APPARATUS FOR IN-LINE DEGASSING AND FILTERING OF ALUMINIUM AND ITS ALLOYS

Abstract: Apparatus for on-line degassing and filtering aluminum and its alloys, constituted by a thermally insulated container body, provided with a removable lid (2) incorporating heating means (7) for heating the metal to be processed, said container body being internally subdivided, by means of a vertical partitioning wall (11) into two chambers communicating with each other only in the nearby of the bottom of the container, wherein in one of said two chambers, provided with an inlet for the liquid metal to be processed, injection means (13, 14) are provided, for injecting inert, and/or active gases, which are so located as to perform a degassing in countercurrent relatively to the entering metal stream, whilst at the bottom of the second chamber at least a substantially horizontal plate, or wall (12) is provided, made of a porous material, such as ceramic, graphite, or the like, which is positioned spaced apart from the bottom of the container, such to allow the metal, coming from the first chamber, to flow upwards, and pass through said porous plate, with a rising movement of laminar type.

Description: The object of the present invention is an apparatus for the combined, on-line treatment of degassing and filtration of liquid aluminum, and/or its alloys. It is known that the processes of degassing and purfication of a liquid metal have the purpose of removing the hydrogen dissolved inside the liquid mass, and also some solid impurities, such as, e.g., oxides and salts, various slagging substances, sodium fluoride, aluminum fluoride and still other fluorides, whose presence in suspension is also favoured by the presence of hydrogen: the degassing is generally carried out by injecting nitrogen free from oxygen, or argon, or another inert gas, which act by entraining, and by mixing the liquid metal. Also known are various processes for purifying liquid aluminum by means of the injection of active gases, such as chlorine, or other gases developing chlorine in situ , such as, e.g., chlorofluorocarbons, to the end of removing the alkali metals coming from the electrolysis of cryolite baths; in fact, chlorine combines with sodium forming sodium chloride which, by being a solid, goes to end in the slag, dragged to the surface by the injected inert gas. In particular, chlorofluorocarbons act as reactants and also act by entraining the suspended particles which, when come to the surface of the liquid metal, are englobed by the scorifiers and are skimmed from the same surface. In order to obtain metal and light alloys endowed with particular characteristics of purity and structural homogeneousness, even the smallest solid particles which remain equally dispersed in suspension, have to be removed. To that end, according to some techniques known from the prior art, chlorine is delivered, through graphite rotors, which act as true stirrers; they, by revolving inside the liquid metal, keep it stirred, and thus facilitate the removal of the solid particles, which rise to the metal surface under the thrust applied by the gas escaping from the rotor. In practice, this technique suffers from the serious drawback that it uses, inside a high-temperature mass, moving parts, which show a rapid decay, and which result difficult to be managed. Also processes for liquid aluminum filtering are known, which use substantially spherical bodies of tabular alumina, which allow the impurities to be adsorbed by the same spherical bodies, on their surface; but the spherical bodies get rapidly clogged, losing their adsorbent characteristics, and hence require expensive operations of cleaning and reclamation. US 3,904,180 (J.A. BASS) and US 3,917,242 (J.A. BASS) relate to a process and apparatus for treating molten metal for removing gas and finely-divided solids therefrom in a two stage apparatus. Hydrogen gas is fluxed from the metal in the first section by means of a fluxing gas injected through tubes including porous heads. The degassed molten metal is passed upwardly through a granular filtering medium. The particles of the filtering medium are so chosen and arranged that they become buoyant upon the occurence of molten metal being passed therethrough, which tendency prohibits the quick clogging of the filter material. The filter medium is supported on a perforated table and a perforated screen of stainless steel is placed over the filter to prohibit the floatation of the material into the casting system. GB 1266500 (BRITISH ALUMINIUM CO. LTD) relates to a process and apparatus for treating molten aluminium for the removal of non-metallic inclusions by causing the aluminium to flow through a bed providing a multiplicity of flux lined tortuous channels, e.g. very coarse granules coated with a flux. US 4,087,080 (A. STEINEGGER) relates to an apparatus for filtering aluminium melts by the use of a loose bed of granulate and with introduction of gases, comprising a filter chamber and a riser chamber. Gas inlet bricks are arranged in the floor or lower side wall of the filter chamber. More recently, processes of filtration of liquid metal have been proposed, according to which the liquid metal is filtered through porous septa, provided inside a chamber, with the liquid metal being fed from the top, and the filtered metal being discharged under the porous septum. EP-0135050 (OLIN CORP.) relates to a process for treating molten metal comprising the step of degassing and the step of removing the particulates from the melt. The degassing is accomplished by injecting an inert gas into the melt by means of a conventional sparge ring or other conventional techniques. The degassed melt is filtered to remove particulates, flowing downwardly through a filter plate positioned in the upper part of the treatment zone so that particles are retained onto the upper surface of said filter plate. US 4,007,923 (E.C. CHIA) and US 4,067,731 (E.C. CHIA) relate to a process and apparatus for treating molten aluminium to remove impurities and gases therefrom in a four stage apparatus. Hydrogen gas is fluxed from the molten aluminium in the second stage by means of