Erosion En Los Refractarios

Paper presented at 58th Electric Furnace Conference, Orlando, Florida, November 13th, 2000. In 58th Electric Furnace Conference Proceedings: 361–78. Warrendale, PA.: Iron & Steel Society. 2000.

An Anatomy of Furnace Refractory Erosion: Evidence from a Pilot-Scale Facility
Paul den Hoed
MINTEK

Private Bag X3015 2125 Randburg South Africa Telephone +27-11-709-4735 Telefax +27-11-709-4564denhoed@mintek.co.za Keywords: DC-arc, furnace, reverberatory, blast, copper, lead, slag, nickel laterite, chrome, magnesia, alumina, spinel, silicon carbide, graphite, refractory, corrosion, erosion, phase chemistry INTRODUCTION Plasma-arc technology—and DC transferred-arc technology, in particular—has its adherents.1, 2 MINTEK is one of them. Since the late 1970s, it has sought to apply thistechnology to the recovery of valuable metals from certain ores and from furnace slags and dusts. Commercial furnaces are now in place for the production of ferrochromium from chromite and for the smelting of ilmenite. Beginning with small-scale DC-arc furnaces, it has demonstrated the application in four other areas:2–7 • • • • The recovery of copper, nickel and cobalt from converter slags Therecovery of nickel from nickel laterite The fuming of lead and zinc from lead blast-furnace slag (LBFS) The removal of zinc and lead from electric-arc furnace (EAF) dusts collected during steelmaking

Then, several years ago, the scale of work leapt with the commissioning of a 5.6 MVA (1–3 MW) facility. The furnaces, although larger (about 2.5 m in diameter), follow earlier designs—a cylindrical shellof water-cooled panels; a conical roof, through which the graphite cathode passes; and the facility to feed charge close to the arc. In most applications, an alumina castable protects the roof and an MgO rammable the hearth. The refractories of the sidewall, being the focus in this matter, vary according to the demands and concerns of each campaign. The right choice of refractory has always beenintegral to the success of a campaign. These days, the assessment of their performance provides invaluable pointers in choosing refractories for large-scale, industrial furnaces—it is for the purpose of designing these furnaces, after all, that campaigns on a pilot scale are conducted. The campaigns themselves have been good vehicles for testing refractories under particular conditions. One cancite two reasons: 1. Conditions in the small, pilot-scale furnace are sometimes more severe than those likely to be encountered in a industrial-scale furnace. The dimensions are such that, more than once, high temperatures have 2000 ELECTRIC FURNACE CONFERENCE PROCEEDINGS 361

prevented a freeze-lining from forming; and in at least one configuration, flaring from the arc impinged on a section ofsidewall in the freeboard. 2. Severity notwithstanding, of all the tests one can devise, a campaign in a pilot-scale furnace best simulates the conditions that will prevail in an industrial furnace. Heat transfer profiles are similar, and both corrosive and erosive forces are at play: • The hot-face is at the refractory-slag interface and temperature drops across the refractory. This stands incontrast to the cup test, in which a crucible of the refractory, or a cavity drilled into a brick of the material, is filled with slag and heated in a furnace. This configuration forces temperature, when conditions have stabilized, to be uniform throughout the slag and refractory. • Continuous feeding and tapping keep the composition of slag in the bath constant. This maintains the chemical potentialsdriving corrosion. In the cup test, by contrast, the ratio of slag to refractory is low, with the effect that chemical potentials equalize when slag reacts with the refractory. The spindle test, in which a rotating rod of refractory is immersed in a bath of molten slag, would circumvent this flaw of the cup test if the bath were large in comparison with the immersed refractory. • Turbulence in...