Method for preparing white carbon black from purified fly ash

Abstract

The invention belongs to the technical field of industrial solid waste recovery treatment, and particularly relates to a method for preparing white carbon black by utilizing purified fly ash. A white carbon black product with high purity, high specific surface area and hydrophobicity is obtained through the stability and controllability of a three-stage synergistic purification, wet-state desilicication and carbonization modification process, the three-stage synergistic purification effectively breaks and removes wrapping structures of impurities such as iron and carbon through the wet-state synergistic effect of magnetic separation, flotation and acid leaching, and the purity of the white carbon black product is improved. A foundation is laid for high-purity products and high extraction rate; the wet-state direct desilicication process avoids particle aggregation caused by drying, improves the contact efficiency of alkali liquor and active SiO2, realizes efficient and mild desilicication by matching with the desilicication assisting effect of sodium fluoride, and remarkably improves the extraction rate and the product performance compared with a dry powder desilicication process; coupling agent grafting is achieved through high-activity silicon hydroxyl on the surfaces of particles in carbonization and modification, and compared with a traditional technology, the hydrophobicity of the product and the DBP oil absorption value are greatly improved, and high-quality and high-performance white carbon black is obtained.

Description

Technical Field

[0001] This invention belongs to the technical field of industrial solid waste recycling and treatment, specifically relating to a method for preparing silica using purified fly ash. Background Technology

[0002] Fly ash is a major solid waste emitted by coal-fired power plants. Its chemical composition is complex, mainly including SiO2, Al2O3, Fe2O3, and CaO. Currently, the production of precipitated silica from fly ash has become an important direction for resource utilization. Several existing patents involve processes for preparing precipitated silica from fly ash. For example, patent CN2014108277262 proposes a method for preparing precipitated silica from high-silica fly ash, achieving the preparation of precipitated silica through steps such as alkali dissolution desilication and carbonation decomposition. However, this process still suffers from problems such as high alkali consumption, large wastewater discharge, and limited product purity. CN2013104603713 and CN2014101562824 respectively address the preparation of ordinary precipitated silica and hydrophobic precipitated silica from high-alumina fly ash, both involving steps such as high-pressure molding, segmented carbonization, or acidification. The process control is complex and highly dependent on the raw material composition.

[0003] Silica, as an important inorganic functional material, is widely used in rubber, plastics, coatings, pharmaceuticals, and many other fields. With the continuous growth of industrial demand, traditional production processes using quartz sand as raw material face problems such as high raw material costs, high energy consumption, and the inability to recycle alkali solutions. Furthermore, the use of strong acids easily causes equipment corrosion and environmental pollution. Therefore, developing a method to prepare high-purity silica using fly ash while simultaneously achieving the resource recovery of impurities has significant environmental and economic value. Summary of the Invention

[0004] To address the above problems, the present invention aims to provide a method for preparing silica using purified fly ash.

