CA2037275A1 - Steam explosion pulping process for papermaking - Google Patents

Abstract

ABSTRACT

This invention relates to an improved process preparing pulp suitable for papermaking, resulting in lower cost, higher yield and brightness consisting of short time saturated steam cooking of chips impregnated with alkaline solutions of sulphites and carbonates, chlorides, sulfates or bicarbonates in the temperature range of 180 to 210°C
followed by explosive decompression and fiber refining.

Description

BACKGROUND OF THE INVENTI(3N ~ ~

Ingruber et al., Pulp and Paper Manufacture, Volume 4, TAPPI, CPPA, p. 160 (1985) define that convention conventional ultra-high-yield chemithermomechanical or 5 chemimechanical pulping is preferably conducted at a pH level between 4 and 9, and involves either liquid or vapor phase cooking with sodium sulphite-bisulphite solutions for about 10 to 30 minutes at a temperature between 60 and 175C. It is generally accepted that the chemical treatment is mainly responsible for permanent fibre softening, increase in long fiber content, fibre specific surface and conformability, as 10 demonstrated by Heitner et al., Pulp and Paper Can., (84)11: T252-T257 (1983).
There is another softening approach which consists of a steam treatment of chips at high temperatures followed by explosive decompression.
The production of pulp using high-pressure and high steam chip softening w011 above glass transition ternperatures of lignin should theoretically lead to lower energy 15 consumption in subsequent refining stages.
The initial research in the field of high-pressure steam cooking, followed by defibration by explosion, was made by Mason, U.S. Pat. 1 824 221; ? 645 623; 2 494 545; 2 379 8290. The masonite pulp obtained according to a two stage Sprout-Waldron refining procedure showed weak physical strength, dark color and yield loss 20 of 16% ~o 20%, and revealed itself simply unsuitable for the production of paper according Koran et al., Pulp and Paper Can., 79(3~: T107~T-113 (1978). Mamers and al., TAPPI, 64(7): 93-96 ~1981); APPITA, 29(5): 356-362 (1976) investigated explosion pulping of pinus elliotti wood chips with the help of high pressure carbon dioxide solutions and bagasse of wheat straw explosion pulping under high pressure 25 of nitrogen. Paper properties which were obtained were similar to that of CTMP/CMP
pulps, but at the expense of brightness. The major problem to overcome are oxidation, as well as hydrolytic degradation of fibers leading to brightness and yield loss.

rJ ~

It has been suggested by Vit et Kokta, Vit et al., Can. Pat. 1 212 505 (1986) that the ultra-high-yield (90%~) pulp suitable for papermaking can be produced by vapor phase steam explosion cooking. The initial properties of papers mads from exploded softwood chips were similar to those of TMP. However, refining energy was about 5 20% to 25% lower. Recently, a pulping process entitled "Process for Preparing Pulp for Paper Making", Kokta B.V., Can. Pat. 1 230 208 (1987); U.S. Pat. 4 798 651 (1989); Can. Pat. Appl. #542 643 (May 1987), referred to as "Steam Explosion Pulping Process" or "S-pulping" has been proposed both for softwoods and hardwoods. In this process, impregnation and cooking conditions ware aimed at minimizing yield and 10 brightness loss, maximizing resulting paper properties and decreasing specific refining energy. The steam explosion pulping process consists of the chemical impregnation of chips, short duration saturated steam cooking at temperatures varying from 180C
to 210C, pressure release, refining and bleaching ~if necessary).
Kokta et al., Paperi Ja Puu - Paper and Tirnber, 9, 1044-105~ (1989), have 15 shown that the specific refining energy of aspen explosion pulps is at least 50% lower than that of CMP pulp of similar yield and ionic content level, while paper strength increases by up to 50%. C:ornpare at similar CSF levels, explosion hardwood pulps (i.e. aspen, maple, hardwood mixtures, eucalyptus) at 90% yield provide similar or higher paper properties then commercial low yield (_ 50/O) bleached hardwood pulps.

OBJECTS

The object of this invention is to provide a process in which additional yield saving and brightness level increase and cost decrease are obtained when compared 25 to previous invention of Kokta, Can. Pat. 1 230 208 (1987) by substituting sodium hydroxide with milder swelling agents like: sodium carbonate, sodium bicarbonate, magnesium carbonate, magnesium chloride, etc.

f~ ~3 t~ ~ ~J ~
THE INVENTIONI

The major problems accompanying previous processes using expiosive decompression are believed to have been the degradation due to the oxidation of 5 woocl and acid hydrolysis leading to loss in brightness, deterioration of fiber and paper properties and loss of yield. The approach adopted by this invention is therefore to attempt to curtail hydrolytic and oxidative wood degradation and thsreby to protect against loss of yield, brightness and fiber strength. The loss of fiber strength will be particularly great if the degree of polymerization of the cellulose falls below the critical 10 value which is about 500-600. Hydrolytic degradation will also cause yield loss due mainly to degradation of hemi-cellulose.
The process of this invention tries to achieve a positive improvement in the strength of the paper that will be produced from the fibers by increasing the number of hydrophilic groups on the fiber surfaces thereby adding to the potential sites for 15 hydrogen bonding.
The conditions for the achiavement of the foregoing objects in accordance with the process of this invention are as follows:
1 ) The wood fra~ments, having fibers suitable for paper making, such as chips, are in a form in which thorough chemical impregnation can be achieved in a 20 reasonable time.

2) There is an initial thorough impregnation of the chips or other wood fragments by an alkaline aqueous liquor having at least one agent acting to produce hydrophilic groups and as an antioxidant which is capable of protecting the chips against oxidation and develops hydrophilic groups during the cooking stage. The same 25 chemical may act as both an agent to produce hydrophilic groups and as an antioxidant or these functions may be performed by separate chemicals. At the end of cooking the pH should not be lower than about 6.0, so that acids released during cooking will be neutralized. Preferably a swelling agent is also used in the case of high density wood.

3) The impregnated chips are cooked using saturated steam in the substantial absence of air at high temperature and pressure.

4) After cooking, the chips that have been steam cooked are subjected to explosive decompression to result in chips which are softened and mostly defibrated.

5) The defibrated chips are preferably washed and then, without undue delay, and preferably immediately, refined to provide pulp.
The steps of the process of this invention which will for convenience be referred 10 to as the improved explosion process, will now be considered in more detail.

The wood trapments The starting material will normaly be chips in chich the fibers are of a length suitable for paper making. Shavings could also be used but sawdust would be 15 undesirable except as a minor part of the total furnish as the fibers are partially cut.
The chips would also, as is well known, be suitable in the sense of being free from bark and foreign matter.
It is desirable for the purposes of this invention that coarse chips be avoided as otherwise the subsequent impregnation may deposit chemicals only on the chip 20 surface, unless impregnation is carried out for a very long time. Another problem with coarse chips is that cooking would not be complete. It is best to use shredded or thin chips of a 4-8 mm thickness. It has been found that this process is applicable to hardwoods, jack pine and larch, blaGk spruce, doublas fir giving stronger papers at lower refining energy compared with conventional chemo-thermo mechanical or chemi-25 mechanical pulping.

