|Publication number||US3245868 A|
|Publication date||Apr 12, 1966|
|Filing date||Sep 6, 1963|
|Priority date||Sep 6, 1963|
|Publication number||US 3245868 A, US 3245868A, US-A-3245868, US3245868 A, US3245868A|
|Inventors||Bridge Adam P, Espenmiller Howard P|
|Original Assignee||Black Clawson Co|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (6), Referenced by (18), Classifications (8)|
|External Links: USPTO, USPTO Assignment, Espacenet|
April 12, 1966 ENMILLER ET AL 3,245,868 OCESS FOR THE RECOVERY OF PAPER BROKE CONTAINING A WET STRENGTH RESIN Flled Sept. 6, 1963 3 Sheets-Sheet 1 CALENDER STACK Fl G zz 2O l4. s|zE PRESS If BREAKER STACK I IO 25 [8 1;, I6 PULPER scREw FEEDER 0 ACID 3 34 PULPERS 23 STEAM STOP VALVE DEWATERING 3| TUBE 27 35 COOKING TUBE VENT WATER cmp CHEST 35 CYCLONE DISCHARGER DII-UTIO/N CHEST SCREEN as j 42 I 37 I \V, ky 1.1L )AI r BLOW c'HEsT u|D cYcLoNE \WA HE 39 4o 43 5 Rs FIRST STAGE I sEcoND STAGE THIRD STAGE SHREDDING DEWATERING COOKING WITH CHEMICALS g. I 1 3! 35' j scREw I FEEDER SHREDDER PRESSURE COOKING TUBE 27 CHIP CHES DEWATER SCR W ING E STAGE FINAL CHEM FIFTH SIXTH STAGE I m sT'sskJE I l- [COMPLETE 1 CLEANING, SCREENING, CYCLONE DEFIBERING THICKENING 0R WASHING I AND RETURN TO SYSTEM. 35 35 FOURTH INVENTORS DISCHARGER HOWARD P. ESPENMILLER a BY ADAM I? BRIDGE STORAGE CHEST 2 w fi gg ATTORNEYS April 12, 1966 p, EspENMlLLER ETAL 3,245,868
CONTINUOUS PROCESS FOR THE RECOVERY OF PAPER BROKE CONTAINING A WET STRENGTH RESIN Filed Sept. 6, 1963 3 Sheets-Sheet 2 FIRST STAGE FIG 3 SECOND STAGE I THIRD STAGE SHREDDING DEWATERING COOKING WITH CHEMICALS 68 55 l is a? 5a SHREDDER 3 J HI DENSITY ATMOSPHERIC STORAGE COOKING TUBE CHIP I CHEST J/J/ A J FouRTfi STAGE EIFTH STA-6E slxTi-l STAGE FINAL CHEMICAL TREATMENTasToRAeE COMPLETE cLEAN|Ne,scREEN|Ne,
, DEFIBERING. l THICKENING 0R WASHING 4 a RETURN To SYSTEM.
STORAGE CHEST FIRST STAGE 4 SECOND STAGE THIRD STAE SHREDDING DEWATERING cooxme WITH CHEMICALS I I CHEMICAL HOT -I EMICAL 55 WATER HOT WATER DEWATERING I 60 scREw PULPER PULPER SHREDDER CHIP CHEST l FOURTH sTAE FINAL c'HEmcAL TREATMENT FIFTI-i STAGE SIXTH STAGE AND $TRAGE COMPLETE CLEANING, SCREENING,
' DEFIBERING vTHICKENINGORWASHING L a RETURN TO SYSTEM.
y L INVENTORs 63 HOWARD P. ESPENMILLER a a BY ADAM R BRIDGE COOK CHESTS STORAGE CHEST W awk 73 ATTORN EYS April 1966 1-1. P. ESPENMILLER ETAL 3,245,868
CONTINUOUS PROCESS FOR THE RECOVERY OF PAPER BROKE CONTAINING A WET STRENGTH RESIN Flled Sept. 6, 1963 3 Sheets-Sheet 5 FIRST STAGE ggcogo TAGE THIRD STAGE T; DEWATERING COOKING WITH CHEMICALS CHEMICAL v HOT wATER DEWATERING SHREDDER scREw PULPER JLE l I SCREEN CHIP CHESTN W Q l FOURTH STAGE F|NAL CREMICAL'TREATMENT AND STORAGE.
