Study On The Enrichment Mechanism And Properties Of Gallic Tannin On Germanium

Germanium,one of the sparse metals,is rare and dispersed in the earth's crust.Germanium is often associated with other minerals and can only be recovered and utilized comprehensively in the mining of main metal deposits.At present,tannin precipitation is the main method to enrich germanium metal in China.Tannin is a kind of plant polyphenols with molecular weight of 500-3000,and its abundant o-diphenol hydroxyl group structure provides sufficient active sites for chelating metal ions,which makes tannin show a high coordination affinity for many metal ions.However,the method of tannin precipitation has a large amount of tannin consumption,and its residue in the process of germanium precipitation not only affects its utilization efficiency,but also tends to cause interference and secondary pollution in the subsequent smelting process.Moreover,the current understanding of tannin precipitation of germanium is insufficient,especially the mechanism of tannic acid's action on germanium is not clear,so the improvement measures are difficult to be targeted.In order to solve this problem,the methods of germanium enrichment and recovery were studied from the aspects of tannin types,precipitation conditions,tannin precipitation mechanism and immobilized tannin,so as to reveal the mechanism of action of tannin precipitation in different forms and to maximize the utilization efficiency of tannin acid.There are three parts in this paper:(1)The effects of tannin types and technological conditions on the precipitation of germanium by free tannin were studied.It was found that the order of germanium precipitation ability of five tannins was:Chinese gallotannin>Tara Tannin>Bayberry tannin>Phyllanthus emblica tannin>Larch tannin.Chinese gallotannin not only has the best germanium precipitation efficiency(99.05%),enrichment ratio(0.055%up to3.6%)and selectivity,but also has the strongest adaptability to the changes of metal ion composition and concentration in acid leaching solution.However,the amount of tannin should not be too large.If using too much tannins,the selectivity of tannin will be affected and the taste of germanium slag will be reduced.Studies on tannin precipitation in pure germanium system by pH value,temperature and germanium concentration show that the increase of pH value is beneficial to improve the tannin's utilization rate without interference of other metal ions.The increase of temperature is also beneficial to improve the tannin's utilization rate,and the efficiency cost of 60?is relatively.The higher the initial concentration of germanium solution was,the higher the tannin utilization rate was.However,when the germanium concentration was greater than 2 mmol/L,there was not obvious effect.(2)In order to improve the utilization rate of tannic acid,the mechanism of germanium precipitation by tannic acid was explored from four aspects:different metal ions;the content and molecular weight distribution of Chinese gallnut tannic;tannic acid form and graded precipitation.Firstly,different metal ions have different interaction with germanium.Among them,Na has synergistic effect on the precipitation of germanium by tannic acid through salting-out effect.When CNa⁺rises from 30 mmol/L to 50 mmol/L,the precipitation rate of germanium by tannic acid can rise from 23.01%to 96.52%;Pb(?)has synergistic effect on the precipitation of germanium by tannic acid through coprecipitation,when CPb²⁺rises from 1 mmol/L to 2 mmol/L,the precipitation rate of germanium by tannic acid can rise fron 12.24%to 98.56%;Fe(?)has synergistic effect on the precipitation of germanium by tannic acid through coprecipitation in low concentration range,but when the concentration is more than 10 mmol/L,it will be transformed into competitive effect,reducing the efficiency of tannic acid precipitation;GA(?)is a complete competitive relationship with germanium,reducing the efficiency of tannic acid precipitation.Secondly,the molecular weight and distribution of tannic acid have great influence on the efficiency of germanium deposition.The lower the average molecular weight of tannic acid and the more the distribution of tannic acid in the low molecular weight range,the better the efficiency of germanium deposition.Generally,the detection methods of tannic acid include chrome skin powder method,Folin phenol method,ferrous tartrate method and HPLC.In terms of germanium deposition,HPLC is the most favorable method to show and express the influence of the difference of tannic acid sample content and molecular distribution on the efficiency of germanium deposition.Thirdly,the aggregation state of tannins has a great influence on the efficiency of germanium precipitation.The critical concentration of tannic acid solution is 11.45%(wt%).When the concentration of tannic acid solution is reduced from 30%to 7.5%,the efficiency of germanium deposition can be increased from 2.78%to 91.46%.With the increase of tannic acid concentration,the better the dispersing effect of coagulant on tannic acid micelles,the germanium precipitation rate increased from2.78%to 49.60%of 30%tannic acid solution.The effect of the coagulant on the germanium deposition efficiency of tannins below the critical micelle concentration is limited.Finally,the graded precipitation is beneficial to improve the efficiency of germanium precipitation,enrichment ratio of germanium(0.16%to 4.37%)and the grade of germanium slag.The optimum amount of tannin used is 16 times of the initial germanium content in a single precipitation of germanium.Compared with one-step germanium precipitation,graded precipitation is beneficial to improve the utilization of tannic acid,and when the amount of tannin is less than 10 times,there is not need to separate.After that,it is beneficial to improve the utilization of tannic acid.With the increase of the number of stages,the grade of germanium slag becomes worse.It is suggested that germanium slags should be stored and treated separately.(3)A solid adsorption material based on Chinese gallotannin and chitosan was generated and applied to adsorb rare and scattered metals,including Ge(?),Ga(?)and In(?),from simulated zinc plant leachates.FTIR,solid state nuclear magnetic resonance(SSNMR),and AFM were performed to characterize the grafting way and apparent morphology of Chinese gallotannin on the chitosan matrix.It was clearly shown that glutaraldehyde reacted with the chitosan matrix and Chinese gallotannin by means of Schiff-base reaction and phenol-formaldehyde condensation,respectively.The adsorption tests showed that under the premise of ensuring the stability of the acid leaching solution,an appropriate increase in pH is more beneficial to enhancing the adsorption of immobilized Chinese gallotannin(ICGT).At a temperature exceeding30?,the immobilized tannin became less sensitive to temperature.Adsorption kinetics studies showed that adsorption of the three scattered metals by ICGT followed a quasi-second-order kinetic equation,with chemisorption as mechanism.The adsorption isotherm was well fitted by the Langmuir model,indicating uniform adsorption sites on the surface of ICGT;the adsorption capacities of Ge(?),Ga(?)and In(?)in solution by monolayer were 61.39 mg/g,63.82 mg/g and 59.92 mg/g,respectively.At this time,the enrichment ratio increased from 7.68%,7.23%,7.40%to 20.52%,21.34%,20.42%respectively.Mechanical property and stability tests showed that ICGT had good resistance to harsh adsorption conditions of scattered metal ions.