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By A. R. Miller

A fuel cell locomotive incorporates the advantages of its competitors, namely catenary-electric and diesel-electric units, while avoiding their disadvantages. It possesses the environmental benefits, at the vehicle, of an electric locomotive but the higher overall energy efficiency and lower infrastructure costs of a diesel locomotive. The natural fuel for a fuel cell is hydrogen, which can be produced from many renewable sources or via nuclear energy, and thus a hydrogen-fuel cell locomotive will not depend on imported oil. Hydrogen produced from renewable primary energies or nuclear energy would provide a totally zero-emissions vehicle, that is, with zero carbon in the energy cycle. A project partnership among Vehicle Projects Inc of the USA, and South African partners Anglo American Platinum, Ltd, Trident South Africa, and Battery Electric, has designed and is launching a series of five prototype fuel cell-powered mine locomotives.

Jan 1, 2012

By А. V. Aksenov, S. G. Rybkin, А. Y. Senchenko

For gold plants in Russia, it is very complicated and costly to use cyanide leaching for efficient processing of gold refractory ores, where gold is closely associated with sulphide minerals and the significant amount of natural carbonaceous matter present. Oxidative roasting of gold refractory concentrates is a well-known method, making it possible to obtain a high recovery rate of precious metals to a target product. The use of oxidative roasting is limited in general by the expense of capturing and neutralising the harmful components of roasted gas, mainly sulphur (SO2, SO3) and arsenic (As2O3) oxides. This paper addresses testing and development of an optimised technology to process gold refractory gravity and flotation concentrates by means of oxidative roasting which stabilises arsenic as calcium arsenate. The process involves roasting of a gravity or flotation refractory ore concentrate in a mixture with lime and calcium chloride, with a supply of oxygen, and at 600-700°С. During the roasting a carbonaceous matter of the concentrate burns out and sulphide components transform into harmless calcium sulphate (CaSO4) and calcium arsenate (Ca3(AsO4)2), and iron passes into the form of hematite (Fe2O3). After disintegration, the roasting cinder is concentrated on a table to obtain a high-grade gravity concentrate and gravity tailings. The high-grade gravity concentrate is subject to smelting in a mixture with fluxing agents, which results in alloyed gold and slag. The gravity tailings are ground and reported to cyanide leaching. In the case of a flotation concentrate, the roast cinder is ground to minus 30 μm and subject to sorption cyanide leaching. High gold recovery (90-95%) of the target product and environmental safety are typical of the improved technology under consideration. Two national patents were granted for the methods developed to treat refractory concentrates bearing precious metals. Keywords: Gold concentrates, oxidative roasting, cinder, cyanide leaching, smelting, alloyed gold

Jan 1, 2020

By S. Langea, H. Lieberwirth

Impact crushers are common machines in comminution plants. Their high size-reduction ratio and relatively simple design, make them a preferred tool even when subjected to harsh wear, and producing a large quantity of fines in some applications. Until now the alignment of the operational parameters was an operator’s responsibility. Due to the random process characteristic this is mostly done based on empirical values and sensor data, such as power consumption. This paper describes a new approach for online process monitoring of the horizontal-shaft-impact crusher. The application of robust, low-cost acceleration sensors enables the permanent observation of the process characteristics. The operational and evaluation methods are determined on the basis of experiments. Subsequently the recorded measurements are analysed in regards of throughput, rotational speed and product particle size distribution. Occurring signal characteristics are described based on physical and technical principles. The measured data of the acceleration sensors at varying operation conditions provide insight in the course of process. The accurate utilisation yields the determination of the preferential process conditions providing the best balance between the pros and cons of an impact crusher. Both the experimental method and the data processing is discussed from a scientific perspective, displaying a completely new approach for the operation of impact crushers. Keywords: Impact crusher, acceleration sensors, data processing, particle size

Jan 1, 2020

By O. T. Konina, V. A. Masloboev, A. V. Svetlov, E. A. Krasavtseva, D. V. Makarov, A. A. Goryachev

