Preparation Of Calcium Sulfate Whiskers From Phosphogypsum By Chlorine-free Salt Solution Method At Atmospheric Pressure

Phosphogypsum is the by-product gypsum produced by the decomposition of phosphate by H₂SO₄ in the process of extracting phosphoric acid..From the perspective of environmental protection and long-term development,the reuse of phosphogypsum has been urgent.Calcium Sulfate Whisker（CSW）is a low-cost,non-toxic and multifunctional inorganic whisker,which is widely used in toughening and reinforcing materials,friction materials and paper making,etc.The preparation of CSW is an effective method to transform phosphogypsum waste into valuable.General conventional preparation methods need to use high concentration of acid as the medium or high pressure environment,so this paper uses a mild salt solution method at atmospheric pressure,But this method generally takes chlorine salt as the medium,and chloride ions will corrode the metal components used in the process of making whiskers and whisker products,increasing the cost of equipment maintenance.In order to prevent the hydration of whiskers and enhance the mutual compatibility with organic materials,whiskers are generally modified.In the conventional modification steps,CSW products are usually prepared first and then modified,that is,prepared first and then modified.The process is cumbersome.In view of this series of problems,this topic mainly studies the optimal process parameters of dissolving phosphogypsum and recrystallization to prepare CSW in chlorine-free salt solution at atmospheric pressure;The effects of nitrate and sulfate on the formation rate and crystal morphology of CSW were discussed.The hydrophobic CSW was prepared and modified by chlorine-free solution in one step at atmospheric pressure.In the study of the best preparation parameters of CSW,adding Ca（NO₃）₂ with mass fraction of 30%can increase the solubility of phosphogypsum by salt effect,and reduce the activity of water through the orientation force and hydrogen bonding.Under the conditions of reaction medium temperature of 103℃,p H of 3.0 and phosphogypsum dosage of 12%,CSW with average length and aspect ratio of 398.12μm and 59.21 can be generated for 4 h.In the study of using nitrate and sulfate to regulate the formation rate and crystal morphology of CSW,the regulation ability of Na₂SO₄,NaNO₃,K₂SO₄,KNO₃,MgSO₄ and Mg（NO₃）₂inorganic salts was compared.The results show that adding a small amount of nitrate and sulfate can shorten the generation time of whiskers,and Na₂SO₄ has the most obvious effect.It was found that both Mg SO₄ and Mg（NO₃）₂ could increase the average length and finish of CSW,and reduce the crystallinity of whisker surface.Mg SO₄ showed better regulation ability,and the average length of CSW was increased from 398.12μm to 513.97μm,and the aspect ratio increased from 52 to 61 when the supplemental level was 1.0%.In the study of one step preparation and modification of CSW by chlorine-free solution method at atmospheric pressure,3 h,respectively after joining by water or ethanol dispersion of oleic acid,sodium oleic acid,stearic acid,sodium stearate modified whisker,the results show that water dispersed modifier of whisker is not modified,and hydrophobic calcium sulfate whiskers cannot be prepared.anhydrous ethanol dispersion in a modifier after the optimal effect of sodium stearate,sodium stearate is the best modifier was finally determined.The hydrophobic CSW with contact Angle of 105.15°can be obtained under the experimental conditions of 85℃,20%anhydrous ethanol slurry,mass fraction of sodium stearate of 0.4%,rotational speed of 300 r/min,reaction time of 5 minutes.The XRD results showed that the CSW did not hydrate during the modification process,the results of infrared spectrum and XPS show that the modification of CSW by sodium stearate is a process of chemical adsorption and physical adsorption.