Neutralization of Acidic Wastewater by the Use of Waste Limestone

Nov 14, 2008 - Waste limestone from marble cutting operations was adopted as a neutralizing agent of acidic wastewater from the glass industry. Hydrog...
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Ind. Eng. Chem. Res. 2009, 48, 399–405

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APPLIED CHEMISTRY Neutralization of Acidic Wastewater by the Use of Waste Limestone from the Marble Industry. Mechanistic Aspects and Mass Transfer Phenomena of the Acid-Base Reaction at the Liquid-Solid Interface Domenico Petruzzelli,* Mario Petrella, Giancarlo Boghetich, Piero Calabrese, Valentina Petruzzelli, and Andrea Petrella Department of CiVil and EnVironmental Engineering, The Polytechnic UniVersity of Bari 4, Via Orabona, 70125 Bari, Italy

Waste limestone from marble cutting operations was adopted as a neutralizing agent of acidic wastewater from the glass industry. Hydrogen fluoride containing wastewater from the glass matting operations was investigated with the final aim of studying the influence of mass-transfer phenomena at the liquid-solid interface of the heterogeneous neutralization reaction as the rate determining step of the overall process. In this context, chemicals interdiffusion at the stationary liquid film (Nernst film) around the particles and/or in the particles themselves may play a relevant role as kinetic rate determining step. Specifically, the influence of the stirring speed, temperature of the liquid-solid mixture, and grain size of the limestone particles during batch neutralization operations were analyzed in order to carry-out a mechanistic study of the acid-base reaction at the liquid-solid interface. Apparently, limestone is not the best material to be used in the neutralization of HF containing wastewater because of molecular interdiffusion hindrances of the involved species. The present work is essentially aimed to the interpretation of mechanistic aspects of the neutralization reaction in the reference heterogeneous solid-liquid system. Introduction The marble industry performs a number of operations including cutting, smearing, and smoothing of the solid surfaces.1,2 Reference operations lead to the production of solid waste in the form of small pieces, chips, powder, and wet cutting sludge, thus exceeding 1 Mt at national level.2 Italy is the world leader in the area with a net production exceeding 10 Mt/year (over 50% exported), and the Apulia Region (S.E. Italy) alone covers more than 20% the mentioned production. Waste limestone (chips and sludge) from cutting activities are generally disposed of in controlled landfills as nonhazardous (inert) solid waste according to the current EU legislation.3,4 Marble waste production at regional level exceeds 200.000 t/y, according to a recent survey covering only 70% of the net national production.5 In compliance with the mentioned EU regulations, the present investigation is aimed to find potential recovery and reuse of reference calcareous solid wastes, for instance, as neutralizing agents for the hydrogen fluoride containing acidic wastewater resulting from the glass matting operations, thus reaching simultaneous management and control of both (solid and liquid) wastes. Neutralization of the acidic industrial wastewater by the use of waste limestone is more economically attractive with respect to conventional technologies based on the use of commercial chemicals such as lime [Ca(OH)2], sodium hydroxide [NaOH], sodium carbonate [Na2CO3], or eventually ammonia [NH3].6-12 It goes without saying that, as a waste material, the only cost associated with the use of waste limestone is transport and handling operations. On the basis of previous investigations, * To whom correspondence should be addressed. E-mail: [email protected]. Tel.: (+39) 5963777. Fax: (+39) 5963635.

neutralization of acidic wastewater by the use of waste limestone leads to formation of lower amounts of sludge with respect to lime,13 when considering the highly pure (>99% CaCO3) and reactive microcrystalline structure of the Apulian limestone, as compared to the use of less reactive derivatives, e.g. dolomite, adopted by other authors.13,14 Main limitation for the extensive and effective application of reference waste materials in environmental applications, such as the case at hand, is associated to the heterogeneity of the reaction system, where the influence of mass-transfer phenomena at the liquid-solid interface may play a relevant role, over the neutralization reaction itself, as the rate determining step of the overall process. It is known that the interdiffusion of the chemicals at the stationary liquid film (Nernst film) around the particles (film diffusion control, fdc) and/or in the particles themselves (particle diffusion control, pdc) may play a relevant role.15,16 In this respect, the influence of the stirring speed, temperature of the liquid-solid mixture, and grain size distribution of the calcareous materials during batch neutralization operations were analyzed in order to have a mechanistic insight of the overall neutralization process. Accordingly, calcium carbonate does not appear to be the best material to be used as the neutralizing agent of hydrofluoric acid containing wastewater because of the formation of CaF2 external shells acting as diffusional barriers to the free interdiffusion of the involved species in and out of the solid-liquid heterogeneous system at hand; nevertheless, the paper is intended to give a mechanistic interpretation of the neutralization reaction. Background. Any kinetic problem poses the following questions: (a) What is the mechanism of reaction? (b) What is the rate determining step? (c) What rate laws are obeyed? (d) How can the rate be predicted?

