Electrostatic separation of ores in its present form is generally known as the Huff process, from the name of Charles H. Huff, of Boston, Mass., through whose constant and persistent labors (with the invention of Clinton E. Dolbear as a basis) the successful commercial process embracing separative machinery and the various electrifying devices has been developed step by step, and the finances for the long period of development provided, and the method finally established and recognized throughout the world as an important and successful addition to the ore-dressing department of metallurgy.

The permanent field-success of electrostatic separation began in 1908 with a 20-ton Huff experimental mill built specially for the purpose by the American Zinc, Lead & Smelting Co., in Plattevilie, Wis., a plant which was a success from the start and was gradually increased in capacity as the market-conditions warranted to 100 tons of concentrates per day. Much credit is due to the above-mentioned company for its initial venture, and for its assistance in applying the process to field use.

Prior to 1908 electrostatic separators had been installed and operated (but for a comparatively short time, however) in a number of places; some under the patents of Mr. Dolbear by himself and associates, and some under the patents of Lucien I. Blake and Lawrence N. Morscher, by W. G. Swart, mining engineer, of Denver, who has always been a courageous advocate of electrostatic separation.

Due to the difficulties experienced in the generation of the electrical charges, the primitiveness of the separators, the wooden construction instead of iron as at present, the lack of control of the electrical fields and other difficulties overcome by the later inventions of the Huff Electrostatic Separator Co., electrostatic separation did not gain favor during those early field-endeavors.

There is an old experiment in physics where an electrified rod is brought close to a suspended pith ball. The pith ball is first attracted, clings for a moment to the rod, and is then vigorously repelled. As the rubber rod approaches the pith ball, a charge of opposite kind, so called, is induced on the side of the pith ball nearest to the charged rod, and as unlike charges of electricity attract one another and as the pith ball is very light, it moves to the rubber rod. But pith, though not a good conductor of electricity, does, because of the moisture contained, conduct electricity appreciably, and it soon becomes, as a whole, charged similarly to the rubber rod, and away it flies. This is the principle which is utilized in electrostatic separation, and to accomplish separation, the differential property is the conductivity of the minerals involved. Thus, if the circuit of an ordinary 110-volt incandescent lamp is broken, and a piece of pyrite inserted in the break, the current will again flow. Similarly, if a piece of quartz be used, the electrical circuit will remain broken. Between these extremes, the conductivities of minerals grade, there being, however, a great difference in conductivity between the so-called better conductors and poorer conductors.

The physical separation may be accomplished in several ways, all utilizing the same underlying principle of difference in conductivity of the minerals. In the simplest form, a mechanical mixture consisting of readily-conducting and poorly-conducting minerals (such, for example, as grains of copper and grains of sand) is dropped on to a metallic plate charged to a high potential. Immediately upon contact with the plate the better-conducting minerals become charged to the potential of the plate, and are thrown vigorously from it. The poorer conductors require a much greater time to reach the electrical condition of the plate, and therefore, if they are not given time to reach this condition, due to being removed from the plate, a separation is obtained. This method was utilized in the simplest form of electrostatic separator, as illustrated in Fig. 1, in which the charged plate is replaced by an electrified roll.

Wherever there is electrical repulsion, there must be an electrostatic field (analagous to a magnetic field), with two terminals to the lines of force. Just as a magnetic particle endeavors to move along the magnetic lines of force, and would were there no other forces acting, so an electrified particle endeavors to move along the electrostatic lines from one terminal of them to the other. In the example just cited, the charged surface is at one terminal of the lines, while the walls of the room are at the other, the electrostatic lines existing as is shown in Fig. 2.

The particle charged from its contact with the metallic roll, may be said to be repelled from the roll, or attracted to the walls of the room.

Suppose now instead of permitting all of the lines to pass to the walls of the room, they are concentrated to another roll, as shown in Fig. 3, by connecting opposite terminals of the charging-device to the two rolls. The field is now much more localized and intense than in the case of the disseminated field of Fig. 2.

However, instead of depending solely upon the removal of the poorly-conducting particles from the surface of the charging-body before they have time to become charged, the length of time during which the metal surface remains charged may be regulated, as the better-conducting particles obtain their charges practically instantaneously upon contact.