a fluxing gas injected through porous diffuser heads. Molten aluminium is filtered in the first deslagging stage through vertically disposed filter elements for filtering out dross and larger particles in the third adsorption stage through the narrow passages between flat adsorption plates over which particulate matter will adhere to the reactive surfaces of the plates and in the fourth filtration stage through a rigid porous material disigned to provide a large surface area, constituted by a plurality of horizontally disposed channels integrally interconnected with each other to form a rigid sinuous or ondulating shape extending from the floor to a point adjacent to the top cover. These porous septa are generally made of graphite, ceramic, and also of various types of agglomerates; in practice, they suffer from the serious drawback that they get clogged after a short operating time, in that the impurities, pressed against the filter by the pressure of the metal, and by the same impurities which have been previously collected on the surface of the filter, tend to clog it, also in an irreversible way; the filter must be therefore removed, after the apparatus being preliminarily emptied, then cleaned, if possible, and then re-assembled, or replaced, with evident financial and practical burdens. US 4,401,295 (T.YOSHIDA) relates to an apparatus for treating a molten metal comprising the gas treating and the filtration step in a vessel divided into two parts. The degassing is accomplished by injecting a gas through a stirring mechanism. The molten metal is then permeated or rather passed from outside into the inside of tube filters through their pipe walls while being filtered there. In the botton of the vessel a blowing nozzle is provided through which a gas is blown to cause the molten metal to flow. Therefore, a purpose of the present invention is to provide an apparatus for the on-line degassing and filtration of liquid aluminum, and/or of its alloys, which is capable of overcoming the drawbacks and the limitations which affect the apparatuses and processes known from the prior art, and, above all, such as to result highly efficacious and reliable in the treatment of the metal. Another purpose of the invention is to provide an apparatus of the above specified type, having such a structure as to result cheap, as relates to the installation costs and the operating costs, and easy to be operated and regulated. These and still other purposes, as they may be better evidenced by the following disclosure, are achieved by an apparatus as defined by claim 1 for on-line degassing and filtering liquid metals, in particular, aluminum and its alloys, by means of the use of inert and/or active gases and of filtering porous plates or septa, which apparatus is constituted, according to the present invention, by a substantially parallelepipedon container body, provided with a removable lid, integrally thermally insulated and incorporating heating means for heating the metal to be processed, said container body being internally subdivided, by means of a vertical partitioning wall, into two chambers communicating with each other only in the nearby of the bottom of the container, wherein in one of said two chambers, provided with an inlet port for the liquid metal to be processed, injection means are provided, for injecting inert, and/or active gases, which are so located as to perform a degassing in countercurrent relatively to the entering metal stream, whilst at the bottom of the second chamber at least a substantially horizontal plate, or septum is provided, of a porous material, such as ceramic, graphite, a ceramic agglomerate, or the like, which is positioned spaced apart from the bottom of the container, such to allow the metal, coming from the first chamber, to flow upwards, and pass through said porous plate, with a rising movement of laminar type, and allow the filtered metal to reach the discharge port. More particularly, said injection means for injecting inert or active gases are constituted by pipes, or the like, which are anchored, in a vertical position, to said removable lid, and are provided, in their bottom portion, with blocks, or cylinders, or cones of a porous material, the length of said injection pipes being such as to position the porous cylinders in the nearby of the bottom of the container, such to make it possible the injected gas to be diffused, and uniformly distributed throughout the mass of molten metal, without causing vortexes, or an irregular mixing of the same metal. The invention is now disclosed in greater detail in the following, according to a preferred, not exclusive form of practical embodiment thereof, by referring to the hereto attached drawing tables, supplied for merely indicative and not limitative purposes, wherein Figure 1 schematically shows a vertical sectional view, taken along the middle thereof, of an on-line degassing and filtering apparatus accomplished according to the invention, and Figure 2 shows a vertical sectional view of the apparatus of Figure 1, taken along the broken line A-A. Referring to such Figures, the apparatus of the present invention is constituted by a container body 1, having a substantially parallelpipedon shape, with thermally insulated walls, which is open in the top, and can be tightly sealed by a flat lid 2, which is also coated with a thermally insulating material. Through a side wall of the container 1, an inlet port 3 for molten metal feeding, and, in a spaced apart position, an outlet port 4 for filtered metal discharging, are provided; both the inlet port 3 and the outlet port 4 are located at substantially the same height from the bottom of the container 1, and are so dimensioned, that the level 5 of the liquid metal substantially corresponds to the middle axis 6 of the inlet port 3. Inside the lid 2, a plurality of electrical resistors 7 are installed, to heat the liquid metal during the degassing and filtration treatment. Atop the vertical walls of the container 1, vents 8 are provided (Figure 1), to allow the treatment gases to escape, as it is better clarified in the following. The interior of the container 1 is