Scholarly article on topic 'Separation of Struvite from Mineral Fertilizer Industry Wastewater'

Separation of Struvite from Mineral Fertilizer Industry Wastewater Academic research paper on "Chemical engineering"

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{struvite / "industrial wastewater" / "continuous reaction crystallization" / "crystallizers with jet pump" / "product quality" / "phosphorus recycling"}

Abstract of research paper on Chemical engineering, author of scientific article — Andrzej Matynia, Boguslawa Wierzbowska, Nina Hutnik, Agata Mazienczuk, Anna Kozik, et al.

Abstract The research results of struvite reaction crystallization process from the phosphorus mineral fertilizers industry wastewater, carried out in DTM type continuous crystallizers with internal circulation of suspension driven by jet pump device fed with mother solution, are presented. The wastewater of pH < 4, contained 0.445 mass % of phosphate(V) ions and typical impurities: aluminium, calcium, copper, iron, potassium, magnesium, titanium, zinc, fluosilicate, fluoride and sulphate(VI) ions. Tests ran in three continuous jet pump DTM crystallizers of increasing working volumes: 1.2, 15 and 36 dm3, in temperature 298 K, assuming both stoichiometric conditions and 20% excess of magnesium ions in relation to phosphate(V) and ammonium ion contents. Influence of technological control parameters (pH, τ) on product crystal size distributions (CSDs) and crystals/mother liquor chemical composition were identified. Crystals of low size-homogeneity (coefficient of variation CV ca. 90%) which mean size varied from ca. 9 to ca. 33 μm were identified. Struvite crystals of the largest sizes were produced in a DTM crystallizer of working volume 36 dm3 at 20% excess of magnesium ions, at pH 9 and for elongated mean residence time 7200 s. Excess of magnesium ions in a process system influenced the struvite reaction crystallization process yield definitely advantageously. Concentration of phosphate(V) ions decreased from 0.445 mass % in a feed to 1.810–3 mass % in a postprocessed mother solution, what can be regarded as a very good result of their removal process. In product crystals, besides main component – struvite, also all impurities originally present in wastewater appeared in a form of hydroxides, phosphates and other salts. Aluminium, copper, iron and zinc ions were the subject of practically total co- precipitation with struvite.

Academic research paper on topic "Separation of Struvite from Mineral Fertilizer Industry Wastewater"

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Procedía

Environmental Sciences

Procedía Environmental Sciences 18 (2013) 766 - 775

2013 International Symposium on Environmental Science and Technology (2013 ISEST)

Separation of struvite from mineral fertilizer industry

wastewater

Andrzej Matyniaa, Boguslawa Wierzbowskaa, Nina Hutnika, Agata Mazienczuka,

Anna Kozika, Krzysztof Piotrowskib*

aWroclaw University of Technology, Faculty of Chemistry, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland bDepartment of Chemical & Process Engineering, Silesian University of Technology, ks. M. Strzody 7, 44-101 Gliwice, Poland

The research results of struvite reaction crystallization process from the phosphorus mineral fertilizers industry wastewater, carried out in DTM type continuous crystallizers with internal circulation of suspension driven by jet pump device fed with mother solution, are presented. The wastewater of pH < 4, contained 0.445 mass % of phosphate(V) ions and typical impurities: aluminium, calcium, copper, iron, potassium, magnesium, titanium, zinc, fluosilicate, fluoride and sulphate(VI) ions. Tests ran in three continuous jet pump DTM crystallizers of increasing working volumes: 1.2, 15 and 36 dm3, in temperature 298 K, assuming both stoichiometric conditions and 20% excess of magnesium ions in relation to phosphate(V) and ammonium ion contents. Influence of technological control parameters (pH, x) on product crystal size distributions (CSDs) and crystals/mother liquor chemical composition were identified. Crystals of low size-homogeneity (coefficient of variation CV ca. 90%) which mean size varied from ca. 9 to ca. 33 ^m were identified. Struvite crystals of the largest sizes were produced in a DTM crystallizer of working volume 36 dm3 at 20% excess of magnesium ions, at pH 9 and for elongated mean residence time 7200 s. Excess of magnesium ions in a process system influenced the struvite reaction crystallization process yield definitely advantageously. Concentration of phosphate(V) ions decreased from 0.445 mass % in a feed to 1.810-3 mass % in a postprocessed mother solution, what can be regarded as a very good result of their removal process. In product crystals, besides main component - struvite, also all impurities originally present in wastewater appeared in a form of hydroxides, phosphates and other salts. Aluminium, copper, iron and zinc ions were the subject of practically total co-precipitation with struvite.

© 2013 The Authors. Published by Elsevier B.V.

Selection and peer-review underresponsibility of Beijing Institute of Techno logy.

Keywords: struvite; industrial wastewater; continuous reaction crystallization; crystallizers with jet pump; product quality; phosphorus recycling.

* Corresponding author. Tel/fax:+48-32-237-14-61. E-mail address: krzysztof.piotrowski@polsl.pl.

Abstract

1878-0296 © 2013 The Authors. Published by Elsevier B.V.

Selection and peer-review under responsibility of Beijing Institute of Technology.

doi:10.1016/j.proenv.2013.04.103

Nomenclature

b - distance between feeding nozzle outlet and mixing chamber inlet, m;

CV - coefficient of (crystal size) variation, defined as 100(L84 - L16)/(2L50), %;

de - feeding nozzle diameter (outlet), m;

dk - mixing chamber diameter, m;

do - confusor diameter (inlet), m;

D - diameter of cylindrical tank of the crystallizer, m;

h - distance between feeding nozzle outlet and crystallizer bottom, m;

H - total height of a crystallizer, m;

lk - mixing chamber length, m;

lo - confusor length, m;

L - mean size of z-th crystal fraction, m;

Lm - mean size of crystal population, defined as I,xlLl, m;

L50 - median crystal size for 50 mass % undersize fraction, m;

MT - crystal content in suspension (suspension density), kgcryst /m3;

[Mg2+]RM - inlet concentration of magnesium ions, mass %;

[NH4+]rm - inlet concentration of ammonium ions, mass %;

[P04 ] crystals - concentration of phosphates in crystal product, mass %;

[P043 ]RM - inlet concentration of phosphate(V) ions, mass %;

[P04 ] solution - concentration of phosphate(V) ions in mother solution, mg/kg;

P 1 eu - unit power of the liquid stream feeding the jet pump, W/kg;

qv - volumetric (out)flow rate of crystal suspension from the crystallizer, m3/s;

u - ejection degree of a jet pump;

V - crystallizer total volume, m3;

Vw - crystallizer working volume, m3;

xi - mass fraction of the crystals of mean fraction size L;.

Greek letters

X - mean residence time of suspension in a crystallizer working volume, defined as Vw/qv,

Abbreviations

CSD - Crystal Size Distribution

DTM - Draft Tube Magma (crystallizer)

1. Introduction

Inexpensive and easily accessible secondary sources of phosphorus can be industrial and municipal wastewaters, liquid manure, etc. [1, 2]. Integrated processes focused on recovery of usable phosphorus forms from such complex liquid systems are generally named "phosphorus recycling". Main idea of these is application of reaction crystallization process followed by separation from wastewaters the sparingly soluble phosphate salts, mainly magnesium ammonium phosphate(V) hexahydrate, MgNH4PO4-6H2O, struvite [3]. Elaboration of optimal design solution, in respect to both technological and apparatus aspects, for the process of phosphates(V) removal from diluted wastewater solutions is not a simple task [1]. Particular attention should be paid, among others, on changeable streams of these solutions, low (below 1.0 mass %, in selected cases even below 0.1 mass %) and varied in time concentration of their main component - phosphate(V) ions, as well as on the presence of various impurity forms [4-6]. Continuous reaction crystallization process course and its results can be also affected by temperature, reagent concentrations, mean residence time of crystal suspension and environment's pH [1-3, 7, 8]. These parameters influence not only the struvite product properties, but also co-precipitation of sparingly soluble salts or/and hydroxides of some metals present in complex wastewater systems. Successful operation of the process is a result of appropriately and rationally selected continuous crystallizer construction and its work parameters - pH, mean residence time of suspension in its working volume, internal suspension mixing and circulation intensity, interior geometry and spatial locations of the reagents inlet ports, reagents contact method, etc. [9-12].

The experimental data concerning recovery of phosphate(V) ions from the phosphorus mineral fertilizers industry wastewater by continuous reaction crystallization of sparingly soluble salt: magnesium ammonium phosphate(V) hexahydrate - struvite, MgNH4PO4-6H2O, are presented. The resulting crystalline product can be directly applied as a mineral fertilizer or component for soil quality improvement, slowly dosing its nutrients content [13]. Research was carried out in three, fully automated experimental plants of diverse sizes, working in continuous regime and controlled by computer system. Original continuous crystallizers with internal circulation of suspension driven by liquid jet pump were applied [14]. Feeding nozzle of each jet pump was located in a bottom part of each apparatus, producing thus ascending flow of suspension in a mixing chamber. Such designed crystallizers can be included into DTM (Draft Tube Magma) type constructions [15]. Jet pump induces, similarly to stirrer or internal pump, intensive circulation of suspension and its good mixing in a crystallizer, simultaneously considerably constraining attrition and breakage within the crystal phase (lack of moving elements inside the apparatus generating shear stresses).

Working volumes of jet pump DTM crystallizers used in the presented research were: Vw 1.2, 15 and 36 dm3. Feed solution in all three plants was real wastewater collected from one of polish phosphorus mineral fertilizers industry plants. Concentration of PO43- ions in this wastewater was 0.445 mass %. Detailed chemical composition of the wastewater is presented in Table 1. Influence of struvite reaction crystallization environment's pH within the 9-11 range, mean residence time of suspension in the DTM crystallizers x from 900 to 7200 s and excess of magnesium ions in relation to phosphate(V) ions (molar ratio of the reagents: [PO43-]rm : [Mg2+]RM : №+]rm = 1 : 1 : 1 and 1 : 1.2 : 1) on the crystal product quality was identified experimentally.

2. Experimental

Recovery of phosphate(V) ions from the phosphorus mineral fertilizers industry wastewater ran according to technological scheme presented in Fig. 1. The reagents: wastewater (from Z.Ch. POLICE S.A., Poland, of pH 3.8 and chemical composition presented in Table 1), magnesium chloride

hexahydrate MgCl2-6H2O and ammonium chloride NH4Cl, were stored in transitional tanks. These were then introduced into the mixer, where substrates dosed in a crystalline form were dissolved in wastewater.

magnesium chloride and ammonium chloride; 4 and 6 - belt feeders; 7 - mixer; 9 - tank of alkaline agent solution; 11 - liquid jet pump DTM crystallizer;12 - overflow; 13 - decanter; 15 - partial vacuum belt filter; 16 - belt conveyor; 17 - storage reservoir of

postprocessed, practically phosphate(V) -free wastewater.

Fig. 1. Simplified technological scheme of process plant for continuous struvite production from mineral fertilizers industry wastewater.

Table 1. Chemical composition of phosphorus mineral fertilizers industry wastewater used.

Component Concentration, mass %

PO43- 0.445

Al 6.4-10-4

Ca 0.044

Cu 0.25-10-4

Fe 8.9-10-4

K 4.6-10-3

Mg 0.0306

Si 5.110-3

Ti 0.2-10-4

Zn 2.2-10-4

F- 4.2-10-3

SO42- 0.0703

Mass streams of these reagents resulted from the assumed molar ratio of substrates [PO43-] RM : [Mg2+] RM : [NH4+] rm (as 1 : 1 : 1 or 1 : 1.2 : 1), required mean residence time of suspension in a crystallizer x (from 900 to 7200 s) and working volume of the crystallizer applied (Vw 1.2, 15 or 36 dm3). Clear

solution of the mixed and totally dissolved components, of pH 3.6 (stoichiometric proportions between the reagents) or 3.5 (magnesium ions excess) was dosed with the pump into the crystallizer. Geometrical parameters of the jet pump crystallizers used in the tests are presented in detail in Table 2. To adjust and control the pH of struvite reaction crystallization environment aqueous solution of sodium hydroxide, of concentration 5 mass % of NaOH, was dosed. For effective enough internal circulation and mixing of suspension in the crystallizers the minimal value of unit power of a jet pump feed stream Peu was assumed, ca. 0.22 W/kg [11]. Intensity of circulation inside the apparatus was thus also minimal, what, however, limited disadvantageous crystal attrition and breakage phenomena. The jet pump systems in the crystallizers used in the presented research were designed to keep the ratio of mass stream of suspension being the subject of suction and solution provided into jet pump's feeding nozzle constant. Thus jet pump's ejection degree u was in each studied case identical, ca. 6.

Table 2. Characteristic of continuous DTM type crystallizers with liquid jet pumps for struvite production from mineral fertilizers industry wastewater. Working volumes of the crystallizers: Vw (A) 1.2, (B) 15 and (C) 36 dm3.

Parameters A Crystallizers B C

Volume of crystallizer:

- total Vt, dm3 2.1 20 50

- working Vw, dm3 1.2 15 36

Crystallizer:

- diameter of cylindrical tank D, mm 90 250 300

- total height Ht, mm 330 600 900

Jet pump:

- ejection degree u ~6 ~6 ~6

- feeding nozzle diameter (outlet) de, mm 2.0 3.4 7.5

- confusor:

diameter (inlet) do, mm 30 50 90

length lo, mm 12 15 30

- distance between feeding nozzle outlet and crystallizer

bottom h, mm 25 40 40-50

- distance between feeding nozzle outlet and mixing chamber

inlet b, mm 0 0 0

- mixing chamber:

diameter dk, mm 15 26 54

lenght lk, mm 125 235 335

- djdk 0.133 0.131 0.139

Continuous reaction crystallization of struvite ran in temperature 298 K, under atmospheric pressure. Crystal suspension was removed from the crystallizer with the pump (crystallizer of Vw 1.2 dm3) or overflow (crystallizers of Vw 15 and 36 dm3) and directed to decanter, in which suspension was a subject of ca. 10-time thickening. Then solid phase was separated from mother solution on a filter. After drying the size distribution of the product crystals (CSD) (solid particle analyzer Coulter LS-230, Table 3) and their chemical composition (atomic absorption spectrometer iCE 3000, spectrophotometer UV-VIS Evolution 300) were determined (Table 4). Process was evaluated on the basis of analysis results of the

products removed from the crystallizers after their continuous process time 5т, starting from the moment of the required work parameters values stabilisation. It can be thus assumed, that crystal concentration in suspension and CSD correspond to the selected control process parameters set (establishment of the steady-state conditions) [16].

3. Results and Discussion

As it was mentioned, final results of the discussed process are dependent on: environment's pH of struvite precipitation and crystallization, mean residence time of crystal suspension in a crystallizer (which represents some unknown, however unequivocally correlated value of mean supersaturation in mother solution), impurities present in wastewater, molar ratio of reagents (e.g. excess of magnesium ions in relation to phosphate(V) ions), as well as working volume of the crystallizer. Influence of all these parameters on product crystals quality is presented in Tables 3 and 4.

Fast and effective separation of the crystals from mother solution depends mainly on their CSD. Statistical parameters of product's CSD are presented in Table 3. From the table it results, that increase in pH of struvite continuous reaction crystallization process environment from 9 to 11 produces more than 2-time decrease of crystals mean size, Lm (from 20.2 to 9.2 ^m for т 900 s in a crystallizer of Vw 1.2 dm3) [17]. With the pH increase struvite solubility decreases, whereas its precipitation potential significantly raises [1, 2]. In these process conditions also the induction time shortens, indispensable for nucleation process initiation. In result population density of struvite nuclei enlarges and in consequence crystal mean size decreases. Contrary, elongation of mean residence time of suspension in a crystallizer results in mean crystal size enlargement, both in a crystallizer of smaller (Vw 1.2 dm3) [17], and in crystallizers of larger (Vw 15 dm3 [18] and 36 dm3 [19]) working volumes (Table 3).

Table 3. Influence of selected technological parameters of continuous struvite reaction crystallization process from phosphorus mineral fertilizers industry wastewater in a DTM crystallizers with liquid jet pumps on the product quality. Process temperature: 298 K.

Crystallizer Process parameters Suspension in a crystallizer Crystal-product characteristic

Vw pH X MT [р043 ]solution [р°43 ]crystals Lm L50 CV

dm3 - s kg crystals/m3 mg/kg mass % ^m ^m %

Molar proportions of reagent ions in a feedi [P043-]rm i [Mg24 ]RM i [NH4 ]rm = 1 i 1 i 1

9 900 10.9 54.1 39.4 20.2 17.2 92.6

10 900 10.9 48.2 39.5 17.1 12.6 93.2

1.2 [17]

11 900 11.3 38.6 39.9 9.2 6.1 90.6

9 3600 11.4 22.6 40.3 25.6 21.1 79.0

9 900 11.1 51.9 39.3 19.2 14.6 91.6

15 [18] 9 3600 11.3 26.1 40.4 26.0 20.6 90.6

9 7200 11.6 20.4 40.9 28.8 23.1 89.9

36 [19] 9 7200 11.6 55.0 41.3 29.6 26.3 89.6

Molar proportions of reagent ions in a feed: [PQ43-]rm : [Mg2+] RM i [NH4 ]rm = 1 i 1.2 i 1

1.2 [17] 9 900 11.1 19.8 40.3 22.3 18.6 90.6

15 [18] 9 900 11.2 19.6 40.7 20.8 16.2 92.0

36 [19] 9 7200 11.7 18.0 40.5 32.6 28.2 90.3

Under such modified process conditions working supersaturation in solution decreases, thus nucleation rate also falls. Linear crystal growth rate goes down with the supersaturation devaluation, however longer suspension residence time, thus its direct contact with supersaturated mother solution, compensates this effect resulting in systematic increase in particle size. From this reason elongation of mean residence time of suspension in the applied crystallizers from 900 to 3600 s (at pH 9) made manufacturing of products of mean crystal size larger by ca. 30% (Lm increased up to 26 ^m) possible. Moreover, in the crystallizers of Vw 15 and 36 dm3 further, 2-time elongation of this time from 3600 to 7200 s caused increase in Lm by the next 10-12% (up to 29.6 |am) [18, 19]. From the DTM crystallizers used solid products of diverse crystal sizes were removed. Coefficient of crystal size variation, CV (Table 3), demonstrated high values, even up to ca. 93%. With the elongation of mean residence time of suspension size-homogeneity within crystal population insignificantly increased (CV ca. 90%), what is connected with lower nucleation rate and more stable process of particle size growth, in spite of disadvantageous effect of longer residence time on crystal attrition [15].

Excess of magnesium ions in respect of phosphate(V) ions concentration in the environment of chemical reaction of struvite precipitation shifts this reaction equilibrium right - towards struvite synthesis. In such modified process conditions concentration of phosphate(V) ions in mother solution decreases (Table 3), thus struvite reaction crystallization process yield increases, as well as the closely related wastewater purification effect. It was accompanied, as it was confirmed experimentally (Table 3), with small increase in mean size of struvite product crystals. It is the effect of complex feedback between higher supersaturation in solution at the crystallizer inlet and CSD of suspension removed from the crystallizer, through nucleation and crystal growth rates, dependent in turn on the resulting supersaturation establishing in mother solution. Increase in mean struvite crystal size by ca. 2-3 ^m (thus by ca. 10%) was thus reported.

