A Process for the Chemical Pulping of

Lignocellulosic Material

This invention relates to an improved process for the chemi- lßcal pulping of lignocellulosic material, particularly wood chips, but it is equally applicable to straw, bamboo, bagas- se etc.

EP-A-12 775 discloses a process for the chemical pulping of 20lignocellulosic material utilizing an acidic nitrate cooking procedure in the presence of aluminum sulfate, or an alkali- ne nitrate cooking procedure in the absence of aluminum sulfate. EP-A-22 881 and WO 81/00267 relate to a si ilar subject matter disclosing aluminum salts other than alumi- 25num sulfate to be used in the acidic nitrate cooking proce¬ dure. In both latter publications the alkaline nitrate cook¬ ing procedure utilizing a pH of 11 to 13 is also carried out in the absence of an aluminum salt.

^ao From US-A-1 , 717,798 a process for t e chemical pulping of lignocellulosic material is known, --. r m cooking ith a neutral pH alkali nitrate solutior. . -. S J. The cooking is accomplished with pressures of aboui o to 3 bar and at tem- peratures of about 150°C for a per: : :f at Least 2 hours gc followed by draining andseveral _^ . „ : .-. a cold solution of chemical leached from the cooking :: . -. -.

1

It has now been found that the alkaline nitrate cooking process can be accelerated and provides a fully defibered chemical type pulp if it is carried out in the presence of an aluminum salt, such as aluminum sulfate. 5

Accordingly, the invention relates to an improved process for the chemical pulping of lignocellulosic material, par¬ ticularly wood chips, straw, ba boo and bagasse, wherein said lignocellulosic material is subjected to an alkaline nitrate cooking procedure at a pH of 11 to 13,5, preferably pH 12 to 13, which process is characterized in that the pulping is carried out in the presence of an aluminum salt, preferably aluminum sulfate.

15

The aluminum salt to be utilized in the present invention should be soluble in the pulping liquor. Aluminum sulfate (alum) is preferred and the commercial products having var ing amounts of crystal water (13 to 18 oles of water __per Al_(SO.)^ ) can readily be used. Other suitable alumi¬ num salts are reported in WO 81/00267 incorporated herein by reference.

With the process of the invention the pulping is carried ₅ -out in an alkaline nitrate solution at a pH of 11 to 13,5 , preferably 12 to 13. To reach the basic pH as mentioned above, usual and normally available alkalis and/or earth alkalis can be used, such as caustic soda, potassiu hydro¬ xide, ammonium hydroxide and calcium hydroxide. Preferably 3 _Q caustic soda is used. The necessary amount is determined by the above stated pH which is to be reached.

The nitrate ion concentration in the alkaline pulping liquor which is neccessary for achieving the intended ob- 35Ject is at least 0.05 percent, generally from 0.1 to 1.0 percent and preferably from 0.1 to 0.5 percent, each by weight of the total pulping liquor. It can be achieved by adding a suitable amount of either nitric acid or nitrate

salts, such as sodium nitrate, potassium nitrate or ammo¬ nium nitrate to the alkaline pulping liquor. '

It is preferred to provide the above stated nitrate ion concentration by the addition of the respective amount of 5 nitric acid to the alkaline pulping liquor. In this case, the aluminum salt, in particular alum, is preferably pre- mixed with the nitric acid before the addition to the pul¬ ping liquor. Alternatively, the aluminum salt can be added to the pulping liquor simultaneously with the nitric acid or immediately thereafter, e.g. within a few minutes, but premixing of the nitric acid with the aluminum salt is preferred.

The choice of alkali and nitrate source usually is determi- lδned by the rate of activity desired, cost, safety and choice of residual chemical ions. Normally the most practical for pulping for paper products will be a combination of NaOH and HNO,.

For obtaining practically desirable results, the amount of 0alum, as the preferred aluminum salt, used is for example 1 part by weight of alum per 10 parts by weight of H O^ (100 %) . In terms of aluminum concentration this means that about 0.08 parts by weight of aluminuπv and aluminum ions, respectively, are present for each 10 parts by weight of 5nitrate (N0. .

While it has been noted that there is a minimum amount of aluminum to achieve a remarkable effect, e.g. 0.01 , 0.02 , 0.03 or 0.04 parts by weight of aluminum per each 10 parts 0of NO.., there is actually no strict upper limit, and the aluminum concentration could go far beyond 0.08, for example up to 0.10, 0.15, 0.2, or even more each per 10 parts of NO.,. It has been found, however, that with exceeding a cer- tain aluminum level no further advantage as to time reduc- 5tion is obtained so that higher amounts are not desirable under economic and environmental pollution point of views. Also, sometimes with a very high aluminum concentration the quality of the end product is affected. Thus, the pre-

ferred ränge of aluminum is from 0.04 to 0.16, more pre¬ ferably from 0.6 to 0.12 parts by weight of aluminum per ~ each 10 parts by weight of 0 ₃ , a value of about 0.08 giving desirable results in the practice of the process of the invention. The latter value of 0.08 parts by weight of aluminum, in ter s of alum (A1 ₂ (S0.)_ x 18 H_0) corresponds to 1 part by weight of alum per 10 parts by weight of

'nitrate.

The alkaline nitrate pulping process of. the invention, depending on the lignocellulosic material to be pulped and the planned end use, is carried out at temperatures of up to 100°C, preferably 85 to 100°C, specifically 95 to 99°C, e.g. 96°C for wood Chips, without the appliance of -pressure or vacuum being necessary, although pressure or vacuum can be applied for Special purposes, if desired. Pressure can be permitted but temperature should be held close to a maximum of 100°C. Higher temperatures will de- grade pulp quality. Depending on the type of fiber to be opulped and the planned end use, pulping time varies from a few minutes to a few hours, e.g. 15 minutes to 4 hours. Pulping time should be selected carefully to opti ize subsequent defibration, i.e. pulp quality levels, energy input, and where necessary, as in printing paper pulps, 5minimal chemical action to maximize pulp brightness and opacity.

The process of the invention can be carried out in open ves- sels as one-step process providing a fiber yield of 85 - 90% of the woody material treated. 0

The process of the present invention provides a high quality pulp in the above mentioned high yield having a high tear and high bursting strength, in particular as shown in the following comparison in regard to the economic and strength benefits of the pulp product of this pulping System expres- sed in the following table by fiber length fractionation, energy requirement and yield percentage.

OMPI

U.S.Southern Spruce Spruce Pine Thermo- Inven¬ Kraft Process mechanical tion Pulp Pulp Pulp

1. Fiber Fractiαn (McNett) Screw R 14 - Long Figer 74.1 % 5.0-7.0 % 65,58 % " R 30 - Medium Fiber 11.5 24.0-27.0 13,62

2. Energy H.P. Days/Ton 10 130 30

3. Yield 55-65 % 90-92 % 91 %

10

In addition, the present invention has the advantage of re- ducing pollution problems to a inimum since the formation of gases or odors which affect the environment can be avoi- ded or, even if produced, can be easily controlled and eli- iδminated. The residual pulping liquor can be recycled and made up again to strength with the necessary amounts of the three necessary Chemicals (alkali, nitrate and aluminum) . After having recycled it 25 times the liquor can be either neutralized and discarded as an effluent (e.g. into the ⁽ - ⁾ biologically treated sewage System) or used again, after concentration, in the paper making process as additive to the fiber furnish or burned as in the conventional kraft pulping System.

⁵ According to the process of the invention the material to be pulped can be passed through an initial impregnation step in water, if desired, at a temperature of 100°C or nearly 100°C, or more expeditiously with steam. Depending on the fibrous material to be pulped and the intended end 0 use, the impregnation time can vary from 15 minutes up to

3 hours, preferably it ranges from 0.5 to 1.5 hours.

With the process of the invention it is preferred to add the alkaline pulping liquor alone to the material to be 5 pulped after carrying out the impregnation step (if any) with hot water. The aluminum salt/nitric acid mixture should then be added immediately or shortly following, e.g..

O PI

¹ within 1 to 2 minutes.

Taking specifically the use of U.S. Southern Pine pulping, for example, takes place at an alkali concentration star- 5ting at 2.88 % and ending at 2.08 %, each calculated as NaOH, with a starting nitrate ion concentration of 0.3 % and an aluminum sulfate concentration of 0.02 % (0.67 parts by weight of alum per 10 parts by weight of nitrate), each by weight of pulping liquor, which is a typical formulation l^for most fibrous structures to be pulped. Preimpregnation with hot water or steam and time period of cooking are the main variables in effecting ease of defibering.

15The following example further illustrates the invention.

E x a m p l e

Spruce wood chips in the amount of 13Q0 g A.D., equivalent

20to 1000 g B.D., are immersed in a pulping solution of 5000 ml and held at 100°C in a non-pressure Container for 4 hours.

The pulping liquor is made up as follows:

NaOH, 50 % 150 ml

25 HN0 ₃ , 50 % (containing

1 g alum per 10 parts of

HN0 ₃ (100 %) ) 15 ml

Water, added to 5000 ml

30After the end of the pulping period the black liquor is drained from the Chips and recycled. The obtained fiber is soft, very light in color and shows a relatively low per- centage of delignification. The yield is 89 %. Test results on defibered, screened pulp (hand sheet) were as follows: 35

Test

Basis weight, g/m ² 117.5 132 122.5 121

Schopper Riegler, degree 40 36 34 43 δApparent Density, g/cm ³ 0.522 0.530 0.506 0.519

Burst, kPa 431 451 416 416

Elmendorf Tear, mN 1815 2266 1844 1972

Tensile, m 5149 4090 4707 5124

Elongation, % 2.2 2.7 2.3 2.3

Ring Crush, kN/m; C .DD..|

M.D.) 1.10 1.23 1.27 1.29