A process for leaching copper and cobalt from finely divided oxidic copper-cobalt slimes

Abstract

The present application relates to the technical field of mineral processing, and particularly discloses a method for leaching copper and cobalt from fine-grained oxidized copper-cobalt slime. The method of the present application first pre-treats the fine-grained oxidized copper-cobalt slime with dilute acid to effectively remove the impurities entrained in the slime; then uses a combined reducing agent for intensified leaching, while adding a surfactant, and controls the temperature of the slurry by programmed temperature rise, so that copper and cobalt are effectively leached; and then solid-liquid separation is performed to obtain a copper-cobalt-containing leaching solution. The method of the present application improves the leaching rate and recovery rate of copper and cobalt, provides a new approach for efficient recovery of copper and cobalt resources from fine-grained oxidized copper-cobalt slime, and has significant economic benefits.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of mineral processing, in particular to a method for leaching copper and cobalt from fine-grained oxidized copper-cobalt slime. BACKGROUND

[0002] Copper and cobalt, as strategic mineral resources, play a crucial role in the development of new energy, 5G communication, aerospace and national defense industry, and are directly related to the national economy and strategic security. In recent years, copper and cobalt enrichment, separation and extraction technology has been continuously developed and has made significant progress, but still faces problems such as low recovery rate, gradient separation and difficulty in deep impurity removal.

[0003] For example, when processing oxidized copper-cobalt ore, fine grinding is necessary to achieve single dissociation due to the fine dissemination size, but fine grinding will cause poor floatability of fine-grained oxidized copper-cobalt ore and reduce the recovery rate, and is accompanied by various interference factors in the flotation process, such as poor recovery index of slime and fine particle chrysocolla, making it difficult for traditional flotation process to effectively recover copper and cobalt from fine-grained oxidized copper-cobalt slime. Especially, cobalt minerals mainly occur in limonite, copper-cobalt hard manganite and water cobalt ore, and trivalent cobalt ions are difficult to be directly acid leached due to their high stability, resulting in low cobalt recovery rate. When using normal temperature or heating stirring acid leaching combined with sodium pyrosulfite reduction leaching process to process oxidized copper-cobalt ore, the leaching rate of cobalt is also low, and when the temperature is increased to 70℃, the leaching rate can reach about 80%. The existing method has generally poorer treatment effect when used to treat fine-grained oxidized copper-cobalt slime, which seriously restricts the recovery and utilization of cobalt resources.

[0004] Based on the above problems, the present application aims to develop a treatment method for fine-grained oxidized copper-cobalt slime to achieve simultaneous and effective leaching of copper and cobalt therein. SUMMARY

[0005] The technical problem solved by the present application is to provide a method for leaching copper and cobalt from fine-grained oxidized copper-cobalt slime, which can effectively recover and utilize copper and cobalt resources in fine-grained oxidized copper-cobalt slime.

[0006] To solve the above technical problems, the technical solution adopted by the present application is: a method for leaching copper and cobalt from fine-grained oxidized copper-cobalt slime, comprising the steps of:

[0007] S1: pretreatment of fine-grained oxidized copper-cobalt slime

[0008] Mixing fine-grained oxidized copper-cobalt slime with dilute sulfuric acid solution and stirring for pretreatment;

[0009] S2: intensified leaching

[0010] The slurry obtained in step S1 is adjusted to a pH value of 1.0-2.0, and then combined reducing agents and surfactants are added, and temperature-controlled stirring leaching is performed;

[0011] S3: separation to obtain a copper-cobalt-containing leaching solution

[0012] The slurry obtained in step S2 is subjected to solid-liquid separation, and a copper-cobalt-containing leaching solution and a leaching residue are obtained respectively.

[0013] The combined reducing agent is at least two or more of sodium pyrosulfite, sodium thiosulfate and sodium sulfite.

[0014] The surfactant is at least one of Tween 20, sodium dodecyl sulfonate and cetyltrimethylammonium bromide.

[0015] As an embodiment of the present application, the combined reducing agent is a mixture of sodium pyrosulfite and sodium sulfite, and the mixing mass ratio is: sodium pyrosulfite:sodium sulfite=(3-6):1.

[0016] As an embodiment of the present application, in step S1, the concentration of the dilute sulfuric acid solution is 0.3-2.0 mol / L, and the dilute sulfuric acid solution is mixed with the fine-grained copper-cobalt oxide slime at a mass ratio of 2:1-5:1.

