DE4306234A1 - - Google Patents

Abstract

A fine-grain oxide powder, in particular based on MgO, 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 by spray drying an aqueous suspension of the oxide powder and a coating agent. The coating agent may be a carboxylic acid or a derivative thereof.

Description

The invention relates to a finely divided oxide powder, ins special based on magnesium and aluminum as well as the Use of this surface-modified fine-particle Oxide powder.

Additions of fine-particle oxide powders, so-called micropowders, for example based on SiO ₂₁ , Cr ₂ O ₃ and Al ₂ O ₃ , are often used to produce ceramic molded parts and high-density materials. The fine particles of these micropowders fill the voids between the coarser grains of the matrix material, which lowers the open porosity and increases the strength and bulk density.

The use of such micropowder is particularly important at thixo tropical refractory casting compounds.  

AT 3 92 464 B1 describes a magnesium oxide micro powder with a particle size smaller than 15 microns, the individual particles a coating of a hydropho possessing substance. Through these substances reduce the internal friction of the magnesium oxide micropowder sets and the particles get in cooperation with a binder a high degree of mobility, so that a high degree of compaction when shaping the ceramic is achieved. The hydrophobic coating still has the purpose of hydrating the oxide particles to magnesium prevent hydroxide as much as possible if this processed into refractory products in an aqueous environment will.

From practice it is known that such top surfaces modified fine particle oxide powder after a relative costly process by mixing the coating with the finely divided oxide powder in one Intensive mixers can be produced at elevated temperatures. The aim of this coating process is one if possible monomolecular layer of the coating agent on the individual powder particles and functional Groups of the coating agent with the surface of the To bring powder particles into reaction. With the well-known However, procedures come about because of the long treatment time and in that the powder is dry mixed with the coating agent to form agglomerates ions of the individual particles. The result is that often Powder agglomerates are coated, but this is not is desired, because then the properties of it manufactured products deteriorate. For improvement the uniformity of the coating (around each one Particles) it is therefore also known from practice the coating agents in the form of dilute solutions Use of non-aqueous, volatile solution  use funds. The solvent evaporates on Coating process, what a corresponding security technical design of the coating system and a Disposal of the solvent vapors is required.

The invention has for its object a finely divided Specify oxide powder that is as completely mono molecular coverage of the surfaces of the individual powders has particles with a coating and in one simple procedures without the use of solvents and can be produced without substantial hydration.

The most general description of the invention describes this leadership form a finely divided oxide powder, in particular based on magnesium or aluminum, with a maximum Degree of hydration of 10 wt .-%, at which the individual Oxide particles with one to prevent further hydration the thin coating are provided by fol receive the steps immediately following one another has been:

• - First, an aqueous suspension from a pure Oxide powder and a coating agent,
• - the suspension is then homogenized,
• - Then the suspension is spray dried subjected to a temperature at which the coating remains stable.

Surprisingly, it has been shown that such a finely divided oxide powder with a relatively low hydra degree of production also in aqueous suspension  that can, provided the slurry of the exit materials immediately the spray drying corridor connects.

From US-A-38 43 380 is a method for treatment of mineral pigment aggregates known for ver serve in colors or plastics. Doing so a pumpable aqueous sludge with 0.1 to 5% by weight of a water-dispersible thickener and the sludge so atomized then becomes one Subjected to spray drying treatment.

The mineral pigments mentioned exist, for example wise from titanium dioxide.

An application of such a spray drying process for those slurried in an aqueous medium and hydration-sensitive oxide powder of the above Art appears because of the susceptibility to hydration of the oxide powder not possible; nevertheless it was found that one immediately following the slurry spray drying performed not only one (undesirable) Largely prevents or stops hydration, but also a monomolecule lare surface coating of the individual oxide particles favored.

In this sense, it is advantageous to use the suspension treatment to make it as short as possible. A slurry over a period of 6 hours may apply as a maximum; according to a preferred embodiment, the treatment less than an hour. The shorter the oxide particles  remain in the aqueous solution before spray absorption, the lower the degree of hydration of the dried Particle that has values far below 10% by weight (partially below 3 wt .-%) can be reduced.

The suspension treatment must at least be like this be carried out carefully that a homogeneous ver mixing takes place to the desired monomolecular Ensure coating.

The task is particularly advantageous Way solved when the suspension before the spray treatment temperature does not exceed 40 ° C. A tem temperature maximum of 10 ° C has been found to be optimal poses.

Surprisingly, it is also the case with hydrations sensitive oxide powders by using the spray drying possible, such oxide powder in an aqueous Coating suspension when all process steps before spray drying at the specified low temperatures and in as short a time as possible.

Another advantage is that a homogeneous, complete coating on the powder particles minimal consumption of the coating agent reached becomes. At the same time, the use of non-aqueous, volatile solvents according to the prior art avoided to achieve the same effect in the previous coating technologies were required and led to the problems mentioned above.  

Finally, it is also important that the spray drying at a temperature is carried out at which the coating tion remains stable. The coating agent works the same as a release agent and largely prevents one Agglomeration of the powder particles during spray drying. Inso the finely divided particles according to the invention are widely distinguished Oxide powder through a full surface coating the primary particles from; Particle agglomerates occur this way well not on. In this respect, there is also a refill superfluous. This also does not apply to such Exposure of uncoated active substances occurring during grinding Powder surfaces what in subsequent use the finely divided oxide powder in turn to undesirable (further) hydration reactions would lead.

