Study On Removal Of Calcium By-product α-high-strength Gypsum From Acid Hydrolysis Solution Of Phosphate Rock Decomposed By Nitric Acid

The process of nitric acid wet process phosphoric acid needs to remove calcium from acid hydrolysate,and there are many ways to remove calcium from acid hydrolysate,among which freezing method and sulfate method are more effective and industrialized.The removal rate of calcium by freezing method is usually controlled at 70～80%（mainly limited by economic feasibility）.The residual calcium ions in the mother liquor of decalcification will generate a large amount of citrate soluble or insoluble calcium phosphate or compound salt in the neutralization process,resulting in the reduction of phosphorus utilization rate,and it is difficult to produce fine phosphate and high water-soluble fertilizer;The high calcium removal rate of sulfate process is beneficial to improve the water solubility of nitrophosphate.The mother liquor of decalcification can be used to produce ammonium phosphate and other phosphate products,but the gypsum generated may still become waste residue and pollute the environment.It is of great significance to change the“solid waste”phosphogypsum into“product”gypsum,minimize the environmental risk caused by a large number of solid waste phosphogypsum,and promote the sustainable development of phosphorus chemical industry.Therefore,this paper proposes and studies a process technology coupled with the nitric acid wet process phosphoric acid process,which uses ammonium sulfate to remove calcium from acidolysis solution,and the by-product isα-type hemihydrate gypsum（α-HH）with gelling property.It is proposed to solve the problem of solid waste phosphogypsum discharge from the source,providing theoretical and technical support for the by-product gypsum in the production process of wet phosphoric acid by nitric acid process.（1）The effects of processing conditions on the formation and morphology ofα-HH crystal were studied.The results show that when the reaction temperature in the system is≥95℃,the dihydrate gypsum（DH）will gradually dissolve and crystallize intoα-HH,and the conversion rate of DH toα-HH will increase as the reaction temperature further increases;Both H⁺and NH₄⁺ions in the acidolysis solution will be adsorbed on the（110）crystal face ofα-HH crystal.Due to the strong electronegativity of H⁺ion,the adsorption of NH₄⁺on（110）crystal face ofα-HH crystal is much lower than that of H⁺ion,and the increase of concentration of both ions will cause the increase of length diameter ratio ofα-HH crystal;Under the experimental conditions,the most suitable process conditions are:reaction temperature 100℃,reaction time 240 min,acid hydrolysis liquid nitric acid content 10%,system P₂O₅ concentration 4.12%,acid hydrolysis liquid Ca O content 6.93%,SO₄^2-/Ca²⁺1.7:1.Under the condition of neutralization degree 1,a short columnarα-HH with a length-to-diameter ratio of 2.6～3.3:1 can be prepared.The physical properties is:the absolute dry compressive strength is 46.6 MPa,and all physical properties are satisfied JCT-2038-2010“α-type high-strength gypsum”standardα40 grade requirements.（2）The influence of impurity ions such as Mg²⁺,Al³⁺,Fe³⁺,F^-in the acid hydrolysis solution on the formation,microscopic morphology and composition ofα-HH crystals was studied.The results show that:Mg²⁺has the effect of promotingα-HH to form polycrystalline,although Mg²⁺,Al³⁺,Fe³⁺plasma will be adsorbed on theα-HH（110）crystal plane to increase the aspect ratio,but the effect is not obvious;In the chemical composition ofα-HH,Mg²⁺and Al³⁺ions have little effect on the purity ofα-HH,while Fe³⁺and F^-ions will reduce the purity ofα-HH;When the Fe₂O₃ content in the acid hydrolysis solution is more than 0.28%,the gypsum phase is composed of moreα-HH and a small amount of flake DH.As the Fe³⁺ion concentration in the acid hydrolysis solution increases,the DH content in the gypsum phase increases first The tendency to decrease later;When the content of acid hydrolysis solution F is 0.32%,α-HH crystals are hexagonal prisms with regular particles;However,when the F content of the acid hydrolyzate increases to 0.52%,F^-ions may not only be adsorbed on the（111）and（110）crystal faces ofα-HH crystals,but may also be adsorbed on the crystal faces of the DH crystals to prevent them.Dissolve and recrystallize,so that the micro morphology of gypsum is mainly long sheet-shaped DH crystals and a small amount of tortoise-shapedα-HH crystals;When the F content of the acid hydrolysis solution is more than 0.52%,as the adsorption of F^-ions on the surface of theα-HH crystal is further enhanced,the tortoise-shapedα-HH crystals gradually grow into irregular short columns,and DH can be completely transformed.（3）The effects of different initial reaction temperatures on the composition and morphology of gypsum minerals were studied.The results show that,except for the phase composition of the gypsum prepared at the initial temperature of 70℃,which isα-HH and DH,the gypsum prepared at the other initial temperature is DH;Under the experimental conditions,the most suitable process conditions are:initial reaction temperature 70℃,initial reaction time 1 h,phase change temperature 100℃,phase change conversion time 6 h,acid hydrolysis solution nitric acid content 10%,system P₂O₅ concentration 4.12%,the content of CaO in acid hydrolysis solution is 6.93%,SO₄^2-/Ca²⁺is 1.7:1,and the neutralization degree is 1 to produce multiphase gypsum in whichα-HH and DH coexist.（4）Using actual factory acid hydrolysis solution as raw material,the technology of removing calcium by-productα-HH from nitric acid wet phosphoric acid hydrolysis solution was tested,and the influence of process parameters on calcium removal rate andα-HH physical properties was investigated.The results show that under the experimental conditions,the decalcification rate of the acid hydrolysis solution reaches 95.14%,and the most suitable process conditions are:neutralization degree 1,the Ca O content of the diluted acid hydrolysis solution 6.93%,the system P₂O₅ concentration 6.73%,SO₄^2-/Ca²⁺1.7:1,the initial reaction temperature is 70℃,the initial reaction time is 1 h,the phase change conversion temperature is 100℃,and the phase change conversion time is 13 h.α-type high-strength gypsum with an aspect ratio of about 2.3:1 can be prepared.The physical properties are:absolute dry compressive strength of 38 MPa,meeting the standard of JCT-2038-2010“α-type high-strength gypsum”standardα30 grade or higher.