Abstract : The insurance powder is a kind of commonly used reducing pulp bleaching agent. This paper introduces several typical processes and application and research status of the insurance powder in the pulp bleaching, and analyzes the bleaching performance, environmental impact and cost of the insurance powder pulp. Keywords: Pulp and Powder Bleaching Application of Sodium Dithionite in Bleaching of Pulps Jiang Yanbin (Chemical Engineering Research Institute, South China University of Technology, Guangzhou, 510640) Qian Yu (Chemical Engineering Research Institute, South China University of Technology, Guangzhou, 510640) Yu Tianxiang (Chemical Engineering Research Institute, South China University of Technology, Guangzhou, 510640) Zhan Huaiyu (State Key Lab. of Pulp and Paper Engineering, South China University of Technology, Guangzhou, 510641) ABSTRACT: Sodium dithionite is a kind of reductive The bleaching agent which is widely used in pulp bleaching. Several typical sodium dithionite bleaching processes, and its current situation of industrial application and research were introduced in this paper. The characteristics, environmental impact and cost of sodium dithonite bleaching were also prompted briefly.KEYWORDS: Pulp, sodium dithionite, bleachingâ–²
Dithionite is a typical reducing bleaching agent. It was used to bleach mechanical pulp in the early 1930s, and was then used to bleach waste paper deinked pulps and some soda pulps. In the early stage, ZnS2O4 with good stability was mainly used. Due to the contamination of water sources by heavy metal zinc ions, sodium dithionite (Na2S2O4, commonly known as insurance powder) was gradually adopted after the 1970s. The foreign countries have made comprehensive and systematic research on the mechanism, process variables and auxiliary chemicals of the insurance powder pulp bleaching. The research and industrial application of early insurance powder in pulp bleaching was mainly focused on the bleaching of mechanical pulp. In the past 10 years, due to environmental protection and cost reasons, the paper industry in the world has focused attention on recycling of waste paper. In the past, the research and industrial application of the insurance powder in the bleached deinking pulp has also been increasingly reported, mainly focusing on multi-stage bleaching, such as PY two-stage bleaching, (EPO) ZY multi-stage bleaching, E is NaOH, P is H2O2, and O is Oxygen, Z is ozone, Y is the insurance powder [1 ~ 4]. A large number of studies and industrial practices have shown that insurance powder bleached deinked pulp and bleached mechanical pulp have basically the same mechanism, bleaching effect and influencing factors. PY or (EPO) ZY multi-stage bleaching is the most cost-effective deinking pulp bleaching. The method has industrial applications in Germany and North America. In contrast, China's research and application of insurance powder pulp bleaching is less.
For the bleaching mechanism of the insurance powder, the classical view is that the hydrolysis of S2O2-4 produces a lot of new ecology [H] to achieve the purpose of improving whiteness. Further research shows that when the insurance powder bleaches the pulp, the trivalent sulfur in the S2O2-4 root is oxidized into the tetravalent sulfur in the HSO-3 root, ie, S3+→S4++e, and the reduction is mainly used to increase the pulp's BaiDu. The insurance powder is used to bleach the pulp to improve the whiteness of the pulp, and its role is mainly reflected in the following aspects: 1Reducing the terminal saturated carbonyl group at the α,β position; 2Reducing the structure of the germanium in the pulp to make it become colorless Phenol derivatives; 3 The structure of coniferaldehyde in reduced pulp; and the flavonoid-type colored components; 4 The reduction of Fe3+ in pulp to Fe2+; 5 The production of HSO-3 and SO2-3 in the bleaching process act on the ring conjugated double bonds The system produces a bleaching effect on isoeugenol and chalcone compounds [5].
