FR2699530A1 - Ceramic for electromagnetic pump bodies with increased working - Google Patents

Abstract

A ceramic is used for the bodies of electromagnetic pumps for strongly reactive molten metals, offering an extremely low thermal conductivity and an enhanced resistance to the aggressive attack of very reactive molten metals such as aluminium and its refining alloys. The ceramic consists of a base with a very high porosity formed from silico-aluminous fibres incorporated in a mixture of tabular alumina and calcium aluminate in which the surface pores are closed by a double impregnation made up of a very thin layer itself coated with a glazed skin, both of which are resistant to attack by the molten metal.The process used for the application of the double protective layer onto the ceramic is also claimed and consists of applying a thin layer of boron nitride or molybdenum disulphide followed by a glazed skin coating of dilute phosphoric acid, mixed with an oxide of very high enthalpy such as alumina or chromium oxide.

Description

CERAMICS FOR BODIES OF ELECTROMAGNETIC PUMPS
The present invention relates to a ceramic for electromagnetic pump bodies for highly reactive molten metals, as well as a manufacturing process applicable to the pump body and its associated parts.

- Prior techniques
It is well known that molten metals such as aluminum and its refining products attack the part of the containers and pipes in contact with the molten metal.

 It is also known that to reduce this attack, these containers and these pipes are made with ceramics.

The contacting of the molten metal with the ceramic leads to metallization and metallic infiltration within the latter, as confirmed by a study by Monsieur
A. STARK, a translation of which appeared in the December 1991 issue of the French review "Hommes et Fonderie".

 The molten metal infiltrates the ceramic through the communicating pores of the refractory mass, extends on the surface and, penetrates deep until the isotherm of the solidification temperature. This penetration of the molten metal into the ceramic, referred to as metallization, is lower the higher the viscosity of the molten metal and the lower the thermal conductivity of the ceramic.

 Furthermore, the phenomenon of metallic infiltration of the ceramic by the molten metal placed in contact with it depends on the extent of the cracks in the mass of the ceramic, themselves caused by a local modification of the coefficient of expansion; this phenomenon also depends on the viscosity of the molten metal.

 In addition to the mechanical actions, the molten metal exerts on the ceramic a chemical action favored by metallization and by the infiltration of the corrosive metal into the mass of the ceramic.

 To reduce chemical attack, several solutions have been considered.

 The first of these was to favor the use of very dense ceramics in which the diameter of the pores is only a few tenths of a millimeter. However, ceramics having a reduced porosity offer a very high conductivity and consequently have an increase in the chances of cracking of the ceramic. In addition, the ceramic surrounding the magnetic masses of the pump must have a sufficiently small thickness so that the air gap of the active part of this pump is not more than a few centimeters. As a result, the ceramic layer at this level must be thin and therefore particularly insulating in order to maintain the magnetic masses at a temperature as far as possible from the Curie point. Such ceramics were then produced by dispersing in a cement resistant to high temperatures, a mixture of tabular alumina and fibers made up of a high percentage of silica (60%), and 40% of alumina. A ceramic with high thermal resistivity is thus obtained, but its resistance to chemical attacks has proved to be disappointing.

 Mullitic fibers have also been used, and here too, high porosity is obtained, but, on the other hand, the mechanical strength is disappointing.

 We therefore see that these ceramics must, on the one hand, have a very high thermal resistivity and therefore have as high a porosity as possible, and that they must, on the other hand, offer a very high resistance to attack. molten metals.

 These two properties required of ceramic for electromagnetic pumps are therefore contradictory.

 I1 therefore seemed necessary to separate the two extremely low thermal conductivity; then, cover such a ceramic with stable products completely protecting it from the chemical action of molten metals.

 These two problems have received a very satisfactory solution from the inventor.

 In a first step, we will examine the constitution and preparation of ceramics with very low thermal conductivity, and, in a second step, we will very significantly improve the resistance of these very porous ceramics to the destructive action of molten aluminum.

- Low conductivity ceramics
Based on research that has shown that in a product resistant to the chemical action of aluminum, the total amount of silica must not exceed 8% of the total composition of dry ceramic products, the author of the present invention judged that it was essential to develop a new ceramic incorporating fibers, in which, the proportion of alumina is higher than that of silica, and in which, these fibers are broken, then associated in small grains dispersed in aggregates and cement.

 So that the invention relates, firstly, to a ceramic for electromagnetic pumps in which silico-aluminous fibers are dispersed in extra pure alumina and calcium aluminate cement, characterized in that the fibers dispersed contain a percentage of alumina greater than 56% of the total weight of the fibers. In addition, this ceramic for electromagnetic pumps is characterized in that the percentage of cement used in its composition is of the order of 15 to 30% of the total weight of the ceramic.

 Finally, according to the invention, the fibers are incorporated into the aggregates and the cement in the form of "grains", of a few millimeters, and made up of agglomerated strands of broken fibers.

 The method of preparation of these ceramics will reveal other properties of these ceramics.

- Fiber preparation
According to the inventor, the silico-aluminous fiber thus chosen is dry chopped by means of a propeller. One thus obtains strands of fibers with a unit length of 1 to 3 millimeters, which spontaneously agglomerate upon breaking by the propeller, and stick together to form "grains of rice" from 2 to 3 millimeters long, and less than 2 millimeters in diameter.

- Ceramic preparation
The "rice grains" immersed in the aggregate mix absorb for their attachment with the cement a large amount of water, because part of the laitance of the cement penetrates inside the "grain of rice". But these "grains of rice", trapped in the ceramic, are only partially agglomerated inside; they therefore form as many points of microcracks promoting the reduction of thermal conductivity, but also the storage inside the ceramic.

 A cement often used in these manufactures is the calcium aluminate cement Secar (Lafarge cements for example). The mixing uses an amount of water between 10 and 20% of the total weight. This mixture is introduced into a mold. During filling, the mold is vibrated. The ceramic cement is allowed to set in its mold for a time which can go up to 2 or 3 days depending on the size of the casting, while cooling the mold to combat the exothermic reaction caused by the lime contained in the cement. .

 The product is then kept for a week in ambient air to perfect the holding of the ceramic.

 Then, the ceramic is subjected to a firing cycle at increasing temperatures comprising, a first stage at 70 "C. corresponding to the extraction of the evaporation calf, then a second stage at 140 C. leading to l extraction of the crystallization calf, finally, at a third level at 340 ° C. leading to the transformation of the cement; the cycle ends with a fourth stage for the ceramization of the product by sintering.

 A ceramic with a density close to 1.4 is thus obtained, having a thermal conductivity of the order of 0.4 W / m C, and a crushing resistance greater than 500 kg / cm 2.

 The product continues to offer a high porosity due to the large quantity of mixing water, so that wetting, metallization and metallic infiltration remain perfectly possible.

 However, this ceramic is already becoming operational, blocking only the surface pores.

- Surface treatment
The surface protection of the ceramic is carried out in two stages
The first step, tends to close only the pores of the surface of the ceramic in contact with the molten metal.

 During the second stage, the part of the ceramic in contact with the molten aluminum is glazed by depositing on the surface of the preceding layer a film which is not attacked by the molten aluminum.

 Clogging of surface pores is obtained by depositing, only on the surface, a very thin layer of boron nitride or molybdenum disulfide.

 To hang this product in the powder state on the surface of the ceramic, an oily support is used intended to limit the penetration of powders, such as for example glycerol. I1 may happen, during cooking, that the layer thus deposited is too thick. Boron nitride being a thermal conductor, the mass of the ceramic would then become conductive. To ensure that the layer remains extremely thin and simply closes the surface pores of the ceramic, demineralized water should be added beforehand, the proportion by volume of which will depend on the dimensions of the pores (for example 1 / 3 water for 2/3 glycerol).

 The protection of the ceramic, by this very thin layer of boron nitride, very seriously prolongs the life of the ceramic, but after a more or less long time, the flow of the molten metal along this layer boron nitride, causes it almost insensitively. The molten metal and more particularly the treatment products associated with it can then penetrate into the surface cracks thus created, and begin to attack the basic product of the ceramic.

 To further extend this surface protection significantly, a very thin film of products is deposited on the exterior surface of the ceramic using the bonding properties of phosphoric acid or its derivatives.

 This surface product intended to protect boron nitride and to prevent any chemical action on the ceramic is produced from mixtures comprising very fine alumina powders. Indeed, the attack on aluminum proceeds by reduction of the compound whose free enthalpy of formation is less negative than its own. This explains our choice for alumina, since few oxides have a more negative enthalpy.

Chromium oxide falls into this category with zirconia, magnesia and lime as common products.

 After many experiments, two solutions were selected.

 The first solution consists in using aluminum monophosphate loaded with 3 to 5 times its weight of very fine alumina, depending on the surface condition of the ceramic. This mixture is diluted in demineralized water so as to obtain a smooth liquid mixture.

As an example, 1000 g of demineralized water can be poured into half a liter. alumina and 250 g. monophosphate
The second solution consists in incorporating into demineralized or permuted water a commercial mixture of phosphorus trioxide p2os and of alumina added with chromium oxide Cr 2 oh,
The attachment of the protective layer against attack by molten aluminum on the ceramic surface is carried out as follows
After cooking and complete cooling of the pump body, the metal inlets in the pump are sealed and the boron nitride mixture is poured into the top outlet of the pump. After a certain time, the mixture is recovered and the pump is left to dry for less than 48 hours.

 After drying, the same operation is repeated, but this time with one of the two products based on phosphoric acid, the composition of which we have given.

 After draining the phosphoric acid mixtures from the pump, allow the impregnated ceramic to dry again for less than 48 hours.

 For the parts of the pump body which are also in contact with the aluminum of the oven, and which cannot be impregnated as above, such as the external faces of the pump body for example, they will be brushed with the brush of 1 3 coats each of the two products, respecting the drying times in between.

