A process for agglomerating by extrusion, metallurgical by-products and waste products having small particle size, is described. A blend of metallurgical by-products, some of which contain silicates previously treated at high temperatures and thereby being capable of chemically and structurally binding water, optionally mixed with portland cement and/or slag cement, and containing water which has been added in amounts that the silicates and cement present can completely absorb and react with, is further mixed with a hydrocarbonaceous substance. The resulting mixture is subsequently agglomerated by extrusion. The extrudates obtained are shape retaining and sufficiently hard to be handled without breaking, crumbling or dusting, and may be charged to extractive pyrometallurgical installations for recovering value metals contained in the metallurgical by-products and waste products. The extrudates may also be stored without dusting and loss of hardness.