Evolution Of Mineralogical Characteristics And Gelling Properties Of Hemi-hydrate Phosphogypsum

Phosphogypsum is obtained as a major solid waste, when producing ortho-phosphoric acid by decomposing apatites with sulphuric acid. About 50 million tons of phosphogypsum are produced annually across China. At present, only 15% of phosphogypsum production has been used. Most phosphogypsum piled up and occupied considerable land resources, which eventually led to serious environmental contamination and damaged to human health. Study on utilization technology of phosphogypsum is always one of the hot spots in the worldwide. According to the concept of broad-sense mineralogy, phosphogypsum is an artificial mineral which contains a small amount of unreacted natural mineral. Up to now, resource utilization technique of phosphogypsum is mainly modification treatment using physical and chemical methods. Macroscopic properties of phosphogypsum and its products were paid too much attention, whereas mineralogical characteristics of phosphogypsum and effect of the mineralogical characteristics on gelling properties of phosphogypsum were paid less attention, which caused utilization technique of phosphogypsum to be still unsatisfactory. According to research status of utilization technique of phosphogypsum, particularly involving a technical difficult problem for making use of hemi-hydrate phosphogypsum(HPG) which is obtained during the hemi-hydrate process of wet-process phosphoric acid and has poor gelling performances, evolution of mineralogical characteristics and gelling properties of HPG was further studied in the paper, which would provide theoretical supports to make a breakthrough in resource utilization technology of phosphogypsum.Firstly, the mineralogical characteristics and gelling properties of industrial HPG were studied. Main factors for poor gelling properties of HPG were explored from the view of chemical composition, mineral composition and crystal morphologies. Results showed that industrial HPG was almost aggregation made of irregular oriented schistose crystals of type-α hemi-hydrate gypsum(α-HH). The crystal morphologies of schistose aggregation caused hydration-hardening rate of HPG to decrease, and then industrial HPG had poor gelling properties as follows, the compress strength 0.38 MPa.Secondly, by simulating the hemi-hydrate process of wet-process phosphoric acid, evolution of mineralogical characteristics and gelling properties of HPG was investigated in different technological parameters. The relationships among technological parameters, mineralogical characteristics, and gelling properties of HPG were also analyzed. Results showed that evolution of mineral composition of HPG was mainly affected by concentrations of liquid-phase SO42-, reaction temperatures, and concentrations of phosphoric acid. Evolution of crystal morphologies and gelling properties of HPG was affected by concentrations of liquid-phase SO42-, reaction temperatures, concentrations of phosphoric acid, apatite powders content, and stirring speeds. When controlling technological parameters as follows: the concentration of liquid-phase SO42- at 20 mg?m L-1, reaction temperature at 95 ℃, P2O5 content of phosphoric acid at 36%(by mass), apatite content at 15.3%(by mass), and stirring speed at 135 r?min-1, HPG were almost aggregation made of irregular oriented hexagonal column shape of α-HH crystals and had good gelling properties as follows, the flexural strength 2.75 MPa and compress strength 5.28 MPa.Crystal-equilibrium morphology was predicted through molecular simulation. The distribution of water channels in the crystal morphology of α-HH was firstly theoretically detected. The growth morphology of α-HH was also analyzed based on connection mode of crystal structure and rough surfaces model. Results showed that the important crystal-growth surfaces were morphologically {10-1}, {101}, {011}, {0-11}, {110}, and {1-10}. Water channels were mainly distributed in the cylinders of α-HH crystal, whereas no water channel existed in the conical surfaces {110} and {1-10} parallel to the z-axis. In phosphoric-acid solution, the crystal growth rate of α-HH was fast along the [010] direction. Thus, the crystal extended along crystal plane clusters {10-1} and {101}, and the growth morphology of α-HH showed a long column shape. When the crystal growth was blocked along the [001] direction, the crystal plane cluster {002} was exposed. Thus the crystal morphology changed from four to six cylinders with conical surfaces.Effect of Mg2+, Al3+ and Fe3+ ions, as additives, on the crystallization of α-HH was studied under simulated conditions of reaction of calcium dihydrogen phosphate and sulfuric acid. Results showed that Mg2+, Al3+ and Fe3+ ions increased the crystallization induction time of α-HH compared with in absence of additives. Under the same concentration of impurity ions, the order of extending the crystallization induction time of α-HH crystal was Fe3+ ions, followed by Mg2+, Al3+ ions. Mg2+, Al3+ and Fe3+ ions were mainly adsorbed on the exposed surface {1-10} of α-HH crystals. Thus crystal morphology of α-HH was wedge shaped or short column.Schematic illustration of HPG morphology under the condition of crystal seeds was firstly studied,by investigating effect of crystal seeds on crystal growth and morphological evolution of HPG during the hemi-hydrate process of wet-process phosphoric acid. Results showed that when crystal seeds were introduced, the parallel intergrowth between α-HH crystals easily appeared in the crystal growth process. The HPG morphological evolution could be identified three time regions: a period for early stage of columnar crystals formation, a period for short columnar crystals formation, and a period for later stage of schistose crystals formation.Finally, it colud be certificated that water channels were mainly distributed in the crystal cylinders of α-HH compared with the hydration reactivity of HPG with typical morphologies. This finding agrees with the theoretical calculations. A critical method that enhanced the hydration reactivity of HPG through a morphology-controlled preparation technology was firstly proposed during the hemi-hydrate process of wet-process phosphoric acid. The important role of water channels in hydration could be investigated through the contrast between the hydration reactivity and the apparent activation energy of HPG with typical morphologies. Results showed that when the cylinders of α-HH crystal were exposed clearly, there were numerous water channels in α-HH crystal. Thus, the speed of the hydration reaction was physically controlled. When the cylinders of α-HH crystal were less evident, there were only few water channels in α-HH crystal. Therefore, the speed of the hydration reaction was chemically controlled.