Effect of HF, H2SiF6, and Al3+ ions on the crystal growth process and morphology of α-type hemihydrate calcium sulfate in phosphoric acid solution

The reaction of calcium dihydrogen phosphate solution with sulfuric acid in the presence or absence of HF, H2SiF6, and Al3+ ions was used to simulate the crystal growth process of alpha-type hemihydrate calcium sulfate (alpha-HH) with 30% P2O5 at a reaction temperature of 95 degrees C. The changes in Ca2+ ion concentration in the solution were measured at different reaction times. Then, the growth rates and reaction steps of alpha-HH crystals were fitted using the crystal growth kinetic equation. The growth morphologies of alpha-HH crystals were characterized using scanning electron microscopy, X-ray diffractometry, X-ray photoelectron spectrometry, and differential scanning calorimetry, and the influence mechanism of HF, H2SiF6, and Al3+ ions on the crystal growth process of alpha-HH were deeply investigated. Results show that the addition of HF, H2SiF6, and Al3+ ions decreased the growth rate constants of alpha-HH. Without HF, H2SiF6 and Al3+ ions, the growth rate constants of alpha-HH was 0.7452 x 10(-3) (mol center dot min(-1)center dot m(-2)). When 0.060 mol center dot L-1 HF was added, the growth rate constants of alpha-HH decreased to 0.4954 x 10(-3) (mol center dot min(-1)center dot m(-2)). When 0.015 mol center dot L-1 Al3+ ions and 0.060 mol center dot L-1 HF were added, the growth rate constants of alpha-HH decreased to 0.4548 x 10(-3) (mol center dot min(-1)center dot m(-2)). The inhibition of alpha-HH growth by HF, H2SiF6, and Al3+ ions can be ranked as follows: HF, H2SiF6, and Al3+ ions > HF and Al3+ ions > H2SiF6 > HF. The reaction steps of alpha-HH in phosphoric acid solution in the presence or absence of HF, H2SiF6, and Al3+ ions were close to 2, and the growth of alpha-HH crystals was mainly affected by surface diffusion, adsorption, and helical growth mechanism. HF, H2SiF6, and Al3+ ions inhibited the growth of alpha-HH crystals along the C-axis, resulting in a decrease in aspect ratio, and the morphology of alpha-HH crystals is short columnar. These impurity ions decreased the intensity of diffraction peaks and crystallinity of the (200), (310), and (400) crystal planes of the alpha-HH crystals.