Technical Paper


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Refractories: Roles and Effects* Ruth Engel, Refractory Consulting Services


* Presented at the 52nd Annual Symposium on Refractories, The Am. Cer. Soc., held in St. Louis, MO, USA, March 30 & 31, 2016 Introduction


Although it may not be explicitly stated, almost all carbon containing refractories have some type of antioxidant added. This material is usually thought of as a “metal addition” although it may not consist of one. Among the most common additions used for this purpose are Al or Si metal, but Mg and different alloys can also be found as are carbides, borides and nitrides (SiC, CaC, AlC, B4C, ZrB2, Si3N4, etc.) singly or in combination with metals. Criteria for selecting what to add is a function of the main refractory constituents and expected field requirements. These additions work not only as antioxidants ensuring carbon retention, but can play a role in the ease of drying of a castable, improve the refractory’s hot strength, decrease its porosity, improve its corrosion resistance or, affect several of these properties at the same time.


The additions can be found in blast furnace tap hole clays, iron trough refractories, alumina-carbon, magnesia-carbon, alumina-magnesia-carbon, etc. refractories. This presentation will review the role of the major metal/ boride/carbide species added to the different refractories, their expected effect on properties and how they manifest themselves.


Background


The refractory wear improvement as a result of having carbon present was explained by Herron et.al. [1], who showed how the carbon in pitch impregnated burned magnesia brick prevented slag from penetrating its pores. Subsequent studies showed that a dense zone formation protected this carbon from oxidation. This led to the development of techniques to encourage an efficient process for its formation which was accomplished by adding metals to the refractory. Although this technology was not implemented until the 1980s the idea was not new. The first mention can be found in a 1935 patent [2] that starts out by saying


“The present invention is related to the last type (coke residue bond), and has for its object the provision of a lasting protection against oxidation


Atmosphere Zone I Zone II Zone III Zone IV Zone V 18 CO2, CO, N2 Al MgO + MA


MA = MgO Al2O3 Spinel


> 1807o CO, N2 C


MgO + AlN + C < 1,807o


C MgO + MA + C > 1327o N2 MgO + AlN + C > 1107o Neutral Atmosphere Unchanged Material


MgO + Mg + C < 1107o


C C


MgO + Al4C3 + C MgO + Al + C


ENGINEER THE REFRACTORIES


M gO + Mg + C < 1327o


C C


MgO + Mg3N2 + C MgO + Mg + C MgO + Mg + C Table 1: Reactions of Al, Mg, or Si additions to MgO brick with some carbon as a function of temperature and atmosphere [5] May 2016 Issue MgO + C MgO


of such a bond, when exposed to oxygen and oxidizing conditions at the various temperatures...”


and goes on to state


“This protection is accomplished, in the course of manufacture, by uniformly incorporating in the mixture one or more inclusions of oxidizable metal, metals, alloys, metalloids or carbides of same. These metallic inclusions, if properly selected, are capable of having their oxides form a molten and viscous glass.”


In 1983 a patent [3] was issued for the addition of metallic Mg to a chemically, pitch, bonded brick. The idea was to increase the amount of Mg gas generated which would lead to a thick dense zone. The refractory could be made out of magnesia, dolomite or alumina. This concept was applied in the laboratory development of brick formulations which showed promise. Then, a commercial product was successfully manufactured: a pitch bonded magnesia brick containing Mg metal.


Other refractory types containing carbon were subsequently developed and the advantage of an addition to protect their carbon from oxidation was equally applicable. Through these studies the role of metals was expanded and the use of other additions explored.


Initial work was carried out on tar/pitch bonded magnesia refractories which were mainly used in BOFs. This steelmaking process was started in the late 1950s, but advancement in refractory performance was needed for it to succeed. High strength burned MgO brick were introduced in the late 1960s and by the 1970s BOFs were being lined with pitch bonded magnesia brick, and pitch impregnated burned MgO brick. Subsequently the amount of carbon, in the form of graphite contained in the brick, was greatly increased and had to be protected from oxidation. This gave rise to the current technology of magnesia carbon refractories. Much work has been carried out studying the reactions that take place in magnesia –carbon (MgO-C) brick, the effect of the different additions and, how they


Mg Si MgO + M2S


M2S = 2MgO SiO2 forsterite


> 1527o


MgO +M2S + C < 1527o


C C


MgO + SiC + C > 1427o


MgO + SiC + C < 1427o


C C


MgO + Si3N4 + C > 1537o


MgO + M2S +Mg + C < 1537o


C C


MgO + SiC + C MgO + Si + C


Anti-oxidant Additions to


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