Composite reducing agent for bleaching titanium dioxide by sulfuric acid method and application of composite reducing agent

Abstract

The invention provides a composite reducing agent for bleaching titanium dioxide by a sulfuric acid method and application of the composite reducing agent, and relates to the technical field of preparation of inorganic chemical materials. The composite reducing agent is prepared from the following components in percentage by mass: 90 to 99 percent of sodium hydrosulfite and 1 to 10 percent of EDTA (Ethylene Diamine Tetraacetic Acid) disodium. The method comprises the following steps: adjusting the acid of metatitanic acid slurry, heating to 45-50 DEG C, and adding the composite reducing agent for reaction. The synergistic effect of sodium hydrosulfite and EDTA disodium is utilized, sodium hydrosulfite is mainly used for reducing Fe < 3 + > to Fe < 2 + >, EDTA disodium effectively inhibits generation of insoluble precipitates by chelating metal ions, and therefore the problem of equipment scaling is thoroughly solved while efficient bleaching is conducted. According to the method, the burning and explosion risk and irritant gas pollution of a traditional process are thoroughly eliminated, the reaction condition is mild, the iron content of the product can be reduced to 15 ppm or below, the equipment scaling amount is smaller than 5 g / m < 2 >, the cleaning period is greatly prolonged, and comprehensive benefits are remarkable.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of inorganic chemical materials, and particularly relates to a composite reducing agent for sulfuric acid method titanium dioxide bleaching and application thereof. BACKGROUND

[0002] In the production process of sulfuric acid method titanium dioxide, the metatitanic acid slurry after hydrolysis must be subjected to key bleaching and secondary washing to remove impurities such as iron, so as to reach high quality standards. The first generation of traditional process (such as aluminum powder or trivalent titanium bleaching) has inherent defects such as flammability and explosion, high temperature and high acid corrosion equipment, large equipment investment and high maintenance cost, and prominent safety and environmental risks.

[0003] In order to avoid the above risks, the second generation of alternative process has begun to attract attention, mainly including: (1) Hyposulfite reducing agent (such as sodium formaldehyde sulfoxylate, formaldehyde sodium bisulfite): however, the applicant found through in-depth research that under the acid and heating conditions of the bleaching process, the reducing agent will decompose to produce SO2, HCHO and other irritating gases. These gas byproducts further react with Ca 2+ , Mg 2+ and other metal impurity ions in the slurry to form insoluble precipitates such as calcium sulfite and formaldehyde polymers. This type of precipitate can cause two serious consequences: first, trace amounts of precipitate are wrapped into titanium dioxide, forming colored centers after calcination, which significantly degrades the whiteness and color removal of the final product; second, the precipitate frequently scales on the inner wall of the equipment, which must be cleaned, seriously damaging the continuity and stability of production.

[0004] (2) Complex bleaching agent (such as EDTA): existing technologies (such as patent CN103553123B) disclose a method of using EDTA-2Na to complex Fe 3+ in an attempt to replace the reduction process. Although this scheme avoids safety risks to some extent, the applicant found that it has two significant limitations: first, the complexation reaction has limited efficiency in removing Fe 3+ , making it difficult to stably reduce iron content to extremely low levels (such as ≤30 ppm), which is difficult to meet the production requirements of high-grade titanium dioxide; second, to achieve a certain complexation effect, the amount of EDTA added is huge (molar ratio far exceeds 1:1), resulting in a sharp increase in production cost, lack of economic feasibility, and difficulty in industrialization.

[0005] Therefore, the real dilemma faced by those skilled in the art is that the traditional reduction process is high-risk, and existing alternative solutions either introduce new product quality and equipment maintenance problems (hyposulfite) or are difficult to balance effectiveness and cost (complexation method).

[0006] Developing a novel bleaching reducing agent and method that is absolutely safe (no risk of combustion or explosion), environmentally friendly (does not produce harmful gases or irritating odors), highly efficient at removing iron and inhibiting scale buildup in equipment, and economically feasible, has become a systemic technical challenge that urgently needs to be solved in this field. This invention addresses this need. Summary of the Invention

[0007] The purpose of this invention is to provide a composite reducing agent for bleaching titanium dioxide using the sulfuric acid process and its application. Through synergistic effects, it can efficiently remove iron while completely solving the problem of equipment scaling and eliminating safety and environmental hazards.

[0008] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A composite reducing agent for bleaching titanium dioxide using the sulfuric acid process is mainly composed of a main reducing agent and a precipitation inhibitor. By mass percentage, the main reducing agent is 90-99% sodium dithionite, and the precipitation inhibitor is 1-10% ethylenediaminetetraacetic acid (EDTA).

