DE2541141C3 - - Google Patents

Description

The invention relates to an improved process for the production of magnesium aluminate spinels high purity.

Stoichiometrically pure magnesium aluminate spinels are made on a small scale by electrical melting and have shown their utility as an excellent refractory material. the The following comparison of the spinel with other common refractory materials shows the reasons for this:

material Clay Mullite Periclase Spinel AhCh 3AbO3 · 2S1O2 MgO Chemical composition MgAbO4 properties 2015 1830 2800 Melting point, ° C 2135 4.0 3.2 3.6 Density, g / cc 3.6 0.270 0.238 0.287 Specific heat, cal / gm ° C 0.257

(20-1000 ⁰ C)

Medium Reversible Thermal 7.6χ 10 "

Expansion, cm / cm ⁰ C

(20-1000 ⁰ C)

Thermal conductivity, cal / cm ² see
(° C / cm)

100 ° C 0.036

1000 ° C 0.014

8.8 xlO- ⁶

5.3x10

-6

13.5x10 '

0.072
0.015

0.014
0.009

0.090
0.017

Furthermore, the spinel shows first deformation at 2 kg / cm ² at 2000 ⁰ C, does not react with silica to 1735 ° C, not with magnesia or calcium oxide, is brought into solid solution at 2000 ° C to them, or with "-clay up to 1925 ° C. It can be used to contain all metals except the alkaline earth metals and shows better resistance to alkali than alumina, better resistance to splinters than refractory chromium magnesite materials and excellent resistance to basic slag.

None of the numerous attempts to produce high purity magnesium aluminate spinels at low cost have resulted in commercial success. The experiments typically comprised the following steps: physical mixing of alumina and magnesia of high purity, calcining at 900 to 1500 ⁰ C, grinding, pelleting and finally sintering at 1600 to 1900 ⁰ C. The technical complexities of these processes are responsible for the magnesium aluminate spinels very little used as a refractory material at the present time.

It has now been found that high purity magnesium aluminate spinel can be produced thereby it is possible that alumina in the form of dust from electrostatic separation systems with finely divided

he mixes magnesia in such amounts as to give an approximate ratio of magnesia to clay ) 4 to 0.8, and the mixture to temperatures ^ On at least about 900 ° C for a period of time »Scratches that for an essentially complete reaction of the mixture to form the spinel sufficient. Preferably the mixing is done with milling and the heating is done at the temperature above the sublimation point of the sodium oxide is made for a period of time sufficient to produce a spinel with a bulk density of at least about 90% of theory and a sodium oxide content of less than 0.1 % By weight. Thereafter, a method is provided according to which a higher quality Spinel is obtained, but which avoids the use of first class raw materials and expensive process steps.

The method consists in mixing the dust defined below from electrostatic dust collectors (hereinafter referred to as "ESA dust") and finely divided magnesia in the desired ratio and then heating the mixture to or above approximately 900 ^° C. until the mixture is formed of the magnesium aluminate spinel has essentially reacted.

The clay used in carrying out the method according to the invention is ESA dust. "ESA ^: Dust" is the name given to the alumina dust produced as a by-product, which is obtained during the calcining of an alumina-containing hydrate such as hydrargillite in the production of aluminum oxide for reduction to aluminum metal. The term "ESA" refers to an "electrostatic dust collector", a method known for collecting such dust. It goes without saying that the term "ESA dust" used in the following includes all types of alumina dust occurring as a by-product, even if it is collected by other methods. This dust, which is considered too fine for effective reuse in calcination, contains approximately 80 to 97.5% by weight of alumina and consists of approximately 75 to 100% by weight of particles that are smaller than 20 μm.

The ESA dust from hydrargillite calcination is a relatively non-homogeneous mixture Alpha-, Eta-, Delta- and Chi-clay, hydrargillite, sodium carbonate and chemically bound sodium oxide may contain. The unpredictable variations in the relative proportions of these components As well as the high sodium oxide content, the untreated ESA seem suitable for use in dust exclude ceramic and refractory applications. However, it has surprisingly been found that the dust is an excellent source of alumina for the manufacture of magnesium aluminate spinels, which very high reactivity of the dust, which causes the compaction of the spinel at temperatures that are lower, than one would normally expect, and the high sodium oxide content of the dust during the normal spinel sintering is significantly reduced, as will be shown below.

