US3705086A - Coal carbonizing in trickling streams - Google Patents

Abstract

FINE-GRAINED COKE IS PRODUCED BY A CARBONIZATION OF FINE-GRAINED COAL BY A TREATMENT WITH A HOT GAS. THE COAL TRICKLES DOWN IN A COUNTERCURRENT TO THE RISING HOT GAS PAST FIXTURES WHICH RETARD THE FREE FALL AND OCCUPY UP TO 50% OF THE CROSS-SECTION OF THE FLOW. THE COAL MAY IN THAT WAY BE MADE SUITABLE FOR USE IN MAKING BRIQUETTES.

Description

Dec. 5, 1972 P. SCHMALFEDD ETAL 3,705,086

COAL CARBONIZING IN TRICKLING STREAMS Filed July 7 1970 2 Sheets-Sheet 1 Invenfor PAUL SCHMALFELD AND HELMUT HAHN W WW Dec. 5, 1972 SCHMALFEDD ETAL 3,705,086

COAL CARBONIZING IN TRICKLING STREAMS Filed July '7, 1970 2 Sheets-Sheet 2 In venlors;

PAUL SCHIIALl-"ELD AND HELZEUT HAHN United States l atent O 3,705,086 COAL CARBONIZING IN TRICKLING STREAMS Paul Schmalfeld, Bad Homburg, and Helmut Hahn,

Hanau, Germany, assignors to Metallgesellschaft Aktiengesellschaft, Frankfurt am Main, Germany Filed July 7, 1970, Ser. No. 52,900 Claims priority, application Germany, July 11, 1969, P 19 35 214.8 Int. Cl. C101) 47/20 US. Cl. 20134 8 Claims ABSTRACT OF THE DISCLOSURE Fine-grained coke is produced by a carbonization of fine-grained coal by a treatment with a hot gas. The coal trickles down in a countercurrent to the rising hot gas past fixtures which retard the free fall and occupy up to 50% of the cross-section of flow. The coal may in that way be made suitable for use in making briquettes.

BACKGROUND Briquetted coke for household use, e.g., house heating, and for metallurgical use can be produced by a briquetting or pelletizing of fine-grained coal, to which binders, such as pitch, waste sulfite liquor or the like may be added, if desired, and a subsequent coking of the briquettes or pellets. It is also known to briquette fine-grain coking coal in its plastic temperature range alone or with an addition of coke and/or another coal. When the tendency of the fine-grained coal to cake, swell or shrink is too strong, the coal must be leaned with coke before it is briquetted. On the other hand, the briquettes or pellets must be heated at a very low rate during the coking step in order to preserve the grain structure of the briquette. This requirement results in an unduly long coking time. The production of the coke used for leaning is an additional operation and in most cases requires a separated special plant. It is usually accomplished by a carbonization of fine-grained coal in a fluidized bed or in a process in which fine-grained coke is produced and used as a circulating heat carrier. -In these processes, the fine-grained coal is heated to the carbonization temperature within a few seconds and is devolatilized very fast. Many coals are loosened and expanded in structure by the rapid temperature rise so that the resulting coke has a high porosity. Such a highly porous coke has a strong leaning effect in a material to be briquetted and increases the binder requirement. Highly porous coke is desired as a leaning agent where large amounts of caking coal and/or binder, such as tar pitch, are available for making the mixture to be briquetted and should be leaned with a minimum amount of coke. If the coal to be converted into shaped coke has such a strong tendency to swell or shrink that the coal cannot be added as such to the mixture to be briquetted or pelletized, or can be added in the form of coal to such mixture only in a small amount, the coal must be converted into coke beforehand. The porosity of this coke should be minimized so that the coke requires only a minimum of caking coal or of tar pitch as a binder.

In view of previous experience it has been believed that a low-porosity coke will be obtained if the coal is heated slowly and the carbonization is carried out for 30200 minutes or more. Known means for carbonizing fine-grained coal at a low rate include, e.g., a vertical chamber kiln as well as a rotary kiln in which the coal is moved in a countercurrent to the heating gases flowing through the rotary kiln. Whereas these kilns permit a continuous temperature rise of the fine-grained coal during a long time, they are highly expensive and do not have a very high production capacity.

