Inorganic pigments and process for their production

Inorganic pigments, especially cobalt pigments, having a spinel structure and higher covering power are obtained if these pigments have a platelet shape. Platelet shape is produced by continuously moving the mixture during the calcining and further intermixing, preferably in a rotary kiln or high temperature cyclone.

BACKGROUND OF THE INVENTION 
The invention is directed to inorganic pigments, especially cobalt 
containing inorganic pigments having a spinel structure and higher 
covering power and a process for their production. 
A considerable portion of inorganic bright colored pigments are derived 
from the crystallographic group of spinels. These have the general formula 
AB.sub.2 O.sub.4 wherein A is an element in its divalent form and B is an 
element in its trivalent form. There is also known the case that A stands 
for a trivalent cation and B for a divalent cation. There come into 
consideration as divalent elements especially the elements Co, Ni, Cu, Zn, 
Cd, Mg, Mn, Fe and as trivalent metal ions Al, Cr, Ga, In, La, V. 
In practice this multitude of elements also means a considerable breadth in 
variation of the colored body, especially in the coloristic pigment 
properties. This means that that inorganic bright colored pigments based 
on spinels represent pure chemical products only in the rarest case. 
A known material is the cobalt-aluminum spinel, CoAl.sub.2 O.sub.4, which 
e.g. can be modified with Cr.sup.III . 
This compound is also known under the trivial names cobalt blue, Leyden 
blue, Kings' blue and Thenard's blue. In ceramics they find use as blue 
coloring components and in the synthetic resin industry they are used as 
pigments for the production of light fast and weather resistant colors. 
The customary processes for the production of cobalt pigments having a 
spinel structure are described in the appropriate literature. Thereby, 
e.g. cobalt oxide and aluminum oxide in powder form are vigorously 
intermixed and calcined at a temperature of around 1000.degree. C. for 
several hours in crucibles. As aids to the reaction there are frequently 
utilized so-called mineralizers, such as e.g., NaCl, CaCl.sub.2, KCl and 
CaSO.sub.4. Calcined products produced in such manner then are ground to 
pigment fineness. In enlargement these pigments show grain forms which 
resemble basalt splinters. 
An alternative precipitation process with subsequent calcination for the 
production of cobalt blue is described in German OS No. 2840870. The 
covering power of this pigment, however, is very poor because of its small 
particle size, there are obtained only transparent blue tones. 
However, for many uses it is necessary for pigments to have higher covering 
power and greater spreadability. Therefore it was the problem of the 
present invention to provide an inorganic pigment, especially a cobalt 
containing inorganic pigment, having a spinel structure which has a high 
covering power and a great spreadability. Besides there should be 
developed a process for the production of such pigments. 
SUMMARY OF THE INVENTION 
This problem was solved according to the invention by having the pigment in 
the form of platelets. 
As pigments there can be employed any of the types mentioned above, i.e. of 
the formula AB.sub.2 O.sub.4 where A is an element in its divalent form 
and B is an element in its trivalent form or where A is a trivalent cation 
and B is a divalent cation. As divalent elements there can be used for 
example the elements Co, Ni, Cu, Zn, Cd, Mg, Mn, Fe and as trivalent metal 
ions Al, Cr, Fe, Ga, In, La, V. 
Advantageously at least 50% of the pigment particles of a batch have a 
hexagonal platelet form. There are obtained through this pigments which 
have a high covering power and a great spreadability. 
Preferably these platelet shaped spinels are produced by mixing the 
components with each other in known manner and calcining at a temperature 
above 1000.degree. C., whereby, however, the mixture is continuously moved 
during the calcining and further intermixing. This advantageously takes 
place in a rotary kiln or a high temperature cyclone. 
In the following examples there are employed two alternative processes of 
production of platelete shaped cobalt blue pigments. These colors can be 
changed in wide range with other cations. The selection of the examples of 
pigments of the system Co--Al--O is not limited to this specific system. 
Unless otherwise indicated all parts and percentages are by weight. 
The process can comprise, consist essentially of or consist of the stated 
steps with the recited materials.

EXAMPLE 1 
Cobalt hydrate and aluminum hydrate were mixed in such manner that there 
was obtained a molar ratio CoO:Al.sub.2 O.sub.3 =1:1. Preferably the 
mixing is carried out under wet conditions in order to obtain a 
homogeneous mixture. After complete mixing of the components the pulp 
obtained was dried in a drying oven. 
The dry crude mixture was then ground to a fineness of less than 5 .mu.m. 
This powder underwent a flash calcination in a high temperature cyclone. 
At a residence time of material of about 1.5 sec and a temperature of about 
1,350.degree. C. there was obtained a platelet shaped cobalt blue pigment 
which contained as the main phase CoAl.sub.2 O.sub.4 (Astm. No. 10-458). 
This pigment batch which far exceeded 50% platelets, preferably in 
hexagonal form is outstandingly suited for coloring ceramic glazes and 
synthetic resin articles. 
EXAMPLE 2 
The crude mixture of Example 1 dried in a drying oven was calcined in a 
rotary kiln without further comminution. The residence time was 20 minutes 
at a calcining temperature of about 1,100.degree. C. Any agglomerates 
formed were disintegrated dry. The pigments are used in ceramic and 
polymer areas. Also in this process of production there is formed almost 
exclusively platelet shaped modifications of the cobalt blue pigments. 
EXAMPLE 3 
24 kg of cobalt hydroxide, 30 kg of aluminum hydroxide and 19 kg of 
chromium oxide were wet mixed with addition of water. After complete 
drying of the moist mixture in a drying oven at a temperature above 
100.degree. C. the procedure was as set forth in Example 2. There was 
obtained a blue-green pigment having good coloristic properties which was 
distinguished by a large portion of platelets. 
The particle diameter in these production examples is in the range of 0.3 
to 1 .mu.m and they had a thickness of 0.05 to 0.2 .mu.m. Hereby it is 
surprising that the pigments normally crystallizing in the cubic crystal 
system form platelets especially hexagonal platelets. 
The entire disclosure of German priority application No. P3225897.6 is 
hereby incorporated by reference.