While the ESA dust from the calcination of hydrargillite is preferred, those from the calcination of other alumina minerals such as north strandite, bayerite and pseudomorphic Trihydrate, used to make spinel.

The following are the analytical data of ESA dust generated during typical calcinations of Hydrargillite was collected, along with the normal range of those found in ESA dust Values put together:

Sample A *) Sample B *

Area

Composition (% by weight) **)
Loss by heating (1325 ° C)
AI2O3
S1O2
CaO
FezCh

Particle size distribution (% by weight)
+ 20 microns
+10 microns
+ 5 microns
-I- 3 microns
+ 2 microns

Particle size distribution (% by weight)
+1 micron
+ 0.5 microns
True density (g / cm ³ )

* \ Samples chosen at random.
**) Element content expressed as oxide.

2.00-14.00 12.00 2-16 almost exclusively almost exclusively 80-97.5 0.01-0.06 0.08 0.01-0.2 - 0.02 0.005-0.2 0.01-0.03 0.04 0.01-0.1 0.90-3.00 1.40 0.5-4.0 4.0 25.0 0-25 16.0 49.0 0-50 46.0 73.0 25-75 73.0 83.0 50-90 86.0 88.0 70-95 94.0 95.0 80-100 97.0 100.0 90-100 3.68 3.33 3.0-4.0

The preferred finely divided magnesia is made from magnesium carbonate and especially magnesium hydroxide both of which can be easily converted into active magnesia by heating. The analytical Data for representative samples as well as the normal range for finely divided magnesium hydroxide are:

Sample A Sample B Area Composition (weight%) *)
MgO 98.0 96.4 94-99 AI2O3 0.2 0.2 0.1-0.5 S1O2 0.4 0.5 0.2-1.0 CaO 1.2 1.5 0.5-2.0 Fe2Ü3 0.2 1.4 0.1-2.5 All in all 100.0 100.0 Particle size distribution (% by weight) + 20 microns 1.5 2.0 0-10 +10 microns 3.2 6.0 0-20 + 5 microns 8.5 20.0 5-30 + 3 microns 30.0 44.0 20-60 + 2 microns 57.8 63.5 40-90 + 1 micron 85.0 81.5 60-100 4- 0.5 microns 92.5 91.5 85-100 True density (g / cm *) ") 2.36 2.36 2.3-2.4 *) Base: heated (1325 ⁰ C). **) As a hydroxide.

Other sources of magriesia are magnesium acetate, magnesium oxalate, magnesium nitrate, magnesium sulfate and magnesium oxide itself. The finely divided magnesia essentially goes through a sieve with 297 μm lights Mesh size.

The ESA dust and the finely divided magnesia are mixed in such proportions that the Weight ratio of MgO: Al2O3 in the mixture is between approximately 0.4 and 0.8. In this area the spinel refractories, the manufacture of which will be described below, have a high level Bulk density and low sodium oxide content. The preferred mixture is one with one Ratio of 0.395, i.e. the stoichiometric ratio of MgO and Al: Oj, because with it a Refractory material is produced with the smallest proportions of two phases. Mixtures with proportions significantly below the stoichiometric ratio lead to materials with low bulk density and relatively high sodium oxide content.

Mixing can be done either dry or wet, as long as the mixing is closed an intimate contact of the particles in a size range below μηι leads. This is preferably done Mixing in the presence of water and, in addition to mixing, also includes milling. Suitable devices Ball mills, Eirich mixers and turbine mixers are available for both dry and wet mixing. Other dry mixing devices include V-type mixers while wet mixers are propeller mixers include. The mixing times depend on the nature of the clay and the magnesia, on the type of Mixing and the device used. When using ESA dust and the above Magnesium hydroxide takes up to about 4 hours while wet mixing in a ball mill Dry blending requires approximately 24 hours. When wet mixing, the mixture is pre-heated at least partially dried. Suitable drying methods include, for example, atmospheric Drum drying. This drying step

a previous filtration of the wet mixture, such as a rotary drum filtration, can also be to remove excess free water from the mixture.

