DE2457324A1 - Heat storage bricks esp for electrical storage heaters - mfd from ferric oxide and a calcium containing additive - Google Patents

Abstract

Heat accumulating medium, esp. a brick for an electrical storage heater, are mfd by heating a mixture of ferric oxide and an additn. principally consisting of a suitable calcium compound, for improving the volumetric heat capacity of the ferric oxide, to a suitable temp. for bringing about reaction between the addtn and the ferric oxide, and thereafter grinding the reaction product to form particles which are then compacted and sintered.

Description

Heat storage stone The invention relates to heat storage media and in particular but not exclusively on heat storage media for use in electrically heated storage stoves. Such ovens have a heat storage medium suitable form, e.g.

in the form of fired stones, produced by a resistance heating element be heated by electricity especially with cheap electricity of periods lower Load is fed and the stored heat at a preselectable speed be able to give up over different periods.

To be suitable for such electric storage stoves, a must such heat storage medium meet certain criteria.

For example, it should have a high volumetric heat capacity, to keep the dimensions of the storage heater as small as possible and on the other hand they should have sufficient mechanical strength to withstand the compressive loads to be able to record, which are imprinted by placing the heat storage material on top of each other will. In addition, a high electrical resistance is desirable in order to withstand the stress to reduce the electrical insulation of the electrical heating elements, namely especially at the high operating temperatures that occur. Also is a high Resistance to thermal shock effects and corrosion Desired for an ideal medium under high temperatures.

It has already been proposed to use ferrite oxide (Fe2O3) as a heat storage medium to use, especially in connection with electrical storage heaters. In the In practice, the ferrite oxide was directly compressed, with or without heat and it was thus brought into the shape that the heating device has, for example in brick-shaped cuboids in order to adapt to the heating device at a sufficient distance To ensure, such stones show an acceptable heat capacity and sufficient corrosion resistance, but they have a relatively high electrical Conductivity, which requires extensive electrical insulation of the heating elements, A reliable long-term operation at the high temperatures in question to ensure. Ferrite oxide alone is a sizable and unpredictable one Subject to shrinkage when heated and it causes considerable difficulties To produce stones with high dimensional tolerances, which are necessary for good To ensure heat transfer with heating elements of standardized dimensions.

The invention is therefore based on the object of a heat storage medium to create which combines the advantages of ferrite oxide, but at least to a certain extent avoids the disadvantages of this material.

According to one feature of the invention there is a method of producing a heat storage medium carried out in such a way that a mixture of ferrite oxide and an additive consisting of a suitable calcium compound at increased Temperature is mixed, which is sufficient to react the additive with the ferrite oxide to let, whereby the volumetric heat capacity is improved and that the Reaction product is reduced to a certain shape by compressing the particles and the pressed pieces are sintered.

A calcium compound suitable for this purpose is calcium oxide represents or a compound / is capable of the oxide at the elevated temperatures to keep.

The additive actually used is expediently guaranteed in such a way that that the electrical resistance of the medium in comparison with the resistance by ferrite oxide alone is improved, while at the same time the volumetric heat capacity is improved. In one embodiment, the additive can be added to the ferrite oxide afterwards be added, it can also be part of this ferrite oxide be mixed in.

According to a preferred embodiment of the invention, the additive consists just made of a suitable calcium compound with no accidental impurities arising can change with the origin of the material. A suitable connection for this is calcium carbonate, which typically contains magnesium hydroxide, namely in addition to magnesium carbonate and calcium hydroxide.

It has been shown that the volumetric Heat capacity of a medium, generated by reacting ferrite oxide, with increased Temperature increases with the selected additive as the amount of additive increases will. While the actual ferrite oxide additive mix ratio that has been selected depends on the calcium compound used and is determined by the three points mentioned a molar ratio of ferrite oxide to additive of at least 1: 1 and ideally Case in the range 1: 1 to 3: 1 has turned out to be expedient value for the proven volumetric heat capacity.

