Representative components of the fly ash (mass fraction, %).
A mixture of fly ash and sodium hydroxide was calcined, which converted mullite (3Al2O3·2SiO2), the high-temperature stability phase containing silicon and aluminum oxides, and quartz into activated silica alumina phase, and they were dissolved by concentrated NaOH solution into soluble SiO32− and AlO2−. The insoluble impurities including Fe2O3, FeO, CaO, and CaSO4 were filtered out. Experiment results show the optimum experimental conditions for the dissolution: temperature is 60°C, 15% NaOH solution is used, liquid–solid mass ratio is 11:1, stirring time is 3 h, and about 78.9% of silicon and 78.1% of aluminum are dissolved. The obtained pure silicon aluminum solution provides the raw material for preparing high-purity molecular sieves, and Fe2O3 content of the prepared P-type molecular sieve is only 0.25%, and the CaO content is only 0.066%. The paper provided a viable method to remove Fe, Ca, etc. in mineral thoroughly.
- melting and dissolving of coal fly ash
- removal of Fe
- P molecular sieve
Fly ash is a kind of solid waste discharged from power plants and various coal-fired boilers. According to statistics, the amount of fly ash accumulation in China is up to 0.2 billion tons, and it is increasing every year. It is the largest output of ash in industrial waste residue. Fly ash can seriously pollute the environment and cause serious harm to people’s lives, animals and plants, and so on. According to local conditions, timely and effective treatment of fly ash and comprehensive utilization of fly ash have far-reaching significance. It not only saves water, saves soil, and turns waste into treasure but also protects the environment.
At present, fly ash is mainly used as building material. It is used in thermal insulation board, slag cement, wall tile, floor brick, etc. . The utilization ratio of fly ash is large, reaching 67%, but the added value of products is not high. The main representative components of fly ash are silica and alumina, which account for as high as 70%, while the silicon and aluminum are also main ingredients of expensive molecular sieve. Preparing high price molecular sieve with cheap fly ash is an important way to promote the use of additional value. The application of molecular sieve is very wide; can be used for gas or liquid dehydration, drying, separation, and purification; and can also be used as adsorbent, catalyst, and ion exchange agent for various types of reactions in the field of petroleum chemical industry, fine chemical industry, agriculture, and environmental protection. The preparation of molecular sieve with fly ash can not only save raw materials but also can simplify the process and equipment and provide the conditions for large-scale production and wide application of molecular sieves.
The study on the synthesis of molecular sieves with fly ash began from 1985 by Holler and Wrisching .
Since then, more and more molecular sieve types  have been developed. The domestic and international research on the synthesis of molecular sieve from fly ash is more and more widely and deeply .
Yang Liyun et al. recently synthesized zeolite 4A using fly ash fused with synergism of NaOH and Na2CO3 . Asifa Iqbal et al. synthesized and characterized pure phase 4A from coal fly ash . Richa Soni and Dericks Praise Shukia synthesized fly ash-based zeolite-reduced graphene oxide composite and evaluated its property as an adsorbent for arsenic removal . A De Rossi et al. synthesized zeolite by geopolymerization of biomass fly ash and metakaolin . Koshy and Singh described applications of fly ash zeolites for water treatment . Fang et al. synthesized high-quality zeolites from coal fly ash and researched mobility of hazardous elements and environmental applications . Tauanov et al. synthesized coal fly ash-derived zeolites doped with silver nanoparticles for mercury(II) removal from water . Lim et al. prepared quasi-solid-state electrolytes using a coal fly ash-derived zeolite-X and zeolite-A for dye-sensitized solar cells . Collectively, researches on zeolites derived from fly ash focus on applications gradually.
However, fly ash is not pure aluminum silicate. In addition to containing valuable elements of silicon and aluminum, fly ash also contains a considerable part of the iron and calcium and other impurities, leading to impure molecular sieve. The impurities not only are easy to plug in the channels of the molecular sieve but also reduce the exchange capacity, catalytic performance, and cycle performance. The drawbacks have not drawn enough attentions so far. Therefore, it is necessary to pretreat fly ash to remove the impurities such as iron, calcium, and so on before preparing the molecular sieve with fly ash.
Acid leaching of fly ash to remove iron and calcium used to be employed by previous patent literature, as the reaction temperature is below 100°C, the process can not destroy the high-temperature phase such as mullite and quartz, so the high-temperature phase included iron and calcium impurities does not dissolve, this process can only get rid of 60% of iron and calcium, is not complete. We once used carbon reduction-magnetic separation-acid leaching method; although the iron removal efficiency is high, the process is slightly lengthy . Alkali melting method was used in literature, but the following alkali dissolving was not used. The impurities still exist in prepared molecular sieve .
