© The Minerals, Metals & Materials Society 2018
Boyd R. Davis, Michael S. Moats, Shijie Wang, Dean Gregurek, Joël Kapusta, Thomas P. Battle, Mark E. Schlesinger, Gerardo Raul Alvear Flores, Evgueni Jak, Graeme Goodall, Michael L. Free, Edouard Asselin, Alexandre Chagnes, David Dreisinger, Matthew Jeffrey, Jaeheon Lee, Graeme Miller, Jochen Petersen, Virginia S. T. Ciminelli, Qian Xu, Ronald Molnar, Jeff Adams, Wenying Liu, Niels Verbaan, John Goode, Ian M. London, Gisele Azimi, Alex Forstner, Ronel Kappes and Tarun Bhambhani (eds.)Extraction 2018The Minerals, Metals & Materials Serieshttps://doi.org/10.1007/978-3-319-95022-8_115

Development of an Encapsulation Process to Extend the Stability of Scorodite Under Wider pH and Redox Potential Range Conditions

Fuqiang Guo1 and George P. Demopoulos1  
(1)
Department of Mining and Materials Engineering, McGill University, 3610 University Street, Montreal, QC, H3A 0C5, Canada
 
 
George P. Demopoulos
Email: george.demopoulos@mcgill.ca

Abstract

Scorodite (FeAsO4·2H2O) is suitable mineral carrier for immobilization of arsenic-rich wastes. Its stability is, however, highly pH dependent (typically at 4 ≤ pH ≤ 7) and satisfactory only under oxic disposal conditions. In this work an encapsulation process using mineralized gels of aluminum hydroxy-oxides is developed to enhance the stability of scorodite under wider pH and redox potential range conditions. The encapsulation involves blending and ageing of synthetic scorodite produced by McGill’s atmospheric scorodite process with aluminum hydroxyl gels derived from controlled hydrolysis of aluminum salts. The amorphous hydrolyzed Al-gel encapsulates the scorodite particles, which upon short-term aging transforms into crystalline Al(OH)3/AlOOH mineral phases providing a robust protection microscopic barrier. Long-term stability testing demonstrates the encapsulation system to be highly effective in suppressing arsenic release under either oxic or anoxic (100 mV < Eh < 600 mV) potential and neutral-alkaline pH (7 ≤ pH ≤ 9) ranges.

Keywords

ArsenicScoroditeEncapsulationStabilizationAluminum hydroxyl gels

Introduction

Scorodite (FeAsO4·2H2O) is advocated as an acceptable carrier for the immobilization of arsenic-rich wastes released during pyrometallurgical or hydrometallurgical processing of arsenic-containing ores, owing to its naturally wide occurrence in oxidized zones of arsenic-bearing ore deposits [1]. Considerable research has been carried out either looking into the experimental determination of its synthesis-production on one hand and its solubility and thermodynamic stability [2–4] on the other. From an industrial application point of view there are basically two routes available for the production of scorodite: the hydrothermal route that involves autoclave processing at elevated temperature (150–180 °C) and pressure [5, 6] and the atmospheric pressure precipitation route at around 80–95 °C first developed in our laboratory at McGill [7–9]. The McGill Atmospheric Process [10, 11] became commercial reality thanks to Ecometales in Chile back in 2012 [12]. At the same time Dowa developed in Japan an oxidation based atmospheric scorodite process variant that demonstrated at pilot plant scale [13–15]. Other companies have also been active testing different scorodite production options [16]. The stability of scorodite is, however, highly pH dependent (typically at 4≤ pH ≤ 7) and satisfactory only under oxic disposal conditions. Its stability may be inadequate under more alkaline conditions (pH = 7–8.5) while it undergoes reductive break down when Eh < 200 mV [17–19]. Therefore, there is strong interest in investigating stabilization technologies to enhance the stability of scorodite over a wider range of disposal conditions.