Dusting phenomena have been examined at the tailings storage facility of Apatit JSC Kirovsk, a mining operation located in the far north of the Russian Federation. The material composition of the tailings was identified. Existing dust suppression methods were examined. Field observations were undertaken and official data was obtained from the Murmansk Administration for Hydrometeorology and Environmental Monitoring, and analysed. A database was built for forecasting dusting according to critical scenarios in order to estimate the peak MPCs of suspended solids in the atmospheric air. Mitigation measures were formulated and recommended to the mining operation. Keywords: Mineral dust particles, apatite-nepheline ore concentration tailings, suspended solids MPC, climate change

Jan 1, 2020

By D. Hu, J. Xu, N. Liao, C. Wu, J. Ye

The vertical stirring mill’s fine grinding equipment is affected by the operating conditions during the grinding process. The grinding media has a great influence on the energy consumption and particle size distribution. This paper investigates the fine grinding performance of nano-ceramic balls and steel balls of the same weight for magnetite by a wet grinding process in a vertical stirring mill, where 15 mm nano-ceramic balls and 12 mm steel balls are used as grinding media to carry out batch grinding tests. We have observed that the newly qualified particle content of the nano-ceramic balls is higher than steel balls of the same weight at the same specific energy. The reason may be the advantage of a greater surface area and volume filling rate with the nano-ceramic balls. Moreover, the specific energy of nanoceramic balls is less than steel balls of the same weight under the same grinding fineness. In addition, energy efficiency indicates that the nano-ceramic ball has a higher energy efficiency than a steel ball of the same weight. This is due to the light bulk density of nano-ceramic balls. The application of a nanoceramic ball in a vertical stirring mill will improve iron ore grinding performance. Keywords: Steel balls, nano-ceramic balls, vertical stirring mills, fine grinding, particle size distribution, energy consumption

Jan 1, 2020

By A. Rahmanab, A. Torpy, M. I. Pownceby, M. N. Zaman, N. Haque, W. Bruckard, J. Tardio

Ilmenite is the principal feedstock for producing TiO2 - used in paints, plastics and paper and the production of Ti metal. The Brahmaputra River system in Bangladesh represents a potentially new source of ilmenite concentrated within sandbar placer deposits and the current study presents results from beneficiation and characterisation studies conducted on Brahmaputra River Fe-Ti oxides. Characteristic of fluvial systems, the ilmenite concentration is low (~0.5 wt-%), however extensive annual dredging for waterway navigation provides the economic impetus to recover saleable ilmenite. The Fe-Ti oxide suite derived from magnetic fractionation comprises homogeneous ilmenite, titanohematite and hematite, although many grains exhibit complex exsolution within the (Ilm-Hem)ss series. The textural variability results in difficulty generating a pure ilmenite feedstock with the bulk concentrate typically being low in TiO2, containing 31.0 wt-%TiO2 and 62.7 wt-% (Fe2O3). Results are interpreted via an analysis of the implications for producing a commercial-grade ilmenite concentrate suitable for TiO2 production. Keywords: Ilmenite, titanohematite, Fe-Ti oxide exsolution, TiO2 production

Jan 1, 2020

The nature of metallurgical engineering?applying and optimising chemical and physical processing of ores into useful materials?is reviewed. Important trends that shape the context in which metallurgical engineers practise are identified ? increased global production of primary metals, declining prices, environmental constraints, and increased property requirements of materials. A successful department of metallurgical engineering is one which employs a team of academic leaders to develop graduate engineers who are productive across the full range of metallurgical processing. Keywords: metallurgical engineering, primary metals, commodity prices, processing

Jan 1, 2003

By M. Arlandi-Rodríguez, and A. García-Berrocal, A. Alonso-Jiménez, C. Lopez-Jimeno

Spain is a major producer of dimension stone, which is one of the most important industrial sectors in the northwest of Spain. This justifies the maintenance of these industries, although the technical and environmental requirements for their continuity are increasingly demanding. The main target of this paper is to present a new methodology in order to obtain k-ratio stress values on the basis of flat jack measurements and finite element numerical back- analysis with Rocscience© RS2 software. Continuous iterations were performed to try to find a relationship between the k₀ and the measured stress in the tunnel roof and sidewalls. It focuses on technical aspects of the method for designing mine chambers in the slate industry that were previously mined by open pit methods. The size of the chambers impacts the operating cost, and the dimensioning of a correct support ensures better efficiency and safety. Correct knowledge of the k₀ therefore is critical for the design of the support. The proposed new method entails a simplification of the classical procedure for estimating the k-ratio.

