Study On Resource Utilization Technology And Application Of Phosphogypsum In Agriculture

Phosphogypsum is known as a kind of industrial by-products which generated from the production of phosphoric acid and phosphate fertilizer.Due to its depot covers,environmental pollution,harmful impurities and strong acid,it is difficult to be comprehensive utilization effectively,moreover,the emission and accumulation of which has growing increased.As reported,the production of phosphogypsum was 80 Mt in 2015 and the accumulation has exceeded 300 Mt so far in China.The existing technology to the low industrialization of the phosphogypsum utilization has hampered the development of the phosphate fertilizer and agriculture.Therefore,there is an urgent need to enhance the phosphogypsum industrialization utilization.Meanwhile,regarding to the abundance of potash in China and the absence of soluble potassium,the exploitation of insolubility potassium resources would be beneficial to the issue of long-term potassium deficiency.Fertilizer,acid deposition and industrial pollution lead to problems such as the soil environment deteriorating in our country.Soil pollution has become one of the issues which were needed to be solved in the field of environmental problems.To figure out the comprehensive utilization of phosphogypsum,combining with the development and utilization of insolubility potassium and soil remediation problem,the present work investigated the removal of heavy metals from phosphogypsum with the method of chelating precipitation.Studies were carried out by KAlSi₃O₈-CaSO₄-CaCO₃ system which combined with purified phosphogypsum,potash feldspar?KAlSi₃O₈?and limestone?CaCO₃?.The mineral soil conditioner was prepared by roast-activation of potash feldspar.At last,soil conditioner was applied in the field test to evaluate the effects of soil remediation,the yield and quality of plant.Compared with various chelating agents,Na₂ EDTA was chosen to be the chelating agents in this study owing to the best leaching effects.Chelating precipitation was applied to study the removal efficiency of heavy metals in phosphogypsum which derived from Guangdong zhanjiang?ZJ?,Anhui tongling?TL?,Henan jiaozuo?JZ?and precipitation effects of heavy metals in the leaching liquid.The Pb removal efficiencies of ZJ and TL phosphogypsum were achieved to be 90.6-100.0%,while Cu and Zn were above 60% when the concentration of Na₂ EDTA was 10 mM.Yet the removal efficiencies of heavy metals obtained to be over 70%,when JZ phosphogypsum subjected to the pretreatment of nitric acid for 4h with the concentration of 0.033 m L/g.The results of chelating heavy metal leaching from phosphogypsum were fitted well by two first-order reactions model.In view of fitting calculation,the total content of extractable Pb can reach 74-100% for the three kinds of phosphogypsum.After the pretreatment of nitric acid,soluble Cu and Zn will measured to be78% and 71% of JZ phosphogypsum.With comparison of phosphogypsum component before and after treatment,results showed that the phosphogypsum after treatment was exerts to be purification to some extent,even the Ca?SO₄?·0.5H2 O were all converted to Ca?SO₄?·2H2O of which the main ingredients were Ca?SO₄?·2H2O and a few of CaHPO₄ ·2H2O.Moreover,according to the software simulation of MINTEQA2,the optimized pH value was 10-12 in the heavy metal leaching system by using Na₂ S and Na₂ EDTA,and the optimized concentration of Na₂ S was 2.0 mM.Experimental results were consistent with the simulation results.It showed that all the heavy metals can be converted to precipitation in the Na₂ EDTA leaching liquid,and soluble calcium in the leaching liquid transformed into precipitation which including calcium phosphate and small amounts of calcium hydroxide,calcium sulfate,when pH value was 11 and the concentration of Na₂ S was 1.5-2.0 mM.According to the analysis of phase diagram?CaO-SiO₂-Al₂O₃?,thermodynamics,XRD and FTIR,For the system of KAlSi₃O₈-CaSO₄-CaCO₃?molar ratio was 1:1:?5-20??,results showed that the optimized molar ratio was 1:1:14,and the chemical reaction equation was K2O·Al₂O₃·6SiO₂ + CaSO₄ + 14 CaO = K2SO₄ + 3CaO·Al₂O₃ + 6?2CaO·SiO₂?,in which the ?G was determined to be minimum in this present system.However,the existence of excessive CaO about roasted products will emerge when proportion of CaCO₃ exceeded 14.The experimental results were similar to that of analysis.The activation rate of K achieved to be maximum?88-89%?while the molar ratio of KAlSi₃O₈-CaSO₄-CaCO₃ was 1:1:12-16 and the optimum reaction temperature was 1100 oC.Na₂SO₄ acted as an addition agent which not only can reduce the reaction temperature of the system but also enhance the activation rate of K,and the optimum addition was 3%,as well as phosphorus.The main decomposition process of potash feldspar was KAS6?potash feldspar?? KAS4?leucite?+ 2C2 S ? CAS2 + 4C2 S ? CA + 6C2 S ? C3 A + 6C2 S.On the basic of the equation,each step of the reaction will consume CaO,which was equivalent to that 14 mol of CaCO₃ were needed when 1mol of potassium feldspar was decomposed completely.Investigation was performed by the KAlSi₃O₈-CaSO₄-CaCO₃ system which was obtained from the replacement of CaSO₄ by phosphogypsum in the KAlSi₃O₈-Phosphogypsum-CaCO₃ system.Results showed that molar ratio of KAlSi₃O₈-Phosphogypsum-CaCO₃ was 1:1:14 when the reaction temperature was 1050 o C,while the activation rate of K and the total amount of activated K was 88% and above 4.05%,respectively.It is illustrated that phosphogypsum can reduce the system temperature since the activation rate of K was invariant when continued to raise the system temperature,while there is no obvious role in the improvement of KAlSi₃O₈-Phosphogypsum-CaCO₃ system by Na₂SO₄.Compared to potash feldspar-CaSO₄-CaCO₃,KAlSi₃O₈-Phosphogypsum-CaCO₃ system showed a faster response to achieve reaction equilibrium.The reaction was detected to be equilibrium at 60 min when the temperature was 1050 o C.But it will decrease to 40 min when the temperature was 1100 o C.The activation rate of K and the total amount of activated K were above 85% and 4% when the reaction was equilibrium;it declared that phosphogypsum can reduce the reaction time.By comparison of results obtained from the optimum proportion of roasting at the temperature of 1100 o C,a few of CaHPO₄ will convert into Ca3?PO₄?2 at high temperature,followed by the reaction with 2CaO·SiO₂ and K2SO₄ to generate Ca2SiO₄·0.05Ca3?PO₄?2 and K2CaP2O7.But a few of residual CaSO₄ may reacted with 3CaO·Al₂O₃,accompanied by the product of Ca4Al6O12SO₄.However,the main reaction products were K2SO₄,2CaO·SiO₂ and 3CaO·Al₂O₃ owing to the low content of phosphorus in phosphogypsum.Based on the above results,mineral soil conditioner is prepared by purified potassium feldspar which activated by phosphogypsum,and the contents of effective constituents like K2 O,CaO and SiO₂ were 4.1%,35.5% and 20.0%,respectively.The mineral soil conditioner was applied to farmland soil in which cultivating plants such as sweet potato,pueraria thomsonii,folium cannabis,balsam pear,rice and grape with application amount was 50-100 kg per mu or more.Results of farmland field test showed that the yield of sweet potato increased by 40.1-53.4%,?-carotene content increased by 43.1-58.8%,but the content of heavy metals was decreased by 13.0-100%.The available K in the soil enhanced by 39.8-135.8%,and the available Al reduced by 2.2-8.7%,the exchangeable and bound to carbonates of Cd and Pb converted into the chemical forms of bound to Fe-Mn oxides and bound to organic matter resulted in reducing the bioactivity of heavy metals.For pueraria thomsonii,the yield increased by 12.5-22.3%,soluble sugar and starch increased by 22.7-27.7% and 4.8-5.0%,respectively.The content of heavy metals decreased by 22.2-96.9%,but the soil organic matter and the contents of available NPK increased in different extent.The pH of soil which planting folium cannabis increased from 5.3 to 6.0-6.4,and the content of available Pb,Cu,Zn and Ni which obtained from TCLP decreased by 37.8-84.9%.Heavy metal contents of leaf and stem for folium cannabis decreased by 41.2-67.8% and 52.0-76.7%,which leading to improve the safety of folium cannabis edible parts.The mean yields of balsam pear,rice and grape increased by 11.4-21.0%,along with the obvious changes of plant growth.Soil physicochemical properties were tested to be improved after 8 days of fertilization;it is evident that the effect of soil reclamation is more obvious after harvest.As the results from farmland field test,for soil conditioner,it can not only neutralize acidic soil,reduce the available heavy metals,supply medium and trace elements such as calcium,silicon and magnesium in the soil,improve soil quality,increase the content of soil organic matter and available NPK,facilitate the soil remediation;but also evaluate the plant grow,enhance the yield and nutrient contents of plants,lower the content of heavy metals,raise significant effects on the yield and quality of plants.