US 3912580 A
A pair of horizontally-disposed water-cooled drums are mounted side-by-side so that the nip between the drums forms a reservoir for receiving molten smelt of the type which is derived from paper pulping processes. The nip gap is set between 0.005 and 0.025 inches, and drums are rotated downward through the reservoir while concurrently cooling the drums to reduce the localized temperature of the smelt at the nip to its freezing temperature and to remove the heat of fusion of the smelt. Accordingly a thin, brittle sheet of solidified smelt is passed between the drums and the sheet is free from the surface of either drum.
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Description (OCR text may contain errors)
United States Patent Casten Oct. 14, 1975 METHOD FOR SOLIDIFYING SNIELT  Inventor: James W. Casten, Mountain Brook, Primary Exammerl zobert Lmdsay Ala Attorney, Agent, or F zrm-Robert E. Krebs; Thomas S.
MacDonald  Assignee: Envirotech Corporation, Menlo Park, Calif.  ABSTRACT  Filed; O t, 23, 1973 A pair of horizontally-disposed water-cooled drums are mounted side-by-side so that the nip between the [2-1] Appl' 408989 drums forms a reservoir for receiving molten smelt of the type which is derived from paper pulping pro-  US. Cl. 162/30; 62/346; 264/175 CeSSeS- The nip gap s Set between 0.005 and 0.025  Int. Cl. D21C 11/00 inches, and r m are rota e ownward through the  Field of Search 162/29, 30, 239, 240; rvoir hile conc rrently cooling the drums to re- 62/346; 264/175 duce the localized temperature of the smelt at the nip to its freezing temperature and to remove the heat of  Referen Cit d fusion of the smelt. Accordingly a thin, brittle sheet of UNITED STATES PATENTS solidified smelt is passed between the drums and the t t 1,327,354 1 1920 Perry 264/175 x ghee free from he Surface of drum 2,590,544 3/1952 Kallok 264/ 175 X 3 Claims, 3 Drawing Figures U.S. Patent Oct. 14, 1975 METHOD FOR SOLIDIFYING SMELT BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an improved apparatus and method for transforming molten salts into solid fragmemts and, more particularly, this invention relates to an apparatus and method for cooling hot liquid smelt of the type which is derived from sodium-base or Kraft paper pulp liquors so as to form brittle, readily shattered sheets.
2. State of the Art In paper pulping processes, such as the Kraft process, it is common practice to burn the spent pulping liquor (i.e., dilute black liquor in a fluidized bed reactor, a furnace or an incinerator. The residual, concentrated molten inorganic salts in such furnaces are known in the industry as smelt and generally comprises the following salts: sodium sulfide (Na S), sodium sulfate (Na So sodium hydroxide (NaOH) and sodium carbonate Na CO Small percentages of Na SO and Na S O may also be present.
A typical procedure for handling such smelt in the paper pulping industry comprises dropping the molten smelt into a water bath which is held in a so-called dissolving tank. The resultant hot liquid solution in the dissolving tank is known as green liquor and it can be recycled or returned to the pulping process. Specifically, the green liquor usually is recausticized to white liquor" by converting the sodium carbonate fraction to sodium hydroxide; the white liquor is then re-used to digest raw materials, usually wood chips.
Molten smelt is generally at a temperature above I500 F. When it strikes the water in a dissolving tank, the result is a noisy, immediate and possibly explosive formation of steam and other vapors. Because of the extreme temperature difference between the water and the molten salt, a fraction of the water vapor may be disassociated into elemental hydrogen and oxygen. To rid the plant of such potentially explosive vapors, large blowers or fans are usually provided to sweep the vapors into a chimney or stack.
Nevertheless, explosions and related accidents may occur. Such accidents have, unfortunately, resulted in the loss of human lives. In fact, it is recognized that black liquor boilers and the associated dissolving tanks constitute on of the most dangerous areas in a paper pulp mill. Furthermore, the vapors from the dissolving tanks usually contain some caustic materials and sulfur gases and, as such, are caustic and maladorus.
To ameliorate such problems, it has been suggested to cool the smelt materials in a dry state. For example, it has been suggested to utilize modifications of devices which are known as drum flakers to cool the smelt. The operation of such drum flakers generally comprises applying a partially-cooled molten material in a thin layer to a revolving drum whereon the material adheres, solidifies, and is thence removed in flake form by a stationary knife. The stationary knife (or doctor blade) which exfoliates-the solidified smelt from the drum surface is also a feature of a proposed process which utilizes a drum and a roller. In that proposed process, hot smelt is fed onto the surface of the drum, is rolled by the roller to adhere the smelt to the drum and, finally, the solidified smelt at a temperature of about 120 F. is scraped by a doctor blade from the drum as a thin peel (about 0.6 0.8mm thick).