FIFTR sTAGEIsIXTH sTAeE COMPLETE CLEANING,SCREENING, DEFIBERING. THICKENING oR WASHING i 8RETuRN To SYSTEM. CHEMICAL CHEMICAL I HOT WATER i 1 HOT WATER I f 1 yr 1 DEWATERING COOK CHESTS STORAGE CHEST RiCOVEREDG *PHYSICAL TEST DATA AVERAGE +REFLECTMCE w T STREN TH FIBER FRM CAN.STD. TEAR MULLEN FREENESS FACTOR FACTOR FINES SHRINK PILOT PLANT 8 LABORATORY 50-80 1.3-1.5 1.1-1.2 80-9012.9-14.2 803 -82.6%
B T s s 80 1.02 0.82 98 11 87.5%
CONTINUOUS 1 ggggf 80-98 154 1.13-1.22 78.5 12-13 82-83% COMPARED TO MgO INVENTORS AFTER 45mm. LAB. BEATING HOWARD P. ESPENMILLER 8 BY ADAM P. BRIDGE ATTORN EYS United States Patent 3,245,868 CONTINUOUS PROCESS FOR THE RECOVERY OF PAPER BROKE CONTAINING A WET STRENGTH RESIN Howard P. Espenmiller, Middletown, and Adam P.
Bridge, Franklin, Ohio, assignors to The Iflack Clawson Company, Hamilton, Ohio, a corporation of Ohio Filed Sept. 6, 1963, Ser. No. 307,084 3 Claims. (Cl. 162-4) This invention relates to broke recovery and more particularly to an improved apparatus and process for the continuous recovery of wet strength broke.
In recent years, more and more paper products have appeared which contain wet strength chemicals such as urea formaldehyde resins, melamine formaldehyde resins, and polya-mide resins, for example. These and other thermosetting resins are commonly employed to provide wet strength and generally operate to bind the fibers of the paper to one another as a supplementary binding action over that usually present in an ordinary sheet which is held together by the interleaving, intertwining or matting of the paper fibers.
Heretofore, recovery of wet strength broke was done on a batch basis using heat and chemicals and usually necessitating the removal of the broke from the area of the machine to a storage area. The broke may either be sold to a processor who recovers the pulp or it may be treated at the plant on a batch basis, and in some instances it is burned because this may be the most inexpensive manner of disposing of it. In any event, considerable manipulation and handling, as well as considerable storage area is required for the broke, all of which presents economical as well as practical problems.
Generally wet strength papers are made on high speed machines with daily tonnage rates in the order of 25 to 500 tons or more of wet strength paper or board per day. These relatively large machines usually operate on an automatic basis thus indicating the desirability of continuously or automatically processing the broke which accumulates during operation of the machine. This broke would include the trim and slabs from the finished reels, culls or off-quality rolls which are cut up by a roll cutting machine, stored baled trimmings from the finishing operation, as well as the broke which accumulates during abnormal operation of the paper machine such as wet end trouble, breaks, and so forth.
Accordingly, it is a primary object of the present invention to provide improved apparatus and process for the continuous recovery of wet strength broke whereby substantially uniform operating conditions of broke recovery are maintained thereby providing substantially uniform quality stock for admixture with the stock at the wet end of the paper machine.
This and other objects have been achieved in accordance with the present invention by an apparatus which continuously receives the broke from the machine and shreds it or reduces it to a size and consistency so that it may be transported to a dewatering assembly which increases the consistency of the stock. Following the dewatering operation, the stock is continuously fed to a cooking station where it is treated with chemicals, prefererably :at elevated temperature and pressure to bring about degradation or breakdown of the wet strength resin.
Following the chemical cooking operation, the stock is discharged and treated to bring about final defibering, and then is washed and thickened and returned to the wet end of the machine, either as slurry or wet lap.
Another object of the present invention is the provision of an improved apparatus for the continuous recovery of wet strength broke including a shredder or pulper to receive the broke from the machine and reduce it to an aqueous slurry, a continuous digester for treating the shredded broke to separate the wet strength resin from the fibers, and an appaartus for difibering the digested pulp mass to recover the useable fiber which is then fed to the broke chest for reuse on the machine.