In addition to a calcium compound, however, the mixture can contain an additive include, which is a suitable compound of barium (Ba) or strontium (Sr) or of lead (Pb), for example barium carbonate, strontium carbonate or Lead carbonate. The mixture can also contain a suitable compound of magnesium zinc or samarium, e.g.

Magnesium oxide (MgO).

This gives the general characteristics of ceramics, i.e. the shaping can be done in a moist state and after burning the body is fireproof. In addition, the body is then chemically inert.

In general, such ceramic heat storage media give a higher volumetric, thermal capacity than the ferrite type media.

After firing, the ferrite, ceramic or fireproof heat storage medium expediently be ground to a particle size that creates the possibility of that the particles after moistening in their final form, for example in the form of Bricks are brought in to be used again. The pressed bodies can then be sintered, whereby the particles fuse into their final shape.

Appropriately, the additive has the form of a flux, which the sintering process is facilitated and it can use calcium oxide or aluminum oxide as the filler to be used.

The storage medium can also contain evenly distributed inert metal particles, e.g. contain aluminum flakes or other particles with high thermal conductivity, to improve the heat transfer from the core of the medium to the environment.

Embodiments of the invention are described below: Example 1 Finely divided iron oxide, typically Fe 2 O 3, obtained as a by-product from a hydrochloric acid etching recovery plant with an average analysis according to Table 1 was intimately with finely divided strontium carbonate commercial quality mixed in a proportion which is characteristic 5: 1 percent by weight.

Table 1.

FeO Je20) 89.8 siO2 0.08 Al: 23 0.04 CaO 0.40 MgO 0.11 S 0.10 P 0.009 Mn 0.26.

The mixture was then placed in open pans for about 45 minutes Burned 10700C to cause a reaction between the ferrite oxide and the strontium carbonate before cooling it to room temperature.

The resulting medium was wet ground for about 8 hours, to reduce it in size to such an extent that it is more finely divided than liquid Porridge was suspended in water. After removing most of the water through conventional means, the medium now has the consistency of moist clay and becomes pressed into the required shape.

In this embodiment, the medium was at approximately 450b pressed in the shape of a seal which was slowly dried to form cracks to prevent.

The block was then sintered in air at 130 ° C. for 30 minutes. Both the heating speed and the cooling speed were graded so that that prevents the formation of cracks as a result of thermally introduced mechanical stresses became. The heat storage medium produced in this way showed a volumetric Heat capacity of 2800 kilojoules per liter, measured by a calorimetric Method.

Example 2 Finely divided ferrite oxide as in Example 1 is in this case again intimately with barium carbonate in a mixing ratio of 5: 1 Weight percent mixed. The mixture is baked at about 10000C for 20 minutes, in order to create a medium which is then treated in such a way that heat storage stones are obtained, and 3ar by a method similar to the method of Example 1 equals. The volumetric heat capacity of the stones produced according to this method was 1650 kilojoules per liter.

Example 3 Finely divided ferrite oxide as in Example 1 became intimate in a 1: 1 weight ratio with laboratory grade potassium carbonate in finely divided form mixed.

The resulting mixture was fired at 10000C for 20 minutes, to create a medium that, as in the previous examples, deforms into stones and was sintered at temperatures 0 of about 1200 C.

The volumetric heat capacity of the medium produced in this way was 2,700 Kilojoules per liter.

Example 4 Finely divided ferrite oxide was as in Example 3, intimately mixed with calcium carbonate, which, according to analysis, has the following components of Table 2: Table 2 According to, 81.1% 3 Mg (OH) 2 10.2 MgCO) 2.2 Fe (OH) or 6 Ca (OH) 2 + Al 5.9 The ferrite oxide and calcium carbonate became intimate mixed in a weight ratio of 6: 1 and at a temperature of 12000C Burned for about 20 minutes. The medium thus produced became, as in Example 1, to Stones deformed and sintered at 13000C for about 5 minutes. One showed up volumetric thermal capacity of about 4000 kilojoules per liter in the heat storage according to this embodiment. It is believed that the improvement in thermal Capacity relative to example 3 of incidental impurities of calcium carbonate originates.

