CA1096559A - Process for pretreating particulate lignocellulosic material - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A process is provided for pretreating particulate lignocellulosic material to remove heavy metals and resin before delignification or defibration, which comprises washing particulate lignocellulosic material;
compressing the washed material to a solids content of at least 40% to remove absorbed liquid; impregnating the compressed material with an alkaline aqueous solution comprising alkali and at least one of a heavy metal ion complexing agent and a heavy metal ion reducing agent; heating the impregnated material at a temperature within the range from about 50 to 170°C for up to approximately one hour; compressing the pretreated material to a solids content of at least 40%; and separating undiluted liquor squeezed out during the compression, while maintaining conditions during the pretreating such that the pH of the squeezed-out liquor is within the range from about 4 to about 9.5; thereby separating heavy metal ions, resins and alkali-extracted substances in solution In the expressed liquor.

Description

6~i5~
SPE CIFICATIC~N
In the pulping and defibration of particulate lignocellulosic material - such as wood chips to reduce the material to celluIose pulp, using eitherchemical or mechanical pulping techniaues, or a mixture of both, heavy metals such as iron, manganese ~nd copper interfere by catalyzing the degradation of the lignocellulosic material, reducing brightness and strength, and the decomposition of certain treating chemicals such as peroxide bleaching agentsO The effect on color in part results from the :~ormation of dark-colored compIexes of the heavy metal ions with lignin and lignin derivatives in the woodO The catalytic effect is enhanced at the elevated -temperatures needed or occurring during the delignification, defibration and refining of wood.
Many efforts have been made to alleviate the deleterious effect OI
heavy metal ions and compoundsO Metal chelating agents such as diethylene triamine pentaacetic acid, ethylene diamine tetraacetic acidj nitrilotriacetic acid, and their salts; magnesium chemicals; and chelating phosphate compounds have been added either to the pulping liquor or in a pretreatment of the lignocellulosic material. Such complexing agents tie up the heavy metal ions in aqueous solution in slightly ion;zed complexes, which can - 20 accordingly be separated from the water-insoluble lignocellulosic materialO
Thus, for example, U.S. patent NoO 3, 023,140 to CO Ko Textor patented February 27" 1962, proposes a meth~d f~r producing wood chip refiner pulp in several steps, complexing agents and peroxide bleaching agents being added in one or more of the refining stepsO -U. SO patent No. 3, 701, 712 to Samuelson and Noreus, patented October 31, 1972, provides a process for treating lignocellulose materials .

55~
with alkali in the presence of oxygen and in the présence of a complex magnesium salt of an amino polycarbox~lic acid or aLkali metal salt thereofO
Before carrying out the oxygen/alkali digestion process of the invention, Samuelson and Noreus suggest that it is suitable to pretreat the wood with an 5 aaueous solution containing sulfur dioxide or a sulfite, and, to produce a pulp which is metal-free, to carry out the pretreatment in the presence of a complexing agent for bivalent and/or polyvalent metal ions such as copper, iron, manganese, cobalt and vanadiumO Such chelating agents include the chelating salts of nitrogen-containing polycarboxylic acids, polyphosphates, 10 and ethylenediamine and ethylenediamine derivativesO The wood can be washed with water between the pretreatment and the oxygen/all~ali digestion, but it is indicated that omission of the washing is usually disad~rantageous.
Samuelson and Noreus, UOS. patent No. 3, 769,152, patented October 30? 1973, describe a process for the production of cellulose pulp of 15 high brightness from wood by digestion with al~ali and oxygen in aqueous solution under moderate o~ygen pressure, limiting the amount of alkali at the start of the digestion to less than that required, and progressively addi~.g aLl{ali as the digestion continues, while maintaining the digestion liquor at a pH within the range from about 9. 2 to about 13 It is also indicated that it is 20 particularly advantageous to pretreat the wood before the digestion with water or an aqueous acidic~ neutral, or alkaline solution, preferably in several stages, and preferably at an elevated temperature within the range from about 30 to about 150C. Sulfur dioxide or a sulfite can be present, as well as a complexing agent for bivalent and/or polyvalent metal ions, and the 25 wood can be washed with water between the pretreatment stage and the oxygen digestion stage, although omission of the washing is indicated to usually be disadvantageousO

Jamieson, Samuelson, Smedman and Sondell, U~S. patent No.
4, 05~, 981, patented September 27, 1977, describe a process for improvin~
the selectivity of delignification of lignocellulosic material in $he presence of oxygen gas and aLl{ali by maintaining a carbon monoxide content in the gas 5 phase within the range from about 1% to about 12% by volume. Prior to the delignlfication process of the invention, the lignocellulosic material optionally but preferably is subjected to a pretreatment with water and/or an aaueous solution in one or more stages, to remove metal ions or compounds -such as copper, cobalt, iron and manganese, by dissolution in the pretreating 10 liquor. lrhe pretreatment can be with an acid or alkaline liquor at an elevated temperature, and a chelatin~ or complexing agent for the metal ions Gan also be present, Jamieson et al indicate that a washing of the lignocellulosic material after pretreatment and prior to the delignification process of the invention is desirable.
The difficulty with these processes is that very dilute solutions o~
complexing agents are obtained as waste liquors, which are difficult to recycle, and difficult as well as expensi~e to purify before being run o~
to waste.
In accordance with the process of this invention9 particulate 20 ligDocellulosic material is pretreated to remove heavy metals and resins before ~elignification and/or defibration, which comprises washing par-ticulate lignocellulosic material; compressing the washed material to asolids content of at least 40~C to remove undiluted, absorbed liquid;
impregnating the compressed material with an alkaline aqueous solution 25 comprising alkali and at least one member selected from the group con-sisting of a heavy metal ion complexing agent and a heavy metal ion reduc-ing agent; heating the impregnated material at a temperature within the range from about 50 to 170C for up to approximately one hour; compressing the ~.o~

pretreated ~aterial to a solids content of at leas-t 40~; ar.d separating un~iluted liquor squeezed out during the compression while maintaining conditions during the pretreating such that the pH of the squeezed-out liquor is within the range from about 4 to about 9.5, thereby separating heavy metal ions, resins and alkali-extracted substances in solution in the expressed liquor.
In accordance with the apparatus aspect of the in~ention, there is provided in combination a chip washer, a chip bin having a Bottom in flow connection with a screw press feeder for compressin~ chip material to a solids content of at least 40% and feeding compressed chip material into a vertical impregnating vessel for impregnating chip material with pretreating liquor and ha~ing a vertical screw conveyor for conveyin~ chips therethrough; a pressure reaction vessel ; having at its bottom a screw press feeder having the ability to express and separate pretrea'_ing liquor from the chip material while compressing the chip material to a solids con-tent of at least 4Q%; means for removing liquor expressed in 2Q the screw press feeder, the screw press feeder being in flow connection to a pressur~ vessel having inlet ducts for admission of both steam and compressed air to regulate pressure and temperature in the vessel, and ha~ing at the bottom a screw feeder for conveying and compressing chip material and feeding it to a defi~ration apparatus.
Following the pretreatment the particulate lignocel-lulosic material can be delignified and pulped and/or defibrated by either chemical or mechanical or combined chemical and mechanical delignifying, pulping and defibrating processess.