!me@s~
The purpose of impregnation is to protect the chips against oxiclation duringcooking and during transfer from the cooking vessel to the refiner. It is also an objective to provide a positive increase in strength by developing hydrophylic groups 5 on the fiber surface during steam treatment. This will then provide additional sites for hydrogen bonding.
The preferred anti-oxidant is sodium sulphite Na2$O3 which also forms hydrophilic groups, and which is available at a low cost. It is used to provide a concentration of absorbed chemical of about 1 to 15%. Concentrations below 4%
10 would be used where brightness protection is unimportant and high strength is not required. Where, however, brightness is important the sodium sulphite should be at least 4%. If physical properties are important these will be improved by using a concentration of at least 4% sodium sulphite and will be further improved as the concentration is furiher increased towards 16%. The concentration of the solution is 15 preferably about tha same as percent of chemical to be absorbed where there are equal quantities of chips and liquor. For example, a ton of chips of 50% consistency mixed with one ton of 8% solution will result in about 8% absorbed on the pulp. Of irnportance is thorough impregnation to distribute the antioxidant evenly rather than depositing it just on the surface. Other antioxidants that can be used are potassium 20 sulphite or magnesium sulphite. Ammonium sulphite could be used if cooking conditions are not severe or with a buffer. Complexing agents such as ethylene diamine tetracetic acid ~EDTA), sodium diethylene triaminepentacetate (DTPA), sodium tripolyphosphate (TPF) and other complexing agents known in the art as being usable under alkaline conditions may be added to minimize the catalytic effect of 25 metals such as iron on oxidative degradation.
It is desirable also to use a swelling agent to assist the antioxidant or hydrophilic agent in penetrating the wood and this contributes also to softening the chip. This is ~J ~ J
of particular value in the case of high density wood. Suitable swelling agents are sodium or potassium hydroxide or ammonium hydroxide or sodium carbonate or sodium bicarbonate or magnesium carbonate or magnesium sulphate which will contribute also to providing hydrophilic groups. Other swelling agents that can be 5 used and which may be desirable as auxiliary swelling agents for high densi~y wood are zinc chloride, sodium chloride, sodium bromide, magnesium chloride, calcium isocyanate, Schweitzers solution, cupriethylenediamne (C.E.D.) tetraethylammonium hydroxide, dimethyldibenzylammonium hydroxide. The concentration of swelling agent and conditions of swelling must be controlled in such a way as to avoid any 10 dissolution of the hollocellulose. Thus the percentage of swelling agent in the impregnating solution will be in the range of about 1 to 4% depending on the agent and the conditions.
The impregnating solution must be alkaline and have enough free hydroxyl to be able to neutralize the liberated wood acids such as formic acid and acetic acid.
15 Normally the starting pH is about 7.5 or higher and the final pH after steam cooking should be at least 6 or higher.
The time of impregnation at atmospheric pressure in holding tanks typically ranges from about 12 hours to 24 hours at a temperature of about 30C to 60C.
Approximately equal weights of chips and of aqueous impregnating solution can be 20 used. For industrial purposes, however, the time may be shortened to an hour or to minutes by irnpregnating with steam under pressure and at a higher temperature. The pressure should be up to about 1 atmospheric extra pressure at a temperature Gf about 100C to 110C. To improve impregnation the chips should be compressed in advance of impregnation in cool solutions of chemicals. Under thase conditions, 25 penetration will be achieved in a shorter time, but penetration is what predominantly occurs. There i5 no significant cooking resulting in no significant sulphonic and carboxylic groups increasa.

Steam cookin~
The impregnated chips are steam cooked at a high temperature and pressure.
Equipmen~ and methods that can be used for preliminary compacting of the impregnated chips, for cooking the chips with steam and for the discharge of the chips 5 under conditions of explosive decompression and described in Canadian Patent 1 070 537 dated January 29, 1980; 1 070 646 dated January 29, 1980; 1 119 033 datsd March 2, 1982 and 1 138 708 dated January 4, 1983, all of which were granted to Stake Technology Ltd. The equipment used in the examples was acquired from ~hat compagny.
The temperature of cooking should be within the range of about 180C to 210C
and preferably within the range 190-200C, which is in excess of the temperatures considered possible according to the publications of Asplund and Higgins previously referred to. These temperatures correspond with a pressure of 10 atmospheres for 180C and 15.5 atmospheres for 200C. It is these high pressures and temperatures 1 ~ which make a very important contribution to ensuring excellent penetration of the chips by the cooking liquor and results in higher efficience of ionic groups formation on fiber surface.
The cookiny may be preceded by steam flushing under low pressure steam at 100C for a short period such as one minute. This is a matter of convenience, in that 20 with a batch reactor the cooking vessel is initially open to the atmosphere, to eliminate air. This air would be disadvantageous in ~hat it would result in oxidation if it were trapped in the cooking vessel. Additional antioxidant may if desired be added at this stage. Steam flushing is desirable with a batch reactor but would not be necessary for a continuous reactor.
This preliminary treatment is then followed by cooking for about 30 seconds to 6 minutes and preferably about 1 to 4 minutes.
It has been found that within reasonable limits there is a property improvement J~

