Mill Scale

Mill Scale

Mill scale is the product of oxidation which takes place during hot rolling. The oxidation and scale formation of steel is an unavoidable phenomenon during the process of hot rolling which involve reheating of steel in a reheating furnace, multi-pass hot rolling and air-cooling in the inter-pass delay times and after rolling.  Mill scale is usually removed by process water used for descaling, roll and material cooling, and by other methods. It is subsequently separated by gravity separation techniques.

The formation of oxide scale not only results in a significant loss of yield of steel, but also deteriorates the surface quality of the steel product caused by rolled-in scale defects or roughened surface. In addition, the presence of a hard scale layer on the steel can have an adverse effect on roll wear and working life. The amount of mill scale generated in a rolling mill depends on the type of the reheating furnace and on the practice of rolling adopted in the mill. It is generally in the range of 1 % to 3 % of the weight of the steel rolled.

Mill scale mill scale is a layered and brittle material, composed of iron oxides with wustite as a predominant phase. It is normally considered as waste material. From the chemical and physical analysis performed on the mill scale, and with respect to the environmental concerns, mill scale is considered to be non-dangerous waste and normally considered as a green waste.

Scale formed during the heating of steel to rolling temperatures in the reheating furnace is known as primary scale. This primary scale is removed generally by hydraulic descaling before hot rolling. The removal of the primary scale formed during the reheating operation before hot rolling is usually done for producing steel products with high surface quality and for reducing roll wear. However, secondary scale continues to form on the descaled steel surface during the inter-pass delay time in the roughing and intermediate rolling mills. The colour of primary mill scale is generally bluish black while that of the secondary scale is blue.  The secondary scale gives the steel an appearance which is similar to that of a lacquer coating finish.

Properties of mill scale

Under visual inspection, mill scale appears as a black metal powder made up of small particles, flake, and chips. Its physical state is solid and powdered. The bulk density of mill scale is in the range of 5.7 tons/cum while the specific gravity is 6.2. The melting point of mill scale is around 1370 deg C and the boiling point is around 2760 deg C. It has a stable state. It is insoluble in water and alkalis but soluble in most of the strong acids. It is normally classified as non-dangerous waste material.

Mill scale particles, flakes, and chips are brittle and have low mechanical strength. They easily break hence the stockpiles of mill scale are likely to be made up of varied particle sizes. Mill scale is not combustible. Under normal conditions, fire and explosion hazards are not associated with the mill scale.

The mill scale does not generally contain any dangerous, inflammable, radioactive or explosive substances. The material is free of dirt, nonferrous metals, or foreign material of any kind, and excessive rust and corrosion.

The iron in the primary mill scale is usually present in different chemical forms. The primary scale has three layers of iron oxides consisting of wustite (mostly FeO), magnetite (Fe3O4), and hematite (Fe2O3) from the metal surface outwards.

Wustite is the inner most phase of the scale which forms next to the metal and is the Fe rich phase. It has the lowest O2. It is represented as FeO, and is not stable below around 570 deg C. However, it?s content in the scale increases with increasing temperature and occupies around 95 % of the scale layer when the steel temperature is above 700 deg C. The density of wustite is around 5.87 g/cu cm. Wustite exists as a thermodynamically stable, single phase structure, over a wide range of composition. The non-stoichiometry of wustite increases with increasing temperature and does not seem to reach the stoichiometric composition FeO. Compared to the other scale phases and the steel itself, the wustite phase has a relatively low melting point, which is from 1370 deg C to 1425deg C. Melting of the wustite layer (washing) accelerates the scale formation rate and increases the grain boundary penetration. This not only reduces the surface quality, but also increases the fuel consumption of the furnace and reduces the yield.

The magnetite phase, Fe3O4 is the intermediate phase of the scale. It is the main equilibrium constituent of scale below 500 deg C. It has a density range of 5 g/cu cm to 5.4 g/cu cm. It exists as a metal deficient oxide but at a much smaller level than wustite. It has been shown from various studies that both cations and anions diffuse in Fe3O4. As the temperature increases to around 700 deg C, wustite formation takes place at the expense of the magnetite phase and at elevated temperatures, magnetite occupies only around 4 % of the total scale layer. Magnetite is harder and more abrasive than wustite.

