How to get rid of ants without harming my tree?

Apply food grade diatomaceous earth and spray with a hort

You need to repot the tree.

Sprinkle dry cornmeal or grits around the base of the tree. Ants will think you've left them a treat and take this home to their queen who will eat it and die. When she's gone, the ants will move on to find a new home.

Also so you can place sliced cucumbers around the base of the tree for ants cannot stand the smell and will move on.

Both of the above. Repot the tree and knock of all the dirt and ants you can, add diatomaceous earth to the bottom and top of the pot. Also as I noted before, set a jar of 4 bean salad from Costco (just the syrup) next to the pot. It will attract the ants and kill them. Weird but true.

Thanks to all of you for your replies.

OR IF YOU JUST WANT THEM AWAY FROM THE TREE WITHOUT HARMING THEM, I READ THAT GROUND CHALK, PLASTER OR BAKING SODA IN A 1/2 INCH THICK LINE AROUND THE TREE TRUNK AND THEY WONT CROSS IT FOR SOME UNKNOWN REASON. THEN MOVE IT FARTHER AND FARTHER OUT EACH TIME. HOPE SOME OF THESE COMMENTS WILL HELP YOU MAKE A DECISION THAT'LL WORK FOR YOU.

Got the perfect thing that includes sugar and boric acid and won't harm the orange tree.

http://homeguides.sfgate.com/kill-ants-live-orange-trees-56142.html

Thanks.

Repot the tree and when you take it out get rid of all the dirt and ants you can, far away from the tree and pot. In the pot, put a layer of food grade diatomaceous earth, they don't like that. Fresh soil in the pot and then top with more of the DE.