High-salt wastewater treatment and fluorine corrosion problems

Industrial wastewater contains a large amount of salt (such as F-, Cl-, SO42- plasma), which belongs to high-salt wastewater.

Although there are different definitions for high-salt wastewater in current industrial production, for example, high-salt wastewater refers to saline wastewater with a salt content of more than 1%; another more general statement is that high-salt wastewater refers to organic matter and at least total Wastewater with a mass fraction of dissolved solids (TDS) greater than 3.5%.

However, regardless of the definition, the treatment of high-salt wastewater is still a difficult problem that chemical companies must solve.

High-salt wastewater is one of the most difficult wastewaters to treat. Currently, the main methods for the treatment of saline wastewater include biological methods, physical methods and physicochemical methods.

Among them, the biological method mainly uses halophilic bacteria to complete the treatment of saline wastewater through domestication and cultivation, which can be subdivided into activated sludge method, contact oxidation method, anaerobic treatment method, etc.;

Physical and chemical methods are divided into evaporation method (evaporation-cooling crystallization and evaporation-thermal crystallization), ion exchange method, incineration, membrane treatment, etc. By comparing and analyzing the treatment methods currently used in the industry, find a reasonable way to treat high-salt wastewater , and solve the problem of fluorine corrosion in industry from the root.

Biological treatment of saline wastewater

Biological method has the advantages of low treatment cost, good effect, stable operation and good effluent quality. It is the most common treatment method in wastewater treatment at present.

In the process of salt-containing wastewater treatment, biological treatment can achieve better treatment effect. In the early days, Song Jing used SBBR to treat salt-containing organic wastewater.

The results show that under the condition of 3.5% salinity, the COD removal rate of SBBR process can reach 95%, and the shock load resistance of organic wastewater is strong.

The combination of pure oxygen aeration system and activated sludge was carried out to conduct experimental research on organic matter degradation and salt tolerance.

It can efficiently remove the pollutants in the polluted water body, reduce the pollutant load of the water body to the greatest extent, and has a good ecological environment effect.

Experiments were carried out using aerobic activated sludge to treat high-salt oil production wastewater. The experiments showed that the domesticated activated sludge could adapt to the high-salt environment, and had a high CODCr removal rate for different concentrations of high-salt oil production wastewater. After mud acclimation, the removal rate of CODCr from oil production wastewater can reach more than 90%.

Through the research on the treatment of pickled wastewater by the contact oxidation method, the optimal organic load, HRT, temperature and the highest salinity that can be tolerated in the treatment of pickled wastewater by this method are obtained. The salt tolerance limit is 51.84g/L.

When the NaCl concentration is less than this value, increasing the salt concentration will not have a great influence on the treatment effect.

By using an anaerobic digestion reactor to treat wastewater with an initial COD concentration of 1500 mg/L and a salt content of 0.6%, 2.5%, and 6%, the removal rates of COD were 85%, 84%, and 63%, respectively; When the amount of COD was 2.5% and the initial COD was 900 mg/L, 1500 mg/L and 3000 mg/L, the removal rates of COD were 89%, 86% and 53%, respectively.

The conventional biological activated sludge treatment technology is used to treat industrial wastewater. The treatment cost is low, the operation speed is fast, and the unit activity is strong. However, organic matter and inorganic salts can inhibit the growth of microorganisms or poison them. This method requires a large amount of wastewater treatment. Dilute and extend processing time.

Wastewater undergoes extensive dilution and extended treatment times.

Although biological methods including anaerobic digestion and aerobic activated sludge can effectively treat a certain degree of saline wastewater, the microbial system is very sensitive to changes in ionic strength, and the increase in salinity affects the metabolic activity of microorganisms, and even reduces the system The kinetic coefficient of the reaction.

Even the acclimated activated sludge system has a limited range of salinity adaptation, and even extremely halophilic bacteria can only survive at 15% to 30% salinity.

Due to the limited environmental adaptability of halophilic bacteria, although low-concentration saline wastewater can be treated by biological methods, the problem of effective treatment of a large amount of concentrated saline wastewater cannot be solved.

In order to complete the treatment of high-concentration salt-containing wastewater, in recent years, physical and physicochemical methods such as ion exchange, membrane treatment, evaporation and incineration methods have been rapidly developed for the treatment of high-salt wastewater.

Ion exchange method to treat wastewater

The ion exchange method was first used in seawater desalination, and the ion exchange method combined with ultrasonic wave was used for water softening technology, and the phenol in water was removed by adsorption combined with ion exchange. The ion exchange method was used to remove dissolved organic pollutants in water, and certain treatment effects were achieved.

