WO2019059835A1

WO2019059835A1 - Removal of inorganic elements from wood chips

Info

Publication number: WO2019059835A1
Application number: PCT/SE2018/050955
Authority: WO
Inventors: Hans Grundberg; Magnus HÖRNSTEN
Original assignee: Domsjoe Fabriker AB
Current assignee: Domsjoe Fabriker AB
Priority date: 2017-09-19
Filing date: 2018-09-19
Publication date: 2019-03-28
Anticipated expiration: 2020-03-19
Also published as: SE1751157A1; SE542430C2

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; (Ill) 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 Iiquor; 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 CHIPS

The present invention relates to removal of inorganic elements from wood chips.

More precisely, the invention relates to a process step jn a production line for chemical pulp wherein wood chips, having entrapped air; are treated with teaching liquor under certain conditions for reduction of the content of undesirable inorganic elements in the chips prior to cooking in a digester. The invention also 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 chips prior to introduction of said chips into a digester for cooking in a production line for chemical pulp. Background

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

This might have a negative impact on process chemistryi 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 from chemicals used to manufacture bleached pulp from wood will accumulate in different process streams, due to recirculation of process streams such as bleach plant filtrates. Accumulation of undesirable inorganic elements in a pulp mill often causes problems with scaling which leads to plugging of pipes, screens, instruments etc, which in turn may result in lost production or other process disturbances. Inorganic elements may interfere with chemical reactions in cooking and cause inferior pulp quality, scaling and decomposition of cooking liquor.

Some elements such as manganese may interfere with chemical reactions in bleaching and cause inferior pulp quality and decomposition of valuable hydrogen peroxide.

Chloride and potassium Ions accumulate in the recovery boiler system and contribute to the plugging and corrosion of the boiler tubes that may lead to lost production.

Cadmium accumulates in the black liquors and ends up in the recovery boiler system, whicn contributes to high environmental load.

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 to successful recovery and recycle of bleach plant effluents, Any pulp mill aiming at advanced closure must anticipate, and strive to prevent, the negative impacts of built-up undesirable inorganic elements.

A number of different treatment methods for removal of undesirable inorganic elements 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 precleanihg Stage prior to cooking, in order to remove process detrimental components, in the precleaning stage, wood chips are treated with e. g. bleach filtrate or evaporation condensate at a pH of 2.5-5.

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

The aqueous leaching liquor is e,g. pulp mill process water with a low content of undesirable inorganic components, such as bleach plant spent liquor or condensate. The aqueous leaching liquor drained from the treated wood chips may be purified and recycled back to the process.

WO 2004/050983 relates to a method for treatment of wood chips for the reduction of non-process elements and/or organic process disturbing substances in a mechanical pulping process or a chemimechanical pulping process comprising the following steps:

a) introduction of cracks In the chips or refining the chips to a coarse fibre structure, in order to considerably shorten the diffusion ways within the wood material;

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 mechanical pulping or chemimechanical pulping - WO 2004/050983 also clearly teaches away from applying the treatment process on wood chips for chemical pulping. Brief description of the invention

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

The present invention is based on the recognition that efficient removal of undesirable metals in wood chips, which are to be further processed in a digester unit, can be achieved by acid leaching treatment at atmospheric pressure using a principle of volumetric compression and expansion. The invention involves multi-stage squeezing with compression means, such as screw feeders, at atmospheric pressure and intermed iate chip expansion in acidic leaching liquors, specifically at pH 1.5-5.Thus, one aspect of the invention relates to a process of treating wood chips for reduction of the content of undesirable inorganic elements in said chips prior to introduction of said chips into a digester for cooking in a production line for chemical pulp, wherein the wood chips, having fiber structures with volumes with entrapped air, are

(I) squeezed at atmospheric pressure by volumetric compression in a first compression means, whereby at least part of said entrapped air escapes from said volumes;

(II) the squeezed wood chips are allowed to expand in a first acidic impregnation liquor, whereby the wood chips absorb part of said first acidic impregnation liquor into said volumes; (Hi) the expanded wood chips are squeezed at atmospheric pressure by volumetric compression in a second compression means, 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 (III) are allowed to expand in a second acidic impregnation liquor, whereby the wood chips absorb part of said second acidic impregnation iiquor into said volumes; and

(V) tile expanded wood chips are squeezed at atmospheric pressure by volumetric compression in a third compression means, whereby at least part of the second acidic impregnation Iiquor absorbed in said volumes escapes from the volumes.

