The Removal of Scale Ions from Circulating Cooling Wastewater by Chemical Coagulation Process

In this paper, the circulating cooling wastewater was treated by chemical coagulation process through adding NaOH/Na2CO3.The effect of NaOH and Na2CO3 dose on removal of scale ions, such as Ca2+、Mg2+、Ba2+、Sr2+、SiO2, was studied and the removal mechanism was discussed. The results showed that the increase of NaOH dose was beneficial to the removal of above-mentioned scale ions. When NaOH was only added, the removal efficiency of Ca2+、Mg2+、Ba2+、Sr2+、SiO2 was 86.3%, 91.6%, 86.5%, 58.1%, 84.2%, respectively. When 680 mg/L of NaOH and 300 mg/L of Na2CO3 were added, and the effluent pH was above 11.2, the removal efficiency of Ca2+、Mg2+ was 95.8% and 89.4%, respectively, and the concentration of Ca2+and Mg2+ was below 20 mg/L, which met the target of wastewater treatment. Finally the possible removal mechanism of Ca2+, Mg2+, Ba2+, Sr2+and SiO2 was discussed.


Introduction
A power plant was designedto apply technology of reverse osmosis membrance to treat its circulating cooling water.And the treated water would be completely reused instead of being discharged.However the high hardness of water impacted the process of reverse osmosis, which had to be soft by a pre-treatment process.Conventional softening methods include chemical coagulation and ion exchange [1][2][3].And as the most widely used method, chemical coagulation include several subentry, such as lime softening method, lime-sodium carbonate softening method, sodium hydroxide-sodium carbonate softening method [4][5].Enterprise prefer sodium hydroxide soften method to lime softening, because it show less material-consumption, lower secondary-pollution and more simply operated [6].
The current literature about chemical coagulation soften methods mainly focus on removing conventional scaling ions such as Ca 2+ and Mg 2+ , and research on removal of unconventional scaling ions, such as Ba 2+ , Sr 2+ and SiO 2 was not many [7][8][9].Though applying "NaOH+Na2CO3" soften method to treat circulating cooling waterof the power plant, this paper studied the removal efficacy of scaling ions, such as Ca 2+ , Mg 2+ , Ba 2+  , Sr 2+ and SiO 2 .The related reaction mechanism was deduced.

Experimental approach
The soften coagulation experiment was carried out in a beaker [10]    declined from 56.2mg/L to 39.9mg/L, and the removal efficiency went up from 77.9% to 86%.Similarly, Table 2 shows that the concentration of Mg 2+ gradually decreased with increasing of pH, the concentration of Mg 2+ of raw water was 140mg/L, when pH raise from 10.5 to 11.3, the concentration of Mg 2+ declined from 110mg/L to 11.7mg/L, and the removal efficiency went up from 21.4% to 91.6%.When pH was greater than 7.5, the removal efficiency of Ca 2+ was greater than 75%.However, the removal efficiency of Mg 2+ was able to exceed 75 when pH razing to 11.1.It states that comparing with Mg 2+ , Ca 2+ was able to be removed in lower alkalinity condition.The pH should be greater than 11 when Mg 2+ forming to Mg(OH) 2 [11].According to the literature of Wang [12], Ca 2+ was mainly removed as CaCO3, and Mg 2+ was mainly removed as Mg(OH) 2 .It can be speculated that removing Ca 2+ and Mg 2+ according to following reactions:

The analytical approach of water quality
(3)

The Influence of dosing quantity of Na2CO3 on removing Ca 2+ and Mg 2+
As mentioned earlier, adding NaOH could lead high removal efficiency of Ca 2+ and Mg 2+ .For higher removal efficiency and more reasonable dosage of NaOH, "NaOH+Na 2 CO 3 "dual-alkali soften method was applied in this research.The dosage of PFS was kept as 100mg/L, while NaOH was 680mg/L and pH was about 11.2.To regulate the dosage of Na 2 CO 3 and the change about concentration of Ca2+ and Mg2+ was shown in Table 3, Fig. 2 and Fig. 3. Fig. 4 shows when the dosage of Na 2 CO 3 raised from 50mg/L to 300mg/L, Ca 2+ decreased from 49.6mg/L to 10.6mg/L, and removal efficiency increased from 80.5% to 95.8%.Though adding the dosage of Na 2 CO 3 , Ca 2+ was no longer change.It is because that CO 3 2-and Ca 2+ reacted to form CaCO 3 , which leading to decrease of Ca 2+ .When CaCO3 reaches solubility equilibrium, the concentration of Ca 2+ reaches its own equilibrium.Fig. 5 states that when dosage of Na 2 CO 3 increased, the removal efficiency of Mg 2+ went up slightly.It is because that Mg 2+ was mainly removed as Mg(OH) 2 , the addition of CO 3 2-made few contribution to pH.It can be speculated that "NaOH+Na2CO3" dual-alkali soften method not only involve in reaction (1) ~reaction ( 4), and there should be reaction (5):

