CN112522985B

CN112522985B - Micro-chemical pulping of agricultural straw and full-component refining method of biomass

Info

Publication number: CN112522985B
Application number: CN202011432587.5A
Authority: CN
Inventors: 王兆江; 韩文佳; 曾劲松; 陈克复
Original assignee: Qilu University of Technology
Current assignee: Qilu University of Technology
Priority date: 2020-12-10
Filing date: 2020-12-10
Publication date: 2023-03-17
Anticipated expiration: 2040-12-10
Also published as: CN112522985A

Abstract

The invention relates to a micro-chemical pulping and biomass full-component refining method of agricultural straws, belonging to the field of biomass refining. The pulping method comprises the following steps of carrying out subacid treatment on straws in a subacid environment, and also comprises a treatment method combining subacid treatment and subalkaline treatment. The method overcomes pulping obstacle of straw heterocyte, can realize complete extraction and utilization of straw chemical components, and realizes comprehensive refining of biomass resources.

Description

Micro-chemical pulping of agricultural straw and full-component refining method of biomass

Technical Field

The invention discloses a micro-chemical pulping and biomass full-component refining method for agricultural straws, and belongs to the field of biomass refining.

Background

The resource utilization of agricultural wastes comprises various aspects, such as biological power generation, corn stalk silage, straw returning, furfural production from corncobs, ethanol fermentation after straw pretreatment, straw pulping and papermaking and the like. Among them, pulping and papermaking belong to chemical industry angle utilization of straw, unfortunately, at present, more than 99% of pulping utilizes wood, and the utilization of straw is limited. A new technology is developed, the existing pulping platform and the technology are scientifically expanded, the straw can be used for pulping and papermaking, and other chemicals and materials can be obtained by utilizing non-fiber components (hemicellulose and lignin) of the straw, so that the method conforms to the concept of full-component refining of biomass and also conforms to the national plan on green utilization of agricultural waste resources.

Pulping is a process of separating fibers from a plant fiber raw material to obtain pulp, and is classified into a mechanical method, a chemical method, and a chemimechanical method according to a pulping method. The reasonable combination of chemical and mechanical action can obtain paper pulp with different yield and performance. From the difference of the paper pulp yield, the method can be divided into high-yield pulp (also called chemi-mechanical pulp, the yield is 85% -95%), semi-chemical pulp (the yield is 60% -85%) and chemical pulp (35% -60%) produced by a chemical mechanical method. At present, the mainstream processes of chemi-mechanical pulp are APMP and BCTMP, the process for producing semi-chemical pulp is mainly an alkaline sulfite technology, and the process for producing chemical pulp is a caustic soda method and a sulfate method. These pulping processes are all carried out in an alkaline environment of sodium hydroxide. In fact, pulping has been performed in an alkaline environment from the zellen invention papermakmg to the present. The acidic sulfite process appeared in the 20 th century, and at present, the yield of the whole pulping industry is less than 1%. Currently, pulping techniques are essentially all performed under a single alkaline condition.

Admittedly, the alkaline process (alkaline condition) is very suitable for pulping of wood raw materials because the non-fibrous components (lignin and part of hemicellulose) of the wood raw materials are quickly dissolved into the waste liquid in the alkaline environment, the fibers are dissociated into pulp, and the scientific rationality of the alkaline pulping process has been verified by the huge-scale wood paper industry. However, for pulping of grass-like agricultural straw feedstocks, alkaline pulping techniques encounter many obstacles and challenges due to differences in feedstock structure and composition. First, the leaves, pith (pulp), straw cuticle and other parts of the crop contain a large number of non-fiber cells, including epidermic cells, parenchyma cells, which are also referred to as hybrid cells. These foreign cells not only consume high alkali, but also cause extremely high content of fine components in the pulp, poor drainability of the pulp, and low extraction rate of black liquor in the pulp washing process, which not only means that the pulp is difficult to wash clean, but also means that the alkali in the pulp is difficult to recover. Secondly, the content of hemicellulose glycan in pith (pulp) parts of straws such as corn and the like is high, the hemicellulose glycan is difficult to degrade in an alkaline environment, the hemicellulose glycan is very viscous in the alkaline environment, so that paper pulp is sticky and difficult to filter water, the paper pulp washing difficulty is high, water filtering and dewatering can not be thoroughly performed on a net part of a paper machine, and the speed of the paper machine is difficult to increase. Thirdly, the ash content (inorganic salt) in the straw is high, especially silicon element, and silicon deposits on pipelines and heating surfaces, which causes various obstacles in the production process. In addition, the inorganic salt is high, the viscosity of the concentrated black liquor (pulping waste liquor) is high, the flowing is difficult, the heat transfer efficiency is low, and the interference on alkali recovery is serious. The above characteristics of straw raw materials not only cause the problems of the process, but also cause the problems of environmental pollution, overhigh consumption of chemicals and the like, which is the crux that the straw is not widely used for pulping, especially the corn straw, has not been really used for pulping and papermaking since history.

By combining the above problems, the key problem of agricultural straw pulping is how to remove the hybrid cells. For corn stalks, sorghum stalks, bagasse and other stalks, the removal of pith (pulp) is very important. The first scheme is to strengthen the material preparation, remove the leaves, pith (pulp) and nodes of the straw by physical means, such as screening, air separation, water separation, extrusion and screening, etc., the straw skin is used for pulping, and the pith (pulp) is used for preparing sugar products (CN 202010255192.6). In patent CN201910028784.1, pulp (pulp) and leaves are squeezed and kneaded into particles or fragments by strong washing and double screws, and then removed by subsequent screening, so as to ensure that the residual straw skin can be processed into chemical mechanical pulp. The second scheme mainly focuses on the pretreatment of biological enzymes, some of which act on hemicellulose, some of which act on lignin, and the final aim is to solve various process problems of straw pulping through enzyme treatment, and related patent technologies include CN201510594486.0, CN202010255061.8 and CN201811001993.9. There are also patents which propose the use of hot water extraction to remove hemicellulose from straw, and the subsequent use of the extracted hemicellulose in chemical or biological processes for the production of ethanol and other chemicals, such as patent US8889384B2.