[0009] In this invention, the precipitation inhibitor is further described as disodium EDTA or tetrasodium EDTA.

[0010] This invention also provides a method for bleaching titanium dioxide using the sulfuric acid process with the composite reducing agent described above, characterized by comprising the following steps: S1. The metatitanic acid slurry after one water washing is transferred to a bleaching pot, stirred, and the total titanium concentration and free acid concentration of the slurry are measured. S2. Control the total titanium concentration of the slurry to be 280-320 g / L and the free acid concentration to be 10-15 g / L; S3. Add concentrated sulfuric acid to the bleaching pot to adjust the free acid concentration of the slurry to 20-30 g / L; S4. Heat the slurry to 45-50℃ and keep it at that temperature; S5. Prepare the composite reducing agent into an aqueous solution with a concentration of 40-50%; S6. Add the reducing agent solution prepared in step S5 to the bleaching pot and carry out a reduction reaction at 45-50℃ to remove Fe from the slurry. 3+ Reduced to Fe 2+ After the reaction is completed, a bleaching slurry is obtained; the total amount of the composite reducing agent added is 0.1%-0.2% of the total titanium mass in the slurry.

[0011] In this invention, further, in step S6, the reduction reaction time is 50-70 minutes.

[0012] In this invention, further, in step S6, all the reducing agent solution is added to the bleaching pot at once and stirred and matured for 60 minutes.

[0013] In this invention, further, in step S6, the reducing agent solution is added in two batches: first, 70-80% of the total volume of the reducing agent solution is added at a uniform rate over 8-12 minutes, and the reaction is kept warm for 20-40 minutes; then, the remaining 20-30% of the reducing agent solution is added at a uniform rate over 3-8 minutes, and the reaction is continued to be kept warm and stirred for 20-40 minutes.

[0014] The present invention also provides a titanium dioxide product, which is obtained by bleaching a slurry by the method described above, followed by secondary washing, calcination, and post-treatment.

[0015] The composite reducing agent and method for the sulfuric acid process of titanium dioxide bleaching provided by this invention have the following significant advantages compared with the prior art: 1. Fundamentally eliminates safety and environmental hazards, achieving green and safe production: This invention completely eliminates the high-risk processes involving flammable and explosive hazardous chemicals (aluminum powder) and hydrogen generation in traditional aluminum powder / trivalent titanium processes, thus eliminating major fire and explosion safety hazards at the source. Simultaneously, the entire process does not produce any irritating or toxic gases such as SO2 or formaldehyde, overcoming the shortcomings of hyposulfite reducing agents that deteriorate the operating environment. It is a green production process that is inherently safe and environmentally friendly, protecting the health of operators and the environment.

[0016] 2. It has both excellent bleaching and scale inhibition effects, significantly improving product quality and production continuity: This invention produces a synergistic effect of "1+1>2" through the innovative combination of sodium dithionite and disodium EDTA. The main effects are: (1) Highly efficient reduction and iron removal: Sodium dithionite, as the main reducing agent, can efficiently remove Fe 3+ Reduced to soluble Fe 2+ This ensures that the iron content of the bleached slurry is stably reduced to below 15 ppm (calculated as Fe2O3), and the iron removal effect is better than that of the traditional trivalent titanium process, providing a guarantee for obtaining high whiteness and high-grade titanium dioxide products. (2) Highly efficient scale inhibition: The core function of disodium EDTA lies in its chelating ability. It can preferentially complex Ca in the slurry. 2+ Mg 2+ Impurity ions and stabilize the reduction product Fe 2+ This effectively blocks these ions from reacting with SO4. 2- The pathway for the formation of insoluble precipitates such as FeSO4·H2O by isocyanates fundamentally solves the equipment scaling problem from a mechanistic perspective (scale amount <5g / m³). 2 This extends the equipment cleaning cycle to more than 30 days, greatly ensuring the continuity and stability of production and improving equipment utilization and capacity.

[0017] 3. Mild and energy-efficient reaction conditions, resulting in significant overall economic benefits: This invention significantly improves the efficiency while greatly reducing the severity of reaction conditions. The bleaching temperature (45-50℃) and acid concentration (20-30g / L after bleaching) are both much lower than those of the traditional trivalent titanium process (65℃, 50-70g / L). This not only reduces corrosion to equipment and extends equipment life, but also significantly reduces steam and acid consumption, resulting in significant energy savings. Furthermore, the reducing agent is a stable solid, easy to store and transport, and allows for automated dosing, saving labor costs and the investment and maintenance costs of specialized equipment required for trivalent titanium preparation. Overall, the operating cost is low, demonstrating outstanding economic efficiency.

