Method for reducing moisture content in titanium gypsum

Abstract

The invention discloses a method for reducing water content in titanium gypsum. The method comprises the following steps: (1) selecting pure calcium sulfate dihydrate as a seed crystal; and (2) adding a titanium gypsum crystallization accelerator and a seed crystal into the stone powder slurry, then adding the acidic wastewater subjected to membrane filtration to carry out a neutralization reaction, and separating the reaction product slurry by a centrifugal machine to obtain titanium gypsum, the titanium gypsum crystallization accelerator being terephthalic acid or phthalic acid. The method has the beneficial effects that the pure calcium sulfate dihydrate and the terephthalic acid crystallization accelerant are added in the reaction, and various conditions in the titanium gypsum reaction process are controlled, so that the water content of the titanium gypsum can be reduced to 15% or below, and meanwhile, crystal grains of the titanium gypsum are increased, so that the yield and landfill treatment capacity of the titanium gypsum are reduced; and a new way is developed for energy conservation and emission reduction of the titanium gypsum.

Description

Technical Field

[0001] This invention relates to the field of titanium gypsum treatment technology, a byproduct of titanium dioxide production via the sulfuric acid process, and particularly to a method for reducing the water content in titanium gypsum. Background Technology

[0002] Titanium dioxide production plays a vital role in national economic development and construction. The sulfuric acid process remains the primary domestic titanium dioxide production technology due to its advantages: readily available and inexpensive raw materials, high product quality, long history, mature technology, and the ability to achieve large-scale continuous production. However, this method has disadvantages: it uses large amounts of sulfuric acid, resulting in low sulfuric acid recycling rates and a large volume of waste acid to process. The waste acid contains a high concentration of Fe ions, which increases the consumption of gypsum during the neutralization process and also increases the adsorption of free water by titanium gypsum, thus increasing the yield of titanium gypsum. Furthermore, the traditional neutralization and crystal growth process involves short neutralization times, and the addition of any auxiliary agents leads to finer gypsum grains, significantly increasing the adsorption of free and crystalline water, further increasing the water content of the titanium gypsum and consequently increasing the processing cost. Current titanium dioxide waste acid treatment processes involve the use of seed crystals and crystallization promoters. The seed crystal is dihydrate titanium gypsum, and the crystallization promoter is sodium citrate. However, the final water content after neutralization and centrifugation is difficult to consistently fall below 15%.

[0003] For example, Chinese invention patent publication number CN106007428A discloses a method for reducing the moisture content of titanium gypsum. This method uses carbide slag or lime as a neutralizing agent, adding 2-5 kg ​​of gypsum morphology modifier per ton of lime or carbide slag solid. The solution is neutralized with acidic titanium dioxide wastewater to a pH of 4.0-4.5, maintained at 30-70℃, cured for 30-120 minutes, and then filtered. The filter residue is calcium sulfate dihydrate with particles of 20-40 micrometers in diameter, and the filtrate is a solution containing ferrous sulfate. The gypsum morphology modifier is selected from three or more of the following: adipic acid, citric acid, polyacrylamide, sodium alkylbenzene sulfonate, sodium polyvinyl sulfonate, sodium polyacrylate, sodium sulfate, ammonium sulfate, polyacrylamide copper sulfate, sodium hexametaphosphate, and ammonium nitrate. The gypsum morphology modifier in this patent uses carboxyl groups to complex with calcium ions to inhibit crystal growth. However, its ability to regulate crystal morphology is limited. As a result, the calcium sulfate dihydrate is transformed from needle-like to granular, and the water content cannot be kept stable below 15%.

[0004] For example, Chinese invention patent publication number CN117776567A discloses a method, system, and application for preparing transcrystalline titanium gypsum using acidic wastewater from the sulfuric acid process for titanium dioxide production. The acidic wastewater from the sulfuric acid process for titanium dioxide production is pretreated, and calcium and seed crystals are added to the resulting acidic waste liquid to induce a transcrystalline reaction. The resulting transcrystalline turbid liquid is then separated into solid and liquid components to obtain transcrystalline titanium gypsum. The water content of the titanium gypsum in this patent is between 15% and 25%. Summary of the Invention

[0005] The technical problem to be solved by the present invention is that the water content in existing titanium gypsum cannot be kept stable below 15%, and therefore a method for reducing the water content of titanium gypsum is provided.

