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Treating Phosphating Wastewater with Desulfurized Ash Modified as Coagulant


Treating Phosphating Wastewater with Desulfurized Ash Modified as Coagulant

Desulfurized ash is a kind of aluminosilicate material, which belongs to the solid waste generated by coal-fired thermal power plants. It is obtained by desulfurization of flue gas generated by thermal power plants. Its main components are alumina and silica.

From the perspective of microstructure, the desulfurized ash has a porous honeycomb structure inside, so the desulfurized ash can remove impurities in water by adsorption.

From the perspective of chemical reaction, since the main components of desulfurized ash are alumina and silica, as well as oxides of calcium and iron, such as CaO, the chemical reaction activity of desulfurized ash is strong, and it can produce Al3+ after encountering acid, Ca2+ and other cations combine with the anions in the liquid to form insoluble salts and precipitate out, and then purify the wastewater.

In summary, desulfurization ash can treat wastewater by physical and chemical adsorption. However, when the desulfurized ash is directly used as a water treatment coagulant, the treatment effect has not yet reached the ideal state.

In order to further improve the treatment efficiency of desulfurized ash in wastewater treatment, this study uses desulfurized ash from power plants as raw materials and uses sulfuric acid and hydrochloric acid to modify it. Finally, the ability of modified desulfurized ash to treat phosphating wastewater is verified by experiments.

Test process and method

1.1 Test materials

1.1.1 Desulfurized ash for test

The fly ash taken from the power plant is used as the desulfurization agent after the power plant flue gas purification, namely the desulfurization ash, after increasing the proportion of additives, and its chemical composition is shown in Table 1 below.


It can be seen from the above table that the total content of SiO2 and Al2O3 in the desulfurized ash is greater than 60%, and these two substances are the main components of the coagulant in water treatment.

1.1.2 Preparation of modified desulfurization ash

Reagent A for modification: sulfuric acid, 1 mol/L; reagent B: hydrochloric acid, 1 mol/L; 100 g of granular desulfurized ash were ground to 200 mesh and added to 500 mL of various acid solutions, and then at room temperature at 200 r/min The rotating speed was stirred for 30min, and the filtered desulfurization ash was dried as a water treatment agent.

1.1.3 Test water samples

Taken from a company’s phosphating wastewater, and its water quality is shown in Table 2.


1.2 Test method

Add a certain amount of modified desulfurization ash to the phosphating wastewater, adjust the pH value, firstly stir quickly (200r/min) for 10min, then slowly stir (100r/min) for 15min, then let stand for 30min, take the supernatant for measurement Absorbance value at 700nm wavelength and give phosphate concentration.

Test results of phosphorus removal effect

2.1 Sulfuric acid-modified desulfurization ash phosphorus removal test

In the test, different amounts of powdered sulfuric acid-modified desulfurization ash were added to six beakers respectively, and were stirred and reacted with the same stirring intensity on a six-coupled timing stirrer. The results are shown in Table 3 and Figure 1 below.


It can be seen from the above test results that the desulfurization ash modified with sulfuric acid has a significant effect on phosphorus removal from phosphating wastewater.

When the dosage reaches 10g/L, the removal rate reaches more than 94%, and the concentration of phosphate radical in the effluent is 0.72mg/L. When the dosage is increased, the removal effect is improved less. Therefore, the desulfurization ash dosage should be 10g/L.

2.2 Phosphorus removal test of hydrochloric acid-modified desulfurization ash

The desulfurized ash modified with hydrochloric acid was used as the coagulant, and other test procedures were the same as above. The test results are shown in Table 4 and Figure 2.


From the test data in the above chart, it can be seen that the desulfurization ash modified with hydrochloric acid is used as a coagulant, and its water purification trend is different from that of sulfuric acid. When the dosage is 15g/L, the PO43- concentration in the treated effluent is 0.65mg/L, and the removal rate is 95%.

When the dosage is between 15 and 50g/L, the PO43-concentration of the effluent does not change significantly with the dosage. When the dosage reaches more than 50g/L, with the increase of dosage, the PO43-concentration of effluent increases to some extent. In summary, the optimal dosage is 15g/L.

2.3 Experiment on the removal effect of chroma

In the test, desulfurized ash modified with sulfuric acid and hydrochloric acid was used as coagulants for decolorization test research. The test data and results are shown in Figure 3. Among them, A represents sulfuric acid-modified desulfurization ash, and B represents desulfurized ash modified with hydrochloric acid.


When the pH value of the water sample is 8, the test results of different modified desulfurization ash on the color of phosphating wastewater show that both modified desulfurization ash have higher decolorization efficiency.

The results shown in Figure 3 above show that the removal efficiency of the two desulfurization ash on chroma varies with the dosage. When the dosage of desulfurization ash is small (≤10g/L), the decolorization efficiency increases with the dosage of desulfurization ash. increase rapidly;

When the dosage of type A is 20g/L, its decolorization efficiency can be close to 90%, and when the decolorization efficiency of type Breaches 90%, its dosage must reach at least 40g/L, and then when the dosage is increased, the color is changed. The degree of removal does not contribute much.

It can be seen that the decolorization effect of type A is better than that of type B, so the desulfurization ash dosage should be 20-40g/L. For other types of water quality, field tests are required to determine the dosage.

Analysis of test results

The above test shows that the modified desulfurization ash has a good effect on phosphorus removal and decolorization of phosphating wastewater, and its mechanism of action mainly includes the following aspects:

(1) The production and production process of desulfurization ash is similar to that of activated carbon, so it has a large specific surface area and high surface energy, and there are many active points such as aluminum and silicon, such as Si-O-Si bonds, Al-O-Al bonds and Harmful molecules with a certain polarity produce dipole-dipole bond adsorption, and the secondary positively charged aluminum silicate, calcium silicate and iron silicate in the desulfurization ash and anions such as PO43- and chromophores are formed. It can form ion exchange or ion pair adsorption, so it has a strong adsorption capacity.

(2) The total content of (SiO2+Al2O3) in the desulfurization ash is more than 60%, and the silicon and aluminum components in it can interact with the harmful substances in the wastewater to cause flocculation and precipitation and form a synergistic effect of adsorption-flocculation precipitation with the desulfurization ash.

(3) The modified desulfurization ash is prepared under acidic conditions, so the dissolved Ca2+ ions can form precipitation with PO43-. In addition, the desulfurized ash after activation with acid also contains Al2(SO4)3, FeCl3, AlCl3, Fe2(SO4)3, H2SiO3, and other components, of which Al3+, Fe3+, etc. can form polynuclear hydroxyl complexions, which are effective for PO43- The ions have adsorption and complexation effects, and have a certain destabilizing effect on the suspended solids in the water, while the bridging net capture of polysilicon acid can aggregate fine colloidal particles into large flocs or alum flowers and rapidly settle.

To sum up, the phosphorus removal and decolorization of the modified desulfurization ash are the results of the synergistic effect of the destabilization and coagulation of the electrolyte, the coagulation aid of the high polymer such as silicic acid gel, and the adsorption and precipitation of the desulfurized ash particles.

4 Conclusion

In this study, sulfuric acid and hydrochloric acid were used to modify the desulfurized ash, and the ability of the modified desulfurized ash to treat phosphating wastewater was verified by experiments.

The test results show that by adding 10g/L of desulfurized ash modified by sulfuric acid to the phosphating wastewater, the removal rate of phosphate in the wastewater can reach more than 94%, and the effluent PO43- concentration is 0.72mg/L. Adding 15g/L hydrochloric acid-modified desulfurization ash to the ash, the removal rate of phosphate in the wastewater reaches more than 95%, and the PO43- concentration in the treated wastewater is 0.65mg/L.

The modified desulfurization ash obtained by the two modification methods has a strong ability to treat the phosphate in the phosphating wastewater. At the same time, since desulfurization ash is a solid waste produced by coal-fired thermal power plants, the acquisition cost is low. Therefore, the modified desulfurization ash has great engineering application value in the treatment of phosphating wastewater.