HU176631B - Process for preparing compact simultaneous sinters of direct bond - Google Patents

Abstract

A process for the manufacture of compact refractory materials is described which comprises (i) caustic burning (e.g. at 500-900 DEG C) a mixture of magnesium oxide (or a magnesium compound which forms magnesium oxide on caustic burning) and chromium ore, or first caustic burning the magnesium oxide or magnesium compound and then forming the mixture; said mixture having an average particle size of less than 50 microns and containing 90-40% by weight magnesium (expressed as magnesium oxide), 10-60% by weight of chromium ore, no more than 1% by weight CaO and no more than 1% by weight SiO2; (ii) shaping the mixture; and (iii) sintering (e.g. at 1750 DEG C or more) and then cooling the shaped mixture; the caustic burning and the heating of the mixture up to the sintering temperature being effected under an inert or reducing atmosphere and the sintering and cooling being effected under an oxidising atmosphere. The beating to sintering temperature and cooling are preferably effected slowly.

Description

SUMMARY OF THE INVENTION The present invention relates to a process for producing a dense, high volume, directly bonded, simultaneous color space from a fine mixture of pure magnesium oxide or a mixture of magnesium compounds and silicon-poor chromium molding that provide it at relatively low temperatures. The use of simultaneous color space refractories consisting of a uniformly dispersed, agglomerated aggregate of periclastic and spinel crystals that are tightly bonded together without a binder is becoming increasingly common in the production of the most commonly used basic refractory bricks and shapes.

It is known that 10-70% of the periclase and 20-90% of the crystalline fish glaze of at least two types of magnesium spinel, at least partially bonded together, are required to produce a refractory particle of the simultaneous color, so that the particle size of the starting blend is below 44 microns. and the total amount of impurities is below 5%. Within this, the amount of 20 CaO and SiO ₂ each should not exceed 2%. The blend is compacted by briquetting or pelleting before firing, and the coloring itself is carried out at high temperatures. (U.S. Patents 2,775,525, 2,775,526 and 2,775,527). 25

It is also known that even relatively simple and easy to produce low-chromium content e.g. it is also necessary to produce simultaneous sinterers consisting of a mixture of 70 to 98% MgO and 2 to 30% chromium, that the MgO component is 90%, the 30 chromium component is 80%, has a particle size below 60 microns and the mixture has a CaO / SiO ₂ ratio below 0.6 so, after the briquetting the raw mixture should be at least 1750 ° C, preferably 1900 ° C 5. (Austrian Patent Nos. 263,612 and 263,614).

There is also a process (British Patent No. 1,256,915) in which the entire amount of chromium molten is dissolved in the periclastic crystals during sintering so that upon cooling, they are released as secondary spinels within the periclastic crystals. Of course, this requires sintering at even higher temperatures (2000-2100 ° C).

There is also known a process (British Patent No. 1,327,749) for the preparation of magnesite-chromium and chromium-magnesite bricks by the preparation of chromium-magnesite-based pre-sintered granules from 65 to 90% chromium molding and 35 to 10% magnesium. pelleting, granulation). A chromium standard having a particle size of less than 63 microns and a magnesium content of less than 0.2 mm, containing at least 20% Cr ₂ O ₃ + Al ₂ O ₃ , containing not more than 2.5% SiO ₂ and not more than 1% CaO, are used. The sintering is carried out at temperatures above 1750 ° C, preferably 1900 ° C.

Also known is a process (US Patent No. 3,817,765) which produces, without prior briquetting and extrusion, a simultaneous level of a mixture of 20-80% MgO and 20-80% chromium in which the MgO component is below 44 microns, with an average of 4 microns grain size is less than 44 microns and 95% less than 10 microns. However, the silicic acid content does not exceed 0.5% in MgO and 3% in chromium. The sintering temperature for this process is preferably 2000 ° C.

It is known that the MgO-Cr ₂ O ₃ based simultaneous somewhat to reduce the sintering temperature of 1750, preferably ° C 1800 color space during production if the bulk density is approaching instead of chromium ore at least partly magnéziumkromitot, or a MgO or Cr ₂ O ₃ - Synthetic mixture of al is used as one component and the other contains pure magnesium oxide containing 97 to 99% MgO, which may contain up to 1% by weight of impurities Fe ₂ O ₃ and Al ₂ O ₃ . (British Patent No. 1,396,525).

It is also known that direct bonded magnesite-chromium or chromium-magnesite bricks can reduce polyol curing by providing a low oxygen / air ratio of 1: 1 or slightly above when burning the bricks at temperatures of 800-1300 ° C. in order to prevent oxidation of the divalent iron content in the chromium standard and the accompanying volume growth processes (U.S. Patent No. 3,829,541 to RJ Leonard, Am. Ceram. Soc. Bull. 54, 662 (1975)). However, this has not yet been used in the production of simultaneous color spaces.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a compact, directly bonded, simultaneous color space from a mixture of fine magnesium oxide or fine magnesium oxide providing it during firing and low silica chromium concentrate by caustic firing followed by cutting, shaping and co-sintering.

In our experiments, we unexpectedly observed that when pure magnesium oxide, nesquehonite (MgCO ₃ · 3Η ₂ Ο), basic magnesium carbonate [ca. A mixture of Μβ ₄ (ΟΗ) ₂ (0Ο ₃ ) ₃ · 3Η ₂ br], brucite [Mg (OH) ₂ ] or other magnesium compound which provides pure MgO during incineration, and low in chromium concentrate of silicic acid, which is a mixture of oxides containing 90-40% by weight of MgO, individually or collectively milled to an average particle size of 5 microns, burnt to a caustic at a temperature of 500-900 ° C in a loose or chopped or inert gas atmosphere, preferably with little or no oxygen , after cooling, pressed into briquettes at a pressure of at least 1000kp / cm ² , slowly heated to a temperature of 1750 ° C or slightly in an inert gas, or more preferably with a slightly reducing effect, and then achieved in an oxidizing atmosphere, for the resulting simultaneous level, it is slowly cooled to a very compact, direct binding for a simultaneous level with a bulk weight of 3.5 to 3.7 g / cm ³ and an apparent porosity of only 3-5%.

The mechanism of action of the inert or slightly reducing gas atmosphere of caustic firing and sintering during the sintering of the magnesium oxide chromium mixture is not yet fully understood. Undoubtedly, the preservation of the divalent iron content of the chromium standard and its deoxidation play a role. However, this does not provide a satisfactory explanation for our observation that, in a slightly reducing gas atmosphere, a compressed briquette of magnesium oxide-chromium standard can be made from a compressed briquette for a denser higher bulk density than under the same conditions. in an argon gas atmosphere. It is believed that the slightly reducing gas atmosphere is beneficial for the formation of simultaneous color spaces, because it accelerates the diffusion processes of the mutual equilibrium of the periclical and spinel crystals by 2o partly by causing lattice defects and partly by the gas phase distillation-decapitation mechanism. This is supported by the observation that, where MgO: chromium ore ratio, the same particle size, in the same way ₂₅ in the preparation of the same extruded raw mix temperature and at the same time with sintered simultaneous color space to the spinel phase is less than when the caustic incineration and heating step sintering in an inert lattice constant rather, it is carried out under a slightly reducing gas in a ₃ θ atmosphere, as if under the same oxidizing conditions and at the same time, the simultaneous color space produced is denser and has a higher volume weight.

The following examples illustrate the process of the present invention.

Example 1

1302 kg of synthetically produced magnesium hydroxide having an average particle size of 6 microns and having the following chemical composition:

45 MgO 68.43% CaO 0.63% SiO ₂ 0.021% 50 Fe ₂ O ₃ 0.014% Al ₂ o ₃ 0.014% 55 izz. losers. 30.90%

when slack caustic burned at temperatures between 600 and 700 ° C, a flue gas atmoszfé stern _60, wherein the amount of Co ₂ + H, 1.5 vol% oxygen and 0.5 vol% The amount of less than -.. at. The resulting approx. 900 kg of caustic magnesium oxide are mixed with 100 kg of dry powder of chromium concentrate, previously ground to an average particle size of 6 microns, of the chemical composition:

Cr ₂ O ₃ 59.37% Fe ₂ O ₃ 14.08% Al ₂ O ₃ 8.57% SiO ₂ 0.89% CaO 0.28% MgO 16.15% izz. losers. 0.97%.

The mixture was homogenized in a ball mill for 2 hours. From the homogenized crude mixture, 2.5 cm 0, 2.0 cm high pellets are pressed dry without addition at 2000kp / cm ² . They are sintered in a gas-fired mine furnace at a temperature of 1750 to 1780 ° C in an oxidizing atmosphere for 1 hour. Meanwhile, a sufficient amount of natural gas (in addition to the amount of natural gas burned in the burner) is continuously introduced into the preheating zone above the combustion chamber so that the gas atmosphere in the entire preheating zone contains at least 1.5 vol% CO + H ₂ . and less than 0.3% vol. The internal dimensions of the furnace are selected and fired at such a rate that the rate of warming of the pastilles and the rate of cooling of the finished simultaneous color space is not greater than 15 ° C / min. The result is approx. The volume of a simultaneous color space of 1000 kg is 3.5 g / cm ³ , its apparent porosity is 0.2% and its chemical composition is as follows:

MgO 90.80% CaO 0.84% SiO ₂ 0.12% Cr ₂ O ₃ 5.94% Fe ₂ O ₃ 1.43% A1 ₂ O ₃ 0.88%.

If caustic firing as well as sintering do not use a reducing gas atmosphere during heating, but both processes are carried out in an oxidizing atmosphere, only 3.3 g / cm ^{3 of} product with an apparent porosity of 0.8% is obtained.

Example 2

2040 kg of synthetically produced nesquehonite (Mg / CO ₃ · 3H ₂ O) having the following chemical composition:

MgO 28.98%

CaO 0.403%

0.041%

0.050%

SiO ₂

Fe ₂ O ₃

Al ₂ O ₃ 0.020% bulb. losers. 70.59% consists of needle crystals having a mean length of 0.15 mm and 0.05 mm thick, mixed with 400 kg of chromium concentrate having a particle size of less than 0.3 mm in the composition of Example 1 and milled in a water-ball mill for 8 hours. The result is approx. The slurry having an average particle size of 10 microns was dewatered by filtration. The filter cake is dried in a lump and burnt to caustic in a flue gas containing less than 0.2% oxygen at a temperature between 650 and 800 ° C. The caustic burned crude mixture is ground to a grain size of less than 0.2 mm in a dry ball mill. The resulting powder blend was pastilized and sintered as described in Example 1. The result is approx. 1000 kg simultaneous color space of 3.68 g / cm ³ , apparent porosity 2.7% and chemical composition

MgO 65.39% CaO 0.93% SiO ₂ 0.44% Cr ₂ O ₃ 23.75% Fe ₂ 0 ₃ 5.73% A1 ₂ O ₃ 3.47% If the caustic firing, and sintering

in the same manner, but under an oxidizing atmosphere, only a simultaneous level of 3.33 g / cm ^{3 by} volume, with an apparent porosity of 7.4%, is obtained.

Example 3

2040 kg of synthesized nesquehonite of Example 2 chemical composition and particle size are mixed with 400 kg of Example 1 chromium concentrate of particle size less than 0.3 mm and water milled in a ball mill for 8 hours. The result is approx. The slurry having an average particle size of 10 microns was dewatered by filtration. The filter cake was dried in lumps, and then heated to caustic at -650 to 800 ° C under argon. The caustic burned crude mixture is ground to a grain size of less than 0.2 mm in a dry ball mill. The resulting powder blend is dry-pressed to a pressure of 2.5 cm, and 2.0 cm high at 1000 kp / cm ² . They are sintered at 1750 to 1800 ° C in an oxidizing atmosphere for 1 hour and cooled. The pastilles were heated to 1750 ° C under an argon gas atmosphere. We make sure that the pastil3

1 / ÖÖJl

The temperature of the slabs during heating should not exceed 10 ° C per minute and the rate of simultaneous cooling of the final product should not exceed 12 ° C / min. It is prepared for a simultaneous level of about 1000 kg, having a bulk density of 5.41 g / cm ³ , an apparent porosity of 3.9% and a chemical composition similar to that of Example 2.

Example 4 10

1360 kg of synthetically produced Example 2 chemical composition and particle size nesquehonite are mixed with 600 kg of Example 1 chemical composition, granule size less than 0.3 mm, and the mixture is water-milled in a ball mill for 8 hours. The approx. The slurry having an average particle size of 13 microns was dewatered by filtration. The filter cake was dried and incinerated in a fluffy state in a flue gas containing 1.8% by volume methane and 20% 0.2% oxygen at 750 to 900 ° C. The caustic crude mixture is ground to a grain size of less than 0.2 mm in a dry ball mill. The powder mixture was compressed at 1500 kp / cm ^{2 to} 25 cm ² , 2.0 cm high lozenges and sintered in an oil-fired shaft furnace at 1750-1800 ° C for 2 hours and cooled. The amount of reducing gas (CO + H ₂ ) resulting from the thermal decomposition and partial combustion of the oil in the whole preheating zone was continuously sprayed with ₂ vol. its oxygen content should be less than 0.2 vol. By properly shaping the furnace shape and interior and controlling the firing rate, the rate of warming of the pastillated blend in the preheating zone and cooling of the finished simultaneous color space in the cooling zone is below 10 ° C / min. It is obtained at a simultaneous level of approximately 1000 kg, having a bulk density of 3.70 g / cm ³ , an apparent porosity of 2.8% and a chemical composition of:

MgO 49.30%

CaO 0.72%

SiO ₂ 0.59%

Cr ₂ O ₃ 35.62%

Fe ₂ O 3 8.52%

Al ₂ O ₃ 5.17%

When firing and sintering are carried out in the same manner but in an oxidizing atmosphere, only a simultaneous level of 3.45 g / cm ³ and an apparent porosity of 9.5% is obtained.

Claims (2)

Patent claims: 1. A process for the production of solid, directly bonded simultaneous colorants with a density of 3.5 to 3.7 g / cm ^{3 from a} crude mixture of a mixture of magnesium oxide (MgO) with a low content of silica and methane, and magnesium compounds and a chromium molding process. containing 90-40% by weight of MgO, 10-60% by weight of chromium, based on the oxide, containing not more than 1% by weight of SiO ₂ and not more than 1,0% of CaO, by caustic combustion of the crude mixture between 500 and 900 ° C at a temperature of at least 1000 kp / cm ² , by sintering the molding material at a temperature of 1750 ° C or greater, characterized in that the caustic firing and heating of the molten material to the sintering temperature are carried out in a gas atmosphere with a slight reducing effect, free of oxygen or up to 0.5 f.% oxygen, at a rate such that the temperature of the material rises to 20 ° C / min or less, and sintering and subsequent cooling is performed in an oxidizing atmosphere, ensuring that the temperature of the resulting simultaneous color space does not exceed 20 ° C. «Ot decrease every minute. 2. The method of claim 1, wherein characterized in that caustic combustion of the crude mixture and heating of the crude mixture to a sintering temperature during sintering is carried out in an inert gas (nitrogen, argon) or in a gas mixture (flue gas) containing at least 1% by volume of carbon monoxide and / or hydrogen and / or or methane, or a mixture thereof, and having an oxygen content of less than 0.5% by volume.