Office waste paper pulp deinking process based on matching of deinking agent and flotation equipment

Abstract

The invention discloses an office waste paper pulp deinking process based on matching of a deinking agent and flotation equipment, which comprises the following steps: (1) primary flotation deinking treatment: adopting a vertical flotation tank as primary flotation equipment, and adding a nonionic surface active deinking agent into the primary flotation equipment; (2) secondary flotation deinking treatment: adopting a horizontal flotation tank as secondary flotation equipment, and adding a composite deinking agent into the horizontal flotation tank; and (3) three-stage flotation deinking treatment: a horizontal flotation tank is adopted as three-stage flotation equipment, the three-stage flotation equipment is provided with a foam generator, the residual composite deinking agent in the step (2) is utilized to play a deinking role, and the deinked office waste paper pulp is obtained. According to the technical scheme provided by the invention, the final total ink removal rate is obviously improved through accurate matching of the deinking agent type and the flotation equipment type.

Description

Technical Field

[0001] This invention relates to the field of papermaking technology, and in particular to a deinking process for office waste paper pulp based on the matching of deinking agent and flotation equipment. Background Technology

[0002] Flotation deinking is a mainstream technology for deinking office waste paper pulp. Its principle is to use a deinking agent to peel ink particles from the fiber surface, and then use the foam generated by the flotation equipment to adsorb and carry the ink particles away from the pulp system. Currently, the flotation deinking process for office waste paper pulp suffers from a mismatch between the deinking agent and the flotation equipment. In existing technologies, the selection of deinking agents largely relies on experience and is not adapted to the characteristics of the flotation equipment (such as the difference between vertical and horizontal structures), resulting in a low ink removal rate (typically 70-75%). Vertical flotation cells have a closed structure and require a large amount of foam to ensure ink capture efficiency, but commonly used composite deinking agents do not produce enough foam, leading to low primary deinking efficiency. Horizontal flotation cells have an open structure and are sensitive to the amount of foam (excessive foam can easily lead to backwashing and equipment damage), but traditional surfactant-based deinking agents produce too much foam, which can easily cause operational malfunctions and affect the stability of subsequent secondary and tertiary deinking.

[0003] In view of the above technical problems, this invention proposes a deinking process for office waste paper pulp based on the matching of deinking agent and flotation equipment. By clarifying the compatibility between the foam requirements of different types of flotation equipment and the characteristics of deinking agent, the problem of low deinking efficiency and unstable equipment operation in existing processes is solved, thereby achieving efficient and stable deinking of office waste paper pulp. Summary of the Invention

[0004] In view of the shortcomings of the prior art, the purpose of this invention is to provide a deinking process for office waste paper pulp based on the matching of deinking agent and flotation equipment, which aims to solve the technical problem of low deinking efficiency caused by the mismatch between deinking agent and flotation equipment in the prior art.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A deinking process for office waste paper pulp based on the matching of deinking agent and flotation equipment includes the following steps: (1) Primary flotation deinking treatment of office waste paper pulp: A vertical flotation cell is used as the primary flotation equipment. A nonionic surfactant deinking agent is added to the primary flotation equipment. The flotation time is controlled at 3~5 min, the flotation temperature at 57~61℃, and the office waste paper pulp concentration at 1.2%~1.6% to complete the primary flotation deinking. The specific gravity of the nonionic surfactant deinking agent is 0.93~0.96, the pH value is 8.0~8.5, and the viscosity is 4~6 mPa·s. (2) The office waste paper pulp treated in step (1) is subjected to secondary flotation deinking treatment: a horizontal flotation cell is used as the secondary flotation equipment. The secondary flotation equipment is equipped with a foam generator. A composite deinking agent is added to the horizontal flotation cell. The flotation time is controlled to be 2~4 min, the flotation temperature is 56~60℃, and the office waste paper pulp concentration is 1.0%~1.5% to complete the secondary flotation deinking. The specific gravity of the composite deinking agent is 1.00~1.05, the pH value is 6.7~7.2, and the viscosity is 23~30 mPa·s. (3) The office waste paper pulp after step (2) is subjected to three-stage flotation deinking treatment: a horizontal flotation cell is used as the three-stage flotation equipment. The three-stage flotation equipment is equipped with a foam generator. The residual composite deinking agent in step (2) is used to play the deinking role. The flotation time is controlled at 2~4 min, the flotation temperature is 56~60℃, and the office waste paper pulp concentration is 0.8%~1.2%. The three-stage flotation deinking is completed to obtain the deinked office waste paper pulp.

