Preorder slurry screening and washing process for reducing alkali loss and consumption of bleaching chlorine dioxide

Abstract

The invention belongs to the technical field of pulp paper production, and particularly relates to a preorder pulp screening and washing process for reducing alkali loss and bleaching chlorine dioxide consumption. The method comprises the following steps: pre-impregnating and cooking a wood chip sample in sequence to obtain coarse pulp; then the coarse pulp is sequentially subjected to joint removal, pressure screening, primary washing, secondary washing, oxygen bleaching, degassing condensation, third washing and fourth washing, and the efficiency of the pulp washer can be effectively improved; meanwhile, multiple times of washing are matched, so that the pulp washing efficiency is maximized, the pulp washing quality is improved, and the alkali loss and the pulp COD (Chemical Oxygen Demand) are greatly reduced. Therefore, according to the technical scheme provided by the invention, the washing alkali loss can be reduced, and the recovery amount of alkaline chemicals is increased; and the COD content in the slurry entering the chlorine dioxide bleaching section can be reduced, and the consumption of a subsequent bleaching oxidant chlorine dioxide is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of pulp and paper production, and particularly relates to a pre-stage pulp screening and washing process for reducing alkali loss and chlorine dioxide consumption in bleaching. Background Art

[0002] In recent years, with the continuous development of consumption upgrading and the e-commerce industry, the demand for pulp and paperboard has been increasing continuously; while the paper-making industry is a resource-consuming and pollution-emitting industry, facing great environmental protection pressure. ECF (elemental chlorine-free) bleaching of pulp is a common link in pulp preparation, generally including oxygen bleaching, chlorine dioxide bleaching, oxygen-enhanced alkali extraction, first-stage chlorine dioxide bleaching and second-stage chlorine dioxide bleaching; alkaline liquor is the main chemical in the cooking and oxygen bleaching processes. Since the pulp after oxygen bleaching will enter the chlorine dioxide bleaching section under an acidic environment, if the washing efficiency in the previous stage is not good, it will not only increase the dosage of chemicals, but also cause the content of residual COD (chemical oxygen demand) in the pulp to be too high.

[0003] Most of the existing technologies improve the ECF bleaching process to reduce the consumption of bleaching chlorine dioxide. For example, in a bamboo dissolving pulp alkali extraction process of Chinese Patent CN 102154883 A, hydrogen peroxide and oxygen are used to strengthen the alkali extraction (EOP) to achieve the effect of reducing chlorine dioxide consumption. However, the above solution does not effectively improve the recovery rate of alkaline chemicals. Therefore, how to reduce the consumption of bleaching chlorine dioxide while reducing alkali loss is a difficult problem for those skilled in the art. Summary of the Invention

[0004] The purpose of the present invention is to provide a pre-stage pulp screening and washing process for reducing alkali loss and chlorine dioxide consumption in bleaching, with a simple method and capable of reducing alkali loss and chlorine dioxide consumption in bleaching.

[0005] The present invention provides a method for pre-stage pulp screening and washing, including the following steps: Sequentially pre-impregnating and cooking a wood chip sample to obtain crude pulp; Sequentially subjecting the crude pulp to knot removal, pressure screening, first washing, second washing, oxygen bleaching, degassing and condensation, third washing and fourth washing to obtain pulp.

[0006] Preferably, the parameters of the knot removal include: the inlet pulp mass concentration is 3.8% - 4.3%, the inlet pulp pressure is 2.8 - 3.1 bar, the knot removal pressure difference is 0 - 0.2 bar, and the reject rate is 10% - 12%.

[0007] Preferably, the pressure screening includes: first-stage pressure screening, second-stage pressure screening and third-stage pressure screening; The parameters of the first-stage pressure screening include: the incoming pulp mass concentration is 3.8% - 4.3%, the incoming pulp pressure is 2.8 - 3.1 bar, and the screening pressure difference is 0 - 0.2 bar; The parameters of the second-stage pressure screening and the third-stage pressure screening respectively include: the incoming pulp pressure is 2.8 - 3.1 bar, and the screening pressure difference is 0 - 0.2 bar.

[0008] Preferably, the parameters of the first washing include: the incoming pulp mass concentration is 3.8% - 4.3%, the washing dilution factor is 1.5 - 2.5 m 3 / adt, the pressing pressure is 75 - 90 kN / m, and the outgoing pulp mass concentration is 25% - 30%.

[0009] Preferably, the parameters of the second washing include: the incoming pulp mass concentration is 8% - 9%, the washing dilution factor is 6 - 8 m 3 / adt, the washing water pressure is 0.3 - 0.8 bar, the tank pressure is 0.2 - 0.5 bar, the rotation speed is 3 - 5 r / min, and the outgoing pulp mass concentration is 10% - 15%.

[0010] Preferably, the parameters of the degassing and condensation include: the condensate water flow rate is 110 - 140 kg / s, the gas suction pressure is -1.0 - -1.5 kPa, and the condensate water temperature is 30 - 40 °C.

