Apparatus, method and system for increasing yield of kraft cooking facilities

Abstract

Apparatus, methods, and systems for increasing the yield of kraft cooking facilities provide a synergistic effect for at least two parallel cooking facilities, for example, one cooking facility produces dissolving pulp in a prehydrolysis kraft process, the other cooking facility produces kraft slurry through a kraft pulping process, and the other cooking facility produces kraft pulp through a kraft pulping process. By recovering hemicellulose dissolved in an acidic hydrolysate of a sulfate prehydrolysis process, the cooking yield of a sulfate cooking facility for producing sulfate slurry can be increased. In some embodiments, there may be no cold alkali extraction step on the dissolving pulp production line because the target pulp purity can be achieved only by performing the pre-hydrolysis step and the acidic hydrolysate stream is reused on the sulfate pulp production line.

Description

[0001] This patent application letter publication describes inventive aspects including various novel innovations (hereinafter referred to as the "disclosure") and contains material that is protected by copyright, mask work, and / or other intellectual property rights. The respective owners of such intellectual property have no objection to the facsimile reproduction by anyone of the disclosure as it appears in the published Patent Office file / records, but otherwise reserve all rights. Technical Field

[0002] The innovation relates to pulp production and more particularly to improving the pulp yield of a kraft cooking process. Background Art

[0003] In recent years, the utilization of biomass as a raw material for various industries and products has increased. In this context, the pulp and paper industry provides low carbon footprint materials, including dissolving pulp, which can supply a large number of industries, such as regenerated cellulose (e.g., viscose and lyocell fibers, cellophane and sponges), cellulose acetate, cellulose nitrate and some other celluloses.

[0004] Wood and other materials used in the pulp and paper industry are formed from four main chemical components - cellulose, hemicellulose, lignin and extractives. Compared with paper grade pulp, dissolving pulp is characterized by higher purity, such as higher cellulose content and lower hemicellulose content. The challenge of the former is to remove most of the lignin and extractives during cooking and bleaching reactions while retaining most of the cellulose and hemicellulose, while the challenge of dissolving pulp is also to remove hemicellulose so that the α-cellulose purity in the product is at least 90%.

[0005] Wood, both softwood and hardwood, is the main raw material used in the pulp industry. Although the macromolecular composition of all species is similar, the ratios between the components can vary, as shown in Table 1 (Sixta, 2006).

[0006] Table 1 Composition (%)

[0007]

[0008]

[0009] The cost of wood affects the cost of pulp production, so pulp yield is an important economic factor and an area of ​​development and research. Because most of the wood components other than cellulose are removed, the yield of the dissolving pulp process can be 35% to 38%, which is lower than the paper grade manufacturing process, where the yield is usually over 50%.

[0010] The kraft process (KP) and the prehydrolysis kraft cooking (PHK) process or prehydrolysis sulfate cooking have been described in literature such as Sixta, H., Handbook of Pulp and are also used for producing paper grade pulp and dissolving pulp, respectively, from lignocellulosic material.

[0011] The prehydrolysis step applied before the kraft cooking process can selectively break down and dissolve short-chain molecules such as hemicellulose, producing an aqueous phase rich in acidic carbohydrates. The severity of the prehydrolysis is controlled by the so-called P factor, a single parameter that combines the reaction temperature and retention time of the prehydrolysis stage, which is manipulated to control the pulp purity. The pulp purity can be determined by, for example, the alpha cellulose test (Tappi T-203) or the alkali solubility method (Tappi T-235) or other standard similar methods.

[0012] During the prehydrolysis process, acetyl groups are released into the aqueous phase due to the cleavage reaction of the hemicellulose molecular chains (mainly (glucuronic acid) xylan in hardwood and (galacto) glucomannan in softwood), which usually reduces the pH to the range of 3-4. In addition, with the addition of catalysts such as mineral acids, the pH can be further reduced to below 2.0 to increase the reaction rate. According to Garrote (1999), up to 95% of the original hemicellulose content present in the wood source can be removed during the prehydrolysis process, with little effect on the lignin and cellulose molecules. The hemicellulose removed from the wood source will exist in the aqueous solution in the form of oligomers, monomers, or converted into by-products such as furfural or acetic acid. The resulting acid solution containing dissolved hemicellulose can be called a hydrolysate or acid hydrolysate.

