WO2002099185A1

WO2002099185A1 - Chemical pretreatment step in pulp washing

Info

Publication number: WO2002099185A1
Application number: PCT/SE2002/001081
Authority: WO
Inventors: Richard Andersson
Original assignee: Metso Paper, Inc.
Priority date: 2001-06-07
Filing date: 2002-06-05
Publication date: 2002-12-12
Also published as: SE0102012D0; SE0102012L

Abstract

By subjecting pulp to a chemical pretreatment step involving the adjustment of the level of cations, preferably Mg2+, the bulk and swelling of the fibers can be reduced, improving the drainage and compression properties of said fibers.

Description

Chemical pretreatment step in pulp washing

The present invention concerns an improved process for washing of fibrous pulp, such as cellulose pulp and in particular a process for influencing the swelling and bulk of the fibers.

Background of the invention

The production of chemical pulp involves several unit operations. The main steps of that part of the process called the fiber line are: cooking, sieving, oxygen deligninfication, bleaching, and washing. Devices for washing the pulp are located at several positions, such as after cooking, before and after oxygen delignification, and in association to the various bleaching steps. Washing of the pulp is an important process step where dissolved or added material is removed in order to recycle chemicals, and to improve the oxygen delignification and bleaching of the pulp with regard to chemicals consumption, yield, process selectivity, and quality of the final pulp. For example, high organic (un-oxidized COD) carry-over to the delignification or bleaching stages will have negative impact on the chemicals consumption arid pulp quality.

There are several different devices for washing pulp: various diffusers, filters in one or several steps, as well as wash presses and dewatering presses. All these devices have that in common, that it is favorable to reach as highly concentrated pulp as possible in order to secure a good result of the wash, low water consumption, improved process control, and a lowered chemicals consumption - if any - in the consequent steps.

Prior art

EP 709 521 Al discloses a method of enhancing the process of pulp washing comprising the use of a surfactant of the formula:

R-X(CH₂)CH₂O)_nYZ

wherein R is alkyl or alkyl aryl, X is O=C-O, O=C-N, or O; n is from about 1 - 30; Y is v hydrogen, sulfate, sulfonate, phosphate, or carboxylate; and Z is a water soluble cation, wherein Z is not present when Y is hydrogen. The water soluble cation is preferably chosen among Na⁺, K⁺, NH₄ ⁺, Ca²⁺, Mg²⁺ or H⁺. Thus, the surfactants according to EP 709 521 Al may be either nonionic or anionic. U.S. 5,273,625 discloses a method of treating sodium containing pulp, wherein the pulp is subjected to an ion exchange with a cation (preferably calcium) having a higher valence than the sodium, followed by washing of the sodium cations from the pulp and then forming the pulp containing the cations in an amount of at least 700 ppm of calcium ions into a sheet thereby to facilitate water removal and increase the production rate of the sheet forming machine.

The desired pulp concentration is not always reached, in particular in wash and dewatering presses. The reasons behind this may be related to the particular device or method in question, i.e. how the wash is performed (flow, rpm etc.), the fiber type (spruce, birch, eucalyptus etc.) as well as fiber properties influencing drainage properties and the compressibility of the fiber.

The present invention makes it possible to reach a higher, desired pulp concentration in the output e.g. from wash and dewatering presses in situations where the drainage properties and/or the compressibility of the fiber would be a limiting factor. In other words, the present invention aims at ensuring a high washing performance. The washing efficiency is by some extent dependent on the consistency of the discharge or outlet from the washer. This outlet consistency is primarily a function of the pressure applied in the process, such as in the wash press, and therefore a mechanical issue.

Summary of the invention

The present inventor has surprisingly found that there is chemical aspect that must be considered, and consequently shown that the wash performance can be improved by a specific chemical pretreatment. The present invention makes available a process for washing pulp wherein the pulp is subjected to a chemical pretreatment, through which the bulk and swelling of the fibers is reduced and the drainage properties and compressibility is improved. This chemical pretreatment involves the addition of at least one cation, preferably Mg²⁺ in specific amounts, and at specific locations in the process, as defined in the following description, examples and claims.

Brief description of the drawings

Fig. 1 schematically shows a wash set-up in which un- washed pulp (A) is led to the wash device (10) following dilution to a desired pulp concentration using filtrate (F) and/or wash water (E). Further, a filtrate tank.(l 1) for collection of the filtrate is shown. Fig. 1 also shows seven different positions 1 - 7, corresponding to a) - g) in the above description, for addition of cation(s), wherein positions 1 - 4 corresponding to a) - d) are preferred for the purpose of reaching a desired cation concentration in position A. C denotes wash water added during pulp wash, and D the filtrate obtained.