[0011] Preferably, the parameters of the third washing and the fourth washing respectively include: the incoming pulp mass concentration is 7.0% - 9.0%, the washing dilution factor is 2.0 - 3.0 m 3 / adt, the pressing pressure is 90 - 100 kN / m, and the outgoing pulp mass concentration is 25% - 30%.

[0012] Preferably, the fourth washing is carried out with the VE intermediate condensate water, and the COD value of the VE intermediate condensate water is 600 - 700 mg / L, and the pH value is 8.8 - 9.5.

[0013] The present invention provides the pulp prepared by the method according to the above technical solution, and the alkali loss of the pulp < 7 kg / adt, and the COD of the pulp < 9 kg / adt.

[0014] The present invention provides the application of the method according to the above technical solution in reducing alkali loss and / or reducing the consumption of bleaching chlorine dioxide in pulp production.

[0015] Beneficial effects: The present invention provides a method for screening and washing the previous pulp, comprising the following steps: successively subjecting a wood chip sample to pre-impregnation and cooking to obtain crude pulp; successively subjecting the crude pulp to knot removal, pressure screening, primary washing, secondary washing, oxygen bleaching, degassing and condensation, tertiary washing and quaternary washing to obtain pulp. By equipping degassing and condensation treatment in the oxygen bleaching and degassing and condensation stages, the present invention effectively reduces the influence of high-temperature water vapor, improves the efficiency of the pulp washer; by washing multiple times, the washing efficiency is maximized, the washing quality is improved, and the alkali loss and pulp COD are greatly reduced. Therefore, the technical solution provided by the present invention can not only reduce the washing alkali loss and increase the recovery amount of alkaline chemicals; but also reduce the COD content in the pulp entering the chlorine dioxide bleaching stage and reduce the consumption of the subsequent bleaching oxidant chlorine dioxide. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required to be used in the embodiments.

[0017] Figure 1 It is the technical route map provided by Embodiment 1 of the present invention; Figure 2 It is the technical route map provided by Comparative Example 1 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention provides a method for screening and washing the previous pulp, comprising the following steps: successively subjecting a wood chip sample to pre-impregnation and cooking to obtain crude pulp; successively subjecting the crude pulp to knot removal, pressure screening, primary washing, secondary washing, oxygen bleaching, degassing and condensation, tertiary washing and quaternary washing to obtain pulp.

[0019] In the present invention, unless otherwise specified, the methods and materials used are conventional selections.

[0020] The present invention subjects the crude pulp to knot removal treatment to obtain the pulp after knot removal and large tree knots. The parameters of the knot removal treatment of the present invention preferably include: the inlet pulp mass concentration is 3.8% - 4.3%, the inlet pulp pressure is 2.8 - 3.1 bar, the knot removal differential pressure is 0 - 0.2 bar, and the slag discharge rate is 10% - 12%; the knot removal treatment is preferably carried out in a knot remover; there are no special requirements for the model of the knot remover; when carrying out the knot removal treatment, the pulp enters the sieve body tangentially from the upper part of the sieve body, and the centrifugal force generated by the rotation of the sieve basket causes the materials with a large specific gravity to be thrown to the inner wall of the machine shell and gradually settle to the lower part of the sieve body and discharge, and the fibers pass through the sieve holes and discharge from the bottom of the sieve; there is dilution water to dilute the materials during this process.

[0021] After the knot removal process is completed, the large knots are returned to the digester for re-cooking, and the pulp after knot removal is subjected to first-stage pressure screening to obtain first-stage screened good pulp and first-stage screened reject pulp. The parameters of the first-stage pressure screening in the present invention preferably include: the feedstock mass concentration is 3.8% - 4.3%, the feedstock pressure is 2.8 - 3.1 bar, and the screening pressure difference is 0 - 0.2 bar; the first-stage screened good pulp is preferably subjected to a washing stage once.

[0022] The first-stage screened reject pulp in the present invention is subjected to second-stage pressure screening to obtain second-stage screened good pulp and second-stage screened reject pulp; the parameters of the second-stage pressure screening are preferably: the feedstock pressure is 2.8 - 3.1 bar, and the screening pressure difference is 0 - 0.2 bar; the second-stage screened good pulp is preferably returned to the first-stage pressure screening for screening.

[0023] The second-stage screened reject pulp in the present invention is preferably treated by a conical desander to discharge the sand and gravel, and the treated second-stage screened reject pulp is subjected to third-stage pressure screening to obtain third-stage screened good pulp and third-stage screened reject pulp; the parameters of the third-stage pressure screening preferably include: the feedstock pressure is 2.8 - 3.1 bar, and the screening pressure difference is 0 - 0.2 bar; the third-stage screened good pulp is preferably returned to the second-stage pressure screening for screening.

[0024] The first-stage pressure screening, second-stage pressure screening, and third-stage pressure screening in the present invention are preferably carried out in a pressure screen, and based on a fully enclosed design, the screening and cleaning of the pulp are achieved by using the pressure difference and the action of the rotor.