OBJECTS OF THE INVENTION There is provided, in accordance with this invention, a novel and improved method and apparatus for transforming molten smelt into solid fragments, where the smelt is of the type which is derived from a paper pulping process. The inventive method and apparatus not only overcomes many of the disadvantages of procedures proposed heretofore, but also provides significant advantages. Among the objects and advantages of the present invention:
A primary object is to provide an improved highcapacity apparatus and method for solidifying molten smelt from a pulping process, where the smelt generally comprises a mixture of sodium sulfide, sodium sulfate, and sodium carbonate;
A more specific object is to provide an apparatus and method for reducing such non-viscous molten smelt into solid fragments;
Another object is to form such molten smelt into brittle, readily fragmented sheets; and
Still another object is to provide an apparatus that can accommodate a relatively uneven flow of molten smelt by providing a reservoir to hold and accommodate abnormally high momentary flows, such as are characteristic of recovery boiler operations.
SUMMARY OF THE INVENTION The method according to the present invention generally comprises transforming pulping process smelt (which is comprised, for example, of sodium or potassium sulfide, sulfate, hydroxide and carbonate) into substantially brittle sheets by extrusion between cold drums. Such extrusion is accomplished by drawing molten smelt through the nip formed between two rotatably-mounted cooling drums, whereby the sheet which leaves the nip is free of both drums, i.e., non-adhering, and, therefore, need not be scraped or otherwise exfoliated from either drum. More specifically, the localized temperature of the smelt at the nip of the drums is lowered to the smelt freezing temperature and, also, the heat of fusion of the smelt is removed.
BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the present invention may be readily ascertained by reference to the following description and appended drawings, which are offered by way of illustration only and not in limitation of the invention, whose scope is defined by the appended claims and equivalents. In the drawings:
FIG. 1 is a side view of a device according to the invention;
FIG. 2 is an end view of the device of FIG. 1; and
FIG. 3 is a side view, partially schematic and drawn to an enlarged scale, of apparatus for use with the device of FIG. I and 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The device which is illustrated in FIGS. I and 2 generally comprises a pair of horizontally-disposed watercooled cylindrical drums 11 and 12 which are mounted side-by-side on a common base 15. The nip between the cylindrical surfaces forms an upper reservoir 17 into which molten smelt is supplied. The drums may be fabricated from various materials, including cast iron, steel, stainless steel or other castable or fabricated materials. The smelt is solidified by the cold drums and is passed through the nip as a thin sheet as the drums rotate in opposite senses downward through the smelt reservoir in the direction indicated by the curved arrows in FIG. 2.
The drums are mounted for rotation with axially directed trunnions 20 and 21 which are hollow and whose outer ends are supported on suitable journal means 25 and 26 which are fixed to the base 15. Conventional drive means, such as an electric motor 29, are operatively coupled to the trunnions to rotatably drive the drums at a preselected speed, say between eight and twenty revolutions per minute. The surface of the drums should be smooth. In some instances, the drum surfaces may be chrome plated but that is generally not necessary.
Means are also provided to deliver cooling liquid, such as water, to preferably but not necessarily fill the interior of the drums. In the illustrated arrangement, liquid inlet conduits 34 are connected, via rotary collars 35, to the outer ends of the hollow trunnions 21. Also connected to the rotary collars are liquid outlet conduits 37 which remove warmed liquid from the drums. Similar drums are used in the process field as dryers; in such devices, steam is circulated in the drying drums instead of cooling liquid as is used here. In the process according to this invention, the circulation of cooling liquid in the drums should be sufficient to reduce the localized temperature of the smelt at the nip to the smelt freezing temperature and to remove the heat of fusion of the smelt. In terms of the surface temperature of the drums, the circulation of the coolant is preferably sufficient to maintain the drum surface temperature below 300 F. although somewhat higher temperatures can be tolerated.
In the preferred embodiment, one of the rotating drums 12 is fixedly mounted with respect to the support frame and the other drum is movably mounted so that the minimum clearance at the nip between the drums can be selectively adjusted.
In the illustrated arrangement, the nip clearance is determined by a pair of pneumatic cylinders 41 connected to opposite ends of the drum 12. More specifically, the journals 26 at each end of the movable drum 12 are supported, respectively, by parallel plate members 38 whose lower ends are pivotably mounted at 39 to the supporting platform 15. The pneumatic cylinders 41 are pivotably linked between the respective plate members 38 and the support frame to urge the movable drum toward the fixed drum. The minimum clearance between the drums is determined by draw bolts 42 which are connected between the plates 38 and the frame 15. The pneumatic cylinders permit the nip clearance to expand to accommodate, for example, variations in drum speed or in the rate of feed of the smelt. When the cylinders are extended, the support plates are pivoted and the drum 12 is thereby moved toward the stationary drum 11 so far as the draw bolts 42 will allow. Other means could be utilized to urge the drums together and to maintain the operating clearance between the drums; for example, springs could be attached between the drums.
