Research On The Clean Preparation Technology Of Sodium Metaborate Via Pressure Alkaline Leaching Of Boron Concentrate

In order to solve the problems of high energy consumption, low resourceutilization rate and severe environmental pollution from carbonate-alkaline methoddealing with boron concentrate existing in boron industry, a new process ofpressurized alkaline leaching purification by causticization crystallization separationwas developed and clean preparation of sodium metaborate was achieved. In thisdissertation, based on intensifying decomposition of boron concentrate via sodiumhydroxide and pressurized leaching method，basic research and process optimizationof the new technology were carried out. Main achievements and innovative progresseswere exhibited as follows:(1)Process of pressurized alkaline leaching purification by causticization crystallization separation dealing with boron concentrate was proposed for the firsttime. B was selectively leached by NaOH solution, effectively separated from Mg andFe. Reaction of liquid solid binary system was fast and leaching rate of boron washigh. Influence of initial concentration of NaOH solution, leaching temperature,reaction time, mass ratio of NaOH solution to ore, stirring speed and particle size ofore on leaching of boron was systematically studied. Results indicated that underoptimal conditions: initial concentration of NaOH solution of25%, alkaline leachingfor2h at140℃with mass ratio of liquid to solid of4:1, stirring speed of500rpmand particle size below74μm,95.91%of boron in boron concentrate was extracted.(2)Macro kinetics of decomposition of boron concentrate was investigated.Results indicated that the shrinking core model with surface chemical reaction controldescribed the decomposition process reasonably well. Under experimental conditions,leaching process of B is described by equation of1(1X)1/3=1.3712×102×t×e(31640/(RT))with apparent activation energy of31.64kJ/mol. Measures such asimproving leaching temperature and initial concentration of NaOH solution can beutilized to intensify leaching process of boron.(3)Causticized purification of leaching solution with CaO and washing of boricsludge were preliminarily investigated. Results indicated that temperature hadprominent effect on causticization. Causticized for2h at100℃, removal rate of Siwas about69.93%. Three staged countercurrent slurry washing was utilized torecycle Na2O and B2O3from wet boric sludge obtained by leaching boronconcentrate under optimal conditions and under the conditions of washing temperature of90℃, mass ratio of liquid to solid of3:1and washing time of1h,Na2O and B2O3were effectively recycled, with content in final boric sludge droppingto0.35%and0.45%respectively.(4)Induction time of nucleation and solubility isotherms of NaOH NaBO2H2Oternary ideal system at40and90℃were determinated. Results indicated the systemcould be easily crystallized with short induction time. Solubility of NaBO2inNaOH H2O system increases obviously as temperature increases and initiallydecreases and then alleviates when NaOH concentration increases. Coolingcrystallization can be utilized to separate sodium metaborate from leaching solution,avoiding problem of high energy consumption caused by evaporating crystallization.(5)Main Factors influencing conversion rate of boron in series leaching reactionwere explored which include amount of NaOH of addition, concentration of boron insolution and leaching temperature. Research on leaching behavior of mainlyassociated elements such as Mg, Fe, Si and Al indicated that, during decomposition ofboron concentrate, partial Si and Al were transferred from solid phase into liquidphase while Mg and Fe were nearly enriched in final boric sludge whose main phasewas Mg(OH)2. Results of two-staged leaching via four cycles indicated that underoptimal conditions of first-stage leaching while150℃in second-stage leaching andother conditions the same as in the first-stage, conversion rate of boron reached86.84~96.26%. Concentration of boron in second-stage leachate reached27.33~29.84g/L while content of Si and Al kept low and wasn’t improved obviously. Causticizedwith CaO and placed hermetically at90℃until faded, second-stage leachate wasnaturally cooled to25℃, crystallized isothermally for6h(stirring speed of200r/min)and nearly reached equilibrium after2h. Washed with anhydrous ethanol andsaturated NaBO2solution successively and dried at40℃, NaB(OH)4was obtained,with purity of almost90%, crystallization rate of over70%and section ofparallelogram or hexagon.