WO1998005411A1 - Non-polluting method for regenerating ceramic parts used in aluminium casting - Google Patents

Abstract

A method for regenerating ceramic parts used in aluminium casting, to enable low-cost removal of the surface build-up of materials while conferring properties to said parts that are equivalent to those of new parts. The method is particularly useful for the ceramic parts of bed filters containing sintered alumina. The method combines a chemical attack with a heat treatment. The chemical attack is used to dissolve the aluminium in metal or oxide form and remove insoluble inclusions retained in the parts, without actually chemically attacking the parts. The heat treatment is carried out at a temperature higher than 300 °C and enables the original properties of the treated parts to be completely restored.

Description

NON-POLLUTANT PROCESS FOR REGENERATING CERAMIC PARTS USED

IN ALUMINUM FOUNDRY

Field of the invention

The present invention relates to a method for regenerating ceramic parts used in aluminum smelting, such as ceramic media used for the filtration of liquid aluminum, in particular filter beds, and filter ht support parts.

State of the art and problem posed

Liquid aluminum filtration techniques are well known. Their purpose is to eliminate solid particles, called inclusions, such as skins of aluminum oxides, which are suspended in the liquid metal and which can cause defects in the finished product.

These techniques involve passing the liquid aluminum through a porous ceramic block or into a filter bed containing ceramic elements.

Filter bed techniques, which are known in particular from US Pat. No. 2,863,558, are often designated by the English expression Deep Bed Filter (DBF) (or thick filter bed). The ceramic elements of the filter beds generally take the form of gravel or balls of different sizes Typically, a filter bed comprises a superposition of gravel beds and balls accumulated in the form of superimposed layers The elements are generally made of high-grade alumina temperature, called tabular alumina, which is of high density (> 3.55) and of essentially closed spherical porosity This alumina is refractory and offers great resistance to corrosion by liquid aluminum and the vast majority of its alloys

Filtration techniques through blocks or pieces of porous ceramics generally use refractories very resistant to liquid aluminum, such as silicon carbide or refractories with a high alumina content, and with controlled open porosity The filtration efficiency is linked, among other things, to the surface condition of the filter blocks or the elements of the filter bed, in particular the surface porosity, and to their geometry.

These filtration methods are sometimes combined with methods of removing hydrogen by passing an inert gas.

However, in use, the ceramic parts become contaminated and accumulate inclusions and solid intermetallic particles. In the case of filter media, this accumulation clogs them and reduces their efficiency. We are then forced to change the used elements, which, after use prolonged and after emptying the filter bag, contain 3 to 8% by weight of aluminum in metallic form, often loaded with impurities from the treated alloys

The industrial solution usually adopted consists in recovering aluminum in salt bath ovens and in landfilling used parts. This approach poses problems of pollution and filtration costs in particular.

In certain cases, the ceramic elements stripped of their metallic aluminum by treatment with molten salts are ground and incorporated as filler in refractory products or anti-wear coatings This approach however leads to a very strong devalorization of the product

In the case of filter beds, it has been proposed to renew the surface of the elements by mechanical abrasion and to remove by sieving the fine particles produced (see the article by MM Niedzinski, published in Light Metals - 1 992, p 1 1 23) However, the main drawback of these treatments is that they cause rapid wear of the elements, a modification of the surface condition and an alteration of the shape of the elements, "and particularly angular gravel. This solution also presents the problem of the landfill of the dust produced by the regeneration treatment This approach is also difficult to apply to parts of determined geometry, such as support parts, which may also be damaged by the treatment

The Applicant has therefore sought an industrial solution which makes it possible to reuse, for a lower cost, the used filter media and to avoid the landfill problems, which are expected to be severely controlled in the near future. Subject of the invention

The object of the present invention is a process for the regeneration of ceramic parts used in aluminum smelting, which allows, at low cost, to eliminate the products accumulated on the surface and to restore to said parts properties equivalent to those new parts.

The second object of the present invention is the device implementing said regeneration method.

Description of the invention

The process for regenerating ceramic parts, in particular filter beds comprising elements based on sintered alumina and porous filters based on silicon carbide, according to the invention, is based on the combination of chemical attack and heat treatment.

