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 SO42- 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]: