Browsing by Author "Stefanova V."
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Item A study of the partial neutralization process of solutions obtained during autoclave dissolution of pyrite concentrate(2017-01-01) Petkov K.; Stefanova V.; Stamenov L.; Iliev P.The process of the partial neutralization of solutions with high concentration of ferric ions and acidity (> 60g l-1 Fe3+ and H2SO4), obtained during autoclave dissolution of pyrite concentrate, has been studied. Two types of neutralizers have been used: limestone from deposit ore ``Velikan`` and a waste from cutting of marble plates - a fine and a coarse fraction. It was found that for the neutralization process limestone and the fine fraction from marble's waste can be used successfully. Both reactants are with a high content of calcium carbonate (> 92 %). The use of the coarse fraction of marble was inappropriate, due to the high content of magnesium carbonate (33.51 %) and the low extraction of gypsum (~ 49 %). The optimal parameters of neutralization process have been established: pH from 0.9 to 1.1, temperature 323K and time of neutralization 60 min. Gypsum, obtained under these conditions, has a high purity (< 0.05 % Fe, < 0.002 % Cu) and a high chemical activity. This makes it a suitable reactant for the conversion to ammonium sulfate, a commercial product that can be used as an artificial fertilizer.Item An analytical study of the neutralization process of solutions with high concentration of Fe(III) ions(2017-01-01) Petkov K.; Stefanova V.; Stamenov L.; Iliev P.With the means of software HSC Chemistry ver.7.1 modules: Equations Reaction and Eh-pH diagrams a thermodynamic assessment of the neutralization process of sulfuric acid solutions with high concentration of ferric ions (> 60 g l-1) with calcium carbonate and calcium hydroxide was carried out. Based on the calculated values of the energy of Gibbs and the equilibrium constants of the possible chemical interactions during the neutralization process with Ca(OH)2 and CaCO3 of a sulfuric acid solutions with a high concentration of ferric ions has been established that the neutralization process without participation of Fe3+ was thermodynamically more probable than in the presence of ferric ions. When Ca(OH)2 (hydrated lime) was used as neutralizer, the probability to obtain a precipitate of CaSO4*2H2O (gypsum) and FeO*OH (goethite) was the most thermodynamically probable, while using a neutralizer CaCO3 (limestone), the most probable was a precipitate of gypsum and Fe(OH)3. With increasing of the temperature from 25 to 60°C, the thermodynamic probability of goethite formation increases. Based on Eh-pH diagrams of the system H2SO4-CaCO3-Fe2(SO4)3-FeSO4-H2O it was established that in the pH range from -2 to 6 and temperature 25°C the most stable compound is calcium sulphate dehydrate. With the increase of temperature up to 60°C the most stable compound is anhydrite. The diagrams were built for molar concentrations of the elements in the solution (expressed as mol/kgH2O): 1,079 Fetotal, 0,622 S, 0,622 Ca and 0,622 C. The composition of the solution corresponds of the total iron and sulfuric acid concentrations and the quantity of CaCO3 necessary for neutralization of 100 % H2SO4. At the oxidation potential (Eh > 0.8 V) and high acidity of the solution (pH from -2 to 0.5), the areas of stability of iron ionic complex FeHSO4 2- and Fe3+ ion were found. In practice this means that the process of neutralization have to be carried out at a high oxidation potential and pH < 0.5 in order to avoid coprecipitation of iron sludge: Fe(OH)3 or FeO*OH.Item Analytical study of the Waelz clinker dissolution in NH3 - (NH4)2SO4 system(2014-02-25) Draganova K.; Stefanova V.; Iliev P.; Lucheva B.In the NH3-(NH4)2SO4 system a thermodynamical analysis of dissolution process of the Waelz clinker, which is a waste product of zinc production, has been carried out. The analytical study has been executed by the help of a professional program HSC Chemistry Ver. 7.1 using both modules ``Reaction Equation`` and ``Eh - pH Diagrams``. The NH3-Fe-S-H2O, NH3-Cu-S-H2O and NH3-Ag-S-H2O systems describing the dissolution of iron, copper and silver compounds (oxides and sulfides) of Waelz clinker in an ammonia medium in the presence of ammonium salt (NH4)2SO4 have been investigated at various temperatures and pressures. The influence of temperature (25 and 110°C), pressure (atmospheric and 0.4 MPa) and the consumption of the reagents on the substance composition of the solutions has been defined on the basis of the calculated and constructed Eh - pH diagrams.Item Application of electron beam melting method for recycling of tantalum scrap(2024-10-01) Vutova K.; Stefanova V.; Iliev P.The paper presents a study on the refining efficiency of tantalum scrap when applying the electron beam melting method to recycle tantalum material. Thermodynamic analysis