Steel scrap also referred to as scrap comes from ‘end of life’ products (old or obsolete scrap) as well as scrap generated from the steel manufacturing processes (new, prime or prompt scrap). It is metal which contains iron. Steel scrap can be processed and re-melted repeatedly to form new products. Because of the value of metal in the steel scrap, it is recycled or reused wherever it is possible. In fact, steel scrap is being recycled long before present awareness of environmental concerns started.
The primary sources for steel scrap are those products, for which steel is the main constituent. These are namely, vehicles (including ships and rail coaches and wagons), white goods, products of construction, machinery, electrical and electronic equipment, and packaging etc.
Steel scrap is considered to be free of alloying elements if the residual content of the certain elements contained in steel do not occur at levels consistent with the purposeful creation of an alloy steel. Residual level of elements contained within the steel scrap which is not to exceed is chromium – 0.2 %, nickel – 0.45 %, manganese – 1.65 %, and molybdenum – 0.1 %. The combined residuals level other than manganese is not to exceed a total of 0.6 %. A scrap is considered to be off grade if it fails to meet (i) applicable size limitations, (ii) applicable requirements for the type of scrap, and (iii) applicable requirement with respect to the scrap quality.
Steel scrap in itself does not pose any risk to the environment, i.e., there are no environmental risks in transportation and storage of the steel scrap itself. However, if the steel scrap is contaminated with oil or mixed with other waste, this can be considered hazardous in relation to the transportation or the storage. For example, oil or any other liquid attached to steel scrap, when exposed to rain, can cause contamination to its surrounding environment.
Steel scrap is generated during the production of steel, fabrication or production of the steel products, or when the product made of steel reaches its end of life. Because of the high value of the metal, the steel scrap is largely being recovered. Given the chemical and physical properties of the material, steel produced from the steel scrap can, in almost all applications, compete with primary steel produced from ore. However, the quantity of steel scrap collected and finally recovered depends on several factors, such as the collection system, the possibility and techniques used for the collection, etc. as well as a variety of legislation.
Steel scrap is a term used to describe that steel which has reached the end of its useful life (known as ‘post-consumer scrap’, or which has been generated during the production of steel products (known as ‘pre-consumer scrap’). While the term ‘scrap’ can lead one to believe that it is a waste product, it is actually a valuable raw material used in every steelmaking process. Because of its inherent magnetism, steel is very easy to separate and recycle, making steel the most recycled material in the world. Melting steel scrap at the end of its useful life allows the creation of new steels, by making adjustments to the chemistry and shaping of the new product. Scrap steel is separated and sorted, and can be used to make any new steel product.
Every steel plant uses steel scrap as part of its raw materials mix, and hence every steel plant is also a recycling plant. In blast furnace (BF) steelmaking plant, each charge of the basic oxygen furnace, in which carbon-rich liquid iron is refined into crude steel, typically contains 15 % to 25 % scrap. Scrap acts as a cooling agent, absorbing excess heat from the exothermic decarbonization process, and also as a source of iron units. In some cases, scrap is added directly to the blast furnace as a source of iron units, reducing greenhouse gas emissions. In electric steelmaking, electrical energy is used to re-melt charges of up to 100 % scrap to make new steel products.
All the steel scrap which is available to the steel industry is used repeatedly to create new steel. Recycled steel maintains the inherent properties of the original steel and the quality can also be improved on recycling. Steel scrap use in the steel industry reduces considerably the use of other natural resources, such as iron ore, coal, and limestone.
In theory, all new steel can be made from recycled steel. However, presently, this is not feasible because of the scarcity of steel scrap. This is because of the long service life of steel products, given steel’s strength and durability. The average life of steel products ranges from a few weeks for steel packaging to up to 100 years for buildings and infrastructure. The average lifespan of a steel product is around 40 years which means there is a considerable delay between steel being produced and being available for recycling.
Continued growth in the demand of steel means that transitioning the industry to entirely scrap based production is unlikely to be possible during this century. Steel demand is growing at a faster rate than the steel scrap is being released from the pool of ‘steel in use’. All the steel scrap presently being collected is recycled. As such there is only limited scope to increase scrap availability. Any future increase in availability is to be drawn from the expected increase of post-consumer scrap availability.
There is a difference between carbon steel scrap and stainless-steel scrap since the carbon steel differs from stainless steel by composition and treatment. Carbon steel scrap is mainly used for the production of steel in induction furnace (IF), electric arc furnace (EAF) and partly in basic oxygen furnace (BOF) as well. Stainless steel scrap is used to produce new stainless steel in the electric arc furnace.
Basically, for the purpose of electric steelmaking, heavy melting scrap (HMS) is used. It is industrial or commercial scrap steel higher than 6 mm thick, such as plates, beams, columns, and channels etc. and can also include scrap machinery or implements or certain steel stampings. Heavy melting scrap is normally broken into two categories namely HMS 1 and HMS 2. The difference between the two is that HMS 1 does not contain galvanized and blackened steel while HMS 2 does contain galvanized and blackened steel.
Steel scrap consists of discarded steel or steel products, normally segregated by composition and size or ‘grade’ suitable for melting. There are three main types of steel scrap which are used by the steel industry as feed stock. These are (i) internal scrap, (ii) prompt scrap, and (iii) obsolete scrap.
Internal scrap is also known as revert or home scrap. It refers to the reject metal within the steel plant or steel foundry which gets generated during steelmaking, steel casting, and steel finishing activities within the plant. Prompt scrap is also known as process scrap and it is the waste generated during the steel product manufacturing by the customers of steel plant and steel foundry i.e., the manufacturing industries for the steel products. Obsolete scrap consists of that scrap which is recovered from discarded industrial and consumer items i.e., from ships to refrigerators and from construction structures such as beams to automobiles.
The first two categories of steel scrap can be returned to the steelmaking process with little or no pre-treatment, while the obsolete scrap is needed to be separated from contaminants, sorted, and prepared for the steelmaking process. Because of the large improvements which have taken place in the steel manufacturing, steel casting, steel finishing, and product manufacturing technologies in the recent past, the quantity of generation of the first two types of steel scrap have reduced a lot. On the other hand, resources of obsolete scrap are increasing as the world is becoming more industrialized and because of the larger quantity of discarded consumer durables and worn-out industrial equipment etc. Steel scrap is also distinguished as (i) home scrap, (ii) new scrap, and (iii) old scrap depending on when it becomes scrap in its life cycle. Steel scrap is further distinguished according to its specific source.
Home scrap is the internally generated scrap during the production of the new steel products in the steel plants and the steel foundries. This form of the steel scrap rarely leaves the steelmaking production area. Instead, it is returned to the steelmaking furnace on the plant site and melted again. Technological advancements have considerably reduced the generation of home scrap which accounts for around 29 % of total steel scrap.
New scrap (also called prompt or industrial scrap) is generated in steel-product manufacturing units and includes such items as turnings, clippings, and stampings leftover when parts are made from steel during the manufacturing processes. This material is typically sold to the scrap metal industry which processes it for recycling to steel plants and steel foundries. The composition of new scrap is well known and in principle new scrap does not need any major pre-treatment process before it is melted, although cutting to size can be necessary. New scrap accounts for around 23 % of total steel scrap.
Old or post-consumer scrap results when industrial and consumer steel products (such as automobiles, appliances, buildings, bridges, ships, cans, and railroad coaches and wagons, etc.) have served their useful life. Old scrap is collected after a consumer cycle, either separately or mixed, and it is frequently contaminated to a certain degree, depending highly on its origin and the collection systems. Since the life time of several products can be more than ten years and sometimes even more than 50 years (for example products of building and construction), there is an accumulation of steel products in use since the production of the steel has started on a large scale. Old steel scrap accounts for around 48 % of total scrap. Fig 1 shows three types of steel scrap.
Fig 1 Types of steel scrap
Another way of classifying steel scrap is to classify it according to the products in which the steel was used before it became scrap. The primary steel scrap sources in this sense are automobiles, ships, railroads, construction buildings, machinery, white goods, packaging, and electric and electronic equipment etc. Fig 2 shows generation and recycling of the three types of steel scraps.
Fig 2 Generation and recycling of steel scrap
Specifications and standards
Presently, specifications and standard classifications for steel scrap exist at all levels, international, European, national, as well as between individual parties. It is clear that for the reason of marketing and trading, standards and specifications are needed not only to set the price but also used as reference for classification and controlling of the quality. In several cases based on the production need, steel scrap is processed according to the bilateral specifications agreed upon between the scrap processor and smelters.
Traded steel scrap is basically classified according to several properties which include (i) chemical composition of metals, (ii) level of impurity elements, (iii) physical size and shape, and (iv) homogeneity or variation within the specification.
