In nature iron never occurs as pure material but only as a compound. These are mainly iron-oxydes which are inter-spersed with mineralic constituents called the gangue. The mixture of iron-oxydes and gangue is called iron ore.
The main task of metallurgical engineering is the seperation of the iron-oxydes from gangue and on the other hand, the reduction of the iron-oxygen compound. By addition of carbon during the various processes, iron is converted to steel and by alloying with different elements the various steel types that are known nowadays are created.
The complete process of steel manufacturing is performed in several steps.
1. Ore Dressing – 2. Reduction of Iron Ores – 3. Refining – 4. Steel Casting
The world-wide ore deposits are estimated up to 500 billions tons. The amount of mined ore within the last decade is estimated 700 millions tons. That means the natural recources of iron-ore will be sufficent for several further generations. In our Earth’s crust we find various iron-ore compounds. The main types are:
Magnetite is a iron ore difficult to reduce with a high concentration of iron (50-70 %) appearing as Fe3O4.
Important natural deposits of magnetite are in Sweden, Norway and Russia.
Hematite is an easily reducable iron ore, within a high concentration of iron (40-60 %) appearing as Fe2O3.
Limonite is an easily reducable iron-ore but it has only got a very poor concentration of iron (20-45 %). Appearing as Fe2O3 + water of crystallization.
Siderite is an easily reducable iron ore with a very poor concentration of iron, appearing as FeCO3.
The primary ore dressing is usually done in the mining countries before the export. This increases the quality and lowers the transportation charges costs because less waste rock material and material of little value has to be shipped.
In ore dressing the specific properties of ore, such as specific gravity and intensity of magnetization are used for seperation. The first step in processing is crushing and grinding the ores. Using of the different specific gravities and centrifugal forces the primary seperation takes place in spiral centrifuge concentrators. During the following process within flotation columns the different specific properties of ore and gangue are used for seperation. Immersing the crushed material in a liquid of high specific gravity and injection of air the iron minerals start floating and can be discharged on top. The magnetic dressing uses the forces of a magnetic field to seperate iron minerals from rock and other minerals. As a result of these processes the concentration of iron ore increases from 30-35 % up to 60-65 %. But these fine grained ores are not appropriate to direct further processing.
In the subsequent reduction processes it becomes necessary to blow gases through the ores. To increase efficiency a larger size of the ore pieces is required, which can be archived by sintering or pelletizing of fine grained ores.
To sinter the fine sized ore it is dampened first and mixed with specific additives and heated in a sinter plant. The high temperatures generated, remove the carbon and fuse the ore particles together to form a porous clinker. This conglomerate is then crushed by special machines into the required particle size.
In pelletizing the concentrate is first dampened with water and mixed with specific additives. This mixture is molded into balls with a diameter of 10-15 mm and roasted at a tempera-ture of above 1000 °C. After this dressing the material is used for steel manufacturing in the blast furnace.
Reduction of Iron Ores
The seperation of iron oxydes is a chemical reduction.
To perform this reduction heat and a strong reduction agent are essential.
There are various types of furnaces in use:
- direct reduction furnaces
- melting reduction furnaces
- blast furnaces last furnaces
The most frequently implemented type of furnace for steel manufacturing in Germany is the blast furnace. Therefore only the various processes within the blast furnace will be mentioned hereafter. Coke and Limestone are continuoulsly dumped in layers into the top of the furnace. During its slow descending to the actual reaction-zone the mixture is heated up gradually.
At the same time the hot blast (600 – 1300 °C) is blown into the bottom of the furnace. This hot blast (+ oil or carbon gas) ascends to the top going through numerous chemical reactions.
Ignited by hot blast the carbon of the glowing coke is converted into CO2 but immediatly reconverted with the coke to CO. The ascending CO-gases remove oxygen from the iron oxydes and are converted to CO2 in a combustion process.
Getting in touch with the glowing coke the convertion into CO starts immediately again.
This process takes place in regular intervals and stops roughly at the middle of the height of the furnace dome, because of the too low temperature in this area. Because not all of the CO is used for the reduction process, the hot dirty gases still contain enough CO, to be inflammable.
In regular intervalls of about every 2 hours the furnace is tapped.
Liquid iron first and then liquid slag flow out of the taphole. Since the slag is less dense than iron, it floats on top of the iron.
The liquid iron flows into ladles, known as torpedo-cars, to be transported to further treatment stages or is drained into hot metal pots and transported to the mixer.
The mixer is a refractroy brick lined, horizontal cylinder, pivoted on the longitudinal axis.
The three main purposes of the mixer are:
- storage container for steel-manufacturing plant
- mixing of the hot tapped material to achieve consecutive material homogenity
- reducing of sulfur (continious converting of MnS+FeS into MnS+Fe, MnS is less dense than Fe and sets down in slag)
To accelerate this process soda, calcium carbide or manganese is added.
The bound material floats up and can be casted off as slag on top.
As a result the crude iron now contains
3 – 5 % C
0.2 – 2 % Si
0.2 – 3 % Mn
0.1 – 2 % P
0.02 – 0.06 % S
For producing steel the amount of these elements has to be reduced evidently.