Corundum Crystal Structure Pigments With Reduced Soluble Chromium Content
The potential for release of soluble chromium (VI) ions in the production or processing of certain pigments is reduced by use of certain additives. The additives include aluminum metaphosphate, aluminum fluoride, tungsten oxide, tungstic acid, and mono-ammonium phosphate. The pigments are those which contain chromium and have a corundum crystal structure.
1 . A method of reducing the amount of soluble chromium from chromium-containing pigments having a corundum crystal structure, the method comprising:
adding an effective amount of an additive to a chromium-containing pigment having a corundum crystal structure, the additive selected from the group consisting of aluminum metaphosphate, aluminum fluoride, tungsten oxide, tungstic acid, and mono-ammonium phosphate;
heating the additive and pigment to a temperature in the range of from about 750° C. to about 1300° C.
2 . The method of claim 1 further comprising:
after heating, subjecting the pigment to a size reducing operation.
3 . The method of claim 2 wherein the size reducing operation is milling.
4 . The method of claim 2 wherein boric acid is also added to the pigment and wherein after subjecting the pigment to the size reducing operation, the pigment has an average particle size of from about 0.5 μm to about 2.5 μm.
5 . The method of claim 4 wherein the average particle size is from about 0.7 μm to about 1.5 μm.
6 . The method of claim 1 wherein heating is performed to a temperature of from about 800° C. to about 1100° C.
7 . The method of claim 1 further comprising:
prior to heating the additive and pigment, mixing the additive and pigment.
8 . The method of claim 1 wherein the effective amount of additive added to the pigment is from about 0.1 wt % to about 5 wt % based upon the weight of the pigment.
9 . The method of claim 1 wherein the pigment having a corundum crystal structure is selected from (i) chromium green-black hematite, (ii) iron brown hematite, and (iii) combinations thereof.
10 . The corundum crystal structure pigments of claim 1 having a reduced level of soluble chromium.
11 . The pigment of claim 10 wherein the soluble chromium is chromium (VI).
12 . A method of reducing the amount of soluble chromium from chromium-containing pigments having a corundum crystal structure, the method comprising:
adding an effective amount of an additive to a chromium-containing pigment having a corundum crystal structure, the additive selected from the group consisting of aluminum metaphosphate, aluminum fluoride, tungsten oxide, tungstic acid, and mono-ammonium phosphate;
heating the additive and pigment to a temperature in the range of from about 750° C. to about 1300° C.;
subjecting the pigment to a size reducing operation.
13 . The method of claim 12 wherein the size reducing operation is milling.
14 . The method of claim 12 wherein after subjecting the pigment to the size reducing operation, the pigment has an average particle size of from about 0.5 μm to about 2.5 μm.
15 . The method of claim 14 wherein the average particle size is from about 0.7 μm to about 1.5 μm.
16 . The method of claim 12 wherein heating is performed to a temperature of from about 800° C. to about 1100° C.
17 . The method of claim 12 wherein the effective amount of additive added to the pigment is from about 0.1 wt % to about 5 wt % based upon the weight of the pigment.
18 . The corundum crystal structure pigments of claim 12 having a reduced level of soluble chromium.
19 . A method of reducing the amount of soluble chromium from chromium-containing pigments having a corundum crystal structure, the method comprising:
adding an effective amount of an additive to a chromium-containing pigment having a corundum crystal structure, the additive selected from the group consisting of aluminum metaphosphate, aluminum fluoride, tungsten oxide, tungstic acid, and mono-ammonium phosphate;
mixing the additive and the pigment;
heating the additive and pigment to a temperature in the range of from about 750° C. to about 1300° C.;
subjecting the pigment to a size reducing operation.
20 . The corundum crystal structure pigments of claim 19 having a reduced level of soluble chromium.