The desulfurization and denitrification wastewater (DDW) from the wet flue gas treatment project is difficult to be treated and recycled because of high chloride ion (Cl−) concentration. Cl− can cause equipment and piping corrosion. However, there is a lack of cost-effective treatment technologies for the removal of Cl− from the DDW. In this research, the feasibility of Cl− removal from the DDW using Friedel’s salt precipitation method was evaluated. Factors affecting the Cl− removal, such as Ca(OH)2 dosage, NaAlO2 dosage, solution’s initial pH, solution’s temperature, reaction time, stirring speed, and anions (SO42−, NO3−, and F−), were investigated, and the optimal experimental conditions for Cl− removal were determined. Experimental results showed that Friedel’s salt precipitation method can remove Cl− effectively and can achieve synergistic removal of SO42−, F−, and heavy metal ions. Under the best experimental conditions, the average removal efficiencies of Cl−, SO42−, F−, and heavy metal ions reach more than 85%, 98%, 94%, and 99%, respectively. The Cl− removal mechanism studies showed that Cl− can be removed by precipitation as Ca4Al2Cl2(OH)12. The purified wastewater and the precipitated solid can be reused to reduce the consumption of water and alkali. Friedel’s salt precipitation method is an effective control technology for the synergistic removal of Cl−, SO42−, F−, and heavy metal ions and has enormous potential to be applied in the industrial wastewater treatment field.
The wet flue gas desulfurization (WFGD) technology is one of the world’s most widely used flue gas desulfurization technology due to its high desulfurization efficiency and low investment and operationg costs [
Currently, various Cl− removal technologies have been developed, mainly including evaporation crystallization [
However, these studies focused on the Cl− removal from the circulating cooling water and did not consider the effect of coexistent anions, such as SO42−, NO3−, and F−, on the Cl− removal. Meanwhile, there is little information about the effect of process parameters, such as the solution’s initial pH, reaction temperature, and reaction time on the Cl− removal. The composition of the ions in the circulating cooling water is relatively simple, and the concentration of different kinds of ions is also lower compared to that of the DDW and the DW, but nowadays little information on the Cl− removal from the DDW or the DW using Friedel’s salt precipitation method can be found in the literatures. Therefore, the research aims to evaluate the feasibility of the Cl− removal from the wet flue gas DDW using Friedel’s salt precipitation method. A series of experiments to evaluate the influence of different factors on Cl− removal were carried out, and the Cl− removal mechanism by this process was hypothesized. In addition, removal of Cl− and other ions in the actual DDW using Friedel’s salt precipitation method was also studied.
NaAlO2, Ca(OH)2, Na2SO4, NaOH, NaF, NaCl, KNO3, and HNO3 were analytical grade, and directly used without purification. The authors tested Cl− concentration in the actual DDW and found that the Cl− concentration in the DDW was about 1000–3000 mg/L. So chloride-rich simulated wastewater used in this study was prepared by dissolving anhydrous NaCl in deionized water to get initial Cl− concentration of 2000 mg/L. The solution’s initial pH was adjusted using HNO3 (0.1 mol/L) and NaOH (0.1 mol/L). The actual DDW was obtained from a ceramic production enterprise located in Guangdong province, China.
The solution’s pH was measured with an MP511 pH detector (Shanghai Precision Instruments Co., Ltd.). Concentrations of NO3−, F−, SO42−, and Cl− were measured with an ion chromatography system (Metrohm 883, Switzerland), and concentrations of heavy metal ions such as Ni2+, Pb2+, and Mn2+ were determined with an inductively coupled plasma emission spectrometer (ICP-AES 710, Agilent technologies). The precipitated solids were collected by filtering. The separated solids were then dried at room temperature. X-ray diffraction (XRD) was performed on the solids using an X-ray diffractometer (XRD-6000, Shimadzu, Japan).
