Research On Penomenon,Mechanism,Dynamics Of Borates Crystallization By Diluting Boron-containing Brine Of Sulfate-type Saline Lakes With Water

There are lots of liquid boron resources in salt lakes in Qinghai-Tibet plateau,China.The chemical type of these salt lakes brine is primarily classified into chloride-type,sulfate-type and chloride-type.Among these three types' boron-containing salt lakes,the borate rich in the sulfate-type brine does not crystallize out but accumulates in the highly concentrated brine during brine evaporation,thus showing up high supersaturation phenomena.However,when the high concentrated boron was diluted with water,different species of magnesium borates would crystallize out from the brine,which is called as “crystallizing by dilution”.Based on the “crystallizing by dilution” phenomena,Boron separation from salt lake brine by dilution method can be used during the exploitation and utilization of salt lake resources.Owing to its simple technological process without introducing into other chemical materials and large scale industrial equipment,the dilution method is a green and environmental protection separation technology.It has a wide developing prospect for extracting boron from sulfate-type brine.To solve the problems such as long time solid-liquid equilibrium,complicated phase transformation mechanism,low crystallization rate and low boron recovery during boron separation by dilution method,the paper,taking Da Qaidam salt lake as a research object,has investigated the crystallization regularity,phase transformation mechanism and kinetics of crystallization during dilution process of sulfate-type salt lake brine by Raman,11 B NMR and kinetic model.The results can provide basic physical chemistry data and technical support for the application of dilution method in boron separation from salt lake.The main research work is as follows:?1?The high concentrated boron-containing brine saturated with magnesium chloride was prepared by evaporation after adding mirabilite ores to remove the Mg2+ion.The effects of boron concentration,dilution ratio and temperature on the crystallization period,solution pH,borate species and boron separation recovery during dilution process have been investigated.Finally,the best process parameters of boron separation by dilution method have been obtained.?2?The high concentrated boron-containing brine was used as raw materials.The synthesis condition and phase transformation mechanism of the new magnesium borate?MgB₆O₇?OH?₆·5H₂O?and Pinnoite have been studied by dilution method.The crystal structure of MgB₆O₇?OH?₆·5H₂O compound was solved and refined using conventional X-ray powder diffraction techniques and Raman characterization.Itbelongs to the monoclinic crystal system and the space group is P21/c.The thermal behavior of this new compound has been investigated.The phase transformation mechanism of the new magnesium borate?MgB₆O₇?OH?₆·5H₂O?and Pinnoite have been obtained.?3?The main species and mutual transformation mechanism of polyborate anions in boron-containing brine during dilution crystallization process and evaporation process have been studied by Raman and polythermal 11 B NMR spectroscopy.The chemical shift of borate anions in solution has been assigned.The results showed that the main species of polyborate anions in boron-containing brine are B₆O₇?OH?₆2-?B₅O₆?OH?4-?B₃O₃?OH?4-?B?OH?3 and so on.Among of which the B₃O₃?OH?4-is the principal species.The phase transformation mechanism of magnesium borate has been revealed.?4?The kinetics of crystallization of magnesium borate during dilution has been studied with the kinetic models based on the surface reaction control mechanism.The crystallization kinetics equations of magnesium borate have been calculated by combining the simplex optimization method and Runge-Kutta method.The reaction order and reaction rate constant have been obtained.And the relationship between crystallization rate and temperature,time,boron concentration and dilution ratios has also been studied.