Institute of Scrap Recycling Industry (ISRI) of USA has developed specifications for steel scrap which are known as ISRI specifications. This American standard provides the norms for classification of steel scrap and is used internationally. EFR (European Ferrous Recovery and Recycling Federation) and EUROFER (European Steel Association) have developed the European steel scrap specifications. These specifications cover the requirements from the safety perspective, the excluded elements for all scrap grades from a cleanliness point of view, and the tolerance for residual and other metallic elements. These specifications also provide a detailed description of the specifications by category, which corresponds to the type of scrap. National industry associations of some countries have developed their own specifications for steel scrap. There are also bilateral contract / specifications made as agreements or contracts in trade between two parties. Such a specification is normally based on a standard classification with additional requirements suitable for the desired production process or product.
Categories of steel scrap
Steel scrap is sorted and processed into grades for re-melting in the steelmaking furnaces. Further, steel scrap is to be cleaned. Cleanliness in case of steel scrap means that all the grades of the steel scrap are to be free of dirt, non-ferrous metals, excessive rust and corrosion, or foreign material of any kind. However, the terms ‘free of dirt, non-ferrous metals or foreign material of any kind’ are not intended to preclude the accidental inclusion of negligible quantities where it can be shown that the quantity is unavoidable in the customary preparation and handling of the particular grade pf the steel scrap involved. Steel scrap is also not to contain any non-metallic or steel by-products such as mill scale, slag, grinding dust or swarf.
Wherever the term ‘free of alloys’ is used in the classifications, it means that any alloying elements contained in the steel are residual and have not been added for the purpose of making alloyed steel.
Steel scrap normally includes carbon steel scrap, and stainless-steel scrap. Different categories of carbon steel scrap which are normally used include (i) heavy melting steel scrap, (ii) number 2 heavy melting steel scrap, (iii) number 1 and number 2 hydraulic bundles, (iv) number 1 and number 2 busheling – prepared, (v) plate and structural steel, (vi) hydraulic silicon bundles, (vii) number 1 bushelling (clips), (viii) short shoveling steel turnings (crushed), (ix) machine-shop turnings, (x) mixed turnings and borings, (xi) cast steel borings, (xii) number 1 machinery – cast, (xiii) mixed cast, (xiv) number 1 and number 2 shredded scrap, (xv) briquetted steel turnings – alloy free, (xvi) briquetted steel turnings – alloyed, and (xvii) foundry steel etc. Some of the common categories of carbon steel scrap are described below.
Punchings and plate scrap – The desirable characteristics of these scraps are that they are normally clean, free from rust, melt with high yield and are of known composition. Materials from stamping plants are normally low in carbon, manganese, phosphorous, and sulphur.
Number 1 busheling scrap – This scrap is similar to punchings and plate scrap. For the most part, this scrap category contains more steel clippings (up to 300 mm in length). The scrap is clean steel scrap and includes new factory busheling (e.g., steel clipping, stamping, etc.). It does not include old auto body and fender stock. It is free of metal which is coated, limed, vitreous enameled, and electrical sheet containing over 0.5 % silicon.
New black steel sheet clippings – This scrap is for direct charging with maximum size 2.5 metre (m) by 0.5 m. It is free of metal which is coated, limed, vitreous enameled, and electrical sheet containing over 0.5 % silicon.
Shredded clippings – This type of scrap represents another factory grade of scrap. It resembles shredded automobile scrap (ferrous fraction) with similar density.
Number 1 scrap bundles – These bundles are also known as hydraulic bundles and are tightly compacted bales of light gauge scrap produced in hydraulic baling machines. The material is collected from press shops, and consists of clean sheets, strips, and trimmings which can also be categorized as punchings and plate scrap or as number 1 busheling scrap. These bundles can also include new black steel sheet scrap, clippings, or skeleton scrap, compressed to charging box size, and have a density of not less than 1.2 tons per cubic meter (t/cum). The bundles are tightly secured for handling with a magnet. The bundles can include tightly secured mandrel wound bundles or skeleton reels, chemically de-tinned material, and old auto body and fender stock. The scrap bundles are free of metal which is coated, limed, vitreous enameled, and electrical sheet containing over 0.5 % silicon.
