Hexagonal-bipyramid crystalline scandium oxide powder and a process for preparing the same

Hexagonal-bipyramid crystalline scandium oxide powder and a process for preparing the same are disclosed. The process comprises forming scandium oxalate from an acid aqueous solution containing scandium in the presence of chloride ions and ammonium ions, collecting the precipitate and heating the collected precipitate. Hexagonal-bipyramid crystalline scandium oxide is useful for making electron gun for cathode ray tubes.

FIELD OF THE INVENTION 
This invention relates to hexagonal-bipyramid crystalline scandium oxide 
powder and a process for preparing the same. The hexagonal bipyramid 
crystalline scandium oxide powder is suitable for making electron guns for 
cathode ray tubes. 
BACKGROUND OF THE INVENTION 
The following processes have been known for preparation of scandium oxide. 
(a) A process for preparing scandium oxide comprising causing hydrolysis of 
an acid aqueous solution containing scandium by adding ammonium hydroxide, 
sodium hydroxide or the like to form a precipitate of scandium hydroxide, 
collecting the precipitate and roasting it to obtain scandium oxide. 
(b) A process for preparing scandium oxide comprising forming a precipitate 
of scandium carbonate by adding sodium carbonate, ammonium carbonate or 
the like to an acid aqueous solution containing scandium, collecting the 
precipitate and roasting it to obtain scandium oxide. 
(c) A process for preparing scandium oxide comprising forming a precipitate 
of scandium oxalate by adding oxalic acid to an acid aqueous solution 
containing scandium, collecting the precipitate and roasting it to obtain 
scandium oxide. 
In the processes of (a) and (b), the obtained precipitates are colloidal 
and their filtration is extremely difficult, and the efficient recovery of 
scandium oxide is impossible. Further, the scandium oxide obtained by 
roasting the scandium hydroxide or scandium carbonate is in the coagulated 
state and it becomes a mixture of coarse particles and extremely fine 
particles when crushed and pulverized, that is, it cannot be obtained with 
a uniform particle size distribution. 
The scandium oxalate obtained by the process of (c) is easy to filtrate. 
However, the scandium oxide powder obtained by thermal decomposition of 
the thus obtained oxalate is planar or columnar. 
At any rate, hexagonal-bipyramid crystalline scandium oxide of powder has 
not been obtained. 
We attempted to improve the process (c) in order to obtain scandium oxalate 
precipitate having good filterability by modifying the composition of the 
mother liquor and appropriate selection of the oxalic acid compound to be 
used, and we have found that hexagonal-bipyramid crystalline scandium 
oxide powder can be obtained by forming scandium oxalate precipitate in 
the presence of chloride ions and ammonium ions. 
DISCLOSURE OF THE INVENTION 
This invention provides hexagonal-bipyramid crystalline scandium oxide 
powder. Also this invention provides a process for preparing 
hexagonal-bipyramid crystalline scandium oxide powder comprising forming 
scandium oxalate from an acid aqueous solution containing scandium in the 
presence of chloride ions and ammonium ions, collecting the precipitate 
and heating the collected precipitate. 
As preferred embodiments of the invention, (a) scandium oxalate is 
precipitated by adding oxalic acid and ammonium chloride to nitric acid 
solution or sulfuric acid solution containing scandium, (b) scandium 
oxalate is precipitated by adding ammonium oxalate to a hydrochloric acid 
solution containing scandium. 
As an acid solution containing scandium, a solution prepared by dissolving 
scandium oxide, scandium hydroxide, scandium carbonate, etc. in a solution 
of hydrochloric acid, nitric acid or sulfuric acid, etc. Scandium oxalate 
is formed in the above described acid solution containing scandium in the 
presence of chloride ions and ammonium ions. 
The aforesaid solution containing scandium should have a pH of not higher 
than 4, and the pH should preferably be close to 1 at the time the 
precipitate is formed. The solubility of scandium oxalate is minimum at a 
pH close to 1, and, therefore, the precipitate is easily formed, and thus 
high yield is achieved. At pH's above 4, the starting material, scandium 
oxide, does not dissolve. 
When a hydrochloric acid solution is used, chloride ions need not be added. 
When the acid solution is of nitric acid or sulfuric acid, hydrochloric 
acid or a chloride salt is added. The most preferred chloride is ammonium 
chloride, since this substance can introduce both ammonium ions and 
chloride ions to the reaction system. Metal salts such as sodium chloride, 
potassium chloride, etc. are not desirable, since involvement of these 
metals in the scandium oxalate precipitate from the mother liquor is 
inevitable and hinders production of high purity scandium oxide. 
Ammonium ions can be introduced into the reaction system by ammonium 
hydroxide which is used for adjustment of pH. Or, when ammonium oxalate is 
used as a precipitant, the ammonium ions are introduced thereby. 
When a hydrochloric acid solution containing scandium is used as the 
scandium source and ammonium oxalate is used as the precipitant, chloride 
ions and ammonium ions are introduced without need for any separate 
procedures. 
The functions of the chloride ions and ammonium ions as described above are 
not clearly understood. But it is fact that hexagonal-bipyramid 
crystalline scandium oxide powder is obtained only when both chloride ions 
and ammonium ions are present, and such scandium oxide powder cannot be 
obtained with only one of them. 
The amounts of chloride ions and ammonium ions to be added are not 
specifically limited. 
The precipitate is formed either at room temperature or at an elevated 
temperature. At room temperature, the precipitate is formed in several 
minutes, but if the reaction system is heated when the precipitate is 
formed, the time required for formation of the precipitate is shortened. 
The formed scandium oxalate precipitate is preferably aged in the mother 
loquor for thirty minutes or more. 
The collection of the precipitate is usually carried out by filtration. 
However, any method, including decantation, can be employed. 
Hexagonal-bipyramid crystalline scandium oxide powder is obtained by 
collecting the precipitate and roasting or thermally decomposing it. A 
heating temperature of 650.degree.-800.degree. C. and a heating time of 
about 2 hours are usually satisfactory. The scandium oxide powder obtained 
by the above-described heat treatment is not sintered but is a fine powder 
having an average particle size of 8-12 .mu.m. 
Hexagonal-bipyramid crystalline scandium oxide powder can be easily 
prepared from scandium-containing acid solution by the process of the 
present invention. 
Especially, the scandium oxide powder obtained from the scandium oxalate 
formed by adding oxalic acid and ammonium chloride to a 
scandium-containing nitric acid or sulfuric acid solution, or the that 
obtained from the scandium oxalate formed by adding ammonium oxalate to 
scandium-containing hydrochloric acid solution is free from contamination 
with metal impurities such as alkali metals and, therefore, it is a high 
purity product.

