DE1571299C - Process for the production of magnesia springs - Google Patents

Description

The invention relates to a method for producing magnesia spinels of high reactivity according to German patent 1275 935, in which a homogeneous mixture of an alumina-containing component made with a magnesia-containing component in a practically stoichiometric ratio is, whereby the alumina-containing component alumina hydrate, schwacli annealed alumina or alpha-alumina is used in finely divided form and as a magnesia-containing component magnesium hydroxide, Magnesium carbonate, basic magnesium carbonate or slightly annealed magnesium oxide in finely divided form Shape.

The expression “spinel” or “magnesium spinel” is to be understood _{in the following to mean the magnesium-aluminate spinel MgO · Al 2} O _3. The term "high reactivity" means that the synthetically produced magnesium spinel can be fired at relatively low temperatures after it has been ground and converted into the final shape of the part to be produced.

Synthetically produced spinels with high reactivity are used in technology as binders for high-purity refractory materials. When such are, for example, refractories, for which the presence of magnesia spinel is beneficial for high resistance to attack by metals, e.g. B. refractories for use in melting furnaces for melting aluminum alloys with a high magnesium content. They are also used to manufacture ceramic parts. The magnesium spinel is used for this ground, pressed into the final shape and fired at a certain temperature. To this end the magnesium spinel must be easy to grind. When firing, you want an end product with a high Density can be obtained, which hardly differs from the theoretical density of the raw material, wherein the temperatures should be as low as possible.

For the production of magnesium spinels and refractory products based on spinel are already various methods known. -

In the German Auslegeschrift 1128 352, a method based on activated magnesium oxide and aluminum oxide for: the production of refractory masses based on magnesium aluminate is described. The refractory masses obtainable by this method at firing temperatures of 1450 to 1550 ^° C are dense, hard, ceramic bodies that are very difficult to grind to a powdery material.

The subject of French patent 350 016 is a process for the production of a magnesium spinel brick at elevated temperature using boric acid as a mineralizer. As the reaction temperature is given as 2400 ° C.

In both processes, however, the high firing temperature is a considerable disadvantage.

The main patent relates to a process for the production of high magnesium spinels Reactivity in which certain specific homogeneous mixtures at a temperature in Range from about 1000 to 1250 ° C are annealed.

It has now been found that high reactivity Magncsiumspinclle can be obtained by annealing these mixtures at a temperature of only 850 ° C. The present invention is therefore characterized in that a mixture is used which also contains a fluorine-containing substance in a quantity ratio of 0.2 to 4 percent by weight (based on the oxide), except when the alumina-containing component consists mainly of alpha-speech, and that the mixture is annealed at a temperature between about 850 and about 1000 ^° C., the fluorine-containing substance being able to form hydrogen fluoride during the annealing process.

ίο The high reactivity of the magnesium spinels produced according to the invention allows Manufacture of ceramic parts that can be compacted at firing temperatures of only 1600 ° C, a non-porous, fireproof magnesium spiral

To achieve a product that has a density of more than 3.45 g / cm ³ , while the theoretical density of the magnesium spinel is assumed to be ^{3.58 g / cm 3.}

The improved magnesium according to the invention

ao product is also suitable as a binding agent in refractory materials, e.g. in refractory materials where the presence of magnesium spinel beneficial for high resistance to attack Metal is, for example, refractory materials for

as used in melting furnaces for melting

Aluminum alloys with a high magnesium content.

It has shown; that a mixture of one

alumina-containing component and a magnesium

containing component - whereby the magnesium

Containing component is present in at least sufficient quantity to combine stoichiometrically and thereby _{form MgOAl 2} O ₃ - can be converted into a reactive form of magnesium spinel by annealing at a temperature of about 850 ° C to about 1000 ° C, provided that the components of the mixture are selected in a certain way, on the one hand to ensure that a practically complete conversion to magnesium spinel takes place and, on the other hand, only

little or no sintering or crystal growth occurs at the generated spinel.

In the production of magnesium spinels according to the invention, it is essential that the alumina-containing Component and the magnesium-containing component

the component must be crushed well and mixed thoroughly, thus an intimate mixture of these solids in dry form as a compact or slurry is made.

