CRYSTAL GROWTH & DESIGN
Effect of Rate of Crystallization on the Continuous Reactive Crystallization of Nanoscale 6-Line Ferrihydrite
2008 VOL. 8, NO. 4 1384–1389
Mitch Loan,*,†,4 O. G. Mike Newman,‡ John B. Farrow,† and Gordon M. Parkinson§,4 AJ Parker CooperatiVe Research Centre for Hydrometallurgy, CSIRO Minerals, P.O. Box 90, Bentley, Western Australia 6982, Australia, Zinifex Ltd., P.O. Box 175, Main Road Boolaroo, New South Wales 2284, Australia, Alcoa World Alumina, Cockburn Road, P.O. Box 161, Kwinana, Western Australia 6167, Australia, and Nanochemistry Research Institute, Curtin UniVersity of Technology, GPO Box U 1987, Perth, Western Australia 6845, Australia ReceiVed September 17, 2006; ReVised Manuscript ReceiVed NoVember 5, 2007
ABSTRACT: Altering the rate of 6-line ferrihydrite continuous reactive crystallization from the control conditions of pH 3.6 and 85 °C was investigated by varying the pH, feed concentration, and mean residence time. In these experiments, the rate of precipitation changed the relative proportion of 6-line ferrihydrite and goethite nanoparticles in precipitates and the aggregates’ physical properties of surface area, impurity concentration, filtration rate, and particle size. Although increasing the rate of precipitation had a negligible effect on bulk supersaturation, it is thought to locally increase the supersaturation at the FeIII feed inlet and precipitate 6-line ferrihydrite and for goethite precipitation to occur in the bulk. Alternatively, decreasing the rate of precipitation increased the relative proportion of goethite in precipitates, and the formation of smaller aggregates with higher impurity concentrations and surface areas. A reduction in pH also increased the proportion of goethite in precipitates and reduced the aggregates particle size distribution. The increased solubility at lower pH is believed to promote goethite formation by improved dilution of the feed solution. This work has demonstrated that the physical properties of 6-line ferrihydrite and goethite precipitates can only be altered via the way primary crystals aggregate and not through altering their crystallinity. Introduction Iron oxyhydroxide precipitation occurs in many natural and industrial situations but is still poorly understood. In an industrial context (e.g., zinc refining), precipitation by neutralization of acidic FeIII solutions generally results in the precipitation of ferrihydrite (5Fe2O3 · 9H2O). The form (crystal structure) and crystallinity (degree of order or perfection) is believed to influence process efficiency by impacting the removal of the precipitated solids from solution. The term “ferrihydrite”1 is often used to describe both 2- or 6-line ferrihydrite, which have either two or six identifiable broad reflections in a diffraction pattern.2 The ferrihydrite precipitation mechanism is dominated by rapid nucleation (inherent high supersaturation due to low solubility) and almost negligible growth. Batch precipitation at a slower rate permits the formation of 6-line ferrihydrite over the less crystalline 2-line form. However, the formation of 6-line ferrihydrite using continuous reactive crystallization is the most straightforward method,3 permitting constant supersaturation to be maintained. Loan et al.3 suggested that during continuous crystallization 6-line ferrihydrite and goethite (R-FeOOH) compete for the supply of added growth units, such that decreasing the rate of ferrihydrite nucleation could promote increased goethite growth. Conceptually, this could be achieved by varying the parameters that reduce supersaturation. In continuous crystallization supersaturation (S) cannot be chosen independently,4 as it is a
* Corresponding author. Phone: + 61 89410 3197. Fax: + 61 89410 3180. E-mail:
[email protected]. † AJ Parker Cooperative Research Centre for Hydrometallurgy. ‡ Zinifex Ltd. § Alcoa World Alumina. 4 Curtin University of Technology.
Table 1. Rate (g L-1 h-1) of Crystallization, Calculated Using Equation 4, for Each Continuous Reactive Crystallization
Experiment, and Values of ∆C, Relative (SR), and Absolute (SA) Supersaturationa experiment control3 MRT 2.1 h MRT 11.5 h feed concentration 24 g L-1 feed concentration 6 g L-1 feed concentration 3 g L-1 feed concentration 0.7 g L-1 pH 3.25 pH 2.83
rate ratio to ∆C (mg goethite/ (g L-1 h-1) control L-1) SR SA ferrihydrite 2.4 5.6 1.0 4.0
1.0 2.3 0.4 1.7
8 8 8 8
4.2 4.2 4.2 4.2
3.2 3.2 3.2 3.2
0.7 0.7 6.1 0.7
1.0
0.4
8
4.2 3.2
1.7
0.6
0.2
8
4.2 3.2
7.8
0.1
0.1
8
4.2 3.2
12.9
2.0 2.1
0.8 0.8
15 31
4.3 3.3 4.4 3.4
2.5 6.5
a Estimated equilibrium solubility (Ceq) was determined from the equation shown in Figure 1 for 6-line ferrihydrite at pH 3.64, 3.25, and 2.83, being 2, 5, and 9 mg L-1, respectively. The goethite/ferrihydrite ratio is the goethite to ferrihydrite peak area ratios for the (101) goethite reflection and the distinctive 6-line ferrihydrite reflection at 57.3° 2-theta (Co KR) measured by XRD in each steady-state sample, which represents an estimation of the relative proportion of goethite in each steady-state precipitate.
