In the flue gas treatment of waste incineration plants, the “scrubber + deacidification tower” secondary absorption device process is usually used to remove pollutants such as soot, acid gas, heavy metals, dioxins and other pollutants in the flue gas. This absorption process will produce two kinds of Wastewater, namely washing wastewater and deacidification wastewater.
Biological treatment of deacidification wastewater, the inorganic salts in the wastewater have inhibitory effect on microorganisms and cannot effectively degrade the organic matter in the wastewater, so the evaporative concentration of deacidification wastewater has certain advantages.
The main components of deacidification wastewater are sulfate, chloride, fluoride, nitrate, phosphate and carbonate ions, and calcium chloride is used to remove sulfate, carbonate, phosphate and fluoride ions. Through evaporation and concentration times, industrial sodium chloride is recovered, and the evaporative water is reused in the incineration workshop, and the deacidification wastewater achieves the purpose of comprehensive utilization.
1.1 Experimental method
Take 1L of deacidified wastewater into a beaker, and after aeration for 1h pretreatment, add calcium chloride of different quality, stir, filter, evaporate and concentrate the filtrate, crystals are precipitated and filtered to obtain sodium chloride product, after dissolving and diluting to constant volume, The purity of sodium chloride was analyzed by ion chromatography.
1.2 Main components of deacidification wastewater
The deacidification wastewater was analyzed by ion chromatography, and the main anions were measured in Table 1.
Experimental results and discussion
2.1 Effect of calcium chloride dosage
A large amount of ions in the deacidification wastewater can be removed by calcium ion precipitation, and the experiment uses the reagent calcium chloride to stir to remove part of the precipitated ions. In the experiment, 1L of deacidified wastewater was taken, and different dosages of calcium chloride were added according to the content of fluoride ions. After precipitation and filtration, ion chromatography was carried out.
Through Table 2, it can be found that the amount of calcium chloride increases, the fluoride ion, sulfate, phosphate and carbonate ions all decrease, and the precipitation effect is more obvious, but when the amount of calcium chloride is increased from 16g to 24g, the ion precipitation effect is reduced, that is, chlorine The amount of calcium chloride is too high, which leads to waste of calcium chloride and scaling during re-evaporation. Therefore, the amount of calcium chloride used in the experiment is 16g, which is 8 times the mass of fluoride ions.
2.2 Influence of evaporative concentration times
In the process of evaporation and concentration, the deacidification wastewater is added in stages. The main reason is: in the process of evaporation and concentration, a small amount of sodium sulfate will be precipitated, and the added deacidification wastewater can just dissolve the deacidified sodium sulfate to achieve the purpose of washing salt.
From Figure 1, it can be found that the purity of sodium chloride decreases with the increase of the concentration ratio. The reason is: in the process of concentration and filtration, a small amount of sodium sulfate is precipitated due to the temperature drop, resulting in sodium chloride. Purity decreased.
The purity of industrialized sodium chloride is ≥99.5%. When the experiment is concentrated by 4 times of evaporation, the purity of sodium chloride can reach 99.6%, so the evaporation and concentration of the experiment is 4 times.
Add the deacidification wastewater to 1L from the evaporation concentrate, go through aeration and calcium chloride pretreatment, continue the evaporation and concentration process, and apply it cyclically. It can be found from Figure 2 that the purity of sodium chloride tends to balance first and then decrease with the increase of reuse times.
wwThe reason: the amount of calcium chloride is calculated according to the mass of fluoride ions, which leads to a small amount of calcium chloride in the subsequent application experiment process, and the mass of sulfate radical plasma increases, which is precipitated during the re-concentration process, which affects the purity of sodium chloride. In the figure, when the evaporation concentration is reused 10 times, the purity of sodium chloride can reach industrial production, so the experiment adopts the evaporation concentration reuse 10 times.
(1) After calcium chloride and aeration pretreatment, sulfate and carbonate ions are removed, and evaporation and concentration are carried out. The purity of the recovered sodium chloride is ≥99.5%, which meets the requirements of industrialization. The ratio of the experimental process is that the amount of calcium chloride is 8 times that of fluoride ions, the concentration by evaporation is 4 times, and the mother liquor is reused 10 times, and the evaporated water can be reused in various workshop departments.
(2) The whole process is simple, easy to operate and low cost. This has positive significance for the resource recycling of deacidification wastewater and reducing its pollution to water treatment.