Having substantially damaged the environment and largely exhausted natural resources, it was only in the 21st century that humankind came to realize the dire need for further restricting tightening of industrial environmental policy. Additional stress is added by the persistent and continuing trend towards a crisis of the world economy, contrary to loud statements. After all, everyone knows that environmental technology and equipment, although vital for surviving of the mankind, loads heavily the production process, chewing off a good chunk of financial resource.
However, in some cases, these vectors can be turned in the right direction. In particular, such a profitable turn can be expected in iron milling. Especially in the situation when the environmental restrictions for metallurgical waste have once again become tougher.
Oily mill scale as an industrial waste
The traditional waste of rolling production is a mill scale (iron oxides). Scale is inevitably formed on the rolled product; when heated to almost 1000°C billet goes through the rolling mill, coming into contact with oxygen from atmospheric air and coolant water. Each pass of the billet through the rolls is accompanied by exfoliation of the scale layer and formation of a new one. During the complete rolling process, about 2.5% of the metal weight of the workpiece is slagged. The scale is discharged into the coolant stream, which contains hydrocarbons. As a result, heterogeneous metallurgical waste (particle size from 1 mm to 1 cm) is generated from the scale, lubricants and water – being oily mill scale.
During the production of cold-rolled steel, oily scale is also formed directly on the rolls. Slurry waste from surface sandblasting, also containing metal, but with much smaller particle size (less than 1 mm), is added to it. The content of iron in oily scale reaches 70-80%, in metallurgical sludge – 30%, which makes both wastes a valuable secondary resource.
Previously, the main method of such waste management was disposal in landfills in a pure form or after mixing with slag. Oily mill scale is a waste of hazard class 3 (moderately hazardous) and is formed in significant quantities.
Iron milling generates waste in the amount of 1/3 of raw material, and the slag makes up the bulk of it. According to the data provided, 1.3 million tons of oily scale has been accumulated in the dumps of the Nizhny Tagil Iron and Steel Works JSC, 1 million tons at the Pervouralsk Novotrubny Zavod JSC, over 2.5 million tons at Severstal PJSC.
In the Nizhny Novgorod region (Vyksa), a special landfill was built to accommodate scale of JSC Vyksa Steel Works. However, today the landfilling of waste containing more than 70% of iron is just not acceptable. Recycling (putting into sintering stage) of oily mill scale is impossible due to high content of oil products: up to 10% in oily scale and up to 50% in sludge. During sintering, which is preparation of the ore for melting, the raw material is heated and mixed with coal and lime, but the heavy hydrocarbons of the grease contained in the oily scale do not burn, and go into a volatile state and get on gas filters, which can lead to their ignition and even explosions when reaching certain concentrations in the electrostatic filter.
At the same time, if oil products are removed from oily scale, it can be loaded together with ore, thus avoiding losses of raw materials and solving environmental problems simultaneously.
Oily mill scale as an industrial waste – processing
There is a sufficient number of technologies for removing oil products from industrial solid waste, but each of them has both advantages and serious disadvantages. Landfilling is, in the short term, the cheapest way to dispose oil-contaminated metallurgical waste, but it not only affects production by taking up some of the raw material resource, but also shortens life of the landfill, while adding to the risk of groundwater contamination.
Incineration – burning out oil products from mill scale, significantly reduces the initial amount of waste. Incineration is carried out in a rotary kiln, with fuel supplied only for the initial ignition of the raw mix. Burning is facilitated by oxidation of iron oxide (II) to iron oxide (III). However, incineration, in turn, creates a problem of gas emissions and air pollution (or requires a powerful and expensive gas cleaning system). Incineration process was tested in Krivoy Rog and its results cannot be considered satisfactory due to high soot formation, which prevents briquetting in large quantities and a significant number of unburned oil products. Moreover, both of the above methods are currently at gunpoint of tightening environmental legislation.
Some amount of oily scale can be disposed of during formation of agglomerates, which combustion temperature reaches 1200-1300°С. However, laid on top of the batch, they form a barrier layer that complicates the charge combustion and leads to incomplete burning out of petroleum products.
Biological methods of oil products removal, which are relatively inexpensive and simple, work in this case too, but do not near 100% efficiency and require a long time.
An interesting results can be obtained from vacuum distillation process, especially in combination with thermal methods. However, oily scale disposal methods, which require serious investment, are still a serious problem for steel mills.
There are also some chemical methods to remove oil products from scale. For example, the removal of grease by washing solution in a forced concrete mixer with vertical shaft. Removal of oil products is quite effective, but at long operation (within several months) the system is clogged. Persistent water suspension with fine scale is formed in the chamber, which is difficult to destroy and requires the addition of chemical reagents, which, in turn, irreversibly decompose the washing medium. Removal of grease with organic solvents is also effective. It is carried out in vessels with active agitation. This process is very unsafe, as all its components are flammable. Solvent regeneration requires distillation equipment, some of the solvent is retained in the scale and ventilated explosion-proof settling tanks should be provided to remove solvent vapours, – all of the said making this method non-feasible.
Pyrolysis at the TDP thermal decomposition plant: waste-free disposal of oily scale
Pyrolysis is a promising method for utilization and neutralization of oily mill scale and sludge. In the process of pyrolysis the scale is heated in the atmosphere of oxygen deficiency. Oil products are removed by evaporating in the reactor and condensing in heat exchanger. The efficiency of oil products removal reaches 100%, also some carbon is formed in the dry residue, which does not prevent the loading of mill scale pyrolysis product into the sintering furnace together with ore. Evaporated and condensed oil products are sent to the burners of the pyrolysis unit furnace together with diesel fuel.
The disadvantage of this method is the small number of technologies with required capacity due to the difficulty of ensuring the tightness of the process. However, the method of continuous pyrolysis, implemented by IPEC (Safe Technologies Industrial Group) can serve as a solution.
The waste is loaded into the pyrolysis reactor and moved all along the continuous pyrolysis unit TDP-2 up to the dry residue receiving hopper with the help of the system of screw conveyors. Once in the reactor, raw mix is gradually heated, giving away water and hydrocarbon compounds. The latter, passing through the separator and condenser, form pyrolysis gas and pyrolysis fuel, which is used to maintain the process temperature.
TDP-2 is one cost efficient solution. Consuming only 37 kW of electricity, the unit with the capacity of 2000 kg/h can do without diesel fuel at all, if the content of petroleum products in the feedstock exceeds 20%. The environmental impact of the unit is also eliminated due to the tightness of the pyrolysis process. Dry residue from oily mill scale processing can be loaded in full into sinter furnaces together with ore.
Model range and the equipment for rolled scale pyrolysis are available in the Catalogue of TDP units. IPEC personnel will answer all questions concerning oily mill scale disposal in pyrolysis thermal treatment plants and will help you to choose an optimal solution that can be tailored according to your specific needs.
Статья «Oily mill scale as an industrial waste: processing and utilization» was published in the Safe Technologies’ corporate magazine “Ecoeng” issue № 19, 2019.
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