In this study, carbonate minerals (calcite, limestone, dolomite) were calcined in a microwave kiln to prepare precipitated calcium carbonate (PCC). Characteristic of CaCO3 using mega-crystalline calcite in electrical furnace and batch microwave kiln. Mega-crystalline calcite(m-CC) breaks apart easily during calcination, and cannot be easily converted to CaO due to its characteristic that requires massive heat consumption. To solve this problem, the calcination characteristics were compared using (a) electrical furnace and (b) batch type microwave kiln. Calcination of (a) was performed with m-CC which maintained the particle size in the range of 32 to 42 mesh at 950℃, for 240 min, and that by (b) at 950℃, for 30 min. The results of the XRD pattern of CaO that was formed due to calcinations indicated that decarbonation reaction by thermal decomposition occurred (a) as 98.2wt% at 950℃, 240 min, and (b) as 97.8wt% at 950℃, 30 min. CaCO3 was synthesized through the carbonation process. CaCO3 was synthesized at temperatures 25, 40, 60 and 80℃. At 25℃, in the case of (a), colloidal-shaped CaCO3 was found, and the more spindle-shaped CaCO3 by cubic-shaped self assembly was synthesized at higher temperatures. However, Ca(OH)2 existed in the carbonation process. In the case of (b), colloidal-shaped CaCO3(25℃) existed without Ca(OH)2, and the more spindle-shaped CaCO3 by cubic-shaped self assembly was synthesized at higher temperatures.
Characteristic of precipitated calcium carbonate by hydrothermal and carbonation process with mega-crystalline calcite. Domestic Mega-crystalline calcite(m-CC), which was hard to be calcined because of physico-chemical characteristics, was calcined and converted to CaO by a rotary microwave kiln. As a consequence, m-CC(size : 1.18 ~ 0.85mm) was thermally decomposed and decarbonated to form CaO(96~97 wt%) at 950oC in 30 minutes using the microwave kiln (rotation : 12cycle/min, feeder : 1.5 step(1kg/hr), 2 step(1.5kg/hr), 3 step(3kg/hr) and 4 step(4.5kg/hr). On the other hand, hexagonal-shaped aragonite PCC was prepared by the hydrothermal process. In this process, colloidal-shaped PCC from the carbonation process reacted with CO2 to form supersaturated Ca(HCO3)2 solution. Then, the solution was fed to hot water(100oC) at the flow rate of 10 ml/min as liquid droplets(size : 2 mm). In the CaO-(H2O:CH3OH(2:8 v/v%))-CO2 system, cubic-shaped PCC(size : 1 μm) was prepared by the suction effect of CH3OH. In case of non-suction, spherical-shaped PCC of vaterite structure(c-axis grown) was prepared.
Domestic medium-grade limestone is calcined to form CaO for the use in steel industries. And low-grade limestone is directly used for stack-gas desulfurization. For additional value, strong-acidic cation exchange resin was used to prepare Ca(OH)2 and CaCO3 of high purities. For the purpose, limestone was calcined to form CaO at 950 oC for 60 minutes in a batch-type microwave kiln. XRD results indicated the calcined product from medium-grade limestone is all CaO(100wt%), and the product from low-grade limestone is composed of CaO(97.6 wt%) and SiO2(2.4wt%). Hydration reactions resulted in 96.7, and 94.0 wt% of Ca(OH)2 with uncalcined CaCO3(2~3wt%) respectively. To prepare PCC, the adsorption of calcium ion and recovery reaction were conducted with formed CaO and cation exchange resin(R-SO3H)(mass ratio 1:24). Relatively high-grade PCC was prepared from the reaction with calcium adsorbed resin(Ca-(R-SO3)2) and NaOH. Although Ca(OH)2 is amorphous, Ca(OH)2 by calcium adsorption is plate-type and can be used as plastic filler. However, the delicate recovery condition of calcium ion is necessary because of difficulties in rate controls of calcium ion resolution and agglomerations.
Separation of Ca/Mg using MgCl2 and ion exchange resin from dolomite by batch microwave kiln. Calcium was separated from prepared CaO-MgO by the strong-acidic cation exchange resin(mass ratio 1:12 wt%). High purity MgO(higher than 94 wt%) with unburned CaCO3(1~2 wt%) was obtained by the separation process. Furthermore, almost pure MgO(96 wt%) was obtained by heating this high purity MgO at 950oC. for 60 minutes. Also, high-grade PCC could be prepared from the separated CaO. Separation of Ca and Mg in dolomite by the strong-acidic cation exchange resin has difficulties in adsorption of calcium ions. dissolution control, separation of MgO from resins and several steps for separations. On the other hand, separation of Ca and Mg by addition of MgCl2 is simple and the purity of produced Mg(OH)2 is high when compared to the ion exchange method. Almost pure Mg(OH)2 could be obtained under the optimum condition (CaO-MgO:MgCl2 = 1:2 wt%). * A thesis for the degree of Doctor in December 2012.