**7. Desilication and chemical recovery of black liquor**

### **7.1. Desilication in wheat straw cooking**

Two different kappa number pulps of wheat straw soda-AQ cooking were studied followed by O<sup>2</sup> cooking for elucidating the effects of alkali charge, temperature and oxygen pressure on the black liquor desilication, pulp yield, and pulp delignification. The results showed that the soda-AQ cooking conditions had an influence on the properties of the pulp and the black liquor in the O<sup>2</sup> cooking stage. And, there were proper alkali charge, temperature and pressure ranges for the desilication, the pulp yield, and the black liquor viscosity in the O<sup>2</sup> cooking system at given other conditions. The results also showed that silica content in the O<sup>2</sup> cooking pulp did not always decrease with the rise of system pH and silica content of black liquor did not always increase with the rise of system pH. The O<sup>2</sup> cooking in soda-AQ/O<sup>2</sup> two-stage cooking cannot only successfully desilicate from the black liquor but also delignify effectively itself. Hence, the soda-AQ/O<sup>2</sup> cooking process could be an environmentally friendly potential pulping process [29].

Desilication kinetics of the O<sup>2</sup> stage in the wheat straw soda-AQ/O<sup>2</sup> two-stage cooking were investigated. In addition, the pulp yield, alkali concentration, and O<sup>2</sup> pressure in the O<sup>2</sup> cooking were studied. The results showed that desilication reaction from the O<sup>2</sup> cooking black liquor could be divided into two phases, a rapid initial desilication phase followed by a slow final desilication phase. They both follow pseudo-first-order reaction toward the silica concentration in black liquor with the reaction energy 64 and 79 kJ/mols, respectively. The differential state expressions of two phases were deduced [8, 29].

Studies also indicated that under the experiment conditions, the pulp yield increased by 0–3.5% and the O<sup>2</sup> pressure and alkali concentration in the system decreased with the extending time at given temperature in the O<sup>2</sup> cooking process.

The successful experiences for chemical recovery in China's wheat straw chemical pulping mills indicate the following: (1) to enhance the loss of wheat straw preparation is helpful to decrease the silica content and viscosity of black liquor; (2) rapid cooking at lower temperature can prevent degradation of cellulose and provide wheat straw chemical pulps with good physic strength and drainability; and (3) high-efficient extraction washer for black liquor is helpful to increase the initial concentration of black liquor, resulting steam savings during thickening black liquor and also decrement of fresh water consumption in pulp washing stage [4, 8].

For instance, Quanlin Paper Group has successfully operated its chemical recovery system for wheat straw chemical pulping black liquor, with an alkali recovery rate of more than 85%, and its mid-stage water can be treated economically to meet an extremely strict effluent discharge standard (COD concentration is less than 60 mg/L). There are many new techniques applied in its wheat straw pulping line, such as application of flying hammer chipper to separate more non-fiber components and to improve uniform cooking effects resulting in short cooking time and lower chemical dosage, decrement of black liquor viscosity from 1000cp to less than 500cp (solid content (55%) and temperature (100°C)); a vertical replacement cooking digester with large ration of chips to water was installed, leading to significant chemical dosage decrement and initial concentration of black liquor increment to save evaporation cost and to obtain more thickened black liquor which is a benefit for chemical recovery boiler [3, 4].
