Experiments were carried out on fly ash (series A) after removing carbon powder and fly ash (series B) after being treated with 20% hydrochloric acid. According to the original Al2O3 content (A50 and B50, respectively) and adding industrial alumina, the Al2O3 content in the mixture reached 60% (A60 and B60, respectively) and 70% (A70 and B70, respectively). According to the orthogonal design scheme of three factors and three levels, each component must be tested for 9 times, so 54 tests were carried out on two series of samples with three different Al2O3 contents. The experimental flow adopted is shown in Figure 5. 6.
Figure 5. Experimental process of sintering mullite with high alumina fly ash.
(1) Iron removal
There are two kinds of iron oxides in fly ash, Fe2O3 and Fe3O4 (generally represented by Fe2O3). The latter is magnetic, and iron can usually be removed by a magnetic separator at 15000 ~20000 Gs (magnetic flux density) to reduce the impurity content of iron. Because the research is in the laboratory environment, we only use a semicircular magnet with the length of 10 cm to remove iron manually, and the effect of iron removal is not ideal. Iron oxide in fly ash is only from 1. 95% to 1. 84%. If industrial production is considered, the usual practice is to use a magnetic separator to remove iron.
(2) carbon removal
Fly ash contains unburned coal particles or carbon residue, which must be removed before the experiment of synthesizing mullite. Otherwise, CO2 produced in the later heating process will reduce the density of sintered products, and even reduce the amount of mullite. There are two ways to remove carbon from fly ash: one is to use density difference for flotation, and the porous carbon after flotation can be used as adsorbent for waste or made into filter material; The other is to remove carbon from fly ash by heating it at a certain temperature. This method is often used when the carbon content of fly ash is low and there is not much extraction value. The decarbonization process can generally be carried out at 800℃ for 2 h. In this study, SX2-4- 10 box resistance furnace produced by Shanghai Yuejin Medical Instrument Factory was used to decarbonize fly ash at 800℃ for 2 h, and the effect was satisfactory. The LOI of fly ash is from 2. 10% to 1. 02%.
(3) Fine grinding and mixed grinding
Fine grinding is an indispensable link in the process of synthesizing mullite, because the synthesis of mullite by sintering method is mainly completed by solid-state reaction between Al2O3 and SiO2. Fine grinding can improve the dispersibility of raw materials, increase the lattice defects of Al2O3 and SiO2, improve the reaction activity, accelerate the solid-state reaction process and reduce the sintering temperature. Usually, the finer the abrasive, the more thorough the mixing, the more complete the mullite reaction, and the more mullite produced, the better the physical properties. However, the degree of fine grinding is related to grinding equipment and economic factors, and generally more than 80% of particles smaller than 10 micron can be guaranteed.
In this experiment, ZJM-20 periodic stirring ball mill produced by Zhengzhou Dongfang Machinery Factory was used, and the ratio of ball: ash: water was 5: 1: 1, and the stirring time was 5 h, which could completely meet the above requirements. Considering the high mullite content in fly ash, corundum (Al2O3) balls with a diameter of 2 ~ 3 mm are used as grinding media. Industrial alumina is imported from Australia by Jiaozuo Wanfang Aluminum Company. The purity of Al2O3 is above 99%, and the same grinding conditions are adopted for industrial alumina. Mix the ground materials according to 60% and 70% Al2O3 content, and then mix and grind them for 65438+/-0h to make them fully mixed. In this process, fly ash and industrial alumina can also be mixed first, then mixed and ground. Because fly ash is ridge material, we don't add any dispersant in the grinding process. When the viscosity of the ground material increases, a small amount of water can be added to dilute it to improve the grinding effect.
(4) calcium removal
Treating fly ash with hydrochloric acid can remove some impurities in fly ash, such as iron, calcium and magnesium. Because hydrochloric acid can react with active CaO (or CaCO3, MgCO3, etc.). ) and Fe2O3 (or hematite, siderite, limonite, etc. ) to form soluble salt, and the reaction formula is as follows:
Calcium oxide (active)+hydrochloric acid = calcium chloride+H2O.
CaCO3+ 2HCl = CaCl2+ CO2+ H2O
Magnesium carbonate+hydrochloric acid = calcium chloride+carbon dioxide+H2O
Fe2O3+ 6HCl = 2 FeCl3+ 3H2O
FeCO3+ 2HCl = FeCl2+ CO2+ H2O
The purpose of this pickling is mainly to remove calcium, and the concentration of hydrochloric acid used is 20%. Generally speaking, with the increase of hydrochloric acid concentration, the iron removal effect will be better and better. However, when the hydrochloric acid concentration increases to 20%, the relationship between hydrochloric acid concentration and iron removal effect is not obvious. The reason is 20. 2% is the evaporation point of hydrochloric acid and water, and hydrochloric acid will quickly escape during heating acid leaching, which will affect the acid leaching effect. In acid leaching, if the temperature is increased, the activity of reactants can be improved, the reaction speed can be accelerated, and the leaching effect can be improved. Generally speaking, a good leaching effect can be obtained by reacting at 80 ~ 100℃ for 3 h, and a similar effect can be obtained by soaking for 2 ~ 3 d at room temperature (Li et al., 200 1).
For the pulverized fly ash, 20% hydrochloric acid was used, and the slurry was put into a plastic bucket at room temperature for 3 days according to the ratio of solid: liquid = 1: 3, and then washed with tap water to pH =7. During the experiment, we found that the clear water poured out after standing for 3 days was light green (mainly iron extract), and the slurry was white and turbid, which was not easy to precipitate (mainly calcium extract). Change the clean water every 2 ~ 3 days, and once a day after 25 days. At this time, the extraction solution is basically removed, and it takes about 30 days for the pH value of the slurry to reach neutrality. Because it is still washed with water instead of running water, it takes more time. The increase of particle fineness also delayed the precipitation process. This step is generally completed by filter pressing in the actual production process. The cleaned material was put into DHG-9000A electric heating constant temperature blast drying oven produced by Shanghai Boxun Industrial Co., Ltd., dried at 65438 005℃ for 2 h, and then naturally cooled to room temperature, so as to prepare the molding material.
(5) forming
Considering that the sintered samples are brittle materials, it is necessary to carry out uniaxial compressive strength experiments and classify the materials according to 1
(6) sintering
Firstly, the molded samples were put into DHG-9000A electric heating constant temperature blast drying oven, and dried at 65438 005℃ for 2 h ... The dried samples were put into SX6- 12- 16 all-fiber rapid heating resistance furnace produced by Zhongshan Instrument Factory in Xiangtan, Hunan Province, and were heated at 1000℃ respectively. Naturally cooling to room temperature under laboratory conditions, and taking out. It takes about 24 h to complete a batch of samples in this experiment, because it usually takes a long time for the resistance furnace to cool naturally in a closed state. Pay attention to the heating rate when sintering, because the heating rate of resistance furnace is faster at low temperature and slower at high temperature, so it is necessary to constantly adjust the current of resistance furnace to ensure the experimental temperature of mullite synthesis. After natural cooling, measure the diameter and height of the samples one by one with vernier caliper method and make records.
So far, the experimental process of synthesizing mullite from sintered fly ash has been basically completed, and then the physical and mechanical properties and mullite content of sintered samples will be determined.