WO1994010376A1

WO1994010376A1 - Low temperature bleaching

Info

Publication number: WO1994010376A1
Application number: PCT/CA1993/000401
Authority: WO
Inventors: Adriaan Reinhard Pieter Van Heiningen; Yonghao Ni
Original assignee: University Of New Brunswick
Priority date: 1992-10-23
Filing date: 1993-10-07
Publication date: 1994-05-11
Also published as: CN1137593A; AU5104493A

Abstract

An ozone bleached pulp substantially free of chlorine and having a viscosity equivalent to a viscosity of at least 30 cP and a kappa no. of 10 ml for Northern sofwood pulp and a method of producing same. The method applies a solution of water and a freezing point reducing agent to the chemical pulp, then cools the pulp to a temperature below 0 °C and then subjects the pulp to an ozone bleaching step at the reduced temperature to produce a bleached pulp.

Description

LOW TEMPERATURE BLEACHING

Field of the Invention

The present invention relates to a bleached chemical pulp and a method of producing same by ozone bleaching of the pulp under low temperature conditions.

Background of the Present Invention

It is conventional practice to bleach chemical pulps using bleaching agents such as chlorine and chlorine dioxide to produce pulps having the desired degree of whiteness. The use of chlorine has been described as detrimental to the environment particularly when contained in the effluent from the pulp mill, and thus efforts have been made to either eliminate chlorine and/or chlorine compounds or to reduce their use to an absolute minimum.

Peroxides such as hydrogen peroxide have been used in different stages of bleaching to obtain the desired brightness of pulp and reduce the amount of chlorine applied.

Ozone is also used as a bleaching agent to obtain the required brightness of chemical pulps. However, ozone has been known to degrade chemical pulps and thereby reduce their quality, in particular the strength characteristics of the chemical pulp. It is generally accepted in the industry that the viscosity of the pulp provides a very good indication of the strength characteristics of the pulp, i.e. the higher the viscosity the better the strength potential at a given kappa or permanganate no.

It has also been suggested to replace the water, which is the medium in which the pulp is normally contained, with a suitable organic solvent such as ethanol or methanol, and to bleach the pulp using ozone gas as described for example in Japanese patent 7849107 published May 4th 1978 by Ueshima. This patent describes a process for recovering methanol from the digestion of wood chips with sodium hydroxide and Na₂S and using this recovered methanol as a protector for the wood pulp during ozone bleaching. In the process described, air dried pulp was impregnated with methanol substantially free of water and was not acidified.

Japanese patent 7890403 published August 9th 1978 to Ueshima et al. describes another application of methanol followed by ozone bleaching of the methanol containing pulp. Again, in this patent, the water free pulp was impregnated with methanol which was free of water and was not acidified. The impregnated pulp was subsequently ozonated. The patent did not show as good a result as those obtained in the earlier patent referred to above. An article entitled " The effect of cellulose protectors on ozone bleaching of kraft pulp" by Kamisima published in the Journal of Japanese Technical Association of the Pulp and Paper Industry, Vol. 31, 9, pp 62-70, Sept. 1977, describes a number of different solvents that may be used to protect the pulp during an ozone bleaching stage. In these teachings, this publication indicates that ethanol is not effective in improving the viscosity of the ozone bleached pulp whereas the use of methanol does produce a positive result.

An article entitled "The use of ozone in bleaching of pulps" by Liebergott et al. 1991 Pulping Conference, TAPPI Proceedings, pp 1-23, provides a review of the literature on ozone bleaching and describes a number of different chemicals that have been tried, i.e. added to the pulp before and in combination with an ozone bleaching stage in attempts to overcome or significantly reduce the detrimental effects of the ozone stage on the quality of the bleached pulp.

In an article entitled "Strength Characteristics of KP Bleached by Multistage Ozone Bleaching" by Kobayashi, T. et al., published in Japan Tappi, 31(12), pp 807- 811 (1977), the effect of lowering the ozone bleaching temperature from room temperature down to approximately to 0°C is described. In the process described the medium surrounding the pulp was water. The results obtained by ozone bleaching at the lower temperature as compared with those obtained by ozone bleaching at the higher temperature indicate that bleaching at the lower temperature produced a somewhat higher viscosity (8.5 versus 5.5 centipoise (cP)) softwood pulp at the expense of a lower brightness (79.3% verses 82.1%). However, compared to a conventionally bleached pulp the viscosity of the low temperature ozone bleached pulp was dramatically lower (8.5 versus 29 cP).

