| Electrolytic manganese residue?EMR?, a by-product of the electrolytic metal manganese process, is produced by the acid leaching, neutralization, and pressure filtration treatment of manganese carbonate powder. The main pollutants of the EMR are Mn and NH4+-N. Currently, the EMR is primarily dumped into the EMR gallery without pretreatment in China. The leachate contains high concentrations of Mn2+ and NH4+-N, and seriously pollutes the surrounding environment. Considering immature technologies for the EMR application on a large scale, the landfill treatment of the EMR is still a primary choice for the electrolytic manganese industry currently. It is significantly necessary that the EMR is treated harmlessly before the landfill treatment. Stabilization of Mn and control NH4+-N are the keys for harmless treatment of the EMR. At present, the focal points of harmless treatment of the EMR are mainly the immobilization of heavy metals ions and the treatment of NH4+-N is seldom studied. In addition, the sedimentary technologies of heavy metals ions make the pH value of the EMR rise, which is unfavorable to the landfill of the EMR. Considering the limitations and shortcomings of current researches in heavy metals stabilization, removal of NH4+-N, treatment capacity and the prevention of secondary pollution, this paper developed some new technologies about the treatment of pollutants from the EMR according to new thoughts such as CO2 carbonization, air oxidation, electrooxidation, electrokinetic enrichment, etc.This study explored some new technologies of harmless treatment and resources recovery of the EMR from an electrolytic manganese plant in Xiushan County, Chongqing City by CO2 carbonization, electrooxidation, struvite precipitation, air oxidation, electrokinetic enrichment, etc. The change of mineralogical characteristics, principles of chemical reactions and kinetics mechanisms were determined in the process of the EMR treatment. Meanwhile, some new technologies of harmless treatment of the EMR were given in this study. Main contents and conclusions were as following.?1?The main pollutants of the EMR were soluble Mn and NH4+-N of which Crystalline phases primarily consisted of MnSO4·H2O,?NH4?2SO4,?NH4?2Mn?SO4?2·6H2O,?NH4?2Mg?SO4?2·6H2O, CaMn2O4, etc. by the analyses of physicochemical properties of the EMR. The extractable amounts of Mn and NH4+-N were 1.55% and 0.55%, respectively.?2?The efficiencies and mechanisms of two ways of stabilizing Mn from the EMR slurry were compared using carbonates and CO2 with alkaline reagents assist, respectively. Soluble Mn could be immobilized by Na2CO3 and NaHCO3, and produced spheroidal MnCO3. The immobilization efficiency with Na2CO3 was greater than with NaHCO3. When the Na2CO3:residue mass ratio was >0.4, CaSO4·2H2O with regular cylindrical particles and strip particles was translated into CaCO3. CaO was suitable as the additive agent used for CO2 immobilizing Mn in comparison with NaOH. MnSO4·H2O and?NH4?2Mn?SO4?2·6H2O were translated into MnCO3 by CaO and CO2. The immobilization efficiency of Mn was 99.99% under the condition of the CaO:residue mass ratio of 0.05, 20 min reaction time and 0.8 L/min CO2 flow rate. The immobilization efficiency of Mn with CO2 was calculated using the theory of chemical equilibrium at different pH values and the values were in accord with experimental data.?3?Prior to removal of NH4+-N of the filtered fluid from the EMR, soluble Mn was stabilized by CO<sup>2+CaO under the optimized experimental conditions. Removal of NH4+-N of the dilute filtered fluid was conducted by Ti/RuO2-TiO2-Ir O2-Sn O2?DSA? anode and graphite cathode under indirect electrooxidation. Results showed that the increases of Cl- concentration and initial pH value resulted in the increases of the removal efficiency of NH4+-N and the current efficiency. The increase of current density raised the removal efficiency of NH4+-N, but was unfavorable to the current efficiency. The lower NH4+-N concentration was favorable to the NH4+-N removal, but led to a lower current efficiency. In view of a great influence of Mn2+ on the NH4+-N removal, Mn2+ should be immobilized from the filtered fluid of the EMR before NH4+-N electrooxidation. Reaction mechanisms and kinetics relationships of the NH4+-N removal were explored; The immobilization of NH4+-N was carried out by magnesium sources and phosphate sources after stabilizing Mn. NH4+-N from the EMR slurry reacted with magnesium sources and phosphate sources and formed struvite precipitation. This study compared the immobilization results of different magnesium sources and phosphate sources. Results showed that the immobilization efficiency of NH4+-N reached a maximum?89.1%? using MgCl2 and Na3PO4 at the Mg:P:N molar ratio of 1.5:1.5:1. The speciation of chemicals, dissolution and balance of chemicals and saturation state of precipitants dissolution from NH4+-N solution were calculated by Visual MINTEQ software. The remaining NH4+-N of the filtered fluid was removed by direct electrooxidation, after most of NH4+-N was immobilized via struvite precipitation. This study indicated that the combined treatment of struvite precipitation and electrooxidation was extremely efficient for NH4+-N control. Working small scale tests, the technology of CO2-magnesium salt, phosphate- electrooxidation was given to stabilize Mn and control NH4+-N for harmless treatment of the EMR.?4?Influential factors and reaction principles of simultaneous stripping recovery of NH4+-N and immobilization of Mn by air were studied. CaO was more suitable as the alkaline additive agent used for stripping recovery of NH4+-N and immobilization of Mn in comparison with NaOH. The increases of temperature and air flow rate resulted in the increase of the stripping efficiency of NH4+-N. The increase of temperature could raise the immobilization efficiency of Mn. Stripped NH4+-N was absorbed by a solution of sulfuric acid and formed?NH4?2SO4 and?NH4?3H?SO4?2. The soluble Mn was oxidized as Mn3O4 which was confirmed by XRD and SEM-EDS analyses. Working small scale tests, the technology of air-magnesium salt, phosphate- electrooxidation was given to stabilize Mn and control NH4+-N for harmless treatment of the EMR. The running cost of the technology of air-magnesium salt, phosphate- electrooxidation was lower than CO2-magnesium salt, phosphate- electrooxidation and it was recommended to operate in the industry of electrolytic manganese.?5?The results and reaction characteristics of Mn enrichment using electrokinetic process were studied. TheCO32- was produced by OH- produced at the cathode space reacting with CO2 under electrokinetic process, which decreased the pH value at the cathode space. TheCO32- reacted with Mn2+ from other positions of the EMR chamber by electromigration and electroosmotic flow to form MnCO3. The enrichment amounts of Mn and manganese carbonate reached maximum values and were respectively 7.5%? 4.5% using 0.1 mol/L H2C2O4 which pretreated the EMR in comparison with other investigative additive agents with 48 h reaction time under electrokinetic process with 0.2 L/min CO2 assist. The preferable enrichment amount of Mn using the H2C2O4 pretreating agent was attributed to the reducing power of H2C2O4 which reduced manganese oxides of high valence to Mn2+ in acid condition. |
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