and experimental tests were carried out at different technological regimes to determine the optimal conditions for obtaining tantalum with high purity. The behavior of the controlled impurities (W, Nb, Mn, Ce, Cr, Mo, Al, Fe, etc) and the influence of the thermodynamic and kinetic technological parameters (temperature, melting time) on the refining efficiency of the studied material were investigated. At a temperature of 3700 K and a melting time of 10 min, the highest overall degree of refinement of 98.82% and the highest tantalum purity of 99.987% were achieved.Item Behaviour of Impurities during Electron Beam Melting of Copper Technogenic Material(2022-02-01) Vutova K.; Stefanova V.; Vassileva V.; Kadiyski M.The current study presents the electron beam melting (EBM) efficiency of copper techno-genic material with high impurity content (Se, Te, Pb, Bi, Sn, As, Sb, Zn, Ni, Ag, etc.) by means of thermodynamic analysis and experimental tests. On the basis of the calculated values of Gibbs free energy and the physical state of the impurity (liquid and gaseous), a thermodynamic assessment of the possible chemical interactions occurring in the Cu-Cu2O-Mex system in vacuum in the temperature range 1460–1800 K was made. The impact of the kinetic parameters (temperature and refining time) on the behaviour and the degree of removal of impurities was evaluated. Chemical and metallographic analysis of the obtained ingots is also discussed.Item Cyclic voltammetry investigations of hydrogen peroxide on Ag/Ni foam electrodes(2013-12-17) Stefanova V.; Lichev I.; Miletiev R.; Lucheva B.The main problem of the hydrogen peroxide fuel cells (HPFC) is to find out appropriate electro-catalytic materials for the electrodes. A possible solution is to use of Ni-foam. By deposition of suitable films onto the electrodes made of Ni-foam can be achieved a significantly increase of the chemical and corrosion stability, an electrocondutivity and a high catalytic activity. Moreover, a considerably decrease in the weight of the electrode and respectively its price with comparison to the massive metal can be also achieved. In the present work, the behavior of stabilized solutions of hydrogen peroxide with concentration 0.15M H2O2; 0.22M H2O2 and 0.45M H2O2 in alkaline medium (1M KOH) and synthesized by hydrolysis of CaO2.xH2O in the presence of KHCO3 peroxide solutions with concentration 0.42M H2O2 and1M KOH onto massive, Ag electrode and Ag/Ni-foam electrodes have been investigated by cyclic voltammetry. The thickness of the electrodeposited coating onto nickel foam was 5 and 30 μm respectively. On the basis of the present study it is established that silver shows a high catalytic activity during reduction process of HO2- ion in alkaline solutions. The electrochemical behavior of H2O2 onto silver film, electrodeposited on Ni foam is analogous to this onto massive silver. The silver coating behaves actively even after prollong (100 cycle) electrochemical treatment. The same behavior of synthesized solution of hydrogen peroxide as compared to the stabilized solution is established and this suggests a possibility for its direct use as reagent in hydrogen peroxide fuel cells.Item Effect of electron beam method on processing of titanium technogenic material(2019-06-01) Vutova K.; Vassileva V.; Stefanova V.; Amalnerkar D.; Tanaka T.This study reveals the efficiency of the electron beam processing of titanium technogenic material with a high level of impurities and the quality of the obtained metal in correlation to process parameters which are discussed. The influence of the beam power and melting time on the composition variation, morphologies, hardness of metal samples and mass losses is investigated. Based on the different technological parameters, the removal efficiency of impurities is also discussed, and the corresponding experiments are carried out in order to make a comparison. Different thermal process conditions are realized during the single-melt operation. Chemical and metallographic analyses are performed, and the results are discussed. The hardness of the titanium decreases by prolonging the time of the electron beam processing. A maximal overall removal efficiency of 99.975% is seen at 5.5 kW beam power for a 40 min melting time and the best purification of Ti (99.996%) is achieved.Item Effect of germanium on zinc electrowinning sulfate solutions(2014-02-25) Mehmed E.; Stefanova V.; Lucheva B.The basic method for the electrolyte zinc production is an electrowinning process based on sulfate solutions. The presence of the impurities in the electrolyte is a major problem for the zinc electrowinning industry. They decrease the current efficiency, increase the energy consumption and deteriorate the quality of cathode deposited zinc. In this work the influence of the concentration of germanium in the synthetic or industrial electrolyte from 0 to 3.17 mg/L and 0.04 to 3.21 mg/L respectively, without and in the present of antimony (4.95 mg/L) on zinc electrodepositing has been studied. For this purpose, two electrochemical techniques have been used: a cyclic voltammetry and a galvanostatic deposition. It has been established that the increase of germanium concentration in the zinc sulfate electrolyte above 0.05 mg/l leds to essential decreasing of hydrogen overpotential and intensive reverse anodic dissolution of zinc. The presence of germanium ions in the electrolyte, significantly decreases the current efficiency and the quality of the electrodeposited zinc. The harmful effect of germanium in the electrolyte is increased in the presence of antimony.Item Effects of process parameters on electron beam melting technogenic materials for obtaining rare metals(2018-10-19) Vutova K.; Vassileva V.; Koleva E.; Stefanova V.; Amalnerkar D.In this paper, effects of electron beam melting process parameters (e-beam power, refining time, etc.) and the number of melting operations for fulfilling the requirements concerning the composition and the structure of metals obtained after e-beam processing are studied. Results obtained at electron beam melting of molybdenum and tungsten technogenic materials are presented and discussed. The refining efficiency is evaluated and effective technological regimes (process parameters) for production of metals which meet specific requirements for high-purity and good-quality structure after electron beam melting of Mo and W technogenic materials are proposed.Item Electron beam melting efficiency at multiple hafnium e-beam processing(2024-01-01) Markov M.; Stefanova V.; Vutova K.; Vassileva V.; Tanaka T.; Kakugawa K.The method of electron beam melting and vacuum refining has clear advantages over other metallurgical methods since it enables manufacturing of refractory and chemically active metals. This study focuses on the efficiency of removing impurities from technogenic hafnium under multiple electron beam melting. Assessments are performed on the efficiency of double and triple e-beam melting processing of refractory metal hafnium. The influence of different e-beam melting technological modes on the refining effectiveness is investigated. A highest hafnium purity of 99.2% was achieved after double and triple e-beam refinements of the investigated materials, with the highest process efficiency reaching 61.58% and 51.07%, respectively.Item Purification of zinc containing Waelz oxides from chlorine and fluorine(2017-01-01) Iliev P.; Stefanova V.; Lucheva B.; Kolev D.The Waelz oxides (WO) obtained upon processing of zinc containing semi products in Waelz kilns are rich in impurities such as alkalines, chlorides and fluorides. After acid leaching of the WO chlorine and fluorine ions dissolve into the zinc electrolyte, causing corrosion of aluminum cathode and lead anode and sticking of the deposited zinc on the cathode. For normal production operation the WO must contain up to 1000 ppm chlorine and 100 ppm fluorine. In the present study the laboratory tests on alkaline scrubbing of zinc containing WO in sodium carbonate solutions are presented. The effect of sodium carbonate concentration, pulp density and test duration on the degree of fluorine and chlorine purification is investigated. Based on the conducted research it was established that at temperature 363 K, pulp density 300 g l-1, concentration of sodium carbonate 100 g l-1 and test duration 180 min, the final concentration of fluorine and chlorine in the treated WO is 90 and 220 ppm, respectively. WO with such concentration of impurities are appropriate for further processing for cathode zinc production.Item Recovery of copper and silver from Waelz clinker wasted from zinc production(2014-02-25) Lucheva B.; Iliev P.; Draganova K.; Stefanova V.Waelz clinker is a waste material from the processing of zinc-lead cake into Waelz furnace in Bulgarian metallurgical plant KCM Ltd, Plovdiv, which contains valuable components such as copper silver, iron and carbon. Previously the clinker was treated by sulfuric acid solution in order to remove iron. The residue which contains 3,8 % copper, 250 g t-1 Ag and 26 % C is an object of this experimental work. The purpose of the work is to investigate the silver and copper recovery from this residue using H2SO4-HNO3 solutions. The influence of dissolution time, temperature and concentration of nitric acid at constant sulfuric acid concentration on the silver and copper recovery degree were investigated. The maximum silver recovery degree - 95 % is achieved at the following conditions: process duration - 1 h; temperature - 90°C; sulfuric acid concentration - 1M; nitric acid concentration - 1M; pulp density - 10 %. At these conditions the copper is practically completely recovered. The remaining insoluble residue contains 0,02 % Cu and 22 g t-1Ag. A technological scheme for two stage processing