Sulfuric Acid Production Research Articles

Characteristics of Solid Mineral Phase Transitions During Sulfuric Acid Production from Gaseous-Sulphur-Reduced Gypsum

The acid co-production of cement is a prominent research focus for the large-scale, high-value utilization of phosphogypsum in the context of dual-carbon strategies. This paper builds on extensive research conducted by its authors on the co-production of sulphoaluminate cement clinker through acid production from gaseous-sulphur-reduced phosphogypsum. The solid mineral phase transformations occurring in the kiln during this process are systematically studied, and the effects of various calcination regimes (temperature, time, and atmosphere) on the evolution of clinker mineral phases are elucidated. This paper provides basic data support for the gas-sulfur-reduced phosphogypsum-acid cogeneration of sulfoaluminate cement clinker processes, and promotes the realization of the large-scale high-value utilization of phosphogypsum resources. The generation of the clinker mineral phase anhydrous calcium sulphoaluminate (C4A3S̅) begins at 1100 °C. Increasing the calcination temperature and extending the calcination time promote C4A3S̅ formation. However, when the calcination temperature exceeds 1350 °C, C4A3S̅ decomposes, leading to the formation of low-activity C2AS. In a CO atmosphere, the main mineral phases in the clinker transform into C2AS and 12CaO·7Al2O3, owing to the decomposition of CaSO4, which inhibits C4A3S̅ formation. At calcination temperatures exceeding 1300 °C, a significant amount of C2AS appears in the calcined material, and 12CaO·7Al2O3 begins to form.

Exergoeconomic analysis of a steam turbine power plant in a sulfuric acid factory

This study presents an exergoeconomic analysis of a steam turbine power plant (STPP) implemented within a sulfuric acid production facility. The plant harnesses waste heat from an exothermic chemical reaction to generate electricity. A detailed mathematical model was developed in Engineering Equation Solver (EES) to perform the exergoeconomic assessment on individual components and the overall system.The STPP generates 4.7 MW of electricity, with an exergoeconomic factor of 33 %. The exergy cost and unit exergy cost of the electricity are 455.8 $/h and 26.94 $/GW, respectively. The turbine represents the highest capital cost within the system. The steam produced incurs an exergy cost of 640.7 $/h, with a unit exergy cost of 2.2 $/GW. The waste heat boiler exhibits the highest exergy destruction cost at 112 $/h, and the major exergy destruction cost ratios are observed for the waste heat boiler (0.23 W/$) and the turbine (0.51 W/$). Additionally, the impact of variations in steam inlet temperature and pressure on the exergoeconomic performance was analyzed.These findings provide valuable, cost-based indicators such as exergy destruction cost, exergoeconomic factor, and exergy destruction cost ratio, which can guide the optimization and practical implementation of STPPs in the sulfuric acid industry.

Enhancing wet phosphoric acid production efficiency with vanadium catalyst waste derived silica

Filtration of phosphogypsum (PG) is an important step in the production of wet phosphoric acid (WPA). In recent years, the phosphate rocks used for WPA production in Morocco started showing deficiencies in terms of their reactive silica content that affects PG crystallization and as such decreases PG filtration efficiency and subsequently the overall efficiency of the WPA production. In this work reactive silica waste from spent vanadium catalysts, used in the production of sulfuric acid for WPA production, was added to the digestion process in an attempt to increase WPA production efficiencies. The laboratory work presented here shows that the added silica improved the quality of the produced dihydrate WPA by 6 %. Besides, the chemical yield of the reaction was improved by 5 % as a result of the shape-change of the PG crystals that allowed for better filtration. The results of this work are promising and we strongly recommend testing this approach on larger pilot- and ultimately industrial scale.