[0006] Further, in step (1), the nonionic surfactant deinking agent is a fatty alcohol polyoxyethylene ether nonionic surfactant.

[0007] Furthermore, in step (1), the bottom of the primary flotation equipment is equipped with a stirring device with a stirring speed of 300~500 r / min.

[0008] Furthermore, in step (2), the composite deinking agent is composed of surfactants, soap compounds and detergents.

[0009] Furthermore, in step (2), the air intake of the foam generator is 0.5~1.5m³. 3 / h.

[0010] Furthermore, in step (2), the amount of composite deinking agent added to the horizontal flotation cell is 0.1% to 0.3% of the oven-dry weight of the office waste paper pulp.

[0011] Furthermore, the liquid level in the secondary flotation unit is higher than that in the tertiary flotation unit, and the pulp enters the tertiary flotation unit from the secondary flotation unit by gravity flow caused by the liquid level difference.

[0012] Beneficial Effects: This invention provides a deinking process for office waste paper pulp based on the matching of deinking agents and flotation equipment. Through precise matching of the type of deinking agent and the type of flotation equipment, it offers the following main advantages compared to conventional deinking solutions in the prior art: (1) Significantly improved deinking efficiency: The first stage flotation uses vertical flotation equipment and is matched with non-ionic surface active deinking agent, while the second and third stages use horizontal flotation equipment and are matched with composite deinking agent, so that the final total ink removal rate is stable at more than 80%, which is significantly higher than the average level of 70-75% of existing technologies, and meets the high cleanliness requirements of office waste pulp recycling.

[0013] (2) The three-stage flotation relies on the composite deinking agent (residual concentration ≥0.05%) remaining from the two-stage flotation to achieve "fine deinking". No additional deinking agent is needed, which avoids waste of deinking agent and reduces the subsequent washing load, thus achieving a balance between deinking effect and cost. Detailed Implementation

[0014] To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the invention is further described in detail below. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of the invention.

[0015] The aforementioned deinking process for office waste paper pulp based on the matching of deinking agent and flotation equipment includes the following steps: (1) First-stage flotation deinking treatment of office waste paper pulp: A vertical flotation cell is used as the first-stage flotation equipment. Non-ionic surfactant deinking agent is added to the first-stage flotation equipment. The flotation time is controlled at 3~5 min, the flotation temperature is controlled at 57~61℃, and the office waste paper pulp concentration (referring to mass concentration) is controlled at 1.2%~1.6% (which can be monitored in real time by an online concentration sensor. When the concentration is lower than 1.2%, dry pulp is added, and when it is higher than 1.6%, hot water at 60~65℃ is added for adjustment). Complete the first-stage flotation deinking; the specific gravity of the nonionic surfactant deinking agent is 0.93~0.96, the pH value is 8.0~8.5, and the viscosity is 4~6 mPa·s; in practical applications, during the first-stage flotation deinking process, the foam state can be monitored in real time through the observation window on the side wall of the tank to ensure that the foam layer thickness is maintained at 10~20cm and the foam is uniform and fine (bubble diameter 1~3mm). If the foam is too coarse, the amount of deinking agent added should be increased appropriately; if the foam is too sparse, the stirring speed should be reduced.

[0016] (2) The office waste paper pulp treated in step (1) is subjected to secondary flotation deinking treatment: a horizontal flotation cell is used as the secondary flotation equipment. The secondary flotation equipment is equipped with a foam generator (preferably a Venturi foam generator). A composite deinking agent is added to the horizontal flotation cell. The flotation time is controlled to be 2~4 min, the flotation temperature is 56~60℃, and the office waste paper pulp concentration is 1.0%~1.5% to complete the secondary flotation deinking. The specific gravity of the composite deinking agent is 1.00~1.05, the pH value is 6.7~7.2, and the viscosity is 23~30 mPa·s. (3) The office waste paper pulp after step (2) is subjected to three-stage flotation deinking treatment: a horizontal flotation cell is used as the three-stage flotation equipment. The three-stage flotation equipment is equipped with a foam generator (preferably a Venturi foam generator). The residual composite deinking agent in step (2) is used to play the deinking role. The flotation time is controlled at 2~4 min, the flotation temperature is 56~60℃, and the office waste paper pulp concentration is 0.8%~1.2%. The three-stage flotation deinking is completed to obtain the deinked office waste paper pulp.

