Silicon-containing residues, as they accumulate especially during the synthesis of organosilanes out of metallic silicon, and especially methyl chloride, are processed according to the invention to briquettes, being additives for the quality adjustment of cast iron being produced in a cupola furnace in order to avoid either costly regeneration for the recovering of the valuable components or the total dumping of said silicon-containing residues. Besides the silicon-containing residue the briquettes contain 1-10 percent by weight of cardboard fiber, 5-40 percent by weight of hydraulic cement and 0-20 percent by weight of additional additives having influence on the quality of the cast iron. The cement- and cardboard fiber-containing briquettes are mechanically and thermally sufficiently stable with respect to the operation of the melting furnace. The yielding rate of the inserted small-sized silicon in the cast iron is about 85% and reaches thereby the same range of lumpy ferrosilicon.