Table 2.

Single factor assays and determination index (extraction rate, y) with different combination of the range of pH value (X₁), content of potassium nitrate (X₂) and the inoculum volume (X₃).

Order	Factor value			Extraction rate (y, %)
	X1	X2 (%)	X3 (mL)
1	6.0(−2)	0.6(−1)	1.0(−1)	0.1672
2	6.5(−1)	0.6(−1)	1.0(−1)	0.1653
3	7.0(0)	0.6(−1)	1.0(−1)	0.1916
4	7.5(1)	0.6(−1)	1.0(−1)	0.0949
5	8.0(2)	0.6(−1)	1.0(−1)	0.0988
6	6.5(−1)	0.4(−2)	1.0(−1)	0.0724
7	6.5(−1)	0.6(−1)	1.0(−1)	0.2579
8	6.5(−1)	0.8(0)	1.0(−1)	0.3157
9	6.5(−1)	1.0(1)	1.0(−1)	0.2016
10	6.5(−1)	1.5(2)	1.0(−1)	0.1067
11	6.5(−1)	0.6(−1)	0.5(−2)	0.1178
12	6.5(−1)	0.6(−1)	1.0(−1)	0.1249
13	6.5(−1)	0.6(−1)	1.5(0)	0.1334
14	6.5(−1)	0.6(−1)	2.0(1)	0.2088
15	6.5(−1)	0.6(−1)	2.5(2)	0.1134