[0017] As an embodiment of the present application, in step S1, the stirring time is 1-2 hours.

[0018] As an embodiment of the present application, in step S2, the pH value is adjusted using concentrated sulfuric acid.

[0019] As an embodiment of the present application, in step S2, the combined reducing agent is added in an amount of 0.01%-0.10% of the mass of the fine-grained copper-cobalt oxide slime in step S1.

[0020] As an embodiment of the present application, in step S2, the surfactant is added in an amount of 10-60 mg per liter of the slurry obtained in step S1.

[0021] As an embodiment of the present application, in step S2, the temperature-controlled stirring leaching is performed by raising the temperature of the reaction system from 18-25°C to 40-60°C within 1-1.5 hours, and then leaching is performed by stirring for 0.5-2.0 hours under temperature maintenance.

[0022] The fine-grained copper-cobalt oxide slime treated in step S1 has a particle size of ≤10 μm.

[0023] The method for leaching copper and cobalt from fine-grained oxidized copper-cobalt slime provided by the application is characterized by the following steps: first, dilute acid pretreatment is performed to effectively remove the carbonate impurities entrained in the slime, so as to prevent a large amount of bubbles from being generated in the subsequent leaching reaction, thereby affecting the reduction leaching effect; then, a combined reducing agent is used for intensified leaching, for example, taking the combined reducing agent of sodium pyrosulfite and sodium sulfite as an example, sodium pyrosulfite is used as the main reducing agent and has strong reducibility, which can effectively reduce the insoluble cobalt minerals and convert them into easily soluble cobalt mineral forms; and sodium sulfite is used as an auxiliary reducing agent, and the combination of the two can further improve the reduction leaching effect of cobalt. The auxiliary reducing agent not only promotes the reduction and dissolution of the cobalt minerals in the reduction process, but also reduces the ineffective decomposition of sodium pyrosulfite to a certain extent, thereby improving the overall reduction leaching efficiency. The surfactant added at the same time can play multiple roles in the oxidation-reduction reaction at the liquid-solid interface, on the one hand, it can enhance the surface activity of the leaching solution, improve the wettability of the liquid and the reducing agent therein to the surface of the fine-grained minerals, thereby increasing the contact area between the reactants and improving the reaction efficiency, and on the other hand, the surfactant can form a foam layer on the liquid surface by a small amount of gas (such as sulfur dioxide and / or carbon dioxide) generated in the reduction leaching process, the foam layer has a certain air isolation effect, which helps to reduce the interference of oxygen on the reduction reaction, and creates a more favorable environment for the reduction reaction, thereby ensuring the effective progress of the reduction reaction. In addition, the reaction temperature of the slurry system is controlled by combining the temperature programming, which can improve the reaction rate while effectively avoiding the ineffective decomposition of the reducing agent, so that copper and cobalt are effectively leached, and efficient recovery of copper and cobalt resources is realized.

[0024] The method improves the leaching rate and recovery rate of copper and cobalt, and provides a new way for efficient recovery of copper and cobalt resources in fine-grained oxidized copper-cobalt slime, which has significant economic benefits. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 The process flowchart used in examples 1-3 of the application is shown in the figure. DETAILED DESCRIPTION

[0026] The technical solutions of the application will be described in detail below through specific examples. The following examples are intended to further illustrate the content of the application, but not to limit the scope of protection of the claims of the application.

[0027] In the following examples, all the reagents used are commercially available. When not specifically stated, the percentage content in each example is the mass percentage.

[0028] Example 1

[0029] The embodiment provides a method for leaching copper and cobalt from fine-grained oxidized copper-cobalt slime, and the particle size of the treated fine-grained oxidized copper-cobalt slime sample ranges from 0.7 to 10 microns, the mass percentage of copper is 3.64%, and the mass percentage of cobalt is 0.096%. The method comprises the following steps: Figure 1 The process shown in the figure is used for treating the fine-grained oxidized copper-cobalt slime sample, and the specific steps are as follows:

