【作者】 杨敏；

【导师】 钱觉时；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2008， 博士

【摘要】 磷石膏是生产磷酸所产生的一种工业废渣,其CaSO4·2H2O含量非常高,但目前未能得到有效利用,主要原因在于杂质对其应用性能会产生不利影响。杂质的影响随着石膏相的不同存在差异,因此,探究杂质对不同相磷石膏性能的影响是磷石膏预处理及资源化应用的一项重要工作。通过常规化学分析并结合采用原子吸收光谱(AAS)、X射线衍射光谱(XRD)、扫描电镜分析(SEM)和差示扫描量热分析(DSC)等微观测试手段结合物理力学性能试验,对原状磷石膏的性质进行了研究,结果表明:由于形成条件的不同,二水磷石膏的晶体形貌和颗粒分布与天然石膏存在较大差异;由于杂质成分的不同,磷石膏和天然石膏的溶解性存在一定差异;可溶杂质和有机物等的存在使磷石膏从二水相转变成半水相的第一次脱水温度较天然石膏低20~35℃不等。磷石膏用作缓凝剂对水泥性能的影响主要是由于杂质对磷石膏溶解和脱水性能的影响引起的。溶解性试验结果表明磷石膏在饱和石灰溶液中的溶解速率和溶解度随着温度升高而降低,因此当掺磷石膏的水泥水化温度变化时,溶液中溶解的硫酸钙浓度随之改变,磷石膏的缓凝作用受到影响。磷石膏在70~130℃的水泥粉磨温度范围内会出现明显的二水相向半水甚至无水相的转变。将磷石膏置于不同温度烘箱内恒温30min处理后取出进行相分析,结果显示磷石膏在70℃的处理温度下脱水产生了4%左右的半水相,天然石膏中则未能见半水或无水相;随着温度的升高,半水相含量增加,到130℃时磷石膏中半水相含量已经达到55%左右,并出现了少量无水相;天然石膏中半水相含量在7%左右。半水石膏的存在会造成水泥发生闪凝现象,因此二水石膏的脱水不利于水泥的凝结硬化。常规的预处理如水洗和石灰中和无法消除磷石膏中的共晶磷,共晶磷在温度升高时发生分解,降低二水石膏脱水温度,即使经过水洗处理后,磷石膏在90℃仍部分脱水生成3%左右的半水石膏,到了130℃时半水石膏生成量约为11%,因此预处理或原样磷石膏在用作水泥缓凝剂时必须注意石膏的溶解和脱水问题。通过对比试验,研究了天然石膏和磷石膏制备的半水石膏在相同石膏相组成条件下的性能。半水天然石膏初凝时间一般在7min左右,而半水磷石膏中由于杂质的存在,凝结时间延缓且硬化体强度降低,杂质对半水磷石膏的影响主要是可溶性杂质与钙离子在水化时生成难溶物质覆盖在二水石膏晶体表面,降低二水石膏的析晶饱和度,即杂质具有半水石膏常用缓凝剂的作用。对于纯半水石膏,杂质的延缓作用因其含量的不同而异,在本研究中杂质对凝结时间可延缓1倍以上。对凝结非常迅速而不利于施工的半水石膏而言,杂质的缓凝具有利用价值。与建筑石膏常用缓凝剂柠檬酸相比,在对强度影响程度相当的情况下,杂质的缓凝效应相对要小一些。对比试验结果显示,杂质对无水磷石膏的水化硬化影响较小,原因在于无水石膏水化非常缓慢,而杂质与钙离子的中和反应则非常迅速,因此无水石膏水化时,水化生成物有足够的时间冲破中和反应生成的难溶物质,从而其水化速率和水化产物的生长受到的影响较小。在无水和半水混合相磷石膏中,杂质对半水磷石膏水化硬化的影响对无水磷石膏的水化反而有利,磷石膏作为无水和半水混合相石膏的原料具有一定优势。本研究表明,杂质对不同相磷石膏的应用性能有不同程度的影响,虽然采用水洗和中和的预处理方式可一定程度上改善不同相磷石膏的一些特性,但从本文的研究结果来看,这些预处理手段并不能有效改善不同相磷石膏的应用性能。从不同相磷石膏的一些应用形式来看,杂质对磷石膏应用性能的不利影响是可以接受的,甚至能够加以利用,从不同角度正确认识杂质的影响机理对磷石膏的应用至关重要。更多还原

