The present invention relates to a process for the recovery of a lower aliphatic acid from the pulp from a pulping process carried out with this same organic acid.

Previously, formic acid was recovered, from the pulp from a pulping process carried out with formic acid, by washing the pulp with water and by recovering the formic acid from the obtained dilute aqueous solution of formic acid by distilla¬ tion. This method has a disadvantage not only in the cumbersom distillation required by the concentrating of a dilute acid an the resulting high cost, but also in the fact that esterified formic acid is left in the pulp. Even in the best case, after wash with pure water there remains formic acid approx. 3 % by weight of the pulp, whereas the acceptable concentration of formic acid in pulp is 1-2 % by weight of the pulp, and achiev¬ ing this level requires very large quantities of washing water in proportion to the pulp.

The object of the present invention is to eliminate the disad¬ vantages involved in this known method and to provide a process for such recovery of aliphatic acid from the pulp from a pulp¬ ing process carried out using this same organic acid that the amount of acid in the pulp will be brought to an acceptable level very economically and without significant amounts of esterified formic acid remaining in the pulp.

The main characteristics of the invention are given in the accompanying claims.

In the process according to the invention, a lower aliphatic acid, such as formic acid, is removed in two steps, first by vacuum evaporation at an elevated temperature and subsequently by a wash or by a stripping with steam.

The vacuum distillation is preferably carried out at a tempera¬ ture of approx. 70-100 °C, and the steam stripping is preferab¬ ly carried out at 100-140 °C, most preferably at 110-120 °C.

In the first step of the process according to the invention, i.e. the vacuum evaporation, more than 90 % by weight of the acid is recovered in the form of a concentrated acid suitable directly for the acid cycle. When the pulp is subsequently washed or steam stripped, a pure pulp is obtained which con¬ tains combined acid 1-2 % by weight and free acid less than 0.5 % by weight of the pulp.

The process according to the invention can be used for treatin pulp from a pulping process in which some lower aliphatic acid has been used, such as formic acid, acetic acid or propionic acid.

The invention is described below in greater detail with the help of examples.

Example 1

63.2 g of formic acid cellulose having a solids content of 18.45 % by weight and a formic acid content (free + combined) of approx. 390 % by weight of the solids was batched into a rotary evaporator. The temperature was raised to 70 °C, and th formic acid was evaporated for 2 h under a pressure of 14 mbar The product obtained was 14.1 g of formic acid cellulose the solids content of which was 82.75 % by weight and which con¬ tained free formic acid 10.3 % by weight and combined formic acid 9.5 % by weight of the solids. The recovered quantity, of concentrated formic acid (89.7 % by weight) was 49.0 g.

The vacuum evaporated formic acid cellulose was batched into a tubular reactor. The reactor temperature was raised to 120 °C, whereafter water was fed into it for 4 h at a rate of 50 ml/h

via an evaporator at 120 °C. The product obtained was 23.0 g o a formic acid pulp the solids content of which was 50.8 % by weight and which contained free formic acid 0.25 % by weight and combined formic acid 1.58 % by weight of the solids. The recovered quantity of dilute formic acid (approx. 1 % by weight) was 180 g.

Example 2

75.2 g of propionic acid pulp having a solids content of 19.97 % by weight and a propionic acid content (free + com¬ bined) of approx. 400 % by weight of the solids was batched into a rotary evaporator. The temperature was raised to 70 °C and the propionic acid was evaporated for 2 h under a pressure of 14 mbar. The product obtained was a propionic acid pulp which contained free propionic acid 0.10 % by weight and com¬ bined propionic acid 0.11 % by weight of the solids. The re¬ covered quantity of propionic acid was 59.4 g.

The vacuum-evaporated propionic acid pulp was batched into a tubular reactor. The reactor temperature was raised to 120 °C, whereafter water at 50 ml/h was fed into it for 4 h via an evaporator at 120 °C. The product obtained was a propionic acid pulp which contained free propionic acid less than 0.001 by weight and bound propionic acid 0.01 % by weight of the solids. The amount of dilute propionic acid recovered was 135 g.

Example 3

131.1 g of a formic acid pulp having a solids content of 18.45 % by weight and a formic acid content (free + combined) of approx. 390 % by weight of the solids was batched into a rotary evaporator. The temperature was raised to 70 °C and the formic acid was evaporated for 2 h under a pressure of 14 mbar The product obtained was 25.5 g of a formic acid pulp the sol¬ ids content of which was 95.05 % by weight and which contained free formic acid 6.0 % by weight and combined formic acid

10.1 % by weight of the solids. The recovered quantity of con¬ centrated formic acid (88.1 % by weight) was 103.5 g.

The vacuum-evaporated formic acid pulp was washed twice with hot (approx. 90-95 °C) water. Water was used in each wash ap¬ prox. 8 times the weight of the formic acid pulp. The product obtained was 142.3 g of a formic acid cellulose the solids content of which was 17.0 % by weight and which contained free formic acid 0.3 % by weight and combined formic acid 2.4 % by weight of the solids. The recovered quantity of dilute formic acid (approx. 1.2 % by weight) was 260 g.