BACKGROUND OF THE INVENTION Cellulose, for use in the production of paper and the like, may be obtained from an organic fibrous material containing cellulose and/or hemicellulose by the pulping of the organic fibrous material to obtain a cellulose pulp. Typically, the organic fibrous material also contains substantial quantities of lignin. Preferably, at least a substantial proportion of the lignin is removed prior to the conversion of the cellulose pulp into a final product. This is particularly preferred for the production of low KAPPA pulps (eg. pulps with a KAPPA number in the range 12-14) for use in the production of bleachable grade of paper. The bleaching processes typically used for these pulps are either chlorine or chlorine dioxide based and produce bleached pulps with a final Kappa number of 1-2 and a brightness greater than 85% ISO.

Various processes have been developed to pulp an organic fibrous material to obtain a cellulose pulp for the production of paper.

Sulfite pulping was first industrialized in about 1866. It was not until about a century later, in the late 1960's, that sulfite pulping was conducted at an alkaline pH (e. g. 11-12) and typically at atmospheric pressure. This process was advantageous as it provided paper strength and ease of bleachability similar to that achieved using the kraft process.

In a more recent development, sulfite pulping processes which operate in the semi-alkaline pH range (e. g. from about 7. 5 to

about 10) and typically at atmospheric pressure. have been developed (semi alkaline sulfite or SAS pulping). Such processes are capable of producing pulps suitable for the production of bleached fine papers (eg. with a Kappa number from 12 to 14 before bleaching and a Kappa number of 1-2 after bleaching). By operating in the semi-alkaline range, higher unbleached yields (for example, 55-65%) and higher bleached yields (for example, 55%) may be obtained from the pulping process. Further, the delignification of the organic fibrous material is improved.

In addition, a further recent development has been the introduction of a yield enhancer into the paper pulping process. Yield enhancers, such as anthraquinone, are beneficial as they can result in an increased yield (for example, yields of about 130% of those obtained without yield enhancers) from the paper pulping process as well as achieving further improvements in the delignification of the organic fibrous material. Anthraquinone is a relatively expensive chemical. Nevertheless, due to the increased yield and improved delignification which may be achieved by its use, it has been adopted in numerous pulping operations. An example of one of these is the semi-alkaline sulfite pulping with anthraquinone process (SASAQ). A further example is the soda-AQ process (the combination of anthraquinone with a soda pulping process).

One disadvantage of anthraquinone is its cost. Due to the unit cost of anthraquinone, it has not achieved wide use in soda pulping operations. In addition it can cause downstream problems in chemical recovery systems, leading to higher maintenance costs.

SUMMARY OF THE INVENTION It has surprisingly been found that by combining a steam explosion process for the pulping of an organic fibrous material with the use of a semi-alkaline pulping liquor, substantially improved

yields may be obtained while at the same time still producing unbleached pulps for use in the production of bleachable grades of paper (eg. for use in the production of printing and writing paper, tissue and newsprint). In particular, according to the process of the instant invention, low Kappa unbleached pulps, i. e. unbleached pulps having a KAPPA number from 12-16, preferably from 12-15, and most preferably from 12 to 14 may be obtained. Further, these yields may be comparable to those which are obtained from the SASAQ process but without the use of anthraquinone. Accordingly, it is possible to obtain yields comparable to those of the SASAQ process with substantially reduced (eg. 25 %) chemical loading.

In accordance with one embodiment of this invention, there is provided a process for producing low KAPPA unbleached pulp from the feed stock comprising subjecting the feed stock to a steam explosion process in a digester at a semi-alkaline pH to produce pulp.

In accordance to a further embodiment of this invention, there is provided a process for producing pulp from a feed stock comprising subjecting the feed stock to a steam explosion process in a reactor in the presence of a semi-alkaline pulping liquor to produce pulp.

In accordance with a further embodiment of this invention, there is provided a process for producing pulp suitable for the production of bleached paper from a feed stock comprising the steps of : (a) exposing the feedstock to a semi-alkaline pulping liquor to produce a treated feedstock ; and, (b) subjecting the treated feedstock to a steam explosion process in a reactor to produce pulp unbleached pulp ; and, (c) recovering the unbleached pulp.

