Air pollution prevention and control within cold rolling mills

Air pollution prevention and control within cold rolling mills is usually related to heat treatment processes (annealing and tempering furnaces), to emissions from surface preparation (pickling bath and mechanical descaling), to emissions arising at the cold rolling and temper rolling section, and possibly to emissions by acid regeneration and neutralisation processes.

A.4.3.1.1 Air pollution prevention and control in the surface preparation section

Before the hot rolled strip enters the cold rolling section, the scale on its surface has to be removed in order to ensure that the surface quality is sufficient for cold rolling. This surface treatment is done both mechanically and chemically.

At first, the surface of the hot strip is usually mechanically pre-descaled in order to enhance the efficiency of chemical descaling [180]. The dust arising during mechanical surface cleaning (e.g. bending / tension levelling or shot blasting) can be evacuated and the dust cleaned by a precipitation system (e.g. bag filter) (1). Dust concentrations in the off-gas of at least <50mg/m³ are achievable by well operated precipitation systems.

Chemical surface preparation (i.e. pickling), which mostly follows mechanical descaling, gives rise to acidic aerosols and possibly gaseous emissions depending on the pickle in use.

The pickle is chosen according the steel grade processed. Measures to prevent and control emissions from pickling stations i.a. depend on the type of pickling installation used for the processed workpieces. This study focuses on continuous type pickling stations where carbon steels or stainless steels are processed. Within these installations usually either sulphuric acid or hydrochloric acid are used to treat carbon and low-alloyed steels and mixed acids (often nitric / fluoric acid mixtures) for stainless steels.

For chemical surface preparation, the measures to prevent emissions include a reduction of the waste gas volume and the contaminant load of the waste gas, which is exhausted from the

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pickling tanks. The contaminant emissions i.a. depend on the equilibrium partial vapour pressure of the components of the pickling bath, but also on the operating conditions. This pressure is dependent on the substance, its concentration in the liquid phase and the temperature. Gaseous emissions might be reduced (2) in particular by a lowering of the bath temperature, by an optimisation of the composition of the solution and possibly by the use of surface active substances [131]. However, these parameters can only be changed within certain limits restricted by quality requirements.

Generally, for all types of pickling departments the following emission control measures can be taken (3) [154]:

• Covering of the baths with tight, well-maintained covers

• Location of exhaust ducts near openings in hoods and tanks

• Minimising of open area with local seals or closures, use of double covers

• Regular maintenance of fan, hoods and ducting and careful, well-balanced duct design

The cleaning process of the exhausted gas depends again on the acid in use. In the following, possible gas cleaning processes (4) for HCl, H₂SO₄ and HF / HNO₃ emissions are briefly⁹ presented. While for H2SO4 pickling departments mist eliminators are usually sufficient, these precipitators serve at the most as pre-precipitators for the cleaning of exhaust gas from hydrochloric acid pickling departments [131]. The use of mist eliminators with intensive spraying zones for gas cleaning in pickling stations using H2SO4 allows easily clean gas concentrations of 5 to 10mg H₂SO₄/m³ [131]. To clean exhausted gases and aerosols from pickling installations employing hydrochloric acids, packed scrubbers are most often used [131, 154]. Multistage absorption, operated in cascades, makes it possible to clean the exhaust gas to a clean gas concentration below 10mg HCl/m³ and allows the recuperation of hydrochloric acid with a concentration ranging between 10 to 15 weight percent [131]. The cleaning of the nitrous fumes arising during pickling with nitric acid depends in particular on the workpiece being pickled and the resulting gas concentration as well as the NO / NO₂-ratio.

In principal, absorption or catalytic processes are available, which have to be selected according the particular case. For the control of fluoric acid emissions absorption processes are suitable. However, if aerosols are present, the operation of an intensive spraying zone is required for an efficient aerosol precipitation [131]. Gas cleaning for nitric / fluoric acid mixtures can be done by a combination of the mentioned different gas cleaning processes.

Figure A.4-10 shows a packed scrubber for absorbtive gas cleaning.

9 Cf. i.a. Rituper 1993 [131] for a more detailed description of cleaning options for exhausted gases from pickling tanks.

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Cleaned off-gas

Mist arrestor

Packing

Off-gas

Water

Waste water

packed.ds4

Figure A.4-10: Diagrammatic view of a packed scrubber for absorptive gas cleaning

Source: [131]

The process of acid regeneration (cf. A.4.3.3.2), that is related to chemical surface cleaning, may also give rise to emissions into the air, depending on the regeneration process. These emissions can also be arrested by a gas cleaning system (5).

A.4.3.1.2 Air pollution prevention and control within the cold rolling section

Cold rolling gives rise to oil mist or fumes. These can be exhausted by a local exhaust ventilation system served by a mist eliminator (6). Mist eliminators can be mechanical (e.g.

using plastic media, filtering fibres, meshes) or electrostatic [168]. Citepa [26] states achievable residual emission levels of 8-50mg/m³ (by a mechanical oil mist eliminator) and below 25mg/m³ (by an electrostatic mist eliminator). A study of the European Union [133], including stainless steel plants, reversing C-steel mills and continuous mills states oil emission figures with concentrations ranging between 0.1mg/m³ and 35mg/m³ with a weighted average of about 4.9mg/m³ from the cold rolling section for the reporting installations. However, it has to be noted, that both imprecision in the terminology and variations sampling / analysis methods led to difficulties in interpreting the data of the mentioned study [133].

A.4.3.1.3 Air pollution prevention at heat treatment furnaces

Combustion products arising within heat treatment furnaces can be minimised by the choice of fuel (7), a good control of the combustion conditions (8) [168] and possibly the use of low

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NO_x-burners¹⁰ (9), as in the case of reheating furnaces (cf. A.4.2.1.2) [26]. Off-gases have to be post-combusted, if necessary. Furthermore, the use of hydrogen instead of nitrogen as an annealing gas may reduce overall emissions within annealing furnaces (10), because hydrogen fosters the heat transfer and reduces significantly heating times and energy input [168]. A comparison between conventional hood-type furnaces and high convection hood-type furnaces has shown savings of 15% of natural gas input (in kWh/t) at high convection hood-type furnaces operated with hydrogen (cf. A.4.3.2.3) [85].

A.4.3.1.4 Air pollution control within temper mills

Depending on the temper mill process (rolling dry / with lubricant), a suitable gas cleaning system has to be chosen to arrest the arising emissions (dusts) (11).

At the moment, no data for achievable values is available for temper mills.