3,705,080 Patented Dec. 5, 1972 THE EINVENTION It has been found that low-porosity coke can be produced from fine-grained coal by a carbonizing process which does not take more than an hour, and may even take less than 30 minutes. It is sufficient to abandon the sudden temperature rise in the fluidized bed or in the process using a heat carrier and to effect the carbonization with a gradual temperature rise within about 0.5-5, or about 13 minutes, e.g., within 1 minute. This requirement can be fulfilled to a certain degree in a known process, in which fine-grained coal is entrained in hot gases flowing through a plurality of series-connected cyclone separators and the coal is carbonized while it is thus entrained. The coal which has been separated in one cyclone is entrained into and subsequently removed from a gas stream which is at a higher temperature so that the coal is carbonized in a plurality of steps, e.g., in 3 or 4 steps, each of which involves a temperature rise. Owing to the carbonization of the entrained coal and the treatment in cyclones, the coal is subjected to very high mechanical stresses so that the resulting coke contains a high proportion of dust.

The invention provides simpler means with which the carbonization can be carried out in a much larger number of steps and the continuity of the temperature 'rise is improved. In accordance with the invention, the finegrained coal to be carbonized is contacted with the hot heating gas; the gas is caused to rise in a reactor, e.g., a shaft furnace, and the coal is caused to trickle down in a countercurrent to the gas through fixtures which are permeable to the gas, such as screen plates.

The fine-grained coal trickles slowly from one plate to the next lower plate, remains on each plate for a short time of 5-20 seconds, and is continuously heated by the heating gases flowing in a countercurrent. The devolatilization is sufiiciently slow and suflicient time is available for the volatile constituents to emerge from the structure of the coal or coke without destroying the particles or without converting a melting or caking coal into a coke having large pores. The coal and heating gas may form on each plate a mixed phase which is similar to a fluidized bed, although this is not required. In most cases, it is more desirable to cause the fine-grained coal to trickle slowly through the plates and to fluidize the coal in a phase of very low density between the plates so that its free fall is retarded by the rising heating gas.

The invention relates to a process of producing finegrained coke by a distillation of fine-grained coal by a treatment with a hot gas, which may be circulated through the carbonizing reactor and a condensing system.

The process according to the invention is characterized in that the coal trickles down in a countercurrent to the rising hot gas past fixtures which retard the free fall and occupy up to about 50, preferably up to about 40% of the cross-section of flow (i.e., free area in fixture for flow of material at least about 50%, preferably at least about 60% The coal trickling down is in a highly loosened, fluidized state. During the known carbonization carried out in a shaft furnace with the aid of a scavenging gas, the hot scavenging gas rises in a descending body of packed lump coal or briquettes. In the carbonization carried out in a fluidized bed, the scavenging gas maintains the finegrained coal in a fluidized state, which is comparable to the state of a boiling liquid.

The coal to be used in the process according to the invention should neither be too coarse nor too fine so that the gas velocities which can be obtained in the column are sufiiciently high for a satisfactory heat transfer rate and carbonization rate, whereas the entrainment of the finest particles by the heating gas and the volatilized distillation products is minimized. The particle size can be 0.2- millimeters. A particle size range of 0.5-3 millimeters is preferred; although the particle size may be below and/or above that range in individual cases. By particle size is meant the smallest size of the opening in a square mesh screen that the particle will pass through.

The gas velocity is maintained between 1.0 meter and 5.0 meters per second, preferably between 1.5 and 3.0 meters per second. By gas velocity is meant the velocity at operating conditions based on the free cross-section of the furnace. Depending on the diameter of the furnace or reactor, the fixtures are spaced apart by a distance of 100-1000 millimeters, preferably 400-600 millimeters. It is preferable to use perforated or screen-like or net-like plates having apertures in a size of about 3-15 millimeters, preferably about 7-10 millimeters. The free area of each screen plate should be as large as possible and in any case at least about 50%, preferably at least about 60%, and better yet, about 6075% of the area of the plates or screens.

The countercurrent operation results in an efi'icient heat exchange so that the flow rate of the heating gas can be minimized and the temperature difference between the heating gas and the fine particles on each screen plate is small. The water equivalent, which is the product of the amount and specific heat of the coal or coke and of the heating gas, may be maintained approximately constant. The devolatilization takes place continuously as a result of a temperature rise of 100-1000 C. per minute, depending on the conditions which are set. It will be understood that the coarse particles trickle somewhat faster through the screen plates than the finer particles. Very fine particles will even be entrained by and exhausted with the heating gas. These facts impose limitations on the process and should be taken into account in selecting the particle size of the coal and the velocity of flow of the gas. It will be remarkable that even fine particles having a terminal velocity which is lower than the gas velocity will trickle down and descend in a fluidized state in the column because the laws which govern the movement of individual particles are not applicable to the relatively dense phase of falling particles.

