Study On The Process Of Preparing ?-calcium Sulfate Hemihydrate Composite Phase Change Energy Storage Materials From Phosphogypsum

Phosphogypsum(PG)is the main by-product of the wet-process phosphoric acid production process and its main component is Ca SO4·2H2O.In the past 20 years,PG has shown a dramatic increase,with the development of high-concentration phosphate compound fertilizers and wet-process phosphoric acid.The annual emissions are close to 80 million tons,and the accumulated stockpiles have exceeded 500 million tons.There is an urgent need for large scale comprehensive utilization of them.Phase change materials(PCM)can store/release thermal energy during the phase change process,PG is made into hemihydrated gypsum and then compounded with PCM for the building maintenance structure,which can increase the thermal inertia of the building and reduce indoor temperature fluctuations.It can not only realize the resource utilization of PG,but also achieve the effect of building energy saving.In this paper,firstly,the process of preparing ?-calcium sulfate hemihydrate(?-CSH)from PG using magnesium nitrate solution instead of chloride salt as the reaction medium was studied.Then,it was studied to use sodium sulfate decahydrate(SSD)and sodium acetate trihydrate(SAT)as phase change materials,adding attapulgite(ATT)to prepare form-stable phase change materials(FSCPM)process.Finally,the process of preparing PG-based phase change materials with ?-CSH as the matrix materials and FSPCM was studied,and the physical and chemical properties of the samples were characterized.(1)PG was used as raw material to prepare ?-CSH in magnesium nitrate solution by atmospheric pressure salt solution method.The influence of magnesium nitrate solution concentration,reaction temperature,initial p H value of the solution and maleic acid concentration on the conversion process of PG was investigated.The adsorption mechanism of maleic acid was analyzed by FT-IR and XPS characterization techniques.The results showed that PG can be converted into ?-CSH in a 25wt% magnesium nitrate solution at 95 °C.With the increase of salt concentration and reaction temperature,the reaction rate of phosphogypsum increased.When the p H value and the concentration of maleic acid were increased,the dehydration reaction of PG was inhibited,but the aspect ratio of the ?-CSH crystal decreases.The studies had shown that the optimal process conditions were 95 °C,p H 3,25wt% magnesium nitrate solution concentration,and 0.1wt% maleic acid,the ?-CSH samples prepared had the best performance,bending/compressive strength was 8.9/36.8 Mpa,SEM results showed that the aspect ratio of ?-CSH was 1.24:1.(2)Sodium sulfate decahydrate(SSD)and sodium acetate trihydrate(SAT)with a molar ratio of 0.71:0.29 were mixed to prepare a binary eutectic hydrated salt(EHS)phase change material.The EHS cooling curves of adding different nucleating agents were compared,and borax was determined as the best nucleating agent.The vacuum adsorption method was used to absorb EHS to prepare FSPCM with ATT as the matrix material,and then added polyurethane(PU)to make polyurethane/form-stable phase change materials(PU/FSPCM)to further reduce EHS leakage.The DSC results showed that the PU/FSPCM phase change enthalpy was 82.6 J/g,and the enthalpy value only decreased by 10.4% after 200 thermal cycles.SEM results showed that EHS was adsorbed in the pores of ATT,and PU was coated on the pore surface of ATT to prevent EHS from leaking.TG results showed that the weight loss rate of FSPCM and PU/FSPCM decreased,which indicated that ATT and PU enhanced the thermal stability of EHS.(3)The three phase change materials of EHS,FSPCM and PU/FSPCM were combined with ?-CSH to prepare PG-based composite materials,and the influence of the addition of phase change materials on the compressive strength of the samples was investigated.The experimental results showed that the compressive strength of the PGbased composite materials decreased with the increase of the content of the three phase change materials.Among them,EHS had the greatest influence on the compressive strength of ?-CSH,and PU/FSPCM had the least influence.The PG-based composite materials board was prepared by mixing 40wt% PU/FSPCM and ?-CSH,covering it on a foam box to make an incubator.Infrared light was irradiated for 7 hours to investigate the temperature change in the incubator.At the same time,it was compared with the pure PG board incubator.The results showed that the peak temperature difference between the two incubators was 3.8 °C,indicating that the PG-based composite materials had a significant effect of regulating temperature changes.