Table 5. Market price of19 sintered mullite
Assuming that the original Al2O3 content in fly ash is m%, in order to make the Al2O3 content in the ingredients reach y%, the amount of industrial alumina to be added, x%, can be obtained by the following formula:
Characteristics of high alumina fly ash and its application in synthesis of mullite and cordierite
Table 5. 20 shows the amount of industrial alumina needed to synthesize M50, M60 and M70 mullite products according to the assumed original Al2O3 content in fly ash. It can be seen that if the Al2O3 content in the original fly ash is 30%, then 75% of industrial alumina is needed to synthesize M60 mullite products, and the cost of synthesizing mullite alone is 3,495 yuan, accounting for 72. 8% of the sales price of mullite. To synthesize M70 mullite, it is necessary to add industrial alumina as high as 133. 3%. Therefore, the cost of synthesizing mullite from fly ash and industrial alumina mainly depends on the amount of industrial alumina added, which is also the key factor that leads to the failure of commercial production of mullite synthesized from fly ash so far. If high-alumina is used instead of industrial alumina, its synthesis cost will be reduced, but the reduction is not significant, because the price of high-alumina in China is also about 3000 yuan/ton, and high-alumina usually contains a certain amount of SiO2 and other impurity oxides.
Table 5. 20 The amount of industrial alumina needed to synthesize M50, M60 and M70 mullite determined according to the assumed original Al2O3 content in fly ash.
If we use high-alumina fly ash from Zhungeer Power Plant as raw material, M50 series mullite can be directly prepared without adding any alumina. For M60 and M70 mullite products synthesized by mixing A (untreated with hydrochloric acid) and B (treated with 20% hydrochloric acid) series high-alumina fly ash with industrial alumina, The amount of industrial alumina added is 18. 2%, 57. 6% and 14. 8%, 53. 0% respectively. According to the latest market sales price of industrial alumina and sintered mullite published in1October 2005, the corresponding alumina costs are 848 and 2684 respectively. They account for 17. 7%, 46. 3% and 14. 4%, 42. 6% of the market price of mullite, respectively (table 5. 2 1), which is different from the previous use of common fly ash and industrial alumina1:/kloc-. Compared with the stoichiometric mullite (Al2O3= 7 1. 8%, SiO2= 28. 2%, and the mass ratio of Al2O3/ SiO2 is 2. 55), the synthesis cost is greatly reduced.
It can also be seen from Table 5. 2 1 that M50 series mullite is directly prepared from high-alumina fly ash of Zhungeer Power Plant, or M60 series mullite is synthesized from high-alumina fly ash and industrial alumina as raw materials, and according to our experimental results, M60 series samples have the highest mullite content and the best physical properties. M50 series mullite is directly produced from high-alumina fly ash of Zhungeer Power Plant and M60 series mullite is synthesized from high-alumina fly ash and industrial alumina. Excluding the production cost, the net profit is about1000 ~1500 yuan/ton, which is also the focus of our next semi-industrial test.
Table 5. 2 1 Cost Accounting of Mullite Synthesis from Zhungeer Power Plant Fly Ash
The impurity content in fly ash is the key factor affecting the quality of synthetic mullite. To improve the purity of synthetic mullite, it is necessary to pretreat fly ash to minimize the amount of impurities in fly ash. There is a mature technology for the treatment of iron in fly ash, which can be directly applied, that is, the magnetic separator is used to separate fly ash and remove the magnetic substances. The effect of removing CaO with 20% hydrochloric acid is quite ideal. Our experiments above have shown that the CaO content in fly ash can be reduced to below 1% after being treated with 20% hydrochloric acid, and the amount of other impurities can also be reduced.
At present, the market price of industrial hydrochloric acid is about 300 yuan/ton, so the cost of treating impurities in fly ash can be greatly reduced if industrial hydrochloric acid is used. The content of MgO in fly ash is generally not high, and its influence on the synthesis of mullite is much less than that of CaO, and it can be partially removed with hydrochloric acid treatment, so there is generally no need to worry too much about MgO in fly ash. The content of fly ash is usually TiO _ 2, which is second only to Fe2O3. The effect of removing TiO _ 2 with hydrochloric acid is not obvious, but it has little effect on the synthesis of mullite, and a small amount of it can also toughen the products. In order to avoid the synergistic effect of TiO2 _ 2 and Fe _ 2O _ 3, the Fe _ 2O _ 3 content in fly ash should be removed as much as possible (such as increasing the magnetic field intensity of magnetic separator, etc.). If fly ash with high TiO _ 2 content (more than 4%, which is actually rare) is encountered, the effect of removing TiO _ 2 by aluminum fluoride method is very obvious (Lin Hecheng et al., 1999), and most of SiO2 _ 2 can be removed to improve the aluminum-silicon ratio. Generally, the contents of K2O and Na2O in fly ash can be removed simultaneously when CaO is removed by hydrochloric acid.
Impurities in fly ash are a double-edged sword, which has both advantages (reducing sintering temperature and volume expansion) and disadvantages (reducing the content of mullite and the physical, mechanical and thermal properties of products) in synthesizing mullite, which requires us to measure and consider it from various aspects in order to achieve the best expectation.
On the whole, there is no technical difficulty in removing impurities from fly ash, and the degree of removal mainly depends on the quality requirements and economic feasibility of synthetic mullite. General refractories and ceramic products do not have high quality requirements for mullite. In order to reduce the synthesis cost, the quality requirements for mullite synthesized from fly ash can be appropriately relaxed. The metallurgical industry standard "Sintered Mullite" (YB/T5267—2005), which was implemented in 2005 1 February1day, has been promulgated. The maximum allowable amounts of Fe2O3, TiO2 and K2O+Na2O are respectively widened to 1. 5%, 3. 5% and 3. 0% (Na2O ≤0. 3%) in the standard, which is formulated based on the comprehensive consideration of the existing resources in China, especially the appropriate relaxation of the impurity allowable amount index is conducive to the rational use and comprehensive utilization of resources. According to our experimental results, it is not impossible for the technical indexes of mullite synthesized by sintering fly ash to reach the industry standard, and some indexes are even far higher than the industry standard.
According to our investigation, if a tunnel kiln with an annual output of 1. 5 × 104t mullite is built, it will probably require kiln investment of1100000 yuan, equipment investment of1200000 yuan, workshop investment of 500000 yuan and warehouse investment of 200000 yuan, plus spare parts supply. According to the lowest price of mullite market in 2003, 2500 yuan/ton, excluding raw materials and production costs, the profit can be110/5 million yuan, and the cost can be recovered in about one year. Considering the resource utilization of industrial waste (saving natural resources) and environmental benefits (the charging standard for fly ash storage is 30 yuan/ton), and the national tax reduction and exemption policy for solid waste utilization, its economic and social benefits are considerable. Zhungeer Power Plant fly ash is an ideal raw material for synthesizing mullite because it uses a single coal-fired source, with relatively stable chemical composition, high alumina content and low impurity content.