Fluorinated wastewater is produced in many industrial productions, such as smelting of non-ferrous metals and rare metals in traditional industries, pickling of stainless steel, pesticides, aluminum electrolytic refining, etc., as well as organic chemical synthesis in modern industries, electronics industry, atomic energy industry, etc. Fluorine is one of the essential trace elements for the human body. An appropriate amount of fluorine is beneficial to human health. Too little or too much fluorine in the human body will cause harm to health. The daily fluoride requirement for adults is 1.0-1.5mg, and the fluoride content in people's daily drinking water should not exceed 0.6mg/L. If the fluoride in drinking water exceeds 1.5mg/L, it is high fluoride water. Drinking high fluoride water can cause great harm to the body. High fluoride water is mainly caused by the illegal discharge of fluorine-containing wastewater produced by industrial production. Therefore, proper treatment of fluorine in industrial wastewater is of great significance to protect the environment and ensure human health. Judging from the current treatment methods of fluorine-containing wastewater, the two most widely used methods are precipitation method and adsorption method. This article mainly discusses the precipitation method.
1. Chemical precipitation
The principle of chemical precipitation treatment of fluorine-containing wastewater is to separate fluorine from water by adding some fluorine-containing wastewater to the fluorine-containing wastewater, which can react chemically with fluoride ions in the wastewater, and can produce precipitation substances that are insoluble in water. The commonly used precipitants in chemical precipitation methods include lime, calcium carbide slag and calcium chloride.
Lime precipitation method
The lime precipitation method is an important method for treating high-concentration fluorine-containing wastewater. The calcium ions generated after the dissolution of limestone can react with the fluoride ions in the water to generate calcium fluoride that is insoluble in water, thereby removing the fluorine in the water. In the actual application process, since the calcium fluoride generated by the reaction will hinder the one-step reaction, it is necessary to add lime. Under the condition of pH 6.5-7.0, the precipitation reaction time is about 1h, and the dosage is more than 2.5 times of the theoretical value, which is the best working condition for defluorination.
Calcium carbide slag precipitation method
When polyethylene is produced by calcium carbide method, waste residue will be generated. These waste residues are generated by the reaction of calcium carbide and water, which are not only cheap but also easy to obtain. It is precisely because of this feature that calcium carbide residue precipitation method is widely used in the actual treatment of fluorine-containing water. application. The main component of calcium carbide slag is Ca(OH)2, so the principle of fluorine removal is similar to that of lime precipitation, but it has better effect, and the required dosage is less, and the sediment is easier to dehydrate and precipitate.
Calcium chloride method
The principle of calcium chloride method is similar to that of lime precipitation method, but calcium chloride has high solubility and can be added to wastewater in solution state, which can react more fully with fluoride ions in water. It has the advantages of less solid slag, less dosing dust, and simple and convenient operation. The disadvantage is that the price is more expensive than calcium hydroxide, and the cost is higher when dealing with high-concentration fluorine-containing wastewater. In addition, since most fluorine-containing wastewater is acidic, after calcium chloride treatment, it needs to be neutralized by adding alkali, which increases the cost of wastewater treatment.
2. Coagulation sedimentation method
The coagulation sedimentation method is a method of adding substances that have coagulation ability or can precipitate with fluoride into the fluorine-containing wastewater, so that the fluorine in the wastewater can generate a large amount of colloids and insoluble substances with it, and then separate by precipitation and muddy water. A method to remove fluoride from water bodies. Compared with the chemical precipitation method, the coagulation precipitation method not only requires less amount of medicine, but also has a larger processing capacity. After one treatment, the fluoride concentration can be lower than 10mg/L. The disadvantage is that in the treatment of high-concentration fluorine-containing wastewater, due to the need to use a large amount of coagulant, the cost is relatively high, so the chemical precipitation method and the coagulation precipitation method are usually used in combination, and the chemical precipitation method is used first. Reduce the concentration of fluorine in the wastewater, and then add a coagulant to settle and adsorb, so that the wastewater can meet the discharge standard.
Inorganic coagulants
Inorganic coagulants mainly include two kinds of aluminum salts and iron salts. The hydroxide alumization produced by the hydrolysis of these two kinds of salts can play the role of adsorbing fluoride ions. In addition, the more Al3+ and Fe3+ can undergo complex reaction with fluoride ions, so as to achieve the effect of removing fluoride ions.
The aluminum salt coagulation precipitation method mainly uses aluminum salts such as aluminum sulfate, polyaluminum sulfate and polyaluminum chloride. Adding them to the fluorine-containing wastewater can effectively remove the fluoride in it. This method has a good fluorine removal effect. The dosage of medicine is relatively small, the cost is relatively low, and the concentration of fluoride in the treated wastewater is less than 10mg/L.
The iron salt coagulation precipitation method mainly uses ferrous sulfate, polyferric sulfate and ferric chloride and other agents. In particular, polymeric ferric sulfate has a very good defluorination effect. Relevant experiments show that it can remove more than 96% of fluoride ions in wastewater, and its modification treatment can further improve its defluorination efficiency and shorten the reaction time. In addition, iron salt coagulants are usually applied together with Ca(OH)2, which can effectively improve the fluorine removal efficiency, but the treated water needs to be adjusted in pH to make it neutral before it can be discharged, so the process is relatively complicated. .
Organic coagulants
Polyacrylamide (PAM) is a water-soluble polymer with very good flocculation properties. There are three main types of PAM used in water treatment, namely non-ionic, anionic and cationic, depending on the pH of the wastewater. to select the appropriate PAM agent type. PAM cannot directly remove fluorine, but it can play the role of adsorption bridging, accelerate the formation of flocs, thereby improving the effect and speed of sedimentation. In practical applications, adding a small amount of PAM can achieve the effect, so it will not cause secondary pollution. In addition to PAM, chitosan and acrylamide-modified chitosan, lignin, etc. also have very good defluorination effects.
At present, the precipitation method is the most important fluorine-containing wastewater treatment method. Among them, the precipitation method can be divided into chemical precipitation method and coagulation precipitation method. Chemical precipitation method is widely used in industrial wastewater treatment, and has the advantages of simple method, low cost and good effect; In the treatment of concentrated fluorine-containing wastewater, it has the advantages of less amount of chemicals to be added and a large amount of treatment, and the discharge standard can be met after one treatment. Therefore, chemical precipitation is usually used to reduce the concentration of fluorine in high-fluoride wastewater, and then it is treated again by coagulation and precipitation to make it meet the discharge standard.