A further object of the present invention is an improved method for the continuous recovery of wet strength broke whereby the broke is reduced to :a size small enough to allow transportation to a continuous digester where it is treated to hydrolyze or bring about solution of the thermosetting resin, and continuously defibering the cooked pulp mass followed by washing to remove the dissolved resins and cooking chemicals to provide substantially uniform quality stock for admixture with the stock at the wet end of the machine.
Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.
In the drawings:
FIG. 1 is a somewhat diagrammatic view of a continuous broke recovery system in accordance with the present invention;
. FIG. 2 is a diagramamtic view similar to FIG. 1 showmg the sequence of steps involved in a continuous broke recovery in accordance with the present invention;
FIGS. 3-5 are views showing modifications of the system illustrated in FIGS. 1 and 2; and
FIG. 6 is a chart showing comparative data of pulp recovered on a batch basis and pulp recovered by the contmuous system of the present invention.
Referring to the drawings, which illustrate preferred embodiments of the present invention, FIG. 1 shows schematically a portion of the dryer section of a paper machine including dryer sections 10, 12 and 14 with a breaker stack 16 located between dryers 10 and 12, :and a size press 18 located between dryer sections 12 and 14. The paper leaving the last dryer 14 passes through a calender stack 20 and from there to a wind-up reel 22. Generally, it breaks are to occur on the dryer end of the machine, they occur either at the breaker stack 16, the size press 18, or the calender stack 20, resulting in the accumulation of a substantial amount of paper in a very short time.
The continuous recovery system in accordance with the present invention is preferably associated with the dry end of the machine and includes pulpers 23, 24 and 25 located below the level of the paper machine to rece1ve the broke from the breaker stack, size press, and calender stack, respectively. Satisfactory results have been achieved using for pulper 25, a pulper available from The Black Clawson Company under the trademark hydrapulper and described more fully in US. Patent 3,073,535 issued January 15, 1953, and assigned to the same assignee. Also, a pulper available from The Black Clawson Company under the trademark Sydrapulper" has proven satisfactory for use as pulpers 23 and 24.
As the broke enters the pulpers, it is mixed with water and shredded to a chip size of about inch square to rectangular pieces about /2 inch wide and 1 inch long, and having a consistency of the pulp such that it can be continuously fed to a chip chest 27 which acts as a storage station. From the chip chest 27, the pulp mass is introduced into an inclined screw operated dewatering tube 30 wherein pulp consistency is increased and from the dewatering tube, the pulp passes through a screw feeder 31 which again increases the pulp consistency While also preferably reducing the chip size to about one third its original size. The dewatering tube and the screw feeder are conventional items, as is well known in the art.
The discharge end of the screw feeder is connected to the horizontally arranged continuous digester tube 33 which includes a variable speed screw flight and which forms a cooking station in which the chip mass is treated to break down the fiber-to-fiber bond and thus permit separation of the fibers. Satisfactory results have been achieved in accordance with the present invention with the use of a cooking or digesting tube available from The Black Clawson Company under the name Continuous Pandia Digester, and described more fully in the Beveridge et al. Patent 2,323,194 issued June 29, 1943.
The chip mass continuously extracted from the screw feeder 31 is treated in the cooking station by a minimum mechanical treatment, and is simultaneously chemically treated to react with the resin on the fiber, as by hydrolysis with chemicals, both treatments being conducted without significant degradation of fiber quality. The continuous digester of the type previously described operates satisfactorily to bring about both types of treatment simultaneously. The cooking station also includes means to supply steam, chemicals and Water for treating the paper chip mass during the digesting operation. Also chemicals can be added after cooking to neutralize the pulp mass if this is desired or necessary.
The nature and/ or degree of chemical treatment varies somewhat depending on the nature and content of the wet strength resin, and wet strength of the particular type paper itself which is directly related to the dry strength of the sheet. Certain operations in the preparation of the stock such as Jordaning and refining assists in developing or increasing dry strength of the sheet, and the increased dry strength tends to increase the wet strength of the sheet for any given resin content. For example, a kraft bag sheet has more dry strength than a tissue sheet, and accordingly, the kraft bag sheet has a greater wet strength based on an equivalent usage of a given wet strength resin. Moreover, the retention of the wet strength resin by the fiber of the pulp is proportional to the purity of the fiber, i.e., the permaganate number; and ground wood, which has a relatively high permanganate number has a low retention of wet strength resin when compared to rag pulps which have a low permagnate number.
