Methods and apparatus for cooling mixes such as carbonaceous particle-pitch masses being blended

United States Patent

<td> [72]</td><td> Inventor</td><td> Dennis A. Wheeler</td>

<td></td><td></td><td> Saginaw, Mich.</td>

<td> [21]</td><td> Appl. No.</td><td> 783,296</td>

<td> [22]</td><td> Filed</td><td> Dec. 12,1968</td>

<td> [45]</td><td> Patented</td><td> Mar. 16,1971</td>

<td> [73]</td><td> Assignee</td><td> Baker Perkins, Inc.</td>

<td></td><td></td><td> Saginaw, Mich.</td>

[54] METHODS AND APPARATUS FOR COOLING MIXES SUCH AS CARBONACEOUS PARTICLEPITCH MASSES BEING BLENDED

Claims, 7 Drawing Figs.

[52]

[51]

[50] u-s-ci........................................................ 165/1,

165/87,165/120,259/9 “^tci......................................................... F28f5/06

Field of Search............................................ 165/1,2

120,109, 87, 88; 259/9, 10 (IHC) ____________ UH 3,570,588

[56] References Cited

UNITED STATES PATENTS 3,163,403 12/1964 Engels.......................... 259/9

Primary Examiner—Charles Sukalo Attorney—Learman, Learman &amp; McCulloch

ABSTRACT: Methods and apparatus for intimately blending a mass such as a carbonaceous particle-pitch mass prior to compacting the mass and forming it into products wherein: the mass is forwarded while it is being intimately blended at an elevated temperature in a continuous mixer, a coolant is then supplied to cool the mix, and the mix is then further blended and mixed prior to discharge. In the form of the invention illustrated a vaporizable liquid coolant is added to the mass being blended and the mass is thereafter forwarded and mixed to blend the coolant with the mass, the mass then being introduced to a region of lower pressure wherein the liquid coolant vaporizes and further reduces the temperature of the mass uniformly, and while the mass is still being mixed.

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Patented March 16, 1971

3,570,588

Sheets-Sheet 1

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Patented March 16, 1971

3,570,588

Sheets-Sheet 2

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INVENTOR DENNIS A. WHEELER

BY '/CulLdi

3,570,588

METHODS AND APPARATUS FOR COOLING MIXES SUCH AS CARBONACEOUS PARTICLE-PITCH MASSES BEING BLENDED

One of the prime objects of the invention is to provide improved apparatus and methods for cooling masses which are being mixed or blended at somewhat elevated temperatures, and which require cooling before they are in condition for further processing operations. The invention is particularly suited to systems for fabricating products such as electrodes for use in aluminum ore reduction furnaces. In the carbon and graphite electrode fabrication industry, carbonaceous particles such as petroleum coke particles and coal tar pitch are blended in a mixing machine which heats them from room temperature to a temperature above the pitch-melting temperature, while blending them into a homogenous mass. Prior art processes for treating such masses, which are conventionally generally blended in batch mixers, have sought to cool the mass, after it is released from the mixer, from the blending temperature of around 165° C. to a lower temperature at which the mix may be compacted to produce a so-called “green” cakes which are then subsequently baked in graphitizing furnaces and machined to form the electrodes or other products desired. Forced air-cooling equipment has been utilized and the pitch fumes produced during the mixing operation have been exhausted to scrubbers. In batch installations, mixing has had to be prolonged beyond the time required for proper blending of the mass to reduce the low boiling pitches and tars in the mass. Further, the free-flowing granulated paste obtained in a continuous blending system which has been used has tended to agglomerate into lumps in the aircooling phase, which have to then be crushed prior to compacting the blended mass in high-pressure presses.

One of the prime objects of the present invention is to utilize a different approach wherein cooling is achieved during the mixing and blending operation so that the product, at discharge, is substantially at a proper temperature for compacting and need not be force air cooled prior to moving to the compacting presses.

