Environmental Impacts Case Study: Production of Iron and Steel
Steel is manufactured by the chemical reduction of iron ore, using an integrated steel manufacturing process or a direct reduction process (IFC). Steel is produced by the conversion of iron into the steel in a Basic Oxygen Furnace. Another method of steel making is in electric arc furnace from scrap steel and sometimes reduced iron is used as well. The three basic steps of steel making are:
Coke is the substance used to reduce iron ore; it is a solid carbon fuel. Coke production begins with pulverized, bituminous coal. The coal is placed into a sealed coke oven which is then heated to extremely high temperatures for approximately 20 hours. Coke will usually be produced in large batches, with many ovens operating at the same time. In order to operate more efficiently heat is often transferred from one oven to another (ISTC).
To make iron- coke, iron ore, limestone and heated air are fed into a blast furnace. The coke combusts when it comes into contact with the hot air, this combusted coke provides the heat and carbon sources for the iron making process. The limestone reacts with the acidic impurities (slag) causing them to float to the top which allows them to be skimmed off easily (ISTC).
Steel Making with the Basic Oxide Furnace
For the final step of the refinement of iron into steel, the iron from the blast furnace-which is in a molten state, is transferred to a basic oxide furnace. After this high purity oxygen will be fed into the furnace, this combusts the carbon and silicon in the molten iron. Other alloys are also sometimes added to enhance the properties of the steel (ISTC). The iron produced is usually cast into slabs, beams or slabs, beams or billets (USEPA, 1995). It can then be re-shaped as desired at steel foundries.
One of the major pollution sources from steel production is coke making. The process pollutes the air with many emissions such as, naphthalene, ammonium compounds, crude light oil, coke oven gas, sulphur and coke dust. Water pollution is also caused by the coke making process, because water is used to cool the coke after baking which results in the water becoming polluted with coke breezes and other substances (ISTC). The main pollutants in this waste water include total suspended solids, lead, chromium, cadmium, zinc, fluoride and oil and grease. As well as this coke making produces solid waste such as furnace slag and furnace dust (IFC).
The iron and steel industry is a major contributor to greenhouse gas emissions, according to the Greenhouse Gas Estimates for Selected Industry Sectors it is responsible for 224.8 Tg of carbon dioxide, 151.7 Tg was down to fuel consumption and the rest was caused by non-fuel uses for example the release of carbon dioxide from the limestone reacting. This means that the industry is the second biggest contributor to greenhouse emissions, with only the petroleum refining industry contributing more. The toxicities of the emissions from iron manufacturing in a blast furnace range from 10kg/t of steel produced to 40kg/t; nitrogen oxides (NOx) mainly from heating and sintering (1.2kg/t of steel); sulphur oxides (SOx) mostly from sintering and pelletizing operations (1.5kg/t of steel); hydrocarbons; carbon monoxide; hydrogen fluorides and in some cases dioxins (IFC).
The emissions from steel manufacturing vary from less than 15kg/t to 30kg/t of steel, nearly all of these come from the dusulphurisation process. The toxicity levels in the wastewater are; total organic carbon (typically 100-200 milligrams per litre); total suspended solids (7000 mg/l); dissolved solids; cyanide (15mg/l); fluoride (1000 mg/l); chemical oxygen demand (500 mg/l) and zinc (35 mg/l). The typical level of process solid waste such as slag and collected dust is between 300 kg/t and 500kg/t of steel produced, 30kg of this is rated as lethal (IFC).
All of these emissions can have serious effects on human health and the environment, I will analyse the main pollutants and their damages:
Sulphur Oxides (SOx)
The health damages of sulphur dioxide consist of eye irritation, asthma attacks, coughing and chest pains. People with chronic and respiratory disorders and cardiopulmonary disease are particularly susceptible. In terms of environmental damage, sulphur dioxide is one of the main contributors to acid rain. Acid rain causes a lot of harm to rivers and lakes affecting the fish and others water creatures, it also harms forest and plant life by removing vital nutrients from the soil (ENVIRONMENTALINDICATORS)
Nitrogen Oxides (NOx)
Nitrogen oxides are also harmful to the environment; it is a big contributor to smog and acid rain. Exposure to smog can result in impaired lung function and can also accelerate deterioration in lung function (ENVIRONMENTALINDICATORS).
Carbon monoxide has no colour or smell so it can be hard to detect sometimes, this makes it even more hazardous. If this gas is inhaled then it can affect the body’s ability to absorb oxygen, this is because it impairs the ability of red blood cells to carry oxygen to body tissue. Carbon monoxide poisoning can often result in death (ENVIRONMENTAL INDICATORS).
It is common knowledge that greenhouse gases are resulting in climate change. The main greenhouse gas is carbon dioxide, of which the production of iron and steel produces a lot of. The impacts of the climate change caused by these emissions include; rising sea levels, heat waves, droughts, and floods. All of which threaten the lives of people all around the world (ENVIRONMENTALINDICATORS).
In order to make sure companies are keeping within the pollution limits allowed then emissions should be regularly monitored. Air emissions should be monitored continuously using a continuous monitoring system with a visual alarm to warn of excess emissions (SOUTHSOMERSET). Sulphur oxides, nitrogen oxides and fluorides are checked annually, this doesn’t include checking of sulphur in the ores as this needs to be done more regularly.
Wastewater discharges also need to be checked every day to ensure that they are within the allowed limits; this excludes metals however which need to be monitored less regularly-at least 4 times a year. During upset conditions and when starting up, sampling of the emissions needs to be carried out a frequent rate. The data collected from monitoring the emissions is regularly reviewed and analysed and then compared with the allowed limits in order to see if any of the releases are exceeding the set boundaries, the necessary action is then taken (IFC)
Pollution caused by the iron and steel industry can be controlled in many ways. First of all, some of the ways by which to control the emissions from iron manufacturing:
* Change the fuel type used, by using coal, oil or gas instead of coke the blast furnace efficiency can be improved therefore the air emissions are minimized.(CLEANERPRODUCTION)
* Use dry SOx removal systems such as caron absorption for sinter plants or lime spraying in flue gases.
* Use low NOx burners to decrease the NOx from fuel burning in ancillary operations.
* Recover the thermal energy from the gas in the blast furnace before using it as fuel, cutting gas emissions by improving the charge distribution of the blast furnace.
* Covering the iron runners when tapping the blast furnace and using nitrogen blankets when tapping in order to decrease the dust emissions.
* Recycle material that contain a lot of iron for example; iron ore fines, pollution control dust and scale in a sinter plant.
* Recuperate the energy from exhaust gases and sinter coolers.
There are also many ways to control the pollutant levels in steel manufacturing:
* Recycle any collected dust. Use dry methods of dust collection and removal systems in order to minimize the generation of wastewater.
* Use Basic Oxygen Furnace gas as fuel.
* Use enclosures for Basic Oxygen Furnaces.
* When casting the steel produced, use a continuous process In order to make the energy consumption more efficient.
These methods help to keep the amount of pollutants down; however the pollutants that are produced still need to be treated. To do this different methods and devices are used for Air emissions, wastewater treatment and solid waste treatment.
There are now air emission control technologies such as scrubbers, electrostatic precipitators (ESP’s) and baghouses for the removal of particulate substances. Baghouses and ESP’s can remove up to 99.9% of the particulate substances and the associated metals such as chromium, nickel, lead and cadmium. Desulphurisation plants are used to remove the sulphur oxides, with a 90% or more removal rate. However despite this high efficiency, using low sulphur fuels and ores may be more cost-effective. By using these methods the levels presented in this table should be achieved. (IFC)
Parameter Maximum Value
PM10 100 g/t of product (blast furnace,
basic oxygen furnace); 300 g/t from sintering process
Sulphur Oxides For sintering: 1,200 g/t; 500 mg/m3
Nitrogen oxides For pelletizing plants: 500 g/t; 250-
750 mg/Nm3; for sintering plants:
Fluoride 1.5 g/t; 5 mg/Nm3
Table 1. Load Targets per unit of production, per unit of production. (IFC)
Wastewater treatment systems normally include sedimentation to remove suspended solids, physical or chemical treatment for instance pH adjustment to precipitate heavy metals and filtration (IFC). The target levels in the table below should be achieved by carrying this out.
Parameter Blast Furnace Basic Oxygen Furnace
Wastewater 0.1m3 0.5m3
Zinc 0.6g 3g
Lead 0.15g 0.75g
Cadmium 0.08g N/A
Table 2. Target Load per unit of Production, Steel Manufacturing (IFC)
Solid Waste Treatment
Solid wastes often contain heavy metals and therefore need to be stabilised, using chemical agents before being disposed of (IFC).