Sep 1, 2022

By K. Esmeli, A. Ozkan

The enhancement of shear flocculation of a galena suspension by ultrasound in the presence of sodium isopropyl xanthate and potassium ethyl xanthate was investigated by studying the effects of surfactant concentration on flocculation efficiency, contact angle, and zeta potential. Strong flocculation was obtained under alkaline conditions (pH > 9) and a high ultrasound power level (150 W). Although the ultrasonic treatment increased the negativity of the zeta potential of the galena particles, this did not decrease the flocculation efficiency, thereby showing that the hydrophobic interactions between the particle were stronger than the electrical double-layer repulsion resulting from xanthate adsorption. The findings indicate that the ultrasonic treatment promoted the adsorption of surfactants onto the mineral surfaces.

Jul 5, 2023

By D. Stone, M. Le Roux, Q. P. Campbell, M. Fofana

Float-and-sink analysis is widely used in the coal industry to obtain washability data, yielding important information about beneficiation potential and performance. This method is associated with health and environmental problems, and research into alternative densimetric methods is important. The RhoVol is a new technology developed by De Beers Group Technology South Africa (DebTech) for the image-based generation of densimetric data from ores. The analysis is done on a per-particle basis, within a size range of +3 mm –8 mm. In this paper we compare the established methods used for the float-and-sink analysis of coal with a new method using RhoVol. The aim of the study was to determine the validity of the information obtained from the RhoVol analyser by performing comparative densimetric analysis on three different coal samples. The results showed that the RhoVol method was more rapid, safe, and precise, but tended to consistently underestimate the density of the coal sample, probably due to varying coal porosity.

Oct 3, 2023

By M. Le Roux, Q. P. Campbell, M. Fofana

The determination of densimetric information to predict coal washabilities is an important step in characterising the feed material, as well as the performance of operating plants. However, traditional methods like float-and-sink analysis are hazardous and time-consuming. This is especially true for finer particle size ranges, where very long settling times are required in a medium. De Beers Technologies recently developed a density analyser, known as the RhoVol, for use on diamondiferous kimberlite. Operating over size ranges from 25 mm down to 3 mm, it can determine the mass, volume, density, size and shape of all the particles in a given population and is thus able to provide information about the densimetric distribution on a single particle basis. A study was carried out to compare uncalibrated and calibrated RhoVol results to densimetric and size data obtained using traditional zinc chloride float-sink procedures, for three different coal samples in the size range 3 mm to 8 mm. The results indicate that the RhoVol can be used to provide simple, rapid and safe size-by-washability information for coal, provided proper pre-calibration is done, mainly to account for the effects of particle porosity in the density determination. Keywords: Coal washability determination, RhoVol, densimetric curve

Jan 1, 2020

By M. Saghaeian, R. Hejazi, L. Hosseini, M. V. Sheikhzadeh

Blaine number has long been used to evaluate the pelletising ability of iron concentrate in the industry. Much information and practical data are available about the effect of Blaine number on pelletising efficiency of different concentrate types. But not many researchers have focused on the effect of Blaine increasing method on pelletising behaviour of coarse concentrates. In this paper the effect of two different grinding methods (i.e. HPGR and Ball Mill) on physicochemical properties of coarse magnetic concentrate green and fired pellets were investigated. Microstructural studies by Field Emission Scanning Electron Microscope (FESEM) showed that the concentrate for which the Blaine number had been increased through HPGR grinding, has flake-like morphology. In contrast, increasing the Blaine number through ball mill grinding results in more granular particles. Two samples were prepared using HPGR and ball mill. The Blaine number of the HPGR product and ball mill product were 900 cm2.gr-1 and 1200 cm2.gr-1 respectively. The results revealed that pellet properties of this HPGR product are comparable to that of the ball mill product; this is due to the fact that the flake-like morphology leads to enhanced pelletising performance of coarse magnetite concentrates. Keywords: Iron pelletising, Blaine number, HPGR, ball mill, coarse particles

