Reduction of phosphate ore by carbon: Part II. Rate limiting steps
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INTRODUCTION
IN earlier work (Mu et al. 1) we showed that the kiln reduction of phosphate ore (KPA process) developed by Occidental Research could proceed at high temperatures. In the range of 1200 to 1400 ~ where the partial pressure of the products exceeds 1 atmosphere, products exit the reactive pellets by convection, the reaction being essentially at equilibrium at the reaction site. Under these conditions, the overall conversion is limited by the resistance to gas flow within the agglomerate. Earlier work L2'3indicates no conversion dependence on overall ball size, but significant dependence on the particle size of the ore used to make the agglomerates. It is, therefore, postulated that any resistance to flow resides at the reaction sites, not in the main pores of the balls. In this pressure regime, when excess silica is present, the conversion data indicate an exponential dependence on reaction time, i.e., pseudo-first order kinetics. This can be explained as a direct consequence of a calcium silicate by-product building up around the reaction site, causing an increase in the resistance to product transport. The hypothesis is tested by comparing the temperature dependence of reaction rates to published thermochemical data. At temperatures below those of commercial interests, the conversion will cease to be limited by transport of product species away from the reaction sites. It is proposed in this work that the rate limiting step at temperatures below 1250 ~ is the diffusion of phosphatic species to a source of carbon for reduction. This reaction appears to take place in a silica induced melt phase ~ where only a fluorapatite derived species is soluble in the melt. II.
of interest in the process (1250 ~ to 1400 ~ in a graphite crucible. Apatite-silica mixtures were placed in the graphite crucible so that walls of the vessel served as the reductant. In this configuration the bulk of the apatite was separated spatially from the reductant, so that transport of a phosphorus-bearing species would have to occur if the reaction was to take place. We assume that such transport occurs by diffusion, and the reaction at the graphite-apatite interface is fast enough so that the diffusion is rate limiting. Note that this needs not be the case when we consider reaction between graphite and apatite in agglomerate balls where, as we have already discussed, 2 situations exist in which transport of the product gases out of the ball can be rate limiting. However, in the present instance, we restrict attention to the interracial reaction, and within that step we assume a mechanism in which condensed state diffusion is rate limiting. The rate at which the interfacial reaction proceeds is available from the gravimetric data accumulated in the TGA measurements, and the postulated mechanism places considerable restriction on the functional form of the kinetic relation. In addition to the rate of weight loss monitored by the TGA method, it is possible to use optical microscopy to observe directly the diffusion zone (within which th
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