Grain size composition and porous structure of carbon hearth block materials for aluminum electrolyzers

Abstract

Results are provided for the design of a carbon refractory structure with a pore size of less than 25 mm. It is established that if in planning the charge for carbon blocks the coarse fraction (10 -5 mm) is excluded, converting the medium and fine fractions to 100%, then the sizes of fractions become close to those calculated by equations used traditionally describing optimum particle packing. In a three-fraction charge carbon material compositions composites are obtained within which the predominant pore size is less than 25 or even 10 mm.The material considered is a continuation of the cycle of articles for evaluating the characteristics of carbon hearth blocks for aluminum electrolyzers [1, 2] it has been shown that the size of permeable pores is a critical property of the structure of blocks from the point of view their service life. This conclusion was drawn on the basis of simulating movement of electrolyte through permeable pores of hearth blocks and was confirmed by experimental observations.In hearth blocks with a pore size of 25 ± 5 mm the rate of electrolyte movement though permeable pores is determined by the rate of movement of the temperature front of the electrolyte liquidus. After 96 -100 h electrolyte reaction with aluminosilicate refractory commences, there is formation "lenses" of reaction products, raising of blocks, and breaking.In hearth blocks with pore diameters less than the critical value the rate of electrolyte filtration is less than the rate of the liquidus movement front. Electrolyte crystallizes in pores of the carbon refractory; reaction of electrolyte and aluminosilicate refractory occurs over many months after the start of operation.The composition and properties considered in this publication are for carbon hearth blocks that were used performing an industrial experiment. The properties of hearth block materials are provided in Table 1; for comparison specifications are also shown for hearth blocks in accordance with TU 1913-109-021-2003 "Carbon hearth blocks for aluminum electrolyzers." ANALYSIS OF THE GRAIN SIZE COMPOSITION OF CHARGE MIXES FOR CARBON HEARTH BLOCKSIn carbon refractory technology (Fig. 1) the producers implement different preparation schemes for molding mixes. Here:-as a rule a screen is used with two to three sieving grids; it is possible for anisotropically drawn out particles to pass through the screen (that is typical for anthracite) and screening accuracy does not exceed 80%;-the fine fraction is a product of grinding in a ball mill of a fraction finer than 0.5 mm in order to obtain not less than 80% fraction finer that 0.075 mm;