by increasing the time (min) X temperature (C) multiple assigned as constant K. By increasing this constant from 285 to 760 in the case of black spruce at about the same freeness (157-167) the burst index increased from 3.15 to 4.41 and breaking length from 6.3 to 7.6 and tear from 5.5 to 5.8. Refining energy dropped from 3.2 to 5 3.1 and brightness dropped from 59.7 to 55.1.

Ex~losive decompression After cooking the pressure is instantaneously released and the chips are exploded into a release vessel. If there is to be a delay between release of the chips 10 and refining it is important to cool the chips down by washing them. Washing may also be desirable for the purpose of chemical recovery.
It is desirable immediately to refine ~he chips after explosive decompression.
Otherwise, if the chips are stored, some oxidation will occur with resultant loss of brightness. The rapidity with which this will occur depends on how much residual 15 antioxidant is present at that time and on the temperature of the chips and the extent of exposure to oxygen. Preferably, therefore, refining is immediate so that it is unnecessary to incur the cost of excess antioxidant. In any event, undue delay should be avoided. Such delay is regarded as bein~ undue if oxidation takes place to an extent that will materially affect brightness.
The chips resulting from the explosive decompression are softened and partially defibrated.

Refininq Refining in the experiments described below standards using an atmospheric 25 laboratory refining was conducted at 2% consistency level using a blender coupled with an energy meter model EW 604.
According to A.C. Shaw "Simulation of Secondary Refining" Pulp and Paper Canada 85(6): T152-T155 (1984) the blender results closely match those obtained with industrial refiners. Properties were evaluated af~er preparing paper sheets according to standard CPPA testing methods.
Refining energies are usually low and can be expected to be in the range of 1.7 5 to a, MJ/kg, hardwoods, CSF _ 100 ml, which is considerably lower than that of conventional CMP and similar to that describad in Kokta, Can. Pat. 1 230 208 and U.S. Pat. 4 798 651 ~1989).
The present invention is described in the enclosed example.

~s Freshly cut and naturally grown aspen trees from the Joliette region of Quebec were debarked, chipped and screened at La Station Forestière Duchesnay, Quebec.
Average chip size after screening, was as follows: length 2.5 to 3.75 cm; width: 1 15 2 cm; thickness: 1 to 9 mm with maximum distribution at 5 mm.

ation 150 g of chips (= 50% siccity) were mixed in P!astic ba~s aloncl with 150 ~ of a solution made up of 8% Na2SO3 alone or with 1% of NaOH or MgCI2 or NaHCO3 or MgCO3. Time of impregnation: 24 hours; temperature of impregnation: 60C.
Liquid/chip ratio during impregnation was equal to 3.
In addition, 0.5% DTPA was used in applied cooking liquors.

Cooking Explosion pulps have been prepared using vapor phase team cooking of chemicaly pretreated aspen wood chips. Pulps have been prepared by using cooking temperature 190C, with cooking time 4 minutes.

7~

Cooking took place using saturated steam in a laboratory batch reactor build by Stake Tech. Co. Cooking was preceded by one minute steam flushing at atmosphsric pressure. After cooking, the pressure was instantaneously released and chips which exploded into the release vessel were washed and cooled down with one liter of tap 5 water, and subsequently refined after being stored in a cold room. The reported amount of steam used for cooking varied from 0.5 to 1 kg of steam for 1 kg of chips.
Yield was measured as follows: exploded chips (75 g) were washed wi~h one liter of tap water and subsequently defibrated for 90 seconds in a laboratory blender at 2%
consistency. The pulp was washed again with one liter of water, dried at 105C to 10 cons~ant weight and the resulted weights were compared to the initial O.D. weight of chips.

