EFFECT OF CONTINUOUS CRYSTALLIZER PERFORMANCE ON STRUVITE CRYSTALS PRODUCED IN REACTION CRYSTALLIZATION FROM SOLUTIONS CONTAINING PHOSPHATE(V) AND ZINC(II) IONS

Hutnik, N.; Wierzbowska, B.; Piotrowski, K.; Matynia, A.

doi:10.1590/0104-6632.20160332s00003385

N. Hutnik

Wroclaw University of Technology, Faculty of Chemistry, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland. Phone: (+ 48 71) 320-3497; Fax: (+ 48 71) 328-0425. E-mail: nina.hutnik@pwr.edu.pl; boguslawa.wierzbowska@pwr.edu.pl; andrzej.matynia@pwr.edu.pl

B. Wierzbowska

Wroclaw University of Technology, Faculty of Chemistry, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland. Phone: (+ 48 71) 320-3497; Fax: (+ 48 71) 328-0425. E-mail: nina.hutnik@pwr.edu.pl; boguslawa.wierzbowska@pwr.edu.pl; andrzej.matynia@pwr.edu.pl

K. Piotrowski

Silesian University of Technology, Department of Chemical Engineering and Process Design, M. Strzody 7, 44-101 Gliwice, Poland. Phone/Fax: (+ 48 32) 237-1461

* To whom correspondence should be addressed

A. Matynia

Wroclaw University of Technology, Faculty of Chemistry, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland. Phone: (+ 48 71) 320-3497; Fax: (+ 48 71) 328-0425. E-mail: nina.hutnik@pwr.edu.pl; boguslawa.wierzbowska@pwr.edu.pl; andrzej.matynia@pwr.edu.pl

* E-mail: krzysztof.piotrowski@polsl.pl

Figures (4)
Tables (3)

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Figure 1
Schematic diagram of the experimental plant. 1 - DT MSMPR type crystallizer with internal circulation of suspension, 2 - thermostat, 3 - PC computer, 4 - feed reservoir: aqueous solution of MgCl₂, NH₄H₂PO₄ and ZnCl₂, 5 - feed proportioner (pump), 6 - alkalinity agent tank: aqueous solution of NaOH, 7 - proportioner (pump) of NaOH solution, 8 - receiver (pump) of product crystal suspension from the crystallizer tank, 9 - storage tank of the product crystal suspension, 10, 11, 12 - electronic balances, M - stirrer speed control/adjustment, pH - alkaline/acid reaction control/adjustment, T - temperature control/adjustment.

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Figure 2
Exemplary differential (left scale) and cumulative (right scale) volumetric (mass) size distributions of struvite crystals produced in the continuous DT MSMPR crystallizer. Process parameters: (a) [Zn²⁺]_RM 0.2 mg kg^-1, pH 9, τ 900 s; (b) [Zn²⁺]_RM 2.0 mg kg^-1, pH 9, τ 900 s; (c) [Zn²⁺]_RM 0.2 mg kg^-1, pH 11, τ 900 s; (d) [Zn²⁺]_RM 0.2 mg kg^-1, pH 9, τ 3600 s (tests No. 2, 5, 7 and 9 in ).

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Figure 3
Scanning electron microscope images of struvite crystals produced in the continuous DT MSMPR crystallizer. Mean size of product crystals L_m: (a) 31.4 µm; (b) 23.1 µm; (c) 22.0 µm; (d) 41.0 µm (crystal size distributions - see ; tests No. 2, 5, 7 and 9 in ).

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Figure 4
Details of habit and surface morphology of a struvite crystal produced in the presence of zinc(II) ions (corresponding to ).

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Table 1
Effect of Zn²⁺ ion concentration in the feed, pH and mean residence time τ of the suspension in the DT MSMPR crystallizer on crystal product characteristics. Concentration of phosphate(V) ions in the feed: [PO₄^3-]_RM = 1.0 mass %, molar proportion of substrate ions in a feed: PO₄^3-: Mg²⁺: NH₄⁺ = 1 : 1 : 1. Reaction crystallization process temperature: 298 ± 0.2 K.

