Failure Mechanism of Alumino Silicate Refractories in Anode Baking Furnace

Abstract

Anode baking furnaces is an integral part of aluminium industry. In this furnace, anodes are baked to use in electrolytic reduction. Depending on the design either open top or closed top furnaces are employ ed. Till date, alumino-silicate refractories are the choicest material due to low cost, low creep under compression, high resistance to thermal cycle, carbon monoxide disintegration and alkali attack. This paper deals with failure mechanism of such type of refractories. Used samples from a Reid Hammer bake oven have been studied to find out the reasons of failure. The failure has occurred due to thermomechanical, pyro-chemical stresses and alkali attack in reducing condition. Out of different reasons of failure, alkali attack in reducing condition is found to be the prime cause for failure.[Keywords : Alumino-silicate, Alkali attack, Aluminium industry] IntroductionAnodes are required for pot lining furnaces where alumina is reduced to metallic aluminium. These anodes are made of petroleum coke along with spent anode bonded with pitch which is formulated at high pressure for baking in furnace before use. The anodes are baked at a temperature range of 1150 o -1200 o C. In the electrolytic pot cryolite (NaAlF 6 ) is being added as electrolyte. During the process the anode absorbs alkali oxide and comes in the spent anode. 1 Alumino-silicate refractories with alumina content ranging from 45-50% are generally used in these types of furnace. For getting higher life, the proper selection of refractory is of paramount importance to refractory manufacturers and users. 2 In the present study, attempts have been made to understand the failure mechanism of refractory used in a Reid Hammer bake oven with future action to augment the furnace life.A number of literatures are available on the study of failure mechanism of alumino-silicate refractories in anode baking furnace. However, till now, significant studies have not been carried out to assess the failure due to alkali attack on the refractory. 3 The present study is attempted to understand the mechanism of refractory attack by alkali.

Alteration analysis of refractories bricks for industrial horizontal anodes baking furnace

“…MPa for unused brick to 32 MPa for used bricks. This observation have been done by Mishra et al [16]; when they charactherized alumina-silica unused and used bricks after 50 baking cycles at 1130 -1160 °C and observed that the apparent porosity of brick increased from 16.9% for unused brick to 19.1% for used brick on both faces (anode side and hot side). The water absorption is related to the porosity of the brick and the density.…”

“…In the same line, authors Trond Brandvik et al [15] observed after 63 heating cycles a densification of the bricks in reducing conditions and they observed a more densification in the anode side. Contrary to this observation authors B. Mishra et al [16] have charactherized unused and used silicaalumina bricks (44.1wt.% Al2O3 and 49.4wt.% SiO2) after 50 baking cycles at 1130 -1160 °C and observed that the bulk density of bricks decreased from 2.34 g/cm 3 for unused bricks to 2.26 g/cm 3 for used bricks on both faces anode side and hot side. From this observation, it could be deduced that the increase or decrease in density depends on the temperature of baking cycle, the composition of bricks, the flue gas and the method used for characterize the bulk density…”

“…The glassy phase and solid particles formed in crack and pores increase the density of brick. The decrease of the bulk density of brick observed by B. Mishra et al [16] should be explained by the evaporation of SiO (g) and solid particles from bricks using mercury intrusion porosimeter and this evaporation was not totally compensated by the precipitation of F, Na, K, Ca, Mg and C inside the pore formed.…”

“…This variation of colour was due to the diffusion of anode components like petroleum coke in the brick. B. Mishra et al [16] observed that in the reducing gas, silica rich mullite 3Al2O3.2SiO2 (secondary mullite) are completely dissociated into glassy phase and corundum, while some alumina rich mullite 2 Al2O3.SiO2 (primary mullite) remains unaltered. After reaction of silica with NaF, SiO(g) evaporate from brick and C, F, Na, K and Ca from anodes readily precipitate in the pores of bricks sometimes in larger amount.…”

Innovative Methodology for Comprehensive Utilization of Spent MgO-Cr2O3 Bricks: Copper Flotation

“…Improving refractory life and developing new chrome-free refractories have nowadays received much attention as potential methods to reduce the amount of spent MgO-Cr 2 O 3 bricks and to avoid chromium pollution, respectively. , Although the lifetime of the refractory can be improved by optimizing the lining concept, engineering the slag, and changing the process conditions, , and thus substantially reducing the amount of the waste, it is unable to fundamentally solve the problems. A variety of chrome-free refractories, such as magnesia-alumina, magnesia-carbon, and alumino-silicate materials, − have been successfully developed and applied to iron metallurgy and refractory linings for rotary kilns in the cement industry, but in many secondary metallurgy processes in ferrous metallurgy as well as in the metallurgy of copper and lead, MgO-Cr 2 O 3 bricks are still used on a large scale because of their outstanding performances compared with these chrome-free alternatives. , A more sustainable method to solve the problem is to recycle spent refractories. The typical recycling process is first to sort the refractory to avoid the mixture of different types, then to crush and grind them for liberating metals, slag, and other impurities, and to magnetically separate the Fe-rich chromite from the MgO-rich fraction .…”