Bismuth vanadate process

An improved solid state process for the preparation of compounds based on bismuth vanadate wherein the improvement comprises subjecting the reaction product to wet grinding in alkaline solution or to sequential wet grinding and alkaline treatment, said process modification resulting in the preparation of bright, high tinting yellow pigments.

Bismuth vanadates have been identified as pigmentary, yellow compounds 
applicable for coloring plastics and paints (see U.S. Pat. Nos. 4,115,141 
and 4,115,142). A variety of precipitation and solid state reactions have 
also been disclosed for preparing such bismuth vanadates and related 
compounds. For example, prior to discovery of the indicated pigmentary 
properties, Gottlieb et al, Therm. Anal. Fourth ICTA, Budapest, 1, 675-679 
(1974) describe synthesis of bismuth vanadates by solid state preparations 
wherein intimately mixed bismuth and vanadium oxides are heated at 
800.degree. C. for 16 hours and by precipitation methods wherein solutions 
of sodium vanadate and bismuth nitrate are reacted at controlled 
concentrations, temperatures, time and pH. Correspondingly, Roth et al, 
Amer. Mineral. 48, 1348-1356 (1963) disclose solid state approaches 
involving heating of the mixed oxides. 
In addition, the approach of the above noted U.S. patents involves first 
precipitating a gel-like precursor from soluble bismuth and vanadium 
compounds, and then converting the precursor into the crystalline, 
pigmentary form either by a heat treatment at 200.degree.-500.degree. C. 
or by an aqueous aftertreatment carried out under specified conditions. 
Alternate approaches are disclosed in German Nos. 3,315,850, 3,315,851, 
U.S. Pat. Nos. 3,843,554 and 4,063,956. In addition, U.S. Pat. No. 
4,316,746 describes bismuth vanadate/molybdate and bismuth 
vanadate/tungstenate pigments which consist in the case of bismuth 
vanadate/molybdate of a crystalline phase having a scheelitelike 
structure, while in the case of bismuth vanadate/tungstenate a two-phase 
product is present. 
A bismuth vanadate/molybdate or bismuth vanadate/tungstenate was also 
proposed in U.S. Pat. No. 4,455,174 and German No. 3,221,338 as a further 
alternative yellow pigment. These are multiphase products which consist of 
a bismuth vanadate phase and a bismuth/molybdate and/or a 
bismuth/tungstenate phase and which are prepared by a process in which a 
solution containing a bismuth salt, a vanadate and a molybdate is 
acidified, then treated with an alkaline solution optionally containing 
the tungsten compound, whereupon the solids are recovered, washed and 
optionally dried and heat treated at 300.degree.-800.degree. C. 
These diverse processes have, however, exhibited certain disadvantages. A 
key disadvantage of the solid state processes is noted in the calcined 
mass wherein a dull yellow brown color or a dirty green color is 
frequently encountered. Such a reduction in the desired bright yellow 
color has, in turn, a significant impact on the use of these materials in 
pigment-related applications. Although the use of oxidizing agents in 
certain of the prior art processes has served to minimize this effect, 
many of the oxidizing agents produce noxious off-gases. A further 
disadvantage of certain of these processes is the need for a multiplicity 
of steps including precipitation, calcining, and the like. 
Accordingly, it is the primary advantage of the invention to develop an 
improved solid state process for the preparation of pigmentary-quality 
bismuth vanadate compounds. 
It is a further object to provide such a process which substantially 
eliminates the adverse color effects on the desired bright yellow color of 
these compounds. 
Various other objects and advantages of this invention will become apparent 
from the following descriptive material. 
It has now been surprisingly discovered that by subjecting the calcined 
bismuth vanadate compounds to a procedure where the compound is wet ground 
in the presence of an alkaline material or is treated with the alkaline 
material subsequent to wet grinding, the disadvantages of the prior art 
approaches have been substantially eliminated. Thus, the off-color is 
believed to be attributable to the presence of the excess vanadium 
frequently utilized in order to insure complete bismuth reaction and/or to 
the presence of lower valent vanadium, namely, trivalent and pentavalent 
vanadium. The instant process thus serves to convert these contaminants to 
the soluble and readily removable alkali vanadate form in order to obtain 
the desired color. In addition, the process allows for the use of low cost 
raw materials and for relatively simple operation particularly in the 
combined alkaline-wet grinding procedure. Finally, the process is 
applicable to a wide variety of bismuth vanadate compounds, including 
bismuth vanadate and bismuth vanadates containing a broad range of bismuth 
and vanadium replacement ions. The key end result is the preparation of 
bright yellow pigmentary bismuth vanadate compounds. 
The bismuth vanadate compounds applicable for preparation according to the 
instant process comprise bismuth vanadate and single phased bismuth 
vanadate compounds resulting from the incorporation of various precursor 
materials or the solubilization of various ADO.sub.4 compounds therein. 
These compounds may be depicted by the general formula 
EQU ADO.sub.4 
wherein A represents trivalent bismuth alone or cation combinations of 
bismuth and at least one other cation; 
and D represents pentavalent vanadium alone or cation combinations of 
vanadium and at least one other non-A cation; the applicable cations being 
compatible with the pigmentary properties of the resulting product. 
Typical bismuth replacement cations include alkaline earth metals and zinc, 
while typical vanadium replacement cations include molybdenum and 
tungsten. 
A sub-group within the above noted formula corresponds to the formula 
EQU (Bi,E)(V,G)O.sub.4 
wherein E is an alkaline earth metal, zinc or mixtures thereof, and G is 
molybdenum, tungsten or mixtures thereof, the molar ratio of E:Bi being 
between 0.1 and 0.4 and the molar ratio of G:V being between 0 and 0.4. 
Molar ratios of 0.1-0.3 for each are preferred. The (Bi,E) and (V,G) 
notations are to be understood as meaning that bismuth is partly replaced 
by one or more E cations and that vanadium can be partly replaced by one 
or more G cations. 
The latter compounds are more fully described in U.S. application Ser. No. 
016,045, filed Feb. 18, 1987, now U.S. Pat. No. 4,752,460, and the 
appropriate sections thereof are incorporated herein. 
The pre-calcining and calcining operations are well-known to those skilled 
in the art, as particularly identified in certain of the aforementioned 
publications. The applicable processes include solid state reactions at 
elevated temperatures starting from the corresponding metal oxides. As 
noted, the usual method is to calcine mixtures of oxides, or any salt 
which yields the corresponding oxide by thermal decomposition, e.g., 
carbonates, nitrates oxalates, hydroxides, etc., in the proper ratios for 
the desired composition. Calcining temperatures vary from about 
300.degree. to about 950.degree. C. The optimum temperature depends upon 
the particular composition being prepared. Higher temperatures are 
preferred in order to facilitate reaction and to assure a homogeneous 
product. A critical upper limit for calcining temperatures is imposed by 
the formation of a liquid phase. 
The time of calcination is not critical; times of 1 to 100 hours may be 
used, but 4-48 hours is preferred. Longer times are required at lower 
temperatures. Calcining times may be shortened and homogeneity of the 
products improved by regrinding between periods of heating. 
The improved process steps of the invention involve subjecting the calcined 
product to wet grinding in the presence of an alkaline material or wet 
grinding followed by alkaline treatment. Thus, subsequent to calcining, 
the product is discharged and cooled to room temperature. Wet milling will 
generally be conducted in a pebble, ball, microball or sand mill for a 
period of time sufficient to achieve pigmentary particle size. The 
alkaline material is added as an aqueous solution to provide the wet 
milling environment. The alkaline material will generally be introduced at 
the onset of the milling operation, although it may also be added in 
solution during milling such that it is in contact with the calcined 
material for sufficient time to form the alkali vanadate. 
An operable but less preferred approach involves stirring the milled 
material in an alkaline solution at room temperature for a sufficient 
period of time for salt formation. 
Applicable alkaline materials include alkali metal hydroxides and 
carbonates, preferably sodium and potassium materials, and alkaline earth 
metal hydroxides, carbonates and oxides, preferably magnesium materials. 
The alkaline materials are added in sufficient amount to provide a pH 
value of from about 7.0-13.0 to the pigment-containing slurry, and 
preferably a pH value of 9.5-12.0. 
Finishing operations for the resulting material will include filtration, 
washing to remove soluble salts and drying, for example, at 
100.degree.-110.degree. C. Subsequent dry grinding may also be an option. 
The resulting bismuth vanadate compounds exhibit quality pigmentary 
properties, particularly the desired bright yellow color and high tinting 
capability. They are highly suited for pigmenting a wide variety of high 
molecular weight organic materials, including resins, oils and organic 
polymers. They can be incorporated into lacquers, paints and printing 
inks. 
To improve certain pigment properties, the prepared compounds can 
additionally be treated with texture-improving agents, for example with 
long-chain aliphatic alcohols, esters, acids or salts thereof, amines, 
amides, waxes or resinous substances, such as abietic acid, hydrogenation 
products, esters or salts thereof, further with nonionic, anionic or 
cationic surface-active agents. 
The following examples further illustrate the embodiments of the invention. 
In these examples, all parts given are by weight unless otherwise 
indicated.

