55. Impurities in the form of iron oxides lower the economic value and usefulness of industrial minerals, such as kaolins, to ceramic and paper-processing industries. A 24 experiment was conducted to assess the effects of four factors on the percentage of iron removed from kaolin samples (“Factorial Experiments in the Development of a Kaolin Bleaching Process Using Thiourea in Sulphuric Acid Solutions,” Hydrometallurgy, 1997: 181–197). The factors and their levels are listed in the following table:

Low High Factor Description Units Level Level 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 listed in the next table.

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

a. Calculate estimates of all main effects and two-factor interaction effects for this experiment.

b. Create a probability plot of the effects. Which effects appear to be important?