Effect of trace metals on reactive crystallization of gypsum

Abstract

The effect of Fe 2+, Fe 3+ , and Cr 3+ ions on crystallization of calcium sulfate dihydrate (gypsum) produced by the reaction between calcium hydroxide suspension and sulphuric acid solution was investigated at 3.5 pH and 65°C in the absence and presence of 2500 ppm citric acid concentration. Crystal size distributions, filtration rates, and morphology of gypsum were determined and discussed as a function of ion concentration. Average particle size of gypsum was not affected significantly by the presence of Fe 2+, Fe 3+, and Cr 3+ ions individually. Variation of gypsum morphology depending on ion concentration affected the filtration characteristics. The presence of Fe 3+ or Cr 3+ ions besides 2500 ppm citric acid influenced both average particle size and filtration characteristics. The effect of citric acid on gypsum morphology was suppressed at high Fe 3+ and Cr 3+ ion concentrations. The change of morphology is related to the complex formation between Fe 3+ or Cr 3+ ions and citric acid at high ion concentrations.

Gypsum Crystallization in the Presence of Cr3+ and Citric Acid

“…As explained in literature, the structure of the complex between Cr 3+ and citric acid contains discrete Cr(Cit) 2 2-units in which the chromium atom is octahedrally coordinated by two tridentated citrate anions [9]. It has been reported that gypsum crystals had a bimodal CSD distribution depending on the Cr 3+ ion concentration [15]. This bimodal CSD behavior was attributed to the crystal breaking at low Cr 3+ ion concentrations and the complex formation between Cr 3+ ions and citric acid resulted in the precipitation of differently formed gypsum crystals at high Cr 3+ ion concentrations.…”

“…In literature, it has been reported that a trace amount (ppm level) Cr 3+ retards the crystal growth rate of various salts in aqueous solutions [14]. In our previous study, we showed that the presence of Cr 3+ ions influenced both average particle size and filtration characteristics at citric acid concentrations over 2500 ppm [15]. In the same study, it was also shown that the effect of citric acid on gypsum morphology was suppressed at high Cr 3+ ion concentrations and the change of morphology was related to the complex formation between Cr 3+ ions and citric acid at high ion concentrations.…”

Geochemical diagenetic trends during phosphorite formation – economic implications: The case of the Negev Campanian phosphorites, Southern Israel

“…However, other components are also important – for example: (i) high CaO/P 2 O 5 in phosphate rocks results in more sulphuric acid consumption during H 3 PO 4 manufacture (Lauriente, 1996b) and in an increased production of phosphogypsum (a harmful by‐product of H 3 PO 4 manufacture); (ii) organic matter (OM) causes foaming problems during wet fabrication of H 3 PO 4 (Theys, 2003) leading to acid loss. Organic matter also hampers suitable crystallization of the precipitated phosphogypsum causing low phosphoric acid filtration (Sayan et al. , 2007); (iii) fluorine (F) causes corrosion problems and F emissions during the sulphuric acidulation of the phosphate rocks (van der Loo & Weeda, 2000).…”

“…However, other components are also important -for example: (i) high CaO/P 2 O 5 in phosphate rocks results in more sulphuric acid consumption during H 3 PO 4 manufacture (Lauriente, 1996b) and in an increased production of phosphogypsum (a harmful by-product of H 3 PO 4 manufacture); (ii) organic matter (OM) causes foaming problems during wet fabrication of H 3 PO 4 (Theys, 2003) leading to acid loss. Organic matter also hampers suitable crystallization of the precipitated phosphogypsum causing low phosphoric acid filtration (Sayan et al, 2007); (iii) fluorine (F) causes corrosion problems and F emissions during the sulphuric acidulation of the phosphate rocks (van der Loo & Weeda, 2000). On the other hand, F contributes to form [AlF n ] (3)n)+ complexes in the acid solution (Va-lancius et al, 2005), causing a clustering of gypsum crystals and thereby improving phosphoric acid filtration and recovery; (iv) trace metals (Cd, Cr and Fe) are also known to affect the nucleation rate and crystal morphology of gypsum (Tinge, 2004;Hamdona & Al Hadad, 2007) influencing phosphoric acid recovery (Morse, 1983;de Vreugd et al, 1994); (v) silica (chert) increases the levels of insoluble materials during H 3 PO 4 fabrication causing erosion problems in the reactor; (vi) high Mg levels increase the slurry viscosity and decrease its filterability (Theys, 2003); (vii) sulphur (when present as sulphides) causes processing problems, especially during the manufacture of sodium phosphate; (viii) low CO 3 2) substitution for PO 4…”

A formulation additive for simultaneously improving flue gas desulfurization efficiency and gypsum quality

“…Filtration characteristics of gypsum like particle size distribution (PSD) and crystal structure are important for economic considerations [11][12][13][14], since these characteristics are closely related with energy consumption during the filtration process and moisture content of the final gypsum [15]. In this regard, the formation of large and uniform size particles is desired [16]. Therefore, to guarantee good gypsum quality, and to reduce the cost for separation, it is necessary to improve the crystal structure of gypsum.…”