Adam Luckos

Mintek, Pyrometallurgy Division, Private Bag X3015, Randburg 2125, South Africa

Abstract – A 0.15 m ID CFB reactor has been constructed at Mintek for high-temperature pre-treatment of a wide range of particulate raw materials used in the metallurgical industry. Initial roasting tests were conducted with typical East Coast ilmenite concentrate at temperatures from 740° C to 800° C. Roasting under oxidising conditions resulted in a sixteen-fold increase in the bulk magnetic susceptibility of the concentrate.


An increasing trend towards processing fine particulate raw materials, such as flotation ore concentrates and electric-arc furnace dusts, has prompted development of high-temperature circulating fluidized-bed (CFB) reactors for numerous large-scale operations in the metallurgical industry (e.g. Peinemann (1991), Peinemann et al. (1992), Reh (1995), Dry and Beeby (1997)). A 0.15 m × 6.0 m tall CFB reactor with solids returned via an L-valve has been designed and constructed in Mintek’s Pyrometallurgy Division for pre-heating and pre-treatment of a wide range of fine raw materials in reducing/oxidizing atmospheres at temperatures up to 1000° C.

Ilmenite concentrates from heavy mineral deposits in KwaZulu-Natal contain large amounts of chromite, which is a contaminant in the production of high-quality ilmenite. Such concentrates typically contain about 90% ilmenite, 5% Ti-hematite, 3% spinel and 2% silicates by weight. The magnetic susceptibility of iron-poor, chrome-rich spinel is not significantly affected by roasting under relatively mild oxidizing conditions. On the other hand, roasting under suitable conditions can increase dramatically the magnetic susceptibility of ilmenite. It is therefore possible to roast a chromite-bearing concentrate to increase the magnetic susceptibility of the ilmenite, leaving the magnetic susceptibility of chrome-bearing spinel unchanged. Magnetic separation of titanium-rich fraction from chromite then becomes possible.

Nell and Den Hoed (1997) roasted 15 g samples of an ilmenite concentrate in a 17 mm ID externally heated bubbling fluidized-bed reactor. Isothermal runs were carried out at temperatures between 700° C and 800° C with air, CO2 and mixtures of air and CO2. They found that roasting in air and air/CO2 mixtures causes a five- to six-fold increase in the bulk magnetic susceptibility of the concentrate in less than two hours, even at a relatively low temperature of 700° C. At 750° C, good magnetic susceptibilities were attained in less than 30 minutes. This time was recommended as a practical residence time for industrial process control.

The small-scale experiments can be used to define approximate operational limits for the oxidizing roasting of ilmenite concentrates. However, industrial roasting conditions may differ, sometimes significantly, from those occurring in laboratory units. In this paper the initial results obtained in the 150 mm ID CFB pilot plant are presented and discussed.


A simplified diagram of the Mintek pilot plant is presented in Figure 1. The principal reactor shaft or riser is composed of six refractory-lined flanged sections, providing a chamber of 150 mm diameter with an overall height 6.0 m. The refractory is erosion resistant and can withstand temperatures up to 1100° C. The bottom section is tapered gradually on the inside from a 90 mm diameter at the bottom to 150 mm over its 1.1 m height. This feature provides a high-acceleration zone which helps to prevent agglomeration and sintering. Pressure taps and thermocouples are located at 545 mm intervals along opposite faces of the column. An external propane burner is used for start-up of the unit and to maintain the reactor at required temperature. Air and propane flowrates are measured by means of ½-inch and 1½-inch orifice plates with DP cells. Off-gas from the burner is introduced to the bottom conical section of the reactor through a stainless steel bubble cap with 12 holes of 10 mm diameter.