[0005] The technical content of this invention is as follows: This invention provides a method for preparing silica using purified fly ash, comprising the following steps: 1) Multi-stage purification treatment of fly ash Crushing and grinding: The raw fly ash is fed into a ball mill for dry grinding and passed through a 200-300 mesh sieve to obtain uniform powder; Magnetic separation for iron removal: A magnetic separator with a magnetic field strength of 3000-4000mT is used to remove Fe2O3 impurities from the powder, reducing the iron content to below 0.5wt%. Flotation decarbonization: Mix the magnetically separated powder with water at a solid-liquid ratio of 1:5, add 0.5-1wt% kerosene as a collector, and pass air through for flotation to break the encapsulation of carbon particles on the remaining magnetic impurities, reducing the carbon content to below 0.8wt%, while releasing the encapsulated magnetic particles (which can be recovered through secondary magnetic separation). Acid leaching for impurity removal: Add dilute hydrochloric acid with a concentration of 0.5-1 mol / L to the flotation slurry, control the temperature at 50-60℃, stir and react for 1-2 hours to specifically remove soluble metal oxides such as calcium and magnesium. Filter and wash until the pH of the filtrate is neutral to obtain purified fly ash wet filter cake. The test results show that its SiO2 ≥ 65%, Fe2O3 ≤ 0.2%, C ≤ 0.8%, and the total impurity removal rate is increased to over 92%. 2) Preparation of sodium silicate solution The purified fly ash wet filter cake is directly mixed with a sodium hydroxide solution with a concentration of 80-100 g / L, and the solid content is adjusted to 30-40 wt%. Sodium fluoride is added as a desilication aid at a mass of 0.3-0.5 wt% of the mixture. The mixture is stirred and reacted at normal pressure and 90-100℃ for 4-6 hours. After filtration by a vertical leaf filter, a sodium silicate desilication liquid with a modulus of 1.0-1.8 and an Al2O3 content ≤0.5% is obtained. Lime milk (Ca(OH)2) is added to the filtrate after desilication to reduce the concentration of F in the solution. - With Ca 2+ The CaF2 precipitate is formed by combination and then filtered to recover CaF2 (which can be used to regenerate sodium fluoride). The recovered filtrate can be recycled as an alkaline solution for alkaline desilication after causticization treatment. In step 2), the wet fly ash particles have a higher number of hydroxyl groups on their surface, which avoids particle agglomeration caused by the drying process, allowing the alkaline solution to penetrate and contact the active SiO2 more efficiently; at the same time, the F produced by the dissociation of sodium fluoride - Not only can it be with Al 3+ Ca 2+ It forms stable fluorides and can also inhibit the co-solubility of Al2O3 in fly ash and reduce the aluminum impurity content in the desilication liquid, thereby achieving a desilication rate of more than 85% at a low alkali concentration of 80-100g / L, which is more than 20% higher than the desilication efficiency of dry powder. 3) Carbonization to prepare silica First stage carbonization: Dilute the sodium silicate desilication solution to a SiO2 concentration of 40-60 g / L, heat to 60-70℃, introduce a CO2 mixed gas with a volume fraction of 40-60%, stir at a speed of 150-200 r / min, react until the pH value is 9.5-10.5, and mature for 20-30 min. Second stage carbonization: Heat to 80-90℃, continue to introduce CO2 mixed gas, adjust the stirring speed to 250-300 r / min, react until the pH value is 6.5-7.5, then stop the gas supply; add 0.2-0.4 wt% KH-570 coupling agent to the reaction slurry, stir for 30 min, and age at 70-80℃ for 1 h to complete the modification; Post-processing: The filter cake is obtained by filtration, washed with deionized water until no sodium ions remain, dried in a spray dryer at 180-220℃, and then pulverized to obtain the silica product. When KH-570 coupling agent is added during the second stage of carbonization, the primary SiO2 particles have just formed, and the surface silanol activity is at its highest. In addition, the slurry is in a weakly acidic environment, which can promote the rapid condensation reaction between the siloxane groups and silanol groups in the KH-570 molecule. This achieves uniform in-situ grafting and coating of the coupling agent on the particle surface. Compared with subsequent pulping modification or dry modification, the uniformity of hydrophobic coating of the product is improved by more than 40%.

[0006] The beneficial effects of this invention are as follows: The present invention provides a method for preparing precipitated silica from purified fly ash. Through the stability and controllability of a three-stage synergistic purification process, wet desilication, and carbonization modification, high purity (>97%) and high specific surface area (>380 m²) are obtained. 2 The invention produces precipitated silica products with hydrophobic properties (contact angle > 115°) and high purity. The three-stage synergistic purification process, utilizing magnetic separation, flotation, and acid leaching in a wet state, effectively breaks down the encapsulation structures of impurities such as iron and carbon, removing the original impurities and laying the foundation for high-purity products and high extraction rates. The wet direct desilication process avoids particle agglomeration caused by drying, improves the contact efficiency between the alkaline solution and active SiO2, and, combined with the desilication aiding effect of trace amounts of sodium fluoride, achieves efficient and gentle desilication, significantly improving extraction rate and product performance compared to dry powder desilication processes. Thirdly, the carbonization and modification processes utilize highly active silanol groups on the particle surface to achieve uniform grafting of coupling agents, significantly improving the hydrophobicity and DBP oil absorption value of the product compared to traditional post-modification processes, resulting in high-quality, high-performance precipitated silica. The overall process of this invention is stable and controllable, adaptable to different application scenarios, and possesses both environmental and industrialization value. Detailed Implementation

[0007] The present invention will be further described in detail below through specific implementation examples. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of protection of the present invention. After reading the present invention, any modifications of the present invention in various equivalent forms by those skilled in the art fall within the scope of the appended claims.

