Effective removal of calcium and magnesium sulfates from wastewater in the rare earth industry

Article abstract of DOI:10.1039/c9ra05615g

The wastewater discharged from the rare earth (RE) industry generally contains a high level of calcium and magnesium sulfates, which confers permanent hardness and causes difficulties in recycling this wastewater. In this study, the alkyl phenoxy acetic acid derivatives including 4-methyl phenoxy acetic acid (M-POAA), 4-tert-butyl phenoxy acetic acid (B-POAA) and 4-tert-octyl phenoxy acetic acid (O-POAA), were synthesized via the Williamson reaction and characterized by nuclear magnetic resonance (NMR), infrared (IR), and ultra-violet (UV) spectroscopy, as well as elemental analysis and X-ray diffraction (XRD). Synthesis of the POAAs were simple and green, and the raw materials used for their production are widely available and low-cost. The potential for removal of Ca and Mg sulfates from industrial wastewater using POAAs as the organic precipitants was assessed. The total precipitation efficiencies of Ca and Mg from wastewater with the use of POAAs increased with the following order: M-POAA < B-POAA < O-POAA. The residual concentrations of Ca and Mg using O-POAA as the precipitant were lower than 0.099 and 0.089 g L^-1, respectively. The O-POAA could be regenerated five times without any significant change in its structure and precipitation performance. Thus, the use of the novel precipitants is a prospective alternative to the conventional processes for softening wastewater.

Full text of DOI:10.1039/c9ra05615g

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Effective removal of calcium and magnesium
sulfates from wastewater in the rare earth industry†
Cite this: RSC Adv., 2019, 9, 33922
Yanliang Wang, Xiangguang Guo, Yan Bai and Xiaoqi Sun *ac
a
a
b
The wastewater discharged from the rare earth (RE) industry generally contains a high level of calcium and
magnesium sulfates, which confers permanent hardness and causes difficulties in recycling this wastewater.
In this study, the alkyl phenoxy acetic acid derivatives including 4-methyl phenoxy acetic acid (M-POAA), 4-
tert-butyl phenoxy acetic acid (B-POAA) and 4-tert-octyl phenoxy acetic acid (O-POAA), were synthesized
via the Williamson reaction and characterized by nuclear magnetic resonance (NMR), infrared (IR), and ultra-
violet (UV) spectroscopy, as well as elemental analysis and X-ray diffraction (XRD). Synthesis of the POAAs
were simple and green, and the raw materials used for their production are widely available and low-cost.
The potential for removal of Ca and Mg sulfates from industrial wastewater using POAAs as the organic
precipitants was assessed. The total precipitation efficiencies of Ca and Mg from wastewater with the
use of POAAs increased with the following order: M-POAA < B-POAA < O-POAA. The residual
Received 22nd July 2019
Accepted 10th October 2019
ꢀ1
concentrations of Ca and Mg using O-POAA as the precipitant were lower than 0.099 and 0.089 g L
,
respectively. The O-POAA could be regenerated five times without any significant change in its structure
and precipitation performance. Thus, the use of the novel precipitants is a prospective alternative to the
conventional processes for softening wastewater.
DOI: 10.1039/c9ra05615g
rsc.li/rsc-advances
5
tonnes. The wastewater derived from the hydrometallurgical
1
Introduction
process generally confers permanent hardness and affects the
6,7
The unique physical and chemical properties of rare earth (RE) subsequent recycling of the wastewater. Techniques for elimi-
elements make them irreplaceable, as they can be used not only in nating permanent hardness in wastewater are of particular impor-
the traditional elds of metallurgy, agriculture, and ceramics, but tance. The four main methods for soening hard wastewater and
also in high-tech elds, such as magnetic materials, catalysts, laser removing Ca and Mg are summarized as follows:
1,2
materials and so on. RE resource are abundant in China,
accounting for 30% of the world's reserves where China dominates
the world's RE production at 85%. However, it is not widely known
(1) Inhibiting scaling with anti-scalants
3
Anti-scalants, also called chemical inhibitors, such as sodium
tri-polyphosphate, organic phosphoric acid and phosphine-
based polyacetic acid, are used to inhibit the growth of scale
that mining RE elements discharges considerable amounts of
wastewater with high hardness. The generation of wastewater
4
containing Ca and Mg during the RE production process is illus-
trated in the ESI (Fig. S1†). RE sulfates can be obtained aer
enhanced roasting of a mixture of bastnaesite and monazite with
concentrated sulfuric acid and leaching with a large amount of fresh
or reused water. The wastewater with Ca and Mg sulfate is generated
in the acid/base neutralization and extraction transformation
processes. It is reported that the annual discharge of wastewater
from RE production in Inner Mongolia, China is about 2.5 million
8
crystals. Studies have shown that anti-scalants can decrease the
particle sizes of precipitates and change the shapes of the
particles. Smaller particles can cause a greater decline in the
9
microltration ux during the solid/liquid separation steps.
