most important applications of magnetism to ore dressing. A plant for the concentration of siderite from gangue has been in operation in France for a number of years, and another is now being constructed in Hungary. Magnetic concentration has also been applied to the treatment of ores carrying chalcopyrite. This mineral has a tendency to slime, when crushed, which gives rise to an important loss in subsequent wet concentration; but after roasting it is readily saved by magnetic attraction, even if in a fine state of division. There are other minor applications of magnetic concentration such as leucite from lava, manganese ores, garnetiferous schists, etc.

In magnetic separation, as distinct from concentration, the applications are more numerous and complex. There occur in nature many combinations of minerals whose specific gravities are too similar to permit of their separation by any of the usual concentrating devices. In such combinations where one of the minerals is magnetic, or may be rendered magnetic by the application of heat, magnetism offers an efficient, and often the only, method of separation.

For reasons connected with the subsequent reduction of zinc ores the presence of iron is highly objectionable, and ores which carry more than a small percentage of iron are severely penalized. This, together with the similarity of the specific gravities of the iron and zinc minerals often found together, gives rise to one of the most important applications of magnetic separation. Zinc blende frequently occurs with pyrite, marcasite or siderite, all minerals of specific gravities too similar to permit a separation by specific-gravity methods. Pyrite and marcasite are not capable of separation in their raw state, but become magnetic on roasting; siderite is separable by magnetic fields of high intensity, and may also be transformed into a strongly magnetic compound by calcination. Oxidized zinc minerals also occur in important ore bodies with limonite, and here again the difference in the specific gravities of the minerals is too slight to permit a separation by milling methods. Limonite is slightly magnetic and may be removed in its raw state by fields of high intensity, and may also be calcined to the strongly magnetic oxide of iron and removed as such. Zinc blende carrying sufficient combined iron to be magnetic occurs in many localities in Colorado and elsewhere in conjunction with pyrite, from which it may be separated by magnetism without preliminary treatment.

At Broken Hill, N. S. W., immense ore bodies carry blende together with rhodonite and garnet, minerals of similar specific gravity. The middling products from water concentration of ores carrying these minerals are separated by magnetism. The peculiar ore bodies at Franklin Furnace, N. J., are treated exclusively by magnetic separation.

Magnetic separation has found application in the treatment of monazite sands, in the separation of tin-tungsten concentrate, for the removal of magnetic contaminations from corundum, in heavy sulphide concentrates, in the separation of chalcopyrite-blende-siderite concentrates, and in other cases.

The principal applications of magnetism to ore dressing as represented by successful installations have been stated above, but there are many other separations which are entirely practicable but not at present in commercial use. The low prices and high standards of iron ores obtaining in the United States do not permit of the exploitation of ore fields which in another country would be of great value. As our purer ores are exhausted, and prices rise, there will be a steady increase in application of magnetism to the concentration of iron ores, not alone in the treatment of natural magnetite, but also for the lean hematites and limonites which cannot now be worked at a profit.

Magnetic Separation as a Process

Where applicable, this process possesses all the advantages held by other separation processes, and, in addition, is independent of gravity. A prerequisite of success in any separation process is the existence of the minerals to be separated as free particles, and in this magnetic separation constitutes no exception. Furthermore, all separating devices work better on sized material than on a mixture of coarse and fine particles. While sizing is necessary in many specific-gravity methods, it is desirable, but not imperative, in magnetic separation. The preparation of the ore for treatment by crushing and sizing represents, in any case, a large proportion of the total cost of the process, whether the final separation be made by jigs and tables or by magnetic separators.

The magnetic separator has been developed into an efficient machine which is economical of power, both for operation and excitation of the magnets, not liable to break down or get out of adjustment, is easily operated by anyone with the intelligence necessary to operate any of the usual concentrating machines, and is not a source of large expense bills for repairs and renewals.

