| Panzhihua is an important vanadium-titanium magnetite metallurgical industry base in China.Vanadium-titanium magnetite metallurgical process produces three types of solid waste:mine tailings,blast furnace slag and steel slag.The amount of mine tailings,blast furnace slag,steel slag in Panzhihua are 125 million,60 million and 8 million tons,respectively.These waste slags were long-term open storage.Their potential environmental risks have attracted much attention in recent years.These slags were the research object of this paper.Their leaching behavior,leaching thermodynamics and kinetics were researched.The paper includes the chemical and mineral composition of these slags,their leaching behavior under different environmental conditions such as water medium,acid rain medium,C02 atmosphere,and leaching thermodynamics and dynamics simulation.1.The slags chemical and mineral composition:the chemical and mineral composition of mine tailings,blast furnace slag and steel slag were investigated by modern analytical and test technology.The chemical composition of mine tailings is mainly of Si,Fe,Al,Ca,Mg,Ti and Na,and the trace elements such as Mn,V and Co,and its main minerals is composed of diopside,anorthose,K-feldspar,quartz,chlorite,epidote,ilmenite and a small amount of pyrite.Blast furnace slag is mainly composed of Si,Fe,Al,Ca,Mg and Ti,and trace elements such as Mn,V and Cr,and its main minerals is composed of diopside,perovskite,magnesium-aluminate spinel and elemental iron.Steel slag is mainly composed of Si?Fe?Al?Ca?Mg and trace elements such as Mn?Ba?Cr?V.Steel slag is an alkaline waste,its main mineral composition includes dicalcium silicate,tricalcium silicate,wustite,calcium ferric aluminate,fayalite and f-CaO.2.The leaching behavior of waste slags includes batch,dynamic and type-column leaching.And the carbonation of steel slag was studied because of its rich calcium property.(1)The batch leaching behavior:the vanadium-titanium mine tailings are relatively stable in pure water medium,but the leaching of Mn,Fe,Co,Ni,V,Zn,Cu,Cd,Cr in mine tailings is significantly improved in acid rain medium.The concentration of these elements in the leachate has exceeded the quality limit value of national Class I surface water,with potential environmental risks.Vanadium is the most trace element in leachate.The concentration of vanadium in pure water leachate is 26.8 times of that in acid rain medium.Because acid rain improves the leaching of iron in blast furnace slag.The hydroxide iron form under weak acid condition,which prevents the vanadium from leaching.The analysis results confirmed that vanadium in leachate is pentavalent vanadium.On the other hand,a large number of Mn in blast furnace slag is leached in acid rain.The Mn concentration is 917 times standard limit value of the national surface water.The most significant feature of steel slag batch leaching is that a large amount of Ca ion leaching leads to a strong alkaline solution.Ba concentration is 96 times standard limit of national surface water.The trace lelements is controlled by second minerals phase of hydroxide ferric(aluminum),which concentration in leachate is very low.(2)Dynamic leaching behavior:the concentration of maro-element Ca,Mg,K and Na in the tailings leachate were high during the initial leaching and then decreased rapidly.Fe migrated from top to bottom with a precipitation-dissolution-precipitation cycle.Co,Mn,Ni a migration peak due to adsorption and dissolution.V,Ti were more strongly precipitated and absorbed,consequently their transmission resistance were greater,and breakdown time were longer.The Ca,K,Na and Mg concentrations in blast furnace slag leachate decreased gradually.However,Fe,A1 concentrations were very low due to precipitation.Vanadium exhibited a gentle leaching peak shape due to unreacted shrinking core diffusion controlling and adsorption effects.The Ca?K?Na?Ba concentrations in steel slag leachate were high during the initial leaching,then rapidly declined.These elements did not precipitated during migration.Ca has reached supersaturated state and formed a large number of calcite secondary minerals with CO2 in air.The trace elements have been kept at a low concentration by the controlling of leaching solution strong alkaline.(3)The type-column leaching behavior:the column leaching of mine tailings,blast furnace slag and steel slag provided the horizon information during the leaching process,which confirmed the time and spatial variation of elements in static leaching and dynamic leaching.Most of the maro-elements,such as Ca,K,Na,etc.showed a continuous growing trend,were not precipitated and absorbed.However,Most of the trace elements,such as V,Mn,Co,Ni,and Fe,Al were precipitated and absorbed during leaching.3.Steel slag carbonation:the reaction rate of steel slag carbonation under atmospheric pressure is not much different from that under elevated CO2 pressure.Whereas the increase of CO2 pressure significantly increased the carbonation conversion of CaO in steel slag.The carbonation conversion of CaO in steel slag is related to the CO2 flow rate,but there is no obvious relationship with the liquid-solid ratio(L/S)of the reaction system,which indicated that carbonation reaction can occur as long as the steel slag in the wet state fully contacts with CO2.The carbon isotope is significantly fractionated during the carbonation of the steel slag.The CaCO3 solid phase is preferentially enriched in 13C,the gas phase and the solution are relatively enriched in 12C,and the carbon isotope equilibrium is synchronized with the carbonation process.Although the carbonation is beneficial to reduce the pH of the steel slag leachate,it significantly increases the release of the heavy metals V,Cr,Mn,U and Ti in slag.4.The leaching thermodynamics and kinetics of waste slags:it is confirmed that the secondary minerals of Fe and Al hydroxide is the main controlling factor of leaching thermodynamics of waste slags by PHREEQC geochemical simulation.The simulation results of V in the leaching solution confirmed that V was mainly pentavalent vanadiun species.The results show that the leaching rate equation of most elements is consistent with Elovich,power function,parabolic,pseudo-two,pseudo-first order rate equation.The leaching kinetics of vanadium in blast furnace slag is consistent with the model of unreacted shrinking core model,and the leaching rate is controlled by ash layer diffusion.The carbonation kinetics of steel slag is also consistent with the unreacted shrinkage nucleus model,and the carbonation rate is controlled by the ash layer diffusion. |
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