=+Hydrometallurgy, 1997: 181–197). The factors and their levels are displayed in the following table:

Low High level level Factor Description Units (21) (11)

A H2SO4 M .10 .25 B Thiourea g/l 0.0 5.0 C Temperature °C 70 90 D Time min 30 150 The data from an unreplicated 24 experiment is given in the table below:

Iron Iron Test extraction Test extraction run (%) run (%)

(1) 7 d 28 a 11 ad 51 b 7 bd 33 ab 12 abd 57 c 21 cd 70 ac 41 acd 95 bc 27 bcd 77 abc 48 abcd 99