(9) Extraction of residue fraction

The cleaned residue obtained from step (8) was air-dried, ground, weighed, and the correction factor was calculated, and then the sample of 0.2000 g was accurately taken out and placed in the PTFE crucible. The sample was wetted in the crucible by water and 5 mL of hydrochloric acid (f), nitric acid, and perchloric acid mixture in the ratio of 1:1:1 was added. The crucible was heated on an electric heating plate until the white smoke of perchloric acid was fully distributed. Then, 3 mL of hydrochloric acid (b) was added to the crucible. After the crucible wall was washed, the crucible continued to heat until the salt was dissolved, and then cooled to room temperature. Finally, the remaining product was transferred to a colorimetric tube, and the volume was kept at 25 mL and shaken well. The contents of lead, zinc, and cadmium were analyzed by the ICP-OES method.