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Table 2
Comparison of the influence of selected impurities present in the solution feeding the continuous DT MSMPR crystallizer on the struvite crystal characteristics. Process parameters: concentration of phosphate(V) ions in the feed: [PO₄^3-]_RM = 1.0 mass %, molar proportions of substrate ions in the feed PO₄^3-: Mg²⁺: NH₄⁺ = 1 : 1 : 1, mean residence time of the suspension in the crystallizer: τ 900 s, pH 9.

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[Cl–]ML concentration of chloride ions in the postprocessed mother solution (mass %) CV coefficient of crystal size variation (%) ddt draft tube diameter (m) dm propeller stirrer diameter (m) D crystallizer diameter (m) hdt draft tube height (m) H crystallizer height (m) kv crystal's volume shape factor Ksp solubility product L crystal characteristic size (m) L50 median crystal size for 50 mass % undersize fraction in the cumulative mass (volume) distribution (m) L84, L16 crystal sizes corresponding to 84 and 16 mass % undersize fractions in the cumulative mass (volume) distribution (m) La crystal length (m) Lb crystal width (m) Lc crystal height (m) Ld crystal mode size (m) Li mean size of the i-th fraction (m) Lm mean crystal size (m) MT solid phase content in the product crystal suspension (kg m-3) [PO43−]ML concentration of phosphate(V) ions in the postprocessed mother solution (mass %) [PO43−]RM feed concentration of phosphate(V) ions (mass %) xi mass fraction of crystals Vcrystal crystal volume (m3) [Zn2+]ML concentration of zinc(II) ions in the postprocessed mother solution (mg kg-1) [Zn2+]RM feed concentration of zinc(II) ions (mg kg-1) Greek Letters τ mean residence time of the crystal suspension (s) Abbreviations CSD Crystal Size Distribution DT Draft Tube IAP Ion Activity Product IWW Industrial Wastewater MSMPR Mixed Suspension Mixed Product Removal crystallizer SI Saturation Index WI Without Impurities

No.	Process parameters				Suspension in a crystallizer		Struvite crystals characteristics^a a After drying, without crystal washing on the filter.					Product^a a After drying, without crystal washing on the filter.
No.	[Zn²⁺]_RM (mg kg^-1)	pH	τ (s)	^SI SI - saturation index, SI = log(IAP) - log(Ksp); IAP - ion activity product; SI values provided by Visual MINTEQ software	M_T (kg m^-3)	[PO₄^3-]_ML (mg kg^-1)	^L_m Lm - mean crystal size,Lm = ∑xiLi, where: xi - mass fraction of crystals of mean fraction size Li; (µm)	^L₅₀ L50 - median crystal size for 50 mass % undersize fraction; (µm)	^L_d Ld - crystal mode size; (µm)	^CV CV = 100(L84 - L16)/(2L50), where: L84, L16, L50 - crystal sizes corresponding to 84, 16 and 50 mass % undersize fractions in the cumulative distribution. (%)	L_a/L_b	[Zn²⁺](mg kg^-1)
1	0.1	9	900	4.625	23.6	146	34.0	26.6	36.9	82.7	8.4	~0.02
2	0.2	9	900	4.626	23.8	148	31.4	24.5	34.6	84.0	8.3	~0.04
3	0.5	9	900	4.626	24.0	144	29.3	21.2	26.1	90.0	7.6	~0.10
4	1	9	900	4.625	24.1	146	27.2	19.3	22.0	90.4	7.3	23.3
5	2	9	900	4.624	24.3	144	23.1	16.1	19.9	92.1	7.0	64.3
6	0.2	10	900	4.951	24.5	122	23.8	16.8	21.0	92.0	7.6	~0.04
7	0.2	11	900	4.541	24.7	101	22.0	14.1	18.9	92.8	6.6	~0.04
8	0.2	9	1800	4.626	24.7	128	35.6	25.0	28.7	83.0	6.9	~0.04
9	0.2	9	3600	4.626	24.8	110	41.0	30.8	41.7	88.3	6.0	~0.04

