| The main composition of phosphogypsum is CaSO4·2H2O.The stacking of phosphogypsum will pollute the environment,and the dehydrated product of CaSO4·0.5H2O has great application potential.Normal pressure salt solution method is a common method to prepareα-CaSO4·0.5H2O.Phosphogypsum is placed in salt solution for reaction without pressure vessel.It has advantages of low pressure,low temperature and low energy consumption.In this paper,α-CaSO4·0.5H2O was prepared from phosphogypsum with salt solution as medium under normal pressure.The type of salt medium,salt medium concentration,temperature,liquid-solid ratio and pH value were selected as the influencing factors for the preparation ofα-CaSO4·0.5H2O crystals.In the experiment,sodium chloride,potassium chloride and lithium chloride were selected as the salt medium to study the effect on theα-CaSO4·0.5H2O crystal.The results showed that sodium chloride solution was suitable for the salt medium solution.The obtained crystal product wasα-CaSO4·0.5H2O with a length-diameter ratio of 8:1 to 10:1,and the crystal was complete.It was found that high concentration of salt solution and high temperature and liquid-solid ratio were conducive to the conversion of phosphogypsum,and the increase of pH value would inhibit the conversion of phosphogypsum,but had no effect on the crystal morphology ofα-CaSO4·0.5H2O.SEM and XRD confirmed that the conversion process from CaSO4·2H2O toα-CaSO4·0.5H2O crystals in atmospheric saline solution was a process of solution-crystallization.In the process of preparingα-CaSO4·0.5H2O from phosphogypsum in normal pressure salt solution,inorganic salts,organic acids and compound additives were added as crystal modifier to study their effects onα-CaSO4·0.5H2O crystals.The results of SEM and XRD showed that the inorganic aluminum sulfate had no effect on the morphology ofα-CaSO4·0.5H2O.Organic acid has obvious influence on the morphology ofα-CaSO4·0.5H2O,and succinic acid has the most obvious control effect on the aspect ratio of crystal,but high concentration of organic acid is not conducive to the conversion of phosphogypsum.The complex additive of aluminum sulfate and succinic acid was added to fix 0.2%wt of succinic acid mass fraction.When no aluminum sulfate was added,succinic acid inhibited the dehydration reaction of phosphogypsum.With the increase of aluminum sulfate mass fraction,the dehydration reaction of phosphogypsum could occur,and the obtainedα-CaSO4·0.5H2O crystal aspect ratio increased.It can be speculated that succinic acid is actually involved in the crystallization,and aluminum sulfate has the effect of preventing succinic acid from performing the crystallization.In this paper,succinic acid was used as the crystallizer to study the control mechanism ofα-CaSO4·0.5H2O aspect ratio in the atmospheric salt solution.The results of FTIR and XPS characterization showed that under the appropriate pH value,the succinic acid ionized an appropriate amount of carboxylic acid ions were chelated and adsorbed on the crystal surface ofα-CaSO4·0.5H2O crystal,which slowed down the growth rate ofα-CaSO4·0.5H2O crystal on the crystal surface and reduced the aspect ratio of the crystal.XRD and TEM characterization results confirmed that carboxylic acid ions were adsorbed on the（204）crystal surface ofα-CaSO4·0.5H2O crystal.This paper also studied the effect of organic acid salts on the crystal morphology ofα-CaSO4·0.5H2O in a one-step of two-water and half-water wet-process phosphoric acid with phosphate powder as raw material.The results show that sodium succinate and sodium citrate have regulatory effects on the aspect ratio ofα-CaSO4·0.5H2O under the condition of appropriate dosage,and when the dosage is too much,the conversion of CaSO4·2H2O will be inhibited.The results of FTIR and EDS indicated that carboxylic acid may adsorb on the crystal surface or complexate with Ca2+to reduce the active growth site of Ca2+on the crystal surface and promote the generation ofα-CaSO4·0.5H2O with short aspect ratio. |