Brief Description of the Present Invention

It is the object of the present invention to provide a high strength ozone bleached pulp substantially free from chlorine. It is another object of the present invention to provide a method of bleaching chemical pulp using ozone at low temperatures to produce a bleached pulp having high strength characteristics, comparable to those produced with chlorine containing delignification bleaching chemicals. Broadly the present invention comprises of ozone bleached pulp substantially free of chlorine and having a viscosity equivalent to a viscosity of about 30 cP and a kappa no. of about 10 ml for Northern softwood pulp (with a chemical pulp from hardwood using the ALCELL^® process (ethanol medium) a viscosity of about 20 at a kappa no. of about 6 is obtained). The present invention also broadly relates to a method of ozone bleaching of a pulp comprising impregnating said pulp to provide a pulp containing a solution of water and a freezing point reducing agent, cooling said pulp and then subjecting said pulp to an ozone bleaching step at a low temperature below 0°C to produce an ozone bleached pulp having a viscosity at least substantially equivalent to a conventionally chlorine bleached chemical pulp.

Preferably, said ozone bleaching step will be carried out at a temperature below -10°C and more preferably at or below -20°C.

Preferably said freezing point reducing agent will be selected from the group consisting of low molecular weight aliphatic alcohols fully miscible in water and more particularly methanol or ethanol.

In another embodiment of the present invention said freezing point reducing agent will comprise dioxane.

In yet another embodiment of the present invention said freezing point reducing agent will comprise (an) inorganic or organic salt(s) such as for example Na₂SO₄ (or the salts collected in the electrostatic precipitator of the waste pulping liquor recovery process) or urea.

Brief Description of the Drawings

Further features, objects and advantages will be evident from the following detailed description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings in which

Figure 1 is a flowsheet illustrating the method of the present invention. Figure 2 is a plot of kappa no. versus ozone consumed at different temperatures.

Figure 3 is a graph showing the effect of temperature on ozone selectivity for kraft softwood pulp. It is a plot of kappa no. versus viscosity.

Description of the Preferred Embodiments

Figure 1 illustrates an arrangement wherein the present invention has been used in combination with an ozone generating plant that incorporates an oxygen generator and is designed to make efficient use of the low temperature gas available from the oxygen generator. As shown schematically a low temperature gas which is likely to be nitrogen is produced in the oxygen generator 10 and used to cool the incoming air as indicated at 12.

The cooling capacity of the gas not completely used in the oxygen generator 10 is delivered via line 14 to the ozonation stage 16 which in the illustrated arrangement functions as both an ozonation zone and a cooling zone as indicated at 18 and 20 respectively as will be described in more detail here below.

The oxygen from the oxygen generator 10 passes via line 22 to the ozone generator 24 and the ozone generated in generator 24 is delivered via line 26 to a separator 28. Oxygen (surplus oxygen) is returned to the ozone generator as indicated by the line 30.

The ozone collected in the separator 28 is transported with the carrier gas supplied via line 29 into the ozonation zone 18 of the ozone stage 16 via line 32. The ozone containing gas may or may not be cooled in the cooler 31

In the illustrated arrangement the pulp enters the system as indicated at 34 and is impregnated with a suitable low melting point, water containing medium introduced as indicated at 36 into an impregnation chamber or stage 38. A suitable pH adjusting additive may also be added to the aqueous impregnation medium or be introduced into the impregnation stage 38 as indicated at 35.

The impregnated pulp leaves the stage 38 and passes via line 40 to a consistency adjusting stage 42 where the consistency is adjusted to the consistency that is intended to be used within the ozonation stage 16. Generally the consistency will be increased to a high consistency somewhere between 20 and 60% (the higher the consistency the less cooling required in the cooling zone 20 and the ozonation zone 18 and thus it is preferred to operate at a relatively high consistency).

The pulp at the proper consistency is then delivered via line 44 to a fluffer 46 where it is fluffed to the required extent and then is passed via line 48 into the cooling zone 20 of the ozonation stage 16.

In the ozonation stage 16 the pulp is first cooled to the desired temperature which as above indicated will be below 0°C preferably less than -10°C and more preferably less than or equal to about -20 (and maintained at this reduced or low temperature as it traverses the ozonation zone 18 and contacts the ozone introduced into this zone via line 32).

Extra cooling may be required in the ozonation stage 16 to compensate for cooling losses and heat released by the ozonation reaction.

Countercurrent contact between the pulp and the ozone containing gas stream is preferred although co-current and other contacting operations may be used. It is extremely important that good contact between the pulp and the ozone be obtained within the ozonation zone. Any excess ozone i.e. ozone not consumed exits with the exit gases via line 54 and the bleached pulp leaves the ozonation zone 18 as indicated via line 52.

The cooling and ozonation might each be performed in more than one stage in order to improve control of the temperature and ozone consumption.

The bleached pulp in line 52 must obviously be brought back up to room temperature and thus may be heated for example in heat exchanger 100 to transfer heat to another medium (cool the other medium) and this other medium conducted via line 102 and used to cool the pulp in line 48 i.e. by heat exchanger 104. Alternatively the cold pulp may be used directly for example in the same heat exchanger to cool the pulp in line 48 or for any other useful purpose.

The cold nitrogen leaving the stage 16 may be used elsewhere or exhausted to atmosphere.

In the illustrated system the operator may use all or part of the cold nitrogen in line 14 for cooling the ozonation stage 16. The excess nitrogen may simply be vented via line 60. The present invention is based in part on the finding that the (lignin- carbohydrate) selectivity and delignification efficiency improves when pulp is ozonated at significantly lower temperatures than conventionally practised.

However, to prevent freezing of the aqueous solution with which the pulp is contacted, a suitable substance or mixture of substances (i.e. a freezing point reducing agent) must be added to the solution to lower the melting (or freezing) point of the resulting aqueous solution. Methanol, ethanol, 1,4 dioxane and Na₂SO₄ are examples of suitable substances which reduce the melting point of the aqueous solution that have been tried, however others may be evident to those skilled in the art. The preferred characteristics for such freezing point reducing agent are a) ability to lower the freezing (or melting) point of the resulting aqueous medium to a temperature well below 0°C, preferably below - 20°C. b) form a single homogeneous liquid phase when mixed with water at the desired low temperature i.e. fully miscible with water. Pulp is impregnated as indicated at 38 with an aqueous solution containing the melting point depressing medium or substance(s) (freezing point reducing agent) introduced as indicated at 36, and is preferably acidified to a pH of about 1.5 to 5, preferably 2 to 3 by an additive added as indicated by line 35 in Figure 1. The wet pulp mass is then brought to the desired consistency as indicated at 42 for the ozone stage which may be any conventionally used in an ozone bleaching stage, but as above described preferably will be in the high consistency range i.e. about 20 to 60 % for example by pressing to the desired consistency, most preferably, at about 35 to 45%. The so impregnated pulp is fluffed as indicated at 46 to obtain the desired open structure of the pulp mass to ensure better contact between the pulp and the ozone in the ozonation stage 16.

The fluffed pulp is then cooled in a suitable cooling zone 20 and passed into the ozonation zone 18 wherein the pulp is maintained at a reduced temperature (i.e. below ambient room temperature) of less than 0°C and preferably at about - 20°C or lower, and is contacted as uniformly as possible with an ozone containing gas stream. Other than the low temperature the conditions in the ozonation stage are those normally used for the ozonation reaction. The pulp may, as above indicated, be bleached at any suitable consistency (i.e. medium (- 12%) and low (- 1%)), but in the present disclosure only examples of high consistency (-40%) ozonation are provided since as above indicated generally only high consistency bleaching will reduce the cooling requirements.

The ozonation zone 18 will normally apply between 0.3 and 3 % ozone based on the oven dry weight of the pulp.

After treatment in the ozonation stage the pulp may be treated in the conventional manner, for example, it may be subject to further bleaching stages, for example one or more further ozonation stages, peroxide stages or brightening stages with a chemical based on hydrogen peroxide to bring the pulp to the final desired brightness.

Examples

Experimental Procedure

The ozone bleaching stage was carried out in a standard gas wash bottle having a fritted glass disk and a side inlet at the bottom. The fluffed pulp was placed in the bottle and formed a column of pulp resting on the fritted glass disk and filling a significant part of the wash bottle. The pulp was contacted with an ozone containing gas stream which entered the side inlet below the fritted glass disk and passed up through the fritted glass disk which distributed the gas uniformly over the lower area of the column of fluffed pulp so that the gas was uniformly applied to and could flow upwardly through the column of pulp.

Four levels of temperature are investigated: room temperature, -20°C (obtained by immersion of the gas wash bottle in a mixture of ice and sodium salts), -40°C and -60°C ( the latter two temperatures are achieved by immersion in a mixture of dry ice and acetone).

About 5 grams of fluffed unbleached pulp at approximately 40% consistency were contacted in the wash bottle with a 3.83% (by weight) ozone in air mixture introduced at a flow rate of 1.01 1/min. The unreacted ozone leaving the flask was captured in a wash bottle filled with a KI solution. The quantity of ozone captured in this manner was measured by iodometric titration.

The ozone charge was varied by changing the time that the ozone-air mixture flowed through the pulp. Example 1

Kraft Hemlock pulp (kappa no. 31.9 ml, viscosity 35.8 mPa.s according to TAPPI standard 230 om-82) was treated with ozone at -60°C ± 2°C using a methanol-water (70/30 w/w) mixture as the liquid medium and at a pH of 1.8. This medium (solution) does not freeze at -60°C.

Three ozone charges, 2, 4 and 8% were used. The kappa no. and viscosity of the thus obtained pulps were analyzed and the results are listed in Table 1. Included in the table are also the respective kappa no. and viscosity of the same unbleached pulp ozonated at room temperature (23°C) and impregnated with acidified water or acidified methanol-water (70/30 w/w) medium both at the above specified pH. The ozone bleaching results at -60°C and 23°C are also displayed in Figures 2 and 3 as respectively the kappa number versus ozone consumption and viscosity versus kappa number. Figure 2 shows that the delignification with methanol-water as liquid medium is much more efficient at -60°C than at 23°C. For example, to delignify to a kappa number of 10.0 ml at -60°C and to a kappa number of 9.2 at 23°C, the corresponding ozone consumptions are 0.94% and 1.73% respectively. Moreover, Figure 3 shows that the pulp viscosity changes only from 35.8 to 30.4 mPa.s at -60°C when the kappa no. decreases from 31.9 to 10 ml, compared to a viscosity decrease from 35.8 to 19.9 mPa.s when the ozonation is performed at 23°C and approximately the same delignification is achieved. Therefore, this example shows that ozone bleaching at -60°C with methanol-water as liquid medium leads to a significant improvement in the (lignin-carbohydrate) ozone selectivity.

TABLE 1

Ozone bleaching with ozone charges of 2, 4 and 8% respectively

Ozone bleaching is performed in three consecutive stages with charges of 1.08, 1.08 and

0.54% in respectively the 1st, 2nd and 3rd stage Example 2

In this example ozone bleaching was performed with the same unbleached Hemlock kraft pulp (see Example 1) but at -40°C ± 2°C. A methanol-water solution (70/30 w/w) acidified to a pH of 1.8 is used for impregnation to prevent freezing. After ozonation the pulp is washed with large amounts of tap water, made into handsheets and air-dried. The kappa number and viscosity of the air-dried sheets are summarized in Table 2.

Table 2 shows that ozone bleaching at -40°C is superior to that at room temperature in terms of minimum viscosity decrease and ozone consumption per kappa number unit removal. At -40°C the pulp viscosity decreases only by about 5 units when the kappa number decreases from 31.9 to 9.4 ml. About 0.05% O₃ is consumed per kappa number unit decrease at -40°C, while about 0.1% of ozone is required to achieve one kappa number unit reduction at 23°C, confirming that delignification by ozone is much more efficient at low temperature.

TABLE 2

* Ozone bleaching with ozone charges of 2, 4 and 8% respectively ** Ozone bleaching is performed in three consecutive stages with charges of 1.08, 1.08 and

0.54% in respectively the 1st, 2nd and 3rd stage

Example 3

In this example, the same Hemlock kraft pulp (kappa no. 31.9 ml, viscosity 35.8 cP according to Tappi standard 230 om-82) was used. Ethanol-water (70/30 w/w) liquid medium was used. The pH was adjusted to 1.8 with 4N sulfuric acid. The temperature was controlled at -18 ± 1°C. Ozone bleaching was performed in four consecutive stages with 0.5% ozone charge in each stage. After each stage the pulp was washed with tap water, reimpregnated with the acidified ethanol-water solution, refluffed, and then again ozonated. The pulp viscosity and kappa no. after 4 stages of ozone bleaching are 29.2 mPa.s and 11.9 ml respectively, for a total ozone consumption of 0.98%. At a similar kappa number the pulp viscosity after ozone bleaching at room temperature with a 70/30 (w/w) ethanol-water mixture as impregnation liquor is about 22 mPa.s, while the ozone consumption is 2.05%. Again this example shows that ozone bleaching at low temperature can dramatically decrease the cellulose degradation and that the ozone consumption can be reduced by a factor of 2 while maintaining the same degree of delignification. Example 4

ALCELL^® pulp (kappa no. 25.5 ml, viscosity 27.5 mPa.s according to Tappi standard 230 om-82), produced by cooking hardwood chips in a mixture of ethanol and water, was treated with ozone in the manner described in the Experimental Procedure. Ethanol-water (70/30, w/w) was used as impregnation liquid to prevent freezing of the aqueous medium (solution). The pH of the ethanol-water solution was adjusted to 1.8 by the addition of sulfuric acid. After ozonation the pulp was washed with large amounts of tap water, made into a handsheet and air-dried. The kappa number and viscosity of the air-dried sheets prepared after ozonation at -40°C ± 2°C and -18°C ± 2°C are listed in Table 3. It shows that ozone delignification at low temperature is very efficient, and that the decrease in pulp viscosity is minimal when compared with pulp impregnated with acidified water and ozonated at room temperature. Example 5

The freezing point of an aqueous solution can also be reduced by addition of salts. In this example, the standard Hemlock kraft pulp was impregnated with acidified (pH ~2) water containing 20% (by weight) Na₂SO₄. The ozonation was performed at -3°C and the results are shown in Table 4. TABLE 3

10

Initial kappa no. 25.5 ml, ozone bleaching with ozone charges of 2, 4 and 6% respectively

Initial kappa no. 38.7 ml, ozone bleaching is performed in three consecutive stages with charges of 1.08% in each stage

15

TABLE 4

* Experiments performed with Hemlock kraft pulp; kappa no. 31.9 ml, viscosity 35.8 mPa.s

When comparing pulps of the same kappa number in Table 4 after ozonation at -3°C and 23°C, it is again obvious that the (lignin-carbohydrate) selectivity and delignification efficiency is better when ozonation is performed at the lower temperature. Example 6

1, 4-dioxane was added to the acidified (pH = 1.8) aqueous solution to prevent freezing of the impregnation liquid. The weight fraction of 1,4-dioxane in the liquid medium (impregnation liquid) was 70%.

The pulp properties after ozonation of the kraft Hemlock pulp at -18 ± 1°C are given in Table 5. Also shown in Table 5 are the pulp properties when ozonation is performed at room temperature. Comparison of the properties of the ozonated pulps produced by bleaching at -18°C and 23°C shows again that the (lignin-carbohydrate) selectivity and delignification efficiency are improved at the low temperature, although the improvements are not as large as with mixtures of ethanol-water and methanol-water presented in earlier examples.

TABLE S

* Experiments performed with Hemlock kraft pulp, kappa no. 31.9 ml, viscosity 35.8 mPa.s,

Example 7

The favourable bleaching characteristics of low temperature ozonation are especially pronounced when relatively large ozone charges are applied and the ozonated pulp is subsequently extracted with caustic. Thus, when the standard Hemlock kraft pulp is impregnated with a methanol-water (70/30, w/w) mixture and then treated with a charge of 12% ozone (2.07% consumed on o.d. pulp) at -40°C and 40% consistency in the manner described in the Experimental Procedure, a pulp of 6.2 ml kappa number and a viscosity of 27.1 mPa.s is obtained. Subsequent caustic extraction of the ozonated pulp with a 2% NaOH charge (on o.d. pulp) at 10% consistency and 60°C for 1 hour results in an ozonated and extracted (ZE) pulp of kappa no. of 2.7 ml and viscosity of 25.0 mPa.s.

