Removal Of Silicon From Green Liquor In The Chemical Recovery Process Of Wheat Straw Pulping

Because of the shortage of wood resources, a large number ofnon-wood materials such as wheat straw, reed and bamboo are still usedas papermaking resources in China, India and many other countries.However, these non-wood raw materials contain much silicon, whichwill react with sodium hydroxide in the cooking process, form sodiumsilicate, and dissolve in black liquor. The silicate causes seriousproblems of scaling in the evaporator in which the black liquor isconcentrated, and the evaporative efficiency decreases. In the process ofcausticizing, sodium silicate reacts with calcium hydroxide to generatecalcium silicate, which makes recycling of lime difficult and reducesthe activity of lime while the lime mud is calcined. By now, there is noadvanced process other than burying or piling up of the lime mud withhigh silicon content, which not only pollutes the environment but alsowastes resources.Due to the problems caused by high silicon content of green liquorin the chemical recovery process of wheat straw pulping, the greenliquor was treated with carbon dioxide to precipitate silica by one-stepprocess and seeding process. The total silicon removal, charge density,the particle size and sedimentation performance of silica wereinvestigated. And the silicon residue was analysed by infrared spectraand scanning electron microscopy （SEM）. Additionally, the siliconremoval of green liquor treated with acetic acid was investigated. Wehad the following conclusions from the study in this paper.In this study, the green liquor of200ml, the results showed that the removal of silicon improved with the amount of carbon dioxideincreasing in the one-step process. About99%of silicon removal wasachieved when the volume of carbon dioxide was15L; In the one-stepprocess, charge density, the particle size and the sedimentation rate ofsilica increased with the amount of CO₂increasing; the reactiontemperature had no significant effect on the removal of silicon, whenthe reaction temperature increased from30℃to90℃, silicon removalincreasing only1.54%. The particle size decreased apparently, thesedimentation rate increased quickly and the drainage rate increasedgradually with the reaction temperature increasing. Stirring speed hadno significant effect on the removal of silicon and charge density ofgreen liquor. Silicic acid generated from green liquid with carbondioxide acidification suspended in the form of colloids. The coagulantwas introduced the system, we found that the addition of Al₂（SO₄）₃washelpful to the settlement of the silicon residue of green liquid. Theoptimal conditions of carbon dioxide to precipitate silica by one-stepprocess: the flow rate was1L/min and the time of15min; the reactiontemperature was80°C, stirring speed was medium, the dosage ofAl₂（SO₄）₃was2.0g/L. About99%of silicon was removed after treatingunder these conditions, the sedimentation rate was1.93cm/min and thedrainage time was shortened to62s.Compared with the one-step process, the particle size generated bythe seeding process was bigger at the same reaction conditions. Withthe increasing of the amount of CO₂in the second step, the particlesize of silica increased, the specific surface area decreased and thesedimentation rate increased. The particle size of silica decreasedapparently, the specific surface area increased greatly and thesedimentation rate increased rapidly with the reaction temperatureincreasing. Compared with one-step process, the sedimentation timewas shortened greatly, the sedimentation rate was increased and thedrainage time was shortened apparently by the seeding process. Withthe increasing of Stirring speed, the particle size of silica increased,the sedimentation rate increased and the drainage time was shortened. The addition of Al₂（SO₄）₃was helpful to improve the sedimentationrate of the silicon residue. The optimal conditions with carbon dioxideacidification by seeding process: The amount of green liquor first stepwas the40percent of total amount of green liquor, the first step theamount of carbon dioxide was3L; the second step, the amount ofcarbon dioxide was12L，reaction temperature was80°C, stirring speedwas medium, and an appropriate amount Al₂（SO₄）₃was added. About99%of silicon was removed，the sedimentation rate was2.32cm/minand the drainage time was shortened to52s after treating by seedingprocess.During the acetic acid acidification for the silicon removalexperiment, green liquid was titrated with40%concentration of aceticacid, The rate of silicon removal increased gradually and charge densitydecreased with the pH decreasing. The reaction temperature had nosignificant effect on the removal of silicon and the charge density. Theoptimal conditions with acetic acid acidification: The concentration ofacetic acid was40%, acid was added until the pH reached9, thereaction temperature was over80°C. After acetic acid acidificationtreatment, a bout97.7%of silicon was removed.From the scanning electron micrographs of silicon residue, we cansee that silicon residue is porous at the higher pH values. With theincrease of acetic acid amount, pH reduced, silicon residue is morecompact, uniform and stable.