Obabkova, N; Gorkunovb, V; Munterb, R; Beketov, A
Results of pilot plant and full scale studies on the elaboration of high temperature protective coatings for graphite in the process of the reductive melting of niobium pentoxide and NbAI alloys production are presented. Graphite has found extensive application in high temperature processes due to its unique thermophysical properties. However, in these conditions it reacts very easily with air oxygen, liquid metals, oxides, and fluorides. As a result, porous graphite gets impregnated with liquid metal; besides, carbonization of melt and formation of a layer of metal carbides on graphite surface takes place. The layer of metal carbides has a low adhesion with graphite and a major part of them is transferred to the melt. The graphite facing of a furnace is quickly destructed. To avoid these harmful processes, special plasma sprayed protective coatings are used. In this work various coatings (Al2O3, Al2O3CaO, ZrO2, NbC, Al2O3 + Nb, ZrO2 + Nb, NbC + Nb, Nb) were tested in pilot plant and industrial conditions. It was established that the best durability of the protective coating on graphite can be achieved in the case of its three- layer composition: Nb (0.07 mm) + NbC (0.4 mm) + Nb (0.3 mm). After industrial melting of 500-2000 kg ingots about 80% of this coating on the belt of the shaft furnace had preserved. Plasma sprayed protective coatings on the graphite facing extended its operation time, reduced the carbon content in the metal ingots to 0.02% (mass), and slag carbon content to 0.05-0.15% (mass). (English) [ABSTRACT FROM AUTHOR] Tänu oma suurepãrastele termofüüsikalistete omadustele on graflit leidnud laialdast kasutamist kõrgtemperatuursetes protsessides. Grafiidil on suur temperatuuripüsivus ja mehaaniline tugevus, kuid kõrgetel temperatuuridel (>1000°C) reageerib ta aktiivselt õhuhapniku, sulametallide, oksiidide fling fiuoriididega. Tulemusena seguneb poorne grafiit sulametalliga ja teisalt toimub metallisulami karboniseerumine fling metallikarbiidide kihi moodustumine graflidi pinnal. Karbiidide kihil on graflidiga nõrk adhesioon ja suur osa nendest kandub tile sulametalli ning šlaki massi. Sulatusahju grafiitvooderdis laguneb ruttu. Selle vältimiseks kasutatakse plasmameetodil grafiidi pinnate kantud kaitsekihte. Artiklis on toodud erinevate kaitsekihtide (Al2O3, Al2O3CaO, ZrO2, NbC, Al2O3 + Nb, ZrO2 + Nb, NbC + Nb, Nb) katsetamise tulemused piloot- ja tööstuslikes tingimustes nioobiumpentoksiidi taandamisel NbAI-sulamiks. Kaitsekihi suurim püsivus graflidi pinnal on saavutatud kolmekihilise struktuuri Nb (0,07 mm) + NbC (0,4 mm) + Nb (0,3 mm) puhul. 500-2000 kg NbAI-sulami tootmisel tööstuslikus šahtahjus on pärast sulatus-protsessi ahju "vöö" kohal säilinud -80% kaitsekihist. Plasmameetodit grafiidile kantud kaitsekiht võimaldab vähendada süsiniku sisaldust metallisulamis kuni 0,02%-ni (mass) ja šlakis 0,05-0,15%-ni (mass). (Estonian) [ABSTRACT FROM AUTHOR] Copyright of Proceedings of the Estonian Academy of Sciences is the property of Teaduste Akadeemia Kirjastus and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts)