FR2525491A1 - Continuous filtration unit for salt baths - in heated enclosure with gas bubble lifting - Google Patents

Abstract

Aggressive liquid at high temperature, e.g. molten salt from a metallurgical treatment bath, is filtered through a porous cartridge in a metal filter pot, enclosed in an independently by heated furnace. Salt is lifted up a continuously rising suction tube, its end immersed in the bath, in sections, interspersed with gas bubbles from a supply pipe discharging into the end. Salt passes over a return bend at the top into the filter. An inclined pipe from the base carries filtered salt back to the bath, and an overflow is provided in case of blockage. This method of lifting the interrupted column of salt is easy to regulate, and has the advantage of no moving parts exposed to heat or salt. Heating the filter enclosure keeps the molten salt fluid, facilitating filtration, and allows it to drain back into the bath at the normal operating temp.

Description

 The invention relates to a filtration unit for aggressive liquids brought to high temperature in a tank, in particular salt baths for metallurgical treatments.

 Metallurgical treatments in a salt bath, such as quenching, tempering, preheating operations, or surface treatments such as carburizing, nitriding, oxidation, almost always give rise to insoluble particles, oxides, sulfides, carbon in the form of soot or graphite. These particles come either from inclusions in the parts, released by a surface attack, or from secondary reaction products between the bath and the parts, or from particle entrainment by the parts, these particles possibly reacting with the bath. These insoluble particles very often have an unfavorable action on the treatment of parts, and sometimes can induce accelerated decomposition of the salts of the bath.

 The particles are removed conventionally, either periodically generally by decanting the contents of the bath, the sedimentation of the sludge formed by the particles being able to be induced or reinforced by a variation in temperature of the bath, heating or cooling, or by circulating the contents of the bath through a filtering body, often a metal cloth, circulation being obtained by a mechanical pump.

 The periodic elimination of insolubles involves delicate handling, the salts having to be moved in the molten state, and long immobilizations of material and significant tonnages of salts. The tendency is therefore to carry out these operations only when they become essential, that is to say very often when the processing waste becomes significant.

 Furthermore, the mechanical pumps which are used to carry out continuous filtration are obviously subjected to the contact of the molten salts at the working temperature. It goes without saying that the pump parts must withstand the contact of molten salts; to avoid frictional adjustments below the surface of the salt bath, pumps are sometimes used where the rotating impellers are driven by long shafts, rotating in small bearings outside the bath. At working temperatures, the trees are subject to deformation, which causes frequent decommissioning. In addition, the flow rate adjustment of centrifugal pumps is uncertain, and generally very much higher than what is necessary for filtration. One is then led, either to use very large filtering surfaces, to ensure a reduced pressure drop through these filtering surfaces, or to use filter bodies of reduced size which are then subjected to excessive pressure drops.

However, the surfaces or filter bodies capable of withstanding the attacks of molten salt baths are either metallic frits or refractory fibers, and can neither be made conveniently in large areas, nor can they withstand forces corresponding to pressure drops high, at least at prices compatible with industrial exploitation.

 Federal German patent application P 29 23 028 dd briefly describes a filter group intended for periodically removing particles in nitriding salt baths, and composed of a filter pot of unspecified structure, and of a suction pump with a suction pipe terminated by a discharge spout above the filter pot, and a nitrogen pipe which descends into the bath parallel to the cane to straighten at its lower part and engage coaxially in the lower part of the cane. The nitrogen injection forms bubbles which rise back into the cane, entraining the salts.

 If this provision avoids the use of mechanical pumps, it does not allow continuous filtration of the salt baths, in particular because the filtration causes cooling of the molten salt bath, which must be compensated by overheating of the tank, cooling and overheating causing unacceptable temperature variations to keep the bath in operation.