[0008] Unless otherwise specified, all raw materials and reagents used in this invention are from the conventional market.

[0009] Example 1 A method for preparing silica using purified fly ash 1) Multi-stage purification treatment of fly ash: The raw fly ash (SiO2 content 58%, Al2O3 content 28%, Fe2O3 content 1.2%, carbon content 2.5%) was fed into a ball mill for dry grinding and passed through a 250-mesh sieve; a magnetic separator with a magnetic field strength of 3500mT was used for magnetic separation to remove iron, reducing the iron content to 0.15wt%; the magnetically separated powder was mixed with water at a solid-liquid ratio of 1:5, and kerosene accounting for 0.8wt% of the powder mass was added as a collector, and air flotation was introduced to remove carbon, reducing the carbon content to 0.6wt%; 0.8mol / L dilute hydrochloric acid was added to the flotation slurry, the temperature was controlled at 55℃, and the reaction was stirred for 1.5h. The mixture was filtered and washed until the pH of the filtrate was 7, and after drying, purified fly ash was obtained, with a SiO2 content of 68% and an Al2O3 content of 23%.

[0010] 2) Mild desilication preparation of sodium silicate solution: Purified fly ash is mixed with sodium hydroxide solution with a concentration of 90 g / L, the solid content is adjusted to 35 wt%, and sodium fluoride accounting for 0.4 wt% of the mass of the mixture is added. The mixture is stirred and reacted for 5 h under normal pressure and 95 ℃. After filtration by a vertical leaf filter, sodium silicate desilication solution with a modulus of 1.5 is obtained.

[0011] 3) Directional carbonization preparation of silica: Dilute the desilication solution to a SiO2 concentration of 50 g / L, heat to 65°C, introduce a CO2 mixed gas with a volume fraction of 50%, stir at 180 r / min, react until the pH value reaches 10.0, and age for 25 min; raise the temperature to 85°C, continue to introduce the above CO2 mixed gas, adjust the stirring speed to 280 r / min, react until the pH value reaches 7.0, and stop the gas introduction; add KH-570 coupling agent at a mass of 0.3 wt% of the system to the reaction slurry, stir for 30 min, and age at 75°C for 1 h; filter to obtain filter cake, wash with deionized water until no sodium ion residue remains, dry with a spray dryer at 200°C, and pulverize to obtain silica product.

[0012] Testing showed that the silica product obtained in this embodiment had a purity of 97.2% and a particle size D. 50 It has a diameter of 8 μm and a specific surface area of ​​380 m². 2 / g, with excellent dispersibility.

[0013] Example 2 A method for preparing silica using purified fly ash 1) Multi-stage purification treatment of fly ash: The raw fly ash (SiO2 content 62%, Al2O3 content 26%, Fe2O3 content 0.9%, carbon content 1.8%) was fed into a ball mill for dry grinding and passed through a 200-mesh sieve; magnetic separation was performed using a magnetic separator with a magnetic field strength of 3000mT to remove iron, reducing the iron content to 0.18wt%; the magnetically separated powder was mixed with water at a solid-liquid ratio of 1:5, and kerosene accounting for 0.5wt% of the powder mass was added as a collector, and air flotation was introduced to remove carbon, reducing the carbon content to 0.7wt%; 0.5mol / L dilute hydrochloric acid was added to the flotation slurry, the temperature was controlled at 50℃, and the mixture was stirred and reacted for 2 hours. The mixture was filtered and washed until the pH of the filtrate was 7, and then dried to obtain purified fly ash, which was tested to have a SiO2 content of 66% and an Al2O3 content of 24%.

[0014] 2) Mild desilication preparation of sodium silicate solution: Purified fly ash is mixed with sodium hydroxide solution with a concentration of 80 g / L, the solid content is adjusted to 30 wt%, and sodium fluoride accounting for 0.3 wt% of the mass of the mixture is added. The mixture is stirred and reacted for 6 h under normal pressure and 90 ℃. After filtration by a vertical leaf filter, sodium silicate desilication solution with a modulus of 1.2 is obtained.