Research and production practices have shown that with the use of bleaching powders, the whiteness of the pulp is increased while there is no negative effect on the strength of the pulp fibers and the opacity of the paper [6], and H2O2 or chlorine-containing bleaches have negative effects to varying degrees. . At the same time, the insurance powder also has a strong role in removing pulp color [7]. 1 Main Process Variables and Typical Process Flows 1.1 Main process variables The process variables for pulp powder bleaching mainly include the pH value, the amount of contaminated powder, bleaching pulp concentration, bleaching temperature, bleaching time, etc. [5]. These variables cannot be considered in isolation. Production should be selected and determined according to specific conditions. 1.1.1 pH It is generally considered that the initial bleaching pH of the suitable insurance powder pulp should be 6-8. It should be determined according to the specific conditions during use. With the decrease of pH value, the ineffective decomposition of the powder is accelerated, but if the pH value is too high, the activity of the powder will be reduced, which is not conducive to pulp bleaching. In the bleaching process, the pH of the pulp will drop by 0.3 to 1.0, depending on the conditions of the treatment, the temperature, the white water circulation system, etc. The more air the pulp touches during bleaching, the greater the pH drop. 1.1.2 The amount of the insurance powder is generally between 0.2% and 2% (for absolute dry pulp). The excessive amount of the insurance powder is of little practical value. At the same time, the concentration of the decomposition powder of the insurance powder in the system circulating white water will increase, resulting in increased corrosion. And the accelerated decomposition of the powder will lead to a decrease in the bleaching effect. 1.1.3 Pulp Concentration The traditional insurance powder pulp bleaching is usually carried out at a low concentration (3% to 5%). Currently, the medium-concentration and high-concentration insurance powder bleaching process has been industrially applied. The insurance powder is easily oxidized by air, and the pulp with high concentration generally contains more air. Therefore, in the middle-high concentration bleaching process, more attention should be paid to the sealing of the equipment and the removal of dissolved oxygen in the pulp. If the pulp concentration is too low, the dissolved oxygen in the water is sufficient to decompose the diluted powder. 1.1.4 Bleaching Temperature The temperature has a significant effect on the bleaching effect. It is not appropriate to use low temperatures. The reason may be that the oxygen content of the pulp suspension is too high at low temperatures. Low-concentration bleaching is generally around 60°C, while medium-to-high-density bleaching is generally performed at about 85°C, and high-concentration bleaching is generally performed at 120°C or higher. In the range of the normal concentration and the amount of the infusible powder, the activity of the chromophoric groups in the reduced powder of the insurance powder increases with the increase of the temperature. At the same time, increasing the temperature is advantageous in eliminating the dissolved oxygen in the pulp and reducing the ineffective decomposition of the insurance powder. Therefore, for each temperature increase of 11 to 14°C, whiteness can be increased by 1% ISO, and the higher the amount of chemical use, the more effective the effect. However, the higher the temperature, the stronger the low-valence sulfur corrosion produced by the decomposition of the insurance powder, and the higher the anti-corrosion requirements for the equipment. 1.1.5 Bleaching time The bleaching time is closely related to the temperature. If the temperature is increased, the time can be shortened. However, the premise is that the bleaching should be carried out in a suitable bleaching equipment. The general bleaching time is 40-65°C for 60 minutes, and the bleaching time for 60-75°C is 30. ~ 45min, 75 ~ 100 °C bleaching time of 10 ~ 15min, more than 100 °C bleaching time is less than 5min. 1.2 Typical process flow and main process variables There are many process flows and equipments for the insurance powder pulp bleaching. The insurance powder can be added directly to the pulp, and the insurance powder can also be added to the mill pit or grinding disc. Single-stage bleaching can be used. Multi-stage bleaching can also be used. The bleaching process is simple enough to mix the chemicals well with the pulp and complete the bleaching process under specific process conditions. After more than 60 years of practice, the bleaching process of the insurance powder pulp has been greatly improved, and several types of process flow and equipment have been gradually developed and formed, and the white water is recycled in various processes. 1.2.1 Traditional Bleaching Tower Process[8]
The bleaching process of the traditional bleaching tower for insurance powder pulp is shown in Figure 1. The main process variables are listed in Table 1. The traditional bleaching tower process has the characteristics of simple equipment and process, but its bleaching effect is relatively poor, and the whiteness value is increased by 10% or less, and is mostly 5% to 8% ISO. In some of the relatively long history of mechanical pulp bleaching of paper mills, the process is mainly used for single-stage bleaching of insurance powder and two-stage bleaching of H2O2-safety powder. It is also mainly used in single-stage bleaching and multi-stage bleaching of waste paper deinking pulp. This process.
