Crystalline-selected synthesis of ferrous oxalate dihydrate from spent pickle liquor

Ferrous oxalate dihydrate with different crystalline structures, polymorphs and sizes have been synthesized through a facile precipitation reaction from spent pickling liquors (SPL) by controlling synthesis conditions. The beta-FeC2O4 center dot 2H(2)O in orthorhombic phase can be formed at room temperature, whereas alpha-FeC2O4 center dot 2H(2)O with a monoclinic unit cell can be obtained after aging at 80 degrees C. With the increasing aging temperature, the morphology of samples can be tailored from flower-like to prismatic-like and the particle-sizes are decreased. Innovatively, thermal decomposition behaviors of products have been investigated in details by X-ray diffraction pattern (XRD) and thermal gravimetric-differential scanning calorimetry (TG-DSC). The results show that both FeC2O4 center dot 2H(2)O samples can be transformed to alpha-Fe2O3 in air atmosphere and Fe3O4 in argon atmosphere at T >= 400 degrees C. In air atmosphere, there are two sharp exothermic peaks for beta-FeC2O4 center dot 2H(2)O (250 degrees C) and alpha-FeC2O4 center dot 2H(2)O (239.5 degrees C) correspond to the decomposition reactions. In argon atmosphere, the decomposition end temperatures are 500 degrees C and 430 degrees C for beta-FeC2O4 center dot 2H(2)O and alpha-FeC2O4 center dot 2H(2)O, respectively. This indicates that beta-FeC2O4 center dot 2H(2)O has higher thermal stability in comparison with alpha-FeC2O4 center dot 2H(2)O.