The regular alkali recovery of wheat straw pulp began in the early 70s, such as the Henan Zhonghao Paper Mill, Zhejiang Minfeng Paper Mill, Shanghai Xinhua Paper Mill, Anhui Huainan Paper Mill, etc., with a scale of 30-50 t/d pulp. Xinhua and Huainan Paper Mill's wheat straw black liquor is also mixed with some (30%) rice straw black liquor, both of which adhere to normal production for many years. Alkali stoves usually do not use auxiliary fuel.
However, due to many reasons, especially the low extraction rate (<70%) and extensive management, slurry alkali operation is not synchronized, resulting in alkali recovery rate of only 40% to 60%.
Under the pressure of governing the Huaihe River under the development and practice of wheat straw pulp recovery for several decades, a batch of 75-100t/d wheat straw pulp alkali recovery project funded by China Energy Conservation Investment Corporation was implemented in 1996. It concentrates on the achievements of this technological advancement and the crystallization of the experience of the new generation and the old generation, so that it can be put into production successfully and it can basically achieve the intended effect. At present, the extraction rate of wheat straw black liquor is 80% to 85%, and the alkali recovery rate is 70% to 75%. This project mainly uses the following technologies:
(a) Wet and dry preparation of raw materials to purify raw materials to reduce the silicon content of heterocells and black liquor in the slurry, which is beneficial to black liquor extraction and alkali recovery. The grass water is recycled after being purified.
(b) Continuous cross-cooking at a cooking temperature of 165°C for a cooking time of 10 minutes to 15 minutes with an alkali amount of 12% (NaOH), a yield of 50% and a Kappa number of 20. Due to the rapid cooking and cold spraying, not only the pulp strength is good, but also the slurry has a good filterability, which facilitates black liquor extraction.
(c) Using a drum vacuum washer, four stages in series, hot water countercurrent washing and black liquor extraction. The design was carried out according to the following conditions: 1.2 to 1.3 tons of coarse pulp was washed per unit area, the concentration of extracted black liquor was 10%, the temperature was 70°C, and the extraction rate of black liquor was 80% to 85%.
(d) The concentration of black liquor exiting the three-tube, two-plate, five-effect evaporation station is 40% to 42%, which can save about 30% of equipment investment compared with the full-plate five-effect evaporation station.
(e) The primary and secondary air supply temperature for the slurry slurry furnace is 250°C, which helps to increase the furnace temperature, facilitates drying, and can burn 48% to 50% of the furnace black liquor. Such as the configuration of a single three-electric field electrostatic precipitator, can make the amount of smoke dust standards.
(f) Causticizing using qualified lime (CaO>80%, MgO <1.5%) and controlling the causticity at 82% to 85%, excess lime at 3% to 5%, making the causticizing emulsion easy Shen Qing, white mud easy to wash, reduce residual alkali. Excessively high degrees of causticity and excess ash are undesirable. During the operation, the volume of pumping mud of the concentrated white liquor clarifier shall be reduced as much as possible, and the extracted mud shall be filtered first, and the filtrate (cloudy white liquor) shall be returned to the inlet of the clarifier, and the filter cake shall be stirred after being scattered to enter the dilute white liquor for clarification. Reduce the total alkali content of diluted white liquor and the concentration of diluted white liquor (which can be controlled within 12g/L), and ensure that the residual effluent lime mud residual alkali is reduced below 1%, which is convenient for comprehensive utilization or heap Storage pits.