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By D. R. Mitchell

This paper discusses some of the fundamental physical and chemical characteristics of coal in Illinois that affect its preparation for the market. At the present time preparation consists almost entirely of sizing and hand-picking. The washing of coal has almost entirely disappeared from the state. This is a rather startling fact when one considers that in the year 1908 Illinois outranked all other states in the quantity of coal washed and a few years later, in 1912, 85 washing plants were reported. Two reasons are apparent for this change. One, which is probably the most important, is that immense areas of comparatively clean coal were opened; the other is that many of the early washing plants were inefficient, so that washed coal was looked upon with disfavor in many trade centers. Objectives of Operators Some of the outstanding preparation aims being given due consideration by operating men are: 1. The greatest possible production of lump and domestic sizes which bring in the market at least $1 per ton more than the fine sizes or screenings. 2. Efficient screen sizing with as little breakage as is consistent with economical operation. 3. The removal of extraneous ash and sulfur. 4. Utilization of low-priced screenings. 5. Suppression of dust in domestic coal shipments. Preparation Underground To get the greatest possible production of the larger sizes, preparation must start at the face. Improvements in blasting and loading have increased the sizes larger than 2 in. by as much as 20 per cent. at some mines. Tiffany and McKitterickl report the results of an investigation they made at a mine in southern Illinois in which, by a carefully controlled

Jan 1, 1931

By Ray W. Arms

The combination wet and dry coal-cleaning process is not an attempt at a compromise between the wets and the drys nor is its merit confined entirely to the limitation of moisture in the smaller sizes. It is more correctly, and more broadly, an application of the sized-coal cleaning principle adapting itself in an ideal fashion to present day multiple-size preparation practice. The purpose of this paper, therefore, is to call attention to the broader significance of the term "combination wet and dry process" and to mention some of the factors that govern its application. In the narrow sense, when considered apart from the sized-coal cleaning principle, the combination process finds its justification chiefly in the ability to limit the moisture in cleaned coal. Larger sizes of coal offer less superficial area compared with their bulk for the attraction of moisture films. They drain more readily in bins or railroad cars, having more interstitial space for the free flow of water. Therefore, they neither attract nor hold objectionable moisture percentages. In fact, there is so keen a demand for certain washed sizes containing some moisture that hygroscopic salts such as calcium chloride are applied to attract and retain it. When coal deliveries are made on a guaranteed moisture percentage the coarser sizes may be washed and mixed back with the fines without adding greatly to the original moisture but the fines must be dry cleaned or dried to control moisture. Thus in all respects moisture in the coarser sizes is less objectionable than in the fines. The implication of this statement, that moisture is always more or less objectionable, is true. It is safe to say that if dry-cleaning apparatus could be perfected to a point at which its technical results, cost and other aspects would be equal to wet washing, water would no longer be used. As it is, we find that good technical results are more difficult to obtain by dry cleaning than by water. Obviously the use of air tables for coarse coal is not ideal, for it involves the attempt to stratify material in a thin bed on a table surface when the load is only one particle deep. Furthermore, the various air cleaning tables on large sizes require a large volume of air for separation, more power to operate, more dust-collecting apparatus and involve more coal breakage.

