SE1751157A1 - Removal of inorganic elements from wood chips - Google Patents

Abstract

A process of treating wood chips for reduction of the content of undesirable inorganic elements prior to cooking in a production line for chemical pulp, wherein the chips are (I) squeezed at atmospheric pressure by volumetric compression in a first compression means, whereby entrapped air escapes; (II) the squeezed chips are allowed to expand in a first acidic impregnation liquor, whereby the wood chips absorb first acidic impregnation liquor; (III) the expanded chips are squeezed at atmospheric pressure by volumetric compression in a second compression means, whereby said first acidic impregnation liquor escapes; (IV) the squeezed chips from step (III) are allowed to expand in a second acidic impregnation liquor, whereby the chips absorb said second acidic impregnation liquor; and (V) the expanded chips are squeezed at atmospheric pressure by volumetric compression in a third compression means, whereby the second acidic impregnation liquor escapes from the volumes.

Description

REMOVAL OF INORGANIC ELEMENTS FROM WOOD CHIPSThe present invention relates to removal of inorganic elements from wood chips.

More precisely, the invention relates to a process step in a production line forchemical pulp wherein wood chips, having entrapped air, are treated withleaching Iiquor under certain conditions for reduction of the content ofundesirable inorganic elements in the chips prior to cooking in a digester. Theinvention also relates to a system for treating wood chips, having fiberstructures with volumes with entrapped air, for reduction of the content ofundesirable inorganic elements in said chips prior to introduction of said chipsinto a digester for cooking in a production line for chemical pulp.

BackgroundAs pulp mill plants strive to further decrease usage of fresh water from currentlevels by means of closed looped systems, the concentration of non-processelements within the manufacturing process, i. e. undesirable inorganic elements, will tend to increase in different process streams.

This might have a negative impact on process chemistry, scale formation,process efficiency and product properties.

Such undesirable inorganic elements are e. g. aluminum, calcium, barium,phosphorus, chlorine, potassium, manganese, cadmium, magnesium, iron, and zinc.

Undesirable inorganic elements from the wood raw material and fromchemicals used to manufacture bleached pulp from wood will accumulate indifferent process streams, due to recirculation of process streams such asbleach plant filtrates. 2Accumulation of undesirable inorganic elements in a pulp mill often causes problems with scaling which leads to plugging of pipes, screens, instrumentsetc, which in turn may result in lost production or other process disturbances.

Some elements such as manganese may interfere with chemical reactions inbleaching and cause inferior pulp quality and decomposition of valuablehydrogen peroxide.

Chloride and potassium ions accumulate in the recovery boiler system andcontribute to the plugging and corrosion of the boiler tubes that may lead tolost production.

Among the different measures to minimize or eliminate the effluent from chemical pulp mills, closure of the bleach plant poses a great challenge.

Therefore, management of undesirable inorganic elements is a key tosuccessful recovery and recycle of bleach plant effluents.

Any pulp mill aiming at advanced closure must anticipate, and strive toprevent, the negative impacts of built-up undesirable inorganic elements.

A number of different treatment methods for removal of undesirable inorganicelements from wood material prior to pulping have been proposed previously.

EP0921228 discloses a method of preparing chemical pulp from wood chips,in which method wood chips are treated in a precleaning stage prior tocooking, in order to remove process detrimental components. ln theprecleaning stage, wood chips are treated with e. g. bleach filtrate orevaporation condensate at a pH of 2.5-5.

WO 03/046276 relates to a process of treating wood chips for reduction of theconcentration of undesirable inorganic elements prior to cooking in aproduction line for chemical pulp, wherein the wood chips, having entrapped 3air, are treated with an aqueous leaching liquor at elevated temperature and pressure, followed by draining at atmospheric pressure or below atmosphericpressure, the pressures being controlled to yield a moisture content in thewood chips as low as possible for adequate leaching result and behavior ofthe chips in a subsequent digester.

The aqueous leaching liquor is e.g. pulp mill process water with a low contentof undesirable inorganic components, such as bleach plant spent liquor orcondensate. The aqueous leaching liquor drained from the treated woodchips may be purified and recycled back to the process.