[0017] In steps (1) to (3) above, the flotation temperature can be controlled by circulating hot water in the tank jacket of the flotation equipment, or by heating the bottom of the tank of the flotation equipment with electric heating tubes. The temperature fluctuation shall not exceed ±0.5℃.

[0018] Further, in step (1), the nonionic surfactant deinking agent is a fatty alcohol polyoxyethylene ether nonionic surfactant. Preferably, the fatty alcohol polyoxyethylene ether nonionic surfactant has an HLB value of 10-12, possessing both good ink stripping ability and foam stability; the amount of this deinking agent added is 0.08%-0.15% of the oven-dry weight of office waste paper pulp. In practical applications, it can be accurately delivered using a metering pump (metering accuracy of ±0.1mL / min) to avoid fluctuations in the amount added affecting the deinking effect.

[0019] Furthermore, in step (1), the bottom of the primary flotation equipment is equipped with a stirring device with a stirring speed of 300~500 r / min. In practical applications, the stirring resistance can be monitored in real time by a torque sensor. If the resistance exceeds the upper limit of the normal range, it is determined that there are sediments at the bottom of the tank. The speed should be increased to 550~600 r / min, the sediments should be removed, and the original speed should be restored.

[0020] Further, in step (2), the composite deinking agent is composed of a surfactant, a soap compound, and a detergent builder. The surfactant can efficiently peel off and disperse the ink, preventing ink agglomeration; the soap compound can enhance the ability to capture dispersed ink particles, ensuring that the ink detaches with the foam; the detergent builder can adjust the pH of the system to a suitable range of 6.7~7.2, while chelating metal ions in the pulp to prevent ink from re-adhering to the fibers. Preferably, the surfactant is sodium dodecylbenzenesulfonate, the soap compound is sodium stearate, and the detergent builder is sodium tripolyphosphate.

[0021] Furthermore, in step (2), the air intake of the foam generator is automatically controlled by an electromagnetic flow valve, and the air intake of the foam generator is 0.5~1.5m³. 3 / h. As a preferred option, the ratio of air intake to pulp feed is maintained at 1:8 to 1:12 (the pulp feed can be collected in real time by an electromagnetic flow meter, and the air intake is adjusted accordingly with the pulp feed) to avoid uncontrolled foaming due to fluctuations in air intake.

[0022] Furthermore, in step (2), the amount of composite deinking agent added to the horizontal flotation cell is 0.1% to 0.3% of the oven-dry weight of the office waste paper pulp. The horizontal flotation cell is sensitive to the amount of foam; excessive foam can easily cause backwashing and equipment failure. When the amount of composite deinking agent exceeds 0.3% of the oven-dry weight of the office waste paper pulp, the total amount of surfactant will still lead to excessive foam; when the amount is less than 0.1%, the amount of foam is insufficient and ink cannot be effectively captured. The amount of 0.1% to 0.3% can accurately control the thickness of the foam layer in the cell, which avoids equipment operation risks and ensures the foam's ability to adsorb ink.

[0023] Furthermore, the liquid level in the secondary flotation unit is higher than that in the tertiary flotation unit. The pulp flows from the secondary flotation unit into the tertiary flotation unit by gravity due to the liquid level difference. Since the tertiary flotation relies on the residual composite deinking agent from the secondary flotation unit, if a pump is used to transport the pulp, the mechanical shear force of the pump will disrupt the stable foam structure, causing ink particles to detach from the foam and re-mix into the pulp. Simultaneously, it may break down the residual deinking agent molecules, reducing its subsequent ink-capturing ability. Gravity flow, naturally transported by the liquid level difference without mechanical interference, can completely preserve the foam state and residual deinking agent concentration in the pulp after secondary flotation, providing stable initial conditions for the "fine deinking" of the tertiary flotation and ensuring a continuous and efficient deinking process.

[0024] To facilitate understanding, the following examples are provided for further illustration.

