DE1911386B2 - PROCESS FOR THE MANUFACTURING OF PRIMERALLY OR FULLY CUBIC CRYSTALIZED ZIRCONIA - Google Patents

Description

It is known so-called stabilized zirconium oxide. that is to say that which is predominantly or completely cubic crystallized, to be prepared by adding suitable zirconium-containing ores, such as zircon sand, baddeleyite or zirconite, with the necessary addition of coal and z. B. calcium oxide and / or magnesium oxide melts as stabilizing oxides in the electric arc furnace. As a result of the reduction that occurs, it is possible to reduce the undesired accompanying oxides of the zirconium ores, mostly SiO ₂ , Fe ₂ O ₃ and TiO _2, to metal. However, the high proportion of SiO ₂ in particular requires a great deal of electrical energy. It is also necessary to correct the reduced metal alloy with metallic iron, which is usually added to the melt in the form of iron filings. This increases the specific weight of the alloy and enables it to sink into the lower part of the melt. In addition, the magnetic properties of the alloy are improved by the addition of iron, because it must be possible to remove it by magnetic separation during the subsequent processing.

But the whole process is difficult to control. It is difficult to reduce the ores, which have a 65 to 80% zirconium oxide content, to the maximum SiO ₂ content of 0.5% permitted for stabilized zirconium oxide. The reduced alloy must be completely removable from the product. The ground material must be oxidized at a temperature of at least 1300 ^{° C.} In spite of all the effort, however, it is very difficult to achieve a uniform end product which, as the very sensitive stabilized zirconium oxide, is of suitable quality (see US Pat. No. 25 35526).

According to another known method Zirconium be basically reacting with oxides, which must be to acidic oxides at a certain ratio, mixed, and the M ischung is melted at about 1800 ⁰ C. The molten material is broken up, finely ground and the ground material is initially classified mechanically into a coarser fraction, which essentially consists of zirconium oxide, and a finer fraction of silicates. The coarser fraction is then treated with hydrochloric acid to destroy the silicate fractions and then with sodium hydroxide solution to dissolve the resulting silica gel, is classified again, washed and dried

This process is also complicated, expensive and difficult to master. It was later modified in the mixture of Zirkonerzen with dolomite, and also possibly with fluorspar, wherein the temperatures between 1350 and 145 ° C were ^0. The further processing is similar to that already mentioned, ie the partially melted mass is finely comminuted. Then a thin slurry is produced with water, from which the cubic zirconium oxide crystals are obtained in two washing processes using hydrochloric acid and sodium hydroxide solution (see USA patents 2578748 and 2721115).

An extensive removal of the silica by hydrofluoric acid has also been attempted, but this is straightforward started from molten zirconium ore, so doing a large amount of relatively concentrated acid was needed. The production of stabilized zirconium oxide was not aimed for (German Patent 647,918).

The use of stabilized zirconium oxide is becoming more and more important, but so are the requirements to rise to this product. For example, it may be advisable to use zirconium oxide in a completely stabilized manner. In many cases, however, mixtures of monoclinic and cubic zirconium oxide, the are much less sensitive to temperature changes. The most common being cubic Zirconium oxide stabilized with CaO, processed. Increasingly, however, the cubic one is also becoming Modification with other oxides stabilized required. These requirements thus make the provision from chemically and physically very different, possibly small batches necessary. These strongly changing requirements are hardly possible with the manufacturing processes known up to now to be fulfilled, but with the procedure described below.

According to the invention one works in the production of predominantly or completely cubically crystallized Zirconium oxide from monoclinic zirconium oxide, as it is obtained by separation from a ZrOj-SiOi mixture is obtained by means of flotation, in such a way that the possibly still moist fine-grained material is diluted with Treated hydrofluoric acid 1 to 3 hours, the same separated, the washed and dried material with 3 to 6% by weight of fine-grain calcium oxide or equimolar amounts of fine-grain, only cubically crystallizing Oxides of polyvalent metals, the ionic radius of which is in the order of magnitude of the zirconium ion, intimately mixed and the resulting mixtures (possibly as

Is reached to form pieces) to temperatures between 1000 and 2000 ° C, preferably, heated from 1400 to 1900 ⁰ C for so long in a solid state reaction to the desired degree of conversion into the cubic crystal form

As oxides, e.g. those of calcium, magnesium, yttrium, thorium, cerium or mixtures' the same can be used, instead of the metal oxides, such metal compounds can also be used which, through decomposition during the process, give rise to metal oxides, such as hydroxides, carbonates, Bicarbonates, oxalates, acetates or the like.

The method according to the invention thus consists of several completely separate and faultless individual levels to be mastered.

The first step is the application of the procedure according to DT-AS 1118178. Zircon sand is melted in an electric arc furnace. The melting material decomposes into zirconium oxide and SiO ₂ . After the melted material has been finely ground, the two phases are separated using a flotation process. A monocoline zirconium oxide with a residual content of about 0.65% SiO _{2 is obtained} .

In the second step, this intermediate product is _{further purified to a residual content of about 0.2% SiO 2.} This purification process is carried out using hydrofluoric acid at room temperature, and surprisingly the relatively short treatment with a 5 to 10% hydrofluoric acid is completely sufficient as 75% of this dilute acid is recovered and used again for the same process. The purified zirconium oxide is decanted and dried.

This material is now in a third Verfat-.rensschritt of the monoclinic predominantly or completely \ r the cubic modification converted But this is not done as previously customary, via a melt, but in a solid state reaction.

