Advances in Pyrometallurgy

Pyrometallurgical furnaces are not conventional structures that fall directly under a single design code. Engineering companies attempt to apply appropriate safety factors for stresses and temperatures to vessel shells, refractory, and cooling elements. The furnace components must be designed to function together to avoid plastic deformation, erosion, corrosion, and fracture. Thermal cycling, intense heat loads, and required design loads are not typically found in design handbooks or codes. The loads can vary significantly as the refractory lining wears. Historical analysis of vessel performance under similar operating conditions and processes are key for developing design loads and limits for heat, stress, corrosion, and movement. Advancements made in furnace design over the past several decades are highlighted.

The paper starts with a discussion on “furnace containment systems”. Recently, there have been several experiences with DC furnaces for continuous processing of particulate feed, some of which have experienced containment challenges. The Kazakh high-carbon ferrochrome DC operation is used to investigate what process mechanisms could take place in such a furnace. Several scenarios have been simulated with a dynamic multi-zone model to see how and especially how rapidly deviations in feed composition, quantity, or charging location can affect the operation. Of interest also are how fast these deviations could be detected and especially what data could be monitored to recognise them. This paper contributes to understanding fundamental mechanisms inside DC furnaces, which are becoming even more important due to an increased interest in open-bath furnaces for smelting DRI. A better understanding of process mechanisms can support in de-risking new equipment and processes, ensuring a safer operation, also for green steel production.

Freeport-McMoRan Inc. (FCX) uses ISASMELT™ top-submerged lance (TSL) technology for the primary smelting of copper concentrates at the Miami smelter. The furnace was installed in 1992 with a capture system designed to cool and contain molten splash, dust, and gases generated from smelting reactions. The capture system is comprised of water radiant roof and wall panels, some of which generate steam. “Waste heat boiler” (WHB) is often used to refer to the primary furnace’s off-gas handling system because of the useful steam it generates. The waste heat boiler system must be available to operate the primary smelting furnace. Issues related to a waste heat boiler often make up a significant portion of a smelter’s downtime. Thus, effectively maintaining and operating the waste heat boiler is critical to a smelter’s overall effectiveness. The Miami smelter’s boiler maintenance and operational improvements to date will be reviewed in this paper.

The corrosion of copper cooling elements in the sidewall of platinum group metal (PGM) furnaces is unique in its severity and nature. ‘Chloride-accelerated sulphidation’ has been identified as the most likely corrosion mechanism and is prominent in the zone directly adjacent to the slag-concentrate interface. Tenova Pyromet developed a new composite copper-graphite cooler for implementation in one of Sibanye-Stillwater’s circular PGM furnaces at their Marikana smelter complex. The cooler design is unique as, in addition to the hot face, the sides, bottom, and top of the cooler are completely covered with graphite, preventing contact between the copper cooler and the sulphur- and chlorine-bearing species thought responsible for the corrosion. The design of the cooler and test work performed to evaluate its effectiveness are discussed. The coolers have been in operation since August 2022. The implementation and performance of the coolers to date are discussed.

The smelter operates a single six-in-line electric AC furnace. This furnace treats nickel sulphide concentrates produced by Glencore and third-party feeds and as such its performance is key to the company’s nickel supply chain. The furnace was constructed in 1978 and upgraded in 1994 to double the power density. The sprung arch roof was rebuilt in 2004, and the walls were rebuilt in 2004 and 2015. The hearth, however, remains the original installation. Work is underway to replace the entire furnace including the hearth in 2026. Since 1994, the smelter has successfully operated with a single furnace. Importance has been placed on the development of furnace integrity practices and equipment improvements. Another key focus has been robust practices on matte and slag tapping operations and maintenance. This paper will review the design considerations of the upcoming rebuild and approach taken to ensure its safe and reliable operation.

Magnesia-chromite (MgCr₂O₄) spinel-based refractories are applied in nickel and copper flash smelting furnaces, where the refractories experience high temperatures but also thermal gradients due to the cooling of the walls. In the gas space, the refractories are subjected to an aggressive environment with high SO₂ concentrations. Furthermore, the increased use of recycled material streams has introduced new reactive impurities, such as halides, to the process. Therefore, the interactions between the refractories and gaseous species as well as the related reaction mechanisms need to be understood. This study presents a comparison between as-received and spent refractories from a nickel flash smelting furnace to identify the reactive species and shed light on their reaction mechanisms. The thermal gradient over the refractory is expected to affect the microstructure and contribute to the diffusion of elements within the structure. Therefore, special attention was paid to the role of the thermal gradient in the abovementioned interactions.

Refractory fiber modules are widely used in new or upgraded high-temperature forging furnaces because of their lightweight, heat-insulating, and heat-retaining properties. The service life and operational efficiency of the furnace is strongly related to the stability of the internal fiber module anchorage structure. We have analyzed the force characteristics of the anchoring structure of refractory fiber modules and established the force equilibrium of the stable and non-dislodging interaction of refractory fiber modules with different anchorage structures and its correlation with the preset force F₀ provided by the anchoring weld. The shrinkage characteristics of the refractory fiber modules in the anchorage structure, the stability and oxidation resistance of the anchorage structure, and the pre-positioned weld material to high temperatures are investigated through high-temperature simulation experiments. Combined with the actual application, we propose the installation layout mode to effectively reduce the high-temperature shrinkage gap between the refractory fiber modules, and optimize the position distribution of the anchored metal structural parts in the refractory fiber anchorage structure and the anchorage welding adjustment scheme. The optimized scheme was applied to the repair and renovation project of a heating furnace in a factory, which achieved significant economic and social benefits.