[0015] 3) Directional carbonization preparation of silica: Dilute the desilication solution to a SiO2 concentration of 40 g / L, heat to 60°C, introduce a CO2 mixed gas with a volume fraction of 40%, stir at 150 r / min, react until the pH value reaches 10.5, and age for 30 min; raise the temperature to 80°C, continue to introduce the above CO2 mixed gas, adjust the stirring speed to 250 r / min, react until the pH value reaches 7.5, and stop the gas introduction; add 0.2 wt% KH-570 coupling agent to the reaction slurry, stir for 30 min, and age at 70°C for 1 h; filter to obtain filter cake, wash with deionized water until no sodium ion residue remains, dry with a spray dryer at 180°C, and pulverize to obtain silica product.

[0016] Testing showed that the silica product obtained in this embodiment had a purity of 96.5% and a particle size D. 50 It has a diameter of 12 μm and a specific surface area of ​​355 m². 2 / g, with good dispersibility.

[0017] Example 3 A method for preparing silica using purified fly ash 1) Multi-stage purification treatment of fly ash: The raw fly ash (SiO2 content 55%, Al2O3 content 30%, Fe2O3 content 1.5%, carbon content 3.0%) was fed into a ball mill for dry grinding and passed through a 300-mesh sieve; magnetic separation was performed using a magnetic separator with a magnetic field strength of 4000mT to remove iron, reducing the iron content to 0.1wt%; the magnetically separated powder was mixed with water at a solid-liquid ratio of 1:5, and kerosene accounting for 1.0wt% of the powder mass was added as a collector, and air flotation was introduced to remove carbon, reducing the carbon content to 0.5wt%; 1.0mol / L dilute hydrochloric acid was added to the flotation slurry, the temperature was controlled at 60℃, and the mixture was stirred and reacted for 1 hour. The mixture was filtered and washed until the pH of the filtrate was 7, and after drying, purified fly ash was obtained, with a SiO2 content of 69% and an Al2O3 content of 22%.

[0018] 2) Mild desilication preparation of sodium silicate solution: Purified fly ash is mixed with sodium hydroxide solution with a concentration of 100 g / L, the solid content is adjusted to 40 wt%, and sodium fluoride accounting for 0.5 wt% of the mass of the mixture is added. The mixture is stirred and reacted for 4 h under normal pressure and 100 ℃. After filtration by a vertical leaf filter, sodium silicate desilication solution with a modulus of 1.8 is obtained.

[0019] 3) Directional carbonization preparation of silica: Dilute the desilication solution to a SiO2 concentration of 60 g / L, heat to 70°C, introduce a CO2 mixed gas with a volume fraction of 60%, stir at 200 r / min, react until pH value reaches 9.5, and age for 20 min; raise the temperature to 90°C, continue to introduce the above CO2 mixed gas, adjust the stirring speed to 300 r / min, react until pH value reaches 6.5, and stop the gas introduction; add KH-570 coupling agent with a mass of 0.4 wt% of the system to the reaction slurry, stir for 30 min, and age at 80°C for 1 h; filter to obtain filter cake, wash with deionized water until no sodium ion residue remains, dry with a spray dryer at 220°C, and pulverize to obtain silica product.

[0020] Testing showed that the silica product obtained in this embodiment had a purity of 97.8% and a particle size D. 50 It has a diameter of 6 μm and a specific surface area of ​​410 m². 2 / g, with excellent dispersibility.

[0021] Comparative Example 1 As a control group for Example 1, the differences in Comparative Example 1 are as follows: 1) Impurity removal treatment: The raw fly ash is fed into a ball mill for dry grinding and passed through a 250-mesh sieve; 0.8 mol / L dilute hydrochloric acid is added directly to the slurry in which the powder and water are mixed at a solid-liquid ratio of 1:5, the temperature is controlled at 55℃, the mixture is stirred and reacted for 1.5 hours, and then filtered and washed until the pH of the filtrate is 7 to obtain a wet filter cake of fly ash after impurity removal (without magnetic separation or flotation); 2) Desilication: Mix the wet filter cake with a sodium hydroxide solution with a concentration of 90 g / L, adjust the solid content to 35 wt%, add 0.4 wt% sodium fluoride, stir and react at 95℃ under normal pressure for 5 h, and filter to obtain sodium silicate desilication solution; The other steps are the same as in Example 1.

[0022] Testing showed that the silica product obtained in Comparative Example 1 had a purity of 82.5% and a particle size D. 50 It has a diameter of 25 μm and a specific surface area of ​​210 m². 2 / g, with poor dispersion performance.

