DE2524646A1 - FIRE-RESISTANT, FIRED STONES OF PERICLAS AND CHROME AND METHOD FOR THEIR PRODUCTION - Google Patents

Description

HOFFMAiMN · Ϊ21ΤΙ.Ε & PARTNER

PATENT LAWYERS DR. .NG. E. HOFFMAN N · D.PL.-.NG. WETLE. DR. RER. N AT. K. HOFFMAN N · D.PL.-.NG. W. LE HN D-8000 MÖNCHEN 81ARABELLASTRASS E 4 (STERN H AUS) TELEPHONE (089) 911087. TELEX 05-29 _S19 (PATHE)

General Refractories Company, Philadelphia / USA

Refractory, fired stones made of periclase and chrome ore and methods of making them

The invention relates to refractory, burned bricks made from mixtures of magnesia and chrome ore with a low porosity.

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It has been found that basic, refractory briquettes with improved porosity are made from a mixture of about 40 to 80% by weight of dead-burned magnesite (periclase) and about 60 to 20% by weight of chrome ore, the mixture being CaO and SiO _? in a weight ratio of at least 1: 1 and the total content of CaO and SiO _? is at most 5 wt .-% can be obtained. A further improvement in the porosity is achieved in that the mixture contains 20 to about 45% by weight of fine grains with a grain size of less than 0.147 mm, the ratio of chrome ore to magnesia (periclase) being at least 1: 1.

Basic refractory bricks are used extensively in the steel industry, not only in Siemens martin furnaces and electric arc furnaces, but also in the newer ones Process for steel production, v / ie'dt-in oxygen-argcr process (AOD process), vacuum oxygen decarburization (VOD process) and in the case of vacuum degassing processes. Furthermore, basic, fire- solid stones also used in furnaces used in other industries, such as the production of non-ferrous metals, especially copper, and in the stone and earth industry, e.g. 3. for the extraction Portland cement and lime.

So far, basic, refractory bricks have been used for such purposes from dead-burned magnesia or dead-burned magnesite (periclase) and natural or processed chrome ore after crushing on certain grain fractions, mixing with a binding agent

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means and forming into the desired shape and used in the unfired state or after a fire. During the past decade, higher purity raw materials have been used and higher firing temperatures have been used to obtain improved bricks with higher hot flex and compressive strengths caused by better direct bonding. However, these stones have the major disadvantage that they have a relatively high porosity. Commercially available direct-bonded magnesite bricks have an apparent porosity of 16 to 19 %. It is known that the resistance of a given type of stone to slag erosion is inversely proportional to the stone porosity, so that a decrease in the apparent porosity results in an increase in the slag resistance. Slag erosion is a major factor in wear and tear in all applications of refractory bricks or other formations.

So far, no effective practical contributions have been found been used to reduce the apparent porosity of directly-bonded magnesite bricks to diminish. A high porosity is due to the expansion of the stones during firing, which is caused by conversions between the magnesia and spinel constituents of the chrome ore. An addition of fluxes or sintering aids to reduce the porosity runs counter to certain principles, since these additives reduce the purity of the stones and their strength properties are deteriorated.

The use of previously converted grains or melt grains (see e.g. US-PS 3,116,156 and 3,328,183) is known to lower the apparent porosity and thereby increase the slag

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resistance, yet makes the process of making the refractory Materials the use of such stones in some cases uneconomical.

Refractory bricks and other refractory bricks according to the invention consist essentially of dead-burned magnesite or magnesia with a preferred MgO content of about 95 % and a CaO-SiOp ratio of at least 2: 1, preferably at least 2.5: 1> and a chrome ore with a low silica content of preferably less than 1.5%. The CaO-SiOp ratio in the stone, which includes the ratios of these two components in the magnesia (periclase) and in the chrome ore component, is at least 1: 1 and preferably even at least 1.5: 1, the maximum content of CaO and SiOp in the stones is about 5 % by weight and can be up to only 1.5% by weight, if high-purity magnesites are used even up to only 1% by weight. -%.

Higher CaO-SiOp ratios, such as 2.5 or 3 J 1 or above, can be used. at However, higher levels of CaO and S1O2 are the maximum permissible The ratio of CaO to SiOg is limited to not more than 2.5: 1.

Stones with a composition falling within the ranges listed usually show little shrinkage in a fire at high temperatures, which leads to the characteristic give low values for the apparent porosity of these products. It is represented without any theory whatsoever is to be assumed that the advantageous results on the interaction or on a conversion of a high CaO-SiO -Ver-

ratio having silicates of the MgO component of the mixture with the silicates of the chrome ore part of the mixture, which have a low

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CaO-SiOp ratio are due. With the middle ones Levels or temperatures of the fire, relatively low-melting silicates are formed, which cause shrinkage and compression of the refractory briquettes. In the final stages of the fire, an almost equilibrium in terms of composition is achieved, whereby silicates with higher fire resistance, which are necessary for the high temperature properties of the products, are formed.