Impurities in a feed			Parameters of struvite CSD^a a Comments - see Table 1				Crystal shape	References
Ion	Concentration (mg kg^-1)	SI	L_m (µm)	L₅₀ (µm)	L_d (µm)	CV (%)	L_a/L_b	References
Ca²⁺	500	4.593	32.6	25.5	27.8	85.1	4.2	Hutnik et al., 2011b
Cu²⁺	0.2	4.626	27.5	19.4	25.5	92.9	6.6	*Hutnik et al., 2011c*Hutnik, N., Matynia, A., Piotrowski, K. and Mazienczuk, A., Influence of copper(II) ions on the quality of struvite crystals produced in continuous reaction crystallization process. Markos, J., (Ed.), Proceedings of the 38th International Conference of Slovak Society of Chemical Engineering, Slovakia, 318 (2011c).
Fe²⁺	5	4.626	33.0	27.4	24.6	81.8	5.0	*Hutnik et al., 2012*Hutnik, N., Piotrowski, K., Wierzbowska, B. and Matynia, A., Continuous reaction crystallization of struvite from water solutions of phosphates(V) in presence of iron(II) ions. Journal of Environmental Science and Engineering, A1, 35 (2012).
K⁺	250	4.624	31.6	25.0	25.6	83.0	5.3	*Hutnik et al., 2013b*Hutnik, N., Wierzbowska, B. and Matynia, A., Continuous reaction crystallization of struvite from solution containing phosphate(V) and potassium ions. Przemysł Chemiczny, 92, 791 (2013b).
NO₃^-	886	4.623	29.4	20.3	27.6	88.5	6.4	*Hutnik et al., 2013c*Hutnik, N., Wierzbowska, B., Piotrowski, K. and Matynia, A., Continuous reaction crystallization of struvite from solution containing phosphate(V) and nitrate ions. The Online Journal of Science and Technology, 3, 58 (2013c).
Zn²⁺	0.2	4.626	31.4	24.5	34.6	84.0	8.3	Present study
Zn²⁺	2	4.624	23.1	16.1	19.9	92.1	7.0	Present study
IWW^b b Phosphorus fertilizer industry wastewater	^* * Concentration of phosphate(V) ions 0.445 mass % and impurities (in mg kg-1): Al - 6.4, Ca - 440, Cu - 0.25, Fe - 8.9, K - 46, Ti - 0.2, Zn - 2.2, Si - 51, F- - 42, SO42- - 703 (Hutnik et al., 2013a)	3.788	25.5	21.6	22.5	79.8	3.8	Hutnik et al., 2013a
WI^c c Without impurities	-	4.625	27.5	21.5	23.6	82.4	6.0	Hutnik et al., 2011a

[Cl^–]_ML	concentration of chloride ions in the postprocessed mother solution (mass %)
CV	coefficient of crystal size variation (%)
d_dt	draft tube diameter (m)
d_m	propeller stirrer diameter (m)
D	crystallizer diameter (m)
h_dt	draft tube height (m)
H	crystallizer height (m)
k_v	crystal's volume shape factor
K_sp	solubility product
L	crystal characteristic size (m)
L₅₀	median crystal size for 50 mass % undersize fraction in the cumulative mass (volume) distribution (m)
L₈₄, L₁₆	crystal sizes corresponding to 84 and 16 mass % undersize fractions in the cumulative mass (volume) distribution (m)
L_a	crystal length (m)
L_b	crystal width (m)
L_c	crystal height (m)
L_d	crystal mode size (m)
L_i	mean size of the i-th fraction (m)
L_m	mean crystal size (m)
M_T	solid phase content in the product crystal suspension (kg m^-3)
[PO₄³⁻]_ML	concentration of phosphate(V) ions in the postprocessed mother solution (mass %)
[PO₄³⁻]_RM	feed concentration of phosphate(V) ions (mass %)
x_i	mass fraction of crystals
V_crystal	crystal volume (m³)
[Zn²⁺]_ML	concentration of zinc(II) ions in the postprocessed mother solution (mg kg^-1)
[Zn²⁺]_RM	feed concentration of zinc(II) ions (mg kg^-1)
Greek Letters
τ	mean residence time of the crystal suspension (s)
Abbreviations
CSD	Crystal Size Distribution
DT	Draft Tube
IAP	Ion Activity Product
IWW	Industrial Wastewater
MSMPR	Mixed Suspension Mixed Product Removal crystallizer
SI	Saturation Index
WI	Without Impurities

[1] * E-mail: krzysztof.piotrowski@polsl.pl

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EFFECT OF CONTINUOUS CRYSTALLIZER PERFORMANCE ON STRUVITE CRYSTALS PRODUCED IN REACTION CRYSTALLIZATION FROM SOLUTIONS CONTAINING PHOSPHATE(V) AND ZINC(II) IONS

Abstract