Solid particulate materials are contained in separate sealed hoppers mounted on load cells. The capacity of each weigh hopper is approximately 200 kg of ilmenite concentrate. Particles are fed to the unit by means of two 50 mm diameter screw feeders. Solids feed rate is controlled in the range of 0 to 150 kg/h by means of variable speed AC drives. Positive pressure is maintained in both hoppers to avoid a back flow of hot gases from the riser. Solid products are removed from the main reactor chamber continuously through a 80 mm ID 2.5 m long stainless steel air-cooled discharge pipe and cast-iron plug valve. Gas and entrained solids leaving the top of the riser enter a refractory-lined high-efficiency cyclone. Solids captured in the cyclone drop into a 100 mm ID externally insulated stainless steel standpipe. The bottom of the cyclone and the top of the standpipe are connected through a bellows-type expansion joint to allow thermal expansion. The solids are returned to the riser through the horizontal section of the L-valve, which is 520 mm long. The circulation rate is controlled by the amount of aeration gas (nitrogen or air) fed to the L-valve. Circulation of solids can be determined by visual observation through a quartz window.

Gas leaving the cyclone is sampled for gas analysis, cooled and directed to the baghouse through a water-cooled duct. Gas samples are analysed for oxygen, CO and CO2 using continuous gas analysers. Other gaseous species such as hydrocarbons, H, NO and SO can be analysed off-line by means of a gas chromatograph.

Data acquisition and operation of the unit are controlled by a PLC, which processes and stores electrical outputs from thermocouples, pressure transducers flow meters gas analysers and load cells. It also provides information on the current status of the start-up burner and screw feeders.

Figure 1. Simplified schematic diagram of circulating fluidized-bed reactor at Mintek.


Commissioning and initial roasting tests were carried out with typical East Coast ilmenite concentrate supplied by Iscor. Beach sand ilmenites have relatively narrow particle size distribution and belong to group "B" solids (sand-like particles) according to Geldart’s classification (Geldart and Abrahamsen (1978)). Physical properties of ilmenite particles determining their behaviour in fluidized-bed systems are presented in Table 1.

Table 1. Physical properties of East Coast ilmenite concentrate.

Average particle size

169 μm


4648 kg/m3

Sphericity (estimated)


Void fraction at minimum fluidization velocity (estimated)


Minimum fluidization velocity (in air at 775 ºC)

0.02 m/s

Terminal fluidization velocity (in air at 775 ºC)

1.19 m/s

Transport velocity (in air at 775 ºC)

6.61 m/s

Magnetic susceptibility

177´ 10-6 cm3/g

Approximately 2000 kg of ilmenite concentrate was roasted during a two-day campaign. In general, the roasting tests with East Coast ilmenite concentrate showed quite stable operation. No problems were experienced with agglomeration or sintering of ilmenite particles.

The residence time in the reactor was controlled in the range of 10 to 20 minutes by adjusting the solids feed rate. The solids inventory was estimated on the basis of pressure drop measurements across the riser. Other parameters were kept at the same constant level. The average operating conditions are presented in Table 2.

Table 2. Average operating conditions for roasting ilmenite concentrate in the CFB reactor.

Ilmenite feed rate, kg/h


Residence time, minutes


Temperature at bottom of reactor, ° C


Temperature at top of reactor, ° C


Temperature in cyclone, ° C


Average temperature in reactor, ° C


Oxygen in off gas, % vol.


CO2 in off gas, % vol.


Suspension density, kg/m3


Fluidization velocity, m/s


Energy consumption, kW


It should be noted that throughout the campaign the reactor was run at a relatively low solids circulation rate and, therefore a low solid suspension density. This resulted in non-uniform temperature distribution in the riser, with maximum difference between temperatures at the bottom and the top of reactor as high as 40° C.

Nell and Den Hoed (1997) showed that, for a given bed temperature, the particle residence time plays a critical role in controlling the magnetic susceptibility of the ilmenite. They found that at 800° C the optimal roasting time is about 15 minutes, increasing to about 75 minutes at 700° C. At 750° C good magnetic susceptibilities were still attained in less than 30 minutes. In Figure 2 the influence of residence time on magnetic susceptibility of ilmenite roasted in the CFB reactor is shown. Even at a relatively low residence time of 10 minutes, a thirteen-fold increase in magnetic susceptibility was achieved. The magnetic susceptibility of roasted ilmenite increases further with residence time, and at 20 minutes reaches a value approximately 16.5 higher than that for unroasted concentrate. Such an unexpectedly large increase in magnetic susceptibility, compared to results obtained in laboratory-scale bubbling-bed reactor, may be attributed to better gas-solid contact and, therefore, higher oxidation rates occurring in CFB roasters.