Although the disclosure has shown examples of softwood kraft pulp and hardwood ALCELL^® pulp, the invention is also believed to be applicable to hardwood kraft pulps, oxygen bleached pulps and other pulps produced by a chemical or organic solvent based pulping process, and is expected to produce equivalent improved results compared to those obtained with a conventional ozone treatment. Obviously the absolute values of the viscosity and kappa no. will reflect the type of pulp being processed thus the term equivalent to a Northern softwood pulp is to be interpreted as requiring suitable scaling of the absolute values normally valid for the other above mentioned pulp types. It is believed that the cold temperature process described above tends to convert the system to approach reaction dependency as opposed to diffusion or access dependency.

Having described the invention modifications will be evident to those skilled in the art without departing from the scope of the invention as defined in the appended claims.

Claims

I. An ozone bleached pulp substantially free of chlorine and having a viscosity equivalent to a viscosity of about 30 cP and a kappa no. of 10 ml for Northern softwood pulp. 2. A method of ozone bleaching of a chemical pulp comprising impregnating said pulp to provide an impregnated pulp containing a medium composed of a solution of water and a freezing point reducing agent, cooling said impregnated pulp and then subjecting said impregnated pulp to an ozone bleaching step at a low temperature below 0°C to produce an ozone bleached pulp having a viscosity at least substantially equivalent to a conventionally chlorine bleached chemical pulp.

3. A method as defined in claim 2 wherein said low temperature is a temperature below -10°C,

4. A method as defined in claim 3 wherein said low temperature is a temperature of at most -20°C. 5. A method as defined in claim 2 wherein said freezing point reducing agent is selected from the group consistmg of low molecular weight aliphatic alcohols.

6. A method as defined in claim 3 wherein said freezing point reducing agent is selected from the group consisting of low molecular weight aliphatic alcohols.

7. A method as defined in claim 4 wherein said freezing point reducing agent is selected from the group consisting of low molecular weight aliphatic alcohols.

8. A method as defined in claim 2 wherein said freezing point reducing agent is selected from the group consisting of methanol and ethanol.

9. A method as defined in claim 3 wherein said freezing point reducing agent is selected from the group consisting of methanol and ethanol. 10. A method as defined in claim 4 wherein said freezing point reducing agent is selected from the group consisting of methanol and ethanol.

II. A method as defined in claim 2 wherein said freezing point reducing agent is dioxane.

12. A method as defined in claim 3 wherein said freezing point reducing agent is dioxane.

13. A method as defined in claim 4 wherein said freezing point reducing agent is dioxane. 14. A method as defined in claim 2 wherein said freezing point reducing agent characteristics include a) ability to lower the freezing (or melting) point of the resulting aqueous medium to a temperature below 0°C, and b) form a single homogeneous liquid phase when mixed with water at the desired low temperature.

15. A method as defined in claim 3 wherein said freezing point reducing agent characteristics include a) ability to lower the freezing (or melting) point of the resulting aqueous medium to a temperature below -10°C, and b) form a single homogeneous liquid phase when mixed with water at the desired low temperature.

16. A method as defined in claim 4 wherein said freezing point reducing agent characteristics include a) ability to lower the freezing (or melting) point of the resulting aqueous medium to a temperature below -20°C, and b) form a single homogeneous liquid phase when mixed with water at the desired low temperature.

17. A method as defined in claim 1 wherein said freezing point reducing agent is a suitable inorganic or organic salt.

18. A method as defined in claim 14 wherein said freezing point reducing agent is a suitable inorganic or organic salt.

19. A method as defined in claim 17 wherein said freezing point reducing agent is salts collected from a precipitator of a waste pulping liquor recovery process. 20. A method as defined in claim 18 wherein said freezing point reducing agent is salts collected from a precipitator of a waste pulping liquor recovery process.