 Finally, we will cook the pump body, impregnated and brushed with these two mixtures, following a cooking cycle with a level to extract the evaporation water, followed by a higher level to perfect the water extraction remaining evaporation, and finally a cooking level above 3600C.

 The basting process with these products is also used successfully to treat the surfaces of ceramics other than those of the pump body. This treatment has been used, for example, on the protective skirt of the level measurement which is made of ceramic based on silicon carbide. It has also been used on other ceramics with alumina aggregates only, and without fibers.

- Conclusion
It can be seen that the present invention finally makes it possible to produce electromagnetic pumps for reactive molten metals which can actually be used in industry.

 Indeed, it is now more than 30 years that we know how to produce electromagnetic pumps in which the pump duct is made of ceramics. The ceramic of these pumps must have two contradictory characteristics: on the one hand, it must have an extremely low thermal conductivity, therefore, it must be porous, and on the other hand, it must resist the destructive action of molten metal, which supposes that molten metal is prohibited from seeping into the pores of the ceramic and attacking it chemically. As a result, all the pumps produced so far had an insufficient service life for industrial use.

 According to the present invention, a very porous ceramic is produced but already having, by virtue of its constitution, a non-negligible resistance to the action of molten metals.

This resistance is considerably reinforced by the attachment to the surface of the ceramic, which will then be brought into contact with the molten metals, with a layer of a product selectively sealing the surface pores and only these.

 To further extend the life of the ceramic, a film enriched with high enthalpy oxides is superimposed on the first thin layer, thus rendering the surface called to be in contact with the molten metals perfectly inert to them.

 So that electromagnetic pumps produced according to the invention can resist attack by molten metals for a period long enough to allow them to respond correctly to their use in industry.

Claims (11)

- CLAIMS  1) Ceramic for electromagnetic pumps offering, on the one hand, an extremely low thermal conductivity and, on the other hand, a prolonged resistance to the aggression of very active molten metals such as aluminum and its refining products, characterized , in that it consists of a very high porosity base formed of silico-aluminous fibers incorporated in a mixture of tabular alumina and calcium aluminate base in which the surface pores are closed by a double impregnation formed, by a very thin layer, itself covered by a layer of frosting, both resistant to attack by molten metals.  2) Ceramic for electromagnetic pumps offering, according to claim (1), a very low thermal conductivity and a prolonged resistance to chemical attack on metals, characterized in that the fibers incorporated in the mixture of tabular alumina and cement contain a percentage of alumina significantly higher than the percentage of silica.  3) Ceramic for electromagnetic pumps offering very low thermal conductivity and prolonged resistance to chemical attack on metals, according to claim (2), characterized in that the fibers are incorporated in the form of "grains of rice" formed from small fragments of fibers stuck together, 1 to 3 millimeters in length and less than 2 millimeters in diameter.  4) Ceramic for electromagnetic pumps having a prolonged resistance to chemical attack by molten metals, according to claim (3), characterized in that it offers a thermal conductivity of the order of 0.4 W / mOC, obtained by incorporating "grains of rice" into the mixture of tabular alumina and cement, and by firing at four thermal levels, the first at 70 "C, the second at 1400C, the third at 3400C, corresponding to the transformation of the cement, and a fourth stage corresponding to ceramization by sintering.  5) Ceramic with very low thermal conductivity and prolonged resistance to the attack of molten metals, according to claims (3) and (4), characterized in that the very thin layer sealing by impregnating the surface pores of the ceramic base consists of a very thin film of boron nitride.  6) Ceramic with very low thermal conductivity and prolonged resistance to the attack of molten metals, according to claims (3) and (4), characterized in that the very thin layer sealing by impregnating the surface pores of the ceramic base consists of a very thin film of molybdenum disulfide.  7) Ceramic according to claims (5) and (6), offering a prolonged resistance to the attack of molten metals, characterized in that the surface glazing film is obtained by impregnation of aluminum monophosphate charged with alumina.  8) Ceramic according to claims (5) and (6) offering a prolonged resistance to the attack of molten metals, characterized in that the surface glazing film is obtained by the impregnation of dilute phosphoric acid, mixed with a or more high enthalpy oxides.  9) A method of depositing a protective double layer on the surface of a very porous ceramic offering, according to claim (4), a thermal conductivity of a few tenths of watts per meter and per centigrade degree, characterized in that l 'impregnation of the said ceramic with a very thin layer sealing the surface pores of the base must precede by less than 48 hours the impregnation with the film resistant to the attack of molten metals.  10) Method according to claim (9), characterized in that after the interior impregnation and the exterior brushing of the pump body, the latter undergoes a cooking cycle comprising a bearing for extracting the evaporation water , a second higher level to extract the remaining water, and finally, a level of cooking at a temperature above 360 "C.  11) Method according to claims (9) and (10), characterized in that the interior impregnation and the exterior basting are applied to the annex parts of the pump.