Number 2 scrap bundles – These bundles are also produced in hydraulic baling machines and contain considerable quantity of steel sheets which have been galvanized or otherwise coated with zinc. With these bundles, not only have the contaminants which result in a poor-quality melt, but also the yield is poor (around 70 %). These bundles consist of old black and galvanized steel sheet scrap, hydraulically compressed to charging box size and have density not less than 1.2 t/cum. The bundles do not include tin coated or lead coated material or vitreous enameled material.
Number 3 scrap bundles – These bundles are also produced in hydraulic baling machines and contain old steel sheet, compressed to charging box size. The bundles have density not less than 1.2 t/cum. It can include all coated ferrous scrap not suitable for inclusion in number 2 bundles.
Electric furnace bundles – These bundles are simply smaller version of number 1 bundles and are so named because of their better suitability for charging through the smaller doors of electric arc furnaces.
Prompt silicon grades – This scrap category consists of punchings, trimmings, and skeletons from high silicon electrical sheet. These represent very pure scrap categories in terms of tramp elements but are high in silicon, which restricts their use.
Flashings – This scrap category is also called forging scrap and consists mainly of croppings, and frequently includes defective forgings. This category of scrap shows the effect of forging temperatures, mainly in the form of scale.
Heavy home scrap from steel plants – This scrap includes items such as ingot butts, billets, blooms, slab crops, and heavy beams and rail crops.
Railroad wheels and track materials – Railroad wheels and track materials represent a class of scrap where impurities are normally low. Users of this scrap are also to consider that railroad wheels and track materials are being alloyed increasingly with chromium and molybdenum. Also, it is to be remembered that majority of the earlier cast iron wheels are being replaced with medium carbon steel forgings.
Cut plate and structural scrap – This type of scrap is covered under several different codes, depending on the size of the pieces. These categories are more commonly used for charging large electric furnaces than for cupolas.
Number 1 heavy melting scrap – This scrap category is characterized by a higher percentage of impurities than is found in cut plate and structural scrap, as well as higher alloy content since it normally consists of high-strength low-alloy (HSLA) steels. It is normally available in lengths under 1.5 m for charging into basic oxygen furnaces and large electric arc furnaces. It can also include wrought iron and / or steel scrap of 6 mm and above in thickness. Individual pieces are normally not above 0.6 m x 1.5 m (charging-box size) prepared in a manner to ensure compact charging.
Number 2 heavy melting scrap – This scrap category differs from number 1 heavy melting scrap mainly in that the lower limit of thickness is 3 mm, and more coated steel is allowed. It includes wrought iron and steel scrap, black and galvanized steel, 3 mm (millimeter) and above in thickness. The scrap is of charging-box size and includes material not suitable as number 1 heavy melting scrap.
Shredded scrap – It is normally similar to shredded clippings except that shredded scrap is likely to contain more shreddings from auto bodies, and can contain more plastics, aluminum, and other contaminants. It includes homogeneous iron and steel scrap, magnetically separated, originating from automobiles, unprepared number 1 and number 2 steel, and miscellaneous baling and sheet scrap. Average density of this category of scrap is 0.8 t/cum.
Shredded clippings – These clippings consist of shredded low carbon steel clippings or sheets. This scrap category is to have an average density of 1 t/cum.
Shredded tin cans for re-melting – This scrap category includes shredded steel cans (tin coated or tin free) and can include aluminum tops but is to be free of aluminum cans, non-ferrous metals except which is used in can construction, and non-metallics of any kind.
Auto slabs – Auto slabs have essentially the same chemical composition as number 2 scrap bundles but they provide more yield since they contain less trash.
Briquetted steel turnings – Briquetted steel turnings are classified in accordance with several codes including a number of types (and grades of purity) which depends on whether or not the turnings are mixed with cast iron borings.
Steelmaking slag scrap – Steelmaking slag scrap is normally considered to be a low-grade melting material. This material consists of irregular steel nuggets which have been separated magnetically from crushed slag. Melting yield with this category of scrap normally ranges from 70 % to 80 %.
Machine shop turnings – These include clean steel or wrought iron turnings, free of iron boring, non-ferrous metals in a ‘free state’, scale, or excessive oil. It does not include badly rusted or corroded material.
Machine shop turning and iron borings – These are the same as machine shop turnings but include iron borings.
Shoveling turnings and iron borings – These are the same as shoveling turnings but include iron borings.