[0030] S1: The fine-grained oxidized copper-cobalt slime is mixed with a dilute sulfuric acid solution with a concentration of 2.0 mol / L, wherein the mass ratio of the dilute sulfuric acid solution to the fine-grained oxidized copper-cobalt slime is 3:1, and the mixture is stirred to form a uniform slurry, and then the slurry is continuously stirred for 1 hour, at which time no bubbles are generated in the slurry, and the pretreatment is completed;

[0031] S2: Concentrated sulfuric acid (98% in mass percentage) is added to the slurry obtained in step S1, and the pH value of the reaction slurry is adjusted to 1.5; a combined reducing agent, sodium pyrosulfite and sodium sulfite (mass ratio of sodium pyrosulfite to sodium sulfite=3:1), is added to the reaction slurry, and the amount of the combined reducing agent is 0.08% of the mass of the fine-grained oxidized copper-cobalt slime in step S1; a surfactant, Tween 20, is added, and the amount of the surfactant is 30 mg per liter of the slurry obtained in step S1; then the temperature of the reaction slurry is slowly increased from room temperature (about 20°C) to 40°C within 1 hour, and the reaction slurry is kept at 40°C for 1 hour, and the leaching reaction is performed;

[0032] S3: After the leaching reaction is completed, the slurry obtained in the reaction is subjected to solid-liquid separation, and a copper-cobalt-containing leaching solution and a leaching residue are obtained; the leaching residue is washed twice with a dilute sulfuric acid solution, and a tailing residue is obtained.

[0033] Through testing of the tailing residue, it is found that the leaching rate of copper in the fine-grained oxidized copper-cobalt slime is 95.1%, and the leaching rate of cobalt is 87.5%.

[0034] Comparative Example 1

[0035] The comparative example provides a method for leaching copper and cobalt from fine-grained oxidized copper-cobalt slime, and the fine-grained oxidized copper-cobalt slime sample is the same as that in Example 1, and the treatment method is direct sulfuric acid leaching, and the treatment steps are as follows:

[0036] S1: The fine-grained oxidized copper-cobalt slime is mixed with a dilute sulfuric acid solution with a concentration of 2.0 mol / L, wherein the mass ratio of the dilute sulfuric acid solution to the fine-grained oxidized copper-cobalt slime is 3:1, and the mixture is stirred to form a uniform slurry;

[0037] S2: The slurry obtained in step S1 is heated from room temperature (about 20°C) to 40°C, and kept at 40°C for 3 hours, and the leaching reaction is performed.

[0038] S3: After the leaching reaction is completed, the slurry obtained in the reaction is subjected to solid-liquid separation to obtain a copper and cobalt-containing leaching solution and a leaching residue, respectively; the leaching residue is washed twice with a dilute sulfuric acid solution to obtain a tailing residue.

[0039] Through testing of the tailing residue, it is known that the leaching rate of copper in the fine-grained oxidized copper-cobalt slime is 90.9%, and the leaching rate of cobalt is 35.3%.

[0040] Comparative Example 2

[0041] The present comparative example provides a method for leaching copper and cobalt from fine-grained oxidized copper-cobalt slime, and the fine-grained oxidized copper-cobalt slime sample treated is the same as in Example 1, and the treatment method is sulfuric acid + sodium pyrosulfite leaching, and the treatment steps are as follows:

[0042] S1: The fine-grained oxidized copper-cobalt slime is mixed with a dilute sulfuric acid solution with a concentration of 2.0 mol / L, wherein the mass ratio of the dilute sulfuric acid solution to the fine-grained oxidized copper-cobalt slime is 3:1, and stirring is performed to obtain a uniform slurry;

[0043] S2: A reducing agent, sodium pyrosulfite, is added to the slurry obtained in step S1, and the amount of sodium pyrosulfite added is 0.08% of the mass of the fine-grained oxidized copper-cobalt slime in step S1, the reaction slurry is heated from room temperature (about 20°C) to 40°C, and the temperature is maintained for 3 hours while stirring, to perform leaching reaction;

[0044] S3: After the leaching reaction is completed, the slurry obtained in the reaction is subjected to solid-liquid separation to obtain a copper and cobalt-containing leaching solution and a leaching residue, respectively; the leaching residue is washed twice with a dilute sulfuric acid solution to obtain a tailing residue.

[0045] Through testing of the tailing residue, it is known that the leaching rate of copper in the fine-grained oxidized copper-cobalt slime is 94.1%, and the leaching rate of cobalt is 81.4%.

