Pulp digestion accelerator and method for producing pulp using the same

Abstract

To provide a digestion accelerator that can efficiently digest lignocellulose material when digesting lignocellulose material to produce pulp. [Solution] A digestion accelerator characterized by containing at least one of lecithin and its derivatives, and phosphatidylcholine, and preferably further containing a sulfur-containing compound.

Description

Technical Field

[0001] The present invention relates to a digestion accelerator and a method for producing pulp using the same, and more particularly to a digestion accelerator for lignocellulosic materials and a method for producing pulp using the same.

Background Art

[0002] In order to produce pulp from lignocellulosic materials such as wood, generally, digestion treatment is performed using an alkali, sulfite, or the like. After dissolving or dispersing unnecessary lignin components, natural resin components, etc. by this digestion treatment, pulp is obtained by removing these components by filtration washing or the like.

[0003] On the other hand, the logging of natural resources such as wood is restricted due to environmental problems, etc., and the price of wood is also rising at present. Therefore, it has become important to increase the production volume of pulp per unit of raw log and produce high-quality pulp products. As a method for solving these problems, a method using a digestion accelerator is known.

[0004] For example, in Japanese Patent Laid-Open No. 53-74101 (Patent Document 1), in a digestion step of digesting a lignocellulosic substance with an alkaline chemical solution or a chemical solution containing sulfite to form pulp, a hydroxyanthracene or a hydroxyanthracene derivative is added as a digestion aid to a digestion solution composed of an alkaline chemical solution or a chemical solution containing sulfite, and the lignocellulosic substance is digested by an alkaline method or a sulfite method. A method for producing pulp is disclosed. When hydroxyanthracene or a hydroxyanthracene derivative is added to the digestion solution, the digestion treatment is promoted, but it is not always sufficient. In addition, anthracenes are suspected as carcinogenic substances, and there are cases where the influence on the human body cannot be eliminated. Therefore, since it is necessary to reduce the usage amount, a digestion aid (digestion accelerator) that replaces hydroxyanthracene and hydroxyanthracene derivatives is required.

[0005] Furthermore, Japanese Patent Publication No. 2020-2481 (Patent Document 2) discloses a pulping accelerator containing at least one of glucose and fructose. Adding at least one of glucose and fructose to the pulping solution accelerates the pulping process, but not necessarily sufficiently. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 53-74101 [Patent Document 2] Japanese Patent Publication No. 2020-2481 [Overview of the project] [Problems that the invention aims to solve]

[0007] In recent years, the increasing demand for timber has led to soaring timber prices, and the need to curb deforestation due to global warming has made efficient pulp production a challenge, requiring further improvements in the efficiency of the pulping process.

[0008] The present invention has been made in view of the problems of the prior art described above, and aims to provide a digestion accelerator that can efficiently digest lignocellulose material when digesting lignocellulose material to produce pulp, and a method for producing pulp using the same. [Means for solving the problem]

[0009] As a result of diligent research to achieve the above objective, the present inventors have discovered that a digestion accelerator containing at least one of lecithin, its derivatives, and phosphatidylcholine exhibits excellent digestion-promoting effects, and that by using this digestion accelerator, lignocellulose material can be efficiently digested to produce pulp, thus completing the present invention.

[0010] In other words, the present invention provides the following embodiments. [1] A digestion accelerator containing lecithin and its derivatives, and at least one of phosphatidylcholine. [2] The digestion accelerator according to [1], further containing a sulfur-containing compound. [3] The pulping accelerator according to [2], wherein the sulfur-containing compound is at least one salt selected from the group consisting of hydrogen sulfide, sulfite, bisulfite, disulfite, thiosulfite, dithionite, dithionite, disulfite, peroxomonosulfate, peroxodisulfate, and polythionite. A method for producing pulp, comprising the steps of preparing a digestate containing a digestion accelerator described in any one of [4][1] to [3], and performing a digestion treatment using the digestate. A method for producing pulp according to [4], comprising the step of preparing a digestate containing the digestion accelerator described in [5], [2] or [3], wherein the lecithin and its derivatives, and at least one of the phosphatidylcholine, are mixed in advance with the sulfur-containing compound before being added to the digestate. [Effects of the Invention]

[0011] According to the present invention, a digestion accelerator capable of efficiently digesting lignocellulose material is obtained when digesting lignocellulose material to produce pulp, and by using this digestion accelerator, it becomes possible to efficiently digest lignocellulose material and produce pulp. [Modes for carrying out the invention]

[0012] The present invention will be described in detail below with reference to its preferred embodiments. The following description of preferred embodiments is essentially illustrative and is not intended to limit the present invention, its applications, or its uses.

[0013] [Digestion accelerator] First, the digestion accelerator of the present invention will be described. The digestion accelerator of the present invention contains at least one of lecithin and its derivatives, and phosphatidylcholine. In this specification, lecithin and its derivatives, and phosphatidylcholine will be collectively referred to as the "main component of the digestion accelerator."

[0014] (Lecithin and its derivatives) In the present invention, lecithin and its derivatives (hereinafter collectively referred to as "lecithins") act as digestion accelerators when digesting lignocellulose materials.

[0015] Lecithin is a general term for mixtures mainly composed of various phospholipids, specifically a mixture of major phospholipids represented by phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, and phosphatidic acid, and other phospholipids (lecithin in a broad sense). There are no particular restrictions on such lecithin; for example, it may be derived from plants, animals, or microorganisms, and it may be used as is extracted from living organisms such as plants, animals, and microorganisms, or purified. Furthermore, synthetic lecithin can also be used. Examples of plant-derived lecithin include those derived from soybeans, rapeseed, wheat, rice, corn, cottonseed, safflower, flaxseed, sesame, and sunflower seeds, while examples of animal-derived lecithin include those derived from egg yolk and milk.

[0016] Furthermore, examples of lecithin derivatives include the hydrogenated lecithin (lecithin with hydrogenated fatty acids), enzymatic hydrolysates (lecithin that has been enzymatically hydrolyzed), and fractionated products (lecithin in which the concentration of specific phospholipids has been altered).

[0017] These lecithins may be used alone or in combination of two or more. Among these lecithins, from the viewpoint of more promoting saccharification, plant-derived lecithin, animal-derived lecithin, and their derivatives are preferable, and from the viewpoints of safety and cost, plant-derived lecithin and its derivatives are more preferable, and soybean-derived lecithin and sunflower seed-derived lecithin are even more preferable.

[0018] (Phosphatidylcholine) In the present invention, phosphatidylcholine acts as a saccharification promoter when saccharifying lignocellulosic materials.

[0019] Phosphatidylcholine is one of the main phospholipids constituting lecithin. Although phosphatidylcholine itself may be referred to as "lecithin" (narrow sense of lecithin), in this specification, phosphatidylcholine is directly denoted as "phosphatidylcholine", and the above-mentioned "broad sense of lecithin" is denoted as "lecithin" to distinguish them.

[0020] Another name of phosphatidylcholine is 1,2-diacyl-sn-glycerol-3-phosphocholine, and it has a structure in which fatty acids are ester-bonded to the 1st and 2nd positions of glycerol respectively.

[0021] The fatty acid is not particularly limited, and for example, a fatty acid having 12 to 22 carbon atoms may be used, such as saturated fatty acids like lauric acid, myristic acid, palmitic acid, stearic acid, phytanic acid, and unsaturated fatty acids like myristoleic acid, oleic acid, linoleic acid, erucic acid. The carbon chain of the fatty acid may be linear or branched.

[0022] The two fatty acids bonded to the 1st and 2nd positions of glycerol in phosphatidylcholine may be the same fatty acid or different fatty acids.

[0023] As phosphatidylcholine, one kind may be used alone, or two or more kinds may be used in combination.

[0024] (sulfur-containing compounds) In the present invention, sulfur-containing compounds do not exhibit a digestion-promoting effect when used alone, but their digestion-promoting effect is further enhanced when used in combination with at least one of lecithin and its derivatives, and phosphatidylcholine. Examples of such sulfur-containing compounds include salts containing sulfur atoms, such as hydrogen sulfide, sulfite, bisulfite, disulfite, thiosulfite, dithionite, dithionite, disulfate, peroxomonosulfate, peroxodisulfate, and polythionate. Examples of salts include sodium salts, potassium salts, and ammonium salts.

[0025] These sulfur-containing compounds may be used individually or in combination of two or more. Among these sulfur-containing compounds, hydrogen sulfide salts, thiosulfates, dithionite salts, dithionates, disulfates, peroxomonosulfates, peroxodisulfates, and polythionates are preferred from the viewpoint of further promoting pulverization, thiosulfates, dithionite salts, dithionates, disulfates, peroxomonosulfates, peroxodisulfates, and polythionates are more preferred, and thiosulfates are even more preferred.

[0026] (Digestion accelerator) The digestion accelerator of the present invention contains at least one of the lecithin and its derivatives, and the phosphatidylcholine, and more preferably further contains the sulfur-containing compound from the viewpoint of further promoting digestion.

[0027] When the pulping accelerator contains at least one of the lecithin and its derivatives, and the phosphatidylcholine (main component of the pulping accelerator) and the sulfur-containing compound, there are no particular restrictions on the ratio of the total mass of the main component of the pulping accelerator to the total mass of the sulfur-containing compound (total mass of the main component of the pulping accelerator: total mass of the sulfur-containing compound). However, from the viewpoint of further promoting pulping, a ratio of 1:4 to 100:1 is preferred, 1:2 to 50:1 is more preferred, and 1:1 to 25:1 is even more preferred.

[0028] Furthermore, the amount of the digestion accelerator used in the present invention (total amount of the digestion accelerator main component and the sulfur-containing compound used) is preferably 0.0001 to 1.0 parts by mass, more preferably 0.0001 to 0.5 parts by mass, even more preferably 0.0001 to 0.2 parts by mass, and particularly preferably 0.001 to 0.2 parts by mass, per 100 parts by mass of the dry mass of the lignocellulose material.