WO 2004/050983 relates to a method for treatment of wood chips for thereduction of non-process elements and/or organic process disturbingsubstances in a mechanical pulping process or a chemimechanical pulpingprocess comprising the following steps: a) introduction of cracks in the chips or refining the chips to a coarse fibrestructure, in order to considerably shorten the diffusion ways within the woodmaterial; b) treatment of the chips with an acidic leaching liquid; c) extensive removal of the acidic leaching liquid; and d) adjustment of the pH to a level suitable for refining, preferably pH 4.

Not only is this process aimed for treatment of wood chips for mechanicalpulping or chemimechanical pulping - WO 2004/050983 also clearly teachesaway from applying the treatment process on wood chips for chemical pulping.

Short description of the invention There is need for a process of treating wood chips for removal of inorganicelements from the wood chips, which process is not burdened with processsteps with long retention or residence times; which process does not dependon expensive and/or complicated process equipment for pressurised processsteps; and which process can be used for treatment of wood chips that are tobe used in chemical pulping. 4One object of the present invention is to provide such process of treating wood chips.

Thus, one aspect of the invention relates to a process of treating wood chipsfor reduction of the content of undesirable inorganic elements in said chipsprior to introduction of said chips into a digester for cooking in a productionline for chemical pulp, wherein the wood chips, having fiber structures withvolumes with entrapped air, are (I) squeezed at atmospheric pressure by volumetric compression ina first compression means, whereby at least part of said entrappedair escapes from said volumes; (ll) the squeezed wood chips are allowed to expand in a first acidicimpregnation liquor, whereby the wood chips absorb part of saidfirst acidic impregnation liquor into said volumes; (lll) the expanded wood chips are squeezed at atmosphericpressure by volumetric compression in a second compressionmeans, whereby at least part of said first acidic impregnation liquorabsorbed in said volumes escapes from the volumes; (IV) the squeezed wood chips from step (lll) are allowed to expandin a second acidic impregnation liquor, whereby the wood chipsabsorb part of said second acidic impregnation liquor into said volumes; and (V) the expanded wood chips are squeezed at atmosphericpressure by volumetric compression in a third compression means,whereby at least part of the second acidic impregnation liquor absorbed in said volumes escapes from the volumes. 5According to another aspect of the present invention relates to a system for treating wood chips, having fiber structures with volumes with entrapped air,for reduction of the content of undesirable inorganic elements in said chipsprior to introduction of said chips into a digester for cooking in a productionline for chemical pulp, which system comprises - first compression means for squeezing out entrapped air from thewood chips; - a first impregnation vessel, for impregnation of the wood chipswith a first acidic impregnation liquor after said squeezing out ofentrapped air from the wood chips; - second compression means for squeezing out first acidic impregnation liquor from the wood chips; - second impregnation vessel, for impregnation of the wood chipswith a second acidic impregnation liquor after said squeezing out ofentrapped air from the wood chips; and - third compression means for squeezing out second acidic impregnation liquor from the wood chips.

