DE1796034C3 - Refractory stones made from stabilized zirconia - Google Patents

Description

The invention relates to the manufacture of fire retardants Building material, especially stone material made of tirconia, which is particularly cheap Resistance to temperature changes (TWB) and insufficient high pressure fire resistance (DFB).

It is known that zirconium dioxide, exists in three modifications wherein the stability areas #ER individual modifications as follows are: Until about IO0O ° C monoclinic, about 1000 to 1900 ⁰ C tetragonal and • over about 1900 ⁰ C to the melting point at about 170O ⁰ C, cubic. In the pure state, these modifications are reversible and involve changes in volume. These changes in volume are very uncomfortable with refractory bricks.

Various attempts have been made to stabilize one or the other structure of zirconium dioxide carried out by introducing various foreign substances into the crystal lattice in order to to keep crystallographic transformations during heating or cooling to a minimum. At the Calcium oxide (e.g. U.S. Patents 32 22 148, 29 10 371 and 31 75 919). So lets Both the cubic and the tetragonal modification of ZrOz stabilize. Through the Zugabs of calcium oxide, optionally with additional other alkaline earth oxides, however, the other properties become of the refractory material, particularly in view of the high temperature strength, thus deteriorates The applicability of zirconium oxides stabilized in this way for furnace linings is very limited These known stabilized refractory materials do not have good thermal shock resistance. As a result, the punching time of the material at high working temperatures is not satisfactory. Besides that it was found that the stabilizing effect of calcium oxide does not last. After some heating and In cooling cycles, the stabilization is already worsened, possibly as a result of diffusion of the Calcium oxide from the grain to the grain boundaries. This is the way to counter these disadvantages stabilized zirconium oxide S1O2 and AI2O3 added (US-PS 29 37 102), which is not only at the expense of stabilization, but also to the formation of Calcium silicates and aluminates leads with all known difficulties.

Finally, solid solutions of rare earth niobates with zirconium oxide have also been tried (US-PS 32 68 349), but without being able to solve the problem, quite apart from the fact that the Application of large amounts of such a stone material in industrial furnace construction because of the high cost of forbids rare earths.

It has now been found that the refractory material based on zirconium dioxide with 3 to 10 percent by weight Calcium oxide and 2 to 15 weight percent niobium pentoxide for a total of 5 to 20 weight percent Calcium oxide and niobium pentoxide, excellent pressure fire resistance and in the refractory material Has resistance to temperature changes. The refractories according to the invention are stabilized, and in the cubic modification, through the joint addition of calcium oxide and niobium pentoxide.

Will be pure zirconium dioxide with a certain amount of calcium oxide or a substance that undet the firing conditions provide calcium oxide, such as calcium fluoride or oxalate, heated with niobium oxide, a partial recrystallization of the zirconium dioxide takes place from the monoclinic to the cubic modification, whereby this is stabilized. It seems that the effective wedge of the niobium oxide shows at least partially at the grain boundaries of the polycrystalline material. It it was found by electron microscopy that calcium oxide is present in the Grain boundaries migrates. This phenomenon is generally found especially in commercial products freshly burned grain material and in some cases also in ternary systems from the oxides of calcium Niobium and zirconium. In most cases, zirconia contains silica and alumina; Impurities that collect at the grain boundary along with the calcium oxide. Is in dei Containing bulk niobium oxide, it shows the same tendency and accompanies to a certain extent Calcium oxide at the grain boundaries. However, some niobium oxide remains with calcium oxide in the grain and thus ensures the stabilization of the cubic modification of the zirconium oxide.

Usual impurities, in particular iron and titanium oxides, do not show such a pronounced tendency to Migration to the grain boundaries such as the oxides of calcium, silicon, niobium and aluminum.

