Sentence:  The bitumen floats to the top of the separation cell and is largely recovered in this top-layer frothnb..., while sand is discarded at the bottom and the <WSD>middlings</WSD> of intermediate density are partly returned to the separator for recycling and partly forwarded to a scavenger separation cell ... for separate treatment in a froth settlernb.... 
Meaning of the word marked with <WSD> tags:  Partially refined ore or petroleum.