Fragmentized scrap from incineration – Fragmentized incinerator scrap is loose steel scrap which is processed through an incinerating plant for household waste followed by magnetic separation, fragmentized into pieces not exceeding 200 mm in any direction and consisting partly of tin coated steel cans. This scrap is required to be prepared in a manner to ensure direct charging. The scrap is to be free of excessive moisture and rust and is to be free of excessive metallic copper, tin, lead (and alloys) and steriles (cleanliness) to meet the aimed analytical contents.
Some of the common categories of stainless-steel scrap are given below.
200 series stainless steel scrap solids – This type of scrap includes all types of clean 200 series of stainless-steel scrap solids, which contain a maximum of 0.5 % copper.
Stainless steel scrap – Stainless steel scrap consists of clean 18-8 type (300 series) stainless steel clips and solids containing a minimum of 16 % chromium, 7 % nickel, a maximum of 0.5 % phosphorous, and 0.03 % sulphur and is otherwise free of harmful contaminants. Typical scrap comes from the manufacture of sinks, tanks, and pipes, etc.
Stainless steel turnings – Stainless steel turnings consist of clean 18-8 type (300 series) stainless steel turnings containing a minimum of 16 % chromium, and 7 % nickel, and are free of non-ferrous metals, non-metallics, excessive iron, oil and other contaminants.
400 series stainless steel scrap – This scrap category consists of clean 400 series stainless steel containing no nickel and 10 % to 17 % of chromium, and is otherwise free of harmful contaminants.
General conditions applicable to all the grades of steel scrap
The general conditions which are applicable to all the grades of steel scrap include (i) all the grades of steel scrap are not to contain steriles more than 1 % by weight of iron, (ii) steel scrap is to be free from excessive rust, corrosion, dirt, oil, or grease, (iii) steel scrap is to be free of hazardous materials, such as, but not limited to oil filled devices, explosives, radioactive materials, military scrap including fire arms, and ammunition casting etc., (iv) steel scrap is not to include pressurized, closed, or insufficiently open containers of all origin since this can cause explosion, (v) steel scrap is not to have material which are having radioactivity in excess of the ambient level of radioactivity, (vi) steel scrap is to be free of non-metallic materials, earth, insulation material, excessive iron oxide in any form, except nominal quantities of surface rust arising because of outside storage of the prepared scrap under normal atmospheric conditions, (vii) steel scrap is to be free of all but negligible quantities of non-ferrous metals and such as aluminum, zinc, tin in any form, and lead and lead containing materials, (viii) steel scrap is to be free of visible metallic copper and free of all material which is having high dissolved copper content, (ix) steel scrap is to be free of alloyed steels as well as stainless steels, (x) steel scrap is to be free of all but negligible quantity of combustible non-metallic materials such as rubber, fabric, wood, and lubricants etc., (xi) steel scrap is to be free of large pieces of materials which are non-conductor of electricity, (xii) steel scrap is to be free of waste arising out of steel melting, surface conditioning, grinding, sawing, welding, and torch cutting operations etc., and (xiii) bulk densities of different types of steel scraps vary in the range of 0.4 t/cum to 1.2 t/cum.
The general conditions as given below are applicable to all grades of steel scrap and as is practically achievable in customary preparation and handling of the grade involved. The definitions of this list of specifications apply only to non-alloy carbon steel scrap as raw material for the steel industry.
Environmental, health and safety considerations
Safety – All grades of steel scrap are to exclude pressurized, closed, or insufficiently open containers of all origins which can cause explosions. Containers are to be considered as insufficiently open where the opening is not visible or is less than 100 mm in any one direction.
All grades of steel scrap are to exclude dangerous material, inflammable or explosive, fire-arms (whole or in part), munitions, and dirt or pollutants which can contain or emit substances dangerous to human health or to the environment or to the steel production process. All the grades of steel scrap are to be checked, within the limitations of accessibility and in strict compliance with appropriate detection equipment for radioactivity, to identify (i) material presenting radioactivity in excess of the ambient level of radioactivity, (ii) radioactive material in sealed containers even if no significant exterior radioactivity is detectable because of shielding or because of the position of the sealed source in the scrap delivery.
