If a black hole is created then it'll have the mass of whatever produced it. So if we create one from proton collisions then the most mass it's going to have is equal to the energy of the collision - so 7TeV on the current version. Actually we'd expect that only a small fraction of the energy would be caught in a black hole. Now that's the equivalent to the mass of only 7000 protons. When you're relying on gravitation to pull in new matter for it to grow, 7000 protons is really a very small mass.

This is to be compared to the mass of a star which is what astrophysical black holes are created from. Now their gravitational force is truly fierce because they contain the mass of the star that formed them. If one of those was orbiting the earth, then it would tear the planet to shreds.

Now the complication is that Hawking radiation says that black holes emit energy and lose mass. For a big black hole this energy loss is negligible compared to the speed at which it accretes new matter. However for a small black hole the energy loss is greater than the mass it gains by sucking in new matter and the black hole would evaporate. However as we haven't seen Hawking radiation people have argued that black holes might not evaporate and might instead grow larger and gobble up the world.

Even if the black holes don't evaporate through Hawking radiation, they would gain mass extremely slowly, and wouldn't pose any threat to the earth until well after the sun had exploded.

So you could say that the black hole is created and travels to the center of the ring. As long as you say that Hawking radiation doesn't exist, the black hole will be stable. It will slowly eat up all the matter around it, until it's sitting in a hole. The stuff around the edge of the hole will be too far away to be pulled in so the black hole would just sit there, slowly eating up any passing air molecule that gets too close. Obviously if you throw more garbage in there then it will grow. But you need to throw a lot of stuff in there for it to get big. And by a lot we're talking continents and suchlike.

So you need to say there's no Hawking radiation, that it stays in the middle of the ring and that for some reason it's gravitational pull is much larger than we'd expect (perhaps some weird quantum gravity effect, extra dimensions or something like that) so that it sucks in matter from further away than we'd expect. With those three things you'd get yourself your disaster.

On a side note, people are very serious about looking for black holes at the LHC. If we saw one it would be very cool indeed and would revolutionise particle physics. It would not pose any threat to anything.