[0025] The present invention performs the first washing on the first-stage screened good pulp to obtain the pulp after the first washing. The parameters of the first washing in the present invention preferably include: the feedstock mass concentration is 3.8% - 4.3%, the washing dilution factor is 1.5 - 2.5 m 3 / adt, the squeezing pressure is 75 - 90 kN / m, and the discharge mass concentration is 25% - 30%; the equipment for the first washing is preferably a double-roll pulp washer; the water used for the first washing is preferably the wastewater from the second washing, that is, after the first washing is completed, the washing wastewater is transported to a storage tank for storage.

[0026] The present invention preferably dilutes the pulp after the first washing to obtain the diluted pulp after the first washing; the diluted pulp after the first washing is subjected to the second washing to obtain the pulp after the second washing. The parameters of the second washing in the present invention are preferably: the feedstock mass concentration is 8% - 9%, and the washing dilution factor is 6 - 8 m 3 / adt, the washing water pressure is 0.3 - 0.8 bar, the tank pressure is 0.2 - 0.5 bar, the rotation speed is 3 - 5 r / min, and the pulp mass concentration of the discharged pulp is 10% - 15%. The water used for dilution and the second washing is preferably the wastewater from the third washing; the equipment used for the second washing is preferably a drum displacement pulp washer.

[0027] The present invention dilutes the secondary washed pulp to obtain the diluted secondary washed pulp; the diluted secondary washed pulp is subjected to oxygen delignification treatment. Oxygen delignification is mainly a process in which lignin is oxidized and removed in an alkaline liquor composed of oxygen as the oxidant, oxidative white liquor, and sodium hydroxide. The pulp is in an alkaline, high-pressure, and high-temperature environment during the reaction. After the pulp reaction is completed, the pulp is degassed and condensed to obtain the oxygen-bleached and degassed-condensed pulp.

[0028] The present invention degasses and condenses the oxygen-bleached and degassed-condensed pulp to obtain the degassed pulp. The parameters of the degassing and condensation of the present invention preferably include: the condensate water flow rate is 110 - 140 kg / s, the gas suction pressure is -1.0 - -1.5 kPa, and the condensate water temperature is 30 - 40 °C. The equipment for degassing and condensation includes a degasser, a condenser, a vacuum system, and a heat exchanger. The oxygen-bleached and degassed-condensed pulp enters the degasser, and the gas in the pulp is separated through vacuum; the steam enters the condenser and becomes liquid water after cooling. Since the temperature of the pulp coming from the oxygen bleaching and degassing and condensation section is relatively high, the steam will affect the washing efficiency during the washing process. Therefore, after the degassing and condensation treatment, the gas generated by the pulp is timely sucked out, which is more beneficial for washing and improves the washing efficiency.

[0029] The present invention conducts the third washing on the degassed pulp to obtain the triple-washed pulp; the parameters of the third washing are preferably: the inlet pulp mass concentration is 7.0% - 9.0%, and the washing dilution factor is 2.0 - 3.0 m 3 / adt, the squeezing pressure is 90 - 100 kNm, and the pulp mass concentration of the discharged pulp is 25% - 30%; the equipment for the third washing is preferably a double-roll pulp washer; the water for the third washing is preferably the wastewater from the fourth washing.

[0030] The present invention conducts the fourth washing on the triple-washed pulp to obtain the pulp; the parameters of the fourth washing are preferably: the inlet pulp mass concentration is 7.0% - 9.0%, and the washing dilution factor is 2.0 - 3.0 m 3 / adt, the squeezing pressure is 90 - 100 kN / m, and the discharged pulp mass concentration is 25% - 30%; the equipment for the fourth washing is preferably a double-roll pulp washer; the water for the fourth washing is preferably VE intermediate condensate; the VE intermediate condensate is in the alkali recovery section. During the evaporation and concentration of black liquor, the secondary steam (gaseous water) generated by the evaporation of black liquor undergoes heat exchange and condensation to form liquid water; the COD of the VE intermediate condensate is 600 - 700 mg / L, and the pH is 8.8 - 9.5. After the above treatment, the alkali loss of the obtained pulp is < 7 kg / adt, and the pulp COD is < 9 kg / adt.

[0031] To further illustrate the present invention, the solutions provided by the present invention will be described in detail below in conjunction with the accompanying drawings and embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0032] Example 1 A method for screening and washing the previous pulp to reduce alkali loss and chlorine dioxide consumption for bleaching (see the reference flow chart in Figure 1 ), the steps are as follows: (1) Stock preparation: The acacia wood chips are obtained by mixing the Acacia mangium wood chips produced in Vietnam and the Eucalyptus nitens and Eucalyptus globulus wood chips produced in Australia in a mass ratio of 2:1; The above acacia wood chips and eucalyptus wood chips are mixed in a mass ratio of 7:3. After the wood chips pass through the shaking screen system, mixed wood is obtained; the moisture content of the mixed wood is 46.06%. The percentage of each substance in the mixed wood is as follows: the content of bark is 0.66%, the content of oversized chips is 0%, the content of over-thick chips is 9.5%, the content of qualified chips is 86.52%, the content of undersized chips is 2.93%, and the content of wood chips is 0.37%.