then subdivided into two chambers 9 and 1⊘, different in volume, by a vertical, substantially "L"-shaped partitioning wall 11, which has such dimensions, as to extend up to a certain distance from the bottom of the container; it then continues with its horizontal portion 11a, up to come into contact with the inner wall of the container. Such partitioning wall bounds the filtration chamber 1⊘ communicating with the outlet port 4, whilst the chamber 9, constituting the degassing chamber, remains in communication with the inlet port 3. On the horizontal portion 11a of the partitioning wall, a plate 12 of a porous material, such as ceramic, graphite, or various conglomerates, is installed, to act as the filtering means for filtering the liquid metal fed into the chamber 9. Inside the chamber 9, injection pipes are furthermore installed, to inject inert and/or active gases, such as nitrogen, argon, chlorine and other gases, such pipes being indicated by the reference numeral 13 in Figure 1; they are anchored to the lid, and extend above it. Said pipes show, at their opposite end, a cone, or a cylinder, 14, of a porous material, such as coal. Furthermore, the arrangement of the pipes 13 is such that the cylinders 14 are maintained in the nearby of the bottom of the container, in such a way that the gas, evenly and homogeneously diffused and distributed by the porous cylinders, may concern the whole volume of liquid metal contained inside the chamber 9, without causing vortexes or any irregular mixing, which would endanger the subsequent operations of filtration. The particular position of the gas injection pipes makes it possible to perform a degassing in countercurrent relatively to the metal flow. Therefore, by providing two chambers, so arranged as to communicate with each other according to the principle of the communicating vessels, the outgassing with filtration is carried out on-line, and the large dimensions of the filtering chamber 1⊘ cause the rising motion of the metal, which flows through the filtering plate 12, to be of substantially laminar type: furthermore, in as much as the filtration takes place from down upwards, through the plate 12, any impurities and solid particles, entrained by the metal, already degassed in the previous chamber 9, are forced to stop against the lower surface of the filtering plate, and, hence, to fall down to the bottom of the container, from which they are periodically removed through a drain channel 15 (Figure 2). The ratio between the volumes of the two chambers 9 and 1⊘ and the surface of the porous septum 12 are such to allow, as already said, a laminar and slow flow of the metal to establish from down upwards, which does not cause any pressures to be applied by the impurities to the porous septum, and, above all, which does not hinder the falling down, and settling of the particles of impurities onto the bottom of the parallelepipedon container, according to the invention. The dimensions of the chambers, as well as of the inlet and outlet ports are such to maintain within pre-established limits the difference in liquid level which is established between the two chambers by the effect of the resistance offered by the filtering plate, such resistance being a function of the degree of clogging of the same plate; if the difference in liquid level occurring during the operation exceeds the pre-established level, the liquid metal overflows from one chamber into the other chamber, through an opening (not shown in the Figures) provided through the partitioning wall 11. Finally, in order to secure the metal to outflow even in case of a complete clogging of the filtering plate, outside the same filtering plate, a discharge channel, not shown in the Figures, is provided. The above disclosed apparatus, thanks to its structural simpleness, to the absence of moving parts, and to the realization of the filtration from down upwards through a filtering plate very simple in structure, made it possible in practice a high efficacy to be obtained in practicing the treatment of liquid aluminum, with a high efficiency, low operating costs, and a long useful life of the filtering plate or septum. Finally, it is obvious that to the invention, as above disclosed according to a preferred accomplishment way, in practice modifications and variants can be supplied, which are structurally and functionally equivalent, without going out of the purview of protection of the same invention.

Claims: Apparatus for on-line degassing and filtering liquid aluminium and its alloys, by means of fluxing gas and of filtering porous plates of the type constituted by a substantially parallelepipedon container body, (1) provided with a removable lid (2) integrally thermally insulated and incorporating heating means (7) for heating the metal to be processed, the container body being internally subdivided, by means of a vertical partitioning L-shaped wall (11), into a first (9) and a second chamber (10), communicating with each other only in the nearby of the bottom of the container, in the first of said two chambers, provided with an inlet port (3) for the liquid metal to be processed, injection means (13) being provided, for injecting inert and/or active gas, characterized in that a horizontal filtering plate (12) made of an inert porous material is located in a portion of said partitioning wall forming the bottom of said second chamber (10) provided with a discharge port (4) for the processed liquid metal, the ratio between the volumes of the two chambers (9) and (10) being so that the molten aluminium or Al-alloy flows downwardly from said first chamber (9) and then flows upwardly through said porous plate (12) in a calm, non-turbulent rising movement of laminar flow into said second chamber and through said discharge port and that said bottom of said container body under said filtering plate (12) comprises a discharge channel (15) for the discharge of the slag filtered by the filtering plate (12) and settling on said bottom of said container body.

Inventors: LAZZARO, GIUSEPPE, PIRAS, LEONARDO
Applicants: ALUMINIA S.P.A.
Application year: 1987
Main classification: C22B21
IPC codes: C22B21