of Waelz clinker is developed.Item Recycling of Technogenic CoCrMo Alloy by Electron Beam Melting(2022-06-01) Vutova K.; Stefanova V.; Vassileva V.; Atanasova-Vladimirova S.In the current work, the possibility of the recycling of technogenic CoCrMo material by electron beam melting is investigated. The influence of thermodynamic and kinetic parameters (temperature and melting time) on the behavior of the main components of the alloy (Co, Cr, and Mo) and other elements (Fe, Mn, Si, W, and Nb) present in it, and on the microstructure of the ingots obtained after e-beam processing is studied. The vapor pressure of the alloy is determined taking into account the activities of the main alloy components (Co, Cr, and Mo). The relative volatility of the metal elements present in the alloy was also evaluated. An assessment of the influence of the temperature and the retention time on the degree of elements removal from CoCrMo technogenic material was made. The results obtained show that the highest degree of refining is achieved at 1860 K and a residence time of 20 min. The conducted EDS analysis of the more characteristic phases observed on the SEM images of the samples shows distinct micro-segregation in the matrix composition.Item Selective autoclave recovery of copper and silver from Waelz clinker in ammonia medium(2017-01-01) Iliev P.; Stefanova V.; Lucheva B.; Kolev D.Upon processing of zinc ferrite cakes in Waelz kilns a waste product, called Waelz clinker, is obtained. The copper and silver contents in this product are comparable to those of their ores which turns it into a valuable raw material. In the present work the experimental results on selective autoclave leaching of Waelz clinker in ammonia medium are presented. The effect of main technological parameters on the degree of value metals recovery and iron precipitation is studied. It is established that the highest extend of copper and silver recovery is obtained under the following conditions: temperature 383 K, Po2 0.4 MPa, ammonia concentration 110 g l-1, ammonium sulfate concentration 70 g l-1, pulp density 100 g l-1and test duration 120 min. At these conditions 86,5 % of copper and 68,5 % of silver are solubilized, whereupon over 99 % of iron is hydrolytically precipitated. The composition of the corresponding leach solution is: 1,22 g l-1Cu, 10 mg l-1 Ag, 90 mg l-1 Zn, 30 mg l-1 Fe and 40 mg l-1 Mn.Item Study of the crystallization process of ferric sulfate hydrate from rich of Fe(III) waste solutions(2017-01-01) Stamenov L.; Stefanova V.; Petkov K.; Iliev P.In present study the crystallization process of ferric sulfate hydrate - Fe2(SO4)3.xH2O from rich in Fe(III) sulfate waste solutions was investigated. These solutions were obtained after autoclave oxidation of pyrite concentrate. They are characterized with high concentrations (> 60 g l-1) of ferric ions and sulfuric acid. Based on the ternary diagram of the Fe2(SO4)3-H2SO4-H2O system and the laboratory tests the necessary compositions and conditions for preparation of saturated solutions for ferric sulfate crystallization process were determined. It was found that the crystallization process takes place with obtaining of bulky sludge containing following phases: FeH(SO4)2.4H2O (rhomboclase), Fe2(SO4)3.8H2O (ferric sulfate with eight molecules water) and Fe4.67(SO4)6(OH)2.20H2O (ferric sulfate hydroxide hydrate). After detention of the sludge for seven days at temperature 373 K two modifications of ferric sulfate hydrate were observed: ferric sulfate hydroxidehydrate(Fe4.67(SO4)6(OH)2.8H2O) and paracoquimbite (Fe2(SO4)3.9H2O).Item Study of the Possibility of Recycling of Technogenic Hafnium during Electron Beam Refining(2022-12-01) Vutova K.; Stefanova V.; Markov M.; Vassileva V.The possibility of removing metallic (such as Zr, Fe, Cr, and Zn) impurities and non-metallic (such as [O] and C) impurities from technogenic hafnium through single and double refining in the conditions of electron beam melting (EBM) has been studied. The influence of thermodynamic and kinetic parameters on the degree of removal of these impurities from the base metal under vacuum conditions and within a temperature interval of 2500 K to 3100 K is defined. The relative volatility of metal impurities and the stability of the oxides and carbides present in the base metal are evaluated. The possibility for complete removal of Fe, Cr, Zn, [O], and C during EBM is shown. In the case of double refining, at a temperature of 2700 K for 20 min, the maximum degree of removal of Zr is 46.8%, the achieved highest hafnium purity is 99.004%, and the overall effectiveness of the refining of hafnium from impurities is 53%. There is a correlation between the degree of removal of Zr and the micro-hardness of the Hf ingots obtained after EBM. The weight losses vary in the ranges of 1.5–5.8% and 1–8% under the studied single and double refining processes, respectively.Item