The present invention relates to a process for preparing oxalic acid from dialkyl oxalate, process in which a dialkyl oxalate and an aqueous liquid are fed into a hydrolysis reactor in order to hydrolyze the dialkyl oxalate, the released alcohol is removed from the hydrolyzed mixture obtained, and the remaining aqueous solution, which contains oxalic acid, is directed to a crystallizer in order to separate the obtained oxalic acid in the form of a crystalline dihydrate.

Oxalic acid is used, for example, as a component in dyes, as a raw material for technochemical products, in metallurgy, and as an analytical reagent. Oxalic acid has been prepared, for ex¬ ample, by heating sodium formate to form sodium oxalate and by treating the latter with sulfuric acid in order to release oxalic acid.

Also known is a process for preparing oxalic acid in which oxalic acid dialkyl ester is partially hydrolyzed so that oxal¬ ic acid and alcohol are produced. The preparation process is implemented in such a way that the hydrolysis yields an organic phase and an aqueous phase, which are separated from each other. The organic phase, which contains the ester which has not been completely hydrolyzed and alcohol, is returned to the hydrolysis step, whereas the aqueous phase is directed to a crystallizer, in which the formed oxalic acid separates as a crystalline dihydrate. Before the crystallization, part of the aqueous phase is separated and fed into a distillation column, in which alcohol produced in the hydrolysis is separated and from which the distillation residue is returned to the solution introduced into the crystallizer. In addition, alcohol may be recovered directly from a reaction mixture portion separated from the hydrolysis reactor.

The prior-art process referred to above is based on higher oxalic acid esters, in the hydrolysis of which the reaction mixture is divided into separate phases. The object of the present invention is to simplify the process based on hydrol¬ ysis so that the said separation of the phases from each other is rendered unnecessary. The invention is characterized in that a dialkyl oxalate soluble in the mixture to be hydrolyzed is used in the process so that the hydrolysis will yield a sub¬ stantially homogeneous aqueous solution, the alcohol released from the solution being removed by transferring it to a gas phase, whereafter the solution is transferred to a crystal¬ lizer.

The essential advantage of the invention is that it is possible to separate alcohol from the mixture obtained from the hydrol¬ ysis step and thereafter to separate the obtained oxalic acid, without the necessity of carrying out a division of the mix¬ ture, a phase separation, and a recycling of the organic phase to the hydrolysis reactor. In this manner the substantial dete¬ rioration of the volume yield of the reaction, due to the re¬ cycling, is avoided. The separation of the alcohol by evapora¬ tion or by distillation can be carried out in one step, the evaporation or distillation residue constituting as such a substantially completely hydrolyzed mixture fraction, which is transferred to the crystallization of oxalic acid.

The dialkyl oxalate used in the process according to the inven¬ tion is preferably dimethyl oxalate which, having a small mole¬ cule size, dissolves well in the mixture to be hydrolyzed and from which methanol is released in the hydrolysis. Owing to its low boiling point, methanol is easy to separate by distillation from the aqueous reaction mixture.

According to a preferred embodiment of the invention, the mother liquor from the oxalic acid crystallization step, con¬ taining both water and uncrystallized oxalic acid, is recycled

to the hydrolysis reactor as the liquid phase for the dialkyl oxalate hydrolysis. For the hydrolysis step, pure water is added to the mother liquor, the water substituting for the water removed from the process along with the oxalic acid dihy¬ drate. The recycling of the mother liquor improves the yield of oxalic acid and enables the process to be implemented as a continuous process.

The temperature of the hydrolysis reactor is preferably at minimum 30 °C, at which dimethyl oxalate is sufficiently solu¬ ble, and at maximum approx. 100 °C, at which an aqueous solu¬ tion boils. The most preferable temperature range is 50-80 °c It is preferable to feed the dialkyl oxalate into the hydro¬ lysis reactor as a liquid, either as a strong solution or in molten state. When molten dimethyl oxalate is used, the reactor temperature is preferably at minimum 54 °C, which is the melt¬ ing point of dimethyl oxalate.

The average retention time of the reaction mixture in the hy¬ drolysis reactor must be at minimum 15 minutes. The amount of dialkyl oxalate fed into the hydrolysis reactor is preferably such that the concentration of oxalic acid in the solution entering the crystallizer will be within the range 20-60 %, preferably 30-50 %.

