Role of the Amorphous Phase during Sodium Aluminosilicate Precipitation

Abstract

In the Bayer process, reactive silica associated with bauxite dissolves into alkaline solution and subsequently precipitates as a sodium aluminosilicate “desilication product” (DSP). Multiple DSP phases can be formed, which include an amorphous solid as well as crystalline zeolite A, sodalite, and cancrinite. Even though the ability to control DSP particle size would be of great practical benefit, there remains limited fundamental understanding of the precipitation process, especially during the early stages of the reaction. In this study, DSP was precipitated from synthetic NaOH–NaAl­(OH)4–Na2SiO3–H2O solution at 75 and 90 °C. Four distinct reaction stages were observed: (1) precipitation of the amorphous solid, (2) dissolution of the amorphous solid, (3) growth of crystalline phases, and (4) maturation of the crystalline phases. In addition to the amorphous phase, the nucleation of both particles and two-dimensional crystal planes occurs in the early stages of the reaction. Subsequently, the amorphous phase dissolves, which feeds the early growth of the crystalline DSP phases. After the amorphous phase disappears, the crystal phases continue to grow with reactants from the bulk solution until equilibrium is approached. In the maturation stage, the solution silicate concentration decreases only slightly; however, during this time the sizes of individual crystals decrease slightly while the overall particle size increases via crystal agglomeration, which is promoted at higher temperature.

Electrolytic iron production from alkaline suspensions of solid oxides: compared cases of hematite, iron ore and iron-rich Bayer process residues

“…This phase rapidly dissolves while forming crystalline phases, e.g. cancrinite whose solid entities gradually Arbitrary units mature in the alkaline solution during the Bayer process [15,16]. Calcium in the red mud is also visible as calcite and calcium titanium oxide (perovskite) CaTiO3, as shown in Figure 2.…”

“…Traces of kaolinite remaining in the red mud after the Bayer process -not quantified -might dissolve in the course of electrodeposition tests. However, the single silicate and aluminate ions produced are also to crystallize in the formation of little soluble cancrinite and sodalite as explained above [15,16].…”

A new method for recovering rare earth elements from the hyperaccumulating fern Dicranopteris linearis from China

“…In addition to sodalite and cancrinite, DSP include the linde aluminosilicates type A and N zeolite, both of which exhibit Na12[Al12Si12O48]•27H2O stoichiometry (Peng et al, 2018a;Peng and Vaughan, 2018). It should also be noted that their mechanism of formation includes the intermediate precipitation of an amorphous aggregate that progressively dissolves to form crystalline DSPs (Peng et al, 2018b).…”

The Sandy Desilication Product Process Concept

“…The main desilication product for atmospheric-pressure reactors and lowtemperature pressure digestion reactors is the sodalite phase, which may involve an intermediate amorphous precipitate. 7,8 The overall reaction for the precipitation of sodalite from Bayer liquors is described by Eq. 2, 9,10 The amount of associated water can vary depending on the precipitation conditions and drying procedures prior to analysis.…”