The unbleached pulp may be bleached and treated to obtain bleached paper either at the same location as the unbleached pulp is produced or at another location.

Preferably, the liquor is removed from the treated feed stock prior to introducing the treated feed stock into the digester.

Preferably, the feed stock is exposed to steam explosion process conditions in the absence of yield enhancers. Examples of yield enhancers which have been used in prior art processes include anthraquinone and polysulfides.

The instant process can produce an unbleached yield of up to 60% with a residence time of about 6 minutes without the use of anthraquinone. Further, such yields may be achieved with the pulp having a kappa number of 14.

In particular, the steam exploded pulp according to the instant invention has a lower shive content and higher fines content than both a Decker (screened) pulp and a belt washer (unscreened) pulp. Accordingly, the steam exploded pulp has a lower average fibre length. Surprisingly, despite the lower average fibre length, the steam exploded pulp produced according to the process of the instant invention demonstrated superior burst strength and breaking length, and slightly lower tear strength versus a decker pulp. Accordingly, the process of the instant invention produces a valuable commercial pulp in a process using as much as 25% less chemical loading.

DESCRIPTION OF PREFERRED EMBODIMENT The feedstock for the process of this invention may be any organic fibrous material which is known in the art for producing a cellulose pulp. The feedstock may consist of any material containing cellulose and/or hemicellulose. Preferably, the feedstock comprises wood or non-wood fibrous material and more preferably bagasse,

cereal straw, soft woods, hard woods, industrial crops such as kenaf, hemp and the like.

The feedstock is subjected to steam explosion process conditions in the presence of the semi-alkaline pulping chemicals.

According to a first preferred embodiment of the instant invention, the feedstock is first exposed to a semi-alkaline pulping liquor to produce a feedstock/liquor mixture. This mixture may then be subjected to the steam explosion process conditions. Alternately, after the feedstock has absorbed the pulping chemicals and, in fact, may be impregnated with them, the liquor may be separated from the feedstock prior to exposing the feedstock to the steam explosion process conditions.

The feedstock is preferably exposed to the semi-alkaline pulping liquor for a sufficient amount of time so that sufficient pulping chemicals from the liquor are absorbed into the feedstock to promote the delignification of the feedstock. The feedstock may be exposed to the semi-alkaline pulping liquor such as by introducing the feedstock and the liquor into a vessel and allowing the feedstock to soak in the liquor for an extended period of time. Processes and equipment for conducting this step of the process are known in the art.

After the feedstock has absorbed a sufficient amount of chemical to promote the delignification of the feedstock, the feedstock may then be subjected to steam explosion process conditions.

In a second preferred embodiment of this invention, the feedstock may not be first soaked in the semi-alkaline pulping liquor.

Instead, the feedstock together with the semi-alkaline pulping liquor may be fed to a reactor (such as a digester) where the feedstock in the presence of the pulping liquor is subjected to steam explosion process conditions. According to this embodiment, the feedstock is exposed to the pulping chemicals as part of the steam explosion process.

Steam explosion processes are known in the art. Steam explosion generally refers to the process of exposing fibres to high pressure steam for a relatively short period of time (eg 0. 5-10 minutes) and then explosively discharging the product to atmospheric pressure. Steam explosion may be conducted on either a batch base or a continuous basis.

In such a process, the feedstock is raised to an elevated pressure (preferably from about 100 to about 500 psi, more preferably from about 150 to about 400 psi and, most preferably from about 190 to about 240 psi) and an elevated temperature (preferably from about 180 to about 240°C, more preferably from about 190 to about 220°C and, most preferably from about 195 to about 210°C). There is an inverse relationship between temperature and residence time. If the temperature is below this range, then the residence time increases resulting in a consequential increase in the size of the equipment. As the steam explosion process equipment must withstand substantial pressures, the increase in the required size of the equipment to obtain the increased residence time makes the equipment very expensive. I f the temperature substantially exceeds 240°C, then the cellulose in the feedstock will commence to degrade thereby decreasing the yield of the steam explosion process.

Due to the substantially elevated temperature and pressure conditions, steam explosion processes have a relatively short residence time. For example, the residence time of the feedstock may be from about 4 to about 8 minutes, more preferably from about 4 to about 7 minutes and, most preferably, about 5 to about 6 minutes.

