Study On The Phase Transformation Process Of Gypsum And The Influence Of Impurities In The Two-step Wet-process Phosphoric Acid

Wet process phosphoric acid is produced by decomposing phosphate rock with sulfuric acid.According to the different content of calcium sulfate crystal water obtained in the production process,it can be divided into dihydrate,semi-aqueous,and anhydrous one-step gypsum crystallization methods.The recovery rate of P₂O₅ obtained by the one-step wet process phosphoric acid process is low,and the by-product phosphogypsum has a high impurity content,which is treated by accumulation and landfill.Caused damage to the surrounding land and the environment.The two-step（recrystallization method）wet process phosphoric acid production process is to filter and wash the slurry in the one-step process,and then put it into a crystallization conversion tank for secondary conversion of gypsum crystal form.In this way,not only the insoluble P and eutectic P impurities in the phosphogypsum can be decomposed,the conversion rate of the phosphate rock is as high as 99%,and the recovery rate of P₂O₅ is improved.At the same time,high-purity phosphogypsum can be prepared by recrystallization,without pretreatment,and can be directly used as a raw material for the production of construction gypsum,gypsum products,cement retarders and other building materials.The key technology of the two-step wet process phosphoric acid recrystallization process is the phase transformation of phosphogypsum.The phase transformation process of phosphogypsum is mainly affected by process parameters and impurities.Therefore,this paper simulates the two-step wet process phosphoric acid process conditions,and studies the influence of process parameters and impurities on the phase transformation process of phosphogypsum,which is of great significance for improving the resource utilization rate of phosphogypsum and environmental protection.First,the kinetic process of hydration transformation fromα-hemihydrate phosphogypsum（α-HH）to dihydrate phosphogypsum（DH）was studied,and the dispersive kinetic model was revised to fit theα-HH—DH hydration conversion process.The influence of temperature,phosphoric acid concentration,and SO₄^2-ion concentration on the hydration process ofα-HH—DH was investigated.The results showed that:α-HH—DH hydration was a process of inducing nucleation and growth restriction.With the decreased of temperature and phosphoric acid concentration,the hydration rate ofα-HH—DH increased significantly.With the increased of SO₄^2-ion concentration,the hydration rate ofα-HH—DH was obviously accelerated,but the reaction was not complete.As the SO₄^2-ion concentration increased from 3%to 7%,the conversion rate decreased from 99%to 93%.The order of the influence of process parameters on the rate ofα-HH—DH hydration reaction was:SO₄^2-ion concentration>phosphoric acid concentration>temperature.The influence of impurities(Fe³⁺,Al³⁺,Mg²⁺,Si F₆^2-)on the hydration kinetics ofα-HH—DH was discussed.The properties of the hydration conversion product DH were analyzed by using SEM,XRD,XPS,and TG-DSC,revealing the interaction mechanism between impurities and DH crystals.The results showed that as the concentration of Fe³⁺ion and Al³⁺ion increased from 0 to 0.07 mol·L^-1,the hydration conversion rate ofα-HH—DH slowed down first and then accelerated.As the concentration of Mg²⁺and Si F₆^2-ion increased from 0 to 0.07 mol·L^-1,both prolonged the hydration conversion process ofα-HH—DH.This was because the introduction of impurity ions changed the activation enthalpy（（35）H^*）and activation entropy（（35）S^*）,resulting in a change in the activation energy barrier（E_a）,which affected the rate ofα-HH—DH hydration conversion.With the introduction of Fe³⁺ions and Al³⁺ions,impurity ions at low concentrations were due to the common ion effect of SO₄^2-,which reduced the solubility ofα-HH and inhibited the hydration process.However,as the concentration of impurity ions increased,the solubility ofα-HH increased due to the salt effect,and the reaction rate gradually becomes faster.Mg²⁺ions enter into the crystal lattice of DH to produce a lattice substitution with Ca²⁺ions,thereby inhibiting conversion.Si F₆^2-ions chemically react with Ca²⁺ions in the solution to form calcium fluorosilicate,which was selectively adsorbed on the surface of DH crystals and hindered the growth of crystals.The kinetic process of dehydration conversion from dihydrate phosphogypsum to hemihydrate phosphogypsum was studied,and the influence of process parameters such as temperature,phosphoric acid concentration and SO₄^2-ion concentration on the dehydration kinetics process was analyzed.The results showed that with the increase of temperature,phosphoric acid concentration and SO₄^2-ion concentration,the dehydration conversion rate of DH—α-HH increased significantly.But the high concentration of SO₄^2-ion increased the collision rate of Ca²⁺ions and SO₄^2-ions in the solution,the DH crystals were wrapped byα-HH crystals,which hindered the conversion and caused the reaction to be incomplete.When the concentration of SO₄^2-ion increased from 3%to 7%,the degree of conversion dropped from 97%to 91%.The order of the influence of process parameters on the dehydration conversion rate of DH—α-HH was:SO₄^2-ion concentration>phosphoric acid concentration>temperature.Finally,the influence of impurities(Fe³⁺,Al³⁺,Mg²⁺,Si F₆^2-)on the kinetic process of DH—α-HH is discussed.The properties of the dehydration conversion productα-HH were characterized and analyzed by using SEM,XRD,XPS,and TG-DSC,in order to reveal the interaction mechanism between impurities andα-HH crystals.The results showed that the dehydration process of DH—α-HH follows the dispersive kinetic model in the wet-process phosphoric acid solution.After Fe³⁺,Al³⁺,Mg²⁺and Si F₆^2-ions were introduced into the system,（35）H^*increased and（35）S^*decreased due to the reduction of the kinetic parametersαandβ,causing E_a ofα-HH crystals to increase,thereby inhibiting the dehydration and transformation process of DH—α-HH.The order of inhibition is:Al³⁺>Si F₆^2->Fe³⁺>Mg²⁺.Impurities Fe³⁺,Al³⁺and Mg²⁺combined with phosphate ions to form phosphate precipitates,which were selectively covered on different crystal surfaces ofα-HH,thereby hindering the growth ofα-HH crystals.And a lot of Ca²⁺ions existed on the（111）crystal plane in the c-axial direction,Si F₆^2-ions combine with Ca²⁺ions,thereby hindering the one-dimensional growth in the c-axial direction,so the aspect ratio ofα-HH crystal decreased,and the morphology gradually evolved to a short columnar structure.