A R C H I V E S O F M E T A L L U R G Y A N D M A T E R I A L S Volume 59 2014 Issue 1 DOI: 10.2478/amm-2014-0012 P. PALIMĄKA ∗ , S. PIETRZYK ∗ RESEARCH ON THE ELECTRICAL CONDUCTIVITY OF FLUORIDE ELECTROLYTES NaF - AlF 3 - CaF 2 IN LIQUID AND SOLID STATE BADANIA PRZEWODNICTWA ELEKTRYCZNEGO ELEKTROLITÓW FLUORKOWYCH NaF-AlF 3 -CaF 2 W FAZIE CIEKLEJ ORAZ W STANIE STALYM The main aim of the present work was to perform conductivity measurement of fluoride electrolytes in binary system Na 3 AlF 6 - CaF 2 and ternary system Na 3 AlF 6 - AlF 3 - CaF 2 . The experiments were done in a wide range of temperature (580-1300 K), including the liquid and solid phase. Temperature dependencies of the conductivity of the liquid and solid cryolite mixtures with CaF 2 (2, 4 and 6 wt %) and ternary solution (88 wt % Na 3 AlF 6 – 8 wt % AlF 3 -4 wt % CaF 2 ) were determined by an A.C. two-electrodes technique. The simultaneous measurements of conductivity and temperature were made in 1min steps during heating and cooling of the sample with rate 0,5 K·min −1 . It was confirmed, that the addition of CaF 2 to liquid electrolyte decreases its electrical conductivity and liquidus temperature, whereas addition of aluminium fluoride up to 4 wt % in solid state has the inverse influence on conductivity. The studied ternary system showed the highest conductivity in the solid state at temperatures up to phase transition α − β in solid cryolite (ca. 780 K). This may be due to the formation of a solid solution of NaCaAlF 6 in β-Na 3 AlF 6 and the formation of cationic vacancies, which facilitate the transport of electrical charge. Keywords: aluminium, electrical conductivity, cryolite electrolytes, aluminium electrolytes Głównym celem pracy było wykonanie pomiarów przewodnictwa elektrycznego elektrolitów fluorkowych w układzie dwu- składnikowym Na 3 AlF 6 -CaF 2 oraz trójskładnikowym Na 3 AlF 6 -AlF 3 -CaF 2 . Pomiary wykonano w szerokim zakresie temperatur (580-1300K) zawierającym fazę ciekłą oraz fazę stałą. Temperaturowe zależności przewodnictwa ciekłych roztworów kriolitu z CaF 2 (2, 4 i 6% mas.) oraz roztworu trójskładnikowego o składzie 88% mas.Na 3 AlF 6 -8%mas. AlF 3 -4%mas. CaF 2 określono dwuelektrodową techniką zmiennoprądową. Jednoczesne pomiary przewodnictwa i temperatury wykonywano w 1 minuto- wych odstępach czasu podczas cykli ogrzewania i chłodzenia z szybkością 0,5 K·min −1 . Na podstawie uzyskanych wyników potwierdzono, iż dodatek CaF 2 do ciekłego elektrolitu obniża jego przewodnictwo elektryczne oraz temperaturę likwidusu. Natomiast wpływ tego dodatku do kriolitu w zakresie stężeń do 4% mas. w stanie stałym jest odwrotny. Z kolei badany układ trójskładnikowy wykazał najwyższe przewodnictwo elektryczne w stanie stałym w zakresie temperatur aż do przemiany fazowej α − β w stałym kriolicie (ok. 780 K). Może to być spowodowane tworzeniem się roztworu stałego NaCaAlF 6 w β-Na 3 AlF 6 i powstawaniem wakancji kationowych ułatwiających transport ładunku elektrycznego. 1. Introduction In the electrowinning of aluminum, part of total electrol- ysis current can be lost because of so-called current escapes. This is possible because the crystallized electrolyte (sideladge) on the carbon side lining can conduct electricity [1]. As a re- sult, this part of the current flows from the anode to the cath- ode bypassing liquid electrolyte, causing loss of current and reduced current efficiency of the electrolysis. The sideledge composition is directly dependent on the composition of the electrolyte. The current flow will depend both on the chemical composition of crystalized layer and its temperature. There- fore, it is important to know the influence of composition and temperature of the sideledge on its electrical conductivity. In earlier work [2], the authors studied the electrical con- ductivity of cryolite – the basic component of the electrolyte and its mixtures with addition of 4, 8 and 12 wt %. AlF 3 . The results showed that the addition of aluminum fluoride decreases the conductivity of molten cryolite. A similar effect of this additive was observed in the solid phase, in which AlF 3 and Na 3 AlF 6 do not create solid solutions. However, in the temperature range including a mixture of liquid and solid phases (between liquidus and solidus), the aluminum fluoride strongly increased the conductivity, which is mainly caused by the presence of highly conductive liquid phase. In the present work the effect of calcium fluoride (2, 4 and 6 wt %) on the conductivity of cryolite and the ternary system Na 3 AlF 6 - AlF 3 (8 wt%) - CaF 2 (4 wt%) in a wide temperature range including the liquid and solid phase was investigated. The electrical conductivity strongly depends on the solid solutions in the studied system. The existence and range of component solubility in the cryolite are crucial for ∗ AGH UNIVERSITY OF SCIENCE AND TECHNOLOGY, FACULTY OF NON FERROUS METALS, AL. A. MICKIEWICZA 30, 30-059 KRAKÓW, POLAND Unauthenticated | 89.73.89.243 Download Date | 4/30/14 11:24 AM