To sum the matter up, the only difference between specific-gravity and magnetic-separation processes is that one utilizes differences in the specific gravities of the minerals to be separated, and the other utilizes the differences in their magnetic permeabilities. Where, however, the ore must be roasted or dried before separation, this item must be charged against the magnetic treatment of which it is a prerequisite.

Principles of Magnetic Separation and Preparation of the Ore for Treatment

To separate successfully a mixture of magnetic and nonmagnetic particles a separator must fulfill the following requirements: It must make a proper presentation to the magnetic field of the mixture to be separated; it must bring about the attraction of the magnetic particles by a uniform field of suitable intensity; it must remove the magnetic particles so attracted from the field and cause their discharge from the separator.

Presentation of the Ore Mixture to the Magnetic Field

A proper presentation of the mixture to be separated to the magnetic field is of primary importance. The ore must enter the field in such a manner that the individual particles will be free to be attracted according to their permeabilities. The ore must, there- fore, be fed in a thin, even layer or sheet in order that the magnetic particles may not be hindered in their attraction toward the separating pole by intervening nonmagnetic particles. Theoretically, this layer should be but one particle deep, and in the separation of very feebly magnetic minerals this is carried out in practice. In the separation of magnetite, either natural or artificial, and the ferromagnetic minerals, a deeper feed is permissible, and consequently a greater capacity for the separator. When the feed is more than one particle deep the upward rush of magnetic particles toward the pole is apt to entrain nonmagnetic particles and carry them into the magnetic product. This loss is not a serious one with fields of suitable intensity; that is, with fields just sufficiently strong to attract the magnetic particles. In many separators provision is made for the removal of entrained particles from the magnetic concentrate by a blast of air or a jet of water while it is still under the influence of the field, or by the turning over of the magnetic concentrate by causing it to pass from one pole to another of opposite polarity, which operation causes the magnetic particles to reverse their individual positions as they pass from one pole to the opposite sign.

The above considerations apply more particularly to the presentation of the ore mixture by conveyor belts, shaking plates, drums, and rolls. When the ore is presented to the magnets as a thin sheet falling past the poles, or when the separation is carried out under water, the feed being introduced in suspension in a stream of water, entrainment is a less serious difficulty.

It is also essential to good work that the feed be constant in amount and presented at a uniform distance from the separating pole, that all parts of it may be acted upon equally by the field, the intensity of which varies with the distance from the separating pole. As the intensity of the field is greatest, and the attraction consequently strongest, at the poles, decreasing directly with distance from them, it follows that the ore mixture should be introduced into the field as near the separating pole as is practicable.

The speed at which the ore is presented to the magnets, or the time the ore is under the influence of the field, is also a factor of prime importance. A definite length of time is necessary for the induction of magnetism, the time required for induction, and consequent attraction, varying inversely with the permeability of the mineral treated. That the speed of passage of the ore through the magnetic field must be regulated according to the permeability of the mineral separated is well illustrated by the following experiment.

A Mechernich separator was fitted with a thin conveyor belt passing between the poles of the magnets and so arranged that its speed might be varied at will. A mixture of minerals crushed to pass a 0.75 millimeter aperture was fed upon the belt and passed through the field at different speeds, the intensity of the field remaining constant. With the belt traveling 100 meters per minute only magnetite was removed by the magnet; at 70 meters rhodonite was partially removed, but ferruginous blende was quite unaffected; at 50 meters the rhodonite was completely removed but the blende still remained unaffected; at 40 meters per minute the blende was partially removed, and at 30 meters completely removed, the intensity of the field remaining constant throughout the test.

Separators whose feed is presented to the magnets as a thin sheet falling in front of the separating poles are limited in their application to minerals of high permeability by the speed of the passage of the ore through the field.

Attraction of the Magnetic Particles

Magnetic attraction in performing a separation is opposed by some other force, usually gravity, the magnetic particles being lifted from the mixture under separation, or prevented from falling when fed, for instance, upon a revolving drum or cylinder. Gravity is often supplemented by some other agency, as centrifugal force, a blast of air or a stream of water acting against the magnetic attraction. The opposing forces of magnetic attraction and gravity, or centrifugal force, may be delicately adjusted, and separations effected between minerals having but slight differences in permeability.