Figure 1 Flow chart of a traditional bleaching tower 1 - unbleached slurry storage tank 2 - white water 3 - steam 4 - temperature control 5 - pulp concentration control 6 - liquid level control 7 - insurance powder bleaching liquid 8 - dosing amount control 9 - flow rate Control 10 - bleaching tower 11 - bleached slurry storage tank Table 1 The main technical parameters of the traditional bleaching tower process pH powder powder /% pulp concentration /% bleaching temperature / °C bleaching time / min 5 ~ 8 0.25 ~ 2 3 ~ 5 40 ~ 75 30～120 1.2.2 Bleaching process of directly adding the infusible powder in wood chip grinding disc[9]
This process is shown in Fig. 2. This process is highly concentrated and bleached. The main process variables are shown in Table 2. It is the main process adopted by the new wood chip ground wood pulp production line in the recent years. This process can be used to add single or multi-stage bleaching in the first, second, and bleaching towers. The process and equipment conditions are very different from the traditional bleaching tower process. The manufacturers vary according to the specific conditions. The main process parameters are shown in Table 2. This process can give full play to the bleaching effect of the insurance powder and has a good bleaching effect. The whiteness increment is generally above 12% ISO. Fig.2 Bleaching process of directly adding the infusible powder in wood chip grinding discs 1 - The powdered powder solution dosing system 2 - One grinding 3 - Two grinding 4 - Bleaching towers Table 2 Bleaching of wood flour directly added with insurance powder Process main process parameters pH powdered powder dosage /% pulp concentration /% bleaching temperature / °C bleaching time / min a grinding 5 ~ 6 0.1 ~ 1.5 18 ~ 40> 120 <3 two grinding 5 ~ 6 ~ 0.3 ~ 20 ~ 120 <3 Bleaching tower 5～7～0.3 3 60 60 1.2.3 High-concentration pulp bleaching process of insurance powder[10]
The process is shown in Figure 3, and the main process variables are shown in Table 3. The process has been researched and applied in bleaching mechanical pulp and waste paper deinked pulp. It has been reported that it can achieve whiteness increment of 8% to 15% ISO. Figure 3 High-consistency Pulp Bleaching Process for Infusible Powder Diagram 1—Unbleached Puddle Storage Vessel 2—Fluidization Pump 3—Mixer 4—Bleaching Tower Table 3 High-Concentrate Pulp Bleaching Process for Infusible Powder Main Process Parameters pH Infusible Powder Amount/% Pulp Concentration /% Bleaching temperature/°C Bleaching time/min 4.5～6.5 0.5～1.5 8～15 35～80 15～120 1.2.4 Bleaching process of directly grinding powder in grind pit of pressure grind mill wood pulp [8]
The process is shown in Figure 4. This process is low-concentration bleaching and is currently in pilot production and pilot plant production. See Table 4 for the main process variables. This process can give full play to the bleaching effect of the insurance powder and has a good bleaching effect. The whiteness is increased by 12% to 14% ISO. At the same time, this process can reduce pollutants, and BOD7 is 3kg/t less dry than traditional bleaching tower processes. Fig. 4 Bleaching process of directly grinding powder in the grinding pit of pressure grindstone wood pulp. 1 - Fused powder solution dosing system 2 - Pressure refining pit 3 - Bleaching tower Table 4 Pressure grindstone grinding Wood pulp directly in the grinding pit The main process parameters of the bleaching process plus the insurance powder pH powder strength powder /% pulp concentration /% bleaching temperature / °C bleaching time / min disc 6 ~ 7 0.4 ~ 1.2 1 ~ 2> 105 ~ 3 bleaching tower 5 ~ 6 0.4 ~ 0.8 3 ~60 60 2 Progress of Insurance Powder Pulp Bleaching Technology 2.1 The optimization investigation of the insurance powder pulp bleaching process shows that [1] most of the paper mills using the insurance powder bleaching currently fail to operate under the optimized conditions, affecting the insurance powder. Bleaching effect. Therefore, according to different conditions, the pulp powder bleaching process should be optimized in terms of process and equipment. In terms of equipment, the high temperature is conducive to the characteristics of the insurance powder bleaching, foreign countries have developed or are developing high-temperature short-bleaching time equipment and processes, such as the introduction of the last three kinds of processes, these processes are conducive to make the insurance powder bleaching to a higher white Degree of appreciation. In terms of process conditions, except for the optimization of bleaching process variables according to different pulp types, foreign countries also use improperly used insurance powders. In the process of pulp bleaching, the ineffective decomposition of insurance powder, corrosion of equipment, and the inability to achieve expected whiteness increase After bleaching, the pulp returned to yellow and other issues, a lot of research has been carried out and has been implemented in the industry. 