As stated earlier, the use of ESA dust leads to the formation of magnesium aluminate spinels to unexpectedly low sodium oxide levels in the product, the sodium oxide content being the Spinel mixture of 2% by weight during sintering of the mixture as described below less than 0.1 wt% was decreased. This reduction in the sodium oxide content is essential

crucial if the product is used in refractory or high quality ceramic applications should be used. In fact, the manufacturers of alumina are making great efforts to ensure that the To reduce the sodium oxide content of high-quality alumina products to this level. The procedures for Reducing the sodium oxide content of alumina includes adding boron, fluorine or chlorine containing compounds that interact with sodium at elevated temperatures with the formation of volatile Sodium compounds, which escape from the clay during heating, react. If such If additions were not made, the sodium oxide content of highly heated clays would generally be at about 0.4 to 0.8 weight percent. This sodium oxic is very difficult to remove because it is in the form of a; very stable sodium aluminate is present.

It is reasonable to expect that magnesium aluminum will be produced from alumina containing sodium minatspinell retains a significant amount of sodium in the highly heated product. However, this is not de Fall and the reduction of the sodium oxide content of the spinels to below 0.1 wt .-% was without dl Use of additives easily achieved. The possible mechanism for this sodium oxide depletion content is the replacement of the sodium in the sodium aluminum with magnesium and the escape Sodium oxide formed at temperatures above its atmospheric sublimation point of approx

! 75 ⁰ C with the resulting conversion of the sodium alumina into Magncsiumaluminatspincll. Possible options include:

2 NaAlO ₂ + MgO -

2NaAl ₅ O ₈ + MgO -

2 NaAl ₇ O _n + MgO -

2NaAl ₁₁ O ₁₇ + MgO -

Na ₂ O + MgAl ₂ O ₄
Na ₂ O 4 MgAl ₂ O ₄ + 4Al ₂ O,
Na ₂ O + MgAl ₂ O ₄ + 6Al ₂ O ₃
Na ₂ O + MgAl ₂ O ₄ + 10Al ₂ O.,

These ideas are supported by the following study, in which the ESA dust, a mixture ■ on ESA dust with magnesium hydroxide and a Mixture of sodium aluminate and magnesium hydroxide separately for 1 hour at 1680 ° C were heated:

'robe*) Composition, Wt .-% *) NajO After Heat AhOj NiuO Before heating 2.6 MgO 98.9 1.0 MgO AhOj 1.9 0.0 71.0 0.02 A. 0.0 97.3 30.2 29.0 68.5 2.3 B. 28.4 69.6 29.2 C. 19.8 50.0

*) A - ESA dust alone.
B - ESA dust plus MgO,
C - sodium aluminate plus MgO.
**) Based on the heated oxide.

The mixture is heated to make the spinel for use as a refractory material or as a calcine. To produce refractory material, the mixture is granulated, pelletized or briquetted, with or without water and / or organic binders to form suitable furnace charges, and is then sintered ^{by known methods for sintering dead-burned refractory magnesium materials at temperatures between approximately 1600 and 2100 ° C.} The sintering time depends on the sintering temperature, but is usually between about 0.5 and 20 hours. The sintered mass is then ground for use in refractory products. A typical spinel mass produced from sintering a stoichiometric mixture of ESA dust and magnesium hydroxide, as described above, for one hour at 1680 ° C., should consist of approximately 99.3% by weight magncsium aluminate (MgAl ₂ O ₄ ), 0 , 47% CaO, 0.16% SiO ₂ , 0.05% Fc ₂ Oj and 0.03% Na ₂ O, a true density of 3.58 g / cm ¹ , a bulk density of 3.30 to 3, 44 g / cm ¹ , a total porosity of 5 to 8 vol .-% and an average crystal size of less than ΙΟμηι have. It is actually a magnesium aliminate spinel of high purity, high bulk density, low porosity, and a crystal size that results in excellent structural integrity.

