Conversion of byproduct carbon obtained from spent pot liner treatment plant of aluminum industries to blast furnace tap hole mass(18)

The paper describes a method for converting carbon powder obtained as byproduct while decontaminating spent pot liner by acid treatment route, into blast furnace tap hole mass. Producing an useful industrial product from waste helps in paying back the cost incurred during decontamination process of spent pot liner and thus improves the overall economics of the process. Blast furnace tap hole mass is a prepared muddy material used to close the tap hole of iron making blast furnace. At iron melting temperature the material carbonizes and becomes hard in order to hold the metallostatic pressure inside blast furnace hearth. Since iron gets oxidized at high temperature this tap hole compound is prepared predominantly from carbonaceous material. Further these carbonaceous materials used needs to be semicrystalline in nature in order to withstand high temperature of the furnace. With rising cost of such carbonaceous material used in tap hole compound, it will be advantageous if a source of cheap carbon material matching the properties of carbon material already in use in the tap hole recipe , could be found in order to bring down cost of production of commercial taphole recipe. The aim of this research was to find such a carbonaceous material from an industrial waste which is difficult to dispose because of its toxic contaminants. Such a material chosen for this work is called SPENT POT LINER of aluminum smelter plant. At present majority of the aluminum smelter plants round the world use Hall-Heraoult's electrolysis process for extracting aluminum metal from molten cryolite. The electrolysis cell used for this purpose constitute large carbon blocks as cathode laid at the bottom and side wall of the cell. These carbon electrodes are known as pot liners and basically made of anthracite, graphite and binder/electrode pitch. Various manufactures uses 20-30% (1) graphite in their recipe in order to meet electrical properties needed for the pot liner. While graphite is a superior electrical conductor than anthracite (which has similar structure to graphite), 100% graphite is never used in the manufacture of pot liners as it is soft and can not withstand turbulent molten cryolite contained in the cell. Anthracite on the other hand provides required mechanical property to the finished pot liner. The prefabricated pot liner produces carbon matrix with short range order. Thus prepared pot liners ultimately contain both crystalline (graphite) and amorphous carbon. During service molten cryolite slowly gets reduced and the sodium fluoride crystal deposits within the fine crevices of pot liner creating defect spots . As time passes, these crystals grow and exert pressure within these crevices resulting in the propagation of crack. As a consequence, with time the pot liner looses its electrical property and ultimately being discarded. These rejected waste pot liners are called "spent pot liners" (in short SPL). Spent pot liners are not only contaminated by fluoride but also by other toxic elements such as cyanides (formed at high temperature reaction with atmospheric nitrogen), alkalis and aluminum. Table-1 below shows typical range of the contaminants in such discarded pot liners along with concentration of these toxic elements in SPL carbon powder after decontamination with oxidizing acids. In practice suitability of a specific component in a commercial recipe is tested by evaluating some gross property of the modified recipe against the production recipe of the compound. For example, in development of foundry chemicals (like mould coating, tundish cover , hot tops etc) which basically a combination of various components in a formulation , various substitutes are being tried with above procedure of gross evaluation of certain properties of the compound in order to determine its suitability. Similarly in present case of developing suitable substitute of carbonaceous material in commercial tap hole compound recipe, following properties are important in order to determine its suitability. Apparent porosity. This property determines the ease with which gas generated in the tap hole compound during carbonization can escape easily without breaking or decreasing strength of the carbonized tap hole mass. This value is generally maintained in the range 25-35%. a) Bulk density. This is maintained in the range 1.3-1.6 gm/cc in all commercial recipe in order to match with standard pushing length required by the equipment to fill the tap hole.