US2070530A - Apparatus for screening ore - Google Patents

Description

Feb. 9, 1937.

C. A. GALLAGHER ETAL APPARATUS .FOR SCREENING on];

Filed May 24, 1933 3 Sheets-Sheet 1 INVENTO Char/e5 l7. Gal/a9 er ATTORNEY 1937- c. A. GALLAGHER ET AL 2,070,530

APPARATUS FOR SCREENING ORE Filed May 24, 1933 3 Sheets-Sheet 2 ATTU RN RY Feb. 9, 1937- c. A. GALLAGHER :1- m. 2,

APPARATUS FOR SCREENING ORE 7.

Filed May 24, 1933 3 Sheets-Sheet 3 l VENTORS Char/e5 Gal/aye! Carl M. fll/en 7moM ' Patented Feb. 9, 193 7 PATENT [OFFICE APPARATUS FOR SCREENING ORE Charles A. Gallagher and Carl M. Allen, California, Pa., assignors to General Chemical Com- Daily, New York, N. York Application May 24,

6 Claims.

This invention is directed to apparatus for roasting metallic sulfides, and more particularly for roasting finely divided sulfide ores, to desulfurlze the same and to produce sulfur dioxide for use in the manufacture of sulfuric acid, or

for any other purpose desired.

Generally speaking, to a large extent present practice in roasting sulfide fines includes the use of mechanically operated multiple hearth constructions, such, for example, as the well known MacDougal, Herreshofl and Wedge burners, and while such burners provide effective roasting, the complicated construction and operation thereof involve considerable initial and maintenance expense.

- As distinguished from bed roasting carried out in such burners, it has been previously suggested to roast fines while in gaseous suspension, in operations in which the fines are either ininto a roasting chamber wherein the fines encounter cross or counter-currents of oxidizing and/or suspending gas. 1

Suspension roasting is best applicable when the fines ores are in a relatively finely divided state, and this method presents the notable advantage over the mechanical multiple hearth operation of considerably lowering the cost of production, by reason of the elimination of complicated and moving parts, expensive rabbling and other apparatus inherent in the construction and operation of mechanical burners. In the practice of suspension roasting, however, and particularly as applied to roasting metallic sulflde fines, difllculties are encountered which must be overcome before theoretical advantages of this type of roasting are made practicably available. Oneof such difilculties involves introduction 40 of fines into the combustion chamber, and the formation of a uniform dispersion of ore particles therein. This phase of the process usually requires, for eflicient conduct thereof, a relatively dry ore, free from lumps and extraneous foreign material. Damp or wet ore tends to clog feed pipes and injector mechanisms thus interfering with operations in this respect. The presence of lumps in the fines, after introduction into the combustion chamber, is detrimental to the formation of a satisfactory ore dispersion in the roasting atmosphere. Furthermore, large lumps once in the roasting chamber are relatively incompletely desulfurized, appreciable quantities of sulfide sulfur being retained therein, thus in- Jected into a roasting chamber in suspension il the oxidizing gas, or are showered downwardly'" Y., a corporation of New 1033. Serial No. 572,550

(on. a-25'n creasing the sulfur content of the cinder, and lowering the efiiciency of roasting.

To avoid such difiiculties, it is desirable to feed into the furnace, dry, or dry and preheated ore free from lumps. This requires screening, and screening should preferably be effected after drying, as when damp ore is screened, the damp material tends to clog the screens. Moreover, if the fines are screened prior to drying, lumps which may form during drying are injected, as such, into the combustion chamber. Hence, for efficient operation of processes for roasting metallic sulfides in gaseous suspension, it is preferable to dry or dry and preheat the fines, and subsequently screen the same before charging into the furnace.

Prior screening devices, while effective in a general way, have been more or less objectionable because of attention required to keep such devices in operation, to prevent clogging with lumps and foreign material and to prevent overflow of fine material therefrom. It has also been proposed to dry fines ores prior to injection into the combustion chamber by causing the ore to pass in heat exchange relation over the top and sides of the combustion chamber thus utilizing radiated and conducted heat to dry the ore.

It is one of the principal objects of the invention to provide satisfactory mechanism for screening ore before feeding the same into the furnace injectors. Another object is to provide a screening device which is automatic in operation and which does not require attention of an operator. The invention contemplates apparatus for preliminary drying or drying and preheating of the fines, and for screening such fines, prior to injection into the roasting chamber.

With respect to the improved screening mechanism some of the important features thereof comprise provision for regulation of the amount of fines fed to the screen surface to that withdrawn from the subjacent screenhopper conducting the screened fines into the injector mechanism; continuous withdrawal from the screen of oversized and foreign material; an auxiliary overflow screen and hopper, positioned beyond the main screen surface and associated hopper, which overflow screen and hopper act to prevent discharge from the screening device of undersized material, thus preventing loss of the latter; and an arrangement by which material is ,discharged from the overflow hopper into the fines injector faster than from the main hopper, so that the overflow hopper cannot become filled and cease to function in the intended capacity.

The objects and advantages of the invention may be more clearly understood from a consideration of the following description taken in connection with the accompanying drawings in which Fig. 1 is a verticalsection of a burner in connection with which the improvements of "the invention may be advantageously employed;

Fig. 2 is an enlarged, vertical section of a fines ore injectormechanism;

Fig. 3 is a vertical section of another type of burner illustrating another adaptation of the screening device of the invention:

Fig. 4 is an enlarged, vertical section of the improved screening device, showing a portion of a burner such as illustrated in Fig. 3;

Fig. 5 is a plan view of the hopper of the screening device;

Fig. 6 is an elevation of the hopper taken substantially on the line 66 of Fig. 4;

Fig. '7 is a development, on a reduced scale, of an ore drying and heating jacket surrounding the walls of the burner of Fig. 1, and

Figs. 8 and 9 are sections of a hopper screen agitator.

Referring particularly to Fig. 1 of the drawings, numeral l0 designates generally a shaft bur er comprising a shell ll constructed of suitable fractory material, such as firebrick, and efining a roasting or combustion chamber l2 0 cylindrical cross-section. Surrounding the shell II is a steel casing I3, acting as a protective reinforcement for the furnace. The upper end of the combustion chamber is closed off by a. crown 15, the top side of which forms a drying and preheating hearth IT. The cylindrical shell II and casing l3 project upwardly beyond the crown, and carrry a framework H! which in turn supports ore feeding and rabbling mechanism for hearth II.

The surface of hearth i1 is slightly cone-shaped and slopes downwardly toward the shell of the burner. Lying above the hearth are rabble arms l9, having downwardly projecting plows 20 pitched to work sulfide fines gradually toward the circumference of the drying hearth. Arms I9 are rotated by a motor 22 through shaft 23, supported in bearings so as to maintain the lower ends of the plows 20 properly spaced with respect to the surface of the drying hearth H. An ore bin 24, mounted on framework l8, discharges ore onto a platform 25 from which fines are intermittently dropped onto approximately the center of hearth H by a sweep 21 rotating with shaft 23.

Surrounding the major portion of easing l3 are a plurality of louvers 30 which with casing l3 form an ore drying and preheating jacket designated generally by reference numeral 32, enveloping a substantial portion of the vertical walls of the burner. Cut in shell ll, near the upper end, are downwardly sloping ore supply passages 34 through which ore is passed from the drying hearth I! into the topmost louver 35 constituting the upper edge of jacket 32. The drawings shows, in section, two passages 34. Preferably, several such conduits may pass through the top of shell II at spaced points about the circumference of the burner so that, on rotation of the rabble arms l9, fines are gradually fed through the openings 34 into the top of the drying jacket.

In the embodiment of the apparatus shown in Fig. l the jacket 32 is provided at the bottom with three outlets, indicated at 38, through which fines are fed into the subjacent screener, designated by reference numeral 40. As shown in section at the left of Fig. 1 immediately above screener 48, and also in elevation in Fig. 7, sloping side 42 of lowermost louver 43 is of greater length than the sides of other louvers so as to provide small hoppers, terminating in discharge openings 38. The bottom of jacket 32 is formed by sloping edges 45 which intersect lower louvers 43 several of the intermediate louvers 30, and are attached to steel casing l3. A development ofthe louvers is shown in Fig. 7. Edges 45 of the jacket are sloped at an angle greater than the angle of repose of the sulfide fines, so that the latter run freely toward outlets 38 through which the material is fed into the screener 40. The louvers are supported by ledge-like projections 48 of verticals 49 which in turn may be suspended from framework l8. The louvers rest loosely on the projections, this arrangement permitting repair or replacement.