Jan 1, 1931

By Thomas Fraser, Robert MacLachlan

The dry cleaning of coal is a relatively new art and, as might have been expected, a number of unforeseen problems have been brought to light in the few years that dry methods of treatment have been in commercial use. The pneumatic concentrators that were first adapted to the cleaning of coal had already been developed to a fair degree of efficiency, although at low ratings, in the field of ore dressing. The concentrating machines, therefore, were in a relatively more perfected state than the general technology of dry cleaning and, for this reason, the major difficulties encountered in the early plants were in the accessory operations involved in the servicing of the cleaners rather than in the functioning of the cleaners themselves. The crudities of a new technology were further aggravated, in some cases, by meagerness of appropriations which precluded even the provision of adequate storage bins and such other facilities as had been long recognized as indispensable to the proper functioning of older types of cleaning plants. The practice of dry cleaning is now sufficiently established so that we may examine critically auxiliary equipment and operations. The cleaning processes have been so simplified, with respect to presizing of the raw coal required and the quantity of air circulated, that stinting of expenditures for auxiliary equipment is not necessary to keep the per ton cost of installation in line with complete and well designed plants of other types. Furthermore, sufficient operating experience has been accumulated to ascertain the conditions most conducive to effective operation of dry-cleaning machines; at least the essentials of a definite technology of dry cleaning has been established. Early Operating Problems The problems that have occupied the builders and operators of dry-cleaning plants from the beginning have been relatively few, but difficult of solution. The principal difficulties have been: 1. The proper sizing of the raw coal to suit the cleaning process. 2. The handling of raw coal when it is delivered wet from the mine. 3. Dust collecting. 4. The maintenance of uniformity in cleaning performance.

Jan 1, 1931

By J. F. McLaughlin

The installation of the Chance flotation system for the preparation of anthracite demonstrated the need for some means by which the specific gravity of the fluid mass in the separating cone could be determined with a fair degree of accuracy. The means used (with the original cones) was a set of wooden balls attached to ropes or chains, each with a different specific gravity. In the Northern Anthracite field, where the cones are generally operated with the specific gravity at 1.65, these gravity balls are provided with a range of 1.6, 1.65, 1.7 and 1.75. The operator of the cone from time to time would place one or more of these balls in the cone and estimate its specific gravity by the action of the balls. The margin of error with this system was very great, because there was the possibility that the test ball might be held in suspension and carried around by material floating beneath the surface of the cone in what is known as "island " formation—a formation resulting from a low sand line in the separating cone. The inaccuracy of this system of determination of specific gravity was indicated also by periodic tests of the final refuse bank, which showed excessive losses of coal to the refuse. Further evidence of the unsatisfactory operation of the cone was the uneven appearance of the market product, which caused spasmodic condemnation. Maintenance of Even Preparation With the critical market condition of today, it is essential that the preparation units operate consistently to maintain an even preparation. If a plant wishes to maintain a 1.65 specific gravity preparation, which means that all material of a specific gravity greater than 1.65 is classed as "reject," if the specific gravity of the fluid mass runs to 1.70, the market product will not be satisfactory; or if the specific gravity of the fluid mass is 1.60, the market product will be entirely satisfactory but the losses at the slate ends will be excessive. The desired specific gravity of the fluid mass in the Chance cone is maintained by agitating the sand within the separating cone by means

Jan 1, 1931

By R. G. Baughman

With the constantly increasing sales competition, coal to be sold today must meet the test of quality in every respect. The producers must be able to make all marketable sizes that will meet such general requirements as follows: 1. Ash content must be consistently low, depending upon the use to be made of the coal. 2. Sulfur content must be controlled accurately according to requirements. 3. All sizes must be positively free from visible impurities. 4. Free moisture of washed sizes must be reduced to a minimum. 5. The freezing of wet coal in cars must be prevented. 6. The percentage of fines in small mixed sizes must be controlled to give the uniform and proper combustion when used on various types of automatic stokers. 7. The undersize particles or degradation caused by handling must be kept to an absolute minimum in all sizes loaded. Many difficulties may be met in bringing any particular coal to meet these quality requirements, because the extent to which the quality of a coal may be controlled is determined largely by its physical characteristics. Preparation of an Impure Coal Bed The application of modern and scientific methods in the controlled preparation of bituminous coal from a bed in the mid-western field will illustrate the results that can be accomplished by adequate preparation: The bed referred to contains coal inherently excellent as to low ash and other desirable characteristics; in fact, the coal itself is one of the best in the field. It is, however, divided into four layers by dirt bands of soft shale 1 to 2 in. thick. It also contains much sulfur in the form of sulfur lenses and laminated sheets. Horsebacks or rolls varying from 6 in. to 12 ft. in width are encountered in the mining of the bed and heretofore have rendered it practically unworkable. The mines operating in this

