Substitution of Calcium with Ce, Nd, Er, and Tb in the Structure of Microcrystals of Calcium Sulfates with Controlled Hydration Water: A Proposed Mechanism

Abstract

To interpret the mechanism of cocrystallization and sorptive properties of rare earth elements (REE) with calcium sulfate dihydrate (CSD) crystallization, synthetic crystals were prepared by the crystallization of CSD from solutions containing Ce, Nd, Er, and Tb as heavy and light REE. The obtained crystals were characterized using an array of characterization techniques, including X-ray diffraction, thermogravimetric analysis, differential thermal analysis, time-of-flight secondary ion mass spectrometry, high resolution scanning electron microscopy, and transmission electron microscopy. The results showed that the heavy REE are less prone to enter the CSD structure than the light REE. Further, the main mechanism of incorporation of REE in the CSD structure is via a surface adsorption process rather than an isomorphous substitution process into the CSD crystal lattice.

Behavior of Light and Heavy Rare-Earth Elements in a Two-Step Fe and Al Removal Process from Rare-Earth Pregnant Leach Solutions

“…As previously discussed in one of our studies, surface adsorption is the main mechanism of REE loss to CSD [20]. Therefore, REE could be partially recovered from the precipitates by an acid leaching process.…”

“…(3) adsorbs or retains a fraction of REE while precipitating, resulting in co-precipitation of REE with CSD. The reasons of the REE loss to CSD were thoroughly discussed in our previous papers [19,20].…”

“…As shown in Table 4, increasing the iron concertation from 0.5 to 4.0 g/L resulted in higher REE loss. This is caused by the larger amount of CSD precipitates that are formed when 4.0 g/L Fe is available in the solution which co-precipitates a higher proportion of the REE in the system by surface adsorption or by incorporation into the crystal lattice [16,20]. Another reason for the increased REE loss likely is the iron hydroxide precipitates in this pH range (1.0 to 3.2) which can adsorb REE species from the PLS [16].…”

“…In most of the flowsheets for REE PLS primary purification, the impurities are removed by a simple neutralization step where pH is adjusted using alkaline reagents such as lime, limestone, magnesium oxide, magnesium, or sodium carbonate [4]. Among the reagents, lime and limestone are the cheapest options; however, using calcium-based reagents in a sulfate media causes gypsum (also known as calcium sulfate dihydrate or CSD) formation and consequently REE loss to CSD [18][19][20].…”

“…In our previous studies [19,20], the effect of Ca(OH) 2 pulp density on REE loss was examined. Using a synthetic, pure solution of REE, the pH of the solution was elevated from 1.0 to 5.0, and the mechanisms of REE loss were investigated.…”