Description of embodiments of the invention Before the present invention is disclosed and described, it is to be understoodthat this invention is not limited to the particular configurations, process steps,and materials disclosed herein as such configurations, process steps, and materials may vary somewhat. lt is also to be understood that the terminologyemployed herein is used for the purpose of describing particular embodimentsonly and is not intended to be limiting since the scope of the present invention will be limited only by the appended claims and equivalents thereof. 6lt must be noted that, as used in this specification and the claims, the singular forms "a", "an", and "the" include p|ura| referents unless the context clearlydictates otherwise. ln this specification, unless otherwise stated, the term "about" modifying, forinstance, the quantity of a reagent or the value of a process parameteremployed in the process or the system according to the present inventionrefers to variation in the numerical quantity that can occur, for example,through typical measuring and liquid handling procedures used for makingconcentrates or use solutions in the real world; through inadvertent error inthese procedures; through differences in the manufacture, source, or purity ofthe ingredients used or the parameters employed; and the like. The term"about" also encompasses amounts that differ due to different equilibriumconditions for a composition resulting from a particular initial mixture. Whetheror not modified by the term "about", the claims include equivalents to thequantities. ln this specification, unless otherwise stated, the term “atmospheric pressure”refers to the pressure within the atmosphere at the location of Earth at whichthe inventive process is carried out. ln this context, it should be pointed outthat “atmospheric pressure” in the sense of the present invention should alsobe understood to include pressures which are reduced or elevated withrespect to the standard pressure of 1013 millibar. Pressure differences ofapproximately 1100 millibar around this normal pressure are still regarded asfalling under the term “atmospheric pressure.” ln this specification, unless otherwise stated, the term “undesirable inorganicelements” refers to inorganic “non-process elements” (NPE) as defined byGrace and Tran 2009 (Grace T.M. & H. Tran. (2009) The effect of dead loadchemicals in the kraft pulping and recovery system. Tappi J., vol. 8, n. 7, pp.18-24.), namely all of the chemicals in the system other than sodium, sulphur,carbon, hydrogen, and oxygen. 7ln this specification, unless otherwise stated, the term "chemical pulping" refers to processes such as the Kraft, soda, soda AQ, Kraft AQ, sulphite,bisulfite, and other similar processes, whereby chemical reagents remove thelignin from the fiber structure of the feed material, and the term "chemicalpulp" refers to pulp obtained from the corresponding processes. Forconvenience, the present specification will focus on the sulphite process, butit will be understood by those familiar with this field of technology, that theconcepts and features described and claimed herein, can readily beincorporated into the other types of chemical pulping processes. ln this specification, unless otherwise stated, the term “degree ofdisintegration” refers to the decrease in percentage points of wood particleshaving a size of >ø3 mm due to conversion into particles of smaller sizeduring a unit operation or in the course of a series of connected unit operations.

By squeezing the wood chips by volumetric compression and then releasingthe compressed wood chips in an impregnation liquor, the impregnation liquoris filling up the void volumes of the fiber structures caused by thecompression where air and liquor are removed. This ensures an efficient impregnation.

Due to a more efficient impregnation prior to the cooking the present processprovides a possibility to increase the pulping capacity as the cooking time canbe reduced due to a more stable temperature and more even moisture content of the wood chips. lt also provides for higher pulp yield and better pulp quality.

The wood chips treated in the inventive process or in the inventive systemmay be wood chips of softwood or hardwood, or a mixture of wood chips ofsoftwood and hardwood. The wood chips may come from any or all parts of atree, such as trunk wood or ramial (smaller branch) wood, or a combinationthereof. ln some embodiments of the invention the wood chips are softwood chips originating from trees of the family Pinaceae, such as cedars, firs, hemlocks,larches, pines, spruces, or mixtures of softwood chips originating from two ormore of cedars, firs, hemlocks, larches, pines, and spruces, specifically pine and spruce. ln some embodiments of the invention the wood chips are hardwood chipsoriginating from trees of any of the families Aceraceae (maple family),Betulaceae (birch family), Fagaceae (beech family), Myrtaceae (myrtlefamily), or Salicaceae (willow family). ln some embodiments of the invention the wood chips are hardwood chipsoriginating from trees of any of the families Betulaceae (birch family), such asBetula (birches), or Myrtaceae (myrtle family), such as Euca/yptus, or a mixture thereof. ln some embodiments of the invention the undesirable inorganic elements arechosen among calcium, magnesium, potassium, manganese, barium, iron,aluminium, copper, nickel, chromium and zinc, or combinations thereof. ln one embodiment of the invention the undesirable inorganic elements are calcium, magnesium, manganese, or combinations thereof. ln some embodiments of the invention the squeezed wood chips from step(V) are allowed to expand in a third acidic impregnation liquor, whereby thewood chips absorb part of said third acidic impregnation liquor into saidvolumes with entrapped air; whereafter the expanded wood chips aresqueezed at atmospheric pressure by volumetric compression in a fourthcompression means, whereby at least part of the third acidic impregnationliquor absorbed in said volumes escapes from the volumes and is used as atleast part of the second acidic impregnation liquor.

