Structural and Distribution of Impurities in the Fouling in Wet-Process Phosphoric Acid at T = 80°C

Abstract

In this investigation, the mineral deposits formed in the digestion step in wet phosphoric acid production were studied. The mineralogical characterization of the fouling precipitates obtained at the temperature 80°C from the digestion step in 16-monthly production cycle was carried out. Observing successive layers, based on their colors, allowed us to distinguish fouling layers. Detailed structural aspects, and subsequently, chemical and physical properties of the fouling were determined using complementary analytical and characterization techniques. Particle size fractionation was then used to gain each fouling layer sample homogenized on the micro-scale (63μm). The major elements in the fouling were determined to be silica (Si₂O) and sodium (Na). Some chemical elements of major impurities, as well as traces, K, Al, Mg, Fe, Cu, and Zn, were detected using XRF which cannot appear in their proper solid phases. The obtained solid phases in different fouling layers are principally composed of the malladrite and gypsum minerals. The distribution of impurities into the present structures was investigated. Indeed, the mesh parameters of malladrite in the fouling layers were calculated and compared with those of the pure phase, and a good concordance is obtained. The FT-IR spectra confirm the presence of mallardite and gypsum phases and reveal a supplementary band that has been assigned to C-O vibrations in acids, esters, or ethers. The appearance of this band indicates the possible presence of organic matter within the fouling layer.