CKEditor and escaping elements

It is removed because it is empty. Put some non-breaking space   or zero-width space ​ within it to preserve your tag.

You can also remove i from CKEDITOR.dtd.$removeEmpty object. This may, however, break other empty <i> tags without class="icon-envelope". To fix this you would need to play with a data processor to filter empty <i>'s without class="icon-envelope". Pretty easy I guess.