[0002] Using existing processes for producing chemical cellulose, cellulose pulp can be produced in an economical and environmental friendly manner when wooden raw materials are employed. However, one has not, either technically or economically, succeeded in producing cellulose that would satisfy the environmental requirements from herbaceous plants, i. e. non-wood materials.

[0003] To eliminate the environmental effects of chemical pulp pro- duction,'organosolv'processes based on organic cooking solvents have been devised. The organosolv processes are based on the utilization of organic sol- vents, typically organic acids, in delignification. Several of these processes are based on sulphur-free and chlorine-free chemicals, which can be regarded as the requirement for the industrial implementation of closed circulation of chemicals. Such processes are described e. g. in WO 96/26403 (Chempolis Oy) and WO 00/60160 (Chempolis Oy).

[0004] Non-wood material is most preferably delignified in acid con- ditions. In that case the silicon included in the non-wood material is in a very poorly soluble form and does not thus cause problems in the recovery of chemicals. In alkaline cooking processes, instead, silicon dissolves into the cooking liquor and causes problems. For this reason, the most environmental friendly and economical way of implementing delignification is to carry it out by organic solvents in acid conditions. It is particularly preferable to utilize a proc- ess where chemicals to be used in the cooking are formed in the process itself.

[0005] Several organosolv processes have been reported to pro- duce furfural during the production of cellulose. In general, it has been argued that production of furfural in acid conditions impedes delignification because furfural may react with lignin and other compounds of the plant material, form- ing caramel-like polymers. This was noticed e. g. in an organosolv process <BR> <BR> based on acetic acid (Zil'bergleit, M. A. & Glushko, T. V. , Products from the Polymerization of Furfural and Hydroxymethylfurfural in Acetic Acid, Khimia Drevesiny (Riga), 1991, no 1,66-68) and in acid preliminary hydrolysis of the

conventional sulphate process (Kosaya, G. S., Koseleva, V. D. & Prokopeva, M. A. , New Process of Preliminary Hydrolysis, Bumaznaja promyslennost, 1982, no 9,12-13). It has been found that furfural also reacts in the sulphite cooking, causing pulp blackening (Oblak-Ramer, M. , Budin, D. & Lipic, B., Concerning the basics of condensation of lignins in magnesium bisulphite di- gestion. Part 1. Influence of processing time, temperature and thiosulphate, Zellstoff und Papier 40 (1991), no. 1,10-13). Utilization of furfural in the actual delignification process has not been described in the literature related to the art; instead, furfural has usually been separated from the process. For exam- ple, it has been suggested that the furfural separated from the Alcell process (WO 93/15261, Lora et al.) and the Formacell process (Lehnen, R. , Saake, B.

& Nimz, H. H., Furfural and Hydroxymethylfurfural as Byproducts of FOR- MACELL Pulping, Holzforschung 55 (2001), no. 2,199-204) be utilized as a commercial product.

[0006] It has been found earlier that high-quality cellulose pulp can be produced advantageously using a mixture of formic acid, acetic acid and water in delignification (WO 99/57364, Chempolis Oy). Surprisingly, it has now been found that pulp can be produced from a delignification mixture based on organic acids using also furfural.

BRIEF DESCRIPTION OF THE INVENTION [0007] The object of the invention is to improve the pulp production process based on organic acids. The invention is based on dissolving lignin from a fibre-based raw material using furfural in delignification in addition to organic acids.

[0008] It was found that the pulp yield could be improved by using furfural, which also facilitated the recovery of chemicals. The hemicellulose content of cellulose pulp can be increased without changing the pulp lignin content. In that case the yield and properties of pulp will improve. The chemi- cals to be used in the process, i. e. organic acids and furfural, are formed dur- ing the actual process, especially during the recovery step of the cooking chemicals, which makes the process even more advantageous.

DETAILED DESCRIPTION OF THE INVENTION [0009] The invention relates to a process for producing pulp from a fibre-based raw material, comprising a cooking step using a cooking reagent based on organic acids at a temperature of 105 to 160°C and a recovery step

of said cooking reagent. The process is characterized in that the cooking re- agent used for delignification also includes furfural, which is derived from the recovery step of said cooking reagent.

[0010] The process of the invention is applicable in connection with various organic acids and their mixtures. Typically, formic acid, acetic acid or their mixture is used. In addition to the organic acids, the cooking reagent usu- ally contains water. The contents of the organic acid are typically from 0 to 90%, calculated from the total weight of the cooking reagent, and the acid may be formed of formic acid, acetic acid or their mixture. When a mixture of formic acid and acetic acid is used as the cooking reagent, it preferably contains 40 to 80% of formic acid and 8 to 50%, preferably 8 to 40% and most preferably 8 to 35% of acetic acid, calculated from the total weight of the cooking reagent.

[0011] According to the present invention, the cooking reagent also includes furfural. The furfural content is typically 0.01 to 10%, preferably 1.0 to 7.5%, calculated from the total weight of the cooking reagent. In practice, the furfural formed in the process compensates for the acid losses of the process and reduces the amount of make-up acid. The optimal furfural content is 2.5 to 7.5%, calculated from the total weight of the cooking reagent. Higher furfural amounts can also be used but in that case the cooking usually needs to be enhanced e. g. by raising the cooking temperature or by increasing the cooking time.

[0012] In addition to furfural, the cooking reagent may also contain other furan compounds.

[0013] Furfural is formed in the actual process, typically in the re- covery phase of cooking chemicals. In a typical situation, the cooking liquor that has been recovered from the process and concentrated with respect to the dissolved solids is allowed to react at an elevated temperature. In that case the organic matter, such as hemicellulose and lignin, included in the cooking liquor reacts, producing furfural as well as cooking chemicals to be used in the proc- ess, i. e. formic acid and acetic acid, as reaction products. The acids may be utilized as such in the cooking. According to the invention, it has been surpris- ingly found that furfural obtained as a reaction product can also be utilized in the cooking. Thus the furfural needs not be separately isolated from the mix- ture of cooking acids but the furfural-containing mixture can be used as such in the cooking. This reduces the load on the separation section of the process.

[0014] The temperature used for the reaction of the concentrated cooking liquor is typically 50 to 250°C.

[0015] Furfural is carried to the cooking with acids in the recovery process of cooking chemicals, i. e. cooking acids. The recovery of cooking ac- ids typically comprises evaporation, drying/separation of lignin and concentra- tion of acids by distillation.

[0016] The recovery system of cooking chemicals typically com- prises an evaporation step where the used cooking liquor obtained from the separation of cooking liquor and pulp is evaporated. The evaporation can be carried out at a temperature of 50 to 180°C under a reduced pressure or an overpressure. As condensate, the evaporation yields a concentrated mixture containing formic acid and acetic acid, which is returned to the cooking, and an evaporation concentrate where acetic acid, formic acid and furfural have formed during the evaporation from the organic matter of the cooking liquor and/or organic acids included in the organic matter. The dry solids content of the evaporation concentrate is typically 20 to 85%, particularly 40 to 80%.

More formic acid, acetic acid and furfural can be formed in the evaporation concentrate thus obtained by allowing it to react at an elevated temperature (e. g. 50-250 °C) without evaporation. The reaction time can be 0,5 min to 24 h, for example. According to the present invention, it has been found that fur- fural can also be utilized in the cooking, and thus the furfural-containing acid mixture obtained can be used as such in the cooking without separating the furfural.

[0017] It is also feasible to separate the mixture thus obtained that includes furfural, formic acid, acetic acid and water into different fractions e. g. by distillation. In that case furfural, a mixture of formic acid and acetic acid, and water are obtained as distillation fractions. The mixture of formic acid and ace- tic acid is returned to the cooking for use as cooking acid. The furfural fraction obtained can also be returned to the cooking for use as the furfural component of the cooking reagent.

[0018] In the process according to the invention, the cooking time is typically 20 to 120 min and the cooking temperature 105 to 160 °C, preferably 105 to 150 °C, most preferably 105 to 140 °C. The use of furfural as a compo- nent of the cooking reagent does not substantially increase the cooking time or raise the cooking temperature.

[0019] A typical ratio of the cooking reagent to the raw material is 2.5 : 1-10: 1.

[0020] Where high formic acid contents are used in the cooking, formic acid binds to the pulp as formiate esters, i. e. the cellulose pulp is formy- lated. Formiate esters can be hydrolyzed utilizing the catalytic activity of formic acid. In a typical situation, the formic acid chemically bound to the pulp is re- acted into free formic acid at a temperature of 50 to 95°C, the original content of free formic acid being 3 to 20%. The process is carried out at the normal pressure, and the reaction time is 0,5 to 4 h, for example.

