EP1602425A1 - Granulated powder for continuous casting of metals and his manufacturing method - Google Patents

Abstract

Granulate as an insulating or separation means for high temperature applications containing Si dioxide obtained from amorphous Si dioxide powder containing at least 90% SiO 2 by granulation with addition of water and drying. An independent claim is included for a method of obtaining the granulate by mixing SiO 2 with water in solid to water ratio 1:0.25-1:1.5, and granulation and drying of the granulate, where the SiO 2 powder contains at least 90% amorphous SiO 2.

Description

The present invention relates to insulating granules which can be used as insulating and / or release agents for high temperature applications, as well as their preparation.

In the current applications of high temperature resistant fillers, For example, in the iron, foundry and construction industry, come sands and ashes or their mixtures are used, which are characterized by temperature stability and reaction passivity. These additives are partly in their dusty accumulating form, partly added in the form of granules. The powdery substances are considered to be due to the dust effect occurring harmful to health and therefore should be successively comparable by granules Functionality to be replaced.

The granules are to come in different particle sizes are used and be by spraying or pelletizing pelletizing with sometimes considerable produced technical effort. For the application areas in the iron and steel industry steel processing industry is in addition to the temperature and transport stability low carbon content is required because the carbon content in the additive Property of iron and steel changed in an undesirable manner. In addition to The mentioned properties is a constant volume in foundry technology the aggregate over the entire temperature range desired as a Volume change of the fillers and release agents used to an unwanted Changing the mold geometry leads. In the construction sector, however, are next thermal stability the sound-insulating properties of outstanding Importance.

From DE 197 28 368 it is known that finely powdered acid or basic insulating agent be combined into granules in a matrix of gel-forming materials which are suitable as insulating covering agents in the steel foundry. The Granules have a reduced bulk density compared to the powdery insulating agents on. As gelling materials are guar gum and hydroxyethyl cellulose called. A disadvantage of these granules is that from the gel-forming material Thus, still significant amounts of carbon may be included, resulting in a Contamination of the steel.

There is therefore still a need for separating and insulating agents based on granulated Pellets with defined chemical and physical properties in procedurally controllable, reproducible grain sizes that are characterized by high temperature and material stability and therefore suitable for applications are suitable in different industries, where powdery Substances or mixtures thereof or granules with insulating, volume-constant Fillers or separating agents / additives are used and associated manufacturing process.

Surprisingly, it has now been found that amorphous silica powder can be processed with the addition of water to granules, the desired Have properties.

The object of the present invention is therefore achieved by insulating Granules consisting of an amorphous silica powder by pelleting with the addition of water in a ratio of solid: water of 1: 0.25 to 1: 1.5 are available.

The granules according to the invention offer the advantage of good environmental compatibility in production controllable grain size or bulk density. In addition to one Manual material dosing is an automated task possible. Experiments have shown that a temperature stability up to 1600 ° C is guaranteed. At the same time, the granules behave inert in terms of reaction and thus offer the certainty that they are not used as a filling material react to the substances to be processed.

Suitable SiO ₂ suppliers are all silicon powders which contain high proportions of amorphous SiO ₂ or silica. Particularly suitable are microsilica, silica filter dust and mixtures of microsilica or silica filter dust and rice husk ash in different proportions by weight. The addition of further additives for defined bulk density or particle size control is possible. The composition of the starting materials and the mixing ratio of the components involved allows the control of the grain size. At the same time, these parameters also influence the carbon content that is important for the iron and foundry industry.

The silicon dioxide used is preferably silica filter dust, which is obtained without carbon in the silicon or silicon-producing industry and has a consistently high SiO ₂ content of at least 90%. Particularly preferred is a filter dust is used, which is obtained from the flue gas scrubbing the residue combustion in the silicone production. It is irrelevant whether the silicon powder is already in the dried state or in wet chunk form as a filter cake. The silicon dioxide can be used without further drying in the form of the resulting filter cake. The already existing moisture is often enough to cause a direct granulation by stirring. It is expedient to pretreat a caked filter cake by digestion, for example by comminution / grinding / screening, in particular by pressing the puffy SiO ₂ through a perforated screen, so that an open-pored structure important for granulation is obtained.

