Experimental research on removal of permanent hardness in water by CO₂-NaOH combined method

Chemical crystallization pellet fluidized bed reactor can remove the permanent hardness and temporary hardness in water. At present, the permanent hardness is generally removed by adding sodium carbonate (Na2CO3), and the combination of carbon dioxide (CO2) and sodium hydroxide (NaOH) can also effectively remove the permanent hardness in water.
The experimental study found that when n(Ca2+): n(HCO3-) = 2:1, n(NaOH): n(HCO3-) = 2:1, CO2 dosage was more than 5 mmol/L, the removal efficiency of Ca2+ can reach more than 60%, and the pharmacy cost was 0.45 yuan/m3. Increase of NaOH and CO2 dosage can continue to improve the removal efficiency of calcium ions, the highest removal efficiency can reach more than 95%.
When n(Ca2+): n(HCO3-) = 3:1, n(NaOH): n(HCO3-) = 3:1, the CO2 dosage was more than 6.5 mmol/L, the removal efficiency of Ca2+ can reach 75%, and the pharmacy cost was 0.45 yuan/m3. Increase the dosage of NaOH and CO2 can continue to improve the removal efficiency of Ca2+, the highest removal efficiency of 98%.
The results showed that the actual dosage of NaOH was generally more than 2 times of the theoretical dosage, and the dosage of CO2 was generally more than 1.5 times of the theoretical dosage, it was can remove the permanent hardness better. In the experiment, the utilization rate of CO2 is more than 80%.
In the later stage, the utilization rate of CO2 can be improved by adjusting the way of adding CO2, temperature and pressure. At the same time, compared with the combined dosage of NaOH-Na2CO3, it can be found that the pharmacy cost of the combined dosage of NaOH-Na2CO3 is higher than 1.0 yuan/m3, which is 2-3 times as much as that of the combined dosage of CO2-NaOH at the same calcium ions removal rate.
Therefore, it was feasible to remove the permanent hardness in the crystallization pellet fluidized bed reactor by using the combined method of CO2-NaOH, which had the advantages of low cost, high efficiency, softening water quality and reducing CO2 emission from industrial enterprises.