The intermediate reactions that lead to shrinkage and compression of the bricks during the fire could occur are based on the following reaction of silicates:

. 2 (2OaO.SiO ₂ ) + MgCSiO ₂ + MgO = GaO.MgO.SiO ₂ + 3CaO.MgO.2SiO ₂

The dicalcium silicate (2CaO.SiOp) is present in the magnesite, the MgO.SiOp lies in the chrome ore before the compounds formed have low melting points and cause shrinkage and densification during the initial firing stages.

In the final stages of the fire, a larger part of the highly calcareous compounds of the coarse-grained magnesia fraction reacts with the intermediate reaction products to form refractory Dicalcium silicate and MgO (periclase), according to the following equation:

CaO.MgO.SiO ₂ + 3GaO.MgO.2SiO ₂ + 2CaO - 3 (2CaO.SiO ₂ ) + 2MgO

A further improvement in terms of lowering the apparent porosity and increasing the slag resistance is determined by a suitable selection of the particle sizes of the components and the ratio of the fine chrome ore to the magnesia or Periclase fine grain attainable. In the conventional, directly bonded magnesite chrome bricks, the fine-grain fraction consists of the refractory

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Composition usually made of magnesia ground in a ball mill without the deliberate addition of very fine chrome ore. It has been found that the addition of chrome ore fines promotes compaction, lowers the apparent porosity and increases the slag resistance. The improvement is noticeable with small additions of fine chrome ore grains in an amount of, for example, about 5 % , but is pronounced when the chrome ore / magnesia ratio in the fines portion of the mixture is at least 1: 1 and preferably 2: 1. The term "fine grain" or "fine fraction" used here is to be understood as meaning that fraction of the refractory composition which has a grain size of less than 0.147 mm. The fine grain can be obtained in any of the usual ways. The fine grain of dead-burned magnesia is obtained by grinding in a ball mill, the chrome ore fine grain by grinding in a ball mill to a grain size where 50 to 90 % are below 0.044 mm, or by jet grinding to 5 / ^u or less or by grinding together xait Magnesite or magnesia obtained up to 90 % grain size * below 0.147 mm.

The invention is explained in more detail using the following examples:

Examples 1 and 2: Example 1 relates to the prior art, Example 2 to the present invention.

Using raw materials of the following composition, sets for stone production were produced:

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SiO ₂
Pe ₂ O ₅

Magnesite 2524646 Magnesite A 1.2 % B chrome ore A 0.7% 0.2 % 2.5 % 0.3 % 0.1 % 25.3 % 0.3 % - 16.2 % - 2.8 % 44.9 % 0.7%. 95.7 % 0.2% 98.0 % 10.9%

CaO

MgO

According to Example 1, a refractory set for stone production using 60 % magnesite A, of which 25 % ball mill fine grain of less than 0.147 mm and 35 ° / ° coarser grain of a grain size of over 0.147 mm, and 40 % chrome ore A was a Grain size of less than 0.589 mm built up. The mixture was prepared with ligninsulfonate as a binder and compressed in a customary manner to stones which were fired at about 1600 ° C, 1640 ° C and I705 ⁰ C.

The stones of Example 2 were made in exactly the same way produced as the bricks according to example 1, with the difference that instead of magnesite A, magnesite B was used in this case.

Example 1 Example 2 CaO-SiO ₂ ratio 0.35: 1 1.02: 1

After firing at 1600 ° C, volume change during firing, % volume weight (RG), g / cm * apparent porosity (PS), % flexural strength (BF), kg / cm ² compressive strength (DF), kg / cm ² hot flexural strength (HBF)

at 1260 ° C, kg / cm2 33.6 72.80

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+2.2 +2.3 3.11 3.06 16.5 17.9 36.75 108.85 197.4 178.50

To. Fire at 1640 ° G

Change in volume during fire, % +2.8 -0.3 RG, g / cnr 3.11 3.15 PS,% 17.6 16.2 BF, kg / cm 45.2 56.0 DF, kg / cm 271.6 . 331.45 HBF, kg / cm ² 1260 ° C 65.45 80.15 HBF, kg / cm at 1370 ° 0 32.55 51.10 After fire at 1700 ° C

Volume change during fire, % +4.1 -1.4

RG, g / cm ⁵ 3.04 3.18

PS, % 19.4 15.0

BF, kg / cm ² 31.2 62.7

DF _v kg / cm ² 170.10 426.3

HBF, kg / cm ² at 1260 ° C 72.24 98.0

Examples 3 to 6: The following examples show the effect of the fine-grain fractions on the properties of the stones. It concerns Example 3 the prior art, Examples 4 to 6 relate pay attention to the teaching according to the invention.

The magnesites A and B listed in Examples 1 and 2 were used as the magnesite; the chromium ore had the following analysis:

SiO ₂ 0.7 %

Fe ₀ O, 26.0 % έ 3

Al ₀ O, 15.9%

Cr ₀ O ₂ 46.4 % ² 3

CaO 0.2%

MgO 10.8 %

The composition of the mixtures used and the properties the stones obtained were as follows:

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Example 3 Example 4 Example 5 Example 6

Composition of CaO / SiO ₂ 40% - - - mixture properties 15 % - , - 47.5 % Magnesite A, coarse grain RG, g / cm ⁵ : - 40% 42.5 % 10 % Fine grain PS, % - 15 % 15% 22.5 % Magnesite B, coarse grain HBF, kg / cm ² 35% 35 % 27.5 % 20% Fine grain at 1480 ° C 10% 10 % 15 % 1.72 Chrome ore B, -0.589 mm at 1600 ° C 0.68 1.67 1.72 Fine grain 3.3O 3.16 3.27 3.29 13.1 17.7 14.7 14.0 3O.5 25.2 . 25.5 39.6 n.b. 13.3 10.9 18.2

The stones according to Example 3 showed a considerable increase in volume during firing, the stones according to Examples 4 to 6 however not.

Additional advantages arise with the stones according to the example. 5 and 6 by increasing the ratio of chrome ore fines to magnesia ball mill fines from 10: 15 (example 4) to 15:15 (example 5) and to 20:10 (example 6). The hot flexural strengths were not changed or improved by the composition according to the invention.

With the stones obtained according to Example 5 were dynamic Rotational slagging attempts with a fairly acidic slag such as those in some of the modern steelmaking vessels occurs using the experimental procedures described by Cash in "Measuring Refractory Resistance to Hot Slags," Ceramic Age,

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August 1956t pages 20 to 29, swept, carried out. The composition of the slag was as follows: SiO

CaO 27%

5%
_25th 4 %

MgO 5 %

MnO 5 %

Approximately 4.53 kg of slag were continuously applied to the stones to be examined at I7ÖO ⁰ 0th The test stones were laid side by side with the usual directly-bound stones, so that all stones were loaded in the same way. The relative erosion (stone thickness that was worn due to the ochlackening effect) of the stones according to Example 5 was 63, if the erosion of the control stones (conventional directly-bonded stones with the same magnesia content) showed an erosion assumed arbitrarily as 100.

Examples 7 and 8: In a similar manner to the previous examples, bricks were made from a mixture containing about 50 % MgO using 52.5 % magnesite B and 7.5 % chrome ore B as starting materials. The ratio of chrome ore fines to magnesia fines was 22.5: 15 or 1.5 ' 1. The only difference between Example 7 and Example 8 was that the set for the production of the stones according to Example 8 was 1% of a concentrated aqueous solution of calcium nitrate (150 g in 100 cnr water) was added. The calculated CaO-SiO ^ ratios of the mixtures according to Examples 7 and 8 were 1.63 and 1.80, respectively. After firing at 1760 ° C, the apparent porosity was 13.0 and 11.6 %, respectively.

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Claims (8)

Patent claims: Refractory, burned bricks from mixtures of magnesia and chrome ore with a low porosity, characterized in that they are composed of about 40 to 80% dead-burned magnesia (periclase) and about 60 to 20 % chrome ore and CaO and SiOg in an amount of up to contain about 5 % , the OaO-SiOp weight ratio being at least 1: 1. 2. A method for the production of refractory bricks according to claim 1, characterized in that a) about 40 to 80% dead-burned magnesia (periclase) are mixed with about 60 to 20% chrome ore to obtain a mixture of 100 % magnesia and chrome ore, wherein b) this mixture contains a fine-grain fraction and also a total amount of up to about 5% by weight of CaO and SiO ₂ , c) the weight ratio of CaO to SiO _{2 is} at least 1 \ 1, d) this mixture is formed into refractory shapes and this e) fired at a temperature of at least about 1620 ° 0 will. 3. Process according to Claim 2, characterized in that a mixture with a total content of CaO and SiO ₂ of no more than about 3.5% by weight is used. 4. The method according to claim 2 or 3, characterized in that a mixture with a CaO-SiO ₂ ratio of at least about 1.3 ι 1 is used. . . - 10 - 509849/0930 5. Process according to one of Claims 2 to 4, characterized in that a mixture is used in which at least about 5 % of the chrome ore has a grain size of less than 0.147 mm. 6. The method according to any one of claims 2 to 5 »characterized in that a mixture is used in which the fine grain fraction of a grain size of less than 0.147 mm is about 20 to 45 % and the weight ratio of chrome ore to magnesia in the fine grain fraction is at least 1: 1 is. 7- method according to any one of claims 2 to 6, characterized in that that a calcium compound is added to increase the CaO content in the mixture, which provides CaO when burning. 8. The method according to any one of claims 2 to 7 »characterized in that the bricks are fired at a temperature of at least ⁰ C I7OO. 9 · A method according to any one of claims 2 to 8, characterized in that mixtures are used in which the CaO-SiOp- ^ erhäütris "tvra 1.7" ^Λ i "· ^ ■ and the ratio of Chromerzfeinkorn Magnesiafeinkorn to about 1: 1 to about 2: 1. 509849/0930