10.1021/ie8014268 CCC: $40.75  2009 American Chemical Society Published on Web 11/14/2008

400 Ind. Eng. Chem. Res., Vol. 48, No. 1, 2009

Extensive literature data indicate that sorption kinetics in heterogeneous liquid-solid systems are controlled by the mass transfer phenomena, that is, the interdiffusion of chemical species in and out of the contacting solid and liquid phases, rather than the involved chemical reaction itself.15,16 In the following background we will consider spherical particles of uniform size (chemical AY), that are contacted with a well-stirred solution of an electrolyte BX. B is the entering chemical species, X the accompanying coion, whereas A undertakes diffusion out of the solid-phase to the bulk solution. Incidentally, A and B may react into the solid phase thus forming a highly insoluble compound AB. Compound AB will reprecipitate over the pre-existing solid-phase AY: AY(s) + BX(l) T AB(s) + XY(l) The overall kinetic rate determining steps will be associated with (a) resistance to interdiffusion of the involved chemical species into the solid phase (particle diffusion control, pdc); (b) resistance to interdiffusion of the involved chemical species into the stationary liquid-film around the particle (film diffusion control, fdc); (c) chemical reaction itself between the involved chemical species (chemical control, chem). Particle Diffusion Control. The following general kinetic equation, relating the concentration of species along the spatial coordinate, r, and time, t, is obtained after integration of the second Fick’s law and mass balance equation under infinite solution volume boundary conditions (i.e., massive amounts of AY with respect to the entering chemical BX; CAYVAY . CBXVBX, where CAY and CBX are respectively the solid-phase and liquid-phase concentration of the species AY and BX) and by assuming as the spatial coordinate the particle radius ro15 ∞

(

Dtπ2n2 U(t) ) 1 - 6/π 1 ⁄ n exp ro2 n)1 2

∑

2

)

(1)

(2)

which is directly proportional to the square of the particle radius and inversely proportional to the solid phase diffusion coefficients. Film Diffusion Control. The following general kinetic equation, relating the concentration of species along the thickness (δ) of the Nernst liquid-film around the particle and time, t, is obtained after integration of second Fick’s law in the film and mass balance equation under infinite solution volume boundary conditions (i.e., massive amounts of AY with respect to the entering species BX; CAYVAY . CBXVBX), and by assuming as the spatial coordinate the same film thickness around the particle:15 U(t) ) 1 - exp(-3D ′ CSt ⁄ (roδC ′ ))

parameter

particle diffusion control

film diffusion control

chemical reaction

particle size (ro) concentration (C) stirring speed (rpm) temperature (T) interruption test

1/ro2 independent independent +6% °C sensitive

1/ro ∝C sensitive +4% °C not sensitive

1/ro ∝C independent function Ea not sensitive

t0.5 ) 0.23roδ C ′ ⁄ (D ′ C)

(3)

where D′ is the diffusion coefficient of interdiffusing chemicals in and out of the liquid film, S represents the exposed surface area at the liquid-solid interface, C′ is the concentration of the entering species at the liquid-film interface, C is the bulk liquidphase concentration of the entering species, and δ is the film thickness. The half-time of reaction (t0.5) is easily calculated from eq 3 by imposing U(t) ) 0.5:

(4)

which is directly proportional to the first power of the particle radius, inversely proportional to the liquid phase diffusion coefficient, to the liquid (bulk) and solid interface concentrations and to the thickness of the liquid film around the particle. Chemical Reaction Control. The chemical reaction is slow and mass transfer in the liquid-film and particle is fast. A shrinking core-shell progressive diffusion mechanism is assumed for the case at hand, with the chemical reaction occurring only at the boundary between the reacted and still unreacted internal core. The entering chemical species BX, initially present in the bulk solution, is not allowed to permeate beyond the external portion of the particle already converted in the AB form, with this latter portion of the particle constituting a spherical outer shell over an inner core still in AY form. As mentioned, the chemical reaction remains confined at the periphery of the inner core. Combination of the flux and mass balance equations leads to the following integrated expression for the kinetic equation:17,18 U(t) ) 1 - (1 - kCt ⁄ ro)3

