The Preparation of Chromium Metal by a Sealed, Cold-Hearth, Plasma-Assisted Aluminothermic Method
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The preparation of chromium metal by a sealed, cold-hearth, plasma-assisted aluminothermic method by L.R. Nelson * applications of aluminothermic-reduction technology to the production of metals. Synopsis Future expansion of aluminothermic metal production depends, in part, on the capability The efficiency and cost-effectiveness of the aluminothennic to prove that metal oxides from new sources reduction process in the production of ferro-alloys and 'pure' are amenable to processing. For evaluation of metals depends strongly on appropriate balancing of the process the suitability to aluminothermic reduction of energy required. Insufficient energy results in a poor metal yield, chromic oxide from a unique source, a sealed, while over-supply can result in excessive fuming and possibly even cold-hearth, plasma-assisted test unit was ejection of the reactor contents in a potentially dangerous 'thennite developed. This provided a means of evaluating bomb' reaction. This situation becomes even more critical when the suitability of limited quantities of oxide from new sources must be the aluminothermic preparation of 'pure' evaluated, dictating smaller-scale testwork. The resulting higher chromium metal from a chromic oxide source. relative heat losses from the system complicates control of the This paper gives details of the technique and energy balance, and usually requires the addition of thermal describes the experimental results. boosters or reactant preheating in order to sustain an autothermic process. This often detracts from the quality of the metal product, Process energy required and adds to the hazards of batch aluminothennic-reduction testing. The development of a small-scale method (producing less than General aluminothermic processes 2,5 kg of metal) for the plasma-assisted aluminothennic reduction of chromium oxide that overcomes such difficulties is described. The key factor common to highly exothermic The technique involves the use of a sealed 50 kVA plasma-arc aluminothermic reactions is the need for tight furnace to provide controlled electrical ignition and supplementary control over the energy balance and resultant energy to the reactants contained in a water-cooled copper cmcible. temperatures in the process. This is, in fact, so The application of the method to the evaluation of the critical that it usually determines the following: aluminothennic production of pure chromium metal from a unique . the form of the metal produced, Le. chromic oxide source is reported. 'pure' metal, or alloy (e.g. the 3407°C melting point of pure tungsten can be lowered through alloying with iron to ensure adequate metal-slag separation) . the means of containment of the liquid Introduction products at temperatures in excess of 2000°C The year 1995 is the centenary of . the form of any alloying additions, Le. Goldschmidt's original German patent on the scrap iron if endothermic cooling is aluminothermic 'thermite' reduction processl. required, or mill scale (Fe304), and even At that time, the process and its variations hematite (FezO3), if the additional input were applied to the preparation and commercial of exothermic energy is requiredS production of numerous 'pure' metals and . the nature and sizing of the aluminium ferro-alloys, including chromiumz-7, reactant, Le. fine powder to ensure rapid manganeseZ-4, molybdenumz.3.s.8.9, reaction kinetics and an associated tungstenZ.S.8,vanadiumz.3.s.lo-l6, temperature rise in the autothermic niobiumz-s.8.l7, tantalumZ.3.S.l7-l9, thermite reactions (as planned by titaniumz.3.s.l6.l7.zo.zl, zirconium3.l7.zl.zz, and VantechZ3), or larger scrap in boronz.3.s. The process has found frequent application in South Africa. Recent reports that Vantra, GENCOR Process Research, P.O. Box 658, * Vantech, and Vametco are to increase the local Krugersdorp 1740. aluminothermic production of ferrovanadium @ The South 4frican Institute if Mining and attest to thisz3. Especially in view of the recent Mw~~1~~~~NOO3~R3Y~OO+ expansion of local aluminium production at 0.00. Paper received Oct. 1995; revised paper the Hillside Smelter, potential exists for future received Mar. 1996. The Journal of The South African Institute of Mining and Metallurgy JULY/AUGUST 1996 135 .... The preparation of chromium metal electrically-assisted furnace reactions (as planned by quantity of product available, produced at great expense in a Vantra and Vametc023) novel pilot-plant process. Aim (2) may arise from a need to ~ the need for, and form of, any supplementary energy reduce the costs of the raw materials. Prefect6 even suggests additions through charge preheating, and the use of that 'using conventional processing procedures, frequently as higher (more exothermic) oxides of the required metal much as a tenth of an entire production run might be or of chemical boosters consumed in experimenting to determine the