In conclusion, the overcoming of the pulping obstacle of the agricultural straws, namely the removal of the hybrid cells, mainly focuses on two aspects of enzyme technology and physical screening, and the products are all focused on high-yield chemical mechanical pulp. On the other hand, the synchronous utilization of lignin and hemicellulose in the straws lacks effective and feasible technical support.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides an energy-saving and environment-friendly method for pulping agricultural straws by a micro-chemical method and refining biomass full components. The method overcomes the pulping obstacle of straw hybrid cells, has low chemical consumption, is convenient for processing and resource utilization of production waste liquid, and can realize full component utilization.

The straws of the invention include, but are not limited to, corn straws, wheat straws, sorghum straws, rice straws, reed straws, cotton straws, sugarcane straws, hemp straws and other agricultural straws.

From the perspective of plant cells, cells of agricultural straws can be divided into fiber cells and hybrid cells, the fiber cells mainly comprise stem parts, chemical components of the stem parts are cellulose, hemicellulose and lignin, pulping is easy, and process problems cannot be caused, the hybrid cells mainly refer to parenchyma cells, are distributed on leaves, piths, pulps and other parts, are short and small in cells, are complex in components, contain polysaccharides, fatty acids, fats, sterols, polyphenols, proteins, inorganic salt ash and the like, cannot be used for pulping, and can cause process problems.

The applicant carries out micro-acid treatment on agricultural straws, the micro-acid treatment mainly acts on the mixed cells to dissolve hemicellulose polysaccharides, fatty acids, proteins, inorganic salts and the like of the mixed cells into liquid, particularly, the pith (pulp) part is subjected to acidolysis and dissolved into small molecules to enter the liquid, and of course, the micro-acid treatment also partially acts on the hemicellulose of the fiber cells.

The invention relates to a micro-chemical pulping and biomass full-component refining method of agricultural straws, which comprises the steps of carrying out micro-acid treatment on the straws in a micro-acid environment, wherein the initial pH value of the micro-acid treatment is 0.5-3.0 (preferably, the pH value is 0.9-1.8, the acid concentration is controlled to be 0.01-0.12mol/L in trace amount), the final pH value is 2.0-4.0 (preferably, the pH value is 3.0-4.0), and the temperature is 100-160 ℃ (preferably, the temperature is 110-160 ℃). The treated agricultural straw can be well applied to the pulping and papermaking process by the micro-acid treatment.

Preferably, the mass percentage concentration of the solid matters of the straws in the slightly acid treatment is 5-30%, the acid dosage is 0.5-1.2% relative to the mass of the straw raw materials, and the treatment time is 10-200min (preferably 100-150 min).

The acid environment for the mild acid treatment of the present invention may be provided by extraneous acids, preferably including sulfuric acid, sulfurous acid, sulfur dioxide, hydrochloric acid, phosphoric acid, nitric acid, formic acid, oxalic acid, maleic acid, and/or acetic acid; alternatively, the acetic acid, formic acid and/or gluconic acid derived from the plant straw is produced by autohydrolysis under hydrothermal conditions.

After the micro-acid treatment, the liquid contains a large amount of sugar, uronic acid, alkaloid, acetic acid and other substances. Among them, sugars are the most predominant species, including monosaccharides and oligosaccharides. The pH value of the liquid is improved compared with the initial pH value of the acid treatment, and the liquid can be used as a feed additive for animals after being neutralized by adding alkali, concentrated and dried. The slightly acid-treated liquid can also be mixed with straw after being moderately concentrated, and then granulated, for example, the concentrated liquid and the corn leaves are mixed according to the weight ratio of 1. The oligosaccharide in the liquid has obvious intestinal environment regulation and improvement effects, so that the liquid is suitable for being processed into a feed additive. In addition, the liquid after the micro-acid treatment can also be used as a raw material for producing oligosaccharide, monosaccharide and furfural.

Preferably, the waste liquid after the micro-acid treatment can be used for producing syrup by evaporation concentration, producing syrup by ultrafiltration/reverse osmosis concentration, producing biogas by anaerobic fermentation, producing feed protein by biological fermentation or adding acid into concentrated solution to convert furan, and the process liquid is reused for micro-acid treatment. The process liquid refers to liquid obtained after ultrafiltration/reverse osmosis or anaerobic fermentation or biological fermentation.

Further, the applicant carries out slightly alkaline treatment on the solid after slightly acid treatment, the treatment mainly degrades the lignin of the fiber cells into soluble small molecules to remove, and fat, sterol, polyphenol and protein in the hybrid cells are further dissolved and removed in the process.

After the straw subjected to the slight acid treatment is dehydrated, the slightly alkaline treatment is carried out in a slightly alkaline environment, the initial pH value of the slightly alkaline treatment is 9-14 (preferably pH value is 11-14), the final pH value is 6.0-10.0, and the temperature is 80-160 ℃ (preferably 85-120 ℃).

Preferably, a proper amount of wood can be added during the slightly alkaline treatment, the addition amount of the wood can be determined according to the performance index requirements such as the strength, the evenness, the bulk and the like of the pulp, and the mass ratio of the wood to the straws dehydrated by the slightly acid treatment is preferably (1-9): 1.

Preferably, the concentration of the solid matters (dehydrated straws or dehydrated straws and wood) in the micro-alkali treatment is 10-20% by mass, the alkali consumption is 2-14% (preferably 2-8%) relative to the mass of the initial raw materials (straw raw materials or straw and wood raw materials), and the treatment time is 20-200min (preferably 90-150 min).

Preferably, the base used comprises sodium hydroxide, potassium hydroxide, magnesium hydroxide, calcium hydroxide, sodium carbonate, potassium carbonate, magnesium carbonate, aqueous ammonia, ammonium sulfite and/or ammonium bicarbonate.

After the slightly alkali treatment, the liquid contains a large amount of lignin, oligosaccharide, fatty acid, phytosterol and polyphenol, wherein the lignin is main, phenolic hydroxyl of the lignin is combined with alkali metal to form potassium lignosulfonate, sodium lignosulfonate, potassium sulfonate or the like, and the liquid can be used as a dispersing agent and an organic fertilizer after being concentrated and dried.

Preferably, the method for treating the waste liquid after the slight alkali treatment can be selected from the following two schemes:

scheme 1, adding a flocculating agent into waste liquid for precipitation, filtering precipitates to obtain lignin (such as plate-and-frame filter pressing), and recycling supernatant for micro-alkali treatment, wherein the flocculating agent is used by combining polyaluminium chloride (0.5-5 g/L) and cationic polyacrylamide (5-50 mg/L) or combining anionic polyacrylamide (5-100 mg/L) and cationic polyacrylamide (5-100 mg/L);

scheme 2, the waste liquid is treated by acid neutralization and alkalescence, then flocculant is added for precipitation, and the precipitate is filtered to obtain lignin, wherein the acid comprises sulfuric acid, phosphoric acid, hydrochloric acid and/or carbon dioxide, and the flocculant is used by combining polyaluminum chloride (0.5 g/L-5 g/L) and cationic polyacrylamide (5-50 mg/L) or by combining anionic polyacrylamide (5-100 mg/L) and cationic polyacrylamide (5-100 mg/L).

Further, after the continuous two-step treatment of the micro acid and the micro alkali, the main components of the residual solid matters of the materials, namely cellulose and hemicellulose, are mechanically ground into pulp or hydraulically dispersed to obtain the paper pulp.

Preferably, the material after the slightly alkali treatment is dewatered to the solid matter percentage concentration of 15-25%, enters a high-concentration double-millstone refiner for refining, the obtained pulp is added with water or a solution containing a bleaching agent and is diluted to the medium-low concentration of 5-15% of the solid matter percentage concentration by mass, and then the material enters a medium-low concentration refining mill to obtain uniform unbleached chemical mechanical pulp or bleached chemical mechanical pulp.

Preferably, the pulp is subjected to a microchemical non-polluting bleaching with oxygen, comprising:

bleaching with oxygen or hydrogen peroxide in slightly alkaline environment at pH of 10-12 and temperature of 80-115 deg.C for 30-90min, adding soluble magnesium salt (0.1-0.5%), soluble silicate (0.2-2%), and metal ion chelating agent (0.2-0.5% EDTA);

bleaching with ozone in slightly acidic environment at 20-50 deg.C and pH of 2-4 for 30-90min.

The above bleaching may be repeated 1-3 times, depending on the pulp brightness requirements.

The paper pulp prepared by the method can be used for acid-base treatment of different degrees and subsequent matching with different physical processing modes to obtain paper pulp with different properties and purposes, such as corrugated medium paper, boxboard paper, kraft paper, coated paper, low-basis weight coated paper, white cardboard, grey white, white board paper, offset paper, cultural paper, household paper, industrial special paper and the like. The paper pulp with the material yield of 35-55% after treatment can be directly defibered and screened to be called chemical pulp, high-whiteness paper pulp can be obtained by bleaching and used as raw materials of toilet paper, white cardboard, coated paper and the like, the paper pulp with the material yield of more than 55% after biomimetic treatment can be obtained by subsequent mechanical pulp grinding to be semi-chemical pulp and chemi-mechanical pulp, and can be used for producing cardboard paper, wall paper, cardboard, kraft paper and culture paper. The pulp is also used for producing pulp molding materials.

Furthermore, the invention also provides a method for producing chemi-mechanical pulp by the micro-chemical treatment of straws, which comprises the following three steps of micro-acid treatment, micro-alkali treatment and mechanical pulp refining:

(1) Micro-acid treatment, namely performing spiral extrusion after crushing the straws, adding dilute acid solution into the materials from a compact state before the spiral terminal loses stress and becomes loose, and performing micro-acid treatment, wherein the solid concentration of the straws is 10-30%, the initial pH of the acid environment is 0.5-3.0, the acid dosage is 0.5-1.2% (relative to the mass of the straw raw materials), the temperature is 100-150 ℃, the time is 30-200min, and the reaction end point pH is 2-4. The acids used include, but are not limited to, sulfuric acid, sulfurous acid, sulfur dioxide, hydrochloric acid, phosphoric acid, nitric acid, formic acid, oxalic acid, maleic acid, acetic acid, and the like, or combinations of these acids. The acid source in the acid environment also comprises acetic acid, formic acid, gluconic acid and the like generated by the self-hydrolysis of the plant straws under the hydrothermal condition.

(2) And (3) carrying out micro-alkali treatment, namely carrying out spiral dehydration on the material subjected to micro-acid treatment, and adding dilute alkali liquor to carry out micro-alkali treatment before the material is changed from a compact state into a loose state after the spiral terminal loses stress. The slightly alkaline treatment conditions are as follows: the solid concentration of the material is 10-30%, the initial pH of the alkaline environment is 9-13, the alkali dosage is 2-8% (relative to the mass of the straw raw material), the temperature is 80-98 ℃, the time is 30-120min, and the final pH is 6.0-10.0. The base includes, but is not limited to, basic materials in aqueous solution such as sodium hydroxide, potassium hydroxide, magnesium hydroxide, calcium hydroxide, sodium carbonate, potassium carbonate, magnesium carbonate, ammonia, ammonium sulfite, ammonium bicarbonate, or combinations thereof. The slightly alkaline treatment can optionally be combined with an unlimited proportion of wood chips. The slightly alkaline treatment can be carried out simultaneously, the method is to add bleaching agent hydrogen peroxide, the dosage of the hydrogen peroxide is 2 to 6 percent, and simultaneously add bleaching auxiliary agent, comprising 0.1 to 0.4 percent of magnesium sulfate, 0.1 to 0.3 percent of sodium silicate and 0.2 to 0.5 percent of EDTA.

(3) Mechanical pulping and refining, after slight alkali treatment, carrying out spiral extrusion dehydration to the concentration of 15-25% of the material, entering a high-concentration double-millstone pulping machine, adding water or chemical solution (0.5-4% of alkali and 0.5-4% of hydrogen peroxide) into the obtained pulp to dilute the pulp to the medium-low concentration of 5-15%, then entering a medium-low concentration refining mill, and adjusting the distance between millstones or the pressure to obtain uniform unbleached (natural color) chemimechanical pulp or bleached chemimechanical pulp.

(4) And (3) carrying out physical treatment such as spiral or double-rod extrusion dehydration, washing, screening, impurity removal and the like on the refined paper pulp to obtain uniform paper pulp.

Furthermore, the invention also provides a method for producing semi-bleached microchemical high-yield paper pulp by using corn straws, which comprises the steps of removing leaves of corn, smashing the corn, and steaming for 10-20min at 90-100 ℃ without pressure in a presteaming bin. Then the straw is put into a horizontal tube digester for cooking, dilute acid liquid is added into the spiral feeding end of the material for slight acid treatment, the solid concentration of the straw is 15 percent after the liquid medicine is added, the sulfuric acid dosage in the acid environment is 0.9 percent (relative to the mass of the straw raw material), the temperature is 145 ℃, and the time is 120min. The material after micro-acid treatment is subjected to spiral extrusion dehydration, enters a horizontal pipe impregnator and is subjected to micro-alkali bleaching treatment, the solid concentration of the material is 15%, the sodium hydroxide alkali dosage is 4% (relative to the mass of the straw raw material), the hydrogen peroxide dosage is 4%, the sodium silicate is 0.2%, the EDTA is 0.2%, the temperature is 95 ℃, and the time is 90min. The slightly alkaline treatment can be carried out simultaneously, the method is to add bleaching agent hydrogen peroxide, the dosage of the hydrogen peroxide is 2 to 6 percent, and simultaneously add bleaching auxiliary agent, comprising 0.1 to 0.4 percent of magnesium sulfate, 0.1 to 0.3 percent of sodium silicate and 0.2 to 0.5 percent of EDTA. After the slightly alkaline bleaching treatment, pressure grinding is carried out, the distance between grinding discs is 0.2mm, and then extrusion dehydration is carried out. And adding a medicine into the dewatered pulp in a pumping process to perform slightly alkaline bleaching again, wherein the process is performed in a pulp tower, the solid concentration is 10%, the sodium hydroxide alkali dosage is 1%, the hydrogen peroxide dosage is 2%, and the magnesium sulfate is 0.1%, the temperature is 80 ℃, and the time is 60min. After the second slightly alkaline bleaching, the pulp is mechanically refined, and the gap between the millstones of the refining mill is 0.1mm. The refined pulp is dewatered by extrusion to obtain semi-bleached microchemical high yield pulp with a pulp yield of 66% and a whiteness of 57% ISO.

The invention discloses a micro-chemical pulping method of agricultural straws and a biomass full-component refining method, which are characterized in that the agricultural straws are properly physically crushed, then are subjected to micro-acid treatment, mixed cells and parenchyma cells in the raw materials, pith and pulp of the straws are removed, then are subjected to micro-alkali treatment, lignin, sterol, fat and other fat-soluble substances are dissolved, and finally, physical mechanical treatment is carried out according to needs to obtain paper pulp taking cellulose and partial hemicellulose as main components. Compared with the prior art, the method has the following outstanding beneficial effects:

the principle and creativity of the slightly acidic environment construction are described as follows:

as mentioned above, the limiting factor of straw pulping is a series of problems caused by hybrid cells and parenchyma cells, leaves and pith of straws are the most concentrated parts of the hybrid cells and the parenchyma cells, the main components of the cells are hemicellulose glycan with a branched chain structure, as shown in formula I, the formula I is the structure of the hemicellulose glycan and is composed of a xylose skeleton, the branched chain is provided with glucuronyl, acetyl and arabinosyl, and the arabinosyl is connected with a lignin monomer. Firstly, the method comprises the following steps: during the process of the micro-acid treatment, the polysaccharide which is originally insoluble in the straws is subjected to acidolysis and is changed into soluble sugar, uronic acid, acetic acid and phenolic lignin monomers. Following acidolysis, parenchyma cells and miscellaneous cells are substantially removed. According to the chemical-kinetic thermodynamics of glycosidic bond cleavage, acid (such as sulfuric acid) is a catalyst for glycosidic bond cleavage and does not disappear as the reaction proceeds, so that the acid concentration during acidolysis can be controlled at a trace level (0.01-0.12 mol/L) and the pH for acidolysis is 0.9-1.8. According to the reaction thermodynamics, the glycosidic bond and the ester bond in the structure shown in the formula I are broken under the slightly acidic environment at the temperature higher than 110 ℃. Acetic acid generated by shedding acetyl in the straw, glucuronic acid generated by shedding glucuronyl and phenolic lignin monomers. These materials are weak acids which buffer the pH of the system and create a slightly acidic environment. Therefore, after the reaction is completed, the final pH is higher than the initial pH, and is about 3.0 to 4.0. Secondly, in the invention, the construction of the slightly acidic environment does not necessarily need the addition of the exogenous acid, the hydrothermal reaction environment is formed by controlling a certain temperature, the acetyl in the straw can fall off, and the acetic acid can promote the continuous acidolysis, and the process is called self-hydrolysis. Therefore, by controlling the temperature, a slightly acidic environment can be constructed, and the aim of eliminating hybrid cells and parenchyma cells in the straws is fulfilled. Thirdly, the strong acid (high acid consumption) can also achieve the purposes of hemicellulose acidolysis and hybrid cell removal, but is not suitable for subsequent straw pulping. This is because the paper needs a certain strength to satisfy the usability, the strength of the paper is derived from the strength of the cellulose fiber on one hand and the connection action (hydrogen bond and physical adhesion) of the hemicellulose on the other hand, the strong acid can carry out a large amount of acidolysis to remove the hemicellulose, on the one hand, the subsequent strength reduction of the paper is caused, and the yield reduction of the paper pulp is also caused, therefore, for the refining of the biomass aiming at the paper pulp, the strong acid environment is not recommended, which is another design principle of the slightly acid environment in the patent. That is, the purpose of the slightly acidic environment is to partially remove the foreign cells and hemicellulose, thus ensuring the yield of the paper pulp and the strength of the paper, and experiments show that the removal rate of the non-fibrous components (hemicellulose, foreign cells, parenchyma cells, ash) is in the range of 15% -30% in the slightly acidic environment with the initial pH of 0.1-0.5 and the final pH of 3.8-4.2.

In conclusion, the micro-acid treatment of the invention eliminates the problems of mixed cells, parenchyma cells, pith and inorganic ash, and can reduce the amount of fine components, improve the drainage of paper pulp, improve the speed of a paper machine, reduce the viscosity of black liquor, improve the extraction rate of the black liquor and reduce the subsequent alkali consumption.

The creativity and the advantages of the construction of the slightly alkaline environment are described as follows:

for the current soda process and sulfate chemical pulping widely used in industry, the dosage of the sodium hydroxide (for the absolutely dry plant material, the same is used below) is in the range of 15-28%, for the semi-chemical pulp with a slightly high yield, the dosage of the sodium hydroxide is in the range of 10-20%, the dosage of the sodium sulfite is in the range of 5-15%, and the high dosage of the alkali corresponds to a higher pulping temperature, and is in the range of 150-170 ℃. These high chemical usage and high temperatures imply a strong chemical environment. In fact, the dissociation of plant materials into fibers does not require such high alkali usage and temperature, which are currently used in the industry for the purpose of thorough lignin removal to obtain relatively pure cellulose fibers, but in fact, the strengthening chemistry causes condensation denaturation of lignin and damage to cellulose, with consequent difficulties in pulp bleaching and a reduction in cellulose fiber yield. The micro-alkaline environment designed by the invention reduces the dosage of the alkali to 2-12%, and the pH value at the end point of the reaction is between 6.0 and 10.0, and the reaction is weakly alkaline. The scientific principle and the technical creativity of the slightly alkaline environment comprise that firstly, after the straws are treated by slightly acid and washed, the acidic groups in the hemicellulose are removed, so that the alkali dosage of subsequent alkaline pulping is greatly reduced, and experiments show that the reduction of the corn straw/wheat straw/hardwood raw material is up to 20-50%. Second, when the goal is to produce high yield pulp, lignin removal is not required and the alkali acts only as a swelling softeningPlant materials, the slightly alkaline environment can meet the target requirement. Experiments show that after the corn straws are subjected to micro-acid treatment, the straws can be completely softened when the alkali dosage of a subsequent micro-alkali treatment section is 2-6%, and then mechanical pulping is carried out, so that the yield of the micro-acid alkalization machine pulp is 70-85% finally. Thirdly, the combination of the slightly alkaline environment and the mild temperature (85-120 ℃) can realize the combination of high pulp yield and high whiteness. In the cooking temperature range (150-170 ℃) of the current chemical pulp, lignin is dissolved into waste liquid at high temperature, but polymerization/polycondensation reaction occurs at the same time, and residual lignin on the pulp also undergoes polymerization/polycondensation reaction, so that a large amount of bleaching agent (chlorine, hypochlorite, chlorine dioxide, oxygen, hydrogen peroxide) is required for bleaching the pulp subsequently. The slightly alkaline environment designed by the invention is combined with the mild cooking temperature (85-120 ℃), and in the temperature range, the lignin can not generate polymerization/polycondensation reaction, and the subsequent paper pulp is easy to bleach. And the lower alkali consumption can not cause the large removal of lignin, and the paper pulp yield is higher. Thus, a slightly alkaline environment coupled with mild temperatures (85-120 ℃) enables the production of semi-chemical pulps with higher yields and then very easy bleaching. Experiments show that after the corn straws are subjected to micro-acid treatment, the corn straws are steamed and cooked under the conditions that the alkali consumption is 8 percent, the temperature is 120 ℃ and the micro-alkali mild environment, the paper pulp yield is 68 percent, and the pulp preparation belongs to the category of semi-chemical pulp. After subsequent oxygen delignification and hydrogen peroxide bleaching (5 percent of total alkali and 6 percent of hydrogen peroxide), the high whiteness of 78 percent can be achieved, and the paper is completely suitable for the requirements of cultural paper and toilet paper. Fourthly, partial hemicellulose in the straws is removed through the micro acid treatment, the porosity of the straws is provided, namely a passage for alkali liquor permeation is provided for the subsequent micro alkali treatment, and the efficiency of the micro alkali treatment is greatly improved. On one hand, if the chemical mechanical pulp is produced, the alkali liquor permeation speed becomes fast and more uniform, the straw softening effect is good, the subsequent pulping energy consumption is favorably reduced, the fiber separation is good, and the generated fiber fragments are few; on the other hand, for the production of chemical pulp, the alkali liquor has the same permeation speed and is more uniform, after the subsequent alkali treatment, the fiber separation is good, the fiber bundle generation amount is small, and the fine pulp yield is improved. Experimental dataShows that the mass of the wheat straw is reduced by 15.6 percent before and after the micro-acid treatment, and the Langmuir surface area of the material is 5.43m ² The/g is increased to 66.5m ² Per g, mean pore diameter

Is increased to

To

The micropore volume is 7.02mm ³ The/g is increased to 24.1mm ³ (iv) g. Therefore, after the micro-acid treatment, the porosity, the pore volume and the specific surface area of the straw are all improved by several times, the penetration speed, the penetration depth and the penetration uniformity of the subsequent micro-alkali treatment are obviously improved, the alkali consumption is reduced, and the paper pulp uniformity is improved.

The creativity and the advantages of the micro-chemical pulping of the agricultural straws and the full-component refining method of the biomass are described as follows:

(1) Micro-acid treatment is designed, and the pulping obstacle of straw hybrid cells is solved.

(2) Belongs to the field of micro chemistry and has low chemical consumption.

(3) The continuous synergism of the micro acid and the micro alkali can be realized, the three main chemical components of the straw can be sequentially obtained, so that the straw can be used for pulping and papermaking, and other chemicals and materials can be obtained by utilizing the non-fiber components (hemicellulose and lignin) of the straw, thus the concept of biomass full-component refining is met, and the national plan on green utilization of agricultural waste resources is also met.

(4) The end point pH of the reaction system is close to neutral, so that the liquid treatment and resource utilization are facilitated.

Detailed Description

The present invention is further illustrated by the following examples, which are not to be construed as limiting the invention.

Unless otherwise specified, the contents of the respective components used below are mass% contents.

[ EXAMPLE one ] removing foreign cells and partial hemicellulose by slightly acid treatment before pulping corn straw

Corn straws are cut short and crushed, sulfuric acid with the mass 10 times that of the corn straws and the concentration of 0.82g/L (the using amount of the relative straws is 0.82 percent), the molar concentration of the dilute sulfuric acid solution is 8.4mMol/L, the pH value is 2.08, the dilute sulfuric acid solution is fully stirred with the straws, the straws are treated at the temperature of 140 ℃ for 120min, and the straws are extruded or centrifugally dehydrated to obtain the straws after micro-acid treatment, and the pulp and pulp of the straws are observed to be dissolved except silk, so that the pulp and pulp of the straws are basically dissolved completely. In addition, the color of the straw skin becomes dark, and the shape is not changed. After drying and weighing, the mass loss of the straw is 20.3%, wherein 14.6% of the straw is carbohydrate substances derived from hemicellulose, and 5.7% of the straw is other organic substances and ash, which indicates that most of the micro-acid treatment solution is carbohydrate. The resulting slightly acid-treated solution had a measured pH of 4.12. The micro-acid treatment solution is neutralized by adding quicklime, concentrated or dried to be used as a concentrated solution of sugar, can be used as a feed additive, can also be used for preparing feed protein, ethanol and lactic acid by biological fermentation, and can also be added with concentrated sulfuric acid to produce furfural by a dehydration reaction.

[ example two ] slightly alkali treatment of corn stalks

Example one, after the corn stalks are extruded or centrifugally dewatered, the corn stalks are subjected to slight alkali treatment according to the parameters in table 1, and the dosage of sodium hydroxide is in the range of 4-12% relative to the mass of the stalks and is far lower than the dosage of conventional pulping (the alkali dosage of an alkali method, a sulfate method and the like is 16-26%). The pH of the reaction end point of the slightly alkaline treatment is between 8.05 and 9.59, and the liquid is slightly alkaline. In addition, after the slightly alkaline treatment, the yield of the material is higher and is in the range of 52-82%, and is obviously higher than the yield (38-65%) of the conventional strong alkali cooking pulping, which is also one of the characteristics of the slightly alkaline treatment.

TABLE 1 corn stover mild alkali treatment conditions, reaction end point pH, physical yield

a, straws after micro-acid treatment, b is relative to the mass percentage of the straws, and c is used for calculating the obtained theoretical pH.

[ EXAMPLE III ] corn stalks after micro-acid-base treatment were used for pulping and papermaking

After the slightly alkaline treatment, the obtained material was further treated according to table 2 to obtain a pulp suitable for papermaking, as can be seen from the data in table 2, the yield of the pulp was between 41% and 65%, and the physical properties of the paper sheet produced by the paper sheet are also shown in table 2. As can be seen from Table 2, the alkali dosage of samples C4, C6, C8 and C4 is very low, the straw after the slight acid and alkali treatment is mechanically acted (ground into pulp), the distance between the ground into pulp is 0.1-0.3 mm, and the pulp is obtained, and the pulp is called chemical mechanical pulp. As the alkali consumption of the samples C10 and C12 is slightly high, the delignification is more, the obtained materials do not need mechanical pulp grinding, the hydraulic pulp can be made into paper pulp, the obtained paper pulp belongs to chemical pulp, the kappa numbers of the paper pulp are 15 and 13 respectively, the delignification is thorough, and the requirements of the chemical pulp are met. In addition, the initial freeness of all pulps was below 20 ° SR, which indicates that the drainability of the pulp is good, and also demonstrates the scientificity of the slightly acid-base method. The data in Table 2 also include the strength performance index of the paper, the tensile index of the paper is as high as 61.79N.m/g and the tearing index of the paper is as high as 6.83mN.m after the corn straws are subjected to the micro acid-base pulping ² The breaking index is as high as 5.3Kpa.m ² The physical strength index is superior to that of pulp produced by a city scale alkali method and a sulfate method, and is superior to the physical properties of hardwood chemical pulp and chemi-mechanical pulp.

TABLE 2 Properties of maize straw slightly acidic and slightly alkaline pulp

a comprehensive yield of micro-acid and alkali treatment, b total paper basis weight of 60g/m ² The beating degree of the used paper pulp is 35 DEG SR.

[ EXAMPLE IV ] advantages of micro-chemical pulping of corn stalks over conventional pulping methods (soda process, sulfate process)

The pulp yield was 56.6% by microchemical pulping (C12) of examples one and two. In order to compare the micro-chemical method with other pulping methods, the conventional soda method, the sulfate method and the neutral sulfite method for pulping the corn straws are developed, and the pulp yield is also controlled to be about 56 percent. The removal rate of foreign cells, the initial beating degree of the pulp, the extraction rate of black liquor, the viscosity of black liquor, the silicon content in black liquor and the calorific value of solid matters in black liquor were analyzed, and all the results are shown in table 3.