Calcinicrter magnesium aluminate spinel, the ceramic for the manufacture of articles or can be used as an additive for refractory products is by about 0.5 to 4stündigcs heating the mixture at temperatures / wipe about 900 and 1600 ⁰ C using known Calcinicrungsvcrfahrcn prepared. The converted spinel can be used as such or ground to a powder. The calcine is not as pure as the corresponding refractory material because the lower heating temperatures lead to a smaller reduction in the sodium oxide content. All other impurities are contained in approximately the same amounts as in the higher heated material. Typically, a stoichiometric spinel calcinate contains about 0.07 to 1.7 wt.% Na ₂ O. The higher proportions of sodium oxide can be reduced to about 0.1 wt.% Or less if the calcine is used during further treatment or when used as refractory material is exposed to elevated temperatures. The calcinate which can be produced according to the invention at low costs shows excellent reactivity for the production of dense pure spinel goods, but the more highly heated calcinate also shows resistance for refractory uses,

If desired, during the Mixing a flux can be added for faster compaction at lower temperatures to achieve. Preferred fluxes are inorganic fluid compounds. especially cryolite

so (NaiAlFt,) and aluminum fluoride (AlFi). The amount of Flux can vary within certain limits and is preferably approximately 0.6 to 2.5 for cryolite and for aluminum fluoride, about 0.5 to 2 wt% dei Mixture.

The following examples further illustrate the invention. In the examples, unless otherwise stated, all temperatures are given in ^D C, all compositions in% by weight or weight ratios and all densities in g / cm '.

example 1

Samples of ESA dust from the dust collector of a rotary calciner used to make clay (15 de with induction quality and powdered Magncs Umhydroxid were dried in air at 105 ° C The dried materials were as fol; analyzed:

709 544/3!

ESA dust 0.05 - 0.02 magnesium - 1.38 hydroxide 100.0 Composition (wt .-%) 100.0 Loss from heating 6.21 31.8 (1325 ° C) 1.5 AbCh 92.34 4.0 0.1 3.2 MgO - 16.0 66.9 8.5 SiO ₂ 46.0 0.3 30.0 CaO 73.0 0.8 57.8 Fe2O3 86.0 0.1 85.0 Na2O 94.0 92.5 All in all 97.0 Particle size distribution (Wt .-%) + 20 microns + 10 microns + 5 microns 4- 3 microns + 2 microns + 1 micron + 0.5 micron

sintered for an hour. The resulting mass was cooled and used in refractory products crushed.

The material obtained had a true density of

3.57 g / cm) compared to the theoretical density of

3.58 g / cm ¹ for stoichiometric magnesium aluminate spinel, a bulk density of 3.33 g / cm ³ (93% of the theoretical value) and an average crystal size of less than 10 μm. The X-ray diffraction analysis confirmed that the spinel formation was complete. The chemical composition of the material was:

MgAl ₂ O ₄ **

MgO *)

CaO *)

SiO ₂

Fe ₂ O ₃

Na ₂ O

Wt% 97.4

1.9

0.47 0.16

0.05 0.03

True density (g / cm ³ )

3.68

2.36

The dried fabrics were mixed with water in a ball mill of 1.136 l capacity in the following Quantities filled:

ESA dust 145.1 g Magnesium hydroxide 84.9 g water 250.0 g

The feed (ratio MgO = Al ₂ O ₃ = 0.423) was milled for 4 hours. The resulting slurry was filtered through a laboratory filter funnel to give a filter cake with 30% water. After drying under atmospheric conditions at 105 ^° C., the cake had a bulk density of approximately 1.35 g / cm ³ . The dried cake was ^{heated at 1680 0} C by heating to this temperature during

Refractory materials *) The specified quantities of the components are based on the assumption that the calcium contained in the material is present entirely as CaO.

Example 2

A refractory material was prepared following the procedure of Example 1 with only milling the mixture for only 0.5 hours to effect mixing with minimal milling. The properties of the material were equivalent to those of the material according to Example 1 with the exception that the bulk density was only 3.10 g / cm ³ (87% of the theoretical value).