As readily understood by the skilled artisan the inventive process may well be extended in an analogous manner, i.e. with expansion in a fourth and further 9acidic impregnation Iiquors followed by squeezing of the wood chips in fifth and further and further compression means and so on. ln some embodiments of the invention the first, second, third, and/or anyfurther compression means is/are screw press(es), extrusion press(es), orscrew pump(s). Specifically, the compression means may be compressingscrew feeder(s), such as, for instance, IMPRESSAFINER® systems fromAndritz, which are disc|osed and described in US 2,975,096, and/or PREX®(pressure-expansion) system from Valmet.

The compression ratio of the compression means may be in a range thatcauses a low degree of disintegration, such as less than about a 20percentage points decrease, specifically less than about a 30 percentagepoints decrease, and more particulary less than about a 50 percentage points decrease.

The first, second, third, and/or any further acidic impregnation liquor(s) maybe any acidic liquid. Examples of acidic impregnation Iiquors are mineralacids, such as sulphuric acid, or mixtures of such acids. Other examples areacidic aqueous Iiquors with a low content of inorganic elements that areundesirable in a pulping process, such as pulp mill process water with a lowcontent of undesirable inorganic components, e. g. bleach plant spent liquoror condensate. ln some embodiments of the invention the acidic impregnationliquor(s) is/are pulp mill effluents, bleach plant process water, or bleach planteffluents. ln some embodiments of the invention, the pulp mill process wateris a condensate, such as an evaporation condensate. ln some embodiments of the inventive process at least part of the secondacidic impregnation liquor, which is absorbed in the volumes and escapesfrom the volumes due to volumetric compression in the third compression means, is used as at least part of the first acidic impregnation liquor. ln some embodiments at least 25 wt-%, specifically at least 50 wt-%, andmore specifically at least 75 wt-%, of the second acidic impregnation liquor, based on the weight of the second acidic impregnation Iiquor, which is absorbed in the volumes and escapes from the volumes due to volumetriccompression in the third compression means, is used as at least 25 wt-%,specifically at least 50 wt-%, and more specifically at least 75 wt-%, of the firstacidic impregnation Iiquor, based on the weight of the first acidic impregnation Hquon ln some embodiments at least 25 wt-%, specifically at least 50 wt-%, andmore specifically at least 75 wt-%, of the third acidic impregnation Iiquor,based on the weight of the third acidic impregnation Iiquor, which is absorbedin the volumes and escapes from the volumes due to volumetric compressionin the forth compression means, is used as at least 25 wt-%, specifically atleast 50 wt-%, and more specifically at least 75 wt-%, of the second acidicimpregnation Iiquor, based on the weight of the second acidic impregnation Hquon As readily understood by the skilled artisan the inventive process may well beextended in an analogous manner, i.e. with at least 25 wt-%, specifically atleast 50 wt-%, and more specifically at least 75 wt-%, of a downstream acidicimpregnation Iiquor, based on the weight of the downstream acidicimpregnation Iiquor, which is absorbed in the volumes and escapes from thevolumes due to volumetric compression in a compression means, being usedas at least 25 wt-%, specifically at least 50 wt-%, and more specifically atleast 75 wt-%, of an upstream acidic impregnation Iiquor, based on the weightof said upstream acidic impregnation liquor.

Acidic conditions are desirable, and required in order to extract out undesirable inorganic elements that are metals. At neutral pH, the metals aretightly bound to negatively charged moieties, mostly carboxylic groups, in thewood chips. However, if pH is too low, the lignin in the wood chips will start to react, which may lead to a non-pulpable chip. 11ln some embodiments of the invention the squeezed wood chips are allowed to expand in the first, second, third, and/or any further acidic impregnation|iquor(s) at a pH of from about 1.5 to 5, specifically at a pH of from about 2 to4, in particular at a pH of from about 2 to 3. ln some embodiments of the invention the squeezed wood chips are allowedto expand in the first, second, third, and/or any further acidic impregnation|iquor(s) at a temperature of from about 60 to 100°C, specifically at atemperature of from about 85 to 95°C.