Based on these considerations, as well as the nature and percentage of wet strength resin, the conditions and time of cooking within the cooking station are coordinatedand correlated so as to bring about partial separation of the fiber by mechanical action with a simultaneous chemical action in which the wet strength resin is brought to an aqueous soluble condition, for example, by the use of an acidic material. Preferably the chemical action is one of hydrolysis in which the functional groups of the resin are hydrolyzed at a predetermined pH to bring about a conversion of the wet strength resin to a soluble polymer, and thus the resin bond is removed from the pulp fiber thereby allowing a substantially complete separation of the fibers constituting the pulp mass.
From the cooking tube or vessel 33, the digested or cooked pulp mass is discharged through discharger apparatus 35 and treated with water while passing through a pressure reducing cyclone 36 lowering the pump mass in consistency and temperature and venting steam to the atmosphere, thus bringing the cooked pulp mass to atmospheric pressure and discharging the pulp mass from the apex of cyclone 36 into blow chest 37. The pulp mass is then pumped with consistency regulation, preferably 4% to 6% through rotary refiner 39 which substantially produces complete defibering of the pulp mass. The defibered pulp mass discharges into a dilution chest 40 where the consistency is lowered, preferably to 0.5% to 1.0% consistency. From the dilution chest 40, the stock is forced by pump to a cyclone 42 wherein high specific gravity foreign materials are removed. Since the stock leaving the cyclone 42 may contain lumps or bundles of fibers, it is preferably treated with a rotary screen 43 which brings about particle size separation in a single pass of the pulp therethrough with a minimum possibility that any lumps or bundles can slip through without being rejected. Satisfactory results have been achieved utilizing a rotary screen available from The Black Clawson Company under the trademark Selectifier Screen, described more fully in the Martindale Re. 24,677 patent issued July 28, 1959, and assigned to the same assignee.
From the screen the stock is washed by vat type washers 44 to remove cooking chemicals and dissolved resin if this is desired or necessary in order to reuse the recovered fiber. It is then returned to the broke chest where it is blended with the normal stock furnish being fed to the wet end of the paper machine according to standard paper making practices.
The dewatering tube 30, the screw feeder 31, and the cooking tube 33 are preferably equipped with variable speed drives so that the retention time therein may be varied to provide a maximum output during the break period, while standard consistency regulators and flow control valves for stock and water are provided so that the system may be operated automatically, if desired. These assemblies are individually well known in the art.
The sequence of operations carried out on the broke may be understood with reference to FIG. 2, wherein like reference numerals have been used wherever possible. In the first stage in which the broke is shredded by a shredder or pulper 25, the broke may be treated with water, and if necessary some chemicals may be added to assist in the mechanical reduction in the size of the broke to such a particle size that it can be transported or pumped to the various other stations for subsequent treatment. With a pulper 25 of the type previously described, the stock may have a consistency of between about 1% and about 10% with a preferred consistency being in the range of about 4%. From the pulper 25 the stock enters the chip chest 27 which is preferably large enough to take about 30% of the daily capacity of the recovery system. Assuming the system is capable of handling 30 tons of broke per day, chip chest 27 would have a capacity of about 18,000 pounds or approximately between about 2% to 5% of the maximum output of the paper machine. Here, the mass of paper chips may be stored, and the consistency thereof adjusted, if necessary, to provide a uniform consistency of between about 3 /2% to 4 /2%.
After shredding, the stock is dewatered by treatment with the dewatering tube 30 which increases consistency to about 10% and the screw feeder 31 so that the stock consistency is increased to between 35% and about 65%, and preferably in the range of 50%. By means of a dewatering tube 30, the stock may be dewatered by free drainage to a consistency of about 10%, and the consistency increased by the screw feeder 31 to between about 35% to about 65%. This increase in consistency is desirable in the event that pressure and elevated temperatures are to be used during the cooking cycle since it operates to prevent blow back or back flow of the chip mass from the cooking station in the event that cooking is conducted under pressure.
Following the dewatering operation the stock consistency is further increased to between 35% and 65% and preferably in the range of about 50% and treated at a predetermined pH to bring about partial separation of the fibers by mechanical working, as well as separation of the resin from the fibers by chemical cooking, In the fourth stage of treatment, the stock discharged from the cooking station is passed through cyclone 36 to a storage chest 40 where the consistency is adjusted for the subsequent defibering operation which forms the fifth stage. In the sixth stage, stock is washed clean of cooking chemicals and dissolved resin, thickened and returned to the system which feeds the wet end of the machine.