[0033] (2) Pre-impregnation: The mixed wood in step (1) is transported by a conveyor belt to a pre-impregnation tower filled with an impregnation liquid (the effective alkali content of the impregnation liquid is 9.34 g / L) at a temperature of 100 °C and soaked for 70 min to obtain pre-impregnated wood chips.

[0034] (3) Cooking: The pre-impregnated wood chips in step (2) are transferred to a cooking tower by a high-pressure feeder. The cooking tower is a continuous cooking type. The wood chips move from top to bottom (first enter the upper cooking area, and then successively enter the lower cooking area, the countercurrent washing area, and the spraying and discharging port). After cooking, the wood chips are decomposed into crude pulp, which is discharged by a bottom scraper and then transported to a blow tank for storage; during the entire cooking process, the sulfidity of white liquor is 36.01%, the consumption of white liquor is 296 kg / adt, and the cooking time is 240 min; Among them, in the upper cooking area: the cooking temperature is 150 - 160 °C, the cooking pressure is 2 - 5 bar, the liquor ratio is 4.2, and the residual alkali content of the black liquor is 11.78 g / L; in the lower cooking area: the cooking temperature is 150 - 160 °C, the cooking pressure is 5 - 7 bar, and the residual alkali content of the black liquor is 7.86 g / L; in the countercurrent washing area: the washing dilution factor is 2.3 adt / m 3 , the discharge port temperature is 80 - 100 °C; in the spraying treatment area: the pulp concentration of the spraying line is 11.9%, the residual alkali of the spraying line is 2.33%, and the Kappa number of the pulp is 18.5.

[0035] (4) Knot removal: Transfer the thick pulp in step (3) to a knot remover for knot removal to remove larger tree knots, obtaining the pulp after knot removal; the parameters of the knot removal include: the inlet pulp mass concentration is 4.09%, the inlet pulp pressure is 3.01 bar, the knot removal pressure difference is 0.15 bar, and the slag discharge rate is 12%.

[0036] (5) Pressure screening includes: Perform primary three-stage pressure screening on the pulp after knot removal in (4) to obtain the screened good pulp; Among them, the parameters of the first-stage pressure screening are: the inlet pulp mass concentration is 3.83%, the inlet pulp pressure is 2.97 bar, and the screening pressure is 0.05 bar; The parameters of the second-stage pressure screening are: the inlet pulp pressure is 2.88 bar, and the screening pressure difference is 0.02 bar; The parameters of the third-stage pressure screening are: the inlet pulp pressure is 2.89 bar, and the screening pressure difference is 0.03 bar.

[0037] (6) Washing: Perform the first washing and the second washing on the screened good pulp in (5) in sequence to obtain the pulp after secondary washing.

[0038] The parameters of the first washing are: the inlet pulp mass concentration is 4.02%, the washing dilution factor is 2.14 m 3 / adt, the squeezing pressure is 78 kN / m, the outlet pulp mass concentration is 26.12%; the equipment is a double-roll pulp washer; the washing water is the wastewater from the second washing.

[0039] The parameters of the second washing are: the inlet pulp mass concentration is 8.8%, the washing dilution factor is 6.3 m 3 / adt, the washing water pressure is 0.54 bar, the tank pressure is 0.42 bar, the rotational speed is 4 r / min, the outlet pulp mass concentration is 12.85%, and the pulp concentration of the dilution riser is 11.6%. The water for the second washing is the wastewater from the third washing; the equipment is preferably a drum displacement pulp washer.

[0040] (7) Oxygen bleaching and degassing condensation (i.e., oxygen delignification treatment) Oxygen delignification treatment: The reaction pulp consistency is 11.9%, the consumption of oxidized white liquor is 10.1 kg / adt, the consumption of NaOH is 8.0 kg / adt, the initial reaction pH is 11.2, the oxygen consumption is 12.0 kg / adt, the pipeline oxygen pressure is 7.8 bar, the reaction temperature is 100 °C and the residence time is 10 min; The second-stage oxygen delignification treatment: The reaction pulp consistency is 12.3%, the oxygen consumption is 17.0 kg / adt, the reaction tower top pressure is 5.5 bar, the reaction temperature is 100 °C, the residence time is 60 min, the pH at the end point of the bleaching liquor reaction is 10.70, the Kappa number after bleaching is 10.2 and the brightness (ISO) of the bleached pulp is 52.61%; Washing treatment: The incoming pulp mass concentration is 7.6%, the washing dilution factor is 2.23 m 3 / adt, the pressing pressure is 97 kN / m and the outgoing pulp mass concentration is 29.87%.

[0041] The degassed condensate is carried out on the oxygen-bleached pulp to obtain the degassed pulp. The parameters of the degassed condensate are: the condensate water flow rate is 122 kg / s, the gas suction pressure is -1.2 kPa, and the condensate water temperature is 30 °C.

[0042] (8)Re-washing The degassed pulp in (7) is washed for the third time and the fourth time in sequence to obtain the pulp; The parameters of the third washing are: the incoming pulp mass concentration is 8.03%, the washing dilution factor is 2.51 m 3 / adt, the pressing pressure is 92 kN / m, and the outgoing pulp mass concentration is 25.04%; The parameters of the fourth washing are preferably: the incoming pulp mass concentration is 7.57%, the washing dilution factor is 2.42 m 3 / adt, the pressing pressure is 89 kN / m, and the outgoing pulp mass concentration is 29.63%; The equipment for the fourth washing is preferably a double-roll pulp washer; The water for the fourth washing is preferably VE intermediate condensate; The VE intermediate condensate is in the alkali recovery section. During the evaporation and concentration of black liquor, the secondary steam (gaseous water) generated by the evaporation of black liquor passes through heat exchange and condensation to form liquid water; The COD of the VE intermediate condensate is 623 mg / L and the pH is 9.0. After the above treatment, the alkali loss of the obtained pulp is <7 kg / adt and the pulp COD <9 kg / adt.

[0043] Example 2 The difference from Example 1 is only in steps (4)-(8), and the steps are as follows: (4) Knot removal: Transfer the thick pulp in step (3) to a knot remover for knot removal to remove large tree knots, obtaining the pulp after knot removal; the parameters for knot removal include: the incoming pulp mass concentration is 4.10%, the incoming pulp pressure is 2.95 bar, the knot removal differential pressure is 0.14 bar, and the reject rate is 12%.

[0044] (5) Pressure screening includes: Perform primary three-stage pressure screening on the pulp after knot removal in (4) to obtain screened good pulp; Among them, the parameters for the first-stage pressure screening are: the incoming pulp mass concentration is 3.92%, the incoming pulp pressure is 2.99 bar, and the screening pressure is 0.04 bar; The parameters for the second-stage pressure screening are: the incoming pulp pressure is 2.81 bar, and the screening differential pressure is 0.02 bar; The parameters for the third-stage pressure screening are: the incoming pulp pressure is 2.99 bar, and the screening differential pressure is 0.03 bar.

[0045] (6) Washing: Perform the first washing and the second washing on the three-stage screened good pulp in (5) in sequence to obtain the pulp after secondary washing.

[0046] The parameters for the first washing are: the incoming pulp mass concentration is 4.05%, the washing dilution factor is 2.22 m 3 / adt, the squeezing pressure is 79 kNm, the outgoing pulp mass concentration is 26.71%; the equipment is a double-roll pulp washer; the washing water is the wastewater from the secondary washing.

[0047] The parameters for the second washing are: the incoming pulp mass concentration is 8.8%, the washing dilution factor is 6.7 m 3 / adt, the washing water pressure is 0.57 bar, the tank pressure is 0.5 bar, the rotation speed is 3.5 r / min, the outgoing pulp mass concentration is 12.95%, and the pulp concentration in the dilution riser is 11.2%. The washing water for the second washing is the wastewater from the third washing; the equipment is preferably a drum displacement pulp washer.

[0048] (7) Oxygen bleaching and degassing condensation (i.e., oxygen delignification treatment) Oxygen delignification treatment: The reaction pulp consistency is 11.3%, the consumption of oxidized white liquor is 10.0 kg / adt, the consumption of NaOH is 6.7 kg / adt, the initial reaction pH is 11.6, the oxygen consumption is 11.0 kg / adt, the pipeline oxygen pressure is 8.2 bar, the reaction temperature is 99 °C, and the residence time is 10 min; Second-stage oxygen delignification treatment: The reaction pulp consistency is 11.3%, the oxygen consumption is 16.3 kg / adt, the reaction tower top pressure is 5.7 bar, the reaction temperature is 96 °C, the residence time is 60 min, the pH at the end point of the bleaching liquor reaction is 10.32, the Kappa number after bleaching is 9.56, and the brightness (ISO) of the bleached pulp is 53.63%; Washing treatment: The incoming pulp mass concentration is 8.1%, the washing dilution factor is 2.31 m 3 / adt, the pressing pressure is 101 kN / m, and the outgoing pulp mass concentration is 29.10%.

[0049] The deaeration and condensation of the oxygen-bleached pulp is carried out to obtain the deaerated pulp. The parameters of the deaeration and condensation are: the condensate water flow rate is 122 kg / s, the gas suction pressure is -1.2 kPa, and the condensate water temperature is 30 °C.

[0050] (8) Re-washing The deaerated pulp in (7) is washed for the third and fourth times in sequence to obtain the pulp; The parameters of the third washing are: the incoming pulp mass concentration is 8.01%, the washing dilution factor is 2.26 m 3 / adt, the pressing pressure is 83 kN / m, and the outgoing pulp mass concentration is 25.11%; The parameters of the fourth washing are preferably: the incoming pulp mass concentration is 8.13%, the washing dilution factor is 2.41 m 3 / adt, the pressing pressure is 89 kN / m, and the outgoing pulp mass concentration is 28.99%; Example 3 The difference from Example 1 is only in steps (4)-(8), and the steps are as follows: (4) Knot removal: The thick pulp in step (3) is conveyed to a knot remover for knot removal to remove larger tree knots, and the de-knotted pulp is obtained; The parameters of the knot removal include: the incoming pulp mass concentration is 3.87%, the incoming pulp pressure is 2.91 bar, the knot removal pressure difference is 0.13 bar, and the slag discharge rate is 12%.

[0051] (5) Pressure screening includes: The de-knotted pulp in (4) is subjected to first-stage three-stage pressure screening to obtain the screened good pulp; Among them, the parameters of the first-stage pressure screening are: the incoming pulp mass concentration is 3.83%, the incoming pulp pressure is 2.89 bar, and the screening pressure is 0.03 bar; The parameters for two-stage pressure screening are: the feed pressure is 2.85 bar, and the screening pressure difference is 0.01 bar; The parameters for three-stage pressure screening are: the feed pressure is 2.98 bar, and the screening pressure difference is 0.03 bar.

[0052] (6)Washing: The screened good pulp in (5) is washed for the first time and then for the second time to obtain the pulp after secondary washing.

[0053] The parameters for the first washing are: the feed mass concentration is 3.94%, the washing dilution factor is 2.01 m 3 / adt, the squeezing pressure is 78 kNm, and the discharge mass concentration is 26.5%; the equipment is a double-roll pulp washer; the washing water is the wastewater from secondary washing.

[0054] The parameters for the second washing are: the feed mass concentration is 8.6%, the washing dilution factor is 6.4 m 3 / adt, the washing water pressure is 0.6 bar, the tank pressure is 0.4 bar, the rotational speed is 3.7 r / min, the discharge mass concentration is 13.1%, and the pulp concentration in the dilution riser is 11.2%. The washing water for the second washing is the wastewater from tertiary washing; the equipment is preferably a drum displacement pulp washer.

[0055] (7)Oxygen bleaching and deaeration condensation (i.e., oxygen delignification treatment) Oxygen delignification treatment: the reaction pulp concentration is 12.3%, the consumption of oxidized white liquor is 11.0 kg / adt, the consumption of NaOH is 8.0 kg / adt, the initial pH of the reaction is 11.8, the consumption of oxygen is 11.4 kg / adt, the pipeline oxygen pressure is 8.0 bar, the reaction temperature is 100 °C, and the residence time is 10 min; for the second-stage oxygen delignification treatment: the reaction pulp concentration is 12.3%, the consumption of oxygen is 17.0 kg / adt, the pressure at the top of the reaction tower is 5.8 bar, the reaction temperature is 98 °C, the residence time is 60 min, the pH at the end point of the reaction of the bleaching liquor is 10.45, the Kappa number after bleaching is 9.80, and the brightness (ISO) of the bleached pulp is 52.48%; washing treatment: the feed mass concentration is 8.2%, the washing dilution factor is 2.50 m 3 / adt, the squeezing pressure is 95 kNm, and the discharge mass concentration is 29.11%.

[0056] The deaeration condensation is carried out on the pulp after oxygen bleaching to obtain the deaerated pulp. The parameters for deaeration condensation are: the condensate water flow rate is 122 kg / s, the gas suction pressure is -1.2 kPa, and the condensate water temperature is 30 °C.

[0057] (8)Re-washing The degassed slurry in (7) is subjected to the third washing and the fourth washing in sequence to obtain a slurry; the parameters of the third washing are: the inlet pulp mass concentration is 7.46%, the washing dilution factor is 2.02 m 3 / adt, the pressing pressure is 97 kN / m, and the outlet pulp mass concentration is 28.97%; The parameters of the fourth washing are preferably: the inlet pulp mass concentration is 8.02%, the washing dilution factor is 2.16 m 3 / adt, the pressing pressure is 90 kN / m, and the outlet pulp mass concentration is 28.4%; Example 4 The difference from Example 1 is only in steps (4)-(8), and the steps are as follows: (4) Knot removal: The thick pulp in step (3) is transferred to a knotter for knot removal to remove larger tree knots, and the knot-removed slurry is obtained; the parameters of the knot removal include: the inlet pulp mass concentration is 4.03%, the inlet pressure is 3.02 bar, the knot-removing pressure difference is 0.14 bar, and the reject rate is 12%.

[0058] (5) Pressure screening includes: The knot-removed slurry in (4) is subjected to first-stage three-stage pressure screening to obtain screened good pulp; Among them, the parameters of the first-stage pressure screening are: the inlet pulp mass concentration is 3.93%, the inlet pressure is 2.59 bar, and the screening pressure is 0.03 bar; The parameters of the second-stage pressure screening are: the inlet pressure is 2.90 bar, and the screening pressure difference is 0.04 bar; The parameters of the third-stage pressure screening are: the inlet pressure is 2.87 bar, and the screening pressure difference is 0.03 bar.

[0059] (6) Washing: The three-stage screened good pulp in (5) is subjected to the first washing and the second washing in sequence to obtain the twice-washed slurry.

[0060] The parameters of the first washing are: the inlet pulp mass concentration is 4.10%, the washing dilution factor is 2.26 m 3 / adt, the pressing pressure is 82 kN / m, and the outlet pulp mass concentration is 27.69%; the equipment is a double-roll pulp washer; the washing water is the wastewater from the second washing.

