WO2018147784A1

WO2018147784A1 - System and method for silica removal in a pulping process

Info

Publication number: WO2018147784A1
Application number: PCT/SE2018/050072
Authority: WO
Inventors: Fredrik Wilgotson; Stefan ANTONSSON; Lars Eriksson; Bengt WERNERSSON ÖSTBERG; Krister Olsson
Original assignee: Valmet Oy; Valmet AB
Current assignee: Valmet Technologies Oy; Valmet AB
Priority date: 2017-02-07
Filing date: 2018-01-31
Publication date: 2018-08-16
Anticipated expiration: 2019-08-07
Also published as: FI3580388T3; EP3580388B1; SE1750106A1; BR112019014396A2; PT3580388T; ES2947013T3; SE540305C2; CL2019002201A1; EP3580388A1; CN110249092A; EP3580388A4; PL3580388T3

Abstract

The invention relates generally to a system and a method for the removal of silica contaminants from wood chips in a prehydrolysis kraft cooking pulp production process, and in particular to a system and a method in which silica is removed from a warm chip slurry containing chips that have been soaked in fluid for an extended soaking time at an elevated temperature.

Description

SYSTEM AND METHOD FOR SILICA REMOVAL IN A PULPING PROCESS

TECHNICAL FIELD

The present invention relates generally to a system and a method for the removal of silica contaminants from wood chips in a prehydrolysis kraft cooking pulp production process, and in particular to a system and a method in which silica is removed from a warm chip slurry containing chips that have been soaked in fluid for an extended soaking time at an elevated temperature.

BACKGROUND

In many systems and methods for producing pulp from lignocellulosic material, contaminations in the form of, for example, sand and clay particles impose a problem. When producing dissolving pulp, where the end customer typically requires a high chemical purity, there is a problem with clogging of nozzles and other production equipment in, for example, viscose processes, due to high silica content if sand and clay particles are not removed from the dissolving pulp.

Many systems for the production of pulp therefore comprise systems and apparatuses for chip washing. For example, the U.S. Patent No. 5,882,509 to Perman discloses a system for treating and recycling wash water, wherein the system comprises a settling tank, a circulation tank, and a hydrocyclone. This system is space consuming, and is therefore difficult to adapt to modern, large-scale pulp production processes. Furthermore, the particle separation appears to be rather incomplete. The U.S. Patent No. 9,333,468 to Nguyen et al. describes a soak vessel for impregnating biomass with a liquid, such as a dilute acid, and for removing contaminants, wherein the soak vessel comprises an impeller assembly, which, in turn, comprises no less than three individual impellers which perform different functions. Thus, the system disclosed in this patent is rather complex and comprises many moving parts, which are exposed to wear. In effect, also this system is difficult to scale to match the requirements of today's large-scale pulp mills, at least in a cost-effective way. An object of the present invention is therefore to provide an improved system and an improved method for removing contaminants in a pulp production process. The system and the method should thereby be easy to implement in large-scale pulp production processes in a cost-effective way. The system and the method should in particular be effective for removing silica, and should also be particularly suitable for use in the production of dissolving pulp.

SUMMARY OF THE INVENTION The above-mentioned objects are achieved with a system and a method according to the independent claims. Preferred embodiments are set forth in the dependent claims.

