GB2264939A

GB2264939A - Coating an oxide powder by spray-drying

Info

Publication number: GB2264939A
Application number: GB9304891A
Authority: GB
Inventors: Andreas Meier; Gerhard Graf; Klaus Doesinger
Original assignee: Veitscher Magnesitwerke AG
Current assignee: Veitscher Magnesitwerke AG
Priority date: 1992-03-10
Filing date: 1993-03-10
Publication date: 1993-09-15
Anticipated expiration: 2013-03-10
Also published as: DE4306234C2; GB2264939B; AT398967B; ITMI930464A1; IT1270867B; GB9304891D0; DE4306234A1; FR2688420A1; ATA45892A; ITMI930464A0

Description

FINE-GRAIN OXIDE POWDER 2264939 The invention relates to a fine-grain

oxide powder, in particular based on magnesium and aluminium as well as the application of this surface- modif ied fine-grain oxide powder.

In order to produce ceramic shaped parts and materials of high density additives of fine-grain oxide powders, so-called micro-powders, based f or example on S'02, Cr.03 and A1203 are often employed. The fine-grain particles of these micro-powders fill up the cavities between the rougher grains of the matrix material, as a result of which the open porosity is lowered and the strength as well as the bulk density is increased.

The use of such micro-powders is known above all with thixotropic fireproof casting materials.

AT 392 464 Bl describes a magnesium oxide micropowder with a particle size which is smaller than 15 pm, wherein the individual particles have a coating of a hydrophobizing substance. The internal friction of the magnesium oxide micro-powder is reduced through these substances and the particles obtain a high degree of mobility in the combined effect with a binding agent, so that when the ceramics are shaped a high compression ratio is achieved. The hydrophobic coating has, furthermore, the purpose of inhibiting, as far as possible, hydration of the oxide particles to form magnesium hydroxide, if these are processed in an aqueous medium to form fireproof products.

It is known from practice that such surf ace-modif ied fine-grain oxide powders are produced according to a relatively costly method by mixing the coating agent with the fine-grain oxide powder in an intensive mixer at an elevated temperature. The object of this coating process is to apply a layer of the coating agent, which is as monomolecular as possible, onto the individual powder particles and bring functional groups of the coating agent to react with the surface of the powder particles. With the known method, however,' because of the long treatment time and because the powder is mixed with coating agent in the dry state, the result is agglomerations of the individual particles. The result is that powder agglomerates are often coated, which is, however, undesirable, since the properties of the products produced therefrom deteriorate. In order to improve the smoothness of the coating (about each individual particle) a method is also known for that reason from practice of employing the coating agent in the form of diluted solutions using non-aqueous, readily volatile solvents. The solvent evaporates during the coating process, which makes it necessary to have a corresponding saf ety-technical design of the coating unit and a disposal of the solvent vapours.

The underlying object of the invention is to indicate a fine-grain oxide powder, which has a covering of the surfaces of the individual powder particles with a coating, said covering being as completely monomolecular as possible, and can be produced in a simple method without using solvents and without substantial hydration.

In addition the invention describes in its most general embodiment a finegrain oxide powder, in particular based on magnesium or aluminium, with a maximum degree of hydration of 10-'. by weight, in which the individual oxide particles are provided with a thin coating inhibiting further hydration and which was obtained by the following steps directly following each other:

- firstly an aqueous suspension of a pure oxide powder and a coating agent were produced, - the suspension is then homogenised, - then the suspension is subjected to a spray drying at a temperature at which the coating remains stable.

Surprisingly it has been shown that such a finegrain oxide powder can also be produced with a relatively low degree of hydration in an aqueous suspension, unless the said spray drying process immediately follows the suspension of the starting material.

From US-A 3 843 380 a method for treating mineral pigment aggregates is known, which serve for use in paints or plastics. In this a pumpable aqueous slurry with 0.1 to 5% by weight of a thickening agent able to disperse in water is added and the slurry-- atomised in this manner is then subjected to a spray drying treatment.

The said mineral pigments consist for example of titanium dioxide.

An application of such a spray drying method for the oxide powders, of the above-mentioned type, reduced to slurry in an aqueous medium and susceptible to hydration, does not appear possible because of the susceptibility to hydration of the oxide powder; however, it was established that a spray drying carried out directly following the suspension, not only halts as extensively as possible or stops an undesired hydrate formation, but moreover favours, to a particular extent, a monomolecular surface coating of the individual oxide particles.

For this purpose it is advantageous to organise the suspension treatment to be as short as possible. A suspension over a time span of 6 hours may count as maximum; according to a preferred embodiment the treatment should last less than an hour. The shorter the oxide particles remain in the aqueous solution before the spray absorption the less the degree of hydration of the dried particles which can then be lowered to values of far below 10% by weight (sometimes below 3% by weight).

The suspension treatment must, however, be carried out at least as carefully as possible, so that a homogeneous mixing takes place in order to ensure the desired monomolecular coating.

The set object is achieved in a particularly advantageous manner if the suspension does not exceed a temperature of 400C before the spray treatment. A maximum temperature of 1WC has proven to be optimum.

Surprisingly it is therefore also possible with hydration- sensitive oxide powders through the use of spray drying to coat such oxide powders in an aqueous suspension, if all method steps have been carried out before the spray drying in the indicated low temperatures and in periods which are as short as possible.

A further advantage consists in that a homogeneous complete coating on the powder particles with minimum consumption of the coating agent is achieved. At the same time the use of non-aqueous readily volatile solvents, according to the state of the art, is avoided, said solvents were necessary to achieve the same effect with the coating techniques used hitherto and which led to the above- mentioned problems.

Finally, it is also important that the spray drying is carried out at a temperature, at which the coating remains stable. The coating agent acts at the same time as release agent and prevents as far as possible agglomeration of the powder particles during the spray drying. In this respect the fine-grain oxide powders according to the invention are distinguished by a holohedral surface coating of the primary particles; particle agglomerates practically do not occur at all. In this respect a subsequent grinding is also superfluous. Also the exposure of uncoated active powder surfaces occurring with such a grinding is omitted, which with the subsequent use of the fine-grain oxide powder would lead again to undesired (further) hydration reactions.

Tests have shown that - starting with identical oxide powders and coating agents - the average particle size d5, can be lowered by approximately half by using the described method. For further reduction of the particle size the suspension can, for example, be subjected in addition before or after addition of the coating agent to a grinding treatment.

Provided that the fine-grain powder oxide powder is magnesium oxide powder, a magnesium oxide is particularly suitable as starting material which was produced by pyrohydrolysis of a purified magnesium chloride solution.

The following criteria apply for selecting the coating agent. The coating agent should not undergo an undesirable reaction with water, its vapour pressure at the preferred temperature of the spray drying (approximately 100 to 130'C product -starting -temperature) should be as low as possible and - as described above - it should be stable at these.maximum temperatures.

In this context carboxylic acids and their derivatives (with reactive and functional groups) such as amino carboxylic acids, for example 6-amino hexanoic acid, crotonic acid and saturated or unsaturated fatty acids are particularly suitable. Tests have also shown that hydrophobic coating agents such as stearic acid or oleic acid as well as water soluble, organic polymeric compounds such as lignin sulphonate, polyacrylates and/or polyvinyl alcohols can be used and applied in the said manner.

The invention also provides that additives are added to the suspension before the spray drying in addition to the coating agent. Binding agents, dispersing agents or quite general oxidic, fine-grain components are meant in this connection. As a result a homogeneous distribution of these components is achieved with the later processing of the coated powder to form shaped parts. This is of particular advantage for the formation of a homogeneously distributed binding phase.

The fine-grain oxide powders are suitable in an excellent manner for producing high-density ceramic shaped parts, for example fireproof ceramic bricks, as well as masses.

Further features of the invention are found in the features of the subclaims as well as the other application documents.

The invention will be illustrated in more detail in the following with the aid of different exemplary embodiments.

Through spray sintering of a purified magnesium chloride solution a magnesium oxide powder A is obtained. This material, the physical and chemical data of which are shown in table 1, is now processed as follows.

Exam-ple 1:

The magnesium oxide powder A is dispersed at 100C in water and mixed homogeneously with 1% by weight 6-amino hexanoic acid (relative to the magnesium oxide).

The suspension obtained is then immediately subjected to a spray drying. A previous grinding for example in a stirring ball mill is optional. The properties of the magnesium oxide powder B thus obtained are also shown in table 1. The degree of hydration after spray drying is only 2-% by weight. Furthermore the degree of hydration is determined once the samples have been exposed in a conditioning cabinet for 48 hours at a temperature of 4WC in an atmosphere with 95% by weight relative air humidity.

