Multi-nuclear magnetic resonance study of Na3AlF6-AlPO4 molten and solidified mixtures

Phosphorus is one of the predominant impurities in the Hall-Heroult process for industrial aluminium production. The nature of the dissolved phosphorus species in the Na(3)AlF(6)-AlPO(4) system has been investigated by in situ high-temperature (HT) (19)F, (23)Na, (27)Al, (17)O, and (31)P NMR. The combination of these experiments enables to define the presence of PO(4)(3-), AlF(5)(2-) and (AlF(4)-O-PO(3))(4-) anions in the melt, and then the formation of Al-O-P bonding. Melts solidified at different cooling rates were characterised using various solid-state NMR techniques including multiple quantum magic angle spinning (MQMAS), rotational echo double resonance (REDOR) and heteronuclear single quantum correlation (HSQC). The glass obtained by the rapid quenching of the hypereutectic melt has been carefully described in order to better understand the structure of the melt.     

Phase Diagram of the System NaF-KF-AlF3

The phase diagrams of the binary system KF-AlF₃ as well as the ternary system NaF-KF-AlF₃ in the range up to 50 mol% AlF₃, were measured using the thermal analysis method. In the system KF-AlF₃ the coordinates of the eutectic points are: E ₁: 8.0 mol% AlF₃, 821.2°C, and E ₂: 45.5 mol% AlF₃, 565.0°C. In the investigated concentration range of the ternary system 2 eutectic points have been found with the calculated coordinates: E ₁: 36.3 mol% NaF, 62.7 mol% KF, 1.0 mol% AlF₃; t=711.2°C; and E ₂: 51.9 mol% NaF, 27.4 mol% KF, 20.7 mol% AlF₃; t=734.5°C. Other eutectic points lie most probably beyond the investigated part of the system. This revised version was published online in July 2006 with corrections to the Cover Date.     

Solubilities of lanthanum oxide in fluoride melts: Part I. Solubility in M3AlF6 (M = Li, Na, K)

Solubility of lanthanum oxide was measured by thermal analysis. The solubility in alkali cryolites is rather high, because of chemical reactions between lanthanum oxide and cryolites. In Li₃AlF₆-La₂O₃, alumina precipitates, in the other systems the mixed oxide LaAlO₃ is formed. In La₂O₃-Li₃AlF₆ the eutectic point is at 9.5 mol% La₂O₃ and 755 °C. The eutectic points in La₂O₃-Na₃AlF₆ and La₂O₃-K₃AlF₆ are at 11.5 mol% La₂O₃, and at 937 and 934 °C, respectively.     

C K‐edge NEXAFS study of fluorocarbon formation on carbon anodes in molten NaF–AlF3–CaF2

Operational instability from processes occurring at the anode during the production of aluminum in the commercial Hall‐Héroult process may lead to an increase in undesirable fluorocarbon emissions, higher energy use, and shorter plant life. One contribution to this instability may be the possible formation of a fluorocarbon film at the electrode interface. Here, the surface composition of graphite anodes after electrolysis in molten NaF–AlF₃–CaF₂ at 990 °C is investigated for evidence of fluorocarbon formation using C K‐edge near edge X‐ray absorption fine structure. Fluorocarbon is identified on an anode surface after prolonged anode effect (very high overpotential with increased cell resistance) and also on an anode surface after normal electrolysis without anode effect. This provides evidence that fluorocarbon formation may occur prior to anode effect lowering the surface tension of the anode and therefore resulting in dewetting to contribute to the onset of the anode effect. Confirmation that such compounds form furthers our understanding of electrochemical reactions of graphite with fluoride and of the fundamental processes that occur in an aluminum smelter cell. Copyright © 2013 John Wiley & Sons, Ltd.     

Enthalpie de Formation dans le Système Na2 O-K2O-SiO2

Abstract

L'enthalpimétrie par réaction directe dans les systèmes silicatés n'est pas possible. La méthode de dissolution qu'il faut alors employer engendre une erreur importante vis-à-vis des faibles énergies mises en jeu dans l'effet alcalin mixte ici étudié. Nous montrons cependant que les composés silicatés liquides comportant Na et K présentent une enthalpie de formation par référence aux binaires de sodium et potassium purs négative lorsque la fraction molaire en alcalin reste inférieure à 0,4.

Nos mesures d'enthalpie de formation à 876 K confirment les observations et estimations de la lacune de demixtion pour les composés riches en silice.

