How to merge back the encrypted folder back to main drive Have one encrypted folder and have erased the content of it. But not getting any option to merge the space back to the main partition.

 This comes after we enter the password or rather mount the encrypted drive.

As we can see that the Disk Image is of 10GB. I want this space to merge with other drive: Macintosh.
You've created a disk image, which is basically just a file. You haven't partitioned your Macintosh disk, as can be seen if you select Macintosh in Disk Utility.

To remove the disk image, unmount the disk image using the Eject button next to Disk Image, then delete the file using Finder. This will reclaim the space on your Macintosh drive in the same way deleting any other file will.