Extraction of Alumina from Fly Ash by Soda Lime Sintering Method

Investigated the role of single sintering agent, sintering agent synergistic effects and dosage ratio of sintering, the sintering temperature and sintering time on agents pulverized coal ash clinker sintered aluminum oxide dissolution rate, and the ratio of alkali, calcium ratio, the sintering temperature And the sintering time was carried out for the orthogonal test of fly ash extraction alumina. The test results show that sodium carbonate has much better dissolution effect on alumina in fly ash than on quicklime when used alone; when the mixture of sodium carbonate and calcium oxide is used as sintering agent, various factors are oxidized in fly ash sintered clinker. The order of influence of aluminum dissolution rate is alkali ratio > sintering temperature > calcium ratio > sintering time; when the alkali ratio is 3:1, the calcium ratio is 1:1, sintering at 850 °C for 30 minutes can dissolve 72.21% of alumina in fly ash. . Fly ash is an industrial waste discharged from coal-fired boilers in thermal power plants. It is one of the world's largest industrial solid wastes, which not only seriously pollutes the environment, but also occupies land. In general, 250-300 kg of fly ash can be produced for every 1 t of coal burned. For a long time, fly ash has been a major source of pollution for coal-fired power plants, seriously affecting the daily life of residents around coal-fired power plants. At the same time, it has had a huge impact on the surrounding natural environment. Fly ash is an industrial solid waste, but at the same time it is a huge renewable resource. Therefore, if the useful components in the fly ash can be utilized, on the one hand, the problems of occupying land and polluting the environment brought by the fly ash can be solved, and the pressure brought by the waste of fly ash to the environment can be alleviated, and at the same time, Become a cheap renewable resource. Fly ash generally contains 15% to 55% alumina, which would be a good alumina resource if it can be used reasonably.
I. Original sample and method The sample is a wet coal ash from a coal-fired power plant. The chemical composition of the sample after iron selection is shown in Table 1.

Table 1 Chemical composition (mass fraction) of fly ash

The test uses sodium carbonate and quicklime and fly ash to sinter at a certain temperature to investigate the synergy of the type of sintering agent and the sintering machine. The effects of sintering temperature, sintering time, alkali ratio and calcium ratio on the dissolution rate of alumina in fly ash. The factors affecting the sintering process were formally tested to optimize the conditions of the sintering process, and the optimum sintering conditions for the extraction of alumina from fly ash were obtained. The sintered clinker is dissolved in a dilute solution of 80 g/L sodium hydroxide at 100 ° C for 5 min. The red mud is first washed twice with a dilute solution of 8 g/L sodium hydroxide, and then washed once with hot water at 100 ° C to dissolve the aluminate. The sodium solution was filtered to discard the red mud, and the dissolved alumina content was determined by hydrochloric acid.

II. Test results and discussion (I) Effect of the type of sintering agent on the dissolution rate of Al ₂ O _{3 in} fly ash Mixing fly ash with sodium carbonate and quicklime at a mass ratio of 1:1, respectively, at a certain temperature Sintering was carried out to investigate the change of alumina dissolution rate with sintering temperature in fly ash sintered clinker. The effect of the type of sintering agent on the dissolution rate of alumina in fly ash is shown in Fig. 1.

Fig. 1 Effect of the type of sintering agent on the dissolution rate of alumina in fly ash As can be seen from Fig. 1, as the temperature increases, the sintering results of sodium carbonate and quicklime gradually become better. However, with the increase of temperature, when sodium carbonate is used for sintering, the dissolution rate of alumina in the fly ash sintered clinker is much faster, and when the quicklime is sintered, the dissolution rate of alumina increases with temperature and is slow. . Sodium carbonate and quicklime were used as sintering agents. At 700 °C, the dissolution rate of alumina in fly ash was only 23.88% and 9.95% respectively. At 850 °C, the dissolution rate of alumina in fly ash increased to 77.53% and 16.28%, respectively. . (2) Synergistic effect test of sintering agent The synergistic effect test of sodium carbonate and quicklime is adopted, and the alkali ratio and calcium ratio in the sintered raw material are mainly considered. The influence of sintering temperature and sintering time on the dissolution rate of alumina in fly ash. 1. Effect of alkali ratio on the dissolution rate of alumina in low-fly fly ash clinker The effect of alkali ratio on the dissolution rate of alumina in fly ash is shown in Fig. 2. The test conditions were a calcium ratio of 0.5:1, a sintering temperature of 850 ° C, and a sintering reaction time of 30 min.