[0061] The parameters of the second washing are: the inlet pulp mass concentration is 8.7%, the washing dilution factor is 7 m 3 / adt, the washing water pressure is 0.43 bar, the tank pressure is 0.35 bar, the rotational speed is 4 r / min, the pulp mass concentration of the discharged pulp is 12.95%, and the pulp concentration of the dilution riser is 11.4%. The water used for the second washing is the wastewater from the third washing; the equipment is preferably a drum displacement pulp washer.

[0062] (7)Oxygen bleaching and degassing condensation (i.e., oxygen delignification treatment) Oxygen delignification treatment: The reaction pulp concentration is 12.0%, the consumption of oxidized white liquor is 9.9 kg / adt, the consumption of NaOH is 7.5 kg / adt, the initial pH of the reaction is 11.2, the consumption of oxygen is 11.0 kg / adt, the oxygen pressure in the pipeline is 7.9 bar, the reaction temperature is 99 °C and the residence time is 10 min; for the second-stage oxygen delignification treatment: the reaction pulp concentration is 12.0%, the consumption of oxygen is 16.9 kg / adt, the pressure at the top of the reaction tower is 5.9 bar, the reaction temperature is 95 °C, the residence time is 60 min, the pH at the end point of the reaction of the bleaching liquor is 10.53, the Kappa number after bleaching is 9.80 and the brightness (ISO) of the bleached pulp is 55.0%; Washing treatment: The inlet pulp concentration is 7.8%, the washing dilution factor is 2.33 m 3 / adt, the squeezing pressure is 92 kN / m and the discharged pulp mass concentration is 27.89%.

[0063] The degassing and condensation of the pulp after oxygen bleaching is carried out to obtain the degassed pulp. The parameters of degassing and condensation are: the flow rate of condensate water is 122 kg / s, the gas suction pressure is -1.2 kPa, and the temperature of condensate water is 30 °C.

[0064] (8)Re-washing The degassed pulp in (7) is washed for the third time and the fourth time in sequence to obtain the pulp; the parameters of the third washing are: the inlet pulp concentration is 7.86%, the washing dilution factor is 2.33 m 3 / adt, the squeezing pressure is 94 kN / m, and the discharged pulp mass concentration is 28.41%; The parameters of the fourth washing are preferably: the inlet pulp concentration is 7.73%, the washing dilution factor is 2.36 m 3 / adt, the squeezing pressure is 92 kN / m, and the discharged pulp mass concentration is 29.0%; Comparative Example 1 A method for screening and washing the previous pulp to reduce alkali loss and the consumption of bleaching chlorine dioxide (see the reference flow chart in Figure 2 ), the steps are: The difference from Example 1 is only in steps (4)-(8), and the steps are as follows: (4)Knot removal: Transfer the thick pulp in step (3) to a knot remover for knot removal to remove large tree knots, obtaining the pulp after knot removal; the parameters for knot removal include: the incoming pulp mass concentration is 4.5%, the incoming pulp pressure is 3 bar, the knot removal pressure difference is 2.9 bar, and the reject rate is 10.5%.

[0065] (5)Pressure screening includes: Perform primary three-stage pressure screening on the pulp after knot removal in (4) to obtain screened good pulp; Among them, the parameters for the first-stage pressure screening are: the incoming pulp mass concentration is 4.0%, the incoming pulp pressure is 3 bar, and the screening pressure is 2.80 bar; The parameters for the second-stage pressure screening are: the incoming pulp pressure is 3.1 bar, and the screening pressure difference is 2.9 bar; The parameters for the third-stage pressure screening are: the incoming pulp pressure is 3.0 bar, and the screening pressure difference is 2.9 bar.

[0066] (6)Washing: Perform the first washing and the second washing on the screened good pulp in (5) in sequence to obtain the pulp after secondary washing.

[0067] The parameters for the first washing are: the incoming pulp mass concentration is 4.3%, the washing dilution factor is 2.2 m 3 / adt, the pressing pressure is 75 kN / m, and the outgoing pulp mass concentration is 25%; the equipment is a double-roll pulp washer; the washing water is the wastewater from the second washing.

[0068] The parameters for the second washing are: the incoming pulp mass concentration is 2.2%, the washing dilution factor is 2.8 m 3 / adt, the pressing pressure is 97.3 kN / m, the outgoing pulp mass concentration is 31.86%, and the pulp concentration in the dilution riser is 13%. The washing water for the second washing is the wastewater from the third washing; the equipment is a double-roll pulp washer.

[0069] (7)Oxygen bleaching and degassing (i.e., oxygen delignification treatment) Oxygen delignification treatment: The reaction pulp concentration is 11.6%, the consumption of oxidized white liquor is 11.0 kg / adt, the consumption of NaOH is 8.2 kg / adt, the initial pH of the reaction is 11.8, the consumption of oxygen is 10.6 kg / adt, the pipeline oxygen pressure is 8.3 bar, the reaction temperature is 100 °C, and the residence time is 10 min; for the second-stage oxygen delignification treatment: the reaction pulp concentration is 11.6%, the consumption of oxygen is 16.2 kg / adt, the reaction tower top pressure is 6.0 bar, the reaction temperature is 95 °C, the residence time is 60 min, the pH at the end point of the reaction of the bleaching liquor is 10.70, the Kappa number after bleaching is 9.62, and the brightness (ISO) of the pulp after bleaching is 55.47%; washing treatment: the incoming pulp mass concentration is 7.6%, the washing dilution factor is 2.38 m3 / adt, the pressing pulp pressure is 99 kN / m, and the discharged pulp mass concentration is 28.68%.

