Chemical Pulping of Straws (IV)

Abstract

Among the countries using nonwood plant fiber as pulp raw materials, China is a leading country to use straw, reed, bamboo and so on (76.1% on total pulp ca. 10 million tons in 1992). However, the use of rice straw is quite small, because of a large amounts of ash, especially silica. Silica causes the scale trouble during the concentration of soda black liquor. Therefore, in order to use more rice straw for pulp production, silica trouble must be solved as soon as possible. On this point, it is also important to find the pulping method leaving silica in pulp.This paper presents the results of studies on the ash properties of straw and the behavior of rice straw ash in black liquor. The results are summarized as follows:(1) Crystalline structure of straw silica is the mixture of crystoballite and quartz type. It is interesting that the generated silica from sodium silicate in black liquor shows a diffuse, broad X-ray spectrum.(2) The ash in black liquor is removed by the addition of precipitant, namely CaO or CaO+CaCl2. The efficiency of CaCl2 addition is observed at the lower amounts of CaO addition (200% on silica) and shorter precipitation time (60 min. at 80°C). The efficiency of CaCl2 addition is not observed at higher CaO addition (300% on silica). The amounts of CaO precipitant and precipitation time can be decreased by the concentration of black liquor.(3) In order to remove separately ash and lignin in black liquor, two stage pH adjustments of black liquor are efficient.