2.1.1 Invalid decomposing of insurance powder and prevention The insurance powder is very unstable, easily decomposed by air oxidation, and part of the sodium dithionite molecules will undergo disproportionation reactions, resulting in a less efficient reducing agent. The oxidation reaction proceeds very quickly, and even before bleaching starts, most of the sodium dithionite is consumed, and the disproportionation reaction increases with the increase of acidity. When the pH is lower than 4, the reaction proceeds almost instantaneously and the heavy metal in the pulp Ions have catalytic acceleration. The main oxidation and disproportionation reaction formula is as follows [5]: 2Na2S2O4+2H2O4NaHSO3Na2S2O4+O2+H2ONaHSO3+NaHSO42Na2S2O4+O2→2Na2S2O52Na2S2O4+4H+→4Na++3SO2↑+H2O+S↓3S2O2
Dithionite is a typical reducing bleaching agent. It was used to bleach mechanical pulp in the early 1930s, and was then used to bleach waste paper deinked pulps and some soda pulps. In the early stage, ZnS2O4 with good stability was mainly used. Due to the contamination of water sources by heavy metal zinc ions, sodium dithionite (Na2S2O4, commonly known as insurance powder) was gradually adopted after the 1970s. The foreign countries have made comprehensive and systematic research on the mechanism, process variables and auxiliary chemicals of the insurance powder pulp bleaching. The research and industrial application of early insurance powder in pulp bleaching was mainly focused on the bleaching of mechanical pulp. In the past 10 years, due to environmental protection and cost reasons, the paper industry in the world has focused attention on recycling of waste paper. In the past, the research and industrial application of the insurance powder in the bleached deinking pulp has also been increasingly reported, mainly focusing on multi-stage bleaching, such as PY two-stage bleaching, (EPO) ZY multi-stage bleaching, E is NaOH, P is H2O2, and O is Oxygen, Z is ozone, Y is the insurance powder [1 ~ 4]. A large number of studies and industrial practices have shown that insurance powder bleached deinked pulp and bleached mechanical pulp have basically the same mechanism, bleaching effect and influencing factors. PY or (EPO) ZY multi-stage bleaching is the most cost-effective deinking pulp bleaching. The method has industrial applications in Germany and North America. In contrast, China's research and application of insurance powder pulp bleaching is less.
For the bleaching mechanism of the insurance powder, the classical view is that the hydrolysis of S2O2-4 produces a lot of new ecology [H] to achieve the purpose of improving whiteness. Further research shows that when the insurance powder bleaches the pulp, the trivalent sulfur in the S2O2-4 root is oxidized into the tetravalent sulfur in the HSO-3 root, ie, S3+→S4++e, and the reduction is mainly used to increase the pulp's BaiDu. The insurance powder is used to bleach the pulp to improve the whiteness of the pulp, and its role is mainly reflected in the following aspects: 1Reducing the terminal saturated carbonyl group at the α,β position; 2Reducing the structure of the germanium in the pulp to make it become colorless Phenol derivatives; 3 The structure of coniferaldehyde in reduced pulp; and the flavonoid-type colored components; 4 The reduction of Fe3+ in pulp to Fe2+; 5 The production of HSO-3 and SO2-3 in the bleaching process act on the ring conjugated double bonds The system produces a bleaching effect on isoeugenol and chalcone compounds [5].
Research and production practices have shown that with the use of bleaching powders, the whiteness of the pulp is increased while there is no negative effect on the strength of the pulp fibers and the opacity of the paper [6], and H2O2 or chlorine-containing bleaches have negative effects to varying degrees. . At the same time, the insurance powder also has a strong role in removing pulp color [7]. 1 Main Process Variables and Typical Process Flows 1.1 Main process variables The process variables for pulp powder bleaching mainly include the pH value, the amount of contaminated powder, bleaching pulp concentration, bleaching temperature, bleaching time, etc. [5]. These variables cannot be considered in isolation. Production should be selected and determined according to specific conditions. 