function of mean residence time (MRT), pH, feed concentratio,n and the nucleation and growth kinetics of the system (eq 1). S ) k1(CIN - COUT) /τAT
(1)
In eq 1, k1 is a growth rate constant, C is concentration, τ is the mean residence time, S is ∆C (i.e., C - Ceq, where Ceq is the equilibrium solubility), and AT is the surface area of crystals present and is dependent on the nucleation and growth kinetics of the system. Classically, eq 2 shows the nucleation rate to be a function of temperature (T), interfacial tension (γ), and relative supersaturation (SR ) C/Ceq), whereas the growth rate (eq 3) is more specifically related to absolute supersaturation (SA )
10.1021/cg060620x CCC: $40.75 2008 American Chemical Society Published on Web 02/27/2008
Crystallization of Nanoscale 6-Line Ferrihydrite
Crystal Growth & Design, Vol. 8, No. 4, 2008 1385
Figure 1. Estimated equilibrium solubilities for 2- and 6-line ferrihydrite as a function of pH, at 85 °C in a zinc sulfate solution (160 g L-1 ZnII).
(C - Ceq)/Ceq), where k3 is a growth rate constant and x is indicative of the rate-determining step. JN ) A exp[-k2(γ3/T(ln SR)2)]
(2)
JG ) k3SAx
(3)
Hence altering variables to change supersaturation is not straightforward as all impact the nucleation and growth rates of the system and in turn also supersaturation. However, changing the rate of precipitation (unit mass precipitated per unit time) by altering the feed addition rate through the residence time or feed concentration is more straightforward to calculate and produce. The mass rate of precipitation can be defined as the amount of FeIII precipitated per L per unit of time, as shown in eq 4, where the total FeIII concentration (g L-1) entering the reactor [FeIII]IN is described by eq 5 (which accounts for dilution of the feed liquor), [FeIII]OUT is the total FeIII concentration exiting the reactor (g L-1), and τ is the mean residence time (h). [FeIII]FEED is the FeIII feed liquor concentration, FRFe is the flow rate of the acidic FeIII feed liquor (L h-1), and FRB is the flow rate of the added base (L h-1). mass rate of precipitation ) ([FeIII]IN - [FeIII]OUT)/τ (4) [Fe ]IN ) [Fe ]FEEDFRFe/(FRFe + FRB) III
III
(5)
Describing the effect of pH on the precipitation rate is not straightforward. Industrial-based studies have shown the propor-
Figure 3. XRD patterns of the steady-state samples, showing the increase in intensity of goethite reflections with a decrease in the FeIII feed concentration. The straight lines are those of the standard goethite pattern.
tion of goethite in residues to increase at lower pH,5 where it was suggested that the rate of precipitation decreased due to the higher equilibrium solubility experienced at lower pH possibly reducing supersaturation. In this paper, the effect of changing the rate of 6-line ferrihydrite continuous reactive crystallization is investigated by alteration in pH, feed concentration, and mean residence time (MRT) so as to impact the relative rates of 6-line ferrihydrite and goethite precipitation, and subsequently the physical properties of the precipitates (surface area, impurities concentration, filtration rate, and particle size) formed at steady state. Experimental Section Reagents, Continuous Crystallizer Operation, and Analytical Methods. Synthetic reagents for continuous crystallization experiments were prepared as described previously.3 Reactive crystallization occurred by the addition and reaction of acidic sulfate feed liquor (x g L-1 FeIII, 160 g L-1 Zn2+, pH 1) and NaOH (8 M, 320 g L-1) for continuous neutralization at the desired pH. Experiments were performed in acidic concentrated zinc sulfate solutions to provide relevance to the zinc refining industry where ferrihydrite is precipitated in large volumes from process liquors.3,5 6-line ferrihydrite crystallization was performed in a 2.15 L (draft tube baffled reactor vessel) mixed suspension mixed product removal (MSMPR) continuous crystallizer and operated as described previously,3 using the control conditions of pH 3.6, 85 °C and an MRT of 5 h, where the acidic FeIII feed liquor is pumped in at ∼6 times the flow
Figure 2. Effect of rate of crystallization (g L-1 h-1) on the properties of precipitated aggregates. The % moisture and surface area (m2 g-1) data refer to the left axis. The error for wt% Zn is ( 0.1% and ( 5% for % moisture and surface area.
1386 Crystal Growth & Design, Vol. 8, No. 4, 2008
Loan et al.
Figure 5. TEM images of feed concentration experiments [6 g L-1 (A, B) and 0.7 g L-1 (C, D)] demonstrating the complex nanometerscale relationship between goethite and 6-line ferrihydrite found in sample aggregates. (A) Representative aggregate from feed concentration samples
