When a peeress (ie a duchess, marchioness, countess, viscountess or baroness) obtains a divorce, the general rule is that she places her forename before her title, for example, Mary, Duchess of Hampshire. This is purely a practical measure to avoid confusion should the peer in question marry again.
In the event of a divorced peer remaining unmarried there is no reason why his former wife should not continue to use her title without the qualification of her forename. A case in point is the Countess of Lichfield.
For further details see Forms of Address/Titles or click on the appropriate link below: