【作者】 米阳；

【导师】 陈德玉；

【作者基本信息】 西南科技大学 ， 材料科学与工程， 2019， 硕士

【摘要】 磷石膏是湿法技术生产磷酸过程中,硫酸分解磷矿时产生的一种以二水硫酸钙为主要成分的副产物。我国磷石膏已经成为著名的大宗工业固体废物,排放量达7000⁸000万吨/年,从环境保护和资源利用角度而言,急需规模化消纳和利用。除了二水石膏以外,石膏相态还包括半水石膏（α型和β型）和无水石膏。其中,二水石膏几乎不具有胶凝性,利用价值低,而α-半水石膏是一种重要的胶凝材料,因其性能优越被广泛用于高档建材、铸造模具、骨修复等领域。因此,以二水石膏为原料制备α-半水石膏,不但可提高我国石膏资源应用价值,也是化学石膏高附加值资源化的有效途径。常压盐溶液法利用二水石膏制备α-半水石膏,反应条件温和、反应过程和产品质量易于控制,是目前最具前景的α-半水石膏制备技术。为实现磷石膏制备α-半水石膏,结晶介质常选用为以CaCl₂水溶液为代表的氯盐体系,但是氯离子会造成设备加速腐蚀等问题。本课题提出了常压无氯盐溶液中磷石膏制备α-半水石膏的新技术,以Ca（NO₃）₂取代氯盐,探究了盐溶液浓度和温度的影响,重点研究了含磷杂质对转化过程和产物特性的影响以及作用机理,并对α-半水石膏晶体形貌和反应过程调控进行了深入的探究。在常压盐溶液中,二水石膏能否转化为α-半水石膏只取决于水的活度。水活度降低可促使二水石膏向α-半水石膏转化。电解质的加入可以降低水的活度,满足转化的热力学条件。本实验在浓度3.3³.8mol/L Ca（NO₃）₂水溶液中,温度91¹02℃及常压条件下,成功地将磷石膏转化成α-半水石膏。增加Ca（NO₃）₂浓度或者提高温度均可加快磷石膏到α-半水石膏转化速率。在实验浓度条件下,磷石膏中的磷酸盐杂质,即Ca（H₂PO₄）₂·H₂O、Ca（H₂PO₄）₂·H₂O和Ca₃（PO₄）₂会延长整个二水石膏脱水转化过程。加入0.21wt.%H₃PO₄促进整个转化过程;含量增加到0.41wt.%以后,H₃PO₄对转化过程表现出抑制作用。含磷杂质对α-半水石膏成核诱导期抑制作用比对晶体生长的抑制更为显著。除Ca₃（PO₄）₂外,其余磷杂质亦会对α-半水石膏晶体形貌产生重要影响,得到的α-半水石膏晶体为更粗大的柱状体,进而对产物颗粒的粒度分布产生影响。此外,较高含量的Ca（H₂PO₄）₂·H₂O和CaHPO₄·2H₂O还会诱导部分α-半水石膏发生相变生成无水石膏。对于α-半水石膏而言,机械强度往往是决定应用价值的一个首要指标,而机械强度与α-半水石膏晶体形貌密切相关。在Ca（NO₃）₂溶液中,丁二酸对α-半水石膏形貌有明显调控作用,随着丁二酸掺量由0增加到0.4%,得到的α-半水石膏晶体由长棒状变为碟状,长径比范围由4.5⁸.5减小至0.3⁰.5。丁二酸还会降低磷石膏的脱水速率,但通过加入适量Na₂SO₄可以缓解甚至消除丁二酸引起的延迟效应。当丁二酸掺量由0增加到0.3%时,磷石膏的完全转化时间由5.0h延长至6.5h;加入1.2%Na₂SO₄后,磷石膏的完全转化时间缩短至约3.75h。通过对比浆体强度,发现掺入0.3%丁二酸时,得到的α-半水石膏长径比约为1.0,颗粒的特征粒径为21.3μm,标准稠度用水量为42%,对应的硬化浆体具有最高强度33MPa。更多还原

【Abstract】 Phosphogypsum（PG）is a waste by-product that is composed mainly of calcium sulfate dihydrate（CaSO₄·2H₂O,CSD）produced during the production of wet process phosphoric acid through dissolving phosphate ore with sulfuric acid.In China,PG has become a famous bulk industrial solid waste and the current production is estimated to be around 70⁸0 Mt per year.It demands urgent consumption as a useful resource in large quantities from the point of environmental protection and utilization.Besides CSD,gypsum also exists in the phases of hemihydrate（CaSO₄·0.5H₂O,CSH,α-andβ-form）and anhydrite（CaSO₄,AH）.Contrary to the low value of CSD and AH,CSH owns superior performance as an important class of cementitious material and has been widely used in modern building materials,molding,bone remedy and other fields.Therefore,the preparation ofα-CSH from CSD shows a great economic benefits and market prospect to improve the grade and value of the gypsum resource and promote the high-value-added utilization of chemical gypsum.Salt solution method is the most promising technology,which features mild conditions,excellent control of the phase-transition process and stable product quality.The preparation ofα-CSH from CSD is commonly conducted in chloride solutions which lead to severe erosion of equipment.A novel method was invented to prepareα-CSH from PG in this paper.Simultaneously,effects of salt concentration,reaction temperature and phosphorus impurities on the on the transformation kinetics and crystal morphology were studied.Moreover,controlling the crystal morphology ofα-CSH and tuning the corresponding phase transformation were deeply investigated.In an aqueous solution,water activity is the only determinant of the feasibility of the transformation from CSD intoα-CSH,and decreasing water activity can promote the phase transformation.The electrolyte aqueous solution has a lower water activity and thus is thermodynamically required for the transformation.In this experiment,in the Ca（NO₃）₂aqueous solution with a concentration range of 3.3³.8mol/L,α-CSH was successfully prepared from PG at 91¹02 ^oC under atmospheric pressure.Increasing both Ca（NO₃）₂concentration and temperature can accelerate the dehydration rate of PG.The phosphates including Ca（H₂PO₄）₂·H₂O,Ca（H₂PO₄）₂·H₂O and Ca₃（PO₄）₂ show totally retarding effect on the phase transformation of CSD toα-CSH under experimental conditions.The presence of 0.21 wt.%H₃PO₄ accelerates the whole phase transformation.Further increase in H₃PO₄ content to 0.41 wt.%retards the phase transformation.The inhibitory effect of phosphorus impurities on the induction period ofα-CSH is more significant than that on the crystal growth.Except for Ca₃（PO₄）₂,all impurities studied were found to result in larger diameters of the as-formedα-CSH as well as bigger particle fineness.Furthermore,the presence of Ca（H₂PO₄）₂·H₂O or CaHPO₄·2H₂O at high concentrations induces partial phase transformation fromα-CSH to AH.The mechanical strength ofα-CSH paste is a crucial index of its application prospect,which is morphology-dependent.In the Ca（NO₃）₂ solutions,the morphology ofα-CSH can be controlled effectively in the presence of succinic acid.As the concentration of succinic acid increases from 0 to 0.4%,the obtainedα-CSH crystals change from long rods to plates and the corresponding aspect ratio range shrinks from 4.5⁸.5 to 0.3⁰.5.It is also found the the presence of succinic acid decreases the dehydration rate of PG,which can be alleviated or even reversed by adding Na₂SO₄.When the concentration of succinic acid increases from 0to 0.4%,the time required to complete the dehydration process of PG prolongs from 5.0 h to6.5 h.However,the time decreases to 3.75 h in the presence of 0.2%Na₂SO₄.From a comparison of mechanical strength,it is found that in the presence of 0.3%succinic acid,the obtainedα-CSH with a aspect of¹.0 and characteristic particle size of 21.3μm shows 42%water requirement for standard consistency and the corresponding paste exhibits the largest mechanical strength of 33 MPa.更多还原