FR2688420A1 - FINELY DIVIDED OXIDE POWDER. - Google Patents

Abstract

The invention relates to a finely divided oxide powder, in particular based on magnesium and aluminum, with a maximum degree of hydration of 10% by weight, in which the individual oxide particles are provided with a coating. thin, preventing stronger hydration. The product is characterized by the following steps: 1.1 preparation of an aqueous suspension from an oxide powder and a coating agent, 1.2 homogenization of the suspension, 1.3 then exposure of the suspension to spray drying , at a temperature at which the coating remains stable. Application to the manufacture of shaped parts and ceramic masses of high density.

Description

The invention relates to an oxide powder

  finely divided, especially based on magnesium and aluminum, as well as the use of this finely powdered oxide

divided to surface modified.

  To ensure the production of shaped parts and ceramic masses of high density, it is common to use additives based on finely divided oxide powder, so-called micro-powders, for example based on SiO 2, Cr 203 and A 1203 The finely divided particles of this micro-powder fill the hollow spaces between the coarse grains of the matrix material, causing the open porosity to decrease and the strength and

  bulk density are increased.

  The use of such micro-powders is best known in the case of thixotropic refractory casting masses. Thus, AT 392 464 B1 describes a magnesium oxide micro-powder with a particle size of less than 1 μm, the individual particles having a coating composed of a hydrophobic substance.

  substances, the internal friction of the micro-

  magnesium oxide powder and the particles receive, in cooperation with a binder, a high mobility, so that, during the forming of the ceramic, a high degree of compaction is obtained The hydrophobic coating is also intended to prevent as much as possible the hydration of the oxide particles by giving magnesium hydroxide, when the oxide particles are treated in a

  aqueous medium to give refractory products.

  In practice, it is known to manufacture such finely divided, surface-modified oxide powders in a relatively expensive process by mixing the coating agent with the finely divided oxide powder.

  divided, in an intensive mixture, at a high temperature.

  The purpose of this coating process is to apply a layer as monomecular as possible of the coating product to the individual powder particles and to react the functional groups of the coating product with the surface of the powder particles in the known process. because of the length of the treatment time and the fact that the powder is mixed in the dry state with the coating product, agglomerations of the individual particles are obtained. The consequence is that the powder agglomerates are frequently covered , which is however undesirable, because

  the properties of the products thus manufactured are degraded.

  Therefore, to improve the homogeneity of the coating (around each individual particle), it is also known, in practice, to use the coating products in the form of dilute solutions, using non-aqueous solvents. , easily volatile The solvent evaporates during the coating process, which makes necessary a corresponding technical design, providing a safety of the installation of coating and

  the elimination of solvent vapors.

  It is an object of the invention to provide a finely divided oxide powder having as much monomolecular use as possible of the surfaces of the individualized powder particles with a coating and which can be manufactured in a simple process without the use of

  of solvents and practically without hydration.

  To this end, the invention describes, in its most general embodiment, a finely divided oxide powder, in particular based on magnesium and aluminum, with a maximum degree of hydration of 10% by weight. wherein the individual oxide particles are provided with a thin coating, preventing greater hydration characterized by being obtained by the following process steps, carried out directly one after the other: first preparing an aqueous suspension from an oxide powder and a coating agent, then homogenizing the suspension, then exposing the suspension to spray drying, at a temperature at which the

coating remains stable.

  Surprisingly, it has been found that such a finely divided oxide powder can be manufactured with a relatively low degree of hydration, also in an aqueous suspension, insofar as the reaction process is carried out without delay. spray drying cited, following the

  put into the form of slurry of initial material.

  US-A-3,843,380 discloses a method for treating inorganic pigmented aggregates for use in paint-stain or synthetic materials. An aqueous, pumpable slurry is mixed with from 0.1 to 5 % by weight of a dispersible thickener in the water and the slurry thus atomized is then subjected to a

  spray drying treatment.

  These inorganic pigments consist for example of

titanium oxide.

  In addition, the maximum degree of hydration of the powder

is 5% by weight.

  A use of such a drying process by

  spraying, for oxide powders, of the type referred to above

  above, put into the form of sludge in an aqueous medium and subject to hydration appears impossible because of subjection to the hydration of the oxide powder; likewise, it has been found that spray drying, carried out immediately after sludging, largely or completely stops any (undesirable) formation of hydration, but that, moreover, it favors in a particular proportion a monomolecular coating. of the surface of different

oxide particles.

