Study On The Mechanism And Application Of The Reaction Between Calcite And Heavy Metal Ions During The Mechanical Activation

At present,China is facing serious environmental problems,and the heavy metal pollution is becoming increasingly serious.The large amounts of metal sulfate wastewater produced in the mining and other industries are the major cause of heavy metal pollution to environment.The solution to the problem requires imminent action.In this regard,two approaches are believed to be important: one is to improve the processing efficiency,reducing the amount of emissions,and the second is the purification of various wastes.Lime(Ca(OH)2)precipitation is currently the main method for purification purpose,but with obvious disadvantages such as high p H of water,high water percentage of the sediment,filtration difficulties and so on.The use of cheaper natural mineral materials for the purification purpose,without these problems if possible,has important practical significance.Calcite is the most common natural calcium carbonate(Ca CO3)minerals,and the level of development and application is limited.Taking into account the Ca2+ and CO32-contained in the material,calcite can be used to precipitate and purify most of the metal sulfate wastes.However,its chemical nature is stable and only has the effect of the surface chemical adsorption on metal ions,with very low adsorption capacity.It is necessary to activate calcite using suitable methods to promote its reactivity,so that a chemical reaction with molar equivalents between calcite and metal sulfates can take place.Mechanochemical is a common method to raise the reactivity of substances,with green and efficient advantages.In addition,the degree of reaction can be controlled by adjusting the intensity of mechanical force.Based on the mechanochemical method,the main purposes of this paper are set at two aspects: one is to achieve the coprecipitation of multi-metal ions by using calcite under a high intensity of action and another is to separate of metal ions using calcite under a low intensity of action.With the newly developed separation technology the emissions of heavy metals to the environment can be reduced.A mechanochemical method was applied to study the interaction between calcite and heavy metal ions.Calcite was activated by a planetary mill,and the mechanochemical reactions between calcite and heavy metal sulfates were studied,including(Fe),zinc(Zn),copper(Cu),nickel(Ni)and cadmium(Cd).The reaction characteristics were summarized,and the reaction mechanism was analyzed and discussed.Based on the characteristics and mechanism of the reaction,detailed studies of application were carried out,including the separation of metal ions,the synthesis of carbonate slow-release fertilizers,and the removal of heavy metal ions and so on.And the physical and chemical properties of the obtained samples were characterized through some instruments including ICP,AAS,XRD,FT-IR,SEM and TG/DSC/DTG.Based on the obtained phenomena,several new processes are developed to sovle the related issurs in resource and environmental fields.The study on the characteristics of chemical reaction showed that the results of the reactions between metal sulfates and calcite were different.The differences were mainly caused by two aspects.One was the characteristics of different metal ions,and the other was the different conditions of reactions.The main influence factors were liquid-solid ratio,milling time,grinding speed and molar ratio of Ca CO3-metal ions.When the intensity of mechanical activation was very high,the coprecipitation of multiple metals would occur.Overall,the order of the reactivity of the ions is Fe2+> Cu2+> Zn2+> Cd2+,Ni2+.The mechanism of reaction was investigated in detail.It was concluded that the thermodynamic stability of hydrated ion M(H2O)n2+ determined the difficulty of the reactions under the mechanical activation and M represented heavy metal elements.When the coordination number of water around was six(Fe2+/Ni2+/Cd2+),the spatial configuration of hydrated ion was octahedron.And the spatial configuration corresponding to 4(Cu2+/Zn2+)was a regular tetrahedron.The water molecules in the form of octahedron are more difficult to be removed than that of regular tetrahedron.It is understood that Cu2+ and Zn2+ were easier to react with Ca CO3 than Fe2+,Ni2+ and Cd2+.The mechanism of reaction between Fe2+ and Ca CO3 was special.Based on the results,the possible applications of the reactions between calcite and heavy metal ions were investigated.Firstly,according to the differences between metal ions under a low intensity of action,the applications of calcite in the separation of metal ions were profoundly investigated,including the Cu-Ni?Cu-Co and Cu-Cd separations.The results showed that the calcite had a significant effect on the separation of them.Secondly,based on the coprecipitation of multiple metals at a high intensity of action,the purifications of metal ions in water,incluing Fe2+,Cu2+,Zn2+,Ni2+ and Cd2+ were investigated by using calcite.The results showed that Fe2+,Cu2+ and Cd2+ ions were almost not soluble in aqueous solution,and the leaching rate was less than 0.1%,while the leaching rates of Zn2+ and Ni2+ were less than 2% and 7% respectively.The removal of metal ions for purification can be achieved through this process,with the clear advantages of low water content in the sediment and neutral p H of the waste liquid for direct discharge.Finally,according to the point of carbonate in some reaction products,the application of calcite in the production of slow-release fertilizer(SRF)was studied.The Fe,Zn and Fe-Zn compound fertilizers with slow release performance were synthesized.And the characteristics of the products were characterized by ICP,XRD,SEM and so on.