[0029] The digestion accelerator of the present invention may be used as is, or it may be dissolved, emulsified, or dispersed in water or an organic solvent before use. There are no particular restrictions on the organic solvent, but examples include: lower alcohols having 1 to 6 carbon atoms such as methanol, ethanol, and propanol; alkylene oxide adducts having 1 to 6 carbon atoms such as ethylene oxide and propylene oxide of the lower alcohols; alkylene glycols having 1 to 6 carbon atoms such as ethylene glycol, diethylene glycol, and propylene glycol; 3-methyl-3-methoxybutanol; diethyl ether; ester solvents such as dimethyl glutarate and dimethyl succinate; and terpenes such as d-limonene and terpineol.

[0030] Furthermore, when dissolving, emulsifying, or dispersing the digestion accelerator of the present invention in water, if the main component of the digestion accelerator is difficult to dissolve, emulsify, or disperse in water, it can be easily dissolved, emulsified, or dispersed by adding a base, acid, or surfactant. Examples of bases include inorganic basic compounds such as sodium hydroxide, potassium hydroxide, sodium carbonate, and ammonia; and organic basic compounds such as triethanolamine, diethanolamine, and monoethanolamine. Examples of acids include inorganic acids such as hydrochloric acid, nitric acid, and phosphoric acid; and organic acids such as acetic acid, formic acid, lactic acid, and oxalic acid. Examples of surfactants include anionic surfactants such as fatty acid salts, alkyl sulfates, alkyl sulfate esters, polyoxyalkylene alkyl sulfate esters, alkyl phosphate esters, and polyoxyalkylene alkyl phosphate esters; nonionic surfactants such as polyoxyalkylene alkyl ethers; and cationic surfactants such as tetraalkylammonium salts.

[0031] Furthermore, the digestion accelerator of the present invention may contain surfactants, mineral oil, orange oil, etc., to the extent that it does not impair the digestion-accelerating effect, from the viewpoint of efficiently penetrating the digestion accelerator into the lignocellulose material, and may also contain defoamers, detergents, etc., from the viewpoint of improving the washability of the pulp.

[0032] [Pulp manufacturing method] Next, the method for producing pulp according to the present invention will be described. The method for producing pulp according to the present invention is a method comprising the steps of preparing a digestate containing the digestion accelerator of the present invention and performing a digestion treatment using the digestate. More specifically, the method comprises a digestate preparation step of preparing a digestate containing the digestion accelerator of the present invention, a digestion step of digesting a lignocellulose material using the digestate to produce pulp, a washing step of washing the obtained crude pulp, and, if necessary, a screening step of removing dust from the washed pulp and a bleaching step of bleaching the dust-removed pulp.

[0033] There are no particular restrictions on the pulping method, but alkaline pulping using a pulp containing an alkaline pulping agent and sulfite pulping using a pulp containing a sulfite pulping agent are preferred. Examples of alkaline pulping methods include Kraft pulping, soda pulping, sodium carbonate pulping, and polysulfide pulping, while examples of sulfite pulping methods include alkaline sulfite pulping, neutral sulfite pulping, and bisulfite pulping. Of these pulping methods, alkaline pulping is preferred from the viewpoint of promoting the pulping of lignocellulose material, and Kraft pulping and polysulfide pulping are more preferred.

[0034] The present invention provides a method for producing pulp, in which, in the digestate preparation step, the digestion accelerator of the present invention is added to the digestion main agent (preferably the alkaline digestion main agent or the sulfite digestion main agent) to prepare a digestate, and in the digestion step, this digestate is used to digest a lignocellulose material. Except for using the digestion accelerator of the present invention, known digestate preparation and digestion steps can be used, and known washing, screening, and bleaching steps can be used.

[0035] (Cooking liquid preparation process) In the digestate preparation step, the digestate is prepared by adding the digestion accelerator, which contains at least one of the lecithin and its derivatives and the phosphatidylcholine, to the digestion main agent (preferably the alkaline digestion main agent or the sulfite digestion main agent). When the digestion accelerator is at least one of the lecithin and its derivatives and the phosphatidylcholine and the sulfur-containing compound, the lecithin and its derivatives and the at least one of the phosphatidylcholine and the sulfur-containing compound may be mixed in advance and the resulting mixture added to the digestion main agent, or the lecithin and its derivatives and the at least one of the phosphatidylcholine and the sulfur-containing compound may be added separately and simultaneously to the digestion main agent. However, the former method of addition is preferred from the viewpoint of further promoting the digestion of the lignocellulose material.

[0036] When adding the digestion accelerator to the digestion main agent, the digestion accelerator may be added directly to the digestion main agent, or it may be added in a state in which it has been dissolved, emulsified, or dispersed in water or an organic solvent. Examples of the organic solvent include the organic solvents exemplified in the description of the digestion accelerator of the present invention. Furthermore, as described above, when dissolving, emulsifying, or dispersing the digestion accelerator in water, it can be easily dissolved, emulsified, or dispersed by adding a base, acid, or surfactant. Examples of the base, acid, and surfactant include the base, acid, and surfactant exemplified in the description of the digestion accelerator of the present invention.

[0037] Examples of the alkaline pulping agent include sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate. Examples of the sulfite-based pulping agent include acidic calcium sulfite and sulfurous acid.

[0038] When the aforementioned alkaline pulping agent is used as the main pulping agent, the pulping of lignocellulose material is promoted even if the pulping accelerator is only at least one of the lecithin and its derivatives, and the phosphatidylcholine. However, from the viewpoint of further promotion, at least one of the following is used in combination: hydrogen sulfide, sulfite, bisulfite, disulfite, thiosulfate, dithionite, dithionate, disulfate, peroxomonosulfate, peroxodisulfate, and polythionate. It is preferable to use at least one of the following in combination: sulfite, bisulfite, disulfite, thiosulfate, dithionite, dithionate, disulfate, peroxomonosulfate, peroxodisulfate, and polythionate; it is even more preferable to use at least one of the following in combination: thiosulfate, dithionite, dithionate, disulfate, peroxomonosulfate, peroxodisulfate, and polythionate; and it is particularly preferable to use thiosulfate in combination.

[0039] The amount of the digestion accelerator added (total amount of the main component of the digestion accelerator and the sulfur-containing compound added) is preferably 0.0001 to 1.0 parts by mass, more preferably 0.0001 to 0.5 parts by mass, even more preferably 0.0001 to 0.2 parts by mass, and particularly preferably 0.001 to 0.2 parts by mass, per 100 parts by mass of the dry mass of the lignocellulose material.

[0040] Furthermore, in the pulping process described later, if the alkaline pulping method is adopted as the pulping method, the pulping accelerator of the present invention is added to the alkaline pulping main agent to prepare the pulping solution. The amount of the alkaline pulping main agent used is appropriately set according to the type of alkaline pulping main agent used and the type of lignocellulose material to be pulped. In the case of the soda pulping method, the amount of sodium hydroxide, which is the alkaline pulping main agent, is generally 1 to 120 parts by mass per 100 parts by mass of lignocellulose material. From the viewpoint of efficiently pulping and fully exhibiting the effect of the pulping accelerator, 3 to 60 parts by mass is preferred, and 5 to 60 parts by mass is more preferred.

[0041] When the Kraft pulping method is adopted as the pulping method, sodium sulfide is added to the alkaline pulping agent, and then the pulping accelerator of the present invention is added to prepare the pulping solution. The amount of the alkaline pulping agent used and the amount of sodium sulfide added are set appropriately according to the type of alkaline pulping agent used and the type of lignocellulose material to be pulped. When the alkaline pulping agent is sodium hydroxide, the amount used is generally 1 to 120 parts by mass per 100 parts by mass of lignocellulose material, and from the viewpoint of efficiently pulping and allowing the pulping accelerator to exert its effect to the fullest, 3 to 60 parts by mass is preferred, and 5 to 60 parts by mass is more preferred. The amount of sodium sulfide added is generally 1 to 200 parts by mass per 100 parts by mass of sodium hydroxide, and from the viewpoint of efficiently pulping, 10 to 100 parts by mass is preferred.

[0042] When employing the polysulfide pulping method, the alkaline pulping agent should contain sodium sulfide and sodium polysulfide (NaS xAdd (x=2~5) and further add the pulping accelerator of the present invention to prepare the pulping solution. The amount of the alkaline digestion agent used and the amounts of sodium sulfide and sodium polysulfide added are set appropriately according to the type of alkaline digestion agent used and the type of lignocellulose material to be digested. However, when the alkaline digestion agent is sodium hydroxide, the amount used is generally 1 to 120 parts by mass per 100 parts by mass of lignocellulose material, and from the viewpoint of efficiently digesting and allowing the digestion accelerator to exert its effect to the fullest, 3 to 60 parts by mass is preferred, and 5 to 60 parts by mass is more preferred. The amount of sodium sulfide added is generally 1 to 200 parts by mass per 100 parts by mass of sodium hydroxide, and from the viewpoint of efficiently digesting, 10 to 100 parts by mass is preferred. The amount of sodium polysulfide added is generally 1 to 200 parts by mass per 100 parts by mass of sodium hydroxide, and from the viewpoint of efficiently digesting, 10 to 100 parts by mass is preferred.

[0043] (Cooking process) In the pulping process, the lignocellulose material is subjected to pulping treatment using the pulping solution prepared in the pulping solution preparation process.

[0044] There are no particular restrictions on the lignocellulose material mentioned above. Examples include wood (coniferous trees (N material), hardwoods (L material), etc.), as well as non-wood materials such as straw, bagasse, reeds, kenaf, mulberry, bamboo, herbaceous plants, and weeds.

[0045] There are no particular restrictions on the equipment used for the pulping process (pulping equipment), and known pulping equipment can be used, whether it is continuous or batch type. In addition, the pulping system can employ methods such as the MCC method (modified pulping), ITC method (isothermal pulping of the entire tank), Lo-solids method (reduction of solids in the tank), and BLI method (use of black liquor in the infiltration stage).

