US2874417A - Method of forming thermoplastic briquettes - Google Patents

Description

Feb. 24, 1959 K. F. L. RAMSE 2,874,417

METHOD OF FORMING THERMOPLASTIC BRIQUETTES Filed June 28, 1954 INVENTOR Inkfizm Fizz/r173 [agfldi Fans:

Z M6Zta-t 4 ATTORNEYS United Stat a e METHOD OF FORMING THERMOPLASTIC BRIQUETTES Kristian Fredrik Langfeldt Ramse, Nordstrand, Oslo, Norway, assignor to Elektrokemisk A/S, Radhusgaten, Oslo, Norway, a corporation of Norway Application June 28, 1954, Serial No. 439,830

1 Claim. '(Cl.1855) Continuous electrodes of the so-called soderbergtype utilize a material which is known in the trade as Paste. The paste is produced from small pieces or particles 'of calcined anthracite or petrol coke or other carbonaceous material which is held together withpitch or tar pitch serving as a bonding agent. The pitch ordinarily em ployed has a melting point of between 40 C. and 80 C. so that the paste is solid at normal temperatures. Ordinarily the paste, is produced in mixing machine'sLat a temperature of between 100 C. and 200 C. and the liquid plastic paste is cast into rectangular blocks weighing about 65 pounds for transport to the consumers.

It is not usually satisfactory to add the large blocks directly to the top of electrode as there is always a risk of the paste not being sufficiently fluid to fill' all the spaces. In most instances the blocks are preheated and melted before being added to the electrode or are crushed and added cold. Either of these expedients demands an added operation involving expense.

An ideal form for the electrode paste is to supply it as briquettes-for example, ovals weighing approximately 2 ounces each. It has been found, however, that attempts to form briquettes from the paste by the ordinary methods are not successful, for if the warm paste is handled in the usual machine it will stick to the rolls and the briquettes will be split.

I have discovered that it is possible to form briquettes from this paste if the paste is warmed so that the binder with a film of water. The rolls can advantageously be' cooled internally and also may be cooled externally by having a spray of waterplay upon the portion of their surface out of contact with the paste, or the rolls may dip into a container of water. It is important that the surfaces of the rolls must have a temperature appreciably below the softening point of the binding agent so that when the paste is in contact with the rolls, it will tend to form a shell over its contacting surface. In his connection it may be noted that in usual electrode pastes for use in smelting furnaces a binder with a softening point of about 50 C. is employed, whereas the electrode paste ordinarily employed for electrolytic cells as, for example,

in aluminum production, has a higher softening pointas, for example, 80 C. In general, the higher the melting point of the paste, the more readily it will solidify and therefore the more rapid will be the permissible op eration of the machine.

When the briquettes leave the forming machine, they will still be hot and should be cooled promptly and solidified so that they will keep their shape. This can advantageously be done by dropping them into a liquid cooling medium such as a tank of water which may be provided with a submerged conveyor which can carry the briquettes off from the discharge point to storage.

While the primary use of this process is in connection with electrode pastes, it may also be used with any other type of paste where a water-insoluble thermoplastic binder is employed that has a softening point such that it is solid at normal temperatures but softens at elevated temperatures ranging up to a temperature in the order of about C.

The operation of my process can readily be understood by reference to the accompanying drawings, in which-- Fig. 1 is a sectional view through a device for carrying out the invention, and

Fig. 2 is another sectional view on line 22 of Fig. 1.

In these drawings 10 and 12 are a pair of cylindrical forming-rolls with egg-shaped cavities in their surfaces. These rolls rotate with the same speed but in opposite directions as indicated by the arrows in Fig. 1. These rolls should be made of material of high conductivity; for example, brass. They are here shown as internally cooled by water sprayed on their interiors from the hol- I pitchwill contact them.

Liquid pastefrom the usual mixingmachine is filled into a container 22 above the rolls 10 and 12. This container is here shown as having slanting walls 24 and the rolls 10 and 12 form the bottom of the container. The walls 24 are provided with jacket spaces 26 for passage of a heating medium such as hot oil and the Walls are well insulated, as by insulation 28.

The forming-rolls 10 and 12 are rotated relatively slowly toward each other and during this slow revolution a shell of solidified paste is formed on the surface of each of the rolls. This shell increases in thickness during the travel of the rolls from the outer walls 24 to the center line where they meet. The travelling time from the outer walls to the meeting line determines the thickness of the shell to be formed for a good type of paste at a given temperature. In any event, the shell should be of a thickness less than the entire amount of paste contained in the recesses of the rolls so that the central material will be soft and cause the two halves of the briquettes to stick together when the rolls meet.

It is advisable that the briquettes be chilled rapidly after they drop from the rolls, and accordingly a tank 30 is provided containing water into which the formed briquettes drop. Tank 30 is preferably provided with a conveyor screen 32 which will carry the briquettes off to any desired storage point.

Further details of operation may be gathered from the following illustrative examples:

Example 1 Paste for use in an aluminum furnace was made up of 70% calcined petrol coke and 30% pitch with a softening point of 80 C. This paste was formed at a temperature of approximately C. (between about 150 C. and C.) and was dumped into the container 22 at approximately that temperature. Rolls 10 and 12 had a diameter of about 70 cm. (between 50 and 80 cm.) and it was found that satisfactory briquettes were obtained when the rolls were operated so that it took about 2 seconds for a point on a roll to move from a side wall 24 to the meeting point between the rolls. In the example shown this involved about 120 of rotation. A pair of rolls that was 25 cm. wide and 70 cm. in diameter, rotated at about 10 R. P. M., gave a production of about 5 tons per hour of the briquettes under the conditions of this example.

Example 2 minum oxide and 30% of piteh with a softening point of 70 C. In this case a roll speed corresponding to a cooling agent. This, of course, is the cheapest and mo st efiicient for most purposes but other cooling fluids may be employed. For example, if pitch with a relatively low melting point is employed (6. g. below 40 C.) brine may be used for cooling which may be maintained at a temperature below 0 C.

It is understood that the examples described are given only by way of illustration and that the process may be applied to many diiferent types of paste. I

7 What I claim is:

-The process of forming briquettes from a 'fiowable liquid paste comprising solid particles mixed witha binder of the type which is solid at normal temperatures and a fiowable liquid at temperatures in excess of 80 C..which comprises mixing the solid particles with the binder at elevated temperature above the melting point of the hinder to form a flowable paste, feeding said flowable paste unde the influenc o r r vi y witho p y g y. s stantial pressure thereto into recesses positioned in the surface of each of a pair of cooperating forming rolls, continuously cooling said rolls internally and cooling said rolls externally in advance of the position where the paste is introduced into said recesses so as to maintain the surface temperature of the forming rolls below that at which the binder s olidifiesso that a solid shell will form on that portion of the exterior of the flowable paste in contact with the surfaces of the recesses of each roll, maintaining the fiowable paste in the recesses until a solid shell is formed-around that portion of the flowable paste in contact with the rolls, bringing a recess in each roll together after the shells have formed but before the liquid paste in the center of the recesses has solidified to establish physical contact between the shells and liquid paste in the center thereof, maintaining physical contact between the said two recesses until the fiowable liquidinterior bonds the two shells together to form a briquette and then discharging the formed briquettes into a coolingrnedium to solidify them throughout.

References Cited in the file of this patent UNITED. STATES PATENTS

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