Process for preparing formic acid

A process for obtaining formic acid by thermal separation of a stream comprising formic acid and a tertiary amine (I) which at a pressure of 1013 hPa abs has a boiling point which is at least 5° C. higher than that of formic acid, in which

• (a) producing a liquid stream comprising formic acid, methanol, water and tertiary amine (I) by combining methyl formate, water and tertiary amine (I) at a temperature of from 50 to 200° C.;
• (b) separating from 10 to 100% by weight of the methanol produced from step (a);
• and
• (c) distilling formic acid from the liquid stream of step (b) comprising formic acid, water and tertiary amine (I) in a distillation apparatus at a bottom temperature of from 100 to 300° C. and a pressure of from 30 to 3000 hPa abs;
• wherein, when methyl formate, water and tertiary amine (I) are combined in step (a),
• (a1) methyl formate, water and optionally tertiary amine (I) are introduced in step (a1), where the molar ratio of the tertiary amine (I) optionally introduced into step (a1), n(amine to a1), to the methyl formate introduced into step (a1), n(mefo to a1), is
• 0≦n(amine to a1)/n(mefo to a1)≦0.1,
• and from 70 to 100% of the hydrolysis equilibrium possible under the prevailing conditions is set at a temperature of from 50 to 200° C. and,
• (a2) in step (a2), tertiary amine (I) is subsequently introduced into the stream obtained in step (a1), where the molar ratio of the tertiary amine (I) introduced in step (a2) into the stream obtained in step (a1), n(amine to a2), to the methyl formate introduced into step (a1), n(mefo to a1), is
• 0.1≦n(amine to a2)/n(mefo to a1)≦2,
• where the molar ratio of the total tertiary amine (I) introduced into step (a) “n(amine to a)” to the total methyl formate introduced into step (a) “n(mefo to a)” is at least 0.1.