[0025] Example 1 (1) Primary flotation deinking treatment of office waste paper pulp: A vertical flotation cell is used as the primary flotation equipment. A nonionic surfactant deinking agent is added to the primary flotation equipment. The flotation time is controlled at 3 min, the flotation temperature at 57℃, and the office waste paper pulp concentration at 1.2% to complete the primary flotation deinking. The specific gravity of the nonionic surfactant deinking agent is 0.93, the pH value is 8.0, and the viscosity is 4 mPa·s. The stirring speed of the primary flotation equipment is 300 r / min, and the amount of deinking agent added is 0.08% of the oven-dry weight of the pulp.

[0026] (2) Secondary flotation deinking treatment of the office waste paper pulp after step (1): A horizontal flotation cell is used as the secondary flotation equipment. The secondary flotation equipment is equipped with a foam generator. A composite deinking agent is added to the horizontal flotation cell. The flotation time is controlled at 2 min, the flotation temperature at 56℃, and the office waste paper pulp concentration at 1.0% to complete the secondary flotation deinking. The specific gravity of the composite deinking agent is 1.00, the pH value is 6.7, and the viscosity is 23 mPa·s. The amount of deinking agent added is 0.1% of the oven-dry weight of the pulp. (3) The office waste paper pulp after step (2) is subjected to three-stage flotation deinking treatment: a horizontal flotation cell is used as the three-stage flotation equipment. The three-stage flotation equipment is equipped with a foam generator. The composite deinking agent remaining in step (2) is used to play the deinking role. The flotation time is controlled at 2 min, the flotation temperature is 56℃, and the office waste paper pulp concentration is 0.8%. The three-stage flotation deinking is completed to obtain the deinked office waste paper pulp.

[0027] Example 2 (1) Primary flotation deinking treatment of office waste paper pulp: A vertical flotation cell is used as the primary flotation equipment. A nonionic surfactant deinking agent is added to the primary flotation equipment. The flotation time is controlled at 4 min, the flotation temperature at 59℃, and the office waste paper pulp concentration at 1.4% to complete the primary flotation deinking. The specific gravity of the nonionic surfactant deinking agent is 0.95, the pH value is 8.3, and the viscosity is 5 mPa·s. The stirring speed of the primary flotation equipment is 400 r / min, and the amount of deinking agent added is 0.12% of the dry weight of the pulp.

[0028] (2) Secondary flotation deinking treatment of the office waste paper pulp after step (1): A horizontal flotation cell is used as the secondary flotation equipment. The secondary flotation equipment is equipped with a foam generator. A composite deinking agent is added to the horizontal flotation cell. The flotation time is controlled at 3 min, the flotation temperature at 58℃, and the office waste paper pulp concentration at 1.3% to complete the secondary flotation deinking. The specific gravity of the composite deinking agent is 1.03, the pH value is 7.0, and the viscosity is 27 mPa·s. The amount of deinking agent added is 0.2% of the oven-dry weight of the pulp. (3) The office waste paper pulp after step (2) is subjected to three-stage flotation deinking treatment: a horizontal flotation cell is used as the three-stage flotation equipment. The three-stage flotation equipment is equipped with a foam generator. The composite deinking agent remaining in step (2) is used to play the deinking role. The flotation time is controlled at 3 min, the flotation temperature is 58℃, and the office waste paper pulp concentration is 1.0%. The three-stage flotation deinking is completed to obtain the deinked office waste paper pulp.

[0029] Example 3 (1) Primary flotation deinking treatment of office waste paper pulp: A vertical flotation cell is used as the primary flotation equipment. A nonionic surfactant deinking agent is added to the primary flotation equipment. The flotation time is controlled at 5 min, the flotation temperature at 61℃, and the office waste paper pulp concentration at 1.6% to complete the primary flotation deinking. The specific gravity of the nonionic surfactant deinking agent is 0.96, the pH value is 8.5, and the viscosity is 6 mPa·s. The stirring speed of the primary flotation equipment is 500 r / min, and the amount of deinking agent added is 0.15% of the dry weight of the pulp.