The presence of anti-scalants during precipitation can also
10
decrease the mass of precipitated calcium carbonate. With the
restrictions on the use of phosphorus chemicals, anti-scalants
without phosphorus have been developed, such as polyacrylic
acid and polyepoxide succinic acid. However, these kinds of
a
CAS Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Key anti-scalants readily generate polymer calcium gels which are
Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter,
difficult to dissolve in water and do not possess scale-inhibiting
Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China. E-mail: xqsun@
11
effects.

irsm.ac.cn; Fax: +86-592-3594019; Tel: +86-592-3594019
b
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China
(2) Chemical precipitation method
c
Ganzhou Rare Earth Group Co.,Ltd., Ganzhou 341000, China
Chemical precipitation is one of the conventional methods for
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c9ra05615g
12,13
1
soening hard water in industrial applications.
Chemical
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additives such as lime or soda induce the formation of insoluble (4) Ion exchange
precipitates that are readily discharged from the water to obtain
so water. Taking lime and soda as example, the precipitation
reaction can be expressed as eqn (1) and (2).
Ion exchange resins are frequently used to remove ions such as
Ca and Mg from aqueous media. It is reported that the hardness
of the industrial wastewater from Inner Mongolia, China was
14
3
ꢀ1
reduced from 5.42 ꢁ 10 to 71.2 mg L with a total removal
MgSO
CaSO + Na CO / CaCO Y + Na SO
4
4
+ Ca(OH)
2
/ Mg(OH)
2
Y + CaSO
4
(1)
(2)
26
efficiency of 98%.
Recently, mono-substituted alkylphenoxy acetic acid deriva-
4
2
3
3
2
27–29
tives (POAAs) were reported.
The former can be used for the
30
Considering that the solubility product constant, Ksp, of separation of high-purity yttrium from RE mixture, whereas
ꢀ
9
ꢀ5
CaCO
3
(8.7 ꢁ 10 ) is far less than that of MgCO
3
(2.6 ꢁ 10 ), the latter plays an important role in the enrichment and
2
+
2+
31,32
Ca is more easily precipitated than Mg in carbonate transformation of RE elements.
medium. One study reported that the use of 12% ammonia the coordination characteristics of Ca and Mg ions with
water as the precipitant to remove Ca and Mg resulted in carboxyl groups are similar to those of RE ions. To date, the
removal efficiencies of 75 and 52%, respectively.
Oxalic acid saponied with an alkali can also be used to reported based on a literature survey. The objectives of this
soen high-hardness wastewater, where Ca can be precipi- study are to synthesize mono-substituted alkyl phenoxy acetic
tated and a small amount of Mg is co-precipitated when oxalic acids including 4-methyl phenoxy acetic acid (M-POAA), 4-tert-
acid used as the precipitant. The particle sizes of CaC
or butyl phenoxy acetic acid (B-POAA) and 4-tert-octyl phenoxy
MgC
were extremely small, resulting in difficulty of the solid/ acetic acid (O-POAA) and to discuss the application to the
To the authors' knowledge,
15
1
6
study of precipitation of Ca and Mg using POAAs has not been
2
+
2
+
2 4
O
2 4
O
liquid separation. Another disadvantage of this method is that selective precipitation of Ca and Mg sulfates from wastewater in
oxalic acid is highly water soluble (9.52 g/100 g water), leading to RE industry.
a high residual concentration in the aqueous phase. Oxalic acid
and the oxalates exert biological toxicity with harmful effects
2
Experimental
upon contact. The reported oral lowest published lethal dose
ꢀ
1
17
(LDLO) of oxalic acid was 600 mg kg
.
The precipitation
2.1 Chemical reagents and wastewater samples
reactions of Ca and Mg with oxalates are presented in eqn (3)
and (4).
4
-Methyl-octylphenol, 4-tert-butyl-octylphenol, 4-tert-octylphe-
nol, chloroacetic acid, dimethyl sulfoxide (DMSO) and NaOH
were purchased from Chendu Xiya Reagent Chemical Tech-
nology Co., Ltd. Concentrated hydrochloric acid, oxalic acid,
calcium hydroxide, sodium phosphate, ammonium chloride,
calcium sulfate and magnesium sulfate were purchased from
CaSO + Na C O / CaC O Y + NaSO
(3)
(4)
4
2
2
4
2
4
4
MgSO
4
2
+ Na C
2
O
4
2
/ MgC O
4
Y + NaSO
4
As is evident from eqn (5) and (6), the precipitation of magne- Sino-pharm Chemical Reagent Co., Ltd., China. All other
sium ammonium phosphate (MAP, MgNH PO $6H O) from chemicals were used without further purication.