According to another aspect of the present invention is also provided a process for cooking of wood chips in a production line for chemical pulp, comprising treating fpretfeating") the wood chips according to the above- described chip-treatment process of the invention and thereafter introducing said chips into a digester and cooking the wood chips in the digester.

Preferably, there is no deaeration, such as steaming^ between the

pretreatment and the cooking step.

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 chips prior to

introduction of said chips into a digester for cooking in a production tine for chemical pulp, which system comprises - first compression means for squeezing out entrapped air from the wood chips; - a first impregnation vessel, for impregnation of the wood chips with a first acidic impregnation liquor after said squeezing out of entrapped air ffbm 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 chips with a second acidic impregnation liquor after said squeezing out of entrapped air from the wood chips; and

- third compression means for squeezing out second acidic impregnation liquor from the wood chips. According to another aspect of the present invention is also provided a cooking system for production of chemical pulp comprising a pretreatment system comprising the above^escribed chip-treatment system of the invention and a digester for cooking the wood chips, said digester being arranged downstream of said pretreatment system. Preferably, there is no deaeratibn equipment such as steaming vessels, between the pretreatmerit system and the digester.

Brief description of the drawing Fig, 1 is a schematic flow chart of a process of treating (pretf eating) wood chips according to an example embodiment of the invention.

Description of embodiments of the invention

Before the present invention is disclosed and described, it is to be understood that this invention is not limited to the particular configurations, process steps, and materials disclosed herein as suqh configurations, process steps, and materials may vary somewhat. It is also to be understood that the terminology employed herein is used for the purpose of describing particular embodiments only 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. It must be noted that, as used in this specification and the claims, the singular forms "a", "an", and "the" include plural referents unless the context clearly dictates otherwise.

In this specification, unless otherwise stated, the term "about" modifying, for instance, the quantity of a reagent or the value of a process parameter employed in the process or the system according to the present invention refers to variation in the numerical quantity that can occur, for example, through typical measuring and liquid handling procedures used for making concentrates or use solutions in the real world; through inadvertent error in these procedures; through differences in the manufacture, source, or purity of the ingredients used or the parameters employed; and the tike. The term "about* also encompasses amounts that differ due to different equilibrium conditions for a composition resulting from a particular initial mixture. Whether or riot modified by the term "abouf , the claims include equivalents to the quantities.

In this specification, unless otherwise stated, the term "atmospheric pressure" refers to the pressure within the atmosphere at the location of Earth at which the inventive process is carried out. In this context, it should be pointed out that "atmospheric pressure" in the sense of the present invention should also be understood to include pressures which are reduced or elevated with respect to the standard pressure of 1013 millibar. Pressure differences of approximately ±100 millibar around this normal pressure are still regarded as falling under the term "atmospheric pressure." in this specification, unless otherwise stated, the term "undesirable inorganic elements" refers to inorganic "non-process elements" (NPE) as defined by Grace and Tran 2009 (Grace T.M, & H, Tran. (2009) The effect of dead load chemicals in the kraft pulping and recovery system. Tappr J,, vol; 8, n. 7, pp, 18-24.), namely at) of the chemicals in the system other than sodium, sulphur, carbon, hydrogen, and oxygen. In this specification, unless otherwise stated , the term "chemical pulping" refers to processes such as the Kraft, soda, soda AQ_t Kraft AQ, sulphite, bisulfite, and other similar processes, whereby chemical reagents remove the lignin from the fiber structure of the feed material, and the term "chemical pulp" refers to pulp obtained from the corresponding processes. For convenience, the present specification will focus on the sulphite process, but it will be understood by those familiar with this field of technology, that the concepts and features described and claimed herein, can readily be incorporated into the other types of chemical pulping processes. in this specification, unless otherwise stated, the term "degree of

disintegration" refers to the decrease in percentage points of wood particles having a size of >03 mm due to conversion into particles of smaller size during a unit operation or in the course of a series of connected unit operations.

By squeezing the wood chips by volumetric compression and then releasing the compressed wood chips in an impregnation liquor, the impregnation liquor is fiilirig up the void volumes of the fiber structures caused by the

compression where air and liquor are removed. The invention proposes a multistage principle, where the wood chips are compressed at least three times two of which in acidk? leaching liquor. The proposed squeezing and multi-stage principle ensures an efficient impregnation of the wood chips and thus an efficient leaching out of meta!s into the leaching liquor absorbed by the wood chips.

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

The Invention is performed at atmospheric pressure, without- the need for pressurized reactors and other equipment associated With conventional pressure-based teaching processes.

The inventive process has intrinsic deaeration through the first squeezing step and basically does hot leave any air in the wood chips. This is a significant advantage since air in the wood chips is detrimental to the subsequent cooking process, and typically requires additional processing, such as steaming of the chips in dedicated steaming vessels.

The wood chips treated in the inventive process or in the inventive system may be wood chips of softwood or hardwood, or a mixture of wood chips of softwood and hardwood. The wood chips may come from any or all parts of a tree, such as trunk wood or ramiaj (smaller branch) wood, or a combination thereof. In some embodiments of the invention the wood chips are softwood chips originating from trees of the family Pinaceae, such as cedars, fins, heirilocks, larches, pines, spruces, or mixtures of softwood chips originating from two or more of cedars, firs, hemlocks, larches, pines, and spruces, specifically pine and spruce.

In some embodiments of the invention the wood chips are hardwood chips originating from trees of any of the families Aceraceae (maple family), Betulaeeae (birch family), Fagaceae (beech family), Myrtaceae (myrtle family), or Saiicaceae (willow family),

In some embodiments of the invention the wood chips are hardwood chips originating from trees of any of the families Betulaeeae (birch family)_* such as Betula (birches), or Myrtaceae (myrtle family)* such as Eucalyptus, or a mixture thereof.

In some embodiments of the invention the undesirable inorganic elements are chosen among calcium, magnesium, potassium, manganese, barium, iron, aluminium, copper, nickel; chromium and zinc, or combinations thereof.

In one embodiment of the invention the undesirable inorganic elements are calcium, magnesium, manganese, or combinations thereof. In some embodiments of the invention the squeezed wood chips from step (V) are allowed to expand in a third acidic impregnation liquor, whereby the wood chips absorb part of said third acidic impregnation liquor into said volumes with entrapped air; whereafter the expanded wood chips are squeezed at atmospheric pressure by volumetric compression in a fourth compression means, whereby at least part of the third acidic impregnation liquor absorbed in said volumes escapes from the volumes and is used as at least 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 acidic impregnation liquors followed by squeezing of the wood chips in fifth and further and further compression means and so on. in some embodiments of the invention the first, second, third, and/or any further compression means is/are screw presstes), extrusion press(es), or screw pump(s). Specifically, the compression means may be compressing screw feeder(s), such as, for instance, IMPRESSAFINER® systems from Andritz, which are disclosed and described in US 2,975,096, and/br PREX^® (pressure-expansion) system from Valmet

The inventors have found that compressing screw feeders may be used in leaching treatment in accordance with the present invention to increase the process yield in chemical pulp production. The compressing/squeezing function of the compressing screw feeders provides for extremely effective processes of air removal, impregnation with leaching liquor, and removal of leaching liquor. Thus, a very effective removal of air and metals from the wood chips can be achieved before introduction into a digester unit.

