Jana Jurišová

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.

Preparation of magnesium hydroxide from nitrate aqueous solution

Nucleation of Mg(OH)2 was investigated by measuring the electrical conductivity and pH of the Mg(NO3)2 reaction solution to which ammonia containing different amounts of NH4NO3 was added. NH4NO3 increases solubility and slows down precipitation of Mg(OH)2 in the system. Data are presented on the influence of NH4NO3 on the solubility of Mg(OH)2 at 25°C. The phenomena observed can be explained by the solvation effect of nitrate ions brought to the system with the addition of ammonium nitrate, which was proved by NMR spectroscopy. When the mass fraction of NH4NO3 exceeds 15 %, homogeneous nucleation does not proceed. It was found that seeding of the system with Mg(OH)2 crystals only influenced the rate of Mg(OH)2 crystallisation, not the size and shape of the crystals. Primary crystals are smaller than 0.1 μm. The large difference in the surface energy of individual crystal planes leads to oriented agglomeration. This process is accelerated in a pressure reactor at 130°C. The resulting polycrystals are hexagonal plates 0.2 μm thin with a diameter of 1–2 μm. Under variable reaction conditions, agglomerates as big as 30 μm can be prepared.

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.

Determination of sulphur species in solidified cryolite melts

Solidified cryolite melts containing a known amount of Na2SO4 (0–713 mg kg−1 SO42−), Na2S, FeS, and CdS (0–400 mg kg−1 S2−) together with industrial electrolyte samples were tested for the content of sulphate and sulphide ions by ion chromatography. Added and analytically determined contents of sulphate and sulphide were compared and processed by means of linear regression analysis. It was found that the method of ion chromatography yields satisfying results (uncertainty below 1.1 %) and that it is especially suitable for the determination of low content of soluble sulphate or sulphide in solidified cryolite electrolytes. The method can be used for the estimation of insoluble sulphide content in cryolite melts as well. Results of industrial samples analysis showed that the content of sulphate and sulphide in the samples is influenced by their treatment before the analysis.

Preparation of needle-like aragonite particles from calcium nitrate solution in batch and flow reactors

Preparation of needle-like aragonite particles from calcium nitrate solution in batch and flow reactors Needle-like aragonite particles for application in paper industry were synthesised from calcium nitrate solution. Calcium nitrate was prepared from waste lime. Samples of precipitated aragonite were prepared both in batch and flow reactors, respectively. Conditions (concentration of calcium nitrate, temperature, and flow rate of CO2) were optimized for achieving high yield of aragonite in the product.

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.

Characteristics of Sorel cement prepared from impure materials

Abstract Production of potassium nitrate from potassium chloride and magnesium nitrate is related with the production of by-product, which is water solution of MgCl2 having concentration (24-30) mass %. Along with magnesium chloride this solution contains also nitrates and potassium and calcium cations. This solution can be used as defrosting agent or as a component for the production of building material based on Sorel cement. Sorel cement is material originating from the reaction of MgCl2 solution with solid MgO of suitable reactivity which depends on the temperature of calcinations of MgCO3 and size of MgO particles. Sorel cement paste is prepared by mixing MgO and MgCl2 in molar ratio 9 : 1 with the addition of water in appropriate amount. This cement settles in 2 hours and it can be treated within 24 hours, depending on the reactivity of MgO. This paper deals with the preparation and properties| of Sorel cement made of by-product (MgCl2) from the production of fertilizer KNO3 and MgO prepared from Slovak magnesite (Jelsava) with high content of impurities. We will show that properties of this material are suitable for application in building industry.

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.