[0013] To terminate the prehydrolysis reaction, in a step called neutralization, the chips are alkalized at a lower temperature than in the prehydrolysis stage by adding a strongly alkaline solution, such as white liquor, black liquor or other alkali-rich filtrate, such as the filtrate from the subsequent cold caustic extraction (CCE) stage, thereby raising the pH to above 11.

[0014] Several implementations of the PHK process have been more extensively discussed in batch cookers and their variations, such as the Continuous Batch Cooking process and the Superbatch process.

[0015] In a batch system, for each cooking cycle, the batch digester is filled by dropping wood chips through an opening in the top of the vessel. The chips may be fed into the digester by screw and / or belt conveyors and passed through a packing device as they enter the digester to help increase the amount of wood loaded in each batch. The packing device may include a set of low-pressure steam nozzles to push and distribute the stream of chips downward. The steam displaces the air from the spaces inside and outside the chips, and gases are continuously exhausted from the digester.

[0016] When the digester is completely filled, the top opening is closed and more steam is injected into the vessel to heat the chips to the target prehydrolysis temperature, for example above 150° C. When the target temperature is reached, the steam valve is closed and the digester is kept open for a period of time until the target P factor is reached.

[0017] In a batch system, as wood chips and steam are fed into the digester, a prehydrolysis step is carried out in the "steam stage" until prehydrolysis is complete. At this stage, the liquid medium in the vessel is a mixture of wood moisture and steam condensate, mainly located in the interstices of the wood chips, with negligible amounts of free liquid between the chips. After the prehydrolysis reaction, as acetic acid is formed and hemicellulose is dissolved in the liquid phase, the total liquid volume per dry metric ton of wood may not exceed 1 cubic meter (m3). 3 / BDtw).

[0018] After pre-hydrolysis, a neutralization step is performed by injecting a strong alkaline solution into the bottom of the digester, which is displaced through the digester as more liquor is added. As the neutralizing liquid impregnates the chips, it simultaneously displaces and mixes with the acid hydrolyzate formed in the interstices within the chips. As a result, the PHK batch system produces virtually no acid stream that can be separated and reused for other purposes.

[0019] The PHK cooking process may also include embodiments comprising a continuous cooking system, and more specifically, a system in which the pre-hydrolysis step is performed in a separate vessel from the alkaline cooking stage.

[0020] In such systems, lignocellulosic material, such as wood chips, and water / condensate are continuously fed into the top of a vessel (eg, a prehydrolysis vessel or PHV) capable of maintaining its contents at the required time and temperature to achieve the desired prehydrolysis reaction intensity.

[0021] The initial prehydrolysis step can be carried out in the steam phase, for example at the top of the PHV, with the biomass heated by direct steam injection, but the majority of the prehydrolysis reactions are carried out in the water phase in such a way that the amount of water present in the vessel is typically 2-5 m3 relative to the amount of wood chips. 3 / BDTw. This is significantly higher than, for example, in a batch digester, where the water volume is usually no more than 1m 3 / BDTw, because at this stage most of the water in the digester is due only to wood moisture and condensate from direct steam heating.

[0022] Neutralization is usually performed inside the PHV (lowest zone / bottom of the vessel) or at the top of a subsequent digester, or even in the pipes transferring the chips from the prehydrolysis vessel to the digester.

[0023] Before the neutralization stage begins, a portion of the free acidic hydrolysate can be partially extracted from the PHV vessel through one or more screens (filters). This early extraction has several purposes, such as removing dissolved hemicellulose from the process to improve pulp purity or to produce hemicellulose-derived by-products such as xylitol or furfural. The extracted acidic hydrolysate can be neutralized with white liquor, mixed with spent cooking liquor, and then sent to a heat recovery system (flash tank or heat exchanger) and then to the recovery island, where the hemicellulose is burned in a recovery boiler along with other organic compounds to produce steam.

[0024] The amount of hydrolysate available for extraction depends on several factors, such as chip moisture, water intake with chip, steam condensate generated during direct steam heating of the PHV vessel top, PHV degassing, prehydrolysis intensity factor, prehydrolysis yield, and whether PHV bottom displacement washing is performed.

[0025] The remaining free and bound hydrolysate fractions in the PHV are neutralized together with the wood chips and conveyed to the digester via a transfer circulation line.

[0026] The chips are transferred from the bottom of the PHV to the top of a cooking vessel (e.g., a digester) for an alkaline kraft cooking process. There are many different digester configurations that differ from each other in, for example, the number of recirculation zones, liquid addition, extraction points, number / location of filters, and whether the digester is a hydraulic or steam-liquid phase type.

[0027] Some mills have already had expansion projects and now have more than one cooking unit on the same site. In addition, many of these existing installations have been converted from paper grade pulp mills to dissolving pulp mills either permanently or flexibly / fluctuatingly.

[0028] Adjacent parallel production lines for the simultaneous production of paper grade pulp and dissolving pulp allow the integration of the two production lines. Side streams, such as liquid extracted from one digester, can be used at strategic locations in another production line to improve pulping yield and / or quality. As an example, BOGREN et al. (WO / 2013 / 178608) discloses a system using such integration. In this system, cold alkali extract filtrate (CCE filtrate) containing high molecular weight xylan is extracted from a pre-hydrolysis kraft pulp production line for the production of dissolving pulp and sent to a parallel kraft pulp production line for the production of conventional kraft pulp to increase kraft pulp yield and improve the process economy and mechanical properties of the final kraft pulp. After the alkali impregnation of the wood is completed, the CCE filtrate is added to make it a residual cooking liquor. SUMMARY OF THE INVENTION

[0030] The apparatus, methods, and systems disclosed herein for increasing the yield of a kraft cooking facility in various embodiments provide synergistic effects for at least two parallel cooking facilities, for example, one cooking facility producing dissolving pulp in a prehydrolysis kraft process and another cooking facility producing kraft pulp by a kraft pulping process. Some embodiments can increase the cooking yield of a kraft cooking facility producing kraft pulp by recovering hemicellulose dissolved in an acidic hydrolyzate. In some embodiments, a hydrolyzate stream that might otherwise have been placed in a recovery island for steam generation and / or for extraction for the production of by-products is reused in an adjacent kraft pulp production line for the production of kraft pulp. In embodiments of the disclosed apparatus, methods, and systems, a cold caustic extraction step can be eliminated from the dissolving pulp production line because only a prehydrolysis step is required to achieve the target pulp purity and reuse the acidic hydrolyzate stream in the kraft pulp production line.

[0031] Under some conditions, a portion of the hemicellulose and other organic compounds dissolved in the acidic hydrolysate will precipitate onto the fibers, thereby increasing the cooking yield, thereby reducing the overall specific wood consumption and / or increasing pulp throughput. Embodiments of the disclosed apparatus, methods, and systems can also increase the hemicellulose content of the final bleached pulp, thereby providing, for example, improved pulping properties, mechanical properties, and / or the like.

[0032] Embodiments of the disclosed apparatus, methods, and systems may include two methods for reusing acidic hydrolysate in a kraft cooking process of kraft slurry.

[0033] One method (A) includes a pretreatment step in which the wood chips are sent to an acid impregnation stage before the alkaline cooking process. The steamed wood chips are mixed with a cooled acidic hydrolyzate extracted from an adjacent PHK line, thereby soaking and saturating the wood chips with the hydrolyzate, and the dissolved hemicellulose is precipitated onto the fibers. The acidic hydrolyzate is cooled to such an extent that the temperature of the resulting mixture of wood chips and hydrolyzate becomes 70°C-125°C or 100°C. After this step, any kraft cooking process may be performed to produce pulp, such as conventional cooking or other modified cooking methods, such as ITC cooking, Lo-Solids process, Compact Cooking process, Super Batch Cooking and / or the like.

[0034] The acid impregnation stage can be carried out in a continuous digester with or without providing a substantial retention time. In other words, the chips can be fed directly to the digester using only the chip feed system, with a short retention time (<5 min) for the impregnation stage, or alternatively the digester top can be used to provide a longer retention time, or another arrangement is to use a separate vessel for the acid impregnation step.

[0035] For batch cooking facilities, the acid chip impregnation and subsequent alkaline kraft cooking steps can be performed in the same digester (eg, between the digester chip filling sequence and the kraft cooking stage).

