Synthesis Of Multi-doped Li（FeM）PO₄from Converter Sludge By Using Acidolysis

Converter sludge is called potential high-value secondary resources due to containinga large number of irons and a variety of valuable metal elements. Because of finegranularity and high content of alkali metal, the comprehensive utilization of convertersludge is limited as the metallurgical charge with low value-added, and the valuableelements can not be effectively utilized. In the present study, Fe and the valuable metalelements （Al, Mg, Zn and Mn） were simultaneously recovered from converter sludge andused as a low-cost source of Fe and dopants to prepare multi-doped LiFePO₄cathodematerial for lithium ion batteries. Structure and electrochemical properties of the objectiveproduct were characterized. Meanwhile, recycling and utilization of liquid waste producedduring acid hydrolysis process were explored.In this work, the valuable elements were obtained from converter sludge by acidacidolysis. After removing the impurity, the muti-doped FePO₄precursor was preparedthrough co-precipitation, and Li（FeM）PO₄/C was finally synthesized by lithiation of theprecursor. The optimal parameter for preparation process of Li（FeM）PO₄/C cathodematerial was ascertained as follows: the precursor was pretreated at550℃for0.5h;sintering process was performed at700℃for8h. XRD and SEM results showed thatLi（FeM）PO₄/C obtained from the optimal experimental conditions deliverd a single phasewith olivine structure and well crystallization, presenting a fine particle size less than100nm and a homogeneous elemental distribution in its contexture. At room temperature,the electrochemical properties of LiFePO₄/C prepared from reagents and Li（FeM）PO₄/Csynthesized from converter sludge were examined by charge-discharge test at differtentcurrent densities. At the current density of15,75,150,300and750mA·g–1, the firstdischarge capacities of LiFePO₄/C prepared from reagents were156.8,140.8,132.5,115.2and100.8mAh·g–1, by contrast, the initial discharge capacities of Li（FeM）PO₄/C are165.9mAh·g–1,162.8mAh·g–1,154.1mAh·g–1,135.2mAh·g–1and122.4mAh·g–1, respectively.The result indicates Li（FeM）PO₄/C synthesized from converter sludge delivered muchbetter high-rate performances than that of LiFePO₄/C synthesized from reagents. Thiscould be attributed to dopants from converter sludge, which was beneficial to improve theelectrochemical properies of LiFePO₄. Recycle of the wastewater generated from the co-precipitation process was discussed.According to a large number of NH4+and SO₄2-existed in the wastewater, the optimalutilization of the wastewater was that it is firstly recycled into acidolysis process partiallyreplacing sulphuric acid, and that cycled over two times is used to produce （NH₄）₂SO₄.The results indicated that Li（FeM）PO₄/C synthesized from the wastewater has a slightlyhigher content of dopants than that prepared from excessive H2SO₄, presenting moreexcellent electrochemical performance; quality of the byproduct （NH₄）₂SO₄could be up tothe requirement of superior quality according to the National Standards GB/T5351995,although containing a certain amount of impurities.In this work, a novel route of comprehensive utilization of the converter sludge with ahigh added-value was proposed. Meanwhile, an economical method for fabricating themulti-doped LiFePO₄as cathode material of lithium ion battery was developed. Highlypurified CaSO₄and superior quality （NH₄）₂SO₄could be obtained during the wholeexperimental process without any waste generated. Meanwhile, the general principlederived from this work is expected to supply a constructive reference for the studies oncomprehensive utilization of other Fe-bearing wastes with high added-value.