General Motors Plasma Cupola

General Motors Plant

Commissioned in 1987
Grey Iron Foundry Process
Marc-11H Torch
– 6 active, 4 spares
Nominal Power :
– Total System : 10.2 MW
– Individual Torch : 800 - 1700 KW
Process Gas : Air
Electrode Life :
– Anode : 1000 Hours
–Cathode : 500 Hours

General Motors' (GM) Powertrain plant, located in Defiance, Ohio, has been operating the plasma cupola since June 1989 for the production of gray iron for making engine blocks and other automotive castings. This was the world's first commercial-scale plasma cupola ever built. The use of plasma heating technology allows the GM plant to melt difficult raw materials in a cost effective manner and produce world-class automotive castings.

Introduction

The cupola is a vertical shaft furnace which has been conventionally used in the foundry industry for the remelting of scrap iron and steel. Unlike the traditional cupola, a plasma torch is fitted at the bottom of the shaft in a plasma-fired cupola (PFC). The plasma torch is a high temperature, high efficiency process heating device designed to operate with minimum maintenance in an industrial environment. The plasma torch converts the electrical energy into thermal energy of the gas thereby raising its temperature. In this application, the torch acts as a combustion air preheater operating at over 2500 deg. F, thereby reducing the amount of coke and combustion air that is charged to the cupola. The resulting gas velocities through the cupola shaft are low enough to allow charging of smaller low cost materials such as loose borings and machining turnings.

In addition, thermal control of the plasma-fired cupola process is independent of the chemical control. The costly addition of alloy elements such as silicon, chromium and manganese is minimized and the higher temperatures in the lower sections establish reducing conditions in which silicon can be produced from sand and iron can be produced from the reduction of low cost iron oxides like mill scales waste material.

TEST PROGRAM

The PFC technology was developed by Westinghouse Electric Corporation under Electric Power Research Institute sponsorship during 1983-1990. The development program included participation from several major US foundries including GM, Ford, Intermet, Tyler Pipe, and others. A small but commercial sized pilot plant was commissioned which included a 30" PFC. Extensive tests were conducted over a seven year period. The results of this test program confirmed the flexibility of operation and the cost effectiveness of the PFC technology.

The test results indicated that the plasma-fired cupola is capable of melting very thin charge material like loose cast iron borings up to 75% of the charge. This type of charge material can be fed directly through the cupola charge door. The melt rate can be increased by increasing the plasma torch power. A productivity increase as much as 60% is observed. The melt temperature can be controlled by varying the torch power level. The response time for such a control is in the order of 3-4 minutes.

The carbon monoxide levels in the top gas can be maintained, if required, at significantly higher levels in the PFC. This results in reducing atmospheres inside the PFC. This results in melt yields as high as 98.5% in the PFC. Also, due to very high temperature levels in the melt zone and higher carbon monoxide levels, silicon can be produced by the reduction of sand in the PFC. The silicon generation is more pronounced when sand is premixed with coke breeze and also when it is injected in front of the plasma torch. The ferro-silicon injection at the tuyere level also results in higher silicon recovery. Metal-to-coke ratios were varied between 8:1 to 70:1 during the test programs. Due to low wind rates, even at high metal-to-coke ratios, the cupola back pressure was observed to be in the moderate range. At high metal to coke ratios, most of the energy for melting iron is supplied by the plasma torch.

COMMERCIAL-SCALE PLASMA CUPOLA

Following is the successful results of the development program, GM's Central Foundry Division placed an order for Westinghouse plasma systems, technology and equipment. GM is using this plasma melter to remelt scrap iron at their plant in Defiance, Ohio. In March 1989, the first castings were made with metal from the plasma melter. The melter was dedicated on June 26, 1989 and released to production.

The GM melter is 13 feet in diameter. It has a throughput of up to 50 tons per hour when operated with charge of loose borings and close to 80 tons per hour with regular charge consisting of sprue and bundles. The melt rate for regular charge is currently limited by the capacity of the charging system. The gray iron is melted on top of the coke bed at about 2800oF. The cupola shell is water cooled. Hot blast air is injected through the six tuyeres located at the bottom of the melter. A 2.5 MW Westinghouse MARC-11 plasma torch is mounted onto the end of each tuyere.

GM validated the physical and structural properties of castings made from the iron produced in the plasma melter. Sample castings were shipped to some of GM's customers and GM engineers monitored their machining operations. These customers have tested the engines assembled from the castings on dynamometers. Results indicate that the iron produced by plasma cupola meets or exceeds established quality standards.

GM is using the plasma melter to melt charges consisting of 50%-60% loose cast iron borings. During periods when demand for hot metal is high, GM runs the plasma melter with regular charge material and gets close to 80 tons per hour throughput. The melter is run on a three-shift operation. The plasma torches are operated in a fully automatic mode. The cupola operator starts all six torches by pressing one button on a master control panel. Also the power level is set by operating a single knob. Because of this feature, no engineer is required to operate the plasma systems.

The maintenance of the plasma systems is done by millwrights and electricians who underwent a short on-site training course. Normal maintenance consists of replacing the torch electrodes which is done every 750-1000 hours of operations.

CONCLUSIONS

Overall operating experience of the plasma melter at GM's Defiance foundry indicates that the plasma melter is cost effective and can be operated with a high degree of reliability in the severe foundry environment. A copy of the American Foundrymen's Society-1998 Casting Congress paper, published on May 10-13, 1998, can be read using Acrobat Reader at GM Plasma Cupola - AFS Paper