4
4
2
wastewater has attracted considerable attention as this technique
Real wastewater sample was collected from the RE industry
18
offers the potential for recovering phosphorus for fertilization. The in Inner Mongolia, China. The typical compositions of the
ꢀ
1
MAP crystallization process includes nucleation and crystal growth wastewater are listed as follows, CaSO
0.0113 mol L , MgSO
4
4
ꢀ
1
ꢀ1
processes that are affected by physicochemical factors such as the 0.124 mol L and (NH
4
)
2
SO
4
0.325 mol L . In order to reduce
pH, super saturation ratio, temperature, and the presence of foreign the inuence of impurities, the synthetic wastewater samples
1
9,20
ions.
It is a promising method for removing Ca and Mg from containing CaSO
4
, MgSO
4 4 2 4
and (NH ) SO were prepared and
ꢀ
13 21,22
wastewater based on the low Ksp value of MAP (2.5 ꢁ 10 ).
used in the parts of mechanism research.
2
+
3ꢀ
3
Ca + 2PO4 / Ca (PO ) Y
(5)
(6)
3
4 2
2.2 Apparatus and analytical methods
2
Mg + NH
+
+
4
43ꢀ
+ PO / MgNH
4 4
PO Y
Elemental (C, H and N) analysis of the organic compounds was
performed using an Elementar Vari EL Cube. Inductively
Coupled Plasma Optical Emission Spectroscopy (ICP-OES) was
performed with a JY-Horiba Ultima 2 instrument to determine
(
3) Membrane separation
2+
2+
the concentrations of Ca and Mg . Proton and carbon nuclear
1
13
Membrane separation methods including reverse osmosis (RO), magnetic resonance ( H and C NMR) spectra were obtained
ultraltration (UF), nanoltration (NF) and so on have been with an AV III-500 Bruker spectrometer. Scanning electron
used for several years to remove suspended particles or dis- microscope (SEM) images were collected on a Hitachi SU1510
23–25
solved salts from aqueous media.
This method not only instrument at 30 kV. Particle sizes were measured with the aid
removes calcium and magnesium ions, but also intercepts other of the back-scattering principle by Brookhaven NanoBrook
inorganic salts, carbohydrates, amino acids, etc., in wastewater. Omni, USA. Fourier-transform infra-red (FT-IR) spectra were
The radii of intercepted particles or salts are greater than 1 nm obtained by using a Nicolet iS50 spectrometer (Thermo Scien-
ꢀ1
and the intercept rates are as high as 10–90%.
tic) in the range of 600–4000 cm .
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The method for analysis of the trace POAAs in the aqueous a wide range of sources and low production costs. Thus, the
phase was developed based on the characteristic ultraviolet POAAs are suitable for large-scale industrial usage.
spectral absorption of the benzene ring in the POAA molecule.
4-Methyl phenoxy acetic acid (M-POAA). Final yield 95%.
1
There is a quantitative relationship between the concentration Purity > 99% (acid–base titration method). H NMR (500 MHz,
of the POAA and the ultraviolet spectral absorbance in the DMSO) d 12.97 (s, 1H), 7.08 (d, J ¼ 8.2 Hz, 2H), 6.85–6.75 (m,
1
3
wavelength range of 274–276 nm (ESI, Fig. S2†). The logarithmic 2H), 4.62 (s, 2H), 2.23 (s, 3H). C NMR (125 MHz, DMSO)
acid dissociation constant (pK
a
) and purity of the POAA can be
d
170.79, 156.10, 130.13, 130.09, 114.68, 64.95, 20.52.
9 10 3
H O (166.18): found (anal. calc.)%, C 64.91(65.05), H
determined by acid–base titration in 75 vol% alcohol using
C
ꢀ
1
ꢀ1
ꢂ
0
.01 mol L NaOH as the standard solution and 0.5 vol% 6.07(6.07). Water solubility 1.51 g L at 25 C (UV-Vis Spec-
33
bromothymol blue as the indicator. The water content of solid trophotometry). UV characteristic peak at 276 nm. pK ¼ 5.05.
a
complexes was determined from the weight change before and
4-tert-Butyl phenoxy acetic acid (B-POAA). Final yield 90%.
ꢂ
1
aer drying at 100 C for 5 h.
Purity > 99% (acid–base titration method). H NMR (500 MHz,
DMSO) d 12.98 (s, 1H), 7.29 (d, J ¼ 8.9 Hz, 2H), 6.85–6.79 (m,
1
3
2
H), 4.63 (s, 2H), 1.25 (s, 9H). C NMR (125 MHz, DMSO)
2
.3 Synthesis and characterization of POAAs
d 170.82, 155.94, 143.58, 126.51, 114.29, 64.90, 34.24, 31.79.
