Effects of crystal habit modifiers on the morphology of calcium sulfate dihydrate grown in strong CaCl₂‐HCl solutions

Abstract

BACKGROUND This work deals with the effect of several additives on the morphology of calcium sulfate dihydrate (DH) crystals, produced by reactive crystallization involving strong CaCl2–HCl solutions. Experiments were conducted at 40 °C under various conditions, namely homogeneous and seeded (heterogeneous) crystallization in CaCl2/HCl solutions ranging from very dilute (0.42 mol L−1/0.04 mol L−1) to very concentrated (1.8 mol L−1/6.3 mol L−1). RESULTS In the absence of additives a distinct difference in crystal morphology was observed depending on solution composition. Homogeneously formed DH crystals from dilute solution were in the form of needles. DH crystals formed in concentrated acid solution were thick elongated plates. The most influential additives in concentrated solutions were CTAB (cetyltrimethylammoniumbromide), PAA (polyacrylic acid) and PVS (polyvinyl sulfonic acid). CTAB favoured formation of industrially interesting thick slab crystals. In contrast PAA favoured the formation of elongated and thinner crystals as a result of preferential crystal plane adsorption. PVS had a detrimental effect on DH crystal nucleation and growth explained by its polymeric structure. CONCLUSION Apart from PVS, CTAB and PAA no influence was seen for the majority of additives tested. This was attributed to lack of ionization caused by the highly acidic solutions. © 2013 Society of Chemical Industry

Effect of CuCl2 on hydrothermal crystallization of calcium sulfate whiskers prepared from FGD gypsum

“…To improve crystallization of hydrothermal products, several chemical agents have been added as a crystal modifier to the reaction solution to control the crystallization [9][10][11]. Presence of certain metal ions or organics has been reported to have effects on crystallization and aspect ratio of the hydrothermal products [3][4][5][10][11][12][13][14][15][16]. To find a crystal modifier which can promote crystallization of CSWs and increase the aspect ratio, when FGD gypsum is used as raw material, is of great interests.…”

“…As discussed above, the previous studies generally focused on the effects of crystal modifier on crystal morphology and size of products [1][2][3][4][5][10][11][12][13][14]. The objectives of this investigation were to (1) further increase the aspect ratio of CSWs prepared from FGD gypsum, (2) reveal their crystallization mechanism and (3) verify that the produced CSWs were of monocrystal type.…”

Estudo da co-cristalização em sistema ternário NaCI-CaSO4-H2O para dessalinização de água.

“…No entanto, inúmeros processos, tais como a produção de ácido fosfórico ou fertilizantes fosfatados, produção hidrometalúrgica de zinco e cobre, recuperação de gás natural e dessalinização de efluentes salinos e água do mar são influenciados pelas distintas formas de cristalização deste sal e suas propriedades. Neste contexto, no último século, um esforço tem sido direcionado para o entendimento do processo de hidratação e desidratação do sulfato de cálcio, além do controle dos processos elementares no seu processo de cristalização e da morfologia dos cristais formados (Feldmann, 2013 Os três polimorfos do sulfato de cálcio possuem diferentes morfologias e isso influencia diretamente nas taxas de filtração de suspenções contendo estes sais. Os cristais de sulfato dihidratado apresentam maiores taxas de filtração, se comparados com o hemihidrato e anidrita.…”

“…As sementes de hemihidrato foram preparadas conforme metodologia descrita por Feldmann (2013). Utilizou-se um reator cilíndrico vertical de vidro borossilicato com fundo abaulado, tampa e camisa de troca térmica.…”

“…Esta fração foi utilizada pois apresenta uma maior proporção de cristais de sulfato de Como a taxa de crescimento do NaCl é maior que a taxa de crescimento do sulfato de cálcio (Mersmann, 2001;Feldmann, 2013) Foi observado também que o teor de hemihidrato nas faixas grosseiras é relativamente pequeno. Estas faixas representam a maior parte da massa do produto.…”