In Figure 3 the results of magnetic separation in the barrier Franz separator are shown. As expected, the best results were achieved with ilmenite roasted at 20 minutes’ residence time with approximately 90% of the ilmenite reporting to the magnetic stream at 100 mA.

In Figure 4 particle size distributions for ilmenite concentrate and roasted material determined through the screen analysis are shown. The average particle size of the roasted ilmenite was slightly smaller (144 μm) than unroasted ilmenite (169 μm). This may be attributed to elutriation of larger and lighter silica particles, as well as the break-up and/or abrasion of ilmenite particles due to thermal shock and high superficial fluidization velocity. A decrease in particle size increases the surface area of roasted ilmenite and can enhance the oxidation rate.

Figure 2. Magnetic susceptibility of roasted ilmenite versus residence time.

Figure 3. Cumulative mass of magnetic fraction versus separator current.

The increase in magnetic susceptibility is due to the oxidation of Fe2+ to Fe3+ in the ilmenite structure. Representative samples of the ilmenite concentrate and roasted material were collected through the campaign and analysed for major elements. The results of chemical analyses are shown in Table 3. Formation of Fe2O3 (hematite) resulted in approximately 50% drop in Fe2+ concentration in roasted material. Except for SiO2 the concentrations of other components did not change significantly. A 50% drop in SiO2 content may, however, be associated with the segregation and elutriation of silica particles originally contained in the concentrate.

Figure 4. Particle size distributions for concentrate and roasted ilmenite.

Table 3. Chemical analyses of concentrate and roasted ilmenite.



Roasted ilmenite











Fe (total)













<100 ppm

<100 ppm











550 ppm

630 ppm


It has been demonstrated that ilmenite concentrate can be successfully roasted in a circulating fluidized-bed roaster under oxidizing conditions. A sixteen-fold increase in the bulk magnetic susceptibility of the ilmenite was achieved in 20 minutes at average reactor temperature of 766° C. No problems with agglomeration or sintering of ilmenite particles were experienced. Initial tests show the superiority of CFB roasters compared to bubbling-bed systems. Further tests are, however, required to confirm these results and optimize operating conditions.


This paper is published by permission of Mintek and Iscor Heavy Minerals Division.


Dry, R.J., Beeby, C.J.: Applications of CFB technology to gas-solid reactions. In: Circulating Fluidized Beds (Grace, J.R., Avidan, A.A., Knowlton, T.M. (Eds.)), Blackie Academic & Professional, London (1997), pp 441-465.

Geldart, D., Abrahamsen, A.R.: Powder Technol. 19 (1978), pp. 133-136.

Nell, J., Den Hoed, P.: Heavy Minerals 1997 (Robinson, R.E. (Ed.)),The South African Institute of Mining and Metallurgy Symp. Ser. S17, Johannesburg, (1997), pp. 75-78.

Peinemann, B.: World Gold’91, AusIMM-SME Joint Conf., Cairns, Australia, (1991), pp. 3-9.

Peinemann, B., Stockhausen, W., McKenzie, L.: Fluidization VII (Potter, O.E., Nicklin, D.J. (Eds.)) Engineering Foundation, New York, (1992), pp. 921-928.

Reh, L.: Chem. Eng. Technol. 18 (1995), pp. 75-79.

Copyright © 2001 Adam Luckos,
19 June 2001

Pyrometallurgy Division, Mintek,
200 Hans Strijdom Drive, Randburg, 2125, South Africa
Private Bag X3015, Randburg, 2125, South Africa.

Phone: +27 (11) 709-4650
Fax: +27 (11) 793-6241