(208.26): found (anal. calc.)%, C 69.50(69.21), H
12 16 3
C H O
Molecular structures of M-, B- and O-POAA are illustrated in
Fig. 1. Crystal data and structure renement for M-POAA (CCDC
1
1
prepared in laboratory scale via Williamson reaction and the
synthetic routes are as following: (1) 0.2 mol 4-methyl/4-tert-
butyl/octyl phenol, 0.21 mol chloroacetic acid, 0.41 mol NaOH
were mixed together in the solvent dimethyl sulfoxide (DMSO).
The mixture was stirred at 110 C for 2 h and a large amount of
white solid phase was formed. A certain amount of NaOH was
added to control pH > 11 throughout the reaction. (2) The
ꢀ1
ꢂ
7.92(7.74). Water solubility 0.872 g L at 25 C. UV character-
istic peak at 274 nm. pK ¼ 5.07.
105571), B-POAA (CCDC 1844628) and O-POAA (CCDC
587656) are available (ESI, Table S1†). The compounds were
a
4
-tert-Octyl phenoxy acetic acid (O-POAA). Final yield 83%.
1
Purity > 99% (acid–base titration method). H NMR (500 MHz,
DMSO) d 0.68 (s, 9H), 1.30 (s, 6H), 1.69 (s, 2H), 4.62 (s, 2H), 6.89
d, J ¼ 8.7 Hz, 2H), 7.27 (d, J ¼ 8.6 Hz, 2H), 12.95 (s, 1H);
NMR (125 MHz, DMSO) d 32.0, 32.1, 32.5, 38.1, 56.8, 64.9, 114.0,
127.3, 142.4, 155.8, 170.8. C H O (264.36): found (anal.
1
3
(
C
ꢂ
16 24 3
calc.)%,
C
72.70(72.69),
H
9.25(9.15). Water solubility
ꢀ
1
ꢂ
33.3 mg L at 25 C. UV characteristic peak at 275 nm. pK ¼
a
ꢀ1
4.88.
resultant solution was sequentially neutralized with 6.0 mol L
HCl, extracted with mineral ether, washed with deionized water,
and evaporated on a rotary evaporator. (3) The products were 2.4 Precipitation and stripping procedure
further puried by recrystallization in high purity toluene.
The precipitation experiments were carried out by mixing the
saponied organic precipitants and Ca/Mg solution together to
form a solid complex phase at room temperature. The detailed
precipitation routes are as follows: (1) precipitants were
As for a large-scale preparation, the irritating solvent DMSO
can be omitted without distinct inuence of the nal yield of
product, which leads to the synthetic routes relatively simple
and green. The raw material including 4-methyl/4-tert-butyl/
octyl phenol, chloroacetic acid and NaOH, of POAAs have
ꢀ1
saponied with 5.0 mol L NaOH which transformed the solid
precipitant into a transparent suspension. (2) Solid complexes
containing Ca and Mg were formed by adding the saponied
precipitants to aqueous phase. The solid complex was separated
from the aqueous phase with liquid/solid centrifugal separator.
The saponication degree can be dened as the proportion of
compounds saponied into mono-carboxylate relative to the
total amount of the compounds. The mixture was centrifuged or

ltered, and the solid phase was separated from the liquid
phase. In order to make the precipitants loaded with Ca and Mg
recyclable, the stripping experiments were conducted by
immersing the solid complex in a series of concentrations of
hydrochloric acid for 0.5 h. To ensure data reliability, the
experiments were repeated at least twice. The precipitation rate
(P), distribution ratio (D), separation factor (b) and stripping
rate (S) are dened as follows:
C
ini ꢀ Ceq
P% ¼
ꢁ 100
(7)
(8)
(9)
C
ini
C
ini ꢀ Ceq
D ¼
C
eq
Fig. 1 Molecular structures of 4-methyl phenoxy acetic acid (M-
POAA), 4-tert-butyl phenoxy acetic acid (B-POAA), and 4-tert-octyl
phenoxy acetic acid (O-POAA).
b
M1/M2 ¼ D
1 2
/D
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C
aq ꢁ Vaq
S% ¼
ꢁ 100
(10)
m
org
where Cini and Ceq are the initial and equilibrium concentration
of the metal in the aqueous phase, respectively. D and D
1
2
represent the distribution ratio of metals 1 and 2, respectively.
Caq is the metal concentration in the stripping section. Vaq and
morg represent the volume of the aqueous phase in the stripping
section and the initial mass of metal in the solid complex,
respectively.
