溴-甲醇非水体系分离-重铬酸钾滴定法测定直接还原铁中金属铁

金属铁含量是直接还原铁质量的主要指标,而直接还原铁是优质钢生产不可缺少的原料。因此测定直接还原铁中金属铁含量对优化钢材结构和提高钢的质量具有重要意义。采用溴-甲醇非水体系溶解直接还原铁中金属铁(MFe),使用聚四氟乙烯(PTFE)微孔过滤膜(孔径0.45 μm)抽滤分离残渣,金属铁以三溴化铁的形式存在于滤液中,从而实现了其与其他价态铁的分离。滤液中加入硫酸和多次加入过氧化氢,加热冒硫酸烟去除溴化物和甲醇以避免其干扰后续金属铁的测定。用氯化亚锡还原滤液中大部分三价铁为二价铁,以钨酸钠为指示剂,三氯化钛还原滤液中剩余的三价铁,用稀重铬酸钾溶液氧化过剩的三氯化钛。以二苯胺磺酸钠为指示剂,用重铬酸钾标准溶液滴定二价铁的含量。据此,建立了溴-甲醇非水体系分离-重铬酸钾滴定法测定直接还原铁中金属铁的方法。选取5个直接还原铁样品,按照实验方法进行精密度试验,测定结果的相对标准偏差(RSD,n=9)为0.24%～0.54%;将实验方法应用于2个直接还原铁标准样品中金属铁的测定,测得结果与认定值的相对误差为0.012%～0.15%。

Determination of metallic iron in direct reduction iron by potassium dichromate titration after separation with bromine-methanol non-aqueous system

The content of metallic iron is the main indicator to evaluate the quality of direct reduction iron, which is an indispensable raw material to produce high-quality steel. Therefore, the determination of metallic iron content in direct reduction iron is of great significance to optimize steel structure and improve steel quality. Bromine-methanol non-aqueous system was selected to dissolve metallic iron (MFe) in direct reduction iron. The residue was separated by suction filtration with polytetrafluoroethylene (PTFE) microfiltration membrane (bore diameter of 0.45 μm). Metallic iron existed in the filtrate in the form of ferric tribromide, realizing its separation from other iron compounds with different valence. Sulfuric acid was added into the filtrate followed by adding hydrogen peroxide for several times. The solution was heated to form sulfuric acid smoke to remove bromide and methanol in the filtrate, which could avoid their interference with the determination of metallic iron. Most ferric irons in the filtrate were reduced to ferrous ions with stannous chloride. The residual ferric irons in the filtrate were reduced with titanium trichloride using sodium tungstate as the indicator. The excessive titanium trichloride was oxidized with dilute potassium dichromate solution. The content of ferrous ions was titrated with potassium dichromate standard solution using sodium diphenylamine sulfonate as the indicator. Consequently, a method for determination of metallic iron in direct reduction iron by potassium dichromate titration was established after separation with bromine-methanol non-aqueous system. Five direct reduction iron samples were selected to perform the precision tests according to the experimental method. The relative standard deviations (RSD, n=9) of the determination results were between 0.24% and 0.54%. The proposed method was applied for determination of metallic iron in two certified reference materials of direct reduction iron. The relative errors between the determination results and the certified values were between 0.012% and 0.15%.