[0046] A specific pulping method involves placing the pulp prepared in the pulp preparation step and the raw material containing the lignocellulose material into a pulping vessel, adjusting the temperature and pressure inside the pulping vessel to pulp the lignocellulose material. This extracts the fibrous component (pulp) from the lignocellulose material. The temperature, pressure, and time of the pulping process are set appropriately according to the type, shape, and size of the lignocellulose material to be pulped. For example, when the lignocellulose material is wood chips, the pulping temperature is generally 50 to 300°C, and 80 to 250°C is preferred from the viewpoint of reducing the load on the pulping vessel and other equipment. The pulping pressure is generally atmospheric pressure to 10 MPa, and atmospheric pressure to 5 MPa is preferred from the viewpoint of reducing the load on the pulping vessel and other equipment. Furthermore, the pulping time is preferably 1 to 5 hours from the viewpoint of reducing the load on the pulping vessel and other equipment.

[0047] Furthermore, oxygen may be introduced into the digester during the digestion process. There are no particular restrictions on the method of introducing oxygen. For example, methods include adjusting the pressure in the digester to set a high oxygen partial pressure and injecting oxygen or air into the digester, or injecting oxygen or air when supplying lignocellulose material or digestate. The oxygen partial pressure is preferably 0.05 to 1 MPa, and more preferably 0.1 to 0.3 MPa. There are no particular restrictions on the oxygen source as long as the oxygen partial pressure can be maintained within the above range. For example, pure oxygen, air, or a mixture of oxygen and air can be used. However, since increasing the pressure resistance of the digester increases equipment costs, pure oxygen or a mixture of oxygen and air is preferred.

[0048] Furthermore, in the pulping process, if necessary, oxidizing agents such as peroxides (e.g., hydrogen peroxide or potassium peroxide), percarboxylic acids (e.g., peracetic acid), or hydrogen peroxide adducts (e.g., sodium percarbonate) may be added to the pulp to generate oxygen under pulping conditions. [Examples]

[0049] The present invention will be described more specifically below based on examples and comparative examples, but the present invention is not limited to the following examples. The lecithin and sulfur-containing compounds used in the examples and comparative examples are shown below.

[0050] (Lecithin and its derivatives) A1: Egg-derived lecithin (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent). A2: Soy-derived lecithin (Yelkin TS, manufactured by ADM Japan Co., Ltd.). A3: Soy-derived lecithin (SLP White, manufactured by Tsuji Oil Co., Ltd.). A4: Soy-derived, enzyme-hydrolyzed lecithin (SLP White Lyso, manufactured by Tsuji Oil Co., Ltd.). A5: Soy-derived, hydrogenated lecithin (SLP White H manufactured by Tsuji Oil Co., Ltd.).

[0051] (sulfur-containing compounds) Sodium thiosulfate: Manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent. • Ammonium thiosulfate: Manufactured by Kanto Chemical Co., Ltd. • Potassium hydrogen sulfide: Manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent. • Potassium sulfite: Manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent. • Sodium bisulfite: Manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent.

[0052] (Example A1) [L-sized material, Kraft thawing method] (Preparation of emulsion of digestion accelerator) Lecithin A1, a digestion accelerator, was emulsified in water to prepare an aqueous emulsion of lecithin A1 (digestion accelerator aqueous emulsion) with a lecithin content of 10% by mass. Since lecithin A1 is difficult to emulsify in water, 1.0 part by mass of sodium hydroxide (manufactured by Nacalai Tesque Co., Ltd., reagent) was added to 100 parts by mass of lecithin A1 to mechanically force emulsify the lecithin A1 in ion-exchanged water.

[0053] (Preparation of evaporated liquid) First, 0.075 g of the pulping accelerator aqueous emulsion (lecithin purity: 10% by mass) prepared above (0.0075 g of pure lecithin) was mixed with deionized water to prepare 5 g of the pulping accelerator aqueous emulsion with a lecithin purity of 0.15% by mass.

[0054] Next, 8.0 g of sodium hydroxide (manufactured by Nacalai Tesque Co., Ltd., reagent) and 6.03 g of sodium sulfide pentahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent) (2.8 g of sodium sulfide purity) were placed in a beaker, and then deionized water was added to prepare 145 g of an alkaline aqueous solution of the main digesting agent. To this 145 g aqueous solution of the main digesting agent, 5 g of the aqueous emulsion of a digestion accelerator with a lecithin purity concentration of 0.15% by mass, prepared above, was added to prepare 150 g of digestate with a sodium hydroxide concentration of 5.33% by mass, a sodium sulfide concentration of 1.87% by mass, and a lecithin purity concentration of 0.005% by mass.

[0055] (Selection of wood chips) For the wood chips, we selected chips made from L-sized hardwoods (eucalyptus) that had been dried at 50°C for 24 hours, choosing those that were as similar in size and color as possible.

[0056] (Digestion) 50g of selected wood chips were placed in a pot of a multi-color rotary pot dyeing test machine (MINI COLOR, manufactured by Techsam Giken Co., Ltd.), and then 150g of the aforementioned pulping solution (equivalent to 0.015 parts by mass of pulping accelerator (lecithin purity) per 100 parts by mass of wood chips) was added. Kraft pulping was then performed at 160°C for 90 minutes.

[0057] (Cleaning process) All of the pulp obtained from the aforementioned digestion process was mixed with deionized water to adjust the total volume to 2 kg, and this was placed in a 10 L disintegrator (manufactured by Kumagai Riki Kogyo Co., Ltd.) and disintegrated for 5 minutes. The disintegrated pulp was filtered using Nutche (filter paper: No. 2 filter paper manufactured by Advantec Toyo Co., Ltd.). The filtered pulp was recovered, deionized water was added to adjust the total volume to 2 kg, and the disintegration process and filtration with Nutche were repeated. This series of operations was repeated a total of three times until the lignin color of the filtrate disappeared to wash the pulp.

[0058] (Recovery process) The pulp that had undergone the above-described digestion and washing processes was sieved using a stainless steel mesh with a mesh size of 710 μm, and the pulp that passed through the mesh was further sieved using a stainless steel mesh with a mesh size of 75 μm. The wood chips remaining on the 710 μm mesh (hereinafter also referred to as "residual wood chips") and the pulp on the 75 μm stainless steel mesh (hereinafter also referred to as "recovered pulp") were collected separately.

[0059] [Residual rate of wood chips] The obtained residual wood chips were dried at 105°C for 10 hours, and their mass was measured. This mass was then used as the "amount of residual wood chips after drying (g)" and the following formula was used: Wood chip residue rate (%) = [Amount of residual wood chips after drying (g) / Amount of wood chips used for pulping (50.0g)] × 100 The residual rate of wood chips was calculated using the following method. The results are shown in Table 1.

[0060] [Pulp yield] The obtained recovered pulp was dried at 105°C for 10 hours, and its mass was measured. This mass was defined as the "amount of recovered pulp after drying (measured value) (g)". Furthermore, assuming that the residual wood chips would be subjected to another pulping treatment, the following formula was used: Amount of recovered pulp after drying (corrected value) (g) = Amount of recovered pulp after drying (measured value) (g) + (Amount of residual chips after drying (g) / 2) As shown above, the measured "amount of recovered pulp after drying (measured value) (g)" is used in the following formula: "amount of residual chips after drying (g) / 2" is added as the yield, and this is used as the "amount of recovered pulp after drying (corrected value) (g)". Pulp yield (%) = [Amount of recovered pulp after drying (corrected value) (g) / Amount of wood chips used for pulping (50.0g)] × 100 The pulp yield (%) was calculated using the following method. The results are shown in Table 1.

[0061] (Preparation of test strips) The recovered pulp, obtained separately in the same manner as the digestion and washing processes described above, is processed using a circular paper machine ("Standard Sheet Machine Paper Machine" manufactured by Kumagai Riki Kogyo Co., Ltd.) in accordance with JIS P8222:2015 "Pulp - Preparation of test handmade paper - Method using a standard handmade paper machine" to produce paper with a basis weight of 150 g / m². 2 The paper was hand-formed under these conditions. Then, it was pressed at a pressure of 700 kPa for 5 minutes, and dried at 105°C for 5 minutes using a drum dryer to obtain the test paper.

[0062] [Copper price] The copper number of the recovered pulp was measured using the aforementioned test paper in accordance with JIS P8211:2011 "Pulp - Copper Number Test Method". The results are shown in Table 1.

[0063] (Examples A2-A8) [L-sized material, Kraft thawing method] First, aqueous emulsions of lecithin A2 to A5 (aqueous emulsions of lecithin accelerator) with a lecithin content of 10% by mass were prepared in the same manner as in Example A1, except that the lecithins shown in Table 1 were used as lecithin accelerators. Since lecithin A2 to A3 are not easily emulsified in water, 1.0 part by mass of sodium hydroxide (manufactured by Nacalai Tesque Co., Ltd., reagent) was added to 100 parts by mass of lecithin A2 to A3, as in Example A1, to mechanically emulsify lecithin A2 to A3 in deionized water. Since lecithin A4 to A5 are easily dispersed in water, they were mechanically emulsified in deionized water without adding sodium hydroxide.

[0064] Next, 150 g of pulp solution (sodium hydroxide concentration: 5.33% by mass, sodium sulfide concentration: 1.87% by mass) was prepared in the same manner as in Example A1, except that the pulp accelerator aqueous emulsion (lecithin purity: 10% by mass) prepared above was used, and the lecithin purity was adjusted so that the amount of pulp accelerator (amount of lecithin purity) per 100 parts by mass of wood chips was as shown in Table 1. Furthermore, 50 g of wood chips made from L-grade hardwood (eucalyptus) were subjected to pulp treatment and washing treatment using 150 g of this pulp solution. The residual rate of wood chips and the pulp yield were calculated using the obtained residual wood chips and recovered pulp, and the kappa number of the recovered pulp was measured. The results are shown in Table 1.