To overcome these drawbacks, the invention proposes a filtration group for aggressive liquids brought to high
temperature in a tank, in particular salt baths for metallurgical treatments, comprising a filter pot, a suction tube uniformly rising from a waterproof end in the tank below the level of the liquid to a spout located in the oven above the filter pot, a return pipe descending from the bottom of the filter pot to a spillway above the liquid in the tank, and a pipe connected to a source of pressurized yaz and immersed in the tank up to one end straightened upwards and engaged coaxially in the submerged end of the suction pipe, group characterized in that it comprises an oven equipped with independent heating means, the filter pot, furnished with a porous cartridge and the spout for discharging the suction pipe being arranged in the furnace.

 There are no moving parts in the group, which, if deformed, could put this group out of service. In the suction pipe, the column of hot liquid is divided into segments separated by bubbles of the gas injected under pressure at the base of this pipe. As a result, the hydrostatic pressure at the base of this suction pipe is determined essentially by the height of the column resulting from the addition of the height of the segments, while the hydrostatic pressure in the tank at the base of the tubing corresponds to the immersion depth of this base.

Also, the driving pressure, which causes the ascent of the liquid in the suction pipe, is the difference between the two hydrostatic pressures considered above. The gas flow adjustment allows you to adjust the liquid flow, especially for very low flow rates. It has been understood that the liquid filters by gravity through the porous cartridge to return also by gravity to the tank, the heating of the oven avoiding that cooling of the liquid, by increasing the viscosity, does not compromise filtration. The circulation rate can be adjusted to the minimum necessary, so that the porous cartridge can be reduced in size, while ensuring efficient filtering, and is achievable at a reasonable cost. Finally, the operation of the group does not significantly modify the temperature of the liquid in the tank.

 Preferably the suction pipe is vertical at least in its submerged part in the tank, to avoid liquid returns on one side of the gas bubbles.

 Preferably the filter pot is equipped with an overflow pipe, to evacuate the liquid which would overflow from the porous cartridge if the latter became engorged.

 In a first arrangement, the porous cartridge is made of sintered metal.

 As a variant, the cartridge is made of agglomerated fibers.

The characteristics and advantages of the invention will become apparent from the description which follows by way of example, with reference to the accompanying drawings in which
Figure 1 is a section of a filter unit according to the invention
Figure 2 is a diagram of suction tubing;
FIG. 3 represents a particular arrangement for the injection of gas.

 According to the embodiment chosen and represented in FIG. 1, the filtering group is associated with a salt bath 1, contained in a crucible, and mounted in a console on the wall 2 of this crucible. The group comprises an oven 3 as a whole, with a tubular refractory wall 4, and an upper cover 5. It is equipped with a substantially tubular heating element 7. The oven 3 rests on a refractory base 6, in which is formed a channel 6a inclined from the base of the interior of the oven 3 to overhang the crucible 2. On the base of the channel 6a is placed a flat heating element 8. A l inside the oven 3, concentrically to its wall 4, is disposed a filter pot 9 of boiled metal, with an inclined bottom from which a return pipe 13, which ends in a spout 13a above the salt bath , following the channel 6a. Inside the filter pot 9 is disposed a porous cartridge 10, which constitutes the filter element itself, and rests on legs fixed to the pot 9. The cartridge 10 has an outside diameter substantially lower than that of the filter pot 9, so that the side wall of the cartridge 10 can participate in filtration. In addition, an overflow pipe 12 is pricked halfway up the pot 9, so that if the cartridge 10 becomes engorged, the molten salts overflow into the pot 9 and can return to the crucible through this pipe 12. In the representation which is given the pipe 12 is masked, in the channel 6a by the return pipe 13.

 A suction tube 11 plunges vertically into the salt bath 1 to one end 11a. Above the salt bath 1 the tubing 11 passes inside the channel 6a then straightens up to follow the filter pot 9 until it overhangs by a spout llb, located above the interior cavity of the porous cartridge 10.

 A pipe 14, connected to a pressure source, with a flow control valve, a pressure reducer and a pressure gauge, not shown and of conventional arrangement, runs along the base of the base 6 to come and attach to the vertical part of the tubing suction 11, and accompany the latter in the salt bath. At the end, the pipe 14 forms a loop 14a, to penetrate coaxially upwards into the end 11a of the suction pipe 11. It will be noted that the pipe 11 has an internal diameter of 22 millimeters, while the pipe 14 is of the 6 x 8 tube. In addition, the pot 9, the pipes 11 and 13 and the pipes 12 and 14 are made of 16-18 0 chromium steel, nickel free.

 The operation of the filtration group is as follows, the heating elements 7 and 8 bring the interior temperature of the oven 3 and of the channel 6a to the vicinity of the temperature of the salt bath 1, and preferably to a temperature slightly higher than that of the bath. , to reduce the viscosity of molten salts for filtration. Compressed gas is admitted into the pipe 14 by gradually increasing the flow rate so that the gas escapes from the end 14a by separate bubbles.

Of course the nature of the gas is compatible with the nature of the molten salts, and the desired reaction with the parts. The gas can be a strictly neutral gas, such as argon, a substantially neutral gas such as nitrogen, or an oxidizing gas, air enriched or not with oxygen, or even a reducing gas (hydrogenated nitrogen, ammonia cracking gas. , hydrogen, carbon monoxide).

Those skilled in the art will know how to choose the gas best suited to the salt bath.

 The volume of the bubbles is determined by the equality of the rising force of the bubble in the salt bath, and the surface tension force on the periphery of the mouth 14a. In fact this is only correct if the bubble is within a liquid in hdrstatic equilibrium; but the metal for forming separate bubbles is modified as soon as 1 bubble makes contact with the wall of the tube 11, the column of molten salts rising in the tube 11 without modifying the pressure that it exerts on the bubble . At this time, a certain quantity of molten salts flows downward along the wall of the tube 11, in compensation for the growth of the bubble. The successive bubbles divide the column of molten salts into segments, as is best seen in Figure 2, with segments 21, 23, 25, 27 ... separated by bubbles 20, 22, 24, 26, 28 ... Note that bubbles 20-28 ...

are of increasing volume upwards, the pressure on each bubble corresponding to the height of column constituted by the addition of the segments 21 + 23 t 25. for the bubble 20, 23 + 25 + ... for the bubble 22 and so right now. When the cumulative height of the segments between the end 11a and the spout 11b is less than the height which separates the end 11a from the surface of the salt bath 1, the resulting pressure difference will be the driving force for the rise of the molten salts .

 Returning to FIG. 1, the molten salts pour into the interior cavity of the porous cartridge 10, and the liquid phase passes through the wall of this cartridge 10 to collect at the bottom of the pot 9 and return to the salt bath 1 by return pipe 13. As we have already seen, if the cartridge 10 becomes clogged and overflows, the salts can be returned to the bath 1 by the overflow pipe 12.

 FIG. 3 represents an alternative arrangement for injecting gas into the lower part 11a of the suction pipe 11, where the end 14a of the gas inlet pipe is frustoconical, to promote the formation of separate bubbles.

 We have already mentioned the constitution of the porous cartridge 10, or filter body. It is clear that the filter material must be adapted to the operating temperature and to the chemical aggressiveness of the liquid to be filtered. In the field of salt baths where the present invention originated, the salt baths can be classified into two categories, according to the alkalinity of the bath, or more exactly according to the pH of a normal solution of bath salts in water, called the reference pH below. By decinormal solution is meant a solution where the sum of the normalities of each constituent of the bath is equal to 1/10. There are thus highly basic baths, whose reference pH is equal to or greater than 12, and baths whose reference pH is between 3 and 12.

 Baths of the first category are generally used at temperatures below 500du. The filter bodies are then metallic, in particular made of sintered iron, the iron resistant well to alkalis at these temperatures; fine metallic fabrics can also be used.