The invention is described below in greater detail with the help of examples by first referring to the accompanying drawing, which is a diagram of one process according to the invention for preparing oxalic acid, and by thereafter describ¬ ing the preparation of oxalic acid with the help of embodiment examples.

The apparatus according to the drawing for the preparation of oxalic acid comprises a hydrolysis reactor 1, an evaporator 2, a crystallizer 3, and a crystal separator 4, which are coupled in series, in the said order. Into the hydrolysis reactor 1

there are introduced an aqueous mother liquor which is recycled in the process from tube 5, dimethyl oxalate from tube 6, and water, which is added from tube 7 to the recycling tube 5. In the hydrolysis which takes place, the dimethyl oxalate and water react which each other, forming oxalic acid and methanol. As a result of the hydrolysis there is obtained a homogeneous aqueous solution which travels from the reactor 1 along tube 8 to the evaporator 2. The evaporator 2 separates from the solu¬ tion methanol, which according to the drawing passes into tube 9, and the aqueous solution obtained as the evaporation residue passes along tube 10 to the crystallizer 3. In the crystallizer 3, oxalic acid separates out from the solution as a crystalline dihydrate, which is. separated from the remaining mother liquor, for example, by filtering in a separator 4, to which the mix¬ ture has passed along tube 11. According to the drawing, the separated oxalic acid dihydrate passes into tube 12, the mother liquor returning via the recycling tube 5 to the hydrolysis step.

Example 1

Dimethyl oxalate at 590 g/h and water at 1380 g/h were fed simultaneously at a temperature of 60 °C to a well agitated 2500-ml reaction vessel. From the said reaction vessel the said mixture was further fed at an even rate into a distillation apparatus, from which a methanol distillate was recovered on average at 325 g/h and a mixture of oxalic acid and water as an underflow at an average rate of 1640 g/h. The oxalic acid was allowed to crystallize out from the said mixture at 25 °C, whereupon a 98.6 weight-% oxalic acid dihydrate was recovered as a filtered product at an average rate of 482 g/h. This gives 75.4 % of the dimethyl oxalate as the average oxalic acid yield in the separated product. The concentration of oxalic acid in the filtrate was on average 11.1 % by weight.

Example 2

Dimethyl oxalate at 590 g/h and filtrate from the experiment of

the previous example, at 1380 g/h, were fed simultaneously at a temperature of 60 °C to a well agitated 2500-ml reaction ves¬ sel. From the said reaction vessel the said mixture was further fed at an even rate to a distillation apparatus, from which a methanol distillate was recovered on average at 320 g/h and a mixture of oxalic acid and water as an underflow at an average rate of 1645 g/h. The oxalic acid was allowed to crystallize out from the said mixture at 25 °C, whereupon a 99.1 weight-% oxalic acid dihydrate was recovered as a filtered product at an average rate of 630 g/h. This gives 99.0 % of the dimethyl oxalate as the average oxalic acid yield in the separated prod¬ uct. The concentration of oxalic acid in the filtrate was on average 11.2 % by weight.

Example 3

Diethyl oxalate at 236.2 g/h and water at 551.1 g/h were intro¬ duced at 60 °C into a well agitated 1000-ml reaction vessel. From the said reaction vessel the said mixture was fed further at an even rate into a distillation apparatus, from which an ethanol distillate was recovered at an average rate of 235.3 g/h and a mixture of oxalic acid and water as an under¬ flow at an average rate of 550.4 g/h. The oxalic acid was al¬ lowed crystallize out from the said mixture at 25 °C, whereupon a 99.9 weight-% oxalic acid dihydrate was recovered as a fil¬ tered product at an average rate of 158.4 g/h. This gives 77.0 % of the diethyl oxalate as the average yield of oxalic acid in the separated product. The concentration of oxalic acid in the filtrate was on average 8.3 % by weight.

For an expert in the art it is evident that the various ap¬ plications of the invention are not limited to those presented as examples above but may vary within the scope of the accom¬ panying claims. Thus it is possible that, instead of recycling, the process according to the invention is implemented as a batch process.