Production of Iron from Mine to Cast Iron and Steel

Introduction

The production of iron and steel comprises 95% of all the tonage of metal produced anually in the world. They are the least expensive of the world's metals.

The art of iron making is an ancient one, dating back to 1200 B.C..

The first high tonage steel making process was invented by, and named after, Sir Henry Bessemer of England, prior to the American Civil War.

Production of Pig Iron

The Principal raw material for all ferrous production is pig iron, the product of the blast furnace.

It is obtained by smelting iron ore with coke and limestone. Often it is necessary to "roast" the ground ore to eliminate contents like sulphur, hydrogen or carbon.

The Blast Furnace

The average blast furnace is about 7 meters or more in diameter and about 60 meters high. The capacities of such furnaces range from 800 to 1700 tons per 24 hours.

The raw materials are brought to the top of the furnace with a hoist, and dumped into the double-bell hopper ("Gicht" in German). The necessary charge to produce 1000 tons of pig iron consists of approximately 2000 tons of ore, 800 tons of coke, 500 tons of limestone and 4000 tons of hot air.

The purpose of the hot air is to enable the coke to burn more efficiently, and to assist in the formation of carbon monoxide, which in turn reacts with the iron ore to produce iron and carbon dioxide. With the exhaust gases of the furnace the air is heated in preheated to about 540°C. The hot blast  enters the furnace through tuyères placed around the furnace just above the hearth.

The limestone serves as a flux to help turning the gangue materials of the ore into fluid slag. This slag floats over the molten iron and may be removed whenever found necessary.

The molten iron is tapped in intervals of 5 to 6 hours.

Furnaces for converting pig iron

Pig iron is useless for immediate use, so it has to be converted into wrought iron, cast iron, malleable iron or steel.

The primary difference in ferrons metals is the amount of carbon that they contain.

Because of this the main purpose of "converting" is to burn the carbon down until a predestined percentage is reached. The more exact this mark is reached the better. There are mostly three furnaces used for converting pig iron and for remelting scrap-metal: Basic Oxygen, Electric Arc and Open-Hearth (Siemens-Martin)

Basic Oxygen Furnace (BOF)

Pig Iron is used as primary raw material (60%-80%) the remaining 20-40% are scrap.

Scrap and hot metal is poured into the mouth of the tilted vessel. A water cooled lance is lowered to about 2 meters above the bath in the vertical vessel.

With oxygen blowing over the surface, igniton starts, and the temperature climbs to about 1650 °C.

Carbon, manganese and silicon are oxidized. Impurities are collected by lime and fluorspat as fluid slag. When the process is over (after about 45 minutes) the vessel is tilted for tapping (first the slag is removed, then the molten metal).

Testing of the metal before tapping ensures the right ammount of carbon and other desired alloys.

Electric Furnace

This furnace is charged with selected steel scrap. By close control of the charge and by adding alloying materials, ingots and castings of stainless steel, many general-purpose alloy steel, heat-resistant steels and tool steels can be manufactured in an electric furnace.

The roof of the furnace is designed to allow three graphite electrodes to sit above the srcap heap. The 3-phase current arcs from one electrode (up to 760mm in diameter and over 24 meters long) to the charge and back to another electrode. These furnaces work with 40 volts and currents that exceed 12000 amperes. For a 125 ton charge approximately 50.000 kWh of power are required. It is also possible to inject high-purity oxygen as a refining step, which reduces the tap-to-tap time.

Open-Hearth furnace (Siemens-Martin)

This was once the most popular processes for making steel.

The open-hearth furnace can be charged with all molten pig iron, all solid steel scrap, or a combination of molten and solid material.

Such furnaces hold from 11 to 600 tons of metal in a shallow pool that is heated by action of gas, tar, or oil flame passing over the charge.

About 10 hours after the initial charge, the furnace is ready to be tapped.

The furnace is said to be reverberatory, because the low roof of the furnace reflects the heat onto the shallow hearth.

It is regenerative because the chambers on either side of the furnace are capable of beeing heated by combustion gases which in turn allow the air and fuel entering the furnace to be increased in temperature, thus allowing an increased combustion efficiency and temperature.

Refining furnaces and vessels

Raw steel is sometimes further enhanced (e.g. high-quality steels are produced) for certain applications.

Induction Furnace

Here an induced current, supplied to the primary water-cooled coil, is used to melt the charge ( from a few pounds up to 4 tons) in about 50-90 minutes. This system is cheap, almost noise free and, because the heat is no higher than needed, scrap alloys can be remelted without "burning out" the valueable alloying materials.

Melting in a Vacuum and other Special Atmospheres

Molten metals tend to absorb gases and even worse, the oxidation rate increases with the temperature. These things are both detrimental to the quality of the steel. For some metals, a slag coating is allowed to accumulate over the molten material to protect it from excessive oxidation.

For some special steels (very high-quality steels) it is necessary to employ a vacuum (or inert gas) atmosphere.

The furnaces for these conditions must be electric-arc or induction.

Products and further Useage

After the ore refining process, the materials are cast into ingots or other shapes depending on the anticipated useage.

Steel

Steel is a cristalline alloy of iron, carbon and possibly some other elements.

It may be rolled, cast or forged. Although steel may be cast into  molds to conform to a definite and complex shape and size, it is most often cast into ingots for use in making pipe, bar stock, sheet steel or structural shapes.

Steel Ingots

The ingot molds are mostly circular in cross section, to prevent columnar grains to meet in corners and form planes of weakness.

The sides are corrugated to speed up cooling, this also reduces the size of columnar grains beeing formed.

There are two types of ingots in use:

The big-end-down type is easy to strip from the ingot, but there is a high loss in metal, because of the shrinkage cavity (pipe) that forms on the top during cooling.

This cavity is explained due to the progressive solidification process which starts at the surface and moves to the center. During this period there is a considerable shrinkage of the metal, as layer after layer solidifies. Because of this decrease in volume a pipe in the center is eventually formed. 

The second type of ingot prevents this cavity. It is called big-end-up, and has a large volume of metal available at the top and further a refractory raiser. Metal in the raiser remains molten until the ingot has solidified, and supplies the ingot during this period with needed metal to compensate for the shrinkage.

Killed Steel

Killed steel has been deoxidized, and it evolves no gas during solidification. The top surface of such ingots solidifies immediately, as do the walls.

All impurities in this type of steel are collected in the center of the ingot, because it solidifies at last. The center (about 20% of overall diameter) is because of this accumulationof limited useage, but the rest of the ingot is of high quality.  

If a big-end-up mold is used then all impurities are collected in the shrink head. This part of the ingot can be removed, and the rest of the material is of high quality.

All steels having over 0,3% carbon are killed. This is obtained by addition of high silicon pig iron or an alloy high in silicon in the primary manufacturing process.

Killed ingots have a minimum of segregation, good structure and a large cavity in the center.

Rimmed Steel (Non- or Semi-Killed)

This steel is characterized by a semiboiling action in the ingot after pouring, due to rapid evolution of carbon monoxide gas during solidification.

This causes the formation of a honeycomb structure, which, if controlled compensates for most shrinkage loss.

Rimmed steel ingots have good surface, and there is little or no opportunity for cavities to form.

The disadvantage of rimmed steel is that impurities are spread throughout the ingot.

Continuous Casting

In addition to teeming hot metal in ingots, there is a process that converts molten steel to a continuous slab.

Solidification begins as the steel cools in passing through the mold. The slab can be cut whereever found necessary.

About 300 tons of metal can be cast into solid slabs in about 45 minutes (Ingot processing would take 12 hours).

Vocabulary