CHAPTER 3: COMPOSITION ANALYSIS AND EVALUATED PROCESSES
3.3. PRECIPITATION TREATMENT AND RECOVERY OF CALCIUM
3.3.2.1.7. Chemical and XRD Analysis on Solids Generated with Mixed
The composition analysis was performed in order to determine the effect that the evaluated pH conditions and organic content of the LLP and PAC-LLP had on the recovered solids and their Ca concentration, precipitated by the addition of the mixed Na2CO3-NaOH reagent. Based on the results, the mass percentage concentrations of
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Al, Ca, Fe, K, Mg, Mn, P and Si in their oxides forms were calculated. The results obtained for pH 9 and 10 and pH 11 and 12 are given in tables 37 and 38 respectively.
Table 37. Oxides in recovered solids with mixed Na2CO3-NaOH reagent at pH 9 and 10
pH 9 pH 10
Substance LLP PAC-LLP CV
(%) LLP PAC-LLP CV (%)
Al2O3 (%) 0,03 0,03 1 0,03 0,03 9
CaO (%) 30,4 32,9 6 30,8 31,8 2
Fe2O3 (%) 0,04 0,04 3 0,07 0,04 42
K2O (%) 0,4 0,4 6 0,5 0,8 37
MgO (%) 2,6 2,8 3 2,3 2,6 9
MnO (%) 0,03 0,03 8 0,02 0,02 22
P2O5 (%) 0,9 3,2 79 0,9 6,0 104
SiO2 (%) <0,1 <0,1 0 1,2 1,4 8
Table 38. Oxides in recovered solids with mixed Na2CO3-NaOH reagent at pH 11 and 12
pH 11 pH 12
Substance LLP PAC-LLP CV
(%) LLP PAC-LLP CV (%)
Al2O3 (%) 0,02 0,02 2 0,04 0,05 18
CaO (%) 34,4 32,2 5 31,1 30,6 1
Fe2O3 (%) 0,05 0,03 41 0,05 0,02 49
K2O (%) 0,8 1,1 22 1,3 1,3 4
MgO (%) 4,6 4,6 0 6,9 5,4 17
MnO (%) 0,03 0,03 5 0,03 0,03 1
P2O5 (%) 0,8 2,1 67 0,7 4,6 105
SiO2 (%) 1,6 1,9 11 1,8 1,8 2
As indicated in tables 37 and 38, based on the coefficient of variation (CV) calculated for each of the oxides at the corresponding pH value and only with the exception of P2O5 it was seen that the concentration of the oxides in the solids recovered from the LLP and PAC-LLP did not differ considerably from each other. For instance, only with the exception of MgO at pH 12 the CV calculated for CaO and MgO at each of the pH conditions were less than 10% with CaO mass percentage ranging in between 30,4 to 34,4% and MgO concentration ranged in between 2,3 to 6,9% being higher at pH values of 11 and 12 in the solids recovered from both permeates. The concentration ranges of CaO and MgO at the different pH conditions generated from either permeates were very close to the values found during the production of cement in the feed of cement kiln, which can be around 44 and 5% for CaO and MgO respectively (Oates, 1998). Moreover, for the case of Fe2O3 the calculate CV were around 45% at
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pH values of 10, 11 and 12 where the concentrations of Fe2O3 were higher for the solids recovered from the LLP. Furthermore, the higher concentration of P2O5
measured in the solids recovered from the PAC treated permeate corresponded to the higher concentration of PO4 observed in the LLP after the adsorption treatment indicated in table 25. The sum of the oxides and the CaO mass percentage are illustrated in figure 52.
Figure 52. Mass percentage of CaO and Oxides in Recovered Solids at different pH conditions
As seen in figure 52, the sum of the analyzed oxides in the recovered solids generated from the LLP and PAC-LLP ranged in between 34 to 44% having the lowest value at pH 9 for the solids recovered from the LLP and the largest value at pH 12 for the solids recovered from the PAC-LLP. However, at pH 9 the percentage of CaO in the oxides were higher with values greater than 80% compared to pH 12 with values around 70%, which indicated larger concentration of oxides in the solids at higher pH values such as MgO. Additionally, the CV calculated for the sum of the oxides at the pH conditions of 11 and 12 were less than 3% and for the pH values of 9 and 10 were about 10%. These indicated that at the pH values of 9 and 10 the concentration of the oxides in the recovered solids from the PAC-LLP were higher compare to the solids generated from the LLP at lower pH values. However, as already stated with respect to CaO the composition did not change considerably at either pH condition and permeate used but the concentrations of CaO in the recovered solids was so much higher than any other of the analyzed oxides.
Moreover, as indicated in table 16, the Ca to Mg ratio in the LLP was about 3 and also as indicated in table 25, this ratio was about 2,5 in the LLP and 2,3 in the PAC-LLP. Thus, the concentration of Ca in the recovered solids was expected to be higher than the Mg concentration, the Ca to Mg ratios calculated in the recovered solids from both permeates at the different pH conditions are illustrated in figure 53.
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Sum Ox. LLP Sum Ox. PAC-LL CaO LLP CaO PAC-LLP
88% 86% 81% 74%
83% 74% 77% 70%
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Figure 53. Ca to Mg ratio in recovered solids from LLP and PAC-LLP at different pH conditions
As seen in figure 53, at pH 9 the Ca to Mg ratios were almost the same for the solids recovered from both permeates but at pH 10 they deviated slightly reaching peak values of about 14 and 16 for the solids obtained from the PAC-LLP and LLP respectively. After reaching the maximum points the Ca to Mg ratios decreased until reaching a minimum value of about 5 and 7 in the solids recovered from the LLP and PAC-LLP respectively but still higher than the source Ca to Mg ratios found in the permeates, which ranged in between 2 to 3. Furthermore, based on the Ca to Mg ratio, pH 10 might be seen as an optimal point for the recovery of CaCO3-rich solids.
However, as already indicated in figure 45, at this pH condition about 30% of the hardness still remained in the treated permeates; hence issues related to scaling might still be a problem in downstream membrane processes.
Additionally, besides the oxide substances, others including the heavy metals regulated by the EU Directive for Sewage sludge and Soil were analyzed at the different pH conditions. The results obtained for the regulated substances are given in tables 39 and 40 for the solids generated from the LLP and PAC-LLP respectively.
Table 39. Concentration of substances regulated by EU Directives in Solids Recovered from LLP
Solids recovered from LLP
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Table 40. Concentration of substances regulated by EU Directives in Solids Recovered from PAC-LLP
Solids recovered from PAC-LLP
Substance pH 9 pH 10 pH 11 pH 12 Hg (ppm) <0,01 <0,01 <0,01 <0,01 Cd (ppm) <10 <10 <1 <1 Pb (ppm) <10 <10 2,2 2,8 Cr (ppm) <10 <10 2,3 3,3 Cu (ppm) <10 <10 <2 <2 Ni (ppm) <10 <10 <2 2,2
Zn (ppm) <50 <50 - -
As indicated in tables 39 and 40 and based on the limit values indicated in the EU Directive of 1986 “on the protection of the environment, and in particular of the soil, when sewage sludge is used in agriculture” described in table 3, the concentration of the analyzed heavy metal in the recovered solids were below the limit values indicated in the regulation. Moreover, the results obtained in the analysis of other substances, which are known to have some effect in the quality of soil, are given in tables 41 and 42 for the solids generated from the LLP and PAC-LLP respectively.
