Estudo da oxidação de pirita em meio alcalino através de modelagem termodinâmica e de análises por difração de raios-x, espectroscopia de infravermelho e raman

Abstract

An investigation of pyrite oxidation in alkaline solutions by thermodynamic modeling and analyses by Xray diffraction, infrared and Raman spectroscopy was carried out. The effect of pH and solution composition on the nature of the oxide product and complex formation isexplained. A mechanism is proposed to explain the increase of the oxidation rates in carbonate/bicarbonate solutions due to the formation of the iron carbonate complexes. According to thermodynamics, hematite is the expected iron oxide phase produced during oxidation. As ferrihydrite is considered to be the initial product of the rapid hydrolysis of Fe+3solutions, the predominance area of this metastable phase was also investigated as a possible product of pyrite oxidation in alkaline solutions. The EhpH diagrams indicated that ferrihydrite predominates in the pH range from 2 to 14 and potentials (Eh) of 0.25 V and higher. In the presence of carbonate, the stability diagrams show that soluble iron-carbonate complexes may be formed at pH below 8-12, coexisting with pyrite in the same region. At pH higher than 11 and oxidizing conditions, only Fe(OH)4 - predominates even in the presence ofcarbonate/bicarbonate ions. X-ray diffraction analyses, infrared and micro-Raman spectroscopy indicate that the products formed are determined by solution composition and pH. In hydroxide solutions (pH 8.5 and 10), hematite and feroxyhite were identified as the major and minor solid phases, respectively. At pH 12.5, only ferrihydrite is identified.Conversely, Fe(III)-green rust/ferrihydrite are found as the main products of pyrite oxidation in bicarbonate/carbonate media. It is proposed that the formation of soluble Fe(II)/Fe(III)-CO3 complexes modify the hydrolysis and precipitation pathways of the iron released from pyrite.Two distinct precipitation pathways are proposed. In hydroxide solutions and in conditions of low iron solubility (pH 8.5 and 10), pyrite oxidation leads to the formation of hematite. As pH increases up to pH 12.5, iron solubility increases due to the formation of Fe(OH)4 - andferrihydrite becomes the predominant oxyhydroxide. Similarly, in the presence of carbonate/bicarbonate, Fe(III)-GRCO3 is the main phase identified. The formation of these products was attributed to the increase of iron solubility in the presence of carbonate associated with the formation of the soluble Fe(II) and Fe(III) carbonate, and hydroxylcomplexes. Ferrihydrite transformation to more stable phases (e.g. hematite or goethite) was prevented due to the high concentration/adsorption of soluble silicate ions. The identificationof carbonate compounds among the products formed during pyrite oxidation was possible, by the first time, by the use of diffuse reflectance infrared spectroscopy. It is also important to note that the conditions where the carbonate compounds are identified correspond to thosein which the fastest rate of pyrite oxidation in carbonate solutions is reported. Concerning the mechanism of pyrite oxidation, the formation of Fe(II)-CO3 soluble complexes favors the oxidation of iron, which in turn favors the overall kinetics of pyrite oxidation in carbonate solutions.