As chemical pulping usually does not involve any refining stage/step, there isusually no reason to adjust the pH to a level suitable for refining. Hence, insome embodiments of the invention no pH adjustment is done between anyone of steps (I) - (V) and prior to introduction of said chips into the digester, inparticular not to a level suitable for refining, specifically not to pH 2 4, morespecifically not to pH in the range of 4-9, even more specifically not by addingwhite water or water mixed with one or more basic compounds. lt should be understood that although pH is not intentionally adjusted in thecourse of the process according to said embodiments pH may of coursechange, even to pH 2 4, due to chemical reactions during the process, suchas ion exchanging, where cations like, for instance, calcium is replaced by hydrogen ions in the wood structure. ln some embodiments of the invention the production line for chemical pulp isa production line in which a sulphite process or a bisulfite process isperformed. ln some embodiments of the invention the digester for cooking in a productionline for chemical pulp is a digester for contiuous cooking. ln some embodiments of the inventive system also comprises means forconveying at least part of second acidic impregnation liquor, that has beensqueed out from the wood chips, to the first impregnation vessel. 12 Analysis relating to certain experiments pertaining to some embodiments ofthe invention have indicated an increase of the level of iron in the wood chipsduring the course of the inventive process. The only possible origin of thisadded iron would be iron released from the process equipment during thoseparticular experiments. Still, the content of undesirable inorganic elements inthe wood chips is reduced in the sense that the content of undesirableinorganic elements originally present in the wood chips, i.e. prior to treatment,is reduced during the course of the inventive process. This fact is notchanged by the use of experimental equipment that issues iron; the ironcontributed by that experimental equipment should be disregarded, becausesuch contribution would not take place if equipment with less or no propensityto issue iron would have been used.

Nevertheless, some embodiments of the invention includes the proviso that itinvolve reduction of the content of undesirable inorganic elements except ironin the wood chips.

The invention will now be illustrated in closer detail in the following non- limiting examples.

EXAMPLESWood chips were run through a PREX® (pressure and expansion) unit of apulping pilot plant in a number of runs each followed by impregnation of the wood chips with warm sour condensate.

Raw material and impreqnation liquor The raw material used was a wood chips mixture of pine and spruce fromDomsjö Fabriker AB, Örnsköldsvik, Sweden. The pine content in the mixturewas maximum 40% as target, and the dry content of the wood chips mixturewas 52%. The wood chips were screened on a continuous vibrating screenequipped with hole plates of ø 35 mm and ø 8 mm for removal of oversizedchips and sawdust. To ensure proper feeding stability during the experimentsonly the accepted chips fraction (ø 8 - 35 mm) was used, since a small pilot 13plant equipment was used, which was more sensitive to oversized chips and sawdust than mi|| scale equipment.

The impregnation liquor used was warm condensate collected from the pulpmill at Domsjö Fabriker one day before the experiments. The pH of thecondensate was about 2.5 and it was collected at a temperature of about90°C. The condensate was samp|ed in insu|ated CIPAX® containers in orderto retain heat before the PREX® leaching, and was heated in a heatexchanger before the respective experiments to reach desired temperature.

PREX® pilot plantThe pilot plant set-up for this experiment consisted of a chip bin with a discharge screw feeding the PREX® unit, a plug screw feeder and animpregnation vessel. ln the PREX® screw, the chips were compressed andthen “released” into the impregnation liquid in an impregnation vessel. Thewood chips were then transported via a transport screw to a retention binbefore being collected at the discharge screw of the retention bin.

Pilot programThe feed rates at the different passes were measured using warm water (90°C) instead of impregnation liquor before the experiment and are set forthin Table 1.

Table 1Dry matter content [%] Feed rate [kgB_D./min] Untreated wood chips 52.0 Pass 1 38.3 0.9 Pass 2 32.6 0.8 Pass 3 31.5 0.7 kgsp. = kg of bone dry materialDry matter content was determined according to SCAN-CM 39:94. 14The process data that were prevailing during the experiments are set forth in Table 2 and Table 3.