Because the heating gas cools as it flows from the lowermost plate to the uppermost plate of the carbonization zone, its volume may be reduced so that the velocity of flow in the upper portion of the reactor is reduced. The reactor shell may taper upwardly to ensure that the velocity of fiow is about constant throughout the height of the carbonization zone. The reactor may be circular, square or rectangular in cross-section.

Combustion air may be added between individual aperture plates if afterburning is desirable to raise the temperature of the rising heating gases.

The number of plates in the carbonization zone may be 5-20. In most cases, 8-12 plates in the carbonization zone are sufiicient.

In one embodiment of the invention, a coke-cooling zone may be provided in the reactor below the carbonization zone. In this case the number of plates in the reactor is doubled. The still cold heating gas which is recirculated from the tar-condensing system is admitted to the reactor below the lowermost plate of the coke-cooling zone and is preheated by a heat exchange with the coke trickling down. Air is admixed to the gas between the uppermost plate of the coke-cooling zone and the lowermost plate of the carbonization zone. This air may be compressed and/or preheated and is admixed at such a rate that the resulting combustion of circulated gas derived from the coking raises the gas to the required inlet temperature.

The combination of the steps of cooling the coke and reheating the heating gas by a partial combustion between two adjacent plates of the carbonization zone enables a production of coke at any desired carbonization temperature and a discharge of the coke at any desired temperature.

Thus, in summary, the invention is concerned with a process of producing fine-grained coke involving carbonizing fine-grained coal, and provides the improvement of countercurrently contacting a hot carbonizing gas with the coal in a contacting zone having disposed therein at spaced intervals, for passage of the gas and coal therethrough, perforated fixtures. The fixtures have at least about 50% free flow area. The gas is passed upwardly through the contacting zone, and the coal is passed downwardly through said zone as a trickling stream. The free fall of the coal stream is retarded by the fixtures.

Two embodiments of the invention are shown diagrammatically and by way of example in FIGS. 1 and 2.

FIG. 1 is a flow scheme showing a plant for producing hot coke.

FIG. 2 is a flow scheme showing a plant for producing coke which has been cooled or moderately heated.

FIG. 1 shows a column 1 having plates 2. Ten screen plates are shown in the drawing by way of example. The fine-grained coal is charged to a distributor 18 in the upper portion of the column 1 from an intermediate bin 3 through a metering feeder 4 and trickles through successive screen plates 2 and flows as hot fine-grained coke through a rotary gate 5 out of the column.

The distributor 18 may be a simple conical cap or a screen plate, or a rotary distributor may be used. It will be sufiicient if the distributor 18 results in a coarse distribution over the cross-section of the reactor. A fine distribution is effected by the uppermost screen plate.

The heating gas is admitted to the lower portion of the column through a connecting pipe 6 at a temperature of, e.g,, 700 C. and flows upward through the screen plates and heats the fine-grained coal continuously as the gas cools. The heating gas is exhausted from the upper portion of the column through a connecting pipe 7 at a temperature of, e.g., 200 C., then flows through a cyclone 8, where the entrained fine dust is separated to a high degree, and is cooled in a condensing system 9, where tar, oil and water condensed from the gas are removed. The cooled gases derived from the coking are compressed in a blower 10, partly supplied through conduit 11 to a burner 12 in a combustion chamber 13 and partly conducted through conduit 14 and admixed to the hot combustion gas in the combustion chamber so as to obtain the desired temperature of, e.g., 700 C. The surplus part of the cooled gases is exhausted through conduit 15. The air required for the combustion of the gases derived from the coking is compressed in a blower 16 and is supplied to the burner 12 through conduit 17. This air may be preheated, if desired. Secondary air may be supplied through an annular manifold 19 and inlet pipes 20 extending into the column between two adjacent plates so that the heating gas can be reheated when this is desired. The carbonized coke which has been heated, e.g., to 650 C. leaves the column through the rotary gate 5.

