湿法合成高致密碱式碳酸钴粉末及Co(Ⅱ)-NH_(3)-CO_(3)^(2)--H_(2)O体系热力学分析

Wet synthesis of high-density basic cobalt carbonate powder and thermodynamic analysis of Co(Ⅱ)-NH_(3)-CO_(3)^(2)--H_(2)O system

以CoCl2和4种工业常用沉淀剂NaHCO_(3)、Na_(2)CO_(3)、NH_(4)HCO_(3)和(NH_(4))_(2)CO_(3)为原料,湿法制备碱式碳酸钴粉末,用激光粒度仪、扫描电镜、X射线衍射仪、振实密度仪等,研究溶液pH值和NH_(3)浓度对碱式碳酸钴粉末粒径、微观形貌和晶体结构与密度的影响,并进行Co(Ⅱ)-NH_(3)-CO_(3)^(2)--H_(2)O体系热力学分析。结果表明:溶液pH值和NH_(3)浓度可对钴离子的络合、沉淀行为产生影响。基于络合沉淀过程,用NH_(4)HCO_(3)和(NH_(4))_(2)CO_(3)沉淀剂制备的碱式碳酸钴粉末振实密度较高。基于同时平衡和质量守恒原理,绘制不同溶液组成下lg[Co]_(total)对pH的热力学图,进一步研究溶液pH值对沉淀过程的影响。结果表明,pH为6~8时,钴离子的沉淀率均达到90%以上。在Na_(2)CO_(3)和NaHCO_(3)为沉淀剂的反应体系中,瞬态下游离Co^(2+)占主导地位,接触沉淀剂后快速沉淀,形成松散的絮凝碱式碳酸钴粉末;而在(NH_(4))_(2)CO_(3)和NH_(4)HCO_(3)两种沉淀剂体系中,Co^(2+)与氨配位,Co^(2+)的沉淀过程伴随着从多配位Co(NH_(3))n ^(2+)(n=1,2,…,6)中释放Co^(2+)而缓慢进行,从而形成致密的碱式碳酸钴粉末。

CoCl2 and four kinds of precipitants commonly used in industry,namely(NH_(4))_(2)CO_(3),NH_(4)HCO_(3),Na_(2)CO_(3) and NaHCO_(3),were used as raw materials to prepare basic cobalt carbonate by wet synthesis.The as-prepared basic cobalt carbonate were studied by means of lazer particle analyzer,scanning electron microscope,X-ray diffractometer and vibrating densitometer.The effects of solution pH value and NH_(3) concentration on particle size,microscopic morphology,crystal structure and density of basic cobalt carbonate powder were evaluated,and the thermodynamic analysis of Co(Ⅱ)-NH_(3)-CO_(3)2−-H_(2)O system was performed as well.The results show that the pH value and NH_(3) concentration of the solution have a great influence on the complexation and precipitation behavior of cobalt ions.Based on the complexing precipitation process,the tap density of the basic cobalt carbonate solid powders prepared with NH_(4)HCO_(3) and(NH_(4))_(2)CO_(3) precipitant is higher.Based on the principle of simultaneous equilibrium and mass conservation,the thermodynamic diagram of lg[Co]total on pH under different solution compositions was drawn,and the effect of solution pH value of different systems on the precipitation process was further studied.The results show that the precipitation rate of cobalt ions is more than 90%when the pH is 6−8.In the Na_(2)CO_(3) and NaHCO_(3) precipitant systems,free Co^(2+)is dominant during the transient,and it precipitates rapidly after contacting the precipitant to form loose flocculated basic cobalt carbonate powders.In the(NH_(4))_(2)CO_(3) and NH4HCO_(3) precipitant systems,Co^(2+)is coordinated with ammonia,the precipitation process procees slowly with the release of Co^(2+)from multi coordinated Co(NH_(3))n^( 2+)(n=1,2,…,6),results in dense powders.