US2976996A

US2976996A - Apparatus and method for wet screening

Info

Publication number: US2976996A
Application number: US591929A
Authority: US
Inventors: Freerk J Fontein
Original assignee: Stamicarbon BV
Current assignee: Stamicarbon BV
Priority date: 1955-06-17
Filing date: 1956-06-18
Publication date: 1961-03-28
Anticipated expiration: 1978-03-28

Description

March 28, 1961 F. J. FONTEI'N 2,976,996

APPARATUS AND METHOD FOR WET SCREENING Filed June 18, 1956 1 Patented Mar. 28, 1961 APPARATUS AND METHOD FOR WET SCREENING Freerk .l. Fontein, Heerlen, Netherlands, assignor to Stamicarbon N.V., Heerlen, Netherlands Filed June 18, 1956, Ser. No. 591,929 Claims priority, application Netherlands June 17, 1955 2 Claims. (Cl. 209-355) This invention relates to particle separation and more particularly to an apparatus and method for wet screening suspended mixtures of fine particles differing in grain size. This application constitutes a continuation-in-part of my copending application Serial No. 475,251, filed December 14, 1954, and issued December 8, 1959, as Patent No. 2,916,142.

In the above mentioned patent, there is disclosed an apparatus for screening suspensions of mixtures of particles differing in grain size which is termed a sieve bend. 'Ihe sieve bend comprises a non-continuous curved separating body or'screening deck having a feed device opening tangentially thereto so as to supply the suspended mixture perpendicular to the generatrix of the screening deck. The feed is connected at one end of the deck and a collecting device for the fraction retained by the screen is connected to the other end. The size of the apertures or slots in the screening deck, measured in the direction of flow of the material to be treated, is equal to or smaller than the size of the same bars measured in the direction normal to the direction of flow. The material to be screened is supplied at the feed side at a rate sufilciently high to effect a fair degree of screening or dewatering, whichever may be required, and to ensure the removal of the coarse particles at the delivery end.

A distinction according to shape is made between the curved sieve bend and the flat sieve bend. The radius of curvature of the latter is infinite. In a preferred embodiment, the screening deck is formed by a bar screen having its bars substantially perpendicular to the direction of flow of the material to be screened. In the curved sieve bend the bars, which run perpendicular to the generatrices of the curved screening deck, are straight. The flat sieve bend is preferably mounted at an angle of 30 to 60 to the horizontal plane. One of the advantages of the sieve bend is that it effectively resists blockage. The diameter of the largest particle that is allowed to pass through the screen will generally not be greater than one-half of the width of the slots between the bars. Only in some cases, especially when screening at very fine sizes, does blockage occur.

I have found that by leading the underflow of a sieve bend over a following sieve bend with a smaller width of slot than that of the first, it is possible to reclassify the finer particles in a simple and convenient manner.

Accordingly, it is an object of the present invention to provide a process of the type described in which separation of very fine sized particles can be obtained without blockage.

Another object of the present invention is the provision of a process for wet screening suspended mixtures of very fine sized particles in which the mixture is separated on a first sieve bend and the undersize received 2 therefrom is subsequently separated on a second sieve bend connected in series with the first, the latter having a slot width at most 1.9 times the slot width of the second sieve bend.

A further object of the present invention is the provision of a process of the type described in which successive wet screening operations are performed without the necessity of using additional liquid.

A still further object of the present invention is the provision of a process of the type described in which the undersize particles obtained in an initial wet screening operation are subsequently separated on a sieve bend having screen apertures larger than the maximum size of the particles supplied thereto so as to exclude the possibility of blockage.

A still further object of the present invention is the provision of an apparatus for separating suspended mixtures of particles of very small size which includes a pair of sieve bends connected in series so that the undersize issuing from one is fed to the other and in which the relative slot widths of the sieve bends are so related as to exclude the possibility of blockage on the following sieve bend or sieve bends.

These and other objects of the present invention will become more apparent during the course of the following detailed description and appended claims.

The invention may best be understood with reference to the accompanying drawing in which a schematic view of one embodiment of an apparatus constructed in accordance with the principles of the present invention is shown.