[0006] The technical solution of the present invention is: a method for reducing the water content in titanium gypsum, comprising the following steps: (1) selecting pure calcium sulfate dihydrate as seed crystals; (2) adding titanium gypsum crystallization promoter and seed crystals to the stone powder slurry, and then adding acidic wastewater after membrane filtration for neutralization reaction, and the reaction product slurry is separated by centrifuge to obtain titanium gypsum, wherein the titanium gypsum crystallization promoter is terephthalic acid or phthalic acid.

[0007] The pure calcium sulfate dihydrate crystals in step (1) of the above scheme have a particle size of 20μm to 40μm.

[0008] The temperature of the neutralization reaction in step (2) of the above scheme is 35℃~50℃.

[0009] The amount of seed crystals added in step (2) of the above scheme is 0.1 wt% to 5 wt%.

[0010] The beneficial effect of this invention is that by adding pure calcium sulfate dihydrate and terephthalic acid crystallization promoter to the reaction, and by controlling various conditions in the titanium gypsum reaction process, the water content of titanium gypsum can be reduced to below 15%, while increasing the crystal size of titanium gypsum, thereby reducing the production and landfill volume of titanium gypsum, and developing a new approach for energy conservation and emission reduction of titanium gypsum. Attached Figure Description

[0011] Figure 1 This is a flowchart of the present invention; Figure 2 These are microscope images of the titanium plaster obtained in Example 1; Figure 3 This is a microscope image of the titanium plaster obtained in Comparative Example 1. Detailed Implementation

[0012] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the examples. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments implemented by those skilled in the art without creative effort are within the protection scope of the present invention.

[0013] like Figure 1 As shown, a method for controlling the water content of titanium gypsum includes the following steps: Selection of titanium gypsum seed crystals: Pure calcium sulfate dihydrate is selected as seed crystals with a particle size of about 20μm to 40μm.

[0014] Titanium gypsum neutralization and crystal growth: Add stone powder slurry, crystallization promoter, and seed crystals to a neutralization reaction vessel. The amount of titanium gypsum seed crystals added is 1wt%–4wt%, and the amount of titanium gypsum crystallization promoter added is 0.01wt%–0.5wt%. Membrane-filtered acidic wastewater is also added. The titanium gypsum crystallization promoter is terephthalic acid or phthalic acid. The temperature is controlled at 35℃–50℃, and stirring is performed. The membrane-filtered acidic wastewater is slowly added to the tank while simultaneously stirring the solution evenly. The reaction time is 0.5h–2h, and the pH is controlled not to exceed 4. Subsequently, lime slurry is added until the pH reaches 7–8.

[0015] Titanium gypsum separation: The solution obtained from the above reaction is initially separated using a solid-liquid separation device. The supernatant is sent to the next process, and the concentrated liquid is centrifuged to obtain titanium gypsum with a water content of less than 15%. At the same time, the supernatant obtained from the separation can be used as a stone powder slurry for pulping.

[0016] Example 1: Pure calcium sulfate dihydrate was used as seed crystals, and the particle size of the pure calcium sulfate dihydrate was 20 μm. The steps were as follows: Titanium gypsum crystal growth reaction: 6 kg of stone powder solution was pulped and the temperature was controlled at 50℃. 550 g of pure calcium sulfate dihydrate (1 wt%) and 44 g of terephthalic acid (0.08 wt%), a titanium gypsum crystallization promoter, were added. The mixture was stirred evenly with a stirring paddle and the reaction system temperature was controlled at 50℃. 55 kg of iron removal waste liquid was added to acidic water using a peristaltic pump over 2.5 hours, and the pH was controlled to be no greater than 4. After the addition was completed, the temperature was maintained, and 0.5 kg of lime slurry was added to adjust the pH to 8. Titanium gypsum separation: The slurry after the reaction was completed was dehydrated using a centrifuge to obtain 2.5 kg of dihydrate titanium gypsum, with an analytical water content of 13.5%. Microscopic images are shown below. Figure 2 As shown, the wastewater obtained from filtration can be reused in the process of producing stone powder slurry.

