Thesis or Dissertation On-site synthesis of schwertmannite and its application for arsenic immobilization at copper mines

Houngaloune, Sengpasith

Arsenic (As), a very poisonous inorganic pollutant is a major toxicant at porphyry copper mines the principal source of copper production worldwide. It is important to find a suitable method to control or stabilize the toxic arsenic species that could be released from the large amounts of waste at copper mines. Among the current treatment processes for arsenic control at copper mine waste, retention of arsenic by schwertmannite (a ferric oxyhydroxy sulfate mineral, Fe8O8(OH)8-2x(SO4)x with x typically 1–1.75) has attracted much attention in recent years due to its strong binding affinity to toxic arsenic species. It may also be cost-effective as it may be possible to synthesize schwertmannite from solutions generated in heap leach operations at copper mines. Such leach solutions generally contain high concentrations of Fe3+ and SO4 2−, the components of schwertmannite. In this study, on-site synthesis of schwertmannite at porphyry copper mines by neutralization technique was proposed. First, synthesis of schwertmannite from the simulated copper heap leach solutions was investigated. The efficiency in arsenic removal by the synthesized schwertmannite and the stability of arsenic-sorbed schwertmannite were then evaluated, respectively. Finally, the applications of schwertmannite in As control of copper mine wastes were demonstrated. In Chapter 1, the background and the objectives of the study were presented. The sources of arsenic contamination resulting from copper production were pointed out. The various approach for arsenic wastes control in mining and metallurgical operations was reviewed and schwertmannite was selected as an ideal technique applying for control the toxic arsenic species that could be presented from such large amounts of waste at copper mines. On-site synthesis of schwertmannite by neutralization of the copper heap leach solutions was subsequently proposed. The synthesis is expected to be performed by neutralizing the leach solutions to pH 3–4. The synthesized schwertmannite can be expected to find application in arsenic immobilization of copper mine waste in tailings and spent ore of either dump or heap leach piles. Chapter 2 provided insight into the production of schwertmannite in porphyry copper mines by investigating the effect of co-existing metal ions (Cu2+ and Fe2+) and the reaction temperatures (25˚C and 65˚C) on the synthesis of schwertmannite by neutralization technique. It is shown that Cu2 + and Fe2 + play an important role for the schwertmannite synthesis at 65˚C. However, Cu2 + and Fe2 + did not affect the synthesis at 25˚C. It was observed that schwertmannite is formed at both 25˚C and 65˚C at all experimental conditions except for the solutions containing Fe2+ at 65˚C, as goethite was generated at these conditions. It was found that goethite is formed by transformation of intermediated schwertmannite during the synthesis at 65˚C. However, Cu2 + has the ability to inhibit the transformation of schwertmannite to goethite in the presence of Fe2 +. Although it is possible to synthesize schwertmannite at both temperatures, their surface characteristics are different. The specific surface area of the schwertmannite synthesized at 65˚C was much larger (147.4-176.9 m2 g-1) than the specific surface area of the schwertmannite synthesized at 25˚C (14.1-21.4 m2 g-1), which this may affect their efficiency for arsenic removal. In Chapter 3, arsenic sorption capacities by the synthesized schwertmannite were evaluated. The results indicate that As(V) in acidic solutions (pH 3-4) can be removed effectively by schwertmannite synthesized in the presence or absence of co-existing metal ions (Cu2+ and Fe2+) at 65˚C with the maximum sorption capacity of 94-133 mg g-1. A lower As(V) sorption capacity is observed in product containing goethite synthesized in the presence of Fe2+ at 65˚C; here, the maximum As(V) sorption capacity is 58 mg g-1. The maximum As(V) sorption capacities by ii schwertmannite synthesized at 25˚C are 17-23 mg g-1, which are much lower than the maximum sorption capacities of the schwertmannite synthesized at 65˚C. This should be taken into consideration for its application on mine sites. In Chapter 4, the stability of As(V)-sorbed schwertmannite (Sch-As) under porphyry copper mining conditions was studied by investigating the effect of Cu2+, Fe2+, pH, and ageing time on the stability of Sch-As. The results indicated that Cu2+ has no significant effect on the stability of Sch- As and that the As(V) incorporated into schwertmannite can retard or significantly inhibit the Fe2+- catalyzed transformation of schwertmannite to goethite under acidic conditions (pH 3–4). The Sch- As aged at different pH ranges from 3 to 11 at 25°C exhibits no mineralogical phase changes even after ageing for 120-days; however the concentration of arsenic released from the solid phase appeared to be strongly pH-dependent also at ageing for 24 h. The release of As was almost negligible at pH 2 to 7, and a high release of As was observed at extremely acidic and alkaline conditions. This indicates that the release of As from Sch-As is controlled by environmental factors such as pH rather than time. In Chapter 5, applications of the synthesized schwertmannite for arsenic immobilization in copper mine wastes were demonstrated. The study was divided into two models experiments. The first part was to demonstrate the application of schwertmannite for immobilization of arsenic in tailings resulting from copper flotation operations. The results indicated that high arsenic concentration was observed for the experiment without the addition of schwertmannite, which this may be due to the oxidation of arsenopyrite. The concentration of arsenic became significantly lower when schwertmannite was added; suggesting that schwertmannite synthesized from copper heap leach solutions may be suitable for arsenic immobilization in copper flotation tailings. The second part of this chapter was focused on the possibility in applying the synthesized schwertmannite for arsenic immobilization in mine waters containing the diluted concentration of arsenic. The high arsenic concentration is essential for long-term stability of schwertmannite. The synthesized schwertmannite was used as the sorbent-desorbent to remove the diluted arsenic concentration at low pH and strip it back in the high pH solution to generate higher arsenic concentration. The results indicated that the diluted concentration of As(V) in acidic solution can be sorbed efficiently by schwertmannite; and the arsenic can also strip back or release from the As(V)-sorbed schwertmannite at alkaline pH condition, suggesting that the synthesized schwertmannite may be suitable for arsenic treatment for the solution with diluted concentration of arsenic. In Chapter 6, the summary and conclusions of the study were presented
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Hokkaido University(北海道大学). 博士(工学)

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