CZ284148B6 - Sintered magnesia, process for producing thereof and a shaft furnace for making the same - Google Patents

Abstract

Sintered magnesia, rich in iron, has a grain density greater than 3.35 g / cm.sup.2. and iron content, calculated as F.sub.2.n.O.sub.3.n. greater than 1.5% by weight, wherein the trivalent iron and trivalent iron-containing minor phases, in particular dicalciumferrite (CF), brownmerite (C.sub.4.n.AF) and magnesium ferrite (MF) are at most 0.5% by weight % wt. The process consists in subjecting the coarse-crystalline, crude iron-rich, magnesite-rich process to at least a three-stage process, passing through a heating and firing zone and passing through a zone of reducing atmosphere and reduced temperature before entering the cooling zone. The apparatus comprises a heating (de-acidifying) zone (12), a firing (sintering) zone (14), a reduction zone (20), and a cooling zone (22). The use of sintered magnesia for the production of magnesium and carbon bricks

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to sintered magnesia with a high iron content, a process for the manufacture of this sintered magnesium having a C / S (CaO: SiO2) ratio in particular greater than 2, and shaft furnaces for carrying out the process.

BACKGROUND OF THE INVENTION

As already published by Germany in “Acta Physical Austriaca”, Volume XVIII, 1964, p. 205, density products are reduced 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 double-ferrous calcium, because at places where the magnesium and double-ferrous ferric ferrites were present in the periclase crystal starting material, the voids are formed after the excretion of these constituents. .

SUMMARY OF THE INVENTION It is an object of the present invention to provide a solution by which sintered magnesia is obtained from a high-iron raw material which has the highest density and does not have the disadvantages mentioned. As high a density as possible is meant an attempt to achieve a density of a high iron content sintered magnesia material that corresponds as closely as possible to the low iron content of sintered magnesia prior to sintering.

In this way, it should also be possible to use magnesium oxide agglomerates with an increased iron content, which both have increased 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 magnesium bricks and carbon-bonded materials results in a significant reduction in high temperature resistance properties due to the MgO-C adsorption bonding system, since the active bonding bridges C-MgO-C ... reduce the reducing effect of Fe ³⁺ .

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

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

The separation 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 Fe ₂ O ₃ content of 1.5% by weight. less Fe ₂ O ₃ (<0.5% w / w), while Fe ₂ O ₃ content in high-iron burnt magnesium is higher (> 3.0% w / w).

- 1 GB 284148 B6

The term "carbon bond" includes 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

The sintered magnesia according to the invention fulfills this task, which is characterized in that it has a crude grain density of more than 3.35 g / m ³ and a total iron content, calculated as Fe 2 O 3, of greater than 1.5% by weight, Fe ³⁺ ), calculated as Fe ₂ O ₃ , and the fraction of secondary phases containing trivalent iron (Fe ³⁺ ), in particular di-calciumferite (CF), brownmilerite (C ₄ AF) and magnesium ferrite (MF), is not more than 0.5% by weight, based on the total weight of sintered magnesium.

Preference is given to clinker ratio of CaO: SiO ₂ greater than the second

The proportion of iron (III) in the clinker can be limited to these secondary phases. It does not interfere in any way if other trivalent iron fractions are dissolved in the periclase, if the total Fe ³⁺ content is less than 10% by weight, based on the total iron content of the sintered material.

This high iron sintered material can be prepared in a one-stage production process if 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 intended 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 by a reduction process upstream of the cooling zone. In this process, it is important that the temperature set in the reduction zone is lower than the temperature in the oxidation firing zone, but on the other hand higher than in the cooling zone itself. In the 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 during the reductive treatment of the fired material is to be in the temperature range between 600 ° C and the firing temperature, which may be, for example, 1800 ° C.

The reducing conditions for the reducing zone can be set in different ways and by different means. In the simplest case, a burner is placed in the reduction zone in which the 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 off a partial amount of fuel supplied to the firing zone and feeding it together with the secondary oxygen-free gas stream through the cooling zone into the reducing zone. The gas for generating the secondary gas stream may be an inert gas such as nitrogen.

While the combustion of both partial 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.

In this fuel feed, divided into two streams, one of which 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 is designated as B1 and the second amount of fuel being fed through

A cooling zone, such as B2, is sufficient to have a B2: B1 ratio of from 0.01 to 0.1 to obtain the desired reduction density in the raw state.

During the tests, it has been found that by the process according to the invention it is possible to prepare sintered magnesia from natural alpine magnesite with a high iron content and a raw density of more than 3.35 g / cm ³ , which does not contain Fe ³⁺ containing secondary phases (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 a gas blower, irrespective of the oxygen, during or after the cooling zone. In this case, it must in any case be ensured that the clinker is not cooled when spraying with a liquid, in particular water, below 250 [deg.] C., to avoid the possibility of brucite formation and to avoid undesirable hydration.

In pure gas cooling (nitrogen N ₂ ), the redox potential is shifted so that partly metallic iron is produced and at the same time elemental carbon (methane decomposition) is produced, especially in larger pores and in amounts up to 0.3% by weight. As a result, it is possible to reduce the proportion of iron (III) below 0.1% by weight.

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

Accordingly, the invention provides a shaft furnace for carrying out the method of the invention, which comprises, in the firing direction, the following zones, namely a heating / deacidification zone, an oxidizing atmosphere firing / sintering zone, a reducing zone and a cooling zone. wherein the fuel line for supplying the fuel stream to the firing zone is provided with a bypass section for discharging a partial fuel stream, the outlet end of which is discharged into the cooling zone, and a device for spraying the fired material with liquid and / or nitrogen.

In the firing zone, burners are usually distributed symmetrically throughout the perimeter and 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 explained in the description of an exemplary embodiment of the invention.

The bypass section of the duct may here be connected to the duct for supplying the cooling gas to the cooling zone, or it may be led directly into the cooling zone independently of this cooling duct.

The other features of the shaft furnace, which can find its application in the process of the invention, are the same as in the known shaft furnaces for the production of burnt magnesium.

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

-3 CZ 284148 B6

This magnesium may therefore be advantageously used in carbon binder materials and bricks. For carbon-bonded bricks made of clinker according to the invention, compared to iron-poor sintered magnesia bricks, similar conditions are available for the carbon-based binder components, so that the behavior of the two 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 ³⁺ or only a small amount (<0.5% by weight) of this secondary phase is formed in the clinker. phase.

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

Other features of the invention will be apparent from the dependent claims, as well as from the following description and drawings.

Overview of the drawings

The invention will be further elucidated by means of exemplary embodiments of a shaft furnace shown schematically in the drawing.

DETAILED DESCRIPTION OF THE INVENTION

The shaft furnace is supplied from above in the direction of arrow 10 in lumpy form, with a grain size of 5-50 mm, in particular a coarse crystalline crude magnesite according to Table 1, generally iron-rich. The raw material to be fired is then fed to a heating zone 12, where the material is de-acidified at about 700 ° C before being fed to the burning zone 14, where the oxidation process takes place under conditions corresponding to an air coefficient greater than 1. by means of which natural gas is supplied to the firing zone 14.

While in the firing zone 14 temperatures are maintained at the highest values (about 1850 ° C) from the entire furnace, this temperature decreases inside the furnace down to its outlet end 18, where the temperature drop down to the lower end of the firing zone 14 is about 500 ° C. a meter of baking length, wherein the fired material that turns into clinker during firing is discharged from the firing zone 14 first to the reducing zone 20, in which the air coefficient is set to a value significantly lower than 1. The reducing zone 20 runs from the end of the firing zone 14 up to a 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 route, 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. The composition of the clinker is shown in Table 1.

It is also evident from the illustrated embodiment that the fuel line 16 is provided with a bypass section 167. As a result, the fuel stream B is divided into two partial streams B1, B2, whose weight ratio B2 / B1 in this case is 0.06.

