Sentence:  One of the main problems with recycling and melting magnesium is the presence of particles in the melt which lead to the formation of <WSD>dross</WSD> and sludge. To be more precise, the necessary removal of <WSD>dross</WSD> and sludge greatly increase the costs of the facility. And, material is lost. 
Meaning of the word marked with <WSD> tags:  A waste product from working with metal.