Reducing the Requirement for Raw Materials Through Recycling at all Stages of Steel Manufacturing
Key Components and Fundamentals
Ferrous materials such as irons and steels are some of the few materials that are considered to be 100% recyclable. They can be infinitely remelted and made into new forms without losing its overall quality. Not only is steel scrap used in steel manufacturing to produce new plates and other structural members, it plays a key role in the foundry industry. The foundry industry makes use of pig iron from the steel manufacturer’s blast furnace, and scrap steel from a number of different sources to manufacture complex components that would otherwise be impossible to form.
Steel scrap comes from a number of different sources, one of the most commonly recycled source of steel is from the automotive industry. In Europe, more than 95% of automotive steel scrap is recycled. The automotive industry, in an attempt to decrease the overall weight of the car has been using more and more high-strength low-alloy steels to manufacture the various components. Another very common source of steel scrap is from cans used to package and store food goods. These steel cans will have an added coating of tin on the surface to prevent corrosion from occurring, which may require further processing to be utilized in steel manufacturing.
Steel scrap can be classified by the size of the material, how it has been processed, as well as the individual elemental ranges. The way the steel was collected is also an important means of classification, scrap steel can be generated internally or it can be purchased from scrap yards, purchased scrap is also referred to as merchant scrap. It critical to have the chemical compositional range well controlled, using material for remelt that is outside of the expected composition can lead to either very expensive processing to eliminate the undesirable elements, or can lead to scrap generation due to having to discard the metal because it is not correctable. In steel manufacturing, as well as the foundry industry, there are batch size limitations. This means that when a correction is needed due to the chemistry being off there may not be enough room in the vessel to take the necessary materials to correct the heat. When molten metal is poured off to create more room this can be very costly and time consuming creating unnecessary delays in heat times.
There are a number of other recyclable goods that are generated as waste in the steel making industry. In the blast furnace the flue dust that is collected is composed of primarily iron oxide and zinc, as well as trace amounts of other elements. There is also waste sludge and filter cakes that are created as waste products from the blast furnace, which contains oxides that are in the off-gases produced as part of the blast furnace operations. Similar waste products are also generated through the use of a basic oxygen furnace, or an electric arc furnace. Another waste product that is produced from a blast furnace, basic oxygen furnace, or an electric arc furnace is the furnace slag. There is some ability to recycle the generated slag back into the steel manufacturing process, but generally it is discarded and utilized in other industries. Off cuts and mill scale can be generated further down stream in the steel manufacturing processing. The process of manufacturing coke will also produce waste materials that will require further processing or recycling.
Importance Outside of Steelmaking
The environmental impact of utilizing steel scrap to manufacture new steel and new iron products is incredibly dramatic. The energy requirements for remelting steel is lower than the requirement for melting raw materials to create the same sized batch of steel. Less energy consumed in the manufacturing of steel means that less energy has to be generated through methods which may produce large amounts of pollution. Reduced power requirements may also mean that the needed power can be generated utilizing renewable methods of power generation and not through non-renewable resources that produce more pollution.
Lower energy requirements does not only reduce the amount of electrical power used, it also reduces the requirement for other components that are added to the furnaces during operation, like the oxygen that is added to the basic oxygen furnace. The oxygen added into the steel making process will react with the carbon in the hot metal creating carbon monoxide and carbon dioxide which naturally heats the melt through an exothermic reaction.
By utilizing scrap steel instead of iron ore there is a reduction in mining operations. There will always be a requirement for new materials to be used in the steel manufacturing process, because the availability of scrap in the market is often not enough to completely subsidize the constantly growing requirement for steel. There has been a dramatic increase in steel manufacturing plants, especially with the comparatively lower capital costs of the mini-mill. Through out the world there has been an increased demand for steel as industrial development occurs.
While iron and steel is an infinity renewable material, there are many materials utilized in the manufacture of steel that have a much more finite availability. Coal and the manufactured metallurgical coke are a finite resource that is being depleted not just through use in the steel industry but also as a part of the constantly growing need for electrical power generation. Burning of coal produces greenhouse gases which are contributing to the global warming phenomena, any reduction of these harmful gases is beneficial to the environment. The extraction process can also be quite environmentally damaging. The usage of natural gases in the steel manufacturing process produces similar damaging greenhouse gases. Natural gas, like coal, is a non-renewable resource and is thus of finite availability.
Capturing and utilizing the off-gases produced in steel manufacturing reduces the amount being released into the environment. The concern with off-gases is not only about the chemical composition of the off-gases, but also the temperature of the gases as they are released into the environment. When a very hot gas is released into a colder climate the gas, especially if it contains water vapours, will begin to condense and can rain down into the surrounding areas. The heat of the gases can damage the surrounding environment, which is also observed at power generation facilities when heated water from the cooling system is added back into a natural water reservoir, like a lake or river, slowly heating up the reservoir and killing aquatic life.
Off-gases and flue gases can be collected and filtered during the steel making process and utilized in different locations within the plant. This prevents the potentially harmful gases from being sent into the environment, while also reducing the requirement for raw materials being added into the manufacturing process. The filtration process the gases undergo is the source of some of the waste products that come out of the steel manufacturing process. The amount of product that may come out after filtration may cause issues for people in the local community that have health and breathing concerns.