Another object of the invention is to provide a process and apparatus for blending and cooling masses which operate substantially as efficiently as conventional batch-mixing operations in terms of the removal of tar products, and require no higher quality and more expensive ingredients than do conventional batch-processing operations, while providing all the advantages of continuous processing systems.

Still another object of the invention is to provide an apparatus and method of mixing and cooling which processes material more efficiently and faster than is possible with available equipment and .processes, and still provides a product which meets requirements, and in fact is improved in some respects.

Other objects and advantages of the invention will be pointed out specifically or will become apparent from the following description when it is considered in conjunction with the appended claims and the accompanying drawings, in which:

FIG. 1 is a partly sectional side elevational view of a mixing or kneading machine incorporating the invention;

FIG. 2 is a transverse sectional view, taken on the line 2-2 of the FIG. 1;

FIG. 3 is an enlarged, fragmentary, transverse, sectional view, taken on the line 3-3 of FIG. 1, and illustrating the manner in which a vaporizable coolant liquid is added to the mixer contents;

FIG. 4 is a top plan view of the discharge end of the mixer or kneader housing only;

FIG. 5 is a perspective view of the discharge housing;

FIG. 6 is a slightly reduced, fragmentary, sectional elevational view illustrating the manner in which the mixer shaft is driven; and

FIG. 7 is a schematic view illustrating the control system for metering coolant in measured quantity to the mixer.

Referring now more particularly to the accompanying drawings wherein a preferred embodiment of the invention only is shown, a letter M generally refers to the mixing or blending machine of the invention which includes a jacketed tubular mixing or kneading barrel B having annular passages 9 and 9a for circulating barrel temperature maintaining fluids, dependent on the mixing operation to be performed. A mixer of this general character is disclosed in the present assignee’s U.S. Pat. No. 3,023,455, granted Mar. 6, 1962 to Herbert F. Geier and Henry F. Irving, but it is to be understood that the apparatus has been modified in the present instance to process different materials. As previously indicated, the apparatus which will now be described is particularly suited to mixing or blending coke and pitch fractions which are to be intimately mixed together under pressure and temperature conditions which provide for the desired partial devolatilization of the material. When discharged in accordance with the teachings of the present invention in the form of a granulated free-flowing paste in which the pitch is in liquid form, the mass is at a considerably lower temperature and may be pressed without further cooling procedures into products in presses which compact the so-called “green” material. Later the products are supplied to dehydrogenating or graphitizing furnaces dependent upon whether carbon or graphite electrodes are being produced, in which the electrodes are slowly baked at high temperatures.

It is to be understood that the material being continuously processed in the mixer to be described completely fills the mixer barrel and is under pressure therein. With the present apparatus, the metered coke and pitch fractions which are to be blended may be continuously processed instead of batch processed. To “wet” or liquefy the pitch and accomplish the partial devolatilization desired, such carbon particle-pitch particle mixes or masses must be heated typically to a temperature of about 160° — 165° C. in order to accomplish the blending desired wherein the pitch fraction melts and suitably coats the solid fractions.

In present batch processes, mixing may be continued for periods of up to 1 hour more than necessary to accomplish the desired blending wherein the pitch binds the coke particles for the purpose of reducing the quantity of low boiling pitches and tars present. Thereafter, conventional processes air-cool the mixture, whether continuously or batch mixed, to around 115° C. before compressing the mix in presses which produce the “green” products. To produce good quality “green” products, it is important that the feed temperature of the carbonaceous mass to the press be uniform throughout its mass, and the temperature level be kept constant between each press stroke. The air-cooling procedures presently practiced are not conducive to obtaining this uniformity of temperature throughout the mass.

