Step 4:- Residual fraction that is strongly associated
to the crystalline structures of the minerals
The residue from step 3 was digested in a 3:1
(v/v) mixture of concentrated HNO3 to HF. The metal
content of the residue from Step 4 was determined
using a microwave-assisted acid digestion
procedure.(6) Digests were centrifuged at 3,000 rpm
(20 cm rotar radius) for 10 min to clear the
supernatant which was then analyzed by ICP-OES.