More specifically, the process for regenerating ceramic parts according to the invention is characterized in that it comprises the following operations: a) a chemical attack by an appropriate attack reagent allowing the aluminum found in solution to be dissolved in the state of metal or oxide, and optionally entrainment of the insoluble inclusions retained by said parts, without however chemically attacking said parts; b) after separation of the attacking reagent, a heat treatment of the parts at a temperature above 300 ° C.

The attacking reagent is advantageously a strong base, such as an alkali hydroxide, or a strong acid, such as sulfuric acid, hydrochloric acid or nitric acid.

Preferably, an attack reagent is used which can be recycled and which allows easy recovery and / or use of the aluminum salts resulting from the attack, such as the aluminum sulfates obtained by an acid attack. sulfuric or alkaline aluminates obtained by attack with an alkali hydroxide. This choice has the advantage of being economical and acceptable for the environment, in particular by recycling the spent attack reagent. In this regard, a sodium hydroxide solution is advantageously used, which allows a dissolution of aluminum in the form of sodium alummate, which is directly usable in a Bayer process for the treatment of bauxite, and which n does not chemically attack tabular alumina or silicon carbide significantly This approach not only allows the recycling of spent attack reagent but also the recovery of the aluminum accumulated by the ceramic parts used in aluminum smelting

For economic reasons, the attack conditions are fixed so that the kinetics and the dissolution rate are high, without however causing problems of control and stability of the process.

The heat treatment operation of the parts freed from the attacking reagent is essential because it completes the chemical attack by eliminating the traces of residual contamination left by said attack. These traces are in the form of fine particles (cf. FIG. 3b) which blacken the surface of the parts and can coalesce to form a sort of "gangue" which covers the grains. Their disappearance, due to the heat treatment of the invention, makes it possible to increase the efficiency of the regeneration of the treated parts.

The heat treatment operation is preferably carried out under conditions which make it possible to reduce the costs and to simplify the device for implementing the process. In particular, the temperature is preferably chosen so as to obtain a rapid treatment, but which n '' does not cause excessive energy costs and does not require prohibitive investments The duration of treatment is advantageously less than 8 hours

The invention will be better understood with the aid of the detailed description and of the figures which are given without any limitation being implied.

Description of the figures

Figure 1 shows schematically the regeneration process according to the invention, which comprises a chemical attack and a heat treatment. Step A comprises the chemical attack on the used parts (1) using the attack reagent (4). spent etching reagent (5) from step A contains aluminum dissolved during chemical etching and insoluble particles dislodged by the treatment According to a variant of the process of the invention, step A also includes an additional attack, which makes it possible to eliminate the polluting elements which would not have been dissolved by the attack reagent, and / or a rinsing and a drying of the treated parts. Step B includes the treatment. heat produced on the parts (2) from step A The parts (3) from the heat treatment are regenerated and can be reused without additional treatment

FIG. 2 shows schematically a variant of the preferred embodiment of the invention which includes the recycling in a Bayer process of the spent attack reagent resulting from the chemical attack The Bayer process, which is shown partially and in a simplified manner, comprises a circuit C for recovery of the attack liquor, which consists essentially of a concentrated solution of sodium hydroxide and sodium aluminate The Bayer process comprises a step D of attack of the ground bauxite (6) at l using the attack liquor (7) and decanting the suspension resulting from this attack The attack liquor which has become supersaturated with alumina (9), after separation of the inert residues (8), is treated in step E so as to precipitate the alumina tπhydrate and to separate it from the mother liquor The precipitate of alumina tπhydrate (10) obtained is intended for the production of alumina (1 2) Part of this precipitate is taken and used as a primer (1 1) for precipitation in stage E The aluminate liquor (1 3) resulting from stage E for precipitation, diluted and depleted in alumina is recovered and, to attack the bauxite again, remission to title by evaporation at F and, optionally, by addition of sodium hydroxide at G