New stabilization approaches other than conventional cement-based ones [20] were conceived and investigated by our group based on the concept of encapsulation. Two types of encapsulation materials were examined, metal phosphates (namely, hydrated aluminum phosphate (AlPO4·1.5H2O) and hydroxyl- or fluoro-apatites (Ca5(PO4)3OH,F)) [21, 22] and aluminum hydroxyl gels [23]. Scorodite was encapsulated by direct deposition of phosphate minerals under controlled supersaturation conditions [21]. The encapsulation appears to be effective in suppressing the release of arsenic under both oxic and anoxic conditions by more than one order of magnitude. Arsenic release from the coated scorodite with hydroxyl/fluoro apatite was partially blocked as it was rather in the form of very thin layer but more importantly was associated with release of soluble phosphorus; as such the coating may eventually disappear [22]. However, the aluminum hydroxyl gel was proved to be highly effective encapsulating material compared with metal phosphates [23]. Thus for example there was only 0.2 mg/L of As released from the sulphate gel/scorodite system (Al:As = 1.0) equilibrated at pH 7.3 under oxic condition, one to two orders lower than naked scorodite.

In the present paper, we review the aluminum gel stabilization system for scorodite providing new data that prove the gels form mineralized products of aluminum hydroxyl-oxides that are protective under both oxidizing and non-oxidizing conditions. The encapsulation process involves blending atmospherically produced scorodite particles with aluminum hydroxyl gels and allowing sufficient ageing for the formation of a protective aluminum hydroxide/oxide matrix that suppresses arsenic release under wider pH and redox potential range conditions than ever reported before for scorodite.

Experimental

Synthesis of Scorodite, Aluminum Hydroxyl Gel and Encapsulation of Scorodite

The scorodite substrate material was synthesized by atmospheric precipitation via supersaturation control, which has been developed at McGill’s HydroMET Laboratory [8, 11]. In this procedure 0.5 L As(V)-Fe(III)-H2SO4 solution containing 40 g/L arsenic(V) and iron to arsenic molar ratio of one were placed in a 1-L Applikon® Bioreactor and heated to 95 °C. When the temperature inside the reactor reached ~65 °C and the pH had dropped to 0.4, 5 g of hydrothermally produced scorodite was added to the reactor as seed. In the presence of seed, precipitation started and was allowed to proceed for 24 h, after which the slurry was filtered using a pressure filter with 0.22 µm pore size membrane filter. Solids were then subjected to several washing and consecutive Toxicity Characteristic Leaching Procedure (TCLP) type leachability steps. The freshly prepared scorodite particles were subsequently used for encapsulation with aluminum hydroxyl gels.

Aluminum(III) hydroxyl gels were produced by partial (molar ratio OH:Al = 2.5) quick neutralization of 2 mol/L Al(SO4)1.5 solutions with 5.0 N sodium hydroxide (NaOH) at room temperature as described in our previous paper [24]. Mild stirring had to be applied during mixing as excessive force was found to be counterproductive causing gel thinning. The freshly prepared aluminum hydroxyl gels were used to encapsulate scorodite particles.

The encapsulated scorodite was achieved by blending scorodite and aluminum hydroxyl gels together and subsequently aged at room temperature for 7 days. A low gel/scorodite ratio (Al:As = 0.1) was applied for blending the two products. Some samples of encapsulated scorodite were subjected to SEM characterization and stability testing. A single washing step was employed prior to stability testing in order to remove any entrained soluble material from the aged samples.

Stability Test

The scorodite particles encapsulated by aluminum hydroxyl gels, as well as the scorodite substrate material, were subjected to oxic and anoxic stability testing with an orbital shaker at a liquid to solid ratio (L/S) of 20.

The oxic stability was evaluated by equilibrating these solids in de-ionized water that was initially adjusted to pH = 9 ± 0.2 with 0.5 mol/L Ca(OH)2 slurry. The anoxic stability testing was conducted at controlled reducing potential conditions (Eh = 200 ± 20 mV) via addition of sodium sulfite (Na2SO3) solution. The pH of the solution was monitored and periodically adjusted to pH 9 ± 0.2 with 0.5 mol/L Ca(OH)2 slurry. The system pH was allowed to drift to pH 7 or lower and subsequently readjusted. Both of oxic and anoxic stability test lasted 167 days.

Analysis and Characterization

Arsenic concentration in aqueous samples was analyzed using a Thermo Jarrell Ash IRIS Inductively Coupled Plasma-Atomic Emission Spectrometer (ICP-AES). Powders were characterized by X-ray powder diffraction using a Bruker D8 Discovery X-ray diffractometer with Cu Kα radiation (λ = 1.5405 Å). Field emission scanning electron microscope (FE-SEM) images were taken on a Hitachi S-4700 electron microscope equipped with an energy dispersive spectrometer for X-ray evaluation. Elemental mapping of particle cross sections was achieved by a Hitachi S-3000 N variable pressure scanning electron microscope (VP-SEM).

