I have the same question. There are a few possibilities:

1. Teck will not take Shaft creek into production on the short term

2. Teck did a bad proposal and refuses a better bid 

3. CF might be confinced that it's more interesting to sell the company as a total package to a major player

4. It's part of the negotiations to get a better price. I can't believe Teck will appreciate it if Shaft creek comes into the hands of another major without the approval of Teck.