Stabilized zirconium oxide is mostly used in a particularly high quantitative proportion in higher fineness roughly in the grain range from about 0 to 70p. required the grinding of a melted and already stabilized However, a product of this fineness is not without considerable contamination from grinding iron or silica to carry out. According to the method which is the subject of this invention. can do that in monoclinic form in high purity and in sufficient fineness from the previous process operations accruing material after mixing with the oxides required for stabilization, z. B. with CaO. at temperatures where at most a slight Sintering takes place, partially or fully converted into the cubic form. At most it is an easy one It is necessary to crush the converted product.

If coarser grains are required, the noted and cleaned in hydrofluoric acid are used with the necessary proportion of stabilizing oxides mixed, fine monoclinic zirconium oxide using an organic adhesive that is volatile at low temperatures by applying light pressure These fittings are produced at temperatures of, for example, 185O ° C. for several hours when heated, sintering occurs.

After grinding the fittings, you get coarser Grains of such compressive strength that they build up a grain mixture containing coarser grains should contain are suitable.

Example? 1

150 kg of a monoclinic zirconium oxide pre-cleaned to 0.65% SiO ₂ according to DT-AS1118178 are mixed with 150 liters of 8% hydrofluoric acid. This mixture is poured into a plastic container resistant to hydrofluoric acid and slowly moved for about 2 hours at room temperature. The hydrofluoric acid is then poured off. It can be used again. The zirconium oxide is cleaned of adhering hydrofluoric acid by repeated washing with salt-free water and decanting and then dried. The SiO ₂ content of the material treated in this way has decreased to 0.19%.

200 kg of the _{monoclinic zirconium oxide, purified to 0.19% SiO 2,} are mixed with 9.2 kg of high fineness burnt lime. The zirconium oxide is in the fineness range of 0-4Op. A little alcohol is added to the mixture as a glue. Thereafter untsr low pressure bodies are pressed from about 100 X 200 X 30mm dimensions and this is then fired in a tunnel furnace, the firing temperature is at 1790 ⁰ C. The moldings are placed in about 24 hours from room temperature to diegenannte temperature. The residence time is about 6 hours. You exit the oven at around SOPC. Hard sintered bodies are created, which are broken, ground and then sieved. The following grain sizes are obtained in almost equal proportions:

0 -0.12 mm
0.12-0.25 mm
0.25-0.50 mm
0.50-1 mm

These grains are sufficiently strong to produce molded parts from them. The X-ray examination shows that more than 90% of the grains are converted from the monoclinic to the cubic modification are.

Using 5% wax, the grains are used to produce test specimens with a cross section of 10 × 10 mm and pressed in length 50 mm. These test specimens are heated to 1200 ° C. in a dilatometer. The measured values show, as can be seen from the following table, a completely uniform thermal expansion, which goes back to the value 0 after the test specimen has cooled down.

2O ⁰ C

200 ⁰ C

400 ⁰ C

600 ⁰ C

800 ⁰ C

1000 ⁰ C

1200 ⁰ C

2O ⁰ C

0% expansion
0.17% expansion
0.36% expansion
0.58% expansion
0.81% expansion
1.04% expansion
1.26% expansion
0% expansion

The chemical analysis of the test product shows the following values in _{addition to ZrO 2:}

Al ₁ O, = 0.26% FcO ₁ = 0.041% MgO = 0.06% TiO, = 0.062% CaO = 4.30% SiO ₂ = 0.21%

Example 2

5 kg of monoclinic, purified zirconium oxide of Ο-70 / ί grain size are dry mixed with fine lime dust according to Example 1. The mixture is poured into a pipe section of 120 mm Φ and heated in a gas-heated oven Declarations:

1. 1500 ⁰ C 4 hours

61% cubic material

2. 1400 ° C

24 hours 92% cubic material

The dilator measurements performed on all samples show the expected expansion curve.

In spite of the high proportion of cubic zirconium oxide, sample 1 still clearly shows the influence of the monoclinic modification, i.e. in the temperature range from 1000-110O ⁰ C, depending on the proportion of monoclinic zirconium oxide, it still shows the phase transformation into the tetragonal structure with a corresponding volume contraction.

Sample 2 has a more uniform, practically linear expansion.

The product is only weakly sintered together after the annealing treatment. After being slightly crushed, it is back in a fineness of around 0-70 μ .

Claims (3)

Patent claims: 1. Process for the production of mainly by heating a mixture of monoclinic zirconium oxide with 3 to 6 wt Zirconia is at temperatures between 1000 and 2000 ⁰ C, preferably between 1400 and 1900 ° C, characterized in that monoclinic zirconium oxide is used, which in a known manner by melting zircon sand in an electric arc furnace, grinding the material to an upper grain size of 70 / A preferably 40μ, and subsequent flotation with the addition of saturated or unsaturated higher molecular weight fatty acids or sulfated higher molecular weight fatty acids and foaming agents, after which a 1 to 3 hour treatment of the possibly still moist fine-grained material with 5 to 10% hydrofluoric acid at room temperature takes place and the material after the dept Run the hydrofluoric acid is washed, dried and, if necessary, shaped. 2. The method according to claim 1, characterized in that the crystal oxides are those of calcium, Magnesium, yttrium, thorium, cerium or mixtures thereof can be used. 3. The method according to claim 1 and 2, characterized in that instead of the metal oxides, such metal compounds are used, which decompose during the process, the metal oxides result.