Fig. 2 Effect of alkali ratio in raw meal on dissolution rate of alumina in fly ash As can be seen from Fig. 2, when sodium carbonate and calcium oxide mixed sintering agent and fly ash are sintered, alkali ratio in raw meal is sintered to fly ash. The effect of alumina dissolution rate in clinker is large. When the alkali ratio is 1:1, the calcium ratio is 0.5:1, and sintering at 850 ° C for 30 min, only 23.22% of the alumina in the fly ash is dissolved, and when the alkali ratio is 3:1 under the same conditions, However, 68.71% of the alumina was dissolved. This is related to the reaction mechanism when sintering with sodium carbonate. When soda lime is sintered, the alumina and sodium carbonate in the fly ash mainly react as follows: Al ₂ O ₃ +Na ₂ CO ₃ →NaO·Al ₂ O ₃ +CO ₂ ↑ When the alkali ratio in the raw material is increased, the contact area between the alumina and the sodium carbonate in the raw material and the supersaturation of the sintering agent are increased, which is beneficial to increase the reaction rate of alumina and sodium carbonate in the fly ash. Therefore, the raw material having a high alkali ratio can dissolve more alumina in the fly ash under the same sintering conditions. 2. Effect of calcium ratio on dissolution rate of alumina in fly ash sintered clinker The effect of calcium ratio on dissolution rate of alumina in fly ash sintered clinker is shown in Fig. 3. The test conditions were 3:1, the sintering temperature was 850 ° C, and the sintering reaction time was 30 min.

Fig.3 Effect of calcium ratio on dissolution rate of alumina in fly ashFig.3 shows the ratio of calcium in raw meal to alumina in fly ash sintered clinker when sintering with fly ash by mixture of sodium carbonate and calcium oxide The dissolution rate has a large effect. When the calcium ratio in the raw material is 0.25:1, the alkali ratio is 3:1, and sintering at 850 °C for 30 min, 65.78% of the alumina in the fly ash is dissolved. With the increase of the calcium ratio in the raw meal, the pulverized coal The alumina solution yield in the ash-sintered clinker also increased, and the alumina dissolution in the fly ash sintered clinker increased to 72.21%. However, as the calcium ratio in the raw meal further increases, the dissolution rate of alumina in the fly ash decreases. Appropriate amount of calcium oxide can promote the sintering process and facilitate the dissolution of alumina in fly ash. When the calcium oxide in the raw material is excessive, the excessive calcium oxide hinders the dissolution of the alumina in the fly ash. 3. Effect of sintering temperature on the dissolution rate of alumina in fly ash The effect of sintering temperature on the dissolution rate of alumina in fly ash sintered clinker is shown in Fig. 4. The test conditions were a base ratio of 3:1, a calcium ratio of 1:1, and a sintering time of 30 minutes.

Figure 4 Effect of sintering temperature on the dissolution rate of alumina in fly ash Figure 4 shows that as the sintering temperature increases, the dissolution rate of alumina in the fly ash sintered clinker increases rapidly. When the raw material ratio of 3:1 and calcium ratio of 1:1 is sintered at 700 °C for 30 min, the dissolution rate of alumina in the fly ash sintered clinker is only 27.68%. At 850 °C, the alumina in the fly ash The dissolution rate was 72.21%. The data show that sodium carbonate and alumina start to react at 500-700 °C, and the temperature can be fully sensed at 800 °C, and the temperature reaction is accelerated. It can be completed in 1100 ° C in 60 min. 4. Effect of sintering time on the dissolution rate of alumina in fly ash The effect of sintering time on the dissolution rate of alumina in fly ash sintered clinker is shown in Fig. 5. The alkali ratio was 3:1, the calcium ratio was 1:1, and the sintering temperature was 850 °C.

Fig. 5 Effect of sintering time on the dissolution rate of alumina in fly ash It can be seen from Fig. 5 that the sintering time has little effect on the dissolution rate of alumina in fly ash sintered clinker. When the sintering time is 30 min, the dissolution rate of alumina in fly ash is 72.21%. With the increase of time, the dissolution rate of alumina increases slowly. Even if the sintering time is extended to 120 min, the dissolution rate is only 73.82%. This shows that when the alumina in the fly ash is dissolved by the soda lime sintering method, when the alkali ratio in the raw material is 3:1 and the calcium ratio is 1:1, the material can be dissolved in the fly ash after being sintered at 850 ° C for 30 min. The majority of the alumina, and the portion of the alumina that cannot be dissolved, is inert under these conditions. (III) Orthogonal test of sintering reaction Orthogonal test on the sintering process to investigate the influence of various factors on the dissolution rate of alumina in fly ash during the sintering process of fly ash, and determine the best dosage of various chemicals in the sintering process. Matching. The test uses an orthogonal test of a sintering reaction using a mixture of sodium carbonate and calcium oxide as a sintering agent. The orthogonal test factor level table is shown in Table 2.

Table 2 Orthogonal test factor level