Abstract
In the mills with wheat straw as raw material in China, the high silica content of raw material resulted in a low ratio of alkali recovery and high maintenance cost for equipment. However, till now, little progress has been made on removal of silica in the alkali recovery process of wheat straw pulping. Many researchers have investigated the feasibility of UF/RO organic membranes to treat black liquor (BL), but the success of UF/RO membranes in practice was discouraging due to their short life-span and low flux. In this study a novel process, the membrane-alkali recovery process, was developed by applying an inorganic MF membrane in the alkali recovery process. The rejection performance of inorganic MF membranes on lignin, silica and sodium salts was investigated, and the feasibility of the membrane-alkali recovery process was discussed in detail. The experimental results showed that the inorganic membranes could efficiently reject 75% of lignin and 80% of silica. A running period of more than 40 days for a 0.2 μ membrane was achieved with the maximum flux, 5001/m2h, under the conditions of TMP 0.2 MPa, crossflow velocity 2.3 m/s, and temperature 30—60°C. This newly developed membrane-alkali recovery process could efficiently reject silica. The interference of silica on the alkali recovery process was greatly decreased. Lime sludge could be reused in the membrane-alkali recovery process while it was discharged in the conventional alkali recovery process. The function of the alkali recovery boiler in the membrane-alkali recovery process was emphasized on the chemical reactor while the conventional furnace was taken as a steam regenerator and also a chemical reactor. The capital and maintenance of the new furnace could be decreased, and the operation could be simple and convenient compared with the conventional furnace. Therefore, the membrane-alkali recovery process overcame many disadvantages of the conventional alkali recovery process and fully utilized the organics and inorganics in the BL.
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