Phosphate Recovery by Reactive Crystallization of ... - ACS Publications

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1997 | doi: 10.1021/bk-1997-0667.ch022

Chapter 22

Phosphate Recovery by Reactive Crystallization of Magnesium Ammonium Phosphate: Application to Wastewater Izumi Hirasawa, Hiroyuki Nakagawa, Osamu Yosikawa, and Masanori Itoh Department of Applied Chemistry, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169, Japan

In this study, reactive crystallization of magnesium ammonium phosphate (MAP) has been studied experimentally in a batch agitated crystallizer. Operating conditions for selective crystallization of M A P were obtained. Under these conditions, batch crystallization experiments were performed, to obtain the behavior of concentration decrease in phosphate and magnesium ions, by changing the initial Mg/P ratio. It was recognized that the concentration decrease pattern affected the properties of produced crystals. A new process for removing ammonium and phosphate ions using a crystallization process was proposed for application to wastewater from sludge treatment processes. The authors have already reported a new phosphate removal process by calcium phosphate crystallization ". However in order to cope with the eutrofication problem, removal of ammonium ion should also be considered. For the removal of ammonium ions, a biological denitrification process was established, but that process had problems needing much installation area for the equipment and not to be able to recover ammonium ion. On the other hand, sludge treatment equipment, to treat sludge in a concentrated way, is planned in the large city of Tokyo, as that city doesn't have much space for the sludge treatment. If such sludge treatment will be in practice, large amounts of wastewater containing ammonium and phosphate ions will be produce during the solid-liquid separation of sludge. We experienced a scaling problem(which consisted mainly of MAP) in the pipeline of the digestion tank in the sewage treatment process. The reason of scaling was considered to be that the solution in the tank gained supersaturation in M A P caused by the release of ammonium and phosphate ions from the sludge in the process of digestion . M A P crystals are known to be produced by the reaction in Eq. (1). 1

2)

Mg +NH4 +HP0 "+OH- +6H 0 — 2+

+

4

2

2

MgNHUPC^ · 6H 0+H 0 2

2

(1)

Focusing on these phenomena, the application of M A P crystallization to the wastewater of sludge treatment was considered. The merit of this process is that ammonium and phosphate ions could be removed and recovered in the form of a slow release fertilizer. © 1997 American Chemical Society

In Separation and Purification by Crystallization; Botsaris, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

267

268

SEPARATION AND PURIFICATION BY C R Y S T A L L I Z A T I O N

In this paper, the reactive crystallization of M A P has been studied in a batch agitated crystallizer, in order to obtain the crystallization characteristics and the effect of Mg/P molar ratio on the properties of produced crystals and the behavior of the concentration decrease. Experiments Experiments were performed by using a batch reactive crystallization apparatus as shown in Fig.l, in order to obtain the properties of the produced crystals in the M g NH4 -PO4 "-OH-H 0 system, the effect of pH on the solubility of M A P , and the effect of Mg/P molar ratio on the behavior of the concentration decrease and the properties of crystals. Phosphate concentration was measured by the molybdenum blue absoφtiometΓy, and magnesium and ammonium ions were measured by cation chromatography. Properties of crystals were analyzed by Xray analysis and S E M photographs. 2 +


+

3

2

(1) Crystallization characteristics of MAP Crystallization experiments were performed by varying temperature, pH and concentrations of the reactants : phosphoric acid, ammonium chloride and magnesium chloride. The pH was adjusted by sodium hydroxide solution. Mixed suspension was sampled, when the concentration of each reactant was observed to be constant. After that, the suspension was filtered through 0.45 μ m membrane filter, to be analyzed by Xray diffractometer. (2) Effect of pH on the solubility of MAP Based on the results of experiment (1), crystallization experiments were done in the presence of excess M A P crystals, changing the pH conditions as shown in Table 1, to obtain the concentration of the reactant after 24h. Also the phosphate removal efficiency was obtained under the same experimental condition , changing pH and Mg/P molar ratio. (3) Effect of Mg/P molar ratio on the behavior of concentration decrease and the properties of produced crystals Experiments were done, using a batch crystallizer apparatus as shown in Fig.l, varying the Mg/P molar ratio from 1-4. The experimental conditions are shown in Table 2. During the process of batch crystallization, the mixed suspension was sampled at the determined crystallization time. The samples were filtered through 0.45 μ m membrane filter, and the concentrations in the filtrate were measured. Also sampled crystals were obtained when the concentration was observed to be constant, to be observed by S E M and be analyzed by the Xray diffractometer. The composition of crystals was analyzed after dissolution by a pH 2 HC1 solution. Results and Discussion Crystallization characteristics of MAP Fig.2 shows on outline of the M A P crystallization characteristics in the M g -NHU P O - O H - H 0 s y s t e m . M g H P 0 · 3H 0 was produced at higher temperatures. As for the effect of pH, crystals were not produced below pH 7, but Mg3(P0 ) · 4H 0 and Mg(OH) coprecipitated together with M A P at higher pH. Also Mg(OH) was produced in the case of excess of magnesium ion. In the case of excess of phosphate ion, another type of crystals was produced. It was recognized that excess of ammonium ion and suitable pH and Mg/P molar ratio were desirable as the operating conditions for selective M A P crystallization. +

4

3

2

4

+

2

4

2

2


2

2

22.

HIRASAWA E T A L .

269

Reactive Crystallization of MAP

Table 1. Experimental conditions

H3P04 NH4C1

MgCb

fmoUll

0.0010

fmol/11 [mol/1]

0.030 0.0010


-0.0020

pH Agitation rate Operational temperature

[-] 8 . 0 — 1 2 . 0 [rpml 300 298 [Kl

Figure 1. Schematic diagram of experimental apparatus

Table 2. Experimental conditions

H3P04 NH4C1

MgCh

rmoi/11

0.0010

[moyil

0.030

[mol/1]

0.0010 —0.0040

PH

Agitation rate Operational temperature

[-1

[rpml

Pq

9.0 300 298


270

SEPARATION AND PURIFICATION BY CRYSTALLIZATION

MgHP0 -3H 0 4

2


Increase in temperature pH » 1 0 1

No precipitation

Mg3(P0 )2-4H20 4

Excess of PO*

pH >10 ν Excess of Mg *

3

2

MgNKUP04-6H20 and others

Mg(OH)

2

Excess of NBr

MgNH P0 -6H 0 4

4

2

Figure 2. Characteristics of M A P precipitation

pHH Figure 3 . Effect of pH on the equilibrium concentration of each component


22.

HIRASAWA E T A L .

Reactive Crystallization of

271

MAP

Effect of pH on the solubility of MAP Fig.3 shows the effect of pH on the solubility of M A P . By increasing the pH, the concentration of each reactant is lowered, to reach the lowest concentration in the range of pH from 9 to 10. However the concentration increased when the operational pH became higher than 10. The reason was thought to be that Mg(OH) precipitated at the higher pH. Fig.4 also shows the effect of pH on the phosphate removal efficiency at two different Mg/P ratios. The highest removal efficiency was obtained in the pH range from 9 to 10. Increasing Mg/P molar ratio increased the phosphate removal efficiency. Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1997 | doi: 10.1021/bk-1997-0667.ch022