Experiments were carried out on a six-league electric blender (ZR4-6, China). The experimental steps of Friedel’s salt precipitation method are as follows: the first step was conducted by adding a certain amount of Ca(OH)2 and NaAlO2 to the NaCl solution (2000 mg/L) with a volume of 1 L at the specified reaction temperature; then stirring for a certain time, at last samples were taken and filtered under vacuum through a 0.45
According to the characteristics of ion composition of actual DDW, the literatures [
Experimental conditions of the individual experiments.
Number | Experiment | Experimental conditions |
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1 | The effect of NaAlO2 dosage |
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2 | The effect of Ca(OH)2 dosage |
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3 | The effect of the solution’s initial pH |
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4 | The effect of the solution’s temperature |
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5 | The effect of reaction time |
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6 | The effect of stirring speed |
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7 | The effect of SO42− concentration |
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8 | The effect of F− concentration |
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9 | The effect of NO3− concentration |
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10 | The effect of precipitated solid reuse |
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11 | Removal of chloride ion and other ions in actual DDW | Two-stage Freund’s salt precipitation method: (1) First stage mainly removed SO42−, |
It was reported that Cl− removal was primarily controlled by the formation of Ca4Al2Cl2(OH)12 [
The effect of NaAlO2 dosage.
Figure
The effect of Ca(OH)2 dosage.
In order to further determine the effect of the dosage of chemical reagents on Cl− removal and to determine the reaction product type, XRD was used to examine the crystalline phases of the precipitated solids produced under different conditions, and the results are shown in Figure
XRD patterns of precipitated solids: (a) different Ca(OH)2 dosage; (b) different NaAlO2 dosage. F(Ca4Al2Cl2(OH)12), Ca3(Ca3Al2(OH)12), Ca4(Ca4Al2(OH)14), Ca(Ca(OH)2).
Examination of the samples indicates the presence of mixed phases, the major crystalline phases are Ca4Al2Cl2(OH)12 (ICDD PDF card # 35-0105, 2
Figure
Based on the experimental results, composition of the solids, and the literatures [
Considering the Cl− removal and economic costs, in the next series of experiments, the molar ratio of Ca(OH)2 to NaAlO2 to Cl− were constant at 6 : 3 : 1.
The effect of the solution’s initial pH ranging from 3.0 to 11.0 on Cl− removal has been studied. Figure
The effect of the solution’s initial pH.
Figure
The effect of the solution’s temperature.
The effect of reaction time on Cl− removal is shown in Figure
The effect of reaction time.
Stirring speed has a significant effect on Cl− removal. As shown in Figure
The effect of stirring speed.
Various types of anions such as SO42−, NO3−, and F− exist in the DDW and DW, and the concentrations of these anions are often high. So, coexistent anions in the solution have a certain effect on Cl− removal. In this paper, the effect of anions on Cl− removal has been studied, and the results are shown in Figures
The effect of SO42− concentration.
The effect of F− concentration.
The effect of NO3− concentration.
Figure
Results show that the presence of SO42− in the solution has a significant inhibitory effect on Cl− removal. In order to achieve high Cl− removal, it is necessary to remove the SO42− from the solution first. Table
Results of Cl− and SO42− removal.
Number | Average | Standard deviation | |||
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1 | 2 | 3 | |||
Average removal efficiencies of Cl− (%) | 85.49 | 87.56 | 86.26 | 86.44 | 0.138 |
Average removal efficiencies of SO42− (%) | 99.06 | 99.10 | 98.60 | 98.92 | 0.278 |
Two-stage Freund’s salt precipitation method. (1) The first stage mainly removed SO42− (4000 mg/L), the molar ratio of Ca(OH)2 to NaAlO2 to SO42− was constant at 4 : 1 : 1; the solution’s temperature was 25°C, reaction time was 30 min, and the stirring speed was 400 r/min. (2) The second stage mainly removed Cl− (2000 mg/L), the molar ratio of Ca(OH)2 to NaAlO2 to Cl− was constant at 6 : 3 : 1, respectively, and other operating conditions were the same as the first stage.