The present invention incorporates a cooling concept which operates continuously to provide uniform temperatures throughout the mass discharged from the machine in a steady stream and permits the use of low-cost pitch products having low boiling tars, even though the mixing time is much shorter in the continuous process practiced in the mixer to be described than in a comparable batch-mix system. In other words, while it would seem than higher quality pitch products might be required in a continuous process, it has been determined that the process which will be described may be practiced with the same pitch products which are presently used in batch processes. As will become apparent, the present system provides a continuous mixing operation wherein a vaporizable coolant liquid is added in metered amounts to the mix and acts throughout the mass being mixed to provide the uniform cooling thereof. Prior to discharge of the mass, the liquid added substantially completely vaporizes or flashes off, just before it passes on to the presses for forming the “green” products, so that substantially none of the coolant remains in the “green” mix and later, during baking, creates any undesired porosity in the products formed. It is important to understand that the pressure maintained in the mixing barrel is such that the liquid added does not vaporize during the time the liquid is being mixed with the coke and pitch at the elevated blending tem

3,570,588 perature. The vaporization occurs when the product is just about ready to be discharged from the mixer in a portion of the mixer which is open to atmosphere and is accordingly at atmospheric pressure.

The present machine includes a feed hopper 10 leading into the chamber or bore 11 of the barrel, generally designated B, for delivering the ingredients to be mixed to the chamber 11. It is to be understood that the ingredients to be blended are fed in metered amounts to the hopper 10 which may have a powered feed screw of the type disclosed in U.S. Pat. No. 3,251,512, for example. Provided in the barrel B are circumferentially spaced, radially inwardly projecting teeth or lugs 12, preferably at 120° intervals, which cooperate with interrupted helical threads or blades 13 provided on a mixer shaft 14 in helical formation in a manner to achieve the intimate blending and kneading of the material introduced to the chamber 11. The threads or flight sections 13 are so pitched that if the shaft 14 is revolved in a clockwise direction as viewed from the right end of the machine (see the arrow a), the material will be moved forwardly from right to left, it being understood that shaft 14 is simultaneously reciprocated in timed relation with its rotation, and the disposition of teeth 12, so that during the stroke of the shaft 14 the teeth 12 in effect are passed through the spaces b between the interrupted blades 13. The blades which form the thread portions 13 are interrupted at predetermined intervals as noted to provide the desired interpassage of the teeth 12 through the spaces or gaps b.

As FIG. 6 indicates, the housing 15 of the mixer, rearwardly of the feed hopper 10, may be provided with a pair of fixed cam follower projections 16 which ride in cam tracks 17, provided in a pair of side-by-side cams 18, which are keyed on the rear end of mixer shaft 14. As the shaft 14 is revolved by a motor (not shown) through a suitable gear reduction unit (not shown), the rotary travel of the cams 18, which are fixed on shaft 14, causes the shaft 14 to have an axial reciprocating stroke. At its rear end, the shaft 14 is supported by bearings 21 which are mounted by the casing section 15. Fixed to the shaft 14 to revolve and reciprocate with it are slide bearing sleeves 20 and 21α, and a gear 22, keyed as at 22α to the rear slide bearing sleeve 21α, is connected to the gear reduction unit and drives the sleeve 21a and shaft 14 at the desired speed in terms of revolutions per minute. The gear reduction unit and motor may be housed in a suitable casing portion 23.

Near the front or discharge end of the barrel B a discharge housing section, generally designated 24, is provided in axial alignment with the barrel section B. The housing 24 includes a flange portion 25 which may be bolted or otherwise secured to the end wall of the barrel B in any acceptable fashion. The housing 24 also includes a curvilinear, U-shaped shaft-accommodating wall 26 having an elongated top opening 27, the wall 26 being provided with a bottom discharge opening 28 intermediate its length beneath the opening 27.

As FIG. 1 clearly indicates, the shaft 14 has a reduced diameter diameter portion 14α extending through the discharge housing 24 which is journaled at its front end in a front bearing designated 29, supported by a post 29α. Provided within the housing 24 is a shaft-accommodating sleeve portion 30, and it will be seen that a flight section 32 of opposite hand to the flights 13 is fixed to the shaft 14α and is of a size to engage the wall portion 30 with proper operating operating clearance only. The flight 32 operates to feed any material entering the sleeve 30 in a rearward direction back toward the discharge opening 28. Provided on the front end of the discharge housing 24 is a shaft seal assembly 33, as shown. The same flights 13 are provided on the shaft portion 14α, but only two teeth 12α coact with them as shown in FIG. 2.