Jan 1, 2020

By M. Getz, G. Asbjörnsson, M. Evertsson, E. Clausen

The mining and minerals processing industry is gradually being digitalised to improve efficiency and reduce waste. One aspect of this transition is to ensure that relevant process-data in sufficient quality and quantity is given to the operator in the right format and at the right time to improve decision-making. Augmented reality (AR) can serve as a potential ICT-based support tool for human operators to achieve this goal. With AR, virtual information is combined with the real world in real-time. Therefore, AR technology has the potential to fulfil the need for improved data visualisation in Industry 4.0. This article is the result of a collaborative effort of Chalmers University of Technology and RWTH Aachen University to identify selected state-of-the-art applications of AR that are currently being applied in the mining and minerals industry or selected related industries. This paper aims to show opportunities for the application of AR in the mining and mineral processing industry with selected conceptual applications. This paper concludes with several requirements identified as necessary to implement the conceptual applications of AR in the mining and minerals industry. Keywords: Augmented reality, mining, mineral processing

Jan 1, 2020

By V. E. Cogho

Optimum Coal Mine comprises numerous defunct, active, and future mining sections. The mine is primarily a large opencast coal mine; however, underground mining activities will be increasing steadily over the next five years. Opencast and underground mining activities have a significant impact on surface and groundwater resources. To ensure that the mine will be able to continue with its mining operations in the catchment, the extent of this impact requires that substantial intervention and mitigation measures need to be implemented to ensure the environmental integrity and economic use of the catchment's water resources. In developing a sustainable long-term mine water management strategy, numerous scenarios had to be analysed. The various scenarios are a combination of water and land management activities. The main components of the mine's integrated water management strategy include: ? Management of water recharge by continual and appropriate rehabilitation of disturbed land ? Beneficial re-use of impacted mine water for coal plant process water and mining operations ? Reclamation and desalination of remaining excess impacted mine water to potable standard. From the mine's long-term water balance, it is evident that the continued implementation of numerous water management actions is required to mitigate the water resources impact in a sustainable manner. In addition, the installation of a water reclamation plant at Optimum Coal Mine (15Ml/day) is a key step for the mine to achieve a zero impact target.

Jan 1, 2012

By E. Keskinkilic

"Electrical steels are used in motors, power generators, and transformers for electrical applications owing to their superior magnetic properties. These properties are attributed to the presence of a significant amount of silicon in the steel composition. Silicon steels are mainly grouped into two categories: grain-oriented (GO) and non-grain-oriented (NGO). The Si content of silicon steels is generally between about 0.50% and 6.5%. Therefore, silicon steels are further classified as low-, medium-, and high- Si steels. In this paper, liquid steel processing practices for certain silicon steel grades are outlined by considering a steelmaking company that integrates a basic oxygen furnace (BOF), a ladle furnace (LF), and an RHtype degassing unit (RH). Possible problems are identified, suggestions are discussed, and conclusions are drawn on the basis of real plant data. IntroductionSilicon steels are widely used in motors, power generators, and transformers for electrical applications owing to their superior magnetic properties. They are mainly grouped into grain-oriented (GO) and non-grain-oriented (NGO) silicon steel grades. However, the differences in the properties and thermomechanical production of GO and NGO grades are outside the scope of this paper. In this study, liquid steel processing of certain silicon steels will be discussed. In the integrated iron and steelmaking process, converters are the primary steelmaking units that are used mainly for decarburization and removal of phosphorus, and for obtaining liquid steel with a specific chemical composition and suitable temperature for secondary steelmaking operations. Ladle furnaces are a type of ladle metallurgy station consisting of electrical and/or chemical heating facilities for adjusting the liquid steel temperature during alloying. Various types of vacuum units are used for decarburization and removal of nitrogen, hydrogen, and sulphur, depending on the metallurgical conditions of the vacuum units. In this study, an actual steel-production company is considered in which the primary steelmaking operation uses a basic oxygen furnace (BOF) converter with N2/Ar bottom blowing, and the secondary steelmaking operations involve (1) a ladle furnace (LF) heated by three graphite electrodes and equipped with Ar and N2 top/bottom mixing facilities, and (2) an RHtype (circulation) vacuum degassing unit equipped with a top-blowing attachment for oxygen blowing and an Ar-purging system from an up-leg snorkel."