The present invention relates also to an apparatus for realizing the said process, in which a number of sieve bends are arranged in, series and the undersize of any preceding sieve bend is again subjected to screening on a following sieve bend. According to the invention, the width of slot of any preceding sieve bend is at most 1.9 times the width of slot of the following screen. By way of example the invention is explained with reference to the drawing, which is a diagram of an arrangement of a number of series-connected sieve bends.

As shown in the drawing, the apparatus consists of three series-connected sieve bends, generally indicated 1, 2 and 3. Preferably, these sieve bends are constructed in accordance with teachings of the aforesaid Patent No. 2,916,142, although it will be understood that the invention also contemplates the utilization of flat screening decks placed at an angle to the horizontal plane. Thus, sieve bend 1 includes a cylindrically curved separating body or screening deck 5 which may be formed by bars or the like spaced apart so as to provide separating or screening apertures in the manner indicated in Patent No. 2,916,142. The raw material, as for example, a suspension of coal in water, is tangentially supplied, as at 4, to the concave side the screening deck 5 at a predetermined minimum initial velocity. The greater part of the liquid, together with that part of the solid particles, having a grain size not larger than one-half of the width of slot, drops into a collecting tank 6 and is discharged at 7 onto the second sieve bend 2. The overflow from the sieve bend 1 is discharged at 8. The underflow of screen 1, which is discharged at 7, is tangentially supplied onto a similarly constructed curved screening deck 9 of the sieve bend 2, which has a width of screening aperture or slot of at least 1/ 1.9 times that of the slot width of the sieve bend-1. It can thus be seen that the slots in the screen deck 9 are larger than the size of the largest particles contained in the underflow of screen 1. The underflow of sieve bend 2 is, in its turn, discharged at 10 onto another similarly constructed screening deck 11 of sieve bend 3. The overflow from the screening deck 9 is discharged at 12. The same process is repeated on sieve bend 3 where the underflow is discharged at 13 and the overflow at 14. The slot width of screening deck 11 bears the same relation to the slot width of screening deck 9 as the latter does to the slot width of the screening deck 5.

Example At 4 fine coal with particle size of G8 mm., suspended in water, was supplied. The screening deck had slots of 1.5 mm, the screening deck 9 had slots of 1 mm. and the screening deck 11 had slots of 0.6 mm.

Sieve bend it operated at a particle size of about 1.5+2 =0.75 mm. Consequentl the underflow was smaller than the width of slot of screening deck 9, which was 1 mm. This sieve bend made a cut at a particle size of about 0.5 mm., which size was smaller than the width of slot of screening deck 11, this being 0.6 mm.

It can thus be seen that there has been provided an apparatus and method for separating suspended mixtures of solid particles of very small size which embodies the utilization of two or more series connected sieve bends in which the feed of the subsequent sieve bends is formed by the underfiow of the succeeding sieve bend. The slot width of the succeeding sieve bend being at most 1.9 times the slot width of the following sieve bend. Preferred ratios between these slot widths are between 1.4 and 1.5. It will be understood that the term slot width as herein used denotes the dimension of an opening measured in the direction of fiow of the material to be treated.

The arrangement outlined above secures definite advantages. Since the underfiovv of the sieve bend contains the greater part of the liquid, in which the solid particles are suspended, a good operation of each of the following sieve bends is ensured and it is not necessary to use an additional amount of liquid. Moreover, due to the abovementioned ratio between the widths of slot, any of the following sieve bends will be supplied with particles of sizes smaller than its width of slot, so that blockage is ex cluded.

It thus will be seen that the objects of this invention have been fully and effectively accomplished. It will be realized, however, that the foregoing specific embodiment has been shown and described only for the purpose of illustrating the principles of this invention and is subject to extensive change without departure from such principles. Therefore, this invention includes all modifications encompassed within the spirit and scope of the following claims.