CaCl2+Na2CO3→CaCO3↓+2NaCl
(5) Theoretical dosages of NaOH and Na 2 CO 3 can be estimated by following reactions NaOH=40(Hz+HMg+CO2+Fe+K+Ac) (7) Na2CO3=53(HCa -2×Hz-CO2+β) ( Hz-the hardness of carbonate in raw water, mmol/L; HMgthe hardness of magnesium in raw water, mmol/L; HCa-the hardness of calcium in raw water, mmol/L; CO2the contentof carbon dioxide in raw water, nmmol/L; Fe-the contentof iron in raw water, mmol/L; K-the dosage of coagulant, 0.1-0.5mmol/L;Ac-the excess alkalinity of NaOH, 0.2-0.4mmol/L;β-the excess amount of Na2CO3, 1.0-1.4mmol/L.According to data of water quality, theoretical dosages of NaOH and Na 2 CO 3 are 380mg/L and 46mg/L respectively.However, experimental dosages of NaOH and Na 2 CO 3 are 680mg/L and 300mg/L respectively, greater than the theoretical value.The researcher Liu [14] agrees the point that the theoretical dosage is different from that of experiment.Therefore the theoretical value should only be intended as a reference, and the optimum dosage need to be ascertained by experiment.Only adding NaOH without Na 2 CO 3, the dosage of PFS was kept as 100mg/L with the gradual increase in NaOH ranging from 400 to 560 mg/L.The pH ranged from 10.5-11.3m,and the change about concentrations of Ba 2+ , Sr 2+ and SiO 2 were shown in Table 5, Fig. 6, Fig. 7and Fig. 8.The result demonstrated that the concentrations of Ba 2+ , Sr 2+ and SiO 2 were gradually decreased with increasing of pH, which consistent with the literature [6].When pH raise from 10.5 to 11.3, the concentration of Ba2+ declined from 0.09mg/L to 0.036mg/L, and the removal efficiency went up from 66.3% to 86.5%; the concentration of Sr 2+ decreased from 2.09mg/L to 1.11mg/L, and the removal efficiency increased from 21.1% to58.1%; the concentration of SiO 2 descend from 27.6mg/L to 5.96mg/L, and the removal efficiency raised from 26.8% to 84.2%.The increase of removal efficiency of Ba 2+ and Sr 2+ may be caused by rising pH.When pH kept rising, HCO3-continuously turned to CO 3 2-, which further made Ba 2+ and Sr 2+ translated to BaCO 3 and SrCO 3 respectively ( be removed as sediment).The rising pH also promoted the reaction that SiO 2 turned to SiO 3 2-, and then the SiO 3 2-reacting with Ca 2+ (Mg 2+ ) to form CaCO 3 (MgCO 3 ) [15][16].
It can be speculated that using NaOH to remove Ba 2+ , Sr

Conclusion
For preferable removal efficiency, the dosage of NaOH and Na 2 CO 3 should be regulated to 680mg/L and 380mg/L respectively, and the pH should be no less than 11.2.The predicting outcomes are that Both Ca 2+ and Mg 2+ will be no less than 20mg/L, which meet the target of wastewater treatment.
This paper deducts the removal mechanism: scale ions are mainly removed by translated to sediment.Ca 2+ is translated to CaCO 3 ; Mg 2+ is turned to Mg(OH)2; Ba 2+ and Sr 2+ become BaCO 3 and SrCO 3 respectively; SiO 2 may be formed as CaSiO 3 and MgSiO 3 .

Fig. 1
Fig.1 Experimental process Influence of Dosing Quantity of NaOH on Removing Ca 2+ and Mg 2+Only adding NaOH without Na 2 CO 3 , the dosage of PFS was kept as 100mg/L with the gradual increase in NaOH ranging from 400 to 720 mg/L.The pH ranged from 10.5-11.3m,and the change about concentration of Ca2+ and Mg 2+ was shown in Table3, Figure2andFigure3.Table3demonstrated that the concentration of Ca2+trend down with increasing of pH, the concentration of Ca 2+ of raw water was 254mg/L, when pH raise from 10.5 to 11.3, the concentration of Ca 2+

Table 1
Circulating cooling water quality of the power plant unit: mg/L (except pH) PFS) with 200r/min stirring for 2 minutes.Next step is adding 1mg/L anionic polyacrylamide (PAM, molecular weight: 12 million) with 50r/min stirring for 20 minutes.After 30 minutes standing, the water was made into simple and analyzed.This paper treated concentration of Ca 2+ , Mg 2+ , Ba 2+ , Sr 2+ and SiO 2 as evaluation index to analyse the experimental result.

Table 2
Analytical approach of water quality

Table 3
Change of Concentration of Ca 2+ , Mg 2+ with influence of dosage of NaOH

Table 4
Change of concentration of Ca 2+ , Mg 2+ with influence of dosage of Na2CO3(pH=11.2)

Table 5
Change of concentration of Ba 2+ , Sr 2+ , SiO 2 with influence of dosage of NaOH