SK12195A3 - Sintering magnesia, method and device for its production and its using - Google Patents

Description

The invention relates to high iron sintered magnesia, to a process for the production of this magnesium having in particular a C / S (CaO: SiO2) ratio greater than 2, to an apparatus for carrying out the process and also to the use of magnesia according to the invention.

BACKGROUND OF THE INVENTION

As already published by the German in Acta Physica Austriaca, Volume XVIII, 1964, p. 205, there is a decrease in density in technical products based on sintered magnesium enriched with iron in a reducing atmosphere. This is due to the ferritic secondary phases of the magnesium ferrite and the calcium pyrophosphate, since at the locations where the ferrocity crystal starting material was found to be ferritomagnesia and calcium pyrophosphate, the voids are formed after the elimination of these constituents of the cavity.

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SUMMARY OF THE INVENTION The object of the present invention is to provide a solution for obtaining sintered magnesia from a high-iron feedstock, which

The highest density is understood to be the effort to achieve a raw material density for the preparation of sintered magnesia with a high iron content, which would correspond as closely as possible to the density of the sintered magnesia with a low iron content before sintering.

In this way, it should also be possible to use agglomerates of increased iron content which have both heat resistance and high resistance to infiltration, in particular in molded or unformed refractory materials.

The solution according to the invention is based on the following consideration:

The use of conventional high-iron burnt magnesia for the preparation of fused bricks and carbon-bonded materials results in a significant reduction in high temperature resistance properties due to the MgO-C adsorption bonding system, as the active C-MgO-C ... bond bridges are reduced reducing effect of Fe? ⁺ .

To date, reductive calcined magnesia has been used prior to further processing to MC products (MgO-C) at about 1000 ° C. This reduction, however, increases the porosity of the clinker, so that the advantage of increased resistance to high temperatures is, on the other hand, adversely offset by a reduction in the material's resistance to infiltration due to the increase in pore volume.

These problems do not occur or appear to a much lesser extent in burnt magnesium with a low iron content.

The distribution of burnt magnesium oxide into high-iron magnesium and low-iron burnt magnesium according to the invention is carried out according to the invention at a total iron content calculated from a Fe2O3 content of 1.5% by weight, with low-iron burnt magnesium usually having less Fe2O3. (<0.5% by weight), while the proportion of Fe2C > 3 in high-iron burnt magnesium is higher (> 3.0% by weight).

The term carbon bond encompasses both products whose particles are bound together by, for example, tar or pitch, as well as products formed using synthetic resin or other non-toxic substances as binders.

SUMMARY OF THE INVENTION

This object is solved by the sintered magnesia of the invention, which consists in that the sintered magnesia has a raw grain size greater than 3.35 g / cm 3 and a total iron content, calculated as Γβ2θ3, of greater than 1.5% by weight, %, calculated as Fe 2 O 3, and the secondary phase containing Fe ⁴⁺ , in particular calcium di-ferrite, Brownmilerite and magnesium ferrite, are each at most 0.5% by weight. In a preferred embodiment of the invention, the fraction of secondary phases is less than 0.2% by weight.

A clinker having a CaO: SiO2 ratio greater than 2 is preferred.

The proportion of iron (III) in the clinker can be limited to these secondary phases. In any respect, it is not a problem if further trivalent iron fractions are dissolved in the periclase if the total Fe? ⁺ Content is less than 10% by weight. based on the total iron content of the sintered material.

This high iron content sintered material can be prepared in a one-stage production process as long as the oxidation firing / sintering zone is followed by a reducing zone whose temperature is lower than the temperature in the firing zone and if the clinker passes through this reducing zone before being introduced into the cooling zone.

In other words, while the actual sintering process, which is to achieve the highest density of sintered particles, proceeds as an oxidation process, in the process according to the invention this sintering process is followed up by a reduction process upstream of the cooling zone. In this procedure, it is important that the temperature set in the reduction zone be lower than the temperature in the oxidation firing zone, but on the other hand, higher than in the cooling zone itself. In this process according to the invention, the temperature in the reduction zone following the firing zone should be approximately 600 ° C. According to another preferred embodiment of the method according to the invention, the temperature in the reduction treatment of the fired material is to be in the range from 600 ° C to the firing temperature, which may be, for example, 1800 ° C.

The reducing conditions for the reduction zone can be adjusted in different ways and by different means. In the simplest case, a burner is placed in the reduction zone in which combustion takes place under reducing conditions, i.e. in the absence of air.

A further advantageous embodiment of the process according to the invention consists in creating reducing conditions for the pyroprocess by taking a partial amount of fuel fed to the firing zone and feeding it together with the secondary oxygen-free gas stream through the cooling zone to the reducing zone. The gas for generating the secondary gas stream may be an inert gas, for example nitrogen.

While the combustion of both amounts of fuel in the sintering zone of the shaft furnace takes place with an air coefficient greater than 1, the air coefficient in the reduction zone is substantially less than 1.

With this fuel feed, divided into two streams, from which it is conducted as a bypass, it is sufficient to separate only a minor proportion of the fuel from the main fuel feed and feed it through the cooling zone to the reduction zone or sintering zone. If the amount of fuel fed directly to the combustion / sintering zone such as B1 and the second amount of fuel fed through the cooling zone such as B2 are indicated, a B2: B1 ratio of from 0.01 to 0.1 is sufficient to obtain the desired reducing clinker with increased density in the raw state.

During the tests it was shown that the method of the invention may be prepared vitrified magnesia Alpine natural magnesite with a high iron content as raw density of more than 3.35 g / cm ^3, which does not contain secondary phase containing Fe ³ + (DIN 51065 2).

According to a further preferred embodiment of the invention, the clinker is quenched by, for example, spraying with a liquid and / or blowing with an oxygen-free gas, during or after passing through the cooling zone. In this case, it must in any case be ensured that the clinker is not cooled below 250 DEG C. when spraying with a liquid, in particular water, in order to avoid the possibility of brucite formation and to avoid undesirable hydration.

With pure gas cooling (nitrogen N2), the redox potential is shifted so that partly metallic iron is produced and at the same time elemental carbon (by methane decomposition) is excreted, especially in larger pores and up to 0.3%. As a result, it is possible to reduce the trivalent iron proportion below 0.1%.

The advantages achieved by the method according to the invention over the prior art processes known in the art are obvious. While the known processes have been carried out in two stages, of which the pyroprocess itself takes place in the first stage and then the reduction annealing treatment in the second stage, the process according to the invention provides the possibility of producing a reduced high-iron and high-density clinker at a selected location. In particular, the process according to the invention can be carried out simply in a shaft furnace.

According to the invention, a shaft furnace designed for carrying out the method according to the invention comprises, in the direction of the firing material, the following zones:

- acidification and acidification zone

- firing / sintering zone (with oxidizing atmosphere)

- reducing zone (with reducing atmosphere)

- cooling zone.

Until now, there are usually symmetrically distributed burners in the firing zone, which are supplied by a plurality of supply lines or a single common supply line for fuel, for example oil or gas. However, an embodiment of the invention proposes to supplement the fuel line leading to the firing zone with a bypass whose outlet end is directed to the cooling zone. In this way, the amount of fuel supplied to the firing zone is reduced and the proportion of the fuel by which this supply is reduced is fed through the cooling zone and the reducing zone to the firing zone from the other side. Further details of this embodiment are set forth in the description of an exemplary embodiment of the invention.

The by-pass section of the pipe can be connected to the pipe for supplying the cooling gas to the cooling zone or can be led independently of the cooling pipe into the cooling zone.

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The shaft furnace may be provided with a spraying mechanism for spraying the fired material with a liquid at its discharge end in connection with the implementation of the additional cooling mentioned in the description of the method according to the invention.

The other features of the shaft furnace which may find application in the implementation of the method according to the invention are the same as for the prior art fired magnesia shaft furnaces.

The sintered magnesium according to the invention has a markedly increased raw density over the reduced sintered magnesium with a higher iron content according to the prior art. The sintered magnesia according to the invention, rich in iron, combines the advantages of high heat resistance with high resistance to infiltration.

This magnesium can therefore advantageously be used in carbon binder materials and bricks. For carbon-bonded bricks made of clinker according to the invention, as compared to iron-poor sintered magnesia bricks, similar conditions are available for carbon-based binder components, so that the behavior of both of these materials is similar.

As has already been said, the behavior of the material depends largely on the fact that during the combined oxidation / reduction pyroprocess no secondary phase containing Fe ^ ⁺ or only a small amount (<0.5% by weight) of this secondary phase is formed in the clinker. phase.

Very advantageously, an iron-rich (cost-effective) clinker can be combined as a coarse fraction with an iron-poor (more expensive) fine clinker fraction, so that technical and economic improvements and cost reductions can be achieved.

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Other features of the invention will be apparent from the independent claims and from the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail by means of the exemplary embodiments of the shaft furnace shown in the drawing.

DETAILED DESCRIPTION OF THE INVENTION

The coarse-crystalline raw magnesite, usually rich in iron, is supplied in lump form to the shaft furnace from above in the direction of the arrow 10. The feedstock to be fired is then fed to the heating zone 12, where the material is de-acidified (decarbonised) before being fed to the baking zone 14, where the oxidation process is carried out under conditions corresponding to an air coefficient greater than 1. with which natural gas is supplied to the firing zone 14.

While in the firing zone 14 the temperatures are maintained at the highest values from the entire furnace, this internal temperature inside the furnace decreases towards its outlet end 18, the fired material which is converted to clinker during firing is first discharged from the firing zone 14 to the reducing zone. The reduction zone 20 extends from the end of the firing zone 14 to the furnace section having a temperature of between 550 and 600 ° C. From there, the fired material passes through the cooling zone 22 to the outlet end 18 of the furnace. On this path, the clinker at 24 is sprayed with sprayed water, but it is ensured that the clinker temperature in this section is also higher than 250 ° C in order to prevent the formation of brucite.

It is also evident from the illustrated embodiment that the fuel line 16 is provided with a bypass section 16 '. As a result, the fuel stream B is divided into two partial streams B1, B2, whose weight ratio B2 / B1 in this case is chosen to be 0.06.

Parallel to the second conduit for conducting the second partial fuel stream B2 there is a cooling conduit C for supplying nitrogen. Both ducts are directed through the furnace outlet end 18 to the cooling zone 22 so that the gas streams pass through the reduction zone 20 and are diverted through the cooling zone 22 into the firing zone 14.

The design and operation of this shaft furnace may be varied within the scope of the claims, for example the method of the invention may also take place in a shaft furnace with an inclined focus.

Claims (18)

PATENT CLAIMS Sintered magnesia, characterized in that it has a crude grain density of more than 3,35 g / cm ^ and a total iron content, calculated as Fe2O3, of more than 1,5% by weight, the proportion of trivalent iron (Fe ^ ⁺ ) calculated as Fe 2 O 3, and the secondary phases containing trivalent iron (Fe? ⁺ ), in particular dicalciumferite (CF), Brownmilerite (C4AF) and magnesium ferrite (MF), are at most 0.5% by weight. Sintered magnesia according to claim 1, characterized in that it has a CaO: SiO 2 (C / S) weight ratio greater than 2. Sintered magnesia according to claim 1 or 2, characterized in that the proportion of Fe 4 ⁺ -containing secondary phases is less than 0.2% by weight. Sintered magnesia according to at least one of claims 1 to 3, characterized in that the total Fe ²⁺ content, calculated as Fe 2 O 3, is less than 10% by weight, based on the total amount of iron. Sintered magnesia according to at least one of claims 1 to 4, characterized in that it has a carbon content of C in an amount of up to 0.3% by weight. Sintered magnesium according to at least one of Claims 1 to 5, characterized in that it has an elemental iron content produced by reduction in the sintering process. Method for producing sintered magnesia according to at least one of claims 1 to 6, characterized in that the coarse-crystalline crude magnesium-rich magnesite is subjected to at least a three-stage pyroprocess, passing through a heating zone, a firing zone and a cooling zone. after leaving the firing zone containing the oxidizing atmosphere and before entering the cooling zone, the material is passed through a reducing atmosphere zone at a temperature that is lowered relative to the firing zone. The process according to claim 7, wherein the reduction treatment is carried out in a temperature range between 500 ° C and a temperature close to the firing temperature. Method according to claim 7 or 8, characterized in that for the reduction of clinker a partial amount of fuel is taken from the fuel fed to the firing zone and fed to the reduction zone together with the secondary oxygen-free gas stream. Method according to at least one of claims 7 to 9, characterized in that the clinker is sprayed with liquid and / or nitrogen during or after passing through the cooling zone. 11. The method of claim 10, wherein the spray liquid spraying is conducted such that the clinker temperature does not fall below 150 ° C. in A shaft furnace for carrying out the method according to at least one of claims 7 to 11, characterized in that it comprises downstream zones downstream of one another in the direction of conveying the fired material: a heating / deaeration zone (12), a baking / sintering zone (14). an oxidizing atmosphere, a reduction zone (20) and a cooling zone (22). A shaft furnace according to claim 12, characterized in that a fuel line (16) for supplying the fuel stream (B) is provided in the firing zone (14), which is equipped with a bypass section (16 ') for discharging the partial stream (B2). fuel, the outlet end of which is discharged into the cooling zone (22). in A shaft furnace according to claim 12 or 13, characterized in that the cooling zone (22) has an oxygen-free cooling gas cooling pipe (C). in A shaft furnace according to at least one of claims 12 to 14, characterized in that a mechanism for spraying the fired material with liquid is arranged at the outlet end (18) of the furnace. in A shaft furnace according to at least one of claims 12 to 15, characterized in that the conduits for guiding the second fuel partial flow (B2) to the cooling zone (22) and the first fuel partial flow (BI) to the firing zone (14) are formed. such that the ratio by weight of the second fuel sub stream (B2) to the cooling zone (22) to the amount of the fuel sub stream (B1) to the firing zone (14) is between 0.01 and 0.1. Use of sintered magnesia according to at least one of claims 1 to 7, produced in particular by the method described in at least one of the claims, for the manufacture of magnesium and carbon bricks and masses. Use according to claim 17 and according to the rule that the sintered magnesia according to at least one of claims 1 to 4 is used as a coarse fraction > 0.5 mm and as its other fraction < 0.5 mm sintered or baked magnesia with a total iron content calculated as ^ ^ε 2θ3 '<% by weight.