The screen plates 2 have a large free area in excess of 60% and result only in a small pressure differential, which is not in excess of 10-30 millimeters water per screen plate. Hence, the process can be carried out under a lower pressure and requires blowers 10 and 16 which result only in a moderate pressure rise. If a fluidized bed were maintained on the plates, the pressure differential per screen plate would be -200 millimeters water so that blowers producing a correspondingly higher pressure rise would be required.

In the embodiment shown in FIG. 2, the carbonization of the fine-grained coal in an upper section 21 of the column is combined with the cooling of the resulting coke in a lower section 22 of the column by the circulating gases. Twelve screen plates 23 are shown in the upper section 21 and eight screen plates 24 are shown in the lower section 22 of the column. The cross-section of the column decreases in each section toward the cooler end thereof so that the gas velocity is maintained approximate- 1y constant in all parts of the column although the volume of the gas decreases as a result of the temperature drop.

The fine-grained coal to be carbonized is charged onto a charging plate 46 in the column section 21 from an intermediate bin 25 through a metering feeder 26 and trickles through the twelve screen plates 23 while being continuously heated by the rising heating gas flowing in a countercurrent to the coal. During this treatment, the finegrained coal is continuously devolatilized but this does not result in a formation of relatively large pores or in a bursting of coal particles as a result of a too rapid devolatilization. The finished coke is at a temperature of, e.g.,750" and subsequently trickles through the screen plates 24 of the column section 22 in a countercurrent to the gas which is supplied through conduit 40 and heated in contact with the coke. The cooled coke is discharged through a rotary gate 27.

The heating gases are cooled, e.g., to 180 C. and exhausted from the upper portion of the carbonizing section 21 of the column. The exhaust gases first flow through a cyclone 28, in which a major portion of the fine dust which has been entrained is removed. The gas then flows through, e.g., three cooling-scrubbing units 29, 30, 31. The condensates formed in the cooling-scrubbing units are circulated through the latter by pumps 32, 33, 34. Cooling is effected in the cooling-scrubbing units 29 and 30 by the evaporation of water condensed from the gas and/ or of condensate of the succeeding cooling stage. The circulating condensate of the last cooling-scrubbing unit 31 is re-cooled by coolers 35 supplied with air or water or by a combined cooling with air and water. The coolingscrubbing units 29, 30, 31 are succeeded by cyclones 36, 37 and 38 for collecting entrained droplets. The coolingscrubbing unit 30 may be replaced by an electrostatic precipitator. A recirculating blower 39 suitably precedes or succeeds the cooling-scrubbing unit 30 to circulate the gases through the column sections 22 and 21 and the cooling-scrubbing units 29, 30 and 31.

The gas which has been cooled to about 30 C. and has been purified is supplied at a suitable rate through a conduit 40 into the lower section 22 of the column, rises through the screen plates and cools the coke trickling down to below 100 C. whereas the gas is heated to, e.g., 650 C.

A space 41 which is free of fixtures is disposed between the column sections 22 and 21. From an annular manifold 42, combustion air is blown through a plurality of inlet pipes 43 into the space 41. This compressed air has been compressed in a blower 44. The combustion air is suitably preheated and should be supplied at such a rate that the gas rising from the lower column section 22 is heated from, e.g., 650 C. to, e.g., 800 C. and at this temperature enters the upper column section 21, where it heats and devolatilizes the coal which is trickling down. The coke from the upper section 21 of the column is at a temperature of, e.g., 750 C. as it enters the cooling zone in the lower section 22 of the column.

The surplus part of the gas is discharged from the cycle through conduit 45.

If the fine-grained coal to be carbonized contains only little water, e.g., less than it need not be dried before being charged to the column 1 or the column section 21. If the fine-grained coal contains more than water, it will suitably be dried before being fed to the column 1 or column section 21. This drying may be effected in known means.

The process illustrated in FIG. 1 will be recommendable if the coke is to be shaped in a hot state in a mixture with coking coal, e.g., in a hot-briquetting process at temperatures between 400 and 500 C. at which the caking coal is plastic. This process may also be used if the hot coke is to be admixed to cold, moist coal so that the heat of the coke is used to dry the coal and heat the mixture to a temperature of, e.g., 100 C.

The process illustrated in FIG. 2 will be of advantage if the coke is to be briquetted or pelletized at temperatures below C. alone or together with fine-grained coal with an addition of tar pitch, bitumen pitch, tar oil, spent sulfite liquor, water or the like. This process will also be recommendable if coke is to be used as a fuel for sintering plants and reducing kilns and as a leaning agent for coking plants or the like.

