CS208508B1 - Method of preparation of the aluminium oxide for ceramic purposes - Google Patents

Description

(54) A method of preparing alumina for ceramic purposes

BACKGROUND OF THE INVENTION The present invention relates to the preparation of an alumina of the ceramic type by controlled precipitation of a solid phase alumina trihydrate from an alkaline solution of sodium aluminate.

The prior art preparation of alumina trihydrate, which is generally an intermediate product for the production of electrolytic aluminum, consists in the fact that from a solution of sodium aluminate of 160 kg total sodium oxide / ³ , 145 kg sodium caustic / m ³ , 145 kg sodium caustic / m ³ , 145 kg of alumina / m ³ and 0.65 silica are precipitated by precipitation of a mixture of carbon dioxide and air with alumina trihydrate. The residence time in the crystallizer is about 50 to 100 hours, the deposition temperature is in the range of 50 to 80 ° C, the temperature regime is non-isothermal. In this way, a relatively coarse-grained product is obtained whose average particle size is of the order of 60.10 ^-6 m.

The alumina trihydrate thus obtained is calcined at alumina temperatures in the presence of mineralizers such as fluorides at temperatures of 1150 to 1250 ° C. The disadvantage of this method is that alumina with an unsatisfactory granulometric composition is obtained, and particles larger than 10 microns, often larger than 20 μΐη, of platelet-like habit (corundum clence) are present in the dispersion system. For some particles, the pseudometamorphic shape of the original particles - alumina trihydrate - is evident. The content of impurities, in particular alkali, metal ions and silica, is considerable in this mineralized alumina, e.g.

0.5% sodium oxide, often 0.6%. The alumina thus prepared is absolutely unsuitable as a raw material for oxide ceramics. According to the invention, these processes are overcome by a process for the preparation of alumina for ceramic purposes by controlled precipitation of the solid phase alumina trihydrate from an alkaline solution of sodium aluminate. It is based on the fact that an aluminate solution of 20 to 100 kg of caustic soda / m ³ and 20 to 100 kg of alumina / m ³ and a molar ratio of caustic soda to alumina of 1.2 to 2 is treated with carbon dioxide optionally mixtures thereof with air with a mean residence time of 0.5 to 3 hours and under an isothermal temperature regime and turbulent hydrodynamic conditions, followed by multiple washing of the solid phase of alumina trihydrate with water and dilute mineral acid and calcination thereof.

The process according to the invention yields a product with a defined, predetermined, granulometric composition of the desired specific surface with an isometric to spherical habit of particles having a high sintering capacity, suitable for various corundum ceramic applications.

The process according to the invention is described in more detail below on several exemplary embodiments. Example 1

The alumina trihydrate is dissolved in a stirred reactor in a NaOH solution such that the concentrated sodium aluminate solution contains 15 to 25% Na ₂ O, preferably 20% and

208 508

208 508 to 25% Al ₂ O ₃ , preferably 20% Al ₂ O ₃ . The dissolution temperature is in the range of 80 to 95 ° C, the dissolution time is 1 to 5 hours. The concentrated aluminate solution was diluted with water to the desired concentration of 40 kg Al ₂ O ₃ / m ³ and filtered to remove impurities and a small insoluble solid.

By introducing 30 ° / o CO ₂ , that is 1 to 3 moles of CO ₂ per mole of Al ₂ O ₃ into a sodium aluminate solution of 40 kg Na ₂ O caustic / m ³ and 40 kg Al ₂ O ₃ / m ³ is eliminated alumina trihydrate. Continuous crystallization takes place under isothermal conditions of 25 + 0.5 ° C. C, the mean residence time is 0.5 hours. Hydrodynamic deposition conditions are turbulent and are ensured by the installation of baffles in the crystallizer and an efficient turbine agitator. Under these conditions,

obtains alumina trihydrate with a granulometric composition: linear weight nadsítný dimension share share fractions fractions M (L) (μΐη) (%) (%) 0.08—0.14 0.45 100.00 0,14—0,20 5.89 99.55 0.20—0.25 13.66 93.66 0,25—0,30 16.42 80.00 0.30—0.35 14.02 63.58 0,35—0,40 12.62 49.56 0.40—0.50 18.19 36.94 0.50—0.60 10.11 18.75 0.60—0.70 8.64 8.64 Mean particle size, i. the rest on the net

for the oversize ratio M (L) - 64.7% is 0.3 μΐη. The degree of elimination of Al ₂ O ₃ from the solution to the solid phase is 40 to 60%.

