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Application of High-Efficiency Reaction Deep Cone Sedimentation Technology and Equipment in the Production of Titanium Dioxide by the Sulfuric Acid Method Authors: Long Xiaobing, Lu Zhenrong, Li Jingsheng, Zhan Hanhui, Zhang Jingjing. To address issues such as incomplete recovery of metatitanic acid from dilute acid during the production process at the titanium dioxide plant of Yongli Chemical Co., Ltd., Hunan Zhuzhou Chemical Group, as well as the large size of the treatment systems used to handle high-concentration acidic wastewater (with a concentration of 20–100 g/L), unstable treatment performance, low treatment capacity, and low concentrations in the effluent – all of which led to overloading of the subsequent filter presses and frequent equipment failures – a set of highly efficient integrated devices was developed based on existing theories of gravity (centrifugal force) sedimentation, along with principles of fluid chemistry and secondary flow. This technology is known as high-efficiency reaction deep cone sedimentation technology and equipment. Several years of field operation have proven that this equipment features stable performance, high processing capacity, thorough recovery of metatitanic acid from dilute acids, good effluent clarification, and a high bottom flow concentration (2–3 times the feed concentration). This technology and equipment fill the gap in the recovery of useful minerals from high-concentration wastewater, as well as in the technologies and equipment for treating such wastewater, offering broad market prospects. As environmental awareness grows and resources become increasingly scarce, the concept of sustainable development has taken root in people’s minds. More and more companies are incorporating green production and improved productivity into their strategic goals. In the production process of titanium dioxide using the sulfuric acid method, how to completely recover metatitanic acid from waste acid and wastewater, and effectively treat the resulting high-concentration wastewater so that it can be reused directly, has always been a goal pursued by manufacturers. The recovery of metatitanic acid from the dilute acid in the titanium dioxide plant of Hunan Yongli Chemical Co., Ltd. mainly relies on traditional filtration units. Due to the high feed concentration (10–35 g/L), the treatment efficiency is extremely unstable, resulting in the waste of metatitanic acid present in the dilute acid as it is discharged along with the overflow water. Additionally, the operation and maintenance of such equipment are complex. Wastewater treatment plants dealing with high-concentration acidic wastewater use traditional staged sedimentation, after which the underflow is fed directly into plate and frame filter presses. Due to the low concentration of the underflow, the (plate and frame) filter presses operate at full capacity, resulting in frequent equipment failures that significantly impact the economic efficiency and enthusiasm of the enterprises. After testing various domestic and foreign products for the recovery of metatitanic acid from dilute acids and for the treatment of high-concentration acidic wastewater, it was decided to jointly develop an efficient reaction-deep cone sedimentation technology and apparatus to address this issue, resulting in significant economic and social benefits. 1 Structural (technical) features and working principle: The high-efficiency reactive deep cone sedimentation technology and equipment build upon existing theories of gravitational (centrifugal) sedimentation. By incorporating fluid chemistry principles and secondary flow concepts that are covered by independent intellectual property rights, it explains the mechanism by which high-concentration fine particles behave during coagulation and sedimentation processes. This design makes full use of the available space in the equipment, integrating coagulation (chemical) reactions, sedimentation, suspended layer filtration, and concentration into one unit. As a result, the entire system is compact, the water treatment process is simple, the equipment requires less space, there are lower investment and operating costs, it is easy to operate, and maintenance is straightforward. A schematic diagram of its structure is shown in Figure 1. Figure 1 Schematic diagram of the structure of the high-efficiency reaction deep-cone sedimentation device. 1.1 Structural features: The high-efficiency reaction deep-cone sedimentation technology and device can not only carry out efficient coagulation (chemical) reactions and sedimentation on high-concentration wastewater, enabling rapid separation of solids from liquids, but also perform concentration and fine filtration of the suspended layer within the device. The concentration of the bottom stream after concentration is 2–3 times that of the feed water, and the quality of the filtered water is further improved, thus serving to clarify high-concentration wastewater. The water treatment process of this device is simple; it occupies less space, has low investment and operating costs, is easy to operate, and convenient to maintain. (1) In the middle of this device lies a coagulation (chemical) reaction and sedimentation chamber: by applying the principles of secondary flow and fluid hydrochemistry, the fluid shear conditions (flow field distribution, shear force, exposure time) are carefully controlled to ensure thorough mixing of the chemicals with the particles, thereby accelerating the interaction between them and enabling the formation of dense flocs more effectively and quickly. The larger flocs sink to the concentration chamber under the action of gravity for concentration, while the smaller flocs rise with the upward flow of water and aggregate in the filtration chamber to form a fluidized suspension layer. (2) Between the outer shell and the coagulation reaction chamber lies a suspension layer filtration chamber: as the water after coagulation (chemical) reaction passes upward through this chamber, it forms a fluidized suspension layer through self-aggregation; thanks to physical adsorption at the interface, electrochemical properties, and van der Waals forces, this floc filter layer traps all impurities such as suspended colloidal particles and flocs present in the water after coagulation, thereby further reducing the content of suspended solids in the water. (3) The bottom of the device is the concentration zone, in which multiple layers of conical inclined plates are installed. The sediment undergoes proper collision and compression within the conical structure of the concentration chamber, resulting in highly concentrated discharge. (4) An automatic control device for the suspension layer, which effectively controls the height of the suspension layer (to ensure the stability of the water filtration efficiency achieved by a fluidized suspension layer) as well as the automatic discharge of the underflow, thereby increasing the automation level and accuracy of the device. 