Study on Hardness of Heat-Treated CoCrMo Alloy Recycled by Electron Beam Melting(2023-04-01) Vutova K.; Stefanova V.; Markov M.; Vassileva V.The hardness of heat (thermally) treated CoCrMo ingots, recycled by electron beam melting and refining (EBMR) of a technogenic CoCrMo material (waste from the dental technology) under different process conditions (temperature and residence time) is examined. The heat treatment consists of two-step heating up to temperatures of 423 K and 1343 K and retention times of 40 and 60 min, respectively. The influence of various loads (0.98 N, 1.96 N, 2.94 N, 4.9 N, and 9.8 N) on the hardness of the CoCrMo alloy, recycled by EBMR, before and after heat treatment is studied. It has been found that regardless of the EBMR process conditions, the obtained samples after heat treatment have similar hardness values (between 494.2 HV and 505.9 HV) and they are significantly lower than the hardness of the specimens before the heat treatment. The highest hardness (600 HV) is measured in the alloy recycled at 1845 K refining temperature for 20 min. This is due to the smaller crystal structure of the resulting alloy and the higher cobalt content. The results obtained show that the heat treatment leads to considerable changes in the microstructure of the CoCrMo ingots recycled by EBMR. With the increase of the e-beam refining temperature, after the heat treatment, the grains’ size increases and the grains’ shape indicates an incomplete phase transition from γ-fcc to ε-hcp phase. This leads to a slight increase in the hardness of the alloy.Item THE EFFECTIVENESS OF ELECTRON BEAM MELTING FOR REMOVING IMPURITIES FROM TECHNOGENIC METAL MATERIALS(2024-01-01) Markov M.; Stefanova V.; Vutova K.Electron beam melting and vacuum refining (EBMR) is a modern metallurgical method that has proven advantages in terms of environmental sustainability and efficiency for recycling refractory and reactive metals and alloys with unique physical, chemical, and mechanical properties. In this work, the effectiveness for removing impurities from technogenic molybdenum, titanium, and hafnium metals using EBMR method is investigated. The thermodynamic and kinetic process conditions and their influence on the possibility of impurities’removal from the studied technogenic materials are discussed. It has been established that there are no thermodynamic limitations for the removal of silicon, antimony, iron, aluminum, and copper impurities at EBMR of molybdenum and the maximal overall removal efficiency is 58 %. In the case of titanium technogenic material, the highest refining efficiency and maximal overall impurity removal level of 99.97 % and 100 % for some inclusions (such as Fe, Cu, Ta, and Cd) are achieved. The studies show that impurities (metallic and non-metallic) can be effectively removed from technogenic hafnium using single e-beam refining processing and the highest refining effectiveness is 52.21 %. The obtained results allow formulating the conditions of the refining process aiming to obtain metal materials with high purity, better structures and properties and demonstrate high effectiveness of the EBMR method.Item Thermodynamic analysis of the processes during electron beam melting and refining of copper(2022-04-04) Stefanova V.; Vutova K.; Vassileva V.In the paper, results are presented and discussed obtained by thermodynamic analysis of the refining processes of copper aimed at removing metal impurities and oxygen via electron beam melting (EBM) in vacuum. Thermodynamic evaluations of the possible chemical interactions between the base metal (Cu) and the metal impurities present in Cu that can proceed during the refining of copper are made based on the calculated values of the Gibbs free energy and equilibrium constants taking into account the physical state of copper and of the metal impurities during EBM.Item Thermodynamic analysis of the sulphatization processes taking place in a dust-gas flow from flash smelting furnace(2016-01-01) Iliev P.; Stefanova V.; Shentov D.; Lucheva B.Analytical studies of the physicochemical interactions and isothermal equilibriums in Cu-Fe-S-O system were conducted at temperatures 1623K, 1373K and 973K by software HSC Chemistry ver.7.1 modules: Reaction Equations and Lpp diagrams. The thermodynamical stability of the phases in Cu-S-O and Fe-S-O systems were determined under certain conditions. The tendencies of their variation upon injection of ``sulphatizing air`` in the volume of the dust-gas flow output of flash smelting furnace (FSF) uptake were also studied. It was established that the formation of cuprum sulphate and iron ferric sulphate is thermodynamically possible only at temperatures lower than 973 K. The results predicted on the basis of the analytical investigation were confirmed by chemical and X-ray analyses of dust samples taken from the convective part of the waste heat boiler (WHB).