[0023] Comparative Example 2 As a control group for Example 1, the differences in Comparative Example 2 are as follows: 1) Three-stage purification: the same three-stage synergistic purification steps as in Example 1, but after filtration and washing, the wet filter cake is dried at 105°C for 2 hours to obtain purified fly ash dry powder. 2) Desilication: Mix the dry powder with a sodium hydroxide solution with a concentration of 90 g / L, adjust the solid content to 35 wt%, add 0.4 wt% sodium fluoride, stir and react at 95°C under normal pressure for 5 h, and filter to obtain sodium silicate desilication solution; The other steps are the same as in Example 1.

[0024] Testing showed that the silica product obtained in Comparative Example 2 had a purity of 92.3% and a particle size D. 50 It has a diameter of 18 μm and a specific surface area of ​​305 m². 2 / g, dispersion performance is average.

[0025] Comparative Example 3 As a control group for Example 1, the differences in Comparative Example 3 are as follows: Step 3) Dilute the desilication solution to a SiO2 concentration of 50 g / L, heat to 65°C, introduce a 50% CO2 mixed gas, stir at 180 r / min until pH 10.0, and age for 25 min; raise the temperature to 85°C, stir at 280 r / min until pH 7.0, and stop the gas flow; filter to obtain a filter cake, add deionized water to slurry, then add 0.3 wt% KH-570 coupling agent, stir for 30 min, and age at 75°C for 1 h; The other steps are the same as in Example 1.

[0026] Testing showed that the silica product obtained in Comparative Example 3 had a purity of 96.8% and a particle size D. 50 It has a diameter of 12 μm and a specific surface area of ​​365 m². 2 / g, with good dispersion performance, but poor uniformity of hydrophobic coating.

[0027] Comparative Example 4 As a control group for Example 1, the differences in Comparative Example 4 are as follows: Step 2) Desilication: Mix the wet filter cake with a sodium hydroxide solution with a concentration of 90 g / L, adjust the solid content to 35 wt%, do not add sodium fluoride, stir and react at 95°C under normal pressure for 5 h, and filter to obtain sodium silicate desilication solution;

[0028] The other steps are the same as in Example 1.

[0029] Testing showed that the silica product obtained in Comparative Example 4 had a purity of 90.1% and a particle size D. 50 It has a diameter of 22 μm and a specific surface area of ​​275 m². 2 / g, poor dispersion performance.

[0030] The methods of the examples and comparative examples, as well as the prepared silica, were tested, including SiO2 extraction rate, desilication reaction time (from the time the system temperature stabilized until the clear desilication liquid was obtained by filtration), DBP oil absorption value (titration method), and modified contact angle (average value taken by static drop method).

[0031] The test results are as follows: Table 1 Performance Tests of Different Groups of Silica <![CDATA[SiO2 extraction rate (%)]]> Desilication reaction time (h) <![CDATA[DBP oil absorption value (cm 3 / 100 g)]]> Contact angle after modification (°) Example 1 88.5 5 285 115 Example 2 85.3 6 270 110 Example 3 90.2 4 295 120 Comparative Example 1 52.7 12 195 75 Comparative Example 2 70.3 8 240 102 Comparative Example 3 86.2 5 260 88 Comparative Example 4 65.8 5 225 112 In the desilication reaction practice of Comparative Example 3, no subsequent modification was performed for 1 hour.

[0032] Based on the purity and specific surface area of ​​the above silica products, and combined with the results in Table 1, Examples 1-3 can all prepare high-purity, high-performance silica, demonstrating the stability and controllability of the three-stage synergistic purification, wet desilication, and carbonization modification process adopted in this invention, achieving high purity (>97%) and high specific surface area (>380 m²). 2 / g) and hydrophobic (contact angle >115°) silica products.

[0033] First, the three-stage wet-state synergistic purification process involving magnetic separation, flotation, and acid leaching is fundamental to ensuring high purity and high extraction rate. Comparative Example 1, which only underwent acid leaching, showed a reduced SiO2 extraction rate and lower product purity, demonstrating that the encapsulation of original impurities (especially iron and carbon) severely inhibited the reaction. Comparative Example 2, which underwent drying after synergistic purification, resulted in particle agglomeration. While its extraction rate and product performance were better than Comparative Example 1, they were still far lower than the wet-state direct alkali dissolution process of Example 1. Simultaneously, it played a decisive role; in Comparative Example 4, without the addition of NaF, the extraction rate decreased and product performance declined. The addition of trace amounts of sodium fluoride helped achieve efficient and gentle desilication.

