I do think its possible, it might just take abit of time.
Im very close to my ex even though ive been in a relationship with another man for over 2 years now. We are extremely close. We still talk about our sex lives and everything and he always looks out for me on a night out if my OH isn't joining us (it's very rare that he does to be honest)
It took us about 4 months to get back to being as close as we were before the split which is quite quickly I think.
I do also think that it depends on the kind of person you are. Some people just don't want to be friends with an ex or if they do find it awkward to rebuild the friendship.
If friends is what you are ment to be, you will be in good time!
I'm friends with loads of my Ex's the only one's that I'm not are 90% because they dont want to be not me.. My Ex from 15 years ago visits us most weekends for coffee and has helped us out with our house etc and we are just friends he spends alot of the time chatting with my hubby who I've been with 10 years and theres no issues.. I think if you can get over the partner feelings and onto feelings of friendship they can become one of your best friends.. good luck