The hematite phase, Fe2O3 is the outer most layer of the scale and has the highest oxygen content. It forms at temperatures above around 800 deg C. The density of hematite is around 5.24 g/cu cm. Hematite occupies around 1 % of the total scale layer at high temperatures. As with the magnetite phase, hematite is hard and abrasive.

The secondary mill scale is composed of iron oxides which predominantly consist of ferric oxide (Fe2O3). The thickness of this oxide layer is normally less than 0.1 mm. It initially adheres to the steel surface and protects it from atmospheric corrosion provided no break occurs in this layer. Since secondary scale layer is electro-chemically cathodic to steel, any break in this scale layer causes accelerated corrosion of steel exposed at the break. The secondary scale layer is thus a boon for a while since it protects the steel against corrosion. However this protection disappears when the coating breaks due to handling of the steel product or due to any other mechanical cause.

Mill Scale has been classified has an UVCB material (unknown or variable compositions, complex reaction products and biological material) since it is made of a large number of constituents and its composition can change according to the steel plant production (reinforcement bar, special steel etc.). It can contain C, Si, Ca, Na, Al, Mn and other metal oxides. The allowable limit for oil content in the mill scale is less than 1 % for all uses except batteries and melting charge for which upto 3 % is allowed. Typical average chemical analysis of mill scale is given in Tab 1.

Tab 1 Typical average chemical composition of mill scale
Sl. No. Element Unit Value
1 Fe % 71.1
2 P % max 0.06
3 S % max 0.1
4 Al2O3 % max 1
5 SiO2+CaO % max 1.5
6 MgO % 0.2
7 Na + K % 0.3
8 Si % 0.61
9 Ca % 0.44
10 Ti % max 0.05
11 V % max 0.05
12 Cr % 0.072
13 Mn % 0.059
14 Co % max 0.05
15 Ni % 0.0034
16 Cu % 0.0011
17 Nb % max 0.05
18 Mo % 0.0008
19 Sn % max 0.05
20 Mg % 0.12

The components and the micrograph of mill scale is shown in Fig 1

Fig 1 Components and micrograph of mill scale

The size of Mill scale normally varies from dust size in microns up to usually 6 mm. The typical size analysis of mill scale is given in Tab 2.

Tab 2 Typical size analysis of mill scale
Sl.No. Size Unit Value
1 More than 8 mm 0.4
2 6 to 8 mm 0.6
3 4 to 6 mm 3.8
4 2 to 4 mm 27.2
5 0.5 to 2 mm 32.0
6 Less than 0.5 mm 36.0

Mill scale is a nuisance when the steel is to be processed. Any coating applied over it is wasted, since it comes off with the scale as moisture laden air gets under it. All mill scale need to be removed to present a uniform and clean surface of the substrate steel for any application of any coating on the steel.

Removal of mill scale is virtually impossible by hand. It is extremely tedious and time consuming using power tool cleaning methods. Neither of these two methods gives a good base to start. Steel from the hot rolling mills has no surface profile, which is most important to the overall adhesion strength and integrity of the coating system. Mill scale is normally removed from steel surface by flame cleaning, pickling or abrasive blasting. These methods remove the mill scale and provide a surface profile that gives the coating system its design requirements. Coating over mill scale, however tempting, is a futile exercise, as the presence of mill scale on the steel surface accelerates the corrosion of the underlying steel.

Uses and recycling of mill scale

Mill scale though has a high level content of iron (68 % -72 %) yet it is still considered as an industrial waste in the form of iron oxide mainly because it gets contaminated with water and oil during the process of rolling. Most of the mill scale in a steel plant is recycled in the production of either iron ore sinter or iron ore pellets which are used either for reduction in iron making furnace. It is also used as a coolant in a steel making furnace. Mill scale recycling is also being done by briquetting it after mixing with a binder.

Mill scale is also used as a raw material in granular refractory. When this refractory is cast and preheated, mill scale provides escape route for the evaporating water vapour, thus preventing cracks and resulting in a strong monolithic structure.