Figure
The effect of NO3− concentration on Cl− removal is shown in Figure
The results indicate that using Friedel’s salt precipitation method can effectively remove Cl− in the solution. Cl− removal depends primarily on the Ca(OH)2 dosage, NaAlO2 dosage, the solution’s temperature, and SO42− concentration, and the solution’s initial pH, reaction time, stirring speed, NO3−, and F− concentrations all have a certain influence on NOx removal, but these factors have a relatively little influence on the Cl− removal. Finally, considering the application to the practical engineering, the optimal conditions for Cl− removal using Friedel’s salt precipitation method were identified: (1) For sulfate-free wastewater, use one-stage Freund’s salt precipitation method to remove Cl−. The optimal conditions were the molar ratio of Ca(OH)2 to NaAlO2 to Cl− 6 : 3 : 1, solution’s temperature of 25°C, reaction time of 30 min, and stirring speed of 400 r/min. (2) For sulfate-containing wastewater, use two-stage Freund’s salt precipitation method to remove Cl−. First stage mainly removed SO42−, the optimal conditions were the molar ratio of Ca(OH)2 to NaAlO2 to SO42− 4 : 1 : 1, solution’s temperature of 25°C, reaction time of 30 min and stirring speed of 400 r/min. Second stage mainly removed Cl−, and the optimal conditions were the same as those used for sulfate-free wastewater.
The XRD test (Figure
The effect of precipitated solid reuse.
Removal of Cl− and other ions in actual wastewater by using Friedel’s salt precipitation method was studied. The wastewater was the actual DDW from a ceramic plant. The NaClO2/NaOH solution was used to remove the NOx and SO2 in the flue gas, and the wet flue gas desulfurization and denitrification system was operated under weak acid condition. So, the effluent from the system contained large amounts of Cl− and other ions (Tables
Results of anion removal.
SO42− | Cl− | F− | NO3− | pH | |
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Actual wastewater (mg/L) | 12309.5 | 1476.7 | 244.9 | 955.3 | 5.3 |
Purified wastewater (mg/L) | 188.34 | 215.16 | 8.84 | 790.70 | 13.2 |
Average removal efficiency (%) | 98.47 | 85.43 | 96.39 | 17.23 | — |
Results of heavy metal ion removal.
Cd2+ | Mg2+ | Mn2+ | Ni2+ | Pb2+ | |
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Actual wastewater (mg/L) | 4.332 | 10.88 | 23.85 | 37.62 | 9.20 |
Purified wastewater (mg/L) | — | 0.086 | 0.006 | — | — |
Average removal efficiency (%) | 100 | 99.21 | 99.97 | 100 | 100 |
In this study, Friedel’s salt precipitation method was used to remove the Cl−, and the effects of different experimental conditions on Cl− removal were mainly studied. Based on the results of the experiments, the following conclusions can be made: Friedel’s salt precipitation method is a very effective Cl− removal technology, and Cl− removal can reach more than 85%. Meanwhile, the method can effectively synergistically remove SO42−, F−, and heavy metal ions. The purified wastewater can be reused to reduce the consumption of water and alkali, and the precipitated solids can be used to replace part of Ca(OH)2. Thus, it has a great potential to be applied in the industrial wastewater treatment field. Ca(OH)2 dosage, NaAlO2 dosage, the solution’s initial pH, the solution’s temperature, reaction time, stirring speed, and anions (SO42−, NO3− and F−) have all effects on the Cl− removal. Finally, considering the application to the practical engineering, the optimal conditions for Cl− removal using Friedel’s salt precipitation method were determined. The removal mechanism of Cl− was deduced based on the experimental results, composition of the precipitated solids, and the literatures. The results showed that Cl− can be removed by precipitation as Ca4Al2Cl2(OH)12.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no competing interests.
This work was supported by the National Key R&D Program of China (2017YFC0210704, 2017YFC0210803), the National Natural Science Foundation of China (NSFC-51778264), the Natural Science Foundation of Guangdong Province (2015A030310344), the Project of Science and Technology Program of Guangdong Province (2015A020220008, 2015B020215008, 2016B020241002, and 2017B020237002), the Youth Top-notch Talent Special Support Program of Guangdong Province (2016TQ03Z576), and the Pearl River S&T Nova Program of Guangzhou (201610010150).