[0021] When acetic acid is used in the cooking, the pulp is acety- lated. This has been found in organosolv processes based on acetic acid (e. g. <BR> <BR> <P>Pan, X-J. & Sano, Y. , Atmospheric Acetic Acid Pulping of Rice Straw IV : Phys- ico-Chemical Characterization of Acetic Acid Lignings from Rice Straw and <BR> <BR> Woods, Holzforschung 53 (1999), 590-596, and Saake, B. , Lehnen, R. , Lum-<BR> mitsch, S. & Nimz, H. H. , Production of Dissolving and Paper Grade Pulps by the Formacell Process. Proceedings of the 8th International Symposium on Wood and Pulping Chemistry, Helsinki 1995,2 : 237-242). The acetylation de- gree is proportional to the acetic acid content used. The more acetic acid is used in the cooking, the more acetate esters are formed in the pulp. The ace- tate esters have to be hydrolyzed for the recovery of bound acetic acid.

Saponification, i. e. alkaline hydrolysis, has been suggested as a feasible proc- <BR> <BR> ess for recovering acetic acid (Pan, X-J. & Sano, Y. , Atmospheric Acetic Acid Pulping of Rice Straw IV : Physico-Chemical Characterization of Acetic Acid Lignings from Rice Straw and Woods, Holzforschung 53 (1999), 590-596, and <BR> <BR> Saake, B. , Lehnen, R. , Lummitsch, S. & Nimz, H. H. , Production of Dissolving and Paper Grade Pulps by the Formacell Process. Proceedings of the 8th In- ternational Symposium on Wood and Pulping Chemistry, Helsinki 1995,2 : 237- 242). In that case, however, the hydrolyzed acetic acid reacts into acetate salts corresponding to the alkali, for which reason the acetic acid cannot be directly re-used.

[0022] The process of the invention may thus also include a step where oganic acids chemically bound to the pulp are released by reacting the pulp at a temperature of 50 to 120°C when the content of free acids is 2 to 90%. In that case formiate esters and acetate esters react into free formic acid and free acetic acid.

[0023] The hydrolysis of acetate esters, however, requires a longer reaction time than the hydrolysis of formiate esters. For this reason, the ace- tate esters are preferably hydrolyzed e. g. in a pulp storage container, where the retention time is sufficiently long.

[0024] The pulp de-esterification is typically carried out during pulp washing in a pulp container between the washing steps.

[0025] The chemically bound organic acids included in the organic matter, typically lignin, separated from the pulp can also be released in the same manner. Thus the process of the invention may also include a step where the chemically bound organic acids included in the organic matter sepa- rated from the pulp are released by reacting the organic matter at a tempera- ture of 50 to 180°C when the content of free acids is 2 to 90%.

[0026] The de-esterification of organic matter, typically lignin, is car- ried out on the concentrate obtained from the evaporation of the cooking liquor or during the evaporation.

[0027] The chemically bound acids included in the lignin separated from the pulp can also be released into the cooking liquor during precipitation of lignin using the wash acid obtained from the pulp washing or some other diluted acid or water. The precipitation of lignin is carried out on the cooking liquor for example when lignin and hemicellulose are to be recovered sepa- rately.

[0028] According to the invention, it has been surprisingly found that the total yield of pulp increases when organic acids are partly replaced by fur- fural in the cooking. On the other hand, addition of furfural to the cooking liquor slows down the delignification to some extent. Cooking of the pulp up to a de- sired lignin content is achieved e. g. by increasing the cooking time or by rais- ing the cooking temperature. Furfural does not seem to have a significant ef- fect on the paper-technical properties of pulp.

[0029] The use of furfural as part of cooking chemicals according to the present invention has an advantageous effect on the recovery of chemi- cals, too. The recovery of cooking chemicals typically comprises separation steps, e. g. distillation where a mixture of cooking acids, furfural and water is separated into acid, water and furfural fractions. When part of the acids used in the cooking are replaced by furfural, the feed flow can be decreased at the dis- tillery and the investment and operation costs of the distillery reduced. In addi- tion to the fact that furfural can be employed as a cooking chemical according

to the present invention, it can be utilized as the additive of distillation to facili- tate the separation of water and acids. Furthermore, it is a known fact that the corrodibility of furfural is lower than that of organic acids.

