|Environmental Handbook Volume II: Agriculture, Mining/Energy, Trade/Industry (GTZ, 1995, 736 p.)|
|Trade and industry|
|49. Iron and steel|
This environmental brief covers iron and steel production and processing with the following activities:
- sinter, pellet and sponge-iron production
- pig iron, cast iron and crude steel production (including continuous or strand casting)
- steel forming (hot and cold)
- foundry and forging operations.
The above activities are carried out in an integrated ironworks or sometimes in separate locations.
After delivery and pretreatment of the ore in the ore preparation, sintering and where applicable pelletising plant, pig iron is smelted in the blast furnace with the addition of coke and admixtures; coke supplies the energy and reduces the ore to pig iron. In the converting mill the molten pig iron is refined to form crude steel by top blowing or purging with oxygen and the addition of scrap. Crude steel is also produced from scrap in electric furnaces, sometimes with the addition of pig iron, ore and lime. The crude steel is either continuously cast as blanks or, after casting as slab ingots or blocks in permanent moulds, rolled in the hot rolling mill to form sheets, billets or profiles. Further processing takes place in the cold rolling mills and forges. Continuous casting which already represents 90% of German and 60% of worldwide steel production improves crude steel utilisation by some 10%, saves energy by rolling operations and reduces the production scrap yield in steel and rolling mills per tonne of finished steel by more than 50%.
The direct-reduction process represents an alternative to traditional steel production. With the addition of reduction gas, e.g. from natural gas or coal, sponge iron is produced as a solid, porous product from which crude steel is then refined in the electric furnace, often with the addition of scrap. 90% of sponge iron is produced by the gas-reduction process.
Cast iron smelting takes place in the cupola furnace, with increasing use of induction furnaces.
Moulds and cores are required for the shaped casting of cast iron; these are mostly of sand but frequently contain an organic binding agent.
The following are classified as major units:
sintering plants 20,000 t/day
blast furnaces 12,500 t/day
steel converters 400 t holding capacity
electric furnaces (arc) 250 t holding capacity
cupola furnaces 70 t/h
induction furnaces 30 t/h
In many countries steel is extensively produced from scrap in electric furnaces.
Since iron and steel production is predominantly based on pyrometallurgical processes, air pollution is a primary consideration. In addition to a multitude of gaseous air impurities, dusts play a special role, not only because they occur in large quantities but also due to the fact that the dusts contain some hazardous substances affecting both man and the environment, e.g. heavy metals. Due to the use of coolant water and wet separation methods, problems of maintaining water purity also occur. Continuous casting plants require high specific water quantities from which the wastewater is considerably contaminated with oil. Casting without spray-water cooling relieves the load on water resources.
Metallurgical processes also produce slags which should be recycled wherever possible. Where no effective recycling and final dumping facilities exist, dusts and sludges separated from the waste gas cleaning systems represent potential pollutants of the ground and water environments.
In blast furnace plants and converting mills, also in rolling mills and forging works, noise and vibration protection is of fundamental importance. Foundries produce large amounts of waste from used sand, broken cores and cupola slag.
For reasons of ecology and economy, work is taking place worldwide on process methods which permit the use of coal instead of coke and the extensive use of lump ore instead of sinter or pellets. This would enable coking and sintering plants to be dispensed with as emission sources in a metallurgical plant.
Other developments concern the casting of rolling feed stock in approximately final dimension form. Shortening the process chain permits reductions in energy requirements, residual substances, waste and emissions.