I claim:

1. An apparatus for separating particles according to size comprising first and second separating bodies each having a feed end, a discharge end, a separating surface generally conforming to a surface generated by moving a line parallel to itself so that a given point on the line moves in a plane perpendicular to the moving line, and

surfaces spaced in the direction of movement of said line defining a plurality of separating apertures in said separating surface, the width of the separating apertures of said first separating body measured in the direction of movement of said line being atmost 1.9 times the width of the apertures of said second separating body, means for feeding solid particles and a suspending liquid in layer formation to the feed end of said first separating body at a predetermined minimum initial velocity in the direction of the separating surface thereof substantially perpendicular to the line generating the same so that the aperture defining surfaces of said first separating body facing in the direction of the feed end will block successive layers of a thickness substantially one-fourth the width of the associated separating apertures and particles in the block layers of a size less than twice the thickness thereof and hence less than one-half the width of the associated separating apertures will pass therethrough and the remaining particles will pass onto the discharge end of said first separating body, and means for collecting an undersize fraction passing through the apertures of said first separating body and feeding the same in layer formation to the feed end of said second separating body at a predetermined minimum initial velocity in the direction of the separating surface thereof substantially perpendicular to the line generating the same so that the aperture defining surfaces of said second separating body facing in the direction of the feed end will block successive layers of a thickness substantially less than the width of the associated apertures and particles in said block layers of a size less than twice the thickness thereof and substantially less than the width of the associated separating apertures will pass therethrough and the remaining particles will pass on to the discharge end of said second separating body.

2. A process of separating a mixture of particles according to size which comprises the steps of establishing at a firstreceiving position a layer formation fiow of particles and a suspending liquid having a minimum initial velocity; supporting one surface thereof to locally define the direction of travel of said layer formation flow; interrupting the support of said one surface a fixed distance measured in the distance of flow while permitting the layer formation flow, due to its velocity, to continue substantially in the said defined direction; separating, by blocking part of the layer formation fiow, a layer of a thickness not reater than A of the fixed distance of support interruption from the said one surface of the layer formation flow; supporting the one surface of the remainder of the layer formation flow to locally define its direction of travel; successively repeating the steps of interrupting the support, separating a layer by blocking, and supporting the remainder between said first receiving position and a spaced first discharge position along a first path generally conforming to a surface generated by moving a straight line parallel to itself so that a given point on the line moves in a plane perpendicular to the moving line while maintaining a predetermined minimum velocity in said layer formation flow along said first path; collecting the particles and suspending liquid blocked from the layer formation flow during its movement along said first path; establishing at a second receiving position a layer formation fiow of the collected particles and suspending liquid having a minimum initial velocity; supporting one surface of the layer formation flow of said collected particles and suspending liquid to locally define the direction of travel thereof; interrupting the support of said one surface a distance measured in the direction of flow of at least 1/1.9 of the distance of support interruption of said first mentioned layer formation fiow while permitting the layer formation flow of said collected particles and suspending liquid, due to its velocity, to continue substantially in the said defined direction; separating, by blocking part of the layer formation flow of the collected particles and suspending liquid, a layer of a thickness approximately A of the fixed distance of support interruption from said one surface of the layer formation flow of collected particles and suspending liquid; supporting the one surface of the remainder of the layer formation how of the collected particles and suspending liquid to locally define its direction of travel; and successively repeating the steps of interrupting the support, separating a layer by blocking, and supporting the remainder between said second receiving position and a spaced second discharge position along a second path generally conforming to a surface generated by moving a straight line parallel to itself so that a given point on the line moves in a plane perpendicular to the moving line While maintaining 'a predetermined minimum velocity in the layer formation fiow of collected particles and suspending liquid along said second path.

(References on following page) References Cited in the file of this patent UNITED STATES PATENTS McDougall July 18, 1905 Inman Aug. 23, 1910 Liggett et a1. Apr. 13, 1915 Smith Apr. 20, 1915 Rakowsky June 26, 1945 OTHER REFERENCES Publication AV4 of the Second International Coal Preparation Congress at Essen, Germany, September 20 to 25, 1954, dealing with The Dutch State Mines Sieve Bend by F. I. Fontein.

UNITED STATES PATENT OFFICE CERTIFICATE OEF CORRECTION Patent No, 2,976,996 R I March 28;, 1 961 I Freerk Fontein It is hereby certified that error appears in the above numbered patent requiring correction and that the saitiisLetters Patent. should read as corrected below.

Column 1,; 'line' 34 strike out "ame".

Signed and sealed this 21st day of November-1961.,

I (SEAL).

Attest:

Attesting Officer Commissioner of Patents USCOMM-DC