A reactor 20 has a plurality of tubular downcomers 32, 34, 36, 38, 40 and risers 42, 44, 46, 48, 50, joined by sections 86, 88 in a continuous serpentine path the tubes dimensioned to provide substantially plug flow conditions for solid and liquid reagents fed into a first downcomer 24 with the products extracted from final riser 52. The reactor 20 is designed for a desired residence time by the number, height and diameter of the tubes. The downcomers 24, 32, 34, 36, 38, 40 may include a bend to improve residence time and to thereby reduce the number of tubes required for a desired overall residence time. The reactor 20 can be used in a leaching operation for producing synthetic rutile, where a pre-treated feedstock including ilmenite, leueoxene or titania slag is leached with hot HCl.