[0018] In summary, this invention successfully solves a series of technical problems such as safety risks, environmental pollution, equipment scaling, product quality, and energy consumption, and provides an innovative solution with comprehensive performance superior to existing bleaching processes, possessing extremely high industrial application value. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments and comparative examples. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention.

[0020] Example 1: Preparation of the composite reducing agent of the present invention Weigh 950 kg of sodium dithionite (sodium hydrosulfite) and 50 kg of disodium EDTA, and mix them thoroughly in a mixer to obtain 1000 kg of the composite reducing agent of this invention. This product is a white to light yellow powder, has stable properties, and has a shelf life of over 12 months when stored in a sealed, light-protected container.

[0021] Example 2: Bleaching method using the composite reducing agent of the present invention Includes the following steps: 1. Material preparation: Prepare 200ml of metatitanic acid slurry after the first water washing during the sulfuric acid process for titanium dioxide production. 3 Pour into the bleaching pot and start stirring. Sampling analysis showed: total titanium concentration 305 g / L, free acid concentration 13 g / L, and iron content (calculated as Fe2O3) 420 ppm.

[0022] 2. Acid adjustment: Calculate and add concentrated sulfuric acid with a mass concentration of 98%, controlling the addition amount to 16 g / L of slurry, and adjust the free acid concentration of the system to about 26 g / L.

[0023] 3. Heating: Introduce steam to raise the temperature of the slurry to 48±1℃ and keep it at that temperature.

[0024] 4. Preparation of reducing solution: Weigh 135 kg of the composite reducing agent prepared in Example 1 (the amount added is 0.15% of the total titanium mass in the slurry), add it to 270 kg of demineralized water, stir until completely dissolved, and prepare a reducing agent aqueous solution with a concentration of 40%.

[0025] 5. Bleaching Reaction: Add the entire reducing agent solution to the bleaching pot at a uniform rate over 10 minutes. After addition, maintain the temperature at 48±1℃ and continue stirring for 60 minutes. During this process, sodium dithionite acts as the main reducing agent to reduce Fe... 3+ Reduced to Fe 2 + Disodium EDTA works by chelating free metal ions (such as Ca) in the slurry. 2+ Mg 2+ (etc.), effectively inhibiting the reduction product Fe 2+ With SO4 2- The anions generate insoluble precipitates such as FeSO2·H2O, thus fundamentally preventing equipment scaling.

[0026] 6. Discharge: After bleaching, the clarity of the slurry phase is significantly better than that of the traditional process, with no obvious suspended sediment. The iron content (calculated as Fe2O3) of the bleached slurry is measured to be 15ppm. The slurry is then transferred to the secondary washing process.

[0027] Comparative Example 1: Sodium dithionite alone Except for the reducing agent being replaced by 135 kg of sodium dithionite (without EDTA), the preparation and addition methods are exactly the same as in Example 2, and the rest of the steps are exactly the same.

[0028] Comparative Example 2: Using disodium EDTA alone (CN103553123B scheme) The operation was carried out strictly in accordance with the conditions disclosed in claim 1 and embodiment 1 of prior art document CN103553123B: The reducing agent was changed to disodium EDTA alone. Its dosage was determined according to the requirements of this document, in the formula EDTA-2Na:Fe. 3+ Based on a molar ratio of 4:1, this experiment requires approximately 385 kg of disodium EDTA (the amount used is 2.85 times the total mass of the composite reducing agent of this invention).

[0029] The reaction temperature was strictly controlled at 40±1℃. The holding time was 1.5 hours. The remaining steps were consistent with those in Example 2.

[0030] Comparative Example 3: Traditional Trivalent Titanium Process The bleaching process employed the company's existing production line and traditional aluminum powder method to prepare a trivalent titanium solution. Bleaching conditions were: temperature 65°C, and free acid concentration of 60 g / L after bleaching. The amount of reducing agent added, based on the effective trivalent titanium, remained consistent with the reduction equivalent in Example 2.

[0031] Effect verification and comparative analysis: After the bleaching process is completed, a comprehensive evaluation of the above examples will be conducted. The method for measuring the amount of scale buildup on the equipment is as follows: after 30 days of equipment operation, a 1m sample is taken from the inner wall of the bleaching pot. 2 In the area, all sediment was scraped off, dried at 105°C for 4 hours until constant weight, and then weighed. The result was the average of three measurements.

[0032] The results are recorded in the table below.