[0065] (Example A9) [L-sized material, Kraft thawing method] First, lecithin A1, a digestion accelerator, and sodium thiosulfate were mixed in a mass ratio of 8 / 1. The resulting mixture was emulsified in water to prepare an aqueous emulsion (digestion accelerator aqueous emulsion) containing lecithin A1 and sodium thiosulfate with a total concentration of 10% by mass of lecithin and sodium thiosulfate. Since lecithin A1 is difficult to emulsify in water, 1.0 part by mass of sodium hydroxide (manufactured by Nacalai Tesque Co., Ltd., reagent) was added to 100 parts by mass of the total amount of lecithin A1 and sodium thiosulfate, and the mixture of lecithin A1 and sodium thiosulfate was mechanically forced to emulsify in ion-exchanged water.

[0066] Next, 150 g of pulp solution (sodium hydroxide concentration: 5.33% by mass, sodium sulfide concentration: 1.87% by mass) was prepared in the same manner as in Example A1, except that the pulp accelerator aqueous emulsion prepared above (total concentration of lecithin and sodium thiosulfate: 10% by mass) was used, and the total concentration of lecithin and sodium thiosulfate was adjusted to 0.005% by mass so that the amount of pulp accelerator (total amount of lecithin and sodium thiosulfate) per 100 parts by mass of wood chips was 0.015 parts by mass. Furthermore, 50 g of wood chips made from L-grade hardwood (eucalyptus) were subjected to pulp treatment and washing treatment using 150 g of this pulp solution. The residual rate of wood chips and the pulp yield were calculated using the obtained residual wood chips and recovered pulp, and the kappa number of the recovered pulp was measured. The results are shown in Table 1.

[0067] (Examples A10-A18) [L-sized material, Kraft thawing method] First, aqueous emulsions containing lecithin and sulfur-containing compounds (aqueous emulsion of lecithin accelerator) were prepared in the same manner as in Example A9, except that lecithin and sulfur-containing compounds shown in Table 1 were mixed in the mass ratio shown in Table 1 as lecithin accelerators, with a total concentration of 10% by mass of lecithin and sulfur-containing compounds. Since lecithin A2 is not easily emulsified in water, 1.0 part by mass of sodium hydroxide (manufactured by Nacalai Tesque Co., Ltd., reagent) was added to 100 parts by mass of the total amount of lecithin A2 and sulfur-containing compounds, as in Example A9, and the mixture of lecithin A2 and sulfur-containing compounds was mechanically emulsified in deionized water. Since lecithin A4 is easily dispersed in water, the mixture of lecithin A4 and sulfur-containing compounds was mechanically emulsified in deionized water without adding sodium hydroxide.

[0068] Next, 150 g of pulp solution (sodium hydroxide concentration: 5.33% by mass, sodium sulfide concentration: 1.87% by mass) was prepared in the same manner as in Example A9, except that the pulp accelerator aqueous emulsion (total concentration of lecithin and sulfur-containing compound: 10% by mass) prepared above was used, and the total concentrations of lecithin and sulfur-containing compound were adjusted so that the amount of pulp accelerator (total amount of lecithin and sulfur-containing compound) per 100 parts by mass of wood chips was as shown in Table 1. Furthermore, 50 g of wood chips made from L-grade hardwood (eucalyptus) were subjected to pulp treatment and washing treatment using 150 g of this pulp solution. The residual rate of wood chips and the pulp yield were calculated using the obtained residual wood chips and recovered pulp, and the kappa number of the recovered pulp was measured. The results are shown in Table 1.

[0069] (Example A19) [L-sized material, Kraft thawing method] First, lecithin A2, a component of the digestion accelerator, was emulsified in water to prepare an aqueous emulsion of lecithin A2 (digestion accelerator aqueous emulsion) with a lecithin content of 10% by mass. Since lecithin A2 is difficult to emulsify in water, as in Example A1, 1.0 part by mass of sodium hydroxide (manufactured by Nacalai Tesque Co., Ltd., reagent) was added to 100 parts by mass of lecithin A2 to mechanically force-emulsify the lecithin A2 in ion-exchanged water.

[0070] In addition, sodium thiosulfate, another component of the digestion accelerator, was dissolved in water to prepare an aqueous solution of sodium thiosulfate (aqueous solution of digestion accelerator).

[0071] Next, 0.075 g (0.0075 g of pure lecithin) of the pulping accelerator aqueous emulsion (lecithin purity concentration: 10% by mass) prepared above was mixed with deionized water to prepare a pulping accelerator aqueous emulsion with a lecithin purity concentration of 0.15% by mass.

[0072] Next, 8.0 g of sodium hydroxide (manufactured by Nacalai Tesque Co., Ltd., reagent) and 6.03 g of sodium sulfide pentahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent) (2.8 g of sodium sulfide purity) were placed in a beaker, and then deionized water was added to prepare 145 g of an alkaline aqueous solution of the main digesting agent. To 145 g of this aqueous solution of the main digesting agent, the aqueous emulsion of the digesting accelerator with a lecithin content of 0.15% by mass, prepared above, and the aqueous solution of the digesting accelerator were simultaneously added, such that the mass ratio of lecithin A2 to sodium thiosulfate was 8 / 1, and the amount of digesting accelerator (total amount of pure lecithin and sodium thiosulfate) per 100 parts by mass of wood chips was 0.015 parts by mass. This prepared 150 g of digested liquor with a sodium hydroxide concentration of 5.33% by mass, a sodium sulfide concentration of 1.87% by mass, and a total concentration of pure lecithin and sodium thiosulfate of 0.005% by mass.

[0073] Except for using 150g of this digestate, 50g of wood chips made from L-grade hardwood (eucalyptus) were subjected to digestion and washing in the same manner as in Example A1. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 1.

[0074] (Example A20) [L-sized material, Kraft thawing method] Except for changing the mass ratio of lecithin A2 to sodium thiosulfate to 2 / 1, 150 g of pulp solution was prepared in the same manner as in Example A19, with a sodium hydroxide concentration of 5.33% by mass, a sodium sulfide concentration of 1.87% by mass, and a total concentration of lecithin and sodium thiosulfate of 0.005% by mass. Furthermore, 50 g of wood chips made from L-grade hardwood (eucalyptus) were subjected to pulpation and washing using this 150 g pulp solution. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 1.

[0075] (Comparative Example A1) [L-sized material, Kraft thawing method] Except for the absence of a digestion accelerator, 150 g of digestate with a sodium hydroxide concentration of 5.33% by mass and a sodium sulfide concentration of 1.87% by mass was prepared in the same manner as in Example A1. Furthermore, 50 g of wood chips made from L-grade hardwood (eucalyptus) were digested and washed using this 150 g of digestate. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 1.

[0076] (Comparative example A2) [L-sized material, Kraft thawing method] Except for using glucose (manufactured by Tokyo Chemical Industry Co., Ltd., reagent) instead of lecithin A1, 150 g of pulp solution with a sodium hydroxide concentration of 5.33% by mass, a sodium sulfide concentration of 1.87% by mass, and a glucose concentration of 0.005% by mass was prepared in the same manner as in Example A1. Furthermore, 50 g of wood chips made from L-grade hardwood (eucalyptus) were subjected to pulpation and washing treatment using this 150 g pulp solution. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 1.

[0077] (Comparative example A3) [L-sized material, Kraft thawing method] Except for using the sodium salt of 1,4-dihydro-9,10-dihydroxyanthracene (SAQ, manufactured by Kawasaki Chemical Industries, Ltd., 21% by mass aqueous solution, sodium hydroxide content: 10% by mass) instead of lecithin A1, 150 g of pulp solution with a sodium hydroxide concentration of 5.33% by mass, a sodium sulfide concentration of 1.87% by mass, and an SAQ concentration of 0.005% by mass was prepared in the same manner as in Example A1. Furthermore, 50 g of wood chips made from L-grade hardwood (eucalyptus) were subjected to pulping and washing treatment using this 150 g pulp solution. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 1.

[0078] (Comparative example A4) [L-sized material, Kraft thawing method] Except for not using lecithin A1, 150 g of pulp solution with a sodium hydroxide concentration of 5.33% by mass, a sodium sulfide concentration of 1.87% by mass, and a sodium thiosulfate concentration of 0.005% by mass was prepared in the same manner as in Example A9. Furthermore, 50 g of wood chips made from L-grade hardwood (eucalyptus) were subjected to pulping and washing treatment using this 150 g pulp solution. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 1.

[0079] (Comparative Example A5) [L-sized material, Kraft thawing method] Except for not using lecithin A2, 150 g of pulp solution was prepared in the same manner as in Example A14, with a sodium hydroxide concentration of 5.33% by mass, a sodium sulfide concentration of 1.87% by mass, and a potassium hydrogen sulfide concentration of 0.005% by mass. Furthermore, 50 g of wood chips made from L-grade hardwood (eucalyptus) were subjected to pulping and washing treatment using this 150 g pulp solution. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 1.

[0080] (Comparative example A6) [L-sized material, Kraft thawing method] Except for not using lecithin A4, 150 g of pulp solution was prepared in the same manner as in Example A15, with a sodium hydroxide concentration of 5.33% by mass, a sodium sulfide concentration of 1.87% by mass, and a potassium sulfite concentration of 0.005% by mass. Furthermore, 50 g of wood chips made from L-grade hardwood (eucalyptus) were subjected to pulping and washing treatment using this 150 g pulp solution. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 1.

[0081] [Table 1]

[0082] Comparing Examples A1 to A8 with Comparative Example A1, it was found that when pulp was produced from wood chips made from L-grade hardwood (eucalyptus) using the kraft pulping method, adding lecithin to the pulp solution reduced the wood chip residue rate, improved pulp yield, and lowered the kappa value. This confirmed that lecithin acts as a pulping accelerator in the kraft pulping treatment of L-grade wood.

