Basic Research On Preparation Of Calcium Sulfate Hemihydrate Whiskers In Phosphogypsum Microwave Alcohol Water System

Phosphogypsum is a solid waste produced when phosphoric acid is produced by phosphate fertilizer enterprises.Its appearance is gray or dark black,and its main component is calcium sulfate dihydrate.The stock of phosphogypsum in China has reached 400 million tons,and it is still increasing at a rate of about 50 million tons per year.The massive accumulation of phosphogypsum not only occupies a large amount of land,but also pollutes the surrounding environment and wastes.At the same time,it also hinders the sustainable development of phosphorus chemical enterprises in China.At present,the comprehensive utilization of phosphogypsum in China has made some progress,but the resource utilization of phosphogypsum is still concentrated in the low added value fields such as cement retarder and gypsum composite board.Calcium sulfate hemihydrate whiskers is usually a fibrous single crystal generated from calcium sulfate dihydrate as raw material by a certain preparation process.It has the characteristics of high strength,high modulus and good flame retardancy.It is a kind of good reinforcement and toughening material,widely used in rubber,paper making,friction materials and medicine and other cutting-edge fields.The preparation of calcium sulfate hemihydrate whiskers from phosphogypsum as raw materials is an important way to increase the economic added value of phosphogypsum resource products,which is also the research hotspot of many scholars.The preparation methods of calcium sulfate whiskers are microwave method,alcohol water method,etc.Among them,microwave method has the advantages of fast heating speed and uniform heating.The alcohol-water method can realize the transformation of calcium sulfate dihydrate to calcium sulfate hemihydrate at a lower temperatur e,and the reaction conditions are mild,and there is no or little corrosion to the equipment.The study also found that adding a small amount of additive in alcohol-water solution can further reduce the reaction temperature and shorten the reaction time.Therefore,in this paper,calcium sulfate whiskers were prepared from pretreated phosphogypsum by microwave and alcohol water.The effects of different reaction temperatures and reaction times on the crystal growth of calcium sulfate hemihydrate in microwave-irradiated alcohol aqueous solution were studied.The effects of different additives（sodium sulfate,magnesium sulfate,copper sulfate,sodium chloride,magnesium chloride and copper chloride）and different dosage on the growth of calcium sulfate hemihydrate fibrous crystals were analyzed.The main research results of this paper are as follows:（1）Calcium sulfate whiskers were prepared from pretreated phosphogypsum in microwave alcohol-water system at different reaction temperatures.The results showed that:no calcium sulfate hemihydrate whiskers are formed in the reaction at80°C for 4 hours,and at 90℃,no calcium sulfate whiskers were formed after reaction for 2 hours,a small amount of calcium sulfate hemihydrate whiskers are formed after2h reaction at 100℃,and the product is basically calcium sulfate hemihydrate whiskers after 2h reaction at 110℃,that is,as the temperature increases,the conversion time of calcium sulfate dihydrate to calcium sulfate hemihydrate whiskers is significantly shortened.The reaction temperature and reaction time have important regulatory effects on the formation of calcium sulfate hemihydrate whiskers.（2）Under microwave irradiation,the conversion time of calcium sulfate dihydrate to calcium sulfate hemihydrate whiskers decre ased with the increase of Na₂SO₄ content in 100℃alcohol-water solution.When the Na₂SO₄ content is 4%,calcium sulfate dihydrate is completely converted to calcium sulfate hemihydrate in40min.When the content of Na₂SO₄ is increased to 6%,the conversion time of DH to HH is about 30min.When the content of Na₂SO₄ is increased to 8%,the conversion time is reduced to 20min.The length-diameter ratio of calcium sulfate hemihydrate whiskers increased first and then decreased with the increase of Na ₂SO₄ content.When the Na₂SO₄ content was 4%,the length-diameter ratio of whiskers reached the maximum of 39.XRD and EDS analysis showed that Na⁺entered or adsorbed on the surface of calcium sulfate hemihydrate whiskers in the reaction system.（3）Under microwave irradiation,calcium sulfate dihydrate was not completely transformed into calcium sulfate hemihydrate in 60 minutes when the reaction temperature was 100℃,Mg SO₄·5H₂O or Cu SO₄·5H₂O were 0%,2%,4%and 6%respectively.According to the analysis of whiskers formed within 60min with Mg SO₄·5H₂O content of 6%,Mg²⁺may be adsorbed on the surface of HH whiskers or enter into the interior of HH whiskers.（4）Under microwave irradiation,the formation of calcium sulfate hemihydrate whisker can be accelerated by adding Na Cl into the alcohol water solution at 100℃.When the content of Na Cl is 2%,when the Na Cl content was increased to 4%,all calcium sulfate dihydrate was converted to calcium sulfate hemihydrate in60min.When the Na Cl content was increased to 6%,all calci um sulfate dihydrate was converted into calcium sulfate hemihydrate after 40min,and the ratio of length to diameter of calcium sulfate hemihydrate whiskers was about 42 after 60min.XRD and EDS analysis show that with the addition of Na Cl,Na⁺in Na Cl may enter or adsorb on the surface of calcium sulfate hemihydrate whisker.（5）Under microwave irradiation,when the reaction temperature is 100℃and Mg Cl₂ content increases from 2%to 4%and 6%,calcium sulfate dihydrate does not completely transform into calcium sulfate hemihydrate whisker,but the conversion rate of calcium sulfate dihydrate to calcium sulfate hemihydrate whisker increases.Therefore,the addition of Mg Cl₂ can promote the preparation of calcium sulfate hemihydrate whiskers under microwave irradiation.（6）Under microwave irradiation,when the reaction temperature is 100℃,the reaction time of calcium sulfate dihydrate to calcium sulfate hemihydrate whisker is obviously shortened,the peak value of calcium sulfate hemihydrate whisker is slightly shifted,and the atomic molar ratio of copper on the surface of calcium sulfate hemihydrate whisker is increased with the increase of Cu Cl ₂·2H₂O content.There may be some Cu²⁺doping in the calcium carbonate whisker or some Cu²⁺adsorbed on the surface of the whisker.Therefore,the addition of Cu Cl ₂·2H₂O to alcohol water under microwave irradiation can promote the preparation of calcium sulfate he mihydrate whiskers.（7）The study of six additives（sodium sulfate,magnesium sulfate,copper sulfate,sodium chloride,magnesium chloride and copper chloride）shows that the effect of each additive is quite different.Under the same experimental conditions,most of the effects of sodium sulfate and sodium chloride are better than the other four.