Table 2 PREX® system process data Passes Discharge Discharge PSF, PSF, load lmpregnator, lmpregnator,screw chip screw chip Speed [kW] lifting screws lifting screwsbin, speed bin, load [rpm] speed load[rpm] [KW] [rpm] [KW] 1st pass Average 20 0.43 45 1.55 58 0.30 Std 0 0.04 0 0.08 0 0.01 2“d pass Average 20 0.37 45 1.51 58 0.30 Std 0 0.04 0 0.10 0 0.01 3” pass Average 20 0.35 45 1.33 58 0.30 Std 0 0.05 0 0.12 0 0.02 4lh pass Average 20 0.34 45 1.24 58 0.30 Std 0 0.06 0 0.15 0 0.02 Std = Standard DeviationPSF = Press Screw Feeder Table 3 PREX® system process data Passes Discharge Discharge Level lmp Temp lmp Temp Chem Chemscrew screw [%] [°C] tank Flowretention bin, retention [°C] [l/min]speed bin, load[rpm] [KW] 1st pass Average 5.3 1.22 81 59 74 1.10 Std 0.0 0.03 1 5 1 0.69 2“d pass Passes Discharge Discharge Level lmp Temp lmp Temp Chem Chemscrew screw [%] [°C] tank Flowretention bin, retention [°C] [l/min]speed bin, load[rpm] [KW] Average 5.3 1.19 81 62 75 1.43 Std 0.0 0.01 1 2 1 0.69 3” pass Average 5.3 1.19 80 64 74 1.48 Std 0.0 0.01 1 1 1 0.79 4ih pass Average 5.3 1.19 81 64 73 1.48 Std 0.0 0.01 1 0 0 1.04 Std = Standard Deviation lmp = lmpregnation vessel Chem = lmpregnation Iiquor AnalysesThe original, untreated wood chips, below related to as “raw material” and filtrate from the experimental process were analyzed at Valmet Fiber Technology Center regarding the parameters, and according to the standard methods set forth in Table 4.

Composite samples (both wood chips and filtrate samples) were collected during all passes.

Table 4 ANALYSIS REFERENCE Raw material Particle size distribution SCAN-CM 40:01 The results from these analysis are set forth in Table 5. 16 Table 5Raw material Before After chip 15* 2"" 3"*chip screening pass pass passscreeningParticle Size [%] [%] [%] [%] [%]>ø45 mm 0.60 0.00 0.00 0.00 0.00>ø8 mm <ø45 mm 4.51 2.97 0.99 0.37 0.12>ø13 mm 73.9 77.7 41.3 20.4 8.13>ø7 mm<ø13 mm 13.9 15.3 36.9 41.6 34.1>ø3 mm<ø7 mm 5.19 3.69 18.3 31.2 44.6Dust <ø3 mm 1.87 0.33 2.49 6.40 13.0 The particle size distribution shows that the wood chips contained more than70% of the material (by weight) at a particle size between 3 - 13 mm after theZnd and 3” pass.

The corresponding values for the untreated material and the 1st pass were 19and 55% respectively.

The dust part (<ø3mm) was rather high for the 2” and 3” passes, between 6- 13%.

The liquor uptake of the wood chips was also measured; the results are setforth in Table 6.

The liquor uptake was based on the condensate flow to the impregnator andthe feed rate data, i.e. the data set forth in Table 1.

Table 6Passes Feed rate Condensate flow Liquor uptake[kgB.o./min] [l/min] [ms/tan] 1st pass 0.9 1.1 1.2 Znd pass 0.8 1.4 1.8 3” pass 0.7 1.5 2.1 41” pass - 1.6 - kgsb. = kg of bone dry material mß/teto. = ms/tonne of bone dry raw material 17 The Iiquor uptake of the wood chips increased with the number of passes,presumably due to a more disintegrated wood structure. 5 The wood chips and the filtrates were analyzed with regard to a number ofparameters as set forth in Tables 7 and 8, respectively.

