A method for extracting nanocellulose and detoxifying adsorbable organic halides (AOX) using papermaking sludge

Abstract

The present invention provides a method for extracting nanocellulose and detoxifying adsorbable organic halides (AOX) using papermaking sludge, which includes soaking the papermaking sludge in an alkaline reagent, subjecting it to oxygen-alkali cooking, filtering to collect the slurry on the membrane, and ultrasonic dispersing to obtain nanocellulose. The conditions for oxygen-alkali cooking are an oxygen pressure of 1-2 MPa, a cooking heating-up time of 60-120 min, a maximum temperature of 150-200 °C, and a heat preservation time of 120-180 min. The present invention realizes a cellulose recovery rate of over 90% in papermaking waste sludge through a one-pot method during the oxygen-alkali cooking process, and at the same time, the AOX removal rate can exceed 95%.

Description

Technical Field

[0001] The present invention relates to a method for realizing the detoxification of adsorbable halides (AOX) and the preparation of nanocellulose by a one-pot method using papermaking sludge, belonging to the technical field of solid waste treatment. Background Art

[0002] Papermaking sludge is the end product of the pulp and paper wastewater treatment process. Approximately 1.2 tons of sludge with a water content of 80% is generated for every 1 ton of paper produced.

[0003] On the one hand, papermaking sludge has a complex composition. Its main organic components include cellulose (about 50% of all organic components), hemicellulose, lignin, etc. Due to its high organic content, papermaking sludge is also considered a valuable biomass resource. On the other hand, adsorbable organic halides (referred to as AOX, mainly composed of chlorophenols, chlorobenzenes such as tetrachlorofuran, and chlorophenols) generated during the pulp bleaching process have been proven to be carcinogenic, teratogenic, and mutagenic.

[0004] Based on the characteristics of papermaking sludge with high organic content, high water content, and toxic components, if it can be harmlessly treated (detoxified) and resourcefully utilized simultaneously, it can not only solve the environmental pollution problem but also generate certain economic benefits, which is of great significance.

[0005] Nanocellulose has broad application prospects in the fields of high-performance composites, electronic products, catalytic materials, biomedical materials, and energy, which has attracted great interest in the academic and industrial communities. The cellulose content in papermaking sludge is high, the lignin content is low, and the fiber crystallinity is low, making it a good raw material for preparing nanocellulose. Papermaking sludge is a very complex heterogeneous colloid mixture. Water and solid suspensions are wrapped by microbial cells to form a special zoogloea structure. Therefore, the sludge has a very high viscosity. The dynamic viscosity of sludge with a water content of 90% at room temperature can reach 100,000 mPa·s, while the dynamic viscosity of water is only 1 mPa·s. How to destroy the zoogloea structure in papermaking sludge and release the cellulose therein has become the key to preparing nanocellulose from papermaking sludge. Summary of the Invention

[0006] The present invention aims at the harmless treatment (effective removal of AOX) and resource utilization (effective extraction of nanocellulose) of papermaking sludge, and gives full play to the effects of the alkali-catalyzed wet oxidation process on the dehydration of sludge flocs, the oxidative degradation of AOX, and the separation of cellulose. By regulating the interaction relationship among the three, the one-pot method simultaneously realizes the efficient dehydration and separation of sludge, the safe detoxification of AOX, and the extraction of cellulose, achieving the purpose of harmless treatment and resource utilization of papermaking sludge.

[0007] The reaction process of the present invention is in a wet oxidation system of a near-critical water environment. Compared with ordinary water, the density of near-critical water increases, while its viscosity and surface tension decrease. Therefore, the solubility of flocculent suspended solids in sludge is increased. Organic matter hydrolysis occurs in the sludge and the structure of the flocculent suspended solids ruptures, turning the bound water in the cells into free water. Therefore, hydrothermal treatment helps with sludge dewatering and the separation of cellulose. In addition, a large number of hydroxyl radicals generated by the synergistic action of oxygen and alkali have the function of oxidatively degrading AOX and lignin, which helps with the separation and purification of cellulose.