According to the invention, the spent attack reagent (5) from step A of chemical attack is introduced into the recovery circuit C of the attack liquor, that is to say at the level of the depleted liquor (point 1 4 ), either after addition of sodium hydroxide (point 1 5), or after concentration of the liquor by evaporation (point 1 8) According to the illustrated variant, the attack reagent (4) used during step A is taken, in all (1 9) or part (1 7 and 1 9), of the recovery circuit C of the attack liquor of the Bayer process and reintroduced into this circuit after use. The sample is taken either after stage F of concentration by evaporation (point 1 6), as illustrated, or after addition of sodium hydroxide (point 1 5), or at the level of the depleted liquor (point 14)

FIG. 3 shows the state of the surface (a) of new elements,

| b) used elements having undergone the chemical treatment of the invention and (c) used elements having undergone the combination of chemical attack and heat treatment according to the invention FIG. 3b shows traces of contamination left by attack FIG. 3c shows a surface condition close to that of the new element presented in FIG. 3a

FIG. 4 illustrates the part of a device which makes it possible to implement the chemical attack operation of the method according to the invention and which comprises a cyclindnque reactor (20) comprising a removable basket (21) in which the used ceramic parts (1) to be regenerated, means for progressive introduction (22) of the attack reagent (23) contained in an auxiliary container (24), said means (22) comprising a variable flow pump (25) and the introduction taking place at the base of the reactor (21), said container (24) serving for the accumulation of the attacking reagent (23), a means for rapid evacuation (26) of the attacking reagent, a overflow (27) located at a level just above the upper part of the basket (21) which allows the return to the auxiliary container (24) of the excess attack reagent, a fan (28) and a vent for exhaust (29) enabling the atmosphere of the reactor to be regenerated into air and thereby diluting the reduced gases units released by the reaction, such as hydrogen, in order to reduce the risk of explosion, a device (30) for measuring the hydrogen content of the internal atmosphere of the reactor which makes it possible in particular to follow the evolution of the attack chemical reaction The auxiliary container (24) can be supplied, in whole or in part, with attack reagent by the aluminate liquor withdrawn from the recovery circuit C of an industrial bauxite treatment installation. by the Bayer process This liquor is withdrawn either before (point 14 of figure 2), or after setting under the title for the attack of bauxite (in points 1 5 or 1 6 of figure 2) In order to maintain the content in reducing gases at a value lower than the explosive limit, the air flow entering the reactor enclosure is preferably at least 30 times greater than the flow of reducing gases released by the reaction, which ensures a concentration avg less reducing gas less than about 3% The hydrogen released can also be used as an energy source or stored

The used attack reagent discharged from the reactor by the rapid evacuation means (26) is either discharged and accumulated in the auxiliary container (24) (FIG. 4a), or introduced into the recovery circuit C of the attack liquor of an industrial installation for the treatment of bauxite by the Bayer process (Figure 4b) Detailed description of the invention

The process for regenerating ceramic parts (1) according to the invention is characterized in that it comprises: a) a step A comprising a chemical attack using a reagent (4) allowing the dissolution of the aluminum accumulated, in the metal or oxide state, by said parts, and possibly the entrainment of the insoluble inclusions retained by them, without however significantly attacking said parts chemically; b) after separation of the attacked parts (2) and the spent attack reagent (5), a step B comprising a heat treatment of the parts (2) at a temperature above 300 ° C.

The chemical attack is advantageously carried out at a temperature above ambient, preferably between 30 and 95 ° C. A temperature below 30 ° C generally leads to insufficient solubility and kinetics of attack. A temperature above 95 ° C leads to rapid evaporation of the reagent and problems with vesicular entrainment (loss of reagent liquid caused by the gases and increased risk of corrosion of the pipes).

The chemical attack can optionally be carried out under pressure in an autoclave.

Before the heat treatment step, step A may optionally include additional steps of washing with a mineral acid, rinsing and / or drying so as to rid the surface, by drainage and / or by solution, of the particles insoluble in the first attack reagent which have accumulated there.

