In recent years, increasing environmental pressure against surface methods of mine waste disposal and concern about safety of mining operations have resulted in an increase in the popularity of filling of underground voids. In different countries, underground filling operations have been carried out using coal combustion by-products (CCBs). The Carbosulcis CCBs disposal plant processes two different kinds of wastes: Pulverized Coal Combustion (PCC) fly ash and Atmospheric Fluidized Bed Combustion (AFBC) fly ash. Our aim in this paper is to assess economic optimization of the Carbosulcis S.p.A. CCBs disposal plant. Actually, the present work is a trial to participate in the efforts to throw more light on feasibility of this kind of plant through an optimization study for the prediction of the minimum CCBs disposal cost as function of the ash feed flow rate for different values of fly ash concentration in the slurry and by evaluating the influence of a rheological modifier.