dm: 1~`'J~ 4-It is especially advantageous in carrying out the process of the invention to control the amount of alkali in the pretreating solution so that the pH of the expressed undiluted and separated pretreating liquor at the conclusion of the pretreatment is within the range from about 4 to about 9.5, and preferably from about 5 to about 7.5. The amount of com-plex;ng agent in the pretreating solution should be within ; the range from Q.05% to about 0.80%, based on the dry weight of the particulate lignocellulosic material.
lQ It is also e$pecially advantageous to have present in the pretreating liquor a reducing agent for heavy metal ions, such as sulfur dioxi~e, sulfurous acid or a sulfite salt. The amount of reducing agent is within the range from about 0.1% to about 3.0%, based on dry weight of the particulate lignocellulosic material.
After impregnation with the pretreating liquor, the particulate lignocellulosic material is heated in a closed vessel at a temperature of at least about 50C up to at most about 170C for a time from about l to about 60 minutes. At ; 20 the conclusion of the pretreating time, the hot particulate s ~4~

~æl lignocellulosic material is subjected to compression to a solids content o~
at least 40%, while the pretreating liquor that is thus expressed is contin-- uously removed. The e2~pressed undiluted liquor contains hea~y metal ions resins, and other al~{ali-soluble substances, extracted from the wood and 5 bound in soluble complex form in the pretreating liquorO No free sulf~lr dioxide is present, If the amount of sulfur dioxide added is optimized in accordance with the invention.
After compression to a solids content of at least about 40%,the ligno-cellulosic material can be digested and thereby partially chemically pulped 10 by heating in a pressure vessel such as a digester with steam and/or compressed air at a temperature within the range from about 20 to about l80C
for from about 1 to about 15 minutes, preferably from about 2 to about 5 minutes, and is then subjected to mechanical defibration such as by refining in a disc refiner or in a screw defibrator of the type sold under the 15 tradename F~OTAPULPER. If comp~essed air is used for pressurizing the vessel, the defibration can be carried out at a lower temperature than is possible when steam is used~ .
Alternatively, the particulate lignocellulosic material compressed to a solids content of at least 40% can be dePibrated directly, without first 20 being heated under pressure.
After the chips have been compressed to a solids content of at least 40~c, and ~he expre~sed undiluted pretreating liquor removed, the lignocellulosic material also can be chemically pulped using any chemical digestion or pulping process, and a suitable aqueous pulping liquor, such as (for example3 an 25 acidic sulfite liquor, abisulfite liquor, or a sulfite liquor, or ~ alkaline liquor such as sodium hydroxide, sodium carbonate, sod;um blcarbonate, or white liquor for sulfate pulping, or sodium hydroxide for oxygen/gas delignification.

~0'~65~9 In a further preferred embodiment of the invention, the pretreated lignocellulosic material aEter treatment Wit]l steam and/or compressed air at a temperature within the range from about 10 to about 180~C under a pressure within the range from about 0. 05 to about 1 h~Pa is bleached simultaneously with defibration in a disc refiner. The bleaching agent is supplied to the disc refiner so that it is mixed with the lignocellulosic material in the vicinity of the circumference of the grinding disc, and in a distance of at least one third of the disc radius from its centerO Any bleachingagent can be used, particularly a lignin preserving bleaching agent such as `a peroxide. It is especially suitable to use a peroxide together with conventional peroxide bleaching agent adjuncts, such as, for example, a mixture of hydrogen peroxide with sodium hydroxide, sodium silicate and magnesium sulfateO When sodium silicate is used, it is especially suitable to apply this separately at the circumference of the grinding discs or at a 15- distance of at most 200 mm from their circumference, and to supply th~
remaining bleaching agent to the center of the grinding discs, or ~t a distance from the center corresponding to at most one quarter of the grinding disc ~
radius. The formation of a hard silicate coating on the surface of the grinding discs can thus be inhibited.
A particular advantage of the process of the invention is that the volume of undiluted wasteliquor containingthe heavy metaI ion compl~ s and aLkali-soluble extracted materials is substantially reduced which means that smaller volumes of liquor have to be handled and discarded or processed for recovery of their chemicals contentO This reduces operating costs, and also ~5 facilitates waste liquor disposalO

A further advantage noted in the bleaching of rnechanical and chemimechanical pulp is the combination OI bleaching with defi~ration, which means that there is no need for a bleaching section in the pulping plantO
The process of the present invention also makes possible the 5 production of a light and strong pulp at a lower energy consumpt~on than has been possible heretofore, using mechanical or chemimechanical pu~ping techniques. The pulp furthermore has a good processability, and is well suited for paper production, giving good dewatering, good sheet formation, and good surface uniformityO It is quite surprising, taking into account the 10 e~perience of the prior art, to obtain such a high brightness and strength at such a low energy consumption, utilizing the process of the invention.
The process of the invention can be applied to any kind of lignocellulosic material, but is especially applicable to woodO Both hardwood and softwood can be pulped satisfactorily using this process. Exemplary 15 hardwoods whieh can be pulped include birch, beech, poplar, cherry, sycamore, hickory, ash, oak, chestnut, aspen, maple, alder ancl eucalyptus. Exemplary softwoods include spruce, fir, pine, cedar, juniper and heml~ckO
The lignocellulosic material should be in particulate form. Wood chips having dimensions that are conventionally employed in pulping processes 20 can be used. The wood can be in the form of nonuniform fragmerlts of the type o~ wood shavings or chips ha~ing an average thickness of at most 3 mm, and preferably wîthin the range from about 0. 2 to about 2 mm Sawdust, wood flour, wood slivers and splinters, wood granules, and wood chunks, and other types of wood fragnnents can also be usedO

The washing of the raw material is carried out und~r condition~
such that impurities a~e removed by dissolution in the water.
It is frequently possible to remove most of the water~soluble impurities by washing the lignocellulo9ic material with water. An improved 5 dissolution is obtained at elevated temperatures.
A suitable washing treatment is carried out using hot water at a temperature within the range from about 60 to about 130C for from 0.1 to about lQ minutes. In the course of the heat treatment in the presence of water, some of the lignocellulosic material is hydrolyzed to give organic 10 acids which dissolve in the solution, for example acetic acid, and the resulting acid solution has an irnproved capacity for dissolution of metal ions or compounds present in the lignocellulosic material. Moreover the wood s~ucture is soEtened.
Aqueous acidic solutions COntaining organic and inorganic acids can 15 also be u~ed or the washing, such as acetic acid, citric acid, formic acid, oxalic acid, hydrochloric acid, sulphurous acid, sulphuric acid, nitric acid, phosphoric acid and phosphorous acid. Such solutiorls canhave apE within the range from about 1 to about 5, suitably from about 1. 5 to about 4, and preferably from abouc 2 to a~out 3. 5, with the contact continued for ~orn about 0.1 to 20 about 10 minutes. Treatment with acidic aqueous solutlons canbe carried out at ambient temperatures, i. e., ~om about 10 to about 30C, but elev~ted temperatures can also be used, ranging from about40 to about 100C. In the case of ral.v lignocellulosic materials, such as wood, such a treatment may be accompanied by hydrolysis of the cellulose, with the formation of additional 25 ac ids .

However, when the delignification process of the invention is applied to paper pulp, it is usually desirable to avoid hydrolysis of the cellulose. In such cases, the time and temperature o the treatment together with the pH should be adjusted 90 that depolymerization of the 5 carbohydrate material in the pulp is kept to a minimum.
For the lmpregnation an aqueous alkaline s~lution is suitably used, such as an aL~ali metal hydroxide or alkali metal carbonate or bicarbonate solution, for example, sodiurn hydroxide, sodium carbonate and sodium bicarbonate solution, the alkaline hydroxides or salts being used singly or lO in admixture.
The alkaline treatment is carried out at an elevated temperature within the range from about 50 to about 170C, suitabIy from about 70 to about 150C, and preferably from about 85 to about 130C, until there has been dissolved in the solution an amount of lignocellulosic material ~ithin 15 the range from about 1 to about 8~c by weight, suitably from about l. 5 to - about 6~c by weight, and preferal~ly from about 2 to about 4~c by weight, kased on the dry weight o~ the lignocellulosic material. The trea$ment time can be within the range from about 0.1 to abollt 1 hour, suitably from about O. 25 to about 0. 75 hour, and preferably from about 0. ~5 to about 0. 75 hour.
- 20 Chelating or complexing agents for the heavy metal ions to be removed are also present. Exemplary complexing agents include the polysulphates, such as pentasodium tripolyphosphate, tetrasodium pyro-phosphate, and sodium hexametaphosphate; isosaccharinic acid, gluconic acid, sodium gluconate, sodium heptonate, lactic acid, dihydroxybutyric 25 acid and aldaric acidj and aminopolycarboxylic acids ha~ing the general formula:

5~
MOOCCH2~
/ N--(C2H ~H)n CH2 COOM

in which A is CH2COOH or CH2CH2O:EI and n is a number within the range 5 from 0 to 5, and M is hydrogen, an alkali metal or ammoniumO
Other suita~le chelating acids include ethylene diamine tetraacetic -acid, nitrilotriacetic acid and diethylene triaminepentaacetic acid, as well as amines9 particularly hydroxy alkyl amines such as mono-, di- and tri-ethanolamine, and diamines, triamines and higher polyamines having complexing 10 properties, as well as heterocyclic amines such as dipicolyIamineO ~i~tures of these complexing and chelating agents can also be used, especially combinations of chela~ing agents that contain nitrogen with chelating agents that do not contain nitrogen.
Particularly useful are the metal complexing agents present in waste 1~ cellulose bleaching li~uors~ which should be alkaiine. Such liquors as indicated above in conjunction with the manganese complexes normally contain complex-ing agents derived from the cellulose, as well as the complexing agents added f~r the purpose of the cellulose process fror~ which the waste liquor is obtained.
Suitable waste liquors are, îor example, waste bleaching liquors, espeoially those from peroxide bleaching processes.
Reducing chemicals which can be employed in the pretreating solution of the invention include sulIur dioxide, aLkali metal bisulfites and bisulfites-, and alXali such as sodium or potassium hydroxide, sodium 25 and zinc dithionite, boron hydride, thiogljTcolic acid, ethanolamine and hydroxylamine.

Figure 1 illustrates in flow sheet form ~n apparatus suitable for carrying out the process of the invention.
In the system shown in Figure 1, the particulate lignocellulosic material such as wood chips is plunged into a chip washer 1, where the 5 chips are washed with water, and then passed to a chip bin 2, the lower end of which opens into a tapered screw feeder 3, which feeds the chips from the bottom of the chip bin to the bottom of the vertical impregnating vessel 5. The screw feeder 3 operates within a housing shell whose walls have a plurality of perforations (not shown) for Eluid to escape from within the she~l, and 10 narr~ws in diameter-towards the outlet, and thus is similar to a continuously working screw press. During passage of the chips through the screw feeder 3, they are compressed, so that impregnated liquid from the washer 1 is e~pressed. This liquor passes forward through the feeder to the bottom of the impregnating vessel 5 where it is drained off via the drain pipe 4 The vertical impregnating vessel 5 has two screws 6 for conveying the chips upwardly through the vesseL The pretreating solution which is impregnated into the chips in this vessel is admitted via the duct 7. The solution does not enter the screw feeder 3 because of the bu~k of compressed chips, which ser~res as a sealing plug,while the e~pressed wash liquor leaves -20 via pipe 4 within the mass of chips before the end of the screw feeder.
After progressing upwardly throwgh the pretreating vessel 6, the impregnated chips enter the top of the vessel 8 and then move downwardly The rate o~ their passage through the vessel is adjusted to give the desired treating ~ 6~
time The temperature in the vessel 8 with the aid of a steam jacket 22 can be held at any elev~ted pretreating temperature within the desired range of from about 50 to about 170Co - The pretreated chips progressing downwardly through the vessel 8 5 eventually reach the bottom of the vessel, and enter a second screw conveyor 9 having a tapering outlet which conveys them to a pressure vessel llo The scre~v conveyor 9 also has a decreasing diameter towards the outlet, so that a sealing plug of chips is formed against the excess pressure in the pressure vessel llo The -feeder 9 is also equipped with a conically shaped 10 ram 9a for compressing the chips to increase the density of the material. The ram is put under pressure with the help of the hydraullc cylinder 21. In this way, the pretreating liquor is e~pressed from the chips during their ~assage through the feeder 9, and drains out through the duct 10, located just before the end of the feeder. This undiluted liquor contains the materials 15 diss`olved in the li~uor in the course of the pretreatment, including chemicals, heavy metal compounds and other all~ soluble ex~acted organic. and - inorganic substances. in the lignocellulosic material.
The outlet of the screw feeder 9 is connected to the top of the pressure vessel 119 in which the chips can be heated with saturated steam admitted 20 through the duct 12. Compressed air can be admitted via the duct 19, to moderate the steam temperatureO The chips progress downwardly through the vessel 11, reaching the screw con~reyor 13 at its-bottom, which feeds the chips to ~he center of the grinder housing of a disc refiner 140 The chips are defibrated and re~ined in the disc refiner 14, so that individual fibers are 25 obtained. Bleaching chemicals are charged to the grinder 14 via duct 15, at a distance from the center of the discs.

The partially defibrate~ rp~9s to the sec~ond disc refiner 16, where defibration is completeclO The defibrated pulp is then screenecl in a pressure screen 17, and cleaned in two steps in a hydrocyclone 18~, The finished pulp is then separated from the systemO
The- following Examples in the opinion o-f the inventors répresent preferrcd embodimcnt~ of the inVe~tion.

-: ' ' . .

13 `

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13~A;~PLE 1 In this Example the plant shown schematically in :Figure 1 was usedO
Spruce logs were made into chips, the average length of which was about 25 mm, average width about 20 mm, and average thickness about 3 mmO
The chips were washed with water at about 85C in the chip washer 1. After the chip washer 1, the chips were conveyed to the chip bin 2, the lower end of which was connected to the screw feeder 3., During the passage of the chips through the screw feeder 3? they were compressed to a solids content of 42~C so tfiat excess liquor was pressed out and escaped through the perfora-tions in the wall of the screw feeder, and withdrawn through the pipe 4 The pretreating solution charged through the duct 7 into the impregnating vessel 5 was an aqueous solution of diethylenetriamine pentaacetic acid, sodium bisulfite and sodium hydro~ideO During impregnation of the chips in the impregnating vessel, each kilogram of chips ~based on its absolute 1 dry weight) absorbed about 1 liter of this solution, and about 0O 3 liter . impregnating liquor accompanied the chips on their surfaces. The impregnation .
was so adjusted that 0O 2% DTPA, 1~o NaHSO3 and 0.. 5% NaOH, calculated as 100% pure, and based on the absolutely dry wood welght,was absorbed in the chipso The impregnated chips then entered the reaction Yessel 8, wherein the temperature was kept at about 9QC with the aid of a steam jacket 22 Af~er a 10 minutes transit time the treated chips reached the second screw conve~or 9, and were compressed to a solids content of 41~C and densified such that about 1. 3 m3 liquor per ton of chips was pressed out, and led away 25 through the duct 10. The liquor contained all~ali-soluble reaction products formed with added chemicals and the organic and inorganic substances in the :'. . ' . .

,.~ , ,~; ,-, ~ 6~9 wo~d, as well as heavy metal ions bonded in the chelate complex. This is also clearly apparent from the compa;rison (Table 1) below, showing the analyses of spruc~ wood ch~ps treated according to the invention (Example 1) and chips not treated at all (Control). The expressed liquor had a pH of 7. 9, 5 and not even a trace of free sulphur dioxide.

TABLE I
Content of metals in spruce wood Control ~untreated) Example 1 Fe SCAN-3 13:62 (mg/kg) 120 58 MnSCAN-C 14:62 (mg/kg) 77 32 - ~ As is appa~ent, th~ pretreatment according to the invention g~ve a considerable reduction of the heavy metal content in the chipso The heavy metal ions remaining in the chips were furthermore complex-bonded.
15 The pretreated chips were fed into the pressure vessel 11, wherein the chips were heated with saturated steam 12 to 95C for 3 minutes.-Compressed air through the duct 19 was applied to regulate the temperature.
The steamed chips were defibrated and ground in the disc refiner 14 so that indivldual fibers were obtained. At a distance from the center of the discs 20 corresponding to half their radius, an aaueous bleaching solution was charged via duct 15, containing 3O0% H202~ 5% Na2SiO3 (42Bé ), 0.03% MgSO~, and 1. 0% NaOH, based on the weight o~ dr~ pulpo Energy consumption for defibration was measured at 0~ 8 MWh per ton finished pulpo The pulp was ~urther treated in a second disc refiner 16~ and in this ^
25 stage 0O 6 MWh per ton pulp was consumed.

The refined pulp was screened in one step in a pressure screen 17, and in two steps in the hydrocyclone 180 The finished pulp was tested -~or brightness, metal content and paper characteristics. The results are shown in Table II, below.
For comparison with the above results, as a control thermomechanical pulp was produced without the pretreatment but otherwise in the plant from the same spruce chips of Figure lo After the chip washer 1, the chips were passed directly to the reaction vessel 8 by means of the screw feeder 3. No.
bleaching chemicals were charged at the first defibrating and refining step 14, and only steam ~2 was used in the pressure vessel 11 in preheating the chips to 125C for 3 minutes Otherwise, the chips were processed in the same way as above~
The energy consumption and analysis results are shown in Table IIo TA~3LE II
:E:xampl 1_ Control Energy consumption (MWh/t) Step 1 0,8 1.1 Step 2 ~ . 0.6 0.7 Fr~eness CSF (ml3 300 275 Shives content of unscreened pulp.
Sommerville (gap width 0< 15 mm) (%) 0O 30 - 0O 65 Brightness SCA~-C 1:62 ~0) 76 6û
Extractives content SC~AN-C 7.62 (DKM %) 0O 55 1. 20 Tensile index ~Nm/kg) 42 30 Tearing indBX (NmZ/kg) lOo O 7.0 Light spreading coefficient (m2/kg) 58 60 Iron (Fe) SCAN-C 13:62 (mg/kg~ 37 101 Mang~nese (Mn) SCAN-C 14:62 (mg/kg) 18 69 The energy consumptlon was reduced by albout 22'3~, by the - pretreatment of the invention with alkali, bisulphite ancl complexing agents, as compared with the Control, which represents the conventional production of thermomechanical pulp. The paper characteristics of the pulp were 5 substantially improvedO
The effect of pretreatment with a complexing agent is esp~cially apparent in a comparison of the brightness of the pulpso The addition of complexing agent has thus increased brightness by 16 units, in comparison with the Control. Especially surprising is the low extractives content 10 achieved using the process of the in~Tention. The method is thus especially valuable for producing pulps which are to be used for absorbent products, e.g., soft paper and fluffed pulp. After impregnation, the liquid canbe recovered in a conventional way, or dissolved-out extracted substances can be regained in the same way as for tall oil extraction.

EX~MPLE 2 In this Example the plant shown schematically in Figure 1 was used.
Spruce logs were made into chips, the average length of which was about 25 mm, average width about 20 mm, and average thickness about 3 mm 5 The chips were washed with water at about 85C in the chip washer 1. After the chip washer 1, the chips were conveyed to the chip bin 2, the lower end of which was connected to the screw feeder 30 During the passage of the chips through the screw feeder 3, they were compressed so that excess liquor was pressed out and es~aped through the perforations in the wall of the screw 0 feeder, and withdrawn thrQugh the pipe 4.
The pretreating solution charged through the duct 7 into the impregnating vessel 5 was an a~ueous solution of sodium hydroxide. During impregnation of the chips in the impregnating vessel9 each kilogram of chips (based on its absolute dry weight) absorbed about 1 liter of this solution, and about 0. 3 liter -~ 5 impregnating liquor accompanied the chips on their surfacesO The impregnation was so adjusted that 0. 5% NaOH, calculated as 100% pure, and based on the absolutely dry wood weight, was absorbed in the chips.
The impregnated chips then entered the reaction vessel 8, wherein - the temperature was kept at about 90C with the aid o~ a steam jacket 22. After ; 20 a 10 mînutes transit time the treated chips reached the second screw conveyor .
9, and were compressed and densified such that about 1. 3 m3 liquor per ton of chips was pressed out, ancl led away through the duct 10. The liquor contained aLkali soluble reaction products formed with added chemicals and the organic and inorganic substances in the wood, as well as heavy metal ions soluble in . ^ .
:25 alkali and in complexes with acids dissolved from the woodO This is also clearly .,: , ~: 18 apparent from the comp~rison (Table m~ below, showing the analyses of sp~lce wood chips treated according to the invention ~33xample 2) and chips not treated at all (CoEItrol)O The expressed liquor had a pH of 8. 2.

TABLE III
Content of metals in spruce wood Control (untreated) Example 2 Fe SCAN-3 13:62 (mg/kg) 120 - 92 Mn SCAN~C 14:62 (mg/kg) 77 64 ~ , As is apparent, the pretreatment according to the invention gave a considerable reduction of the heavy metal content in the chips.
The pretreated chips were fe~ into the pressure vessel 11, wherein the chips were heated. with saturated steam 12 to 95C for 3 minutesO
Compressed air through the duct 19 was applied to regulate the temperature.
15 The steamed chips were defibrated andreflned in the disc refiner ~4 so that individual fibers were obtaine~. At a distance from the center of the discs - corresponding to half their radius, an aqueous bleachin~ solution was charged via duct 15, ~ontaining 3O 0% H2O2, 5% Na2SiO3 (42 Bé), 0. 03% MgSO4, and L 0% NaOH, based on the weight of dry pulp. Energy consumption for 20 defibration was measured at 0. 9 MWh per ton finished pulp The pulp was further treated in a seco~d disc refiner 16, and in this stage 0.7 M~ per ton pulp was consumedO
The refined pulp was screened in one step in a pressure screen 17, and in two steps in the hydrocyclone 180 The finished pulp was tested for , brightness, metal content and paper characteristi~s~ The results ~re sh~
in Table IV, helow.
For comparison with the above results, as a control thermomechanical pulp was produced without the pretreatment but otherwise in the plant from 5 tlle same spruce chips of Figure lo After the chip washer 1, the chips were passed directly to the reaction ~ressel 8 by means of the screw feeder 3. No bleaching chemicals were charged at the first deibrating and re~ining step 14 and only steam 12 was used in the pressure vess~l 11 in preheating-the chips to 125C for 3 minutes. (3therwise, the chips wer0 processed in the same way 10 as above.
The energy consumption and allalysis results are shown in Table IV:
TABLE IV_ Example 2 Control Energy consumption (MWh/t) Step1 0~9 1.1 Step 2 Oo? OD7 Freeness CSF (ml) 305 275 Shives content of unscreened pulp.
Sommerville (gap width 0015 mm) (%) 0~ 43 0. 65 20 Brightness SCAN-C 1:62 (%) 72 60 Extractives con$ent SCAN-C 7:62 (DKM %) 00 68 1~ 20 Tensile index ~Nm/kg~ 39 30 Tearing index (Nm2/kg) g. 5 700 Light spreading coefficient (mZ/kg) ~ 59 60 25 Iron (Fe) SCAN-C 13:62 (mg/kg) 89 101 `~ Manganese (Mn) SCAN-C 14:62 (mg~kg) 61 69 '' ~ .

~96~
The energy consumption ~as reduced by a~out 11~ by the pre-treatment of the invention with aL~ali as compar~d with the t!ontrol, which represents the conventional production of thermomechanical pulp. The paper characteristics of the pulp were substantially imlproved.
The effect of pretreatment is especially apparent in a comparison of the brightness of the pulps. The extraction and bleaching with a~kali has increased brightness by 12 units, in comp~rison with the Control.
Especially surprising is the low e~tractives content achieYed using the process of the invention. The method is thus especially valuable for producing pulps which are to be used for absorbent products, e. g. ? soft paper and flu~Eed pulp. After impregnation, the liquid can be recovered in a conventional way, or dissolved-out extracted substances can be - -~-regained in the same way as for tall oil extr-rlion.

. ' ~.
. ' . ' r EX~MPLE 3 In this Example the plant shown schematically in Figure 1 was used Spruce logs were made into chips, the ~verage length of which was about 25 mm, average width about 20 mm, and average thickness about 3 mmO
5 The chips were washed with water at about 85C in the chip washer lo ~fter the chip ~vasher 1, the chips were conveyed to the chip bin 2j the lower end of which was connected to the screw feeder 3O During the passage o~ the chips through the screw feeder 3, they were compressed so that excess liquor was pressed out and esca~ed thro~gh the perEorations in the wall of the screw 10 feeder, and withdrawn through the plpe 4.
The pretreating solution charged through the duct 7 into the impregnating vessel 5 was an aqueous solution of sodium bisulfite and sodium ~ . . .
hydroxide. During impregnation of the chips in the impregnating vessel, each kilogram of chips (based on its absolute dry weight) absorbed about 1 liter of 15 this solution, and about 0. 3 liter impregnating liquor accompanied the chips . . , ~; on their surfaces. The impregnation was so adjusted that 1% NaHSO3 and - O. 5% NaOH, calculated as lQ0% pure, and based on the absolutely dry wood weight, was absorbed in the chips.
The impregnated chips then entered the reaction vessel 8, w~ierein 20 the temperature was kept at about 90C with the aid of a steam jacket 22 After a 10 minutes transit time the treated chips reached the second screw conveyor g, and were cornpressed and densified such that about 1~ 3 m3 liquor per ton of chips was pressed out, and led away throu~h the duct 10. The li~uor contained alkali-soluble reaction products ~ormed with added chemicals and the organic 25 and inorganic substances in the wood, as well as heavy metal ions soluble in a~ali and in complexes with acids derived irom the woodl This is also clearly , apparent from the comparison (Ta~le V) below, showing the analyse~ ~
spruce wood chips treated according to the invention (Example 3) and chips not treated at all ~Control). The expressed liquor had a pH of 7~ 6, and not even a trace of free sulphur dioxide.
~ TABLE V
Content of metals in spruce wood Control (untreated3 Example 3 Fe SCAN-3 13:~2 (mg/kg) 120 B8 Mn SCAN-C 14:62 (mg/kg) 77 51 ~;~ '~ '' ''.' '' ;
As is apparent, the pretreatment according to the invention gave a considerable reduction of the heavy metal content in the chips.
The pretreated chips were fed into the pressure vessel 11, wherein the chips were heated with saturated steam 12 to 95C for 3 minutes.
15 Compressed air through the duct 19 was applied to regulate the tem~erature.
~;~ The steamed chips were defibrated and ground in the disc refiner 14 so that indi~idual fibers w0re obtained. At a distance from the center of the discs corresponding to half their radius, an aqueous bleaching solution was charged via duct 15, containing 3.0% H202, 5% Na2SiO3 (42 Be), 0.03% MgS0~9 and 2û 1~ 0% NaOH, based on the weight of dry pulp. Energy consumption for defibration was measured at 0~ 8 MWh per ton finished pulp The pulp was further treated in a second disc refiner 16, and in this stage 0. B MWh per ton pulp was consumed.
The refined pulp was screened in one step in a pressure screen 17, 25 and in two steps in the hydrocyclone 18. The finished pulp was tested for brightness, metal content and pape-r characteristicsO The results are shown in Table VI, below.
For cornparison with the above results, as a control thermomechanical pulp was produced without the pretreatment but otherwise in the plant from 5 the same spruce chips of Figure L4 After the chip washer 1, the chips were passed directly to the reaction vessel 8 by means of the screw ~eeder 3. No bleaching chemicals were charged at the first defibrating and refinin~r step 14, and only steam 12 was used in the pressure vessel 11 in preheating the chips to 125C for 3 minutesO Otherwise, the chips were processed in the same way 10 as above.
The energy consumption and analysis results are shown in Table YIo ; . TABLE VI
Exarnple 3 Control . Energy consumption (MWh/t) Stepl 008 lol Step 2 . O. 6 00 7 Freeness CSF (ml) 290 2q5 ~- ~hives content of unscreened pulp.
Sommerville (gap width 0.15 mm) (%) O. 30 00 65 Brightness SCAN-C 1:62 (%) 73 60 -tractives content SCAN-C 7:62 (DKM %) 00 60 1. 20 Tensile index ~Nm/kg) 41 30 Tearing inde~ (Nm2/kg) 9. 8 700 Light spreading coeficient (m2/kgj 59 60 - 25 Iron (Fe) SCAN-C 13:62 (mg/kg) 56 101 Manganese (Mn) SC~N-C 14:62 (mg/kg) 43 69 ;5~i~
The energy consumption was reduced by a~out 22% by the pretreatment of the invention with alkali and bisulphite as compared with the Control, which represents the conventional production of thermomechanical pulp. The paper characteristics of the pulp were 5 substantially improved.
The effect of pretreatment is especially apparent in a comparison of the brightness of the pulps. The addition of alkali ancl bisulfite has increased brightness by 13 units, in comparison with the Control. Especially surprising is the low extractives content achieved using the process of the 10 invention. The method is thus especially valuable for producing pulps which are to be used for absorbent products, e. g., soft paper and fluffed pulp.
After impregnation, the liquid can be recovered in a conventional way, or dissolved-out extractive substances can be regained in the same way as for tall oil extractionO

