| Precipitation is a very important method which is widely used in chemical industry in the production of ultrafine powders. Usually, micromixing affects the performance of the powders in the precipitation. Four types of micromixers, straight flow impinging streams micromixer (Ml), conical flow impinging streams micromixer (M2), straight streams vortex micromixer (M3), and twice vortex streams micromixer (M4) were designed on the base of impinging streams mixer according to flow forms and impinging manners of fluids. Then the mixing characteristics of the four types of micromixers were studied by experimental research and theoretical simulation method. The dissertation had a detailed research that the synthesis of barium sulfate (BaSO4), aluminum hydroxide (A1(OH)3) and cerium-doped yttrium aluminum garnet (YAG:Ce3+) yellow phosphor powders by precipitation methods in the four types of micromixers.1. The mixing performance of liquid-liquid in the micromixers. The residence time distribution experiments were performed in the four micromixers and saturated KC1solution was used as a tracer. The results showed fluid in the micromixers was close to full Francis. The inlets diameter of micromixers had little effects on mean residence time and its distribution. The mean residence time delayed and peak width became narrower in the micromixer M4because vortex improved macromixing process of the fluids. The iodide-iodate fast parallel competing reaction was used to quantitatively determine the segregation index Xs in the four different micromixers. The micromixing performance was characterized with segregation index. The smaller the Xs is, the better the micromixing effects are. The effects of volumetric flow, volumetric flow ratio and micromixer configuration on the micromixing performance Xs were studied in details. The experimental results showed the value of Xs was about3.5×10-3～6.0×10-3and Xs had the following rule, XsM4<XsM3<XsM2<XsMi among the four type micromixers. Xs decreased with increasing flow rate, increased with increasing flow ratio, and increased with increasing inlet diameter of nozzles. Macro properties of velocity field, concentration field, turbulent kinetic energy k and turbulent kinetic dissipation rate ε distribution fields were simulated by Fluent software. Distributions of macro flow field results were consistent with the residence time distribution results from experiments. The micromixing time was both1millisecond grade according to Xs experimental data and s simulated values.2. The mixing performance of gas-liquid in the micromixers. The averaged gas-liquid interfacial specific surface area (a) and liquid-mass transfer coefficient kL were experimentally measured by chemical absorption method, in which NaOH-CO2-N2and H2O-CO2-N2reaction systems were used. The experimental results showed a value was about1.2×105～1.0×106m2·m-3, the relation of the averaged gas-liquid interfacial specific surface area a and operating parameters was, The a value of M1, M2, M3and M4gradually increased at the same technological conditions, a had the proportional relations with d-2.22～2.02, qv,L0.55-o.62and qv,g0.38～0.50.The KL value was about0.02-0.12cm·s-1and it increased at least one or two orders of magnitude higher liquid side volumetric mass transfer coefficients compared with the conventional gas-liquid contactors. Vortex resulted in extra contact areas which can guarantee high mass transfer rates in the subsequent micromixer. The kL value decreased with the increase of liquid and gas flow rate, as well as the decrease of the inlet diameter. When the liquid flow rate increased from5L·h-1to25L·h-1, kL increased about2.69-3.23times. When the inner diameter decreased from2mm to1mm, kL increased about1.08～1.36times. According to the vortex model, the theoretical mass transfer coefficient kL was consistent with the experimental results.3. BaSO4was prepared by liquid-liquid mixing in the micromixers. The experiments were performed with two reactants, BaCl2solution and Na2SO4solution to prepare BaSO4in the four micromixers. The results indicated the particles prepared by impinging streams micromixer M4had smaller mean size and narrower size distribution than the other three micromixers because Vortex could intensify micromixing in the precipitation process. The mean size decreased with the increase of liquid flow rate, initial concentration, as well as the decrease of volumetric ratio of reactants and the inlets diameter size. Barium sulfate superfine ellipsoidal particles were prepared with the size of about100nm and the specific surface area of the particles29.25m·g-1at the best technological conditions.4. Al(OH)3was synthesized by gas-liquid mixing in the micromixers. Fibrous pseudoboehmite particles with average diameter about10nm,200nm in length, surface area of390.6m·g-1and large pore volume of2.54cm3·g-1, were prepared by the carbonation method from sodium aluminates solution and carbon dioxide at the optimum conditions. Vortex resulted in smaller particle size and narrower distribution than straightly impinging streams. The mean size and dimensionless variance decreased with the increase of liquid flow rate, gas flow rate, as well as the decreasing of initial concentration and the inlets diameter size.5. Micromixers were further optimized for the preparation of YAG:Ce3+yellow phosphor, used ammonium bicarbonate as precipitant. The results indicated that YAG:Ce3+phosphor particles with a ellipsoidal shape, a mean particle size of about100nm and a narrow particle size distribution could be successfully prepared in them. The YAG powders without any impurity phases could be obtained at1000℃for2h and its crystalline phase transition temperature was about500℃lower than solid-phase sintered method. The luminescent intensity gradually decreased with the increase of the initial concentration of raw materials. The luminescent intensity first increased then decreased with the increase of Ce%doped concentration, and the best Ce%was2.6%. The luminescent intensity gradually increased the increasing flow rate and which changed little when the flow rate exceeded20mL·min-1. The emission intensity of YAG:Ce3+phosphor prepared by shaped impinging streams micromixer M3had about18.6%higher than that by M1and that from M423%higher than that from M1. The results show that the impinging streams micromixers can intensify micromixing effects among reaction materials. So the YAG:Ce3+phosphor with small particle size and a uniform distribution were prepared, and which has excellent luminescence properties. |
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