Near the discharge end of the barrel B, one of the teeth 12 is formed in the manner indicated in FIG. 3 at 12', the shorter tooth 12' being provided with a through bore 34 communicating with a liquid coolant supplying hose 35. As FIG. 7 indicates, a conventional coolant injection pump 36 continuously supplies a coolant liquid such as water at a tap tempera ture which may be about 15° C. to the line 35 through a conventional flow-control valve 37, which may be pneumatically or electrically operated by a temperature-indicating controller 38, which is connected with a temperature sensor 39, provided at the discharge opening 28. Alternatively, the sensor 39 may be located downstream of the continuous mixer-cooler at 39' in the feed hopper of the press 42 which is supplied with material by the conveyor 41. The sensor 39' is also considered to sense the discharge temperature of the material since the temperature drop in the material traveling on conveyor 41 is slight and a constant value. The controller 38 may be the A/D proportional, reverse acting, indicating pneumatic controller (series 624) manufactured by the Bristol Company of Waterbury, Conn., U.S.A., which is a controller which utilizes air pressure in a connecting line 40 to operate the valve 37 and thereby meter the water supply for from pump 36. The valve 37 may be the M-H series 800 diaphragm motor valve manufactured by Honeywell, Inc. of Minneapolis, Minn., U.S.A. In the operation of the mixer, so long as the temperature bulb 39 (or 39') indicates that the paste is being discharged at the desired temperature, the flow control valve 37 will remain in a certain partly opened position, metering the required amount of water through the line 35 to the tooth 12'. However, should the sensor 39 (or 39 ') indicate that the discharge temperature of the blended past is too high, controller 38 will so indicate and operate to bleed enough air form the connecting line 40 to cause the valve 37 to open up slightly and admit more water through line 35 to the tooth 12'. Conversely, of course, when the temperature sensor 39 (or 39') indicates that the temperature of the material being discharged is lower than desired, which would cause undue adherence of the pitch to the jacketed walls of the mixer, the controller 38 will receive an electrical signal from the sensor bulb 39 (or 39') and admit enough additional air under pressure through the line 40 to close valve 37 and admit less coolant through the line 35 to the tooth 12'.

As has been indicated, in the operation of the mixer M in an electrode ingredient processing operation, measured amounts of petroleum coke and coal tar pitch are continuously a supplied to the hopper 10 which feeds them to the interior of the mixing chamber 11. The mixer shaft accepts the feed from hp hopper 10 and the flights compress the mass to generate an operating pressure in excess of the vapor pressure of the coolant at the corresponding product maximum temperature. Typically the mixer is operated so that that the mix is maintained under a pressure of approximately 100—150 p.s.i. as it proceeds from the right end of the mixer to the discharge housing 24. The shaft flights 13 and the screw flights 41 provide an effective pressure seal. A suitable heating fluid is circulated through the annular passage 9 to heat the ingredients to a temperature of approximately b 160° — 165° C., and as the shaft 14 is continuously reciprocated and rotated, the mixing and kneading action provided by the screw flight section 13 and teeth 12 achieves a thorough blending of the materials. The coal tar pitch softens or melts when a temperature of about 110° C. is reached but those hydrocarbons which normally would be removed as vapors at atmospheric pressure when the temperature of the pitch is increased to 16 165° C., remain in the the pitch at the pressure maintained in the mixer mass as the temperature of the mass is elevated to 160° — 165° C. By the time the temperature of 165° C. has been reached, the petroleum coke particles and pitch residues have been reached, the petroleum coke particles and pitch residues have been blended into a homogeneous mass in which the pitch acts as a binder to bind the coke particles. At this point, the mass is in a “paste” state and is easily advanced through the mixer from right to left in FIG. 1.

