Process for the preparation of acicular .alpha.-iron oxide hydrate

A process for the preparation of acicular .alpha.-iron oxide hydrate which consists of

• a) mixing an aqueous solution of 0.1-1 mole per liter of a ferrous salt containing at least one of a nickel salt and a manganese salt in the amount of up to 0.1 mole per liter of said nickel salt and up to 0.1 mole per liter of said manganese salt with an aqueous solution of an alkali hydroxide in a molar excess with respect to the molar amount of said ferrous salt, to obtain a first slurry of ferrous hydroxide, at least one of nickel hydroxide and manganese hydroxide and unreacted sodium hydroxide;
• b) partially oxidizing said first slurry from step a) with an oxygen containing gas at a temperature of 20.degree.-40.degree. C. to partially oxidize said ferrous hydroxide to obtain a second slurry containing acicular .alpha.-iron oxide hydrate of formula FeOOH;
• c) interrupting the oxidation at a stage when the ratio ##EQU1## is 10-50% whereby said acicular .alpha.-iron oxide hydrate grows only in the direction of the diameter and does not grow in the length direction and adding to said second slurry from step b) an aqueous solution of an aluminate salt in an excess of 6 atomic per cent of aluminum with respect to the total iron present as Fe+++ and Fe++ whereby a third slurry is obtained;
• d) homogenizing, stirring and oxidizing said third slurry with an oxygen containing gas whereby a fourth slurry containing free aluminate ion and .alpha.-iron oxide hydrate is obtained;
• e) homogenizing and stirring said fourth slurry from step d), neutralizing said slurry with an acid and then aging to obtain acicular .alpha.-iron oxide hydrate containing at least one of nickel and manganese oxide and having aluminum deposited on the outer surface thereof.

A process for the preparation of acicular .alpha.-iron oxide hydrate which consists of

• a) mixing an aqueous solution of 0.1-1 mole per liter of a ferrous salt containing at least one of a nickel salt and a manganese salt in the amount of up to 0.1 mole per liter of said nickel salt and up to 0.1 mole per liter of said manganese salt with an aqueous solution of an alkali hydroxide in a molar excess with respect to the molar amount of said ferrous salt, to obtain a first slurry of ferrous hydroxide, at least one of nickel hydroxide and manganese hydroxide and unreacted sodium hydroxide;
• b) partially oxidizing said first slurry from step a) with an oxygen containing gas at a temperature of 20.degree.-40.degree. C. to partially oxidize said ferrous hydroxide to obtain a second slurry containing acicular .alpha.-iron oxide hydrate of formula FeOOH;
• c) interrupting the oxidation at a stage when the ratio ##EQU2## is 10-50% whereby said acicular .alpha.-iron oxide hydrate grows only in the direction of the diameter and does not grow in the length direction and adding to said second slurry from step b) an aqueous solution of an aluminate salt in an excess of 6 atomic per cent of aluminum with respect to the total iron present as Fe+++ and Fe++ whereby a third slurry is obtained containing free aluminate and .alpha.-iron oxide hydrate;
• d) homogenizing, stirring and oxidizing said third slurry from step c) with an oxygen containing gas whereby a fourth slurry is obtained;
• e) homogenizing said fourth slurry from step d) and adding at least one of an aqueous solution of phosphoric acid or a salt thereof in an amount of 0.01 liter in the form of a solution of concentration up to 1.0 mole/liter and an aqueous solution of silicic acid or a salt thereof in an amount up to 0.01 liter in the form of a solution of concentration up to 1.0 mole/liter, per liter of said fourth slurry, stirring, neutralizing said slurry, aging to obtain acicular, .alpha.-acicular, .alpha.-iron oxide hydrate containing aluminum and at least one of phosphorus and silicon deposited on the outer surface thereof.