Results and Discussion

Synthesis of Scorodite, Aluminum Hydroxyl Gel, and Aluminum Gel/Scorodite Mixture

Prior to actual encapsulation procedure the scorodite substrate material and aluminum hydroxyl gels were synthesized and characterized. The scorodite substrate material was precipitated atmospherically and the appearance of scorodite wet cake is shown in Fig. 1a. XRD analysis (pattern not shown) confirmed the substrate material to be crystalline scorodite. According to SEM examination as shown in Fig. 2a the scorodite particles appeared to be uniformly spherical with smooth surface and dense appearance and an average diameter of 22 μm. The aluminum hydroxyl gels possessed a sufficiently viscous consistency with a translucent appearance (shown in Fig. 1b), which enable scorodite substrate particles to be easily physically blended with the gels by mechanical mixing without subsequently settling. The gels were weakly acidic with pH of 5.4. The SEM image of a dried sample of the aluminum hydroxyl gel (Fig. 2b) shows smooth particles with a moderate porosity and a gearwheel-like structure. The encapsulated scorodite was obtained by blending scorodite substrate material with aluminum hydroxyl gels manually. After blending the two components, the mixture was noted to become less viscous than the starting gel (Fig. 1c), but sufficiently thick to prevent scorodite from settling. Figure 1d shows encapsulated scorodite after 7 days of ageing.
../images/468727_1_En_115_Chapter/468727_1_En_115_Fig1_HTML.gif
Fig. 1

Typical appearance of a scorodite substrate material, b aluminum hydroxyl gel, c blend of scorodite and aluminum hydroxyl gel, and d encapsulated scorodite after ageing for 7 days

../images/468727_1_En_115_Chapter/468727_1_En_115_Fig2_HTML.gif
Fig. 2

SEM images of a naked scorodite substrate material, b dried aluminum hydroxyl gel, and c blend of scorodite and aluminum hydroxyl gel (Al:As = 1.5, aged for 28 days)

Characterization of Aluminum Gel Coating

Figure 2c shows a typical SEM image of the blend of scorodite and aluminum hydroxyl gel (Al:As = 1.5, aged for 28 days). The encapsulated scorodite particles are quite different in morphological features from the substrate materials as shown in Fig. 2a. There appears the gel to have deposited in a non-uniform “aggregated” form on the smooth surface of the substrate scorodite particles. Figure 3 shows a backscattered electron (BSE) image of the cross section of the aluminum hydroxyl gel/scorodite composite (Al:As 0.1) aged for 1 day along with As, Fe and Al elemental X-ray maps. The image contains several particles and it is evident from the arsenic and iron elemental X-ray maps that the large particles are scorodite. The aluminum X-ray map reveals that there is no aluminum inside the large particles, but several ‘islands’ of aluminum hydroxide phases are evident to be present on or near the scorodite particle surface. These features suggest that the aluminum hydroxyl gel has formed a mineralized matrix around the scorodite particle.
../images/468727_1_En_115_Chapter/468727_1_En_115_Fig3_HTML.gif
Fig. 3

Back-scattered electron (BSE) image of scorodite encapsulated with aluminum hydroxyl gel (Al:As 0.1) and elemental X-ray maps (3000x, 25 keV)

Stability Evaluation

Figure 4 compares near-equilibrium arsenic concentration levels for different systems: naked scorodite and encapsulated scorodite under oxic (640–660 mV) and anoxic environments, respectively. The superior performance of the encapsulated scorodite vis-à-vis the naked scorodite is clearly demonstrated. The release of arsenic from naked scorodite substrate was in the order of 8.8 mg/L at pH 7.2 after 167 days of oxic stability testing. The encapsulated scorodite with aluminum hydroxyl gels exhibited a negligible amount of arsenic release (<0.1 mg/L) after 167 days at a final pH of 7.6. Long-term stability testing up to 900 days were conducted on encapsulated scorodite without post-wash. There was less than 2.0 mg/L of arsenic release at pH 7.1 indicating that the system is equally effective even without prior washing [23]. Under wider pH and redox potential range conditions, the 4.2 mg/L of As solubility by the encapsulated scorodite system (Al:As = 0.1, pH 8.3, Eh was adjusted from 340 mV to 210 mV, 167 days at 22 ̊C) is 33 times less than the solubility of unprotected scorodite for the similar anoxic environment (pH 8.6, Eh = 200 mV). Considering that the suppressed arsenic release level was achieved with low gel/scorodite ratio (Al:As = 0.1) and brief 7 days ambient ageing, this system is identified as excellent candidate for further investigation as a potentially effective hazardous material stabilizer.
../images/468727_1_En_115_Chapter/468727_1_En_115_Fig4_HTML.gif
Fig. 4

