Selection of aggregates, binders and other admixtures for Cold ramming paste
Cold ramming paste is generally used to make the lining of thermal equipment. It can be directly baked without firing process. Compared with common refractory carbon bricks, cold rammed paste has the advantages of simple preparation process, low cost, low energy consumption, easy mechanized construction, excellent product performance and long service life. In recent years, the application proportion of Cold ramming paste in furnace lining manufacturing in submerged arc furnace, metallurgical furnace, yellow phosphorus furnace and other industries has been increasing, which has brought huge financial benefits. In aluminum reduction cell, cold ramming paste can be divided into bottom paste and side paste according to different parts. The side paste usually requires low ash content, good insulation, resistance to molten salt and aluminum corrosion, otherwise the gap filled by the cathode paste is easy to become the weak point of the cathode furnace lining, and the furnace lining will be eroded and damaged from then on, resulting in the damage of the whole electrolytic cell furnace lining.
Cold ramming paste is usually used to fill the gap between cathode carbon block and carbon block, surrounding and conductive steel rod. It is also called tamping paste. It is generally used together with cathode carbon block. Usually, a certain proportion of materials are weighed after crushing and screening, added to preheating to a certain temperature, and kneaded in a kneading pot for 20 ~ 30 minutes. According to the production requirements, appropriate additive materials are selected for wet mixing, and finally discharged for cooling. The preparation materials of Cold ramming paste are usually aggregate, modified asphalt binder and additives, and the type and proportion of constituent materials are adjusted according to the use position and production environment.
1. Selection of aggregate
The selection of cold rammed aggregate is very important. Firstly, through the research on the thermophysical behavior and erosion process of Cold ramming furnace lining materials in the process of aluminum electrolysis, and according to the production requirements of cathode lining side of aluminum reduction cell, select appropriate aggregate, determine the process ratio of Cold ramming materials, and analyze the standard performance. In the process of aluminum electrolysis, the metal sodium in the electrolyte can continuously diffuse to the cathode through the carbon lattice and pores to generate the embedded compound cxna, which causes the expansion and rupture of the carbon block, and finally leads to the serious damage of the cathode lining, which is called Rapoport effect. Therefore, the cathode raw materials should be reasonably selected to reduce the precipitation of metal sodium. It is found that the sodium corrosion resistance of graphite is much greater than that of electric calcined coal at 960 ℃, in which the sodium expansion rate of graphitized carbon block is 0.05% ~ 0.15%, and that of amorphous carbon block is 0.6% ~ 1.5%. Therefore, electric calcined coal and artificial graphite are mainly selected as aggregates for preparing cold rammed paste.
2. Selection of binder
Binder is usually used to bond aggregate, bond cold rammed paste into a whole, and fill voids during thermal decomposition. The binder commonly used in carbon materials for aluminum is mainly coal tar pitch, with a density of 1.25 ~ 1.35g/cm3 at room temperature, in black solid form. When heated to a certain temperature, it becomes softened, the viscosity decreases with the increase of temperature, and the softening point is about 75 ~ 120 ℃. Coal tar is usually added to reduce the softening point of the binder and improve the fluidity, so that the coal tar is easy to be immersed in the pores of the roasted semi-finished products. In addition, in order to meet the requirements of environmental protection, synthetic adhesives such as epoxy resin, furan resin and phenolic resin are widely used. The adhesives that are not easy to graphitize can polymerize with these resins into the cross-linked carbon carbon network
3. Add material selection
According to different use requirements, it is usually necessary to flexibly select the types of added materials. Through the study of cathode lining of aluminum reduction cell, it is found that the performance of cathode lining has a great influence on thermal expansion rate, conductivity and corrosion resistance. In addition, there are certain requirements for flexural strength and porosity. The difference is that the bottom of the cathode requires good conductivity and the side requires good insulation, so as to form a directional current from the side to the bottom, so as to reduce the generation of horizontal eddy current. Therefore, different additives should be used for bottom Cold ramming paste and side cold ramming paste to meet the production requirements. For the side carbon lining, the continuous erosion of high-temperature electrolyte will continuously lift the side block, resulting in the damage of furnace lining. At present, TiB2 is usually used as the bottom Cold ramming paste and SiC is usually used as the side cold ramming paste in the aluminum reduction cell.