Example 3

The procedure of Example 1 was used to prepare a series of refractories in which the amounts of ESA dust and magnesium hydroxide charged to the ball mill were varied to have MgO: Al ₂ O ₃ ratios between 0.200 and 0.800 to obtain. The following results were obtained:

ratio Bulk weight according to Heat Wt .-% Na2Ü Not as MgO: AIjOi Spinel pre- g / cm ³ % of i lying phase theoretical 0.06 Worth 0.02 0.800 3.27 91 0.03 MgO 0.429 3.39 95 0.06 MgO 0.395 *) 3.39 95 0.17 MgO **) 0.386 2.25 63 1.01 none 0.376 1.76 49 none 0.200 1.59 44 AbOj

*) Stoehiometric ratio.
**) Very small amount of MgO.

Example 4

The constituents of Example 1, together with aluminum fluoride crystals, were converted into those in Example I. used ball mill filled in the following quantities:

ESA dust 141.2 g

Magnesium hydroxide 85.3 g

Aluminum fluoride (AIFi) 3.5 g

Water 250.0 g

do The feed (MgO; Al ₂ O ₃ = 0.437) was milled for hours then air dried to 105 ° C. The dried cake was ^{burned at 1680 °} C. for one hour, cooled and chopped up. The material obtained, the X-ray diffraction of which has the vol

It showed constant conversion to spinel with a small excess of magnesium oxide, had a bulk density of 3.39 g / cm ³ (95% of the theoretical value) and the following chemical composition:

MgAl ₂ O ₄ *)

MgO *)

CaO *)

SiO ₂

Fe ₂ O ₃

Na ₂ O

Wt%
96.7

2.6

0.47

0.16

0.05

0.03

*) The stated amounts of the components are based on the assumption that the calcium content of the material is complete is present as CaO.

Example 5

The procedure of Example 1 was repeated with the exception that the dry filter cake was calcined at 1425 ° C. for 4 hours instead of sintering at 1680 ° C. for one hour. The calcined cake, which was easily crushed, consisted of fully reacted spinel with a small excess of MgO and had a true density of 3.56 g / cm ³ and a sodium oxide content of 0.58%.

Example 6

The substances of Example 1 and cryolite were in the ball mill described there in the following amounts filled:

ESA dust
Magnesium hydroxide
Cryolite (Na ₃ AlF ₆ )
water

142.4 g
83.3 g
4.3 g
250.0 g

The feed (MgO: Al ₂ O ₃ = 0.423) was ground, filtered and dried according to the procedure described in Example 1. The dried cake was calcined at 900 ° C. for 4 hours and then the resulting caicinate was dry ball milled for 8 hours. X-ray diffraction analysis of the milled product indicated that spinel formation was complete and a small excess of magnesium oxide was present. The sodium oxide content of the calcine was 1.70% by weight.

The ground calcine was treated with 5% by weight of water and ^{pressed at 703 kg / cm 2} into blanks with an edge length of 2.5 cm × 2.5 cm × 5.1 cm. These blanks were baked for one hour at 168O ⁰ C. The bulk density of the fired blanks was 3.37 g / cm ³ (94% of the theoretical value), while the NaO content was 0.6% by weight.

Example 7

A material was prepared according to the procedure described in Example 1, with only the firing for 4 hours at a temperature of 1600 ⁰ C instead of 1680 ° C lasted. The bulk density of the material was 3.09 g / cm ³ (86% of the theoretical value) and the sodium oxide content was 0.04% by weight.

Examples ^{() 0}

A sample was prepared by following the procedure of Example 7 except that the calcined cake was treated as a 1600 ° C calcine: it was crushed and ball milled for 8 hours. The powder obtained had a sodium oxide content of 0.04% by weight and, according to the X-ray diffraction analysis of the spinel, had an excess of magnesium oxide. The calcine was then treated with 5% by weight of water, pressed into blanks 2.5 cm × 2.5 cm × 5.1 cm at 703 kg / cm ² and fired at 1680 ^° C. for one hour. The bulk density of the fired blank was 3.42 g / cm ³ (96% of the theoretical value) and the sodium oxide content was 0.02% by weight.

