Temporary preservative binders for lime and lime-periclase refractories

Abstract

It is established that in order to increase the atmospheric resistance of compacts from lime and lime-periclase clinkers the temporary binder should contain high-molecular-weight resin and fatty acids, which form water-insoluble salts on the CaO surface, and surface-active substances in addition to a hydrophobic plasticizer. The reqifisite amounts of the acid-containing componcnt and the binder itself are calculated.The problem of temporary binding agents for the formation and preservation of lime and lime-periclase refractories has been discussed in the literature 2 more than once [1 -4]. Its complexity consists in that the binders should not only ensure a satisfactory formation process, but also protect the powders, the forming mixtures, and pressed and fared articles from atmospheric hydration. The mechanism of hydration, or, more exactly, hydroxylation, of lime consists in that at fwst a layer of physically adsorbed moisture forms very rapidly on the surface of CaO. The moisture transforms gradually into chemically bound water and forms portlandite Ca(OH)2 with CaO. The specific volume of portlandite is 1.5 times greater than that of CaO. The crystallization pressure of portlandite causes cracking and fracture of articles in storage or in the course of firing at 900 -1000 K. Consequently, the use of high-strength binders such as the basic processing binder for refractories (BPBR) [3,5,6] does not guarantee that the articles will be intact after molding and firing, because the physically bound water remains in the article and binder components containing [OH-] groups react with CaO and form portlandite [4,7].These facts imply that the development of temporary binders for refractories containing CaO should be based on the following considerations:-the temporary binder should contain a hydrophobic plasticizing component, which should protect CaO from contact with atmospheric moisture, have sufficient strength and plasticity, and be ecologically safe; on the one hand be insoluble in water and on the other hand suit the plasticizer; -at a minimum content of the temporary binder it should mix well with CaO particles and wet them properly without foaming, i.e., it should contain a surface active substance in an amount of 0.1 -1.0%; -the temperature of the beginning of intense decomposition or evaporation of the binder should be at least 700 K, when the equilibrium CaO + H20 ~:~ Ca(OH)2 is already strongly shifted to the left; -the processes of removal of physically bound water and the water formed in the interaction of CaO and Ca(OH)2 with the binder components should be completely finished during the preparation, mixing, and soaking; this determines the duration of mixing or soaking at a temperature of at least 380 -400 K. The upper temperature is limited by the beginning of intense evaporation and decomposition of the binder constituents.Even impure higher alkanes (from ceresin to petrolatum) can be a suitable least toxic and most available hydrophobic plasticizer having sufficient strength and thermal stability. ...

New Unconventional Additives in Concrete Technology for Expansion its Functionality

“…Silicon carbide is widely used in the production of refractory products [13][14][15][16], the content of which ranges from 20% to 99% in the composition, it is the main carrier of refractory and specific properties of the products and determine their applicability for the provided service conditions, so the content of silicon carbide in refractories -is one of their main indicators.…”

Features of temporary binder-preservative removal from moulded lime refractories during firing

“…As already repeatedly noted [1,2], one of the main reason for scrap of refractories based on CaO is hydration. The uniform reaction of CaO hydration is shifted to the right up to 560°C, and this reaction proceeds actively in the warm-up stage in any furnace unit, apart from a vacuum furnace.…”

Effect of Addition of Al₂O₃-SiO₂ Gel Powder on Strength of Al₂O₃-MgO Unfired Bricks

“…Besides, high thermal conductivity tends to decrease the temperature of molten steel. Hence, researchers [1][2] give attention to the development of carbon-free Al 2 O 3 -MgO unfired bricks by adopting additives such as sodium silicate, Al 2 (SO 4 ) 3 , phosphate, cement as binders. However, the additives bring some disadvantages such as low melting substances and pollution on molten steel, which limit its usage.…”