The coke produced in accordance with the invention is distinguished by a low porosity and a high particle strength. Its particle size distribution differs only slightly from that of the feed coal so that it is apparent that the carbonization has not resulted in an appreciable disintegration of particles.

The invention will now be explained in more detail with reference to the following examples.

Example 1 Referring to FIG. 1, heating gas at a temperature of 750 C. is supplied at a rate of 3000 standard cubic meters per hour through the conduit 6 into the lower portion of the circular column 1, which has an inside diameter of 1200 millimeters. The gas rises in the column, which contains 10 screen plates, which are spaced 350 millimeters apart. The openings in the screen plates have a size of 10 millimeters. As the screen plates consist of wire in a thickness of 2.5 millimeters, the free area of the screen plates Slightly caking, long-flaming gas coal having a particle size of 0.5-3 millimeters is supplied into the column 1 through the rotary feeder 4 at a rate of 4000 kilograms per hour and is charged onto the uppermost screen plate by means of the distributing cone 18. The coal passes in a freely fluidized state through the several screen plates in succession and during this movement is heated and carbonized by the rising heating gas flowing in a countercurrent to the coal. The coke formed at a rate of 2700 kilograms per hour is discharged at a temperature of 700 C. from the column 1 through the rotary gate 5.

The heating gas takes up the volatile constituents of the long-flaming gas coal and flows at a temperature of 220 C. through conduit 7 into the cyclone 8, where a major portion of the entrained dust is removed. The heating gas is cooled in the unit 9, where tar, oil and water removed by distillation are condensed and separated. The cooled gases, derived from the coking, at a temperature of 35 C., are compressed by the blower 10 and are reheated to 750 C. by being partly burnt in the combustion chamber 13. Surplus is discharged through conduit 15.

The long-flaming gas coal fed to the process had a particle size of 0.5-3 millimeters and an average particle diameter of 1.2 millimeters. The stepwise temperature rise and carbonization of the coal results only in a slight swelling so that the coke has an average particle diameter of 1.5 millimeters. A fast carbonization within a few seconds would have resulted in a coke having an average diameter of 2.0 millimeters. 12% tar by weight must be added to the coke formed by a stepwise temperature rise in order to produce a strong briquette. By comparison, 20% by Weight of tar pitch would have been required to bond coke formed by a fast carbonization.

Example 2 Referring to FIG. 2, forge coal having a particle size range of 0.04-4 millimeters is charged at a rate of 10 metric tons per hour through a feeder 26 to a combined carbonizing and cooling unit. The forge coal flows initially through the twelve screen plates of the carbonizing section 21 and the resulting coke then flows through the eight screen plates of the cooling section 22. Through the conduit 40, gas at a temperature of 30 C. is supplied at a rate of 5500 standard cubic meters per hour to the cooling section 22 and is circulated through the carbonizing section and the condensing system. The gas rises through the cooling section and the carbonizing section in succession. The gas is heated to about 700 C. in contact with the coke whereas the coke is thus cooled from 800 C. to 100 C. The inlet conduits 43 supply air at a rate of about 500 standard cubic meters per hour into the space 41. This air mixes with the heated gas. The resulting after-burning raises the temperature of the gas from 700 to 850 C.

At this initial temperature, the gas rises through the carbonizing section and heats the coal flowing in a countercurrent so that the coke is obtained at a temperature of 800 C. and the gas is cooled to 250 C. The gas carries the volatile constituents of the forge coal along into the cooling-scrubbing units 29, 30, and 31, where tar, oil and water removed by distillation are condensed and separated and the gas is cooled to 30 C. The gas is returned to the lower portion of the cooling section 22. Surplus gas is discharged through conduit 45.

The cooling section 22 has an inside diameter of 1500 millimeters in its lower portion and of 2100 millimeters in its upper portion. The carbonizing section has an inside diameter of 2300 millimeters in its lower portion and of 1700 millimeters in its upper portion. The two column sections have heights of 4.0 meters and 7.2 meters, respectively, and the entire column, inclusive of the combustion space, has a height of 13 meters. The plates are uniformly spaced 60 centimeters apart in the upper section and 50 centimeters apart in the lower section.