[0008] Wet oxidation is a method based on conventional hydrothermal technology, adding oxygen as an oxidant to oxidize and decompose dissolved or suspended organic matter in water into carbon dioxide, water and non-toxic and harmless substances. Catalytic wet oxidation helps to simultaneously exert the catalytic induction effect of the oxygen-alkali reaction on the generation of oxygen active groups and the cell wall breaking effect of the hydrothermal state on the microbial colloid. Therefore, this synergistic effect can not only safely detoxify AOX, but also realize the separation and purification of the cellulose component in the sludge.

[0009] The present invention provides a method for extracting nanocellulose and detoxifying adsorbable halides (AOX) using papermaking sludge, including soaking the papermaking sludge in an alkaline reagent, subjecting it to oxygen-alkali cooking, filtering and collecting the slurry on the membrane, and ultrasonic dispersing to obtain nanocellulose. The oxygen pressure of the oxygen-alkali cooking is 1-2 MPa, and the material is heated from room temperature to 150-200 °C within 60-120 min and kept at 150-200 °C for 120-180 min.

[0010] Furthermore, the moisture content of the papermaking sludge is 80 wt% - 90 wt%.

[0011] Even further, the time for soaking the papermaking sludge in the alkaline reagent is 10-30 min.

[0012] Even further, the alkaline cooking agent used in the oxygen-alkali cooking process is selected from at least one of NaOH, KOH and Na 2 CO 3 among others.

[0013] Even further, during the oxygen-alkali cooking process, the dosage of the alkaline cooking agent is 15 wt% - 40 wt% of the dry weight of the sludge.

[0014] Furthermore, the filtration is carried out by vacuum filtration using a commercial organic cellulose membrane with a pore size of 0.4 - 0.5 μm. After filtration, the slurry remaining on the filter membrane is washed with warm water, and vacuum filtration is carried out again until the pH of the filtrate collected below the filtration is close to 7 and the AOX content is measured.

[0015] Furthermore, for the ultrasonic dispersion, the slurry on the membrane after filtration is collected, and then water is added to adjust it to 10wt%-15wt% of the total mass of the slurry based on dry weight, followed by ultrasonic treatment.

[0016] Furthermore, the power of the ultrasonic dispersion is 1000w - 1800w, the frequency is 45 - 55Hz, and the treatment time is 45 - 75min.

[0017] Through the oxygen-alkali cooking process, the characteristic pollutant AOX in papermaking sludge can be degraded and removed, and the treated wastewater meets the discharge standard. Moreover, during the cooking process, the cellulose in the papermaking sludge is purified, and through the ultrasonic process, micro-nano cellulose can be produced.

[0018] Compared with the prior art, the present invention has the following advantages:

[0019] 1. For the oxygen-alkali cooking of the present invention, under the conditions of an oxygen pressure of 1 - 2MPa, the material is heated from room temperature to 150 - 200°C within 60 - 120min and kept at 150 - 200°C for 120 - 180min. As a homogeneous catalyst, the alkali can cooperate with oxygen to generate superoxide radicals, which is beneficial to the degradation of pollutants, and can retain cellulose to the greatest extent, with the cellulose recovery rate exceeding 90%. Especially for papermaking sludge containing AOX, free radical groups with strong oxidation performance are more needed, which promotes the oxidative degradation of AOX, lignin, and the separation and purification of cellulose.

[0020] 2. The present invention obtains nano-cellulose through simple and low-power ultrasonic treatment, with the whole process being simple, efficient, green, and environmentally friendly.

[0021] 3. After the one-pot low-temperature wet oxidation treatment of the papermaking sludge of the present invention, the efficient removal of AOX can be achieved, with the removal rate exceeding 95%. Moreover, nano-cellulose can be obtained in a high yield, realizing the recycling of cellulose resources in the papermaking sludge. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for the description of the specific embodiments or the prior art. Obviously, the following drawings are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0023] Figure 1 Process flow chart of the steps for the harmless treatment and resource utilization of papermaking sludge. SPECIFIC EMBODIMENTS

[0024] Example 1

[0025] Bagasse papermaking sludge: The sludge is the waste sludge after anaerobic fermentation in the sewage treatment system of a local pulp and paper mill, with a water content of 85% (wt% whole) and an ash content (dry basis) of 30% (wt% dry material).

[0026] Load 1 kg of bagasse papermaking sludge into a mixing bin, seal it, and start mechanical stirring for 15 min. Then add NaOH solid at 25 wt% (dry basis of raw material) of the sludge dry weight, allow it to penetrate at room temperature for 30 min, and then introduce oxygen to start heating and cooking. The oxygen pressure is 1 MPa, and the material is heated to 180 °C within 100 min, and then kept at 180 °C for 150 min. After the cooking is completed, perform vacuum filtration using a commercial organic cellulose membrane with a pore size of 0.45 μm. After filtration, wash the slurry remaining on the filter membrane with warm water, and perform vacuum filtration again until the pH of the filtrate collected below the filtration is close to 7 and the AOX content is measured. Collect the slurry on the filter membrane after filtration, then add water to adjust it to 13 wt% of the total mass of the slurry dry weight, and then add it to a contact ultrasonic crusher for ultrasonic dispersion treatment according to the following parameters: power 1500 w, frequency 50 Hz, treatment time 60 min, and finally obtain micro-nano cellulose.

[0027] Example 2

[0028] This example provides a method for extracting nano-cellulose and detoxifying adsorbable halides (AOX) using papermaking sludge, which is basically the same as Example 1, except that: NaOH is a solid at 40 wt% (dry basis of raw material) of the sludge dry weight, the oxygen pressure is 2 MPa, and the material is heated to 200 °C within 120 min, and then kept at 200 °C for 180 min.

[0029] Example 3

[0030] This example provides a method for extracting nano-cellulose and detoxifying adsorbable halides (AOX) using papermaking sludge, which is basically the same as Example 1, except that: the sludge penetrates at room temperature for 10 min, NaOH is a solid at 10 wt% (dry basis of raw material) of the sludge dry weight, the oxygen pressure is 1 MPa, and the material is heated to 150 °C within 60 min, and then kept at 150 °C for 120 min.

[0031] Example 4

[0032] This example provides a method for extracting nano-cellulose and detoxifying adsorbable halides (AOX) using papermaking sludge, which is basically the same as Example 1, except that: the raw material is papermaking sludge with a water content of 80%; collect the slurry on the filter membrane after filtration, then add water to adjust it to 15 wt% of the total mass of the slurry dry weight, and then add it to a contact ultrasonic crusher for ultrasonic dispersion treatment according to the following parameters: power 1000 w, frequency 45 Hz, treatment time 75 min.

[0033] Example 5

[0034] This example provides a method for extracting nanocellulose and detoxifying adsorbable halides (AOX) using papermaking sludge, which is basically the same as Example 1, except that the raw material is papermaking sludge with a water content of 90%; the slurry on the membrane after filtration is collected, then water is added to adjust it to a slurry dry weight accounting for 10 wt% of the total mass of the slurry, and then it is added to a contact ultrasonic crusher for ultrasonic dispersion treatment according to the following parameters: power 1000 w, frequency 45 Hz, treatment time 75 min.

[0035] Example 6

[0036] This example provides a method for extracting nanocellulose and detoxifying adsorbable halides (AOX) using papermaking sludge, which is basically the same as Example 1, except that when performing ultrasonic treatment in a contact ultrasonic crusher, the ultrasonic power is 1800 w, the frequency is 55 Hz, and the treatment time is 45 min.

[0037] Comparative Example 1

[0038] This comparative example provides a method for extracting nanocellulose and detoxifying adsorbable halides (AOX) using papermaking sludge, which is basically the same as Example 1, except that no oxygen is added during the cooking process.

[0039] Comparative Example 2

[0040] This comparative example provides a method for extracting nanocellulose and detoxifying adsorbable halides (AOX) using papermaking sludge, which is basically the same as Example 1, except that the oxygen pressure is 3 MPa, the material is heated to 180 °C within 120 min, and then kept warm for 120 min.

[0041] Comparative Example 3

[0042] This comparative example provides a method for extracting nanocellulose and detoxifying adsorbable halides (AOX) using papermaking sludge, which is basically the same as Example 1, except that the oxygen pressure is 1 MPa, the material is heated to 250 °C within 120 min, and then kept warm for 150 min.

[0043] Comparative Example 4

[0044] This comparative example provides a method for extracting nanocellulose and detoxifying adsorbable halides (AOX) using papermaking sludge, which is basically the same as Example 1, except that the oxygen pressure is 1 MPa, the material is heated to 100 °C within 120 min, and then kept warm for 120 min.

[0045] Comparative Example 5

[0046] This comparative example provides a method for extracting nanocellulose and detoxifying adsorbable halides (AOX) using papermaking sludge, which is basically the same as Example 1, except that: after suction filtration, ultrasonic treatment is not carried out.

[0047] The method for determining lignin is the NREL standard for lignocellulose content determination by the US Department of Energy. First, the fibers are converted into oligosaccharides using concentrated sulfuric acid at 25 °C, and then the oligosaccharides are further converted into monosaccharides using 3% (wt%) dilute sulfuric acid at 121 °C. The content of acid-soluble lignin is determined by detecting the acid hydrolysis solution with a spectrophotometer.

[0048] The determination of cellulose recovery follows the method for determining holocellulose TAPPI T249cm-00 and is analyzed and measured by ion chromatography. The fiber length is measured using a scanning electron microscope. The detection of AOX is carried out using a MultiX2500 elemental analyzer from Analytik Jena AG, Germany. After sampling the waste liquid, its pH is first adjusted to about 2 using nitric acid. After dilution, 100 mL is measured into a conical flask, and a certain amount of activated carbon is added for shaking adsorption for 30 minutes. The chlorides in the waste water are adsorbed by the activated carbon, and then the activated carbon is transferred to an adsorption column for filtration. The adsorption column is washed twice with a nitric acid solution to remove the adsorbed inorganic chlorine. After the adsorption column is dried, it is put into a combustion furnace for combustion treatment at 950 °C, and the content of AOX is calculated by microcoulometric titration.

[0049] The summary of the product conditions obtained in the above examples and comparative examples under different conditions is shown in the following table.

[0050] Table 1 Lignin content % (wt / wt) of each example and comparative example

[0051]

[0052] Table 2 Cellulose recovery rate (wt / wt sludge dry weight) of each example and comparative example

[0053]

[0054] Table 3 Nanocellulose size (nm) of each example and comparative example

[0055]

[0056] Table 4 AOX content (mg / L) of each example and comparative example

[0057]

[0058] Obviously, the above embodiments are merely examples given for clear illustration and not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation manners here. And the obvious changes or modifications derived therefrom still fall within the protection scope of the present invention.

Claims

1. A method for extracting nanocellulose and detoxifying adsorbable organic halides (AOX) using papermaking sludge, characterized in that, it includes soaking the papermaking sludge in an alkaline reagent, performing oxygen-alkali cooking, filtering and collecting the slurry on the membrane, and ultrasonic dispersion to obtain nanocellulose. The oxygen pressure of the oxygen-alkali cooking is 1-2 MPa, the material is heated from room temperature to 150-200 °C within 60-120 min, and kept at 150-200 °C for 120-180 min; the power of the ultrasonic dispersion is 1000w-1800w, the frequency is 45-55Hz, and the treatment time is 45-75 min; the time for soaking the papermaking sludge in the alkaline reagent is 10-30 min; for the filtration, after the slurry is cooled, a commercial organic cellulose membrane with a pore size of 0.4-0.5μm is selected for vacuum filtration. After filtration, the slurry remaining on the filter membrane is washed with warm water, and vacuum filtration is performed again until the pH of the filtrate collected below the filtration is close to 7 and the AOX content is measured.

2. The method for extracting nanocellulose and detoxifying adsorbable organic halides (AOX) using papermaking sludge according to claim 1, characterized in that, the papermaking sludge has a moisture content of 80wt% - 90wt%.

3. The method for extracting nanocellulose and detoxifying adsorbable organic halides (AOX) using papermaking sludge according to claim 1, characterized in that, during the oxygen-alkali cooking process, the dosage of the alkaline cooking agent is 15wt% - 40wt% of the dry weight of the sludge.

4. The method for extracting nanocellulose and detoxifying adsorbable organic halides (AOX) using papermaking sludge according to claim 3, characterized in that, The alkaline cooking agent used in the oxygen-alkali cooking process is selected from at least one of NaOH, KOH, and Na 2 CO 3 .

5. The method for extracting nanocellulose and detoxifying adsorbable organic halides (AOX) using papermaking sludge according to any one of claims 1-4, characterized in that, for the ultrasonic dispersion, the slurry on the membrane after filtration is collected, then water is added to adjust the dry weight of the slurry to account for 10wt%-15wt% of the total mass of the slurry, and then ultrasonic treatment is performed.

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