Kinetics of roasting of chromium oxide with sodium nitrate flux
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I.
INTRODUCTION
ALTHOUGH alkali roasting of chromite at high temperatures (~1300 K) is a long-established process, not much is known about its low-temperature reaction kinetics; it has been generally assumed that the reaction does not occur at low temperatures say, in the region of 900 K. 11-4] The alkali roasting reaction can be written as 3
Cr203 (s) + Na2CO3 (s) + 2 02 (g) Z = 2Na2CrO 4 (s) + 2CO2 (g)
[1]
When carried out at high temperatures the flux, Na2CO3, reacts with, in addition to Cr203, the other acidic and amphoteric oxides present in the ore/concentrate. As a result, the impurity content of the product, Na2CrO4, is higher and alkali consumption is larger. In an attempt to circumvent these shortcomings and in view of the paucity of information on these lowtemperature reactions, the feasibility of performing the chromate conversion at relatively low temperatures, 673 to 873 K, was investigated, ts] Besides, low temperature operation will be commercially more attractive. In contrast to the general belief, the yield and the rates of reaction were observed to be reasonably high, as can be seen in Figure 1.[5] Reaction [ 1] involves a gaseous phase (oxygen) and therefore is strictly not a solid-state reaction. Partial pressure of oxygen in the reaction atmosphere has a dominant effect on both the rate and extent of reaction. Alkali roasting conducted in vacuum showed that there was hardly any reaction between Cr203 and Na2CO3 in the absence of oxygen. On the other hand, increasing Po2 from 10 to 101 kPa raised the yield from about 25 pct to over 65 pct in two hours of reaction time at 873 K (Figure 2). ]6] In 4 hours, the yield improved from about 30 pct at a Po2 of 10 kPa to over 75 pct at 101 kPa. It was postulated that an oxidizing fluxing reagent, generating nascent oxygen, would accelerate the A.K. TRIPATHY, Scientist, Pyrometallurgy Group, and H.S. RAY, Director, are with the Regional Research Laboratory, Bhubaneswar 751013, Orissa, India. P.K. PATTNAYAK, Professor, is with the Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay, Powai, Bombay 400 076, India. Manuscript submitted May 17, 1993. METALLURGICALAND MATERIALSTRANSACTIONSB
reaction rate and improve the yield. In the alkali roasting of Cr203, therefore, Na2CO3 flux was progressively replace by NaNO3. Wadier reacted Cr203 with an excess (1:4) of NaNO3 and obtained Na2Cr2OT.tT] Udupa conducted differential thermal analysis/thermogravimetric analysis (DTA/TGA) studies on KNO3-Cr203 mixture in 2:1 ratio, where K2Cr207 was the predominant reaction product. ]81 The function of molten solvent, NaNO 3 or KNO3, in the reactions leading to dichromate formation has not been ascertained. However, Duke and Iversion found that dichromate ion reacts with nitrate ion in molten solvent to produce CrO]--ion along with NO2 and 02 gases, tgl In the light of this reaction scheme, the reaction between Cr203 and NaNO3 can be written as 4NO3 = 4NO2 + 2[0] + 2021
Cr203 (s) + 2[0] + 2 0 2 - + - 02 (g) = 2CrO 2-
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