Still another method of applying and utilizing the charge may be used. It has long been known that from points or sharp edges of conductors, when charged to a high potential, there emanates an electrical spray, which consists of charged air passing from the point or edge to the surface of opposite potential. Assume now there are passing over the roll a number of grains of good conductors and poor conductors,

as illustrated in Fig. 4. The charged air traveling from the point to the roll will strike upon the backs of these particles which are in contact with the roll and deliver charges to the particles. In the case of good-conducting particles this makes no difference, as the charge is immediately transferred from each particle to the roll with which it is in contact; but with the poorly-conducting particles the charge leaks away but slowly to the roll, with the result that the charges on the roll and on the backs of the particles are different, and the particles are held firmly to the roll until the charge leaks away. Assume the roll is grounded. The charges on the backs of the poorly-conducting particles induce charges on the face of the roll near them, and the endeavor of the two kinds of charge to unite holds the particles closely and tightly to the

roll. When the roll has made a partial revolution the particles are removed by a brush, while the differently conducting minerals may be separated by the use of gravity or centrifugal force.

For years these various methods of application of the principle have been studied, and a process developed to combine the applications in the manner which seemed to give the best results in the field under the conditions of mill-practice. It should be borne carefully in mind that in all the cases above cited the separative effect is accomplished by taking advantage of the difference in electrical conductivity of the particles.

Several types of Huff separators have been developed for application to different materials. The following description is of two types only—those in most common use and most generally applicable. A type consisting of six separating-fields and a feeder is illustrated in Fig. 5, which does not, however, show the large feed-hopper and the heavy base. The entire machine, with the exception of the attracting-rods and their supports, is made of metal, and is electrically grounded, so that all portions

which come in contact with the ore are metal, and all parts with which the operator is likely to come in contact are grounded so that the operator gets no electric shocks in touching the machines. As it is desired to pass a thick sheet of ore through the separators, there is more or less interference of particles at each separating-roll; hence it has been found advisable to use several rolls in succession, each contributing a part towards the complete separation. This is especially necessary in those cases in which it is desired to keep the ore coarse at the beginning, and thus have many attached particles to be reground. In order to have all the metallics come in contact with the separating-surface, a number of contacts at different angles are required. In this machine, the only moving parts are seven steel rolls revolving in babbitted and greased bearings, so that the power necessary to drive the separator itself is extremely small.

Fig. 6 is a diagrammatic sketch of a second form of separator, which is finding excellent favor in the separation of washed zinc-iron middlings below 20-mesh, which flow very readily. The advantage of this type is that there are no moving parts whatever, except the feed-roll, and therefore there is a minimum of wear and required power. Not all material, how-

ever, will flow sufficiently freely without external assistance to be applicable to this type of separator.

The size of particles which can be successfully treated by the present forms of Huff machines is from 6- or 8-mesh down to the limit of granularity of the material. Those slimes which are so cohesive that they do not move uniformly over inclined chutes cannot as yet be successfully handled. It is possible that this difficulty can be overcome by a special design of feeder or separator. In Utah a table-middlings is being treated wherefrom the impalpably fine material has been removed during wettable treatment. However, of that portion going to the “ C ” (finest size) machines, all of which is through 80-mesh (aperture 0.0082 in.), 20 per cent, passes a 240-mesh (aperture 0.0020 in.) screen. Huff machines have been built to handle material much coarser than 6-mesh, but up to the present the demand for such a machine has not warranted the investigation necessary to develop it completely.

The action produced in the usual type of electrostatic separator consists in electrically giving the relatively conductive particles a horizontal component of motion in addition to the motion produced by gravity. The less conductive particles, not being thus affected, are acted upon by gravity alone. The heavier the conductive particle, the stronger must this repellent force act to cause it to fall on the outer side of the divider of the machine. The repellent force is dependent upon the intensity of the electrostatic field. As electrostatic separation treats particles varying greatly in size, therefore, with a field sufficiently strong to repel the very heavy particles, there is danger of the finer less conductive particles being thrown over also. Hence it is advisable not to have a feed in which the particles are too widely diverse in size, and therefore in weight. In practice the feed is screened into a few sizes, of which screening the following is an example: from 8-to 12-; from 12-to 20-; from 20- to 50-, and through 50-mesh. When the difference in conductivity of the minerals is small, it is sometimes advisable to size somewhat more closely.