In the embodiments described above, the additives passed of ferrite oxide only consists of calcium carbonate, however the carbonates can be one or several metals mentioned above can also be used. When the carbonates of two or more metals are used, the fired products can be the Examples 1 and 2 are instead reduced to finely divided parts by a suitable combination is milled and mixed before briquetting and the Use final sintering.

For example, the products of Examples 1 and 2 were milled and intimately mixed in a 1: 1 weight ratio before pressing at 470 bar and final sintering at 130 ° C for 30 minutes in air. The end product show a volumetric thermal capacity of 3050 kilojoules per liter.

In all of the above examples, suitable binders can be used together with metal or other particles added relatively high thermal conductivity will.

There have been specific combinations of temperature and time above specified for firing and sintering, but both temperature and time can be changed be to optimal obtain volumetric heat capacity, when the components of the medium show different analyzes or in different Conditions are used.

The heat insulating agent which according to the present invention has certain advantages, e.g.

a high electrical resistance and robustness in comparison with ferrite oxide alone and the inventive agent can be used to remove stones high volumetric heat capacity, with values that match the Approach values of ferrite oxide alone or, in certain cases, even exceed these values.

The present invention also encompasses heat storage stones or others Heat storage elements made from this medium. In the case of stones these expediently have grooves so that adjacent stones form channels, in which suitable heating elements can be inserted.

Since the inventive heat storage medium also has a high electrical Has a resistance of more than 3 to 6 MA at room temperature, so is the insulation the heating elements are not critical and considerable costs can be saved.

Patent claims:

Claims (13)

Claims: l 1) Method for producing a heat storage medium. in that it is noted that ferrite oxide and an additive be mixed, which consists of a suitable calcium compound that the Mixing is carried out at an elevated temperature at which the additive with the ferrite oxide can react to improve the volumetric heat capacity and that the reaction product is reduced to macro parts which are then compressed and that eventually the compact is sintered. 2) Method according to claim 1, characterized in that g e k e n n n z e i c h n e t, that the molar ratio of ferrite oxide residue of the additive is in the range between 0.6: 1 and 3.9: 1. 3) Process according to claims 1 or 2, characterized in that g e k e n n -z Note that the additive is calcium oxide or a compound found in the Is able to deliver calcium oxide at an elevated temperature. 4) Method according to claim 3, characterized in that g ek e n n z e i c h n e t, that the additive is calcium carbonate. 5) Method according to claims 1-4, thereby g e k e n n -z e i c Note that the additive is a suitable combination of one or more of the following Metals include: magnesium, zinc, samrium, strontium, barium or lead. 6) Method according to claims 1-4, characterized in that g e k e n n -z e i c Not that the additive consists entirely of a potassium compound. 7) Method according to one of claims 1-6, characterized in that g e k e n n -z E i c h e t that the additive serves as a flux at elevated temperature. 8) Method according to claims 1-7, characterized in that g e k e n n z e i c h n e t that particles of a material of high thermal conductivity in the medium be included. 9) The method according to claim 8, characterized in that g e k e n n n z e i c h n e t, that the particles consist of aluminum 10) method according to claims 1-9, as a result, that the increased temperature through which the Ferrite oxide activation reaction is generated within a range between 1000 and 12000C. 11) Method according to claims 1-10, characterized in that g e k e n n -z e i n e t that the compact is produced under a pressure of approximately 450 bar before sintering takes place. 12) Method according to claims 1-11, characterized in that g e k e n n -z e i c h n e t that the compact is sintered at a temperature between 12000C and 13000C will. 13) Method according to one of claims 1-12, characterized in that g e k e n n -z E i c h n e t that the production is carried out according to one of Examples i to 4.