[0036] In various embodiments, the extent of hemicellulose recovery may depend on several factors, such as, but not limited to, the wood species and the process conditions applied, with retention time having a significant impact on the results obtained. This retention time effect has been demonstrated in the laboratory by pilot-scale cooking with Eucalyptus Urograndis. The pulp yields at different acid impregnation times were compared with reference cookings without a hydrolysate recovery step and at the same Kappa number level. It was found that the retention time that maximized the hemicellulose recovery was 40-100 minutes, as shown in Table A. However, due to the lower implementation costs, the process with the shortest impregnation time (without additional containers or areas) is also economically feasible.

[0037] The second method (B) involves injecting the acid hydrolyzate into the alkaline cooking zone within the digester, rather than having a dedicated acid impregnation vessel or area. Continuous cooking digester embodiments may include various cooking zones separated by filters in the vessel wall, in which liquor circulation, liquor extraction and injection, temperature and alkaline characteristic changes occur.

[0038] According to the second method (B), the acidic hydrolyzate is added to a low alkali concentration cooking zone, for example to the last cooking zone, wherein the residual effective alkali is less than 10 g EA / l, so that hemicellulose is precipitated onto the fiber surface during the cooking reaction.

[0039] For batch digesters and process (B), the acidic hydrolysis liquor is introduced at the end of the digestion cycle and / or during the subsequent cold displacement phase.

[0040] In one embodiment, a process for increasing pulp yield in a kraft cooking facility is disclosed, comprising: utilizing an acidic hydrolyzate stream from an adjacent pre-hydrolysis kraft process producing dissolving wood pulp.

[0041] In another embodiment, a method for producing kraft pulp is disclosed, comprising: extracting an acidic hydrolysate from a prehydrolysis kraft process for producing dissolving wood pulp; and applying the acidic hydrolysate to a kraft cooking process of a kraft cooking facility.

[0042] In another embodiment, a system for producing a kraft slurry is disclosed, comprising: a kraft cooking facility; and an acidic hydrolysate source providing acidic hydrolysate to a kraft cooking process of the kraft cooking facility.

[0043] BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The appendices and / or drawings illustrate various non-limiting, exemplary, innovative aspects according to the present specification:

[0045] Figure 1 A flow diagram of a PHK cooking facility is shown, representing a two-vessel continuous digester for dissolving grade pulp production, wherein a side stream of acidic hydrolysate is generated containing said dissolved hemicellulose;

[0046] Figure 2 A flow diagram showing an embodiment of process A for kraft pulping in a kraft cooking facility, the flow diagram representing a two-vessel continuous digester configuration with a separate acidic chip impregnation vessel;

[0047] Figure 3 A flow diagram showing an embodiment of process A for grading kraft pulp in a kraft cooking facility, the flow diagram representing one vessel configuration (continuous or batch digester);

[0048] Figure 4 A flow chart showing an embodiment of method B for a kraft pulp in a kraft cooking facility (continuous or batch digester); and

[0049] Figure 5Shown is a comparison of the cooking screened yields for both methods A and B for kraft pulps in one embodiment with a reference cooking process over a wide range of Kappa numbers.

[0050] Detailed description of the drawings

[0051] Embodiments of the disclosed apparatus, methods, and systems include two parallel continuous cooking facilities, one line producing prehydrolyzed kraft dissolving pulp (PHK) and a second line producing kraft pulp (KP) via a kraft process (KP). Alternative embodiments include two parallel production lines, wherein dissolving pulp is produced in a continuous PHK process and kraft pulp is produced in a batch kraft process. The PHK process may include one or more vessels.

[0052] Figure 1 The configuration of a prehydrolysis kraft (PHK) continuous cooking facility for dissolving grade pulp production is shown in one embodiment. In this embodiment, wood chips 101, water 105 and steam 110 are fed into a vessel 115 where a prehydrolysis reaction is carried out. Water is added and / or evaporation facility clean condensate is added, for example, in an amount of 0.5-5 m3 relative to the wood input flow. 3 / BDtw, or 1-3m 3 The prehydrolysis temperature in the PHV vessel can be controlled by steam flow to achieve a target prehydrolysis strength (P factor) at a given chip retention time, for example, a temperature range of 140-175°C for a P factor in the range of 50-1000 units.

