Research Progress of Treating Titanium Dioxide Wastewater by Membrane Method

Titanium dioxide (chemical composition TiO2) is a basic chemical raw material widely used in coatings, plastics, paper, chemical fiber and cosmetics industries. Till now, there are more than 20 countries in the world producing titanium dioxide, including DuPont, Midland Inorganic Chemicals, Kronos, Cole-Meggie, Titanium Oxide, and Kemira Finland. Its production capacity accounts for 78% of the world's titanium dioxide production capacity. Compared with foreign countries, China's titanium resources are rich and its reserves rank first in the world. However, China's titanium dioxide industry started relatively late. Although there are many manufacturers, but the production capacity is small, and the production of more than a single method of sulfuric acid. This process produces waste acid that contains varying amounts of titanium dioxide particles, hydrated titanium dioxide, sulfuric acid, ferrous sulfate, metatitanic acid, etc. If it is directly discharged into the environment, it will not only seriously pollute the environment, but also cause large Economic loss. Therefore, the treatment of titanium white waste occupies an important position in the titanium white industry. This article describes the current status of the treatment of titanium dioxide waste in China, and focuses on the research progress in the operation conditions, membrane fouling control, and cleaning in the study of membrane-treated titanium white waste.

1. Titanium Dioxide Wastewater Sources and Traditional Treatment Methods At present, there are roughly three sources and conventional treatment methods for titanium dioxide waste liquids in China: (1) A large amount of sulfuric acid mists and titanium dioxides are generated during the calcination process of titanium dioxide production. Exhaust gas exhaust gas, its treatment is generally used gravity settlement - spray - electric defogging treatment process. In this process, acid washing waste liquid containing a large amount of titanium dioxide particles is produced. At present, gravity sedimentation process is mostly used to treat and discharge the waste liquid. Because of the small particle size of the titanium dioxide particles, the sedimentation is slow, resulting in incomplete recovery. If the waste liquid containing titanium dioxide particles is discharged directly, a large amount of titanium dioxide will be lost, causing greater economic losses and serious environmental pollution. (2) One of the key steps in the titanium dioxide post-treatment process is the washing of the powder: the water washing step, the purpose of which is to remove water-soluble salt impurities. At present, the leaf-washing washing process or the drum washing process is adopted. One of the two processes will occur the phenomenon of leaching of titanium dioxide and metatitanic acid particles, and the second is that the meta-titanic acid has a small particle size (1 to 4 μm) and the sedimentation rate is slow. The incomplete recovery not only caused environmental pollution but also lost the expensive metatitanic acid particles. (3) The airflow comminution recovery liquid produced in the pulverization process also contained a large amount of titanium dioxide particles. The conventional method was to use a sloping plate settlement tank. This part of the titanium dioxide particles is recovered, but due to the small particle size of the titanium dioxide particles, the sedimentation speed is slow, and the recovery efficiency is low, about 1% to 2% of the titanium dioxide particles are lost each year, which not only seriously pollutes the environment, but also causes economical huge loss. Therefore, it is necessary to develop suitable fine particle recovery technology.

With the fierce competition in commodities and the stringent state emission standards, the traditional separation method has failed to meet the requirements. At present, precision filtration tube filtration technology and membrane separation technology are widely used for the liquid-solid separation of micron and submicron particles. Among them, the membrane separation technology is a new separation technology that has been rapidly developed in the past decade. Because of its advantages such as high separation efficiency, low energy consumption, simple device, and easy operation, it has been widely used in many fields [1].

2. Membrane microfiltration titanium dioxide waste microfiltration membrane is usually made of special fiber resin or high molecular polymer and inorganic material, and its pore size range is generally between 0.1 ~ 10μm. If the pore size of the membrane is between different molecular diameters, the smaller diameter molecules can pass through the pores of the membrane while the larger diameter molecules are trapped, which is the screening effect. Microfiltration is the most widely used and most economical technology in the membrane process. It has been widely used in the sterilization and filtration of the pharmaceutical industry, ultrapure water preparation for the electronics industry, and various industrial wastewater treatment processes. Since the particle size of titanium dioxide in the suspension is within the μm range, membrane microfiltration titanium white waste is currently used. In addition, since the titanium white waste solution is acidic (the waste solution contains a large amount of **), the inorganic film is often used to treat the titanium white waste solution by the membrane method.

In the early 1990s, Bauer et al. [2] adopted a carbon fiber composite membrane microfiltration method to remove titanium dioxide particles from the titanium white waste solution and discussed the feasibility of the carbon membrane treatment suspension. Since then, with the increasing importance of environmental issues in countries around the world, titanium white waste treatment has also received increasing attention. In the past ten years, the domestic and international research work on the membrane treatment of titanium white effluent has mainly proceeded from two aspects: one is the optimization of the operating conditions; the second is the membrane contamination cleaning and control methods.

2.1. Optimization of operating conditions for membrane-based microfiltration of titanium white waste liquid The optimization of membrane conditions for the microfiltration of titanium white waste liquid is mainly focused on the pore size and particle size ratio, operating pressure, cross-flow velocity, temperature, and solution environment. .

2.1.1 Effect of membrane pore size and particle size ratio According to the relationship between the ratio of dp and membrane pore size (dm) and flux, the appropriate membrane pore size is selected to obtain higher flux and reduce membrane The area of ​​use. As industrial needs, if the membrane flux can be increased at a suitable rejection rate, it is very beneficial for reducing equipment investment and operating costs. Mikulasek et al. [3] used a ceramic membrane with a pore size of 0.1 μm to treat a titanium dioxide suspension with particle sizes in the range of 0.17 to 0.24 μm. Experiments have shown that the membrane has a higher ratio of dp/dm in the range of 2.5 to 4.0. Permeation flux. Zhong Wei et al [4] determined the membrane fouling mechanism corresponding to different dp/dm by measuring the permeation flux and pollution resistance of ceramic membrane microfiltration titanium white waste liquid, and pointed out that when dp/dm is less than 2.4, the membrane filtration titanium The process of white waste is blocked by membrane pores and bridging control; when dp/dm is between 2.4 and 10.0, the membrane fouling is mainly determined by the granular deposition on the membrane surface, and the membrane flux is controlled by the structure of the cake layer; With the increase of particle size, the proportion of membrane resistance increases, the proportion of filter cake resistance decreases, and membrane flux increases. When dp/dm is greater than 10.0, membrane fouling still dominates filter cake deposition. Membrane fouling is controlled by "loose" filter cake control towards "dense" filter cake control.