Steriles (cleanliness) – All grades of steel scrap are to be free of all but negligible quantities of other non-ferrous metals and non-metallic materials, earth, insulation, excessive iron oxide in any form, except for nominal quantities of surface rust arising from outside storage of prepared scrap under normal atmospheric conditions. All grades of the steel scrap are to be free of all but negligible quantities of combustible non-metallic materials, including, but not limited to rubber, plastic, fabric, wood, oil, lubricants, and other chemical or organic substances. All grades of steel scrap are to be free of larger pieces (brick-size) which are non-conductors of electricity such as tyres, pipes filled with cement, wood, or concrete. All grades of steel scrap are to be free of waste or of by-products arising from steel melting, heating, surface conditioning (including scarfing), grinding, sawing, welding, and torch cutting operations, such as slag, mill scale, baghouse dust, grinder dust, and sludge.
Residual and other metallic contents
Copper – All grades of steel scrap are to be free of visible metallic copper which means free of copper – wound electric motors, sheets and copper coated materials, bearing shells, winding, and radiator cores. All grades of steel scrap are to be free of all but negligible quantities of wire, insulated wire, and cable tubing, and other copper, brass items mixed with, attached to, or coated steel scrap. All grades of steel scrap are to be free of material with high dissolved copper content such as rebars and merchant bars which are to be grouped in the high residual grades.
Tin – All grades of steel scrap are to be free of tin in any form such as tin cans, and tin coated materials etc. as well as bronze elements such as rings, and bearing shells etc.
Lead – All grades of steel scrap are to be free of lead in any forms such as batteries, solder, wheel weights, terne plate, cable ends, bearings, and bearing shells etc.
Chromium, nickel, and molybdenum – All grades of steel scrap are to be free of alloyed steels and stainless steels as well as of mechanical parts (which mainly contain these elements) such as motors, drive gears for trucks, axles, gear boxes, gear wheels, tools, and dies as well as non-magnetic pieces.
EU-27 and ISRI specifications indicate the maximum levels for certain of these metallic elements in the different grades of steel scrap. Supply of steel scrap not falling within the analytical limits of these specifications, however, is permitted with specific prior agreement between supplier and purchaser based on the knowledge of the real nature / content of the material in question. Specific or contractual maximum contents are subject to agreement between supplier and purchaser and are to be specified when ordering.
Mixtures of grades – No delivery of steel scrap is to contain a mixture of grades, unless by joint agreement.
Facts regarding recycling of steel scrap
The facts associated with steel scrap and its recycling include (i) steel scrap plays a key role in suppressing industry emissions and resource consumption, with every ton of scrap used for steel production avoids the emission of around 1.5 tons of CO2 (carbon di-oxide) and the consumption of around 1.4 tons of iron ore, around 740 kilograms (kg) of coal, and around 120 kg of limestone, (ii) almost 40 % of the global steel production is made from steel scrap, (iii) carbon di-oxide emissions are reduced by 58 % with the use of steel scrap, (iv) recycling one ton of steel scrap saves 2.3 cum of landfill space, (v) recycling of steel scrap uses 75 % less energy compared to creating steel from raw materials, (vi) steel scrap recycling uses 90 % less virgin materials and 40 % less water, (vii) steel scrap recycling produces 76 % fewer water pollutants, 86 % fewer air pollutants, and 97 % less mining waste.
Steel automobile frames contain at least 25 % recycled steel scrap and a typical electrical appliance is normally made of 75 % recycled steel scrap. Steel cans consist of at least 25 % recycled steel scrap.
Steel scrap sorting and preparation techniques
Some of the steel scraps (prompt scrap) accrue in production itself and are directly fed back into production, others (obsolete scrap) are returned to the industry from other industries or after use of steel containing end use products. Typically, prompt scrap is of a more uniform quality and more readily available than obsolete scrap which frequently contains larger quantities of impurities and accrues after significant time lags between production and return to the industry.
Steel scrap quality is a limiting factor for increased scrap use. Certain types of steel scrap contain high quantities of trace elements. High-quality steel needs a low trace element content. In the present practice, steel scrap is used for lower quality steel grades, such as reinforcement bars. However, electric arc furnace steel producers are gradually moving toward the production of higher quality products. In the longer run, declining steel scrap quality and increasing steel quality can pose serious challenges for the use of steel scrap.
The lower is the quality of the steel scrap, the lower is the yield, and hence the higher is the energy requirement per ton of desired output. Since prompt steel scrap is a waste product of the industry itself and does not replace raw material inputs, increases in prompt steel scrap use or quality does not noticeably improve energy efficiency. Only expansion of obsolete steel scrap use can lead to an overall reduction of energy requirements to the extent that obsolete steel scrap substitutes for steel production from ores.