Jan 1, 1931

By H. F. Yancy

PooR results in coal washing and in ore concentration are sometimes attributed to the shape of the particles in the feed. It is well known that the shape of a particle influences its rate of fall in water. This is evidenced by the value of the constant C in the Rittinger equation for the velocity V of settling in water, V = C-D(d- z ), where C has the following values: 2.73 for roundish grains, 2.37 for long grains and 1.92 for flat grains. This report, which is based upon data obtained during the course of coal-washing investigations conducted by the U. S. Bureau of Mines in cooperation with the College of Mines of the University of Washington, gives the results of a method or procedure of screening in which the particles are separated into three shapes and shows the deportment of these shapes when coal is treated on a coal-washing table. The method consisted in first screening the coal with Tyler standard square-hole sieves, followed by screening tests of each of these products with screens of rectangular opening. For this purpose Tyler "Ton-Cap" screens were used; each square-hole product was subsequently sized with two Ton-Cap sieves so selected that their shortest dimensions were approximately 75 and 50 per cent., respectively, of the opening of the square-mesh sieves. For example: A product which is through 8 mesh (0.093 in.) and on 10 mesh (0.065 in.) is screened on slotted sieves which have openings 75 and 50 per cent. of the 0.065-in. opening in one direction and an opening of not less than 0.065 in. in the other direction. For convenience in discussion, the particles retained on the first slotted screen are termed "cubical"; those retained on the second slotted screen are termed "prismatic" and those passing the second screen are called "flat." (See Fig. 1.) The square-mesh screens separate particles according to two dimensions, length and breadth; the slotted screens divide them further according to the third dimension, or thickness. Prismatic particles have a mass of about two-thirds, and flat particles of about one-half, that of cubical particles of the same square-mesh size. The screen-

Jan 1, 1931

By Kenelm C. Appleyard

In introducing to an American audience a description of the work done in dry coal separation in England and in Europe generally, it is perhaps desirable to give a short history of the development outside the United States of the particular processes with which the author is connected, and perhaps to stress a little the differences in the nature of the conditions to be faced. England actually presents one set of problems and the Continent of Europe another, since the character of the coals to be treated is entirely dissimilar, and the attitude of the coal industries towards coal preparation is also dissimilar. England for generations has been working by hand good seams of clean coal of the highest possible quality and has not felt any pressing need to clean its coal. Most Continental countries, however, especially in Central and Western Europe, have been working friable and often very dirty coals for so long that a washery is accepted as a part of the normal equipment of a colliery, with the result that the art of wet washing has reached a high state of perfection, particularly in Belgium and Germany. The advent of machine mining has intensified the problems to be faced in all countries, and in England has been coupled with the working out of the best seams in the older coal fields, thus bringing about the necessity of working thinner and dirtier seams with resultant difficulties as regards marketing the products, while Poland, with clean and well prepared coal, has in recent years been attacking established British markets in Scandinavia. In recent years, therefore, interest in coal preparation has been steadily gaining ground in England, although it has not yet reached Continental dimensions. Broadly speaking, the differences in preparation problems between England and the Continent boil themselves down to one of size. In England, for example, with the exception of the Kent coal field, the proportion of coal of a size less than ? in. to the coal less than 2 in. is very much less than on the Continent, and since the principal difficulties in dry cleaning lie in the fines it is easier to produce good figures on

