PURPOSE: The titled additive giving a highly conc. slurry excellent in standing stability and shear stability, which contains as an effective component a condensate between a mixt. of naphthalenesulfonic acid with a lignin deriv. and formaldehyde, or its salt.

CONSTITUTION: An additive for a coal/water slurry contg. as an effective component a condensate between a mixt. of naphthalenesulfonic acid with a lignin deriv. such as lignin or ligninsulfonic acid and formaldehyde or its alkalimetal salt, ammonium salt or lower amine salt such as methylamine or triethanolamine. The additive is effective in such an amt. that a slurry contains about 0.01W5wt% effective component. A high-conc. and low-viscosity, homogeneous coal/water slurry with excellent standing stability and shear stability, is obtd.