Ammonium removal from waste solutions by ... - Science Direct

14 downloads 2898 Views 475KB Size Report
Ammonium removal from waste solutions by precipitation of MgNH4PO4. II. Ammonium removal and recovery with recycling of regenerate. Tadeusz Stefanowicz ...
Resources, Conservation and Recycling, 6 (1992) 339-345

339

Elsevier Science Publishers B.V.

Ammonium removal from waste solutions by precipitation of MgNH4PO4 II. Ammonium removal and recovery with recycling of regenerate Tadeusz Stefanowicz, Stefania Napieralska-Zagozda, Matgorzata Osittska and Katarzyna Samsonowska Institute of Chemistry and TechnicalElectrochemistry Pozna8 Technical University, PoznaS, Poland (Received $ April 1991; accepted after revision 17 February 1992) ABSTRACT Stefanowicz, T., Napieraiska, S., Zagozda, S., OsiAska, M. and Samsonowska, K., 1992. Ammonium removal from waste solutions by precipitation of Mg~H4PO4. II. Ammonium removal and recovery with recycling of regenerate. Resour., Conse:'v. Recycl., 6: 339-345. It was found that the ammonium ion removal product MgNH4PO4"6H20 after drying and roasting becomes converted into the regenerate Mg3(PO4)2 suitable for recycling in the ammonium ion removal/recovery process. The gaseous ammonia expelled during roasting can be utilized according to known methods while the regenerate can be used as a source of PO]- and Mg2+ ions in MgNH4PO4"6H20 formation. After adding the regenerate to ammonium-laden waste solution, strong acid (HC! or H3PO4) is introduced to dissolve the regenerate (pH 1-2 ) and then pH is adjusted with NaOH to 9-10. Stirring is carried out preferably for 24 hours and the precipitated MgNH4PO4"6H,O is separated and returned for recycling. By this method the ammonium ion concentration in treated solutions can be reduced to below I mg/l.

METHODS

Regeneration and recyclingof magnesium phosphate in NH~ removal The investigations performed with the ammonium ion removal from the waste solutions with H3PO4 and MgO were described in Part I of this series. In this section the precipitated MgNH4PO4.6H20 was subjected to regeneration into Mg3(PO4)2, with the regenerate being recycled in the ammonia recovery trials. In order to reduce the amount of reagents and to recover the ammonium ion for practical use the investigations were undertaken with expulsion of ammonia from MgNH4PO4 by heating and with reuse of roasted regenerate in the ammonium ion removal procedure. To determine the heating temperature and time required, the separated (by filtration) MgNH4PO4"6HeO sedCorrespondence to: T. Stefanowicz, Inst. of Chemistry and Technical Electrochemistry, Poznafl Technical University, 60-965 Poznafi, Poland. 0921-3449/92/$05.00 © 1992 ElsevierScience Publishers B.V. All rights reserved.

340

TADEUSZSTEFANOWICZETAL.

iment was dried at various temperatures for 24 hours to expel ammonia. For determination of NH~ content in the dried sediment it was subjected to distillation from the alkalinized (NaOH) solution with determination by titration of NH3 absorbed in HCI solution [ 1 ]. The results obtained are shown in Table 1. Different amounts of the regenerated sediment were added to 1000 ml of solutions with 1000 mg NH~"/1. By adding an acid (HCI or H3PO4) pH was TABLE 1

Content of NH~" in magnesium phosphate sediment after drying or roasting in various temperatures for 24 hours Heating temperature

Content of NH~ (~)

(°C) 50 I00 150 250

2.7 1.2 0.4 0

TABLE 2 Trials on ammonium ion removal with use of various batches of regenerate Mg3(PO4), masted at 150°C. Initial concentration ofNH~": 1000 mg/l. Stirring time: 30 min No of trial

Regenerate batches (gll)

pH

Concentration of NH~ (mg/l)

I 2 3 4

30.0 29.5 25.0 24.6

!0.03 9.56 9.60 9.60

218.0 0.0 0.0 0.0

5

20.0

8.50

6

20.0

9.16

27.25

0.0

7 8

20.0 20.0

9.21 9.40

0.0 0.0

9 10 II 12 13 14 15 16 17 18 19

20.0 20.0 20.0 17.0 15.0 15.0 15.0 15.0 15.0 15.0 10.0

9.58 9.83 10.20 10.19 8.78 9.20 9.22 9.25 9.60 9.83 9.32

54.5 54.5 27.25 163.5 27.25 0.0 0.0 0.0 0.0 109.0 54.5

AMMONIUMREMOVALWASTESOLUTIONSBYPRECIPITATION

341

TABLE 3

Trials on ammonium ion removal with use of regenerate Mgs (PO4)2 dried or roasted at different temperatures and with different time of stirring. Initial concentration od NH~ • 1000 mg/l No of trial

Batch of regenerate (g/l)

Temperature of regenerate drying °C

Stirring time (h)

pH

Concentration of NH~" (mg/l)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

170 135 135 100 I00 60 30 25 20 30 20 15 15 15 15 8 8 10 10 13.4

50 50 50 50 50 150 150 150 150 150 150 150 150 150 200 200 200 250 250 250

9.27 9.26 9.20 9.36 9.30 9.30 9.30 9.30 9.30 9.30 9.30 9.30 9.30 9.30 9.30 9.30 9.30 9.30 9.30 9.5

21

13.4

250

2 5 2 5 2 5 5 2 2 5 5 4.5 5 5 5 5 24 5 24 1 2 4 1 2 3 4 24

68 0 583 0 420 0 0 136 136 0 0 13 13 0 0 307 119 68 0 204 68 0 408 204 136 68 0

9.5

reduced to pH 1-2 in order to dissolve sediment then pH was adjusted to pH 9-10 with NaOH solution. The suspension was agitated for 30 min and after filtration the filtrate was analyzed for NH~ content. The results obtained with use of sediment roasted at 150 °C are collected in Table 2. The trials confirmed expect~tion~ tilat the regenerate could be used for ammonium ion removal from solutions and, consequently, can replace the commercial reagents required to supply phosphate and magnesium ions. The influence of final pH was also clearly proved. Notwithstanding that sigr~ificant amounts of regenerate (Mg3 (PO4)2) were used (greatly exceeding the s~oichiometry i,~t;le effectiveness of NH~ ions removal was very different. H~vin~, in mind the substantial influence of suspension stirring time upon the c~.ciency of NH~ ions removal, the poor results obtained in ~ome trials probably can be ~tttributed to insufficient stirring time

342

TADEUSZ STEFANOWICZ ET AL.

(30 min). This view was confirmed by the further experiments conducted by using the regenerate dried or roasted at different temperatures, and with different times of stirring (Table 3 ). The following conclusions may be drawn from the results shown in Table 3: When the regenerate was dried or roasted at lower temperatures greater amounts of it should be used for the regeneration process, e.g. the regenerate dried at 50 °C was effective when used in substantial excess ( 100 g of regenerate per I g of NH~ ) and when stirring time was approximately 5 hours. The minimum amount of the regenerate roasted at 250°C during 24 hours was 10 g per ! ~ of NH~" to be removed; however, stirring time must be longer than 5 hours (preferably 24 h). In all cases, the longer stirring times gave better results. The optimum result seems to be example 19, where a high sediment roasting temperature and a long stirring time (24 h) permitted removal of NH~ ions thoroughly at the regenerate-to-ammonium ion ratio of 10: l (by weight). Larger batches of regenerate (examples 20 and 21 ) seems to be indiscriminate in accelerating of MgNH4PO4.6H20 crystallization.

Check trial. In the check trial, 1000 ml solution contained 1039 mg of NH~'. 13.9 g of the regenerate roasted at 250°C for 24 hours was added and pH adjusted to 1.33 with HCI, resulting in thorough dissolution of the suspension. The pH was then increased to 9.5 with NaOH solution and the resulted suspension was agitated for 24 hours, taking the samples in the particular

I000I

' ° °If i" .oor/ &

4

8

12

16

20

24

Time [hi

Fig, I. Ammonium ions concentration changes as a function of time. Regenerate Mg3 (PO4)2 roasted at 250°C was used as a source of magnesium and phosphate and pH was adjusted with HCI and NaOH.

AMMONIUM REMOVAL WASTE SOLUTIONS BY PRECIPITATION

343

time periods, filtering them and analyzing the filtrate for the ammonium ion concentration. The obtained results were plotted in Fig. 1. The curve shows that the stirring time required for MgNH4PO4crystallization is longer than when pure reagents H3PO4 and MgO were used (Part I). The additional trial was performed to check how much the "zero" concentration found by the distillation method deviates from the value determined by Nessler's method [2 ]. A batch of 13.4 g of regenerate (as above) was added to solution ( 1000 ml) with 1039 mg NH~/l. After acidifying with HCI, the pH was adjusted with NaOH to 9.3 and stirring was continued for 24 hours. Ammonium analysis by the distillation method gave the result of zero concentration, while determined by Nessler's method the result was 0.3 mg NH~'/I. PRACTICAL PRESCRIPTION