The construction of the screener is best illustrated in Figs. 4, 5 and 6. The screener 48 includes principally a funnel-like casing 58, preferably having the shape of an isosceles trapezoid in horizontal cross-section, and terminating at the bottom in an outlet pipe 5| for feeding material into an observation funnel 52 on the top end of an injector feed pipe 53 havinga control valve 54. Projecting upwardly from the top edges 56 of the sloping sides 51 of casing 58 are vertical sides 59 converging in a direction away from the roaster shell, and forming a passage 60 through which oversized material is discharged, over a lip 6|, into a collecting pipe 62. Set into casing 50 is a transverse baiile 63 attached at the side edges to sides 51 of casing 50. The bailie slopes downwardly toward the burner shell as shown in section in Fig. 4 and in plan in Fig. 5.

It is to be observed the lower edge 64 of bafile 63 crosses the axis of outlet pipe 5| so that the passage 65 between edge 64 and the back 61 of casing 50 is appreciably more restricted than the passage 10 between the lower edge of baffle 63 and the inner face of front 1| of the casing 50. Baiiie 63 forms with back 61 what may be considered a primary hopper l3, and forms with front ii an overfiow hopper 14. The inner top edge of casing 50 may be made arcuate as at 12 to permit the hopper to fit snugly against shell I 3.

A screen 11 covers hoppers l3 and I4, and is supported in the position shown in Fig. 4 by rear edge 12, the top edge of the baffle 83, front lip iii of casing 50, and by the sloping sides 5'! thereof.

Referring again particularly to Figs. 1 and 2, the injectors for introducing fines into the furnace comprise a principally metallic pipe section 80, the inner end of which is set into the shell of the burner at the lower end of the combustion chamber, pipe providing an ore inlet 8|. Tube 80 is held securely in position by a circular plate 82 and a flange 83 bolted to steel shell l3. In the preferred construction, the axis of tube 80 is directed upwardly at a relatively sharp angle, for example about 75 to the horizontal.

As shown in Fig. 2, the valve-controlled pipe 53 projects into the upper side of tube 80. A gas inlet pipe or jet 85 passes through end plate 86 and terminates beneath the discharge end of pipe 53. Admission of air or other gas to jet 85 is controlled by a valve 81. A plug 89 affords means for cleaning out conduit 8| should the same become stopped up.

Surrounding the burner near the injectors is a bustile 90 for supplying gas to injector jets 85, the bustle being connected to agas supply through a pipe 3| controlled by valve 92. oxidizing gas to support the roasting operation is introduced into the shell I2 through circumferentially spaced ports 33, positioned adjacent the ore injectors, and connected through short pipe sections not shown with a bustle 04. The bustle 94 may be provided with an inlet pipe connection, having a control valve, either open to the atmosphere or connected to a blower when it is desired to operate the burner under positive pressure.

The bottom of the combustion chamber is formed by a hopper-shaped hearth 05 pitched at an angle preferably slightly less than the angle of repose of the oxide cinder, and terminating in an outlet 96 through which cinder is continuously discharged into a conveyor. A gas main 31 for withdrawing gaseous products of combustion from the chamber I2 opens into the latter just beneath the crown I5.

In the specific embodiment of the invention described, the height of the combustion chamber of the furnace, exclusive of the hopper-shaped bottom, is about equal to the diameter. While such particular proportion of the combustion chamber is preferred when carrying out the roasting operation per se in a burner of the general type illustrated in the drawings, it is to be understood that the diameter of the chamber may in some instances advantageously exceed the height by a substantial amount, and may also be less than the vertical dimension. Preferably the diameter of the chamber is not less than the height. It will be understood, however, that the apparatus relative to the means for drying or drying and preheating the fines, prior to introduction into the combustion chamber and relative to the improved screening device may be employed in connection with any kind of roasting chamber.

Fig. 3 represents a different type of burner in connection with which the screener 40 may be used to advantage. In this apparatus, the general construction thereof is substantially the same as already described in connection with Figs. 1 and 2. However, in Fig. 3, the ore injectors I00 are positioned a relatively short distance beneath the crown I5 so that the finely divided ore is fed into the top of the combustion chamber I2. Screener 40 feeds ore from supply passages IOI through inlet pipes I02 into the injectors I00. When employing this type of apparatus, it is preferable to position injectors I00 relatively close to the underside of crown I5, and hence desirable to shorten the distance between the injectors and the screeners as much as possible to conserve head room. Accordingly, angle A (Fig. 4), representing the angle between ,the surface of screen 11 and the horizontal may be less than would otherwise be the case. In this situation, it may be advantageous to intermittently agitate or shake screener 40 in order to work coarse material over the surfacee of the screen, and through discharge opening 60. For this purpose, there may be provided a knocker I05 rotatably journaled in a bearing I06 carried in the top end of the shell II. The knocker I05 includes an arm I01 the lower end of which projects into the path of rabble arms I0, and a second arm I08 normally resting against one side 59 of screener 40.