Jan 1, 1931

By Charles H. J. Patterson

When coal is deposited on the decks of pneumatic tables, all fine particles clinging to the larger pieces are blown free by the air. Inasmuch as the air retains an appreciable residual velocity after passing through the bed of coal, it is capable of suspending particles of goodly size. This velocity is dissipated gradually and, unless confined, can readily extend beyond the limits of the plant. Today the ideal in the design and operation of pneumatic cleaning plants is not that they be no dustier than the ordinary tipple, but rather that they be as clean as an office or shop. This degree of perfection may not yet have been attained in any commercial plant, but it is not impossible of achievement. Not until recently has the recirculation principle been applied to pneumatic separating plants. In the writer's estimation, it has not yet been proved entirely satisfactory in commercial installations. A serious obstacle lies in the fact that it is practically impossible to provide a truly closed circuit. The air circuits must be interrupted at certain points, if only for the introduction of raw material and for the recovery of the finished product. It is feasible to prevent the escape of polluted air at these points, but this can scarcely be accomplished without admitting additional air into the circuit. These additions of air are cumulative, which necessitates means for emitting to the atmosphere the excess while still carrying its objectionable burden, if provisions are not made for conditioning. If it must be provided at all, conditioning is frequently simpler and more economical if applied to the entire volume of air involved. This procedure eliminates the extra initial cost and operating expense of the closed system. Crane Cheek Experiments The first noteworthy commercial installation of pneumatic equipment was made in 1922, at the Crane Creek plant of the American Coal Co. of Allegheny County, at McComas, W. Va. Six tables for cleaning coal between the limits of and 3 in. comprised the initial installation. Dust was collected by hoods connected by a system of ducts to exhauster fans. In this installation the unit dust burden was small, owing to the removal of fines from the feed by vibrating screens.

Jan 1, 1931

By S. M. Parmley

Experience has shown that there are some factors connected with the drying of fine washed coal that are not present in drying of slack coal as normally practiced at cement kilns or pulverized coal plants. It is the purpose of this paper to show some of these variations as they have been encountered under actual operation. The Champion No. 1 cleaning plant of the Pittsburgh Coal Co. uses the wet system of cleaning coal. The plant is equipped to produce commercial sizes or mixtures, the primary sizes being plus 6 in. and 4 by 6 in., which are hand-picked. Minus 4-in. sizes are all cleaned by the wet process. To produce a commercial product as free of moisture as possible, the washed coal is first passed over sizing and dewatering screens, then each size except the — ?-in. coal is conveyed to loading booms by slow-moving draining conveyors. As the +?-in. sizes are not heat-dried, no data will be given, as it is the purpose of this paper to describe the heat drying of — ?-in. coal as practiced at the Champion No. 1 plant of the Pittsburgh Coal Company. To fully appreciate the conditions, it must be remembered that the — ?-in. product is now shipped at a lower moisture content than when received from the mine. The incoming coal in the — ?-in. size will average 5 per cent. moisture and the shipped product 3 per cent. moisture. This is a case where a wet process gives a reduction of both ash and moisture in the fine sizes. The shipped product, therefore, is more uniform in all characteristics, and uniformity of product is the most important single factor in coal preparation. The 0 by ?-in. coal is first dewatered on slow-moving continuous bucket dewatering elevators, running at the rate of 40 ft. per minute, with 48-in. perforated buckets with ¼ by 1½-in. slots, spaced ? by 2?-in. centers staggered. The moisture in the coal discharged from the elevators runs 20 to 28 per cent., depending on the fineness of the product. The discharge from the dewatering elevator is further drained on a drag-type conveyor equipped with ½-mm. wedge wire screen. The conveyor discharges coal with an 18 to 25 per cent. moisture to three centrifugal dryers of the Carpenter type. The Carpenter dryers

Jan 1, 1931

By J. R. Campbell

During the past few years, the writer has had occasion to take several excursions into the realms of the washability of beds 3 and 4 of the Pocahontas coal and the proper handling of these coals in prepared sizes Fig. 1.—Washability curve for 4 by 2½-in. egg, PocahoKtas coal. Note that definite break occurs at about 1.60 sp. gr., and washing at this gravity removes all heavy rock and slate, amounting to about 5 per cent. Curve 2 indicates a bony coal, and washed egg coal might not have a good appcarance, and would be condemned by visual inspection, although it would be well prepared mechanically. to attain the greatest realization for the operator and yet produce a market product acceptable to the consumer. Preparation is not new in the southern field, but some new angles have presented themselves in recent years on account of the progress of mechanical preparation. Mechanical methods are entirely devoid of the five senses given to