Jan 1, 2018

By N. Baloyi, P. Popoola, M. Tlotleng, S. L. Pityana

"The advantages of using Nb and Ti-Nb coatings for improving the hardness and corrosion resistance of Ti6Al4V were investigated. It has been reported that Ti6Al4V used in orthopaedic implants tends to release toxic Al and V ions into the surrounding tissue. Thin layers of Nb and Ti-Nb were deposited on Ti6Al4V substrates using a laser metal deposition technique. The deposited material was analysed using optical microscopy, scanning electron microscopy coupled with energy dispersive spectroscopy, and X-ray diffraction. The corrosion behaviour of the deposited layers was investigated using a Metrohm Autolab PG Stat101 compact potentiostat at 25°C in simulated body fluid. A Vickers hardness system was used to study the mechanical properties. Both Nb and Ti-Nb coatings exhibited good metallurgical bonding with the substrate. The microstructure and the XRD analyses for the Nb system showed that the ??-Nb phase was most dominant, while the Ti-Nb system comprised a mixture of the ?? and ?? phases. The average hardness of the Ti6Al4V substrate was 350 HV0.3, with a slight increase for the Nb coating (363 HV0.3) and a much higher hardness from the Ti-Nb coating at 423 HV0.3. The corrosion results show that the deposited Nb was more corrosion-resistant in the solution than either the Ti6Al4V substrate or the Ti-Nb coating. IntroductionMetals such as CoCrMo, 316L stainless steel, and NiTi alloy are used for manufacturing biomedical implants because of their distinct properties, including low Young’s modulus and resistance to corrosion (Ryan, Pandit, and Apatsidis, 2006). These implants are required to remain in the body of the patient for many years. Due to a favourable combination of mechanical and corrosion properties, titanium (Ti) and its alloys have emerged as the material of choice for biomedical implants. Nevertheless, their use is not entirely without challenges. For example, in total hip replacement, surface degradation and release of metal debris from metal erosion caused by friction limit the service life of the implant (Oldani and Dominguez, 2012; Manivasagam, Dhinasekaran, and Rajamanickam, 2010). Ti6Al4V is generally regarded as the material of choice, but it has been found that over extended periods of time, the combination of surface corrosion and erosion releases toxic aluminium (Al) and vanadium (V) ions into the body (Khan, Williams, and Williams, 1996). This has led to the investigation of titanium alloys containing niobium (Nb), zirconium (Zr), tantalum (Ta), and molybdenum (Mo) as substitutes for the aluminium- and vanadium-containing Ti6Al4V (Okazaki et al., 1997)."