Comparison of near-equilibrium (167 days at 22 °C) arsenic concentrations for different systems under oxic and anoxic conditions

Post Stability Testing Characterization

XRD analysis of the aluminum hydroxyl gels after the stability testing demonstrated a mineralization of amorphous aluminum gel into crystalline Al(OH)3/AlOOH phases (amorphous-to-crystalline phase transformation). After 167 days of stability testing under oxic condition as shown in Fig. 5a, sharp lines developed that were all due to the amorphous-to-crystalline phase transformation. The amorphous aluminum hydroxyl gel was converted into a mixed Al(OH)3 phase of gibbsite and bayerite. The amorphous to boehmite phase transition was observed in Fig. 5b when the aluminum hydroxyl gels were exposed to 167 days of stability testing under chemically generated anoxic condition. The mineralization increases the adhesion of the scorodite substrate and the encapsulating aluminum gels coating, which facilitates the coating to remain on the surface of scorodite. The well-crystallized Al(OH)3/AlOOH mineral phases provide robust protection barrier against arsenic release.
../images/468727_1_En_115_Chapter/468727_1_En_115_Fig5_HTML.gif
Fig. 5

XRD patterns of NaOH derived-aluminum hydroxyl gel after 167 days of stability testing under a oxic condition and b chemically generated anoxic condition

Process Flow Diagram for Industrial Application

Based on the results obtained in the present work, a conceptual process flow diagram for scorodite stabilization with aluminum hydroxyl gel was prepared and depicted in Fig. 6. The process requires two parallel circuits, including one for the atmospheric production of crystalline scorodite and the other for the preparation of amorphous aluminum hydroxyl gel. According to this flowsheet, arsenic solid residue is dissolved in weak acid (H2SO4) solution [25] followed by oxidation of As(III) with an oxidant (SO2/O2) in the case of As(III) materials [26]. Then the resultant arsenic(V)-rich solution is subjected to scorodite precipitation [10]. The resultant slurry is transferred to a solid/liquid separation stage and the filtrate is recycled back to the dissolution step, while the wet scorodite cake is sent to encapsulation stage. In the parallel circuit of Al-gel preparation, aluminum sulphate salt solution is partially neutralized via controlled hydrolysis with sodium hydroxide solution. The atmospheric scorodite and aluminum hydroxyl gel are subsequently blended at a low gel/scorodite ratio (Al:As = 0.1). The encapsulated scorodite is transferred to waste disposal sites either directly as it is or after being aged at ambient temperature for a defined amount of time (at least 1 day).
../images/468727_1_En_115_Chapter/468727_1_En_115_Fig6_HTML.gif
Fig. 6

Conceptual process flow diagram for scorodite stabilization with aluminum hydroxyl gel

Conclusions

Aluminum hydroxyl gels were proposed to extend the stability of scorodite under wider pH and redox potential range conditions. Aluminum hydroxyl gels were blended with atmospherically produced scorodite at a low gel/scorodite ratio (Al:As = 0.1). After sufficient ageing under ambient conditions, a very thin Al-gel coating was present on the surface of scorodite particles along with the agglomerate depositions. The stability testing revealed that the encapsulation of scorodite particles with aluminum gels appears to be effective in suppressing the release of arsenic under wider pH and redox potential range conditions. Under anoxic condition, only 4.2 mg/L of arsenic released from the aluminum hydroxyl gel/scorodite system equilibrated at pH = 8.3, Eh = 210 mV that is 33 times lower than the solubility of unprotected scorodite. After being exposed to 167 days of stability testing under oxic and anoxic conditions, the aluminum gel was shown to form mineralized Al(OH)3/AlOOH phases that provide robust protection to scorodite. The results of the present work show aluminum hydroxyl gels to be a promising option in stabilizing iron arsenate solids under wider pH and redox potential range conditions and deserve further development for large-scale and industrial application.

Acknowledgements

The support of NSERC and industrial partners is gratefully acknowledged. Similarly, the contributions of several past members of our group in this work is proudly acknowledged.