Vladimír Danielik

Content of sodium and lithium in aluminium during electrolysis of cryolite-based melts

The content of sodium and/or lithium in polarized and nonpolarized aluminium in contact with cryolite melts was determined in a laboratory cell. The cryolite-based melts contained 0 to 20mass% excess AlF3 and 0 to 2.5mass% LiF. The cathodic current density ranged from 0 to 0.5Acm−2. The lithium content in aluminium increases linearly with increasing concentration lithium fluoride in the␣melt. It also increases with increasing cathodic current density and decreasing cryolite ratio. On the other hand the sodium content decreases with increasing concentration of LiF in the melt. This effect is more notable at higher current densities.

Electrochemical behaviour of lanthanum fluoride in molten fluorides

The electrochemical behaviour of lanthanum fluoride dissolved in molten lithium fluoride and in eutectic mixture LiF-CaF2 was investigated by cyclic voltammetry and laboratory electrolysis. The cyclic voltammetry experiments were carried out at 900°C and 800°C, respectively, in a graphite crucible (counter electrode). Several types of working electrodes (Mo, W, Ni and Cu) were used. Ni/Ni(II) was used as a reference electrode. Laboratory electrolysis was carried out in the system LiF-CaF2-LaF3 at 800°C in galvanostatic (jc = −0.21 A cm−2) and potentiostatic (E = 0.87 V) regimes. In both cases, nickel served as the cathode and graphite as the anode. It was found that no new separate reduction peak occurred on the molybdenum or tungsten electrodes in the investigated systems. When copper or nickel electrodes were used, new peaks corresponding to the reduction of lanthanum(III) to lanthanum metal appeared. This can be explained by the formation of alloys or intermetallic compounds of lanthanum with copper or nickel. X-ray microanalysis showed that lanthanum was electrodeposited together with calcium under formation of intermetallic compounds with the electrode materials in the galvanostatic regime. In the potentiostatic regime, mainly lanthanum was deposited, which enabled its separation.

Alkali and earth alkaline metal contents in the aluminium cathode in aluminium electrolysis

The contents of sodium, lithium, calcium and magnesium in aluminium in contact with NaF–AlF3-based melts in laboratory and in industrial aluminium cells were investigated in the temperature range 950–1030 ∘C. The experimental data were compared with a thermodynamic model. It was found that the addition of alumina or CaF2 to the NaF–AlF3 melts has only a minor effect on the equilibrium content of sodium in aluminium. Cathodic polarization enhances the content of sodium in aluminium. However, polarization has a smaller effect on the concentrations of lithium, calcium and magnesium in aluminium in industrial cells.

Electrical conductivity of molten fluoride-oxide melts with high addition of aluminium fluoride

Abstract Electrical conductivity of NaF-AlF3 melts with an addition of 2 wt % Al2O3 and/or 5 wt % CaF2 was measured using a pyrolytic boron nitride tube-type cell with a constant distance of electrodes. The molar cryolite ratios MR = n(NaF)/n(AlF3) were 1.8 and 1.6, and the temperature was varied from 865 °C to 1005 °C. Ac-techniques with a sine wave signal with small amplitude in the high frequency range were applied. Electrolyte resistance was obtained from nonlinear regression analysis according to equivalent circuit. Experimental data were used to describe the dependence of the electrical conductivity in fluoride melts with lower temperature on the amount of various additions and temperature.

Preparation of potassium nitrate from potassium chloride and magnesium nitrate in a laboratory scale using industrial raw materials

Abstract Preparation of potassium nitrate from magnesium nitrate and potassium chloride was investigated. Prepared potassium nitrate contains less than 0.5 % chlorides and it can be applied as environmentally friendly fertilizer in hydroponic systems. After filtration out potassium nitrate crystals from the reciprocal system K+, Mg2+//Cl-, NO3- - H2O, the mother liquor still contains reasonable amount of potassium cations. By evaporation of the mother liquor, carnallite (MgCl2・KCl・6H2O) with admixture of MgCl2・6H2O crystallizes out. Decomposition of carnallite with cold water makes it possible to separate potassium chloride from this compound. When this KCl is returned back to the process of KNO3 making, utilization of potassium as high as 97 % can be achieved.