Particularly solid stone material can be achieved by adding additional amounts of unfired zirconium dioxide, namely 1 to 20% and / or niobium oxide 1 to 5% in a total amount of up to 25%, based on the Grain material which is already 3 to 10 percent by weight Contains niobium oxide. The stabilized according to the invention; o Compared to the known materials, zirconium dioxide shows outstanding properties. this applies especially in the case of materials with calcium oxide that are particularly well stabilized.

The manufacture of the refractories according to the invention Stone is generally done according to procedural measures known in the relevant art. the Starting products are mixed and the powder mixture is compacted with a binder if necessary or pressed, these compacts are burned and ground up again and added to the appropriate grain fractions of the grain material sifted. The grain material is now optionally with other reactants or binders are added, shaped and finally subjected to stone firing. The preparation of the invention Stones happens z. B. in the following way.

The raw materials, namely, zirconia, niobium pentoxide and calcium oxide, or in place of the latter, a substance which is capable of forming at Kornbrand calcium oxide, calcium carbonate such as, preferably, yo to have a sufficient purity and a particle size between 0.1 and 2 μ, have. The three components are e.g. B. mixed by dry grinding or wet grinding in conventional devices.

The grain mixture is then optionally w with a binder such as glycerol, starch, gum arabic, dextrin or an adhesive in the form of an elastomer in a ^solvent: dressed e carbon tetrachloride and in the customary manner under pressure of about 350 to 1050 kg / cm ² (5000-15 000 psi). Molded bricks of larger dimensions are expediently shaped in impact pressing under a pressure of about 140 kg.

These moldings are heated to a temperature of about 1600 to about 2000 ° C and at this 4s Maintained temperature for about 3 to 24 hours. The heating rate from room temperature to the firing temperature should be around 100 ° C / h for larger shaped bricks and up to 400 ° C / h for smaller parts. To The material can be fired in the furnace at the desired temperature in the appropriate time cooling down. If lower temperatures are used, longer burning times are required and vice versa. In general, the grain fire takes about 8 hours at 1600 ° C or about 3 hours at 2000 ° C.

After cooling, the material to be fired is broken up and, if necessary, ground in a ball mill. There is now a sieving into individual grain fractions z. B. into the four fractions: 1. 0.84 to 2 mm, 2. 0.3 to 0.84 mm, 3. 0.15 to 0.3 mm and 4. <0.15 mm (-10 + 20; -20 +50; -50 + 100; -100).

For the stone production, a mixture of the desired grain classes is now made and that Grain material in the presence of moisture or an organic binder such as an elastomer adhesive, Deformed dextrin or an epoxy resin.

Unfired, pure monoclinic zirconium dioxide powder can optionally be used before molding to add. It is also sometimes useful to add unfired, pure niobium oxide before molding. In the case of stone fires, inorganic compounds such as alkali hydroxides, alkali carbonates, Sodium phosphate, which were present in the binders with the low moisture.

Shaping can now be carried out in a similar way to the production of the grain material above. In the case of small stones, a pressure of 1000 kg / cm ² is preferably used. However, if desired, a compression pressure between 35 and 8450 kg / cm ² (500 and 120,000 psi) can be used. When producing larger shaped bricks, impact pressing is expedient.

The stone branding is generally carried out at a temperature preferably above 1700 ° C. and above the burning temperature for the grain brandy. It is advisable to carry out the stone firing at about 1700 to 2100 ^{° C.} The higher the temperature in the stone fire, the stronger the building materials are. The burn time is between about 2 and 5 hours, generally about 3 hours. The items to be fired can cool down in the air in the oven.

When carrying out the process for the production of the products according to the invention, the following have been found Working conditions proven to be appropriate.

The starting materials should be relatively pure and fine-grained. Zirconia should preferably be a fine one or precipitated powder with a grain size below 44 μ.