The magnesium-containing component is

homely already by the production very fine, d. H., the large mass of the particles is less than 10 μ. If raw material is used, it must be ground to a similar fineness; The alumina Component usually has to be

will grind; although these do not like the delicacy to have the magnesium-containing component needs. The magnesium-containing component is preferably to be comminuted, up to 90% of all particles are smaller than 10 μ, and the alumina-containing compo-

Component should be comminuted to such an extent that at least 40% of the particles are smaller than 10 μ. When Alumina-containing substance can be used in the mixture of alumina hydrate, for example alumina trihydrate, lightly burned alumina and vt-aluminum

minium oxide or mixtures thereof; Magnesium hydroxide can be used as a magnesium-containing component, Magnesium carbonate, basic magnesium carbonate and lightly burned magnesium dioxide (including

Very lightly burned magnesium carbonate, e.g. burned at 900 ° C) or mixtures of these Components are used.

Under "weakly calcined alumina" is to be understood alumina hydrate, which is so high Temperature is burned that essentially all the ■ water is driven off, but not at one Temperature by which it turns into «-aluminium oxide (Corundum) is converted; -

It has been found to be desirable for the magnesium-containing component to be present in a slight excess beyond the _{proportion stoichiometrically required for the formation of MgO-Al 2} O _{3 in} order to prevent the accidental formation of an alumina-rich spinel. Magnesium oxide itself is insoluble in magnesium spinel and can be leached out.

Furthermore, unless the alumina-containing component consists essentially of ■ * -aluminium oxide, a small percentage by weight of aluminum fluoride or an equivalent fluorine-containing substance added * which is converted into hydrogen fluoride during the firing process, usually in the presence of water can be. The proportion of aluminum fluoride is preferably in the range between about as 0.2 and about 4 percent by weight, based on oxide, and should preferably not be more than 2 percent by weight be. The particularly preferred range for the addition of aluminum fluoride is between 0.5 and 1.5%; it is most expediently at least 0.75 percent by weight.

It was found that the influence of the crystal form of the alumina component in the for Spinel mixture used is important. When the alumina component is used as a Alumina hydrate or lightly burned alumina occurs, is the addition of aluminum fluoride or one equivalent fluorine compound required to produce a sufficient conversion to magnesium spinel at low temperature as required to achieve Magnetic spinel high responsiveness is required to achieve. However, if the alumina Component in the form of aluminum oxide, also known as corundum, is present; is the present not of aluminum fluoride or its equivalent only unnecessary, but even annoying because it sintering the mixture at relatively low temperatures promotes.

The addition of fluorides in the firing of spinel-like products is already mentioned in the French patent 350 01 (5. However, the addition is made "in the process known from" this patent document only for the purpose of removing the boric acid, which acts as a mineralizer. ^ Eihe ^ catalytisphe ^ Wlrl ^ ng'iäes'Puon ^ .i ^ meaning of the present invention is ruled out, since even if fluorides are added to the batch before burning, the fluorides are added due to the low proportion of 0.2% of these substances the heights process temperature of 2400 ° C immediately react with the boric acid to a volatile compound. a catalytic Wirlcung the Uimyandlung of Jq, Al ₂ O _3, such as is present in the inventive .A, can therefore in which, unknown yje. rides are not expected.

^ ie / mentioned, the, magnesia-containing component out; Calcium hydroxide, magnesium carbpnate, basic magnesium carbonate or weakly burned magnesium oxide. "Dead burned" Magnesium oxide is undesirable, because it interacts with aluminum oxide only reacts weakly. ^!

In the process according to the invention in which the mixture of the alumina and magnesium components is heated to convert it to magnesium spinel, the annealing temperature is too low to allow substantial sintering of the newly formed magnesium spinel, with the result that it is very easily broken down into particles Can be crushed to the micron size Kanuj / Sintered magnesium spinel is difficult to grind; however, for use as a ceramic material in slip casting or certain other manufacturing processes, it is necessary to grind it to micron size. In general, an annealing temperature were obtained from at least 85O ⁰ G satisfactory results through the application. . · ■■ ■■ - ■ .- ■■

The invention is illustrated by the following example:

example

Mixtures of high purity were used Magnesium hydroxide from the mineral Brück and Alumina hydrate produced by the Bayer process. These substances had the following chemical composition:

Table!
Analysis of the raw materials used

oxide

Al ₂ O ₃ , ..:, ..

