D. K. Bose

Molybdenum metal by the aluminothermic reduction of calcium molybdate.

This investigation deals with a process for extracting molybdenum metal in well consoli-dated form and in good yield from calcium molybdate, by open aluminothermic reduction. Aluminothermic molybdenum has been further processed both by electron-beam melting and by molten salt electrorefining for the removal of aluminum. The final metal compares very favorably with commercial molybdenum obtainable by other routes.

Development of Al–5%Ti–1%B master alloy

Abstract The enhancement of grain refining action by the addition of boron along with titanium in aluminium and aluminium alloys is well known. This paper describes a process for the development of Al–5%Ti–1%B master alloy by the alloying of aluminium with titanium and also with boron which was produced in situ by the reduction of KBF 4 with excess aluminium. Alloying efficiencies as high as 92 and 80%, respectively, for titanium and boron computed on the basis of elemental recovery were obtained at 1200°C. The decrease in grain size of aluminium by the addition of a calculated quantity of master alloy was also studied.

Development of carbothermy for the preparation of hepta chromium carbide

Abstract Chromium carbide (Cr 7 C 3 ) is a versatile industrial material and is used as wear-resistant coating, as a catalyst a wear resistant ceramic material, and also as an additive in welding electrode flux. Extensive studies have been carried out on the preparative route for the production of hepta chromium carbide (Cr 7 C 3 ) and a process based on carbothermic reduction of Cr 2 O 3 has been optimised. The process essentially involves thorough blending of the oxide and petroleum coke, compaction of the blend and heating soaking of the compacts under dynamic vacuum to yield the carbide. Heating is done in the temperature range of 1173–1473 K. The resulting product was analyzed for phase identification by XRD technique, for oxygen and carbon content by chemical analysis and for surface morphology by scanning electron microscope.

Preparation of high purity niobium

Abstract The commercial methods of niobium production involve carbothermic or aluminothermic reduction of niobium pentoxide. Recently, alternative preparative techniques based on decomposition of niobium nitride have been investigated. The carbothermic reduction of niobium has been extended to the direct preparation of Nb-Ti alloys. The as-reduced niobium product obtained by the above processes contains residual impurities such as carbon, oxygen and nitrogen as well as metallic elements such as tantalum and tungsten. The consolidation and purification of niobium is effected by solid state pyrovacuum treatments and electron beam melting. A variety of vaporization reactions occur during these treatments resulting in the purification of the metal with respect to the interstitial impurities. Ultrapurification methods are used for the removal of tantalum and tungsten and for further reduction of interstitial impurity contents. In this paper the reduction and refining techniques of niobium and Nb-Ti alloys (starting from their oxides) are discussed with emphasis on the more recent techniques and physicochemical principles.

Processing of low-grade Indian molybdenite concentrate by chlorination in a fluidised-bed reactor

Abstract Recovery of molybdenum by chlorination of low-grade Indian molybdenite concentrate in a fluidised bed is described. Using a fluidising gas mixture of N 2 -O 2 -Cl 2 at 275°C, the molybdenum fraction from the concentrate was recovered as oxychloride. The residue after chlorination, containing Cu, Ni and Fe chlorides, was subjected to an oxidative roasting treatment in the same reactor at 500°C in the presence of sodium chloride. This treatment helped in the preferential oxidation of iron chloride. On leaching with water, the roasted residue resulted in a Cu-Ni-rich chloride solution with very low iron.

Electro-extraction of molybdenum from Mo2C-type carbide

A process for the preparation of molybdenum from molybdenum carbide was investigated.It involved fused salt electrolysis of the carbide in an inert atmosphere electrolytic cell using a KCl-K3MoCl6 electrolyte. The preferred conditions for electrolysis carried out in a 0.075 m (3 in.) diam cell were: voltage 0.2 to 0.5 V; cathode current density 8000 A/m2 (720 amp/f2);bath temperature 1203 K; and electrolyte composition 7.5 pct molybdenum. Under these conditions, electrolysis in a 0.15 m (6 in.) diam cell charged with 1.5 kg of the carbide yielded a total metal recovery of 71 pct at an average current efficiency of 60 pct. The metal purity was better than 99.9 pct. The electron beam melt hardness for the electro-extracted molybdenum was in the range of 150 to 160 DPH.