Table 7Wood Chips Sample Ash Fe Ca Si Mg Mn525°C % mg/kg mg/kg mg/kg mg/kg mg/kg Raw material 0.29 9.7 631 34 131 88.11st pass 0.33 15.4 667 25 136 88.52“d pass 0.3 18.1 540 - 93.4 67.93” pass 0.25 18.3 389 22 54.6 44.64"“ pass 0.21 20.7 276 16 29.4 28.6 10 As seen in Table 7 the level of metals are reduced in the chips, except foriron, for which the level on the contrary increases; the only logical explanationfor this would be that that iron was released from the process equipmentduring the experiments. 15 Table 8Filtrate Sample Ca COD Dry matter content Suspended solidsmg/I mg/I % mg/Ilmpregnation Liquor(Condensate) 18.2 10800 0.1 131st pass 73.2 8140 1.3 85962“d pass 130 8800 0.9 42933ld pass 112 10150 0.7 34134"* pass 88.3 10550 0.6 2790

Claims (10)

18Claims

1. A process of treating wood chips for reduction of the content of undesirableinorganic elements in said chips prior to introduction of said chips into adigester for cooking in a production line for chemical pulp, wherein the woodchips, having fiber structures with volumes with entrapped air, are (I) squeezed at atmospheric pressure by volumetric compression ina first compression means, whereby at least part of said entrappedair escapes from said volumes; (ll) the squeezed wood chips are allowed to expand in a first acidicimpregnation liquor, whereby the wood chips absorb part of saidfirst acidic impregnation liquor into said volumes; (lll) the expanded wood chips are squeezed at atmosphericpressure by volumetric compression in a second compressionmeans, whereby at least part of said first acidic impregnation liquor absorbed in said volumes escapes from the volumes; (IV) the squeezed wood chips from step (lll) are allowed to expandin a second acidic impregnation liquor, whereby the wood chipsabsorb part of said second acidic impregnation liquor into said volumes; and (V) the expanded wood chips are squeezed at atmosphericpressure by volumetric compression in a third compression means,whereby at least part of the second acidic impregnation liquorabsorbed in said volumes escapes from the volumes.

2. The process according to claim 1, wherein said at least part of the secondacidic impregnation absorbed in said volumes that escapes from the volumesim step (V) is used as at least part of said first acidic impregnation liquor.

3. 193. The process according to claim 1 or 2, wherein the squeezed wood chips from step (V) are allowed to expand in a third acidic impregnation Iiquor,whereby the wood chips absorb part of said third acidic impregnation Iiquorinto said volumes with entrapped air; whereafter the expanded wood chipsare squeezed at atmospheric pressure by volumetric compression in a fourthcompression means, whereby at least part of the third acidic impregnationIiquor absorbed in said volumes escapes from the volumes and is used as atleast part of the second acidic impregnation Iiquor.

4. The process according to any one of the preceding claims, wherein saidfirst, second, third and/or fourth compression means is/are compressingscrew feeder(s).

5. The process according to any one of the preceding claims, wherein no pHadjustment is done between any one of steps (I) - (V) and prior to introductionof said chips into said digester.

6. The process according to any one of the preceding claims, wherein thewood chips are softwood chips originating from trees of the family Pinaceae.

7. The process according to any one of the preceding claims, wherein theundesirable inorganic elements are chosen among calcium, magnesium,potassium, manganese, barium, iron, aluminium, copper, nickel, chromium and zinc, or combinations thereof.

8. The process according to claim 7, wherein the undesirable inorganic elements are calcium, magnesium, manganese, or a combination thereof.

9. A system for treating wood chips, having fiber structures with volumes withentrapped air, for reduction of the content of undesirable inorganic elements in said chips prior to introduction of said chips into a digester for cooking in aproduction line for chemical pulp, comprising - first compression means for squeezing out entrapped air from the wood chips; - a first impregnation vessel, for impregnation of the wood chipswith a first acidic impregnation Iiquor after said squeezing out ofentrapped air from the wood chips; - second compression means for squeezing out first acidic impregnation Iiquor from the wood chips; - second impregnation vessel, for impregnation of the wood chipswith a second acidic impregnation Iiquor after said squeezing out ofentrapped air from the wood chips; and - third compression means for squeezing out second acidic impregnation Iiquor from the wood chips.

10. The system according to c|aim 9, which also comprises means forconveying at least part of second acidic impregnation Iiquor, that has beensqueed out from the wood chips, to the first impregnation vessel.