Mill scale, a waste product of steel manufacturing, is a hidden treasure that can be utilised in various sectors to save a significant amount of money. This is a mixture of iron oxides that possesses certain features, making it indispensable not only as a raw material for creating cement, but also for recycling and use in the construction of new buildings. Understanding mill scale’s types, applications, and safe usage is crucial for maximising its potential in modern industrial processes and in sustainable production practices.
What is Mill Scale?
Mill scale is a layer of iron oxide that develops on the metal surface during the high-temperature processes of hot rolling to make steel or iron. In the process of steel production, at high temperatures, in the presence of oxygen, iron oxides like FeO, Fe2O3, and Fe3O4 are formed. It can have a content of iron as high as 65-75%. The presence of mill scale in the production of cement, steel recycling, construction work, etc., has become important as it is one of the common ways to meet iron requirements and to reduce the cost of steel production through raw material replacement.
Properties of Mill Scale
- Formation: Besides the primary sources of reheating, multi-pass rolling, and air cooling, mill scale also forms upon steel oxidation in hot rolling. Such a type of scale is a by-product of the most common processes and is generally removed/washed away with water in descaling operations.
- Composition: Millscale is composed of iron oxides with wustite (FeO) as the main phase besides magnetite (Fe3O4) and hematite (Fe2O3).
- Physical State: A noncombustible mill scale is formed in a solid, powdered form and consists of small particles and flakes. It has a bulk density in the range of 5.7 tons/cum and a specific gravity of 5 – 5.2.
- Melting and Boiling Points: Mill scale has a melting point of about 1370°C and boiling point of about 2760°C. Under normal circumstances, mill scale is stable.
- Solubility: Water and alkalis cannot dissolve it, but most strong acids can. Though mill scale is categorised as green waste with no danger of causing fire, it can still be recycled.
- Mechanical Properties: The mechanical properties of the particles are poor, and thus, one will find different particle sizes in stockpiles. Mill scale is non-combustible and, consequently, a fire or explosion hazard of this material does not occur under normal conditions.
- Environmental Impact: Since no harmful substances such as nonferrous metals, radioactivity, or explosive materials are included in mill scale, it is environmentally safe.
Applications of Mill Scale
- Cement Clinker Production: While manufacturing, mill scale is returned to the cement clinker to lower the emissions of hazardous gases and increase the thermal efficiency. It also serves as a raw material that is mixed with the feedstock before it is sent to the rotary kiln.
- Iron and Steel Production: Being a rich iron source, mill scale is utilised to supply iron in the steelmaking process either by a reducing agent or by being mixed with industrial waste to generate the feedstock that yields low-slag alloys.
- Iron Oxides & Pigments: Heating the mill scale with iron oxides at high temperature results in the formation of black, red, or brown pigments. The high-temperature leaching is capable of recovering the ferric oxides for industrial consumption.
- Heat Management: Mill scale can bring about a very high energy efficiency in power arc furnaces because it is a material that causes the necessary exothermic reaction without the use of energy.
- Fertilizers, Heavy Concrete & Radiation Shielding: It is used as a magnetising additive, heavy aggregate, and a radiation-absorbing component for industrial applications.
Pros and Cons of Mill Scale
Pros
- Recyclable: Mill scale is reusable in steel manufacturing, so the amount of waste is less, and the conservation of resources is automatically ensured.
- Cost-Effective: Due to its high iron content, it can be a considerably less expensive alternative to raw iron.
- Eco-friendly: By using mill scale, the need for mining iron is significantly reduced, which in turn lowers environmental impact.
Cons
- Handling Difficulty: Mill scale is very fragile, and its breaking into small pieces is common during transportation; thus, special handling equipment is necessary.
- Moisture Sensitivity: If quality degradation occurs due to an exposure of mill scale to moisture, the performance will be negatively affected.
- Inconsistent Quality: Differences in the chemical composition and sizes of the material can influence its quality, making its use challenging in some high-precision industries.
How to Remove Mill Scale
It is necessary to remove the mill scales from steel surfaces if welding, painting, or coating is to be done, as it can act as an impurity and negatively affect the material’s finish and also lead to accelerated corrosion. The removal can be done by mechanical, chemical, or thermal methods, which are each suited for different applications.
Mechanical Methods
The mechanical methods depend upon abrasion or force to remove mill scale.
- Sandblasting works by applying high-speed sand or abrasive particles to the metal surface to remove thick mill scale on a wide area in a time-saving way. In this process, the use of protective gear is highly recommended.
- Grinding with abrasive wheels or ceramic flap discs gives the operator more accurate control of the surface area to be worked on or for finishing operations.
- Wire brushing uses hand-held or powered wire brushes for light to moderate scale removal and is the most suitable method for irregular and small surfaces.
Chemical Methods
Chemicals dissolve the metal or loosen the scale coatings.
- Acid pickling, which mainly uses hydrochloric or sulfuric acids, is very effective in scale removal, after which neutralisation is done to prevent corrosion.
- Phosphoric acid treatment not only removes the scale but also deposits a protective film that serves as a primer for paint.
The use of safety equipment, adequate ventilation, and other safety precautions is a must because of the chemical hazards involved in these methods.
Thermal Methods
The thermal method of mill scale removal is to heat the steel so that the mill scale can be broken and separated. It should be noted that methods like flame cleaning or induction heating can help scale removal only in small areas, thus making the procedure quite slow. One must also be careful to avoid any kind of distortion of the metal.
How to Safely Use Mill Scale in Construction Projects
The safe and effective use of mill scale can be achieved by practicing the basic safety measures discussed below.
- Safe Storage: Keep mill scale in a place that is dry and well-ventilated. This will ensure that the product will not get wet and its quality remains intact. Moreover, this will prevent safety hazards, such as the occurrence of rust, which is caused by water vapor.
- Protective Equipment: In case mill scale is being moved, workers should put on their protective equipment (PPE) which includes gloves, masks, and safety goggles to avoid getting into direct contact with the harmful particles or dust generated.
- Safe Handling: Limit the number of times the mill scale is handled, especially when it is in its raw form, as it is very dangerous if the dust particles are inhaled. You should have a proper dust extraction or ventilation system when processing.
- Quality Control/ Reliability in Production: Proper sieving or briquetting of mill scale should be carried out to meet the requirements of the material used for making concrete or other applications. Quality control is the key to consistency.
Conclusion
Mill scale is an important source of value for the industrial sector that includes the construction, cement, and steel industries. Besides being a rich iron source and having multiple uses, mill scale also offers both monetary and environmental benefits. Therefore, by recognising its kinds, features, and taking proper precautions while handling the material, the industrial sector can harness its full capacity. The involvement of mill scale in eco-friendly activities and the efficient use of available resources has made it an indispensable item in contemporary industrial processes.
Apoorva H P Achar