(5)

where k ) chemical reaction rate coefficient. Half-reaction time is t0.5 ) 0.21ro ⁄ (kC)

where D ) diffusion coefficient of interdiffusing chemicals in and out of the solid-phase. U(t) is the fractional attainment of equilibrium, that is, fraction of conversion of reactants toward products: C(t)/C(t)∞). The half-time of reaction (t0.5) is easily calculated from eq. 1 by imposing U(t) ) 0.5: t0.5 ) 0.03ro2 ⁄ D

Table 1. Dependence of the Process Kinetics on Experimental Conditions

(6)

Kinetics in this model appear to be strictly dependent on the first power of the particle radius, ro, (as in the case of film diffusion) because the chemical reaction is restricted to the interface boundary between the reacted and unreacted core. Table 1 summarizes the influence of the main experimental conditions on the process kinetics as expected from the outlined theory. Materials and Methods Batch experiments were carried-out by the use of a thermostatted plexiglas reactor (ID ) 12.5 cm; H ) 17.5 cm) equipped with a paddle stirrer from Eurostar Ika-Werke, Germany, operating in the range 50-2000 rpm (Figure 1). A 3 g lump portion of different particle sized dry waste limestone was added to 250 cm3 synthetic 0.004 M HF (38% from J.T. Baker) solution at pH 3.5, reproducing the average composition of the acidic wastewater from glass matting operations. Solution pH was monitored until neutralization was reached. Kinetic experiments were performed at different stirring speeds (150-1200 rpm), temperatures (20-40 °C), and grain size distribution (0.01-0.001 mm). Chemical composition of the waste limestone sludge from marble cutting operations is reported in Table 2; Figure 2 shows the typical grain size distribution of the limestone sludge from marble cutting operations. After Stokes sedimentation of 75 g limestone sludge sample the following fractions were separated: (first fraction) average particle size in the range 0.01-0.074 mm (obtained by sieving);

Ind. Eng. Chem. Res., Vol. 48, No. 1, 2009 401 Table 3. Summary of Kinetic Experiments Carried-out under the Indicated Operative Conditions

test no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Figure 1. Experimental apparatus adopted for kinetic experiments. Table 2. Chemical Composition of the Waste Limestonea

a

parameter

(%)

thermal weight loss chloride sulfate iron calcium magnesium sulfide P total ammonia nitrite Pb Ni Hg As Cd Cr(VI) Cr(III)

43.220 0.023 0.054 0.031 54.700 0.600 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. traces

stirring speed (rpm)

temperature (°C)

grain size distribution (mm)

150 250 500 750 1000 1200 250 250 750 750 1200 1200 1200 1200 1200 1200 750 (interruption test)

20 20 20 20 20 20 30 40 30 40 30 40 20 20 20 20 20

0.1–0.0074 0.1–0.0074 0.1–0.0074 0.1–0.0074 0.1–0.0074 0.1–0.0074 0.1–0.0074 0.1–0.0074 0.1–0.0074 0.1–0.0074 0.1–0.0074 0.1–0.0074 0.1–0.074 0.074–0.052 0.052–0.013 0.013–0.0074 0.1–0.0074

weight t0,5 (g) (min) 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3

4.7 4.1 1.6 1.05 0.6 0.4 1.3 1.25 0.4 0.4 0.25 0.25 1.8 1.6 1.1 0.8 1.0

after 15 min sedimentation); (fourth fraction) average particle size in the range 0.013-0.0074 mm (obtained after 45 min sedimentation). “Interruption tests” were also performed to confirm the influence of particle diffusion control as the rate-determining step of the overall kinetic process. For this purpose, the kinetic experiments carried-out in conditions favorable for particle diffusion control were interrupted at a pre-established reaction time by removal of the limestone particles from the solution for a period of time (e.g., 10-15 min) and then reimmersed. The pause allows for the concentration gradients in the particles to level-out, and, accordingly, with particle diffusion control the rate after reimmersion is faster than the rate prior to the interruption. With film diffusion control, no concentration gradients in the particle exist, the rate depending solely on the concentration gradients across the stationary liquid-film around the particle, and as a consequence, these liquid-film gradients immediately re-establish after reimmersion of the solid phase into the batch, therefore the interruption tests do not affect the rate. Table 3 summarizes the experiments carried-out under different operative conditions. Half-conversion times (t0.5) for the single tests are also summarized in the table.

n.d. ) not detected.

Results and Discussion

Figure 2. Grain size distribution of a typical waste limestone sludge sample.