characteristic of ~ the need for slag fluxing agents such as lime or a particular lot of ore'. fluorspar (and even soda ashl0) to modify the physico- Small-scale aluminothermic testwork (with less than chemical properties of the slag and so promote 2,5 kg of product metal) is therefore frequently desirable, metal-slag disengagement. especially in the evaluation of metal grade and new oxide sources. However, by virtue of the higher ratio of surface area It should be appreciated that a specific metal's affinity for to volume in smaller reactors, the heat losses are propor- oxygen and aluminium (including any tendency to form tionately higher than in larger vessels, requiring the addition aluminides) will frequently determine the need for, and of chemical boosters to provide supplementary energy. This extent of, secondary processing to yield a 'pure' metal introduces a safety risk since the system becomes increasingly product. In such cases, arc-5.18.19,vacuum-5.12.13.18.19, unstable as proportionately more chemical boosters are added electron-beam5.12.13.14.18.19.21.22,or even molten-salt electro- to smaller batches. This effectively limits5 thermite batch sizes refiningl0 may be necessary to effect simultaneous refining to between25 and 40 000 kg. of these (and other) impurities by vaporization (especially as AbO and Ala sub-oxides14.21.22). Aluminothermic reduction of chromic oxide Special requirements for small-scale aluminothermic Source tests The primary objective of the work described here was to Small-scale aluminothermic testwork normally has two aims: evaluate the potential of a novel source of chromic oxide to yield commercial on-grade chromium metal. Two grades of (1) to prove the purity and amenability of a metal oxide chromic oxide (about 100 kg of each in total) were available from a new source for aluminothermic production for laboratory-scale investigation: chromic oxide 10 and (2) to establish suitable charge recipes and production chromic oxide 6000, having chromium-to-iron ratios of guidelines for a new commercial production lot. approximately 10:1 and 6000: 1 respectively (Table I). The In both cases, the rationale is to use the minimum source aim was to test the raw material in individual lots of under of metal oxide. In aim (1), there may be only a limited 5 kg each (Table11). Table I Raw materials analyses in mass percentages Type of feed Chromic oxide 10 Chromic oxide 6000 Alumina Type of feed AI powder Table 11 Charge recipes and key experimental conditions Design recipe Mass of Cr metal, kg %'Cr2<>3' in heel Comments 0.5 20 No evacuation and Ar backfill 1.0 10 No evacuation and Ar backfill 1.5 20 1.5 30 AI layer at base of heel 2.5 20 Large batch-Q.070 kg of un reacted feed 1.5 30 Feed pipe melt-Q.143 kg of unreacted feed 1.5 30 0.538 kg of unreacted feed 1.0 100 'Bomb'-Q.322 kg of un reacted feed ~ 136 JULY/AUGUST 1996 The Journal of The South African Institute of Mining and Metallurgy The preparation of chromium metal Fundamentals Basic thermodynamics The reduction of chromic oxide displays all the key character- The reaction is highlyexothermic (Mf'25°C= -272,80 k}/(molAI), istics of a typical aluminothermic process, including those Table IV), but insufficiently exothermic to be autothermic, related to the energy requirements, and to the fact that the requiring the input of additional energy. As a general rule, purity of the final product is strongly determined by the VolkertZ suggests that, for a self-sustaining reaction and purity of the reactants. The key reaction is as follows: adequate product metal-slag separation, Mfz5°C should be less than -302 k}/(mol AI). CrZO3 (s) + 2 Al (s) =2 Cr (1)+ AIZO3(I), The standard change in the free energy of the reaction is where, the equilibrium constant, strongly negative (t1G'Z5'C= -266,89 kJl(mol AI), Table IV), K, = (aCrZ' aAlz~)/(acrzO3' aAlZ), which yields a high value of K. An analysis of the type The complete reaction in production batches of up to presented by AlcockZ4,involving the application of Raoult's 40 000 kg is exceptionally rapid, typically occurring within law to the alloy phase and Temkin's law to the slag phase, 1 to 10 minutes3.4. During this time, the process temperatures suggests that the extent of the aluminothermic reaction is must exceed 21 OO°Cto permit adequate separation of the sufficient to permit 'pure' chromium metal to be produced product metal and slag (pure chromium and alumina melt at concurrently with an alumina-rich slag (Figure 1). The 1857°C and 2072°C respectively, Table Ill). analysis also shows that, for the ASTM aluminium specifi- cation in chromium (less than 0,3 per cent AI) to be metZ5, some chromium loss to the slag is inevitable. As would be Table 11/ expected from the application of Le Chatelier's principle to an exothermicreaction,K decreaseswith increasing Basic physical data pertinent to the aluminothermic temperature, so that the equilibrium extent of the reaction is production of chromium metal most favourableat lowertemperatures (Figure2).