The compression ratio of the compression means may be in a range that causes a low degree of disintegration, such as less than about a 20

percentage points decrease, specifically Jess than about a 30 percentage points decrease, and more particularly less than about a 50 percentage points decrease.

The first, second, third, and/or any further acidic impregnation liquor(s) may be any acidic liquid. Examples of acidic impregnation liquors are mineral acids, such as sulphuric acid, or mixtures of such acids, Other examples are acidic aqueous liquors with a low content of inorganic elements that are undesirable in a pulping process, such as pulp mill process water with a low content of undesirable inorganic components, e. g. bleach plant spent liquor or condensate, ih some embodiments of the invention the acidic impregnation fiqudr(s) is/are pulp mill effluents, bleach plant process water¹, or bleach plant effluents. In some embodiments of the invention, the pulp mill process water is a condensate, such as an evaporation condensates

In some embodiments of the inventive process at least part of the second acidic impregnation liquor, which is absorbed ih the volumes end escapes from the volumes due to volumetric compression in the third compression means, is recirculated and used as at least part of the first acidic

impregnation liquor. The proposed leaching principle with vofumelrfc compression and expansion has proven very effective in a two stage leaching process with Such countercurrent application of leaching liquor.

With multi-stage leaching (at least two stages of compression plus expansion) and a counter current flow of teaching liquors, the efficiency Of the transport phenomena increases as the chemical potential is recovered In counter current direction, so as to provide refreshment arid enhancement of leaching chemical strength and chemical driving force in the last absorption stage.

In some embodiments at least 25 wt-%, specifically at least 50 wt-%, and more specifically at least 75 wt-%, of the second acidic impregnation liquor, based on the weight of the second acidic impregnation liquor, which is absorbed in the volumes and escapes from the volumes due to volumetric compression in the third compression means, is used as at least 25 wt-%, specifically at least 50 wt-%, and more specifically at feast 75 wt-%, of the first acidic impregnation liquor, based on the weight of the first acidic impregnation liquor.

In some embodiments at least 25 wt«%, specifically at least 50 wt-%, and more specifically at least 75 wt-%, of the third acidic impregnation liquor, based on the weight of the third acidic impregnation liquor, which is absorbed ih the volumes and escapes from the volumes due to volumetric compression in the fourth compression means, is used as at least 25 wt-%, specifically at least 50 wt-%, and more specifically at least 75 wt*%, of the second acidic impregnation liquor, based on the weight 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 at least 25 wt-%, specifically at least 50 wt-%, and more specifically at least 75 wt-%, of a downstream acidic impregnation liquor, based on the weight of the downstream acidic

impregnation liquor, which is absorbed in the volumes and escapes from the volumes due to volumetric compression in a compression means, being used as at least 25 wt-%, specifically at least SO wt-%, and more specifically at feast 75 wt-%, of an upstream acidic impregnation liquor, based on the weight of 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 are tightly bound to negatively charged moieties, mostly carboxylic groups, in the wood 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. In some embodiments of the invention the squeezed wood chips are allowed to expand in the first, second, third, and/or any further acidic impregnation liquor(s) at a pH of from about 1,5 to 5, specifically at a pH of from about 2 to 4, in particular at a pH of from about 2 to 3. In some embodiments of the invention the squeezed wood chips are allowed to expand in the first, second, third, and/or any further acidic impregnation liquors) at a temperature of from about 60 to 10Q°G, specifically at a temperature of from about 85 to 95"C. in contrast to conventional pretreatment processes, which are typically burdened with process steps with long retention or residence times, the present invention offers a process with potentially very short retention times in the compression means and impregnation vessels. The