~' -EXAMP~:E 4 In this Example the plant shown schematically in Figure 1 was usedO
Spruce logs were made into chips, the average length of which was about 25 mm, average width about 20 mm, and average thickness about 3 mm.
5 The chips were washed with water at about 85C in the chip washer lo After the chip washer 1, the chips were conveyed to the chip bin 2, the lo~ver end of which was connected to the screw feeder 3O During the passage-of the chips through the screw feeder 3, they were compressed so that excess liquor was pressed out and escaped through ~he perforations in the wall o~ the screw 10 feeder, and withdrawn through the pipe 4O
The pretreating solution charged through the duct 7 into the im-pregnating vessel 5 was an aqueous solution of DTPA, sodium bisulfite ~`. and sodium hydroxide. During impregnation of the chips in the impregnating . vessel, each kilogram of chips (based on its absolute dry weight) absorbed 15 about 1 liter of this solution, and about 0. 3 liter impregnating liquor accompanied the chips on their surfaces. The impregnation was so adjusted that 0. 2 Yc DTPA, l~c NaHSO3 and 0. 5~c NaQH7 calculated as 100~C pure, and based on the absolutely dry wood weight, was absorbed in the chips.
The impregnated chips then elltered the reaction vessel 8, wherein 20 the temperature was kept at about 90C with the aid of a steam jacket 22.
~fter a 10 minute transit time the treated chips reached the second screw conveyor 9, and were compressed and densified such that about 1 3 m3 liquor per ton of chips was pressed Ollt, and led away through the duct 10.
The liquor contained alkali-soluble reaction products Iormed with added 25 chemicals and the organic and inorganic substances in the wood, as well.
as heavy metal ions bonded in the chelate complexes. This is also clearly ~ 6~
a}~parent from the comparison (Table VII) below, showing the analysis of spruce wood chips treated according to the invention (Example 4) and chips not treated at all (C~ntrol). The expressed liquor had a pH of 7. 9, and not even a trace of free sulphur dioxide.
TABLE V~

Content of metals in spruce wood Control (untreated) Example 4 Fe SCA~-3 13:82 (m~/kg) 120 57 Mn SCAN-C 14: 62 (m~/kg) 77 3~
As is apparent, the pretreatment according to the invention gave a considerable reducti~n of the heavy metal content in the chips. The heaYy metal-ions remaining in the chips wer0 furthermore complex-bonded.
The pretreated chips were fed into the pressure vessel 11, wherein the chips were heated with saturated steam 12 to 95C for 3 minutesO
Compressed air through the duct 19 was applied to regulate the temperature.
The steamed chips were defibrated and~ ground in the disc refiner 14 so that individual fibers were obtained. ~t a distance from the center of the discs corresponding to half their radius, an aqueous bleaching solution was charged via duct 15, containing 3.0% lI202, 5% Na2SiO3 (~2 Bé), 0O03% MgSO4, and lo 0% NaOH, based on the weight of dry pulpo Energy consumption for defibration was measured at 0O 8 MYVh per ton finished pulp The pulp was further ~eated in a second~ disc refiner 16, and in -this stage 0. 6 MWh per ton pulp was consumed.
The refined pulp was screened in one step in a pressure screen 17, and in two steps in the hydrocyclone 18. The finished p~llp was tested for 55~
brightness, metal content and paper characteristicsO The results are sh~Jn in Table ~TIII, belowl, For comparison with the abo~re results, as a control thermomechanical pulp was produced without the pretreatment but otherwise in the plant from 5 the same spruce cllips oE Figure lo After the chip washer 1, the chips were passed directly to the reaction vessel 8 hy means of the scre~V feeder- 3O No bleachmg chemicals were charged at the first defibrating and refining step 14, and only steam 12 was used in the pressure vessel 11 in preheating the chips to 125C for 3 minutesO The pulp was urther treated in a second disc 10 refiner 16, but in this stage an aqueous bleaching solution was charged containing 3. '3~ 2()2~ 5~c Na2SiO3 (42 Bé)9 0. 03~C MgSO~, 1. ~c NaOH and 0.2~C DTPA. Energy consumption in the two defibration stages was 0. 9MWh and 0. 7 MWh respectively per ton pulp. Otherwise, the chips were processed in the same way as above.
The energy consumption and analysis results are shown in Table VllI.

; TABLE V:lll -Example 4 Control - - (refiner~bleached3 Energy corrsumption ~MWh/t3 Stepl 0.8 0.9 Step 2 0~, 6 0. 7 Freeness CSF ~ml) 300 310 Shives content of unscreened pulp.
Sommerville (gap width 0. 15 rnm~ (%) 0. 30 0. 45 ~5 Brightness SCAN-C 1:~2 (%) 76 71 Extractives content SCAN-C 7:6Z (DKM %) Tensile index ~Nm/kg) 42 37 - . Tearing index (Nm2/kg) lOoO 9.2 Light spreading coefficient (m2/kg) 58 59 3~ Iron (Fe) SCAN-C 13:62 (mg/kg) 35 62 Manganese (Mn) SCAN-C 14:62 (mg/kg) 16 49 S~9 The energy consumption was reduced by 12. 5~ by the pre~eatment of the invention as compared with the Conb~oi, which represents the conven-tional production of refiner-bleached thermomechanical pulp. The pape~
characteristics of the pulp were substantially improved.
The pulp produced according to the present invention is thus con-siderably stronger, and above all, considerably brighter, than the Control, 5 refiner bleached thermomechanical pulp. It is thus not solely the addition o~ bleaching chemicals in the first defibrating and refining step that is responsible for the strong and bright pulp of the invention.
No certain e2~planation of the surprising result can be supplied at present. Possibly however the chips during their passage through the two 10 screw feeders are subiected to some kind of predefibration which results in a stronger pulp. The addition of reducing agents and complexing agent, combined with compression, may redu~e the decomposition of the peroxide, which in turn contributes to the high brightness of the pulp.
EXA~PLE 5 Spruce sulphite pulp of the dissolving type was produced according to the prior art, without pretreatment, and also according to the process of the invention. The chips were taken from the plant shown in Figure 1, the con~ol nonprel~reated batch being wood just reduced to chips, while the batch of the invention had ~een treate~ with the alkaline liquor of cornplexing agents, aLkali 20 and bisulphite, according to Example- 1, and was taken out through a hatch 20 on the pressure vessel 11. The steam sup1?ly 12 to the pressure vessel 11 was interrupted, to tal~e out the treated chips. Each batch was 3 kg chips based on dry weight.
The two batches were then subjected to sulphite digestion in a labora-25 tory digester having a volume o-f 25 liters. The untreated control batch was di-geæted first, and then the batch which had been pretreated according to the in~7ention.

