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Study on Treatment of Wastewater Containing Heavy Metals by Reaction Crystallization Method

Author: ZhangFang
Tutor: ZhouQiuSheng
School: Central South University
Course: Metallurgical Engineering
Keywords: wastewater containing heavy metal ions reactioncrystallization additives Ostwald ripening
CLC: X703
Type: Master's thesis
Year: 2013
Downloads: 23
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Wastewater containing heavy metal ions (WHM) causes serious environmental pollution. The economic and efficient treatment of WHM has become a hot issue in the field of global environmental protection. Chemical precipitation is one of the main methods to treat WHM. However, the particles formed in the chemical precipitation process are very fine and thus difficult to separate, resulting in low efficiency of solid-liquid separation and high concentration of heavy metal ions in the effluent which is difficult to reach the standard steadily. Under this consideration, the treatment of simulated wastewater containing heavy metal ions (SWHM) has been investigated by the reaction crystallization based on crystallization principles. The influences of traditional batch operation, continuous operation without overflow and continuous operation with overflow on the reaction crystallization process of SWHM were first studied. The influencing factors of reaction crystallization process and crystallization behavior of particles were then investigated separately during the purification process of simulated wastewater containing zinc ions and relatively complicated simulated wastewater containing several heavy metal ions such as Zn2+, Cd2+and Cu2+. Additionally, the methods of controlling crystal growth so as to improve the purification effect were attempted. The research shows that large particles with compact structure and regular morphology can be obtained under control of reaction crystallization process by using continuous operation with overflow. The main results are as follows:1) The analyses of the particle size of products show that mean particle size(D(4,3), similarly hereinafter) of products are1.45μm,7.67μm and13.42μm by batch operation, continuous operation without overflow and continuous operation with overflow respectively. The products obtained by batch operations are flocculent with small particle for size owing to explosive nucleation. The calculation of the particle number per unit mass of products shows that the primary nucleation is dominant in the continuous operations without overflow, and that continuous operations with overflow favor the crystal growth and agglomeration.2) The purification study of simulated wastewater containing zinc ions shows that the mean particle size of the product obtained at reaction pH value of9is13.63μm, which is bigger than that of10(7.68μm). Prolonging reaction time favors the particle growth and crystalline morphology, however the particle size distribution becomes wide with too long reaction time. The Zn2+removal efficiency is increased significantly with Zn2+content in effluents under0.82mg·L-1which below the lead, zinc industrial pollutant discharge standard (GB25466-2010) by adding additive M during the reaction crystallization process. Additives P with moderate molecular weight can also increase Zn2+removal efficiency, promoting the crystal growth and improving the crystallization morphology. The purification effect with Zn2+content in effluents below0.33mg·L-1of the simulated wastewater containing zinc ions with adding the mixture of additives M and P is better than that of adding M only, but small particles increase. In addition, Ostwald ripening can increase the mean particle size and improve the particle size distribution.3) The research on the simulated wastewater containing several heavy metal ions shows that feeding points have little influence on the reaction crystallization process. The particle size of products is small with no seeds and particles will agglomerate and grow by adding seeds. The influences of additive P and Ostwald ripening on the complicated wastewater are similar to those of the simple one only containing Zn2+The results show that with control of reaction crystallization progress, not only wastewater meet discharging standard but also effects of promoting the crystal growth and improving the crystallization morphology can be achieved by continuous operations with overflow

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CLC: > Environmental science, safety science > Processing and comprehensive utilization of waste > General issues > Wastewater treatment and utilization
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