Table 5.

Four levels of five factors in orthogonal experiments

Level	Factor
	A: initial Fe concentration (mM)	B: alkalization pH	C: alkalization temperature (°C)	D: alkalization duration (minutes)	E: random*
1	600	13	30	15	1
2	300	12	40	30	2
3	150	11	50	60	3
4	75	10	60	120	4

Note:

^*Random is the control factor of orthogonal experiment.