business vs salt mine

business and salt mine both are nouns.

Word	Noun	Adjective	Verb	Adverb
business	Yes	No	No	No
salt mine	Yes	No	No	No

As nouns, salt mine is a hyponym of business; that is, salt mine is a word with a more specific, narrower meaning than business:

business (noun)	salt mine (noun)
the principal activity in your life that you do to earn money	a job involving drudgery and confinement
the activity of providing goods and services involving financial and commercial and industrial aspects	a mine where salt is dug
the volume of commercial activity
incidental activity performed by an actor for dramatic effect
an immediate objective
a rightful concern or responsibility
a commercial or industrial enterprise and the people who constitute it
customers collectively
business concerns collectively