Table 41.Other analyzed substances in Recovered Solids generated from LLP with effects in soil quality
Solids recovered from LLP
Substance pH 9 pH 10 pH 11 pH 12 Na (ppm) 10400 7420 17400 24300 Cl- (ppm) 3100 4700 13000 25000 S (ppm) 1920 <800 1750 2780 N (ppm) <1000 <1000 <1000 <1000 Mo (ppm) <5 <5 <1 <1
Table 42. Other analyzed substances in Recovered Solids generated from PAC-LLP with effects in soil quality
Solids recovered from PAC-LLP
Substance pH 9 pH 10 pH 11 pH 12 Na (ppm) 8940 9350 19500 24400 Cl- (ppm) 3600 3400 17000 25000 S (ppm) 2180 <800 1970 2510 N (ppm) <1000 <1000 <1000 <1000 Mo (ppm) <5 <5 <1 <1
The results obtained in tables 41 and 42 were important specially for the potential application of the recovered solids in soil such as liming material for the correction of soil acidity (ISO, 2015) or as neutralizer of acid rain fall (Oates, 1998). For instance,
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due to its large size the presence of Na+ ions in soil has been known to affect soil structure since they induce dispersion within the soil matrix (MSU, 2003). Thus, in order to further decrease the amount of soluble substances such as Na+ and Cl- from the recovered solids a solid to washwater ratio larger than 1 to 10 might be required.
Additionally, for the most part the concentration of S, N and Mo made up about 0,2 less than 0,1 and less than 0,0005% of the recovered solids respectively. As indicated in table 25 despite the high concentration of SO4 in the permeates with values around 9500 mg/L, the low concentration of sulfur in the recovered solids correlated well with the low removal of SO4 in the permeates illustrated in figure 46 during the precipitation treatment at the different pH conditions. Furthermore, for the case of the macronutrient N, low concentrations were also expected to be present in the precipitated solids since as indicated in table 16 about 90% of the N in the permeate is in the form of NH4-N whose salts are characterized by its high solubility.
The diffractograms obtained from the XRD analysis performed in the recovered solids obtained at the different pH conditions from the LLP and PAC-LLP are given in figures 54 through 61.
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Figure 54. X-ray diffraction pattern for recovered solids at pH 9 from LLP
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Figure 55. X-ray diffraction pattern for recovered solids at pH 9 from PAC-LLP
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Figure 56. X-ray diffraction pattern for recovered solids at pH 10 from LLP
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Figure 57. X-ray diffraction pattern for recovered solids at pH 10 from PAC-LLP
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Figure 58. X-ray diffraction pattern for recovered solids at pH 11 from LLP
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Figure 59. X-ray diffraction pattern for recovered solids at pH 11 from PAC-LLP
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Figure 60. X-ray diffraction pattern for recovered solids at pH 12 from LLP
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Figure 61. X-ray diffraction pattern for recovered solids at pH 12 from PAC-LLP
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As observed in figures 54 through 61, at each pH condition the diffractograms obtained from the XRD analysis were almost identical for the solids recovered from the LLP and PAC-LLP and as expected from the CaCO3 polymorphs calcite was the identified crystal structure in the analyzed solids. Other CaCO3 polymorphs that can form in aqueous medium include aragonite and vaterite however calcite is the most stable CaCO3 crystal form (Koutsoukos & Chen, 2010). The identified calcite in the recovered solids at the different pH conditions from the LLP and PAC-LLP are given in tables 43 and 44 respectively.
Table 43. Identified calcite in the recovered solids at different pH conditions from the LLP
pH: pH 9 pH 10 pH 11 pH 12
Calcite, magnesian
(Ca,Mg)CO3 ✓ ✓ ✓ ✓
Monohydrocalcite CaCO3.
H2O - ✓ ✓ ✓
Table 44. Identified calcite in the recovered solids at different pH conditions from the PAC-LLP
pH: pH 9 pH 10 pH 11 pH 12
Calcite, magnesian
(Ca,Mg)CO3 ✓ ✓ ✓ ✓
Monohydrocalcite CaCO3.
H2O ✓ ✓ ✓ ✓
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3.3.2.2. Recovery of calcium carbonate-rich solids and explored