Effect Of Crystal Shape Regulator In Wet-process Phosphoric Acid On Morphology And Properties Of α-hemihydrate Gypsum

Phosphogypsum（PG）is waste produced by wet-process phosphoric acid industry,contains many impurities,leading to its low utilization rate,and the accumulation of a large amount of phosphogypsum seriously pollures the environment.In the absence of crystal regulator or admixture,the mechanical properties ofα-hemihydrate gypsum（α-HH）are poor,resulting in limited use or random discarding of products,resulting in environmental pollution.This paper is mainly based on the wet process phosphoric acid process,taking H₃PO₄-H₂O solution as crystal conversion medium and phosphogypsum as raw material,by adding different kinds of composite crystal regulator to the solution,the samples with controllable morphology and excellent mechanical properties were preparedα-HH.Firstly,the effects of process parameters such as phosphoric acid concentration,liquid-solid ratio,stirring rate and reaction temperature on the synthesis ofα-HH reaction time,crystal morphology and mechanical properties.And analysis the effects of organic crystal like regulators onα-HH on crystal morphology and properties.Under the optimal conditions of phosphoric acid concentration of 30%,liquid-solid ratio of 3.0:1,stirring speed of 200 rpm,conversion temperature of 95℃,obtained the reaction time ofα-HH is 105 min（medium）,length-diameter（L/D）ratio of 25.8（minimum）,and the crystal is long rod.By adding crystal form regulators such as sodium citrate,EDTA-2Na,potassium tartrate,tartaric acid and succinic acid respectively,obtained the initial setting time ofα-HH is 3～6 min,the final setting time is within 12 min,the drying bending strength is between of 1～1.6MPa,and the drying compressive strength is basically maintained within 5～9 MPa.It can be seen that the standard water demand of whiskerα-HH after hydration is very large,and its compressive properties and bending properties are poor.Secondly,PG1 was used as raw material and complex CR was added to prepareα-HH1.When the complex CR content increases from 0%to 0.6%,The average diameter changed from5.1μm to 37.6μm ofα-HH1;The average length changed from 131.6μm to 45.5μm ofα-HH1;The average L/D ratio changed from 25.80 to 1.21 ofα-HH1;The reaction time was prolonged from 105 min to 400 min ofα-HH1;Afterα-HH1 hydration the standard consistency water demand of hardened body decreased from 0.64 to 0.35;The initial setting time increased from 4 min to 12 min ofα-HH1;The final setting time increased from 10 min to 24 min ofα-HH1;The 2h bending strength increased from 1.36MPa to 5.28MPa ofα-HH1;The drying compressive strength increased from 7.5MPa to 36.8MPa ofα-HH1.The performance index can meet JC/T 2038-2010《α-High strength gypsum》about of levelα30 requirements.When the complex CR content is 0.3%,PG1 is used as raw material and industrial phosphoric acid solution is used as crystal conversion medium the performance indexes ofα-HH meet the requirements levelα30 requirements.Using PG2 as raw material,α-HH2 sample performance index meets levelα40 requirements.Using white PG3 as raw material,the transformation process of calcium sulfate dihydrate（DH）crystal to calcium sulfate hemihydrate（HH）crystal in phosphoric acid aqueous solution was studied by SEM,XRD and crystal water content.The results show that after 15 minutes,the SEM image of the sample shows a very small amount of long needle crystals,and the characteristic peak of hemihydrate gypsum appears in the XRD image.After 90 min of reaction,the SEM image of the sample shows that the DH crystal has completely disappeared,the XRD of the sample shows that the diffraction peaks of DH all disappear,and the other characteristic peaks completely correspond to HH.The content of crystal water is 6.25%.In the whole reaction process,SEM images,XRD patterns and crystal water content changes show that only DH and HH are involved in the reaction process,which indicates that in H₃PO₄-H₂O solution DH transform to HH also follows the"dissolution recrystallization"theory.With the increase of the content of complex CR（wt 0%～1.3%）,the average diameter ofα-HH3 crystal increased by647%,the average length decreased by 972%,and the average length diameter ratio decreased by 62 times.The XRD diffraction analysis of the crystals prepared with different content of complex CR shows that the complex CR inhibits the growth ofα-HH crystals parallel to the lattice planes（200）,（020）and（400）of the c-axis,and promotes the crystal growth of crystal planes parallel to the a-axis and b-axis（204）,（422）,（424）and（604）.With the increase of the content of complex CR,afterα-HH3 hydration,the 2h bending strength of the hardened body increased from 0.98 MPa to 6.56 MPa,the drying compressive strength increased from 5.9 MPa to 52.4 MPa,and the whiteness ofα-HH samples was higher than 92.When the content of complex CR is 1.1%,the performance index ofα-HH3 hardened body after hydration meets JC/T 2038-2010《αHigh strength gypsum》meets levelα50 requirements.Finally,the regulation mechanism ofα-HH3 crystal morphology by complex CR was studied.XRF test shows that the contents of F and Al inα-HH3 samples increased with the increase of the content of composite CR.Both F 1_S and Al2p orbitals of XPS show strong induction spectra peak at 685.5e.V and 74.9e.V,indicating that F^-,Al³⁺onα-HH3 crystal surface occurred adsorption reaction.In XPS spectrum,a new Ca peak appears at Ca 2p orbit of the binding energy of 353.01e.V.In XPS spectrum,the binding energies of 3/2 of S 2p orbit and1/2 of S 2p orbit are 171.52e.V and 168.95e.V,respectively.And all of them have negative displacement.When 1.2%CR is added,the content of F element（0.21%,0.42%）in crystal face is obviously higher than that in cylinder face（0.02%,0.04%）,and the content of Al element（0.10%,0.13%）in crystal face is also higher than that in cylinder face（0.04%,0.06%）,which is consistent with EDS scanning results.F^-and Al³⁺selectively adsorbed on the crystal top plane.As F^-and Al³⁺selectively adsorb on different crystal planes,the relative growth rate of these crystal planes changes.Finally,under the joint action of F^-and Al³⁺,the crystal diameter increases,the length decreases,and the L/D ratio decreases to present a short columnar morphology.