Improvement Of Iron Removal From Silica Sand Using Ultrasound-assisted Oxalic Acid

Silica sand is essential raw materials for glassmaking and a wide range of other industrial applications. However, iron impurities usually present in the silica sand render it completely unsaleable, or saleable at only at a reduced price. Therefore, considerable efforts have been devoted to the problem of removing impurities by physical, chemical, and biological methods. Sometimes various combinations of these methods are required to upgrade silica sand. Until now, inorganic acids are the most widely used reagents for the removal of impurities due to their high removal efficiency, 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 SO42- and Cl-. Thus, selecting eco-friendly reagent and avoiding low removal efficiency are the main effort in the silica sand research and development.This paper deals with the removal of iron from silica sand in aqueous oxalic acid, in the absence and presence of ultrasound. Some meaningful results were introduced as follows:1) Oxalic acid is found to be the most promising because of its low-cost, high environmental acceptability, good complexing characteristics and high reducing power. Furthermore, using oxalic acid, the dissolved iron can be precipitated from the leach solution as iron(Ⅱ) oxalate dehydrate, which can be represented a useful potential feedstock for added-value products.2) The conditions for the maximum removal by using probe based with ultrasound were determined as follows:reaction temperature,95℃; stirring speed, 500 rpm; ultrasound power,150 W; acid concentration,4 g/L; reaction time 30 min. Under these conditions, removal efficiency of iron from silica sand is 75.4%. It should be noted that the remaining iron impurities are in silica lattice and are extremely difficult to remove. 3) The method induces a remarkable acceleration for the iron leaching process, the leach acid concentration reducing dramatically and the removal efficiency increasing considerably, compared to conventional stirring method. Additionally, probe based with ultrasound is better than ultrasonic cleaning machine in iron removal.4) The advantages of ultrasound-assisted leaching were also confirmed by characterizing the silica sand and the leached solid samples using particle size, FESEM and color measurement analysis. When ultrasound is exerted on the system, the particles will become more porous, loose, and be easier to break down. This makes oxalic acid is able to penetrate into the silica sand matrix and undergo the reaction with iron impurities. Furthermore, a powerful impulse wave created by ultrasound can peel off iron impurities from silica sand into the liquid phase and then iron impurities can react with oxalic acid easily. Sonobleaching with oxalic acid may therefore be considered as a competing technology for industry.