Jan 1, 1931

By Edgar Schweitzer

The original Dorrance breaker of the Lehigh Valley Coal Co. was erected in 1883. The coal beds were clean and dry, consequently a dry preparation system was used, consisting of revolving cylindrical screens for sizing and hand-picking, to remove the few impurities from the run of mine coal. The picking or cleaning room contained long parallel rows of chutes with seats mounted on them, for the use of the breaker boy, who bent over the chute and picked out the refuse material from the coal as it gravitated down the chute. The introduction of dry mechanical pickers, of, the sliding frictional type, in the early nineties lessened the number of boys, but hand-cleaning on the coal and refuse discharge ends of the mechanical picker was necessary. The Pardee spiral pickers, which were brought out about 1900, were better than the gravity pickers and gradually replaced them. In 1912 the breaker was entirely remodeled, although the original frame was retained. The revolving screens were replaced by shaking screens, and Pardee spirals were installed on all sizes of coal from egg to pea coal, inclusive. Hand-picking still was necessary on the coal and refuse ends of the spirals, although the changes did result in a reduction of the former breaker force. As mining advanced, and the thick clean bed became exhausted, it was necessary to mine veins containing a high percentage of impurities, which complicated the preparation problem. The most serious difficulty was the increase in the quantity of damp or wet run of mine, which could not be fully cleaned on the spirals. When this quantity exceeded approximately 5 per cent. of the feed, it was necessary to send the excess of the wet coal to the company's Prospect colliery for wet washing. This plan worked well for a time, but as the amount approached 15 per cent., with the prospect of a further increase, the company considered the building of a wet-washing plant. The Dorrance breaker was the only dry-cleaning plant operated by the company. At the seventeen other collieries, wet washing was employed, and jigs, both plunger and pan, were used. All the practical methods used in the anthracite fields were investigated, and it was finally decided to adopt the Rheolaveur system. The decision was due largely to the experience gained from the the operation of a Rheo-

Jan 1, 1931

By W. L. Dennen, V. H. Wilson

THIS paper is a description of the opera¬tion and results of a Humphreys Spiral Silt Cleaning Plant at the Powderly Colliery of The Hudson Coal Co. during the first nine months of operation and follows a pre¬liminary description given at the Anthracite Institute in May, 1947, in a paper titled "Preparation of Anthracite Sift for Boiler Fuel in a Humphreys Spiral Test Plant" by H. H. Otto, V. H. Wilson and W. L. Dennen. This plant is considered a full size operating test plant and, like all new plants, numerous minor changes in the methods of operation have been made from time to time to secure the results desired. Views of the silt preparation plant and of the screening house are given in Fig 1 and 2. This plant consists of 48 spirals in 8 batteries of 6 spirals each, 36 of which were to be operated as primary spirals and the remaining 12 as secondary spirals. A brief description of the plant and its operation follows. The only material cleaned at the plant during 1947 was that carried in the underflow from the Thickener which handles the Breaker waste water. This underflow, amounting to approximately 2000 gpm of water and containing 12.6 pct of solids, is pumped .to vibrating screens where the plus 316-in. material is removed. (Fig 3). This oversize material drops into a pocket from which it is trucked to the run-of-mine

Jan 1, 1948

By William Reynolds

THIS paper deals with the results of a study of the application and development of mining machines for cutting out and removing dirt bands in bituminous coal beds. FACE PREPARATION When one or more dirt bands1 occur in a seam of coal, the problem of elimination of impurities becomes manifest. The degree of complete-ness to which these impurities are removed at the face has an important bearing upon labor costs, haulage costs and outside cleaning costs, and often the ash content of the product is markedly affected. Yet, because the present-day miner is paid for the number of tons he loads he often lacks interest in the quality of the coal. To insure a clean product, a system of penalties for dirty coal is enforced, but control of extraneous impurity content in the loaded coal is often inadequate and ineffectual when coal and banded impurity are shot as one. It seems only logical that to reduce the amount of impurities by proper face preparation before the men are permitted to load the coal is the most efficient method of preparation.