The calculation of the degree of hydration is taken from the increase in weight as a result of absorption of water, whereby a degree of hydration of 100% correspondto the complete conversion of the magnesium oxide to magnesium hydroxide.

The values d1O. d5O, and dgc) indicate the particle diameters with which 10, 50 or 90% of the material is smaller than the given value.

Example 2:

The magnesium oxide powder A is dispersed in water at 100C and mixed with 1-'o' by weight hexanoic acid (relative to the magnesium oxide). The suspension is subjected directly thereafter to a spray drying. The chemical analysis and test results of the coated powder C obtained thus are shown in table 1.

7 - TABLE 1

MgO-Powder A B c Chemical Analysis (% by weight) Mgo 98.4 97.4 97.5 S'02 0.002 0.002 0.002 CaO 0.51 0.49 0.50 Fe203 0.006 0.006 0.006 Cl- 1.03 1.02 1.01 S04--- 0.021 0.019 0.020 Coating Agent - 1 1 Loss at red heat 1.32 2.95 3.34 (10000C, 2 hours) Degree of hydration after production 2 2 Analysis of grains (pm) d10 0.74 0.60 0.64 dso 2.20 1.94 1.5 d90 9.18 5.97 4.0 Degree of hydration (%) after damp storage 60 30 15 The test results prove that the magnesium oxide particles according to the invention have a high degree of fineness. They show, furthermore, a clearly reduced inclination to hydration, even after longer damp storage in comparison with powder A. A low polarity of the coating agent reduces, moreover, the inclination to hydrate.

The following example serves to illustrate the advantages of the described oxide powder.

Exam-ple-3:

Brick mixtures were produced from 10 by weight magnesium oxide powder A or 10-'0,> by weight magnesium oxide powder B and in each case 90-% by weight sintering magnesium oxide in a granular size of less than 5mm, produced from low-iron natural magnesite. The mixtures are pressed to form bricks and fired at 1,8500C. Table 2 shows the test values determined from these bricks.

TABLE 2

Bulk density (g/CM3) 2.95 Open porosity (Vol.) 17.4 Compressive N/mM2 48.1 strength at room temperature With untreated With coated MgO-Powder A MgO-Powder B 3.02 13.5 66 It can be seen clearly that the bricks produced using the magnesium oxide powders according to the invention have a high bulk density, a clearly lower open porosity and a clearly increased compressive strength.

- 9

Claims

CLAIMS 1. A fine-grain oxide powder, in particular based on magnesium and aluminium, with a maximum degree of hydration of 10% by weight, in which the individual b.-z.lde, particles are provided with a thin coating inhibiting further hydration, obtained by the following steps directly following each other:

1.1 an aqueous suspension of a pure oxide powder and a coating agent are produced, 1.2 the suspension is homogenised, 1.3 then the suspension is subjected to a spray drying at a temperature at which the coating remains stable.

2. An oxide powder according to claim 1 with a maximum degree of hydration of 5% by weight.

3. An oxide powder according to claim 1 or 2, obtained by a maximum period of contact of 6 hours of the oxide particles and of the coating agent in the aqueous suspension before the spray drying.

4. An oxide powder according to claim 3, obtained by a maximum period of contact of one hour of the oxide particles and of the coating agent in the aqueous suspension before the spray drying.

5. An oxide powder according to one of claims 1 to 4, obtained with a maximum temperature of WC imposed on the suspension before the spray drying.

6. An oxide powder according to claim 5, obtained with a maximum temperature of 100C imposed on the suspension before the spray drying.

7. An oxide powder according to claims 1 to 6, in which the coating agent consists of a carboxylic acid or derivatives thereof or of water-soluble polymeric compounds.

8. An oxide powder according to one of claims 1 -9--o 7 with a monomolecular surface covering of the coating agent.

9. An oxide powder according to one of claims 1 to 8, obtained by adding a binding agent or dispersing agent in the suspension before the spray drying.

10. An oxide powder according to one of claims 1 to 9, obtained by a wet grinding of the suspension.

11. An oxide powder according to ona of claims 1 to 10, produced by spray drying at a maximum of 13WC.

12. An oxide powder substantially as herein described with reference to powders B and C of the described examples.

13. Application of a surfacemodified fine-grain oxide powder according to one of claims 1 to 11, for producing high-density ceramic shaped parts and materials.