鸟粪石晶体生长速率关键影响因素的定量分析

doi:10.11949/0438-1157.20210702

摘要/Abstract

摘要：

鸟粪石（MAP）结晶法处理氨氮废水具有去除效果好、反应速度快等特点，但MAP生长过程缓慢，晶粒细小，限制了MAP副产品的回收率，经济价值降低。利用基于黏附型生长机制建立的化学势梯度模型结合德拜-休克尔极限公式计算活度系数，研究了不同因素（温度、搅拌速度、pH、离子摩尔比）对MAP晶体生长速率的影响。结果表明，温度、搅拌速度和摩尔比的提升导致动力学生长速率常数 $k t$ 提高，从而导致鸟粪石生长速率上升，pH的升高使得反应初期的生长速率常数 $k t$ 和热力学推动力 $? μ$ 都得到了增强，到了反应后期，主要影响生长速率常数 $k t$ ，最终导致整个反应过程中MAP生长速率上升。对MAP晶体的平均生长速率计算发现，其生长速率分布在10^-9～10^-8 m/s之间；在pH=9.5时，生长速率提高至1.02×10^-8 m/s；离子摩尔比[Mg²⁺]∶[ $P O 4 3 -$ ]∶[ $N H 4 +$ ]=1∶1∶1提高至1.2∶1.2∶1时，MAP生长速率最高达到了1.29×10^-8 m/s。综上分析表明，温度、搅拌速度、pH和离子摩尔比提升，MAP晶体的生长速率均有不同程度的提高，其中，离子摩尔比影响最大，pH次之，温度和搅拌速度影响较小。因此，在实际工业中，pH和离子摩尔比的改变有望调控MAP的结晶过程。

关键词: 鸟粪石, 氨氮废水, 生长速率, 速率常数, 推动力

Abstract:

The treatment of ammonia nitrogen wastewater by struvite (MAP) crystallization method has the characteristics of good removal effect and fast reaction speed. However, the slow growth processes and small grain sizes of MAP limit the recovery rate of MAP by-products and reduce their economic value. The effects of different factors (temperature, stirring rate, pH, ion molar ratio) on the growth rate of MAP crystals were studied by using the chemical potential gradient model based on the adhesive-type growth mechanism and the Debye-Hückel limiting law to calculate the activity coefficient. The results showed that the increase of temperature, stirring speed and molar ratio led to the enhancement of growth rate constant $k t$ , which led to the increase of struvite growth rate. The higher pH enhanced the rate constant $k t$ and thermodynamic driving force $? μ$ at the beginning of the reaction. At the later stage of the reaction, the growth rate constant $k t$ was mainly affected. Finally, the growth rate of MAP increased during the whole reaction process. The average growth rate of MAP crystals were calculated, and it was found that the growth rates were between 10^-9 and 10^-8 m/s. When pH=9.5, the growth rate increased to 1.02×10^-8 m/s, as the ionic molar ratio [Mg²⁺]∶[ $P O 4 3 -$ ]∶[NH₄⁺] =1∶1∶1 increased to 1.2∶1.2∶1, the highest growth rate of MAP reached 1.29×10^-8 m/s. In conclusion, with the increase of temperature, stirring speed, pH and ion-molar ratio, the growth rates of MAP crystals increased to varying degrees. Among them, the influence of ion-molar ratio was the largest, followed by pH, and the influence of temperature and stirring speed was small. Therefore, changes in pH and ion molar ratio are expected to regulate the crystallization process of MAP in practical industry.

Key words: struvite, ammonia nitrogen wastewater, growth rate, rate constant, driving force

中图分类号:

X 703.1

孙雅, 周通, 陈广源, 季琳安, 吉远辉, 陆小华, 王昌松. 鸟粪石晶体生长速率关键影响因素的定量分析[J]. 化工学报, 2021, 72(11): 5831-5839.

Ya SUN, Tong ZHOU, Guangyuan CHEN, Linan JI, Yuanhui JI, Xiaohua LU, Changsong WANG. Quantitative analysis of key factors affecting struvite crystal growth rate[J]. CIESC Journal, 2021, 72(11): 5831-5839.