The reactor may be any steam explosion digester known in the art. Due to the relatively short residence time, steam explosion processes are preferably conducted on a continuous process as opposed to a batch process. For example, as is known in the art, a steam explosion process may be conducted in a digester comprising a

longitudinally extending cylindrical member having a screw auger centrally positioned therein. The auger transports the feedstock from the feed end of the digester to the discharge end of the digester. The feedstock may be fed to the digester for example, by an apparatus comprising, in combination, a screw conveyor mounted in a conduit and a reciprocating annular position which is coaxial with and partially surrounds, the screw conveyor. An example of such a device is disclosed in United States Patent Numbers 4, 186, 658 and 4, 947, 743, which are incorporated herein by reference.

The feedstock may have a moisture content of from about 10 to about 65, more preferably from about 30 to about 65 and, most preferably about 50 wt. % water, based upon the weight of the water and the feedstock. If the feedstock contains too much water, then the feedstock commences to have liquid flow characteristics and, with current technology, it is not practical on a commercial scale to introduce a liquid feedstock into a continuous steam explosion digester. If the feedstock contains too little water, then the autohydrolytic effect of steam explosion may not be achieved.

The semi-alkaline pulping liquor comprises pulping chemicals in a sufficient concentration to obtain a semi alkaline pH from about 7. 5 to about 10, more preferably from about 7. 5 to about 9. 5, and most preferably from about 8 to about 9. As discussed above, the pulping liquid preferably does not contain yield enhancers such as anthraquinone and polysulfides. The semi-alkaline pulping liquor may comprise one or more of sulfites, carbonates, and hydroxides.

These may be any Group I or Group II sulfite, carbonate and/or hydroxide such as calcium, potassium, sodium, magnesium and ammonium.

The semi-alkaline pulping liquor may comprise from about 4 to about 25, more preferably from about 6 to about 20 and, most preferably from about 8 to about 16 wt. % pulping chemicals based

upon the total weight of the pulping liquor. In one particularly preferred embodiment of the invention, the pulping liquor comprises from about 0. 5 to about 25, more preferably from about 1 to about 15 and, most preferably from about 2 to about 12 wt. % pulping chemicals based upon the total weight of the pulping liquor, expressed as Na2O applied on an oven dried fibre basis.

The product of the steam explosion process is a low KAPPA pulp. The pulp may have a KAPPA number from 12-16, preferably from 12-15, and most preferably from 12 to 14.

Example 1 Wet depithed bagasse was air dried to 50 wt. % moisture.

The sample was clean and uniform in terms of cut size and appearance. The pH of a 10% slurry of the bagasse was determined to be about 6. 2. Sample lots of the bagasse at 50 wt. % moisture were impregnated with a semi-alkaline pulping liquor by exposing the bagasse to the liquor (which was at a temperature of 60°C) for 20 hours.

The liquid ratio was maintained at 4 : 1. Prior to steam explosion, all impregnated samples were filtered to remove excess liquor.

The severity of a steam explosion process may be calculated according to the following formula : Ro = t x exp where, Ro = the severity factor inminutes t = the residence time in minutes T= the steam temperature in degrees C

Samples of the impregnated bagasse where then subjected to steam explosion treatment according to the instant invention as well as a soda-AQ process and a SASAQ process. The process conditions and the yield are set out in Table 1 : TABLE 1 Parameter Steam Soda-AQ SASAQ Explosion Process severity (log Ro) 3.7 3.1 3.5 Process Temperature (°C) 200 170 176 Process Pressure (kPa/psig) 1450/210 720/105 790/115 Residence Time (minutes) 6 12 20 Caustic loading (on o. d. fibre as Nua20)-10- Sulfite loading (on o. d. fibre as Na2O) 10. 0-13. 0 Carbonate loading (on o. d. fibre as Na2O) 2. 0-2. 5 AQ loading (% w/w on fibre) no AQ added 0. 05 0. 05 Unbleached Yield (%) 60 50 60 The steam explosion process had the highest severity factor (3. 7 compared with 3. 1 and 3. 5 for the soda AQ and the SASAQ process respectively). Further, the steam explosion process did not use any anthraquinone. Despite this, the steam explosion process produced an unbleached yield of 60% which was comparable to that of the SASAQ (which used 0. 05 wt. % AQ loading). Further, this was 10% higher than the unbleached yield of the soda-AQ process.

Importantly, the 60% yield was achieved in substantially less time than the SASAQ process. In particular, to obtain a 60% yield, the SASAQ process required a residence time of 20 minutes. In comparison, the steam explosion process utilized a residence time of

only six minutes. Accordingly, to obtain the same yield, the residence time of the steam explosion process is less than 1/3 that of the SASAQ process. Therefore, using the instant process, it would be possible to process approximately three times as much material as in the SASAQ process or, alternately, the process equipment could be about 1/3 the size.

Example 2 The strength and optical properties of the pulp which were achieved using the steam explosion process of the instant invention were compared with the same properties of a Decker pulp and a belt washer pulp which are known in the art. The brightness, freeness and the kappa number of the pulp were obtained using TAPPI Standard methods. The results are set out in Table 2.

TABLE 2 Bagasse Pulp Brightness Freeness (CSF) Kappa (% ISO) Number Belt washer pulp (unscreened) 43. 867319. 1 Decker pulp (screened 45. 0 590 13. 3 Steam Exploded Pulp (unscreened) 44. 4 470 14. 0 1 As can be seen from the forgoing table, the steam exploded pulp had a much lower kappa number versus the belt water pulp.

Further, the steam exploded pulp had a comparable brightness to that of the belt washer pulp and the Decker pulp.

The pulp fibre classification of the three pulps was also determined using a Bauer McNett classifier. Further, the bulk, burst strength, breaking length and tear strength of the steam exploded pulp as compared to the belt washer pulp and the Decker pulp were determined according to TAPPI Standard methods. The results are set out in Tables 3 and 4.

TABLE 3 Bagasse Pulp 850 tm 425 am 150 um 75 Nrn Fines Fibre length Belt washer pulp 12. 77 18. 63 35. 82 10. 80 22. 00 1. 13 (unscreened) Decker pulp (screened) 6. 09 18. 51 44. 78 9. 90 20. 72 1. 01 Steam Exploded Pulp 2. 18 10. 76 32. 24 16. 41 38. 41 0. 77 (unscreened) TABLE 4 Bagasse Pulp Bulk Burst Breaking Tear (Zm/gsm) (kPa/gsm) Length (mN/gsm) (km) Belt washer pulp (unscreened) 2. 11 1. 54 2. 26 5. 01 Decker pulp (screened) 2. 18 1. 72 3. 10 6. 48 Steam Exploded Pulp 1. 76 2. 90 4. 26 5. 49 (unscreened) As can be seen from Table 3, the steam exploded pulp had a lower shive content (> 850, um, versus both the Decker pulp and the belt washer pulp). Further, it also had a higher fines content, thus resulting in a lower average fibre length. Nevertheless, as can be seen from Table 4, the lower available fibre length did not adversely affect the strength of the pulp. The steam exploded pulp showed superior burst strength and breaking length compared to the belt washer pulp and the Decker pulp with only a slightly lower tear strength compared to the Decker pulp.

In summary, the forgoing examples demonstrate that steam explosion pulping, at higher pressure and lower residence time, produced a pulp with strength, brightness and kappa number properties comparable to those of the soda-AQ and the SASAQ processes, and at a 60% unbleached yield. Further, this was achieved with lower chemical requirements compared to the SASAQ process.

Example 4 The effect of anthraquinone was investigated at two chemical loadings and two process severities to determine the effect of anthraquinone on pulp yield during steam explosion pulping. The chemical loading and yield results which were achieved are set out in Table 5.

TABLE 5 Run # Na2SO3 Na2CO3 Severity A! Yield (%) (%) (LogRo) (%) 1 4. 0 2. 0 3. 2 0. 05 85 2 4. 0 2. 0 3. 2 0 86 3 8. 0 2. 0 3. 2 0. 05 79 4 8. 0 2. 0 3. 2 0 80 5 4. 0 2. 0 3. 5 0. 05 60 6 4. 0 2. 0 3. 5 0 61 As can be seen by comparing runs 1 and 2, 3 and 4 and 5 and 6, the addition of the anthraquinone did not significantly change the yield which was achieved. Accordingly, at the tested conditions, the addition of anthraquinone had no effect on pulp yield.