New research advances melt control.

Results of the first full-scale experiment of a 2.5 MW transportable plasma torch system in a production foundry cupola line (90-152) at the Peugeot Auto factory in 1986-87 were presented by G. Crouzillat, Aerospatiale Produits Nouveaux, France. He noted that the French use of plasma torches differs from others in that the French tests were made as part of an actual production cycle as opposed to experimental pilot runs, such as those done in the United States.

Crouzillat described the three months of cupola testing under actual production conditions as:

* increasing production 30-40%;

* reducting coke rate;

* increasing recarburizing rate;

* reducing effluent volume by 30%;

* offering a rapid return on investment.

After the initial test, Peugeot boosted their cupola with a 4.5 MW torch. According to Crouzillat, first results confirm the gains of the previous production tests, and showed an even better production return. Production was up 45%, coke use was reduced and an all-scrap melt charge proved acceptable.

In steel casting production, he added, metal temperature decreased from about 1650C at the furnace to 1550C at the continuous casting line. To prevent the temperature from going too low to pour, a vertically-positioned plasma torch was used with excellent results for temperature maintenance in transfer ladles.

Auburn University's K. Nyamekye discussed his work (done with the late A. Draper of Pennsylvania State Univ Industrial Engineering Department) on cupola wall heat loss as it affects furnace efficiency (90-120).

Heat loss is a major problem in the operation of cupolas, Nyamekye said, and controlling it can raise efficiency and improve furnace yield. The thermal efficiency of a cupola is about 37% compared to 70% for an induction furnace, and with coke costs rising, it is important to know where heat loss occurs in the various cupola zones in order to improve thermal efficiency.

Getting a balanced heat loss through the wall in the preheating, melting and liquid flow zones requires careful monitoring of the sensible combustion heat in the coke bed, heat in the air blast and the chemical reactions taking place in the furnace. Heat loss sources to offset the sensible heat of combustion are those by radiation, convection and heat lost out the top of the furnace.

Nyamekye used a coldblast, four-tuyered, 24-inch cupola lined with one row of silica fire brick and a monolithic 85% high density alumina lining. Thermocouples were mounted through the wall and on the shell surface at each furnace zone, showing that the average heat loss through the cupola wall was about 23% of the net combustion energy.

A panel comprised of E. Benion, Wells Mfg Co, R. Hunt, Alabama Ductile, N. Sullivan, Roberts Foundry, and G. Kirby, Wagner Casting Co, discussed procedures for bottom patching silica lined coreless furnaces (90-103). Panelists agreed that bottom patching of silica lined coreless furnaces is practical, but methods varied with the most obvious differences being patch cure time and temperature before startup and bottom surface preparation prior to patching.

It was the consensus of the panelists that furnaces up to 1516 tons were the upper limit for best results with bottom patching, but that larger ones could be successful. Other areas of general agreement were the importance of ridding the patch of any moisture content before charging, and maintaining the taper of the furnace bottom.

The advantages of the relatively new technique of refractory spraying (90-12), or gunning, of ladles and tundishes was explained by A.W. Powdrell and D. Helliwell, Harbison-Walker Refractories. A low density, sprayable refractory system allows ladles and tundishes to be lined effectively and quickly. Some of the major benefits are less metal temperature loss, ease of installation and tear-out and resultant clean metal. The system is applicable to iron, steel and nonferrous metals.

The technology of sprayed magnesite tundish refractory was developed in Europe, and is now licensed for use in North America. All equipment is owned by the refractory supplier, the user required only to purchase the sprayable refractory material.

According to F. Wagener of Technometal, GmbH, and N.G. Bliss, Milltech HOH, inc, secondary metallurgical processes are defined as postmelting treatments of liquid steel, changing from a melt furnace to a ladle or converter (90-105). The requirements expected from secondary metallurgy units have increased greatly in the last few years; however, these developments have only recently taken hold in mini-milis and steel foundries primarily because of the large temperature losses from these processes when treating small heats. Slag-free tapping is essential for postmelt treatment of steel.