It has also been shown that one of the drainage with a chamber filter press accumulating filter cake with a moisture content of 60% directly without further water addition can be granulated after being previously digested has been and in a fluffy, open-pored (potting-like) Consistency exists.

For filter slurries with low moisture contents, the dried and possibly ground filter cake by defined addition of water first in the Granulierungsbereich be brought, with a crushing or grinding for Achieving the open-pore structure described is also preferred.

In any case, the granulation must be carried out from a defined moisture, which may be based on the used SiO ₂ at a water content of up to 150% based on the total mass. Useful water contents are 25 to 150%; Typically, proportions of 30 to 80%, in particular 30 to 70% are particularly suitable. This slightly empirically determined moisture content must be met quite accurately for a given presented mixture, since too little water does not give rise to granules, only local lumps form in the powder. Further addition of water causes the granules to germinate and grow more and more. This happens until, in a moist granulation equilibrium, the added water combines to form an earth-entrained mass with the germs. By further addition of water, the granulation equilibrium is again shifted to an unfavorable range: the granules lose their structure and, depending on the amount of water, a viscous to light-bodied pasty pulp, which can be moved back into the granulation only by re-addition of solids.

By the stirring time, the granules can be further increased, if necessary. reach their final spherical shape after transfer to the granulator. Is the Quality of the resulting granules in the stirrer already sufficient, can on a Pelleting in the plate can be dispensed with.

If additional rice husk ash is used as the SiO ₂ supplier, it must be taken into account that the ash does not contain any clumps or husks, since this leads to unwanted, uncontrolled formation of chunks in the production of granules. When adding rice husk ash, a special low-carbon ash with about 5% C content is preferred. The proportion of Reisschalenasche is for cost reasons, preferably up to 30%. This leads to a total carbon content of less than 2%, preferably less than 1%, in the finished granules. Such granules are particularly suitable for use as an additive or insulating agent in the iron and foundry industry.

In the formation of granules, the components used in a targeted manner cake with water to make granules. The silica and optionally present additives have expediently a particle size on, which is below, preferably well below, the desired granule size. For a granule size of e.g. 0.5 mm is a particle size of the starting materials of a maximum of 0.1 mm. One with higher technical Effort operated finer grinding remains at this granule size without significant impact.

In one embodiment, substances which are used as thickening agents in food chemistry are used as additives. For application, these are first prepared in a low-percentage solution in order to influence the size of the granules after already made ateigung of the components to be used with water and an incipient granulation. The addition of dilute thickening agent solution allows the granules to be enlarged as required so that, depending on the area of application, granules of up to several centimeters in diameter can be produced in a targeted manner. Due to the agglomerating property of the additive, an increase in the bulk density is simultaneously observed, which in turn depends on the used and percentage composition of the SiO ₂ component (s). It should be noted that this addition is not required for granule formation.

High-melting oxides or carbonates of the alkali and alkaline earth metals, such as MgO, CaO, CaCO ₃ , Fe ₂ O ₃ , can also be used as additives. In this case, due to the basic or acidic properties of the components in aqueous solution, the pH of the granules produced changes, and the formation of eutectics lowers the melting point of the mixtures in comparison to pure SiO ₂ . To control the pH, it is also possible, if this is not to be achieved by adding an additive, to influence the silicic acid to be used in the aftertreatment by aqueous solutions of acidic or basic precipitation additions so that the granule preparation is already preceded by a filter dust with a defined pH Value takes place. The effects of the precipitants used in filter dust precipitation, which lead to accelerated formation of solids, must be taken into account.

The granules according to the invention are preferably made of silicon dioxide powder prepared with the addition of water and optionally additives in a stirrer. As far as e.g. for an optimal flow behavior as even as possible Ball shape is desired, completes further processing in a pelletizing at.

It is also possible, the granules by other agglomeration or granulation as the pelleting in a stirrer. Preferably should No or very little compressive forces are used in the granulation. Suitable are e.g. Screen granulators or vertical granulators and the Fluidized bed granulation. The well-known Kompaktierungsverfahren are only suitable if a slightly higher bulk density is acceptable.

When producing the granules, initially a SiO ₂ , for example, a silica filter slurry is introduced. This can be immediately pasted while stirring with water; Alternatively, rice husk ash in variable proportions can be added to the filter slurry before the addition of water, which is then mixed with water in the next step as a homogeneous mixture.