EXAMPLE 1 
Bismuth oxide (46.6 parts) and vanadium pentoxide (18.2 parts) are wet 
milled, dried and heated at 778.degree. C. for four hours. The resulting 
calcined material is thereafter introduced into a pebble mill together 
with sufficient sodium hydroxide solution to provide a pH in the area of 
9.5 to the system and milling is conducted until pigmentary particle size 
is attained. The resulting product is then washed and dried. The product 
exhibits a bright yellow color. 
Pigmentary qualities are determined utilizing rubout in an acrylic lacquer 
and subsequent color readings utilizing a colorimeter. In each case, 22.8 
parts of dry pigment and 100 parts of lacquer are prepared as an ink 
dispersion, referred to as the masstone ink, and drawn down. In order to 
assess color strength, 11.4 parts of pigment and 11.4 parts of pigmentary 
titanium dioxide are blended and drawn down. The results noted below are 
obtained utilizing L, a, b colorimeter measurements wherein "L" refers to 
lightness, "a" refers to red-green ratio with "+a" denoting redness and 
"-a" denoting greeness, and "b" refers to yellow-blue ratio with "+b" 
denoting yellowness and "-b" denoting blueness. 
______________________________________ 
Masstone 
Tint 
______________________________________ 
L 82.2 88.3 
a -8.6 -11.0 
b +50.1 +42.9 
______________________________________ 
These data thus indicate the quality pigmentary properties of the resulting 
pigment. 
EXAMPLE 2 
The following pigments are prepared according to the general procedure of 
Example 1. 
______________________________________ 
TS 
b c d 
______________________________________ 
Bismuth subnitrate 
40.15 -- -- 
Bismuth oxide -- 46.6 46.6 
Vanadium pentoxide 
9.10 18.2 18.2 
Molybdic oxide 3.7 7.5 3.75 
Calcining temp. (.degree.C.) 
667 778 778 
Calcining time (hrs.) 
4 4 4 
pH value 9.5 9.5 9.5 
Color bright bright bright 
yellow yellow yellow 
______________________________________ 
Summarizing, it is seen that this invention provides an improved process 
for preparing pigmentary bismuth vanadate compounds. Variations may be 
made in procedures, proportions and materials without departing from the 
scope of the invention as defined by the following claims.