In operation, assuming there is no break on the paper machine, the trim and slabs from the finished reels may be shredded by pulper 25 which continuously feeds chip chest 27. Off quality rolls or culls which have been cut in half by a roll splitting mechanism may be shredded by either one of pulpers 23- and 24 and similarly fed to the chip chest 27. In this no break condition stored baled trimmings may also be reprocessed. The stock from the pulpers 23, 24 and 25 flows through a consistency regulator (not shown) to the chip chest 27 and into the other apparatus forming the continuous recovery system, and during this time the rate of production is relatively low.
In the event of a break, the entire output of the machine must now be handled by the recovery system, and as the paper enters the pulpers rapidly, the amount of water fed to the pulpers is increased. As the shredded stock is extracted, the consistency regulator between the pulpers and the chips chest maintains the proper stock density to the chip chest, wherein the level begins to rise. The sudden increase in the level in the chip chest indicates that the full production of the paper machine is being handled by the recovery system, and provides an indication by an alarm system so the operator may know that there is in fact a break on the machine. By suitable adjustments, the rates of flow of liquid, consistency of the stock, and retention time within the various tubes may be controlled. For example, the number of revolutions per minute of the screw feeder is increased, the valve on the discharger opens up slightly to take care of the extra capacity while still maintaining the proper cooking conditions within the digester 33, and the entire sequence of operations is increased to maintain constant and uniform treatment of the fiber during the maximum production period.
Once the break is over, this will be indicated by a reduction in the level of stock within the chip chest, and the entire system may be slowed down to the capacity it had during the no break condition of the paper machine.
One of the features of the present invention is the provision of a continuous system for treating the paper chip mass to eifect mechanical separation of the fibers along with chemical separation of the wet strength chemical which tends to bind the fibers to one another. The chemical treatment is provided by the horizontally arranged digester tube previously referred to and generally includes a hydrolysis of the wet strength resin at an elevated temperature and pressure. For example, in the case of urea formaldehyde resins the pulping may be accomplished at a pH from about 3.0 to 4.5 and at a temperature in the range of 150 F. to 212 F., and with this particular type resin it is preferred that the pulp con sistency be relatively high for example above about In the case of papers using a melamine or urea formaldehyde resin, the resin may be hydrolyzed at a pH of between about 2.0 to 4.5, obtained with sulphuric acid, at a pressure of about psi. and a temperature of about 300 F. for a period of time sufficient to bring about chemical degradation of the resin. This may require from about 5 to 20 minutes depending on the nature of the base stock and the percentage of resin utilized.
One of the more diflicult wet strength resins to handle are the polyamide series, and best results are obtained at a pulping consistency of about 8% at a pH of about 11.5 and at a temperature of about 120 F. using caustic soda in an amount equal to about 4% based on the oven dry weight of the fiber. Pulping of this type resin may also be accomplished in the pH range of 10 to 11.5 and at temperatures ranging from 100 F. to 200 F.
Since the chemical action is primarily one of hydrolysis, any acid or acid generating material may be utilized, and as a general rule five pounds of concentrated acid per ton of air dried fiber provide satisfactory results. The amount of chemical used for hydrolysis is dependent upon the percentage of wet strength chemical present in the wet strength broke and the type and grade of broke being processed.
It is possible in accordance with the present invention to utilize a somewhat different form of apparatus and diiferent cooking conditions to bring about separation of the wet strength resin and provide a mass of fibers which have not been degraded to the point where they are no longer useful. Referring specifically to FIG. 3, the first stage is basically the same as the first stage previously described. However, the second stage or dewatering stage is now accomplished by a dewatering screw with a high density storage chest 56 and a horizontally arranged digester tube 58 receiving the pulp mass for the simultaneous mechanical and chemical separation of the fiber. In this particular modification, cooking is carried out under atmospheric conditions but at an elevated temperature (212 F.) Approximately 60% of the dwell time of the cooking stage would be in the high consistency storage chest 56 with the remaining 40% needed to hydrolyze the wet strength resin being in the cooking tube. The remaining portion of the system is similar to that described previously.
