Research On Thermodynamics And Kinetics Of Carbon-Containing Chromite Ore Fines Voluminally Reducted By Microwave Heating

Chinese chromite ore resources are very scarce, input dependence degreebeing already up to 95%. As chromite ore resources in the world are excessivelymined, lumpish mine fit for metallurgical need decreases year after year, thuspowdery one increases year after year. It is reported that currently the annualexploitation quantity of chromite ore in the world already reaches 17 milliontons, powdery mine (granularity＜8mm) accounting for three fourths andlumpish one only one fourth. Therefore, it is very significant to study thetechnological flows by using low-cost chromite ore fines to produceferrochrome.At present, there are two kinds of methods to produce the ferrochrome fromthe powdery chromite ore: the direct furnace smelting and the pretreatmentsmelting. The former includes submerged arc furnace smelting and plasmafurnace smelting; the latter includes sintering, pelletizing and briquetting. The merit of plasma technology is to produce high-carbon fenochrome by using thepowdery raw materials and sleazy carbon reducer. The various plasma furnaceshave been developed in USA, South Africa, Britain and so on, some of whichhave been applied to the industrial field. But now, there are also technique faultsin the large-scale application to the industry such as a short life of the plasmagun, high energy consumption and inapplicability to the production ofmedium-carbon and low-carbon ferrochrome. After sintering, pelletizing andbriquetting, chromite ore fines can be used in the traditional submerged arcfurnace or blast furnace to smelt carbon ferrochrome, but the agglomerationscale and the update of equipments are strictly restricted. Sintering chromite orehas the merits of high strength, good permeability, good reducibility and so on,but their applications are also largely restricted because of high capitalinstruction cost, energy consumption and environmental pollution. Althoughdirectly reducing chromite ore to the alloy may largely decrease the cost andspeed up the reduction course, it is unable to use the fine ore in smelting,because the raw materials of chromite ore need a definite granularity. Since the1980's, the substantive researches on the use of chromite ore fines have beenmade in China, but how to find a much better method to produce theferrochrome or molten iron containing chrome from chromite ore ones,especially the low-carbon ferrochrome, is still an important researchmetallurgical staff facing.Microwave is an especial electromagnetic wave and its frequency is 300 MHz～300 GHz (wavelength 100cm～1mm), locating between infraredradiation and radio wave of the electromagnetic spectrum. The elementaryproperties of microwave are similar to that of the sunshine, the velocity of wavebeing the same as that of light (3×10⁸ m/s) Microwave has an obviousheating effect on the fine ore and its heating characteristics are as follows withthe comparison of the Conventional heating:(1)voluminal heating; (2)selectiveheating; (3) non-contact heating; (4) instantaneous heating; (5) non-pollutingheating. Microwave energy is a radiative heating one and its notable merit is thatit can heat up metallurgical powdery materials quickly, selectively andvoluminally. The quantity of heat transfers not by conduction between granulesbut dielectric property of materials converting microwave energy to the quantityof heat, thus eliminating the nonuniform phenomena of heat and mass transfercaused in the conventional heating way. In view of the characteristics ofvoluminal heating of microwave and self-reduction of carbon-containingchromite ore fines, it can be quickly reduced to sponge ferrochrome of certainmetallic ratio by microwave heating in half-closed system.The experimental raw materials used in this article is carbon-containingchromite ore fines made up of Indian chromite ore fines, anthracite and calciclime. Heated in the microwave field of 10kW and 2.45GHz, thetemperature-rising characteristics of 1kg carbon-containing chromite ore fineswith different carbon fitting ratios were analyzed and contrasted in 20kWadjustable microwave metallurgical furnace at the heating temperatures 1000℃, 1100℃, 1200℃and 1300℃(at atmospheric pressure and without protectivegas), and its thermodynamics and kinetics law was also further investigated.The experimental results show that carbon fitting ratios (C: O isapproximately 0.8561, 0.9527 and 1.1213) of carbon-containing chromite orefines were changed. With increasing gradually carbon fitting ratios, thetemperature-rising rates of carbon-containing chromite ore fines by microwaveheating also increase. When materials heated up. to 1200℃, temperature-risingrates of different samples are 66.67℃/min·kg, 80.00℃/min·kg and85.71℃/min·kg, respectively; carbon fitting ratios (C: O is approximitely 0.842,1.1941 and 1.5941 )of carbon-containing chromite ore fines were varied with theaddition of 5% calcic lime. After calcic lime added, the initial temperature-risingrate of materials decreases at low carbon fitting ratio, but it increases as a wholewith carbon fitting ratios gradually increasing. When materials heated up to1100℃, temperature-rising rates of different samples are 73.33℃/min·kg,68.75℃/min·kg and 91.66℃/min·kg, respectively.Carbon-containing chromite ore fines have better temperature-risingcharacteristics in the microwave field of 2.45GHz, and theirtemperature-increasing rate equations can be approximately derived as follows:T=at+b (the first stage) andT=(ct+d)^1/2 (the second stage) ; carbon-containingchromite ore fines were easily voluminally reduced by microwave heating.When the ratio of C: O atom mole is 0.842 and the ratio of CaO: SiO₂ moleculemole is 0.3939, the temperature-rising rates of carbon-containing chromite ore fines reducted by microwave heating are respectively 62.5℃/min, 68.75℃/min,70.59℃/min and 72.22℃/min at the heating temperatures 1000℃, 1100℃, 1200℃and 1300℃. Solid-solid state reaction order is the first order forcarbon-containing chromite ore fines voluminally reduced by microwaveheating and the apparent activation energy of the reaction is 51.48kJ/mol. Thetotal reaction is controlled by the mass transfer of CO gas among productionlayers between ore-coal dielectric particles.