The heat treatment is carried out at a temperature above 300 ° C, and preferably between 600 ° C and 900 ° C. A temperature below 600 ° C. generally leads to prolonged treatment times which are hardly compatible with industrial rates. A temperature above 900 ° C requires special installations which entail prohibitive investment costs and induce generally high energy costs. The effect of this heat treatment is spectacular: after chemical attack (and possible rinsing), the surface of the ceramic parts is gray because it is still covered in places by a sort of gangue. During the heat treatment, it seems that this gangue detaches like an onion skin and crumbles, revealing the perfectly white surface of the ceramic pieces. According to the preferred embodiment of the process of the invention, the chemical attack is carried out using a concentrated solution of sodium hydroxide and the spent attack reagent is recycled in a Bayer process.

The concentration of the aqueous sodium hydroxide solution is preferably between 40 and 400 g / liter. The attack is advantageously carried out in a regeneration reactor, either by immersion or by circulation of the attack reagent. The duration of the attack is between 30 n and 5 hours, preferably between 2 and 3 hours, so as to satisfy at the same time the requirements of cost and efficiency of the process.

According to a variant of the method of the invention, so as to eliminate the polluting elements which would not have been attacked by the attacking reagent, an additional attacking step is carried out using a mineral acid, such as than hydrochloric acid, sulfuric acid, nitric acid, or a mixture thereof, different from the attack reagent used for basic chemical attack.

In order to ensure a high dissolution rate of metallic or oxidized aluminum, the concentration of the chemical reagent is preferably high enough so that it can ensure a high dissolution kinetics of aluminum and its oxides and that it can maintain in solution the alkali aluminates formed during the attack, that is to say a solubility of the alkali aluminates preferably greater than 20 g of alkaline aluminate per liter of solution.

The treated elements are then advantageously rinsed and dried, then subjected to heat treatment.

According to a first variant of the preferred process, the chemical attack on the parts is carried out using a new sodium hydroxide solution which, after use, is injected into the Bayer process at the level of the recovery circuit C of the attack liquor, that is to say that the spent attack reagent is mixed with the aluminate liquor intended to attack bauxite.

According to a second variant of the preferred method, the attacking reagent (4) comes in all (1 9) or part (1 7 and 1 9) of the recovery circuit C of the attack liquor, that is to say say that a part (an aliquot) of the aluminate liquor is taken from recovery circuit C. After the parts attack phase, the spent reagent (5) is mixed with the aliquot not withdrawn from the aluminate liquor circulating in the recovery circuit C

In order to avoid contamination of the alumina, the spent etching reagent (5) can be filtered in order to remove the insoluble particles therefrom. However, the reagent can be injected as it is into the aluminate liquor of the Bayer process, the insoluble particles being generally easily collected during the decantation phase (step D) and rejected with the inert residues (8)

The etching reagent can be taken from the Bayer process and / or reintroduced into it at different points in the recovery circuit C

Examples

Batches of used tabular alumina filter bed elements from a DBF bag were regenerated using the method of the invention, on a laboratory scale (tests n ° 1) and at industrial scale (n ° 2 and 3) These elements were in the form of gravel and logs

Regeneration testing # 1

The chemical attack was carried out using a sodium hydroxide solution at different concentrations between 40 and 400 g / hr and at a temperature greater than or equal to 30 ° C. In the tests, the chemical attack was carried out by immersion and by circulation of the attack reagent, which made it possible to determine the attack kinetics The quantity of dissolved aluminum reached an asymptote after a period of time T which was depending on the attack temperature, the concentration of the attack reagent and the origin of the used elements Typically, at 30 ° C., the time T was 8 hours for an attack reagent at 40 g / liter and 1 hour at 360 g / liter. The rate of dissolution increased rapidly with temperature

The spent etching reagent typically contained between 10 and 150 g / liter of alumina in solution Regeneration testing # 2

A new solution of 367 g / liter of sodium hydroxide, of density 1, 3, was prepared from a concentrated solution at 705 g / liter of density 1, 5. About 3800 liters of this solution were placed in an auxiliary container to serve as a reserve of attack reagent.