Modification of Sulfur Cake—Waste from Sulfuric Acid Production

In the production of sulfuric acid, sulfur cake—a waste product of the sulfur purification process—is formed in large quantities, which requires its disposal and use. For its use in composite materials, modification is necessary to convert sulfur into a polymer form. The aim of the study was to develop a method for modifying sulfur cake—a waste product of sulfuric acid production—for its disposal. Available reagents—styrene, glycerol, and oleic acid—were tested as modifiers in the work. The sample compositions consisted of 100% sulfur cake (no. 1) and its mixtures: 97% sulfur cake + 3% styrene (no. 2), 97% sulfur cake + 3% glycerol (no. 3), 97% sulfur cake + 3% oleic acid (no. 4), 95% sulfur cake + 3% styrene, 1% glycerol, and 1% oleic acid (no. 5). Modification of sulfur cake was carried out at a temperature of 140 °C for 30 min. The composition, crystal structure, and thermal properties of the samples of the original and modified sulfur cake were studied using X-ray phase and X-ray structural analyses, IR spectroscopy, differential scanning calorimetry, differential thermal and thermogravimetric analysis. The optimal modifier for sulfur cake was a mixture of styrene, glycerol, and oleic acid, which led to the formation of acetal (polyoxymethylene) and an improvement in the structure due to a decrease in the content of impurities. Modification of sulfur cake with styrene resulted in the appearance of a CAr–S bond band at 571 cm−1, and modification with oleic acid a C–S band in the region of 694 cm−1 in the IR spectra. The results of differential scanning calorimetric analysis showed an increase in the heat of fusion of sulfur by 12.45 J/g in the samples of sulfur cake modified with glycerol and styrene. Modification of sulfur cake with oleic acid and a mixture of reagents resulted in the appearance of a third peak with maxima at 244.2 and 264.0 °C, which demonstrated a significant effect of the indicated additives on the thermal behavior of the sulfur cake. Proposed schemes for modifying sulfur cake with styrene and oleic acid are presented.

Assessing Carbon Capture and Carbonation in Recycled Concrete Aggregates: A Holistic Life Cycle Assessment Perspective with Simulation at Industrial Scale

This work explores a life cycle assessment (LCA) concerning carbon capture and carbonation of recycled concrete aggregates (RCA), addressing the growing challenges posed by increased carbon emissions in power generation and ecological concerns tied to construction waste. We propose an integrated solution involving the capture of low-concentration carbon dioxide (CO2) from natural gas combined cycle (NGCC) power plants through aqueous ammonia absorption and sequestration by mineralization of RCA. The cradle-to-gate LCA focuses on an industrial-scale scenario for capturing 200 kilotonnes of CO2 annually, with functional unit per tonne of carbonated RCA output. The findings reveal a net positive carbon abatement of 21.42 kg CO2 eq., with heat production and electricity consumption being the major contributors to global warming potential (GWP). Within the spectrum of environmental impact categories under study, Marine Aquatic Ecotoxicity Potential (MAETP) exhibits a significant impact, primarily influenced by sulfur production. Human Toxicity Potential (HTP) follows as the second-worst contributor, with 96.80% of its impact attributed to heat and sulfur production. Subsequently, Acidification Potential (AP) follows, arising from sulfur and sulfuric acid production. Sensitivity analysis and Monte Carlo simulation introduce a ±20% standard deviation to electricity and thermal energy consumption. Scenario analysis demonstrates a net carbon-positive abatement potential of 286.66 kg CO2 eq. per tonne of CO2 input. Employing carbon mineralization for CO2 capture not only reduces emissions but also repurposes waste materials, providing a comprehensive and sustainable solution for effective CO2 management, contributing to reduced sand mining, and offering cost advantages through the production of carbonated RCA.

A novel and clean utilization of flue gas desulfurization ash coupled with converter sludge to produce calcium ferrate