[0046] By comparing Example 1 with Comparative Examples 1 and 2, it is known that the method of the present application has significant advantages and improves the leaching rates of copper and cobalt.

[0047] Example 2

[0048] The present example provides a method for leaching copper and cobalt from fine-grained oxidized copper-cobalt slime, and the fine-grained oxidized copper-cobalt slime sample treated has a particle size range of 0.7-10 microns, wherein the mass percentage content of copper is 3.73%, and the mass percentage content of cobalt is 0.055%, and the method adopts Figure 1 The process flow shown is used to treat the fine-grained oxidized copper-cobalt slime sample, and the specific steps are as follows:

[0049] S1: The fine particle oxidized copper-cobalt slime is mixed with a dilute sulfuric acid solution with a concentration of 2.0 mol / L, wherein the mass ratio of the dilute sulfuric acid solution to the fine particle oxidized copper-cobalt slime is 4:1, and a uniform slurry is prepared by stirring, and then continuous stirring is performed for 1 hour, at which time no bubbles are generated in the slurry, and the pretreatment is completed;

[0050] S2: Concentrated sulfuric acid (98% by mass) is added to the slurry obtained in step S1, and the pH value of the reaction slurry is adjusted to 1.4; a combined reducing agent of sodium pyrosulfite and sodium sulfite (mass ratio of sodium pyrosulfite to sodium sulfite = 3:1) is added to the reaction slurry, and the amount of the combined reducing agent is 0.05% of the mass of the fine particle oxidized copper-cobalt slime in step S1; a surfactant, hexadecyl trimethyl ammonium bromide, is added, and the amount of the surfactant is 30 mg per liter of the slurry obtained in step S1; then the temperature of the reaction slurry is slowly increased from room temperature (about 20°C) to 50°C within 1.5 hours, and the reaction slurry is kept at 50°C for 1 hour, and the leaching reaction is performed;

[0051] S3: After the leaching reaction is completed, the slurry obtained in the reaction is subjected to solid-liquid separation, and a copper-cobalt-containing leaching solution and a leaching residue are obtained; the leaching residue is washed twice with a dilute sulfuric acid solution, and a tailing residue is obtained.

[0052] Through testing of the tailing residue, it is known that the leaching rate of copper in the fine particle oxidized copper-cobalt slime is 97.1%, and the leaching rate of cobalt is 86.6%.

[0053] Comparative Example 3

[0054] The present comparative example provides a method for leaching copper and cobalt from fine particle oxidized copper-cobalt slime, and the fine particle oxidized copper-cobalt slime sample is the same as that in Example 2, and the treatment method is direct sulfuric acid leaching, and the treatment steps are as follows:

[0055] S1: The fine particle oxidized copper-cobalt slime is mixed with a dilute sulfuric acid solution with a concentration of 2.0 mol / L, wherein the mass ratio of the dilute sulfuric acid solution to the fine particle oxidized copper-cobalt slime is 4:1, and a uniform slurry is prepared by stirring;

[0056] S2: The slurry obtained in step S1 is heated from room temperature (about 20°C) to 50°C, and kept at 50°C for 3.5 hours, and the leaching reaction is performed;

[0057] S3: After the leaching reaction is completed, the slurry obtained in the reaction is subjected to solid-liquid separation, and a copper-cobalt-containing leaching solution and a leaching residue are obtained; the leaching residue is washed twice with a dilute sulfuric acid solution, and a tailing residue is obtained.

[0058] Through testing of the tailing residue, it is known that the leaching rate of copper in the fine particle oxidized copper-cobalt slime is 94.9%, and the leaching rate of cobalt is 31.8%.

[0059] Comparative Example 4

[0060] The comparative example 3 provides a method for leaching copper and cobalt from fine-grained oxidized copper-cobalt slime. The fine-grained oxidized copper-cobalt slime sample to be treated is the same as that in Example 2, and the treatment method is sulfuric acid + sodium pyrosulfite leaching. The treatment steps are as follows:

[0061] S1: The fine-grained oxidized copper-cobalt slime is mixed with a dilute sulfuric acid solution with a concentration of 2.0 mol / L, wherein the mass ratio of the dilute sulfuric acid solution to the fine-grained oxidized copper-cobalt slime is 4:1, and stirring is performed to obtain a uniform slurry;

[0062] S2: The slurry obtained in step S1 is added with a reducing agent sodium pyrosulfite, and the amount of sodium pyrosulfite added is 0.05% of the mass of the fine-grained oxidized copper-cobalt slime in step S1. The reaction slurry is heated from room temperature (about 20°C) to 50°C, and the temperature is maintained for 3.5 hours while stirring, so as to perform leaching reaction;

[0063] S3: After the leaching reaction is completed, the slurry obtained by the reaction is subjected to solid-liquid separation, and a copper-cobalt-containing leaching solution and a leaching residue are obtained, respectively. The leaching residue is washed twice with a dilute sulfuric acid solution to obtain a tailing residue.

[0064] Through testing of the tailing residue, it is known that the leaching rate of copper in the fine-grained oxidized copper-cobalt slime is 95.5%, and the leaching rate of cobalt is 80.7%.

[0065] By comparing Example 2 with Comparative Example 3 and Comparative Example 4, it is known that the method has significant advantages and improves the leaching rates of copper and cobalt.

[0066] Example 3

[0067] The example provides a method for leaching copper and cobalt from fine-grained oxidized copper-cobalt slime. The fine-grained oxidized copper-cobalt slime sample to be treated has a particle size range of 0.7-6.5 microns, wherein the mass percentage content of copper is 1.99%, and the mass percentage content of cobalt is 0.050%. The fine-grained oxidized copper-cobalt slime sample is treated by using the process flow shown in the figure, and the specific steps are as follows: Figure 1

[0068] S1: The fine-grained oxidized copper-cobalt slime is mixed with a dilute sulfuric acid solution with a concentration of 2.0 mol / L, wherein the mass ratio of the dilute sulfuric acid solution to the fine-grained oxidized copper-cobalt slime is 4:1, and stirring is performed to obtain a uniform slurry, and then continuous stirring is performed for 1 hour, at which time no bubbles are generated in the slurry, and the pretreatment is completed;

[0069] ​S2: Add concentrated sulfuric acid (mass percentage concentration of 98%) to the slurry obtained in step S1, and adjust the pH value of the reaction slurry to 1.3; add combined reducing agents sodium pyrosulfite and sodium sulfite (mass ratio of sodium pyrosulfite to sodium sulfite = 4:1) to the reaction slurry at this time, the amount of the combined reducing agents added is 0.05% of the mass of the fine-grained copper-cobalt oxide slime in step S1, and add a surfactant cetyltrimethylammonium bromide, the amount of the surfactant added is 30 mg per liter of the slurry obtained in step S1; then slowly heat the temperature of the reaction slurry from room temperature (about 20°C) to 60°C in 1.5 hours, and keep the temperature for 1 hour with stirring to carry out leaching reaction;

[0070] S3: After the leaching reaction is completed, the slurry obtained by the reaction is separated into solid and liquid to obtain a copper-cobalt-containing leaching solution and a leaching residue, respectively; the leaching residue is washed twice with a dilute sulfuric acid solution to obtain a tailing residue.

[0071] Through testing of the tailing residue, it is known that the leaching rate of copper in the fine-grained copper-cobalt oxide slime is 94.9%, and the leaching rate of cobalt is 83.5%.

[0072] Comparative Example 5

[0073] The comparative example provides a method for leaching copper and cobalt from fine-grained copper-cobalt oxide slime, and the fine-grained copper-cobalt oxide slime sample is the same as that in Example 3, and the treatment method is direct leaching with sulfuric acid, and the treatment steps are as follows:

[0074] S1: Mix the fine-grained copper-cobalt oxide slime with a dilute sulfuric acid solution with a concentration of 2.0 mol / L, wherein the mass ratio of the dilute sulfuric acid solution to the fine-grained copper-cobalt oxide slime is 4:1, and stir to obtain a uniform slurry;

[0075] S2: Heat the slurry obtained in step S1 from room temperature (about 20°C) to 60°C, and keep the temperature for 3.5 hours with stirring to carry out leaching reaction;

[0076] S3: After the leaching reaction is completed, the slurry obtained by the reaction is separated into solid and liquid to obtain a copper-cobalt-containing leaching solution and a leaching residue, respectively; the leaching residue is washed twice with a dilute sulfuric acid solution to obtain a tailing residue.