The numerous sources and forms of steel scrap needs the use of different scrap sorting and preparation techniques to remove the contaminants and / or recover other valuable materials (i.e., non-ferrous metals, and other materials) prior to entering the steelmaking process.
Large items such as ships, automobiles, appliances, railroad cars and structural steel are to be cut to allow them to be charged into a furnace. This can be done using shears, hand-held cutting torches, crushers, or shredders.
Size reduction of steel scrap is normally done before it is supplied to steel plant for melting. Size reduction processes use a wide range of equipment for reducing the size of large scrap material into pieces small enough to enable consolidation, shipment, and subsequent feeding into furnaces. The equipment used to accomplish this includes shears, flatteners, and torch-cutting and turning crusher. This equipment is normally operated by dealers and processors who prepare the scrap to be fed into the steelmaking furnaces.
Processing of steel scrap involves size reduction, cleaning, sorting, shredding, pressing, shearing, and crushing. Steel scrap processing also results into increasing of the density of the scrap mass. Schematics of typical scrap processing cycle is at Fig 3.
Fig 3 Schematics of typical scrap processing cycle
Sorting – Sorting is the process of separating the different metals and other materials from steel scrap.
This is done using magnets, eddy current separators, screening, blowing / suction (air classifier), flotation (gravitational separation), optical separation, and manual separation. Manual sorting involves obviously the removal of components from the scrap by hand. By this method the separation of steel scrap from non-metallics is accomplished manually.
It is the most suitable when miscellaneous attachments have to be removed from the scrap (i.e., radiators from scrap cars, plastic end tanks from radiators) or when manual off-loading is unavoidable.
Pressing – Pressing means compacting the press thin sheet steel scrap into bales. Loose steel scrap which has a high surface area and low density (i.e., lathe turnings) is required to be compacted by baling or briquetting. The pressed sheet steel scrap can be waste from a punching press or cutter. In the press, the thin, light steel sheet is compressed into cubes. Pressing is also used e.g., to reduce the volume of junk cars (logging). The purpose of pressing is in fact to reduce the volume of the light scrap sheet metal and to increase the volume weight to a weight suitable for the electric steel making. Moreover, pressing can also reduce transport costs and facilitate storage. Pressing can also include briquetting, i.e., pressing lathe chips into briquettes.
A baling press is a heavy piece of processing equipment which uses up to three hydraulic rams to compress the steel scrap which needs higher density before its remelting. With 450 kilowatts, the largest baling press can take three flattened autos without engines and in less than two minutes produce a 2.5 tons bale of size 900 mm x 600 mm x 1,500 mm.
In case of a briquetting machine, small scrap is compacted into pockets as it passes between two counter rotating drums. Compaction can be assisted with heat depending on the material.
Crushing – Crushing is done with different types of metal crushers. Crushing is used in the processing of thin or dirty sheet steel scrap and junk cars. The purpose of crushing is to break down the steel scrap metal into smaller pieces so that the different metals can be separated and the impurities removed. For example, crushing cars produces clean, palm-sized, rust-free clumps that can be picked off later with a magnet. In this phase, also the plastics, wood, rubber, sand and other impurities are crushed so small that they can be removed.
Shearing – Shearing means the cutting and pressing of thicker scrap steel. Shearing takes place in big, guillotine-like scrap shears. Steel beams and miscellaneous scrap are cut into 600 mm or 800 mm pieces. Shearing increases the bulk density of the scrap, making it easier to handle and portion out. A hydraulic guillotine shear slices heavy pieces of steel including ‘I’ and ‘H’ beams, ship plate, pipe, and railroad car sides. Shears vary in size from 300 tons to more than 2000 tons of head force.
Shredding – Shredders incorporate rotating magnetic drums to extract steel from the mixture of metals and other materials. Shredders or fragmentizers can reduce old automobile hulks into fist-sized pieces using massive hammer-mills. A medium-size shredder uses 36 hammers weighing 120 kg each to pound auto hulks to pieces. Although the predominant raw material for the shredder is automobile hulks, ‘white goods’ (household appliances such as stoves, washers, dryers, and refrigerators) and other large items can also be shredded. Depending on its size, a shredder can process from 1,500 tons to more than 20,000 tons of steel scrap per month.