Jan 1, 1936

By Arthur Boynton

IN THE preparation of this paper the writer has been influenced by the fact that descriptions of various means of cleaning blast-furnace gas have been published and that further descriptive treatment of the subject will probably be less interesting than a statement of conclusions with regard to preferable methods which have been reached by him as a result, of experience and observation in this country and also as a result of somewhat detailed investigation of the subject made toward the end of the year 1927 on the Continent of Europe and particularly in. Germany. Owing to various circumstances, including a cost of fuel that is extremely high in relation to the wages of labor, the German iron and steel industry has accumulated experience in gas washing which is as yet in part lacking in the United States of America. The paper begins with a statement regarding the economics of the use of blast-furnace gas, then considers the principle's and practices employed in its purification. USES OF BLAST-FURNACE GAS For many reasons having to do with the utilization of coke-oven and blast-furnace gases in steel manufacture and with the use of hot metal, the blast furnace is now usually a part of a steel-producing unit, which also includes coke plant and rolling mill. Therefore both blast-furnace and coke-oven gas are usually applicable to any of the iron and steel-making processes, including the generation of power, and their allocation in any given works should be considered as a single problem. The uses to which blast-furnace gas, either alone or mixed with coke-oven gas may be put, include the following: Regenerative Heating of Air for the Blast Furnace.-This is the oldest and metallurgically the most important of all uses. It is universally performed with unmixed blast-furnace gas and under proper conditions should require not over 23.5 per cent. of the total gas generated for a blast temperature of 1350° F. For Steam-raising Purposes.-This is almost universally done with unmixed blast-furnace gas. From the standpoint of quantity used, it is at the present time the most important application in American steel-works practice.

Jan 1, 1928

By A. C. Dittrick

THE Simon-Carves washer, originally known as the Baum jig washer, was first built in Europe about 1892. It was probably the first washer to use intermittent pressure of air on water in one part of the wash box to obtain water pulsations in another part. This type of washer was also the first to successfully clean various sizes in the same unit. Previous practice was to size closely before washing. Today there are several large firms abroad, and a few in the United States, that are constructing the Baum type washer. The design of all is practically the same, their differences being only in details. In 1928, the Hanna Coal Co. first gave consideration to the question of coal cleaning. It was believed at that time that by efficient removal of so-called noncombustibles appearing in the Pittsburgh No. 8 seam, certain markets that had been lost due to poor preparation might be recovered. Mechanical cleaning and mechanical loading were under consideration. This type of cleaning was considered the most practical means of removing impurities in the sizes below 4 in. It was decided to adopt mechanical cleaning and develop a system of mechanical loading that would be successful on No. 8 coal.

Jan 1, 1935

By C. B. Barlow, T. A. Mekelburg

Due to a tight ash constraint on the clean coal product and difficult middlings washability characteristics, a considerable amount of coal was being lost in the jig middlings at the Monterey No. 2 Mine. An investigation was conducted into circuitry to recover this lost coal with minimal effect on the total clean coal ash. A reverse two-stage hydrocyclone system cleaning at low specific gravities was selected. This system is currently recovering approximately 5.8 kg/s (23 stph) of 11.5% to 12.5% ash coal. The specific gravity of separation for the system is about 1.5 and the probable error is 0.13.

Jan 1, 1986

By R. E. Zimmerman

Wide attention is being placed upon various methods for cleaning the fine sizes of bituminous coals. The author describes and analyzes the results achieved on wet concentrating .tables of modern design on unclassified feeds of - 1/2 in. particle sizes, including tests on high-sulphur coals.