SPECIFIC DESCRIPTION OF THE INVENTION 
Now the invention will be explained in detail by way of preferred working 
examples and comparative examples. 
EXAMPLE 1 
To 12 g of scandium oxide, 120 ml of hydrochloric acid and 60 ml of water 
were added and heated to dissolve it. The solution was boiled down to 40 
ml, water and hydrochloric acid being evaporated. Thereafter, water was 
added to the concentrated solution to make a 1 l solution having a pH of 
1. This solution was warmed to 60.degree. C., and 37.8 g of ammonium 
oxalate monohydrate was added to form scandium oxalate precipitate. After 
the precipitate was aged in the mother liquor for 1 hour, it was collected 
by filtration and roasted at 900.degree. C. for 2 hours in the air. A 
scanning electron microphotograph of the thus obtained scandium oxide 
powder is shown in FIG. 1. From this photograph it is recognized that the 
powder consists of hexagonal bipyramid crystals. Also, the substance was 
confirmed to be scandium oxide by X-ray diffraction analysis (Philips 
PW-1700). 
EXAMPLE 2 
To 12 g of scandium oxide, 120 ml of concentrated nitric acid and 60 ml of 
water were added and heated to dissolve it. The solution was boiled down 
to 40 ml, water and nitric acid being evaporated. Thereafter, 900 ml of 
water was added to the concentrated solution, and further 40 g of ammonium 
chloride was added to adjust the pH of the solution to 1. Further 33.6 g 
of oxalic acid dihydrate was added to precipitate scandium oxalate. After 
the precipitate was aged in the mother liquor for 1 hours, it was treated 
in the same manner as in Example 1 and scandium oxide powder was obtained. 
A scanning electron microphotograph of the thus obtained scandium oxide 
powder is shown in FIG. 2. From this photograph it is recognized that the 
powder consists of hexagonal bipyramid crystals. Also, the substance was 
confirmed to be scandium oxide by X-ray diffraction analysis. 
EXAMPLE 3 
To 12 g of scandium oxide, 120 ml of hydrochloric acid and 60 ml of water 
were added and heated to dissolve it. The solution was boiled down to 40 
ml, water and hydrochloric acid being evaporated. Thereafter, aqueous 
ammonia solution were added to adjust the pH of the solution to 1. This 
solution was treated in the same manner as in Example 1 and scandium oxide 
powder was obtained. The scanning electron microphotograph of this product 
was almost the same as that in FIG. 1, and thus the powder consisted of 
hexagonal bipyramid crystals. Also, the substance was confirmed to be 
scandium oxide by X-ray diffraction analysis. Comparative Example 1 
To 20 g of scandium oxide, 100 ml of concentrated nitric acid and 60 ml of 
water were added and heated to dissolve it. The solution was boiled down 
to 60 ml, water and nitric acid being evaporated. Thereafter, water was 
added to the concentrated solution to make 900 ml, and the pH of the 
solution was adjusted to 1 by addition of ammonia water. Further, 56 g of 
oxalic acid dihydrate was added to the solution and the solution was 
stirred for 1 hour. The mixture was treated in the same manner as in 
Example 1 and scandium oxide powder was obtained. A scanning electron 
microphotograph of the thus obtained scandium oxide powder is shown in 
FIG. 3. As seen in this photograph, the scandium oxid powder obtained by 
this comparative example is of columnar and is inferior in 
crystallization. Comparative Example 2 
To a scandium-containing hydrochloric acid solution having a pH of 1 
prepared in the same manner as in Example 1, ammonium hydroxide was added 
to adjust the pH thereof to 8. Thereafter, the formed scandium hydroxide 
was collected by filtration, and roasted at 700.degree. C. in the air. A 
scanning electron microphotograph of the thus obtained scandium oxide is 
shown in FIG. 4. As seen in this photograph, the obtained scandium oxide 
was inferior in crystallization and that the particle size was not 
uniform. Comparative Example 3 
To a scandium-containing hydrochloric acid acid solution having a pH of 1 
prepared in the same manner as in Example 1, ammonium carbonate was added 
to adjust the pH thereof to 8. Thereafter, the formed scandium carbonate 
was collected by filtration, and roasted at 700.degree. C. in the 
atmosphere. A scanning electron microphotograph of the thus obtained 
scandium oxide is shown in FIG. 5. As seen in this photograph, the 
obtained scandium oxide was inferior in crystallization.