Abstract:

Magnesium hydroxide sulfate hydrate (MHSH-512) whiskers with one-dimensional structure were synthesized by sol-gel-hydrothermal method. The obtained MHSH-512 whiskers were characterized by XRD and SEM. The effects of Mg(OH)₂ precursor, the concentrations of Mg²⁺, \mathrm{S}{\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}-} and Cl^- together with the hydrothermal conditions on the growth and morphology of MHSH-512 whiskers were systematically investigated. In addition, the growth mechanism of MHSH-512 whiskers before and after removal of Cl^- ions was also proposed. The results indicate that the freshly prepared Mg(OH)₂ gel with good dispersibility and high reactivity is liable to form [Mg(OH)₆]^4- during the subsequent dissolution and recrystallization under hydrothermal conditions. A moderate excess of \mathrm{S}{\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}-} ions play an important role in the formation and growth of MHSH-512 whiskers. With a concentration ratio of c(Mg²⁺) to c(\mathrm{S}{\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}-}) of 3, MHSH nuclei can be formed between \mathrm{S}{\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}-} and all the structural units [Mg(OH)₆]^4- by Mg-O-S bonds. Massive Cl^- ions in the solution may hinder the mutual interaction between OH^- and Mg²⁺ and delay the formation of [Mg(OH)₆]^4-. At the sametime, it competes with \mathrm{S}{\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}-} on the surface of [Mg(OH)₆]^4-, which cause the whisker growth to lag, but has no substantial effect on the final morphology and dispersion of whiskers. Nevertheless, the influence of Cl^- on the morphology and dispersibility of the end product can be negligible. The synthesized whiskers exhibit excellent properties including high crystallinity and aspect ratio (150—200), smooth surface and good dispersibility. It has been determined that the optimal hydrothermal treatment is reacting at 190℃ for 10 h with c(Mg²⁺)/c(\mathrm{S}{\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}-}) of 3, which is easy to realize industrial-scale production of MHSH-512 whiskers. This research provides an effective approach for high-value utilization of magnesium resources in salt lake area of Qinghai.

Key words: magnesium hydroxide sulfate hydrate whiskers, sol-gel method, hydrothermal treatment, controllable synthesis, ions, nucleation, effect mechanism

摘要：

以氯化镁、氢氧化钠和硫酸镁为原料，采用溶胶-凝胶-水热法制备了碱式硫酸镁（MHSH-512）晶须，研究了前驱物Mg(OH)₂、Mg²⁺和\mathrm{S}{\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}-}的浓度和杂质离子Cl^-对MHSH-512晶须的生长机制和微观形貌的影响。研究表明，溶胶-凝胶法得到的Mg(OH)₂在水热条件下的溶解再结晶过程中易形成结构单元[Mg(OH)₆]^4-。当c(Mg²⁺)/ c(\mathrm{S}{\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}-})为3时，几乎全部的[Mg(OH)₆]^4-可与\mathrm{S}{\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}-}形成晶核，并以b轴为主轴生长为MHSH-512一维晶须。大量Cl^-的存在可能阻碍OH^-与Mg²⁺相互作用，延缓[Mg(OH)₆]^4-生成，同时与\mathrm{S}{\mathrm{O}}_{\mathrm{4}}^{\mathrm{2}-}在[Mg(OH)₆]^4-表面发生竞争吸附，导致晶须生长滞后，但对晶须的最终形貌和分散性没有实质影响。190℃下水热反应10 h制得的MHSH-512晶须结晶度高、长径比大（150～200）、表面光滑且分散性好，易于实现工业化生产，为青海盐湖镁资源的高值化利用提供了一条有效途径。

关键词: 碱式硫酸镁晶须, 溶胶-凝胶法, 水热处理, 可控制备, 离子, 成核, 影响机制