Carbochlorination of Fly Ash in a Fused Salt Slurry Reactor

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CARBOCHLORINATION OF FLY ASH IN A FUSED SALT SLURRY REACTOR M.S. DOBBINS* and G. BURNET** *Chemical Engineering, Corning Glass Works, Corning, NY **Ames Laboratory, U.S.D.O.E. and Department of Chemical Engineering, Iowa State University, Ames, IA 50011. Received 17 October,

14831.

1986; refereed

ABSTRACT Carbochlorination of the metal oxides in fly ash by suspending the solid reactants in a NaCl-AlCl 3 melt at 530-850 0 C and then sparging chlorine into the melt has been investigated. A mechanically agitated, semi-batch reactor was used to test the effects of temperature, oxide and carbon loading, salt composition and gas flow on the reaction rate. The process was modeled using the carbochlorination of pure alumina, the rate of which was found to be chemical reaction controlled at temperatures below about 650 0 C and gas-liquid mass transfer controlled at higher temperature. The carbochlorination rate of the mixed oxides in coal fly ash was also mass transfer controlled at higher temperatures when aluminum recoveries were less than about 50%. At higher aluminum recoveries, the overall rate was limited by the rate of ash dissolution into the melt. INTRODUCTION Carbochlorination has been demonstrated as a method for recovering metals from coal combustion wastes and other polymetallic materials [1]. In this method, C12 reacts with metal oxides in the presence of C to produce volatile metal chlorides which are recovered and separated. Prior work on the recovery of metals from coal fly ash by carbochlorination has been limited to gas-solid reaction systems [2,3]. Fly ash is a mixture of metal oxides, the composition of which depends on the coal source and method of combustion. The ash is being generated at an annual rate which exceeds 52 MM Mg/yr [4]. Recovery of 80% of the aluminum, titanium and iron present in this amount of fly ash could provide 80% of the United States demand for primary aluminum, and 47% and 5% of the demand for titanium and iron respectively. At the same time, utilization would substantially reduce the problems of disposal. The overall goal of the chlorination research has been a process for recovery of aluminum. Titanium and iron are secondary products that are obtained during the separation and purification of the AlCI 3 . Germanium, gallium, uranium, molybdenum and other trace elements might also be recovered if a particular ash were rich enough in them. Previous work with A12 0 3 indicates that slurrying the ash and the carbon reductant in molten NaCl-AlCI 3 could result in a high level of reactivity and good yields [5]. REACTIONS As normally carried out, carbochlorination is a reaction between two solids and a gas when solid carbon, the preferred reductant, is used. Such reactions tend to be rate limited by interaction between the two solids. The reactions can be changed if the reactants are slurried in a molten salt medium that also acts as a solvent. Dissolved oxide species react with the molten salt or with the carbon and chlorine to produce volatile metal chlorides. When molten NaCl-A