Slag is the waste or undesired leftovers in the process of refining gold or other metals. Yahweh removes the wicked people as if they were trash. (See: [[rc://en/ta/man/translate/figs-simile]])
It is implied that the writer is afraid of God's righteous decrees because the writer knows that God punishes those who disobey his decrees. You can make the full meaning of this statement explicit. (See: [[rc://en/ta/man/translate/figs-explicit]])