Preparation And Stability Of CaSO₄·0.5H₂O By Two-Half Water Method Phosphoric Acid

In this investigation,one-step two-stage process was adopted in this experiment,using mine from Yun’an city of Hubei province as the raw material.The first stage used traditional sulfuric acid leaching process to prepare CaSO₄·2H₂O,and the second stage changed the process conditions to prepare CaSO₄·0.5H₂O by atmospheric acidification.The paper focused on the second stage of the reaction,taking sulfuric acid concentration,phosphoric acid concentration and reaction temperature as independent variables,and using single factor method to explore the influence of process conditions on the preparation of short columnar CaSO₄·0.5H₂O by dihydrate conversion.The research results showed that the optimal reaction conditions for the first stage were the reaction temperature of 85°C,the sulfuric acid concentration of 24wt%,the liquid-solid ratio of1.5:1,and the reaction time of 4h,which prepare flake CaSO₄·2H₂O;the optimal reaction conditions for the second stage were the reaction temperature of 120℃,the added H₂SO₄ concentration of 7wt%and the added H₃PO₄ concentration of 1wt%,therefore a length of 50-100μm,a width of 15-20μm,and an aspect ratio of 4:1 can be obtained.CaSO₄·0.5H₂O.Through this research,it was found that the increase of reaction temperature,phosphoric acid concentration and sulfuric acid concentration was beneficial to accelerate the conversion of CaSO₄·0.5H₂O.In addition,the increase of phosphoric acid concentration and sulfuric acid concentration increased the aspect ratio of CaSO₄·0.5H₂O.In order to improved the stability of CaSO₄·0.5H₂O,this article investigated the effected of adding sodium oxalate on the stability of CaSO₄·0.5H₂O,and used XRD,TG,ion beam cutting crystals and SEM-Mapping to characterize the product,meanwhile,CaSO₄·0.5H₂O synthesized in the process of dihydrate-semi-aqueous wet phosphoric acid was used as raw material,sodium oxalate was used as stabilizer,and the reaction temperature was 100℃and the reaction time was 20min.The experimental results showed that the stability time of CaSO₄·0.5H₂O increased with the increase of sodium oxalate;if the sodium oxalate is 2.5wt%,the stability time of CaSO₄·0.5H₂O in water was the longest,reaching 120min.Through the characterization results from ion beam cutting and Mapping,it was conducted that Ca C₂O₄·H₂O covers CaSO₄·0.5H₂O completely.The mechanism of action was that C₂O₄^2-would be ionized when sodium oxalate in water,which precipitates with Ca²⁺in the solution.Therefore,a precipitate,Ca C₂O₄·H₂O,would be formed on the surface of CaSO4·0.5H2O.It reached the surface modification of CaSO₄·0.5H₂O,thus the stability time of CaSO₄·0.5H₂O in water would be increased.In addition,the influence of sodium carbonate on the stability of CaSO₄·0.5H₂O was studied.When sodium carbonate was added at 1.5wt%and the modification temperature was 100℃,the morphology retention time of CaSO₄·0.5H₂O in water was the longest,from 20min to 150min.Besides,in the paper,the calcination temperature and calcination time were used as control conditions to study the effect of calcination on the stability of CaSO₄·0.5H₂O.The research results showed that as the calcination temperature increased,the morphology retention of calcium sulfate crystals in water increased.In the experiment,when the calcination temperature was 400℃,all CaSO₄·0.5H₂O was converted into anhydrous calcium sulfate,which could exist stably in water.With the increase of calcination time,the stability time of CaSO₄·0.5H₂O in water was the trend of first increasing and then unchanged.Characterization methods such as XRD and TG showed that when the calcination temperature was 200℃and the calcination time was 2h,the resulting product was a mixture of CaSO₄·0.5H₂O and CaSO₄,and the stable time of its crystal morphology in water was 2h.