Technical or Economic Limitations
The increase in steel demand through out the world has created supply deficits due to increased manufacturing of steel. The increase in electric arc furnaces within the market share has also affected the availability of steel scrap and other resources as the electric arc furnace can use almost 100% scrap material to make a heat of steel, which can reduce the availability of steel for other steel manufacturing avenues like the basic oxygen furnace. This can also result in less scrap materials being available to the foundry industry as well which also relies on scrap materials being available.
There can be a lot of variety in steel scrap, in its chemical composition, as well as its physical characteristics, size and condition. Specialty alloys can contain a large amount of elements that may be undesirable in the manufacturing of low alloy steels or other material goods. Stainless steels will contain large amounts of chromium, and may also include nickel, molybdenum, copper, and other elements in large concentrations. Specialty abrasion resistant materials, or alloys designed for work hardening may also include high concentrations of chromium, nickel, or manganese. Re-utilizing this material can be difficult in some occasions, especially when these elements are the elements to be minimized in a certain batch of steel. This is issue is not localized to steel scrap, there can also be a large variety in iron scrap that can be purchased. White irons in particular will contain large amounts of chromium, and possible nickel and molybdenum. Some grades of grey iron may be extremely high in phosphorous or sulfur, and others may have niobium or other elements deliberately added for specific characteristics.
Sorting a material based on its chemistry requires quality equipment to make these determinations. Common equipment includes light spectrometers, mass spectrometers, or x-ray analyzers. All can be quite costly to purchase, maintain, and train personnel in the utilization of. Certain sample sizes are required to ensure accurate testing.
There are many waste streams in the steel manufacturing industry that are presently too difficult to recycle, the infrastructure to recycle these materials is not yet fully mature or readily available in the market. In some instances, it can be more expensive to use the recycled materials than it is to utilize new materials. This is also seen in the refinery industry, specifically in nickel, silver, gold, and copper refining. The slag that comes off the refining process still contains a small but valuable amount of the material, but the extraction process has been exhausted so the slag is collected and sometimes stored in anticipation of advancements.
Many steel products that are recycled as normal household recycling may need special processing to make it safe for use in the steel manufacturing processes, or to remove unnecessary materials that would involve costly measures to mitigate or reduce during heat production. These materials can include appliances, building materials, and storage containers. Some of the most commonly recycled steel materials will need to undergo dezincing or detinning to make the material usable in standard processing. It can be a good practice to heat scrap material to remove materials that have become adhered to the surface, like machining oils and lubricants, or paints and adhesives. Some materials may need to be shredded to a more manageable size so it can be easily added into a commercial furnace for remelting. All these processing steps add additional costs to the recycled material, possibly deterring its bulk usage.
Through improvements to the recycling industry there has been a dramatic change in the amount of raw materials required to manufacture one ton of steel. In the 1970s and 1980s there was an average requirement of 1.44 ton of raw materials needed to produce the 1 ton of steel, the current requirements are averaging to 1.15 ton of raw materials to make 1 ton of steel. This 21% decrease in required raw materials can be attributed to both process improvements as well as an increase in the recycling industry.
There are still a number of different places in the steel manufacturing process where waste is generated and recycling of that waste is difficult or impossible, generally due to size and chemistry constraints. This includes the fine particulates that are filtered out of the off-gases of the blast furnace, basic oxygen furnace, or the electric arc furnace. These fine particles are often collected in waste sludge, which may be condensed into filter cakes. These waste products still contain valuable elements which are thrown away, collecting in garbage dumps. New processing methods are being developed to amalgamate the fine particles into usable materials. This new process uses binding agents to create special briquettes or pellets out of the fine particles, which can then be added back into the steel manufacturing process. These manufactured briquettes and pellets have relatively uniform physical and chemical properties. The addition of these waste materials in this form is provides a much more controllable variable going into the melt. The uniform size allows for good predictability of the required power for incorporation into the melting operations, and the rate of melting. The uniform and controlled chemical composition allows for a reduction in raw materials added into steel manufacturing.
New sorting technologies are going to be required to ensure optimization of merchant scrap. There has been an increase in micro alloying in steel manufacturing in an attempt to promote high strengths without requiring large additions of costly alloying materials. These micro additions, while not accounting for a large compositional chemistry of the steel, may still be considered as a tramp element, and be undesirable for certain applications and heats.
New technology for dealing with the off-gases from steel manufacturing must be considered to prevent the carbon dioxide from going into the atmosphere. A small percentage of these gases are currently being used as a source of power or heat in normal operations, however there is laboratory work into capturing all these gases and converting them into a fuel source. The proposed methodology would result in a diesel fuel that can be converted to a kerosene fuel that could be utilized for aircraft.
With the current demand on the steel industry, coupled with the ever growing market of steel manufacturers there is more and more opportunity to improve recycling in the steel manufacturing process. Reduction in raw materials utilized throughout the plant will allow for a more sustainable industry and the sub industries that are fed from the success of steel manufacturing.
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