While for the sake of convenience in FIG. 1, the mixer M is shown as fragmented, it is to be understood that it will have a substantial overall length and the initial feeding zone and the mixing zone d will each be on the order of the length of the cooling zone which is designated by the letter c in FIG. 1. The flight 13 which receives the ingredients from the hopper 10 in

3,570,588 the initial feeding zone portion of the mixer will be a continuous or nongapped advancing helical flight in the preferred embodiment of the invention and there will be no teeth 12 located in the so-called feeding zone. In the following mixing zone d, the flights 13 with gaps b are used. It will also be noted that the annular, temperature-maintaining, circulating liquid passage 9 terminates at the cooling zone c. A similar liquid at a lesser temperature of about the desired discharge temperature of the mass, 110°—115° C., is circulated through the annular passage 9a surrounding the cooling zone c.

A suitable liquid coolant such as water at tap temperature, or liquid carbon dioxide, is continuously added through the bore 34 of tooth 12' in accordance with the operation of the temperature controller 38 which is connected to the temperature sensor 39 or 39' as described. Because the material being mixed completely fills the chamber 11 and is maintained under pressure the water added through tooth 12', which at atmospheric pressure would vaporize immediately when subjected to temperatures of 165° C., remains in liquid form and is uniformly intermixed with the mass as the mass proceeds through the cooling portion c of the mixer toward the discharge housing 24. The water or other coolant added at tooth 12' is a introduced under a pressure gauged to be higher than the vapor pressure of the coolant at 165° C. For instance, water may be introduced at a pressure of 85 p.s.i. The temperatures of the circulated fluids in passages 9 and 9a a are determined with the amount of heat produced by working of the material in the mixer taken into consideration so that the temperatures desired in the mixer are maintained.

During thorough intermixing of the liquid coolant with the mass as the mass proceeds through zone c of the mixer, the mass is, of course, cooled to some extent. At the time the mass reaches the discharge housing 24 and is subjected to atmospheric conditions, the liquid coolant uniformly dispersed throughout the mass immediately flashes off together with the tar distillates or pitch volatiles and drastically reduces the temperature of the mass. It is thought that about one-third of the heat is removed by the water in zone c and the remainder during during the flash off by evaporative cooling. Continued mixing and agitation is carried on by the flights 13 and the teeth 12α in the discharge portion 24 of the mixer so that all vapor is releases released and porosity will not be a problem in the “green” products which are formed from material discharged through the discharge opening 28. Mixing in discharge section 24 also insures uniform discharge temperatures throughout the mass discharged. At the discharge opening the temperature of the material will be in the neighborhood of 110°—115° C. which is the proper temperature for pressing the material into “green” cakes. In addition to the evaporative cooling effect achieved by the flash off, a steam distillation effect is achieved and desired volatiles flash off with the steam. The hot pitch fraction is steam distilled and tar volatiles are thereby very satisfactorily removed at the lower temperature of discharge to compensate for the fact that mix pressure in the mixer has inhibited devolatilization. Thus continuous mixing can be accomplished with the same inexpensive grade of raw materials as batch mixing.

EXAMPLE

A petroleum coke and coal tar pitch mixture a of about 15—20 parts by weight of pitch to 80—85 parts by weight of petroleum coke is supplied to the mixer hopper 10. The size of the coke and pitch particles may be 10-below 300 mesh. The pressure maintained in the mixing chamber 11 is in the neighborhood of 100 p.s.i. Approximately 3 percent —5 percent by weight of water relative to the weight of the mix is added continuously through the tooth 12'. The material discharged through discharge opening 28 has been reduced in temperature from 165° C. to 110°— 115° C.

It is to be understood that the drawings and descriptive matter are in all cases to be interpreted as merely illustrative of the principles of the invention rather that than as limiting the same in any way since it is contemplated that various changes may be made in the various elements to achieve like results without departing from the spirit of the invention or the scope of the appended claims.