A batch of 3 tonnes of used tabular alumina elements, in the form of balls and gravel, between 0.5 and 20 mm in size and containing 3 to 5% of aluminum alloy, as well as skins and aluminum oxide particles, was placed in a perforated basket with an approximate volume of 2 m

This basket thus loaded was placed in a cyclindnque reactor similar to that described in Figure 4a)

The reactor is hermetically sealed after the introduction of the basket loaded with the elements to be regenerated

The attacking reagent was then gradually introduced into the reactor, at a rate of 150 liters per minute until the device for measuring the hydrogen content indicates that the liquid had reached the base of the batch of elements. , then at a rate of 20 liters per minute so as to reduce the rate of evolution of hydrogen from the reaction. The flow rate was subsequently regulated so as to maintain a hydrogen content of less than 3% in the reactor enclosure. The time required for the complete immersion of the elements was approximately 2 hours.

When the hydrogen content fell below 0.5%, the rate of introduction of etching reagent was further increased to 150 liters / minute so as to achieve a relatively high reagent circulation rate between the reactor and the auxiliary container and, thus, entraining the insoluble particles possibly present in the feed. This recirculation and washing operation lasted 1 hour

The reactor was then drained of the attack reagent by the rapid evacuation means and the basket transferred to a washing vessel with water recirculation in order to remove the residual alkaline liquor impregnating the ceramic elements The etching reagent reached a temperature of around 30 ° C during the test, in particular as a result of the exothermic nature of the reaction for dissolving aluminum and its oxides. The spent reagent contained 280 g / liter of Na ₂ 0 and 58 g / liter of AI ₂ 0 ₃ . The hydrogen content integrated over the entire duration of the reaction corresponded to an amount of metallic aluminum dissolved in solution equal to approximately 108 kg.

The spent etching reagent was introduced into a Bayer process without any particular difficulty.

The washing water was reused to develop a new reserve of attack reagent by adding sodium hydroxide at 705 g / liter.

In these tests, it was found that the subsequent heat treatment phase rids the surface of the parts of the residual gangue left by chemical attack and gives the elements surface characteristics comparable to those of new parts. The elements resulting from the heat treatment had taken on a white coloration comparable to that of the new elements. Microscopic analyzes of the surface showed in particular that the final surface condition of the parts was comparable to that of new parts (Figure 3).

The heat treatments were carried out in air, in a gas oven, at a temperature of 900 ° C. for 1 hour.

Regeneration testing # 3

Tests similar to tests n ° 1 were carried out using as a starting reagent a mixture of concentrated sodium hydroxide solution at 705 g / liter and depleted aluminate liquor taken from the liquor recovery circuit d attack on an industrial installation of the Bayer process. This depleted liquor was at a temperature of 75 ° C, with a density of 1.104, and contained 1 56 g / liter of Na ₂ 0 and 104 g / liter of dissolved I ₂ 0 ₃ . The etching reagent obtained was then at a temperature of 50 ° C and had an effective Na ₂ 0 content of 260 g / liter.

The results obtained using this attack reagent were identical to those obtained during test No. 2. Liquid aluminum filtration tests

Comparative tests of filtration of liquid aluminum were carried out using new filter elements and using elements regenerated according to the process of the invention These tests were carried out on an AA 1070 alloy

The filter bed of the DBF filter bag was composed of a superposition of three layers of tabular alumina elements, the upper and lower layers, with a thickness of 50 mm, consisted of beads of 1, 3 cm and 0, 6 cm in diameter, the intermediate layer, 400 mm thick, consisted of 3-6 mesh gravel The filtration surface was 0.16 m ² The metal flow rate was 10 tonnes / hour

A device for analyzing the purity of the metal, known by the acronym LiMCA (Liquid Metal Cleanliness Analysis), was placed upstream and another downstream of the filter bag in order to continuously measure the number of inclusions contained in liquid metal PoDFA (Porous Disc Filtration Apparatus) analyzes were also carried out in order to determine the particle size distribution of the inclusions

The inclusion rate observed at the entry of the DBF bag varied between 5,000 and 10,000 inclusions / kg during casting. The rate observed at the outlet of the DBF bag fluctuated around an average value of less than 200 inclusions / kg, and no significant difference was observed between the values observed with the filter bed formed from new elements and that formed from d regenerated elements The particle size distributions of the residual inclusions observed by PoDFA analysis on pions taken at the outlet of the DBF pocket were comparable, and the absence of inclusions larger than 50 μm was noted in particular.