[0053] As the hemicellulose is gradually degraded and dissolved, the liquid phase of the reactor is converted into a hydrolyzate. Under the above conditions, up to 5 m3 of cellulose can be separated from the wood chip stream, for example, via a filter on the pre-hydrolysis vessel 115. 3 / BDtw or up to 2m 3 The hydrolyzate of 1000 g / BDtw is then sent to a parallel kraft pulp production line for recovery. The wood chips are transferred to a second vessel (cooker) 120 where they are cooked to produce a dissolving grade pulp 125.

[0054] Embodiments of the disclosed apparatus, methods, and systems include implementing methods A and / or B to reuse hydrolyzate in a second parallel production line, which can help reduce overall specific wood consumption.

[0055] Figure 2The embodiment of method A is shown in Figure 2. The wood chips 201 and the hydrolyzed liquid 205 can be continuously fed into the container 210 and acid impregnated at a temperature of 70-125° C. for up to 180 minutes or 40-100 minutes. In some embodiments, the hydrolyzed liquid can be in a solution containing up to 5 m 3 / BDtw or 0.5-2m 3 / BDtw wood is added, and can be cooled by flash evaporation and / or in an indirect heat exchanger to reach the target impregnation temperature. The hydrolyzed liquid impregnated wood chips 215 are transferred from the container outlet to the subsequent digester 220 for continuous kraft cooking to produce papermaking grade wood pulp 225.

[0056] Figure 3 represents another variation of method A. In this embodiment, the chips and hydrolysate 301 are fed directly into the top of the digester 305, rather than into separate vessels. In this embodiment, acid impregnation occurs in the topmost region of the digester, with the same mass quantities, impregnation times and temperatures as described above. At the start-up of the designed section of the alkaline cooking process, the excess hydrolysate can be extracted and replaced with white liquor 310 and / or other alkaline liquor to neutralize and alkalize the acidic chips and remaining hydrolysate. After neutralization, the subsequent cooking zones are typical of any kraft cooking process and will not be described in detail.

[0057] The above method A can be further derived into such embodiments: wherein the KP production line comprises a batch cooking system. In these embodiments, the hydrolyzate can be, for example:

[0058] - cooled and fed into the top of the digester simultaneously with the chips; or

[0059] - After being loaded with wood chips, it is injected into the bottom of the digester and further displaced through the digester by injecting liquid (such as white liquor and / or other alkali liquor) to neutralize and alkalize the acidic wood chips and remaining hydrolyzate.

[0060] Figure 4 An embodiment of method B is shown. In a continuous cooking digester 401 comprising multiple zones, the dry wood input flow is up to 2 m 3 / bdt, the hydrolyzate 405 is added, for example, to the lowest cooking zone (e.g., the final cooking zone). A matching amount of black liquor 410 can be extracted from the digester so that the liquid to wood flow ratio is not adversely affected by the addition of the hydrolyzate. In this embodiment, the retention time in the combined precipitation / cooking stage can be 30-90 minutes, the residual effective alkali calculated as NaOH is less than 10 g / 1, and the typical temperature of the kraft cooking process (140-170°C).

[0061] In an alternative embodiment of method B for a batch cooking system, the hydrolyzate 405 can be added to the digester 401 at an intermediate time in the cooking stage, for example, mixed with the cooking liquor in the digester and circulated for the remaining cooking time, displaced through the digester (in a system without a circulation pump), and / or the like.

[0062] Figure 5 A comparison of the cooking screen yield 501 for embodiments of methods A and B for kraft pulps with a reference cooking process over a wide range of Kappa numbers 505 is shown in one embodiment.

[0063] Table 2 shows the absolute increase in screen cook yield for embodiments of Method A over a wide range of retention times and at comparable Kappa numbers.

[0064] Table 2

[0065] Method A - Screening for Increased Cooking Yield at Different Retention Times

[0066]

Claims

1. A method for increasing pulp yield in a kraft cooking facility, comprising: The acidic hydrolysate stream from the adjacent pre-hydrolysis kraft process for producing dissolving wood pulp is utilized.

2. The method of claim 1, wherein the acidic hydrolysate stream containing dissolved hemicellulose and produced by prehydrolyzing wood chips in an aqueous phase is reused in the kraft cooking facility.

3. The method according to claim 2, wherein the wood chips in the kraft cooking facility are pretreated and impregnated with the acidic hydrolysate prior to the alkaline cooking process.

4. The method according to claim 3, wherein the acid impregnation is carried out in a vessel located before the alkali digester.

