Treated titanium dioxide pigment with high magnesium oxide addition, 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 a third step, a layer of magnesium oxide and alumina is precipitated on the layer of alumina. Also included are titanium dioxide pigments made by the disclosed process and method using said pigments in paper manufacturing.
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;
(c) precipitating a layer of alumina over the layer of alumina phosphate comprising the steps in the following order:
(i) adding a first alumina source to the aqueous suspension;
(ii) adding a second alumina source to the aqueous suspension, wherein the second alumina source is different from the first alumina source; and
(iii) adding a third alumina source to the aqueous suspension;
wherein steps (b), (c)(i), (c)(ii), and (c)(iii) are each followed by a maturation time;
(d) precipitating a layer of magnesium oxide over the layer of alumina, wherein the precipitating of the layer of magnesium oxide comprises adding a magnesium source to the aqueous suspension after step (c) and adding a basic alumina source to the suspension during the addition of magnesium; and
(e) recovering the titanium dioxide pigments from the suspension.
2. The process of claim 1 , wherein in step (d) the basic alumina source is added so that the suspension does not become more acidic during the addition of the magnesium source than the suspension was at the end of step (c).
3. The process of claim 2 , wherein the pH of the suspension is kept at or below 8.5 during step (d).
4. The process of claim 3 , wherein step (d) is followed by a maturation time.
5. The process of claim 4 , wherein the maturation time is from 1 minute to 30 minutes.
6. The process of claim 2 , wherein the first alumina source, the second alumina source, the third alumina source, and the basic alumina source are each selected from the alumina-source group consisting of aluminum sulfate (Al 2 (SO 4 ) 3 ), sodium aluminate (Na AlO 2 ), aluminum chloride (AlCl 3 ) and combinations thereof.
7. The process of claim 6 , 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.