[0030] (2) Secondary flotation deinking treatment of the office waste paper pulp after step (1): A horizontal flotation cell is used as the secondary flotation equipment. The secondary flotation equipment is equipped with a foam generator. A composite deinking agent is added to the horizontal flotation cell. The flotation time is controlled at 4 min, the flotation temperature at 60℃, and the office waste paper pulp concentration at 1.5% to complete the secondary flotation deinking. The specific gravity of the composite deinking agent is 1.05, the pH value is 7.2, and the viscosity is 30 mPa·s. The amount of deinking agent added is 0.3% of the oven-dry weight of the pulp. (3) The office waste paper pulp after step (2) is subjected to three-stage flotation deinking treatment: a horizontal flotation cell is used as the three-stage flotation equipment. The three-stage flotation equipment is equipped with a foam generator. The composite deinking agent remaining in step (2) is used to play the deinking role. The flotation time is controlled at 4 min, the flotation temperature is 60℃, and the office waste paper pulp concentration is 1.2%. The three-stage flotation deinking is completed to obtain the deinked office waste paper pulp.

[0031] Example 4 (1) Primary flotation deinking treatment of office waste paper pulp: A vertical flotation cell is used as the primary flotation equipment. A nonionic surfactant deinking agent is added to the primary flotation equipment. The flotation time is controlled at 3 min, the flotation temperature at 57℃, and the office waste paper pulp concentration at 1.2% to complete the primary flotation deinking. The specific gravity of the nonionic surfactant deinking agent is 0.93, the pH value is 8.0, and the viscosity is 4 mPa·s. The stirring speed of the primary flotation equipment is 300 r / min, and the amount of deinking agent added is 0.08% of the oven-dry weight of the pulp.

[0032] (2) Secondary flotation deinking treatment of the office waste paper pulp after step (1): A horizontal flotation cell is used as the secondary flotation equipment. The secondary flotation equipment is equipped with a foam generator. A composite deinking agent is added to the horizontal flotation cell. The flotation time is controlled at 3 min, the flotation temperature at 58℃, and the office waste paper pulp concentration at 1.3% to complete the secondary flotation deinking. The specific gravity of the composite deinking agent is 1.03, the pH value is 7.0, and the viscosity is 27 mPa·s. The amount of deinking agent added is 0.2% of the oven-dry weight of the pulp. (3) The office waste paper pulp after step (2) is subjected to three-stage flotation deinking treatment: a horizontal flotation cell is used as the three-stage flotation equipment. The three-stage flotation equipment is equipped with a foam generator. The composite deinking agent remaining in step (2) is used to play the deinking role. The flotation time is controlled at 3 min, the flotation temperature is 58℃, and the office waste paper pulp concentration is 1.0%. The three-stage flotation deinking is completed to obtain the deinked office waste paper pulp.

[0033] Example 5 (1) Primary flotation deinking treatment of office waste paper pulp: A vertical flotation cell is used as the primary flotation equipment. A nonionic surfactant deinking agent is added to the primary flotation equipment. The flotation time is controlled at 4 min, the flotation temperature at 59℃, and the office waste paper pulp concentration at 1.4% to complete the primary flotation deinking. The specific gravity of the nonionic surfactant deinking agent is 0.95, the pH value is 8.3, and the viscosity is 5 mPa·s. The stirring speed of the primary flotation equipment is 400 r / min, and the amount of deinking agent added is 0.12% of the dry weight of the pulp.

[0034] (2) Secondary flotation deinking treatment of office waste paper pulp after step (1): A horizontal flotation cell is used as the secondary flotation equipment. The secondary flotation equipment is equipped with a foam generator. A composite deinking agent is added to the horizontal flotation cell. The flotation time is controlled at 2 min, the flotation temperature at 57℃, and the office waste paper pulp concentration at 1.0% to complete the secondary flotation deinking. The specific gravity of the composite deinking agent is 1.00, the pH value is 6.7, and the viscosity is 23 mPa·s. The amount of deinking agent added is 0.2% of the dry weight of the pulp. (3) The office waste paper pulp after step (2) is subjected to three-stage flotation deinking treatment: a horizontal flotation cell is used as the three-stage flotation equipment. The three-stage flotation equipment is equipped with a foam generator. The composite deinking agent remaining in step (2) is used to play the deinking role. The flotation time is controlled at 2 min, the flotation temperature is 56℃, and the office waste paper pulp concentration is 0.8%. The three-stage flotation deinking is completed to obtain the deinked office waste paper pulp.