5. The method of claim 3, wherein the acid impregnation is performed in the alkali digester.

6. The method of claim 3, wherein the wood chips are impregnated with the acidic hydrolyzate for 2-180 minutes.

7. The method according to claim 3, wherein the wood chips are impregnated with the acidic hydrolysis liquid at a temperature of 70-125°C.

8. The method according to claim 7, wherein the wood chips are impregnated with the acidic hydrolysis liquid at a temperature of 90-100°C.

9. The method of claim 2, wherein the acidic hydrolyzate is injected into the intermediate alkaline zone of the digester.

10. The process according to claim 9, wherein the acidic hydrolysate is injected into the last alkaline cooking zone.

11. The method according to claim 9, wherein the residual effective alkali concentration in the intermediate alkaline region is 10 g / 1 or less.

12. The method according to claim 9, wherein the wood chip retention time in the intermediate alkaline zone is 30-90 minutes.

13. A method for producing a sulfate slurry, comprising: Extraction of acid hydrolysate from the prehydrolysis kraft process for producing dissolving wood pulp; The acidic hydrolyzate is applied to a kraft cooking process in a kraft cooking facility.

14. The method of claim 13, wherein the acidic hydrolysate is produced by pre-hydrolyzing wood chips in an aqueous phase.

15. The method according to claim 13, wherein applying the acidic hydrolyzate to the kraft cooking process further comprises: Prior to the alkaline cooking process, the wood chips are acid impregnated with the acidic hydrolysis liquor in the kraft cooking facility.

16. The method of claim 15, wherein the wood chips are acid impregnated in a vessel located before the alkali digester.

17. The method of claim 15, wherein the wood chips are acid impregnated in an alkali digester.

18. The method of claim 17, wherein the acid pickling is performed in an acid pickling zone near the top of the caustic digester.

19. The method according to claim 15, wherein the acid impregnation is carried out for 2 to 180 minutes.

20. The method of claim 15, wherein the acid impregnation is performed at a temperature of 70-125°C.

21. The method according to claim 20, wherein the acid impregnation is carried out at a temperature of 90-100°C.

22. The method of claim 13, wherein applying the acidic hydrolyzate to the kraft cooking process further comprises: The acidic hydrolyzate is injected into the middle alkaline zone of the alkaline digester.

23. The method of claim 22, wherein the acidic hydrolysate is injected into the last alkali cooking zone of the alkali digester.

24. The method according to claim 22, wherein the residual effective alkali concentration in the intermediate alkaline region is 10 g / 1 or less.

25. The method of claim 22, wherein the wood chip retention time in the intermediate alkaline zone is 30-90 minutes.

26. A system for producing a kraft slurry, comprising: Kraft cooking facilities; and A source of acidic hydrolysate provides acidic hydrolysate to the kraft cooking process of the kraft cooking facility.

27. The system of claim 26, wherein the source of acidic hydrolysate comprises a pre-hydrolysis kraft process for producing dissolving wood pulp.

28. The system of claim 26, wherein the wood chips from the kraft cooking facility are impregnated with the acidic hydrolysis solution prior to the alkaline cooking process.

29. The system of claim 28, wherein the kraft cooking facility further comprises: Alkali digester; and an impregnation vessel located before the alkali digester; wherein the acidic hydrolyzate is provided to the impregnation vessel.

30. The system of claim 28, wherein the kraft cooking facility further comprises: an alkali digester, the alkali digester comprising an impregnation zone near the top of the alkali digester; wherein the acidic hydrolyzate is provided to the impregnation zone.

31. The system of claim 28, wherein the wood chips are impregnated with the acidic hydrolysis solution for a period of 2-180 minutes.

32. The system of claim 28, wherein the wood chips are impregnated with the acidic hydrolysis solution at a temperature of 70-125°C.

33. The system according to claim 32, wherein the wood chips are impregnated with the acidic hydrolysis liquid at a temperature of 90-100°C.

34. The system of claim 28, wherein the kraft cooking facility further comprises: An alkali digester including an intermediate alkali zone; The acidic hydrolyzate is provided to the intermediate alkaline zone.

35. The system according to claim 34, wherein the residual effective alkali concentration of the intermediate alkaline zone is 10 g / 1 or less.

36. The system of claim 34, wherein the chip retention time in the intermediate alkaline zone is 30-90 minutes.

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