| In my country (Vietnam), smelting industry is the pillar industry of the national economy. With the rapid development of my country, smelting industry also becomes more developed and generates large amounts of wastewater. Because smelting wastewater has complex composition, large amounts, high concentration of sulfuric acidity, high toxicity, etc. If wastewater treatment process is non-standard, it will cause a serious threat to ecological environment and human health. Therefore, there were many researchers all over the world investigating the treatment of heavy metals from industrial wastewater. Currently in the world, the most common method for removal of heavy metal ions from wastewater is lime precipitation. However, this method has several disadvantages, such as large amounts of sludge are generated, the residual concentration of calcium ion is very large, the removal efficiency of fluoride ion is very low. To address the above considerations, the study of a new method to treat smelting wastewater is necessary.In recent years, there were many scientific researches (especially in our laboratory study) using the Fe/C micro-electrolysis method to treat zinc and lead smelting wastewater. The results shown that the removal efficiency of lead and copper is effective (its separation mechanism is mainly based on electrochemical action). However, the removal efficiency of zinc and fluoride ions is inefficient (its separation mechanism is mainly based on the effect of flocculation). In addition, our laboratory studied some of methods for removal of heavy metal ions from zinc and lead smelting wastewater, such as electrolysis-micro-electrolysis, micro-electrolysis-flocculation and micro-electrolysis-electrocoagulation (using iron electrodes)-flocculation, etc. However, the treatment of fluoride from this wastewater has not been studied. The use of micro-electrolysis method for removal of smelting wastewater has many advantages, such as removal efficiency of Pb and Cu ions is efficient, action is simple and does not use electric power. However, this method also has many disadvantages, such as removal efficiency of F and Zn ions is inefficient, Fe/C is very easy clumping and a hydraulic retention time is long. The electrocoagulation method has many advantages, such as a hydraulic retention time is short, sludge is not large, but energy consumption is large. To address the above considerations, the study of a new process with the combination of electrocoagulation and fluidized micro-electrolysis will promote the advantages and reduce disadvantages of each method. Thus, this thesis studied using "electrocoagulation and fluidized micro-electrolysis coupled method" for simultaneous removal of heavy metals and fluoride ions from smelting wastewater. The main contents and results of this thesis are as follows:(1) The equipment of the electrocoagulation and fluidized micro-electrolysis coupled method was used in experiments. Electrocoagulation method was used with two types of electrodes, include aluminum electrodes and iron electrodes. Particles of iron-carbon (Fe/C) were put between positive electrodes and negative electrodes, Fe/C particles at fluidized state. Simultaneously, the orthogonal experimental method was used to investigate the influence of factors on the removal efficiency of heavy metals and fluoride. After a set of optimal condition were found. Actual wastewater was treated by these optimal conditions, and then compared to the other methods.(2) Results shown that the electrocoagulation and fluidized micro-electrolysis coupled method using iron electrodes for simultaneous removal of lead, zinc and copper ions from simulated wastewater are highly effective, but removal efficiency of fluoride ion is ineffective. However, when using aluminum electrodes for removal of lead, zinc, copper and fluoride ions is highly effective and the results are stable. Because the mechanism of defiuoridation is mainly due to the formation of precipitate AlnFm(OH)3n-m .(3) The optimal conditions of the electrocoagulation and fluidized micro-electrolysis coupled method for treatment of simulated wastewater are:using four aluminum electrodes, an initial pH of 4, an applied voltage of 5 V, a hydraulic retention time of 30 minutes, a mass of Fe/C of 45 grams, a particle diameter of Fe/C of 20-27 mesh and KCl of 0.5g/L. With these conditions, the removal efficiency of Pb, Zn, Cu and F ions are 99.0%,98.8%,99.6% and 94.1%, respectively. After treatment, the residual concentration of Pb, Zn, Cu and F ions are lower than level 1 of GB8978-1996 "Integrated wastewater discharge standard".(4) Results shown that using electrocoagulation and fluidized micro-electrolysis coupled method for removal of Pb and Cu ions from wastewater was hardly affected by the initial concentration of Pb, Zn, Cu and F ions. But the initial concentration of Pb, Zn, Cu and F ions were large influence on the removal efficiency of Zn and F ions. Because of the changes of the initial concentration of Pb, Zn, Cu and F ions caused changes of pH in solution after treatment (change of pH is from 4 to 7). Thus, pH affected the removal efficiency of Zn and F ions. With the pH large value to remove Zn ion is effective. With the pH value of 6 to remove F- is effective. If the pH is greater much than 6 or less much than 6, the removal efficiency of fluoride will be not good.(5) Using electrocoagulation and fluidized micro-electrolysis coupled method at the optimal condition for treatment of actual zinc and lead smelting wastewater was achieved the residual concentration of Pb, Zn, Cu, F, Cd, total of As (Ⅲ) and As (Ⅴ) ions with <0.001mg/L,1.643mg/L,<0.001mg/L,3.833mg/L,0.064mg/L and 0.004mg/L, respectively; removal efficiency with nearly 100%,96.7%, nearly 100%,98.4%,99.0% and 99.9%, respectively. After treatment, the residual concentrations of ions are lower than level 1 of GB8978-1996 "Integrated wastewater discharge standard", In addition, the treated wastewater by the electrocoagulation and fluidized micro-electrolysis coupled method was separated solid phase before using CaO to adjust the solution pH value of 9 (with purpose prevents the solution of AlnFm(OH)3n-m↓). After all treatment processes have been completed, the residual concentration of Pb, Cu, total of As (Ⅲ) and As (Ⅴ) ions are lower than level 1 of GB3838-2002 "Environmental quality standards for surface water", the residual concentration of Zn ion is lower than level 2 of GB3838-2002, the residual concentration of F and Cd ions are lower than level 1 of GB8978-1996 "Integrated wastewater discharge standard"The main innovation points of the thesis:(1) A novel equipment of the electrocoagulation and fluidized micro-electrolysis coupled method was designed.(2) The influence factors and the simultaneous removal efficiency of heavy metal and fluoride ions by using electrocoagulation and fluidized micro-electrolysis coupled method with aluminum electrodes and iron electrodes were compared. Thence, the optimum process parameters were found.(3) The mechanisms in wastewater treatment process are as follows:Pb and Cu ions are mainly removed through reduction to generate Pb and Cu metals. Zn ion is mainly removed through colloidal flocculation and co-precipitation. When the electrodes are used with aluminum plates, the removal efficiency of fluoride is highly effective. The mechanism of defluoridation is mainly due to the formation of precipitate AlnFm(OH)3n-m . |
PDF Full Text Request