Figs. 8 and 9 show a modified construction in which the screen is vibrated electrically. Screen I25, having outwardly projecting sides I26, is pivoted at the rear edge at I21 to the back 61 of casing 50, the opposite edge of the screen exmaterial thereon is carried by bracket through nut 35 and vibrator rod I36. The up- .edges of sides I28 of the casing is a bracket I30 carrying a vibrator I3I the casing I32 of which may be rigidly connected to support I30 as at I33. In the center of the bracket I30 is a circular opening I35 through which passes a screen vibrating and supporting rod I36. Attached to the upper end of rod I36 is a vibrator disk I31 spaced from fixed core I38 of the vibrator. Underneath the disc I31 is a nut I40 attached to rod I36 and arranged to bear on the upper side of bracket I30 when the core I38 is demagnetized. The lower end of rod I36 is connected to screen I25 by set nuts I4I which are adjusted so that the outer end of the screen is held spaced slightly above the upper surface of lip 6i of the casing. When the core I30 of the vibrator is demagnetized, the weight of screen I25 and the I30 per end of core I38 is threaded into the top of the casing I32 and is provided with a lock-nut I44 by means of which the air gap between disc I31 and the lower end of core I36 may be adjusted. Alternating current is connected to coil I45 by wires not shown.

The invention is applicable to the roasting of metallic sulfide fines such as iron pyrites, pyrrhotite, zinc sulfide or arsenopyrite, but for convenience, and by way of illustration only, the operation of the apparatus may be described in connection with the roasting of iron pyrites.

A supply of relatively finely divided pyrites is maintained in the bin 24 by suitable conveyor or elevator mechanism, not shown. roasting is begun, combustion chamber I2 is preheated to temperatures above the ignition point i the particular ore to be roasted, as by the use of oil burners inserted through conveniently located workholcs, not shown. When the desired degree of preheat is obtained in the combustion.,chamber, the motor 22 is started, and rabble'arms I9 and sweep 21 may be rotated at a rate of say one revolution in two minutes. Fines run continuously out of the bin 24 onto platform 25, and, on each revolution of shaft 23, a regulated quantity of ore is swept off the platform to approximamly the center of hearth I1.

During rotation of rabble arms IS, the fines are gradually worked across the heated surface of hearth I1 into passages 34. The dry or partly dry ore then runs into the louver 35 and the top of jacket 32. The louvers 30 are spaced sufficiently close so that material, while running from one louver into the one immediately below does not overfiow the upper horizontal edge of the lower louver, that is, the surface of material between the lower edge of a louver and the upper edge of a subjacent louver, for example, as indicated at H5, is substantially at the angle of repose. The numerous exposed surfaces of the fines between the several louvers permit intermittent exposure to the atmosphere of at least some of the fines and thereby provides for the ready escape of water vapor formed during drying or drying and preheating of the fines.

When roasting is under way, hearth I1, and the vertical walls of the combustion chamber become relatively highly heated, and sulfide fines are dried and preheated to some extent during movement over the drying hearth I1 and through jacket 32 surrounding preferably at least a major portion of the burner shell. This operation blankets the top and sides of the burner with ore which acts as a heat insulator, retaining heat in the combustion chamber, and employing heat Which'is transmitted through shell II, to the ore, to dry and heat thelatter. This preliminary heating serves to dry theflnes, thus facilitating the formation of more satisfactory dispersion of ore in the combustion chamber, and at the same time preheats the fines to temperatures, say 300 to 400 F., so as to promote ignition shortly after introduction into the combustion chamber.