[0035] The detection results of the above five embodiments are shown in the table below.

[0036] Testing items Example 1 Example 2 Example 3 Example 4 Example 5 Ink residue after primary flotation (ERIC value) 62 59 57 63 60 Ink residue after secondary flotation (ERIC value) 43 39 36 40 42 Ink residue after three-stage flotation (ERIC value) 22 18 16 20 21 Total ink removal rate 81% 86% 88% 84% 84% Equipment operating status Stable operation Stable operation Stable operation Stable operation Stable operation The test results above show that the total ink removal rate of each embodiment is ≥81%, with Example 3 reaching 88% and Example 2 reaching 86%, which is significantly better than the 70-75% total ink removal rate caused by the mismatch between the deinking agent and the equipment in the prior art. This indicates that the matching logic of vertical flotation equipment with nonionic surfactant deinking agent and horizontal flotation equipment with composite deinking agent in this application is effective.

[0037] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any way. It is understood that those skilled in the art can make equivalent substitutions or modifications based on the technical solution and inventive concept of the present invention, and all such modifications or substitutions should fall within the protection scope of the present invention.

Claims

1. A deinking process for office waste paper pulp based on the matching of deinking agent and flotation equipment, characterized in that, Includes the following steps: (1) Primary flotation deinking treatment of office waste paper pulp: A vertical flotation cell is used as the primary flotation equipment. A nonionic surfactant deinking agent is added to the primary flotation equipment. The flotation time is controlled at 3~5 min, the flotation temperature at 57~61℃, and the office waste paper pulp concentration at 1.2%~1.6% to complete the primary flotation deinking. The specific gravity of the nonionic surfactant deinking agent is 0.93~0.96, the pH value is 8.0~8.5, and the viscosity is 4~6 mPa·s. (2) The office waste paper pulp treated in step (1) is subjected to secondary flotation deinking treatment: a horizontal flotation cell is used as the secondary flotation equipment. The secondary flotation equipment is equipped with a foam generator. A composite deinking agent is added to the horizontal flotation cell. The flotation time is controlled to be 2~4 min, the flotation temperature is 56~60℃, and the office waste paper pulp concentration is 1.0%~1.5% to complete the secondary flotation deinking. The specific gravity of the composite deinking agent is 1.00~1.05, the pH value is 6.7~7.2, and the viscosity is 23~30 mPa·s. (3) The office waste paper pulp after step (2) is subjected to three-stage flotation deinking treatment: a horizontal flotation cell is used as the three-stage flotation equipment. The three-stage flotation equipment is equipped with a foam generator. The residual composite deinking agent in step (2) is used to play the deinking role. The flotation time is controlled at 2~4 min, the flotation temperature is 56~60℃, and the office waste paper pulp concentration is 0.8%~1.2%. The three-stage flotation deinking is completed to obtain the deinked office waste paper pulp.

2. The office waste paper pulp deinking process based on the matching of deinking agent and flotation equipment according to claim 1, characterized in that, In step (1), the nonionic surfactant deinking agent is a fatty alcohol polyoxyethylene ether nonionic surfactant.

3. The office waste paper pulp deinking process based on the matching of deinking agent and flotation equipment according to claim 1, characterized in that, In step (1), the bottom of the primary flotation equipment is equipped with a stirring device with a stirring speed of 300~500 r / min.

4. The office waste paper pulp deinking process based on the matching of deinking agent and flotation equipment according to claim 1, characterized in that, In step (2), the composite deinking agent is composed of surfactants, soap compounds and detergents.

5. The office waste paper pulp deinking process based on the matching of deinking agent and flotation equipment according to claim 1, characterized in that, In step (2), the air intake of the foam generator is 0.5~1.5m³. 3 / h.

6. The office waste paper pulp deinking process based on the matching of deinking agent and flotation equipment according to claim 1, characterized in that, In step (2), the amount of composite deinking agent added to the horizontal flotation cell is 0.1% to 0.3% of the dry weight of the office waste paper pulp.

7. The office waste paper pulp deinking process based on the matching of deinking agent and flotation equipment according to claim 1, characterized in that, The liquid level in the secondary flotation unit is higher than that in the tertiary flotation unit. The pulp flows from the secondary flotation unit into the tertiary flotation unit by gravity due to the liquid level difference.