Modification Of Phosphogypsum By Roasting And Preparation Of Calcium Sulfate Whisker And Application

Based on the fact that there is a vast amount of phosphogypsum in Mianyang,Sichuan Province,and the treatment and disposal effect is weak,especially the soluble phosphorus and the soluble fluorine have a substantial environmental risk to the surrounding area,combined with the current development trend of comprehensive utilization of phosphogypsum resources in China,this paper systematically studies the fundamental properties of phosphogypsum in a chemical plant in Anzhou,Mianyang,based on which roasting and lime have been adopted.The influence of modification and calcination on soluble phosphorus,soluble fluorine content,basic properties of phosphogypsum,and cementitious property of phosphogypsum were studied.α-hemihydrate calcium sulfate whisker was prepared by the atmospheric salt solution method,and the influence of organic and inorganic additives on crystal form were studied.Anhydrous calcium sulfate whisker was prepared by the atmospheric acidification method.And phosphogypsum-quicklime-fly ash composite cementitious materials were prepared.These would provide valuable technology for reducing soluble phosphorus and soluble fluorine content in phosphogypsum and realizing the comprehensive utilization of phosphogypsum.The study on the fundamental properties of phosphogypsum showed that the main minerals of phosphogypsum were gypsum（CaSO₄·2H₂O）and bassanite（CaSO₄·0.5H₂O）,containing a small amount of quartz.The diffraction peaks of gypsum and bassanite were firm,and the characteristic diffraction peaks were sharp.The samples were grey-white,mostly rhombic plate crystals,with the particle size concentrated in 10-150μm.The internal exposure index(I_Ra)was 1.1,which meets the requirements of class B decoration materials.Phosphorus and fluorine in phosphogypsum were concentrated in phosphate compounds and fluorosilicate compounds respectively,among which soluble phosphorus（dicalcium phosphate,etc.）and soluble fluorine（fluoride,etc.）have a significant impact on the performance of phosphogypsum.The resource utilization of phosphogypsum could be carried out by roasting quicklime modification and impurity removal.The content of soluble phosphorus and soluble fluorine in the calcined product was studied by using quicklime as the modifier.The results showed that the content of soluble phosphorus and soluble fluorine in phosphogypsum increased with the increase of temperature from 100℃to 200℃,and the content of soluble phosphorus and soluble fluorine decreased with the increase of quicklime.The optimum technological conditions for reducing the content of soluble phosphorus and soluble fluorine in phosphogypsum were as follows:adding 1.0%quicklime and calcined at 150℃for 2 hours.At this time,the soluble phosphorus content in the sample was about 0.005%,and the soluble fluorine content was about 0.017%.The effects of calcination modification on the phase,morphology,particle size and other properties of the products were also studied.It was further confirmed that soluble phosphorus and soluble fluorine in phosphogypsum were mainly concentrated on the surface of gypsum crystal and some particles with smaller particle size.The change of cementitious property of phosphogypsum after calcination was studied.It was found that the water consumption of standard consistency of the sample was basically the same,floating up and down at 70%,and the setting time of the original phosphogypsum sample was the shortest,all less than 10 minutes.With the increase of quicklime content,the setting time of the sample was prolonged.The flexural strength of the specimens calcined at150℃for 2 hours is the best of the three,which is 3.0 MPa.The flexural strength of 1.0%quicklime modified calcined specimens is lower than that calcined at 150℃for 2 hours.Considering the gelling property,the best technological condition was calcination of phosphogypsum at 150℃for 2 hours,the water requirement of normal consistency of the product was 70%,the initial setting and the final setting time were 10 min and 17 min respectively,the compressive strength was 6 MPa,and the flexural strength was 3 MPa.α-hemihydrate gypsum was prepared by the dissolution-recrystallization method in the atmospheric salt solution.The morphology and property ofα-hemihydrate gypsum prepared by different concentration of sodium sulfate,reaction temperature,solid-liquid ratio,reaction time,organic crystal control agents（succinic acid,malic acid,L-aspartic acid,maleic acid）and inorganic additives（magnesium sulfate,ferric sulfate,aluminum sulfate,sodium phosphate）were studied.The best reaction condition was 95℃,0.9 mol/L sodium sulfate,the solid-liquid ratio is 1:10.The morphology of the product was uniform,the average length was200μm,and the average diameter was 20μm.The addition of succinic acid made the crystal form a slender column,while the addition of malic acid promoted the crystal to develop into a thin column.L-aspartic acid was added to decrease the crystal length.The product’s morphology by adding maleic acid was a short column.In the range of 0.1-0.5%,with the increase of maleic acid content,the aspect ratio of hemihydrate crystal gradually decreased.When magnesium sulfate was added,the end of the crystal became sharper.After adding ferric sulfate,the final product was an elongated needle.Aluminum sulfate was added to promote the dissolution of phosphogypsum.The addition of sodium phosphate could inhibit the transformation of gypsum phase toα-hemihydrate gypsum.Anhydrite whisker was prepared by acidizing phosphogypsum under atmospheric pressure.The optimum conditions for the preparation of anhydrous calcium sulfate whiskers by nitric acid acidification were 80℃,8.0 mol/L nitric acid and react for 4 hours.Finally,anhydrous calcium sulfate whiskers were obtained.The optimal experimental conditions were selected:80℃,hydrochloric acid concentration 6.5 mol/L,react for 2 hours,and the product of long and thin acicular whiskers was obtained.The ratio of length to the diameter was 20:1.The yield of nitric acid acidification was higher,but the ratio of length to diameter of anhydrite whisker prepared by hydrochloric acid two-step acidification was larger and the whiteness is higher.The strength change law of phosphogypsum-quicklime-fly ash composite cementitious material system was studied.It was found that the ratio of fly ash and quicklime was closely related to the strength of the test piece.When the ratio was 2³,the test piece had better strength.The optimum ratio of 3%quicklime and 15%phosphogypsum was obtained.The test piece under this condition met the performance requirements of GB/T 9776-2008 building gypsum class 3.0.At the same time,reducing the humidity was not conducive to the increase of the strength of the specimen.Under the condition of standard humidity,the change of compressive strength and flexural strength of specimens under different curing time of 2 hours,7 days and 28 days were studied.The strength could be maintained and further enhanced with the increase of curing time.Finally,phosphogypsum could be used in bulk in the form of phosphogypsum based composite cementitious materials.