In order to investigate the effect of impurity (oxides other than Al2O3 and SiO2) content on the physical properties and mullite production of synthetic mullite, two series of parallel experiments were carried out, one of which did not do any treatment to the impurities in the fly ash, and the other used 20% hydrochloric acid to treat the impurities in the fly ash samples, which was mainly to remove the CaO in fly ash. The other series used 20% hydrochloric acid to treat the impurities in the fly ash samples, mainly to remove the CaO in the fly ash, because the CaO content in the fly ash was the highest among the several impurities, which was 4. 22%. In the process of mullite lithification, calcium ions do not partially enter into the mullite lattice like iron and titanium ions. Moreover, during the sintering process, the coefficient of linear expansion factor of CaO is many times larger than that of MgO ( Gu Xingyong et al., 2001), which causes a decrease in the physical properties of the sintered products.
The fly ash after carbon removal and grinding (A series) and fly ash after carbon removal and grinding and treated with 20% hydrochloric acid (B series) were tested according to the original Al2O3 content (A50 and B50, respectively) and the addition of industrial alumina to make the Al2O3 content of the ingredients reach 60% (A60 and B60, respectively) and 70% (A70 and B70, respectively), respectively. The experiments were carried out. According to the 3-factor, 3-level orthogonal experimental design scheme, 9 experiments were conducted for each ingredient, so 54 experiments were conducted for the two series of 3 samples with different Al2O3 contents. The experimental flow is shown in Fig. 5.6.
Fig. 5.6 Experimental flow of sintering high alumina fly ash to synthesize mullite
( 1) Iron removal
The iron oxides in the fly ash have two kinds, namely, Fe2O3 and Fe3O4 (generally expressed as Fe2O3), and the latter one has magnetic property, so it can be removed by the magnetic separator under the magnetic flux density of 15000~20000 Gs to remove the iron. density) to reduce the iron impurity content. Due to the laboratory setting of the study, we only used a semicircular magnet with a length of 10 cm to remove iron by manual methods, which was not satisfactory, and the iron oxide in the fly ash was only reduced from 1.95% to 1.84%. If the industrialized production considerations, it is necessary to use a magnetic separator to remove iron, which is a common practice.
( 2) Removal of carbon
The presence of incomplete combustion of coal particles or residual charcoal in the fly ash must be removed before the synthesis of mullite experiments, otherwise the CO2 produced in the later heating process will cause a reduction in the density of burnt products, or even reduce the amount of mullite generation. There are two ways to remove carbon from fly ash: one is to use the density difference for flotation, the porous carbon after flotation can be used as a waste adsorbent, or prepared into a filter material; the other is to remove carbon from the fly ash by heating at a certain temperature, which is commonly used when the carbon content in the fly ash is relatively low and does not have much extraction value. The decarbonization process is usually carried out at a constant temperature of 800 ℃ for 2 h. In this study, we used the Shanghai Yueyue Fly Ash Co. In this study, we used the SX2-4-10 box-type resistance furnace produced by Shanghai Yuejin Medical Equipment Factory to decarbonize the fly ash at a constant temperature of 800℃ for 2 h. The effect was ideal, and the loss on ignition (LOI) of the fly ash was reduced from 2.10% to 1.02%.
( 3) Fine grinding and mixing
Fine grinding is an important and indispensable link in the process of synthesizing mullite, because the synthesis of mullite by sintering method is mainly accomplished by the solid-phase reaction between Al2O3 and SiO2. Through fine grinding can improve the dispersion of raw materials, increase the lattice defects of Al2O3 and SiO2, and improve the reaction activity to accelerate the solid-phase reaction process and reduce the sintering temperature. Normally, the finer the abrasive, the more adequate the mixing, the more thorough the mullite-ization reaction, the more mullite generated, and the better the physical properties, but the degree of fine grinding is related to the milling equipment and economic factors, and it is generally ensured that particles smaller than 10 μm account for more than 80% of the particles.
The ZJM-20 cyclic stirred ball mill produced by Zhengzhou Dongfang Machine Manufacturing Factory was used in this experiment, and the ball:ash:water =5:1:1 ratio was used to grind the fly ash for 5 h, which could meet the above requirements. Considering that the content of mullite in fly ash is already high, the grinding medium used is corundum (Al2O3) small ball, the diameter of the ball is 2-3 mm. industrial alumina is imported from Australia by Jiaozuo Wanfang Aluminum Company, and the purity of Al2O3 is above 99%, and the same grinding conditions are adopted for industrial alumina. The milled materials were dosed according to 60% and 70% Al2O3 content, and then mixed and ground for 1 h to achieve full mixing. This process can also be done by dosing fly ash with industrial alumina and then mixing and grinding. Because fly ash is a ridged material, so we did not add any dispersant in the grinding process, when the viscosity of the finely ground material increases, a small amount of water can be added to dilute to improve the grinding effect.