[0017] Example 2: Pure calcium sulfate dihydrate was used as seed crystals, and the particle size of the pure calcium sulfate dihydrate was 30 μm. The steps were as follows: Titanium gypsum crystal growth reaction: 200g of 20wt% stone powder solution is slurried and the temperature is controlled at 50℃. 5g of 1wt% pure calcium sulfate dihydrate and 0.5g of 0.1% titanium gypsum crystallization promoter terephthalic acid are added. The mixture is stirred evenly with a stirring paddle and the reaction system temperature is controlled at 50℃. 500g of iron removal waste liquid is added to acidic water using a peristaltic pump within 2 hours, and the pH is controlled to be no greater than 4. After the addition is completed, the temperature is maintained, and 20g of lime slurry is added to adjust the pH to 8. Titanium gypsum separation: The slurry after the reaction is completed is dehydrated and separated using a Buchner funnel to obtain 25g of dihydrate titanium gypsum with an analytical water content of 14.8%. The filtered wastewater can be reused in the process to produce stone powder slurry.

[0018] Example 3: Pure calcium sulfate dihydrate was used as seed crystals, and the particle size of the pure calcium sulfate dihydrate was 40 μm. The steps were as follows: Titanium gypsum crystal growth reaction: 5.5 kg of stone powder solution was pulped and the temperature was controlled at 50℃. 500 g of 1 wt% pure calcium sulfate dihydrate and 50 g of 0.1 wt% titanium gypsum crystallization promoter terephthalic acid were added. The mixture was stirred evenly with a stirring paddle and the reaction system temperature was controlled at 50℃. 50 kg of iron removal waste liquid was added to acidic water using a peristaltic pump within 2.5 h, and the pH was controlled to be no greater than 4. After the addition was completed, the temperature was maintained and 0.5 kg of lime slurry was added to adjust the pH to 8. Titanium gypsum separation: The slurry after the reaction is completed is dehydrated using a centrifuge to obtain 2.5 kg of dihydrate titanium gypsum with an analytical water content of 13.5%. The filtered wastewater can be reused in the process of producing stone powder slurry.

[0019] Example 4: Pure calcium sulfate dihydrate was used as seed crystals, and the particle size of the pure calcium sulfate dihydrate was 30 μm. The steps were as follows: Titanium gypsum crystal growth reaction: 200g of 20wt% stone powder solution is slurried and the temperature is controlled at 50℃. 5g of 1wt% pure calcium sulfate dihydrate and 0.5g of 0.1% titanium gypsum crystallization promoter phthalic acid are added. The mixture is stirred evenly with a stirring paddle and the reaction system temperature is controlled at 50℃. 500g of iron removal waste liquid is added to acidic water using a peristaltic pump within 2 hours, and the pH is controlled to be no greater than 4. After the addition is completed, the temperature is maintained, and 20g of lime slurry is added to adjust the pH to 8. Titanium gypsum separation: The slurry after the reaction is completed is dehydrated and separated using a Buchner funnel to obtain 25g of dihydrate titanium gypsum with an analytical water content of 14.5%. The filtered wastewater can be reused in the process to produce stone powder slurry.

[0020] Example 5: Pure calcium sulfate dihydrate was used as seed crystals, and the particle size of the pure calcium sulfate dihydrate was 40 μm. The steps were as follows: Titanium gypsum crystal growth reaction: 5.5 kg of stone powder solution was pulped and the temperature was controlled at 50℃. 500 g of 1 wt% pure calcium sulfate dihydrate and 50 g of 0.1 wt% titanium gypsum crystallization promoter phthalic acid were added. The mixture was stirred evenly with a stirring paddle and the reaction system temperature was controlled at 50℃. 50 kg of iron removal waste liquid was added to acidic water using a peristaltic pump within 2.5 h, and the pH was controlled to be no greater than 4. After the addition was completed, the temperature was maintained and 0.5 kg of lime slurry was added to adjust the pH to 8. Titanium gypsum separation: The slurry after the reaction is completed is dehydrated using a centrifuge to obtain 2.5 kg of dihydrate titanium gypsum with a water content of 14.1%. The filtered wastewater can be reused in the process of producing stone powder slurry.