The shredding process produces three types of material namely (i) steel scrap, (ii) shredder residue (light fraction), and (iii) shredder residue (heavy fraction). The two residue fractions, either singularly or collectively, are frequently referred to as automotive shredder residue (ASR). ‘Shredder fluff’ is the term given to the low density or light materials, which are collected during the shredding process for cyclone air separation. Each ton of steel which is recovered produces around 300 kg of ASR, comprised of plastics, rubber, glass, foam and textiles, contaminated by oil and other fluids. The steel scrap is recovered by the shredder operator using magnetic separation and used as charge for the steelmaking furnaces. The ASR heavy fraction contains primarily aluminum, stainless steel, copper, zinc and lead.
Magnetic separation – Magnetic separation is used when large quantity of steel scrap is to be separated from other materials. Permanent magnets and electromagnets are used in this process. The latter can be turned on and off to pick-up and drop items. Magnetic separation can be of either the belt-type or the drum-type. In the drum, a permanent magnet is located inside a rotating shell. Material passes under the drum on a belt. A belt separator is similar except that the magnet is located between pulleys around which a continuous belt travel. Magnetic separation has some limitations. It cannot separate steel from nickel and magnetic stainless steels. Also, composite parts containing iron will be collected and can contaminate the melt. Manual sorting can be used in conjunction with magnetic separation to avoid these occurrences.
Eddy current separation – Eddy current separators are used to separate non-ferrous metals from waste and ASR. The process normally follows the primary magnetic separation process, and it exploits the electrical conductivity of non-magnetic metals. This is achieved by passing a magnetic current through the feed stream and using repulsive forces interacting between the magnetic field and the eddy currents in the metals. The simplest application of the process is the inclined ramp separator. This uses a series of magnets on a sloped plate covered with a non-magnetic sliding surface such as stainless steel. When a feed of mixed materials is fed down the ramp, non-metallic items slide straight down, while the pieces of steel scrap are deflected sideways by the interaction of the magnetic field with the induced eddy current. The two streams are then collected separately. Variations of the eddy current separator include the rotating disc separator, in which magnets are arranged around a rotating axis. Yet another system uses a conveyor with a head pulley fitted with magnets. Both systems rely on the varying trajectories of materials either affected or unaffected by magnetic fields, to make the separation.
New steel scrap processing technologies have been developed. Portable optical emission spectrometers are evolving as important tools for the on-site sorting and identification of metals. Their analytical precision and accuracy, while not as good as laboratory systems, are more than adequate for sorting mixes and most grade verification requirements. A portable spectrometer is capable of separating different types of steel in addition to separating at least 90 % to 95 % of the individual grades which make up each type of steel.
Colour sorting is one of the first automated sorting processes to be used industrially, and it was developed by the Huron Valley Steel Corporation (HVSC), which is the world’s largest non-ferrous scrap sorter. HVSC has used this technique to sort zinc, copper, brass and stainless steel. Colour sorting is based on computer image analysis where the colour of each metallic piece is detected. Pieces whose colour lies within a specified range are automatically directed out of the feed material. In order for this to work properly, a singling mechanism is used to produce a chain-like profile of scrap particles before the image detector. HVSC’s colour sorter has proven to be very accurate, producing metal purities over 98 %. This purity is possible since this sorting method is independent of particle size and shape. The technological advancement of computers over the last decade has greatly increased the speed of real-time image analysis. Because of the advancement of industrial colour sorters over the last few years, the ability to effectively sort different metals with slight colour variations has improved dramatically.
Decoating techniques – There are presently a number of processes used in scrap processing industry for decoating of steel scrap. Any zinc-bearing scrap included in the charge results in discharge of zinc oxide in the flue dust. The main source of zinc is galvanized steel sheet scrap. The removal of zinc using thermal methods can be accomplished using different techniques such as (i) the galvanized parts are heated to a high temperature (higher than 900 deg C) at which the zinc evaporates., (ii) the galvanized parts are heated to a temperature sufficient to embrittle the coating which is then removed by abrasion, and (iii) heating and subsequent removal of the coating is accomplished by shot blasting. Zinc removal can also be carried out using chemical techniques in which ammonia leach or caustic soda is used to dissolve the zinc coating from galvanized scrap.
In case of detinning, tin-bearing scrap (i.e., food containers and auto bearings) in steel scrap melting affects the surface quality of the steel products since tin segregates to the grain boundaries and causes surface scabs during working. Some of the processes which are being used for detinning the tin-plate scrap, include electrolytic and alkaline detinning.
Further, incineration is being used by some scrap processors to remove combustible materials including oil, grease, paints, lubricants and adhesives.