TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Process: The temperature at which the Si ions are precipitated is not less than 20 degreesC below the dissolution temperature and not less than 200 degreesC. Dissolution is carried out at elevated pressure, which preferably at least corresponds to (and especially is higher than) the saturation vapor pressure of the aqueous alkaline solution (I). (I) is recirculated and its water content is kept constant by replacing (part of) the water consumed in the reaction. (I) is supersaturated with Si ions. SiO2 is precipitated by adding seeds, especially SiO2 seeds, and (part of) the SiO2 precipitate is separated, preferably by sedimentation. Particulate Si (alloy) is used. The amount of H2 generated is regulated, preferably by regulating the amount of (I) available for reaction, especially by regulating the amount of (I) by the pressure of the H2 generated. The process is operated (semi)continuously.Preferred Apparatus: The H2 reactor is thermally insulated. The pressure at least of the H2 reactor is controllable. There is also a direct connector between the H2 reactor and crystallizer. The apparatus includes a pump and pipe for recirculating (I); and an overflow valve between the reactor and crystallizer. The pipe preferably passes through the connector and is in the form of a heat exchanger, at least where it passes through the connector. The crystallizer, which is cooled, has a base holding a bed of seed crystals and the SiO2 crystals precipitated and a valve for adding fresh (I). The elements forming the crystallization surfaces are interchangeable. The reactor and/or crystallizer is connected to a reservoir with valve for replacing the water used, preferably a water reservoir below the crystallizer, connected by a rising pipe. The crystallizer base is inclined and has a controllable closure, especially a clack valve, on the outlet side, connecting the crystallizer to the water reservoir, optionally via a connector. The water reservoir had an inclined base with valve for discharging sediment. The H2 pressure in the reactor is controlled by a gas valve, preferably a level regulator controlling the amount of (I) available. The displaced volume of (I) is compensated by interaction with a volume equalizer, especially in the water reservoir; or a pipe connecting the water reservoir to the gas space of the reactor or to a pipe for discharging H2. The H2 reactor is followed by a gas purifier.