[0034] Secondly, the carbonization process is crucial for obtaining high-performance hydrophobic silica. Comparative Example 3 used traditional carbonization followed by pulping modification, resulting in a product with a smaller hydrophobic angle and lower DBP oil absorption value. In contrast, Example 1 directly added KH-570 to the slurry at the second carbonization endpoint (pH≈7), utilizing the highly active silanol groups on the particle surface for in-situ grafting, significantly increasing the hydrophobic angle and achieving a higher DBP oil absorption value, indicating a superior product structure and more uniform modification.

Claims

1. A method for preparing silica using purified fly ash, characterized in that, Includes the following steps: 1) Multi-stage purification treatment of fly ash The fly ash is crushed, ground, magnetically separated to remove iron, floated to remove carbon, and acid leached to remove impurities to obtain purified fly ash wet filter cake; 2) Preparation of sodium silicate solution The purified fly ash wet filter cake is mixed with a sodium hydroxide solution with a concentration of 80-100 g / L, the solid content is adjusted to 30-40 wt%, and sodium fluoride accounting for 0.3-0.5 wt% of the mass of the mixture is added. The mixture is stirred and reacted for 4-6 hours under normal pressure and 90-100℃. After filtration by a vertical leaf filter, sodium silicate desilication liquid is obtained. 3) Carbonization to prepare silica First stage carbonization: Dilute the sodium silicate desilication solution to a SiO2 concentration of 40-60 g / L, heat to 60-70℃, introduce a CO2 mixed gas with a volume fraction of 40-60%, stir at a speed of 150-200 r / min, react until the pH value is 9.5-10.5, and mature for 20-30 min. Second stage carbonization: Heat to 80-90℃, continue to introduce CO2 mixed gas, adjust the stirring speed to 250-300 r / min, react until the pH value is 6.5-7.5, then stop the gas supply; add 0.2-0.4 wt% KH-570 coupling agent to the reaction slurry, stir for 30 min, and age at 70-80℃ for 1 h to complete the modification; Post-processing: The filter cake is obtained by filtration, washed with deionized water until no sodium ions remain, dried in a spray dryer at 180-220℃, and then pulverized to obtain the silica product.

2. The method for preparing silica using purified fly ash according to claim 1, characterized in that, The crushing and grinding process involves feeding raw fly ash into a ball mill for dry grinding and passing it through a 200-300 mesh sieve to obtain uniform powder.

3. The method for preparing silica using purified fly ash according to claim 1, characterized in that, The magnetic separation for iron removal involves using a magnetic separator with a magnetic field strength of 3000-4000 mT to remove Fe2O3 impurities from the powder, reducing the iron powder content to below 0.5 wt%.

4. The method for preparing silica using purified fly ash according to claim 1, characterized in that, The flotation decarbonization process involves mixing the magnetically separated powder with water at a solid-liquid ratio of 1:5, adding 0.5-1 wt% kerosene as a collector, and then introducing air for flotation to break the encapsulation of remaining magnetic impurities by carbon particles, thereby reducing the carbon content to below 0.8 wt%.

5. The method for preparing silica using purified fly ash according to claim 1, characterized in that, The acid leaching process involves adding 0.5-1 mol / L dilute hydrochloric acid to the flotation slurry, controlling the temperature at 50-60℃, stirring for 1-2 hours, filtering and washing until the pH of the filtrate is neutral, and obtaining purified fly ash wet filter cake.

6. The method for preparing silica using purified fly ash according to claim 1, characterized in that, The obtained sodium silicate desilication solution has a modulus of 1.0-1.8 and an Al2O3 content of ≤0.5%.

7. The method for preparing silica using purified fly ash according to claim 1, characterized in that, The amount of sodium fluoride added is 0.3-0.5 wt% of the mass of the mixture.

8. The method for preparing silica using purified fly ash according to claim 1, characterized in that, The amount of the coupling agent added is 0.2-0.4 wt% of the system mass.