Table 3 Factors and levels that were used for first order 23 full factorial design.

From: An improved microwave assisted sequential extraction method followed by spectrometric analysis for metal distribution determination in South African coal samples

Variable

Minimum (−)

Central (0)

Maximum ( +)

CA (g) A

0.05

0.075

0.1

T (°C) B

100

150

200

Time (min) C

5

17.5

30

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