[0077] Through testing of the tailing residue, it is known that the leaching rate of copper in the fine-grained copper-cobalt oxide slime is 94.8%, and the leaching rate of cobalt is 65.3%.

[0078] Comparative Example 6

[0079] The comparative example 5 provides a method for leaching copper and cobalt from fine-grained oxidized copper-cobalt slime, and the treated fine-grained oxidized copper-cobalt slime sample is the same as that in the example 3, the treatment method is sulfuric acid + sodium pyrosulfite leaching, and the treatment steps are as follows:

[0080] S1: mixing the fine-grained oxidized copper-cobalt slime with a dilute sulfuric acid solution with a concentration of 2.0 mol / L, wherein the mass ratio of the dilute sulfuric acid solution to the fine-grained oxidized copper-cobalt slime is 4:1, and stirring to obtain a uniform slurry;

[0081] S2: adding a reducing agent sodium pyrosulfite to the slurry obtained in step S1, wherein the amount of sodium pyrosulfite added is 0.05% of the mass of the fine-grained oxidized copper-cobalt slime in step S1, the reaction slurry is heated from room temperature (about 20℃) to 60℃, and the leaching reaction is carried out by keeping the temperature and stirring for 3.5 hours;

[0082] S3: after the leaching reaction is completed, the slurry obtained in the reaction is separated into a solid and a liquid, and a copper-cobalt-containing leaching solution and a leaching residue are obtained; the leaching residue is washed twice with a dilute sulfuric acid solution to obtain a tailing residue.

[0083] Through the test on the tailing residue, it can be known that the leaching rate of copper in the fine-grained oxidized copper-cobalt slime is 94.8%, and the leaching rate of cobalt is 78.2%.

[0084] By comparing the example 3 with the comparative example 5 and the comparative example 6, it can be known that the method has significant advantages, and the leaching rates of copper and cobalt are improved.

[0085] The above only describes the embodiments of the present application, and does not limit the patent scope of the present application, and any equivalent transformation or direct or indirect application in other related technical fields by using the content of the present application is included in the patent protection scope of the present application.

Claims

1. A method for leaching copper and cobalt from fine-grained copper-cobalt oxide ore slime, characterized in that, Including the following steps: S1: Pretreatment of fine-grained copper-cobalt oxide slime Fine-grained copper-cobalt oxide ore slime was mixed with dilute sulfuric acid solution and stirred for pretreatment; the particle size of the fine-grained copper-cobalt oxide ore slime was ≤10μm. S2: Enhanced leaching The pH of the slurry obtained from the pretreatment in step S1 is adjusted to 1.0~2.0, and then a combination of reducing agent and surfactant is added. Temperature-controlled stirring leaching is carried out. The temperature-controlled stirring leaching is as follows: the temperature of the reaction system is raised from 18~25℃ to 40~60℃ within 1~1.5 hours, and then the temperature is maintained and stirred for 0.5~2.0 hours for leaching. S3: Copper-cobalt leaching solution was obtained by separation. The slurry obtained in step S2 is separated into solid and liquid components to obtain copper-cobalt leaching solution and leaching residue, respectively. The combined reducing agent is a mixture of sodium metabisulfite and sodium sulfite, with a mixing mass ratio of sodium metabisulfite:sodium sulfite = (3~6):1; The surfactant is selected from at least one of Tween 20, sodium dodecyl sulfonate, and hexadecyltrimethylammonium bromide.

2. The method according to claim 1, characterized in that, In step S1, the concentration of the dilute sulfuric acid solution is 0.3~2.0 mol / L, and the dilute sulfuric acid solution is mixed with fine-grained copper-cobalt oxide ore mud at a mass ratio of 2:1~5:

1.

3. The method according to claim 1, characterized in that, In step S1, the stirring time is 1 to 2 hours.

4. The method according to any one of claims 1-3, characterized in that, In step S2, concentrated sulfuric acid is used to adjust the pH value.

5. The method according to claim 4, characterized in that, In step S2, the amount of the combined reducing agent added is 0.01% to 0.10% of the mass of the fine-particle copper-cobalt oxide ore slime in step S1.

6. The method according to claim 5, characterized in that, In step S2, the amount of surfactant added is based on the volume of the slurry obtained from the pretreatment in step S1, with 10-60 mg of surfactant added per liter of the slurry obtained from the pretreatment in step S1.

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