"For every one shall be salted with fire, and every sacrifice shall be salted with salt"-Mark 9:49 KJV
Salt is good and symbolized as sanctification,
"Ye are the salt of the earth: but if the salt have lost his savour, wherewith shall it be salted? it is thenceforth good for nothing, but to be cast out, and to be trodden under foot of men."-Matthew 5:13 KJV
and the fire is tribulations.
"These things I have spoken unto you, that in me ye might have peace. In the world ye shall have tribulation: but be of good cheer; I have overcome the world."-John 16:33 KJV