Expansion/Shrinkage of Hearth Mixes During Firing
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Vol. 61, No. 2, July, 2020
EXPANSION/SHRINKAGE OF HEARTH MIXES DURING FIRING G. D. Apal’kova1,2 Translated from Novye Ogneupory, No. 3, pp. 61 – 64, March, 2020.
Original article submitted October 13, 2019. The urgency of the problem of shrinkage phenomena during hearth firing, contributing to joint opening and crack formation, is demonstrated. Contemporary methods and means of studying expansion/shrinkage of hearth mixes during firing are analyzed. Expansion/shrinkage of hearth mixes, differing in the ratio of the carbon component (anthracite) and artificial graphite within their composition, is investigated. It is established that shrinkage phenomena decrease in a series from anthracite to graphite-anthracite. Studies show that the difference in the scale of linear changes during structural shrinkage is quite significant, i.e., about 20%. It is noted that minimization of shrinkage phenomena contributing to opening of joints and crack formation is one of the factors for reducing the risk of leakage at the bottom of a cell. In this connection it is advisable to optimize the shrinkage rate of hearth mixes during firing in the stage of structural adjustment taking account of operating conditions. Keywords: carbon lining materials, hearth mixes, joints between blocks, hearth mix expansion/shrinkage.
In spite of many years of continuous improvement of cathode lining material indices, breakdown of an electrolyzer, including due to nonconformity of hearth mix quality for joints between blocks under powerful electrolyzer operating conditions, gives rise to concern [1]. A hearth mix is the weakest component of an electrolyzer carbon hearth. This is connected with the fact that the hearth mix experiences during firing complex phase transformations, i.e., from an original plastic heterogeneous pitch and coke composition to a coked monolith formed during firing in an electrolyzer. These transformations are accompanied by liberation of volatile components of the mix pitch component and structural shrinkage processes. With respect to physicochemical properties and also the method and means of monitoring hearth and side blocks in the direction of both the user and also the producer there is no marked difference of opinion concerning hearth mix quality. Plastic hearth mixes are transferred into a non-plastic phase in the range from 400 to 600°C (from 200 – 300°C for binder resins). Shrinkage phenomena are typical for binder and give rise to coking; in this case the filler experiences thermal expansion without structural changes. Structural shrinkage of a hearth mix in the range between the tempera1 2
ture with which the mix becomes non-plastic and the working temperature (950°C) is an important factor specifying operating life. High shrinkage during firing is undesirable since it may lead to occurrence of electrolyzer fettling [2]. One of the main requirements laid down for a hearth mix is preparation during firing of a joint between blocks of high density with low shrinkage [1 – 3]. Questions of reaction of binder and filler
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