Temperature and seeding effects on the precipitation of scorodite from sulfate solutions under atmospheric-pressure conditions

Abstract

Arsenic is a major contaminant in the nonferrous extractive metallurgy. In the past 20 years, many studies have shown that it can be precipitated as relatively stable crystalline scorodite (FeAsO 4 • 2H 2 O) by precipitation under ambient or elevated pressures. In the present study, an extensive program of scorodite precipitation tests under ambient pressure has shown that the rate of scorodite formation increases dramatically by a small increase in temperature from 85 °C to 100 °C. The beneficial effects of temperature are attributed to the higher thermodynamic stability of scorodite at elevated temperatures, but also to higher rates of secondary nuclei formation and crystal growth. In any case, irrespective of the precipitation temperature, the leachability of all scorodite precipitates observed in toxicity characterization leaching procedure (TCLP) tests is below 5 mg/L As. Another parameter examined in this study was seeding. It was observed that the higher the initial concentration of seed, the faster the precipitation. Precipitation of well-crystallized scorodite can be effected equally well on heterogeneous seed such as hematite (Fe 2 O 3 ) or gypsum (CaSO 4 • 2H 2 O) added externally or formed in situ.

Factors to Enable Crystallization of Environmentally Stable Bioscorodite from Dilute As(III)-Contaminated Waters

“…Hollow bioscorodite seed particles were found increasingly filled with newly formed scorodite (Figure 4(a1-a6)), whilst solid chemical seeds induced their surface to be thoroughly coated with new scorodite precipitates (Figure 4(b1-b6)). Other abiotic studies under atmospheric pressure condition using As(V) and Fe(III) as starting species also have reported a positive effect of seeds with high specific surface area, e.g., the better scorodite precipitation kinetics were observed by using finer seeds such as hematite and hydrothermal scorodite [6]. Caetano et al (2009) reported that a specific surface area higher than 270 m 2 /g was required to achieve 85% As removal [16].…”

“…Chemical scorodite syntheses are generally conducted at pH 1.0 or lower [2,6,8,9,25]. Fujita et al (2009) chemically synthesized stable scorodite particles at the pH range of 0.3-1.0, but higher pH levels (pH > 1.2) negatively affected scorodite stability [17].…”

“…Early studies on scorodite synthesis employed hydrothermal processes targeting high As(V) concentrations of 170-460 mM (13,,000 mg/L) [2,3]. They were later followed by studies using atmospheric pressure (mostly at around 95 • C), again targeting high As(V) concentration ranges of 130-670 mM (10,000-50,000 mg/L) [4][5][6][7][8][9][10]. While these chemical approaches are generally effective for high As(V) concentrations, even after treatments with a good As removal efficiency, residual soluble As(V) must yet meet the effluent standard (e.g., 95% As removal from [As(V)] ini = 10,000 mg/L would still leave 500 mg/L of residual As(V) in solution [7]).…”

“…From studies of conventional abiotic scorodite syntheses, seed feeding (scorodite or other heterogeneous crystals) was shown to be one of the influential factors to promote scorodite formation [4,6,16]. Initial pH was also reported to affect the stability and Fe/(As + S) ratio of the resultant scorodite product [17].…”

Vibrational Spectroscopy of Complex Synthetic and Industrial Products

“…This can be done for example at elevated temperatures, near the boiling point of water (80-95 °C) and under controlled supersaturated conditions. (Singhania et al, 2005;Fujita et al, 2008) Crystalline Scorodite is at least 100 times less soluble than its amorphous counterpart (FeAsO4 · xH2O[am]) (Krause and Ettel, 1989;Langmuir et al, 2006;Bluteau & Demopoulos, 2007) and given its high arsenic content (in comparison to the Fe (III)-As(V) co-precipitates as mentioned above) has been advocated for the fixation of arsenic-rich wastes. (Filippou & Demopoulos 1997;Fujita et al, 2008) At even higher temperatures (>100 °C, the hydrothermal precipitation range) during autoclave hydrothermal processing of copper (Berezowsky et al, 1999) and gold (Dymov et al, 2004) sulphide feedstock's, other crystalline phases than Scorodite are reported to form some of which exhibit equal or better stability than Scorodite (Swash & Monhemius, 1994;Dutrizac & Jambor, 2007.…”