When stirring the pasted mixture nuclei are formed after a short time, already containing the structure of the granules to be formed. As the stirring time increases, the diameter of the seeds increases until granules of the desired size are formed. If the amount of water added is too large, the granules lose their consistency again and it forms a pasty porridge, which can be brought back into the necessary for granulation SiO ₂ water ratio only by adding more Festsoff (filter dust or rice husk ash).

For further control of the granule size, the already formed Granules are treated with the described additives; also by one Addition of rice husk ash is a granule enlargement possible. Because the Granules already formed during stirring, is a subsequent treatment the same in the granulator not necessarily necessary, but leads to a regular spherical shape, which is desirable for certain applications.

Regardless of the follow-up, the increase in size can be due to Powders are finished with rice husk ash or filter dust, taking has shown that the rice husk ash is particularly good for this process suitable is. A Reisschalenaschezugabe of about 0.5% based on the Total mixture is sufficient. It produces dull black granules with a Size of 0.5 - 5mm, depending on the stirring time and Reisschalenaschezugabe.

Spraying the granules formed with weakly concentrated thickener liquid As an additive, the size continues to increase, so that the granules can be built up to spheres with several centimeters in diameter. In contrast to the thickener-free granules, shiny black ones are produced Granules powdered again through filler or rice husk ash can be.

The process of enlarging the resulting granules is the same in the granulating plate feasible; the granules differ only in the rounder shape. An alternative in granulating dishes is a final one Spraying the granules with water that softens the surface of the granules and through the rotation in the plate leads to a perfect spherical shape.

It is advantageous to filter dust from the beginning in a predetermined ratio to mix with rice husk ash, thus the granule size is better controllable. The rice husk ash can be used as required in any Weight ratios are added; in practice shares have up to 30% Ashes led to a good result. At the same time, the addition of the Rice husk ash but increase the bulk density.

Another important aspect for some applications is the pH neutrality of the Granules that are at pH 8. An aggregate should become pH neutral behave as changes in the pH negatively affect the processing substance or the forms in which the substance is processed and is transported, can carry.

In contrast to the method known from DE 197 28 368, the present production process without combining the powders used forms a matrix with the aid of a gel-forming material. The use of amorphous SiO ₂ allows direct granulation.

By variation of the starting materials, inclination by defined addition of water and controlling the reaction conditions by residence time and Inclination / rotational speed of the pelletizing plate can be a granulate in defined Grain size of 0.5 - 5 mm size can be produced, which is characterized by thermal and mechanical stability up to the high temperature range.

The granule size can also be determined by the residence time in the stirrer or later in the Pelletizing be influenced. In both cases, an extension leads to an enlargement of the granules, whereby by the evaporation of the water the granules surface increasingly dries and the granulation for Standstill comes. An enlargement of the granules is only possible as long as the Outside of the granules has a residual moisture, through their pasty Surface allows attachment of additional material. The same applies to the Stirring or Granulating: Increasing speed leads to a Magnification of the granules, wherein in particular when stirring an exceeding a defined number of revolutions of the stirrer as a function of container size destructive effect on the granules may have. By changing the Inclination of the granulation plate is given another possibility of growth to control the granules. A larger angle of inclination lets through the steepness the granules by the greater gravity at the same time less friction with Increased speed fall to the lower edge of the container, the Caking process is strengthened. This also leads to a Enlargement of the granule diameter. By a downstream sieving the grain size range can continue according to the customer's request be limited.

This is important in the filler industry, depending on the application a certain reproducible grain size is required, according to the invention is achieved by varying the production parameters. The grain size is crucial in combination with bulk density, since when applying or introducing the granules in or on the forms Independent bleeding is necessary for the dosage.

The powdered materials used so far have the here Disadvantage of missing flow properties and are therefore less favorable to handle and dosage. Especially the production of granules with a defined Grain size leads to a possibility of customer and application specific Adaptation for the respective application.

The granules according to the invention have a bulk density of 0.2 to 0.7 kg / l, e.g. of 0.5 kg / l and thus meet the demand for a light granules with a low bulk density.

The finished product can vary according to customer requirements in bags or BigBags Size, transported in troughs or silos and abandoned.

