**1. Introduction**

The need for better process monitoring, control, and optimization of industrial reactors, and to secure environmental sustainability, has driven the development of new technologies in metallurgical processes. For example, many conventional reactors in the metallurgical industry have undergone improvements and optimization in their design and operation. Advanced sensing techniques, such as those presented in this work, increase understanding of the physical chemistry phenomena that take place in the process, facilitating these process improvements.

Flash smelting technology produces over 50% of the primary copper in the world [1] and it is widely used by the top producers of primary copper, such as China, Japan, Chile, and Russia, who represent more than 60% of the copper production through the pyrometallurgy processes. The flash furnace used in this type of process originated in Finland at the end of World War II and presented an excellent alternative to the energy shortage that existed in post-war Europe since, at that time, the intensive demand for thermal energy was mainly supplied by the combustion of hydrocarbons [2]. Over the years, flash smelting furnaces have become the most widely used technology in new smelting companies due to their ability to take advantage of the heat released through combustion reactions.

At present, new processes and technologies have emerged as competitors to flash combustion [3,4]. The flash furnace continues to be widely used for both its energy efficiency and its environmental performance. Since these furnaces appeared on the market, numerous studies have been reported to improve the understanding of physicochemical

phenomena and thus optimize their performance. One example is the work of Jorgensen in 1981 [5], who measured the temperature of pyrite particles reacting inside a laminar flow reactor, all of which was facilitated by optical pyrometry techniques and particularly by the two-color pyrometry method.

Over the years, it led to the improvement of measurement techniques as can be seen in the work of Tuffrey et al. [6], who also measured the temperature of pyrite, registering much higher values (maximum 3127 ◦C) than those reported by Jorgensen. The latter was attributed to the higher speed of the pyrometer data acquisition system used by Tuffrey. Subsequently, many studies on combustion kinetics, reaction mechanisms, and particle size measurement, among others, have applied two-color pyrometry as a support in their experiences [7–10].

The latest works reported on spectral measurements of sulfide combustion in laboratory tests have been carried out by a group of researchers from the Metallurgical and Electrical Engineering Departments of the Universidad de Concepción, Chile. This group has made use of optical pyrometry techniques to retrieve the flame temperature of concentrates and pure mineral species under combustion conditions. In these investigations, different spectral ranges of all the captured radiation were used and a cooled optical fiber was specially designed for high-temperature applications [11,12].

The application of spectroscopy at an industrial level is scarce, and one of the few examples is the mentioned by Sun et al. [13], who measured a coal-fired flame in a boiler, while the only reported measurements in an industrial flash furnace were performed at the Chagres smelter, Anglo American [12]. These tests were carried out by introducing a cooled optical probe inside the furnace through a sight glass positioned on the roof of the reaction tower, which allowed this optical system to capture flame combustion similarly to those found in laboratory experiments [14,15].

#### **2. Spectral Emissions of Iron and Copper Oxides**

In the last 50 years, different researchers in the field of emission spectroscopy have found the ranges of wavelengths in which certain chemical elements and molecules emit radiation, thus determining their spectral profiles. Among them are the emission spectra profiles of iron oxide and copper oxide. Although in these investigations, these emission profiles have not been obtained from sulfide combustion tests, they can be used as a reference when determining the presence of these oxides in a copper and iron sulfide emission spectrum, as Toro et al. did in their research on high-grade pyrite combustion spectra [16].

#### *2.1. Spectral Emission of Iron Oxide*

In the case of iron oxide (FeO), West and Broida [17] conducted a study in the visible spectral range (VIS). In their work, they reported that the FeO molecule emits a continuous spectrum between 500 and 700 nm. The emission profile contains wide spectral bands centered at 570, 590, and 620 nm. In the case of the wavelength of 590 nm, it is usually not perceived due to the interference of the sodium emission [18]. Toro et al. applied multivariate techniques on high-grade pyrite emission spectra. Through principal component analysis (PCA) and multivariate curve resolution method alternate least squares (MCR-ALS) analysis, the combustion spectrum was deconvolved obtaining pure emission profiles of Na, FeO, and Fe3O4 [16].

#### *2.2. Spectral Emission of CuOx*

The study presented by Gole in the early 1990s was the first to report the emission profile of copper oxide, with spectral bands located in the range of 580 to 720 nm [19]. For their part, Knapp et al. proposed a spectral profile of copper oxide and aluminum oxide in their study of the emission spectroscopy of the termite combustion flame [20]. In this study, they presented four peaks located at 606, 616, 629 and 640 nm, which represent CuOx emissions.

This research shows the application of an optoelectronic technique to measure the combustion flame produced by copper concentrates. The obtained spectral information was related to the physicochemical processes. The industrial process control could be performed with the application of this optoelectronic technique, allowing the operators to visualize important indicators of the processes online and in real-time.

The objective of this research is to study the emission spectra of chalcopyrite and a real copper concentrate when it is exposed to combustion conditions using an optical measurement system.