[0083] Furthermore, comparing Example A9 with Example A1, Examples A10-A14, A19-A20 with Example A2, and Example A15 with Example A7, it was found that adding a sulfur-containing compound along with lecithin to the pulp solution further reduced the wood chip residue rate, further improved the pulp yield, and further reduced the copper value, indicating that the kraft pulping of L-grade wood was further promoted. On the other hand, when only a sulfur-containing compound was added to the pulp solution (Comparative Examples A4-A6), the kraft pulping of L-grade wood was not promoted.

[0084] Furthermore, when lecithin and sulfur-containing compounds were pre-mixed and added to the pulp (Examples A10 and A12), the wood chip residue rate decreased, the pulp yield improved, and the copper value decreased compared to when lecithin and sulfur-containing compounds were added separately and simultaneously to the pulp (Examples A19 and A20). This indicates that pre-mixing lecithin and sulfur-containing compounds and adding them to the pulp further promotes the kraft pulping of L-grade wood.

[0085] (Example B1) [N material, Kraft thawing method] (Preparation of emulsion of digestion accelerator) Lecithin A1, a digestion accelerator, was emulsified in water to prepare an aqueous emulsion of lecithin A1 (digestion accelerator aqueous emulsion) with a lecithin content of 10% by mass. Since lecithin A1 is difficult to emulsify in water, 1.0 part by mass of sodium hydroxide (manufactured by Nacalai Tesque Co., Ltd., reagent) was added to 100 parts by mass of lecithin A1 to mechanically force emulsify the lecithin A1 in ion-exchanged water.

[0086] (Preparation of evaporated liquid) First, 0.075 g of the pulping accelerator aqueous emulsion (lecithin purity: 10% by mass) prepared above (0.0075 g of pure lecithin) was mixed with deionized water to prepare 5 g of the pulping accelerator aqueous emulsion with a lecithin purity of 0.15% by mass.

[0087] Next, 10.8 g of sodium hydroxide (manufactured by Nacalai Tesque Co., Ltd., reagent) and 8.61 g of sodium sulfide pentahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent) (4.0 g of sodium sulfide purity) were placed in a beaker, and then deionized water was added to prepare 145 g of an alkaline aqueous solution of the main digesting agent. To this 145 g aqueous solution of the main digesting agent, 5 g of the aqueous emulsion of a digestion accelerator with a lecithin purity concentration of 0.15% by mass, prepared above, was added to prepare 150 g of digestate with a sodium hydroxide concentration of 7.20% by mass, a sodium sulfide concentration of 2.67% by mass, and a lecithin purity concentration of 0.005% by mass.

[0088] (Selection of wood chips) For the wood chips, we selected chips made from N-grade coniferous trees (Japanese red pine) that had been dried at 50°C for 24 hours, choosing those that were as similar in size and color as possible.

[0089] (Digestion) 50g of selected wood chips were placed in a pot of a multi-color rotary pot dyeing test machine (MINI COLOR, manufactured by Techsam Giken Co., Ltd.), and then 150g of the aforementioned pulping solution (equivalent to 0.015 parts by mass of pulping accelerator (lecithin purity) per 100 parts by mass of wood chips) was added. Kraft pulping was then performed at 160°C for 90 minutes.

[0090] (Cleaning process) All of the pulp obtained from the aforementioned digestion process was mixed with deionized water to adjust the total volume to 2 kg, and this was placed in a 10 L disintegrator (manufactured by Kumagai Riki Kogyo Co., Ltd.) and disintegrated for 5 minutes. The disintegrated pulp was filtered using Nutche (filter paper: No. 2 filter paper manufactured by Advantec Toyo Co., Ltd.). The filtered pulp was recovered, deionized water was added to adjust the total volume to 2 kg, and the disintegration process and filtration with Nutche were repeated. This series of operations was repeated a total of three times until the lignin color of the filtrate disappeared to wash the pulp.

[0091] (Recovery process) The pulp that had undergone the above-described digestion and washing processes was sieved using a stainless steel mesh with a mesh size of 710 μm, and the pulp that passed through the mesh was further sieved using a stainless steel mesh with a mesh size of 75 μm. The wood chips remaining on the 710 μm mesh (hereinafter also referred to as "residual wood chips") and the pulp on the 75 μm stainless steel mesh (hereinafter also referred to as "recovered pulp") were collected separately.

[0092] Using the obtained residual wood chips and recovered pulp, the residual rate of wood chips and the pulp yield were calculated, and the copper number of the recovered pulp was measured. The results are shown in Table 2.

[0093] (Examples B2-B8) [N material, Kraft thawing method] First, aqueous emulsions of lecithin A2 to A5 (aqueous emulsions of lecithin accelerator) with a lecithin content of 10% by mass were prepared in the same manner as in Example B1, except that the lecithins shown in Table 2 were used as lecithin accelerators. Since lecithin A2 to A3 are not easily emulsified in water, 1.0 part by mass of sodium hydroxide (manufactured by Nacalai Tesque Co., Ltd., reagent) was added to 100 parts by mass of lecithin A2 to A3 to mechanically emulsify lecithin A2 to A3 in ion-exchanged water, as in Example B1. Since lecithin A4 to A5 are easily dispersed in water, they were mechanically emulsified in ion-exchanged water without adding sodium hydroxide.

[0094] Next, 150 g of pulp solution (sodium hydroxide concentration: 7.20% by mass, sodium sulfide concentration: 2.67% by mass) was prepared in the same manner as in Example B1, except that the pulp accelerator aqueous emulsion (lecithin purity: 10% by mass) prepared above was used, and the lecithin purity was adjusted so that the amount of pulp accelerator (amount of lecithin purity) per 100 parts by mass of wood chips was as shown in Table 2. Furthermore, 50 g of wood chips made from N-grade coniferous trees (Japanese red pine) were subjected to pulp treatment and washing treatment using 150 g of this pulp solution. The residual rate of wood chips and the pulp yield were calculated using the obtained residual wood chips and recovered pulp, and the kappa number of the recovered pulp was measured. The results are shown in Table 2.

[0095] (Example B9) [N material, Kraft thawing method] First, lecithin A1, a digestion accelerator, and ammonium thiosulfate were mixed in a mass ratio of 8 / 1. The resulting mixture was emulsified in water to prepare an aqueous emulsion (digestion accelerator aqueous emulsion) containing lecithin A1 and ammonium thiosulfate with a total concentration of 10% by mass of lecithin and ammonium thiosulfate. Since lecithin A1 is difficult to emulsify in water, 1.0 part by mass of sodium hydroxide (manufactured by Nacalai Tesque Co., Ltd., reagent) was added to 100 parts by mass of the total amount of lecithin A1 and ammonium thiosulfate, and the mixture of lecithin A1 and ammonium thiosulfate was mechanically forced to emulsify in ion-exchanged water.

[0096] Next, 150 g of pulp solution (sodium hydroxide concentration: 7.20% by mass, sodium sulfide concentration: 2.67% by mass) was prepared in the same manner as in Example B1, except that the aqueous emulsion of pulp accelerator prepared above (total concentration of lecithin and ammonium thiosulfate: 10% by mass) was used. The total concentration of lecithin and ammonium thiosulfate was adjusted to 0.005% by mass so that the amount of pulp accelerator (total amount of lecithin and ammonium thiosulfate) per 100 parts by mass of wood chips was 0.015 parts by mass. Furthermore, 50 g of wood chips made from N-grade coniferous trees (Japanese red pine) were subjected to pulp treatment and washing treatment using 150 g of this pulp solution. The residual rate of wood chips and the pulp yield were calculated using the obtained residual wood chips and recovered pulp, and the kappa number of the recovered pulp was measured. The results are shown in Table 2.

[0097] (Examples B10-B19) [N material, Kraft thawing method] First, aqueous emulsions containing lecithin and sulfur-containing compounds (aqueous emulsion of lecithin accelerator) were prepared in the same manner as in Example B9, except that lecithin and sulfur-containing compounds shown in Table 2 were mixed in the mass ratio shown in Table 2 as lecithin accelerators, with a total concentration of 10% by mass of lecithin and sulfur-containing compounds. Since lecithin A3 is not easily emulsified in water, 1.0 part by mass of sodium hydroxide (manufactured by Nacalai Tesque Co., Ltd., reagent) was added to 100 parts by mass of the total amount of lecithin A3 and sulfur-containing compounds, as in Example B9, and the mixture of lecithin A3 and sulfur-containing compounds was mechanically emulsified in deionized water. Since lecithin A5 is easily dispersed in water, the mixture of lecithin A5 and sulfur-containing compounds was mechanically emulsified in deionized water without adding sodium hydroxide.

[0098] Next, 150 g of pulp solution (sodium hydroxide concentration: 7.20% by mass, sodium sulfide concentration: 2.67% by mass) was prepared in the same manner as in Example B9, except that the pulp accelerator aqueous emulsion (total concentration of lecithin and sulfur-containing compound: 10% by mass) prepared above was used, and the total concentrations of lecithin and sulfur-containing compound were adjusted so that the amount of pulp accelerator (total amount of lecithin and sulfur-containing compound) per 100 parts by mass of wood chips was as shown in Table 2. Furthermore, 50 g of wood chips made from N-grade coniferous trees (Japanese red pine) were subjected to pulp treatment and washing treatment using 150 g of this pulp solution. The residual rate of wood chips and the pulp yield were calculated using the obtained residual wood chips and recovered pulp, and the kappa number of the recovered pulp was measured. The results are shown in Table 2.

[0099] (Example B20) [N material, Kraft thawing method] First, lecithin A3, a component of the digestion accelerator, was emulsified in water to prepare an aqueous emulsion of lecithin A3 (digestion accelerator aqueous emulsion) with a lecithin content of 10% by mass. Since lecithin A3 is difficult to emulsify in water, as in Example B1, 1.0 part by mass of sodium hydroxide (manufactured by Nacalai Tesque Co., Ltd., reagent) was added to 100 parts by mass of lecithin A3 to mechanically force-emulsify the lecithin A3 in ion-exchanged water.

[0100] In addition, ammonium thiosulfate, another component of the digestion accelerator, was dissolved in water to prepare an aqueous solution of ammonium thiosulfate (aqueous solution of digestion accelerator).