3
Results and discussion
3.1 Comparison of oxalic acid and POAAs
As mentioned above, oxalic acid can be used for soen waste-
water with high hardness. However, the precipitant cannot be
reused for the next time. In this paper, we designed the aqueous phase. Aqueous phase: CaSO ¼ 0.0141 mol L , MgSO4
Fig. 2 Total precipitation efficiency of Ca and Mg with oxalic acid, M-,
B- and O-POAA and residual concentration of organic precipitants in
ꢀ
1
4
¼
ꢀ
1
0
0
.0434 mol L , VFeed ¼ 20.0 mL. Solid phase: oxalic acid ¼
.0155 mol, M-POAA ¼ B-POAA ¼ O-POAA ¼ 0.00310 mol, saponi-
precipitation strategy as follows. Firstly, the novel precipitants
were saponied into anionic surfactants with alkali, such as
sodium hydroxide and ammonia, which could be contacted
with the aqueous phase thoroughly. Secondly, insoluble solid
complexes containing Ca and Mg sulphates were formed by
adding the saponied precipitants into wastewater with high
hardness. Therefore, Ca and Mg were transferred from aqueous
phase into solid phase. Solid complexes could be separated
from wastewater easily with liquid/solid centrifugal separator,
plate frame lter, or other lter equipment, leaving water with
low hardness. Finally, the precipitants were designed to be
regenerated by the stripping of solid complexes with concen-
trated HCl. Thus, low concentration of slightly soluble Ca and
Mg sulphates transitioned to high concentration of soluble Ca
and Mg chloride.
Total precipitation efficiency of Ca and Mg sulphates with
oxalic acid and POAAs were compared. As can be seen in Fig. 2,
POAAs indicates better precipitation abilities for the Ca and Mg
sulphates than oxalic acid. With the increase of alkyl chain
carbon atoms in POAAs, the precipitation rates of Ca and Mg
sulphates increased. The results indicate that the total precip-
itation rates of Ca and Mg sulphates from hardness wastewater
reached 97.7% by the use of O-POAA and the residual concen-
fication degree ¼ 80%.
molecular structure, the hydrophobicity of the molecule is
increased. Correspondingly, lower water content can lead to the
faster separation of solid complexes from aqueous phase.
To evaluate the precipitation abilities and selectivities using
M-, B- and O-POAA, their precipitation rates for Ca and Mg ions
were compared. As shown in Table 1, oxalic acid reveals better
precipitation abilities for Ca than Mg with the precipitation
rates of 95.8% and 45.8%, respectively. The separation factor
between Ca and Mg, bCa/Mg, can be deduced as 26.9, corre-
spondingly. This can be explained that, the solubility product
ꢀ
9
constant, Ksp, of CaC
2
O
4
(2.57 ꢁ 10 ) is far less than that of
ꢀ6
MgC
2
O
4
(4.83 ꢁ 10 ). In a diametrically opposite way, O-POAA
indicates better precipitation abilities for the Mg than Ca and
demonstrated bCa/Mg value of 0.293. Comprehensive consid-
ering the maximum precipitation rate, minimum water content,
we can assume that O-POAA among POAAs might be used in
parallel to remove Ca and Mg from wastewater streams.
tration of Ca and Mg sulphates were lower than 27.8 and 3.2 Precipitation mechanism
ꢀ
1
1
5.6 mg L , respectively. Total precipitation efficiency of Ca
Unlike oxalic acid, the POAAs are completely non-dissociated in
water and the precipitation reactions between POAA and Ca/Mg
and Mg sulphates with oxalic acid reached only 58.1%.