These starting products are either mixed dry or, after mixing, wet-ground and dried at about 100 to 100 ° C, so that you can easily friable, non-dusting material

The calcium oxide used is preferably obtained by decomposing calcium carbonate in situ. So you can use any commercially available, relatively pure calcium carbonate. When it's expedient appears, calcium oxide, calcium fluoride or calcium oxalate can also be used. For the invention The purpose is any commercially available pure niobium oxide that is not necessarily tantalum-free. It should however, be fine-grained, in particular below 44 μιτι.

The dry grain material is then z. B. formed on body 50 χ 50 mm diameter and ^{burned in a gas-heated furnace at 1600 0} C for about 8 to 24 hours or in such a furnace with oxygen-enriched air at about 1700 to 2000 ° C.

These bricks can either cool down in the oven or are discharged and can be left in the air cool. They are then broken up and sieved to the desired grain fractions.

The desired grain structure is now put together and possibly not yet fired Zirconium dioxide and / or niobium oxide added. Further additives can also be introduced. the The amount of different grain fractions for the grain structure can vary. The strength properties of the refractory bricks depend to some extent on the grain structure. In the Individual masses with a special grain structure are described in the examples. However, this is not intended to be a Limitation of the possibilities of variation with regard to grain fractions and proportions of the individual Grain fractions take place. The grain structure will always be designed with the desired strength properties in mind. This is in the refractories industry common practice.

In many cases it is advisable to add an organic binder to the grain structure to facilitate further handling and, in particular, deformation. The stone shaping is carried out in a similar way to the production of the shaped bodies for the grain brandy. In general, a compression pressure of about 1000 kg / cm ^{2 is} considered satisfactory and preferred.

The stone can be fired in any furnace ζ. Β. as mentioned above, preferably carried out at 1700 to 2100 ^{° C. in 3 hours.}

In the grain structure, preference is given to about 4 to 7 percent by weight calcium oxide, preferably added as calcium carbonate, about 3 to 10 percent by weight niobium pentoxide, the remainder being zirconium dioxide. As mentioned, it is sometimes expedient to introduce 1 to 20 percent by weight of zirconium dioxide and / or 1 to 5 percent by weight of niobium oxide, based on the weight of the grain structure, for stone production. The same material that was used as the starting product for the manufacture of the refractory grain is suitable for this addition.

In some cases, especially with large shaped stones, no unfired zirconium dioxide should be added in order to keep the firing shrinkage small.

A standard stone 114 χ 15.9 χ 12.7 mm is most suitable for carrying out the tests.

The density of the fired bricks depends on the grain structure , as well as on the pressing pressure and the maximum firing temperature. This dependency is clearly shown in the examples.

The density of most of the small test specimens is 4.9 to 5.2 g / cm ³ , the porosity is 5 to 20%. The apparent density of most test specimens is around 5.6 g / cm ³ . The density of large shaped bricks, i.e. those with 203 mm in at least one dimension after deformation with the aid of an impact press under a pressure of about 136 kg + pressure is about 4.4 to 4.7 g / cm ³ .

The crystal structure, both of the grain material with about 5.9 percent by weight of calcium oxide, 5.2 percent by weight of niobium oxide, the remainder of zirconium oxide and of the stone material corresponds mainly to the cubic modification. The remaining approximately 10 to 20 percent by weight of the zirconia is monoclinic.

As the high-temperature X-ray examinations showed, the conversion of the monoclinic modification into the tetragonal modification begins at around 800 ° C and is completed at 1200 ° C. When cooling down , the reverse transformation takes place, namely from _s0 tetragonal to monoclinic. The cubic modification remains unchanged during heating and cooling.

According to the above procedure, some test bodies were now made by in order to determine the flexural strength of these. The flexural strength results from the maximum load until breakage when the test specimen is supported at two ends and loaded in the middle. The flexural strength m is calculated from the formula

f> 0

IP L, 2B-I) ²

P = load,

B = width,

L = width,

D = thickness.