The disadvantage is that it is combined with other processes, and the processing cost is high.

The use of reverse osmosis to treat high-salt wastewater can realize the reuse of saline wastewater, and the removal rates of COD and TDS can reach more than 90% and 99%, respectively. Yang Keyin introduced the application technology of membrane separation of high-salt wastewater. Compared with thermal concentration process, membrane separation technology has the characteristics of low treatment cost, large scale and mature technology.

The disadvantage is that the concentration ratio is not high, usually about 3 times. Although the membrane separation ratio can be greatly improved after enhanced pretreatment, it requires a long pretreatment process.

At present, membrane separation technologies include microfiltration (MF) membrane separation technology, ultrafiltration (UF) membrane separation technology, nanofiltration (NF) membrane separation technology and reverse osmosis (R0) membrane separation technology, among which it is used to treat high-salt wastewater. The main ones are nanofiltration membrane separation technology and reverse osmosis membrane separation technology.

Ion exchange and membrane treatment have high treatment costs and strict equipment requirements. At the same time, the treatment membrane is easily polluted, and frequent backwashing and replacement of the treatment membrane are required, which causes inconvenience to the treatment, and the generated concentrated water needs further treatment by subsequent methods.

Evaporation and incineration method

Although ion exchange and membrane treatment can be used in actual production, the labor and cost input are too high, so evaporation and incineration methods have been developed.

At present, the high-concentration salt-containing wastewater treated by evaporation and incineration methods has a salt content of more than 8% to 20%. It needs to undergo a certain pretreatment before entering the equipment, and the final treatment has achieved good results.

Fluorine corrosion problem

At present, the treatment of high-salt wastewater by evaporation and incineration technology has a good treatment effect, but there are also disadvantages in the treatment process, that is, the corrosion problem of equipment is increasingly prominent, and the actual life of many equipment cannot reach the design life. Therefore, high-salt wastewater Dealing with equipment issues has also been given attention.

The commonly used industrial equipment is made of stainless steel, which is low in price and good in shape. However, because high-salt wastewater usually has high chlorine content and strong corrosiveness, it has anti-corrosion requirements for equipment materials. In order to prevent equipment from corroding, alternative materials with better anti-corrosion properties have been considered. , such as titanium materials and titanium alloys.

Titanium metal materials and titanium alloys have the advantages of good corrosion resistance, light weight and long service life, and have been widely used in evaporation and incineration treatment in recent years.

Unfortunately, the good times did not last long. Many titanium and titanium alloy equipments were found to be corroded after several years of use, or even shorter time. caused by ions.

Since an oxide film with good stability and strong bonding force is automatically formed on the surface of titanium, titanium alloys have good corrosion resistance in alkaline solutions, most organic acid solutions, inorganic salt solutions and oxidative media.

However, in a reducing acid solution, fluoride is easily combined with hydrogen ions to form hydrogen fluoride, which is preferentially adsorbed on the oxide film on the surface of the titanium material, and the expulsion of oxygen atoms causes the passive film on the surface of the titanium alloy to form soluble fluoride and corrode and be destroyed. , among which HF solution has the strongest corrosion effect on titanium metal.

At the beginning, the fluorine content is very low, which will not cause equipment corrosion, but as the treatment time prolongs, after the concentration and enrichment, the fluorine content continues to increase, exceeding the corrosion resistance of titanium materials, and finally causes fluorine corrosion. The main reactions that occur in the fluorine corrosion process are as follows:

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Through the main reaction formula of fluorine corrosion, the fluorine corrosion mechanism can be found, so as to find a solution to the fluorine corrosion equipment.

Conclusions and recommendations

Evaporation and incineration technology is currently a widely used and cost-effective method, which is worthy of industrial production. At the same time, it is pointed out that the limit of fluorine corrosion resistance of titanium metal equipment is 30ppm.

Therefore, fluorine corrosion can be prevented from two aspects in chemical production. One is to reduce the fluorine content in industrial fluorine-containing wastewater, and obtain calcium fluoride products by calcium precipitation;

The second is to solve the problem of fluorine corrosion of titanium equipment. It is urgent to develop a set of low-cost and high-efficiency deep fluoride removal technology for deep fluorine removal, improve equipment use environment, extend its service life, reduce the cost of high-salt wastewater treatment, reduce Wastewater treatment accident risk and promote chemical safety production.

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