According to stoichiometry, in order to bind 1000 mg of NH~" 7.3 g of Mg3(PO4)2 is required. With reference to trial 19 (Table 3) it can be assumed that, in practice, 10 g of the roasted regenerate (250°C, 24 h ) is sufficient. However, with reference to Part I of the article (commercial reagents) it would be better to apply a larger batch, not less than 14 g per g of NH~" to be removed. After adding regenerate, the suspension is dissolved by adding e.g. HCI (pH 1-2) and the pH is then increased to 9-10 by adding NaOH. Next, the suspension should be agitated for some hours for the thorough removal of NH~ (the exact stirring time should be determined experimentally for particular solutions taking into account the composition of solution, its volume, agitating rate, temperature etc. ). Filtrate (free of ammonia) can be rejected to sewage or reused if suitable for industrial tasks, while sediment can be recycled, i.e. dried and roasted to expel ammonia. For this stage, drumtype driers prepared for roasting of sludge can be considered suitable [ 3 ]. It is obvious that this method of ammonium ion removal from waste solu. tions can be combined with the recovery of ammonia or with production of ammonium compounds (e.g. NH4CI or NH4NO3). From this point, the method based on the magnesium-ammonium phosphate formation seems to be similar to an ion exchange method; however, it is more attractive because it does not produce additional waste solutions. To support such a view let us consider the ion exchange method with respect to ammonium removal from a waste solution. First of all, the ion exchange method can be used for this purpose effectively under conditions where other cations are absent from solution, to avoid fast exhaustion of the resin. The exchange capacity of some commercial cationic resins fluctuates near 1 val/l: Wofatit F, 1.0 val/l; Wofatit C, 0.8 val/l; Wofatit CN, 1.0 val/l; Lewatit PN, 0.7 val/l; Lewatit KSN, 1.3 val/l; Lewatit S-100, 2.75 val/l; Lewatit CNO,

344

TADEUSZ STEFANOWICZ ET AL.

1.25 val/l; Lewatit CNS, 1.0 val/l; Permutit G, 0.75 val/l; Permutit FG, 1.0 val/I; Permutit S, 0.75 val/l. It means that, in order to remove 1000 mg ofNH + from 10001 of solution 55.61 of cationic resin with the exchange capacity of I val/l should theoretically be used. It is worth noting that the usual concentration range for the ion exchange method is considered to be 100-500 mg/l [ 4 ]. This method is economically unacceptable when the concentration of ion exceeds 2500 mg/l due to rapid exhaustion of the resin and too expensive regeneration [ 5 ]. Hence, the removal ofammonium ions by the ion exchange method when its concentration is I000 m8/l seems to be rather impractical. For example, let us to consider waste solutions produced by the Electric Bulb Plant "POLAM" in Pila (Poland), in amounts of 70 m 3 per day with an average ammonium ion concentration of 1000 mg NH~/l. If we assume that resin will be regenerated once per day then the bed should contain 4 m 3 of resin in the column. Usually, for regeneration of the resin, up to 300% of NaCI consumption should be considered with respect to the theoretical requirement. Consequently, 700 kg of NaCl may be required per day. By passing 70 m 3 of waste solution through the bed the effluent becomes purified of ammonium ions. However, afterwards, due to bed regeneration, 24 m 3 of new wastes are produced per day: • 1.5 bed-volumes of regeneration solution (5-10% NaCl solution), • 4-6 bed-volumes of postregeneration wash solution (2-4 bed-volumes for backwash are not considered, because water from postregeneration wash can be reused). As a consequence, approximately 6 m 3 of solution with 10 g NH~/1 and approximately 18 m 3 ofwastes containing 0.5 g NH~/1 are produced. The final result of the ion exchange process, according to the above example, is rather unfavourable: the water consumption increases by approximately 35%, and, notwithstanding that 3/4 of waste was free of ammonium ion, the said ion accumulates in the remaining amount of waste, which is additionally charged with chloride ions. Despite the fact that a more concentrated ( 10 g/l) solution of NH~ is more attractive for recovery, even by ammonia evaporation, the problem of the diluted solution (0.5 g/l) remaiE~:; unsolved. By contrast, the method based on precipitation of MgNH4PO4 is free ot' such drawbacks. As was noted in Part I, for final purification of effluent the treated waste-waters after NH~ removal can, in cases of excessive concentration of phosphate ion, be subjected to an additional treatment stage with addition of Ca (OH) z to precipitate Ca3 (PO4) 2.

AMMONIUM REMOVAL WASTE SOLUTIONS BY PRECIPITATION

345

CONCLUSIONS

The method of ammonium ion removal from waste solutions by precipitation of MgNH4PO4"6H20 seems to be an effective means of waste purification. The product can be used as a fertilizer in agriculture or recycled, permitting recovery of gaseous ammonia or production of ammonium salts. The final polishing of effluent, with removal of phosphate ions, can be effected according to the known method of precipitation of Ca3 (PO4)2.

REFERENCES 1 Minczewski, J. and Marczenko, Z., 1978r. Chemia Analityczna, PWN, Warszawa, 1978r., Tom II. p. 225. 2 Chariot, G., 196 I. Les Methodes de la Chimie Analitique, Masson et Cie, Editeurs, p. 613 3 McCornick,P.Y., 1988. Chem. Eng., 15: 113. 4 Palmer, S.A.K., Breton, M.A., Nunno, T.J., Sullivan, D.M. and Surprenant, N.F., 1988. Metal/Cyanide Containing Wastes Treatment Technologies, Noyes Data Corp., Park Ridge, New Yersey, USA, p. 114. 5 Ref. 4, p. 256.