Electrochemical behaviour of the LiF-(CaF2)-La2O3 system

The electrochemical behaviour of the LiF-La2O3 and LiF-CaF2-La2O3 systems was investigated by means of cyclic voltammetry. Several types of working electrodes (spectrographic pure graphite, W, Mo, Ni, Cu) were used. It was found that chemical reactions take place in the system during the dissolution of lanthanum oxide. The reduction of lithium cations occurred at the most positive potential from the species formed in the melt on ‘inert’ cathodes (W, Mo). The reactive cathodes (Cu, Ni) allowed the lanthanum deposition with depolarisation.

Determination of the reactivity of CaSO4 · 2H2O

Abstract Reactivity of energy gypsum was investigated. The method is based on the conversion reaction of aqueous suspension of gypsum with ammonium carbonate solution. Rate of the conversion of all investigated samples, including gypsum originating from flue gas desulphurization (FGD), was high and 80 % conversion was achieved during one hour. It was found that there is a correlation between BET specific surface and the rate of the conversion reaction.

Phase diagram of the system CaSO4—K2SO4—KNO3—Ca(NO3)2—H2O

Abstract Potassium nitrate as a fertilizer suitable for greenhouse and hydroponic applications can be prepared by the reaction of potassium sulphate with calcium nitrate. However, it may happen that simultaneously with the precipitation of gypsum (CaSO4·2H2O) also two other binary salts, viz. syngenite (K2SO4·CaSO4·H2O) and görgeyite (K2SO4·5CaSO4·H2O) can crystallize. This would lower the yield of KNO3. For minimization of potassium loss we have to determine the conditions under which syngenite and görgeyite crystallize. As a useful tool for the quantitative determination of specific hydrates, simultaneous DTA/TG technique appeared. Each hydrate decomposes at a certain temperature. The loss of water at dehydration can be used for a quantitative determination of the amount of the hydrate in the precipitating solid phase. Based on the experimental data several conclusions can be drawn: (i) excess of calcium cations lowers the concentration of sulphate ions in the liquid phase together with lowering of contents of syngenite and görgeyite in the solid phase; (ii) higher content of water results in a higher solubility of sulphate ions; (iii) joint crystallization of syngenite and gypsum occurs in the composition area interesting from the point of KNO3 production; (iv) area of the primary crystallization of görgeyite does not exist in the phase diagram at 80 °C. However, görgeyite crystallizes at the molar ratio Ca(NO3)2:K2SO4 = 1:1 by ternary crystallization; (v) area of crystallization of pure gypsum is shifted to lower ratio Ca(NO3)2:K2SO4 by the addition of water to the system.

Absorption of ammonia in the melt of ammonium nitrate

The paper deals with the absorption of ammonia in a model fertilizer of ammonium nitrate. Volumetric overall mass transfer coefficient quantitatively characterizing the process was estimated on the basis of experimental data and kinetics modeling. The volumetric overall mass transfer coefficient depends on the temperature, content of water in the fertilizer and hydrodynamic conditions. Different hydrodynamic conditions were simulated by different speeds of the propeller stirrer. The empirical equation describing the volumetric overall mass transfer coefficient was proposed.

Corrosion of metals in zinc nitrate hexahydrate and calcium chloride hexahydrate

Abstract Materials used for heat accumulation are substances with the phase change at temperatures below 80 °C. In such substances, a high amount of energy can be stored due the phase change. Because of possible corrosion of the heat container components, it is necessary to know their resistance in the given medium. In this work, the corrosion of aluminum, copper and carbon steel was studied in two liquid media: zinc nitrate hexahydrate and calcium chloride hexahydrate. Corrosion tests have shown that steel is the least resistant to corrosion in both media. Aluminum has been proved as the most durable material in zinc nitrate hexahydrate media. On the other hand, pitting corrosion on aluminum occurred in the calcium chloride hexahydrate making it unsuitable for the use in this media. From the comparison of two studied PCMs follows that zinc nitrate hexahydrate is a more aggressive medium in comparison with calcium chloride hexahydrate. From the point of view of corrosion, zinc nitrate hexahydrate is not suitable for heat accumulation when using the studied metals. When using the calcium chloride hexahydrate as PCM, copper is suitable as a construction material; aluminum and carbon steel show pitting corrosion.