Treated titanium dioxide pigment, process of making thereof and use thereof in paper manufacture
Process for the surface treatment of a titanium dioxide pigment, characterized in that it comprises the following steps: an aqueous suspension of titanium dioxide pigments is formed, in a first step, a layer of alumina phosphate is precipitated on the surface of the pigment, in a second step, a layer of alumina is precipitated over the first layer of alumina phosphate, and in an optional third step, a layer of magnesium oxide is precipitated on the layer of alumina. Titanium dioxide pigments made by the disclosed process and method using said pigments in paper manufacturing are also disclosed.
1. A process for the surface treatment of titanium dioxide pigments, comprising the steps in the following order:
(a) forming an aqueous suspension of titanium dioxide pigments;
(b) precipitating a layer of alumina phosphate on the surface of the pigments in said aqueous suspension formed in step (a) to form an aqueous suspension of titanium dioxide pigments having a layer of alumina phosphate precipitated on the surface of the pigments;
(c) precipitating a layer of alumina over the layer of alumina phosphate in said aqueous suspension formed in step (b) to form an aqueous suspension of titanium dioxide pigments having a layer of alumina phosphate precipitated on the surface of the pigments and a layer of alumina precipitated over the layer of alumina phosphate, wherein the precipitating of the layer of alumina over the layer of alumina phosphate comprises the following sub-steps in the following order:
(i) adding a first alumina source to said aqueous suspension formed in step (b) to form an aqueous suspension of titanium dioxide pigments having a layer of alumina phosphate precipitated on the surface of the pigments and alumina precipitated over the layer of alumina phosphate;
(ii) after sub-step (c)(i), allowing said aqueous suspension formed is sub-step (c)(1) to mature for a maturation time;
(iii) after sub-step (c)(ii), adding a second alumina source to said aqueous suspension formed in sub-step (c)(i) to form an aqueous suspension of titanium dioxide pigments having a layer of alumina phosphate precipitated on the surface of the pigments and additional alumina precipitated over the layer of alumina phosphate, wherein the second alumina source is different from the first alumina source;
(iv) after sub-step (c)(iii), allowing said aqueous suspension formed is sub-step (c)(iii) to mature for a maturation time;
(v) after sub-step (c)(iv), adding a third alumina source to said aqueous suspension formed in sub-step (c)(iii) to form an aqueous suspension of titanium dioxide pigments having a layer of alumina phosphate precipitated on the surface of the pigments and additional alumina precipitated over the layer of alumina phosphate; and
(vi) after sub-step (c)(v), allowing said aqueous suspension formed in sub-step (c)(v) to mature for a maturation time, wherein said maturation time associated with steps (c)(ii), (c)(iv) and (c)(vi) is from 1 minute to 30 minutes, and wherein the first alumina source, the second alumina source and the third alumina source are each selected from the alumina source group consisting of aluminum sulfate (Al 2 (SO 4 ) 3 ), sodium aluminate (NaAlO 2 ), aluminum chloride (AlCl 3 ) and combinations thereof; and
(d) after sub-step (c)(vi), recovering the titanium dioxide pigments from said aqueous suspension formed in step (c)(v).
2. The process of claim 1 , wherein the maturation time associated with steps (c)(ii), (c)(iv) and (c)(vi) is from 2 minutes to 10 minutes.
3. The process of claim 1 , wherein after step (c) and before step (d) a layer of magnesium oxide is precipitated over the layer of alumina to produce titanium dioxide pigments.
4. The process of claim 1 , wherein the second alumina source does not include a member of the alumina-source group that is included in the first alumina source, and wherein the third alumina source includes at least one member of the alumina-source group that is included in the first alumina source or the second alumina source.