1.1.1 pH It is generally considered that the initial bleaching pH of the suitable insurance powder pulp should be 6-8. It should be determined according to the specific conditions during use. With the decrease of pH value, the ineffective decomposition of the powder is accelerated, but if the pH value is too high, the activity of the powder will be reduced, which is not conducive to pulp bleaching. In the bleaching process, the pH of the pulp will drop by 0.3 to 1.0, depending on the conditions of the treatment, the temperature, the white water circulation system, etc. The more air the pulp touches during bleaching, the greater the pH drop. 1.1.2 The amount of the insurance powder is generally between 0.2% and 2% (for absolute dry pulp). The excessive amount of the insurance powder is of little practical value. At the same time, the concentration of the decomposition powder of the insurance powder in the system circulating white water will increase, resulting in increased corrosion. And the accelerated decomposition of the powder will lead to a decrease in the bleaching effect. 1.1.3 Pulp Concentration The traditional insurance powder pulp bleaching is usually carried out at a low concentration (3% to 5%). Currently, the medium-concentration and high-concentration insurance powder bleaching process has been industrially applied. The insurance powder is easily oxidized by air, and the pulp with high concentration generally contains more air. Therefore, in the middle-high concentration bleaching process, more attention should be paid to the sealing of the equipment and the removal of dissolved oxygen in the pulp. If the pulp concentration is too low, the dissolved oxygen in the water is sufficient to decompose the diluted powder. 1.1.4 Bleaching Temperature The temperature has a significant effect on the bleaching effect. It is not appropriate to use low temperatures. The reason may be that the oxygen content of the pulp suspension is too high at low temperatures. Low-concentration bleaching is generally around 60°C, while medium-to-high-density bleaching is generally performed at about 85°C, and high-concentration bleaching is generally performed at 120°C or higher. In the range of the normal concentration and the amount of the infusible powder, the activity of the chromophoric groups in the reduced powder of the insurance powder increases with the increase of the temperature. At the same time, increasing the temperature is advantageous in eliminating the dissolved oxygen in the pulp and reducing the ineffective decomposition of the insurance powder. Therefore, for each temperature increase of 11 to 14°C, whiteness can be increased by 1% ISO, and the higher the amount of chemical use, the more effective the effect. However, the higher the temperature, the stronger the low-valence sulfur corrosion produced by the decomposition of the insurance powder, and the higher the anti-corrosion requirements for the equipment. 1.1.5 Bleaching time The bleaching time is closely related to the temperature. If the temperature is increased, the time can be shortened. However, the premise is that the bleaching should be carried out in a suitable bleaching equipment. The general bleaching time is 40-65°C for 60 minutes, and the bleaching time for 60-75°C is 30. ~ 45min, 75 ~ 100 °C bleaching time of 10 ~ 15min, more than 100 °C bleaching time is less than 5min. 1.2 Typical process flow and main process variables There are many process flows and equipments for the insurance powder pulp bleaching. The insurance powder can be added directly to the pulp, and the insurance powder can also be added to the mill pit or grinding disc. Single-stage bleaching can be used. Multi-stage bleaching can also be used. The bleaching process is simple enough to mix the chemicals well with the pulp and complete the bleaching process under specific process conditions. After more than 60 years of practice, the bleaching process of the insurance powder pulp has been greatly improved, and several types of process flow and equipment have been gradually developed and formed, and the white water is recycled in various processes. 1.2.1 Traditional Bleaching Tower Process[8]
The bleaching process of the traditional bleaching tower for insurance powder pulp is shown in Figure 1. The main process variables are listed in Table 1. The traditional bleaching tower process has the characteristics of simple equipment and process, but its bleaching effect is relatively poor, and the whiteness value is increased by 10% or less, and is mostly 5% to 8% ISO. In some of the relatively long history of mechanical pulp bleaching of paper mills, the process is mainly used for single-stage bleaching of insurance powder and two-stage bleaching of H2O2-safety powder. It is also mainly used in single-stage bleaching and multi-stage bleaching of waste paper deinking pulp. This process.
