Check out this post about the ckeditor removing div tags. This can also be applied to font awesome icons.
Essentially you need to modify the config.js file for the ckeditor to not remove those. You can find the config.js file in /CMS/CMSAdminControls/CKeditor/config.js
Once you've added the code, make sure to clear your cache in the site and within your browser to see the changes.