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

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

-4GB 284148 B6

Table 1 Crude clinker magnesia (% by weight) (% by weight)

SiO ₂ 0.3 0.6 Fe ₂ O ₃ 2.9 5.8 * K ₂ CO ₃ 0.2 0.4 MnO 0.3 0.6 CaO 1.1 2.2 MgO 44.7 90.4 burning loss -50.5

* of which as Fe ³⁺ : 0.1%, as Fe (metal): 1.7% and as Fe ²⁺ : 4.0%.

PATENT CLAIMS

Claims (14)

Sintered magnesium having a crude grain density of more than 3.35 g / cm ³ and a total iron content, calculated as Fe 2 O 3, of greater than 1.5% by weight, wherein the proportion of iron (Fe ³⁺ ), calculated as Fe2O _3, and the proportion of secondary phase containing trivalent iron (Fe ³ '), in particular dikalciumferitu (CF) brownmileritu (C ₄ AF) and magnesium ferrite (MF) is not more than 0.5% by weight based on the total weight of the sintered magnézie. Sintered magnesia according to claim 1, characterized in that it contains CaO and SiO ₂ and the weight ratio of CaO to SiO ₂ is greater than 2. Sintered magnesia according to claim 1 or 2, characterized in that the proportion of Fe ³⁺ containing secondary phases is less than 0.2% by weight, based on the total weight of the sintered magnesium. Sintered magnesia according to one of claims 1 to 3, characterized in that the total Fe ³ 'content, calculated as Fe ₂ O ₃ , is in an amount of less than 10% by weight, based on the total amount of iron. Sintered magnesium according to 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 one of claims 1 to 5, characterized in that. It comprises elemental iron formed by reduction in the sintering process. Method for the production of sintered magnesia according to one of claims 1 to 6, wherein the coarse-crystalline crude iron-rich magnesite is subjected to at least a three-stage pyroprocess, wherein it is guided successively through a heating zone, a firing zone with an oxidizing atmosphere and a cooling zone. The method according to claim 1, characterized in that, after leaving the firing zone and before entering the cooling zone, the fired material is guided through a reducing zone in which the temperature prevails, which is lowered compared to the firing zone. Method according to claim 7, characterized in that the reduction treatment of the fired material is carried out in a temperature interval between 500 ° C and a temperature close to the firing temperature. - 5 GB 284148 B6 Method according to claim 7 or 8, characterized in that a partial amount of fuel is taken from the fuel supplied to the firing zone, which, together with the secondary oxygen-free gas stream, is fed to the reduction zone through the cooling zone. Method according to at least one of Claims 7 to 9, characterized in that the fired material is sprayed with liquid and / or nitrogen during or after passing through the cooling zone. Method according to claim 10, characterized in that the spraying of the liquid and / or nitrogen with the spray is carried out so that the temperature of the shade does not fall below 250 ° C. A shaft furnace for carrying out the method according to any one of claims 7 to 11, comprising downstream zones in the conveying direction of the fired material - heating de-acidification zone (12), firing sintering zone (14) with oxidizing atmosphere, reducing zone (20). ) and a cooling zone (22), characterized in that the fuel line (16) for supplying the fuel stream (B) to the firing zone (14) is provided with a bypass section (16 ') for discharging the partial fuel stream (B2), the end is discharged into the cooling zone (22), and at the outlet end (18) of the furnace there is a device (24) for spraying the fired material with liquid and / or nitrogen. The shaft furnace of claim 12, further comprising an oxygen-free secondary gas cooling line (C) flowing into the cooling zone (22). A shaft furnace according to claim 12 or 13, characterized in that the conduits for conducting the second fuel partial flow (B2) to the cooling zone (22) and the first fuel partial flow (B1) to the firing zone (14) are formed such that the ratio by weight of the second fuel partial stream (B2) to the cooling zone (22) to the weight of the first fuel partial stream (B1) to the firing zone (14) is between 0.01 and 0.1.