Besides the above uses, several other uses for mill scale have been developed. The main uses of mill scale include (i) negative electrode for alkaline storage batteries, (ii) preparation and use of catalysts, (iii) in the production of cement clinker, (iv) in the preparation of heavy concrete and heavy weight aggregates, (v) composite counterweights for washing machines, (vi) in the production of ferro phosphorus and ferro molybdenum, (vii) in steel foundry and heat treatment of castings, (viii) in making flux for welding electrode coating, (viii) synergistic agent for mixed fertilizer and material for phosphate fertilizer, (ix) in the production of iron powder for powder metallurgy, (x) in the production of friction materials, (xi) in the manufacture of colored glass, (xi) in the production of Iron oxide pigments, (xii) in the method of making mineral wool, (xiii) in the iron oxide paint pigment precursor, (xiv) as an electromagnetic radiation shielding material, (xv) as a component of materials for road construction, (xvi) in the preparation of some refractory mixes, (xvii) in the treatments of water and soil, (xviii) in the production of exothermic powders. The different uses of the mil scale are shown in Fig 2.

Fig 2 Different uses of mill scale

Matters associated with the shipping of mill scale

Mill scale is being increasingly exported as a bulk cargo in 5he international trade. However in order to have mill scale fit for sea carriage, the stockpile is always typically accumulated at the port from different sources. This mill scale, having different particle sizes due to the way the material has been previously handled results into the individual stockpile at the port not homogeneous. This indicates that no two consignments do have same characteristics, even if these consignments have come from the same port or shipper. Further, mill scale is also a cargo which tends to drain water easily, accumulating at the bottom of a stockpile to form a ?wet base?.

Mill scale is traded effectively in the same way as primary mined iron ore fines since the physical nature of mill scale is similar to primary mined iron fines.  However, it is to be noted that the mill scale is not a proper ?bulk cargo shipping name? (BCSN) under the IMSBC (International Maritime Solid Bulk Cargoes) Code. It is a cargo that is not listed in the IMSBC Code.  IMO (International Maritime Organization) Circular DSC.1/Circ.63 of 12 October 2010 states that iron ore fines are a cargo that may liquefy and are therefore a ?Group A? cargo. ?Group A? cargo under the IMSBC code is a cargo that may liquefy if shipped at moisture content in excess of its transportable moisture limit (TML). The terms of this circular also applies to mill scale even though mill scale is normally carried in a dry condition with moisture content below its TML.

Though the mill scale cargo is not listed in the IMSBC Code, Section 1.3 of the IMSBC Code specifies that unlisted cargoes, such as mill scale, should only be accepted for loading provided it is accompanied by a certificate issued by the national competent authority, stating the commodities suitability for seagoing carriage, of the country of the port of loading.

The IMSBC Code also warns about the potential liquefaction hazard of all fine-grained mineral cargoes shipped with inherent moisture content, regardless of whether or not the cargo is specifically identified as a ?Group A? cargo in the Code.  Appendix 3, Para 2.1 of the Code states; “Many fine-particle cargoes if possessing a sufficiently high moisture content are liable to flow”.  Thus any damp or wet cargo containing a proportion of fine particles should be tested for flow characteristics prior to loading.

Mill scale does have fine particle sizes with significant inherent moisture content, and fall therefore within the scope of this provision.  Therefore, mill scale cargoes are treated as cargoes that may liquefy unless testing shows otherwise.  For ?Group A? cargoes, SOLAS (safety of life at sea) and the IMSBC Code require shippers to provide the Master with a certificate of the moisture content and the TML prior to loading.

Mill scale frequently shows a ?wet base?.  This occurs when the material drains well and accumulates water at the bottom of the stockpile.  Because of this drainage, the upper sections can appear quite dry. The wet base nature of mill scale makes it vital that any moisture sampling prior to loading does not just focus on the surface area of any stockpiles, and that a fully representative sample is taken. The need for representative sampling is also vital for the determination of the TML, particularly due to the variable nature of the material.  In order to sample stockpiles mechanical excavators are necessary as manually digging into the piles is impossible.

Also, because of the high density of mill scale cargo, the trimming requirements as detailed in the Code are that it should be trimmed flat for the voyage to distribute the weight evenly across the tank top.  Wet base cargoes are prone to cargo shift as the bottom liquefies and the top of the high-density stow becomes free to slide around over the wet base.  The only remedy for this is to trim the cargo properly.