| As an important basic chemical raw material,boron and its compounds play an important role in development of national economy and industrial manufacturing.Its raw materials are mainly derived from solid boron ore and brine ore.During exploitation of salt lake brine,high-efficiency separation of boron can lead to high-quality boric acid products,while minimizing influence of boron on the quality of valuable elements such as lithium and magnesium.Solvent extraction method is the most preferred means for industrial practices of boron separation from salt lake brine.This technique has many advantages,such as low effect by boric acid content in the solution,short process and simple process equipment,easy operations,etc.In this paper,solvent extraction method was used to study the mechanism and process application of boric acid extraction.A new process for efficient extraction of boric acid was developed.The main research scope and conclusions are as follows:(1)Two monohydric aliphatic alcohols A1416 and 2-butyl-1-n-octanol and a dibasic fatty alcohol 2,2,4-3-methyl-1,3-pentane(TMPD)were used as the complex extractant,and the boron extraction systems were constructed.The optimal process parameters of three extraction systems for extracting boric acid were obtained by single factor experiment.The optimized extractant concentration were 50 vol.%A1416,2.0mol/L 2-butyl-1-n-octanol,0.4 mol/L TMPD,phase ratio O/A=1/1,p H=1.Under this condition,the single-stage extraction efficiency of boric acid were 88.60%,89.21%,and 89.56%,respectively.The extraction efficiency of boric acid after three-stage countercurrent extraction exceeded 99%;The stoichiometry of the extracted complex is about 1.5.The stoichiometry of TMPD to boric acid is 1.17 at the initial concentration c(TMPD):c(B(OH)3)<1,and 2.17 at the initial concentration c(TMPD):c(B(OH)3)>1.(2)The kinetics of extraction of boric acid by 2-butyl-1-n-octanol and TMPD extractants were investigated by constant interface cell method.The interface area,stirring speed,reactant concentration and reaction temperature were investigated.The results showed that the extraction process of the two extraction systems occurs in the two-phase interface region.The extraction reaction is controlled by diffusion and chemical reaction,and the main diffusion resistance zone is concentrated in the water phase.At 298 K,the kinetic rate equations for the extraction of boric acid from brine by 2-butyl-1-n-octanol and TMPD were R=7.231×10-3[B(OH)3][R(OH)]2and R=2.928×10-2[B(OH)3][TMPD],respectively.The apparent activation energies of the two systems were 19.038 k J·mol-1 and 21.244 k J·mol-1,respectively,which were both mixed control types with diffusion and chemical reactions.The results of thermodynamic experiments show that the extraction of boron by 2-butyl-1-n-octanol/kerosene extraction system is a spontaneous exothermic reaction,and TMPD/CCl4 extraction is a spontaneous endothermic reaction.(3)The extraction mechanism of 2-butyl-1-n-octanol and TMPD extraction system was studied by means of IR spectroscopy and 1H NMR spectroscopy.The alcoholic hydroxyl group of the extractant complexed with boron to form the extractant into the organic phase.Raman spectroscopy was used to study the existence forms of boron in different boron-containing liquids and their relationship with each other.The results show that boron converted from B(OH)3 to[B(OH)4]-and poly-boron anion with increasing p H.TMPD is mainly complexed with B(OH)3 and[B(OH)4]-in solution,but not with[B3O3(OH)4]-.The organic phase in the extraction process was monitored by React IR spectrometer.As the extraction progressed,the absorption peak of boric acid B-O appeared in the organic phase and the intensity gradually increased.When 2-butyl-1-n-octanol is complexed with boron,both mono-and dibasic borate esters are formed.(4)The main chemical species in the process of extracting boric acid by 2-butyl-1-n-octanol and TMPD were calculated by B3LYP/6-311(d,p)DFT method in GAMESS package.The results show that the B-O bond in B(OH)3 is a double bond,which is more stable than the single B-O bond of[B(OH)4]-.In the reaction of 2-butyl-1-n-octanol with boron,the B-O bond levels and stability of the complexes BR and B2R decrease in turn.The Gibbs free energy change of the reaction indicates that the main resistance of the reaction is the migration of B(OH)3 from the aqueous phase to the organic phase.The enthalpy of the reaction indicates that the extraction reaction is exothermic.In the complexation reaction between TMPD and boron,the Gibbs free energy of TMPD and B(OH)3 is lower than that of[B(OH)4]-,indicating that TMPD is easier to interact with B(OH)3.The reaction between TMPD and B(OH)3 mainly produces B3L and BDC.The extraction reaction is endothermic.(5)The multi-stage countercurrent extraction experiment and the stripping regeneration process of A1416/kerosene,2-butyl-1-n-octanol/kerosene and TMPD/CCl4 were carried out,respectively.And on this basis,a full process experiment was carried out.With A1416/kerosene as the extraction system,the hydraulics and mass transfer performance of the centrifugal extractor were studied,and the cycle cascade full-scale process amplification experiment was carried out on the mixer-settler.The extraction rate of boric acid reached more than 99%,the concentration of the stripping solution H3BO3 was 45?50 g/L,and the purity of the untreated stripping solution was>97%.Through the analysis of the post-equilibration extraction and stripping stages,the distribution of boric acid and Li+,Na+,K+,Ca2+and Mg2+ions during the extraction cycle was revealed,which provided a theory for the cascade extraction of boric acid from salt lakes.It provides a theoretical basis for process design and process enhancement in the future process enlargement. |
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