In this study, through thermodynamic calculation, the possible reactions of carbon composite bricks in a high-temperature water vapor environment were analyzed. The morphology of carbon composite bricks after water vapor erosion was investigated through a water vapor oxidation experiment. In addition, a damage investigation was carried out on a blast furnace using hydrogen-rich gas smelting. During the period, a green-white phase with a thickness of 150 mm–200 mm was found inside the carbon composite brick in the taphole area. The carbon bricks in this area were sampled, and XRD, chemical analysis, and SEM–EDS detection were carried out. The test results show that there is blast furnace slag erosion and harmful element Zn erosion in carbon composite bricks. The erosion of harmful elements caused the expansion and ring cracking of carbon composite bricks, resulting in further slag erosion, which eventually led to the macroscopic slag phase erosion in the taphole area. The service life of carbon composite bricks can be effectively improved by optimizing the structure of carbon composite bricks, reducing the number of pores, optimizing the pore structure, and promoting the formation of a slag-rich protective layer on the hot surface of carbon composite bricks by improving the structure of blast furnace slag system.

Design of wear-resistant refractories necessitates an in-depth understanding and accurate quantification of the continuous wear. However, the experimental methods reported in the literature are mostly phenomenological and unable to reveal the physicochemical background of continuous wear. Main goals of this work are scientific investigation of continuous refractory wear and acquisition of data for quantitative simulation of continuous wear to design wear-resistant refractories. A modified rotating-finger test (RFT) device was equipped with high-resolution laser to scan the sample surface for dimension measurement. Generally, refractory dissolution in molten slag is controlled by diffusion through a boundary layer and diffusivity is the most important parameter to quantify dissolution. The data obtained from modified RFT studies were applied to accurately determine effective binary diffusivity using simulation method or mass transfer equation. Also, results of erosion studies were applied for inverse calculation of erosion parameters. Continuous wear of alumina in silicate slag will be exemplified here.

Flexospheres are a recent product development successfully implemented in fired magnesia-chromite bricks at RHI Magnesita. This technology focuses on inhibiting the corrosion mechanism and increasing flexibility of the refractory product. During development, the experimental investigations included chemical and physical analyses as well as standardised corrosion testing at RHI Magnesita’s Technology Center Leoben (Austria). The results showed that the Flexosphere technology not only improves corrosion resistance, but also contributes to the formation of a more flexible structure, which results in better thermal shock resistance. Furthermore, a RH degasser field trial validated the laboratory results, revealing a clear performance increase compared to the standard product. Thus, the outstanding properties achieved with the patented Flexospheres make this development suitable for applications under aggressive conditions, such as the tuyere area in copper furnaces or the RH degasser in the steel industry, by guaranteeing a significant quality improvement of the magnesia-chromite product.

The key challenge for pyrometallurgical furnace operators is to maximize production whilst balancing the risks of failure, i.e., ensure the molten materials remain contained in these furnaces until the next rebuild. To understand this risk, a deep understanding of the various failure modes is necessary, as well as the barriers that should be in place to prevent and mitigate them. The information needed to inform this risk, such as process and equipment conditions, changes over time and is often distributed across several people and systems within an organization. These circumstances make it difficult to get a “comprehensive picture” of the asset condition to evaluate whether the risk of molten material containment is becoming unacceptably high at any point in time. Hatch has applied the digital twin concept to online monitoring of risk barrier status using the risk-bowtie methodology and real-time asset and operational information to visualize the live risk profile of a particular unwanted event; in this case, a runout.

The corrosion behavior of MgO-C refractory in electric arc furnace (EAF) slags was investigated by adopting a rotating immersion method (25 r/min) at 1823 K. The EAF entirely uses direct reduced iron as raw materials, and the change of refractory radius, MgO content in slag, and contact angle were studied at various slag basicity conditions. The results showed that the reduction of radius diminished as the slag basicity increased from 1.6 to 2.47. The slag with higher basicity has a larger contact angle compared to lower basicity slags. In addition, the MgO solubility increased with a decrease in basicity and an extension of exposure time and subsequently stabilized after 60 min. The reduction of FeO_x and the formation of a magnesiowüstite (MW) intermediate layer occurred within the refractory. The MW layer is thicker in cases of low-basicity slag, but the slag penetration is deeper, leading to severe erosion.

When using pusher kiln to produce vanadium nitrogen alloy, alkali metals in raw materials will volatilize and deposit in the kiln, which will corrode refractories and hinder the smooth production. Therefore, it is of great significance to study the corrosion of refractory by sediment. In this paper, SiC-Si₃N₄ composite brick was used as the research object, and sodium carbonate and potassium carbonate were used as corrosion reagents to carry out corrosion experiments in a vacuum tube furnace. The results indicate that the surface of the composite brick occurs microporous and spalling phenomenon after being corroded by sodium carbonate or potassium carbonate for 3 h. The thicknesses of spalling layer and corrosion layer caused by sodium carbonate are 3.0 mm and 2.67 mm, respectively. The thicknesses of spalling layer and corrosion layer caused by potassium carbonate are 0.5 mm and 0.77 mm, respectively. It was found that the corrosion ability of sodium carbonate to SiC-Si₃N₄ composite brick is stronger than that of potassium carbonate. Combined with theoretical calculation and experimental characterization, it is found that the corrosion process is divided into three stages: corrosion reaction, slagging, and reduction of alkali metal oxides.