Extraction towers for treating phenol-containing wastewater in the coke production process

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Extraction towers are used to treat phenol-containing wastewater generated during coke production. Coking wastewater is a type of organic wastewater that is difficult to biodegrade, with high concentrations of ammonia nitrogen and organic substances; the main toxic pollutants in such wastewater are phenolic compounds. In recent years, various biological treatment methods have been developed for coking wastewater. This article focuses on the use of extraction towers for treating phenol-containing wastewater from coke production. Phenols are classified as Category 2 pollutants according to the GB8978-1996 Standard for Comprehensive Wastewater Discharge, with allowable discharge concentrations of 0.5 mg/L for both Category 1 and Category 2 discharges. The approach to dealing with phenol-containing wastewater in industrial production is first to actively promote clean production, that is, to reform production processes and improve operational management in order to minimize the phenol concentration in the wastewater discharged from production facilities. Or recycle phenol-containing wastewater to reduce waste volume ； Secondly, it is necessary to strengthen end-stage treatment; once phenol-containing wastewater is discharged from the production facilities, it should be recycled and reprocessed as much as possible to ensure that it is discharged in compliance with standards. Wastewater containing phenols at a concentration of 1000 mg/L or higher is generally referred to as high-concentration phenol-containing wastewater; such wastewater must have the phenols recovered before it can be treated ； Phenol-containing wastewater with a concentration of less than 1000 mg/L is referred to as low-concentration phenol-containing wastewater; such wastewater is usually recycled, with the phenol being concentrated and recovered for further treatment ； Wastewater with a phenol concentration of 300 mg/L or less can be treated using methods such as adsorption, biochemical treatment, and chemical oxidation to meet discharge standards. With the development of current extraction equipment and the increasing maturity of extraction technology in our country, the extraction method is widely used in the treatment of phenol-containing wastewater at present. Research on using this method for wastewater treatment has been carried out both domestically and internationally, with certain results achieved. Given the current situation in our country, the main problem is the disposal of concentrated COD components and color components. Due to our national conditions, we cannot use extractants such as liquid CO2; moreover, common extractants generally require back-extraction, after which the solvent must be reused while the back-extraction liquid needs to be treated. The wastewater treated in China shares one common feature: a large proportion of the organic pollutants in it serve as raw materials or products for reactions. Therefore, it is possible to start by studying extraction methods in order to improve the selectivity of extraction, thereby linking the back-extraction liquid to the original production process. If this is achieved, solvent extraction for treating high-concentration colored wastewater will be widely used due to its economic efficiency and practicality. In the treatment process of phenol-containing wastewater, pulse vibration is employed to create pulses between the extractant and the phenol-water mixture, thereby ensuring thorough contact between them. This approach prevents emulsions from accumulating in the sieve pores of the extraction tower, avoids clogging of those pores, and maintains their openness. However, the amplitude in the pulse must be appropriate; an excessive amplitude can cause flooding of the liquid, leading to mutual entrainment between the extractant and the phenol solution, and resulting in a significant loss of extractant. Foaming can also cause water to enter the alkali scrubber tower, affecting the quality of sodium phenolate. The pulse amplitude is generally kept within 35 mm. 　　During the extraction process, emulsions are formed, and these emulsions accumulate on the screens inside the extraction tower, blocking the sieve pores. This prevents the extractant from coming into full contact with the phenol-containing water. Additionally, the large amount of emulsion makes it difficult to separate oil from water, leading to significant loss of the extractant. To break down the emulsion, it is necessary to remove the hydrophilic substance in the phenol water – tar. 　　First, the phenol water is allowed to settle statically in the storage tank, with tar being discharged from the bottom of the tank on a regular basis. Next, the phenol water free of tar is passed through a coke filter filled with 25mm coke particles, thereby removing the gelatinous tar from the phenol water as well. 　　The amount of water used for phenol treatment should be appropriate; too much water can lead to liquid flooding, and it is generally necessary to keep this amount at 8–12 T/h. After being used in extraction for a period of time, the extractant should be regenerated promptly. Passing the extractant through hot water at 75–85°C breaks down the emulsified oil in the extractant, allowing its quality to be restored. 　　An excessively high two-phase interface in the extraction tower can result in water being carried in the oil output from the tower, which affects the quality of sodium phenolate in the alkaline washing tower. An excessively low two-phase interface in the extraction column results in a too low extraction height within the column, which affects the extraction efficiency. Therefore, it is necessary to accurately control the height of the two-phase interface inside the extraction tower, keeping it below the middle part of the tower.