Residual concentration of organic precipitants in aqueous
phase during the precipitation process have also been investi-
gated. As the simplest dicarboxylic acid, oxalic acid is the
organic compound with the saturated solubility in water of 143 M-, B- and O-POAA. CaSO4
g/100 g (20 C). In the current precipitation process, residual 0.0434 mol L , VFeed ¼ 20.0 mL. Solid phase: oxalic acid ¼
Table 1 Precipitation rate of mixed Ca and Mg ions with oxalic acid,
ꢀ1
¼
0.0141 mol
L
,
MgSO4
¼
ꢂ
34
ꢀ1
ꢀ
1
0.0155 mol, M-POAA ¼ B-POAA ¼ O-POAA ¼ 0.00310 mol, saponi-
fication degree ¼ 80%
concentration of oxalic acid reached 5.67 g L . With the
increase of alkyl chain carbon atoms in POAAs, the residual
concentration in water signicantly decreased. The solubility
order of M-POAA, B-POAA and O-POAA at room temperature
P%
D
ꢀ
1
Precipitants
Ca
Mg
Ca
Mg
0.844
1.70
5.54
b
Ca/Mg
follows the reverse sequence as, M-POAA (3.93 g L ) > B-POAA
ꢀ
1
ꢀ1
(
1.28 g L ) > O-POAA (0.033 g L ). Water contents of solid
complexes including oxalic acid and POAA complexes were
tested as 78.8, 82.2, 71.1 and 54.8%, respectively. It can be B-POAA
explained that, with the increase of the alkyl chain in the O-POAA
Oxalic acid
M-POAA
95.8
63.1
92.8
95.1
45.8
63.0
84.7
98.5
22.7
1.72
12.8
19.3
26.9
1.01
2.32
65.8
0.293
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sulfates cannot be directly carried out through direct contact. As Table 2 The relationship between the formation of solid complex,
2+
+
+
DM , and the reduction of H in the solid precipitants, DH
the typical weak mono acids, logarithmic acid dissociation
constant, pK , values of M-, B- and O-POAA were determined as
+
ꢀ4
DM2+ ꢁ 10ꢀ4, mol
+
2+
a
Sap. rate%
0
DH ꢁ 10 , mol
DH /DM
5.05, 5.07 and 4.88 in 75 vol% ethanol, respectively, much lower
than rst-order dissociation of oxalic acid (pKa1 ¼ 1.2). The
0
0
N/A
structures of the POAAs showed dimer compounds via XRD 15
2.31
3.85
7.70
9.24
10.0
10.8
11.6
12.3
13.1
1.20
1.85
4.32
5.01
5.35
5.44
5.65
5.94
6.49
1.92
2.08
1.78
1.85
1.87
1.98
2.04
2.08
2.02
2
5
6
6
5
0
0
5
single crystal characterization (Bruker D8 Venture diffractom-
eter). In the solvent extraction system, the acidic extractants
were diluted in inert solvents to reduce the dimerization
tendency. In the current precipitation strategy, quantitative 70
precipitation of Ca and Mg could be achieved when the POAAs 75
8
8
0
5
were saponied by acid–base neutralization reactions to break
the dimeric connections between the molecules.
Taking O-POAA as an example, the effect of saponication
degree on the precipitation of Ca and Mg was investigated and
the results are shown in Fig. 3. The precipitation efficiencies of
Ca and Mg with the POAA increased quantitatively with an
increase in the degree of saponication. In addition, the
2
+
ꢀ
M
+ 2HL + OH / ML
2
2
Y + H O
(13)
Therefore, the precipitation of Ca and Mg with POAA
2
+
formation of the solid complex, DM (M ¼ Ca or Mg), and the
proceeds via a cation exchange mechanism. In practical oper-
ation, the saponication degree of the precipitants and the
acidity of the aqueous phase are the key factors that impact the
effectiveness of the precipitants and the precipitation efficien-
cies of Ca and Mg, respectively.
+
reduction of the H concentration in the solid precipitants,
+
DH , were calculated; the results are also shown in Table 2. The
+
2+
ratio of DH /DM remained at about 2 for different saponi-
cation degrees, which means that the precipitation efficiencies
of Ca or Mg was quantitative under the current saponication
conditions. The water solubility of O-POAA was determined as
ꢀ
1
ꢂ
3
.3 Temperature effects
3
3.3 mg L at 25 C. With an increase in saponication degrees
during the precipitation processes, the water solubility of O-
POAA increased slightly. The saponication and precipitation
reactions can be expressed in terms of the following equations:
In earlier publications, temperature was found to affect the
35
equilibrium of the extraction reaction.
Increasing the
temperature should result in an increase in the reaction rate in
many cases. In industrial production, extraction and separation
ꢀ
ꢀ
OH + HL / L + H
2
O
(11)
(12)
ꢂ
are generally carried out at temperatures above 15 C. However,
the effects of temperature on the precipitation rates of Ca and
Mg have seldomly been reported. The effect of temperature on
the precipitation of Ca and Mg with O-POAA was thus investi-
gated. As shown in Fig. 4, the total precipitation rate decreased
2
+
ꢀ
+ 2L / ML Y
2
M
The total reaction can be represented as:
ꢂ
ꢂ
slightly with an increase in the temperature from 5 C to 65 C,
indicating the insignicant effect of temperature on Ca and Mg
precipitation. However, with an increase in the temperature
during the precipitation processes, the water solubility of O-
POAA obviously increased. The recommended working
ꢂ
temperature is below 35 C, where the water solubility of the
ꢀ
1
precipitant can be controlled to below 242 mg L . This
precipitation method is especially suitable for cold areas with
ꢂ
annual temperatures below 35 C.
3
.4 Stripping studies
Conventional precipitating agents, i.e. oxalic acid or a mixture
of NH Cl and Na PO , cannot be recycled for Ca and Mg
4
3
4
precipitation. The denite advantage of this method is the
possibility of reducing the production cost by recycling the
precipitant. Therefore, stripping Ca and Mg from the loaded
solid complex using hydrochloric acid is the key step to
achieving recyclability of the precipitant. The effect of HCl
concentration on the stripping of Ca and Mg from the loaded O-
POAA was investigated as shown in Fig. 5. When the concen-
Fig. 3 The effect of saponification degree on the precipitation of Ca
and Mg with O-POAA. Synthetic wastewater sample: CaSO
4
¼
ꢀ
1
ꢀ1
0
0
0
.00951 mol
L
,
MgSO
4
¼
0.0223 mol
L ,
4 2 4
(NH ) SO
¼
ꢀ1
.0584 mol L , VFeed ¼ 24.0 mL, pH ¼ 6. Solid phase: O-POAA ¼
.00154 mol, saponification degree ¼ 0–85%.
ꢀ
1
tration of HCl exceeded 1.2 and 2.4 mol L , stripping
33926 | RSC Adv., 2019, 9, 33922–33930
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environment. In order to further recycle the ammonium sulfate
wastewater from the process of RE smelting, the ammonium
sulfate wastewater can be mixed with Ca(OH) and more than
2
17% of the ammonia can be obtained in the distillation column.
If the content of Ca and Mg is too high, this will directly affect
the recycling of wastewater.
Pilot tests were carried out to verify the efficacy of current the
precipitation strategy for the removal of high-concentration Ca
and Mg from wastewater in the RE industry (process-I); the ow
chart presented in Fig. 6. The residual concentration of Ca and
Mg in the treated water (process-I) were lower than 0.099 and
ꢀ
1
0
.089 g L , respectively, satisfying the requirements of local RE
production enterprises (Baotou Iron and Steel Co., Ltd., Ca <
ꢀ
1
ꢀ1
0
.1 g L , Mg < 0.5 g L ). The precipitants were regenerated by
Fig. 4 The effect of temperature on the precipitation of Ca and Mg
ꢀ1
stripping the solid complexes with 6 mol L HCl. The mass-to-
volume ratio of the solid phase and aqueous phase was 1 : 1.
The total concentration of 0.656 mol L CaCl + MgCl dis-
2 2
with O-POAA. Synthetic wastewater sample: CaSO
4
¼
ꢀ1
ꢀ1
0
0
0
.00945 mol
L
,
MgSO
4
¼
0.0226 mol
L
,
(NH
4
)
2
SO
4
¼
ꢀ1
ꢀ1
.0586 mol L , VFeed ¼ 24.0 mL, pH ¼ 6. Solid phase: O-POAA ¼
.00154 mol, saponification degree ¼ 80%.
charged from the regeneration process can be locally used as
snow melting agent aer further purication.
For comparison, the precipitation of Ca and Mg from
wastewater with oxalic acid was also evaluated (process-II).
Detailed material balance calculations of Ca and Mg are pre-
sented in Table 3. The residual concentration of Ca and Mg in
the treated water (process-II) were determined as 0.0981 and
efficiencies of Ca and Mg from the loaded solid complex could
be achieved as 94.5 and 99.7%, respectively. The water solubility
ꢀ
1
of POAA during the stripping step was lower than 31.7 mg L
ꢀ1
when [HCl] > 1.2 mol L . O-POAA could be regenerated aer
the stripping steps and there was no signicant change in its
precipitation performance.
ꢀ1
2.41 g L , respectively. Thus, the residual Mg from process-II
does not meet the requirements of the local RE production
enterprises.
The morphologies and sizes of the particles of the POAA
complexes and Ca + Mg C O produced in process I and II were
3.5 Pilot tests
2
4
measured, and the results are shown in Fig. 7. The gure shows
the surface morphologies of POAA-Ca(Mg) with particle sizes of
There are abundant RE resources in Bayan Obo in Inner Mon-
golia, China. However, the industrial production of REs,
including the decomposition of bastnaesite and monazite
concentrates, separation of the RE elements, and manufacture
of high-purity RE products requires the use of a large amount of
chemicals, resulting in a large amount of waste gas, wastewater
and radioactive residue, which are serious threats to the
about 50 mm. In the case of Ca(Mg)C
2 4
O , particles with sizes of
2–5 mm accounted for only 1/10 or lower of the proportion. The
larger particle size of POAA-Ca(Mg) signicantly facilitated
separation of the solid phase from the liquid phase.
Further experiments were carried out to remove Ca and Mg
from wastewater with Ca(OH)
2 2 3
+ Na CO (process-III) and
Na PO + NH Cl (process-IV). The total precipitation efficiencies
3
4
4
of Ca and Mg were tested as 37.6 and 60.1% in process III and
IV, respectively.
Fig. 5 The effect of HCl concentration on the stripping of Ca and Mg
from loaded POAA. Aqueous phase: VHCl ¼ 6.0 mL, HCl concentration
ꢀ
1
¼
0.24–6.0 mol L . Solid phase: 2.0 g O-POAA loaded with Fig. 6 Process flow chart used for the removal of Ca and Mg sulfates
0
.000302 mol Ca and 0.00278 mol Mg.
with POAA from wastewater in RE industry.
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Table 3 Material balance calculation of Ca and Mg during the current precipitation strategy. The molecular weight of Mg (24) is set to that of Ca
(
40) according to the definition of the hardness of water. (a) Process-I. O-POAA ¼ 0.0742 mol, saponification degree ¼ 80%. Real wastewater
ꢀ1
ꢀ1
ꢀ1
sample: CaSO
4
¼ 0.0113 mol L , MgSO
4
¼ 0.124 mol L , (NH
4
)
2
SO
4
¼ 0.325 mol L , VFeed ¼ 200 mL. Mass-to-volume ratio of solid and
aqueous phase ¼ 1 : 1 in stripping section. (b) Process-II. Oxalic acid ¼ 0.0371 mol. (c) Process-III. Ca(OH)
2
+ Na CO
2 3
¼ 0.0810 mol. (d) Process-
IV. Na
3
PO
4
+ NH
4
Cl ¼ 0.0720 mol
Precipitation/stripping
efficiency%
ꢀ1
(
a)
Procedure
Feed
Raffinates
Solid complex
Stripping solution
Ca + Mg, mol L
V, mL
Material balance%
Precipitation section
Stripping section
0.135
0.00620
0.586
200
200
44.1
40
(100)
4.60
(100)
101.9
95.4
101.9
0.656
Precipitation
efficiency%
ꢀ1
ꢀ1
ꢀ1
(b)
Procedure
Ca + Mg, mol L
V, mL
Material balance%
Precipitation section
Feed
Raffinates
0.135
0.103
200
200
(100)
76.0
24.0
Precipitation
efficiency%
(c)
Procedure
Ca + Mg, mol L
V, mL
Material balance%
Precipitation section
Feed
0.132
200
200
(100)
62.4
37.6
Raffinates
0.0824
Precipitation
efficiency%
(d)
Procedure
Ca + Mg, mol L
V, mL
Material balance%
Precipitation section
Feed
Raffinates
0.132
0.0528
200
200
(100)
39.9
60.1
POAA was recycled ve times on the basis of process I, which analysis of the original and recycled POAA, analyzed using an
lasted for one month, and the result is shown in Fig. 8. The total Elementar Vario Micro Cube instrument showed absolute
precipitation efficiency of Ca and Mg was above 95% and the discrepancies of less than 0.4% between the experimental and
precipitant concentration in the treated (soened) wastewater calculated values. IR, UV and XRD characterization showed that
ꢀ1
was below 37.5 mg L . The average weight loss of POAA in the the crystal structure, infrared and UV spectra of the original
regeneration process was determined as 3.9%. Original POAA sample were all consistent with that of the recycled sample. The
(a) and POAA regenerated ve times (b) were compared aer the acid values were determined by acid–base titration and the
stripping steps and the results are shown in Fig. 9. Elemental
Fig. 8 Comparison of (a) the initial O-POAA and (b) regenerated O-
ꢀ
1
POAA for five times. Real wastewater sample: CaSO
4
¼ 0.0113 mol L
,
ꢀ1
ꢀ1
MgSO
4
¼ 0.124 mol L , (NH
4
) SO
2 4
¼ 0.325 mol L , VFeed ¼ 200 mL.
Fig. 7 Particle morphologies and sizes of the solid precipitates. (a) Solid phase: O-POAA ¼ 0.0742 mol, saponification degree ¼ 80%.
POAA complexes loaded with Ca and Mg in process I. (b) Ca + Mg Mass-to-volume ratio of solid and aqueous phase ¼ 1 : 1 in stripping
C
O
2 4
produced in process II.
section.
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wastewater generated by the RE industry in Inner Mongolia,
China is anticipated, and the problem of the water shortage in
the area is expected to be effectively alleviated.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by National Key R&D Program of
China (2017YFE0106900), Science and Technology Major
Projects of Fujian Province (2015HZ0001-3), Natural Science
Foundation of Fujian Province (2016J05058), ‘Hundreds of
Talents Program’ and Science and Technology Service Network
Initiative from Chinese Academy of Sciences.
Fig. 9 IR and XRD characterization of (a) the initial O-POAA and (b)
regenerated O-POAA for five times.
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evaluation of POAA was also carried out, and the median
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