To address the environmental pollution from industrial wastes like desulfurization ash (DA) and converter sludge(CS), this study introduces a novel method for synthesizing calcium ferrate. This method involves reacting converter sludge with desulfurization ash at high temperatures, facilitating the concurrent elimination of toxic elements. The solid byproduct is suitable as a sintered ore additive, while the gaseous SO2 serves as a precursor for sulfuric acid production. Additionally, the metal-rich secondary dust offers a source for valuable metal recovery. This research employs simulation software (FactSage 8.1) and thermal analysis to explore the high-temperature reaction dynamics of converter sludge and desulfurization ash. Subsequent in-situ experiments and coupled high-temperature tube furnace-infrared flue gas analysis further elucidated the phase transitions, morphology changes, flue gas emissions, and migration of harmful elements (e.g., S, K, Na, Pb, Zn). Analysis using the Predom dominant zone module calculation indicates that low SO2 concentration and reduced oxygen levels promote the stability of Ca2Fe2O5. Infrared flue gas analysis reveals high concentrations of CO2 and SO2. Sulfur removal rates rose from 62.15 % to 98.48 % between 800 °C and 1100 °C. Removal efficiencies for alkali and heavy metals, namely K, Na, Pb, and Zn, improved significantly, reaching 99.95 %, 99.49 %, 99.71 %, and 86.7 % respectively. At 1100 °C under an N2 atmosphere, the main reaction products from Flue Gas desulfurization ash (FGDA) and converter sludge were CaFe3O5 and Ca2Fe2O5. This study’s comprehensive analysis of synthesizing calcium ferrate from Flue Gas desulfurization ash and converter sludge introduces a novel waste utilization method, offering substantial environmental and economic advantages.

Preparation of Oxidized Pellets from Sulfuric Acid Residue Containing Zinc and Lead by Chlorination Roasting and Its Mechanism of Dezincing and Lead Removal

The utilization of sulfur acid residue is an urgent problem confronting sulfuric acid production enterprises, especially the application of sulfur acid residue (SAR) containing lead and zinc. A method combining chlorination roasting using CaCl2 with pelletizing for processing SAR containing lead and zinc was used in this study, and the effect of calcium chloride on pelletizing performance was studied; in addition, the removal behavior of lead and zinc was also studied by using polarized light microscopy (Zeiss double Axioskop 40A), X ray diffraction, SEM and EDS. The results showed that CaCl2 migrated to the surface of the pellets during drying, and this phenomenon resulted in a lower removal rate of lead and zinc inside the pellets than outside the pellets during the preheating phase. When the roasting temperature was 1220 °C, with an increase in the basicity of pellets, the silicate minerals in the pellets gradually decomposed, the hematite particles were gradually refined, and more lead or zinc minerals were exposed, which further increased the removal rate of lead and zinc in the pellets. Finally, the SAR pellets with Pb and Zn removal rates up to 91.33 and 97.88%, and a compressive strength of 2789 N, could be obtained, which is very beneficial to the sustainable development of sulfuric acid mills.

Production of effective materials on the basis of sulfuric acid industry wastes

Introduction. Solving the urgent problem of the production of special concretes using certain varieties of substandard natural raw materials and a number of industrial wastes will support introducing the principles of resource saving and improving the technological features of production.Aim. To make a comprehensive assessment of the impact of sulfuric acid production wastes and substandard natural clay raw materials on the production of high-quality and durable concretes for special purposes.Materials and methods. Easily fusible clay rocks serve as a raw material for the production of expanded clay gravel. Organic (solar oil) and ferruginous (pyrite cinders) additives were used. X-ray analysis was carried out to determine the crystalline phases in the clay of the Nikolskoe oil-field, in the resulting expanded clay gravel, as well as to analyze the factory bulk samples of pyrite cinders. A tubular electric furnace was used for firing in a two-stage mode of 700 and 1100 °C to obtain high-strength expanded clay gravel under laboratory conditions. The research involved studying parameters of phosphate binder formation and properties of iron phosphate concrete based on H3PO4 and sulfuric acid industry wastes – pyrite cinders.Results. It was found that Nikolskoye field clay forms a good raw material for the production of high-strength heavy expanded clay gravel with a strength of up to 550 kg/cm2, water absorption of 1.5 % and resistance to aggressive environments, thereby making it suitable for manufacturing special concretes. The results obtained specify the test methods and technical requirements, control methods and product acceptance rules, thus improving the quality and durability of the concretes considered.Conclusions. The investigated non-bloating clay can be used with organic and ferruginous additives to obtain high-strength expanded clay gravel for the manufacture of structural concretes. A mixed iron phosphate binder serves for producing heat-resistant concretes for linings of melting and casting units in non-ferrous metallurgy with high resistance and durability.

