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Recovery of tungsten from tungsten bearing compounds
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Date
2013
Author
Erdoğan, Metehan
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Extensive research in recent years has failed to develop any essentially new method of large scale tungsten production. A new tungsten powder production technique from calcium tungstate (CaWO4) has recently been reported. In this thesis, this technique was further explored from the aspects of electrochemical reduction mechanism and kinetics, applicability to scheelite concentrates and industrial production. Cyclic voltammetry, constant potential and constant current electrochemical reduction tests were performed to determine the reversible cell potential. Analyses of the experimental results revealed that at least 2.2 V was required to compensate the potentials for the accompanying cell reaction and the electrode polarizations. A cell reaction was proposed by associating the experimental results and the Gibbs Energy changes of the possible reactions. An experiment (mixture) design was created to optimize the process parameters of the electrochemical reduction of CaWO4 to W in molten CaCl2-NaCl eutectic mixture. Temperature, applied voltage and the length of Kanthal wire winding of the CaWO4 pellets were selected as the process parameters and allowed to vary between the predetermined minimum and maximum values. The rates of the electrochemical reductions were interpreted from the variations of current and total charge vs. time graphs under different conditions. The analysis pointed out 640oC and 2.81 V from the created mixture design for the fastest reduction and it was seen that the effect of Kanthal wire winding on the output current was less pronounced when compared to the other two parameters. Another set of experiments was performed by full factorial design to investigate the cleaning procedure needed to remove calcium containing byproducts after electrochemical reduction experiments. Three levels were determined prior to the experiments for the selected three parameters; temperature, acid concentration and exposure time. Main effect and interaction graphs for calcium percent as a function of process parameters were plotted. Calcium contents of the samples were determined by XRF measurements. A 300 g/day capacity tungsten production line was manufactured to take the process one step closer to industrialization. Problems at larger scale were addressed as incomplete reduction, oxidation of graphite and corrosion of cathode materials. After careful research, AISI 316 Ti steel was found to impart sufficient resistance to highly corrosive environment. Oxidation of graphite anode inside the cell was lowered to acceptable levels by continuous nitrogen flow. Metallic tungsten powder was obtained from rich and flotation concentrates of Uludağ Etibank Volfram Plant (closed in 1989) together with mainly iron. It was seen that tungsten and iron do not make compounds at the temperatures used for reduction (600-750oC). A basic diffusion model in the electrolyte was developed to better understand the decrease in current values and incomplete reduction encountered during large scale production. The model was used to simulate the recorded current vs. time graphs of selected experiments.
Subject Keywords
Tungsten.
,
Tungsten
,
Tungsten compounds.
URI
http://etd.lib.metu.edu.tr/upload/12615540/index.pdf
https://hdl.handle.net/11511/22262
Collections
Graduate School of Natural and Applied Sciences, Thesis
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Tungsten is mainly produced by H-2 reduction of WO3 which makes its production difficult and costly. A promising method was recently reported for direct electrochemical production of tungsten from CaWO4 (scheelite) which is estimated as the seventy percent of the all tungsten reserves of the world. This paper investigates the application of pulse current in the electrochemical reduction of CaWO4 pellets attached to a current collector. The electrolyte was composed of calcium chloride and sodium chloride sal...
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Hydrogen reduction of WO3 is the major industrial process in tungsten production. A promising cost and energy efficient method was recently reported [1-3] for direct electrochemical production of tungsten from CaWO4 (scheelite) which is estimated as the two third of the all tungsten reserves of the world. Following the above mentioned patent, several studies verified the production of metallic tungsten by electrochemical reduction of calcium tungstate in the laboratory. This study investigates the applicati...
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A novel production method for tungsten by an electrochemical process was reported in 2010. The process involved electrochemical reduction of calcium tungstate (CaWO<SUB>4</SUB>) in calcium chloride containing molten salt solutions. Although the process promised remarkable advantages over the traditional method relying on the laboratory scale experiments, the sintering behavior of the powder needs to be characterized properly. This study aimed to investigate the liquid phase sintering of the elec...
Characterization of the Liquid Phase Sintered Tungsten Heavy Alloys Prepared by an Electrochemically Produced Tungsten Powder
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M. Erdoğan, “Recovery of tungsten from tungsten bearing compounds,” Ph.D. - Doctoral Program, Middle East Technical University, 2013.