1.2 Technical Features 1) Recovery of useful minerals from high-concentration wastewater and treatment of wastewater with high concentrations (20–90 g/L): For a long time, the treatment of wastewater with concentrations in the range of 20–90 g/L has been a problem that needed to be addressed urgently. Since direct filtration cannot achieve the required concentration of sludge water for use in filtration machines, this results in low treatment efficiency, which is neither economical nor efficient ； Traditional chemical dosing sedimentation requires large amounts of chemicals, results in bulky sedimentation equipment, and leads to high treatment costs. Although it can achieve a certain degree of solid-liquid separation as long as economic conditions permit, its efficiency is very low; moreover, the concentration of the underflow is quite low, which makes subsequent treatment more difficult. 2) Advanced principle: Building on existing theories of gravity (centrifugal force) sedimentation, it incorporates fluid chemistry and secondary flow principles with independent intellectual property rights to explain the mechanism by which high-concentration fine particles behave during coagulation (chemical) reactions and sedimentation processes. This ensures that the device requires less chemical reagent when treating high-concentration wastewater, offers high concentration efficiency, large treatment capacity, and excellent clarification results. 3) The device has a unique structure: it makes full use of the available space, integrating coagulation (chemical) reaction, sedimentation, suspended sediment filtration, and concentration into one unit. As a result, the entire equipment is compact, the water treatment process is simple, the equipment occupies less space, the investment and operating costs are low, operation is easy, and maintenance is convenient. 4) Innovation and advancement of the results: Conclusion of the novelty search: No literature reports at home or abroad have been found that possess the technical features and equipment characteristics of the project under review. The evaluation conclusion is that this technology and device are the first of their kind in China, representing a leading level in the country. And two patents were obtained. 1.3 Working Principle: The coagulant (such as polyacrylamide) and high-concentration wastewater are introduced into the coagulation reaction chamber via the dosing pipe and the wastewater inlet pipe, respectively. Mechanical stirring is used to control the fluid shear conditions (flow field distribution, shear force, contact time) in accordance with principles of fluid chemistry, ensuring thorough mixing of the coagulant with the particles. The effect of secondary flow accelerates the interaction between the coagulant and the particles, enabling the formation of denser flocs more effectively and rapidly. The coarse-grained flocs form and, under the action of gravity, sink to the concentration chamber where they are concentrated, and are finally discharged through the concentration tube ； Under the action of upward flowing water, fine particle flocs self-agglomerate within the filtration chamber to form a fluidized suspended layer. When the effluent resulting from the coagulation (chemical) reaction passes upward through the filtration chamber, where it self-aggregates to form a fluidized suspension layer, this floc filter layer uses physical adsorption at the interface, electrochemical properties, and van der Waals forces to trap all impurities such as suspended colloidal particles and flocs in this suspension layer, thereby further reducing the concentration of suspended substances in the water. The excess water then flows into the overflow tank and is discharged through the overflow pipe. 2 Process flow before application 2.1 Process flow for recovering titanic acid from dilute acid in titanium dioxide plants before application, see Figure 2. Figure 2 shows the process flow for recovering metatitanic acid from diluted acid in the titanium dioxide plant prior to its application. Before this application, Hunan Yongli Chemical Co., Ltd.’s titanium dioxide plant relied on traditional filtration units to recover metatitanic acid from diluted acid; however, a drawback of these traditional filters is that they require a strict concentration limit for the incoming liquid – if the concentration exceeds 2 g/L, their filtering capacity decreases and their performance deteriorates. Since the actual feed concentration on site is 10–35 g/L, the high acidity of the feed wastewater results in a lower treatment capacity per unit area for the filtration units, as well as unstable effluent quality. A portion of metatitanic acid present in the dilute acid is wasted along with the effluent, causing losses to the factory’s economic performance. 2.2 Process flow for treating high-concentration acidic wastewater from sewage treatment plants prior to application is shown in Figure 3. Figure 3: Process flow for treating acidic wastewater before application. The process used for treating high-concentration acidic wastewater in the wastewater treatment plant before application is the traditional method—stepwise neutralization and sedimentation. After multiple reaction steps and sedimentation, the bottom stream is fed into a plate and frame filter press for filtration. Due to the high water content in the underflow, the plate and frame filter operates at full capacity, which leads to frequent equipment failures, incomplete automatic unloading, and the need for frequent replacement of the filter cloth, thereby significantly affecting the economic efficiency and enthusiasm of the enterprise. However, as the company’s titanium dioxide production facilities continue to expand, the amount of wastewater generated from titanium dioxide production also increases accordingly. Effective treatment of this wastewater to ensure it meets regulatory standards has become a strategic goal for Hunan Yongli Chemical Co., Ltd. Although the company already has environmental protection treatment equipment such as diaphragm filter presses, which are effective in treating wastewater, their drawbacks include high labor intensity, frequent failures, and high maintenance and operating costs. 