SiO ₂ ; ...

Fe ₂ O ₃ .......:%

MgO

Loss on ignition.

Mg (OH) ₂ annealed AI2O3 At first
worth ■ '' ■ ν.- · - - ■■ ⁰ Zo - . ■ _- ^ -—. "V. ^-:; . ._-. 0.3 65.6 0.2 1.3 0.02 0.9 97.1 0.02 68.0 - ^: - _ 1.3 0.9 ' -'' 30.0 34

3 H ₄ O

99.35
0.03
0.035

0.55

The aluminum oxide-magnesium oxide ratio of the mixture was _{calculated to achieve a stoichiometric magnesium spinel (AljO 3} ZMgO ratio 2.55). To avoid the accidental formation of high alumina spinel, a slight excess of MgO was present in the mixture. Excess MgO is insoluble in Magnesiumspineli and, (alls erwiinscht, "ground away 'by treatment with dilute HCl after _annealing; - ν ■

The raw material mixture was ground in a laboratory ball mill. Before grinding, 1 ° / p AlF _{3 was} added. The ground mixture became: compacts in, the, size 1ÖÖ · 25 · 30 mm with a. Pressure; About 280 kg / cm ² pressed. The PressinEe were annealed at temperatures between 700 and 140 ° C.

The products obtained were determined by X-ray diffraction analysis for their mineral composition

composition examined. ' No noticeable proportions of spinel were found at temperatures below 800 ^{J C.} Between 800 and 900 ° C the conversion was incomplete, while at 900 ° C and above the conversion to spinel was practically complete.

The annealed materials were ground again in a laboratory ball mill, pressed into discs 25 mm in diameter and baked for 1 hour at 1600 ⁰ C. The density achieved by firing at 1600 can serve as a measure of the reactivity of the product. The higher the density achieved by firing, the greater the reactivity. Table II gives the X-ray diffraction values and the burning density values of the samples as a function of the annealing temperature.

From the values given in Table II it can be clearly seen that magnesium spinel with optimal properties can be obtained by calcining a mixture of alumina trihydrate and magnesium hydroxide at 900 ° C. or above with the addition of 1% AlF ₃ as ao mineralizing agent. The density of ceramic parts obtained by firing, which are made from ^{mixtures annealed at temperatures below 900 °} C., decreases - presumably due to the presence of unreacted aluminum oxide and magnesium oxide in the powder _: - with decreasing temperature. For mixtures annealed at 850 ° C, the density achieved by firing is only 3.46 g / cm ^s , which is a limit value, while ceramic parts made from mixtures annealed at 825 ° C are no longer satisfactory and one achieved by firing Have a density of 3.19 g / cm ³ .

Tables

Composition and density of magnesium spinel parts before and after firing *

35

Line intensity in X-ray diffraction *

Glow temperature
rature
in ⁰ C

700

800

825

850

900

1000

.1050

1100

1200

1400

Spinel Periclase </ = 2.86A </ = 2, llA 2 66 α ^x 72 20th 58 64 17th 82 6th 98 1 97 5 105 1 112 2 , 128 3

Density in g / cm ¹ before firing

1.41
1.50
1.70
2.05
2.13
2.12
2.14
2.18
2.17
2.20

density

in g / cm ^s

after the

Burn

at 1600 ⁰ C

45

2.46
2.97
3.19
3.46
3.50
3.50
3.51
3.49
3.50
3.30

55

* Spinel produced by annealing aluminum hydrate and magnesium hydroxide with the addition of 1 '/ · AlFs at the specified temperatures.

· * Line intensities are comparable with regard to the samples, but g _o not with regard to the minerals.

Albeit the emphasis is placed on the use of aluminum trifluoride as a mineralizing agent has been, so are other fluorine-containing compounds, obtained by pyrohydrolysis in the presence of moisture during the conversion or des Convert the annealing process into hydrogen fluoride, usable. Such, within the scope of the invention Usable fluorine-containing compounds are hydrogen fluoride, fluorine, ammonium fluoride and calcium fluoride. Hydrogen fluoride or fluorine can be introduced directly into the furnace.

