Reclaim lost disk space after deleting partition
From this article it looks like what you're doing is a 35-pass secure wipe of the unpartitioned space.

You might have better luck resizing the existing Macintosh HD to the full disk capacity. 

If you try to resize Macintosh HD using disk utility and you encounter an error please update your question with a screenshot of the error, and the output of running diskutil list from a terminal window. Terminal can be found in the utilities folder.