Fig. 2 shows the results from an experiment applying the process according to the invention, and using oxygen bleached softwood pulp from Sweden. The pulp concentration (%) is plotted against both added Mg²⁺ (mmol/kg dry fiber) and Mg²⁺ concentration (mmol/1).

Fig. 3 shows the results from an experiment applying the process according to the invention, and using oxygen bleached hardwood pulp (eucalyptus) from Portugal. The pulp concentration (%) is plotted against both added Mg²⁺ (mmol/kg dry fiber) and Mg²⁺ concentration (mmol/1) .

Detailed description of the invention

The term "pulp" is used herein to define fibrous pulp, preferably chemical pulp such as softwood pulp or hardwood pulp, or mixtures thereof. An example of hardwood pulp is eucalyptus pulp.

The term "chemical pretreatment" is used herein to define a pretreatment through which the bulk and swelling of the fibers is reduced, thereby improving the drainage properties and compressibility of the fibers.

The term "cation" herein denotes any cation, including univalent, divalent, and trivalent cations (e.g. K , K , K ). According to the present invention, Mg is the preferred cation, and it is preferably used in the form of MgSO₄ .

According to a preferred embodiment of the present invention, the chemical pretreatment is performed by addition to the pulp, before the pulp reaches the wash apparatus in question, one or more cations (K⁺, K²⁺, or K³⁺), preferably at least one cation being Mg, more preferably Mg²⁺ and most preferably in the form of MgSO₄. The cation is added in an amount resulting in the concentration of free added cation in the pulp, before the pulp reaches the wash apparatus, is in the following intervals:

0.1 - 1000 mmol/1 for K⁺, preferably 0.5 - 200 mmol/1 0.05 - 1000 mmol/1 for K²⁺, preferably 0.05 - 100 mmol/1, most preferably 0.1 - 15 mmol/1

0.01 - 1000 mmol/1 for K³⁺, preferably 0.01 - 50 mmol/1, most preferably 0.05 - 5 mmol/1

According to a preferred embodiment of the present invention, the addition of cation and the wash is performed at a pulp concentration of 0.5 - 15 % (w/w), preferably 1 - 10 % (w/w).

According to a preferred embodiment of the present invention, the in-flowing pulp has low consistency (normally a pulp concentration of about 1 to about 8 %, or so called LC pulp) and the out-flowing (washed) pulp has high consistency (normally a pulp concentration from about 25 to about 40 %, or so called HC pulp).

According to a preferred embodiment of the invention the wash of the pulp is performed according to a displacement method, e.g. using a press of filter, preferably a wash press or dewatering press. In order to remove or transport the pulp and filtrate and/or wash water, pumps or corresponding apparatuses can be used.

According to a preferred embodiment of the invention, one or several cations are added either to:

a) the un- washed pulp before addition of wash water,

b) the un- washed pulp after addition of wash water,

c) the filtrate before this is added to the un-washed pulp

d) the wash water before this is added to the un-washed pulp,

e) the wash water during addition of the same to the pulp during wash,

f) the filtrate obtained during wash of the pulp, or

g) the filtrate when in a collection tank (e.g. a filtrate tank).

Among the above alternatives, a), b), c) and d) are more preferred, wherefore one of these preferably is used. The Mg²⁺ may according to the invention also be added in the end of the brown stock washing, if condensate or fresh water is used in the last washer. Alternatively, Mg²⁺ may be added in the end of a high kappa pulp line, if condensate or fresh water is used as wash water.

According to an embodiment of the invention, Mg²⁺ is added on the last post oxygen washer, if condensate or fresh water is used as wash water. The Mg²⁺ may also be added in the bleach plant in low ionic strength positions.

The Mg²⁺ is preferably added just before the pump, which feeds the pulp to the press in question.

As an alternative mode of adjusting the level of cations, the addition of C0 is used. It has been found that the addition of CO dissolves precipitated calcium and magnesium carbonates, present in the pulp, and thereby releases Ca²⁺ and Mg²⁺.

The present invention will now be described in closer detail with the aid of examples and figures, but these are for illustrative purposes only, and are not to be construed as limiting the invention as defined in the attached claims.

Examples

Example 1

As can be seen from Fig. 1, the process according to the invention can be applied as follows:

Un-washed pulp (A) is led to the wash apparatus (10) after dilution to the desired pulp concentration using filtrate (F) and/or wash water (E). Cation(s) can be added at seven different positions, 1 - 7, corresponding to a) to g) above, wherein position 1 - 4

(corresponding to a) to d)) are preferred, in order to reach a desired concentration of added cation in position A. C is wash water added during washing of the pulp. D is filtrate obtained during washing of the pulp.

Example 2

In a series of screening trials, various pulp samples were pressed with a pre-defined pressure and time. The results are summarized in Table 1. Table 1. Screening trials

1) low = pre- washed pulp, medium = mill pulp, high = sodium-rich filtrate added

The conclusion from the above is inter alia that the addition of metal ions to the pulp prior to pressing increased the consistency if the ionic strength was low. If the pulp was laboratory washed, the ionic strength was lowered even further which gave an even more swollen fiber and the pressed pulp consistency was reduced.

Following the screening trials, further experimentation was undertaken. Fig. 2 shows the results from an experiment applying the process according to the invention, and using softwood pulp (Sweden). The pulp was pressed in a laboratory scale wash press. Samples were taken continuously and the pulp concentration determined. Samples were taken when the pulp was well washed, well washed with added cations, and when un-washed. As can be seen in Fig. 2, the pulp concentration increases after cation addition, which illustrates the effect of the present invention.

Example 3

Fig. 3 shows the results from an experiment applying the process according to the invention, and using hardwood pulp (eucalyptus, Portugal). The pulp was pressed in a laboratory scale wash press. Samples were taken continuously and the pulp concentration determined. Samples were taken when the pulp was un-washed at 20°C, un-washed at 85°C, un-washed (S) + 10 mmol MgSO₄, well washed, well washed (N) w + 10 mmol MgSO₄. A clearly positive effect of the added cations can be seen. A pulp concentration of 28.5 % was reached in the last mentioned sample, compared to 26.0 % for well-washed pulp.

Example 4 ^■ -

In an experiment applying the process according to the invention, softwood pulp from the Czech republic was used. The pulp was pressed in a laboratory scale wash press. Samples were taken continuously and the pulp concentration determined. The kappa value for the pulp was initially 70. Samples were taken for un-washed pulp at 20°C, un-washed pulp (S) + 10 mmol MgSO₄, well- ashed pulp and well-washed pulp (N) + 10 mmol MgSO₄. As can be seen, a clearly positive effect was obtained.

Although the invention has been described with regard to its preferred embodiments, which constitute the best mode presently known to the inventor, it should be understood that various changes and modifications as would be obvious to one having the ordinary skill in this art may be made without departing from the scope of the invention as set forth in the claims appended hereto.

Claims

1. Process for washing of fibrous pulp, characterized in that the pulp is subjected to a chemical pre-treatment involving the adjustment of Mg²⁺ to a level effective to improve the drainage properties and compressibility of the fibers by reducing the bulk and swelling of said fibers, whereupon the fibrous pulp is washed.

2. Process according to claim 1, characterized in that Mg²⁺ is added to the pulp in the form of MgSO₄ before the wash, the level of free added Mg²⁺ in the pulp before this reaches the wash step being in the interval of 0.05 to 1000 mmol/1.

3. Process according to claim 2, characterized in that the level of free added Mg²⁺ is in the interval of 0.05 to 100 mmol/1, preferably 0.1 tol5 mmol/1.

4. Process according to claim 1, characterized in that Mg²⁺ is added to one of the following:

a) the un-washed pulp before the addition of wash water,

b) the un-washed pulp after the addition of wash water,

c) the filtrate before addition of the same to the un-washed pulp,

d) the wash water before addition of the same to the un-washed pulp,

e) the wash water during addition thereof to the un-washed pulp,

f) the filtrate obtained during the washing of pulp, or

g) the filtrate when present in a collection vessel.

5. Process according to claim 1, characterized in that Mg²⁺ is added to one of the following:

a) the un-washed pulp before the addition of wash water,

b) the un-washed pulp after the addition of wash water,

c) the filtrate before addition of the same to the un-washed pulp, or d) the wash water before addition of the same to the un-washed pulp.

6. Process according to claim 1, characterized in that the level of free Mg -2+ in the pulp is adjusted through the addition of CO₂.