| Rare earth has become a strategic resource for transforming traditional industries and developing high-tech,cutting-edge technologies and new energy materials due to its unique physical and chemical properties.The weathered crust elution-deposited rare earth ores is the main source of medium and heavy rare earth.In industry,ammonium sulfate is usually used to recover rare earth through in-situ leaching process.However,due to the complexity of actual geological conditions of mine,the huge difference in the structure of ore body,the accumulation and enrichment of leaching agents and residual ammonium,the in-situ leaching process of ammonium sulfate has many problems to be solved urgently,such as low rare earth leaching efficiency,large consumption of ammonium sulfate,and serious ammonia nitrogen pollution.In order to better solve these problems,the application of aid leaching agent has become the key.Therefore,the environmentally friendly natural surfactant humic acid was selected as the aid leaching agent in this study.The interfacial properties and structural composition of humic acid were studied.The effect of humic acid compound ammonium sulfate on rare earth leaching process was discussed to reveal the synergistic mechanism of rare earth leaching activated by humic acid combined with ammonium sulfate.The calculation model of ammonium sulfate dosage based on the law of mass action was constructed to study the ammonium reduction effect of humic acid combined with ammonium sulfate to leach rare earth ore.The feasibility of humic acid as an aid leaching agent to optimize the dosage of ammonium sulfate and reduce the residual ammonium salt was clarified to realize the source control of ammonia nitrogen pollution.The main research contents and conclusions are as follows:(1)The results of interfacial properties and structural composition of humic acid show that the surface tension of humic acid solution decreases first and then tends to be stable with the increase of humic acid concentration.When the concentration of humic acid reaches 0.2 g/L,the surface tension of the solution hardly changes.The critical micelle concentration of humic acid is 0.22 g/L calculated by surface tension two-line method.The results of Zeta potential test show that the interface of humic acid is electronegative,and the Zeta potential value decreases first and then stabilizes with the increase of humic acid concentration.When the concentration of humic acid exceeded 0.2 g/L,the Zeta potential of humic acid solution remains basically unchanged.The results of functional group determination show that the total acidic group content of humic acid is 7.21 mmol/g,the carboxylic acid group content is 5.24 mmol/g,and the phenolic hydroxyl group content is 1.97 mmol/g.Elemental analysis,ultraviolet and infrared characterization show that there are a large number of C=C double bonds,alkane structures and aromatic fat structures in the humic acid in addition to the carboxylic acid,alcohol hydroxyl and phenolic hydroxyl functional groups.(2)The leaching process results of rare earth ore leached by humic acid-ammonium sulfate show that the optimal leaching process assisted with humic acid is as follows:the optimal concentration of humic acid is 0.2 g/L,the liquid-solid ratio is 2:1,the pH of the leaching solution is 6.5,the flow rate of the leaching solution is 0.6 mL/min and the leaching temperature is 298 K.The leaching efficiency of rare earth under the optimal conditions is 95.27%.The leaching kinetics study shows that the leaching process of weathered crust elution-deposited rare earth ores is controlled by the internal diffusion kinetic model.The rare earth leaching kinetic equations related to the concentration of humic acid,0～0.2 g/L and 0.2～0.5 g/L,are as follows:1-2/3α-(1-α)2/3=(0.00430C01.87776+0.000389)t(0～0.2 g/L)and 1-2/3α-(1-α)2/3=(0.0000307C0-1.56881+0.000389)t(0.2～0.5 g/L).When the concentration of humic acid is 0～0.2 g/L,the kinetic equation of rare earth leaching related to temperature and humic acid concentration is:1-2/3α-(1-α)2/3=0.00433C01.87776e-15390/RT t+0.000389t.The study on the mass transfer leaching process shows that when the concentration of humic acid is 0.2 g/L,the leaching temperature is 323 K,and the pH of the leaching solution is 6.5,the peak concentration of rare earth is the highest and the corresponding HETP is the smallest,suggesting the best mass transfer efficiency of rare earth leaching.It can be seen that the optimal addition of humic acid,the moderate high temperature and the weak acid environment are beneficial to improve the mass transfer efficiency of rare earth.The surface tension of humic acid combined with ammonium sulfate solution shows that the presence of humic acid with different concentrations can reduce the surface tension of ammonium sulfate solution.The lower the surface tension,the faster the seepage of the leaching agent solution as well as the more conducive to the rare earth leaching.Infrared spectroscopy analysis shows that the hydroxyl group in humic acid is adsorbed into the ore body,when the humic acid is applied with ammonium sulfate.(3)The dosage optimization results of ammonium sulfate show that when humic acid combined with ammonium sulfate is used as leaching agent,its mass action quotient Kt value is larger than that of single ammonium sulfate,and the reaction molar ratio(n)is smaller than that of single ammonium sulfate.The unit consumption equations of ammonium sulfate for the single ammonium sulfate leaching process and the humic acid-ammonium sulfate leaching process are as follows:m(NH4)2SO4=ε2.368+1(2.368)2.368βαMM(NH4)2SO4/2(1-ε)0.864MREO×mRE and m(NH4)2SO4=ε2.267+1(2.267)2.267βαM(NH4)2SO4/2(1-ε)0.993MREO×mRE.The colum leaching results show that the presence of humic acid can reduce the consumption of ammonium sulfate,and improve the actual leaching efficiency of rare earth than that of single ammonium sulfate.The simulated leaching of 288 t actual rare earth ores was carried out with the fixed rare earth leaching efficiency of 80%.The results show 26%of the amount of ammonium sulfate can be saved using the synergistic leaching technology of humic acid combined with ammonium sulfate than that of single ammonium sulfate.Furthermore,the content of residual ammonium in tailings decrease especially the content of water-soluble ammonium and exchangeable ammonium decrease to 88%and 58%of single ammonium sulfate,respectively.The optimal dosage ratio is 0.2 g/L humic acid combined with 0.08 mol/L ammonium sulfate as leaching agent,with a dosage decrease of ammonium sulfate from 0.1 mol/L in the traditional process to 0.08 mol/L,where the leaching efficiency of rare earth is 95.18%.Compare with the traditional ammonium sulfate leaching method,humic acid-ammonium sulfate has a significant synergistic effect,which can not only improve the rare earth leaching efficiency,but also reduce the ammonium sulfate concentration to 80%of the original. |
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