5S~
Both batches were digested in the liquid phase, under the foll~i/ing conditions:
- Ratio o~ liquicl (wood moisture included) to dry wood = 4. 5:1 The digestion liquor provide~ a charge of 4~ û% Na20 and 24. 0%
5 S02, calculated by weight of absolutely dry woo~0 Before the addition of the digestion liquor in each batch, the chips were treated with saturated steam at atmospheric pressure for 30 minutes.
Chips and cooking liquor were heated by circulating, the cooking liquor through a heat exchanger. The content of the diges~er was heated m this way to 95C~ for a period of 45 minutes, and then at from 95C to 110C for a further 3 hours, after. which the temperature was increased - to 145C for a period of 60 minutes. The chlps were subse~uently digested at 145C for 3 hours? whereon the excess pressure in the digester was gradually reduced t~ atmospheric pressure during a period of 20 minutes.
15. The digested chips were screened in a laboratory screen of the Wennberg type, with a mesh width of 0 .25 mm, whereon the screened.
pulp was dewatered in a c.entrifuge to. a dry contenl; of about 30%, and was- torn to small pieces before drying at 35C for 16 hoursO The pulp yield and other characteristics of the pulps from each batch are shown belowO
20 . TA13LE IX
Control Example 5 -- - .
Pulp yield (%) 46D 2 47. ~

Shives content (%) û. 83 0.36 Kappa number SCAN-C 1: 59 6. 7 50 8 Extractives content SCAN-C 7:62 (DKM%) 1; 33 00 61 Viscosity SC~N~C 15:62 (Tappi cP) 410 8 560 5 Fe SCAN-C 13:62 (mg/kg) 14 12 Mn SCAN-C 14:62 (mg/kg) 8 - 5 ;55~
As is apparent from the results above, the batch according to the invention gave a pulp with substantially lower extractives con~ent and somewh~ lower content of heavy metal ions. Surprisingly enough, the batch according to the illvention also gave a higher pulp yield and lower shives content than the control batch for a comparable Kappa number.
Possibly in the pretreatment with alkali, inter alia, sodium ions are introduced into the chips, which have a positive effect during digestion.
Another explanation may be that a glucomannan stabilization is obtained during the pretrea~ment of the chips, which has contributed to increasing the pulp yieldO A thir~ possible explanation is that the chips have been exposed to mechanical working during the passage through the screw ~eedérs, resulting in crack formation and points of stress concentration, which during subsequent digestion facilitated the digesting liquor pene~cr~tion into the pieces of chips, and thereby resulted in a more homogeneous sulphonization than that which can be obtained in digestion of chips which have not been pretreatedO
For comparison wi~h Example 5, control runs with pretreatments according to the Samuelson et al patents Nos. 3,~01,712 and 3,769,152 and Jamieson et al No. 4, 05Q, 9~1 were carried out.
Spruce sulphite pulps of the dissolving type were produced accord-ing to these patents using their pa~ticularly su~table and preerred pre-trea~ment processes.
About 6 kg of spruce chips, calculated as oven dry, were taken from the plant shown in Figure 1, the 6 kg batch being wood just reduced to chips. The whole batch of chips was then filled into a pressure vesse~
having a volume ~f about 40 liters. To the vessel was further ch~rged &;5~9 25 liters of a hot (70C) aqueous solution of sodium bisulfite and ~TPA.
The solution contained 30 g/l of NaHSO3 and 2 g/l of DTPA chelating agent.
AEter closillg the vessel nitrogen gas was added until a pressure of 1 MPa was obtained inside the vessel. In order to achieve a good liquor flow and 5 good contact betwee~ the chips and the pretreating solution, the solution was pumped from the bottom of the vessel and fed into the top of the vessel during the entire period o-f treatment, which lasted 60 minutes.
A third, about 2 kg, of the pretreated batch of chips (designated Control 1) was then immediately subjected to sulphite digestion in a 10 laboratory digester having a volume of 25 liters. The digestion was per-formed in liquid phase, under the following conditions:
Ratio liquid, including wood moisture = 4. 5dry wood The digestion liquor contained 4. ~c Na20 and 2~. ~c SO2, calsulated by weight of absolutely dry wood.
Bef~re adding the digestion liquor to the digester, the chips were treated with saturated steam at atmospheric pressure for 30 minutes.
The batch of spruce chips (Control 1) and the cooking liquor were heated by circulating the liquor through a heat exchanger. The content of the digester was heated in this way to 95C for a period of 45 minutes, and then at from 95C to 110C for a further 3 ho~rs, after which the tempera~
ture was increased to 14~C for a period of 60 minutes. The chips were subsequently digested at 145C for 3 hours, whereon the excess pressure in the digester was gradually reduced to atmospheric pressure during a - period of 20 minutes.
The digested ships were scréened in a laboratory screen of the ;559 Wennberg type with aperture widths of 0. 25 mm, whereon the screened pulp w~s dewatered in a centrifuge to a dry content of about 3U~, and was torn to small pieces before drying at 35 (~ for 16 hours. The pulp yield and other characteristics of the Con~ol 1 pillp are shown in Table ~ below.
Another third (2 kg) of the pretreated batch of spruce chips, designated Co}ltrol 2, was washed with tap water for 3 hours. The volume of clean tap water used for the washing amounted to 1800 lLters, i. e., about 9Q0 liters per kg of dry wood washed. In spite of the considerable amount of-water used, traces of DTPA could still he found in the pre-10 treated-chips, which may be explained by the dense structure of the chips alld the fact that the DTPA molecule is comparatively large.
The washed chips of Control 2 were then subjected to sulphite digestion, screening, dewatering and drying in a manner similar to that used for the Control 1 chips.
The last third of the pretreated batch of spruce chips, 2 kgj designated Control 3, was washed in the same way as Control ~, with the exception that the water used was not tap water, but deionized clean water.
Traces of DTPA could, however; also here be found subsequent to the extremely vigorous washing with deionized water. Also the chips of 20 Control 3 were then digested, screened, dewatered and dried in exactly the same manner as the chips OI Controls 1 and 2. ~aeld and other characteristics of Controls 2 and 3 pulps are shown in Ta~le X below. For comparison, the characteristics of the Example 5 pulp-which was pretreated according to the process of the invention, but digested, screened, dewatered 25 and dried in a manner similar to that used for the controls- are also shown in Table X.

6S~

Control 1 Control 2 Control 3 Example 5 Pulp yield ('~c ) 47 3 46 . 4 46 . 1 47 . 3 Shives content (~c) 0.88 O.B0 0. 91 0.36 Kappanumber, SCAN-Cl:59 6.1 6.8 6.5 5.8 Extractives content (DKM~c) 0-79 0. 96 O.q7 0.61 SCAN-C7: 62 ViscosityJ SCAN-Cl5:62 48.0 40.2 50.0 ~6.5 ; (TAPPI cP) Fe, SCAN-C13:62 (mg/k~) 14 18 12 12 Mn, SCAN-C14:62 (mg/kg) 8 9 6 5 As is e~Tident from the results above, the pulp pretreated according to the process of the invention had a substantially low~r shives content and in addition a lower extractives content and a considerably higher viscosity 15 than the conl~ols.
The fact that the Control 2 pulp had such a high content of metals indicates that possibly the chips may have functioned as an ion exchanger, and absorbed keavy metal ions *om the~ wash water. Furthermore, the fact that Control 3, in spite of the e2~ceedingly expensi~e washing with deioniied 20 water, resulted in an inferior pulp compared with Example 5 is surprising, and results in all probability from the absence of compression in hvo stages beo~e and after the pretreatment. This is probably also the case with the pulps of Controls 1 and 2.
- The process of the invention utilizes compression of the ligno-25 cellulosic material before and after the impregnation in the pretreating process of the in~ention. Compression is not disclosed in the Samuelson et al and Jamieson et al references. The data show that the compression steps ;5~
undoubtedly give useful and unexpected results. The process of the inYentLon may thus be chaxacterized by compressing the lignocellulosic material, i. e., the wood chips, before and after impregnation to a solids content of between 40 and 70~c, preferably of between 45 and 55~c, the compression being carried 5 out to such an extent that the material subsequent to said compression is capable of instantaneously absorbing a volume of liquid corresponding at least to 80~C of the dry weight of the wood.
The process of the invention can thus be used to advantage for producing chemical pulp as well. In this connection it is not always 10 necessary to wash the chips beore the impregnation. It can be sufficient to steam the chips and thereafter take them to the screw feeder 3, which is connected to the impregnating vessel 5. For continuous digestion, the impregnating liquid containing complex-bonded heavy metals can be substantially removed by compression in the digester screw feeder 9.
In pr~ducing high-yield pulp according to the invention, complexing agents can also be supplied to the process at other places, e. g. In the refiner 14. Furthermore, the temperature is not limited to what i~ stated for the executed trial. The temperature can thus vary between 20C and 170C.
Compressed air can be used to advantage for regulating the temperature, 20 in accordance with Swedish patent No. 318 ,178.
Neither is the invention limited to the addition of bleaching chemicals in the first defibrating a~t refining step for the production of high-yield pulps. Bleaching chémicals can thus very well be added in a later refining step also.
~5 Bleaching can furthermore be carried out in a separate step, the residue chemicals being recycled to the first defibrating step according to the Swedish pate~t No. 363, 650.

Claims (21)

Having regard to the foregoing disclosure, the following is claimed as inventive and patentable embodiments thereof:

1. A process for pretreating particulate lignocellulosic material to remove heavy metals and resin before delignification or defibration, which comprises washing particulate lignocellulosic material; compressing the washed material to a solids content of at least 40% to remove undiluted, absorbed liquid; impregnating the compressed material with an alkaline aqueous solution comprising alkali and at least one member selected from the group consisting of a heavy metal ion complexing agent and a heavy metal ion reducing agent, heating the impregnated material at a temperature within the range from about 50 to 170°C for up to approximately one hour;
compressing the pretreated material to a solids content of at least 40%; and separating undiluted liquor squeezed out during the compression; while maintaining conditions during the pretreating such that the pH of the squeezed-out liquor is within the range from about 4 to about 9.5; thereby separating heavy metal ions, resins and alkali-extracted substances in solution in the expressed liquor.

2. A process according to claim 1, in which following the pretreatment the particulate lignocellulosic material is delignified and pulped by a chemical pulping process.

3. A process according to claim 1, in which following the pretreatment the particulate lignocellulosic material is defibrated by a mechanical defibrating process.

4. A process according to claim 1, in which following the pretreatment the particulate lignocellulosic material is delignified, pulped and defibrated by a combined chemical and mechanical delignifying, pulping and defibrating process.

5. A process according to claim 1, in which the amount of alkali in the pretreating solution is so controlled that the pH of the expressed and separated pretreating liquor at the conclusion of the pretreatment is within the range from about 5 to about 7.5.

6. A process according to claim 1, in which the amount of complexing agent in the pretreating solution is within the range from 0.05%
to about 0.80%, based on the dry weight of the particulate lignocellulosic material.

7. A process according to claim 1, in which the heavy metal ion reducing agent is selected from the group consisting of sulfur dioxide, sulfurous acid and sulfite salts, and the amount of reducing agent is within the range from about 0.1% to about 3.0 %, based on the dry weight of the particulate lignocellulosic material

8. A process according to claim 1 in which after compression to a solids content of at least about 40% the lignocellulosic material is pulped by heating in a pressure vessel in the presence of at least one of steam and compressed air at a temperature within the range from about 20 to about 180°C for from about 1 to about 15 minutes, and is then subjected to mechanical defibration.

9. A process according to claim 1 in which the particulate lignocellulosic material after compression to a solids content of at least 40%
is defibrated directly.

10. A process according to claim 2 in which the lignocellulosic material after compression to a solids content of at least 40% is chemically pulped using a chemical pulping process and an aqueous pulping liquor selected from the group consisting of acidic sulfite liquor, bisulfite liquor, sulfite liquor, alkaline sodium hydroxide liquor, alkaline sodium carbonate liquor, alkaline sodium bicarbonate liquor, white liquor, and sodium hydroxide/oxygen gas.

11. A process according to claim 1 in which after compression to a solids content of at least about 40% the lignocellulosic material is digested and thereby partially chemically pulped by heating in a pressure vessel in the presence of at least one of steam and compressed air at a temperature within the range from about 20 to about 180°C for from about 1 to about 15 minutes, and is then subjected to mechanical defibration in a disc refiner, in the presence of a bleaching agent supplied to the disc refiner so that it is mixed -with the lignocellulosic material in the vicinity of the circumference of the grinding disc, and at a distance of at least one third of the disc radius from its center.

12. A process according to claim 11, in which the bleaching agent is a lignin preserving bleaching agent.

13 A process according to claim 12, in which the bleaching agent is a mixture of a peroxide bleaching agent, sodium hydroxide, sodium silicate and magnesium sulfate.

140 A process according to claim 13, in which the sodium silicate is applied separately at the circumference of the grinding discs or at a distance of at most 200 mm from their circumference, and the remaining bleaching agent is applied at the center of the grinding discs, or at a distance from the center corresponding to at most one quarter of the grinding disc radius.

15 A process according to claim 1 in which the heavy metal ion reducing agent is SO2, and all of the SO2 supplied via the alkaline aqueous solution is consumed so that the expressed liquor contains substantially no free SO2.

16 A process according to claim 1, in which the alkaline aqueous solution comprises alkali.

17. A process according to claim 1, in which the alkaline aqueous solution comprises alkali and a heavy metal ion complexing agent

18. A process according to claim 1, in which the alkaline aqueous solution comprises alkali and a heavy metal ion reducing agent

19. A process according to claim 1, in which the alkaline aqueous solution comprises alkali, a heavy metal ion complexing agent, and a heavy metal ion reducing agent.

20. An apparatus for pretreating wood chip material to remove heavy metals and resin before delignification or defibration, comprising in combination, a chip washer, a chip bin having a bottom in flow connection with a screw press feeder for compressing chip material to a solids content of at least 40% and feeding compressed chip material into a vertical impregnating vessel for impregnating chip material with pretreating liquor and having a vertical screw conveyor for conveying chips therethrough; a pressure reaction vessel having at its bottom a screw press feeder having the ability to express and separate pretreating liquor from the chip material while compressing said chip material to a solids content of at least 40%; means for removing liquor expressed in the screw press feeder, the screw press feeder being in flow connection to a pressure vessel having inlet ducts for admission of both steam and compressed air to regulate pressure and temperature in the vessel, and having at the bottom a screw feeder for conveying and compressing chip material and feeding it to a defibration apparatus.

21. An apparatus according to claim 20, in which the screw press feeder is arranged to form a sealing plug of chip material at the bottom of the pressure reaction vessel, preventing backflow from the pressure vessel.