A process is disclosed for extracting pure, coarse-grain silicic acid crystals from silicic acid-containing spent lye in cellulose production, comprising adding coarse-grain silicic acid to alkalized spent lye, lowering the pH of the alkalized spent lye to about 9 such that a sediment is formed, separating the sediment formed into coarse grain and fine grain silicic acid with little lignin and lignin-containing spent lye free from silicic acid, and separating the coarse grain and fine grain silicic acid with little lignin into course grain silicic acid and lignin. At least part of the course grain silicic acid separated is fed back to the alkalinized spent lye to be desilicified.

Process for extracting pure, coarse grain silicic acid crystals from spent lye

Process for extracting coarse-grain pure silicic acid crystals

In a process for extracting coarse grain silicic acid from the spent lye to be burned from a process for extracting cellulose from annual plants, the pH value of the alkalised spent lye, which is equal to 11, is gradually reduced below 10, in particular 9. The resulting lignin is washed out and the silicic acid is largely fed back upstream of the first crystallisation stage (2) for inoculation purposes. The fine crystals are dissolved and the coarser crystals are recrystallised and further enlarged. The excess silicic acid to be recorered is preferably discontinuously removed and washed once again. The silicic acid is washed during sedimentation, for example by using desilified spent lye to reduce its water content.

Process for extracting pure, coarse grain silicic acid crystals

of EP0431337In a desilication process, the pH of the waste liquor is lowered in two stages (3 and 3') by mixing in CO2, care being taken that no crystalline silica is yet formed or precipitated silica is redissolved, and that no local overacidification takes place. The silica is then precipitated by further lowering of the pH in a third, substantially identical reactor 3'' at pH = 10.2. The precipitated silica is discharged and, to allow the crystals to grow further, partially recycled. The discharged silica is washed and dehydrated in a filter press 12, the liquid separated off is partially evaporated and incinerated as waste liquor or fuel and partially recycled into the reactors to maintain the viscosity, whereby the foaming problem is also reduced.

Glasner Alfred Dipl-Ing

Papers

Process for the desilication of waste liquid