( 4) Calcium removal
Treatment of fly ash with hydrochloric acid can remove some of the impurities in the fly ash, such as iron, calcium, magnesium, etc., because hydrochloric acid can react with the active CaO (or CaCO3, MgCO3, etc.) and Fe2O3 (or hematite, rhodochrosite, limonite, etc.) in the fly ash to form soluble salts, the reaction formula is as follows:
CaO ( active) + 2HCl = CaCl2+ H2O
CaCO3+ 2HCl = CaCl2+ CO2+ H2O
MgCO3+ 2HCl = CaCl2+ CO2+ H2O
Fe2O3+ 6HCl = 2 FeCl3+ 3H2O
FeCO3+ 2HCl = FeCl2+ CO2+ H2O
The main purpose of our pickling is to remove calcium, and the concentration of hydrochloric acid used is 20%. Generally speaking, with the increase of hydrochloric acid concentration, the effect of iron removal will be better and better, but when the concentration of hydrochloric acid increased to 20%, the relationship between the concentration of hydrochloric acid and the effect of iron removal is not very obvious, the reason is that 20. 2% is the evaporation of hydrochloric acid and water **** point, hydrochloric acid in the process of heating, acid leaching will quickly escape, affecting the effect of acid leaching. Acid leaching, if the temperature can be increased to increase the activity of reactants, speed up the reaction rate, is conducive to improving the leaching effect. In general, a good leaching effect can be obtained by reacting at 80-100℃ for 3 h. Similar effect can be achieved by long time leaching (2-3 d) at room temperature (K.K. Li et al., 2001).
For the finely ground fly ash, 20% hydrochloric acid was used, and the slurry was placed in a plastic bucket at room temperature for 3 d according to the solid:liquid = 1:3, and then rinsed with tap water to pH = 7. During the experiment, we found that the water poured out after 3 d of resting showed a light green color (mainly iron leachate), and the slurry showed a white turbidity that was difficult to precipitate (mainly calcium leachate), and it took about 2 to 3 d to replace the water. It takes about 2-3 d to replace the water once a day after 25 d. At this time, the leachate is basically removed, and it takes about 30 d for the pH value of the slurry to reach neutrality. Because of the static water rinsing instead of flowing water rinsing, a lot of time is consumed. The increased fineness of the particles also slows down the sedimentation process. This step is usually accomplished by filter presses in actual production. The cleaned material is put into the DHG-9000A type electric constant temperature blast drying oven produced by Shanghai Boxun Medical Equipment Factory, dried at 105℃ for 2 h, and then naturally cooled to room temperature, in order to prepare the material for pressing and molding.
( 5) Molding
Considering that the sintered specimen is a brittle material and the uniaxial compressive strength test should be carried out, the material is molded according to 1 < L/D ratio <2. In order to meet the requirements of the specimen molding, we specially machined a steel mold with an inner diameter of 19 mm at the upper end and 19.5 mm at the lower end (to facilitate the removal of the molded specimen), a length of 600 mm, and a base with an outer diameter of 50 mm. The specimens were molded by uniaxial pressurization in a WE-30B hydraulic universal testing machine, and the pressures were selected to be 100, 150, and 200 MPa at three levels. Because fly ash is a ridged material, the molding is semi-dry (containing 5% moisture), so the friction between the specimen and the inner wall of the mold during sampling will easily cause the specimen to produce brittle peeling and brittle fracture phenomenon. In order to avoid the above phenomena, on the one hand, the inner wall of the mold was polished, and on the other hand, 2% of dextran (starch) was added into the material as a binding agent, and the molding effect was quite satisfactory. In order to examine the shrinkage of the samples after sintering, the diameter and height of the molded samples were measured one by one with vernier calipers and recorded.
( 6) Sintering
The molded specimen was firstly put into DHG-9000A electric constant temperature blower drying oven, and dried at 105℃ for 2 h. The dried specimen was put into SX6-12-16 all-fiber fast heating resistance furnace produced by Hunan Xiangtan Zhongshan Instrument Factory, and heated up at a rate of 10℃/min before 1000℃, and at a rate of 5℃/min after 1000℃. After 1000℃, the temperature was increased at the rate of 5℃/min to 1300, 1400 and 1500℃, and then the temperature was kept constant at different temperatures for 2, 3 and 4 h. The specimens were cooled down naturally to the room temperature and then taken out under the laboratory conditions. It took about 24 h to complete each batch of specimens in this experiment, because it usually takes a long time for the resistance furnace to cool naturally in a closed state. The rate of heating during the sintering process must be noted because the furnace heats up quickly at low temperatures and slows down at high temperatures, so the furnace current is constantly adjusted to ensure the experimental temperature of the synthesized mullite. After the natural cooling of the sample, the same vernier caliper method to measure its diameter and height one by one, and make a record.
So far, the use of fly ash sintering synthesis of mullite experimental process is basically completed, and then it is necessary to sintered samples of physical and mechanical properties and mullite content determination.