Figure 1 Flow chart of a traditional bleaching tower 1 - unbleached slurry storage tank 2 - white water 3 - steam 4 - temperature control 5 - pulp concentration control 6 - liquid level control 7 - insurance powder bleaching liquid 8 - dosing amount control 9 - flow rate Control 10 - bleaching tower 11 - bleached slurry storage tank Table 1 The main technical parameters of the traditional bleaching tower process pH powder powder /% pulp concentration /% bleaching temperature / °C bleaching time / min 5 ~ 8 0.25 ~ 2 3 ~ 5 40 ~ 75 30～120 1.2.2 Bleaching process of directly adding the infusible powder in wood chip grinding disc[9]
This process is shown in Fig. 2. This process is highly concentrated and bleached. The main process variables are shown in Table 2. It is the main process adopted by the new wood chip ground wood pulp production line in the recent years. This process can be used to add single or multi-stage bleaching in the first, second, and bleaching towers. The process and equipment conditions are very different from the traditional bleaching tower process. The manufacturers vary according to the specific conditions. The main process parameters are shown in Table 2. This process can give full play to the bleaching effect of the insurance powder and has a good bleaching effect. The whiteness increment is generally above 12% ISO. Fig.2 Bleaching process of directly adding the infusible powder in wood chip grinding discs 1 - The powdered powder solution dosing system 2 - One grinding 3 - Two grinding 4 - Bleaching towers Table 2 Bleaching of wood flour directly added with insurance powder Process main process parameters pH powdered powder dosage /% pulp concentration /% bleaching temperature / °C bleaching time / min a grinding 5 ~ 6 0.1 ~ 1.5 18 ~ 40> 120 <3 two grinding 5 ~ 6 ~ 0.3 ~ 20 ~ 120 <3 Bleaching tower 5～7～0.3 3 60 60 1.2.3 High-concentration pulp bleaching process of insurance powder[10]
The process is shown in Figure 3, and the main process variables are shown in Table 3. The process has been researched and applied in bleaching mechanical pulp and waste paper deinked pulp. It has been reported that it can achieve whiteness increment of 8% to 15% ISO. Figure 3 High-consistency Pulp Bleaching Process for Infusible Powder Diagram 1—Unbleached Puddle Storage Vessel 2—Fluidization Pump 3—Mixer 4—Bleaching Tower Table 3 High-Concentrate Pulp Bleaching Process for Infusible Powder Main Process Parameters pH Infusible Powder Amount/% Pulp Concentration /% Bleaching temperature/°C Bleaching time/min 4.5～6.5 0.5～1.5 8～15 35～80 15～120 1.2.4 Bleaching process of directly grinding powder in grind pit of pressure grind mill wood pulp [8]
The process is shown in Figure 4. This process is low-concentration bleaching and is currently in pilot production and pilot plant production. See Table 4 for the main process variables. This process can give full play to the bleaching effect of the insurance powder and has a good bleaching effect. The whiteness is increased by 12% to 14% ISO. At the same time, this process can reduce pollutants, and BOD7 is 3kg/t less dry than traditional bleaching tower processes. Fig. 4 Bleaching process of directly grinding powder in the grinding pit of pressure grindstone wood pulp. 1 - Fused powder solution dosing system 2 - Pressure refining pit 3 - Bleaching tower Table 4 Pressure grindstone grinding Wood pulp directly in the grinding pit The main process parameters of the bleaching process plus the insurance powder pH powder strength powder /% pulp concentration /% bleaching temperature / °C bleaching time / min disc 6 ~ 7 0.4 ~ 1.2 1 ~ 2> 105 ~ 3 bleaching tower 5 ~ 6 0.4 ~ 0.8 3 ~60 60 2 Progress of Insurance Powder Pulp Bleaching Technology 2.1 The optimization investigation of the insurance powder pulp bleaching process shows that [1] most of the paper mills using the insurance powder bleaching currently fail to operate under the optimized conditions, affecting the insurance powder. Bleaching effect. Therefore, according to different conditions, the pulp powder bleaching process should be optimized in terms of process and equipment. In terms of equipment, the high temperature is conducive to the characteristics of the insurance powder bleaching, foreign countries have developed or are developing high-temperature short-bleaching time equipment and processes, such as the introduction of the last three kinds of processes, these processes are conducive to make the insurance powder bleaching to a higher white Degree of appreciation. In terms of process conditions, except for the optimization of bleaching process variables according to different pulp types, foreign countries also use improperly used insurance powders. In the process of pulp bleaching, the ineffective decomposition of insurance powder, corrosion of equipment, and the inability to achieve expected whiteness increase After bleaching, the pulp returned to yellow and other issues, a lot of research has been carried out and has been implemented in the industry. 2.1.1 Invalid decomposing of insurance powder and prevention The insurance powder is very unstable, easily decomposed by air oxidation, and part of the sodium dithionite molecules will undergo disproportionation reactions, resulting in a less efficient reducing agent. The oxidation reaction proceeds very quickly, and even before bleaching starts, most of the sodium dithionite is consumed, and the disproportionation reaction increases with the increase of acidity. When the pH is lower than 4, the reaction proceeds almost instantaneously and the heavy metal in the pulp Ions have catalytic acceleration. The main oxidation and disproportionation reaction formula is as follows [5]: 2Na2S2O4+2H2O4NaHSO3Na2S2O4+O2+H2ONaHSO3+NaHSO42Na2S2O4+O2→2Na2S2O52Na2S2O4+4H+→4Na++3SO2↑+H2O+S↓3S2O2
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