The crystallization of sodium oxalate (Na2­C2O4) crystals was studied in the presence of different inorganic anions of the Bayer liquor in the alumina industry, including OH–, CI–, Al­(OH)4 –, CO3 2–, and SO4 2–. The morphology of the obtained Na2­C2O4 crystals depends on the type and concentration of anion species under similar oxalate supersaturation. The presence of divalent anions (CO3 2–, SO4 2–) has a greater influence on the morphological variability, including cyclic twins, spherulites, and double-leaf structures, through the branching growth process. The specific surface areas of Na2­C2O4 crystals grown in the presence of divalent anions (CO3 2–, SO4 2–) are more than 2 times higher than those in the presence of monovalent anions (OH–, CI–, Al­(OH)4 –). X-ray photoelectron spectra results indicate that the inorganic anions (CI–, Al­(OH)4 –, CO3 2–, SO4 2–) are adsorbed onto the surface of Na2­C2O4 crystals, thereby inducing internal stress (defects) of crystals and modifying the crystal morphology.

The shape changes of sodium oxalate (Na2C2O4) crystals in the presence of polyacrylic acids (PAA) with different molecular weights (1800, 5100, 450 000, 1 250 000 g/mol) and propionic acid (74 g/mol) was investigated in a highly alkaline solution (5 M NaOH). Over the entire concentration range tested (0–100 mg/L), propionic acid had no influence on Na2C2O4 shape, indicating that the number of carboxylate functional groups is not the vital factor for Na2C2O4 shape control, despite playing a role in the adsorption. In contrast, with increasing PAA concentrations, Na2C2O4 crystals were significantly modified through distinct shape changes proceeding through a coexisting family of rods, bundles, flower-like structures, irregular spherulites, and eventually spherulites. PAA anions with medium molecular weights (1800, 5100 g/mol) were the most effective in promoting the spherulitic shape. Thus, the mechanisms of Na2C2O4 shape changes can be attributed to PAA adsorption on Na2C2O4 surfaces and their subsequent incorporation into the Na2C2O4 crystal matrix. PAA adsorption is responsible for the inhibition of step train growth on prismatic {110} and {200} faces and nuclei propagation on their top {001} faces. PAA incorporation can lead to multistep-branching growth based on a noncrystallographic crystal branching model.

To mimic the process of gibbsite nucleation on sodium oxalate crystals and their intergrowth in alumina Bayer process, such a coprecipitation process has been broken down into three distinct stages and studied by in situ AFM and ex situ SEM under static conditions. In the first stage, a solution saturated in aluminate and supersaturated in oxalate has been used for investigating the oxalate growth on its {110} face, which is controlled by a layer-by-layer mechanism. In the second stage, in a solution saturated in oxalate and supersaturated in aluminate, gibbsite secondary nucleation was preferred along macrosteps of sodium oxalate {110} faces and appeared as two-dimensional (2D) islands with pisolitic shapes. There does not appear to be a preferred epitaxial relationship at the heterointerface between gibbsite crystals and sodium oxalate substrates, indicating that the magnitude of the surface free energy for nucleation is greater than that associated with strain energy caused by lattice mismatch. In the third stage following gibbsite secondary nucleation, in a solution saturated in aluminate and supersaturated in oxalate, the inclusion of gibbsite crystallites into the bulk of sodium oxalate crystals through step growth on {110} faces of sodium oxalate was observed.

In situ atomic force microscopy (AFM) was used to study the growth mechanism of the gibbsite basal (001) face and prismatic (100) face in high ionic strength () and highly alkaline solution (pH=13.4). The height of the steps on the gibbsite (001) faces can vary from less than one nanometre (elementary steps) to several nanometres (microsteps) and finally to dozens of nanometres (macrosteps). The lateral advancement rates of micro/macrosteps arerelated to the height of steps. Step coalescence occurs during lateral advancement, driven by Ostwald ripening. There are three possible pathways for the formation of micro/macrosteps on basal (001) face, all of which are controlled by layer-by-layer growth mechanism. Gibbsite structure suggests that the frequent and random formation of the elementary steps on basal (001) face can be attributed to weak inter-layer hydrogen bonds along the c-axis of gibbsite unit cell. The prismatic (100) faces are characterised by microstep trains along gibbsite [001] direction. These microsteps grow higher along gibbsite [100] direction, advance laterally and step coalescence also occurs. Two dimensional islands often form on the terrace adjacent to the edges of micro/macrosteps, suggesting that the step edge should be a preferred site for island nucleation due to the Ehrlich-Schwoebel effect.

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