Life cycle assessment of lithium carbonate production: Comparing sedimentary deposits

Lithium sedimentary deposits which were once considered impractical to extract, have become increasingly attractive for exploiting and producing high-quality lithium compounds, due to the surge in demand for batteries and from other markets. However, potential environmental impacts are yet to be evaluated for this emerging lithium production route. Therefore, this paper presents a comparative Life Cycle Assessment study for three prominent and near-to-opening lithium clay projects globally: Sonora Mexico, Falchani Peru, and Thacker Pass USA. Specifically, this study used literature, statistical data, expert interviews, and technical reports to develop cradle-to-gate models covering the mining to refining processes. The results suggest that lithium carbonate production in the Thacker Pass project has higher impacts than the two other selected sedimentary projects. Additionally, the impact categories of the Sonora project are significantly influenced by the source of electricity. The sensitivity analysis highlights the pivotal role of a transition to clean energy sources for these emerging lithium production routes. Especially, the Thacker Pass project would benefit significantly from on-site sulfuric acid production and power generation to reduce the associated environmental impacts.

PRB technology incorporating acidic ground conditions and bio-geochemical clogging – A critical review

Groundwater acidity resulting from pyrite oxidation in acid sulphate soil terrain presents a severe threat to the environment. The exposure of low-lying acidic coastal belts to the atmospheric oxygen, exacerbated by phreatic surface lowering in dry seasons and activities like infrastructure development and agriculture, leads to pyrite oxidation and sulfuric acid production in soil. This paper reviews the challenges posed by acid sulphate soils by emphasising the environmental and infrastructure damage caused by acidic soil leaching into water bodies. Permeable reactive barriers (PRBs) have emerged as a promising method of passive treatment for mitigating groundwater acidity in pyritic terrain. This review mainly focuses on the effectiveness of PRBs in low-lying floodplains by addressing the bio-geochemical clogging that diminishes the reactivity and porosity of PRBs over time. This paper also summarises the numerical methods needed to design PRBs in acidic terrains by identifying gaps in current research that could enhance the accuracy of future PRB designs. This comprehensive review contains valuable insights into the ongoing efforts of addressing the challenges associated with groundwater contamination in regions containing acid sulphate soil.

Numerical analysis of a protective coating for mining industry feed chute

The selection of materials for the protective coating of a crusher’s feed chute significantly influences the mean time between failures (MTBF) and operational costs associated with the mass production of sulfuric acid at MMG/Kinsevere. Neglecting the condition of the protective coating can pose a serious challenge to production efficiency and result in increased downtime. This study focuses on enhancing industrial productivity in Sulfuric Acid production with minimal maintenance. The investigation explores the quality and type of materials used for the protective coating of the feed chute in the mining industry, particularly at MMG/Kinsevere, utilizing Ansys Fluent and SOLIDWORKS Software for analysis. The paper proposes a Ceramic protective liner for the crusher feed chute due to its superior resistance to erosion compared to other liners. Different feed chute models, constructed with materials like halogenated butyl rubber, 316 stainless steel, and ceramic, underwent analysis. The evaluation of the potential impact on the mean time between failures (MTBF) for different materials used in a protective coating, considering the rate of erosion through computational fluid dynamics (CFD) with ANSYS FLUENT software, is a central aspect of this study. The detailed modeling and simulation reveal that the ceramic material exhibits the most favorable protective coating characteristics, with an erosion rate density of 06.636753 kg/m2s, outperforming halogenated butyl rubber (3.326576 kg/m2s) and 316 stainless steel (2.186633 kg/m2s). The simulation yielded results corresponding to flow rates of 284.19 kg/s for the solid phase and 123.14 kg/s for the liquid phase.