[0030] The process of the invention can be carried out e. g. by using a tube reactor whose dimensions are 0.5 < L/D < 25, preferably 1 < L/D < 15.

The reactor is typically lined with a zirconium or a teflon coating.

[0031] Preferably herbaceous plants are used as the raw material in the process according to the invention. Herbaceous plants generally refer to non-wood fibre sources. The most important fibre sources include straw, e. g. grain straw (rice, wheat, rye, oats, barley) ; hay, e. g. esparto, sabai and lemon hay; reeds, e. g. papyrus, common reed, sugar cane, i. e. bagasse, and bam- boo; bast fibres, e. g. stems of common flax and oil flax, kenaf, jute and hemp; leaf fibres, e. g. manilla hemp and sisal, and seed hair, such as cotton and linter fibres of cotton. One important raw material that grows in Finland is reed canary grass.

[0032] The process of the invention is also applicable to wood mate- rial.

[0033] The following examples describe the process of the inven- tion.

Example 1 [0034] Wheat straw was cooked using a mixture containing 44% of formic acid, 35.2% of acetic acid and 2.5% of furfural as the cooking reagent (the rest of the mixture was water). The cooking temperature was 125°C and the cooking time 35 min. The ratio of the cooking reagent to the raw material was 5: 1.

[0035] The cooking was carried out in a Zr-lined cooking reactor whose volume was 1 litre and whose dimensions were L/D = 2.56. External electric heating was used in the reactor.

[0036] Lignin was washed from the delignified pulp obtained with an acid whose concentration was 44% of formic acid and 35.2% of acetic acid.

Then the pulp was washed at a temperature of 65°C with peracid, which was prepared in situ by adding 1% of hydrogen peroxide from the weight of the fi- bre raw material. After the washing, the pulp was de-esterified at a tempera- ture of 70°C, the acid concentration being 10% (4 h). Finally, the pulp was washed with water.

[0037] The pulp obtained was extracted with alkali and bleached twice with hydrogen peroxide. The brightness of the bleached pulp was 81.7 ISO (measured using the standards SCAN-CM 11: 75 and SCAN-P 3: 93). Its tensile strength was 57.4 kNm/kg (measured using the standard SCAN-P 67: 93), and its SR was 35 (measured using the standard SCAN-CM 26: 99).

Example 2 [0038] Brown (unbleached) pulp was produced from wheat straw in the same manner as in example 1. Table 1 presents the cooking conditions with different furfural contents (% by wt.) while the kappa number of pulp is the same. The contents of furfural, formic acid (HCOOH), acetic acid (CH3COOH) and water shown in the table have been calculated from the total weight of the cooking reagent (% by wt.).

[0039] It appears from the results of Table 1 that furfural improves the yield of pulp.

Table I Furfural HCOOH CH3COOH Water Cooking Cooking Yield *) tempera-time ture 0 73, 8 10,2 16 115 28 41, 0 2,5 71,6 9,9 16 115 32 42,2 5 694 = 16 = 16 430 0 52 30 18 125 30 43, 5 2,5 50,4 29,1 18 125 27 44, 5 5, 0 48, 8 28,2 18 125 27 45, 0 5,0 33, 5 44 17, 5 130 35 44, 0 *) Screened yield

Example 3 [0040] Reed canary grass was cooked by a laboratory cooker using a mixture containing 42% of formic acid, 40% of acetic acid and 0. 1% of fur- fural as the cooking reagent (the rest of the mixture was water). The cooking temperature was 125°C and the cooking time 55 min.

[0041] The resulting pulp was washed and de-esterified using an acid mixture containing 13% of formic acid, 12% of acetic acid and 75% of wa- ter at a temperature of 95°C. The contents of the bound acids as per cents from the weight of the pulp are presented as a function of time in Table 2.

Table 2 De-esterification time Bound HCOOH Bound CH3COOH (h) 0 2, 8 2,6 2 0, 8 2, 2 10 0,4 1 1 [0042] It appears from the results of Table 2 that the contents of the bound acids decrease as a function of time, i. e. the acids are released from their esters due to the catalytic effect of the acids.

[0043] It is obvious to a person skilled in the art that, as the technol- ogy advances, the concept of the invention can be implemented in various ways. The invention and its embodiments are thus not limited to the examples described above, but they may vary within the scope of the claims.