A study into the hydrodynamic behaviour of heavy minerals in a circulating fluidized bed

To understand the hydrodynamic behaviour of titania-bearing solids undergoing chlorination in CFB reactor at 1000°C, tests were conducted in a Perspex replica of the CFB. The cold model stands 5-m tall; the internal diameter of the riser is 80 mm. Circulating solids in the CFB return to the riser through an L-valve. Four materials were tested, three of them heavy minerals (ilmenite, rutile and titania slag); the fourth, silica sand. Superficial gas velocities were run at values of 2.8–7.4 m/s; solids-circulation fluxes, at values of 5.3–75 kg/m2·s. At fixed gas velocities, increasing rates of solids circulation increased both the average pressure at each point in the CFB and the pressure gradients at the top and bottom of the riser. At constant rates of solids circulation, increasing gas velocities raised the average pressure at each point in the CFB, but decreased the pressure gradient in the lower sections of the riser. Solids concentrations were higher at the top and bottom of the riser than they are half way up. Slip velocities between the gas and particles in the riser ranged from 4 to 11 times the terminal velocities of single particles. The dimensionless slip velocity, Usl/Ut, was independent of the solids-circulation flux. Usl/Ut, correlates linearly with the dimensionless velocity, U/Ut. The solids-circulation flux through the L-valve correlated independently with pressure drop across the valve and with superficial aeration velocity. The study answered puzzling questions about the operation of the CFB chlorinator. INTRODUCTION Titanium tetrachloride is a valuable precursor to titania (TiO2) pigments and titanium metal. It is produced commercially by chlorinating titaniferous ores and slag in bubbling fluidized beds (BFB) at high temperature. The process is established and profitable in developed countries. South Africa does not run a chloride process, yet it produces a quarter of the world’s titaniferous raw materials. As a hopeful step in redressing the balance MINTEK recently designed, built and commissioned a pilot facility for producing TiCl4.1 The design was that of a circulating fluidized bed, a choice made to take advantage of certain benefits: a CFB chlorinator—so it was argued—would run at higher levels of productivity, could treat finer (sulfate-grade) slag, and could tolerate more alkaline-earth elements without defluidizing.2 Estimates showed the concept to be as profitable as the conventional BFB process. To contain a corrosive chlorine at 1000°C the design included an L-valve and called for graphite as the material of construction, but it lacked instruments such as pressure sensors, which can be used to monitor the flow of solids in a CFB. Unable to observe the internal dynamics of our reactor, we built a replica, a cold model, of the CFB. Understanding the hydrodynamics of solids in the cold model, we reasoned, would help us to run the chlorinator effectively. Our study served two purposes. First, it showed up conditions that could affect the kinetics of chlorination. In fluidized systems gas-solid interactions influence rates of heat and mass transfer, which in turn may speed up or slow down rates of reaction, and raise or lower the conversion of reactants. Three conditions bear particular consideration. The fraction of solids in