compression/expansion principle and the extremely tow content of air inside the chips make the leaching liquor entrain far within the chips, which provides for very efficient leaching. The retention times can for example be in minute^ ranges, but would ultimately be determined by design considerations relating to the surrounding system/equipment. As chemical pulping usually does not involve any refining stage/step, there is usually no reason tp adjust the pH to a level suitable for refining. Hence, in some embodiments of the invention no pH adjustment is done between any one of steps (!) - (V) and prior to introduction of said chips into the digester, in particular not to a level suitable for refining,: specifically not to pH≥ 4, more specifically not to pH in the range of 4-9, even more specifically not by adding white water or water mixed with one or more basic compounds. It should be understood that although pH is not intentionally adjusted in the course of the process according to said embodiments pH may of course change, even to pH as 4, due to chemical reactions during the process, such as ton exchanging, where cations like, for instance, calcium is replaced by hydrogen ions in the wood structure.

In some embodiments of the invention the production line for chemical pulp is a production line in which a sulphite process or a bisulfite process is performed.

In some embodiments of the invention the digester for cooking in a production line for chemical pulp is a digester for continuous cooking.

The invention applies both to processes and systems where the Chips are directly introduced into a digester after the leaching treatment (pretreatment) of the invention and to processes and systems with some kind of intermediate equipment, such as transporters, chip bins, etc., between the leaching treatment and the digester. "Digester" here means a digester unit comprising one or more digesters/reactors with associated equipment. The digester unit may be batch or continuous, single vessel or two vessel, etc. For example, the invention applies also to processes and systems with a two-vessel continuous digester of the type comprising an impregnation vessel and a digester vessel. In some embodiments, the inventive system also comprises means for conveying at least part of second acidic impregnation liquor, that has been squeezed out from the wood chips, to the first impregnation vessel.

Fig. 1 is a schematic flow chart of a process of treating wood chips

according to an example embodiment of the invention. Wood chips, having fiber structures with volumes with entrapped air, are in accordance with the invention pretreated with a combined deaeratipn and leaching process for efficient reduction of the content of undesirable inorganic elements in said chips prior to introduction of said chips into a digester for cooking in a production line for chemical pulp. Ail steps are performed at atmospheric pressure. In step S1 , the wood chips are squeezed at atmospheric pressure by volumetric compression in a first compression means, whereby at (east part of said entrapped air escapes from said volumes. This can with advantage be achieved by means of a compressing screw feeder, which ensures extremely low levels of remaining air in the wood chips.

The wood chips are thereafter processed with a first acidic leaching liquor in a first leaching stage (S2-S4). The squeezed wood chips are in step S2 allowed to expand in the acidic leaching Hquor at a pH of from about 1.5 to 5 for metals to leach out from the chips. The wood chips absorb leaching liquor into said volumes and inorganic elements that are metals are extracted out into the leaching liquor. Due to the deaeration in step Sli the wood chips are efficiently impregnated with leaching liquor in step S2. in step S3, the expanded wood chips are squeezed at atmospheric pressure by volumetric compression in a second compression means, whereby acidic leaching liquor, with extracted (i.e. leached out) metals, absorbed in said volumes escapes from the chips. The leaching liquor with extracted metals are removed/withdrawn from the second compression means in step S4, and may for example be further processed or purified within or outside the chemical pulping production line. The second compression means can with advantage be a compressing screw feeder for very efficient removal (pressing out) of the liquid stream with the leached out metals.

The wood chips are thereafter processed with another acidic leaching liquor in a second leaching stage (S5-S7). The squeezed wood chips from the first leaching stage are in step S5 allowed to expand in a second acidic leaching liquor at a pH of from about 1 ,5 to 5 for metals to leach out from the chips. The Wood chips absorb leaching liquor into sard volumes and inorganic elements that are metals are extracted out Due to the squeezing and withdrawal in steps S3, S4, the wood chips are efficiently impregnated with leaching liquor in step S5. In step S6, the expanded wood chips are squeezed at atmospheric pressure by volumetric compression in a third compression means, whereby acidic leaching liquor, with extracted (i.e. leached out) metals, absorbed in said volumes escapes from the volumes. The leaching liquor with extracted metals are removed/withdrawn from the third compression means in step S7, The third compression means can with advantage be a compressing screw feeder for very efficient removal of the liquid with the leached out metals.

In some preferred embodiments, second acidic leaching liquor withdrawn from the third compression means in step S7 is in an optional recirculation step S8, partly or wholly, returned to step S2 and used as first acidic leaching liquor in the first leaching stage. This is in Pig. 1 illustrated with dashed arrows for recovered leaching liquor and incoming leaching liquor,

respectively. By using recovered leaching liquor from step S7 as the first leaching liquor in step S2 and cleaner leaching liquor, e.g. fresh acidic liquor input to the system, downstream, as the second leaching liquor in step S5 a very efficient leaching process is achieved making maximum use of the chemical potential of the leaching liquor. This recirculation has proven especially advantageous in the proposed leaching process of the invention, since the deaeration in step S1 gives a high impregnation and leaching potential.

Analysis relating to certain experiments pertaining to some embodiments of the invention have indicated an increase of the level of iron in the wood chips dunng the course of the inventive process. The only possible origin of this added iron would be iron released from the process equipment during those particular experiments. Still, the content of undesirable inorganic elements in the wood chips is reduced in the sense that the content of undesirable inorganic elements originally present in the wood chips, i.e. prior to treatment, is reduced during the course of the invehtivd process. This fact is not changed by the use of experimental equipment that issues iron; the iron contributed by that experimental equipment should be disregarded, because such contribution would not take place if equipment with less or no propensity to issue iron would have been used. Nevertheless, some embodiments of the invention includes the proviso that ft involve reduction of the content of undesirable inorganic elements except iron in the wood chips.

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

EXAMPLES

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

Raw material and impregnation liquor

The raw material used was a wood chips mixture of pine and spruce from Domsjo Fabriker AB, 0mskoldsvik, Sweden, the pine content in the mixture was maximum 40% as target, and the dry content of the wood chips mixture was 52%. The wood chips were screened on a continuous vibrating screen equipped with hole plates of 035 mm and 08 mm for removal of oversized chips and sawdust. To ensure proper feeding stability during the experiments only the accepted chips fraction (08 - 35 mm) was used, since a small pilot plant equipment was used, which was more sensitive to oversized chips and sawdust than mill scale equipment.

The impregnation liquor used was warm condensate collected from the pulp mill at Domsjo Fabriker one day before the experiments. The pH of the condensate was about 2.5 and it was collected at a temperature of about 90°Q, The condensate was sampled in insulated CI PAX? containers in order to retain heat before the PREX^® leaching, and was heated in a heat exchanger before the respective experiments to reach desired temperature. PREX® pilot plant

The 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 an impregnation vessel- In the PREX* screw, the chips were compressed and then "released* into the impregnation liquid in ah impregnation vessel. The wood chips were then transported via a transport screw to a retention bin before being collected at the discharge screw of the retention bin. Pilot program

The feed rates at the different passes were measured using warm water (90^eC) instead of impregnation liquor before the experiment and are set forth in Table 1. Table 1

kge.o. - kg of bone dry material

Dry matter content was determined according to SGAN-CM 39:94.

The process data that were prevailing during the experiments are set forth in Table 2 and Table 3.

Table 2 PREX® system process data

Analyses

The 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 wodd chips and filtrate samples) were collected during all passes.

Table 4

The results from these analysis are set forth in Table 5.

Table 5

The particle size distribution shows that the wood chips contained more than 70% of the material (by weight) at a particle size between 3 - 13 mm after the 2^nd and 3^rd pass.

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

The dust part (503mm) was rather high for the 2*¹ and 3* passes, between 6 - 13%. The liquor uptake of the wood chips was also measured; the results are set forth in Table 6.

