从钢铁厂烧结机头灰中选择性配位提取银

Selective Coordination Extraction of Silver from Sintering Dust in Steel Plants

根据钢铁厂烧结机头灰富含银、氯、铁、碳等多种有价元素的特点,采用烧结机头灰自身含有的氯元素为配位剂,通过配位浸出的方式提取回收银。本实验研究溶液pH、反应温度、配位剂浓度、反应时间、液固比等反应条件对银浸出率的影响规律。采用X射线衍射仪(XRD)、扫描式电子显微-光微区(SEM-EDS)、粒度和化学成分(ICP-MS)分析手段分别对原料和浸出渣进行表征可知:烧结机头灰中的银主要以AgCl形式存在,通常被铁氧化合物、碳颗粒等物质包裹,铁主要以Fe_(2)O_(3)和Fe_(3)O_(4)物相存在;浸出渣的主要成分是铁氧化合物及元素碳等。实验结果表明:在弱酸性和中性溶液中,配位浸出过程中氯化银主要以AgCl43-络合离子形态被浸出,而铁、碳等对烧结有利的元素不被浸出进入渣中,浸出过程具有选择性。在溶液pH值为3.0、反应温度80℃、氯离子浓度6 mol·L^(-1)、浸出时间2 h、液固比10:1 ml·g^(-1)、搅拌速度400 r·min^(-1)的最佳条件下,银的浸出率可达93.90%,铁的浸出率仅为1.28%,银的选择性分离提取效果好。

As one of the solid wastes produced in steel plant,sintering dust had a huge annual output and its chemical composition was complex,so it was not suitable to directly return to sintering ingredients to avoid damaging equipment,nor was it suitable to direct--y stockpile,so as to avoid the waste of resources caused by heavy metals seeping into the land and polluting the environment.Therefore,it was urgent to develop new recovery and treatment technologies.According to the fact that the insoluble compound silver chloride could be dissolved in hydrochloric acid,and could also form the coordination ion dissolved in a high concentration of halogen ion solution,combined with the high content of chlorine element in the sintering dust.Therefore,chlorine was used as coordination agent in the experiment to change the silver in the sintering dust into a soluble complex and increased its solubility into the solution,so as to realize the chlorination coordination leaching of silver.Firstly,quantitative sintering dust was weighed,and then quantitative leaching agent(ammonium chloride)and deionized water with predetermined concentration were added.Finally,the temperature in the flask was heated to the preset experimental temperature,and stirred continuously at a constant temperature for 3 h.During stirring,pulp pH was measured every half hour until the end.After leaching,liquid-solid separation was completed by pumping filtration,and the leaching solution and leaching residue were obtained.In this experiment,the effect of reaction conditions on silver leaching ratio was investigated,such as the pH of solution,reaction temperature,coordination agent concentration,reaction time,liquid-solid ratio and so on.With the increase of pH value,the leaching ratio of silver first increased and then decreased.When pH was 3.0,the maximum silver leaching ratio was 93.9%.As pH continued to increase,the concentration of hydrogen ions in the solution decreased,the dissolved silver chloride decreased,and coordination complexation weakened.With the increase of reaction temperature,the leaching ratio of silver showed an increasing trend.As the temperature rose,the reactivity of reactants was enhanced,and the diffusion ratio of reactants and products was accelerated,the dissolution ratio of silver was accelerated,and the reaction ratio was faster.When the temperature was 80℃,the silver leaching ratio reached the maximum of 93.9%.The leaching ratio of silver increased with the concentration of coordination agent increasing.When the concentration of Cl-was 2 to 6 mol·L^(-1),the leaching ratio of silver was from 32.6%to 93.9%.The higher the concentration of Cl-,the greater the probability of the formation of a complex coordinated with free Ag+and the greater the solubility of silver chloride,the higher the concentration of silver in the leaching solution and the higher the leaching ratio of silver.With the increase of leaching reaction time,the leaching of silver first increased and then decreased.When the leaching time was from2 to 4 h,the leaching ratio was from 94.8%to 84.5%.With the extension of leaching reaction time,the volatilization loss of leaching agent would increase,the coordination effect of Ag-Cl would be weakened gradually,and the silver leaching ratio would decrease.The silver leaching ratio increased with the increase of the liquid-solid ratio.When the liquid-solid ratio was from 10 to 12 ml·g^(-1),the leaching ratio was from 93.9%to 95%.From the use of experimental raw materials and reagents,the larger the liquid-solid ratio was,the greater the consumption of raw materials and reagents would be,resulting in higher cost and complicated subsequent treatment of the solution.The compositions and phase results of raw material and leaching residue were characterized by X-ray diffraction(XRD),scanning electron microscope-energy dispersive spectrometer(SEM-EDS),particle size analysis and chemical composition analysis(ICP-MS).According to the characterization results,the silver content in the sintering dust reached 610.4 g·t^(-1)and mainly existed in the form of AgCl,which was usually wrapped by ferric oxides,carbon particles and other substances.The chlorine content was 1.5%;the iron content in the sintering dust was 28.5%and mainly existed in the form of Fe_(2)O_(3)and Fe_(3)O_(4).The average diameters of raw materials and leaching residue were 9.63 and 3.59μm respectively,and the average diameters of specific surface areas were 3.74 and 0.78μm respectively.Compared with the size distribution of the raw material,the size distribution of the leaching residue was smaller,which proved that the soluble material in the raw material could get a more adequate reaction.The leaching residue was iron oxide in phase,and its chemical composition was as follows:Ag content decreased to 78.2 g·t^(-1),chlorine content was only 0.096%,iron content increased to 52.6%,and element carbon content was 4.5%.The metal elements which were harmful to sintering process were almost all leached out,while the iron leach ratio was low and remained in the leaching residue.The leaching ratio of chlorine element was the highest,indicating that in the process of leaching of sintering dust,chlorine ion and silver element were selectively coordinated to form soluble compounds into the solution.High content of iron and carbon in the leaching residue,heavy metals such as zinc and alkali metals such as potassium were dissolved into the solution,the efficient leaching of zinc and alkali metals could avoid the cycle enrich?ment of zinc and alkali metals in the recycling process of leaching residue,and reduced the harm of zinc and alkali metals to sintering,blast furnace and other ironmaking processes.The experiment results showed that in weak acidic and neutral solution,silver chloride was leached mainly in the form of AgCl43-complex ions during the process of coordinate leaching,while iron and carbon were not leached into the residue,which meant that the leaching process was selective.Under the optimal conditions of the solution pH of 3.0,the reaction temperature of 80℃,chloride ion concentration of 6 mol·L^(-1),the reaction time of 2 h,liquid-solid ratio of 10-1 ml·g^(-1)and stirring velocity of 400 r·min^(-1),the leaching ratio of silver was up to 93.90%,and the leaching ratio of iron was only 1.28%,the selective separation and extraction of silver was effective.Compared with other methods,chlorine self-coordination method was effective in extracting silver from sintered ash,and it was a new method to treat sintered dust.The extraction of silver from silver-containing solution was also a worthy research direction.Meanwhile,other ionic applications,such as potassium ion could be used to produce potash fertilizer,could also be studied.