At this point attention is called to a matter which properly belongs later. Crushing some varieties of ore to 10-mesh, for example, will expose practically all the mineral, but will free but little of it. Because of the numerous separation-fields to which the material is subjected while passing through the separator, all sides of every particle are brought into contact with the charging-surface, and nearly every particle which contains an appreciable portion of exposed mineral will be repelled, thus eliminating the rock as coarsely as possible. All the mineral can be thrown into a concentrate, or the better part into the concentrate from the first two or three rolls, and the balance into a middlings for recrushing.

The question of the electrification of the separators in a manner which should not be affected by varying atmospheric conditions was a serious problem at the beginning of the electrostatic development. The frictional or induction generators are in themselves exceedingly susceptible to changes of humidity of the atmosphere, and also their capacity is so small that any source of electrical loss on the line or in the separators is likely to throw the whole system out of adjustment. It is unfortunate that this difficulty should have been so widely advertised in the earlier stages of the work in connection with the “Blake” machines. It has restrained many from investigating the Huff machines. There is no longer any difficulty whatever from this source, the machines being adaptable, if properly installed, to any climate or any atmospheric conditions. The electricity is furnished the Huff machines by electromagnetic generators instead of frictional machines. These generators work as independently of atmospheric conditions as does the ordinary lighting-dynamo, and are capable of supplying any line- or machine-losses which may occur and yet hold the potential absolutely the same from day to day. Contrary to general belief, the difference of potential which is used in electrostatic separation is only from 18,000 to 25,000 volts, instead of about 100,000 volts, earlier supposed to be necessary. There has been developed a small, compact, 3-h-p. electrical set, which is sufficiently large to care for the requirements of a 100-ton mill, and probably a much larger plant. Protective devices are placed between the generators and the separators to prevent injury to workmen in case of accident at the separators or accidental contact with the high-potential line. In the four years of its field-operation, no one has been injured from this source.

As the effectiveness of any electrostatic separation depends on the differences in conductivity of the particles, it follows that there must be no extraneous factor interfering to affect the conductivity. Water is, in the sense of electrostatic separation, a good conductor. Therefore the particles must be dry. Some minerals dry very readily and remain so. Some are hygroscopic, and if allowed to stand cold for some time in a moist atmosphere, tend to collect on their surfaces an extremely thin film of moisture. However, experience has shown that by properly constructing the mill equipment so that the ore from the drier passes immediately through its various paths, and is not permitted to lie for some time cold, and exposed to the atmosphere of a room, very few minerals, and none of the common ones, offer any serious difficulty from this standpoint.

With regard to the minerals and their conductivities, as stated previously, the condition is entirely relative, some minerals being better conductors than others. However, the minerals may be divided quite closely into two classes: one, those which are very easily charged and repelled, and the other, the very poor conductors which act as non-conductors. A complete list of the conducting minerals has been compiled by G. W. Pickard. Table I. presents the more common of both classes.

Table I.— Conducting and Nonconducting Minerals.
Important Conductive Minerals
Argentite,	Galena	Psilomelane
Arsenic, Native,	Graphite	Pyrargyrite
Arsenopyrite,	Hausmannite	Pyrite
Bismuth, Native,	Hematite	Pyrolusite
Bismuthinite,	Ilmenite	Pyrrhotite
Bornite,	Jamesonite	Redruthite
Brookite,	Leucopyrite	Silicon
Calaverite,	Linnaeite	Smaltite
Carborundum, (artificial)	Magnetite	Sperrylite
Chalcopyrite,	Manganite	Stannite
Chalcocite,	Marcasite	Stephanite
Cobaltite,	Mercury, Native,	Sylvanite
Copper, Native,	Millerite	Tellurium
Covellite,	Molybdenite	Tetrahedrite
Enargite,	Niccolite	Wad
Ferrosilicon,	Pentlandite	Wolframite
Franklinite,	Proustite	Zincite
Poorly-Conductive Minerals
Zinc-Blende	Epidote	Quartz
Quartz	Garnet	Rutile
Feldspar
Nearly all the silicates, carbonates, and sulphates.
Most of the siliceous rocks.