(second fraction) average particle size in the range 0.074-0.052 mm (obtained after 1 min sedimentation); (third fraction) average particle size in the range 0.052-0.013 mm (obtained

Neutralization kinetics of the hydrofluoric acid containing liquid effluents from glass matting operations by the use of waste limestone may include the following mass-transfer phenomena with the associated chemical reactions (Figure 3): (1) Diffusion of hydrofluoric acid (H+F-) from the bulk solution through the Nernst stationary liquid film around the limestone particles. (2) Chemical reaction of the hydrofluoric acid with calcium carbonate with simultaneous reprecipitation of an external CaF2 layer over a still unreacted calcium carbonate core. Reaction will carry-on at the CaCO3/CaF2 boundary. (3) Diffusion of hydrofluoric acid into the external CaF2 porous shell. (4) Backdiffusion of carbon dioxide through the newly formed CaF2 external porous shell. (5) Backdiffusion of carbon dioxide through the stationary liquid-film around the particles.

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Figure 3. Schematic view of the neutralization reaction mechanism of hydrofluoric acid with limestone particles.

Figure 4. Kinetics of limestone neutralization at the indicated stirring speed of the batch solution (T ) 20 °C; grain size of the limestone particles, 0.1-0.0074 mm).

It goes without saying that the slower step(s) will be the rate determining of the overall process. Film Diffusion Control. Figure 4 shows kinetic curves for tests no.1-6 (Table 3) carried-out at different stirring speeds in the range of 150-1200 rpm, constant temperature (20 °C), and grain size distribution of the limestone particles from 0.1 to 0.0074 mm. In the figure are also indicated the halfconversion times, t0,5, that is, the time needed to reach pH 5.25 an intermediate value between the initial (pH 3.,5) and final neutralization (pH 7.0). As expected, higher stirring speeds of the batch solution lead to faster neutralization kinetics. If it is assumed that diffusion of hydrogen fluoride in the stationary liquid film around the particles would control the overall kinetics, an increase of the stirring speed would lead to a sensible reduction of the liquidfilm thickness, δ, around the limestone particles and, as a consequence, to higher fluxes of the involved species through the Nernst film and to faster kinetics. Once the minimum value for the film thickness at a given stirring speed and temperature is reached, the chemicals flux through the liquid film reaching the maximum value, and the half-exchange time converges to its minimum value (eq 4) with corresponding faster kinetics of neutralization. This trend is clearly shown in Figure 5 where t0,5 is correlated to the stirring speed (rpm) of the batch solution.

Figure 5. Correlation of the half-exchange times of neutralization, t0.5, with stirring speed (rpm) of the batch solution (T ) 20 °C; grain size of limestone particles, 0.1-0.0074 mm).

Figure 6. Kinetics of neutralization at the indicated grain size distribution of the limestone particles (T ) 20 °C; stirring speed, 1200 rpm).

To better substantiate film diffusion control as the rate determining step of the kinetic process, experiments at different grain size of the limestone particles, ro, were carried out (tests 13-16 in Table 3) at constant temperature (20 °C) and stirring speed (1200 rpm) (Figure 6). Figure 7 shows linear correlations of t0.5 versus particle radius (ro), thus confirming theoretical expectations based on credited film diffusion control models (see eq 4).

Ind. Eng. Chem. Res., Vol. 48, No. 1, 2009 403

conditions with the backdiffusing calcium ions thus inducing calcium fluoride precipitation due to the very low solubility product of this latter salt (KpsCaF2 ) 4 × 10-11). Ca2+ + 2F- T CaF2(S)

Figure 7. Correlations of t0.5 vs ro according to eq 10 (T ) 20 °C; stirring speed, 1200 rpm).

On these premises it is likely that an external porous shell of solid CaF2 is formed over a still unreacted limestone core (Figure 3). Accordingly, the neutralization reaction occurring at reacted-unreacted boundary may be conditioned by the HF (H+ and F-) independent migration resistance through the newly formed CaF2 external porous shell. Porosity of this latter external shell is associated with simultaneous gas evolution (i.e., carbon dioxide) while solid CaF2 is under formation. Above reactions summarize the overall kinetic process. Figure 6 shows kinetic curves for experiments carried-out at different grain size distribution (tests no.13-16 in Tab. 3) by

It goes without saying that the increase of the solid-to -liquid exposed surface area associated with the smaller particle size, and the thinner liquid-film around the particles, associated with the higher stirring speed of the batch solution, synergistically lead to higher HF fluxes through the liquid-film with the related faster neutralization kinetics. Chemical Control. Figure 8 shows kinetics of limestone neutralization for tests no. 2, 4, and 6-12 (Table 3) carried-out at different temperatures in the range from 20 to 40 °C, stirring speeds of 250, 750, and 1200 rpm, and constant grain size distribution of the limestone particles (0.1-0.0074 mm). Applying the Arrhenius Equation to the the earlier data gives k ) A exp(-Ea ⁄ RT)

(7)

-1

where k ) kinetic constant (s ), A ) frequency impact factor, Ea ) activation energy (J/mol), R ) gas constant (J/(mol K)), and T ) temperature (K). The activation energy, Ea for the neutralization reactions carried out at different stirring speeds was calculated. The representative kinetic constants were assumed to be the half-reaction times, and these were correlated to the temperature. Figure 9 shows linear correlations (t0.5 vs 1/T) for tests carried-out at the indicated stirring speed. The activation energies were calculated from the corresponding slopes of the representative Arrhenius curve and are given in Table 4. On the basis of the reported low figures for the activation energies, the chemical reaction of neutralization may be considered sufficiently fast to control the overall kinetic process. In this context it has to be considered that a typical figure for the activation energies for chemical reactions range in the hundredth of kJ/mol. On these premises, mass tranfer phemonema, that is, the diffusional resistance to chemicals migration through the liquid-film around the particle and/or the diffusional resistance to chemicals migration through the solid phase itself may play a more relevant role as rate determining step of the overall kinetic process. Particle Diffusion Control. Once hydrogen fluoride (both hydrogen and fluoride ions in equilibrium) has completed its migration through the liquid-film, thus reaching the liquid-film/ limestone particle interface, the neutralization reaction occurs with release of backdiffusing calcium ions and carbon dioxide (Figure 3).19 + 2+ CaCO3(s) + H(1) f Ca(I) + HCO3(I) + HCO3(I) + H(I) f H2O + CO2(g)

The incoming fluoride co-ions may soon reach saturation

Figure 8. Kinetics of limestone neutralization for experiments carried-out at different temperatures (20, 30, 40 °C), stirring speed of the batch solution (250, 750, 1200 rpm), and constant grain size of limestone particles: 0.1-0.0074 mm.

404 Ind. Eng. Chem. Res., Vol. 48, No. 1, 2009

Figure 9. Arrhenius correlations for Ea determination (9, 250 rpm; 2,750 rpm; b, 1200 rpm); grain size distribution of the limestone particles, 0.1-0.0074 mm).

Figure 11. Positive interruption test for kinetics carried-out in the most favorable conditions for particle diffusion control (T ) 20 °C; 750 rpm; particle grain size, 0.1-0.0074 mm).

Table 4. Activation Energies Calculated for the Neutralization Reaction on the Basis of the Arrhenius Equation

reaction playing a minor role. Possibly in the early stages of the kinetic process film diffusion control may play a major role thus evolving to particle diffusion at a later stage in proximity of equilibrium.20

stirring speed (rpm)

activation energy (kJ/mol)

250 750 1200

18 35 47

Conclusions From the systematic kinetic investigation carried-out under different operative conditions it was confirmed that mass transfer phenomena may control neutralization rates of hydrofluoric acid containing effluents from the glass matting operations by the use of waste limestone from the marble industry, with the neutralization reaction between calcium carbonate and hydrofluoric acid playing a minor role. Correlations based on a credited theoretical model presented confirm that the rate determining step of the overall process is associated with simultaneous diffusional resistance to migration of ions in and out of the stationary liquid-film around the particle and/or in the particle themselves. A specific model for simultaneous film + particle diffusion control should be setup for the system at hand, and this is the matter of new investigations. Figure 10. Plot of t0.5 vs ro2 correlation (T ) 25 °C; 1200 rpm, limestone particle size, 0.1-0.0074 mm).

Literature Cited

adopting sufficiently fast stirring speed in order to minimize the liquid-film thickness, δ, and related liquid-film contribution to determination of the overall kinetics. Figure 10 shows the correlation of t0,5 vs ro2 (the square of the particle radius) thus confirming potential particle diffusion control in true agreement with theoretical expectations based on eq 2. To better substantiate particle diffusion control contribution to the overall kinetics the “interruption test” was carried out under favorable experimental conditions for pdc control (test no. 17 in Table 3). For this purpose, while the kinetic experiment was underway (i.e., 50% conversion) the neutralizing agent (CaCO3 sludge) was separated from the liquid-phase for 15 min and then reimmersed into the batch. Figure 11 shows the positive result obtained as confirmed by the faster kinetics monitored (steeper slope of the kinetic curve) after reimmersion of the neutralizing agent, as a consequence of the relaxation of the solid-phase concentration profiles. Neutralization kinetics of hydrofluoric acid containing wastewater by the use of limestone apparently seem to be controlled by both resistance to chemicals diffusion through the liquid film around the particles and particles themselves, with chemical

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ReceiVed for reView April 18, 2008 Accepted October 13, 2008 IE8014268