[0101] Next, 0.075 g (0.0075 g of pure lecithin) of the pulping accelerator aqueous emulsion (lecithin purity concentration: 10% by mass) prepared above was mixed with deionized water to prepare a pulping accelerator aqueous emulsion with a lecithin purity concentration of 0.15% by mass.

[0102] Next, 10.8 g of sodium hydroxide (manufactured by Nacalai Tesque Co., Ltd., reagent) and 8.61 g of sodium sulfide pentahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent) (4.0 g of sodium sulfide purity) were placed in a beaker, and then deionized water was added to prepare 145 g of an alkaline aqueous solution of the main digesting agent. To 145 g of this aqueous solution of the main digesting agent, the aqueous emulsion of the digesting accelerator with a lecithin content of 0.15% by mass, prepared above, and the aqueous solution of the digesting accelerator were simultaneously added, such that the mass ratio of lecithin A3 to ammonium thiosulfate was 8 / 1, and the amount of digesting accelerator (total amount of pure lecithin and ammonium thiosulfate) per 100 parts by mass of wood chips was 0.015 parts by mass. This prepared 150 g of digested liquor with a sodium hydroxide concentration of 7.20% by mass, a sodium sulfide concentration of 2.67% by mass, and a total concentration of pure lecithin and ammonium thiosulfate of 0.005% by mass.

[0103] Except for using 150g of this pulping solution, 50g of wood chips made from N-grade coniferous trees (Japanese red pine) were subjected to pulping and washing in the same manner as in Example B1. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 2.

[0104] (Example B21) [N material, Kraft thawing method] Except for changing the mass ratio of lecithin A3 to ammonium thiosulfate to 2 / 1, 150 g of pulp solution was prepared in the same manner as in Example B20, with a sodium hydroxide concentration of 7.20% by mass, a sodium sulfide concentration of 2.67% by mass, and a total concentration of lecithin purity and ammonium thiosulfate of 0.005% by mass. Furthermore, 50 g of wood chips made from N-grade coniferous trees (Japanese red pine) were subjected to pulpation and washing treatment using this 150 g pulp solution. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 2.

[0105] (Comparative Example B1) [N material, Kraft thawing method] Except for the absence of a digestion accelerator, 150 g of digestate with a sodium hydroxide concentration of 7.20% by mass and a sodium sulfide concentration of 2.67% by mass was prepared in the same manner as in Example B1. Furthermore, 50 g of wood chips made from N-grade coniferous trees (Japanese red pine) were digested and washed using this 150 g of digestate. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 2.

[0106] (Comparative example B2) [N material, Kraft thawing method] Except for using fructose (manufactured by Tokyo Chemical Industry Co., Ltd., reagent) instead of lecithin A1, 150 g of pulp solution with a sodium hydroxide concentration of 7.20% by mass, a sodium sulfide concentration of 2.67% by mass, and a fructose concentration of 0.005% by mass was prepared in the same manner as in Example B1. Furthermore, 50 g of wood chips made from N-grade coniferous trees (Japanese red pine) were subjected to pulping and washing treatment using this 150 g pulp solution. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 2.

[0107] (Comparative Example B3) [N material, Kraft thawing method] Except for using the sodium salt of 1,4-dihydro-9,10-dihydroxyanthracene (SAQ, manufactured by Kawasaki Chemical Industries, Ltd., 21% by mass aqueous solution, sodium hydroxide content: 10% by mass) instead of lecithin A1, 150 g of pulp solution with a sodium hydroxide concentration of 7.20% by mass, a sodium sulfide concentration of 2.67% by mass, and an SAQ concentration of 0.005% by mass was prepared in the same manner as in Example B1. Furthermore, 50 g of wood chips made from N-grade coniferous trees (Japanese red pine) were subjected to pulping and washing treatment using this 150 g pulp solution. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 2.

[0108] (Comparative example B4) [N material, Kraft thawing method] Except for not using lecithin A3, 150 g of pulp solution was prepared in the same manner as in Example B10, with a sodium hydroxide concentration of 7.20% by mass, a sodium sulfide concentration of 2.67% by mass, and a sodium thiosulfate concentration of 0.005% by mass. Furthermore, 50 g of wood chips made from N-grade coniferous trees (Japanese red pine) were subjected to pulping and washing treatment using this 150 g pulp solution. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 2.

[0109] (Comparative Example B5) [N material, Kraft thawing method] Except for not using lecithin A1, 150 g of pulp solution was prepared in the same manner as in Example B9, with a sodium hydroxide concentration of 7.20% by mass, a sodium sulfide concentration of 2.67% by mass, and an ammonium thiosulfate concentration of 0.005% by mass. Furthermore, 50 g of wood chips made from N-grade coniferous trees (Japanese red pine) were subjected to pulping and washing treatment using this 150 g pulp solution. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 2.

[0110] (Comparative example B6) [N material, Kraft thawing method] Except for not using lecithin A3, 150 g of pulp solution was prepared in the same manner as in Example B15, with a sodium hydroxide concentration of 7.20% by mass, a sodium sulfide concentration of 2.67% by mass, and a potassium hydrogen sulfide concentration of 0.005% by mass. Furthermore, 50 g of wood chips made from N-grade coniferous trees (Japanese red pine) were subjected to pulping and washing treatment using this 150 g pulp solution. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 2.

[0111] (Comparative Example B7) [N material, Kraft thawing method] Except for not using lecithin A5, 150 g of pulp solution was prepared in the same manner as in Example B16, with a sodium hydroxide concentration of 7.20% by mass, a sodium sulfide concentration of 2.67% by mass, and a sodium bisulfite concentration of 0.005% by mass. Furthermore, 50 g of wood chips made from N-grade coniferous trees (Japanese red pine) were subjected to pulping and washing treatment using this 150 g pulp solution. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 2.

[0112] [Table 2]

[0113] Comparing Examples B1 to B8 with Comparative Example B1, it was found that when pulp was produced from wood chips made from N-grade coniferous trees (Japanese red pine) using the kraft pulping method, adding lecithin to the pulp solution reduced the wood chip residue rate, improved the pulp yield, and lowered the copper value. This confirmed that lecithin acts as a pulping accelerator in the kraft pulping treatment of N-grade wood.

[0114] Furthermore, comparing Example B9 with Example B1, Examples B10-B15, B20-B21 with Example B3, and Example B16 with Example B8, it was found that adding a sulfur-containing compound along with lecithin to the pulp solution further reduced the wood chip residue rate, further improved the pulp yield, and further reduced the copper value, indicating that kraft pulping of N wood was further promoted. On the other hand, when only a sulfur-containing compound was added to the pulp solution (Comparative Examples B4-B7), kraft pulping of N wood was not promoted.

[0115] Furthermore, when lecithin and sulfur-containing compounds were pre-mixed and added to the pulp (Examples B11 and B13), the wood chip residue rate decreased, the pulp yield improved, and the copper value decreased compared to when lecithin and sulfur-containing compounds were added separately and simultaneously to the pulp (Examples B20 and B21). This indicates that pre-mixing lecithin and sulfur-containing compounds and adding them to the pulp further promotes the kraft pulping of N wood.

[0116] (Example C1) [L-type material, soda pulping method] (Preparation of emulsion of digestion accelerator) Lecithin A2, a digestion accelerator, was emulsified in water to prepare an aqueous emulsion of lecithin A2 (digestion accelerator aqueous emulsion) with a lecithin content of 10% by mass. Since lecithin A2 is difficult to emulsify in water, 1.0 part by mass of sodium hydroxide (manufactured by Nacalai Tesque Co., Ltd., reagent) was added to 100 parts by mass of lecithin A2 to mechanically force emulsify the lecithin A2 in ion-exchanged water.

[0117] (Preparation of evaporated liquid) First, 0.075 g of the pulping accelerator aqueous emulsion (lecithin purity: 10% by mass) prepared above (0.0075 g of pure lecithin) was mixed with deionized water to prepare 5 g of the pulping accelerator aqueous emulsion with a lecithin purity of 0.15% by mass.

[0118] Next, 10.8 g of sodium hydroxide (manufactured by Nacalai Tesque Co., Ltd., reagent), which is the main digestion agent, was placed in a beaker, and then deionized water was added to prepare 145 g of an alkaline aqueous solution of the main digestion agent. To this 145 g aqueous solution of the main digestion agent, 5 g of the aqueous emulsion of a digestion accelerator with a lecithin purity concentration of 0.15% by mass, which was prepared above, was added to prepare 150 g of digestate with a sodium hydroxide concentration of 7.20% by mass and a lecithin purity concentration of 0.005% by mass.

[0119] (Selection of wood chips) For the wood chips, we selected chips made from L-sized hardwoods (eucalyptus) that had been dried at 50°C for 24 hours, choosing those that were as similar in size and color as possible.

[0120] (Digestion) 50g of selected wood chips were placed in a pot of a multi-color rotary pot dyeing test machine (MINI COLOR, manufactured by Techsam Giken Co., Ltd.), and then 150g of the aforementioned pulverized liquid (equivalent to 0.015 parts by mass of pulverization accelerator (lecithin content) per 100 parts by mass of wood chips) was added, and soda pulverization was performed at 160°C for 90 minutes.

[0121] (Cleaning process) All of the pulp obtained from the aforementioned digestion process was mixed with deionized water to adjust the total volume to 2 kg, and this was placed in a 10 L disintegrator (manufactured by Kumagai Riki Kogyo Co., Ltd.) and disintegrated for 5 minutes. The disintegrated pulp was filtered using Nutche (filter paper: No. 2 filter paper manufactured by Advantec Toyo Co., Ltd.). The filtered pulp was recovered, deionized water was added to adjust the total volume to 2 kg, and the disintegration process and filtration with Nutche were repeated. This series of operations was repeated a total of three times until the lignin color of the filtrate disappeared to wash the pulp.

[0122] (Recovery process) The pulp that had undergone the above-described digestion and washing processes was sieved using a stainless steel mesh with a mesh size of 710 μm, and the pulp that passed through the mesh was further sieved using a stainless steel mesh with a mesh size of 75 μm. The wood chips remaining on the 710 μm mesh (hereinafter also referred to as "residual wood chips") and the pulp on the 75 μm stainless steel mesh (hereinafter also referred to as "recovered pulp") were collected separately.

[0123] Using the obtained residual wood chips and recovered pulp, the residual rate of wood chips and the pulp yield were calculated, and the copper number of the recovered pulp was measured. The results are shown in Table 3.

[0124] (Example C2) [L-type material, soda pulping method] First, lecithin A2 and ammonium thiosulfate, which are digestion accelerators, were mixed in a mass ratio of 8 / 1. The resulting mixture was emulsified in water to prepare an aqueous emulsion (digestion accelerator aqueous emulsion) containing lecithin A2 and ammonium thiosulfate with a total concentration of 10% by mass of lecithin and ammonium thiosulfate. Since lecithin A2 is difficult to emulsify in water, 1.0 part by mass of sodium hydroxide (manufactured by Nacalai Tesque Co., Ltd., reagent) was added to 100 parts by mass of the total amount of lecithin A2 and ammonium thiosulfate, and the mixture of lecithin A2 and ammonium thiosulfate was mechanically forced to emulsify in ion-exchanged water.

[0125] Next, 150 g of pulp solution (sodium hydroxide concentration: 7.20% by mass) was prepared in the same manner as in Example C1, except that the aqueous emulsion of pulp accelerator prepared above (total concentration of lecithin and ammonium thiosulfate: 10% by mass) was used, and the total concentration of lecithin and ammonium thiosulfate was adjusted to 0.005% by mass so that the amount of pulp accelerator (total amount of lecithin and ammonium thiosulfate) per 100 parts by mass of wood chips was 0.015 parts by mass. Furthermore, 50 g of wood chips made from L-grade hardwood (eucalyptus) were subjected to pulp treatment and washing treatment using 150 g of this pulp solution. The residual rate of wood chips and the pulp yield were calculated using the obtained residual wood chips and recovered pulp, and the kappa number of the recovered pulp was measured. The results are shown in Table 3.

[0126] (Example C3) [L-type material, soda pulping method] Except for changing the mass ratio of lecithin A2 to ammonium thiosulfate to 2 / 1, 150 g of pulp solution was prepared in the same manner as in Example C2, with a sodium hydroxide concentration of 7.20% by mass and a total concentration of lecithin and ammonium thiosulfate of 0.005% by mass. Furthermore, 50 g of wood chips made from L-grade hardwood (eucalyptus) were subjected to pulpation and washing using this 150 g pulp solution. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 3.

[0127] (Example C4) [L-type material, soda pulping method] First, lecithin A2, a component of the digestion accelerator, was emulsified in water to prepare an aqueous emulsion of lecithin A2 (digestion accelerator aqueous emulsion) with a lecithin content of 10% by mass. Since lecithin A2 is difficult to emulsify in water, as in Example C1, 1.0 part by mass of sodium hydroxide (manufactured by Nacalai Tesque Co., Ltd., reagent) was added to 100 parts by mass of lecithin A2 to mechanically force-emulsify the lecithin A2 in ion-exchanged water.

[0128] In addition, ammonium thiosulfate, another component of the digestion accelerator, was dissolved in water to prepare an aqueous solution of ammonium thiosulfate (aqueous solution of digestion accelerator).

[0129] Next, 0.075 g (0.0075 g of pure lecithin) of the pulping accelerator aqueous emulsion (lecithin purity concentration: 10% by mass) prepared above was mixed with deionized water to prepare a pulping accelerator aqueous emulsion with a lecithin purity concentration of 0.15% by mass.

[0130] Next, 10.8 g of sodium hydroxide (manufactured by Nacalai Tesque Co., Ltd., reagent), which is the main digesting agent, was placed in a beaker, and then deionized water was added to prepare 145 g of an alkaline aqueous solution of the main digesting agent. To this 145 g aqueous solution of the main digesting agent, the aqueous emulsion of the digesting agent with a lecithin content of 0.15% by mass and the aqueous solution of the digesting agent prepared above were simultaneously added, so that the mass ratio of lecithin A2 to ammonium thiosulfate was 8 / 1, and the amount of digesting accelerator (total amount of lecithin content and ammonium thiosulfate) per 100 parts by mass of wood chips was 0.015 parts by mass. This prepared 150 g of digested liquor with a sodium hydroxide concentration of 7.20% by mass and a total concentration of lecithin content and ammonium thiosulfate of 0.005% by mass.

[0131] Except for using 150g of this digestate, 50g of wood chips made from L-grade hardwood (eucalyptus) were subjected to digestion and washing in the same manner as in Example C1. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 3.

[0132] (Comparative Example C1) [L-type material, soda pulping method] Except for the absence of a digestion accelerator, 150 g of digestate with a sodium hydroxide concentration of 7.20% by mass was prepared in the same manner as in Example C1. Furthermore, 50 g of wood chips made from L-grade hardwood (eucalyptus) were digested and washed using this 150 g of digestate. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 3.

[0133] (Comparative Example C2) [L-type material, soda pulping method] Except for using fructose (manufactured by Tokyo Chemical Industry Co., Ltd., reagent) instead of lecithin A2, 150 g of pulp solution with a sodium hydroxide concentration of 7.20% by mass and a fructose concentration of 0.005% by mass was prepared in the same manner as in Example C1. Furthermore, 50 g of wood chips made from L-grade hardwood (eucalyptus) were subjected to pulpation and washing treatment using this 150 g pulp solution. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 3.

[0134] (Comparative Example C3) [L-type material, soda pulping method] Except for not using lecithin A2, 150 g of pulp solution with a sodium hydroxide concentration of 7.20% by mass and an ammonium thiosulfate concentration of 0.005% by mass was prepared in the same manner as in Example C2. Furthermore, 50 g of wood chips made from L-grade hardwood (eucalyptus) were subjected to pulping and washing treatment using 150 g of this pulp solution. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 3.

[0135] [Table 3]

[0136] Comparing Example C1 with Comparative Example C1, it was found that when pulp was produced from wood chips made from L-grade hardwood (eucalyptus) using the soda pulping method, adding lecithin to the pulp reduced the wood chip residue rate, improved the pulp yield, and lowered the kappa number. This confirmed that lecithin acts as a pulping accelerator in the soda pulping treatment of L-grade wood.

[0137] Furthermore, comparing Examples C2-C4 with Example C1, it was found that adding a sulfur-containing compound along with lecithin to the pulp further reduced the wood chip residue rate, improved the pulp yield, and further reduced the kappa number, indicating that soda pulping of L-grade wood was further promoted. On the other hand, when only a sulfur-containing compound was added to the pulp (Comparative Example C3), soda pulping of L-grade wood was not promoted.

[0138] Furthermore, when lecithin and a sulfur-containing compound were pre-mixed and added to the pulp (Example C2), the wood chip residue rate decreased, the pulp yield improved, and the copper value decreased compared to when lecithin and the sulfur-containing compound were added separately and simultaneously to the pulp (Example C4). This indicates that pre-mixing lecithin and a sulfur-containing compound and adding them to the pulp further promotes the soda pulping of L-grade wood.

[0139] (Example D1) [N material, polysulfide pulping method] (Preparation of emulsion of digestion accelerator) Lecithin A3, a digestion accelerator, was emulsified in water to prepare an aqueous emulsion of lecithin A3 (digestion accelerator aqueous emulsion) with a lecithin content of 10% by mass. Since lecithin A3 is difficult to emulsify in water, 1.0 part by mass of sodium hydroxide (manufactured by Nacalai Tesque Co., Ltd., reagent) was added to 100 parts by mass of lecithin A3 to mechanically force emulsify the lecithin A3 in ion-exchanged water.

[0140] (Preparation of evaporated liquid) First, 0.075 g of the pulping accelerator aqueous emulsion (lecithin purity: 10% by mass) prepared above (0.0075 g of pure lecithin) was mixed with deionized water to prepare 5 g of the pulping accelerator aqueous emulsion with a lecithin purity of 0.15% by mass.

[0141] Next, 10.8 g of sodium hydroxide (manufactured by Nacalai Tesque Co., Ltd., reagent), 6.03 g of sodium sulfide pentahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent) (2.8 g of sodium sulfide purity), and 4.0 g of sodium tetrasulfide solution (manufactured by Nagao Co., Ltd., 30% by mass solution) (1.2 g of sodium tetrasulfide purity) were placed in a beaker, and then deionized water was added to prepare 145 g of an alkaline aqueous solution of the main digesting agent. To this 145 g aqueous solution of the main digesting agent, 5 g of the aqueous emulsion of a digestion accelerator with a lecithin purity concentration of 0.15% by mass, prepared above, was added to prepare 150 g of digested liquid with a sodium hydroxide concentration of 7.20% by mass, a sodium sulfide concentration of 1.87% by mass, a sodium tetrasulfide concentration of 0.80% by mass, and a lecithin purity concentration of 0.005% by mass.

[0142] (Selection of wood chips) For the wood chips, we selected chips made from N-grade coniferous trees (Japanese red pine) that had been dried at 50°C for 24 hours, choosing those that were as similar in size and color as possible.

[0143] (Digestion) 50g of selected wood chips were placed in a pot of a multi-color rotary pot staining test machine (MINI COLOR, manufactured by Techsam Giken Co., Ltd.), and then 150g of the aforementioned pulverized solution (equivalent to 0.015 parts by mass of pulverization accelerator (lecithin purity) per 100 parts by mass of wood chips) was added. Polysulfide pulverization treatment was then carried out at 160°C for 90 minutes.

