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Low alkali decomposition of refractory tantalum niobium ore study on new technology and Application

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Tutor: WeiZuo
School: Kunming University of Science and Technology
Course: Non-ferrous metallurgy
Keywords: Tantalum-niobium ore,Low-alkali decomposition,Low-acid pulp extraction,Thermodyn
CLC: TF841.6
Type: PhD thesis
Year:  2013
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Abstract:
Tantalum and niobium are the rare metals with high melting point, which have been applied widely in the fields of steel,electronic and other high-tech industries. The key to tantalum-niobium metallurgy is the decomposition of tantalum-niobium ore and the separation of tantalum and niobium. Now the most extensive used method is hydrofluoric acid method, but this method is not right for a low-grade refractory tantalum-niobium ore and the decomposition rate is only85%.1ton of tantalum-niobium ore can consume6-8tons of hydrofluoric acid, which leads to serious fluorine pollution.Therefore, sub-molten method was proposed by Institute of Process Engineering, Chinese Academy of Sciences, by which the decomposition rate of tantalum-niobium ore was10%higher than hydrofluoric acid method and the fluorine pollution was cut from the source. Based on the previous research, the reaction behavior of tantalum and niobium in sodium hydroxide was further explored and the new clean and energy-saving process for decomposing tantalum-niobium ore with low-alkali was proposed, in which the fundamental research and process optimization of low-alkali decomposition of tantalum-niobium ore and tantalum-niobium low-acid extraction separation from pulp were carried out. The innovative research results are summarized as follows:(1) The decomposition thermodynamics of tantalum-niobium ore with sodium hydroxide was studied.The results showed that in sodium hydroxide Ta2O5and Nb2O5can be changed into NaTaO3and NaNbO3, which provided theoretical reasons for low-alkali decomposition of tantalum-niobium ore.(2) The decomposing kinetics of niobium from a refractory tantalum-niobium ore with low alkali was studied. It indicated that increasing alkali-ore mass ratio, decreasing particle size of the ore or increasing reaction temperature significantly improves the conversionrate of niobium and the decomposing process was well interpreted with a shrinking core model under surface chemical reaction control. According to the Arrhenius expression, the apparent activation energy was estimated to be78.82kJ/mol and the kinetic equation was1-(1-x)1/3=4.803X103exp(-78820/RT)t. Combining the kinetic experimental results with the results of XRD, it was proved that, a solid product layer is formed and the main composition is sodium metatantalate, sodium metaniobate, topaz, tin oxide and iron-manganese oxides. Additionally, the technical parameters of the decomposing process were optimized. The results showed that the conversion rate of tantalum and niobium exceeded99%, which were10%higher than the conversion rate of tantalum and niobiumin the hydrofluoric acid method, so that the resource utilization of tantalum and niobium was increased.(3) The low-acid extraction pocess of tantalum and niobium from the pulp was studied. The single extraction rate of tantalum exceeded99%and the single extraction rate of niobium was less than5%in low-acid extraction unit of tantalum. The obtained organic phase including tantalum was washed six times by5mol/L sulfuric acid and the pure organic phase including tantalumwas obtained. The tantalum was stripped by pure water. The tantalum oxide was obtained from the pure tantalum liquid by neutralization, precipitation, filtering and roasting. The content of tantalum oxide is99.6%.The single extraction rate exceeded99%in niobium extraction unit. The obtained organic phase including niobium was washed six times by6mol/L sulfuric acid and and the pure organic phase including niobium was obtained. The niobium oxide was obtained from the pure niobium liquid by neutralization, precipitation, filtering and roasting. The content of niobium oxide is99.5%.(4) The low-acid extraction kinetics of tantalum from the pulp was studied. The results showed that the reaction was controlled by interfacial chemical reaction when the stirring speed was350-400rpm. The extraction rate constant was10-2.35±0.1and the extraction flux is F(kmol·m-2·s-1)=10-2.35[TaF72-][H+][MIBK](o). The control step of overall reaction wasH-MIBK+(i)+TaF72-→TaF7-MIBK-(i). According to the Arrhenius expression, the apparent activation energy was estimated to be31.8kJ/mol. According to the transition state theory, the activation enthalpy and the activation entropy were29.1kJ/mol and-185.7J/(mol-K), respectively.
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