The following examples are intended to illustrate the invention in more detail without, however, auf to limit the specific embodiments described. All information Unless otherwise indicated,% by weight.

example 1

60 g of microsilica (Elcem, Norway) having the following composition: SiO ₂ : 91.1%, Fe ₂ O ₃ : 2.5%, Al ₂ O ₃ : 0.9%, MgO: 1.1%, K ₂ O. : 1.7%, Na ₂ O: 0.4%, SO ₃ : 0.4%, residual moisture 0.5% and total carbon content. 1.65% was mixed with 30 g of water. The mass was thoroughly mixed in the stirrer, it formed granule nuclei, which increased in size with lasting stirring time. To achieve a smooth surface, the granules were powdered with 5g rice husk ash. The granules had a particle size of 0.5-3 mm and a bulk density of 0.6 kg / l.

Example 2:

48 g of the silica powder from Example 1 were mixed with 12 g of rice husk ash having the following composition
SiO ₂ : 87%, CaO: 1%, Al ₂ O ₃ : 0.2%, Na ₂ O: 0.2%, K ₂ O: 2%, Fe ₂ O ₃ : 0.15%, C _ges : 6.2%, residual moisture 1.1%
and 25 g of water mixed. Subsequently, the mass was stirred in the stirrer, which forms open-pored pellets after a short time, which grow with slowly rotating Rüher. Continued stirring leads to clumps of about 3 mm, which constitute the formation of the granules. Further treatment in the pelletizing plate leads to smooth pellets, which continue to grow. Powdering with SiO ₂ stops the enlargement and leads to a uniform surface and a round structure. The granules had a grain size of 2 - 5 mm and a bulk density of 0.62 kg / l.

Example 3:

160 kg of silica powder with the composition
SiO ₂ : 96.7%, CaO: 1%, Al ₂ O ₃ : 0.2%, Na ₂ O: 0.03%, K ₂ O: 0.01%, Fe ₂ O ₃ : 0.2% , C _tot : 0.75%, residual moisture 1%
were packed with 40 kg of rice husk ash having the following composition:
SiO ₂ : 87%, CaO: 1%, Al ₂ O ₃ : 0.2%, Na ₂ O: 0.2%, K ₂ O: 2%, Fe ₂ O ₃ : 0.15%, C _ges : 6.2%, residual moisture 1.1%
and mixed 138 kg of water in an Eirich mixer. About a doser and a screw conveyor, the conglomerate is fed to a granulating and dried in a belt dryer. By powdering with SiO ₂ or spraying with water, the pellets in the plate can be influenced in size. Screening in the dryer returns dust and large chunks to the Eirich mixer. The pellets had a bulk density of 0.5 kg / l and a softening temperature of 1,600 ° C.

Claims (12)

Granules as insulating or release agent for high temperature applications containing silica, characterized in that the granules of amorphous silica powder containing at least 90% SiO ₂ are obtainable by granulation with the addition of water and subsequent drying. Granules according to claim 1, characterized in that the granules are obtainable by pelleting. Granules according to claim 1 or 2, characterized in that microsilica or silica filter dust is contained as silica powder. Granules according to claim 1 or 2, characterized in that mixtures of microsilica and / or silica filter dust with rice husk ash in different proportions by weight are included as silica powder. Granules according to one of claims 1 to 4, characterized in that additives for adjusting bulk density or grain size are included. Granules according to one of claims 1 to 5, characterized in that the bulk density is in the range of 0.2 to 0.7 kg / l. Granules according to one of claims 1 to 6, characterized in that the grain size is in the range of 0.5 - 5 mm. Process for the preparation of granules as insulating or separating agent for high temperature applications, wherein a) silicon dioxide powder is pasted with water in a ratio of solids: water of 1: 0.25 to 1: 1.5, b) the mass is granulated in a conventional manner and c) the granules are dried characterized in that the silica powder contains at least 90% amorphous SiO ₂ . A method according to claim 8, characterized in that the mass is pelleted. A method according to claim 9, characterized in that the grain size of the granules over the residence time and / or the inclination and / or the rotational speed in the pelletizing to 0.5 - 5 mm is set. A method according to any one of claims 8 to 10, characterized in that the grain size is controlled by the amount of water and / or the composition of the silica powder. A method according to any one of claims 8 to 11, characterized in that the bulk density is controlled by the amount of water and / or the composition of the silica powder.