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Ind. Eng. Chem. Res. 2006, 45, 796-802
Fluoride Recovery in a Fluidized Bed: Crystallization of Calcium Fluoride on Silica Sand Rube´ n Aldaco,* Aurora Garea, and Angel Irabien Departamento de Ingenierı´a Quı´mica y Quı´mica Inorga´ nica, UniVersidad de Cantabria, AVda. Los Castros, s/n ESTIIYT, 39005 Santander, Spain
The removal of fluoride in a fluidized bed reactor by crystallization on the surface of sand grains has been studied as an alternative to chemical precipitation, which generates huge amounts of a water rich sludge that is not possible to recover due to the high water content and the low quality of the sludge. Water recycling is required in order to decrease the fluoride concentration and prevent primary nucleation. However, water recycling may increase the secondary nucleation and, therefore, the efficiency of the process is lower due to the formation of fines, which cannot be recovered. The environmental benefits of the reuse of raw materials and the reduction of wastewater make it necessary to improve the efficiency of the process, reducing the formation of fines. The influence of supersaturation has been experimentally studied, leading to a quantitative model. The combination of a fluidized bed reactor and a sand filter bed in the process has been able to achieve a reduction in the formation of fines for technical applications. Under these conditions, process efficiencies close to 80% and pellets with a calcium fluoride content higher than 97%, which are able to be reused as raw material in several applications, have been obtained. Introduction Fluoride containing wastewater is an industrial effluent requiring neutralization due to acidity and fluoride concentration control. Fluoride is a regulated pollutant, and therefore, some treatment is necessary to reduce its concentration below the required limits. Several methods to remove fluoride from industrial wastewater have been described and applied. Precipitation is the most common treatment.1-4 However, the fluoride precipitation process generates large amounts of a water rich sludge, which must be disposed of with increasing costs. The high water content (50-60%) and the low quality of the sludge (40-60% of CaF2) prevent, technically and economically, the application of the fluoride precipitation process. Many alternatives for fluoride removal from industrial wastewater have been suggested.5 An alternative process, which utilizes the reaction of fluoride with granular calcite particles, was developed in the 1970s. The bed packed with granular calcite allows high efficiencies of fluoride removal without sludge generation.6-9 However, the effluent needs to be treated further by using a conventional process to satisfy wastewater discharge regulations. In addition, low effective conversion rates of calcite and the formation of lumps in the fixed bed have been reported. Flotation10 of the precipitates and other processes involving recycling to reduce the sludge and running costs11 have been also reported, but they are not able to recover the product. Fluidized bed crystallization (FBC) has been used in different water and wastewater treatment plants. The use of a pellet reactor, which is a reactive fluidized bed growth-type crystallizer, has been reported for water softening of drinking water,12,13 phosphate and fluoride removal,14,15 and heavy metal recovery from wastewaters.16-19 When it is compared with the precipitation process, the major advantages of this technology are the elimination of sludge formation, the recovery of materials, and the reduction of solid wastes. * To whom correspondence should be addressed. Tel.: +34942201586. Fax: +34-942201591. E-mail:
[email protected].
The chemistry of the process is similar to that of conventional precipitation. By dosing calcium hydroxide to the wastewater, the solubility of CaF2 is exceeded and fluoride is converted from the aqueous solution to solid crystals, according to the following reactions:
H+ + F- T HF
(1)
Ca2+ + 2OH- T Ca(OH)2
(2)
Ca2+ + 2F- T CaF2(s)
(3)
The process is based on the crystallization of calcium fluoride upon silica sand grains instead of mass precipitation in the liquid phase. During the operation, the grains increase in diameter in the fluidized bed reactor and fluoride covered grains are removed from the bottom of the reactor and replaced by fresh seed grains. Chen et al.17 describe a mechanism to explain the crystallization of lead hydroxide from synthetic wastewater upon sand grains. The mechanism may be applied to the calcium fluoride system in the same way. Calcium fluoride crystallization upon silica sand is referred to as the electrical attraction between the sand surfaces, with negative charges when the system is operated at basic pH, and the calcium ions, Ca2+. The calcium ions on the sand surface react with the fluoride ions in the bulk solution, leading to calcium fluoride precipitation on the surface of the sand. The mechanism is shown in Figure 1. The growth of sand-calcium fluoride takes place by molecular growth and aggregation between the silica grains and the formed calcium fluoride in the liquid phase (nucleated precipitation).14 The molecular growth and aggregation on the sand grains takes place while competing with discrete precipitation in the liquid phase (primary and secondary nucleation) and mineral layer abrasion. Nucleation in the liquid phase and abrasion of the grains in the fluidized bed lead to small particles (referred to as fines), which leave the reactor from the top and form, together with the remaining fluoride in solution, the fraction of the fluoride that is not possible to recover in the reactor. Abrasion is expected
10.1021/ie050950z CCC: $33.50 © 2006 American Chemical Society Published on Web 11/19/2005
Ind. Eng. Chem. Res., Vol. 45, No. 2, 2006 797
Figure 1. Precipitation mechanism upon silica sand in a fluidized bed reactor.
to be significant near the inlet nozzles due to the high-energy dissipation. The formation of fines by abrasion in the reactor is difficult to avoid; a proper selection of the hydrodynamical conditions may reduce abrasion. However, the formation of fines by primary nucleation may be minimized by the adequate selection of the supersaturation conditions at the inlet of the fluidized bed reactor. Low supersaturation implies nucleated precipitation. On the contrary, when the system is operated at higher supersaturation, primary nucleation occurs,20 leading to the formation of many nuclei (discrete precipitation), which are not possible to retain in the reactor, not even by aggregation with silica sand. Under these conditions, the efficiency of the process is lower, increasing the turbidity of the effluent. Appropriate supersaturation conditions at the inlet of the reactor are not possible when the fluoride wastewater presents a high fluoride concentration. In this case, it is necessary to dilute the fluoride wastewater to prevent primary nucleation. The objective of this work is the study of the FBC behavior when recycling is required to obtain low supersaturation and to minimize the generation of fines, achieving high efficiencies of fluoride recovery. Materials and Methods Experimental Setup. The experimental facility is shown in Figure 2, where the following pieces of equipment are presented: (i) a fluidized bed reactor (FBR) with recycling [the first recycle is to dilute the fluoride wastewater (1), and the second recycle is to feed calcium hydroxide (2)] and (ii) a fluidized bed reactor (FBR) with recycling and filter sand, which has been used in order to improve the efficiency of the process through the filter sand at the outlet of the fluidized bed reactor. The reactor consists of a methyl methacrylate cylindrical vessel with a height of 500 mm and a diameter of 36 mm partially filled with silica sand (0.15-0.30 mm) as the seed material into which the fluoride water and the calcium reagent solution are pumped upward through the reactor at a velocity that ensures the fluidization of the pellets so that the cementing of grains is prevented. The flow is such that the seed material will not settle down and flow out of the crystallization vessel. The superficial velocity, SV, and the particle Reynolds number, Rep, are 30 m‚h-1 and 1.9, respectively. Figure 3 shows the geometrical configuration of the bottom of the reactor in order to clarify the micromixing pattern of the process. The fluidized bed reactor is provided with two inlet nozzles. The main nozzle (6 mm in diameter) is located vertically at the symmetry axis of the reactor. The fluid velocity of the fluoride solution through this nozzle is in the range between 0.15 and 0.18 m‚s-1. The calcium solution is pumped into the reactor through a secondary nozzle (6 mm in diameter) placed horizontally at the wall at a height of 15 mm from the
Figure 2. Experimental setup: (i) fluidized bed reactor with recycling to dilute fluoride (1) and to feed calcium hydroxide (2); (ii) fluidized bed reactor with recycling and a sand filter bed.
Figure 3. Geometry of the bottom of the fluidized bed reactor.
bottom of the reactor. The fluid velocity of the calcium solution is 0.12-0.15 m‚s-1. In experimental facility (ii), a sand filter bed was used to filter the fines formed in the fluidized bed reactor before fluoride dilution and calcium feeding. The filter sand bed consists of a cylindrical vessel filled with sand (0.5-1.0 mm).
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Table 1. Experimental Conditions To Determine the Influence of Supersaturation (SM.1-4), Recycling (1R and 2R), and the Sand Filter (SF)a exp no. SM.1 SM.2 SM.3 SM.4 1R 2R SF a
CF,in,system (mg‚L-1)
FF,in,system (mL‚min-1)
CF,in,reactor (mg‚L-1)
FCa,in,reactor (mL‚min-1)
FF,in,reactor (mL‚min-1)
CF,TOT,reactor (mg‚L-1)
34 34 34
200 300 400 600 240 240 240
250 250 250 250 200 200 200
250 250 250 250 300 300 300
100 150 200 300 144 144 144
2000 2000 2000
FF,recir (mL‚min-1)
FCa,recir (mL‚min-1)
sand filter
200 200
no no no no no no yes
266 266 266
Superficial velocity (SV) ) 30 m‚h-1; Ca/F ) 1.1; Rep ) 1.9.
The calcium reagent and fluoride solutions were injected into the fluidized bed reactor using different peristaltic pumps (Watson Marlow 323, 313S, and 101U/R). The experimental facility is completed with several regulation valves and flow meters. Materials. The fluoride solution used as the feed stream was obtained by diluting a concentrated hydrofluoric acid solution. Synthetic fluoride solutions in the range 200-2000 mg‚L-1 were used in the experiments. Chemical grade reactants (hydrated lime as the calcium reagent (Ca2+)) and demineralized water were used. Several studies show the hydrated lime as an adequate source of calcium for the neutralization of fluoride containing wastewater.1,2 In addition, the Ca(OH)2 particles are likely to dissolve along the bed length, distributing the supersaturation more evenly throughout the bed.14 Synthetic fluoride wastewater with the above-mentioned concentration was introduced in the experimental setup shown in Figure 2 at room temperature following the experimental plan shown in Table 1. Analytical Techniques. Samples were taken from the effluent of the reactor at different time intervals. Turbidity and pH measurements were conducted with a turbidity meter (Turbiquant 3000IR, Merck) and a pH meter (Crison GLP 22), respectively. The efficiency of fluoride removal can be calculated by measuring the concentration of fines and dissolved fluoride in the outlet stream. The concentrations of the inlet and outlet streams were measured. Dissolved fluoride was analyzed by a specific fluoride ion electrode (Crison 120/S7) provided with a reference electrode (Crison 5240). Suspended solids in the outlet stream were filtered with a 0.45 µm filter, and a mass balance was applied to the solid in order to estimate the amount of fluoride released as fines. The dimensionless conversion (X), fines fraction (XF) and efficiency (XR) are calculated as conversions in terms of molar rates, w (mol‚min-1):
X)
wF,dis,in - wF,dis,out wF,dis,in
(4)
XF )
wF,fines wF,dis,in
(5)
XR )
wF,grains wF,dis,in
(6)
Analysis of Fluoride-Coated Sand. The fluoride-coated sand grains were removed from the bottom of the reactor at different intervals. The pellets were air-dried and analyzed to determine the morphology using a scanning electron microscope (SEMJSM 5800LV, JEOL).
The composition of the calcium fluoride sand grains was analyzed after distillation by a potentiometric method using a fluoride-ion sensitive electrode (Crison 120/S7) according to the standard method (ISO 5439:1978) to determine the available fluorine content in acid grade fluorspar. The white crystals obtained in the process were analyzed with an X-ray diffraction analyzer (XRD-PW 1729, Philips). Results and Discussion Influence of Supersaturation: Primary Nucleation and the Formation of Fines. The influence of supersaturation on the the formation of fines in fluidized bed crystallization has been reported by several authors.14,17,22 To describe the formation of fines, it is very important to establish the relationship between supersaturation and the rate of fines’ formation, allowing one to control and to increase the efficiency of the process. Levels of supersaturation in aqueous solutions of sparingly soluble electrolytes are expressed in terms of the solubility product as
S)
() IP Ka
1/ν
(7)
where IP is the ion activity product of the lattice ions in solution, Ka is the activity solubility product of the salt, and ν is the number of ions in a formula unit of the salt.21 Supersaturation can be defined for a calcium fluoride system at room temperature as
S)
(
[Ca2+][F-]2
3.4 × 10-11
)
1/3
(8)
At a given pH value and under an excess of calcium, the supersaturation defined by means of eq 8 depends only on the fluoride concentration and, therefore, the influence of supersaturation can be studied in terms of the inlet fluoride concentration in the reactor. Table 2 shows the effect of the inlet fluoride concentration on the fluoride conversion, the fines’ generation, and the process efficiency, according to the experiments shown in Table 1 (experiment nos. SM.1-4), without recycling. The fluoride conversion increases from 87% to 95% when the fluoride inlet concentration increases. The conversion difference is related to the fluoride inlet concentration and the calcium fluoride solubility when the Ca/F ratio is in excess, according to eq 4. In addition, it is shown that a high fluoride inlet concentration in the reactor increases the generation of fines in comparison with fluoride precipitation upon the sand surface. A fluoride concentration of 100 mg‚L-1 leads to 8% of fines, while a concentration of 300 mg‚L-1 leads to 51% of fines.
Ind. Eng. Chem. Res., Vol. 45, No. 2, 2006 799 Table 2. Results of the Supersaturation Influence Experiments process parameter
effluent parameter
exp no.
CF,in,reactor (mg‚L-1)
S (-)
X
XF
rF (mg‚L-1‚min-1)
XR
CF,out,reactor (mg‚L-1)
pH
SS (mg‚L-1)
turbidity (NTU)
SM.1 SM.2 SM.3 SM.4
100 150 200 300
12.8 19.2 25.7 38.5
0.87 0.91 0.93 0.95
0.08 0.19 0.33 0.51
2.52 10.34 21.52 51.19
0.79 0.72 0.60 0.44
13.2 14.3 14.6 13.9
8.1 7.9 7.4 7.7
15.6 64.2 133 315
9.2 17.1 51.3 112.7
The theory of primary homogeneous nucleation assumes that in supersaturated solutions solute atoms or molecules combine in a series of bimolecular reactions to produce ordered aggregates or “embryos”. The overall free energy of these embryos goes through a maximum at some critical size, which can be shown to be inversely proportional to the logarithm of the supersaturation. Embryos larger than the critical size are “stable nuclei” and grow to form macroscopic crystals. The kinetic nucleation equation of new nuclei formation is of the following form:20
[
B1 ) C1 exp -
3 2
]
16π6 υ 3κ3T3(ln S)2
(9)
where B1 is the number of formed nuclei per unit time per unit volume (nuclei‚m-3‚min-1), γ is the interfacial tension (J‚m-2), υ is the molecular volume, κ is the Boltzmann constant (1.3805 × 10-23 J‚K-1), T is the temperature (K), S is the supersaturation, and C1 is a constant. The nucleation rate can be related to the fines’ formation per unit reactor volume neglecting heterogeneous nucleation, secondary nucleation, and abrasion at the bottom of the reactor according to the following equation:
[
]
mF KSγs3 ∝ KF exp - 3 rF ) VR T (ln S)2
[
CS
]
(ln S)2
[
rF ) 786.03 exp -
37.65 (ln S)2
]
(12)
The correlation coefficient of the fitting was 0.995. The wellcorrelated data demonstrates that primary nucleation seems to be responsible for the formation of fines in the fluidized bed reactor. It is, therefore, important to control the system under suitable supersaturation values. The fluoride concentration at the bottom of the reactor must be kept below a critical value in order to prevent discrete precipitation. From a technical point of view, a fluoride concentration lower than 150 mg‚L-1 needs to be introduced in the reactor in order to avoid the formation of fines.
(10)
where rF is the fines’ formation rate per unit of reactor volume (mg‚L-1‚min-1), mF is the mass flow of fluoride (mg‚min-1), VR is the reactor volume (L), and KF and KS are constants. This assumption can be taken into account considering the particle size of silica sand (0.15-0.3 mm), which is big enough to make the formation of fines by heterogeneous nucleation negligible.21 Secondary nucleation has been considered negligible taking into account the low concentration of particles of crystallized calcium fluoride in the supersaturated solution, that could favor secondary nucleation.20,21,23 Abrasion at the bottom of the reactor is considered negligible in the fines’ formationsupersaturation relationship due the constant hydraulic load in the reactor. Under these conditions, two experimental variables may influence the rate of nucleation and, therefore, the formation of fines: temperature, T, and supersaturation, S. The temperature and interfacial tension from eq 10 can be included in a constant, CS, taking into account a constant interfacial tension in the studied supersaturation range (S ) 1040)21 and a constant temperature, according to the following equation:
rF = KF exp -
low values of S but increasing extremely rapidly once some critical supersaturation is reached. The experimentally determined linear relationship between ln rF and (ln S)-2 allows one to determine the constants KF and CS, as can be seen in Figure 5. From these results, it is possible to define the equation that describes the formation of fines in the fluidized bed reactor as a function of the fluoride inlet concentration in the reactor in terms of the supersaturation:
Figure 4. Influence of supersaturation and fluoride inlet concentration on the formation of fines in the fluidized bed reactor.
(11)
Figure 4 shows the fluoride inlet concentration, supersaturation, and fines’ generation relationships. From this figure, it is possible to verify the strong influence of the supersaturation on the generation of fines. The formation of fines is, therefore, highly nonlinear in solution supersaturation, being near-zero for
Figure 5. Linear fitting of the fines’ formation rate and supersaturation according to the primary nucleation model.
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Table 3. Results of the Recycling and Sand Filter Bed Influence Experiments process parameter
effluent parameter
exp no.
X
XF
XR
CF,out,reactor (mg‚L-1)
1R 2R SF
0.98 0.99 0.97
0.51 0.63 0.17
0.47 0.36 0.80
9.2 10.1 10.3
pH
SS (mg‚L-1)
turbidity (NTU)
7.6 8.0 7.8
303 2793 61.2
141.9 597.8 19.3
Influence of Recycling: Secondary Nucleation. To get a fluoride concentration lower than 150 mg‚L-1 starting from a concentrated wastewater, a recycling ratio is applied to dilute the fluoride concentration in the feed stream of the reactor. Consequently, the effluent concentration is lowered and it is possible to control the supersaturation in the reactor according to the previous results. A second recycling is required in order to feed the calcium reagent with process water. The studied recycling has been carried out in the experimental facility drawn in Figure 2 (i) with the experimental conditions shown in Table 1 (experiment nos. 1R and 2R). In Table 3, it can be observed that the fluoride conversion (X) is the same in the studied cases for a Ca/F molar ratio 1.1. This parameter depends only on the fluoride inlet concentration and the calcium fluoride solubility at a given calcium overdose. Therefore, recycling does not show any influence on the total fluoride conversion. The fines fraction (XF) and, therefore, the efficiency (XR) of the process are different. At steady state, with one and two recyclings, the fines fraction is 51% and 63%, respectively, while without recycling this variable lies around 20%. Furthermore, at the same time that the formation of fines increases, the outlet stream of the reactor increases the turbidity, which may influence the crystallization process. From these results, it is possible to deduce that the nucleation in the supersaturated solution is much faster when crystals of the recycled calcium fluoride are present. These parent crystals have a catalyzing effect on the nucleation phenomena, and thus, nucleation occurs at a lower supersaturation than that needed for primary nucleation.20 Consequently, the recycling of fines could be the reason for the additional nucleation of new fines, contributing thereby to secondary nucleation, which results from the presence of crystals in the supersaturated solution. Secondary nucleation is a complex phenomenon, and it is not well understood. A general theory for the prediction of the nucleation rate does not exist; however, the empirical evidence pointing out the importance of secondary nucleation in crystallization is contained in the influence of three operating variables: the supersaturation, the magma density, and the fluid mechanics interactions. These effects can be described by an empirical power law relation:20
B2 ∝ SbM jTN h
(13)
where B2 is the secondary nucleation rate (nuclei‚m-3‚min-1), S is the supersaturation, MT is the magma density, N is used as a measure of the fluid mechanics phenomena (pump rotation rate), and b, j, and h are the secondary nucleation orders referred to the supersaturation, magma density, and fluid mechanics interactions, respectively. Influence of the Sand Filter: Minimization of Secondary Nucleation. A sand filter bed has been used after the reactor in order to reduce discrete precipitation (secondary nucleation) and to improve the efficiency of the process. The experimental study has been carried out in the experimental facility shown in Figure 2ii and with the experimental conditions of Table 1 (experiment no. SF).
Figure 6. Comparison of the conversion (X), fines’ generation (XF), and process efficiency (XR) of the crystallization process without recycling, with recycling, and with recycling and a sand filter bed.
Figure 7. Composition and particle size of calcium fluoride pellets from the crystallization process as a function of time.
The sand filter bed retains suspended solids, increasing the efficiency of the process, as has been suggested previously. After filtration, the fines are negligible and, therefore, secondary nucleation is reduced. The available fluoride for crystal growth in the fluidized bed reactor is greater, increasing the calcium fluoride precipitation upon the silica sand. The use of a sand filter bed allows one to obtain similar efficiencies without recycling, as is shown in the comparative results of Figure 6. Under these conditions, process efficiencies close to 80% have been obtained. Characterization of Calcium Fluoride-Coated Sand Grains. Low supersaturation and the filtration of fines increase the precipitation on the seed, favoring crystal growth, i.e., the enlargement of the size of the pellets. In this work, a characterization of the recovered pellets is performed in order to evaluate the reuse of synthetic fluorite as a raw material in technical applications. The calcium fluoride pellets coming from the fluidized bed crystallizer have been submitted to qualitative and quantitative analysis. Figure 7 shows the calcium fluoride composition and the particle size of the pellets removed from the reactor as a function of time. The composition of the pellets is strongly dependent on the size of the pellets. When calcium fluoride precipitation takes place upon the seed material, the pellet growth and the silica composition are reduced, increasing the calcium fluoride composition. Under these conditions, the maximum pellet growth will be limited by the adequate selection of the hydraulic load allowing the fluidization of the particles in the reactor and avoiding pellets being washed out.
Ind. Eng. Chem. Res., Vol. 45, No. 2, 2006 801 Table 4. Comparison of the Chemical and Physical Characteristics of the Calcium Fluoride from the Crystallization Process and Classical Precipitation parameter
crystallization
morphology
pellets (0.8-1.0 mm)