A new method, alkali melting, or alkali dissolving, of fly ash, to remove iron, calcium, and other impurities, was proposed in the paper. Firstly, fly ash and NaOH solid were mixed and roasted to convert the mullite and quartz into glass phases. The roasted clinker was leached by NaOH solution to dissolve silicon and aluminum components and to filter out Fe2O3, CaO, and CaSO4 insoluble impurities, etc., so as to obtain purified Na2SiO3 and NaAlO2 solution. The purified solution can be used for the preparation of high pure molecular sieve to improve the performance and the service life.
2. Experimental part
In this study, the fly ash was gotten from a power plant in Shandong; the main chemical composition is shown in Table 1.
The specific steps are as follows. A certain amount of fly ash was weighed, placed in a mortar, and fully ground before putting through a 200 mesh sieve prior to be mixed with NaOH powder with a mass ratio of 1 to 2. The mixture was transferred to a crucible and was roasted at 600°C in a muffle furnace for 2 h. The clinker was then leached with a high concentration of NaOH solution to dissolve the silicon aluminum phase, and the impurities such as iron and calcium are filtered out to obtain a pure solution containing the silicon aluminum phase merely. The contents of Si and Al in the filtrate were analyzed by ICP analyzer, and the dissolution ratios of silicon and aluminum were calculated according to the total amounts. At the same time, the dissolution ratio of fly ash in clinker was also measured. The filtrate was added with hydrochloric acid and the alkalinity (OH/Si) was controlled. A certain proportion of Na2SiO3 solution and the seed crystal directing agent was added to regulate the ratio of silicon and aluminum for a special molecular type. After that, the mixture was stirred evenly, transferred to an autoclave, and crystallized at a certain temperature for a certain time. The derived solid was just a molecular sieve and was filtered and dried, measured by SHIMADZU X-ray diffractometer XRD-6000. Zeolite P was synthesized according to the document . Ion concentration is analyzed by 3600A inductively coupled plasma atomic emission spectrometer made by Keje company in China.
3. Results and discussion
3.1 Calcination of NaOH and fly ash
SiO2and Al2O3 existing as quartz and mullite in fly ash have low activity; high-temperature alkali roasting method can greatly increase their activity and improve the efficiency of the fly ash conversion and crystallization synthesis of the molecular sieve. There are commonly two ways to calcine fly ash, one is to add Na2CO3 to calcine at 800°C, and the other is to add NaOH to calcine at 600°C. In the experiment, fly ash and NaOH reacted after roasted in a muffle furnace at 600°C for 2 h. Figure 1 is the contrast diagram of XRD for fly ash before and after calcination with NaOH.
From Figure 1, the diffraction peaks of quartz and mullite are strong before calcination, which show the main minerals are quartz and mullite. Quartz and mullite are difficult to react with NaOH at room temperature due to their low activity. Therefore, calcination of fly ash and NaOH at high temperature is necessary and can stimulate its activity. The diffraction pattern of calcined fly ash and NaOH shows that the form of the material is mainly aluminosilicate when calcined at high temperature. At this time, there are few quartz and mullite, and the diffraction peak almost disappeared. This is because as the reaction proceeded, high-temperature roasting destroyed crystal structure, thus releasing the active SiO2 and Al2O3. These substances react with NaOH and generate amorphous aluminosilicate that is able to participate in the zeolite framework structure.
What is more, high-temperature roasting can get rid of the organic impurities in fly ash and amorphous carbon, thereby improving the purity of raw materials. In addition, alkali melting of fly ash provided a large amount of NaOH for the following leaching process since real mass amount of NaOH reacted is equal to the of fly ash according to the experimental result.
3.2 Effect of mass ratio of solid clinker to alkali solution
Figure 2 shows that the NaOH solution can dissolve alkali melting residue, which includes Na2SiO3 and NaAl(OH)4 derived from mullite, quartz, and other silicon aluminum phases of coal fly ash. Dissolving ratios of Si and Al decrease with increasing mass ratio of solid clinker to alkali solution. The maximum dissolving ratio of Si is 56.6%, while the maximum dissolving ratio of Al is 33.8% under the experimental condition. 100 g of water can dissolve nearly 37 g Na2SiO3; however, 56.6% is not high.
3.3 Effect of temperature on dissolving ratio of Si and Al
From Figure 3, low leaching temperature is beneficial to high dissolving ratio of Si and Al. Dissolving ratios of Si and Al reach 78.9 and 78.1%, respectively, as the leaching temperature is 20°C. Similar research showed dissolution ratio of Si is 75%, while dissolution ratio of Al is only 25% during alkali leaching from titania slag . Both the dissolving ratios dropped sharply with increasing leaching temperatures because hydrolysis reaction occurred at higher temperatures. H4SiO4 and Al(OH)3 precipitates stayed in solid residues, which led to the low dissolving ratios.
3.4 Effect of mass ratio of NaOH to solution on dissolving ratio of Si and Al
Figure 4 shows higher NaOH concentration is more viable in order to dissolve more Si and Al. When the alkali concentration is increased from 5 to 10%, the dissolution rate of aluminum decreases. The leaching rate fluctuated, just like the acid leaching of titanium-bearing minerals; at present it is thought that the evolution of AlO2− causes the fluctuation because the AlO2− is more soluble than Al(OH)4−, and the general trend is still rising with the increase of NaOH concentration.
The possible following reaction occurs when NaOH concentration is larger than 10%.
Thus, insoluble Na2O · Al2O3 · xSiO2 in water dissolved in concentrated NaOH solution. The tendency indicates OH- plays an important role during the process, which takes part in the coordination process. The large amount of use of NaOH causes big cost. Reusing the redundant NaOH solution is significant and economical. The condensation and the seed can make the reuse of NaOH solution possible.
Figure 5 shows Na2O-Al2O3-H2O system phase diagram . The area of OBCO belongs to the dissolving process of Al2O3 into the NaOH solution according to Figure 3. The solubility of Al2O3 in NaOH solution is increasing along the NaOH concentration until 20% (line OB); after the summit of 20% of the solubility, the dissolving ability decreases with the increasing NaOH concentration [line BC]. Therefore, a highly efficient NaOH concentration is around 20%; the number is consistent with our experimental result in Figure 4 where 0.20 mass ratio of NaOH made the highest dissolving ratio of Al. 15% NaOH is used, and NaOH is surplus after the roasting process. Overall, the concentration of NaOH is around 20%.
3.5 Effect of stirring time on dissolving ratios of Si and Al
Figure 6 indicates too long time is disadvantageous for high dissolving ratio of Si and Al. The dissolving ratio decreases with adding stirring time. The tendency is obvious for Al. The dissolving ratio drops steeply during 2 h or so. The gradual decreasing tendency is due to the precipitate of Si and Al. Reducing stirring time is essential so as to get high dissolving ratios of Si and Al. 1 h is enough for the stirring time.
Figure 7 shows the filtration of slag after alkaline solution filtration. The color of the slag is red, which proves that the slag contains iron oxide, which can be used for carbon reduction to obtain sponge iron for steel-making. The filtered solution can be used for the preparation of the molecular sieve. The molar ratio of Si to Al in the leach solution is adjusted to 4.6; pH drops to 9 by adding HCl solution. Then, the solution is put in an autoclave after aging for 1 h and taken out after crystallization for 16 h at 100°C. The p-type molecular sieve is obtained after filtration, washing, and drying. The cost of raw material and the environmental pollution were reduced because of using fly ash as raw material.
The Fe2O3 content of the prepared molecular sieve drops to 0.25% after the alkali treatment from 2.87% by merely acid leaching of fly ash, while the content of CaO drops to 0.066% from 1.18%. The chemical formula of the P molecular sieve prepared is confirmed as Na6Al6Si10O32·12H2O through ICP examination. The purity of the P molecular sieve prepared by the method from coal fly ash is 99.06% (seen in Table 2), while the whiteness level is 96 (seen in Figure 8). Both purity and whiteness level of the molecular sieve can meet the standard of market molecular sieve.
Figure 9 shows XRD diagram of the product; there are five characteristic peaks at 2θ = 12.48°, 17.71°, 21.68°, 28.15°, and 33.37°, respectively, and miscellaneous peaks are few, indicating the product is P molecular sieve (JCPDS NO 39-0219). Pure P molecular sieve can be used as petroleum catalyst, support, and detergent additive to soften hard water in washing process.
At 600°C, the alkali melting activated quartz and mullite in the fly ash into the aluminosilicate glass phase, which can be dissolved in the alkali solution. The optimum conditions of alkali dissolving of clinker were obtained by optimizing experiments, that is, the reaction temperature is 20°C; using 15% NaOH solution, the liquid solid ratio is 10:1, and the stirring time is less than 1 h; the whole dissolution ratio of silicon reaches 78.9%, while that of Al reaches 78.1%. The method provides pure solution for preparing molecular sieves from fly ash. The Fe2O3 content of the prepared molecular sieve through actual verification drops to 0.25% after the alkali treatment from 2.87% by merely acid leaching of fly ash, while the content of CaO drops to 0.066% from 1.18%.