Refinin~
Laboratory refining was also done using a domestic blender Osterizer B-8614 at 15 a consistency level of 2%. Defibration and refining energy was measured using a HIOKI model 3181-01 powermeter with an integrator. Relative specific refining energy was calculated by substracted bl0nding energy of fully beated pulp from the total energy needed to defibrate and blend the fiber suspension.

20 Propertv evaluation Paper sheets were prepared and tested according to standard CPPA testing rnethods on 1.2 g sheets. Brightness (Elrepho) was evaluated on sheets made with deinosized water. Ionic content (sulfonate and carboxylate ions was determined by means of conductometric titration.

Bleachinq Bleaching was carried out wsing 2% of hydrogen peroxide, 2% NaOH; 0.05% of MgSO4; 2% of sodium silicate; 0.5% DTPA; pulp concentration: 2()%; bleaching time:
2 hours; bleaching temperature: 80C; neutralization with Na2S2O5 to pH _ 5 5;
washing with de-ionized water.

CONCLlJSlONS

In the following Figures 1 to 4, the paper proper~ies of improved steam explosion process are compared to that of ordinary explosion process as defined in Kokta, Can.
Pat. 230 208 (19~7) and Kokta, U.S. Pat. 4 798 651 (1989~ using either 8% Na2SO3 10 or 8% Na2SO3 ~ 1% NaQH.
It is obvious from Figure 1, that substituting 1% NaOH with 1% of MgCI2 or 1%
NaH(::03 resulted in 3% pul~yield increase. Futhermore, this incresement was six percent when 1% MgCO3 was used.
The effect of chemical pre-treatment on brightness is shown in Figure 2. It it 1~ obvious, that brown stock brightness increases from 59.9% of that obtained with 8%
Na2SO3 + 1/c~ NaOH to 64.3%; 65.4%; 64.5% when 1% NaOI I is substituted either with 1% of Mg(:~12 or NaHCO3 or M~CO3.
The increase of yield as well as brightness is obtained in the case of NaOH
substitution by NaHCO3 or MgCO3 without any breaking length lost as demonstrated 20 in Figure 3. On the other hand, system with MgCI2 gave lower strength.
The results in Figure 4 indicates also ~hat by substituting NaOH with either NaHCO3 or MgCO3 leads to the same low level of relative specific refining energy.
The comparable physical strength as well as relative specific refining energy of the above indicated systems can be explained by similar ionic content as indicated in 2~ Figure 5.
In addition, the cost of NaHCO3 (US$ 0.19$/lb) or MgCO3 (US$ 0.75$/lb) is cheaper than that of NaOH (US~ 1 .13$/lb).

Therefore the present invention, consisting in substituting NaOH with carbonatesor bicarbonates results not only in the yield and brightness advantage but also in cost decrease.

Claims (3)

1. A process for ultra-high-yield pulping providing yield in excess of 85% to produce pulp suitable for making paper, which comprises the steps of thoroughly impregnation wood fragments with an alkaline aqueous liquor including a soluble sulphite or other hydrophylic agents capable of acting to provide hydrophylic groups and/or act as an antioxidant, for example water soluble sulphite, sodium sulphite, potassium sulphite, magnesium sulphite alone or in combination with sodium carbonate, sodium bicarbonate, magnesium carbonate, magnesium sulphate, preferably in an amount of.1-16% absorbed by the wood fragments, or other hydrophylic agents, steam cooking the impregnated wood fragments with saturated steam at superatmospheric pressure and at an elevated temperature; subjecting the cooked wood fragments to explosive decompression to partially defibrate same; transferring the partially defibrated fragments to a refiner and refining the softened and defibrated fragment to provide pulp, characterized in that the steam cooking is conducted at the cooking temperature in the range of about 180°C to 210°C, preferably 190°C to 200°C whereby the cooking pressure is about 10 atm to about 15.5 atm.

2. The process of claim 1, wherein the step of impregnation wood fragments is also carried out in the substantial absence of air achieved by replacing air with saturated steam.

3. The process as claimed in claim 1 or claim 2, wherein the time of cooking is in the range of about 30 seconds to about 6 minutes, preferably 1 minute to 4 minutes.