Development of a slow-release CO2 absorbent material for alkaline activation of persulfate oxidation of TCE and CO2 capture

Control of carbon dioxide (CO2) emission during chemical oxidation processes of organic-chemical contaminated sites has become a challenge. In this study, a novel slow-release CO2 absorbent material (SCAM) was developed for a slow release of Ca2+ and OH–, which were used for CO2 capture, pH adjustment, and alkaline activation of persulfate (PS) oxidation for trichloroethylene (TCE) degradation. The SCAM was produced using basic oxygen furnace slag (BOFs) and calcium sulfate hemihydrate (CSH) as the components for granulation and modification. Batch experiments were conducted to determine the optimal mass ratio of BOFs to CSH and evaluate CO2 capture capacity and SCAM performance. An increase in BOFs proportion led to a higher CO2 capture capacity, with the maximum capture amount reaching 156 g CO2/kg SCAM. An excessive addition of BOFs resulted in an overly alkaline pH value (pH = 12.4), causing the diminished PS oxidation efficiency. The optimal mass ratio of BOFs to CSH (1 to 1) could capture produced CO2 after TCE oxidation without inhibiting PS oxidation. The SCAM could serve as the catalyst and activate PS oxidation, resulting in 93.1 % of TCE oxidation (initial TCE concentration = 2.32 mM) and 95.4 % of CO2 emission reduction due to the formation of CaCO3 precipitates under alkaline conditions. Produced sulfate and hydroxyl radicals confirmed the PS oxidation of TCE using SCAM as an activator. The developed SCAM is a green catalyst for PS oxidation and it is also an effective CO2 absorbent. SCAM can reduce CO2 emission due to the formation of CaCO3 precipitates under alkaline conditions. The released OH– could also prevent the acidification problem caused by the production of sulfuric acid through PS decomposition.

Spatial distribution and health risk assessment of soil heavy metals in an abandoned sulfuric acid production site

Spatial distribution and health risk assessment of soil heavy metals in an abandoned sulfuric acid production site

Do ferrous iron-oxidizing acidophiles (Leptospirillum spp.) disturb aerobic bioleaching of laterite ores by sulfur-oxidizing acidophiles (Acidithiobacillus spp.)?

The extraction of nickel, cobalt, and other metals from laterite ores via bioleaching with sulfur-oxidizing and ferric iron-reducing, autotrophic, acidophilic bacteria (e.g. Acidithiobacillus species) has been demonstrated under anaerobic as well as aerobic conditions in experiments in different laboratories. This study demonstrated the bioleaching of laterites from Brazil with the addition of elemental sulfur in 2-L stirred-tank bioreactors with pure and mixed cultures of Acidithiobacillus and Sulfobacillus species under aerobic conditions. In particular, a potential disturbance of mineral dissolution under aerobic conditions by ferrous iron-oxidizing acidophiles likely introduced as contaminants in an applied bioleaching process was investigated with Leptospirillum ferrooxidans at 30°C and Leptospirillum ferriphilum at 40°C, at maintained pH 1.5 or without maintained pH leading to an increase in acidity (with pH values <1.0) due to the biological production of sulfuric acid. Despite the proportion of ferrous iron to the total amount of extracted iron in the solution being drastically reduced in the presence of Leptospirillum species, there was a negligible effect on the extraction efficiency of nickel and cobalt, which is positive news for laterite bioleaching under aerobic conditions.

MATHEMATICAL MODELING AND EVALUATION OF THERMODYNAMIC PERFECTION OF A CHEMICAL REACTOR

The development of a formalized mathematical model for quantitative and qualitative assessment of the thermodynamic perfection of a gas chemical reactor is described in the article. The reactor was considered as an energy technology system, that is, in the unity of the transformation of matter and energy. Mathematical modeling and evaluation of the thermodynamic perfection of a chemical reactor is generalized to a class of known chemically active gas systems characteristic of ammonia synthesis, sulfur dioxide oxidation, methanol synthesis, nitric oxide oxidation, etc. The equilibrium degree of transformation of a substance and exergy are taken as the determining parameters for modeling and evaluating the thermodynamic perfection of a chemical reactor. These parameters are functions of temperature and reflect the opposing tendencies of the process when it changes, the evaluation of which allows us to quantify the finding of the maximum exergy in the coordinates (E-T) when justifying the optimal temperatures of the process. The model was tested on a specific example of a sulfur dioxide oxidation reactor in the production of sulfuric acid. The public Octave programming environment was used for computer modeling. The developed mathematical model is recommended to be used in the operational preparation and optimization of the initial data for the design of gas reactors. The mathematical model can be supplemented with a mathematical model developed by the authors earlier for quantifying the current profile of the degrees of transformation of the target component of a chemically active reactive gas mixture in the course of its movement in a polytropic tubular reactor of the "pipe in a pipe" type. After testing and setting up the model in practice, it can be used in an automated control system for chemical reactors, including when creating adaptive control systems. For citation: Andreev A.S., Aksenchik K.V. Mathematical modeling and evaluation of thermodynamic perfection of a chemical reactor. ChemChemTech [Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol.]. 2024. V. 67. N 5. P. 114-120. DOI: 10.6060/ivkkt.20246705.6964.