Experiments have shown that - starting from identical oxide powders and coating agents - the average particle size d ₅₀ can be reduced by about half by using the described method. To further reduce the particle size, the suspension can also be subjected to a grinding treatment, for example, before or after the addition of the coating agent.

If the finely divided oxide powder is magnesium oxide powder, is suitable as a starting material ins special a magnesium oxide, which by pyrohydrolysis of a purified magnesium chloride solution was obtained.

The following apply to the selection of the coating agent Criteria. The coating agent should not be undesirable React with water, its vapor pressure at the  preferred temperature of spray drying (about 100 to 130 ° C product outlet temperature) should be as low as possible and it should - as explained above - with these maxi paint temperatures should be stable.

Carboxylic acids are particularly suitable in this context and their derivatives (with reactive and functional groups) such as aminocarboxylic acids, for example 6-aminohexanoic acid, Crotonic acid and saturated or unsaturated fatty acids. Experiments have shown that also hydrophobic coating agents such as stearic or oleic acid and water-soluble, organic polymer compounds such as lignin sulfonates, poly acrylates and / or polyvinyl alcohols used and on the way mentioned can be applied.

The invention also provides the suspension in addition Additives to the coating agent before spray drying to admit. Here is binder, dispersant or more generally oxidic, fine-particle components thought. This creates a homogeneous distribution of these Components in the later processing of the coated Powder to molded parts reached. This is especially for the formation of a homogeneously distributed binding phase of Advantage.

The finely divided oxide powders are excellent Way of making high-density ceramic mold parts, for example refractory ceramic stones, but also masses.

Further features of the invention emerge from the note paint the subclaims and the other registration documents.  

The invention is based on various examples of management explained in more detail.

By spray roasting a cleaned magnesium chloride solution a magnesium oxide powder A is obtained. This material, whose physical and chemical data are shown in Table 1, is now processed as follows:

example 1

The magnesium oxide powder A is dispersed in water at 10 ° C. greed and with 1 wt .-% 6-aminohexanoic acid (based on the magnesium oxide) mixed homogeneously.

The suspension obtained is then immediately sprayed subject to drying. For example, a previous grind in an agitator ball mill is optional. The properties of the magnesium oxide powder B thus obtained are also listed in Table 1. The degree of hydration after Spray drying is only 2% by weight. Furthermore was The degree of hydration is determined after the samples are in a Climatic cabinet for 48 hours at a temperature of 40 ° C of an atmosphere with 95 wt .-% relative humidity had been exposed.

The degree of hydration was calculated from the weight gain due to ingestion of water, whereby a degree of hydration of 100% of full implementation corresponds to the magnesium oxide to magnesium hydroxide.

The values d ₁₀ , d ₅₀ and d ₉₀ indicate those particle diameters in which 10, 50 and 90% of the material are smaller than the specified value.

Example 2:

The magnesium oxide powder A is dis at 10 ° C in water pergiert and with 1 wt .-% hexanoic acid (based on the Magnesium oxide) mixed. The suspension becomes immediate then subjected to spray drying. The chemical Analysis and test results of the coating obtained in this way Powder C is shown in Table 1.

The test results show that the inventive Magnesium oxide particles have a high degree of fineness. They also show a significantly reduced hydration inclination, even after long periods of moisture storage, in comparison to powder A. A low polarity of the Coating agent additionally the tendency to hydration.

To illustrate the benefits of the oxide described powder is used in the following example.

Example 3

There are stone mixtures from 10 wt .-% of magnesium oxide powder A or 10 wt .-% of magnesium oxide powder B and 90% by weight of sintered magnesia in a grain size less than 5 mm, obtained from low iron Natural magnesite. The mixtures become stones pressed and fired at 1850 ° C. Table 2 shows the test values determined on these stones.

It can clearly be seen that the using of the magnesium oxide powder according to the invention Stones have a higher bulk density, a significantly lower one open porosity and a significantly increased compressive strength exhibit.

Claims (12)

1.Fine-particle 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 which prevents further hydration, are obtained by the following, immediately successive steps:

• 1.1 An aqueous suspension is produced from a pure oxide powder and a coating agent,
• 1.2 the suspension is homogenized,
• 1.3 the suspension is then subjected to spray drying at a temperature at which the coating remains stable.

2. Oxide powder according to claim 1 with a maximum hydra degree of 5 wt .-%. 3. oxide powder according to claim 1 or 2, obtained by a maximum contact time of the oxide particles and the coating detergent in the aqueous suspension before spraying drying of 6 hours. 4. oxide powder according to claim 3, obtained by a maximum Contact time of the oxide particles and the coating means in the aqueous suspension before spray drying hour. 5. oxide powder according to any one of claims 1 to 4, obtained through a maximum temperature exposure of the Sus pension before spray drying at 40 ° C. 6. oxide powder according to claim 5, obtained by a maximum Temperature exposure of the suspension before spraying drying at 10 ° C. 7. oxide powder according to any one of claims 1 to 6, in which the coating agent from a carboxylic acid or their derivatives or water-soluble polymer compound exists. 8. oxide powder according to one of claims 1 to 7, with a monomolecular surface coverage of the coating by means of. 9. oxide powder according to any one of claims 1 to 8, obtained by adding a binder or dispersant in the suspension before spray drying.   10. oxide powder according to any one of claims 1 to 9, obtained by wet grinding the suspension. 11. oxide powder according to one of claims 1 to 10, Herge provides by spray drying at a maximum of 130 ° C. 12. Use of a surface-modified fine particle Oxide powder according to one of claims 1 to 11 for Manufacture of high-density ceramic molded parts and Masses.