Characteristics of reaction and product microstructure during light calcination of magnesite in transport bed

doi:10.11949/0438-1157.20200530

Abstract

Abstract:

A gas-heating laboratory transport bed was adopted to simulate the industrial transportbed reactor for flash calcination of magnesite, and a method based on TG analysis of a reacted sample was further developed to calculate the decomposition rate or conversion of its containing MgCO₃. The study investigated how the conversion and product reactivity as well as microstructure vary with reaction conditions including temperature, particle size and times of re-calcination for powder magnesite. Magnesite powder (<150 μm) calcination is a quick reaction that reaches 98% decomposition of its containing MgCO₃ in 1—2 s,corroborating the feasibility of magnesite flash-calcination in transport bed reactors. The coloration time given by the citric acid chromogenic method was 17—55 s and 294 s for the obtained products from transport and fixed beds, respectively. This proves the obviously higher activity and thus improved microstructure of the product from transport bed. During the calcination process, the MgO grain size of the product gradually increases, and the surface structure changes from loose and porous to dense and smooth. This structural change can be completed within a few seconds.

Key words: magnesite, light calcination, fluidization, reactivity, microstructure

摘要：

利用高温烟气加热实验室规模输送床模拟菱镁矿闪速轻烧工业输送床反应器，建立基于热重分析计算菱镁矿中MgCO₃分解率方法，研究了煅烧条件和原料粒径对菱镁矿细粉输送床分解的转化率和产品活性的影响，揭示了煅烧过程中产物微观结构变化特性。菱镁矿粉（<150 μm）轻烧为秒级快速反应，仅需1~2 s菱镁矿中MgCO₃分解率即可达98%以上，验证了输送床闪速轻烧技术的可行性。输送床煅烧产物的柠檬酸显色时间17~55 s，活性显著高于固定床煅烧产物（显色时间294 s），且产物活性由菱镁矿分解率和微观结构共同决定；煅烧过程中产物MgO晶粒尺寸逐渐增大，表面结构由疏松多孔变为致密光滑，该结构变化可在数秒内完成。

关键词: 菱镁矿, 轻烧, 流态化, 活性, 微观结构

CLC Number:

TQ 026.7

SUN Cong,YAN Bowei,CAI Changyong,HAN Zhennan,XU Guangwen. Characteristics of reaction and product microstructure during light calcination of magnesite in transport bed[J]. CIESC Journal, 2020, 71(12): 5735-5744.

孙聪,闫博威,蔡长庸,韩振南,许光文. 菱镁矿输送床轻烧过程反应与产物微观结构特性[J]. 化工学报, 2020, 71(12): 5735-5744.

Figures/Tables 14

Fig.1 XRD patterns of magnesite

Table 1 Chemical compositions of magnesite

Composition	Content/%(mass)
MgO	46.12
SiO₂	0.58
Al₂O₃	1.27
CaO	0.53
MnO	0.01
Fe₂O₃	0.24
LOI^①	51.24

Table 2 Calcination conditions in muffle furnace and product property

Particle size/μm	Temperature/℃	Residence time /h	LOI/%(mass)	Coloration time/s
106~150	900	2	0.1	294

Fig.2 Diagram of gas-heating transport bed calcinatory

Fig.3 Temperature distribution of gas-heating transport bed reactor

Table 3 Operation parameters of transport bed under different calcination conditions

Case	Feeding rate of magnesite/ (kg/h)	Bottom temperature of calcinator /℃	Flow rate of propane/ (m³/h)	Flow rate of air / (m³/h)	Content of CO₂ in flue gas/ % (vol.)	Flow rate of flue gas/ (m³/h)	Gas velocity in calcinatory/ (m/s)	Residence time /s
Case 1	1.2	850	1.05	64.18	4.18	75.36	9.7—12.3	0.91
Case 2	1.2	900	1.13	68.74	4.32	78.4	10.5—13.5	0.84
Case 3	1.2	950	1.25	71.25	4.71	79.67	10.9—14.2	0.79
Case 4	1.2	1000	1.33	74.83	4.98	80.96	11.4—15.2	0.75

Fig.4 TG analysis of magnesite and calcined product

Fig.5 Influence of calcination conditions and particle size on decomposition ratio of magnesite

Fig.6 TG analysis of calcined magnesite products under different calcination conditions

Fig.7 Influence of calcination conditions on coloration time (a) and activity index (b) of products

Fig.8 Influence of particle size on coloration time (a) and activity index (b) of products

Fig.9 XRD patterns of calcined magnesite products

Table 4 Half-peak width of peak at 2θ=42.9° in XRD pattern and grain size of MgO crystal

Number of calcination cycles	B^①	D^②/nm
1	0.81	10.5
2	0.72	12.3
3	0.64	13.3
4	0.63	13.8

Fig.10 SEM images of calcined magnesite products

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