Multi‐model predictive control of converter inlet temperature in the process of acid production with flue gas

SummaryThe smelting of non‐ferrous metals produces substantial quantities of sulfur dioxide (SO)‐laden flue gas, which is seriously harmful to environment and humans. To improve the conversion ratio of SO and minimize environmental pollution, controlling converter inlet temperature during acid production has proven to be an efficient approach. However, unsteadiness of smelting procedure leads to frequent changes in the concentration of SO, which affects the catalytic conversion of SO and the production of sulfuric acid. To regulate converter inlet temperature, a proposed method of multi‐model predictive control is introduced. First, working conditions are divided and characterized according to the range of SO concentration. Then, the mathematical model is established for each working condition and the model predictive controller is designed. Finally, an effective switching mechanism is established to realize smooth switching under different working conditions and closed‐loop control of the whole system. Through simulation validation, compared with traditional single‐model predictive controllers and multi‐model PID controllers, the proposed approach demonstrates improved transient performance and steady‐state performance. Simulation outcomes clearly highlight the superiority of the proposed algorithm.

Probing the dynamics and bottleneck of the key atmospheric SO2 oxidation reaction by the hydroxyl radical

SO2 (Sulfur dioxide) is the major precursor to the production of sulfuric acid (H2SO4), contributing to acid rain and atmospheric aerosols. Sulfuric acid formed from SO2 generates light-reflecting sulfate aerosol particles in the atmosphere. This property has prompted recent geoengineering proposals to inject sulfuric acid or its precursors into the Earth's atmosphere to increase the planetary albedo to counteract global warming. SO2 oxidation in the atmosphere by the hydroxyl radical HO to form HOSO2 is a key rate-limiting step in the mechanism for forming acid rain. However, the dynamics of the HO + SO2 → HOSO2 reaction and its slow rate in the atmosphere are poorly understood to date. Herein, we use photoelectron spectroscopy of cryogenically cooled HOSO2- anion to access the neutral HOSO2 radical near the transition state of the HO + SO2 reaction. Spectroscopic and dynamic calculations are conducted on the first ab initio-based full-dimensional potential energy surface to interpret the photoelectron spectra of HOSO2- and to probe the dynamics of the HO + SO2 reaction. In addition to the finding of a unique pre-reaction complex (HO⋯SO2) directly connected to the transition state, dynamic calculations reveal that the accessible phase space for the HO + SO2 → HOSO2 reaction is extremely narrow, forming a key reaction bottleneck and slowing the reaction rate in the atmosphere, despite the low reaction barrier. This study underlines the importance of understanding the full multidimensional potential energy surface to elucidate the dynamics of complex bimolecular reactions involving polyatomic reactants.

Thermal decomposition mechanism and characterization of phosphogypsum in suspension under multifactor coupling effect

Phosphogypsum (PG) is solid waste generated by the phosphate fertilizer industry. However, by high-temperature thermal treatment, it can be effectively decomposed into byproducts like SO2 and CaO, which are valuable raw materials for the production of sulfuric acid and construction materials, respectively. In this study, we investigated the thermal decomposition properties of suspended PG, as well as the factors governing the PG decomposition procedure. The results show that the PG decomposition rate was influenced by several parameters, with temperature being the most significant, followed by time, atmosphere, and charcoal powder dosing. Similarly, the desulfurization rate was primarily influenced by time, followed by temperature, atmosphere, and charcoal powder dosing. The PG exhibited maximum decomposition and desulfurization rates of 97.73% and 97.2%, respectively, at a calcination temperature of 1180 °C, a calcination period of 15 min, a carbon powder dosage of 4%, and a CO concentration of 6%. Kinetic studies revealed that the activation energy of the PG decomposition reaction decreased from 303.45 to 254.28 kJ/mol as the CO concentration increased from 4% to 6% upon addition of charcoal powder as a reducing agent. Furthermore, higher CO concentrations had a more pronounced effect on lowering the activation energy of PG pyrolysis. When the contents of SiO2 and Al2O3 impurities in the PG content were higher, they reacted with CaO in the decomposition products to produce silicate and aluminate minerals, respectively, decreasing both the CaO content in the product and the PG decomposition rate.

Evaluation of welded joints of austenitic stainless steels at high temperatures in the presence of SO2, SO3, and O2

This study undertakes a comprehensive examination of crack formation in welded joints of austenitic stainless steel 304, particularly in high-temperature corrosive environments prevalent in sulfuric acid production units. Initially, an investigation was conducted on a steam superheater that had been operational for about 19 years, where a crack formation was discovered. This unit, subjected to a working temperature of approximately 620 °C and exposure to gases composed of SO2, SO3, and O2, allowed for the analysis of corrosion products formed during the crack propagation. Building on the initial findings, a more controlled study was undertaken, focusing on the cracks formed in welded joints using the coated electrode E308L for joining 304H stainless steel plates under similar corrosive conditions. The specimens, constructed using materials typically utilized in sulfuric acid production units, were designed based on the double-beam principle, enabling exposure to both tensile stresses and a corrosive environment at high temperatures. Over a span of 122 days, or roughly 3000 h, microstructural analyses were conducted, revealing that the formation and propagation of cracks preferentially occurred through fragile microconstituents, notably the sigma phase. These findings mirrored the sigma phase formation observed in the initial superheater study and point to a stress corrosion process acting synergistically with sigma phase development, and consequently seen as a probable cause of failure. This comprehensive evaluation, therefore, indicates a significant role of the sigma phase and corrosive environments in influencing crack formation and propagation in welded joints of austenitic stainless steels, which require further studies for preventative strategies in industrial setups.

Bioleaching of a lateritic ore (Piauí, Brazil) in percolators

Heap leaching of laterites for extraction of nickel and cobalt is an attractive alternative to capital and energy intensive high pressure acid leaching, the dominant hydrometallurgical processing technology for limonitic laterites. Conventional approach for heap leaching of laterites is leaching with sulfuric acid. Consumption of sulfuric acid during heap leaching is substantial and industrial-scale operations require construction of a sulfuric acid production plant on site. In this study, heap bioleaching of laterites was simulated in laboratory scale column percolators and bioleaching of nickel and cobalt from lateritic material was successfully demonstrated for the first time. The process is based on biooxidation of the bacterially modified “wet sulfur” inside column percolators by sulfur-oxidizing acidophilic bacteria Acidithiobacillus thiooxidans. The “wet sulfur“ was generated in a bioreactor with the bacterial culture, harvested, and mixed with lateritic ore before forming agglomerates to be filled in the percolator columns. Liquid was circulated with a flow rate of 8 mL/min. Maximum metal extraction was 66% nickel, 95% cobalt, 10% iron, 55% magnesium and 89% manganese from the Piauí lateritic ore after one month bioleaching. For comparison, chemical leaching with 1 M sulfuric acid with or without addition of 10 g/L of ferrous sulfate heptahydrate as reductant resulted in extraction of approximately 80% nickel, 86% cobalt, 33% iron, 50% magnesium and 81% manganese. With bioleaching a higher cobalt but lower nickel and iron extraction was achieved, i.e. a better selectivity of nickel over iron extraction, as well as a relatively higher pH of the pregnant leach solution requiring less limestone and, consequently, lower CO2 emission and generation of iron cake waste in case of laterite bioleaching. Overall, the results are promising and show potential of laterite heap bioleaching to be further developed to application on industrial scale.