3 Content and economic benefits of key applications: The three high-efficiency reaction deep-cone sedimentation technologies and devices are used in the titanium dioxide plant of Hunan Yongli Chemical Co., Ltd. to recover metatitanic acid from dilute acid. In December 2006, the first high-efficiency reactive deep cone settler was used in a titanium dioxide plant to recover titania from dilute acid. In March 2007, the second and third high-efficiency reactive deep cone settlers were put into use in the titanium dioxide plant. On one hand, this ensured a low content of metatitanic acid in the overflow water, enabling the recovery of most of it and thus increasing the overall yield of titanium dioxide. On the other hand, it increased the concentration of the underflow slurry by more than 2.1 times, reducing the operational load on the diaphragm filter press. In this way, annual energy savings, consumption reduction, and the recovery of metatitanic acid result in annual savings of over 3.5 million yuan ； The two-high-efficiency reaction deep cone sedimentation technology and equipment are used in the environmental protection treatment plant of Hunan Yongli Chemical Co., Ltd. to treat highly concentrated acidic wastewater. The two settlers handle 3.2 million tons of wastewater per year (with each setter processing 200 tons per hour). The amount of wastewater that overflows after treatment is 1.6 million tons, and 50% of this overflow water is reused directly for replenishing circulating water and for surface washing purposes. At a cost of 2.61 yuan per ton of industrial tap water, the cost of treating 1 ton of recycled water is approximately 1.22 yuan. The annual savings resulting from this reuse amount to: 1.6 million tons × 50% × (2.61 – 1.22) = 111,200 yuan ； The concentrated bottom flow of 1.6 million tons is then sent to a plate and frame filter press for further treatment, which improves the efficiency of treating wastewater using this type of filter press, reduces its electricity consumption and filter cloth usage, as well as the amount of chemicals required. Additionally, it eliminates the need to operate two filter presses, resulting in cost savings of approximately 2 million yuan. The total savings from these energy-saving measures amount to 3.112 million yuan. 4 Process flow and test results after application The process flow for recovering titanic acid from dilute acid in titanium dioxide plants after application is shown in Figure 4. Figure 4 shows the process flow for recovering metatitanic acid from dilute acid in titanium dioxide plants after application; the process flow for treating high-concentration acidic wastewater after application is shown in Figure 5. Figure 5 shows the process flow for treating acidic wastewater after application, as well as the monitoring and analysis results along with evaluations. The monitoring and analysis methods and instruments are listed in Table 1. The test results of applying the high-efficiency reactive deep cone sedimentation technology and equipment to the recovery of titania from dilute acid in titanium dioxide plants are shown in Table 2. The feed concentration is generally between 10 and 35 g/L. The monitoring results of the high-efficiency reactive deep cone sedimentation technology and equipment applied to the treatment of high-concentration acidic wastewater are shown in Table 3. Table 2 shows the test results of applying the high-efficiency reaction deep-cone sedimentation technology and equipment to the recovery of metatitanic acid from dilute acid. As indicated by the monitoring results in Table 3, when using this technology and equipment with a treatment flow rate of 200 m3/h, the SS concentration in the treated effluent ranges from 6.4 to 10.3 mg/L; the SS removal rate is between 99.89% and 99.94%. The ratio of the bottom flow concentration to the inlet concentration (concentration multiplication factor) is between 2.4 and 2.9. 5 Conclusion The high-efficiency deep cone sedimentation technology and equipment integrate the \"fluidic chemistry\" principles of independently innovated coagulation reactions, as well as the principles of secondary flow in the sedimentation process, along with suspension layer filtration and concentration principles, to carry out the entire process of sewage sedimentation and separation. This approach simplifies the overall process flow. Moreover, this equipment is reliable in operation, easy to use, and simple to maintain, with excellent performance as reported by users on site. The high-efficiency reactive deep cone sedimentation technology and equipment have performed well over the past few years at Hunan Yongli Chemical Co., Ltd. They have passed the evaluation of scientific and technological achievements conducted by the Hunan Provincial Department of Science and Technology. The use of this technology and equipment not only ensures that the clear effluent resulting from the treatment of high-concentration wastewater can be discharged directly or reused, but it also increases the recovery rate of titanium dioxide ； On the other hand, it can increase the slurry concentration by more than 2.1 times, thereby improving the operating efficiency of the subsequent filter press. The high-efficiency reactive deep cone sedimentation technology and its associated equipment feature unique design and advanced principles. They provide a reliable means to increase the overall yield of titanium dioxide in titanium dioxide production, as well as to handle acidic wastewater effectively, offering significant economic benefits and broad market prospects.