Direct enthalpimetry is not suitable for liquid silicate systems study. Then, dissolution calorimetry becomes a necessity but leads to inaccuracy in the measurements which is great with respect to the weak energy effects observed when studying mixed alkaly effect.

By our measurements at 876 K we show that liquid Na-K silicates exhibit a negative enthalpy of formation referenced to Na and K liquid silicates, for alkaline molar concentration less than 0, 4. We confirm also the evidence of a miscibility gap for rich SiO₂ compounds.     

Conductivity of low-temperature KF-AlF<Subscript>3</Subscript> electrolytes containing lithium fluoride and alumina

Electroconductance of molten electrolytes KF-AlF₃ (cryolite ratio 1.3) containing LiF or/and Al₂O₃ is evaluated using empirical equations. The electroconductivity of molten mixtures KF-AlF₃, KF-AlF₃-Al₂O₃, KF-AlF₃-LiF, and KF-AlF₃-LiF-Al₂O₃ is measured in the temperature interval 687 to 797°C. The electroconductivity is determined in cells with a capillary of pyrolytic boron nitride and parallel molybdenum electrodes. Comparing calculated and experimental data on the electroconductivity of cryolite-alumina melts confirms that the model proposed for the calculations reflects the temperature and concentration dependences adequately enough and makes it possible to forecast variations in the electroconductivity upon adding different admixtures into the electrolyte.     

火焰原子吸收光谱法测定冰晶石中的铁

建立了火焰原子吸收光谱法测定冰晶石中铁的分析方法。样品用高氯酸溶解后,在0.48mol/L的盐酸介质中用空气-乙炔火焰测定了样品中的铁,测定结果与有色金属行业标准方法(邻二氮杂菲光度法)的分析结果一致。检出限为0.004 11mg/L,相对标准偏差(RSD)为0.95%~4.5%,加标回收率为98.00%~101.49%。     

包头磁尾矿稀土浸出和制备冰晶石的研究

包头白云鄂博矿床产生大量磁尾矿,会产生很大的环境问题。采用纯碱焙烧法对其进行了稀土浸出实验和氟的资源化利用研究。实验结果表明,包头磁尾矿与纯碱混合物于700℃焙烧1.0 h,经稀盐酸洗涤后,以3.0 mol.L-1稀硫酸于65℃下浸出4.0 h,氟的洗脱和稀土浸出效果较好,其中稀土浸出率为97.97%,氟浸出率为99.54%,氟洗脱率为77.32%。以该条件下产生的含氟废水为原料,于90℃下制备了冰晶石,溶液中氟回收率为85.63%,并推算出磁尾矿中氟的总利用率为66.21%。该法使磁尾矿由废弃物变为二次资源成为可能。     

火焰原子吸收光谱法测定电解铝用冰晶石中锂

含锂冰晶石取代锂盐添加到电解质中被广泛应用于电解铝工业,其质量直接影响到电解铝工业的电解效率、吨铝能耗高低和电解铝产品质量优劣。控制冰晶石中锂含量是保证产品质量和应对废旧电解质再生对氟化盐市场冲击的一个重要举措。通过在铂坩埚中加入一定量高氯酸在高温条件下加热冒烟赶氟后再加入盐酸酸化至盐类完全溶解,建立了波长670.80 nm处采用火焰原子吸收光谱法测定铝用冰晶石中锂含量的方法,避免了大量氟基体的干扰,选择一定量的铝钠基体匹配来保证测试灵敏度和准确性;通过加标回收实验得知锂的回收率为102%;锂元素校准曲线线性相关系数≥0.9994;锂的检出限为0.0065μg/mL;不同含量含锂冰晶石的锂标准偏差均小于0.01%,与电感耦合等离子体原子发射光谱(ICP-AES)法测定结果相一致。方法准确可靠、简单易操作,满足铝电解工业生产分析的需要。     

稀土氧氯化物在冰晶石—氧化铝熔体中的溶解度

用等温饱和法测定了单一稀土氧氯化镧（LaOCl)和混合稀土氧氯化物（REOCl)在冰晶石（Na3AlF6)-氧化铝（Al2O3）熔体中的溶解度。结果表明：LaOCl和REOCl的溶解度变化规律一致,即其溶解度随温度升高而增大;随着冰晶石分子比增加而增大;当有4．0％（质量分数）Al2O3存在时,其溶解度相应减少。