The fines dried, or dried and preheated, on

passage'thru jacket 32 are directed-by sloping edges 45 toward outlets 38 through which the material is fed to' the subjacent screeners 40, be-

fore injection into the combustion chamber. The

' as to tend to continuously feed fines ontoscreen TI. The slope of the latter, represented by angle A, is preferably such that oversize material is gradually worked over the surface of the screen and discharged through opening 60. Generally speaking, the upper horizontal edge II 6 of baffle 63 is spaced with respect to the burner shell I3 so that the remote edge or toe I I! of the mound II8 of material on top, of the screen I1 lies about at the position of horizontal edge I I6 of the baffle 63, and so that the upper surface of the mound II8 assumes the angle of repose. According to the nature and particular size of the particular fines, the position of the toe III may be adjusted by vertically slidable gate I20 so that toe III lies, substantially above the upper edge of the baffle 63. By this arrangement, there is maintained on the screen above hopper I3 a given amount of material which acts in turn to lute off passage IM and control the amount of ore fed therefrom to the screen.

The bulk of the material passes through screen 11 and into the primary hopper 13 from which the fines are withdrawn through passage 65, and pipe 5I into observation funnel 52 on the top of feed pipe 53. From a consideration of Fig. 4, it will be seen that if material less than screen size works down over the surface of the screen and passes the upper edge of baiiie 63, such undersized material will pass through the lower end of the screen and into the overflow hopper I4. This construction prevents discharge over lip 6| of material of less than screen size. It will be observed lower edge 64 of bafiie 63 crosses the axis of outlet pipe 5I so that the passage I is larger than passage 66. For this reason, screened material will feed out of hopper 14 into pipe I at a rate faster than out of hopper I3. Accordingly, material collecting in hopper I4 is quickly fed out through opening I0 into the funnel 52 thus insuring an empty 'or at least partially empty terial off screen, I1 and out through opening '80.

In the operation of the screener 40, only the equivalent of theore withdrawn through pipe 53 can be fed to screen I1. As the same amount of material is fed to the screen as is withdrawn continuously through pipe 53, segregation of relatively fine and coarse undersize material is prevented. The feed conduits to the screen, such as passages I0 I, are sealed by material on the screen, and overflow of the screen is avoided. Coarse oversize finaterial is discharged without blocking the hopper, and material of less than screen size which might be discharged along with the oversized material is caught in overflow hopper I4, and directed into the ore injector through pipe 53.

The operation of the screener of Fig. 1 is substantially the same as described in connection with Fig. 4. In Fig. 1, it will be understood the construction .is sothat the horizontal spacing between the upper edge of battle 63 and the lower edge of bottom louver 43 is such that a line drawn between the two represents with the horizontal approximately the angle of repose of the particular material being used for roasting. If desired, a small adjustable gate corresponding with gate I20 of Fig. 4 may be attached to the lower edge of louver 42 so as to permit horizontal adjustment of the position of the toe of fines on the surface of screen 11 relative to the upper edge II6 of bafile 63.

In the operation of the modified apparatus shown in Figs. 8- and 9, a mound of material II8 is maintained on screen I 25 as already described in connection with Fig. 4. In the construction of Figs. 8 and 9, however, provision is made for continuously vibrating screen I25. When coil I45 of vibrator I3I is connected to a source of alternating current, it will be understood core I33 is alternately magnetized and demagnetized and disc I3'I carrying screen I25 is raised to engage the lower end of core I38, and then drops back to the position shown in Fig. 8. It will be understood this construction continuously vibrates screen I25 about pivot I 21 thus insuring continuous feed to the hoppers 13 and I4, and avoid ing clogging of the screen. This construction may be employed to advantage where the angle formed by the surface of screen I25 and the horizontal is not sharp enough to effect prompt discharge of oversized material from the screen.

Valves 54 in feed pipes 53 are adjusted so that a substantially steady stream of fines runs into feed conduits 8I. into the combustion chamber I2 by means of air,

. steam, or other gas not adversely afiecting oxidation of the sulfide. It is preferred to employ air for this purpose, and when using the latter, air may be introduced into the lower ends of conduits 8| through valve-controlled jets 85 at pressures of about 5 pounds per square inch. When starting up roasting operations, it will be understood the combustion chamber I2 is preheated to above ignition temperature of the fines by oil burners or other convenient means not shown.

The axes of inlet conduits 8 I, Fig. 1, the amount of fines fed into the injectors through pipes 53, and the air pressure in jets 85, regulated by valves 81, are all adjusted with respect to the particular size of the roasting chamber so that the ore particles from each injector rise through the combustion chamber to an elevation just below the undersign of crown I5. Ore particles then fall to hearth 35, and while passing through an atmosphere increasingly rich in oxygen, oxi- The fines are then injected dation of the fines is completed. It will be understood in the embodiment of the invention shown in Fig. 1, the major portion of the air for roasting is drawn into the combustion chamber through ports 93 by a suction fan in the gas outlet conduit 91 through which the gaseous prodnets of roasting are withdrawn. The roasting operation per se is no part of the present invention, and hence need not be further detailed.

The operation of the apparatus of the invention when used in connection with the burner illustrated in Fig. 3 is thought to be apparent from the description already given relative to the enlarged section shown in Fig. 4. However, when employing the apparatus of Fig. 3, it will be understood the fines are injected into the upper,end

of the combustion chamber and fall thro h-a rising current of air or other oxidizing gas ntroduced into the bottom of the combustion zone as in the apparatus of Fig. 1.

The sulfur dioxide gases produced may be used, for example, in the manufacture of sulfuric acid. The sulfur dioxide content of the burner gas may be regulated as desired by adjusting the amount of air fed -into the combustion chamber as is known by those skilled in the art. Gases containing 10-13% sulfur dioxide may be readily made by the present process.

We claim:

1. Apparatus for screening material comprising a casing, a sloping screen covering the casing, an outlet at the bottom of the casing, a bafile in the casing forming primary and overflow hoppers, the upper edge of the bafiie lying against the screen and the lower edge of the baffle being positioned, with reference to the axis of the casing outlet, toward the material feed end of the screen, means for feeding material to the higher end of the screen, means for discharging oversized material over the lower end of the screen, and means for withdrawing material from the bottom of the casing.

2. Apparatus for screening material comprising a casing, a sloping screen covering the casing, an outlet at the bottom of the casing, a baffle in the casing forming primary and overflow hoppers, the baffle sloping downwardly in a direction toward the high end of the screen, the upper edge of the baffle lying against the screen and thelower edge of the baffle being positioned, with reference to the axis of the outlet conduit, toward the material feed end of the screen, means for feeding material to the higher end of the screen so as to maintain the toe of the mound of material thereon adjacent the edge of the baffle, means for discharging oversized material over the lower end of the screen, and means for withdrawing material from the bottom of the casing. I

3. Apparatus for screening material comprising a screen, a casing provided with a discharge opening beneath the screen, abafliein the casing forming with the walls of the casing a primary hopper and an overflow hopper, each of said hoppers having discharge openings formed by the lower edge of the bafile and the walls of the casing communicating with the discharge opening of the casing, the discharge opening of the overflow hopper being larger than the discharge opening of the primary hopper, means for feeding material to the screen above the primary hopper, means for discharging oversize material from the end of the screen beyond the overflow hopper and means for withdrawing material from the discharge opening of the casing.

4. Apparatus for screening material comprising a casing having side walls, the said walls having upper edges forming an angle with the horizontal, a screen disposed above the casing at the same angle as the upper edges of the side walls of the casing, a baflie in the casing dividing the easing into a primary hopper having as side walls the higher portions of the side walls of the casing and an overflow hopper having as side walls the lower portion of the side walls of the casing, means for introducing material to be screened onto the screen above the primary hopper, and means for withdrawing oversized material from the screen beyond the overflow hopper, the angle of the upper edges of the side walls of the casing and the screen being such as to maintain a mound of the material on the screen so that the toe of the mound is adjacent the upper edge of the bafile and the upper surface of the mound of material is substantially at the angle of repose of the material. v

5. Apparatus for screening material comprising a casing, a screen covering the casing, an outlet at the bottom of the casing for withdrawing material therefrom, a baflle in the casing forming primary and overflow hoppers, the upper edge of the baflle lying against the screen and the lower edge of the bafile being positioned, with reference to the axis of the casing outlet, toward the outer end of that portion of the screen covering the primary hopper, means for feeding material to said outer end of the screen, and means for discharging oversized material over the opposite end of the screen.

6. Apparatus for screening material comprising a casing, a screen covering the casing, an outlet at the bottom of the casing, for withdrawing material therefrom, a baffie in the casing forming primary and overflow hoppers, the upper edge of the baffle lying against the screen and the lower edge of the baffle being positioned, with reference to the axis of the casing outlet, toward the outer end of that portion of the screen covering the primary hopper, means for feeding material to said screen so as to maintain the toe of the resulting mound of material on the screen covering the primary hopper not substantially beyond the edge of the baflie, and means for discharging oversized material over the opposite end of the screen.

CHARLES A. GALLAGHER. CARL M. ALLEN.

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