[0021] Comparative Example 1: The difference from Example 1 is that sodium citrate is used as a crystallization promoter instead of terephthalic acid. Titanium gypsum crystal growth reaction: 6 kg of stone powder solution is slurried, the temperature is controlled at 50°C, 500 g of 1 wt% pure calcium sulfate dihydrate is added, and 55 g of 0.1 wt% sodium citrate is added. The mixture is stirred evenly with a stirring paddle, and the reaction system temperature is controlled at 50°C. 55 kg of iron removal waste liquid is added to acidic water using a peristaltic pump over 2.5 hours, controlling the pH to be no greater than 4. After the addition is complete, the temperature is maintained, and 0.5 kg of lime slurry is added to adjust the pH to 8. Titanium gypsum separation: The slurry after the reaction was completed was dehydrated using a centrifuge to obtain 2.6 kg of dihydrate titanium gypsum. The water content was analyzed to be 16.3%, and its microscopic photograph is shown below. Figure 3 As shown.

[0022] The stone powder solutions in the above examples and comparative examples refer to 25 wt% stone powder solutions. 90% of these solutions are calcium carbonate, with the remainder being calcium oxide and impurities.

[0023] As can be seen from the above embodiments and Comparative Example 1, the innovation of this invention lies in exploring a new approach to control the moisture content of titanium gypsum to remain below 15% using terephthalic acid or phthalic acid as a crystallization promoter. This is a direction that has not been previously studied and was unexpected by those skilled in the art. If sodium citrate is used as a crystallization promoter, the moisture content of titanium gypsum may exceed 15%.

[0024] Compared with the inorganic salts or citric acid (sodium) crystallization promoters commonly used in the prior art, the terephthalic acid or phthalic acid selected in this invention have the following advantages: 1. Unique Mechanism of Action: The rigid structure of the aromatic ring and the directional coordination ability of the two carboxyl groups can form a specific interaction with the gypsum crystal face and can undergo specific adsorption with calcium ions, effectively regulating the growth direction of gypsum crystals rather than inhibiting crystal growth. This promotes the formation of larger, more uniform, and denser crystals (such as short columnar or plate-like crystals) and reduces the formation of needle-like or fine fibrous crystals, thereby significantly improving the strength, purity, and dehydration performance of gypsum. In contrast, inorganic salts and citric acid, as promoters, mainly affect solubility by changing the ionic strength of the solution and inhibit crystal growth by complexing with calcium ions through carboxyl groups. Their ability to regulate crystal morphology is limited, which can easily lead to uneven crystal size or impurity inclusions.

[0025] 2. Higher process compatibility and impurity tolerance: Industrial wastewater from purified terephthalic acid is recycled as a byproduct during the neutralization and crystal growth process of titanium gypsum, achieving "waste-to-waste treatment." Its effectiveness is influenced by coexisting ions (such as Fe). 3+ Al 3+It exhibits minimal interference and good stability under acidic conditions. However, inorganic salts will introduce additional cations (such as Na+). + K + This could exacerbate the burden on subsequent wastewater treatment, and chloride ions pose a risk of equipment corrosion. Citric acid is easily decomposed in the strongly acidic environment of titanium dioxide waste acid and forms stable complexes with various metal ions, which may interfere with the neutralization process and increase the residue of metal impurities.

[0026] 3. Economic and Environmental Advantages: As a byproduct of the purified terephthalic acid industry, its acquisition cost is far lower than that of commercial-grade citric acid. Compared with inorganic salts, although its unit price may not be advantageous, its addition amount is small, its effect is significant, and its overall treatment cost is advantageous. As an organic compound, terephthalic acid poses a far lower long-term cumulative risk to the environment than inorganic salts (such as chlorides).

Claims

1. A method for reducing the water content in titanium gypsum, characterized in that, The process includes the following steps: (1) Selecting pure calcium sulfate dihydrate as seed crystals; (2) Adding titanium gypsum crystallization promoter and seed crystals to the stone powder slurry, and then adding acidic wastewater after membrane filtration for neutralization reaction. After the reaction product slurry is separated by a centrifuge, titanium gypsum is obtained. The titanium gypsum crystallization promoter is terephthalic acid or phthalic acid.

2. The method for reducing the water content in titanium gypsum as described in claim 1, characterized in that: The pure calcium sulfate dihydrate crystals in step (1) have a particle size of 20μm to 40μm.

3. The method for reducing the water content in titanium gypsum as described in claim 1, characterized in that: The temperature of the neutralization reaction in step (2) is 35℃~50℃.

4. The method for reducing the water content in titanium gypsum as described in claim 1, characterized in that: The amount of seed crystals added in step (2) is 0.1 wt% to 5 wt%.

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