Fig. 2. Scanning electron microscope images of crystals (mainly struvite) produced from mineral fertilizers industry wastewater in a continuous liquid jet pump DTM crystallizers: (a) Vw 15 dm3, (b) Vw 36 dm3. Corresponding process parameters and characteristic of product crystals - see Table 3: (a) pH 9, x 3600 s, stoichiometric conditions; (b) pH 9, x 7200 s, magnesium ions excess. Scanning electron microscope JEOL JSM 5800LV.

Solid products of the process, without water washing of crystals on a filter (ca. 20-25 mass % of mother solution in a filter cake), after drying consisted of mainly struvite, but covered also phosphates and hydroxides of calcium, iron, aluminium, zinc, as well as sulphates, fluosilicates, fluorides and other impurities derived from mother solution (indirectly - from wastewater). Scanning electron microscope

images of the exemplary product crystals are presented in Fig. 2. Diverse sizes within struvite crystals are clearly visible. Also other solid particles can be identified, co-precipitated from wastewater in the common process conditions applied. These usually form agglomerates on the parent struvite crystal surfaces. Struvite crystals are just covered with precipitated hydroxides of metal impurities, hydroxyapatite and other salts. The best shaped struvite crystals, of the most advantageous CSDs, were produced at pH 9, elongated mean residence time x 7200 s and at 20% excess of magnesium ions in a feed, as well as in a crystallizer of the largest working volume Vw 36 dm3 (Fig. 2b). Based on scanning electron microscope images analysis one can conclude, that struvite crystals surface was covered and blocked by simultaneously co-precipitating and co-growing particles of hydroxides and impurity salts, what finally generated large tensions within the struvite crystal structures. Thus numerous crystal cracks and fractures, irregular surface morphology, deformed edges, presence of characteristic tubular and trough crystals, etc. can be identified (Fig. 2). It can be assumed, that final struvite CSD and individual crystal habits are the net result of complex action of impurities present in the investigated wastewater, as well as of continuous reaction crystallization process parameters. From the scanning electron microscope images it also results, that agglomeration within struvite crystals was not significant. It generally speaks advantageously about process conditions established in the DTM crystallizers for interrelated nucleation and struvite crystals growth processes. However, taking under consideration all components of struvite continuous reaction crystallization process in modern constructions of DTM crystallizers with liquid jet pump one can notice, that main factor influencing directly or indirectly the process run is solution supersaturation, very strongly dependent (at constant composition of the feed stream, constant temperature and constant mixing/circulation intensity) on environment's pH and mean residence time of suspension in a working volume of the crystallizer.

Table 4. Chemical composition of solid phase (see also Table 3) and mother solution after filtration of crystal suspension removed from the continuous DTM crystallizers with liquid jet pumps.

Concentration in:

Component Mother solution, mg/kg Solid phase*, mass %

PO43- 52.1 - 180** 39.3 - 41.3**

Mg2+ 30 - 242 9 - 10.1

nh4+ 74 - 120 6.6 - 7.2

Al 0.1 - 0.3 (4.8 - 5.5)-10-2

Ca < 50 2.9 - 4.0

Cu 0.02 - 0.11 (0.6 - 1.3)-10-4

Fe 0.03 - 0.08 0.10 - 0.20

K 24 - 37 0.12 - 0.22

Si 23 - 40 (8.6 - 9.7)-10-2

Ti < 0.2 < 2-10-5

Zn < 0.5 (1.6 - 1.9)-10-2

F- 3 - 24 0.36 - 0.48

SO42- 400 - 580 1.5 - 2.0

* after drying, without water washing of crystals in a filter ** see Table 3

In Table 4 there are presented the concentration ranges corresponding to individual components of postprocessed mother solution and solid phase (without water washing of crystals on a filter and after their drying) removed from the DTM crystallizers (see Table 3). Crystalline product, as it results from Table 4, except main component MgNH4PO4-6H2O, contained also all impurities originally present in wastewater, among others: metal hydroxides, phosphates, fluosilicates, fluorides and sulphates. From the data analysis it results, that at magnesium ions excess practically total precipitation of aluminium, calcium, copper, iron and zinc ions is observed (compare these ion concentrations in raw wastewater (Table 1) and in postprocessed mother solution (Table 4)). One may also notice, that phosphate(V) ions concentration in a postprocessed mother solution varied from 54.1 mg/kg (pH 9, x 900 s) to 22.6 mg/kg (pH 9, x 3600 s) (tests in crystallizer of Vw 1.2 dm3, Table 3). This concentration values decreased regularly with the pH increase and with elongation of mean residence time x of struvite crystal suspension in a crystallizer. From the comparison it results, that concentration of phosphate(V) ions decreased even 2.5-time. It is connected with the decrease of struvite solubility with the increase in reactive mixture's pH, whereas longer contact time of crystals with the supersaturated solution in a crystallizer provides more thorough discharge of the generated supersaturation. The [PO43-]solution values can be regarded small, thus the efficiency of phosphate(V) ions removal from the feed solution as a fully satisfactory.

Excess of magnesium ions in relation to phosphate(V) and ammonium ions concentrations influenced the process yield advantageously. Concentration of phosphate(V) ions in a postprocessed mother solution was ca. 2-3-time lower compared to stoichiometric conditions results (see Table 3).

4. Conclusions

The sparingly soluble salt, MgNH4PO4-6H2O, struvite, was produced from the phosphorus mineral fertilizers industry wastewater. For continuous reaction crystallization of this salt crystalline magnesium and ammonium chlorides were applied. Process was carried out in three experimental plants. Original crystallizers with internal circulation of suspension driven by liquid jet pump, of working volumes Vw 1.2, 15 and 36 dm3, were applied. Feeding nozzle of a jet pump was set in a bottom of each apparatus, providing thus ascending suspension movement in their mixing chambers. Influence of process control parameters (pH, x) on the quality of product manufactured in these crystallizers was identified experimentally.

Struvite crystals of mean size Lm from ca. 9 to ca. 33 |am were removed from the crystallizers. It was demonstrated, that increase in pH (from 9 to 11) of struvite reaction crystallization process environment produced decrease of crystal mean size by more than 50% (Lm 20.2 ^ 9.2 |am, x 900 s, Vw 1.2 dm3). Contrary, elongation of mean residence time of suspension in a crystallizer from 900 to 7200 s produced considerable increase in this size by ca. 18% (Lm 29.6 |am at pH 9, x 7200 s, Vw 36 dm3). Products of small size-homogeneity (CV ca. 90%) were removed from the crystallizers. It is a net effect of complex action of pH and mean residence time of suspension, as well as crystal attrition and breakage on the supersaturation level self-establishing in mother solution.

Excess of magnesium ions in a process system influenced the struvite continuous reaction crystallization process yield definitely advantageously. Concentration of phosphate(V) ions decreased from 0.445 mass % in a feed to 1810-4 mass % (18 mg/kg) in a postprocessed mother solution, what can be regarded as a very good result of the process of their removal. It was, however, accompanied by growth of mean size of crystal product (Lm up to 32.6 |am).

In a product, except main component - struvite, also all impurities originally present in wastewater appeared in the form of hydroxides, phosphates and other salts. Aluminium, copper, iron and zinc ions were a subject of practically total co-precipitation with struvite. Practical utilization of such product in

agriculture is limited, however part of these impurities can be regarded as the advantageous, soil enrichment components.

Acknowledgements

The work was supported by the Ministry of Science and Higher Education of Poland within a frame of statutory activity grant realized in Faculty of Chemistry, Wroclaw University of Technology.

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