NO127572B - - Google Patents

Description

Method for the production of predominantly or

fully cubic crystallized zirconia.

It is known to produce so-called stabilized zirconium oxide, i.e. which is predominantly or completely cubically crystallized by melting in an electric arc furnace suitable zirconium-containing ores such as zircon sand, baddeleyite or zirkite, with the necessary addition of coal and e.g. calcium oxide and/or magnesium oxide as stabilizing oxides. With the resulting reduction, it is possible to reduce the unwanted by-products in the zircon ore, mostly SiO2, Fe^ O^ °S

TiOg to metal. Thereby, however, above all it requires the high amount

of SiO and a lot of electrical energy. It is also necessary to correct the reduced metal alloys with the help of metallic iron, which is mostly supplied to the smelter in the form of iron shavings. By doing so, you increase medical

nobody's specific gravity and enables it to sink into the lower part of the melt. Furthermore, the alloy's magnetic properties are improved by the addition of iron, because it must be able to be completely removed during the subsequent preparation by magnetic separation.

However, the overall process is difficult to master. The reduction of the ores that have a 65 to 80% zirconium oxide content,

to the maximum permissible SiO2 content of 0.5% for stabilized zirconia is difficult. The reduced alloy must be completely removable from the product. The painted goods must be oxidizingly annealed at a temperature of at least 1300°C. After all, however, it is very difficult to achieve a uniform end product, which, like the very sensitive stabilized zirconium oxide, is of suitable quality, etc. US

patent no. 2,535,526..

According to another known method, zircon ores are mixed with basic reactive oxides, which must be in a specific ratio to acidic reactive oxides, and the mixture is melted at approx. l800°C. The smelting material is broken, finely ground and the ore material is primarily classified mechanically into a coarser part which essentially consists of zirconium oxide,

and in a finer part of silicates. The coarser part is then treated to destroy the silicate quantity with hydrochloric acid and then to dissolve the formed silica gel with caustic soda, classified again,

washed and dried.

This procedure is also complicated, expensive and difficult

to master. It was later modified to a mixture of zircon ores with dolomite and possibly with fluorspar, the temperatures being between 1350 and 1450°C. The further processing is therefore similar to that already mentioned, i.e. the partially melted mass is finely crushed, then a thin-flowing sludge is prepared with water, from which the cubic zirconia crystals, etc. are extracted in two washing processes using hydrochloric acid and caustic soda. US Patent Nos. 2,578,748 and 2,721,115.

Also a strong removal of silicic acid using flux-

acid was tried, starting directly from molten zircon ore, so that a large amount of relatively concentrated acid was thereby required. The production of stabilized zirconium oxide was therefore not pursued,

coll. German patent no. 647,918.

The use of stabilized zirconium oxide is steadily gaining importance, but the requirements for this product are also increasing. It can e.g. be appropriate to use completely stabilized zirconia. Often, mixtures of monoclinic and cubic zirconium oxide can also be used, which are significantly less sensitive to temperature changes. Cubic zirconia, which is stabilized with CaO, is most often processed. Increasingly, however, the cubic modification stabilized with other oxides is also demanded. These requirements thus necessitate the preparation of small charges, which are chemically and physically very different. These highly varying requirements can hardly be met with the previously known production methods, but can be met with the following described method.

The invention therefore relates to a method for producing predominantly or completely cubic crystallized zirconium oxide from monoclinic zirconium oxide with a residual content of approx. .

a) that the still moist goods, which have an average grain size of approx. 40 micron, treated 1 to 3 hours with 5 to

10# cold hydrofluoric acid to a residual content of approx. 0.2% SiO2,

after which the hydrofluoric acid is removed,

b) that the washed and dried material is then mixed with

3 to 6 wket# fine-grained calcium oxide or equimolar

quantities of fine-grained, only cubically crystallized oxides of polyvalent metals, whose ionic radius lies in

the order of magnitude of the zirconium ion and

c) that the mixture is converted into the cubic crystal form in a solid-state reaction by heating to temperatures between

1000 and 2000°C, preferably 1400 to 1900°C to achieve the desired degree of conversion.

As oxides, those of calcium, magnesium, yttrium, thorium, cerium or mixtures thereof can then be used. Instead of the metal oxides, such metal compounds can also be used which, when split during the process, give the metal oxides, such as e.g. hydroxides, carbonates, bicarbonates, oxalates, acetates and the like.

The method according to the invention thus consists of a<y>

several completely separate and easily manageable single steps.

The first step then consists in the application of the method according to DAS 1,118,178. Zircon sand is melted in an arc furnace. Thereby, the melt splits into zirconium oxide. and SiO 2 . The two phases are separated after refining the melt using a flotation process. A monoclinic zirconium oxide is extracted with a residual content of approx. 0.6555 Si02: In the second step, this intermediate product is purified further, to a residual content of approx. 6.2% SiCl3. This cleaning process is carried out using hydrofluoric acid at room temperature, the surprisingly short treatment time being completely sufficient with a 5 to 10# hydrofluoric acid. More than 75% of this diluted acid is recovered and reused for the same process. The purified zirconium oxide is decanted and dried.

This material is now converted in a third process step from the monoclinic predominantly or completely into the cubic modification. However, this does not take place as usual,

over a melt; but in a solid state reaction.

Stabilized zirconium oxide is required in particularly high quantity-wise parts in higher finenesses, mostly approximately in the grain range from approx. 0 to 70 m^u. The grinding of a melted and already stabilized product to this fineness cannot, however, be carried out without considerable contamination by grinding iron or silicic acid. In the method which is the subject of the invention, the material or mixture formed in monoclinic form in high purity and in sufficient fineness from the preceding process steps can be partially or completely in the cubic form reacted with the oxides necessary for stabilization, e.g. with CaO, at temperatures where light sintering takes place in all cases.; In any case, a slight compression of the converted product is necessary.

If coarser graining is required, molded pieces are made from it

the floated and cleaned in hydrofluoric acid, mixed with the required amount of stabilized oxides, fine monoclinic zirconium oxide using a volatile organic adhesive at lower temperatures by light printing. If these shaped pieces are heated at temperatures of, for example, 1850°C for several hours, sintering occurs.

After painting the mold pieces, coarser grains are obtained

of such compressive strength that they are suitable for building up a grain mixture, which must contain coarser grains.

Example 1.

1^0 kg of a monoclinic zirconium oxide pre-purified according to DAS I.II8.178 up to 0.65$ SiO2 is mixed with 150 liters of 8% hydrofluoric acid.

This mixture is filled into a plastic container resistant to hydrofluoric acid and moved slowly approx. 2 hours at room temperature. The hydrofluoric acid is then poured off. It can be reused. The zirconium oxide is purified 1 ■ ■

by means of repeated washing with salt-free water and decantation of adhering hydrofluoric acid and then dried. The Si02 content of the thus treated material has returned to 0.19$.

200 kg of monoclinic zirconium oxide purified to 0.19$ SiO2 is mixed with 9.2 kg of quicklime of high fineness. The zirconium oxide is in the fineness range of 0-40^u. Some alcohol is added to the mixture as an adhesive. Then, under low pressure, bodies of approx.

100 x 200 x 30 mm dimensions and these are then fired in a tunnel kiln. The burning temperature is 1790°C. The worm bodies are brought into the race

of approx. 24 hours from room temperature to said temperature. - The downtime amounts to approx. 6 hours. They leave the oven at approx. 80°C. Hard sintered bodies are produced which are broken, ground and then sieved.

The following grains appear in approximately equal amounts:

0 - 0.12 mm

0.12 - 0.25 mm

0.25 - 0.50 mm

0.50 by 1 mm

These grains are sufficiently firm to produce shaped parts from them. The X-ray examination shows that more than 90$ of the grains have been converted from the monoclinic to the cubic modification.

The grains are pressed using 5$ wax test bodies of 10 x 10 mm cross section and 50 mm length. These specimens are heated in a dilatometer to 1200°C. The measured values show, as can be seen from the following table, a completely uniform thermal expansion which, after the test object has cooled, returns to the value 0.

The chemical analysis of the test product shows the following values in addition to ZrOp:

Example 2.

Per 5 kg of monoclinic purified zirconium oxide of 0-70/U grain

size is mixed dry corresponding to example 1 with fine lime dust.

The mixture is filled into a pipe of 120 mm diameter and heated

in a gas-heated oven. Burning temperature, burning time and amounts of cubic zirconium oxide show the following information. 1. 1500°C

4 hours

61$ cubic material.

2. i4oo°c :

'24 hours

92$ cubic material.

They carried out dilatometer measurements on all samples

shows the expected expansion curve.

Sample 1 allows despite the high amount of cubic zir-

conoxide yet clearly recognize the impact of the monoclinic modifica-

tion, i.e. in the temperature range of 1000-1100°C, depending on the amount of monoclinic zirconium oxide, it also shows the phase transformation in the tetragonal structure with a corresponding volume contraction.

Sample 2 has a smoother practically linear expansion.

After the annealing treatment, the product is only concentrated

weakly. After light printing, there is again a fineness of approx.

0-70/u.' ,

Claims (1)

Process for producing predominantly or completely cubic crystallized zirconium oxide from monoclinic zirconium oxide with a residual content of approx. 0.65$ SiO2, as obtained by separating a previously electrothermally entangled ZrC^-SiC^ mixture by means of flotation, characterized by a) that the still moist goods, which have an average grain size of approx. 40 microns, treated for 1 to 3 hours with 5 to 10% cold hydrofluoric acid to a residual content of approx. 0.2% SiO2, after which the hydrofluoric acid is removed, b) that the washed and dried material is then mixed with 3 to 6% by weight of fine-grained calcium oxide. or equimolar quantities of fine-grained, only cubically crystallized ok of multivalent metals, whose ionic radius is in the order of magnitude of the zirconium ion, since instead of the aforementioned metal oxides can be used metal compounds such as wood cleavage during the process gives these metal oxides, and c) that the mixture is converted into the cubic crystal form in a solid-state reaction by heating to temperatures . between 1000 and 2000°C, preferably 1400 to 1900°C to achieve the desired degree of conversion.