The flexural strength was determined at increasing temperatures up to 1500 ° C.
The following examples illustrate the invention.

example 1

100 g calcium oxide, 50 g niobium pentoxide and 850 g zirconium dioxide were slurried in water and mixed thoroughly in a conventional high-speed mixer. The water was then filtered off, the moist filter cake was ^{dried in an oven at about HO 0} C, the dry product was broken up and, after the addition of an organic binder, shaped. The briquettes were heated in a gas-heated oven to 1600 ° C. at a rate of 100 ° C./h, held at this temperature for 8 hours, after which the briquettes were allowed to cool in the oven.

The briquettes were then broken open and crushed in a rubber-lined ball mill with balls made of zirconium dioxide and sieved into four grain fractions, namely grain fraction 1) 0.84 to 2 mm, grain fraction 2) 0.3 to 84 mm, grain fraction 3) 0, 15 to 0.3 mm and finally grain fraction 4) below 0.15 mm.

The following grain structure was now made. 50 percent by weight of grain fraction 3, 40 percent by weight of grain fraction 4 and 10 percent by weight of unfired zirconia powder as used to produce the refractory grain material. The whole thing was made up with an organic binder and made from approximately 20 g of 31.7 χ 12.7 y 9.5 mm (1.25 0.5 χ 3/8 in.) Rods. This was done by pressing under a pressure of about 4200 kg / cm ² .

The rods were then heated to 2000 ° C. in a gas-heated oven with oxygen-enriched air and kept at this temperature for 3 hours. The heating rate was 400 ° C./h. The stones obtained were allowed to cool in the oven. The flexural strength was determined on two bars as follows. The rod was placed at both ends and the load was applied until it broke. Test temperature 1500 ° C. The rupture occurred at a load of 760 and 675 kg / cm ^2, respectively. These two test bars are referred to as sample A and B in the following.

The fragments from this test, each about 19 mm long, were tested for thermal shock resistance investigated, namely by 50 changes between room temperature 25 and 1200 ° C, with cooling took place with air. The test specimens were kept at 1200 ° C. for a total of 500 hours. The invention After this examination, stones showed no damage. They were free of cracks.

In a modification of this experiment, a stone material was produced from a grain structure without the addition of unfired zirconium dioxide powder and with different proportions of the grain fractions (sample C) and the flexural strength was also determined. This was 460 kg / cm ² .

Example 2

In the sense of Example 1, a grain structure with 50 percent by weight of grain fraction 2, 10 percent by weight of grain fraction 3 and 30 percent by weight of grain fraction 4 was produced together with 10 percent by weight of unfired zirconium dioxide powder, with the help of various inorganic additives, test specimens in size 31.7 χ 12.7 χ 9, 5 mm at a pressure of about 4200 kg / cm ² .

These samples D and E with 2 and 1%, respectively, of unfired niobium oxide were ^{fired for 2 hours at 2100 °} C., the flexural strength was determined at 15,000 ^° C. and values of 1095 kg / cm ² and 454 kg / cm ^{2 were} found.

A 114 mm long rod of the composition corresponding to sample D after a stone fire of 2 h at 2100 ^° C. withstood 50 temperature changes from 200 ^° C. to room temperature, the total time at 1200 ^° C. being 800 h. The test specimen obtained could not be broken by hand. The stones were formed under a pressure of around 1000 kg / cm ² .

For the other samples H to L, various additives were used, namely alkali hydroxide, alkali carbonates, alkali phosphates. The press pressure was again around 4,200 kg / cm ² and the bunt 3 hours at 200O ⁰ C. The bending strength at 1500 ⁰ C was between 3.5 and 5.5 kg / mm ^2.

Example 3

In the sense of Example 1, a grain material was produced, but this time the grain fire in 3 hours 1725 ° C.

The following grain structure was carried out: 50 percent by weight, grain fraction 3, 30 percent by weight Grain fraction 4 and 10 weight percent unfired zirconia powder, of the same grade as it was used to produce the grain material.

The whole was mixed dry, made into a paste with a binder and made into 20 g test specimens shaped according to example 1.

