AbstractSilicon is the second most abundant element in the earth's crust; thus, familiarity with the behavior of silica in wet processing systems is important to the hydrometallurgist. Plant problems originating from silica are numerous. These problems often occur in the high-tonnage, high-volume portion of the flowsheet. A plant bottleneck caused by silica is often not recognized until substantial loss of production has occurred. Fundamentals of silicon chemistry are therefore reviewed, focusing on metallurgical hydrochemistry. Examples are given for controlling silica while processing zinc and nickel silicates, refractory metals (W, V, U, Mo, Zr, Ta), copper-dump leach liquors, mine water, high pressure slurries, and acid-bake residues. Production of silicon-based by-products is discussed.