图/表 8

图1 实验装置示意图1—pH计;2—搅拌桨;3—高精度温度计;4—300 ml夹套烧杯;5—恒温水浴槽

Fig.1 Schematic diagram of experimental equipment

表1 MAP结晶的实验条件

Table 1 Experimental conditions of MAP crystallization kinetics

序号	温度/℃	pH	搅拌速度/(r/min)	[Mg²⁺]∶[ $P O 4 3 -$ ]∶[ $N H 4 +$ ]
1	20	9.0	400	1∶1∶1
2	25	9.0	400	1∶1∶1
3	30	9.0	400	1∶1∶1
4	25	9.0	300	1∶1∶1
5	25	9.0	400	1∶1∶1
6	25	9.0	500	1∶1∶1
7	25	8.5	400	1∶1∶1
8	25	9.0	400	1∶1∶1
9	25	9.5	400	1∶1∶1
10	25	9.0	400	1∶1∶1
11	25	9.0	400	1∶1.2∶1
12	25	9.0	400	1.2∶1.2∶1

表1 MAP结晶的实验条件

Table 1 Experimental conditions of MAP crystallization kinetics

序号	温度/℃	pH	搅拌速度/(r/min)	[Mg²⁺]∶[ $P O 4 3 -$ ]∶[ $N H 4 +$ ]
1	20	9.0	400	1∶1∶1
2	25	9.0	400	1∶1∶1
3	30	9.0	400	1∶1∶1
4	25	9.0	300	1∶1∶1
5	25	9.0	400	1∶1∶1
6	25	9.0	500	1∶1∶1
7	25	8.5	400	1∶1∶1
8	25	9.0	400	1∶1∶1
9	25	9.5	400	1∶1∶1
10	25	9.0	400	1∶1∶1
11	25	9.0	400	1∶1.2∶1
12	25	9.0	400	1.2∶1.2∶1

图2 不同因素对MAP晶体生长动力学的影响(a) 温度;(b) 搅拌速度;(c) pH;(d) 离子摩尔比

Fig.2 Effect of different factors on the kinetics of MAP crystal growth(a) temperature; (b) stirring speed; (c) pH; (d) ion molar ratio

表2 不同因素影响下的生长速率常数kt

Table 2 Growth rate constant kt under the influence of different factors

序号	k_t/(10^-4 mol/min)	R²
1	2.51	0.995
2	3.15	0.991
3	3.78	0.981
4	1.98	0.948
5	3.15	0.991
6	3.64	0.984
7	2.95	0.988
8	3.15	0.991
9	5.00	0.972
10	3.15	0.991
11	6.54	0.997
12	9.68	0.989

图3 不同因素对热力学推动力?μ的影响：(a)温度；(b)搅拌速度；(c)pH；(d)离子摩尔比

Fig.3 Influence of different factors on thermodynamic driving force ?μ: (a) temperature;(b) stirring speeds;(c) pH;(d) ion molar ratio

图4 晶种的粒径分布图（a）和XRD衍射图（b）

Fig.4 Grain size distribution (a) and XRD diffraction (b) of crystal seed

图5 不同因素影响下MAP晶体的粒径分布图：(a)温度；(b)搅拌速度；(c)pH；(d)离子摩尔比

Fig.5 Size distribution of MAP crystals under the influence of different factors: (a) temperature;(b) stirring speeds;(c) pH;(d) ion molar ratio

表3 不同实验条件下晶体的平均生长速率

Table 3 Average growth rate of crystals under different experimental conditions

序号	L_product/μm	r/(m/s)
1	73.99	5.95×10^-9
2	73.99	5.95×10^-9
3	73.99	5.95×10^-9
4	62.22	4.32×10^-9
5	73.99	5.95×10^-9
6	87.99	7.90×10^-9
7	73.99	5.95×10^-9
8	73.99	5.95×10^-9
9	104.6	1.02×10^-8
10	73.99	5.95×10^-9
11	104.7	1.02×10^-8
12	124.4	1.29×10^-8

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