Gray-black encapsulated pigments and method for their preparation

Gray-black pigments are prepared by encapsulating iron titanates of formula FeTiO.sub.3 in zirconium silicate crystals. Manufacture is by calcining a mixture of zirconium oxide having a particle size distribution of 6 to 10 .mu.m and a specific surface area of 2 to 5.2 m.sup.2 /g, silicon dioxide and iron and titanium oxides or Fe and Ti compounds forming oxides in the presence of mineralizers under oxidizing conditions at 800 to 1300.degree. C. Finely divided FeTiO.sub.3 can also be used.

INTRODUCTION AND BACKGROUND 
The present invention relates to gray-black encapsulated pigments based on 
zirconium silicate as the encasing substance for encapsulating pigmenting 
or colorant compounds in the form of a discrete phase. Further, the 
present invention relates to a method for making these encapsulated 
pigments. 
Encapsulated pigments also known as inclusion pigments in general are known 
in the art, as for example from the German Patent 23 12 535. They consist 
of transparent crystals of glaze-stable substances such as zirconium 
silicate, zirconium oxide or tin oxide which encase or encapsulate 
inorganic anhydrous pigmenting or colorant compounds in a discrete phase. 
Pigmenting compounds are illustrated by Thenard's blue, titanium yellow 
and especially cadmium yellow and cadmium red. Moreover, zirconium iron 
rose with Fe.sub.2 O.sub.3 encased in a zirconium silicate casing also is 
known. The manufacture of these encapsulated pigments is carried out by 
methods well known in the art, e.g. heating the casing substances, or its 
initial precursors and the pigmenting substances to be encased, or their 
initial precursors, in the presence of mineralizers, to temperatures up to 
1200.degree. C. However, it has been found in the art that this procedure 
does not allow encapsulation of all conceivable pigmenting compounds into 
the casing substances with significant yields. 
The palette range of gray-black colors is not very wide. Essentially such 
pigments are obtained from iron oxides and chromium oxides, optionally in 
combination with other oxides such as manganese oxide, copper oxide, 
nickel oxide or cobalt oxide. Up to this time, no gray-black encapsulated 
pigments have been known. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide gray-black 
encapsulated pigments of transparent zirconium-silicate crystals, which 
encase the pigmenting/colorant compounds as a discrete phase. A further 
object of the invention is to provide a method for making these 
encapsulated pigments and to achieve relatively high yields thereof. 
In attaining the above and other objects, a feature of the invention 
resides in encapsulating crystalline iron titanates of the FeTiO.sub.3 
composition as the pigment/colorant component. 
These encapsulated pigments provided by the present invention exhibit a 
gray-black color. Different color nuances or shades can be the result of 
different amounts of the encapsulated colored crystallites as well as 
varying particle form and particle size of these crystallites. 
The gray-black encapsulated pigments are prepared by calcining a previously 
ground mixture of zirconium oxide, silicon oxide and precursor compounds 
of the pigmenting compounds to be encased in the presence of mineralizers 
and at temperatures of 800.degree. to 1300.degree. C. In carrying out the 
method, zirconium oxide is emphasized with a particle size distribution 
(D50 values) of preferably 6 to 10 .mu.m and with a specific surface area 
of 2 to 5.2 m.sup.2 /g. As the starting materials for the pigmenting 
compound to be encased iron oxides and titanium oxides or iron compounds 
and titanium compounds which form oxides at elevated temperatures under 
reducing conditions can be employed. The compacted mixture is then 
calcined under reducing conditions from 0.5 to 8 hours in conventional 
furnaces. 
The iron and titanium compounds used are either the oxides such as FeO, 
Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4 and TiO.sub.2, or compounds that at 
higher temperatures and under reducing conditions are capable of forming 
oxides, for instance titanyl compounds. In principle the iron titanate of 
the formula FeTiO.sub.3 also can be used directly as the pigment/colorant 
provided such products be available in the required degree of finenesses. 
The mineralizers are well known in the art and ordinarily are alkali metal 
or alkaline earth metal halides, preferably alkali metal fluorides or 
alkaline earth metal fluorides and alkali metal silicofluorides. 
In this manner gray-black encapsulated pigments containing FeTiO.sub.3 are 
obtained. The yields of useful pigments are 90% and higher. Preferably the 
mean particle size of the encapsulated pigments is in the range of 5 to 10 
.mu.m. The pigmenting compound FeTiO.sub.3 that is completely encased by 
the ZrSiO.sub.4 crystal casing exhibit a substantially smaller particle 
size; i.e. as a rule less than 1 .mu.m. 
Preferably fine-particulate iron oxides (Fe.sub.2 O.sub.3) with D50 values 
of 0.2 to 0.7 m and surfaces of 13 to 24 m.sup.2 /g (determined as for 
ZrO.sub.2 in geometric manner from the particle distribution) are used for 
purposes of the invention. The Ti compound preferably is titanium dioxide, 
in particular as fine-particulate titanium dioxide as possible, most 
preferably having a particle size less than 1 .mu.m. In the event 
pyrogenic titanium oxides are used, then mixtures of anatase and rutile 
are especially advantageous. On account of its method of manufacture, 
pyrogenic TiO.sub.2 frequently exhibits a primary particle size of 
approximately 20 to 30 nm. 
Zirconium oxide and silicon dioxide are used in essentially equivalent 
amounts. The mixture to be calcined is ground before calcining, for 
instance in a ball mill or another intensive grinder or mill apparatus. 
Calcination can be carried out in conventional furnaces such as chamber, 
tunnel or piston furnaces (furnace for getting the product to be calcined 
heated up within the shortest time). Preferably the heating rate shall be 
800.degree. to 1000.degree. C./hour, however, the heating rate also may be 
outside those limits. When calcining in the especially preferred range of 
1000.degree. to 1200.degree. C./hour a calcination time of 1 to 2 hours is 
generally adequate. 
The new gray-black encapsulated pigments are suitable for coloring glazes. 
In contrast, non-encapsulated iron titanates are not glaze-stable, that 
is, they dissolve during vitrification.

DETAILED EMBODIMENTS OF THE INVENTION 
The examples which follow are illustrative of the invention: 
EXAMPLE 1 
A mixture of 25 g zirconium oxide (D50 value=7.6 .mu.m; surface area=3.1 
m.sup.2 /g), 13.5 g silicon dioxide; 3.5 g potassium chloride; 2.5 g 
magnesium fluoride; 1.5 g sodium hexafluorosilicate; 5 g iron (III) oxide 
and 2.5 g pyrogenic titanium dioxide with about 30% by weight rutile 
content is ground for 30 minutes and then heated at the rate of 
1000.degree. C./hour and following covering with sugar is heated to 
1100.degree. C. and at that temperature is then calcined for 1 hour. A 
gray-black product is obtained, consisting of FeTiO.sub.3 crystals 
encapsulated in zirconium silicate. The yield is 95%. 
EXAMPLE 2 
A mixture such as described in Example 1 with the zirconium oxide 
exhibiting a D50 value of 9.3 .mu.m and a specific surface area of 2.2 
m.sup.2 /g, is heated at a rate of 900.degree. C./hour to 1200.degree. C. 
and calcined for 1 hour. Again a gray-black product is obtained. The yield 
is 92%. 
EXAMPLE 3 
A product similar to that of Example 1 is obtained when using zirconium 
oxide with a D50 value of 6.6 .mu.m and a specific surface area of 5.2 
m.sup.2 /g. 
Further variations and modifications of the foregoing will become apparent 
to those skilled in the art and are intended to be encompassed by the 
claims appended hereto.