Heat resistant yellow iron oxide pigment

A method of preparing a yellow iron oxide pigment having improved heat stability, comprising

• (a) forming a first aqueous slurry of from about 30 to about 200 grams of yellow iron oxide per liter of a dilute aqueous alkaline solution,
• (b) subjecting the slurry to a first hydrothermal treatment carried out at a temperature of from about 100.degree. C. to about 240.degree. C., for a time greater than 30 minutes,
• (c) separating the resultant solid phase from the liquid phase,
• (d) forming a second aqueous slurry of from about 30 to about 200 parts of the treated yellow iron oxide of step (c), from about 1 to about 100 parts of a soluble ferric salt selected from the group consisting of ferric sulfate, ferric chloride, ferric nitrate, ferric oxalate and ferric thiocyanate, an amount of alkali metal hydroxide to provide the slurry with a pH of greater than 10, and from about 20 to about 100 parts of a water-soluble or alkali-soluble aluminum salt selected from the group consisting of sodium aluminate, potassium aluminate, aluminum chloride and aluminum nitrate,
• (e) adding to the above slurry from about 20 to about 100 parts as a third metallic component, a metal ion or ions having the characteristics of
• (1) being capable of forming a solid solution with yellow iron oxide and
• (2) having a valence of +3, or wherein a combination of ions is used, having an average valence of +3, and
• wherein the expression
• 2. 5<.SIGMA.X.sub.1 .multidot.V.sub.1 <3.5
• is satisfied, in which X.sub.1 =mole fraction of each metal ion added as the third component and V.sub.1 =valence thereof, the parts of (d) and (e) being based on 1000 parts of water,
• (f) subjecting the resultant mixture to a second hydrothermal treatment carried out at a temperature of from about 100.degree. C. to about 240.degree. C., for a time greater than 30 minutes, to form, on the surface of the iron oxide particle, a layer comprising (Fe Al)OOH and at least one other metal compound, in the form of a solid solution,
• (g) separating the treated mixture into a solid phase and a liquid phase, and
• (h) recovering the solid phase.

A method of preparing a yellow iron oxide pigment having improved heat stability, comprising

• (a) forming a aqueous slurry of from about 30 to about 200 parts of yellow iron oxide (.alpha.-FeOOH), from about 1 to about 100 parts of a soluble ferric salt selected from the group consisting of ferric sulfate, ferric chloride, ferric nitrate, ferric oxalate and ferric thiocyanate, an amount of an alkali metal hydroxide to provide the slurry with a pH of greater than 10, and from about 20 to about 100 parts of a water-soluble or alkali-soluble aluminum salt selected from the group consisting of sodium aluminate, potassium aluminate, aluminum chloride, and aluminum nitrate,
• (b) adding to the above slurry from about 20 to about 100 parts as a third metallic component, a metal ion or ions having the characteristics of
• (1) being capable of forming a solid solution with yellow iron oxide and
• (2) having a valence of +3, or wherein a combination of ions is used, having an average valence of +3, and wherein the expression
• 2.5<.SIGMA.X.sub.1 .multidot.V.sub.1 <3.5
• is satisfied, in which X.sub.1 =mole fraction of each metal ion added as the third component and V.sub.1 =valence thereof, the parts of (a) and (b) being based on 1000 parts of water,
• (c) subjecting the resultant mixture to a first hydrothermal treatment carried out at a temperature of from about 100.degree. C. to about 240.degree. C., for a time greater than 30 minutes, to form, on the surface of the iron oxide particle, a layer comprising (Fe Al)OOH and at least one other metal compound, in the form of a solid solution,
• (d) separating the treated mixture into a solid phase and a liquid phase,
• (e) forming an aqueous slurry of the solid phase,
• (f) subjecting the slurry to a second hydrothermal treatment carried out a temperature of from about 250.degree. C. to about 350.degree. C., for a time period greater than 30 minutes, at a temperature higher that that of the first treatment,
• (g) separating the treated mixture into a second solid phase and a second liquid phase, and
• (h) recovering the second solid phase.