Abstract: The present invention relates to a method for purifying silica waste obtained in the manufacture of aluminium fluoride from hexafluosilicic acid and aluminium hydroxide, whereby the silica, is subjected to an acidic leaching at a pH below 3 to leach out part of the fluorine and substantially all the aluminium present in said waste, and is thereafter subjected to an alkaline leaching at pH 8.5-11.5 to leach out remaining fluorine present in said waste. The leachings are preferably carried out at elevated temperatures, normally 60.degree.-90.degree. C., and for a time sufficient to considerably reduce the contents of aluminium and fluorine, normally 1-3 hrs.The silica obtained is highly reactive and can be used as such or transformed into sodium silicate by a reaction with sodium hydroxide. The contents of fluorine and aluminium can be transferred to a process for the production of cryolite or similar compounds.

Abstract: A process for increasing the feed value of lignocellulosic materials by alkali-treatment and subsequent neutralization with an acid in a closed system under circulation of the treating agents. Said treating agents are supplied or prepared during the course of the treatment at a rate essentially corresponding to the liquid uptake of the lignocellulosic material, the quantities of residual solutions resulting from the treatment being kept at a minimum. Moreover, an apparatus for carrying such process into effect.

Abstract: The invention relates to a process for increasing the feed value of lignocellulosic materials by contacting the material with an alkaline liquid. The alkaline liquid is applied on top of a mass of lignocellulosic material within a limited zone, said zone being moved across the whole upper surface of said mass. The invention also provides for an apparatus suitable for carrying out such process.