Removal Iron from Silica Sand to Get High Purity of Silica using Sonication Method Assited Acid Leaching

Silica sand is one of the most abundant minerals. It occurs in many different settings throughout the geological record. The presence of iron compounds in silica sand is prohibitive to the production of optical fibers, glass, ceramics and refractory materials. Considerable efforts have been devoted to the problem of removing contaminants by physical^[1], chemical^[2], and biological methods^[3]. Sometimes various combinations of these methods are required to upgrade silica sand. The most appropriate method depends on the mineralogical forms and distribution of iron in the particular ore. The photovoltaic manufacturing of cells requires the use of a very pure silica (purity >99.9999 %) for obtaining a silica solar grade^[4].

Chemical methods involve upgrading of such minerals with inorganic and organic acids. The most commonly used inorganic acids are sulphuric and hydrochloric , but these are generally costly and the ensuing effluents are environmentally unacceptable. Furthermore, inorganic acids such as sulphuric or hydrochloric acids easily contaminate the minerals with SO₄^2- and Cl^-. Thus there is considerable interest in the development of alternative technological means such as organic acid leaching which may be more effective and eco-friendly. Additionally, oxalic acid is found to be the most promising because of its acid strength, good complexing characteristics and high reducing power, compared to other organic acids. Using oxalic acid, the dissolved iron can be precipitated from the leach solution as iron(II) oxalate dihydrate, which can be represented a useful potential feedstock for added-value products. The removal of iron from silica sand with oxalic acid has been studied by several workers. The chemical reactions can be summarized as follows^[5]:

Fe₂O₃ +  6H₂C₂O₄ à 2Fe(C₂O₄)₃^3- + 6H⁺ + 3H₂O   (1)

2Fe(C₂O₄)₃^3- + 6H⁺ +4H₂O à 2FeC₂O₄ . 2H₂O _(s) + 3H₂C₂O₄ + 2CO₂  (2)

Fe₂O₃ + 3H₂C₂O₄ + H₂O à 2FeC₂O₄ . 2H₂O_(s) + 2CO₂   (3)

Oxalic acid reacts with surface Fe(II) ions to form complex. Once the surface complex has performed, the dissolution mechanism differs depending on the iron mineral concerned^[6]. Fe(III) will be reduced become Fe(II) and will be reacted with oxalic acid and form complex. The reaction temperature is found to be critical as confirmed by many researcher. As an example, it is found at varying temperatures between 90 and 100^oC the maximum iron extraction that can be achieved is approximately 40%^[2]. Recently, sonication as an auxilary energy has ben succesfully applied in the mining industry. It is found that the iron on the surface of silica sand and matrix of silica sand can be eliminated more efficiently by sonication than by mechanical scrubbing. Furthermore the iron elimination rate can be raised when the sonication combine with chemical process, such as the iron elimination use acid leaching process^[5].

When an sonication field is used in the acid leaching process, the mechanical interaction between the sound waves and the liquid would lead to the phenomena of cavitation and acoustic streaming. The mechanism of sonication assisted acid leaching is schematically shown in Fig.1. Because many gas molecule gather in defects and cracks of silica sand, they can easily turn into the nuclei for sonication cavitation. The cavitation bubbles near the deffects and cracks will first grow by directional diffusion. As they reach the resonance size range, they collapse, resulting in the generation of a high temperature near the bubbles. The acoustic cavitation is also accompanied by other mechanical and physical effects, such as the formation of shock waves and turbulent motion of the liquid. Due to these sonication field effects, the wettability of the impurities was enhanced, and the interfacial tensions were broken during the acid leaching process, allowing the impurities, which are exposed in the defects and slits, to react with the acid solution^[7].

Figure 1. Reaction Mechanism of Acid Leaching under an Sonication Field

References:

[1] Yildirim, K., Cho, H., & Austin, L.G. 1999. The modeling of dry grinding of quartz in tumbling media mills. Powder Technology. 105,  210-221.

[2] Taxiarchou, M., Panias, D., Douni, I., Paspaliaris, I., & Kontopoulos, A. 1997. Removal of iron from silica sand by leaching with oxalic acid. Hydrometallurgy. 46,  215-227.

[3] Tyriakova, I., Tyriak, I., Malachovsky, P.L., Vec.era, Z., & Kolou.ek, D. 2007. Bacterial clay release and iron dissolution during the quality improvement of quartz sands, Hydrometallurgy. 89, 89-106..

 [4] Braga A.F., Moreira, S.P., Zampieri, P.R. Bacchin, J.M.G., & Mei, P.R. 2008. New processes for the production of solar-grade polycrystalline silicon, Solar Energy Materials & Solar Cells 92, 418–424.

[5] Feihu, D., Jingsheng, L., Xiaoxia, L., & Zhang, Z. 2010. Improvement of Iron Removal Silica Sand Using Sonication Assisted Oxalic Acid. Ultrasonics Sonochemistry. 18, 389-393.

[6] Veglio, F., Passariello, B., & Abburuzzese, C. 1999. Iron Removal Process for High-Purity Silica Sand Production by Oxalic Acid Leaching. Ind. Eng. Chem. Res. 38, 4443-4448.

 [7] Jian Zhang, Tingju, L., Xiadong, M., Dawei, L., Ning L., & Dehua L. 2009. Optimization of the Leaching Process by Using an Ultrasonic Field for Metallurgical Grade Silicon. Journal of Semiconductors. 30, 53002;1-6.