## 1 Introduction  
Blast furnace (BF) technology is currently evolving towards operative regimes involving smaller and smaller quantities of reducing agents in order to limit carbon dioxide (CO\({}_{2}\)) emissions. At the same time, the need to increase productivity has led to increasing BF volumes. Operating conditions characterized by low levels of reducing agents and increasingly large BFs lead to occurrence of undesirable phenomena such as slipping and channeling [1, 2]. Since these irregularities in the functioning of BF ultimately result in a loss of production and materials and cause variations in the quality of the iron [3, 4], the analysis and the prevention of the above-mentioned phenomena is of great interest. The authors of [5] give a list of measures for preventing violations in the run of a specific BF, identified through the analysis of scull slipping and channeling in the BF prior to implementation of these measures. This has encouraged a reduction in the specific consumption of coke and an increase in the productivity of the furnace. There are several studies available in literature that have led to the formulation of predictive methodologies able to identify the phenomena at an early stage [6, 7, 8, 9, 10] before they have the opportunity to manifest themselves. For instance, in [9], a visualization technology mainly made of pressure distribution was developed to predict BF channeling phenomena. With this technology, a prediction can be obtained 10 to 15 min ahead of channeling, allowing sufficient time for the operator to adjust the BF operation in order to avoid the occurrence of channeling. In [10], statistical processing and pattern classification techniques are applied to various types of sensory information. A plurality of feature quantities, correlated highly with the risks of gas channeling, were defined by data and combined to provide operators with a quantitative index of gas channeling risks. These methodologies allow the operators to implement countermeasures for preventing major violations in the run of blast furnaces, so as to reduce the frequency with which such episodes of slipping and the channeling occur, however, without being able to prevent them completely. During the furnace slipping, the charged materials fall uncontrollably toward the hearth of the furnace and force the hot gases upward with a sudden overpressure in correspondence of the BF throat (stock line), with the consequent intervention of the emergency valves (bleeders) which have the purpose of venting this overpressure in order to safeguard the integrity of the system. The opening of these valves leads to the release of BF gas to the atmosphere with associated pollutant emission of CO gas and dust. Due to the high temperature of the top gas, flames often generate once in contact with the atmosphere. The result is a very conspicuous emissive phenomenon (Figure 1), although of short duration (generally less than one minute). In the study on [11], it is stated the effects of these emissions on the environment are globally negligible, and this source emits less total particulate than other in-plant air pollution sources. Nevertheless, these emissions pollute the air in the proximity of the plant, hence the need to minimize them. A possible practice for reducing air pollution involves arrangements for venting gas pressures due to slips and channeling by opening a bleeder communicating directly with the clean side of the gas washer except for BF top overpressures of greater intensity, which require pressure relief directly through the bleeders on the top of the blast furnace [12]. Inspired by the aforementioned solution, this article reports on the adoption in an existing plant of a controlled release system of the BF mouth overpressure, which makes it possible to minimize the intervention of the emergency valves and the associated uncontrolled release of BF gas into the atmosphere.