There are a few important minerals whose conductivity is variable, being dependent upon the composition of the particular specimen. Among these are notably blende and garnet. Pure sphalerite is a very poor conductor, ranking among the best insulators. It seldom occurs pure, however, but is usually contaminated chemically by varying amounts of iron sulphide or manganese sulphide. When these impurities are present in very large quantity in the blende, the resulting mineral is commonly called marmatite. The behavior of blende in electrostatic separation is somewhat dependent upon the amounts and character of impurities in the mineral. Small amounts of iron or manganese, and sometimes up to several per cent, of these impurities, have no seriously deleterious effect upon the behavior of the blende, but when the impurity is present in sufficient amount the resultant mineral changes over into the class of conductors. The relation of impurity and conductivity seems to follow no definite law, but each sample must be examined independently. From the above, however, it is readily seen that it is theoretically impossible to produce the same grade of finished blende-product in any manner from all samples of ore. Whereas the Joplin or Wisconsin blende crystals will analyze from 66 to 67 per cent, of zinc, those of some regions when mechanically free from all impurity contain only from 40 to 45 per cent, of zinc, and perhaps from 10 to 20 per cent, of iron. As a rule, electrostatic separation is very successful in separating the zinc-minerals from other minerals.

Similarly, the conductivity of garnet is sometimes dependent upon the amount of iron present. A high-iron garnet is likely to be more conductive than a low-iron garnet. A study of the data in Table I. will show the general field for electrostatic separation or concentration.

Considering a few specific problems and the specific reasons for the peculiar adaptability of the Huff process to these problems, it should be noted, as shown in Table I., that most of the copper sulphide minerals are excellent conductors, while most gangue-rocks are poor conductors. Therefore, electrostatic separation is applicable to the general concentration of the sulphide ores of copper. It is particularly well-adapted to the concentration of copper-minerals from the heavier gangue-minerals, such as garnet, epidote, barite, the heavy basic rocks, etc. Large bodies of ores of this character exist, usually as altered contact-deposits, though, except in cases where the grade is sufficiently high for direct smelting, they have been developed but comparatively little, because difficulty has heretofore been found in concentrating them.

Often a combination of methods is more efficient than a single process, the governing factor being whether the size of operations warrants complexity of procedure. Very large copper-mills employing gravity-concentration produce a concentrate which has been carried to the economical limit of purification by the method employed, but which is yet capable of mechanical improvement if a different property of the ore than its specific gravity be worked on. Electrostatic separation offers a means of reducing the silica and other objectionable constituents of the gravity-mill concentrates.

Zinc-blende usually occurs in mechanical association with pyrite or marcasite, chalcopyrite, galena, and one or more gangue-materials. The specific gravities of these minerals are approximately as follows: galena, 7.5; pyrite, 5.0; marcasite, 4.8; chalcopyrite, 4.2; blende, 4.; ordinary gangues, 2.7. From the smelters’ standpoint, it is essential (for maximum recovery of the metal at minimum cost) that the material going into the lead-smelter be as free from zinc as possible; that into the copper-smelter, free from zinc and lead; and that into the zinc-smelter, as high in percentage of zinc as possible. Of the latter, theoretically, 67 per cent, is the maximum, but only in exceptional cases (as in the Joplin and Wisconsin districts) is 60-per cent, zinc-blende product obtained commercially by any method, while 50-per cent, zinc is for most complex ores considered excellent and 45-per cent. good. With ores in which the various mineral ingredients dissociate at a reasonable (30-mesh or coarser) degree of crushing, gravity-separators (jigs or water-tables) will usually effect a reasonably efficient separation of minerals which differ in specific gravity by 1.5 points, but as the difference in specific gravity becomes smaller the effectiveness of separation in this manner becomes less, so that with a difference of but one point, the effectiveness of the separation is poor. By putting such a complex ore, suitably crushed, through jigs and over reciprocating tables, the galena (with a portion of the pyrite or marcasite) is efficiently concentrated from the rest of the minerals. Likewise, the elimination of the gangue-rocks is also reasonably well effected. There are left together a greater portion of the pyrite or marcasite, the blende, and the chalcopyrite (with of course a little gangue and a small amount of galena). Fortunately, the galena (which is usually so small in amount as to be negligible except for the associated silver which often occurs in it), the pyrite or marcasite, and the chalcopyrite are excellent conductors of electricity, while the blende and the small amount of gangue- rock are very much poorer conductors. In the utilization of this difference of conductivity is seen the application of the electrostatic process to this problem. This mixture, after drying (but not roasting), is passed through the Huff machines, and two products made, one for the zinc-smelter and one for the copper-smelter.

Blende often occurs in association with certain heavy gangue-rocks of a specific gravity so similar to that of the blende that separation by any gravity means is very ineffective. A method has been developed by the Huff company whereby the surface of the blende can be made conducting, after which the blende can be separated electrostatically from the associated rock-minerals. Two important instances of the above are blende and barite, and blende and fluorspar.

In addition to the concentration or separation of the sulphides mentioned heretofore, the Huff process has proved that it also accomplishes the following results very satisfactorily: concentration of gold and silver pyritic ores, of antimony, arsenic, and molybdenum sulphides; concentration of graphite, of pyrite for sulphuric acid manufacture, hematite, manganese- ores; the separation of galena and barite; purification of abrasives, natural and artificial; and the solution of many problems relating to the more rare minerals. Copper oxides, carbonates, and silicates can sometimes be concentrated, by first roasting to the conductive oxide, or reducing to the metal.

The process is essentially a dry one, obviating the troubles due to drought or freezing. The finished products require no further drying-treatment prior to shipment or smelting to decrease freight-expenses or fuel-costs. Loss in slimes is avoided, as the dust may be collected and is available for use if desired.

There are no shaking or vibrating parts to the machines with the attendant wear and repair on the mechanism. The attendance necessary, after primary adjustment, is small and ordinary mill-labor is suitable. The machines are readily sectionalized for light transportation and easily assembled. There are no complicated or intricate parts to get out of order and the separators are readily adjustable while operating.

The Huff electrostatic process was started into field-operation in the spring of 1908, at the beginning of the present general slump in the mining business. Nevertheless, the method has made progress, and wherever installed has been very successful. The first plant mentioned has been already described.

At Midvale, Utah, the United States Smelting Co. has a very complex lead-zinc-copper-gold-silver ore, for which has been installed an electrostatic plant producing 50 tons daily of zinc- and iron-concentrates, the procedure in use being that above described for complex ores containing zinc. This plant also is described in the references quoted.

A further instance of the eminent adaptability of the Huff process to zinc-ore problems is the newly-constructed electrostatic mill at the Sunnyside mine, Silverton, San Juan county, Colo. This is treating about 40 tons per day, improving the zinc-concentrates obtained in the gravity-milling of the ores of the district.

There is also a 40-ton plant recently installed in Sonora, Mexico, which is giving excellent satisfaction in the separation of blende-chalcopyrite concentrates, producing a blende-product containing 55 per cent, of zinc and a copper-product assaying about 16 per cent, of copper.

A plant recently installed by the Carborundum Co. at Niagara Falls, N. Y., is separating an impurity from artificial abrasive.

The cost of construction and of operation depends upon the general costs in the given region, upon the size of operations, upon whether the plant is run as an independent concentrator or as an adjunct to other operations. Therefore total-cost figures are of but little value unless presented with a complete description of the conditions of operation. The following detailed items are presented, which, by addition to suit the circumstances, will give a proper total.

Only the concentration- or separation-department is here considered, as the grinding-department is the same, independent of the method of concentration. A flow-sheet for the treatment

of an ordinary crude ore in the concentrating-department, shown in Fig. 7, is as follows: rotary drier (not roasting), screens, roughing-machines to make a finished tailings; finishing-machines making finished concentrates, and a small middlings for return to the general system. The dust is drawn from each piece of apparatus and collected (to be used if sufficiently rich, and discarded if of little value). As the separators are upright, they occupy a space 6 by 1.5 ft., and are about 7 ft. high. Leaving sufficient space for attendance, a machine occupies a space 8 by 5 ft., or 40 sq. ft. of floor-space per separator. From 9 to 12 tons per separator can be taken as an average on the initial machine-floor, so that the tonnage-area of the building is about 4 sq. ft. per ton for the separators and about 6 sq. ft. per ton including elevators, belts, etc. The Midvale plant, with a daily capacity of 75 tons of very fine material, occupies a building but 40 by 40 ft. in area, but there is a very considerable amount of space unused at present. The drier is usually situated in the basement, and the screens in a tower above the concentrator-floors. For a crude mill of 1,000 tons daily capacity, approximately 6,000 sq. ft. of building is necessary, or about 60 by 100 ft. The other costs of installation are similar to those of the ordinary gravity-concentration, except that the elevators do not have to handle water. Also, because of the small size of the building, there is a minimum of distributing-machinery.

The operating-costs outside of crushing-costs consist of: drying, power, labor, and repairs. The drying has been mentioned already, as has also the electrical power necessary for separation. The mechanical power for driving the separators is figured at 1/3 h-p. per separator, including line-losses, so that in large installations the entire power, including elevators, screens, etc., amounts to approximately 1/6 h-p. per ton of daily capacity. As everything in a properly-designed mill is automatically handled, with the exception of firing the drier, the labor required is needed only for properly keeping watch of the mill and for loading the products.

In general, the costs of operation in an electrostatic mill do not differ materially from those of a similar mill using reciprocating tables.

Electrostatic Separation

The Huff electrostatic plant of the United States Smelting Company operated in conjunction with its wet concentrator at Midvale, Utah, was the second plant of substantial size installed using the Huff process, and the first plant to be put in operation on the so-called Western complex ores.

In spite of the machinery being of early design and consequently embracing many of the mechanical weaknesses incident to a pioneer plant the mill has operated steadily and uniformly since the summer of 1909, about five years now, without any material change other than some re-arrangement of the machinery for better handling of the sizes.

The ore, at present coming almost entirely, from the company’s mines in Bingham, is of practically the same composition as that upon which the plant was first put in operation.

The crude ore, consisting of the sulphides of copper, lead, zinc, and iron, and containing small amounts of gold and silver, is brought by train and delivered to the hoppers of the wet concentrator, where by jig and table treatment, there are produced a shipping lead concentrate, a tailing and a middling product, the latter being conveyed in push cars to the adjoining electrostatic mill.

The crude ore delivered to the wet concentrator contains from 6 to 9.5 per cent. zinc. At times the plant is used also for the treatment of custom ores from the Bingham district and elsewhere, and at such times the results of the plant vary according to the material in use, but as by far the larger portion of the product is that from the company’s own mines, the results given below are fairly indicative of the general work obtained.

The accompanying diagram illustrates graphically the flow of the ore through the electrostatic mill. The middling coming from the wet mill containing from 15 to 18 per cent, moisture is hoisted while on the cars by an elevator, and delivered from the top of the elevator to a hopper over the drier. The drier first installed was of too low capacity to take care of the moisture present in the tonnage to which the mill was later

raised, and a second drier was placed in series with it. Drying material of this nature requires in the neighborhood of 1 lb. of coal for 5 to 7 lb. of water to be evaporated, the higher the moisture the somewhat more efficient the use of the fuel, due to the heat wasted in raising the temperature of the ore as a whole, being a constant.

The drier delivers the bone-dry material to the boot of the main elevator, where it is raised, and delivered at the top of the mill to two Newaygo screens set in series, each having a double vibrating surface. The feed to these screens is practically all through 16 mesh, and mostly through 30 mesh. The top vibrating screen of the first Newaygo is a 16-mesh scalper, removing the oversize material, chips, etc., which are delivered to a bin and returned again to the wet mill. The Newaygos produce four sizes, through 16 on 40 mesh, through 40 on 60 mesh, through 60 on 100 mesh, and through 100 mesh. Of the total feed, 16 per cent, passes a 200-mesh screen; and of the material in the through 100-mesh size, which is later treated by the so-called fines machines, 46 per cent, is through a 200-mesh screen.

From the Newaygos, which deliver on to the second machine floor of the mill, each size of material passes to its separate elevator and is delivered on the third floor to small hoppers which in turn empty into the separators below.