  In this sense, it is advantageous to provide a slurry treatment as short as possible. Sludge for 6 hours should be considered as a maximum; according to a preferred embodiment, the treatment should last less than one hour. The longer the oxide particles remain in the aqueous solution before the spray absorption, the lower the degree of hydration of the dried particles, this degree being possible. be reduced to values well below 10% by weight (partially below 3% by weight). The suspension treatment must then be carried out so at least so carefully that a homogeneous mixture takes place,

  to provide the desired monomolecular coating.

  The problem is solved particularly advantageously when the suspension is not heated to more than 400 before the spray drying.

  maximum of 10 ° C was optimal.

  Surprisingly, it is also possible, in the case of using hydration-sensitive oxide powders, by applying spray drying, to coat such oxide powders in an aqueous suspension, when all process steps are performed prior to spray drying at the indicated low temperatures and in times as short as

possible.

  An additional advantage lies in the fact that a complete, homogeneous coating is obtained on the powder particles with a minimal consumption of coating material. At the same time, the use of non-aqueous solvents, which are very volatile, is avoided. state of the art, necessary to achieve the same effect in the case of conventional coating technologies and which led to

to the problems mentioned above.

  Finally, it is also important that the spray drying be carried out at a temperature at which the coating remains stable. The coating product simultaneously acts as a separating agent and largely prevents agglomeration of the powder particles during spray drying. the finely divided oxide powders according to the invention are distinguished by a complete surface coating of the primary particles; it does not easily occur, or not at all agglomeration of the particles To this extent also, any subsequent grinding is superfluous There is also disappearance of the release, occurring in the case of such grinding, active surfaces not covered with the powder, which would lead again, when subsequently using the finely divided oxide powder, to

  undesirable (additional) reactions of hydration.

  Tests have shown that starting from the same oxide powders and coating products the average particle size can be reduced by about half by using the described process. To obtain a further reduction of the particle size, the suspension may for example be subjected in addition to a grinding treatment, before or after addition of the product

coating.

  As far as the finely divided oxide powder of magnesium oxide powder is concerned, a particularly suitable starting material is in particular a magnesium oxide obtained by pyrohydrolysis of a solution.

purified magnesium chloride.

  With regard to the choice of the coating product, the following criteria are considered: The coating product must not induce any adverse reaction with the water, its vapor pressure at the preferred temperature of the spray drying (initial product temperature of approx. at 1300 C) should be as low as possible and as stated above it should be stable at these maximum temperatures. Particularly suitable are carbon-containing acids and their derivatives (with reactive and functional groups), such as aminocarbon acids, for example 6-amino caproic acid, crotonic acid and saturated fatty acids. or unsaturated Tests have also revealed that hydrophobic coating products, such as stearic or oleic acids, as well as water-soluble organic polymeric compounds, such as lignin sulphonate, polyacrylate and / or polyvinyl alcohols can be used and

applied in the manner mentioned.

  The invention also provides for the addition of additives to the suspension, in addition to the coating product, prior to spray drying. Binders, dispersants or, very generally, finely divided oxidizing compounds are provided. as a result, a homogeneous distribution of these compounds during the subsequent treatment of the coated powder by producing shaped parts. This is advantageous, in particular for

  performing a homogeneous divided binder phase.

  The finely divided oxide powders are surprisingly suitable for the manufacture of high density ceramic shaped parts, for example of brick, but

  also masses, refractory ceramics.

  The invention is explained below with the help of

  different examples of realization.

  A magnesium oxide powder A is obtained by calcination-sputtering of a purified magnesium chloride solution. This material, whose physical and chemical properties are shown in Table 1, is now restated as follows.

Example 1

  The magnesium oxide powder A dispersed in water at 100 ° C. and is homogeneously mixed with 1% by weight

  of 6-amino caproic acid (relative to magnesium oxide).

  The suspension obtained is then immediately subjected to spray drying. Pre-grinding, carried out for example in a ball mill with stirrer, proves to be optimal. The properties of the magnesium oxide powder B thus obtained are also indicated in the table. The degree of hydration after spray drying is only 2% by weight. In addition, the degree of hydration was determined after the samples had been exposed in an air-conditioning cabinet for 48 hours at a temperature of 50.degree. 400 C, at a

  atmosphere with a relative humidity of 95%.