【Abstract】 Phosphogypsum(PG), an industrial byproduct of phosphoric acid manufacture, has high content of CaSO4·2H2O. However, the utilization ratio of PG is very low due to the negative effect of impurities on the properties of PG plaster. The present studies on PG of its application in cement and plaster revealed that impurities in PG have harmful effects on the setting of cement and gypsum plasters. It is therefore, thought to be essential to beneficiate PG to reduce harmful impurities in it. In fact, effects of impurities on PG differ with differences in the form, type of gypsum and its applied cementing systems. For some binding materials, influences of impurities may be ignored. Pretreatments of PG should be different according the form of gypsum and its applied systems.The constitution of PG and natural gypsum(NG) was investigated by atomic adsorption spectra(AAS) in this study and by a combination of x-ray diffraction(XRD), scanning electron microscope(SEM), differential scan calorific(DSC) and tests on chemical and physical performance, the properties of raw PG were studied. The results show: PG differs significantly from natural gypsum in crystal morphology and grading due to differences in formation process between them. PG has distinct solubility compared with NG. Thermal behaviour of NG and PG was determined and transformation temperature of dihydrate to hemihydrate of PG is 20~35℃lower than that of NG and washed PG, which indicates that water soluble impurities reduce the dehydration temperature of dihydrate.The effect of PG on the performance of cement is mainly due to the influcens of impurities on the solubility and dehydration behaviour of PG. The results of test on solubility of PG showed that with increases in ambient temperature, the solubilization rate and solubility limit of PG in saturated lime solution reduce. Changes in the hydration temperature of cement leads to variation in the concentration of calcium sulfate, thus the retardation of PG was affected. Due to the lower transformation temperature of dihydrate to hemidrate, PG appears to be prone to loss its combined water at various isothermal conditions between 70 and 130℃in a first effort to simulate milling conditions. About 4% hemihydrate was present in PG at 70℃and in NG no hemihydrate was found. The content of hemihydrate was about 55% in PG and 7% in NG at 130℃. The ratios of calcium sulphate forms occur in gypsum can have a profound effect on the setting behaviour of the cement, because their respective solubilities in water are significantly different from one another. The dihydrate form is the preferred form, so the present of hemi and anhydrate form is harmful to the setting of cement. P2O5 in crystal lattices of gypsum can’t be removed through pretreatment of water washed and lime neutralized, and decomposition of P2O5 in crystal lattices of gypsum because of being heated may reduce the dehydration temperature of PG, and at 90℃about 3% hemihydrate was present in washed PG and at 130℃, the content of hemihydrate was about 11%. Great care should be exercised under production conditions when using PG treated or not treated as a set retarder.Properties of hemihydrate made from PG and NG with same content of gypsum form were determined. The initial setting time of hemihydrate from NG is about 7 min, and setting of hemihydrate from PG was retarded and the strength of the hardenite was reduced. Insoluble products formed by the reaction of Ca2+ and soluble impurities covered at the surface of gypsum crystal, which leads to the coarsening of the crystal.The effect of retarding was correlated to the content of impurities. Setting time of hemihydrate made from PG was aobout twice than that of hemihydrate from NG. The fast setting of most building gypsum is unfavourable for construction. So the retarding effect of impurities can be utilized positively. Though the retarding effect of impurities was not better than the citric acid, it can still be used in some cementing systems.Impurities have hardly affected performances of anhydrate form of PG due to its slow hydration. Hydration products of anhydrate have enough time to break out the overburden layer formed by reaction of soluble P, F and Ca2+ to grow up. Compared with NG, PG has advantages to be use as raw materials for the hemihydrate and anhydrate mixing cementing systems.Impurities are thought to be harmful in almost all studies on PG, but in this paper some effects of impurities are discussed tentatively to be utilized and PG should not be necessarily pretreated in some cementing systems. Pretreatment of PG can’t always effectively improve the properties of PG. In some cases effect of impurities on the properties can be ignored and even be utilized. To find out the mechanism of effect of impurities on PG according to the applied systems of gypsum is very important.更多还原