The intensity of a magnetic field should be adjusted to the permeabilities of the minerals it is to be called upon to separate, and the field should be uniform throughout its separating zone in order that all portions of the ore fed may be equally acted upon. The air gap between the poles should be as narrow as is permitted by the conveying device and the ore sheet passing between them. The intensity of the field is determined by the ampere-turns of the exciting coils, the cross section, the length and the material forming the magnetic circuit, the distance between the poles and the shape of the pole pieces. The intensity of the field is controlled in practice by the current allowed to flow through the exciting coils and the distance between the poles, which in most separators is adjustable.

In magnetic separators, for minerals of feeble permeability especially, it is desirable to produce a dense field, or concentration of the lines of force along the separating zone. This may be obtained by beveling the pole pieces, by the device of two parallel magnetized cylinders, by a series of sharp projections on the separating pole or armature placed between the poles, by a laminated construction of pole pieces, or by an armature made up of alternate disks of magnetic and nonmagnetic material. The reason for this concentration is that the lines of force, in their passage across the gap of the separating field between the poles, seek to travel as far as possible through the iron of the pole pieces or armature, as offering less resistance to their passage than air, resulting in a concentration of these lines of force where the air gap is shortest.

In separators employing but one separating field it is usual in order that no magnetic particle may escape attraction to introduce the feed at the strongest part of the field. Where more than one separating field is employed the ore should be passed through fields of gradually increasing strength. The effect of this is to remove minerals of different permeabilities as separate products, the most strongly magnetic by the first and weakest field, and the most feebly magnetic by the last and strongest field, and to prevent entrainment by avoiding the rush of strongly magnetic particles in a field of greater intensity than is necessary for their attraction. If separators having only one separating field are employed it is usually necessary to operate two or more machines tandem, with fields of progressively increasing intensity.

Removal of the Attracted Particles from the Magnets

The removal of the attracted particles from the magnets may be accomplished in several ways, depending upon the form and kind of magnet employed.

With separators which draw the magnetic particles against the magnet itself, these particles must be removed either by force or by interrupting the attraction by breaking the current on the exciting coils. With the old permanent-magnet separators, and with separators employing revolving magnets which do not change their polarity during revolution, scrapers or brushes must be resorted to in order to effect the removal of the attracted particles. With wet separators a jet of water may be employed. With electromagnets the exciting current may be automatically interrupted and the attracted particles allowed to fall; this is only possible with certain constructions and has not been in general use.

With separators which employ secondarily induced magnets to effect the separation these may be caused to pass beyond the field of the primaries and the attracted particles so dropped. A construction which has found extensive application employs a rotating cylinder, or armature, revolving between the primary poles to effect the separation. Here any point on the cylinder changes its polarity during revolution, and, at a position 90 degrees from the separating zone, passes from one sign to the opposite, where the attracted particles are dropped. The cylinder may retain sufficient residual magnetism to hold strongly magnetic particles, even at the neutral point, in which case brushes or scrapers may be necessary to overcome the feeble attraction due to this cause.

In another construction advantage is taken of a property of the lines of force emanating from a magnet pole to concentrate upon points of magnetic material—in other words, employing secondarily induced magnetic points to remove the particles attracted by the primary magnet. Here the secondary magnet points are caused to pass out of the influence of the primary magnet, and, upon losing their magnetism, drop the attracted particles.

With separators which act by deflecting the magnetic particles from a falling sheet of ore adjustable diaphragms are used to divide the particles according to their degree of deflection from the verticle: any particles which may have become attached to the magnet poles may be dropped by breaking the current for an instant.

In separators which employ but one separating zone, the magnet, and means of removing the particles attracted by the same, should be so arranged that at least three different products are obtained—a concentrate, a middling and a tailing. This may be accomplished by gradually decreasing the strength of the field at the discharge and employing adjustable diaphragms to separate the products, the most weakly magnetic falling first and the most strongly magnetic last.

Necessity of Making a Middling Product

In the crushed ore submitted to any process for separation or concentration there is always a certain proportion of composite particles containing both the valuable mineral and waste, and this may not be avoided, even by excessively fine crushing, which is usually undesirable on account of the quantity of dust or slime produced. These particles are too rich to be allowed to go into the tailing, and too lean to be included in the concentrate; in any scheme of treatment, therefore, provision should be made for the recovery of such particles as a middling product. With magnetic separators it is usually advisable to carry on the first magnet encountered by the ore the lowest current which will separate the pure magnetic particles, and a sufficient current on the last magnet to remove all the particles carrying a portion of the magnetic mineral. The result of this is a clean magnetic concentrate from the first magnet and a clean nonmagnetic product, with a middling product, for the retreatment of which provision should be made. Where separators are used which do not yield a middling product two machines should be operated tandem, the first delivering the magnetic product and the second a middling product and non-magnetic discharge. The retreatment of middlings should, of course, be preceded by crushing, and where the ore is roasted for magnetism, a reroast may also be necessary.

Cleaning Magnetite Concentrate

In the separation of strongly magnetic minerals, especially on separators of large capacity, some provision should be made for cleaning the magnetic concentrate from entrained particles of waste. Such cleaning is accomplished in some separators by subjecting the concentrate to the repeated action of magnets of alternate polarity, the magnetic particles forming loops between the poles, which loops are broken and remade in passing from one pole to the next, and the nonmagnetic particles allowed to fall. In some other constructions a blast of air or jet of water is directed against the concentrate while held by the magnets and the entrained particles blown or washed out. Repeated treatment of the magnetic product as exemplified by the Edison deviation separator accomplishes the same result.

Treatment of Fine Material

In crushing ore a variable amount of dust or slime is produced which may not be separated advantageously in conjunction with the coarser sizes. No especial difficulty is met in the separation of strongly magnetic minerals in a state of fine division either wet or dry; several wet separators are designed to treat ore which has been reduced to slime. The separation of feebly magnetic minerals in a state of fine division is a more difficult problem, as the capacity of the separator is cut down by the thinness of the ore layer which may be treated.

In dry-crushing plants the several crushing and separating machines are usually housed in, and the flying dust removed from within the casings by exhaust fans and settled in a dust chamber. Dust is a source of danger to the workmen employed about the machines, and is a hindrance to the separation as well.

Electric machinery should be installed in a separate building, or dust-tight room, as magnetic dust collecting on magnetized bearings, etc., and on motors and dynamos is troublesome.

Nonmagnetic dust has a tendency to adhere to magnetic concentrate, which may be a source of loss, notably in the separation of magnetite and apatite. The dust may be removed by an air blast, or if the trouble be aggravated, resort may be had to wet separation.

If the separator is capable of fine adjustment and the ore is accurately sized, fine material may be separated readily, the capacity of the separator becoming less the lower the permeability of the magnetic mineral and the finer the material treated.

Feeding Devices

A usual origin of separator feed is some form of roller or reciprocating feeding device placed beneath a feed hopper. Such feeder should be absolutely automatic, and so connected with the separator mechanism that, should the separator stop, the feeder will stop also. The feeder should spread a thin, even layer of ore upon the conveyor belt, shaking plate, drum or cylinder employed to transport the ore to the separating zones, and the rate of feed should be capable of regulation. The feeder should be so constructed that if a large piece of ore or other material should find its way past the screening apparatus the feeder will not stop, or necessitate stoppage, for cleaning out. It is usual to place a screen at the feeder, either above or below it, which will eliminate from the feed any oversize particles. In separators which employ conveyor belts to present the ore to the magnet the feeder should spread a uniform layer across the width of the belt, a couple of brushes being set at the edges of the belt to turn back toward the center any particles which might be shaken off and lost. If a feeder works poorly and does not distribute a uniform ore layer, a piece of canvas so fastened that its lower end will drag on the conveyor belt will be found useful to distribute the feed properly.