According to one embodiment of the present invention, a system for silica removal in a process for pulp production, for example a process for the production of dissolving pulp, comprises a vessel; a chip inlet and a steam inlet, which both are arranged in a top section of the vessel; a first fluid inlet arranged below the top section and adapted for inlet of a fluid, such as water or a diluted mineral acid, e.g. diluted sulphuric acid, to provide a chip slurry having a specified chip slurry temperature in a chip soaking zone disposed below the top section; a chip slurry outlet arranged in a bottom section of the vessel and connected to a chip slurry transportation pipe, which extends out from the vessel for transport of the chip slurry for further processing outside of the vessel; a second fluid inlet arranged in the bottom section of the vessel for dilution of the chip slurry transported (or to be transported) in the chip slurry transportation pipe to a liquor-to-wood ratio of at least 10: 1 with a dilution fluid, such as water or a diluted mineral acid, e.g. diluted sulphuric acid, having a specified dilution fluid temperature that is higher than the chip slurry temperature; a dewatering device connected to the chip slurry transportation pipe for dewatering of the chip slurry transported therein and for providing a first flow of chip slurry having a liquor- to-wood ratio of less than 3 : 1 and for providing a second flow of silica-containing fluid; a return pipe connected to the dewatering device for transport of the second flow of silica- containing fluid and connected to the first fluid inlet or to the second fluid inlet; and at least one silica separator, which is arranged in the return pipe. As defined herein, a chip soaking zone is a zone disposed below a top section of the vessel, and in which chips introduced into the vessel via the chip inlet are submerged in or covered by fluid, e.g. water or a diluted mineral acid, e.g. diluted sulphuric acid, which is introduced into the vessel via the first fluid inlet and/or the second fluid inlet. Further, to provide an efficient removal of silica, the chips should dwell in this chip soaking zone for a specified soaking time of about 30 minutes to 120 minutes. Within this time interval, silica-containing contaminants, such as sand and clay, which adhere to the surfaces of the chips or which are more or less embedded in the chips, should be dissolved and released from the chips. Further, the method and the system according to the invention are particularly suited for use in an open system, i.e. the vessel containing the chip slurry, is an open, i.e. non-pressurized, vessel, which means that the temperature of the chip slurry in the soaking zone cannot be too high, to avoid excessive boiling of the chip slurry, and the chip slurry temperature should therefore preferably be about 95 °C to about 105 °C. Furthermore, the dilution fluid temperature should be chosen such that the dilution fluid, when added to the chip slurry contained in the vessel, does not cause excessive boiling of the chip slurry, and in practice a dilution fluid temperature of about 110 °C to about 130 °C is useful.

As can be appreciated, the chip slurry is typically pumped out from the vessel, and the consistency of the chip slurry being transported in the chip slurry transportation pipe, or being contained in the bottom section of the vessel for transport in the chip slurry transportation pipe, should therefore allow effective pumping, whereby a liquor-to-wood ratio of at least 10: 1 is useful for the chip slurry in the chip slurry transportation pipe, i.e. the liquor-to-wood ratio of at least 10: 1 relates to the consistency of the chip slurry in the transportation pipe, where the liquor-to-wood ratio - if one so wishes - can be measured.

In another embodiment of the present invention, said second fluid inlet, which alternatively may be referred to as a dilution fluid inlet, is not arranged in a bottom section of the vessel, but is instead arranged in the chip slurry transportation pipe. In still another embodiment, said second fluid inlet is divided, such that a second fluid inlet is arranged both in a bottom section of the vessel and in the chip slurry transportation pipe. Similarly, in embodiments of the invention, the return pipe for transport of the second flow of silica containing fluid is connected to both the first fluid inlet and to the second fluid inlet. To ensure effective mixing of the chip slurry and the dilution fluid, a mixer can be arranged in the chip slurry transportation pipe; and to enhance the silica-separating action of said at least one silica separator, an inlet for adding an agglomerating and/or flocculating agent, such as polyacrylamide, can be arranged in the return pipe before, as seen in flow direction, the first one of said at least one silica separator. Moreover, if more than one silica separator is provided, these silica separators can be arranged in parallel or in series; and to ensure effective heating of the dilution fluid, the return pipe can be provided with a heater, e.g. a heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be further explained hereinafter by means of non-limiting examples and with reference to the appended drawings, wherein: Fig. 1 is a schematic illustration of a first embodiment of a system for silica removal in a pulp production process according to the present invention.

Fig. 2 is a schematic illustration of a second embodiment of a system for silica removal in a pulp production process according to the present invention.

Fig. 3 is a schematic illustration of a third embodiment of a system for silica removal in a pulp production process according to the present invention.

Fig. 4 is a schematic illustration of a fourth embodiment of a system for silica removal in a pulp production process according to the present invention.

Fig. 5 is a schematic illustration of a fifth embodiment of a system for silica removal in a pulp production process according to the present invention. Fig. 6 is a schematic illustration of a sixth embodiment of a system for silica removal in a pulp production process according to the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

For all embodiments presented below and illustrated in the figures, corresponding parts and components have been given the same reference numerals.

Fig. 1 illustrates schematically a first embodiment of a system 1 for silica removal in a pulping process according to the present invention. The silica-removal system 1 comprises at least a vessel 2, which has a top section 3 and a bottom section 4; a chip inlet 5 provided in the top section 3 for introduction of wood chips, a steam inlet 6 provided in the top section 3 for introduction of steam; a first fluid inlet 7 provided in the vessel and disposed below the top section 3 for introduction of a fluid, such as water or a diluted mineral acid, e.g. diluted sulphuric acid, coming from a fluid source 19, e.g. a fluid tank 19, to provide a chip slurry having a chip slurry temperature in a chip soaking zone 8, which is disposed below the top section 3, the chip soaking zone 8 being a zone in which chips are submerged in or covered by fluid for a specified soaking time; a chip slurry outlet 9 provided in the bottom section 4 and connected to a chip slurry transportation pipe 10 for transport of the chip slurry out from the vessel 2; a second fluid inlet 11 arranged in the slurry transportation pipe 10 for dilution of the chip slurry to a liquor-to-wood ratio of at least 10: 1, as measured in the chip slurry transportation pipe 10, with a dilution fluid, such as water or a diluted mineral acid, e.g. diluted sulphuric acid, coming from a dilution fluid source 18, e.g. a dilution fluid tank 18, and having a dilution fluid temperature that is higher than the chip slurry temperature; a dewatering device 12 connected to the chip slurry transportation pipe 10 for dewatering of the chip slurry contained therein and for providing a first flow 13 of chip slurry having a liquor-to-wood ration less than 3 : 1 and providing a second flow 14 of silica-containing fluid; a return pipe 15 connected to the dewatering device 12 for transport of the second flow 14 of silica-containing fluid and connected to the second fluid inlet 11; and at least one silica separator 16 provided in the return pipe 15. Further, in this embodiment, the soaking time for the chips in the chip soaking zone 8 is about 30 minutes to 120 minutes, the chip slurry temperature in the chip soaking zone 8 is about 95-105 °C, and the dilution fluid temperature is about 1 10-130 °C. Optionally, a heater 17, e.g. heat exchanger 17, can be provided in the return pipe 15, to warm the dilution fluid to the specified dilution fluid temperature, if needed. In the first embodiment shown in Fig. 1, the heater 17 is disposed close to the dilution fluid source 18, such that the heater 17 essentially only warm up the dilution fluid coming from the dilution fluid source 18 and does not warm up the second flow 14 of silica-containing fluid. Further, also optionally, a mixer 20 can be provided in the chip slurry transportation pipe 10 before - as seen in flow direction - the dewatering device 12, to ensure efficient mixing of the dilution fluid and the chip slurry.

As stated above, the first flow 13 of chip slurry has a liquor-to-wood ratio of at least 3 : 1. This rather low value, i.e. a relatively large amount of wood in relation to fluid, indicates that the dewatering device is effective and that the first flow 13 contains a relatively small amount of silica. A low value also implies that the chip slurry contains a relatively small amount of fluid that has to be heated in following processing steps. Such processing steps are not part of the present invention, but should nevertheless be considered when designing an efficient system for silica removal. Dewatering devices that are suitable for use in all embodiments presented herein are the dewatering devices manufactured and marketed by the company Valmet AB, for example the top separators marketed by Valmet AB. Also, the sand separators manufactured and marketed by Valmet AB are suitable for use as silica separators in all embodiments presented herein. In Fig. 2, a second embodiment of a system 1 for silica removal according to the invention is schematically illustrated. The silica-removal system 1 comprises at least a vessel 2, which has a top section 3 and a bottom section 4; a chip inlet 5 provided in the top section 3 for introduction of wood chips, a steam inlet 6 provided in the top section 3 for introduction of steam; a first fluid inlet 7 provided in the vessel and disposed below the top section 3 for introduction of a fluid, such as water or a diluted mineral acid, e.g. diluted sulphuric acid, coming from a fluid source 19, e.g. a fluid tank 19, to provide a chip slurry having a chip slurry temperature in a chip soaking zone 8, which is disposed below the top section 3, the chip soaking zone 8 being a zone in which chips are submerged in or covered by fluid for a specified soaking time; a chip slurry outlet 9 provided in the bottom section 4 and connected to a chip slurry transportation pipe 10 for transport of the chip slurry out from the vessel 2; a second fluid inlet 11 arranged in the bottom section 4 for dilution of the chip slurry to a liquor-to-wood ratio of at least 10: 1, as measured in the chip slurry transportation pipe 10, with a dilution fluid, such as water or a diluted mineral acid, e.g. diluted sulphuric acid, coming from a dilution fluid source 18, e.g. a dilution fluid tank 18, and having a dilution fluid temperature that is higher than the chip slurry temperature; a dewatering device 12 connected to the chip slurry transportation pipe 10 for dewatering of the chip slurry contained therein and for providing a first flow 13 of chip slurry having a liquor-to-wood ration less than 3 : 1 and providing a second flow 14 of silica-containing fluid; a return pipe 15 connected to the dewatering device 12 for transport of the second flow 14 of silica-containing fluid and connected to the second fluid inlet 11; and at least one silica separator 16 provided in the return pipe 15. Further, in this embodiment, the soaking time for the chips in the chip soaking zone 8 is about 30 minutes to 120 minutes, the chip slurry temperature in the chip soaking zone 8 is about 95-150 °C, and the dilution fluid temperature is about 1 10-130 °C. Optionally, a heater 17, e.g. heat exchanger 17, can be provided in the return pipe 15, to warm the dilution fluid to the specified dilution fluid temperature, if needed. In the second embodiment shown in Fig. 2, the heater 17 is disposed in a position such that the heater 17 warm up both the dilution fluid coming from the dilution fluid source 18 and also the second flow 14 of silica-containing fluid. Further, also optionally, a mixer 20 can be provided in the chip slurry transportation pipe 10 before - as seen in flow direction - the dewatering device 12, to ensure efficient mixing of the dilution fluid and the chip slurry.

Fig. 3 illustrates schematically a third embodiment of a system 1 for silica removal in a pulping process according to the present invention. The silica-removal system 1 comprises at least a vessel 2, which has a top section 3 and a bottom section 4; a chip inlet 5 provided in the top section 3 for introduction of wood chips, a steam inlet 6 provided in the top section 3 for introduction of steam; a first fluid inlet 7 provided in the vessel and disposed below the top section 3 for introduction of a fluid, such as water or a diluted mineral acid, e.g. diluted sulphuric acid, coming from a fluid source 19, e.g. a fluid tank 19, to provide a chip slurry having a chip slurry temperature in a chip soaking zone 8, which is disposed below the top section 3, the chip soaking zone 8 being a zone in which chips are submerged in or covered by fluid for a specified soaking time; a chip slurry outlet 9 provided in the bottom section 4 and connected to a chip slurry transportation pipe 10 for transport of the chip slurry out from the vessel 2; a second fluid inlet 11 arranged in the slurry transportation pipe 10 for dilution of the chip slurry to a liquor-to-wood ratio of at least 10: 1, as measured in the chip slurry transportation pipe 10, with a dilution fluid having a dilution fluid temperature that is higher than the chip slurry temperature; a dewatering device 12 connected to the chip slurry transportation pipe 10 for dewatering of the chip slurry contained therein and for providing a first flow 13 of chip slurry having a liquor-to-wood ration less than 3 : 1 and providing a second flow 14 of silica-containing fluid; a return pipe 15 connected to the dewatering device 12 for transport of the second flow 14 of silica-containing fluid and connected to the second fluid inlet 11; and at least one silica separator 16 provided in the return pipe 15. Further, in this embodiment, the soaking time for the chips in the chip soaking zone 8 is about 30 minutes to 120 minutes, the chip slurry temperature in the chip soaking zone 8 is about 95-105 °C, and the dilution fluid temperature is about 110-130 °C. As can be noted, in this embodiment, there is no extra source of dilution fluid, such as a dilution fluid source 18, which was optionally arranged in the first embodiment and the second embodiment, respectively. Optionally, a heater 17, e.g. heat exchanger 17, can be provided in the return pipe 15, to warm the dilution fluid to the specified dilution fluid temperature, if needed. Also optionally, a mixer 20 can be provided in the chip slurry transportation pipe 10 before - as seen in flow direction - the dewatering device 12, to ensure efficient mixing of the dilution fluid and the chip slurry.

Fig. 4 illustrates schematically a fourth embodiment of a system 1 for silica removal in a pulping process according to the present invention. The silica-removal system 1 comprises at least a vessel 2, which has a top section 3 and a bottom section 4; a chip inlet 5 provided in the top section 3 for introduction of wood chips, a steam inlet 6 provided in the top section 3 for introduction of steam; a first fluid inlet 7 provided in the vessel and disposed below the top section 3 for introduction of a fluid, such as water or a diluted mineral acid, e.g. diluted sulphuric acid, coming from a fluid source 19, e.g. a fluid tank 19, to provide a chip slurry having a chip slurry temperature in a chip soaking zone 8, which is disposed below the top section 3, the chip soaking zone 8 being a zone in which chips are submerged in or covered by fluid for a specified soaking time; a chip slurry outlet 9 provided in the bottom section 4 and connected to a chip slurry transportation pipe 10 for transport of the chip slurry out from the vessel 2; a second fluid inlet 11 arranged in the bottom section 4 for dilution of the chip slurry to a liquor-to-wood ratio of at least 10: 1, as measured in the chip slurry transportation pipe 10, with a dilution fluid having a dilution fluid temperature that is higher than the chip slurry temperature; a dewatering device 12 connected to the chip slurry transportation pipe 10 for dewatering of the chip slurry contained therein and for providing a first flow 13 of chip slurry having a liquor-to-wood ration less than 3 : 1 and providing a second flow 14 of silica-containing fluid; a return pipe

15 connected to the dewatering device 12 for transport of the second flow 14 of silica- containing fluid and connected to the second fluid inlet 11; and at least one silica separator

16 provided in the return pipe 15. Further, in this embodiment, the soaking time for the chips in the chip soaking zone 8 is about 30 minutes to 120 minutes, the chip slurry temperature in the chip soaking zone 8 is about 95-105 °C, and the dilution fluid temperature is about 110-130 °C. As can be noted, in this embodiment, there is no extra source of dilution fluid, such as a dilution fluid source 18, which was optionally arranged in the first embodiment and the second embodiment, respectively. Optionally, a heater 17, e.g. heat exchanger 17, can be provided in the return pipe 15, to warm the dilution fluid to the specified dilution fluid temperature, if needed. Also optionally, a mixer 20 can be provided in the chip slurry transportation pipe 10 before - as seen in flow direction - the dewatering device 12, to ensure efficient mixing of the dilution fluid and the chip slurry. In Fig. 5, a fifth embodiment of a system 1 for silica removal according to the invention is schematically illustrated. The silica-removal system 1 comprises at least a vessel 2, which has a top section 3 and a bottom section 4; a chip inlet 5 provided in the top section 3 for introduction of wood chips, a steam inlet 6 provided in the top section 3 for introduction of steam; a first fluid inlet 7 provided in the vessel and disposed below the top section 3 for introduction of a fluid, such as water or a diluted mineral acid, e.g. diluted sulphuric acid, coming from a fluid source 19, e.g. a fluid tank 19, to provide a chip slurry having a chip slurry temperature in a chip soaking zone 8, which is disposed below the top section 3, the chip soaking zone 8 being a zone in which chips are submerged in or covered by fluid for a specified soaking time; a chip slurry outlet 9 provided in the bottom section 4 and connected to a chip slurry transportation pipe 10 for transport of the chip slurry out from the vessel 2; a second fluid inlet 11 arranged in the chip slurry transportation pipe 10 for dilution of the chip slurry to a liquor-to-wood ratio of at least 10: 1, as measured in the chip slurry transportation pipe 10, with a dilution fluid, such as water or a diluted mineral acid, e.g. diluted sulphuric acid, coming from a dilution fluid source 18, e.g. a dilution fluid tank 18, and having a dilution fluid temperature that is higher than the chip slurry temperature; a dewatering device 12 connected to the chip slurry transportation pipe 10 for dewatering of the chip slurry contained therein and for providing a first flow 13 of chip slurry having a liquor-to-wood ration less than 3 : 1 and providing a second flow 14 of silica-containing fluid; a return pipe 15 connected to the dewatering device 12 for transport of the second flow 14 of silica-containing fluid and connected to the first fluid inlet 7; and at least one silica separator 16 provided in the return pipe 15. Further, in this embodiment, the soaking time for the chips in the chip soaking zone 8 is about 30 minutes to 120 minutes, the chip slurry temperature in the chip soaking zone 8 is about 95-150 °C, and the dilution fluid temperature is about 1 10-130 °C. Optionally, a heater 17, e.g. heat exchanger 17, can be provided before the second fluid inlet 1 1, to warm the dilution fluid coming from the dilution fluid source 18 to the specified dilution fluid temperature, if needed. Further, also optionally, a mixer 20 can be provided in the chip slurry transportation pipe 10 before - as seen in flow direction - the dewatering device 12, to ensure efficient mixing of the dilution fluid and the chip slurry.

In Fig. 6, a sixth embodiment of a system 1 for silica removal according to the invention is schematically illustrated. The silica-removal system 1 comprises at least a vessel 2, which has a top section 3 and a bottom section 4; a chip inlet 5 provided in the top section 3 for introduction of wood chips, a steam inlet 6 provided in the top section 3 for introduction of steam; a first fluid inlet 7 provided in the vessel and disposed below the top section 3 for introduction of a fluid, such as water or a diluted mineral acid, e.g. diluted sulphuric acid, to provide a chip slurry having a chip slurry temperature in a chip soaking zone 8, which is disposed below the top section 3, the chip soaking zone 8 being a zone in which chips are submerged in or covered by fluid for a specified soaking time; a chip slurry outlet 9 provided in the bottom section 4 and connected to a chip slurry transportation pipe 10 for transport of the chip slurry out from the vessel 2; a second fluid inlet 1 1 arranged in the bottom section 4 for dilution of the chip slurry to a liquor-to-wood ratio of at least 10: 1, as measured in the chip slurry transportation pipe 10, with a dilution fluid, such as water or a diluted mineral acid, e.g. diluted sulphuric acid, coming from a dilution fluid source 18, e.g. a dilution fluid tank 18, and having a dilution fluid temperature that is higher than the chip slurry temperature; a dewatering device 12 connected to the chip slurry transportation pipe 10 for dewatering of the chip slurry contained therein and for providing a first flow 13 of chip slurry having a liquor-to-wood ration less than 3 : 1 and providing a second flow 14 of silica-containing fluid; a return pipe 15 connected to the dewatering device 12 for transport of the second flow 14 of silica-containing fluid and connected to the first fluid inlet 7; and at least one silica separator 16 provided in the return pipe 15. Further, in this embodiment, the soaking time for the chips in the chip soaking zone 8 is about 30 minutes to 120 minutes, the chip slurry temperature in the chip soaking zone 8 is about 95-105 °C, and the dilution fluid temperature is about 110-130 °C. Optionally, a heater 17, e.g. heat exchanger 17, can be provided before the second fluid inlet 11 to warm the dilution fluid coming from the dilution fluid source 18 to the specified dilution fluid temperature, if needed. Further, also optionally, a mixer 20 can be provided in the chip slurry transportation pipe 10 before - as seen in flow direction - the dewatering device 12, to ensure efficient mixing of the dilution fluid and the chip slurry.

The embodiments described above and discussed in conjunction with the figures are only exemplifying, and many variations are possible. In particular, it is possible to combine all features of a particular embodiment with other features of other embodiments. For example, it is possible to arrange a return pipe in both a first fluid inlet and a second fluid inlet, which is connected to a bottom section of a vessel and/or connected to a chip slurry transportation pipe. Further, for all embodiments, but not shown in the figures, an inlet for adding an agglomerating and/or flocculating agent, such as polyacrylamide, can be arranged in the return pipe 15 before, as seen in flow direction, the first one of said at least one silica separator 16. Such an agglomerating agent can enhance the efficiency of the silica separator. The common feature for all embodiments presented herein is that the efficiency of the silica removal is enhanced by soaking the chips for a predetermined chip soaking time at rather high, specified temperature. Although the present invention has been described with reference to specific embodiments, also shown in the appended drawings, it will be apparent to those skilled in the art that many variations and modifications can be done within the scope of the invention as described in the specification and defined with reference to the claims below. For example, the invention has been described with reference to wood chips, but could be applicable for any lignocellulosic material, such as bamboo.

Claims

1. A system (1) for silica removal in a process for prehydrolysis kraft pulp production, the system (1) comprising at least:

a vessel (2) having a top section (3) and a bottom section (4);

a chip inlet (5) arranged in the top section (3) for introduction of wood chips into the vessel (2);

a steam inlet (6) arranged in the top section (3) for introduction of steam into the vessel (2);

a first fluid inlet (7) arranged in the vessel (2) for introduction of fluid into the vessel (2), to provide a chip slurry having a chip slurry temperature in a soaking zone (8) disposed below the top section (3), the chip soaking zone (8) being a zone in which wood chips are submerged in fluid for a specified soaking time;

a chip slurry outlet (9) arranged in the bottom section (4) and connected to a chip slurry transportation pipe (10) for transport of the chip slurry;

a second fluid inlet (1 1) arranged in the bottom section (4) and/or in the chip slurry transportation pipe (10) for dilution of the chip slurry in the chip slurry transportation pipe (10) to a liquid-to-wood ratio of at least 10: 1 with a dilution fluid having a temperature higher than the chip slurry temperature;

a dewatering device (12) connected to the chip slurry transportation pipe (10) for dewatering the chip slurry and creating a first flow (13) of chip slurry having a liquid-to-wood ratio of less than 3 : 1 and a second flow (14) of silica-containing fluid; and

a return pipe (15) connected to the dewatering device (12) for transport of the second flow (14) of silica-containing fluid and connected to the first fluid inlet (7) and/or to the second fluid inlet (1 1),

characterized in that at least one silica separator (16) is arranged in the return pipe (15), and that the system is arranged so that the soaking time is about between 30 minutes to 120 minutes, that the chip slurry temperature is about 95-105 °C, and that the dilution fluid temperature is about 1 10-130 °C.

2. The system (1) according to claim 1, characterized in that the process for pulp production is a process for dissolving pulp production.

3. The system (1) according to claim 1 or claim 2, characterized in that a mixer (20) is arranged in the chip slurry transportation pipe (15).

4. The system (1) according to any preceding claim, characterized in that an inlet for addition of an agglomerating agent is arranged in the return pipe (15).

5. A method for silica removal in a process for prehydrolysis kraft pulp production, the method comprising at least the following steps:

introducing wood chips into a vessel (2) via a chip inlet (5) arranged in a top section (3) of the vessel (2), said vessel comprising said top section (3) and a bottom section (4);

introducing steam into the vessel (2) via a steam inlet (6) arranged in the top section (3);

introducing fluid into the vessel (2) via a first fluid inlet (7) arranged in the vessel

(2), to provide a chip slurry having a chip slurry temperature in a soaking zone (8) disposed below the top section (3), the chip soaking zone (8) being a zone in which wood chips are submerged in fluid for a specified soaking time;

transporting the chip slurry out of the vessel (2) through a chip slurry outlet (9) arranged in the bottom section (4) and connected to a chip slurry transportation pipe (10);

diluting the chip slurry in the chip slurry transportation pipe (10) to a liquid-to- wood ration of at least 10: 1 with a dilution fluid having a temperature higher than the chip slurry temperature via a second fluid inlet (11) arranged in the bottom section (4) and/or in the chip slurry transportation pipe (10);

dewatering the chip slurry with a dewatering device (12) connected to the chip slurry transportation pipe (10) and creating a first flow (13) of chip slurry having a liquid-to-wood ratio of less than 3 : 1 and a second flow (14) of silica-containing fluid; and

transporting the second flow (14) of silica-containing fluid via a return pipe (15) connected to the dewatering device (12) and connected to the first fluid inlet (7) and/or to the second fluid inlet (11), characterized in that silica is removed from the silica-containing fluid by at least one silica separator (16) arranged in the return pipe (15), and that the soaking time is about between 30 minutes to 120 minutes, that the chip slurry temperature is about 95-105 °C, and that the dilution fluid temperature is about 110-130 °C.

6. The method according to claim 5, characterized in that the process for pulp production is a process for dissolving pulp production.

7. The method according to claim 5 or claim 6, characterized in that a mixer (20) is arranged in the chip slurry transportation pipe (15).

8. The method according to any preceding claims 5-7, characterized in that an inlet for addition of an agglomerating agent is arranged in the return pipe (15).