2

Effect of Mg/P molar ratio on the behavior of concentration decrease and the properties of produced crystals S E M photographs of produced crystals are shown in Photos 1,2 and 3, where the Mg/P molar ratio is varying from 1 to 4. Also Xray diffraction analysis of each crystal are shown in Figs. 5,6 and 7. At Mg/P molar ratio 1, the shape of crystals was observed to be one of typical M A P . However the crystals were agglomerated at Mg/P molar ratio 2. When Mg/P molar ratio became 4, the shape of crystals became needlelike and in addition, fine crystals were observed. These crystals were found to be M A P by Xray diffraction analysis, although the shape of the crystals was different. But the results of the composition analysis shown in Table 3, indicate the amount of ammonium ions in the case of Mg/P molar ratio 2 and 4, was higher than in the case of Mg/P molar ratio 1. The reason for the excess uptake of ammonium ion was not clear, but this phenomenon is important when considering the ammonium ion removal. Figs.8 and 9 show the time course of concentration decrease at different Mg/P molar ratio. At the condition of Mg/P molar ratio 1, the time needed for the concentration to start decreasing was longer, and after that the concentration gradually decreased. But at the condition of Mg/P molar ratio 2 and 3, concentration rapidly decreased from the start of the experiment, to reach the almost constant concentration. These phenomena were thought to come from the increasing nucleation rate with increasing Mg/P molar ratio. This fact is thought to affect the properties of the crystals. Proposal of a new process for the application of MAP crystallization Fig. 10 shows the application of the concept of M A P crystallization in to sewage treatment process. If M A P crystallization is applied, about 90% of phosphate and 20% of ammonium ion is calculated to be removed and recovered in the form of M A P . The residual ammonium ion is removed by the biological ammonium removal process. Conclusion Experiments of reactive crystallization of M A P were performed to obtain the optimal operational conditions for M A P selective crystallization, which were an excess of ammonium ions, pH 9-10, and Mg/P molar ratio 1-4. It was also found that Mg/P molar ratio affected the behavior of the concentration decrease, and changed the properties of the produced crystals. Based on the results of these experiments, M A P crystallization is proposed as an efficient method for the removal of phosphate and ammonium ions from the wastewater of the sludge treatment.



272

SEPARATION AND PURIFICATION BY C R Y S T A L L I Z A T I O N

I . g 60.0Kf

'U ω

1

40.0*f

Q 20.Ο·Λ(-

6

7

8

9

10

11

12

13

ρΗΗ Figure 4. Removal efficiency of PO/depended on pH

Photo 1. S E M photograph of produced crystals (Mg /P0 "=l) 2+

4

3



22.

HIRASAWA E T A L .

273


Photo 2. S E M photograph of produced crystals (Mg /P0 "=2) 2+

4

3

Photo 3. S E M photograph of produced crystals (Mg /P0 =4) 2+

4

3



SEPARATION AND PURIFICATION

BY

CRYSTALLIZATION

Figure 5. Xray analysis of produced crystals (Mg /P04 '=l) 2+

3

Figure 6. Xray analysis of produced crystals (Mg /P04 "=2) 2+

3

Figure 7. Xray analysis of produced crystals (Mg /P04 '=4) 2+

3


22.

HIRASAWA E T A L .

275


Table 3. Composition of M A P

Mg /P04 " 1 2 4 2+

3

Mg P04 ~ N H 1.00 1.00 1.03 1.00 1.25 1.09 1.00 1.27 1.06 3

4

2+

+


0.0010 Starting conditions PO -0.0010, NH =0.030[mol/l] pH=9 [-] T=298 [K] volume=1800 [ml] 4

0.0008

3

+ 4

• Mg/P=l A Mg/P=2 • Mg/P=4

* 0.0006

ο g 0.0004

I δ 0.0002

0.0000 60

90

150

120

180

Elapsed Time [min.] Figure 8. Time course o f P 0 ' cone, in solution (1800ml) 4

3

0.0050

0.0040

S

• Mg/P=l A Mg/P=2 • MgflP=4

•

Starting conditions Ρθ4 -0.0010, NH< =0.030[mol/l] pH=9 [-] T=298 [K] volume=1800 [ml] 3

+

ο

Β' 0.0030 ϋ ο

g 0.0020 υ

α ο ϋ

0.0010

•

••• •

0.0000 30

60

90

120

Elapdsed time [min.] Figure 9. Time course of M g cone, in solution (1800ml)


276

SEPARATION AND PURIFICATION

i

Waste water

Waste water treatment process Sludge


BY CRYSTALLIZATION

TI-IMIUMI

jSludgc treatment process Separated wulcr (High cone.)

wnLur

Dehydrated Hindoo

MAP Precipitation

Treatment

M A P Crystal

- 1 — • Reuse

'routed waLcr ; Treated water Biological IdcGnicaLion process

1st step

2nd stop

3rd step

Figure 10. Schematic diagram of a process using M A P precipitation

Literature Cited 1) Hirasawa. I. etal, Crystallization as a Separation Processes, 1990, ACS Symp. Ser.438 Chapter 26, pp353-363. 2) L.Murrel etal, Effluent Water Treatment Journal, 1972, Oct, pp509-519.


Phosphate Recovery by Reactive Crystallization of ... - ACS Publications

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