Adjustments

A magnetic separator should be capable of easy adjustment to suit different ores. A rheostat should be provided to regulate the current on each magnet, and in separators in which the ore is introduced between the poles, the distance between the poles should be capable of adjustment. The amount of feed, the speed at which the ore is presented to the magnets, and the distance of the ore sheet from the separating poles should be capable of regulation, as well as the positions of diaphragms for dividing the separated products.

Requirements a Magnetic Separator Should Fulfill

Besides the ordinary requirements for any steadily operating machine—such as automatic operation, economy of power, durability and simplicity of construction, and visibility of working parts—a magnetic separator should be provided with a thin, even, regular feed that will present the ore at proper speed as close as may be to the separating poles of the magnets, which should have a concentrated and homogeneous field. The separator should make at least three products: magnetic concentrate, middling, and nonmagnetic tailing; should embody some provision for the cleaning of the magnetic concentrate from entrained nonmagnetic particles, if of high permeability, and should be capable of complete and accurate adjustment.

Capacity

The capacity of a magnetic separator is controlled by the kind of ore treated, by the percentage of magnetic product removed, and by the size to which the ore has been crushed. The effect of the size of the particles treated upon the separator capacity is well illustrated by the results obtained at Ems, Germany, in the removal of raw siderite from blende, where the average capacity of a Humboldt-Wetherill separator is 12 metric tons per 10 hours on material between ½ and 4 millimeters, but only 3.5 metric tons per 10 hours on the fines passing a ½-millimeter aperture. In general, the more strongly magnetic the mineral removed the greater the capacity of the separator. The Ball-Norton belt separator, operating on magnetite ore crushed through 6 mesh, has a capacity of about 20 tons per 10 hours. The capacity of the Dings or the Cleveland-Knowles separator may be taken as 1 ton per hour on roasted pyrite-blende concentrate of average grade. The above figures are taken from representative plants and are generalizations only; the capacities of the several separators are given, when it is possible to do so, in the descriptions of mills in the following chapters.

Cost of Magnetic Separation

The cost of magnetic separation consists of the cost of preparing the ore for treatment plus a few cents per ton for supervision, excitation, and repairs. When the ore must be roasted the cost of this should, of course, be charged against the separation of which it is a prerequisite; the cost of roasting pyrite or siderite to the magnetic oxide should not, under average conditions, exceed 50 cents per ton in a well-equipped plant operated at capacity. Wherever it has been possible to do so, the cost of treatment has been given in the descriptions of mills in the following chapters.

Testing

Where it is intended to employ magnetic separation a preliminary test of the ore is even more important than with other processes, on account of the difference in the magnetic behavior of the same mineral from different localities. Most manufacturers of magnetic separators maintain testing establishments, and will make small scale tests without charge except for any assaying that may be desired. Such tests, if yielding satisfactory results, should be followed by a large scale test under working conditions and personal supervision. While it is impossible to standardize schemes of testing to suit all ores, the following points should be covered: (1) An accurate sample should be used, sufficient in amount to partake of the nature of a mill run; in other words, to be indicative of the results which may be expected from commercial operation. (2) Determination should be made of the size to which the ore should be crushed to yield the best results. (3) If there is a choice between direct separation of the raw ore and separation after roasting for magnetism, both methods should be tried and results compared; which might, perhaps, end in a decision to employ a combination of the two methods. (4) Separation of the ore with different amperages on the magnets, different belt or drum speeds, etc., should be made to determine the adjustments necessary to attain the greatest efficiency and capacity. (5) Determination should be made of the amounts and grades of all products separately, from which data any desired combination of results may be computed. (6) Accounting for all the values in the feed and determination of the sources of loss.