Study on the Uranium Form Controlling Mechanism during Coal Combustion

Author:Tang Zuo

Supervisor:li yi lian


Degree Year:2019





Nowadays,the economic and social development of China have entered a new stage,so as the energy development.It is urgent to reform the traditional energy system and promote energy transformation in this critical period.As is known to all,nuclear energy is one of the relatively clean energy sources which will be in greater demand in the next decade.However,present uranium ore resources can not be meet the demand,and it is necessary to develop and utilize uncoventional uranium resoures.Compared with extracting uranium from seawater or other unconvential uranium resoures,extracting it from uranium-bearing coal is of relatively high content and long-term mining plan.Moreover,uranium can be enriched under special geological conditions or through combustion.Therefore,extracting uranium from uranium-bearing coal ash as a supplement to nuclear raw material has a certain application prospect.During coal combustion,uranium is easily encapsulated by quartz and silicate,which will affect its recovery and utilization efficiency.In this study,the experimental methods(combustion,sequential extraction and leaching)and characterization methods were used to study the control technique on the uranium occurrence form and its transformation mechanism.(1)Effects of combustion conditions on the uranium occurrenceTo investigate the effects of combustion conditions on the uranium occurrence in coal ash,Tessier sequential extraction method was taken to decide the urnaium occurrece.Results showed that,when the coal particle size decreased from 20 mesh to300 mesh,the active uranium contents were firstly increased then decreased.Moreover,the refractory quartz decreased,and the hematite which can be leached by iron-manganese oxide bound extractant increased.As combustion temperature rose,carbonate uranium content showed a trend of firstly decreasing and then increasing,while the residual uranium increased as exchangeable and carbonate bound uranium decreased.Herein,the increase of combustion time and temperautre can both decrease the active uranium in coal ash,but the effect of coal size can bring a maxium active uranium content in coal ash.Considering the uranium leaching results,100 mesh(coal particle),850℃(combustion temperature)and 96 min(combustion time)was the optimal combustion condition.(2)Effect of additives on the uranium occurrenceTaking transition metal additives(Fe2O3,MnO2,Fe3O4),alkali metal additives(KCl、K2CO3,NaCl,Na2CO3)and alkaline earth metal additives(CaO,CaCO3,CaCl2,Ca(OH)2)as researching additives.Based on the results of sequential extraction and leaching experiments,combined with analysis method,this work studied the additive influencing mechanism on uranium occurrences and mineral compositions in coal ash with different dosages.The results showed that Fe2O3 had little effect on exchangeable uranium,carbonate bound uranium and iron-manganese oxide bound uranium.When n(Fe2O3):n(SiO2)=1:2 and 1:1,Fe2O3 reacts rapidly with quartz or aluminosilicate to form ferrosilicates(such as pyroxene and aluminosilicate),which would wrap on the surface of coal ash and increase the residual uranium in coal ash.When the dosage was 2:1,it may react with quartz to form orthosilicates(olivine),increase the acid solubility of the coal ash and reduce residual uranium content.Manganese oxide and Fe3O4 has no obvious controlling effect on uranium occurrence.However,the control mechanism of alkali metal additives was mainly the increase of residual uranium content.With the increase of KCl dosage,exchangeable,iron-manganese oxide and organic matter bound uranium gradually transform to residual uranium,forming amorphous phase or silicate-aluminate mixture,which would reduce the uranium leaching.NaCl shared similar controlling effect with KCl.Moreover,its ability to increase the uranium uranium is greater.Low dosage of K2CO3 and Na2CO3 can promote uranium migration to residual state.When the dosage was larger than 1:2,the ash became melting and led to coal ash sticking.Compared with the above two kinds of additives,alkaline earth metal additives can reduce the residual uranium due to the presence of active calcium ions.With the increase of the addition dosage,CaO,CaCO3 and Ca(OH)2 can increase carbonate uranium and active uranium;CaCl2 can promote the transformation of residual uranium to iron-manganese oxide bound urnaium.During combustion,new minerals formed with additives involoved,which increased the acid solubility of coal ash by increasing calcium orthosilicate and anorthite.Furthermore,the additives can promote residual uranium transformed to active uranium,and increase the uranium leaching content.(3)Uranium leaching from additive-coal combustion ashIn this chapter,coal ashes,derived from different additive co-combustion,were investigated to select the highest uranium leaching within the additives added combustion.CaCO3 was chosen as its higher uranium leaching content and rich reserves.Further,the effects of initial concentration of nitric acid,solid-liquid ratio and leaching time on uranium leaching were investigated.On this basis,the leaching kinetic was explored to recover its leaching process.The optimum conditions are as follows:nitric acid 1 mol/L,solid-liquid ratio 20 mL/g,and leaching time 12 h,which increased uranium leaching rate of coal ash by 27%.Leaching kinetic result showed that the leaching process accorded with the diffusion in the particle-size-invariant shrinkage core model.Calculations show that the apparent activation energy of leaching uranium with CaCO3 treated was 9.23 kJ/mol,while that without CaCO3 was13.26 kJ/mol.It was judged that the leaching reaction of uranium is controlled by the diffusion of solid product layer,and the leaching process of CaCO3-coal combustion ash was relatively not that sensible to temperature,which meant that the leaching of ash at a relatively lower temperature can get a relatively higher uranium recovery rate.Sequential extraction results of ash samples showed that the content of carbonate-bound,iron-manganese oxide-bound and organic-bound uranium increased with calcium carbonate treated.After leaching,the residual uranium of the ash was lower than it in raw coal ash.Combined with XRD and SEM image analysis,it was found that mineral compositions changed before and after leaching.Without adding CaCO3,mineral changes are mainly reflected in the decrease of microcline and a small amount of quartz.After adding CaCO3,new minerals such as calcium hydroxide,wollastonite,gypsum,anorthite,lime and srebrodolskite occurred in the ash.Further,some quartz,hematite,iron silicate,gypsum and mica were remained in residue after leaching.Generation of active CaO resulted in the occurrence transformation from residual form to carbonate-bound form.Based on the uniform distribution of uranium in minerals,the main mechanism of CaCO3 is to reduce the content of amorphous phase by reacting with hematite,quartz or aluminosilicate to produce acid-soluble minerals,which is conducive to the recovery and utilization of uranium.