A panoramic chemical analysis was also carried out on metal samples taken at the outlet of the DBF bag in order to check the possible pollution of the metal coming from the filtering beds. This analysis was carried out by spark emission spectroscopy and focused on 10 elements, namely Si, Fe, Cu, Mn, Mg, Cr, Ni, Zn, Na and Ti No significant difference could be observed between the samples of metal filtered by new elements and the samples of metal filtered by elements regenerated according to the process of the invention These tests have confirmed that the regeneration process of the invention makes it possible to restore worn elements to their initial filtration properties without causing residual contamination.

Benefits

The regeneration process according to the invention has the advantage of allowing a revalorization and a reuse of the used ceramic parts used in aluminum smelting. In its preferred embodiment, it makes it possible to recycle the attack reagent and also to recover the accumulated products, in particular by recovering the accumulated products based on aluminum.

Claims

1 - Process for the regeneration of ceramic parts used in aluminum foundry characterized in that it comprises the following operations a) chemical attack by a reagent allowing the dissolution of the aluminum being in the state of metal or oxide, and possibly entrainment of the insoluble inclusions retained by said parts, without however significantly attacking said parts chemically, b) after separation of the attacking reagent, heat treatment of the parts at a temperature above 300 ° C 2- A method according to claim 1, characterized in that the attacking reagent is a strong base or a strong acid 3- Method according to one of claims 1 or 2, characterized in that the chemical attack is carried out at a temperature between 30 and 95 ° C. 4- Method according to one of claims 1 to 3, characterized in that the attacking reagent is a sodium hydroxide solution of concentration between 40 and 400 g / liter 5- Method according to claim 4, characterized in that the spent attack reagent is recycled in a bauxite treatment installation by the Bayer process 6- A method according to claim 5, characterized in that the spent attack reagent is recycled in the recovery circuit (C) of the attack liquor of the Bayer process for the treatment of bauxite 7- A method according to claim 5 or 6, characterized in that the attack reagent comes in whole or in part from the recovery circuit (C) of the attack liquor of the Bayer process 8- Method according to one of claims 1 to 7, characterized in that the chemical attack operation also comprises additional steps of washing with a mineral acid, rinsing and / or drying, so as to rid the surface , by drainage and / or by dissolution, particles insoluble in the first attack reagent which have accumulated there 9- Method according to one of claims 1 to 8, characterized in that the heat treatment is carried out in air. 10- Method according to one of claims 1 to 9, characterized in that the heat treatment is carried out at a temperature between 600 ° C and 900 ° C 1 1 - Device for implementing the method according to one of claims 1 to 10, characterized in that it comprises a reactor comprising - a removable basket in which are placed the used ceramic parts to be regenerated, a means for progressive introduction of the attacking reagent contained in an auxiliary container, said means comprising a variable flow pump and the introduction taking place at the base of the reactor, said container serving for the accumulation of the attacking reagent , - a means of rapid discharge of the attack reagent, - an overflow located at a level just above the upper part of said basket which allows the return to the auxiliary container of the excess attack reagent, a device for measuring the hydrogen content of the internal atmosphere of the reactor which makes it possible in particular to follow the evolution of the chemical attack reaction, - a fan and an exhaust vent making it possible to regenerate the atmosphere in the reactor atmosphere and to keep the content of reducing gases at a value below the explosive limit 12- Device according to claim 1 1, characterized in that the auxiliary container is supplied, in whole or in part, with attack reagent by an ahquote of aluminate liquor taken from the circuit for recovering the attack liquor d '' an industrial bauxite treatment plant using the Bayer process 1 3- Device according to one of claims 1 1 and 12, characterized in that the spent attack reagent discharged by the rapid discharge means is accumulated in the auxiliary container - Device according to one of claims 1 1 and 12, characterized in that the spent attack reagent discharged by the rapid evacuation means is introduced into the circuit for recovering the attack liquor from an industrial installation of treatment of bauxite by the Bayer process.