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 coppermines the principal source of copper production worldwide. It is important to find a suitablemethod to control or stabilize the toxic arsenic species that could be released from the largeamounts of waste at copper mines. Among the current treatment processes for arsenic control atcopper 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 itsstrong binding affinity to toxic arsenic species. It may also be cost-effective as it may be possibleto synthesize schwertmannite from solutions generated in heap leach operations at copper mines.Such leach solutions generally contain high concentrations of Fe3+ and SO42−, the components ofschwertmannite. In this study, on-site synthesis of schwertmannite at porphyry copper mines byneutralization technique was proposed. First, synthesis of schwertmannite from the simulatedcopper heap leach solutions was investigated. The efficiency in arsenic removal by the synthesizedschwertmannite and the stability of arsenic-sorbed schwertmannite were then evaluated,respectively. Finally, the applications of schwertmannite in As control of copper mine wastes weredemonstrated.In Chapter 1, the background and the objectives of the study were presented. The sources ofarsenic contamination resulting from copper production were pointed out. The various approachfor arsenic wastes control in mining and metallurgical operations was reviewed andschwertmannite was selected as an ideal technique applying for control the toxic arsenic speciesthat could be presented from such large amounts of waste at copper mines. On-site synthesis ofschwertmannite 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. Thesynthesized schwertmannite can be expected to find application in arsenic immobilization ofcopper 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 minesby 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 thatCu2 + and Fe2 + play an important role for the schwertmannite synthesis at 65˚C. However, Cu2 + andFe2 + did not affect the synthesis at 25˚C. It was observed that schwertmannite is formed at both25˚C and 65˚C at all experimental conditions except for the solutions containing Fe2+ at 65˚C, asgoethite was generated at these conditions. It was found that goethite is formed by transformationof intermediated schwertmannite during the synthesis at 65˚C. However, Cu2 + has the ability toinhibit the transformation of schwertmannite to goethite in the presence of Fe2 +. Although it ispossible to synthesize schwertmannite at both temperatures, their surface characteristics aredifferent. 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 byschwertmannite synthesized in the presence or absence of co-existing metal ions (Cu2+ and Fe2+) at65˚C with the maximum sorption capacity of 94-133 mg g-1. A lower As(V) sorption capacity isobserved in product containing goethite synthesized in the presence of Fe2+ at 65˚C; here, themaximum As(V) sorption capacity is 58 mg g-1. The maximum As(V) sorption capacities byiischwertmannite synthesized at 25˚C are 17-23 mg g-1, which are much lower than the maximumsorption capacities of the schwertmannite synthesized at 65˚C. This should be taken intoconsideration for its application on mine sites.In Chapter 4, the stability of As(V)-sorbed schwertmannite (Sch-As) under porphyry coppermining conditions was studied by investigating the effect of Cu2+, Fe2+, pH, and ageing time on thestability 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 evenafter ageing for 120-days; however the concentration of arsenic released from the solid phaseappeared to be strongly pH-dependent also at ageing for 24 h. The release of As was almostnegligible at pH 2 to 7, and a high release of As was observed at extremely acidic and alkalineconditions. This indicates that the release of As from Sch-As is controlled by environmental factorssuch as pH rather than time.In Chapter 5, applications of the synthesized schwertmannite for arsenic immobilization incopper mine wastes were demonstrated. The study was divided into two models experiments. Thefirst part was to demonstrate the application of schwertmannite for immobilization of arsenic intailings resulting from copper flotation operations. The results indicated that high arsenicconcentration was observed for the experiment without the addition of schwertmannite, which thismay be due to the oxidation of arsenopyrite. The concentration of arsenic became significantlylower when schwertmannite was added; suggesting that schwertmannite synthesized from copperheap leach solutions may be suitable for arsenic immobilization in copper flotation tailings. Thesecond part of this chapter was focused on the possibility in applying the synthesizedschwertmannite for arsenic immobilization in mine waters containing the diluted concentration ofarsenic. The high arsenic concentration is essential for long-term stability of schwertmannite. Thesynthesized schwertmannite was used as the sorbent-desorbent to remove the diluted arsenicconcentration at low pH and strip it back in the high pH solution to generate higher arsenicconcentration. The results indicated that the diluted concentration of As(V) in acidic solution canbe sorbed efficiently by schwertmannite; and the arsenic can also strip back or release from theAs(V)-sorbed schwertmannite at alkaline pH condition, suggesting that the synthesizedschwertmannite may be suitable for arsenic treatment for the solution with diluted concentration ofarsenic.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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