The impurity cell removing treatment of the microchemical method is 78 percent, which is twice that of the conventional other paper pulp methods, and is beneficial to the hydrolysis effect of the slightly acid treatment on polysaccharide substances, the initial beating degree of the microchemical method is 17 DEG SR which is far lower than that of the other conventional methods, the beating degree is the free degree of fibers, the water filtering speed of the paper pulp is reflected, the beating degree is low, the fibers are complete, the water filtering speed is high, the beating degree is high, a large amount of fine fibers and fragments are generated, the water filtering is blocked, and the water filtering speed is low. The micro-chemical method has low beating degree, indicates that the fiber is kept complete, the fine fiber and the fragments are few, the water filtration is fast, and the paper making can be carried out at high speed. In addition, the water filterability of the pulp is good, which indicates that the pulp is easy to wash and clean, namely the waste liquid is easy to extract. Due to low beating degree of a micro-chemical method and the benefit of micro-acid treatment, hybrid cells and parenchyma cells are removed in advance, and after subsequent micro-alkali treatment, small fibers and small fragments are few. Because the green water property of the pulp is good (the beating degree is low), as can be seen from table 3, the extraction rate of the black liquor of the microchemical method after pulp washing is as high as 96 percent, which is much higher than that of other conventional pulp methods, which indicates that if the black liquor is combusted to recover the caustic soda therein, the extraction rate of the black liquor of the microchemical method is high, and the corresponding alkali recovery rate can be ensured. The silicon content in the black liquor of the micro-chemical method is lower than that of other pulping methods, namely, a part of silicon is dissolved in the micro-acid treatment process, and the silicon content in the black liquor of the subsequent micro-alkali treatment is lower than that of other pulping methods. Silicon deposits on the walls of the tubes and heat exchange devices, impeding flow and mass and heat transfer, and low silicon content is clearly advantageous. Finally, due to the low silicon, sugar and salt content of microchemical processes, the lignin content is relatively high, and therefore, the calorific value of the solids of the microchemical black liquor is higher than that of other pulp processes. High heat value and combustion value, and the black liquor can be treated by alkali recovery process.

TABLE 3 advantages of microchemical pulping over conventional soda, sulfate and neutral sulfite processes in the process

Remarking: the yield of paper pulp of each listed pulping method is about 56 percent by condition screening, the removal rate of b hybrid cells is evaluated by the quality reduction rate of pith (pulp) parts of corn straws, and the initial beating degree of c refers to the beating degree of the paper pulp after hydraulic dispersion and before mechanical beating. The D black liquor is the extraction rate of the black liquor in the pulping waste liquor and the paper pulp after being extracted by a 4-section vacuum pulp washer. e, the concentration of solid matter in black liquor is 10%, and the test value is at 20 deg.C. d black liquor combustion value under the condition of equivalent solid concentration.

[ EXAMPLE V ] slightly acid-treated straw and other wood were mixed and used together for the production of chemi-mechanical pulp

Example 1 micro-acid treated corn stover was mixed with steam treated (95 ℃ for 5 minutes) fast growing poplar chips and then used together to make high yield pulp. Corn stalk fiber is longer, can play the reinforcing effect, and the fibre of poplar is short, but the evenness is high, and the bulk is also high. The mixing proportion of the materials is not limited and is determined according to the performance indexes of the strength, the evenness, the bulk and the like of the paper pulp. The temperature of the mixed materials is about 90 ℃, the mixed materials pass through a double-rod spiral thread rolling machine, the liquid medicine is added before the materials are changed into loose materials from a tight state after losing stress, the amount of alkali (NaOH, kaOH, caO and ammonia water) is 1 to 8 percent, and at the moment, 2 to 6 percent of hydrogen peroxide, 0.1 to 0.4 percent of magnesium sulfate, 0.1 to 0.3 percent of sodium silicate and 0.2 to 0.5 percent of EDTA can be simultaneously added. The materials mixed with chemicals stay in the dipping tower for 30-90min, and the materials are fully softened and moistened. Then the pulp enters a high-concentration double-millstone pulp grinder after being extruded and dehydrated in a spiral way to reach the concentration of 15-25%, the obtained pulp is diluted to the medium-low concentration of 5-15% by adding water or chemical solution (the dosage of alkali is 2-6%, and the dosage of hydrogen peroxide is 0-4%), and the pulp is soaked for 0-40min. Then the material enters a medium-low concentration refining mill, the distance or pressure between the mill millstones is adjusted, and the distance between the mill millstones is 0.1-0.3 mm, so that uniform chemi-mechanical paper pulp is obtained. According to the process, the slightly acid treated corn stalk (example 1), the wood chips of the fast growing poplar wood are used as raw materials, the mixture of the slightly acid treated corn stalk (50%) and the wood chips of the fast growing poplar wood (50%) is used as raw materials, and the refining energy consumption and the performance of the obtained chemimechanical pulp are listed in table 4. As can be seen from Table 4, the energy consumption of the pulp of the resulting pulp was reduced from 1256kWh/t to 786kWh/t compared to that of fast-growing poplar after mixing of slightly acid-treated straw with fast-growing poplar chips 1. The tensile strength of the obtained paper is obviously improved, the tensile strength is improved to 33.5N.m/g from 6.4N.m/g of the fast-growing poplar pulp, the tearing index is also improved, and the tensile strength is improved to 5.2mN.m2/g from 4.2mN.m2/g. These results demonstrate the feasibility of mixing the straw with fast-growing hardwood after slight acid treatment to produce high strength chemi-mechanical pulp.

TABLE 4 energy consumption, whiteness, tensile index and tear index of chemi-mechanical pulps obtained from different raw materials

Remarking: a, 6 percent of alkali for pulping and 6 percent of hydrogen peroxide. b, the sum of the energy consumption of thread rolling, high-consistency grinding and low-consistency refining.

[ EXAMPLE VI ] slightly acid treatment of wheat straw

The wheat straw is treated by the weak acid, the treatment process is the same as the first embodiment, the specific conditions are that sulfuric acid with the concentration of 0.82g/L (0.656% of the straw dosage) is added, the mass of the sulfuric acid is 8 times that of the wheat straw, the pH value is 1.91, the mixture is fully stirred with the straw, then the mixture is treated for 60min at the temperature of 140 ℃, and the straw after the weak acid treatment is obtained by extrusion or centrifugal dehydration. After drying and weighing, the mass loss of the straw is 15.6 percent, and the Langmuir surface area of the material is 5.43m ² The/g is increased to 66.5m ² Per g, mean pore diameter

Is increased to

To

Micropore volume of 7.02mm ³ The/g is increased to 24.1mm ³ (ii) in terms of/g. The porosity, pore volume and specific surface area of the straw are improved by several times. Theoretically, the penetration speed, penetration depth and penetration uniformity of the liquid medicine can be improved in the subsequent alkaline pulping process.

EXAMPLE VII micro-chemical pulping of wheat straw after micro-acid treatment

The raw materials used are from example six, i.e. slightly acid treated wheat straw, slightly alkaline treatment conditions are as shown in table 5, slightly alkaline treatment is from 2% to 12% in alkali, and the pH at the end of the reaction is slightly alkaline, from 8.25 to 9.76. After the treatment under the treatment conditions of the table 5, the materials with the labels W2\ W4\ W6 are mechanically ground and refined, the grinding distance is 0.1-0.3 mm, and the chemi-mechanical pulp is obtained, wherein the yield is 67-82%. The materials with the labels W8\ W10 are subjected to hydraulic defibering to obtain chemical pulp, the yield is 56% and 62% respectively, and the kappa number is 17 and 14 respectively. The paper pulp after the wheat straw slight alkali treatment is used for papermaking, and the paper strength is listed in table 5. Compared with the conventional soda pulping (grass fiber pulping and papermaking, well-known in plum, china light industry Press, beijing, 2019.6, page 151), under the same kappa number target, the micro-chemical (micro-acid-micro-alkali) pulping of the embodiment reduces the alkali consumption from 14-15% of the conventional pulping to 8-10% and improves the pulp yield from 40-47% of the conventional pulping to 52-56% of the embodiment. The cooking temperature was also reduced from the conventional 160 ℃ to 150 ℃ in this example.

TABLE 5 alkali treatment conditions and pulp properties of wheat straw

Claims

1. The micro-chemical pulping and biomass full-component refining method of the agricultural straw is characterized by comprising the following steps of micro-acid treatment, micro-alkali treatment and paper pulp preparation:

carrying out micro-acid treatment on the straws in a micro-acid environment, wherein the initial pH value of the micro-acid treatment is 0.9-1.8, the final pH value is 3.0-4.0, the temperature is 100-160 ℃, the acid consumption is 0.5-1.2 percent relative to the mass of the straw raw material, the acid environment of the micro-acid treatment is provided by external acid, and the external acid is sulfuric acid, sulfurous acid, sulfur dioxide, hydrochloric acid, phosphoric acid, nitric acid, formic acid, oxalic acid, maleic acid or acetic acid; or, the acid is acetic acid, formic acid or gluconic acid which is generated by the self-hydrolysis of plant straws under the hydrothermal condition;

the waste liquid after the micro-acid treatment is used for producing syrup by evaporation concentration, syrup by ultrafiltration/reverse osmosis concentration, biogas by anaerobic fermentation, feed protein by biological fermentation or furan by adding acid into the concentrated solution for conversion, and the process liquid is reused for micro-acid treatment;

dewatering the straws after the micro-acid treatment, and then carrying out micro-alkali treatment in a micro-alkaline environment, wherein the initial pH of the micro-alkali treatment is 9-13, the final pH is 6.0-10.0, the temperature is 85-120 ℃, the treatment time is 90-150min, the alkali consumption is 2-8% relative to the mass of the initial raw materials, and the alkali used is sodium hydroxide, potassium hydroxide, magnesium hydroxide, calcium hydroxide, sodium carbonate, potassium carbonate, magnesium carbonate, ammonia water, ammonium sulfite or ammonium bicarbonate;

the method for treating the waste liquid after the micro-alkali treatment comprises the following steps:

adding a flocculating agent into the waste liquid for precipitation, filtering the precipitate to obtain lignin, and reusing the supernatant for micro-alkali treatment, wherein the flocculating agent is prepared by combining polyaluminum chloride and cationic polyacrylamide or combining anionic polyacrylamide and cationic polyacrylamide;

or:

neutralizing the waste liquid with acid and alkali, adding a flocculating agent for precipitation, and filtering precipitates to obtain lignin, wherein the flocculating agent is prepared by combining polyaluminum chloride and cationic polyacrylamide or combining anionic polyacrylamide and cationic polyacrylamide;

and mechanically grinding or hydraulically dispersing the material after the micro-alkali treatment to obtain paper pulp.

2. The micro-chemical pulping and biomass full-component refining method of agricultural straws as claimed in claim 1, which is characterized in that: the mass percentage concentration of the solid matters of the straws is 5-30 percent during the micro-acid treatment.

3. The micro-chemical pulping and biomass full-component refining method of agricultural straws as claimed in claim 1, which is characterized in that: during the slightly alkaline treatment, a proper amount of wood is added, and the mass ratio of the wood to the dehydrated straw is (1-9): 1.

4. The micro-chemical pulping and biomass full-component refining method of agricultural straws as claimed in claim 1, which is characterized in that: the solid mass percentage concentration is 10-20% when the micro-alkali treatment is carried out.

5. The micro-chemical pulping and biomass full-component refining method of agricultural straws as claimed in claim 1, which is characterized in that: dewatering the material after the slight alkali treatment to the solid matter content of 15-25%, pulping in a high-concentration double-millstone pulping machine, diluting the obtained pulp with water or solution containing bleaching agent to the medium-low concentration of 5-15% solid matter content, and then refining the material in a medium-low concentration mill to obtain uniform unbleached chemimechanical pulp or bleached chemimechanical pulp.

6. The micro-chemical pulping and biomass full-component refining method of agricultural straws as claimed in claim 1, which is characterized in that:

the method for performing micro-chemical pollution-free bleaching on paper pulp by using oxygen element comprises the following steps:

bleaching with oxygen or hydrogen peroxide in slightly alkaline environment at 80-115 deg.C and pH of 10-12, and adding soluble magnesium salt, soluble silicate and metal ion chelating agent;

bleaching with ozone in slightly acidic environment at pH2-4 and temperature 20-50 deg.C.