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

Table 6

The liquor uptake of the wood chips increased With the number of passes, presumably due to a more disintegrated wood structure.

The wood chips and the filtrates were analyzed with regard to a number of parameters as set forth in Tables 7 and 8, respectively;

Table 7

As seen in Table 7 the level of metals are reduced in the chips, except for iron, for which the level on the contrary increases; the only logical explanation for this would be that that iron was released from the process equipment during the experiments.

Table 8

Claims

1. A process of treating wood chips with leaching liquor for reduction of the content of undesirable inorganic elements in said chips prior to introduction of said chips into a digester for cooking in a production line for chemical pulp, wherein the wood chips, having fiber structures with volumes with entrapped air, are

(!) squeezed at atmospheric pressure by volumetric compression in a first compression means, whereby at least part of said entrapped air escapes from said volumes;

(II) the squeezed wood chips are allowed to expand in a first acidic impregnation liquor at a pH of from about 1,5 to 5, whereby the wood chips absorb part of said first acidic impregnation liquor into said volumes and inorganic elements that are metals are extracted Out;

(III) the expanded Wood chips are squeezed at atmospheric pressure by volumetric compression in a second compression means, whereby at least part of said first acidic impregnation liquor absorbed in sajd volumes escapes from the volumes:

(IV) the squeezed wood Chips from step (ill) are allowed to expand in a second acidic impregnation liquor at a pH of from about 1.5 to 5, whereby the wood chips absorb part of said second acidic impregnation liquor into said volumes and inorganic elements that are metals are extracted out; and

(V) the expended wood chips are squeezed at atmospheric pressure by volumetric compression jn a third compression means, Whereby at least part of the second acidic impregnation liquor absorbed in said volumes escapes from the volumes.

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

3. 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 liquor, whereby the wood chips absorb part of said third acidic impregnation liquor into said volumes with entrapped air; whereafter the expanded wood chips are squeezed at atmospheric pressure by volumetric compression in a fourth compression means, whereby at least part of the third acidic impregnation liquor absorbed in said volumes escapes from the volumes and is used as at least part of the second acidic impregnation liquor.

4. The process according to any one of the preceding claims, wherein said first, second, third and/or fourth compression means is/are compressing screw feeders).

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

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

7. The process according to any one of the preceding claims, wherein the undesirable 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 process for cooking of wood chips in a production line for chemical pulp, comprising pretreating the wood chips according to any of claims 1-8 and thereafter introducing said chips into a digester for cooking in a production line for chemical puip and cooking the wood chips in the digester, without any step of deaeration between the pretreatment and the cooking step.

10. A system for treating wood chips, having fiber structures with volumes with entrapped air, with leaching liquor for reduction of the content of undesirable inorganic elements in said chips prior to introduction of said chips info a digester for cooking in a production line for chemical pulp, comprising

- a first compression means for squeezing out entrapped air from the wood Chips;

- a first impregnation vessel, for impregnation of the wood chips With a first acidic impregnation liquor at a pH of from about 1.5 to 5 after said squeezing Cut of entrapped air from the wood chips; whereby inorganic elements that are metals are extracted out;

- a second compression means for squeezing out first acidic impregnation liquor from the wood chips;

- a second impregnation vessel, for impregnation of the wood chips with a second acidic impregnation liquor at a pH of from about 1.5 to 5 after said squeezing out of entrapped air from the wood chips, whereby inorganic elements that are metals are extracted out; and

- third compression means for squeezing out second acidic impregnation liquor from the wood chips.

11. The system according to claim 10, which also comprises conveying means for recirculating at least part of the second acidic impregnation liquor, that has been squeezed out from the wood chips, to the first impregnation vessel.

12. A cooking system for production of chemical pulp comprising a pretreatment system for treating the wood chips according to claim 10 or 11 and a digester for cooking the wood chips, said digester being arranged downstream of said pretreatment system without any deaeration equipment between the pretreatment system and the digester.