The separators are for the most part arranged in units of three, one being above the other two. Other arrangements are indicated on the flow sheet, the idea being to make a rough split on the first machine of the minerals to be separated, and the final cleanup on the separators below. The arrangement of the units is largely dependent on the percentage of the minerals in the given material to be separated, and the machines are arranged as nearly as possible to keep a uniform load.

Each unit produces three products, a finished “iron,” a finished “zinc,” and a middling product. This middling product is returned to the boot of the main elevator and again passed into the Newaygo screens. This middling is due partly to mechanical entanglement, and partly to the fact that abrasion in passing through the mill has broken up some of the minerals; so by passing all the material again over the screens, it unites with the warm material coming from the drier and is therefore again thoroughly dried and screened into its proper sizes.

The finished products fall into bins under the main floor of the mill, and are thence conveyed by hand cars to the railroad cars outside the mill building.

The separators are electrically energized by a small self-contained electric unit maintained in a dust-free room at the side of the mill. This unit consists of a 3-h.p. motor, belted to a special generator and exciter. The potential is raised by transformer and rectifying device to 18,000 to 22,000 volts which is the potential used on the machines (at this plant). The potential generated in this way is practically uniform, and varies only with such slight variations as are present in the speed of the motor. One terminal of the potential line is grounded direct and the machines are likewise grounded. From the other terminal, a highly insulated wire runs through the mill, being connected to the so-called “attracting” electrodes of the separators, of which there is one to each roll or separating element. The electrical equipment in this plant is considerably more massive than the equipments now being put on the market.

The principle of the process and the general apparatus in use have been described in some detail during the past four years in several publications. It was given by the writer at the New York meeting of this Institute in February, 1912. Numerous improvements have been made since that time, one of the most important from an operating standpoint being the substitution of bare-metal “electrodes,” which has been made possible by the institution of a regulating device in the circuit to the machines. This substitution of the metal for the wooden electrodes has at Midvale reduced the cost of maintenance of the electrodes 75 per cent. Further improvement along these lines has been made, and it is believed that shortly any cost for electrode maintenance will be entirely eliminated. Since the construction of the Midvale plant, a separator has been designed called the “Toboggan” type. The only moving part in this is the feed roll; belts and bearings being eliminated. One of these separators is at present in operation at Midvale and accomplishes about the same result as a unit of three of the roll type. Elsewhere this type is in operation handling all the material through 20 mesh.

The quality of the work in the Midvale plant is largely dependent upon the character of the blende which varies from time to time. The zinc sulphide crystals in the ores from the United States company’s mines contain chemically combined from 3 to 5.5 per cent. iron. When the iron content of the zinc sulphide crystals is under 4 per cent., it is not difficult to make a 48 to 49 per cent, blende and at the same time keep the zinc content of the pyrite product around 11 per cent, or under. When, however, the zinc sulphide crystals contain much over 4 per cent, iron, it is impossible to keep the, grade of the blende up above 48 per cent, unless considerable zinc is run into the pyrite product. The zinc present in the pyrite product consists partly of attached particles and partly of an intimate mixture of zinc-lead mineral, and partly the blende high in iron. The impurities left in the zinc product are largely gangue, left in the middling in the wet mill, and small amounts of attached minerals. That the percentage of iron chemically combined with the blende is not a criterion in judging the conductivity of the blende, is illustrated by the fact that some blendes, containing as high as 14 per cent, iron in the crystals, are separated electrostatically. It is not known definitely what determines this conductivity.

Typical analyses of the wet-mill feed, and the head and resulting products of the static mill are as follows (the impurities of minor metals in each of the products being largely as minute attached particles):

The capacity of the present mill when running full is approximately 65 tons, and on the various sizes the units have the following capacity: on 40 mesh, 12 tons; on 60 mesh, 10 tons; on 100 mesh, 8 tons; through 100 mesh, 5 tons. The Midvale ore is a comparatively difficult one with which to obtain the best results, and the tonnage per unit is therefore lower than with a more simple ore. The separators for crude ore are of much larger capacity.

Six mills of this general type are in operation in the Western zinc field, some modifications being required to meet local conditions in each case; as well as plants in operation on other classes of work as described in the articles above mentioned.