 Second category baths can work either in the 2500-350DC range or in the 5000-9500C range. The filter bodies will then be made of fibers, in particular ceramic fibers, stable at high temperatures and resistant to most chemical reagents, with the exception of concentrated alkalis (whose reference pH is above 13).

 EXAMPLE 1 A nitrosulphurization bath, with a weight content of 38 m of cyanate ions, 17 of carbonate ions, the cations being sodium, potassium and lithium, with 5 ppm of sulphide ions, is used to treat cast iron parts at 57O0C. A cartridge 10 of ceramic fiber 20 m thick is used.

 The gas blown through the pipe 14 is air, and the flow rate is regulated so that the circulation of the volume of the bath is carried out in less than 4 hours. While the parts treated in the unfiltered bath had numerous graphite inclusions in their surface layer, filtration made it possible to remove these inclusions.

 Example 2. An oxidizing bath having a composition by weight potassium hydroxide 60 ~ sodium nitrate 30 Ó and sodium carbonate 10 Ó, to which 5 5 of a mixture of equal parts of permanganate and potassium dichromate was added, is used at 4000C. for a treatment to complement nitriding in the presence of sulfide, to improve corrosion resistance. This oxidizing bath being stable only If the particles of sulphides, oxides and oxysulphides which are formed during the treatment of the nitrided parts are removed quickly, they are filtered, with a flow rate corresponding to the volume of the bath in less than 6 hours, over a sintered iron cartridge. The blown gas is air, the stability of the bath requiring that the bath remains practically saturated with dissolved oxygen.

 Example 3. A nitriding bath similar to that of Example 1, and working at 5700C, is filtered. The density of the bath is around 1.7, and the reference p is between 10.5 and 11.5. The compressed air pressure at the top of the pipe 14 is 2 bars. The wall thickness of the ceramic fiber filter cartridge is 25 mm.

 The salt flow rates were noted as a function of the flow rate under normal compressed air conditions, the diameter of the tube 11 being 22 mm, and that of the tube 14 8 mm.

Air flow (1 / mon) 1.5 2 3 4
Salt flow (1 / mon) 1.05 3.18 4 8
It is quite clear that the filtration group described, although it was developed for the continuous filtration of molten salt baths, can be used for the filtration of all kinds of aggressive liquids at high temperature; a person skilled in the art will be able to choose the nature of the constituents of the group, pipes, conduits and filter pot, the filter cartridge and the propellant gas.

Claims (5)

 1. Filter unit for aggressive liquids (1) brought to high temperature in a tank (2), in particular salt baths for metallurgical treatments, comprising a filter pot (9), a suction pipe (11) uniformly rising from a end (lla) immersed in the tank below the liquid level up to a discharge spout (llb) located in the oven above the filter pot, a return pipe (13) descending from the bottom of the filter pot to '' to a weir (13a) above the liquid in the tank, and a pipe (14) connected to a source of pressurized gas and immersing in the tank to an end (14a) straightened upwards and engaged coaxially in the submerged end (11a) of the suction pipe (11), group characterized in that it comprises an oven (3) equipped with independent heating means (7, 8), the filter pot (9 ), furnished with a porous cartridge (10) and the discharge spout (llb) of the suction pipe (11) being arranged in the oven (3).  2. Filter unit according to claim 1, characterized in that the suction pipe (11) is, at least in its part immersed in the tank (2) substantially vertical.  3. Filter unit according to claim 1 or 2, characterized in that the filter pot (9) is equipped with a pipe (12) of overflow returning to the tank (2).  4. Filtration group according to any one of claims 1 to 3, characterized in that the end (14a) of the pipe (14) engaged in the immersed end (lla) of the suction pipe (11) is shaped in converging nozzle.  5. Filter unit according to claim 1, characterized in that the porous cartridge (10) is made of sintered metal.  G. Filter unit according to claim 1, characterized in that the porous cartridge (10) is made of agglomerated fibers.