[0070] The oxygen - bleached pulp is degassed and condensed to obtain the degassed pulp. The degassing parameters are: the condensate water flow rate is 122 kg / s, and the gas suction pressure is - 1.2 kPa.

[0071] (8)Washing again The degassed pulp in (7) is washed for the third and fourth times in sequence to obtain the pulp. The parameters for the third washing are: the incoming pulp mass concentration is 7.9%, the washing dilution factor is 2.47 m 3 / adt, the pressing pulp pressure is 100 kN / m, and the discharged pulp mass concentration is 26.97%; The parameters for the fourth washing are preferably: the incoming pulp mass concentration is 8.0%, the washing dilution factor is 2.66 m 3 / adt, the pressing pulp pressure is 102 kN / m, and the discharged pulp mass concentration is 23.42%; The pulps prepared in Examples 1 - 4 and Comparative Example 1 and the pulps bleached in the same subsequent bleaching stage are respectively detected, and the results are shown in Table 1.

[0072] Table 1 Detection data of different treatments

[0073] It can be seen from Table 1 that after adopting the technical solution provided by the present invention, the alkali loss is less than 7 kg / adt, the COD of the washed pulp is < 9 kg / adt, and the savings of chlorine dioxide are between 2 - 4 kg / adt. Through examples, it shows that the invention plays an important role in improving pulp washing, can significantly reduce the subsequent consumption of chlorine dioxide, and reduce the manufacturing cost.

[0074] Although the above - mentioned embodiments have described the present invention in detail, they are only a part of the embodiments of the present invention, not all embodiments. People can also obtain other embodiments without creative efforts based on these embodiments, and these embodiments all belong to the protection scope of the present invention.

Claims

1. A method for screening and washing the previous pulp, characterized in that, It includes the following steps: Pre-impregnate and cook the wood chip samples in sequence to obtain the thick pulp; Subject the thick pulp to knot removal, pressure screening, primary washing, secondary washing, oxygen bleaching, degassing and condensation, tertiary washing and quaternary washing in sequence to obtain the pulp.

2. The method according to claim 1, wherein The parameters for knot removal include: the inlet pulp mass concentration is 3.8% - 4.3%, the inlet pulp pressure is 2.8 - 3.1 bar, the knot removal differential pressure is 0 - 0.2 bar, and the reject rate is 10% - 12%.

3. The method according to claim 1, wherein The pressure screening includes: primary pressure screening, secondary pressure screening and tertiary pressure screening; The parameters for the primary pressure screening include: the inlet pulp mass concentration is 3.8% - 4.3%, the inlet pulp pressure is 2.8 - 3.1 bar, and the screening differential pressure is 0 - 0.2 bar; The parameters for the secondary pressure screening and the tertiary pressure screening respectively include: the inlet pulp pressure is 2.8 - 3.1 bar, and the screening differential pressure is 0 - 0.2 bar.

4. The method according to claim 1, characterized in that, The parameters of the first washing include: the incoming pulp mass concentration is 3.8% - 4.3%, the washing dilution factor is 1.5 - 2.5 m 3 / adt, the pressing pressure is 75 - 90 kN / m, and the outgoing pulp mass concentration is 25% - 30%.

5. The method according to claim 1, wherein The parameters of the secondary washing include: the incoming pulp mass concentration is 8% - 9%, the washing dilution factor is 6 - 8 m 3 / adt, the washing water pressure is 0.3 - 0.8 bar, the tank pressure is 0.2 - 0.5 bar, the rotational speed is 3 - 5 r / min, and the outgoing pulp mass concentration is 10% - 15%.

6. The method according to claim 1, wherein The parameters for degassing and condensation include: the condensate water flow rate is 110 - 140 kg / s, the gas suction pressure is -1.0 - -1.5 kPa, and the condensate water temperature is 30 - 40 °C.

7. The method according to claim 1, characterized in that, The parameters of the third washing and the fourth washing respectively include: the incoming pulp mass concentration is 7.0% - 9.0%, the washing dilution factor is 2.0 - 3.0 m 3 / adt, the pressing pressure is 90 - 100 kNm, and the outgoing pulp mass concentration is 25% - 30%.

8. The method according to claim 1 or 7, characterized in that Use the VE intermediate condensate water for the fourth washing, and the COD value of the VE intermediate condensate water is 600 - 700 mg / L, and the pH value is 8.8 - 9.

5.

9. The slurry prepared by the method according to any one of claims 1 to 8, characterized in that, The alkali loss of the pulp < 7 kg / adt, and the COD of the pulp < 9 kg / adt.

10. Application of the method according to any one of claims 1 - 8 in reducing alkali loss and / or reducing the consumption of bleaching chlorine dioxide in pulp production.

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