[0144] (Cleaning process) All of the pulp obtained from the aforementioned digestion process was mixed with deionized water to adjust the total volume to 2 kg, and this was placed in a 10 L disintegrator (manufactured by Kumagai Riki Kogyo Co., Ltd.) and disintegrated for 5 minutes. The disintegrated pulp was filtered using Nutche (filter paper: No. 2 filter paper manufactured by Advantec Toyo Co., Ltd.). The filtered pulp was recovered, deionized water was added to adjust the total volume to 2 kg, and the disintegration process and filtration with Nutche were repeated. This series of operations was repeated a total of three times until the lignin color of the filtrate disappeared to wash the pulp.

[0145] (Recovery process) The pulp that had undergone the above-described digestion and washing processes was sieved using a stainless steel mesh with a mesh size of 710 μm, and the pulp that passed through the mesh was further sieved using a stainless steel mesh with a mesh size of 75 μm. The wood chips remaining on the 710 μm mesh (hereinafter also referred to as "residual wood chips") and the pulp on the 75 μm stainless steel mesh (hereinafter also referred to as "recovered pulp") were collected separately.

[0146] The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the copper number of the recovered pulp was measured. The results are shown in Table 4.

[0147] (Example D2) [N material, polysulfide pulping method] First, lecithin A3, a digestion accelerator, and sodium thiosulfate were mixed in a mass ratio of 8 / 1. The resulting mixture was emulsified in water to prepare an aqueous emulsion (digestion accelerator aqueous emulsion) containing lecithin A3 and sodium thiosulfate with a total concentration of 10% by mass of lecithin and sodium thiosulfate. Since lecithin A3 is difficult to emulsify in water, 1.0 part by mass of sodium hydroxide (manufactured by Nacalai Tesque Co., Ltd., reagent) was added to 100 parts by mass of the total amount of lecithin A3 and sodium thiosulfate, and the mixture of lecithin A3 and sodium thiosulfate was mechanically forced to emulsify in ion-exchanged water.

[0148] Next, 150 g of pulp solution (sodium hydroxide concentration: 7.20% by mass, sodium sulfide concentration: 1.87% by mass, sodium tetrasulfide concentration: 0.80% by mass) was prepared in the same manner as in Example D1, except that the pulp accelerator aqueous emulsion prepared above (total concentration of lecithin and sodium thiosulfate: 10% by mass) was used, and the total concentration of lecithin and sodium thiosulfate was adjusted to 0.005% by mass so that the amount of pulp accelerator (total amount of lecithin and sodium thiosulfate) per 100 parts by mass of wood chips was 0.015 parts by mass. Furthermore, 50 g of wood chips made from N-grade coniferous trees (Japanese red pine) were subjected to pulp treatment and washing treatment using 150 g of this pulp solution. The residual rate of wood chips and the pulp yield were calculated using the obtained residual wood chips and recovered pulp, and the kappa number of the recovered pulp was measured. The results are shown in Table 4.

[0149] (Example D3) [N material, polysulfide pulping method] Except for changing the mass ratio of lecithin A3 to sodium thiosulfate to 2 / 1, 150 g of pulp solution was prepared in the same manner as in Example D2, with a sodium hydroxide concentration of 7.20% by mass, a sodium sulfide concentration of 1.87% by mass, a sodium tetrasulfide concentration of 0.80% by mass, and a total concentration of lecithin and sodium thiosulfate of 0.005% by mass. Furthermore, 50 g of wood chips made from N-grade coniferous trees (Japanese red pine) were subjected to pulping and washing treatment using this 150 g pulp solution. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 4.

[0150] (Example D4) [N material, polysulfide pulping method] First, lecithin A3, a component of the digestion accelerator, was emulsified in water to prepare an aqueous emulsion of lecithin A3 (digestion accelerator aqueous emulsion) with a lecithin content of 10% by mass. Since lecithin A3 is difficult to emulsify in water, as in Example D1, 1.0 part by mass of sodium hydroxide (manufactured by Nacalai Tesque Co., Ltd., reagent) was added to 100 parts by mass of lecithin A3 to mechanically force-emulsify the lecithin A3 in ion-exchanged water.

[0151] In addition, sodium thiosulfate, another component of the digestion accelerator, was dissolved in water to prepare an aqueous solution of sodium thiosulfate (aqueous solution of digestion accelerator).

[0152] Next, 0.075 g (0.0075 g of pure lecithin) of the pulping accelerator aqueous emulsion (lecithin purity concentration: 10% by mass) prepared above was mixed with deionized water to prepare a pulping accelerator aqueous emulsion with a lecithin purity concentration of 0.15% by mass.

[0153] Next, 10.8 g of sodium hydroxide (manufactured by Nacalai Tesque Co., Ltd., reagent), 6.03 g of sodium sulfide pentahydrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent) (2.8 g of sodium sulfide purity), and 4.0 g of sodium tetrasulfide solution (manufactured by Nagao Co., Ltd., 30% by mass solution) (1.2 g of sodium tetrasulfide purity) were placed in a beaker, and then deionized water was added to prepare 145 g of an alkaline aqueous solution of the main digesting agent. To 145 g of this aqueous solution of the main digesting agent, the aqueous emulsion of the digesting accelerator with a lecithin content of 0.15% by mass, prepared above, and the aqueous solution of the digesting accelerator were simultaneously added, such that the mass ratio of lecithin A3 to sodium thiosulfate was 8 / 1, and the amount of digesting accelerator (total amount of pure lecithin and sodium thiosulfate) per 100 parts by mass of wood chips was 0.015 parts by mass. This prepared 150 g of digested liquor with a sodium hydroxide concentration of 7.20% by mass, a sodium sulfide concentration of 1.87% by mass, a sodium tetrasulfide concentration of 0.80% by mass, and a total concentration of pure lecithin and sodium thiosulfate of 0.005% by mass.

[0154] Except for using 150g of this pulping solution, 50g of wood chips made from N-grade coniferous trees (Japanese red pine) were subjected to pulping and washing in the same manner as in Example D1. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 4.

[0155] (Comparative Example D1) [N material, polysulfide pulping method] Except for the absence of a digestion accelerator, 150 g of digestate with a sodium hydroxide concentration of 7.20% by mass, a sodium sulfide concentration of 1.87% by mass, and a sodium tetrasulfide concentration of 0.80% by mass was prepared in the same manner as in Example D1. Furthermore, 50 g of wood chips made from N-grade coniferous trees (Japanese red pine) were digested and washed using this 150 g of digestate. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 4.

[0156] (Comparative example D2) [N material, polysulfide pulping method] Except for using glucose (manufactured by Tokyo Chemical Industry Co., Ltd., reagent) instead of lecithin A3, 150 g of pulp solution was prepared in the same manner as in Example D1, with a sodium hydroxide concentration of 7.20% by mass, a sodium sulfide concentration of 1.87% by mass, a sodium tetrasulfide concentration of 0.80% by mass, and a glucose concentration of 0.005% by mass. Furthermore, 50 g of wood chips made from N-grade coniferous trees (Japanese red pine) were subjected to pulpation and washing treatment using this 150 g pulp solution. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 4.

[0157] (Comparative Example D3) [N material, polysulfide pulping method] Except for not using lecithin A3, 150 g of pulp solution was prepared in the same manner as in Example D2, with a sodium hydroxide concentration of 7.20% by mass, a sodium sulfide concentration of 1.87% by mass, a sodium tetrasulfide concentration of 0.80% by mass, and a sodium thiosulfate concentration of 0.005% by mass. Furthermore, 50 g of wood chips made from N-grade coniferous trees (Japanese red pine) were subjected to pulping and washing treatment using this 150 g pulp solution. The residual wood chips and recovered pulp obtained were used to calculate the residual rate of wood chips and the pulp yield, and the kappa number of the recovered pulp was measured. The results are shown in Table 4.

[0158] [Table 4]

[0159] Comparing Example D1 with Comparative Example D1, it was found that when pulp was produced from wood chips made from N-grade coniferous trees (Japanese red pine) using the polysulfide pulping method, adding lecithin to the pulp solution reduced the wood chip residue rate, improved the pulp yield, and lowered the copper value. This confirmed that lecithin acts as a pulping accelerator in the polysulfide pulping treatment of N-grade wood.

[0160] Furthermore, comparing Examples D2-D4 with Example D1, it was found that adding a sulfur-containing compound along with lecithin to the pulp solution further reduced the wood chip residue rate, improved the pulp yield, and further reduced the copper number, indicating that polysulfide pulping of N wood was further promoted. On the other hand, when only a sulfur-containing compound was added to the pulp solution (Comparative Example D3), polysulfide pulping of N wood was not promoted.

[0161] Furthermore, when lecithin and a sulfur-containing compound were pre-mixed and added to the pulp (Example D2), the wood chip residue rate decreased, the pulp yield improved, and the copper value decreased compared to when lecithin and the sulfur-containing compound were added separately and simultaneously to the pulp (Example D4). This indicates that pre-mixing lecithin and a sulfur-containing compound and adding them to the pulp further promotes polysulfide pulping of N-grade wood. [Industrial applicability]

[0162] As described above, according to the present invention, a digestion accelerator that can efficiently digest lignocellulose material is obtained when producing pulp by digesting lignocellulose material. Therefore, the pulp production method of the present invention is useful as a method that can efficiently pulp lignocellulose material because it uses such a digestion accelerator.

Claims

1. A digestion accelerator characterized by containing at least one of lecithin and its derivatives, and phosphatidylcholine.

2. The digestion accelerator according to claim 1, further comprising a sulfur-containing compound.

3. The digestion accelerator according to claim 2, characterized in that the sulfur-containing compound is at least one salt selected from the group consisting of hydrogen sulfide, sulfite, bisulfite, disulfite, thiosulfate, dithionite, dithionate, disulfate, peroxomonosulfate, peroxodisulfate, and polythionate.

4. A method for producing pulp, comprising the steps of: preparing a digestate containing a digestion accelerator according to any one of claims 1 to 3; and performing a digestion treatment using the digestate.

5. The method for producing pulp according to claim 4, characterized in that, in the step of preparing a digestate containing the digestion accelerator according to claim 2 or 3, at least one of the lecithin and its derivatives, and the phosphatidylcholine, is mixed with the sulfur-containing compound beforehand and then added to the digestate.

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