Studies on the electric discharge compaction of metal powders

Consolidation and densification of metal powder are usually attained by compaction followed by prolonged high-temperature sintering or by hot pressing. However, these methods are not suitable where grain growth is to be avoided. So electric discharge compaction is being studied so as to establish a fast, cheap, and reproducible process. In this process, high current pulse for a few millisecond duration is passed through metal powder compacts held under pressure in a die. This paper gives the details of the investigation being carried out to establish the process parameters for the densification of titanium, tin and zinc powders. Remarkable improvements in the density and microstructure are noticed after subjection to the electric discharge.

Preparation of carbon incorporated NbAl alloy and its subsequent conversion to pure niobium by electron beam melting

Abstract Aluminothermic reduction of niobium pentoxide coupled with either direct electron beam melt refining of thermit niobium or a combination of pyrovacuum treatment of thermit niobium followed by electron beam melt refining of niobium sponge are the most widely adopted processes for the production of high purity (greater than 99.9 wt.%) niobium metal. Prior to pyrovacuum treatment, the thermit niobium ingot is crushed to smaller size of the order of 6 to 8 mm. Owing to very high hardness of thermit ingot, the crushing has been found to be quite difficult and energy intensive. The effectiveness of electron beam melt refining of thermit niobium is directly dependent on residual aluminum and oxygen concentration in the feed, which are inversely related. In the present studies, it has been found possible to eliminate problems encountered during crushing of thermit niobium as well as the pyrovacuum step by incorporating a judicious amount of carbon in the thermit niobium. A modified charge composed of niobium pentoxide, aluminium reductant corresponding to 10 wt.% excess over the stoichiometric amount and carbon amounting to 1 wt.% of Nb2O5 when reacted, resulted in a product alloy of NbAlC with higher niobium recovery (equal to or greater than 99 wt.%) and low residual aluminium concentration (less than or equal to 2 wt.%). It has been observed that the presence of carbon has not affected the overall yield. Carbon in the form of NbC preferentially precipitated at the grain boundaries of thermit alloy and rendered it easily crushable. Crushed alloy product could be subjected to direct electron beam melt refining without undergoing a pyrovacuum treatment step. No operational problems have been encountered, and in fact residual carbon assisted deoxidation in the final stage of electron beam melt refining. The electron beam melted button exhibited a hardness value of 65–70 VPN and a total impurity concentration of less than 0.05 wt.%.

Elemental analysis by radioisotope-excited XRF during thermit smelting of ferrovanadium and niobium

The energy-dispersive x-ray fluorescence (EDXRF) technique was applied to the determination of major and trace elements present in aluminothermically smelted ferrovanadium and niobium. Rapid and multielemental analysis of thermit feed and product aided in the appropriate alterations of the charge constituents to obtain the optimum charge composition. The results of analysis by the EDXRF method were verified using wet chemical analysis for a few typical samples of the product and were found to be in close agreement.

Thermal decomposition of ammonium polymolybdate in a fluidized bed reactor

Abstract Studies on thermal decomposition of ammonium polymolybdate were carried out in a fluidized bed reactor using air as a fluidizing medium. The ammonia vapour released during the decomposition process was passed directly into hydrochloric acid solution. The change in pH of the solution was monitored with respect to time and temperature and the degree α of decomposition was calculated at intermediate stages of the process. The kinetics of the overall decomposition process were found to fit well into a ‘power law’ model. The overall activation energy E a and the rate constant k were determined to be 16.3 kJ mol −1 and 1.24 × 10 −3 s −1 respectively. The value of the activation energy suggests the process to be surface controlled. The preparation of MoO 3 from ammonium polymolybdate in a fluidized bed reactor was found to be accomplished at a lower temperature and in a lesser time than that needed in a static bed reactor.