This time the stone fire took place with oxygen-enriched fuel gas. The heating rate from room temperature to 2100 ⁰ C was 400 ° C / h, firing time for 2 hours at 2100 ° C.

The stones could cool down in the oven. The flexural strength was then determined. Parallel samples of the stones showed a flexural strength of 3.46 and 4.11 kg / mm ^{2 at a temperature of 1500 ° C.} These samples are designated with F and G. Test specimens of similar composition and dimensions were produced and their resistance to temperature changes at a temperature of up to 1200 ° C. was also investigated. In addition, the stones were ^{heated three times to 2000 °} C. and then cooled to room temperature. Even after this more difficult test, the test specimens showed no damage.

Example 4

According to Example 1, a granular material was produced and burned at 1600 ° C. for only 1 hour

Table I.

Sifting into the grain fractions was a grain build-up of 50 percent by weight of grain fraction 2, 10 percent by weight of grain fraction 3, 20 percent by weight of grain fraction 4 and 20 percent by weight of unburnt zirconium dioxide powder as it was used to produce the grain material made of an elastomer under a pressure of about 1000 kg / cm ² . The stone fire took place in

ίο 2h at 2100 ° C.

The test specimens were subjected to the following tests: A test specimen was ^{kept at 1200 °} C. for 500 hours and subjected to 50 changes in between by cooling in air to room temperature and heating again, etc. Finally, they were heated 5 times to 2000 ° C. and then cooled back down to room temperature. After these difficult conditions for thermal shock resistance, parallel samples with a length of 31.7 mm were cut from the stones and the flexural strength at 1500 ^° C. was determined on them. Samples R and S had flexural strengths under these conditions of 3.23 and 2.29 kg / mm ^2, respectively.

Example5

In a modification of the conditions of Examples 1 to 4, different heating programs were used on the Samples T to Z made and tested accordingly. The results are summarized in Table I. It follows that the refractory bricks according to the invention are essential to the known materials are superior.

Finally, samples M to P were also made under different conditions for the stone fire.

Finally, comparative experiments 1 to 4 were carried out with commercially available refractory Steiner and the flexural strength are given in Table H. Comparative product 1 is a rod that from a commercially available dense zirconium dioxide stone with the usual calcium oxide stabilization would. The parallel samples examined had a cross section of 15.9 χ 19 mm.

Comparative product 2 is the comparative product 1 because; 3 h before the examination was heated to 2000 ° C. egg three parallel experiments were made.

Comparative product 3 is a commercial special: dense zirconium dioxide stone above cross section.

Comparative product 4 is a commercially available yttrium oxide stabilized zirconium dioxide stone with a Cross-sectional area of 19x19 mm. The investigation genes were carried out on two parallel samples.

sample Rod. grain Nb2O-. Grain brandy (H) Grain structure until mm U.N- anorg. additions Stone fire (H) Flexural strength (Weight percent) 52 8th 03 used 3 at 1500 C (Weight percent)
03 0.15 <0.15 mm ZrO; 52 8th until 50 40 3 CaO 52 (° Q S. 0.84 10 - (° C) 2 (kg / mm-) AWAY 53 52 1600 8th mm 10 40 2000 2 7.6; 6.75 5.2 3 10 30th - - 2 (10800; 9640} G 53 1600 10 30th 10 2w / oNb2Os 2000 4.6 (6540) D. 53 1600 50 10 30th 10 1w / oNb2O5 2100 11.0 (15600) E. 5.9 1600 50 10 - 2100 4.5 (6440) F, G 5.9 1725 50 2100 4.1; 3v5 50 (5860; 4940) 609 651/9 '

ίο

continuation

sample Rod. grain NbiOi Grain brandy (H) Grain structure 0.15 <O, I5 U.N- anorg. additions Stone fire (H) Flexural strength (Weight percent) 5.2 8th until mm used 3 at 1500 ⁰ C 5.2 8th (Weight percent) 0.3 Ζ1Ό2 3 5.2 8th 0.3 mm 3 5.2 8th until 10 30th 10 1% NaOH 3 CaO 5.2 Γ C) 8th 0.84 10 30th 10 1% Na2CO3 ( ⁰ C) 3 (kg / mm ² ) H 5.9 5.2 1600 8th mm 10 30th 10 1% U2CO3 2000 5 ■ 5.15 (7340) ! 5.9 1600 50 10 30th 10 2% NasPO " 2000 5.08 (7230) J 5.9 5.2 1600 8th 50 10 30th 10 2% Na2HPO4 2000 3 5.5 (7810) K 5.9 5.2 1600 1 50 10 30th 10 - 2000 2 4.25 (6040) L. 5.9 1600 50 2000 3.51 (5000) M, N 5.9 3.0 1600 3 50 50 40 10 - 1850 3 3.52; 3.18 5.0 3 50 10 20th 20th - 2 (5020; 4560) P. 5.9 1600 1850 3.9 (5550) R, S ·) 5.9 10.4 1600 2 - 10 30th 10 - 2100 2 3.23; 2.49 50 10 30th 10 - (4600; 3260) U 5.9 10.4 1650 2 2000 2 2.81 (4000) T ₁ V 4.0 1600 50 10 30th - - 2100 2.5; 2.13 50 (3270; 3030) W ₁ X 5.9 1650 10 30th 10 - 2100 2.27; 2.35 - (3240; 2950) Z ₁ Y 5.9 1650 2100 1.5; 1.48 50 (2130: 2110)

*) These samples are from a 100 mm long bar which has been subjected to the TWB-examination (50 exchange by air quenching of 1200 ° C. for 500 hours at 1200 ⁰ C).

Table Il

Flexural strength at 1500 ^° C. kg / mm ²

Comparative product 1 Comparative product 2

Comparative product 3 Comparative product 4

0.0218; 0.0555 (31; 79) 0.220; 0.556; 0.61 (313; 792; 867) 1.33 (1900)

0.225; 0.246 (320; 350)

Claims (6)

Patent claims: : - .. 1. Refractory material in the form of calcium oxide stabilized zirconium dioxide, marked by a stabilizer content of 3 to 10 percent by weight calcium oxide and 2 to 15 percent by weight of niobium pentoxide, but a total of 5 to 20 percent by weight of stabilizers. based on the refractory material. ι ο 2. Refractory materia! according to claim 1, characterized by a content of 4 to 7 percent by weight calcium oxide and 3 to 10 percent by weight niobium pentoxide. 3. Refractory material according to claim 1 to 2, characterized by a content of 5.9% Calcium oxide and 5.2% niobium pentoxide. 4. A method for producing the refractory material according to claim 1 to 3, characterized in that that 3 to 10% calcium oxide is added to the zirconium dioxide or calcium compounds which provide calcium oxide under firing conditions and 2 up to 15 percent by weight niobium pentoxide. however, a total of 5 to 20 percent by weight, based on refractory. Material, clogs. 5. The method according to claim 3, characterized in that the zirconium dioxide 4 to 7 Weight percent calcium oxide and 3 to 10 weight percent Adds niobium pentoxide. 6. Method of making the refractory jo Materials according to Claims 1 to 3, characterized in that 1. a grain material is made from Zirconium dioxide and 3 to 10 percent by weight calcium oxide and 3 to 10 percent by weight Niobium pentoxide, the proportion of calcium oxide and niobium pentoxide totaling 5 to 20 percent by weight makes up by mixing, shaping, and firing manufactures, 2. the grain material crushed and sieved and 3. the desired grain structure of the individual Grain fractions of the grain material 1 to 20 percent by weight »commercial, unfired zirconium dioxide and / or 1 to 5 percent by weight niobium pentoxide is added, 4 forms and 5. the stone fire in about 2 to 5 h at about 1700 to 2100 ° C. 45