GB2159805A

GB2159805A - Method of producing a sinterable gamma -LiAlO2 powder

Info

Publication number: GB2159805A
Application number: GB08509431A
Authority: GB
Inventors: Dieter Vollath; Horst Wedemeyer; Elmar Gunther
Original assignee: Kernforschungszentrum Karlsruhe GmbH
Current assignee: Forschungszentrum Karlsruhe GmbH
Priority date: 1984-04-25
Filing date: 1985-04-12
Publication date: 1985-12-11
Also published as: DE3415324C2; GB8509431D0; FR2563512A1; BE902191A; DE3415324A1; GB2159805B

Abstract

A method of producing a sinterable gamma -LiAlO2 powder having a 99% phase purity comprises dissolving metallic aluminium in an aqueous lithium hydroxide solution. The suspension which is formed by the precipitation produced during the dissolving reaction is directly spray-dried by means of air at a temperature of from 200 DEG C to 450 DEG C, without any additional, intermediate treatment; and the dried material is then directly calcined at a temperature of from 850 DEG C to 1100 DEG C and thereby converted to gamma -LiAlO2.

Description

SPECIFICATION Method of producing a sinterable y-LiAI O2 powder and use thereof.

The present invention relates to a method of producing a sinterable y-LiA1 02 powder having a 99% phase purity, whereby metallic aluminium is dissolved in an aqueous lithium hydroxide solution.

It has already been proposed to use lithium-containing, oxidic ceramics materials-lithium aluminate, for example--as breeder materials for fusion reactors to obtain tritium. In most of the cases for producing LiA102, solid Li2CO3 and A1203 powders were finely ground and mixed, and such mixtures were subjected to heat-treatmet either when they were in their dry state-as pressed pellets, for exampleor when they were in the form of aqueous sludges, said heattreatment including drying and calcining at high temperatures. Several hours were needed to grind the starting substances. The ground product was then compressed and diffusion-annealed for one to two days for calcination purposes.The reaction product then had to be reground so that it was possible to obtain the desired, sinterable powder.

Impurities may result from the grinding and pulverising of not only the starting substances, but also the calcined reaction product. In addition, these solids reactions generally only account for 90% to 95% of the desired phase in the end product. Finally, the thermal conductivity of the LiA102 product-which is required to obtain tritium-is adversely affected by the presence of other phases.

In consequence, it was considered desirable to provide a simple method of producing y LiA102 so that the impurities in the end product can be eliminated and so that the considerable amount of time needed for the prior art methods (approximately three of four days) can be shortened. To eliminate the grinding and diffusion-annealing steps, it was proposed that as much of the method as possible should be effected in an aqueous solution. It is known that aluminium is dissolved by aqueous LiOH solutions (Gmelin's "Handbuch der Anorganischen Chemie", System No. 35, Aluminium, Part A (1934, reprinted in 1953), Page 408, published by Verlag Chemie GmbH, Weinheim). It is reported there that a difficultly soluble aluminate of the formula LiH(Al 02)2 . 5H2O is precipitated after such a solution has been boiled.However, there is no disclosure about the production of y-LiA102.

The invention seeks to provide a simple, improved method of producing y-LiA102, whereby as much of the method as possible is effected in an aqueous solution and whereby only a short period of time is required to obtain a highly sinterable powder of the desired lithium aluminate in an almost phase-pure manner.

According to the invention, the object is achieved in that: a) the suspension which is formed by the precipitation produced during the dissolving reaction is directly spray-dried by means of air at a temperature of from 200"C to 450"C without any additional, intermediate treatment; and b) the dried material is then directly calcined at a temperature of from 850"C to 1 1 00 C and thereby converted to y-LAl 02.

The metallic aluminium in the LiOH solution is advantageously dissolved with constant stirring. It is not necessary to heat the solution, but the expected precipitation of the lithium hydroxo aluminate according to the reaction equation: 2LiOH + 2A1 + aq = Li [A1 2(OH)7] . nH2O + LiOH + aq does not become too coarse-grained when the solution has been previously heated from approximately 40"C to 50"C. If the spray-drying step is effected in the temperature range of from 350"C to 400"C, and if the calcining step is effected at the preferred temperature of 900"C, powder particles having an outer diameter of between 5 ,um and 1 5 ym are obtained.

By using the y-LiAi02, which is produced in accordance with the method of the invention, as a breeder material for fusion reactors, unexpected advantages are also achieved for obtaining tritium since the obtained powder is highly sinterable and can be processed to form moulded bodies having densities of up to 95% of the theoretical density.

The invention is explained more fully hereinafter with reference to one procedural example.

Example: In accordance with the reaction equation: 2LiOH + 2A1 + aq Li [A12(OH)7] . nH2O + LiOH + aq, the deposits listed in the Table were produced by dissolving metallic aluminium in an aqueous lithium hydroxide solution with constant stirring at room temperature.

Table: Deposits for spray-drying lithium aluminate No. LiOH Al metal H2O Li [A12(OH)7] Number of (9) (9) (cm3) (s/l) Deposits 1 23.95 26.98 1000 90 1 2 47.90 53.96 1750 100 1 An industrially produced sheet aluminium was used for Deposit No. 2, and an extremely pure aluminium strip (made by MERCK) was used for Deposit No. 1. LiOH may be used both as a water-free hydroxide and as a hydroxide containing water of crystallisation (LiOH . H2O) to produce the aqueous solution.The resultant suspension, comprised of very finely distributed lithium heptahydroxo aluminate in lithium hydroxide solution, was spray-dried in the temperature range of from 250'C to 400go. The yield of spray-dried powder a stoichiometric mixture of lithium hydroxo aluminate and lithium hydroxide, was always more than 90%. by Air-treating the lithium hydroxide solutions and the suspensions, the powders contained up to 5% by weight of carbonate components which were removed during the calcining step.

During the calcining of the powders (two hours), the desired lithium monoaluminate was produced from the finely-dispersed, stoichiometric powder mixture which was initially present. In dependence upon the calcining temperature, mixtures of a- and ssLiA102 (at 600"C), a- and y Liy102 (at 800"C) and pure y-LiA102 (above 900"C) were observed. Powders calcined at 900"C could be compressed to between 80% and 85% of the theoretical density by being pressed and sintered at approximately 1250"C, and such powders could be compressed to between 90% and 95% of the theroetidal density by being pressed and sintered at approximately 1450 C in six hours. The sintered samples always produced monophase y-LiA1 02.

Claims

1. A method of producing a sinterable y-LiA1 02 powder having a 99% phase purity, whereby metallic aluminium is dissolved in an aqueous lithium hydroxide solution, in which: a) the suspension which is formed by the precipitation produced during the dissolving reaction is directly spray-dried by means of air at a temperature of from 200"C to 450"C without any additional, intermediate treatment; and b) the dried material is then directly calcined at a temperature of from 850"C to 1100 C and thereby converted to y-LiA 102

2. A method as claimed in claim 1, in which spray-drying is effected at 350"C to 400"C and calcining is effected at 900"C.

3. A method of producing a sinterable y-LiA102 powder having a 99% phase purity, as claimed in claim 1 or 2, substantially as hereinbefore described and exemplified.

4. A breeder material for a fusion reactor for producing tritium, comprising sinterable, pure LiA102 powder produced in accordance with the method as claimed in claims 1, 2 or 3.