In another modification shown in FIG. 4, which is somewhat similar to that shown in FIG. 3, cooking is carried out in two pulpers 60 and 62 which are arranged in series, pulper 60 reducing the shredded mass of paper chips to a somewhat smaller size than provided by shredder or pulper 25, and pulper 62 operating to reduce the size of the chip even further. Pulper 62 then discharges alternately into cook chests 63 and 64, with both of the chests emptying into a common storage chest 65. Approximately 30% of the dwell time would be in pulper 60, 30% in pulper 62 and 40% in the cook chests. The subsequent stages of defibering and cleaning, screening, thickening or washing are the same as those previously described.
Referring now to FIG. 5, the chip mass from the dewatering screw 55 is received by a pulper 68 wherein the chip mass is treated with chemicals and hot water, and the extracted pulp is fed to a rotary mechanical screening assembly 70 whose operation is similar to that of the rotary screening assembly 43 described in connection with FIG. 1. The rejects from screen 70 may be fed back into the pulper 63 for further processing. From the screen 70 the extracted pulp is dewatered by dewatering screw 72 to thicken the stock by free drainage, and the pulp is alternately fed to cooking chests 73 and 74 to complete chemical separation of the wet strength stock and resin. In this system, approximately 30% of the dwell time would be in the pulper 68, 10% in the screen 70 and about 60% in the cooking chests 73 and 74, the pulp from the cook chests being pumped to storage chest 65. The fifth and sixth stages may be of the type previously described.
One of the important advantages of the continuous recovery system of the present invention may be under stood with reference to FIG. 6 which shows a comparison of pulp recovered on a continuous basis from a pilot plant and laboratory system pulp recovered on a batch basis, and pulp recovered on a continuous basis in a large scale installation. In addition .to the elimination of storage and handling of the broke, which in itself is a substantial advantage, the system in accordance with the present invention unexpectedly produces pulp which is far better than the pulp produced on a batch basis. Significant is the fact that the pulp from the continuous large scale installation was better in quality than either the batch or pilot plant system.
These unexpected and superior results of the continuous system may, in part, be attributable to the fact that the volume of material being handled at any one point in the system is substantially less than that handled on a batch basis, and thus more precise control of various conditions is possible. While the amount of pulp or raw material being handled at any one time by the continuous system may be less than the amount handled on a batch system, the total output of the continuous system is equal to, and may exceed the total output of a batch recovery system. Not only is the pulp from the continuous system of improved quality, but there is also the advantage of eliminating the storage and handling as previously described.
While the methods and forms of apparatus herein described constitute preferred embodiment of the invention, it is to be understood that the invention is not limited to these precise methods and forms of apparatus, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.
What is claimed is:
1. A method for the continuous recovery of paper broke from a paper machine wherein the paper has been treated with a wet strength thermosetting resin for imparting said strength characteristics thereto, comprising the steps of continuously shredding the broke received directly from said machine in the presence of water to provide a chip mass dispersed in water of consistency between 1% and 10%, continuously dewatering said chip mass to increase the consistency thereof to between 10% and 65%, adding chemical to hydrolyze the thermosetting resin for solubilizing a substantial portion thereof, said chemical being added in an amount to provide a pH of between 2.0 and 12.0, continuously cooking said chip mass at a consistency of between 4% and 65% at a temperature between 100 F. to 300 F. for a period of time suflicient to hydrolyze the thermosetting resin thereby forming a pulp mass wherein the resin is substantially completely separated from the pulp fibers, and thereafter continuously washing said pulp mass to remove the dissolved resins and the chemicals to provide a pulp mass free of thermosetting resin and chemical.
2. A method as set forth in claim 1 wherein said cooking chemical is added in an amount sufiicient to provide a pH of between about 10.0 and 11.5 for hydrolysis of said thermosetting resin.
3. A method as set forth in claim 1 wherein said cooking chemical is added in an amount sufficient to provide a pH of between 3.0 and 4.5 for hydrolysis of said thermosetting resin.
References Cited by the Examiner UNITED STATES PATENTS 2,394,273 2/ 1946 Thomas 1624 2,697,661 12/ 1954 Hollis 1624 2,872,313 2/1959 House et a1. 162-6 2,910,398 10/ 1959 Durant et a1 1624 2,977,274 3/1961 Hollis 1624 3,057,769 10/ 1962 Sandberg 1624 DONALL H. SYLVESTER, Primary Examiner. HOWARD R. CAINE, Examiner. S. L. BASHORE, Assistant Examiner.
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|U.S. Classification||162/7, 162/8, 162/191, 241/28, 162/264|