Experimentation has shown that the nip clearance between the drums is quite important. Generally speaking there must be enough space between the drums so that a thin sheet of smelt will pass therebetween but not so much space as to allow the molten liquid to flow freely and escape. In practice, the preferred spacing at the nip between the drums is in the range of 0.005 inches and 0.025 inches and the typical sheet thickness is between 0.005 and 0.025 inches.
It should be appreciated that the molten salts which are derived from the black liquor in paper pulping processes are very thin and of relatively low viscosity. Such salts typically melt in the l200- 1800 F. range and do not stick to a cool drum surface.
Black liquor smelt, as previously mentioned, generally comprises a mixture of sodium sulfide, sodium sulfate, and sodium carbonate. The relative percentages of these components can vary widely. A typical black liquor smelt may comprise, by weight, one-third Na one-sixth Na SO and one-half Na CO with the remainder being made up of other salts. It should be understood that the chemical composition of the smelt after solidification may differ from the presolidification composition because of oxidation and other reactions which occur during cooling.
To contain the molten feed in the nip area between the drums, a pair of dam plates 46 are mounted at each end of the drums and are in sliding contact with the drum heads. These dam plates may be fabricated, for example, of steel or cast iron to withstand the high temperatures of molten smelt. It usually is advisable to extend the darn plates downward past the nip so that the smelt sheets which leave the nip are guided downward.
In practice it may be desirable to mount light-duty, spring loaded metallic wipers 49 below the nip of the drums as a safety device to remove traces of solidified smelt which may adhere to the drum surfaces. It should be emphasized, however, that such wiper blades are optional and are significantly different from the doctor blades which are found on conventional drum flaker devices. In those conventional devices, the doctor blades act almost like chisels to exfoliate solidified smelt which has frozen to the drum surface, thereby to form flakes. In the present device, the wiper blades merely brush occasional bits of smelt or foreign materials from the drum surface.
The smelt which is processed according to this invention issues from the nip as a thin sheet or as a ribbon. Although the smelt is solidified, its temperature usually ranges from 600 to 800 F. Usually the sheets cool within a few feet after leaving the nip. The sheets themselves are quite brittle, and, as will be described hereinafter, it is relatively easy to shatter or fracture the sheets into small shards.
Illustrated in FIG. 3 is one configuration of a device for further cooling and carrying the sheets of smelt away from the drums. That device generally comprises a covered, elongated slide or chute 51 which extends at an incline from beneath the nip of the drums. The floor 53 of the chute is perforated so that air can flow therethrough. An appropriate duct 55 communicates with the interior of the chute to carry cool air upward through the perforated floor 53. A fan can be used to induce a draft so that the smelt sheets flow down the chute on a cushion of air.
At the bottom of the chute, a conventional conveyor 61, say of the paddle-type, is provided to shatter the sheets into small fragments and thence to carry the shards to discharge. In some instances, a water jacket 63 is provided about the lower half of the conveyor.
An experimental model of the previously described device has been built and tested extensively. The
drums were watercooled, 12 inches in length, and 12 inches in diameter. The nip clearance was maintained by springs at an average of 0.025 inches. The average thickness of the smelt sheets was 0.024 inches. Molten smelt at a temperature of about l600 F. was poured into the pinch to a pool depth of to 4 inches. The drums rotated at seventeen revolutions per minute. It was calculated that the test device could process about one thousand pounds of smelt per hour per foot of drum length. The temperature of the solidified sheet leaving the pinch varied between 600 and 900 F. By was of comparison, devices which have been proposed previously in this field would process only about 400 pounds of smelt per linear foot of drum per hour by using one drum to which the product is reported to be stuck and subsequently removed by a doctor knife.
l. A process for forming sheets of solidified smelt from molten smelt such as the type which is derived above the nip between a pair of closely spaced drums having a minimum but expandable preset nip clearance of 0.005 to 0.025 inches;
b. rotating the drums into the liquid stream and resiliently urging one of the drums toward the other;
c. concurrently cooling the drums to reduce the localized temperature of the smelt at the nip to its freezing temperature and to remove the heat of fusion of the smelt, and forming a thin, brittle sheet of solidified smelt which issues at a temperature below about 900F vertically downward from the nip and which is free of the surface of the either drum.
2. A process according to claim 1 wherein the temperature of the solidified smelt which issues from the nip exceeds 300 F.
3. A process according to claim 1 wherein the smelt is admitted into the reservoir at a temperature ranging from the combustion of black liq lid i paper from l200-1800F and the solidified sheet of smelt is pulping processes comprising:
a. admitting a stream of such molten liquid smelt at a temperature above about 1200F into a reservoir issued from the nip at a temperature ranging from 600-800F.