US3525510A - Continuous vacuum degassing apparatus with reverse drainage means - Google Patents

Description

Aug. 25, 1970 H. RICHARDSON I 3,

' CONTINUOUS VACUUM DEGASSING APPARATUS WITH REVERSE DRAINAGE MEANS Original Filed May 24. 1966 FIG.2

HARRY L. RICHARDSON INVENTOR.

AGENT United States Patent 3,525,510 CONTINUOUS VACUUM DEGASSING APPARATUS WITH REVERSE DRAINAGE MEANS Harry L. Richardson, New York, N.Y., assignor to Chemical Construction Corporation, New York, N.Y., a corporation of Delaware Original application May 24, 1966, Ser. No. 552,551, now Patent No. 3,457,064, dated July 22, 1969. Divided and this application Mar. 3, 1969, Ser. No. 803,716

Int. Cl. C21c 7/10 U.S. Cl. 266-34 2 Claims ABSTRACT OF THE DISCLOSURE Molten metals are subjected to' vacuum for degassing purposes by flowing the molten metal stream through a plurality of chambers in the form of thin horizontal films, with successively higher vacuum or lower absolute pressure being provided in succeeding chambers.

The present application is a division of U.S. patent application No. 552,551 filed May 24, 1966 and now U.S. Pat. No. 3,457,064 issued July 22,1969.

The present invention relates to the continuous vacuum degassing of liquids such as molten ferrous metal,

and provides an improved method and apparatus for continuously removing a constituent such as a dissolved impurity from the process liquid as a desorbed gaseous component by the influence of vacuum. The method may also be applied as described herein in order to subject the liquid to a chemical or physical force or influence which may be varied in order to obtain a change in a characteristic of the liquid as a result of the vacuum atmosphere. This change may consist of physical, chemical, stoichiometric or other natural phenomena and may be caused by chemical or physical means.

One application of the invention relates to the technique and equipment to allow molten metals or metallic compounds such as ferrous products, as for instance steel,

to be subjected to an artificial atmosphere in which the absolute pressure is greatly reduced below ambient atmospheric pressure. In the production of metals, and particularly in the steel industry, it has been established that a desired improvement in the quality of the finished product can be materially enhanced by so called vacuum degassing. This is used as an intermediate step to improve the quality of any and all types of steel and alloys. The objective of such a step is the removal of gases which are absorbed, or rhay be generated by reaction of included materials which are deleterious to the desired physical characteristics of the finished product.

The established techniques, advantages, and gaseous com-" ponents are Well known in the industry.

The conventional practice to achieve thebenefits derived from the vacuum degassing of steel involved intermittent, or so-called batch processing, in which a quantity of the semi-finished material, on the order of perhaps 200 tons, is subjected to an atmosphere which approaches, as nearly as practical limits will allow, an absolute vacuum. This results in a reduction of the relative vapor pressure of the gases contained in the liquid and causes them to be discharged into the evacuated gas area by well known laws of physics. At present this result is obtained by several means, all of which require a vessel to be enclosed in a chamber which can be evacuated. This vessel may contain the raw liquid to be degassed in which case only the surface of the liquid is exposed. The bulk of the liquid is under ferrostatic pres-.

sure at varying depths below the surface of the liquid and this pressure will partially or totally overcome the effect ice generated by the evacuated atmosphere depending on the ferrostatic head, which is similar to a hydrostatic fluid head or pressure. With more complex equipment, an non-magnetic or stainless steel vessel may be surrounded by a complex electrical field to generate induction stirring. Another method in standard practice, as generally illustrated in U.S. Pat. No. 2,937,790, is to place an empty ladle in a chamber which can be evacuated and to introduce a stream of the liquid into the evacuated chamber, allowing it to fall freely through a limited vertical height before entering the receiving ladle. This method is known as stream degassing. There are several other deviations from these basic procedures, all of which attempt to secure the exposure of the maximum area of liquid surface to the evacuated area for the maximum length of time. Typical developments in this field include the procedures of U.S. Pat. Nos. 2,997,760; 2,893,- 715; 2,882,570; 2,859,262; 2,587,793 and 2,054,923.

The present invention is an improvement based on conclusions relative to knowledge of, and existing technicalfaults of, existing techniques. Thus, the installed and operating costs for maintaining an evacuated area, and the removal of gases from this area to maintain the predetermined absolute pressure, increase in an exponential progression as the absolute pressure is lowered. In addition, the time required for vacuum removal of the gases from the liquid under ideal conditions is only an infinitesimal segment of the total time now required for the overall process. Gaseous equilibrium under ideal conditions is reached in less than one second in an overall 'treating time of some 35 minutes. Further, sequential atmospheres progressing from ambient to the final desired ultimate vacuum are desirable in order to reduce costs and time of exposure. Finally, a continuous system ten ferrous metal. The invention generally involves the provision of a plurality of aligned chambers, which are connected through hydrostatic liquid seals. The liquid stream flows into each chamber from the previous chamber as a thin liquid film and thereafter flows horizontally in the chamber as a thin liquid film on a horizontally disposed surface. The liquid flows from the base of the chamber and'thr ough a lower hydrostatic liquid seal to the next succeeding chamber. The chambers are maintained at sub-atmospheric pressures with each succeeding chamber being at a lower absolute pressure than the previous chamber. Consequently, impurity is evolved in the gaseous state from'the' thin liquid film within each chamber and removed due to the vacuum effect and a liquid I stream of reduced impurity content is removed from the" final chamber. All evacuating areas are so designed that the hydrostatic head of the liquid during treatment ap-j proaches zero to the nearest practical limit.

The principal advantage of the present invention is that the desired objec'tive of vacuum degassing of a liquid is attained with a minimum of installed equipment and oper-" ating costs. The liquid stream is effectively dispersed into I horizontally flowing thin liquid films in each chamber, and thus equilibrium vacuum degassing is eflectively and rapidly attained in each chamber. Another advantage is.

that the initial chambers may be maintained at a vacuum much higher than that finally desired. This will allow the bulk of the gasesor gaseous impurity to be removedat a relatively low operating cost. The liquid stream is thus discharged through any number of succeeding chambers with steady progression toward the ultimate optimum vacuum atmosphere for final removal of small amounts of residual impurity. An added advantage is that the system may be designed to match plant capacity as a continuous process. Interruptions of the continuous flow are of no consequence as the vacuum and hydrostatic liquid seals are maintained without liquid flow. In addition, when treating a liquid at elevated temperature, the system and chambers are easily preheated with a countercurrent gas fiow before hydrostatic liquid seals are established. The liquid discharge may be easily plugged to allow evacuation of the entire system by the first stage evacuator or vacuum pump, with each higher vacuum evacuator or pump being activiated as the hydrostatic liquid seals are established. The hydrostatic liquid seals may be evacuated for complete removal of material from the chambers by a reversal of the process, or through external tapholes, if desired.

Each hydrostatic seal also serves as a flotation slag removal skimmer. This feature cannot be incorporated into existing processes. Finally, the relatively small size of the installation allows the use of external emergency heaters to be incorporated into the system at a very nominal cost.

. It is an object of the present invention to provide an improved method and apparatus for the continuous vacuum degassing of liquids.

Another object is to remove impurity from a liquid stream in an improved manner by vacuum degassing.

A further object is to degass a liquid by disposing the liquid as a thin film which is subjected to vacuum.

An additional object is to attain vacuum degassing of a liquid in a plurality of stages within chambers maintained at successively lower pressure levels and in which the liquid is disposed as a thin film.

Still another object is to degass a liquid in a plurality of connected chambers in which the liquid is exposed to vacuum as a horizontal film and a vertical falling film.

An object is to provide an improved method and apparatus for the continuous vacuum degassing of molten ferrous metal in a plurality of connected chambers maintained at successively lower pressure levels.

These and other objects and advantages of the present invention will become evident from the description which follows. The invention will be described relative to a preferred application, consisting of the continuous vacuum degassing of molten ferrous metal such as steel.

Referring tothe figures,

FIG. 1 is a sectional elevation view of the apparatus of the invention, and FIG. 2 is a sectional elevation view of FIG. 1, taken on section 2-2.

Referring now to FIG. 1, an arrangement involving substantially horizontal flow of the liquid steel as a thin liquid film is illustrated. The molten steel is passed via tundish 76 into an opening in the room 77 of vessel 78, which consists of a suitable molten steel retention vessel such as a ladle. The molten steel flows downwards from tundish 76 into vessel 78 and joins a pool of molten steel in unit 78, which serves to provide a hydrostatic head for liquid flow through the system. A vacuum is maintained within unit 78 by the provision of suitable vacuum means as described supra, which serves to remove volatile impurities stream 79 via nozzle 80. Vessel 78 is the first of four vacuum chambers. The gross removal of impurities occurs in this hogging chamber. Gas evolution is usually of such a magnitude that an explosive etfervescence requires a relatively large area or volume and may require the provision of spray shields, not shown, adjacent to the inlet of nozzle 80. The liquid flows past lower taphole 81 disposed in the bottom of unit 78, and next fiows through an opening in the lower part of the wall of unit 78. The molten steel thus flows into the first chamber of the preferably cylindrical and horizontally aligned container 82. The first chamber is defined by container 82, the wall of vessel 78, and the substantially vertical partition 83. The

liquid flows upwards into the first chamber past step 84, and then fiows substantially horizontally across bottom section as a thin liquid film. A vacuum is maintained within the first chamber by the provision of suitable vacuum means as described supra, which serves to remove volatile impurities stream 86 via nozzle 87. The vacuum level is greater in the first chamber than in the vessel 78, that is, a lower absolute pressure is maintained in the first chamber than in vessel 7 8.

The liquid molten steel next flows under the lower end of partition 83 and upwards past the step 88, and into the second chamber defined between partition '83 and the substantially vertical partition 89. The liquid then flows substantially horizontally across bottom section 90 as a thin liquid film. A vacuum is maintained within the second chamber by the provision of suitable vacuum means as described supra, which serves to remove volatile impurities stream 91 via nozzle 92. The vacuum level is greater in the second chamber than in the first chamber, that is, a lower absolute pressure is maintained in the second chamber than in the first chamber.

The liquid molten steel next flows under the lower end of partition 89 and upwards past the step 93, and into the third chamber defined between partition 89 and the outlet end 94 of the container 82. The liquid then flows substantially horizontally across bottom section 95 as a thin liquid film. A vacuum is maintained within the third chamber by the provision of suitable vacuum means as described supra, which serves to remove volatile impurities stream 96 via nozzle 97. The vacuum level is greater in the third chamber than in the second chamber, that is, a lower absolute pressure is maintained in the third chamher than in the second chamber. The third chamber will preferably be maintained at an absolute pressure of less than 0.015 kg./sq. cm.

The fully degassed and substantially impurity-free liquid now flows from the third chamber past level controller 98, and is collected in atmospheric seal leg 99 for subsequent product utilization. The atmospheric seal leg 99 may consist of a balanced U-seal, or the rising leg of the seal 99 may be truncated to allow the level controller 98 to control the discharge flow of the substantially impurity-free liquid by suitable means such as a stopper rod, not shown.

The area adjacent and external to seal leg 99 may be flooded with inert gas to prevent recontamination during conventional casting or during flow into a continuous casting device.

On shut-down of the system, which may be necessitated by a change of heats or alloy composition, the facility is readily drained free of residual molten metal and slag, by the opening of taphole 81. In this case, the liquid flow will be reversed, and the residual liquid steel and slag will flow down the steps 93, 88 and 84. The slag originally present in the molten steel fed to the tundish 76 is efiectively skimmed off by the partitions 83 and 89, which also cooperate with the steps 88 and 93 respectively to provide hydrostatic liquid seals between the chambers. Residual metal in leg 99 may be removed through an auxiliary taphole, not shown.

FIG. 2 is a sectional elevation view of the container 82, taken on section 2-2 of FIG. 1, and shows the preferable cylindrical nature of the container 82, as well as the partition 89. FIG. 2 also demonstrates the minute amount of liquid retained in the vessels to maintain interchamber seals.

Numerous alternatives within the scope of the present invention will occur to those skilled in the art. Thus, al though the method and apparatus are particularly applicable to the vacuum degassing of molten steel, other applications in the degassing or selective removal of a specific component or impurity from a liquid stream, either at elevated ambient or sub-ambient refrigerated temperatures, will be evident to those skilled in the art. Additives may be added to the liquid stream in each of the chambers if desired. While a maximum vacuum or lowest absolute pressure of less than 0.015 kg./sq. cm. is preferred in the last and lowest vacuum chamber, a higher pressure for final vacuum degassing may be pro vided in this chamber in suitable instances. The number of vacuum chambers to be provided in practice, with successively reduced absolute pressure levels in succeeding chambers, will depend on the circumstances of a particular installation. Thus, three vacuum chambers are provided in the assemblage of FIG. 1. In some instances, more than three chambers may be provided, however due to the exposure of the liquid to continuous vacuum degassing as a thin horizontal film in the present invention, rapid impurity removal and equilibrium is attained in each chamber, and consequently three chambers will usually suffice in most instances.

An example of an industrial application of the present invention to the continuous vacuum degassing of steel will now be described.

EXAMPLE The apparatus of FIG. 1 was designed for a process stream consisting of 9,000 kg./minute of molten steel at a temperature of 1620 C., which was subjected to vacuum degassing as illustrated in FIG. 1. The initial process stream contained dissolved impurities consisting of ap proximately 6.8 ppm. hydrogen, 60 p.p.m. nitrogen and 275 ppm. oxygen. The four vacuum chambers were designed for absolute pressures of approximately 0.112, 0.042, 0.014 and 0.0014 kg./ sq. cm. respectively. The four vacuum chambers removed about 60, 20, 8 and 4% of the total dissolved impurities respectively. The final fully degassed molten steel product contained only very minor residual proportions of impurities, and was a high quality finished steel suitable for continuous casting.

I claim:

1. An apparatus for the continuous vacuum removal of an impurity and slag from a molten metal stream containing dissolved impurity and slag which comprises a substantially horizontally oriented cylindrical container, at least one substantially vertical partition disposed within said container and serving to divide said container into a plurality of chambers, said partition terminating above the bottom of said container, means to pass a molten metal stream containing dissolved impurity and slag into an inlet end of said container, whereby said molten metal stream flows substantially horizontally through each of said chambers in series as a thin molten metal film, and whereby said molten metal stream flows through the opening defined between the lower end of said partition and the bottom of said container and thereby maintains a hydrostatic molten metal seal between said chambers, the level of the bottom of said container being successively elevated in stepwise progression from the molten metal stream inlet end of said container to the molten metal stream outlet end of said container, with the lower end of said partition terminating adjacent to and below the top of a step, whereby said hydrostatic molten metal seal is formed between said chambers and said slag is retained by said partition, means to maintain the initial molten metal inlet chamber at a sub-atmospheric pressure, means to maintain each succeeding chamber at a lower subatmospheric pressure than the previous chamber, means to remove said molten metal stream of reduced impurity and slag content from the outlet end of said container, a lower taphole at the inlet end of said container, said taphole including valve means adapted to be closed during the flow of said molten metal stream through said chambers, and means to terminate the flow of said molten metal stream, whereby said taphole valve may be opened by adjustment of said valve means and a reversal of molten metal flow takes place within said chambers to produce reverse drainage flow of substantially all residual molten metal and slag from all of said chambers through said taphole when the input flow of said molten metal stream is terminated and said taphole valve is opened.

2. An apparatus for the continuous vacuum removal of an impurity and slag from a molten metal stream containing dissolved impurity and slag which comprises a plurality of connected cylindrical chambers, said chambers being juxtaposed in a substantially horizontal plane with each of said chambers being separated from the next succeeding chamber by a molten metal seal, means to flow a molten metal stream containing dissolved impurity and slag through said plurality of connected chambers, the first of said chambers being provided with a lower taphole, said taphole including valve means adapted to be closed during the flow of said molten metal stream through said chambers, whereby said molten metal stream flows horizontally as a thin molten metal film on the floor of each chamber, and thereafter flows through said molten metal seal to the next succeeding chamber, with the floor of each succeeding chamber being elevated above the floor of the previous chamber, whereby said slag is retained in each of said chambers by said molten metal seal,

means to maintain a sub-atmospheric pressure level within each chamber, with each succeeding chamber being at a lawor absolute pressure than the previous chamber, whereby said impurity is evolved from the thin molten metal film within each chamber in the gaseous state, means to remove a molten metal stream of reduced impurity and slag content from the final chamber, said final chamber being maintained at lowest absolute pressure, and means to terminate the flow of said molten metal stream, whereby said taphole valve may be opened by adjustment of said valve means and a reversal of molten metal flow takes place within said chambers to produce reverse drainage flow of substantially all residual molten metal and slag from all of said chambers through said taphole when the input flow of said molten metal stream is terminated and said taphole valve is opened.

References Cited UNITED STATES PATENTS Re. 11,737 4/1899 Wainwright.

2,859,262 11/1958 Harders er al 266-34 x 3,193,892 7/1965 Sickbert 266-34 X FOREIGN PATENTS 683,996 3/1930 France.

ROBERT D. BALDWIN, Primary Examiner

Images