[0033]

[0034] Based on the data in Table 1, the following conclusions can be drawn: (1) Data from Comparative Example 2 (strictly simulating CN103553123B) demonstrates that even when strictly following the teachings of the prior art and using its upper limit molar ratio (4:1), relying solely on EDTA complexation cannot effectively achieve the core purpose of bleaching and iron removal (iron content as high as 380ppm). This indicates that the "complexation substitution reduction" scheme claimed in the prior art may have significant flaws in practical applications, or that its claimed effects are based on specific preconditions that have not been fully disclosed. The present invention unexpectedly discovers that placing a small amount of EDTA in a reduction system does not primarily function to remove iron, but rather to inhibit scaling, providing a novel and non-obvious use for this substance.

[0035] (2) This proves that the "compound formulation" produced a synergistic effect of "1+1>2", rather than a simple additive effect: Although Comparative Example 1 (single sodium hydrosulfite) had a certain reduction effect, it produced obvious scaling (50g / m³). 2 This demonstrates that using sodium hydrosulfite alone introduces new equipment maintenance problems. This invention, by combining a small amount of EDTA (5%) with sodium hydrosulfite, not only achieves optimal reduction (15ppm) but also completely solves the scaling problem (<5g / m³). 2 This demonstrates that EDTA acts as a "cleaner" in the reduction system by chelating with metal ions, creating a reaction environment that inhibits precipitation. Its effect is far more than a simple sum of the functions of the two components; rather, it produces a genuine synergistic effect. This is something that those skilled in the art could not have predicted from prior art or common knowledge.

[0036] (3) Compared with the closest prior art (CN103553123B), unexpected technical effects have been achieved: The present invention and the prior art are fundamentally different in terms of technical means, functional purpose and effect: Means: The present invention is a composite system of "main reducing agent (sodium hydrosulfite) + auxiliary precipitation inhibitor (small amount of EDTA)"; the prior art is a "single complexing agent (large amount of EDTA)". Purpose: The present invention aims to "efficiently reduce iron removal and simultaneously inhibit equipment scaling"; the prior art aims to "replace reduction through complexation". Effect: The present invention achieves excellent iron removal and scale inhibition effects at the same time; Comparative Example 2 proves that the iron removal effect of the prior art scheme is poor, and its limited scale inhibition effect is only due to the inherent dispersing effect of EDTA itself, and is not an innovative solution with synergistic effect designed for the specific chemical environment of 'reduction bleaching'.

[0037] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A composite reducing agent for bleaching titanium dioxide using the sulfuric acid process, characterized in that, It mainly consists of a primary reducing agent and a precipitation inhibitor. By mass percentage, the primary reducing agent is 90-99% sodium dithionite, and the precipitation inhibitor is 1-10% ethylenediaminetetraacetic acid (EDTA).

2. The composite reducing agent according to claim 1, characterized in that, The precipitation inhibitor is disodium EDTA or tetrasodium EDTA.

3. A method for bleaching titanium dioxide using the sulfuric acid process with the composite reducing agent as described in claim 1 or 2, characterized in that, Includes the following steps: S1. The metatitanic acid slurry after one water washing is transferred to a bleaching pot, stirred, and the total titanium concentration and free acid concentration of the slurry are measured. S2. Control the total titanium concentration of the slurry to be 280-320 g / L and the free acid concentration to be 10-15 g / L; S3. Add concentrated sulfuric acid to the bleaching pot to adjust the free acid concentration of the slurry to 20-30 g / L; S4. Heat the slurry to 45-50℃ and keep it at that temperature; S5. Prepare the composite reducing agent into an aqueous solution with a concentration of 40-50%; S6. Add the reducing agent solution prepared in step S5 to the bleaching pot and carry out a reduction reaction at 45-50℃ to remove Fe from the slurry. 3+ Reduced to Fe 2+ After the reaction is completed, a bleaching slurry is obtained; the total amount of the composite reducing agent added is 0.1%-0.2% of the total titanium mass in the slurry.

4. The method according to claim 3, characterized in that, In step S6, the reduction reaction takes 50-70 minutes.

5. The method according to claim 3, characterized in that, In step S6, all the reducing agent solution is added to the bleaching pot at once and stirred for 60 minutes.

6. The method according to claim 3, characterized in that, In step S6, the reducing agent solution is added in two batches: first, 70-80% of the total volume of the reducing agent solution is added at a uniform rate over 8-12 minutes, and the reaction is kept warm for 20-40 minutes; then, the remaining 20-30% of the reducing agent solution is added at a uniform rate over 3-8 minutes, and the reaction is continued to be kept warm and stirred for 20-40 minutes.

7. A titanium dioxide product, characterized in that, It is prepared by bleaching the slurry according to any one of claims 3-6, followed by secondary washing, calcination, and post-treatment.

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