Apr 1, 2019

By J. G. M. Mukoya, B. Adebayo

"This work was carried out to determine the influence of rock properties and drilling machine parameters on the penetration rate at the SJ pit of Rössing Uranium Mine, Namibia. Rock properties (uniaxial compressive strength, tensile strength, and modulus of elasticity) of samples collected were determined in the laboratory. Drilling experiments were conducted in which feed pressure, air pressure, rotary speed, weight on the bit, and torque were varied to measure their effect on the penetration rate. The uniaxial compressive strength varied from 90–180 MPa for layered marble-quartzite and banded gneiss. Increases in feed pressure, weight on the bit, and rotary speed beyond the optimum level led to a decrease in penetration rate and caused the drill bit to ‘stall’. Results of the study revealed that penetration rate increases with an increase in the feed pressure and air pressure. After reaching a maximum value, the penetration rate begins to decrease despite increasing feed pressure. A very high torque causes the drill bit to stall, since the feed pressure is too high and the air pressure is not sufficient to remove the cuttings from blast-hole at maxium bailing velocities. The average penetration rate varied from 19 to 45 m/h for phase 2 of the pit, and from 17 to 68 m/h for phase 3. The optimum drilling machine parameters obtained in phase 2 and phase 3 were slightly lower than those currently being used at the mine. The trials of the optimum machine parameters will assist in reducing the cost of drilling, which varied from N$29.48 to N$36.31 per metre for the tricone bit. IntroductionDrilling and blasting contribute most to the operational cost in opencast mining and have the potential to influence the cost of downstream operations (Aipanda, 2011). The drillability of rocks depends not only on the rock properties, but also on the drilling tools and operational variables. In rotary drilling, rotational speed, thrust, and flushing are the operational variables, and are known as the controllable parameters. Rock properties and geological conditions are the uncontrollable parameters. Although, many attempts have been made to correlate drillability with rock properties, the rock characteristics affecting rotary drilling have not been entirely defined (Hartman, 1962; Kahraman, Bilgin, and Feridunoglu, 2003). The drill bit is forced into a rock surface from the rotary power head, hence creating stresses and cracks at the bit-rock interface. The performance of a particular bit in any formation i"

May 1, 2019

By H. Cevizci

"Numerical simulation of the plaster stemming method (PSM) was performed and compared with the conventional drill cuttings stemming method (DCSM). Many earlier in situ tests have proved that PSM can use the blast energy more efficiently than DCSM. Despite PSM generating more blast vibrations, it has advantages over DCSM such as better fragmentation and lower cost per unit volume of rock blasted. In this study, numerical simulation with Autodyn software using a 2D tool was employed to prove the efficiency of plaster stemming by comparing parameters such as pressure, Y-velocity, Y-force, internal energy, acceleration-Y, and compression. For example, the maximum pressure attained at the top of explosive column was 7 395 MPa for DCSM whereas it was as high as 11 945 MPa for PSM. Most of the computed parameters were significantly higher in PSM than those obtained for DCSM. This paper is the first study elucidating the efficiency of PSM by numerical simulation. It is concluded that PSM can save substantial amounts of money and effort. IntroductionThe stemming of blast-hole collars in surface mines with an inert material redirects blasting energy to the rock more efficiently, thus the blast-induced energy is utilized more effectively in breaking rock. With proper stemming, the gases should not escape due to loose stemming material. More efficient stemming with better confinement therefore increases the blast fragmentation. In addition, the distance of scatter is increased, giving rise to a looser rock pile that can be more easily loaded and transported. In order to improve the performance of a blast, stemming is used to help maintain the gas pressure over time (Konya and Konya, 2018). Proper stemming has been shown to increase the explosive efficiency by over 41% (Snelling and Hall, 1912).The most common stemming material in open pits and quarries is drill cuttings because of their ready availability at blast sites and the drill cuttings stemming method (DCSM) is a low-cost method. However, it has a major disadvantage in that dry drill cuttings eject very easily from blast-holes during an explosion, thus a great percentage of the blast energy is wasted and lost to the atmosphere. Cevizci (2012, 2014, 2013, 2017) studied blasting parameters in open pits and obtained better results with the plaster stemming method (PSM) in many limestone, basalt, and clay quarries. Moulding plaster is preferred for stemming because of the fast hardening time of 25–30 minutes."