To illustrate the manufacture of magnesium spinel powder from the product of the invention 500 g of magnesium spinel produced according to the invention were obtained using 3500 g of high-alumina Grind balls in a 6.1 liter ball mill for 3 hours at 60 rpm. The resulting milled magnesium spinel had the following Size distribution on:

Weight percent Size in microns larger than the specified
size 10 8th 5 26th 3 47 2 63 1 87 0.5 97 0.3 100

Regardless of their production by arc melting or by sintering in one or more kilns are commercially available magnesium spinels tight and not readily usable for the production of ceramic precision parts. In order to make such spinels usable for such purposes, they must be comminuted and ground and often treated with acid before pressing and at temperatures of at least 1800 ° C can be fired.

With the method according to the invention is a Magnesium spinel powder is made which further turns into a dense spinel by burning at a lower rate Temperature of about 1600 ° C can be processed. As an explanation for the special Properties of the magnesium spinel produced by the process according to the invention should be one Compared to the annealed clays, which are produced according to the Bayer process and the dense sintered clay tablets used in the ceramic industry. . ■. · - ■ ;; ■. ■ ·

While alumina tablets a. Lean grain of suitable To represent density for refractory parts, they must be finely ground before they are sintered be able. The alumina annealed according to Bayer for Ceramics consist of collections of micron-sized crystallites, which can be easily ground by grinding can be turned into a micron-fine powder be able. This can be brought into the desired shape by pressing and at a minimum of 1600 ° C be fired to zero porosity with a shrinkage between 11 and 20%. Just like commercially available dense spinel is comparable in its physical state to clay tablets, this corresponds Spinel powder according to the invention of Bayer clay. In addition, it equates to a very responsive one Alumina it sinters at a lower temperature. Just like Bayer clays, it has to be ground to develop its responsiveness. This can easily be done because of spinel compacts according to the invention soft and crumbly

are. After grinding, the powder can be pressed and fired at 1600 ° C to zero porosity, and densification to 97% of theoretical occurs Density with a shrinkage of about 17%.

'■ δ

Claims (10)

Patent claims: 1. Process for the production of magnesia spinels of high reactivity according to German Patent 1,275,935, in which a homogeneous mixture of an alumina component to made with a magnesia-containing component in a practically stoichiometric ratio is, whereby the alumina-containing component alumina hydral, lightly annealed alumina or Alpha clay in finely divided form is used «5 and magnesium hydroxide as a component containing magnesia, Magnesium carbonate, basic magnesium carbonate or slightly annealed magnesium oxide in finely divided form, characterized in that a mixture ver ao which also contains a fluorine-containing substance in a quantitative ratio of 0.2 to 4 percent by weight (based on oxide), unless the alumina component consists mainly of alpha alumina, and that the mixture at as a temperature between about 850 and about 1000 ° C is annealed, wherein the fluorine-containing substance capable of forming hydrogen fluoride during the annealing process. 2. The method according to claim 1, characterized in that the mixture is at a temperature is annealed at a temperature of at least 900 ° C. 3. The method according to claim 1 or 2, characterized in that the magnesium-containing component is present in a slight excess over the amount required for the connection in the stoichiometric ratio of the same with the entire existing, alumina-containing component for the formation of MgO · Al ₂ O ₃ . 4. The method according to any one of claims 1 to 3, characterized in that at least 90% of the magnesium-containing substances present in the mud Component has a particle size of less than 10 μ and that at least 40% the alumina-containing component have a particle size of less than 10 μ. 5. The method according to any one of claims 1 to 4, characterized in that the fluorine-containing Substance is aluminum fluoride. 6. The method according to claim 5, characterized in that the aluminum fluoride in the Mixture makes up a proportion of about 0.5 to 1.5 percent by weight, based on oxide. 7. The method according to any one of claims 1 to 4, characterized in that the alumina-containing Component made from ct-aluminum oxide and the magnesium-containing component from weak annealed magnesium oxide. 8. The method according to any one of claims 1 to 6, characterized in that the alumina-containing Component made from alumina hydrate and the magnesium-containing component made from magnesium hydroxide or weakly annealed magnesium oxide. 9. The method according to any one of claims 1 to 6, characterized in that the alumina-containing Component made from lightly annealed alumina and the magnesium-containing component weakly annealed magnesium oxide. 10. The method according to any one of claims 1 to 6, characterized in that the alumina- containing component consists of lightly annealed alumina and the magnesium-containing component consists of magnesium hydroxide. 109609/158