Example 9

Dried ESA dust as described in Example 1 and calcined nesquehonite as described below were filled with water in the ball mill described in Example 1:

Calcined nesquehonite
component

MgO
AbO3
S1O2
CaO

98.35 0.5

feed
ESA dust

Calcined nesquehonite
water

111.11g

80.89 g

318.00 g

The feed (MgO: Al ₂ O ₃ = 0.781) was processed into the refractory material according to the procedure of Example 1 with the following results:

Moist cake
Water content (wt .-%)

Dry cake before firing bulk density (g / cm ³ )

Refractory material
Composition (wt .-%)

MgAhO ₄

MgO

CaO

S1O2

FezOi

N 320

Bulk density (g / cm ³ )

Example 10

39 1.14

78.6 21.2

3.5

(85% of theory)

The materials of Example 1 were placed in a 1.136! Capacity ball mill in the following amounts:

ESA dust

Magnesium hydroxide

water

118.257 kg

68.258 kg

192.825 kg

The feed (MgO: AbOj = 0.44) was Milled hours and the resulting slurry pans dried at IO5 "C. The dried Material was crushed and mixed with the addition of v (water in a high-speed mixer-pell tiercr pelletized. The size distribution of the spherical pellets obtained was:

Clearance MasL'hcnwcitc (mm)

Gcw .- 'v'o

to 4.00
4.00-2.83
2.83-1.19
over 1.19

40 30 28

composition
MgAl ₂ O ₄ *)
MgO *)
CaO *)
SiO ₂
Fe ₂ Oj
Na ₂ O

Bulk density (g / cm ³ )

The bulk density of the dry pellets was 1.67 g / cm ^J.

The pellets were ^{burned at 1680 °} C. for one hour and gave a product with the following properties:

Particle distribution

Clear mesh size
to 4.00
4.00-2.83
2.83-1.19
over 1.19

Weight%

11th

24

59

Wt% 95.32

3.15

0.66

0.74

0.08

0.05

3.25 (91% of theory)

*) The stated amounts of these components are based on the assumption that the calciunig noise of the material is present exclusively as CaO.

Example H

Magnesium aluminate spinel is prepared according to the method described in Example 1 with the exception that the charge contained no water and was milled for 24 hours. Then, it was pressed mii 703 kg / cm ² in blanks and one hour lanj fired at 2100 ⁰ C. The crushed blanks produced refractory material of comparable quality.

Claims (10)

Ll patent claims: 1. Process for the production of magnesium aluminate spinels high purity, characterized in that alumina is obtained in the form of electrostatic dust collectors Dust is mixed with a finely divided magnesia-producing material in such proportions that the Weight ratio of. Magnesia to clay is between 0.4 and 0.8, and this mixture up heated to a temperature of at least 900 ° C for a period of time sufficient for the complete reaction of the Mixture to form the spinel is sufficient. 2. The method according to claim 1, characterized in that the heating to temperatures takes place between 900 and 1600 ° C. 3. The method according to claim 1, characterized in that the mixing with simultaneous Milling and heating takes place at a temperature above the sublimation point of sodium oxide takes place for a period of time necessary for the production of a spinel with a bulk density of at least 90% of the theoretical value and a sodium oxide content below 0.1% by weight is sufficient. 4. The method according to claim 3, characterized in that the heating to temperatures takes place between 1600 and 2100 ° C. 5. The method according to claim 1, characterized in that the magnesia-supplying material Magnesium hydroxide is. 6. The method according to claim 1, characterized in that that the mixing is done in the presence of water. 7. The method according to claim 1, characterized in that d; e clay and the magnesia-supplying Material should be mixed in approximately stoichiometric amounts. 8. The method according to claim 1, characterized in that the mixture contains a fluoride Contains flux. 9. The method according to claim 8, characterized in that the flux is cryolite. 10. The method according to claim 8, characterized in that aluminum fluoride is used as the flux is used.