Jan 1, 1950

By A. A. Beard

ABOUT three years ago, I. L. Dunn, of Marietta, Ohio, found that wells could be cleaned by blowing with compressed air; this method has been used with great success on many wells in the Marietta, Ohio, district, where compressed air is being used to increase production, by forcing oil through the depleted sands. About a year ago, the process was successfully used to clean out wells in the Lenapah pool in Oklahoma, where an air plant had been installed, under the supervision of I. L. Dunn. Wells from 900 to 1000 ft. deep, with 40 ft. of sand and cavings in them, were cleaned out in from two to three days. The use of this process had, so far, been limited to properties using the Smith-Dunn process, where compressed air, under suitable pressure and volume, was available. For other properties, a portable compressor seemed essential; this was built and successfully demonstrated by Mr. Dunn, for the Tidal Oil Co. in September, 1924. The process of cleaning out with air has been described.1 It consists of lowering the tubing into the well, with a shoe or nipple of heavy pipe substituted for the perforated anchor on the bottom. Into the tubing, compressed air is introduced through a rotary swivel hose at the top, or by means of swing connections. The tubing is lowered to the top of the cave to be cleaned out, several barrels of water are put into the hole, and the compressed air is turned on. This blows out the material between the casing and the tubing, the tubing being lowered as the material is removed.

Jan 9, 1925

By W. M. Bertholf

IN 1945 tests were made in the coal washery of the Colorado Fuel and Iron Corporation, at Pueblo, Colo., to determine the value of the Humphreys spiral. So far we have demonstrated that it is of definite value to us in rewashing a portion of our table middlings; and we are fairly certain that this is not necessarily the only place in which it can be used to advantage. Because of manpower and material shortages, experimental work has been discontinued for the present. Three of the spirals are being installed for use in rewashing the middlings, and testing and experimental work will proceed when this installation is completed. All evidence secured so far points to the fact that the spiral can make an effective separation of heavy material below 8-mesh; but that it is ineffective in removing larger, heavy material. This, of course, will limit its use to fine coal or a classified feed containing no heavy material larger than 8-mesh. In considering the results of our tests, it should be noted that the feed was our table middling, and that any real separation of this material is a "moral victory," as there is little material that could be properly called coal and practically no heavy rock, the consequence being that previous attempts to clean the middlings have not been at all successful. Several articles dealing with the spiral and its use in treating heavy minerals have already been published,1 but for the benefit of those who have not seen these we include a brief description of the spiral and of the way it operates, taken from Bulletin 4 of The Humphreys Investment Co.: The Humphreys Spiral Concentrator consists of a spiral conduit of modified semicircular cross section. Ore pulp is fed to the top and, as it flows downward, the heavier particles concentrate in a band along the inner side of the pulp stream (Fig. I). Outlets for removal of products are located at the lowest points in the cross section of the conduit. Wash water, added at the inner edge of the stream, flows outwardly across the concentrate band. The width of concentrate band removed at the outlets is controlled by adjustable splitters. Concentrate is usually removed from the upper portion of the spiral and middling from the lower portion. Tailing is discharged from the lower end. Tests are usually made in closed circuit, since this provides a maximum of information with a minimum amount of sample. Fig. 2 shows the spiral set up for closed-circuit testing, and gives a good idea of the general arrangement of such an installation. The small cyclone separator is used to provide clean wash water without introducing additional water into the circuit.

Jan 1, 1946

By M. R. Geer

THE increase in the number of coal-cleaning plants employing dense-medium processes occurring since 1946 is especially interesting when viewed historically. Both sand and magnetite were introduced as material for heavy mediums at about the same time, sand in the Chance process in 1921 and magnetite in the Conklin process in 1922, but from that point on their records diverge. The Chance process enjoyed a steady growth from its inception, whereas no additional magnetite plants were built in the United States for over 20 years. Then, following the close of the World War 11, magnetite was again introduced, this time with marked success. During the following years some 47 plants employing magnetite medium were built.

Jan 7, 1953