450 kilograms tar are recovered. The yield of coke is 84%. The resulting coke has a low porosity and consists of strong particles. Its particle size distribution differs only slightly from that of the feed coal. The coke contains 2.5% by Weight of volatile constituents.

The forge coal being processed has a particle size range of 0.4-4 millimeters and,an average particle diameter of 2.0 millimeters. Due to the stepwise temperature rise and carbonization and the moderate swelling capacity of the forge coal, the coke has substantially the same particle size distribution as the coal and the average particle diameter increases only to 2.1 millimeters. A fast carbonization of the forge coal would have resulted in an increase of the particle diameter to 2.5 millimeters. A fast carbonization involving higher mechanical stresses would have resulted in a bursting or abrasion of part of the particles. This would have resulted in smaller particle sizes but in a higher porosity. The amount of tar pitch which must be added to the coke to produce a strong briquette is by Weight in the case of the coke obtained by stepwise carbonization and in the case of the coke obtained by fast carbonization.

What is claimed is:

1. In a process of producing fine grained coke which comprises carbonizing fine grained coal, the improvement which comprises countercurrently contacting a hot carbonizing gas with the coal in a coking, contacting zone having disposed therein at spaced intervals, for passage of the gas and coal therethrough, stationary perforated fixtures having about 60-75% free flow area, passing the hot carbonizing gas upwardly at a velocity of 1.0-5.0 meters per second through said perforated fixtures in said zone and passing the coal downwardly through said perforated fixtures in said zone, said coal trickling slowly from one fixture to the next lower fixture, remaining on each fixture for 5-20 seconds and being continuously heated by said hot carbonizing gas, said coal having a particle size in the range of 0.2-5 millimeters, the free fall of which is retarded by said perforated fixtures and said upwardly passing gas, the pressure differential being not in excess of 10-20 millimeters water per fixture.

2. A process according to claim 1, wherein the fixtures are spaced apart by a distance of about 100-1000 millimeters.

3. A process according to claim 1, wherein the plates have apertures in a size of about 5-15 millimeters.

4. A process according to claim 1, wherein volatile carbonization products are formed during the contacting, and the step of adding heat to the carbonizing gas by at least partially burning said volatile carbonization products during passage of said gas through the contacting zone.

5. A process according to claim 1, and contacting as aforesaid the coke produced and cold combustible gas suitable for production of the coking gas by heating and at least partial combustion thereof, in a coke cooling zone, communicating with the coking zone for receiving the coke therefrom and delivering gas from the cooling zone thereto, thereby heating the combustible gas, and at least partially burning the hot combustible gas to provide said carbonizing gas.

6. A process according to claim 5, hot combustible gas being produced by said carbonizing, cooling at least part of the hot combustible gas from carbonizing, and using the resulting cooled combustible gas as the cold combustible gas used in said coke cooling zone.

7. A process according to claim 1, wherein the coke produced is mixed with coking coal at temperatures between 400 and 500 C.

8. A process according to claim 1, wherein the gas velocity is about 1.0-5.0 meters per second, the coal fed to the process has a particle size in the range of about 0.2-5 millimeters, the fixtures are spaced apart a distance of about 100-1000 millimeters, the free flow area of the plates is about -75%, and the plates have apertures of about 5-15 millimeters.

References Cited UNITED STATES PATENTS 1,955,025 4/1934 Sabel et a1. 201-36 3,318,798 5/1967 Kondis et al 20l37 X 2,635,949 4/1953 Fenske et al 202-121 X 2,832,725 4/ 1958 Scott 201-44 X FOREIGN PATENTS 574,892 1/ 1946 Great Britain 201-31 NORMAN YUDKOFF, Primary Examiner D. EDWARDS, Assistant Examiner US. Cl. X.R. 201-35, 36, 42

CERTIFICATE OF CORRECTION December 5, i972 UM D "s'lwms iwricNT 01mm;

Patent No. 3 705 Dated PAUL SCHMALFELD and HELMUT HAHN Invent0r(s) It is Certified that error appears in the above-identified patent. and that said Letters Patent are hereby corrected as shown below:

Col. 6, line 26, after "screen plates" insert -is 64%."

C01. 6, line 62, "om-4" should be --o.4-4-

Signed and sealed this 1st day v of May 1973- (SJJAL) z Attest: ED -MED M. FLETCHER, JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents

Images