May 1, 2019

By T. Steyn, M. Fofana

"One of the most important performance indicators of a dense medium cyclone (DMC) circuit is the Tromp curve, and by extension the separation density and Ecart Probable (Ep) values. The densimetric profiles of DMC product streams have been traditionally acquired using heavy liquid sinkfloat analysis, which has certain disadvantages, such as the associated safety and health risks. More recently, non-toxic media such as lithium hetero-polytungstates (LST) have been used, with the desired densities being achieved by maintaining the solutions at specific temperatures. However, the high costs of these liquids can be prohibitive. The long turnaround time of the sink-float analysis is a further disadvantage for timeous interventions to the operating set-points of the DMC process.The RhoVol technology can generate the density distribution of a batch of particles in a rapid, accurate, repeatable, and safe manner. Additional data of interest, such as particle size and shape, are also measured and reported on a per-particle basis. Furthermore, samples can be sorted into discrete sorting bins based on any of the measured parameters of the particle, making further analyses of the material possible.This technology has applications across all commodities that use the DMC, particularly in the size fractions –25 +8 mm and –8 +3 mm. To date, laboratory results have proved very encouraging – separation densities are within 5% of traditional sink-float results, and the technology is being introduced to diamond DMC plants. IntroductionDense medium separation is a commonly applied beneficiation technique for the density separation of valuable products from crushed ores. The industrial applications of dense medium separation have been summarized and reviewed by various authors (Scott and Napier-Munn, 2000; Bosman, 2014; Napier- Munn, 2018). Dense medium cyclones (DMCs) are widely used in the coal and diamond industries to produce clean coal or to preconcentrate kimberlitic ores, respectively. Cyclone performance (separation density monitoring) techniques are varied. In the mineral industry. Density tracer tests (Napier- Munn, 1985, 2014; Davis, Wood, and Lyman, 1985) without mineral material, are widely used, although their accuracy in predicting operating cut-points becomes limited once mineral material is introduced. More recently, radio frequency tracers have been also used for the determination of cyclone efficiency (Partition Enterprises, 2017)."

Feb 1, 2019

By L. A. Cornish, J. W. van der Merwe

"The hydrofluoric acid (HF) industry in South Africa uses normalized mild steel (SA 516 Gr 70) for the storage and distribution of its technical-grade product (70% HF). The technical-grade acid is split from the anhydrous hydrogen fluoride (AHF) product during distillation just after HF is produced, in a stainless-steel-lined kiln, from the reaction of calcium fluoride (CaF2) with sulphuric acid (H2SO4). Uniform corrosion of the storage tanks is mitigated during commissioning by contacting the steel with 70% HF. A corrosive reaction takes place (2H+ + 2F- + Fe - H2 + FeF2) to form scale inside the tank which diminishes the attack of the steel by fresh HF, thus prolonging the service life expectancy of the vessels. This iron fluoride scale in the vessel grows continually, resulting in corrosion of the vessels continuing at a predictable rate (approx.0.5 mm/a) since the first commissioning of the tanks at Necsa in 1993. In early 2012, an increase in the average corrosion rate of the tanks to 3 mm/a measured at and below the liquid band in the storage vessels was noted. Three months later the corrosion rate had increased to 30 mm/a, just before the first leak from the tank was detected. The tanks were decommissioned shortly afterwards and an investigation revealed that the rapid corrosion was due to the presence of increased levels (>50 ppm) of nitric acid (HNO3) in the tanks, which attacked the fluoride layer protecting the steel.The oxidation characteristics of high concentrations of HNO3 with low concentrations of HF on stainless steel surface treatment are well documented. The effects of low levels of nitric acid on the corrosion of steels in sulphuric acid are also known, but no suitable published data currently exists on corrosion by high HF concentrations with low HNO3 concentrations. Establishing the rate and mechanism of corrosion by HNO3 (0.1 to 1%) in 70% HF is currently a high priority for the HF industry in South Africa and will become increasingly important in the near future due to depleting fluorite reserves and cheaper, but less efficient, processes producing H2SO4 with higher impurity levels, which were not anticipated in the original plant design. The aim of this study is to simulate the corrosion conditions in the plant on a laboratory scale in order to establish the corrosion characteristics of the steels used in the HF plant, which are increasingly exposed to the HNO3 impurities that tend to concentrate in the final technical-grade HF acid product."

Jan 1, 2016