  Calculation of the degree of hydration was done from the increase in weight following water retention, a degree of hydration of 100% corresponding to a complete conversion of magnesium oxide to magnesium hydroxide . The values d 10, d 5 o and d 9 go each indicate the particle diameter for which 10, 50, respectively

  % of the material is below the indicated value.

Example 2

  The magnesium oxide powder A is dispersed at 10 ° C. in water and mixed with 1% by weight of caproic acid (relative to magnesium oxide). The suspension is then

  immediately spray-dried.

  Chemical analysis and test results of powder C

  covered, thus, obtained are shown in Table 1.

Table 1

  Mg powder OABC Chemical analysis (% by weight) Mg O 98.4 97.4 97.5 Si O 2 0.002 0.002 0.002 Ca O 0.51 0.49 0.50 Fe 2 03 0.006 0.006 0.006 Ci 1.03 1 , 02 1.01

SO 4 0.021 0.019 0.020

  Coating product 1 1 Loss on fire 1.32 2.95 3.34

(1000 C, 2 hours) -

  Degree of hydration (%) after manufacture 2 2 Particle size analysis (lm) d 10 0.74 0.60 0.64 d 50 2.20 1.94 1.5 d 50 9.18 5.97 4.0 Degree The results of the tests show that the magnesium oxide particles according to the invention exhibit a very fine particle division. They also have a distinct tendency to hydration. reduced, even after long storage in moisture, compared to powder A. There is in addition a limitation of the tendency to hydration, due to the low polarity of the coating product. The following example is given to illustrate the benefits

of the oxide powder described.

Example 3

  Brick mixtures, composed of 10% by weight of the magnesium oxide powder A, respectively,% by weight of the magnesium oxide powder B and in each case 90% of sintering magnesia, with a smaller particle size are prepared. at 5 mm, obtained from low iron natural magnesite The mixtures are compressed to form bricks and calcined at 1850 ° C. Table 2 shows the test values determined from

of these bricks.

Table 2

  With powder With Mg OA powder of untreated Mg OB covered Bulk density 2.95 3.02 (g / cm 3) Open porosity 17.4 13.5 (% by volume) Resistance to pressure 48 1 66 at room temperature (N / mm 2) It is clearly seen that the bricks manufactured using the magnesium oxide powder according to the invention have a higher bulk density, a much lower open porosity and a resistance to the

significantly higher pressure.

Claims (11)

  1 finely divided oxide powder, in particular based on magnesium and aluminum, with a maximum degree of hydration of 10% by weight, in which the individual oxide particles are provided with a thin coating, preventing greater hydration characterized by the process steps, carried out directly following one another: 1.1 preparation of an aqueous suspension from an oxide powder and a coating agent, 1.2 homogenization of the suspension, 1.3 thereafter exposure of the suspension to spray drying, at a temperature at which the coating remains stable.   2 oxide powder according to claim 1, characterized in that its maximum degree of hydration is 5% by weight.   3 oxide powder according to claim 1 or 2, characterized in that it is obtained by a maximum contact time of six hours of the oxide particles and the coating agent in the aqueous suspension, before spray drying.   4 oxide powder according to claim 3, characterized in that it is obtained by a maximum contact time of one hour of the oxide particles and the coating agent in the aqueous suspension, before spray drying.   5 oxide powder according to one of claims 1 to   4, characterized in that it is obtained by heating the suspension to a maximum temperature of 400 before the spray drying.   6. The oxide powder according to claim 5, characterized in that it is obtained by heating the suspension to a maximum temperature of 10 ° C. spray drying.   7 oxide powder according to one of claims 1 to   6, characterized in that the coating product is composed of an acid containing carbon or its derivatives,   or water soluble polymeric compounds.   8 oxide powder according to one of claims 1 to   7, characterized in that the superficial covering of   the covering agent is monomolecular.   9 oxide powder according to one of claims 1 to   8, characterized in that it is obtained by adding a binder or dispersant in the suspension before drying by spraying.   Oxide powder according to one of Claims 1 to   9, characterized in that it is obtained by wet milling of the suspension.   11 oxide powder according to one of claims 1 to   characterized in that it is manufactured by drying   by spraying at a maximum temperature of 130 ° C.   12 Use of a finely divided oxide powder,   modified surface according to one of claims 1 to 11,   for the manufacture of shaped parts and masses high density ceramics.