The invention relates to a fluidised-bed reactor (10, 20, 30, 40) comprising two zones (11,12, 21, 22; 31, 32; 41, 42), in which the upper zone (11, 21, 31, 41) has a different cross-section to the lower zone (12, 22, 32, 42). A zone with a reducing atmosphere is obtained in one of the two zones (11
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