| P507 is an extracting agent and is widely used for the extraction of rare earth elements.But it needs to be saponified with ammonia before extraction,leading to the production of large amounts of waste water containing ammonia-nitrogen.The extractant used in this paper is a non-saponified system consisting of P507,N235 and kerosene.In this paper,the influences and process conditions of extraction and separation of light rare earths,the stability of the organic phase recycling and the effect of water on the extraction was studied.The organic phases before and after extraction were also determined by spectrum analysis.The main contents and results of this paper are as follows:(1)In P507-N235 system,the extraction rates and distribution ratios of La,Ce,Pr,Nd increased with increasing the concentration of P507 when the pH was greater than 3,but decreased when pH was less than 3.The extraction rates,the distribution ratios and the raffinate pH increased with the increase of the dosage of N235.It was easy to form a third-phase when increasing the dosage of P507 or N235 to the result that too high extraction rate was obtained.The extraction rates and the distribution ratios increased with increasing phase ratio when the composition of the organic phase was constant.The time of phase separation was prolonged as the phase ratio was too high.When the organic phase was P507 30%,N235 25%,kerosene 45%and the phase ratio was 3:1 or 4:1,the time of phase separation of LaCe/PrNd separation was relatively short and the high separation coefficient was obtained.Nd/Pr separation coefficient in P507-N235 system was close to that in saponified-P507 system,while Ce/La separation coefficient in P507-N235 system was slightly high than that in saponified-P507 system.(2)In P507-N235 system,the process of extraction and separation of the mixed light rare earths is as follows:The feed solution of mixed light rare earth liquid was separated by LaCe/PrNd grouping extraction.After three stages of extraction,the raffinate was obtained containing La and Ce,while Pr,Nd,and a small amount of La,Ce were enriched in the loaded organic phase.Then through four stages of washing with water,La and Ce entering into the organic phase were washed into the washing liquid.After washing,the loaded organic phase was stripped by 1mol/L hydrochloric acid and the mixture of Pr and Nd was moved into the stripping solution.Pr can be separated from Nd through further procedures of extraction,washing and stripping.The washing liquid and the raffinate from LaCe/PrNd grouping extraction were mixed,and La and Ce can also be separated through further processes of extraction,washing and stripping.(3)Hydrogen bond may exist between P507 and N235 after P507 mixed with N235,and the kerosene may weaken the hydrogen bonds.In P507-N235 system,N235 extracted acid of the feed solution and formed N-H bond,and thus,the main role of N235 is to regulate the acidity of the extraction process,but do not involve in the extraction reaction.Extraction reaction occurred at P-OH group in P507,and H in P-OH exchanged with rare earth cations,leading to the changes of the local environment of P-OH and P=O(adjacent to P-OH)groups:in the infrared spectra,when P507 mixed with N235,P=O stretching vibration,P-OH stretching vibration and P-OH bending vibration shifted to lower wave numbers,whereas these characteristic peaks moved to higher wave numbers when the extraction reaction occurred between P507-N235-kerosene organic phase and light rare earths.The same changes were observed for the saponification reaction of P507 and the extraction reaction between saponified-P507 and light rare earths.For the extraction of mixed light rare earths and the extraction of single light rare earth,the infrared spectra and NMR spectra of the organic phase are similar,and only the shift amplitude of P-OH stretching vibration and P-OH bending vibration was slightly different after extraction.(4)For the extraction reaction in P507-N235 system,the molar ratio of extraction reaction of P507 and Ce3+ was close to 1.5:1,suggesting the reaction may be mainly cation exchange mechanism;The molar ratio of P507 and Nd3+ was close to 1:1,suggesting the production of Cl--coordination complex;The extraction between P507 and Pr3+ or La3+ may not only occur cation exchange reaction,but also produce the Cl--coordination complex.Extraction of Nd in mixed light rare earths may be mainly cation exchange mechanism.Extraction of La,Ce and Pr in mixed light rare earths may not only occur cation exchange reaction,but also produce the Cl--coordination complex.Because of the competitive extraction of Nd>Pr>Ce>La,the extraction mechanism of Nd or La in mixed light rare earths and the extraction mechanism of single Nd or La are different.(5)The extraction ability of P507-N235 system remains unchanged after repeated recycling.P507 and N235 changed to the original structure after stripping.The structure of the recycled organic phase and the structure of the initial organic phase are the same.(6)Water can be extracted into the organic phase when the extraction between P 507 and rare earths,resulting in the emulsification and difficulty in phase separation.P507 can extract water and water in acidic solution,and N235 only extracts water in acidic solution.Hydrochloric acid inhibits water entering into P507 but promotes water entering into N235.NH4+ promotes water entering into P507.Saponified-P507 is easier to extract water than P507.In the acidic environment,N235 is easier to extract water than P507.Water is easy to be extracted into the organic phase in saponified-P507 system,the probability of emulsification and difficulty in phase separation in sapcnified-P507 system is much higher than that in P507-N235 system,and thus,only saponification rate of 40～60%were obtained for the ammonia saponification of P507,while the difficulty in phase separation occurred under the other saponification rates.In P507-N235 system,water enters into the organic phase during the first extraction,and no more water enter into the organic phase during the processes of washing and stripping and the subsequent recycling processes.Water entering into the organic phase will not be washed or stripped into the aqueous phase during washing,stripping and recycling processes.Water has hardly interferences to the phase separation during extraction of rare earths.Therefore,the P507-N235 system has the advantages of no consumption of alkali liquor,no waste water discharge and non-saponification of P507,and will be used for the separation of light rare earth elements. |
