Mettech Industrial Alloys of India claims to be the only company in that country capable of recycling mill scale using a German technology known as ‘Thermite’. Thermite is used for track welding and the production of low carbon ferro alloys. The process uses plain ‘mill scale’ as a raw material to recover clean metal, which in turn is poured into molds for the manufacture of different types of steel castings. In this article S N Gupta* and Sandeep Gupta* explain the origins of the process and its advantages.
THE Iron and steel industry is without doubt one of the most important foundations of any industrial economy. However, the industry is also a significant contributor to environmental pollution. It would, therefore, need to adopt more efficient and cleaner technologies to minimise its environmental footprint and ensure the long term sustainability of the industry.
Some years back, China started asking for supplies of mill scale. Steel rolling mills took note of this. Something that was thrown out as waste was being bought in whatever quantity available. Thus the talk of mill scale circulated. Some governments, like India and Turkey, also took note of this development and they stopped exporting this so-called waste material. The earliest well known use of this so-called waste material was by Dr.Hans Goldsmith of Germany who developed the compound ‘Thermit’ in 1895.This mixture of aluminium and mill scale was used to create an exothermic reaction to weld rail joints. It is still being used in many parts of the world.
Somewhere in the middle of 2000 A D the process of alumino-thermy came to be used in many countries for the production of low-carbon ferro alloys from their respective ores. In the early 70s the National Metallurgical Laboratory in Jamshedpur, India, took the lead and so did a few scientists from the Metallurgical Department of I.I.T.Kanpur and Bombay. They developed low carbon ferro alloys by using thermit, the exothermic compound.In India, ferro Titanium was the first low carbon ferro alloy produced from ilmenite, a sand found on a beach in Kerala.
The high cost of power and furnaces were a deterrent for setting up units for producing steel and alloy steel castings, the demand for which was accelerating owing to the massive growth of the construction, heavy engineering and steel industries. Smaller steel casting units looked for alternatives to overcome this problem and thus the rise of this new process. One idea was the use of exothermy to melt a required quantity of steel scrap. This in turn brought in problems like the design of melts pot, the lining, method of pouring, time required for this, removal of slag and so on. Persistence, however, paid off. The issues were satisfactorily resolved. The yield of iron from mill scale rose from 10% to 30%-40%. Today the yield of clean iron from mill scale can be higher than 50% by pre-treating it. The clean metal composition can be as high as 99.6% Fe. Suitably alloyed it can be used for the manufacture of very high value steel castings. This process is still making castings from the steel scrap available. The quantity of Thermit required to melt large scale steel scrap also brought to notice the yield of molten metal from mill scale. Hence the next effort was to melt mill scale only to recover clean molten metal.
The process for melting mill scale through exothermic reaction today stands to work as an alternative. Heavy Industries, particularly steel rolling mills, in so many countries throw out large quantities of mill scale as waste. Large amounts of this waste can be recycled using an exothermy compound to melt it.
The Benefits of recycling mill scale:
1. Waste dumped by steel rolling mills, which is recycled using technology that minimises pollution.
2. Dust pollution is also minimised.
3. It saves on transportation and the cost of disposing of this waste.
4. It also saves on the cost of waste storage.
5. The clean metal recovered can be sold to other steel foundries.
6. Finally, and most importantly, the waste is recycled within the country of origin, thus giving an initiative to others to find ways of recycling other types of waste.
7. It generates employment opportunities for locals.
The economic feasibility of the process is very attractive. It does not require costly plant and equipment. It is an exceptionally clean process and causes no pollution to the environment. It leaves no residue since the slag, hard like cinder, can be used in place of stone pebbles to fill up pits and potholes. Also the size of a steel foundry using this process can vary from 100Mt/ month to 200Mt/month. That said, caution has to be taken for this purpose.
To the best of my knowledge there is nobody else in India who is using this process to make castings on a commercial basis. There are bound to be a few people who may have tried melting mill scale, but no known party is using it for commercial production.
A few years ago there was a sudden demand for very large quantities of mill scale from China. Nobody knows for sure, but the little information gained showed that China was mainly using it for sintering and pelleting. India and a few other countries had put a ban on the export of mill scale to China. I do not know the present position, but I do know that steel mills are happy if we can take away the mill scale from their premises and save them the cost of disposal. I must stress that recycling mill scale to recover clean metal reduces the cost of producing castings and takes care of the pollution caused by it.