Example 2

Dissolution of alumina trihydrate (hydrargite to sodium aluminate as in Example 1. Dilute the aluminate solution to 80 kg Na ₂ O caustic / m ³ and 80 kg Al ₂ O ₃ / m ³ , filter to remove impurities and small undissolved solids by filtration phase.

By introducing 30% CO ₂ (1 to 3 moles of CO ₂ per mole of Al ₂ O ₃ ) into a 80 kg Na ₂ O caustic / m ³ and 80 kg Al ₂ O ₃ aluminate solution under turbulent hydrodynamic conditions see Example 1, isothermal Continuous deposition at 25 + 0.5 ° C and a mean residence time in the crystallizer of 1 hour gives alumina trihydrate having a granulometric composition:

linear weight nadsítný dimension share share fractions fractions M (L) (μπι) (%) (%) 0,4-0,8 2.07 100.00 0,8-1,2 9.19 97.93 1,2-1,6 14.61 88.74 1.6-2.0 10.54 74.13 2,0-2,5 17.19 63.59 2,5-3,0 9.39 46.40 3,0-3,5 12.45 37.01 3,5-4,0 14.37 24.56 4,0-5,0 10.19 10.19

The mean particle size, ie the residue on the mesh for the oversize fraction M (LJ - 64.7%, is 2 μπι). The degree of elimination of Al ₂ O ₃ from the solution into the solid phase is 80 to 95 ° / o.

The precipitated Al ₂ O ₃ .3 H ₂ O is separated from the mother liquor (soda solution and partially decomposed sodium aluminate) on a vacuum filter or centrifuge. The alumina trihydrate thus separated contains a considerable amount of adsorbed alkali from the solution. The solid phase A1 ₂ O ₃ .3 H ₂ O is washed several times at an elevated temperature of 30 to 90 ° C with water, thereby reducing the alkali content to 0.2 to 0.5% Na ₂ O. Further alkali is removed from the solid phase by washing Al ₂ O ₃ .3 H ₂ O with dilute strong mineral acid, for example 0.01 to 0.1 N HCl, or the separated solid phase is transferred to a stirred reactor, where the remaining adsorbed Na ₂ CO _{3 is} preferably reacted with dilute mineral acid, e.g. :

the precipitated Al ₂ O ₃ .3 H ₂ O containing T 0.5% Na ₂ O in a stirred reactor reacts with an excess of acid; the composition of the suspension is, for example, kg Al ₂ O ₃ .3 H ₂ O

10.10 ^-3 m ³ 0.05 N HCl.

In this way, alumina trihydrate containing less than 0.1% Na ₃ O is obtained after separation. Further washing of the solid phase with distilled or demineralized water at an elevated temperature of 30 to 90 ° C washes off the residues of Cl and Na ⁺ ions.

From the pure alumina trihydrate of a predetermined granulometric composition, suspensions containing 20 to 50% Al ₂ O ₃ .3 H ₂ O are prepared in demineralised or distilled water. From this suspension, a granulate having a grain size of 0.02 to 1.10 is prepared in a spray drier ^{. 2} m.

The granulated intermediate, finely dispersed A1 ₂ O ₃ .3 H ₂ O is calcined at 1200 to 1400 ° C, the mean residence time at the calcination temperature of 1 to 5 hours for special alumina type IV, with the required mean particle size of 0.3 to 2 μΐη and a predetermined surface area value of 5 to 20.10 ³ m ² kg ^-1 . Calcination can be carried out with or without the addition of mineralizers. Suitable mineralizers are, for example, boric acid and the like. magnesium salts such as sulphates or nitrates in concentrations of 0.02 and 0.5% based on alumina trihydrate.

Special alumina of suitable granulometric composition, specific surface area, isometric habitat of particles with high sintering capability prepared in this way fits well with various applications of corundum ceramics, for example energy.

Claims (1)

SUBJECT OF THE INVENTION Process for the preparation of alumina for ceramic purposes by controlled precipitation of solid phase alumina trihydrate from an alkaline solution of sodium aluminate, characterized in that an aluminate solution having a concentration of 20 to 100 kg of caustic sodium oxide / m ³ and 20 to 100 kg of alumina / m ³ and a molar ratio of sodium caustic to alumina of 1.2 to 2, treated with carbon dioxide, or a mixture thereof with air, with a mean residence time of 0.5 to 3 hours and under an isothermal temperature regime and turbulent hydrodynamic conditions, followed by multiple washing the solid phase alumina trihydrate with water and dilute mineral acid and calcining it.