This paper studies various processes of wastewater treatment, and compares and analyzes the advantages and disadvantages of various wastewater processes, which provides a theoretical basis and reference for the treatment of salt-containing wastewater in the titanium dioxide production industry.
Production process of surface modification of titanium dioxide base material by chloride method
The titanium dioxide base material produced by the chlorination process is neutralized to reduce the viscosity and then introduced into a sand mill for grinding and classification. After the particle size reaches the requirements, it is introduced into the surface treatment process for surface treatment.
In order to ensure the quality of the product, the titanium dioxide product is subjected to pressure filtration and washing after surface treatment. The operation process includes five stages: filtration, washing, pressing, drying, and discharging. The filter cake is processed through subsequent drying and steam pulverization. The finished product of titanium dioxide is obtained, and the waste water enters the waste water treatment process for treatment.
Second, the characteristics of titanium dioxide production in salt-containing wastewater
In the production process of titanium dioxide surface treatment, chemical substances such as sulfuric acid, sodium silicate and aluminum sulfate are added. Therefore, the main components of wastewater are Cl-, SO42- and TiO2. The composition of wastewater is affected by the process, equipment, and personnel. The interval value of the change, but on the whole, the water quality and quantity of the saline wastewater are relatively stable.
Salt-containing wastewater treatment technology
Membrane treatment technology
Membrane separation technology utilizes the selective permeability of membranes to separate mixtures. According to different separation functions, it can be divided into microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO). Among them, MF and UF are usually used as pretreatment units for NF and RO. A company uses secondary RO technology to treat the salty wastewater of chlorination titanium dioxide, and the quality of the reclaimed water produced meets the “Water Quality Standard for Reclaimed Water for Circulating Cooling Water” HG/T3923-2007 water quality standard.
Membrane treatment technology has become a hot spot in the research and application of saline wastewater treatment in recent years due to its advantages such as good selectivity, no pollution, simple operation, convenient maintenance, structural use, and no phase change. However, the high cost of the membrane treatment process and the easy pollution of membrane components, which reduce the effect of wastewater treatment, greatly limit the application of membrane separation technology in wastewater treatment. If we can develop low-cost, anti-pollution, self-cleaning membrane materials, or use the “membrane + evaporation” improved process, membrane separation technology will be more applied in the field of wastewater.
Multi-effect evaporation technology
The evaporation method is to heat the solvent of the solution to vaporize, and the solute remains in the unevaporated solvent, and the vapor is collected and then condensed.
The purpose of evaporation operation is the separation of solute and solvent, but its essence is heat exchange and transfer. According to the flow direction of the secondary steam and the solution, the multi-effect evaporation process can be divided into the co-current process, counter-current process, and cross-flow process.
Evaporation can be divided into single-effect and multi-effect according to the number of stages. Single-effect evaporation is to use raw steam to provide heat to directly condense the evaporated steam, that is, the secondary steam, without using its condensation heat, and using the former effect of the secondary steam as the next steam. The heating medium of the evaporator makes full use of thermal energy and saves raw steam. The boiling point and pressure of the solution of the aftereffect in the multi-effect evaporation must be lower than that of the former effect.
This is a relatively common, mature, and reliable evaporation technology. Combined with different forms of evaporators, it can concentrate materials to a concentration of more than 60%, or directly evaporate to supersaturation and crystallization. Multi-effect evaporation has relatively high requirements on water quality, otherwise, it will cause frequent blockage of the system, reduce processing capacity, and affect product development.
The main energy consumption of this technology is on raw steam: if a four-effect evaporator evaporates one ton of water, the amount of raw steam required is 250-350kg;
The three-effect evaporator evaporates one ton of water, and the amount of raw steam required is 350~450kg. The energy consumption of technology is mainly closely related to the technical level of system operators. Improper operation can easily lead to the failure of the entire system to meet the design targets. in addition,
From the perspective of personnel allocation, the multi-effect evaporation device requires many operating personnel, and the production management cost is relatively high.
MVR evaporation technology
Mechanical compression heat pump evaporation technology (referred to as MVR) is a new type of high-efficiency evaporation technology. Its principle is to introduce a steam compressor or a compressor fan to recycle a large amount of latent heat of the secondary steam. \MVR evaporation technology does not rely on the temperature difference between the effects to realize the multiple utilization of steam, but uses the compressor to increase the temperature of the secondary steam, and uses the heated secondary steam as the evaporation heat source, thereby reducing the demand for outside steam. A certain amount of power is required.
In recent years, with the increasingly strict national environmental protection industry policies, the reduction of salty wastewater, and even zero discharge has become an inevitable trend. As an energy-saving and high-efficiency evaporation treatment technology, MVR has broad application space and development prospects. The main energy consumption of the MVR system is the power consumption of the compressor. Usually, 30~78kg of raw steam is required to evaporate one ton of water, and the power consumption is 30~55KW (compressor power consumption).
At present, the device level of MVR evaporation technology is mainly closely related to the design ideas and equipment manufacturing level of compressor manufacturers: energy-saving compressors have stricter requirements on the quantity and quality of wastewater, and cannot operate under conditions that deviate from the design indicators, while the energy consumption is slightly lower. A taller compressor will sacrifice some efficiency to improve compressor stability under harsh conditions.
Electrodialysis technology is a new technology developed in the 1950s. It was originally used for seawater desalination and is now widely used in chemical, light industry, metallurgy, papermaking, and pharmaceutical industries, especially for the preparation of pure water and the treatment of three wastes in environmental protection. The most important, such as acid and alkali recovery, electroplating waste liquid treatment and recovery of useful substances from industrial wastewater, etc.
Since the membrane used in electrodialysis technology is an ion exchange membrane, it is very sensitive to impurities such as iron ions, BOD and COD.
At present, the actual operation of the project faces the following situations:
(1) The comprehensive investment cost is large, the operation and operation energy consumption is large (usually 17~20kWh/m3), and the service life is short (usually the replacement cycle does not exceed 3 years);
(2) The pretreatment of the solution is insufficient, and the membrane is prone to poisoning and failure;
(3) Through this technology, the material can only be concentrated to 13% salt content, and it lacks the successful operation engineering performance in the higher salt content system.
The titanium dioxide industry is an emerging industry, and my country’s economic development is inseparable from the stable development of titanium dioxide. However, with the rapid development of titanium dioxide, a large amount of salty wastewater is also discharged, which not only causes great harm to the safety of the water environment but also It also aggravated the increasingly tense situation of my country’s water resources.
With the enhancement of people’s awareness of environmental protection, the treatment of salty wastewater has received more and more attention. Although many processes have been proved to be effective in solving environmental pollution problems, not all processes can economically carry out the engineering practice of chlorination titanium dioxide production wastewater treatment.
Therefore, in the process of wastewater treatment, it is necessary to effectively select economical and feasible wastewater treatment measures in combination with water quality and local policies.