Scielo RSS <![CDATA[Journal of the Southern African Institute of Mining and Metallurgy]]> vol. 113 num. 8 lang. en <![CDATA[SciELO Logo]]> <![CDATA[<b>Refractories and sulphuric acid</b>]]> <![CDATA[<b>Report for ITA World Tunnel Congress 2013, February 2013</b>]]> <![CDATA[<b>SO<sub>2</sub> - an indirect source of energy</b>]]> Global sulphur dioxide (SO2) emissions peaked around the mid-1970s, after which they declined. However, with the growth of specifically China, emissions are on the rise again. In 2008, global anthropogenic SO2 emissions totalled 127 Mt, with energy production accounting for 63.2 Mt and metal-related processes 12.8 Mt. As a well-known gaseous pollutant, SO2 is not per se known as a source of energy. However, in the presence of water SO2 can be electro-oxidized at the anode of an electrolyser to produce hydrogen ions, which in turn can be reduced at the cathode of the electrolyser to produce hydrogen gas. Gaseous emissions of SO2 can therefore be cleaned up with the simultaneous production of hydrogen, an energy store or carrier, which provides an economic offset to the overall cost of this potential remediation process. This process forms part of the Hybrid Sulfur (HyS) cycle as well as the once-through HyS (OTHyS) cycle. Indications are that the greatest stride towards the development of an effective electrolyser for the electro-catalytic oxidation of SO2 requires the development of an anode electrocatalyst exhibiting enhanced activity for the electro-oxidation of SO2 A critical review will be presented on the research and development of such an anode electrocatalyst, and a strategy for a more effective research and development effort will be discussed. This will include theoretical studies on the electro-catalytic oxidation of SO2 on different metal-based catalytic surfaces (for which some preliminary results are presented) in conjunction with combinatorial (simultaneous multi-metal multi-electrode) electroche6mistry studies as well as single-electrode electrochemistry studies. <![CDATA[<b>Integrated production of liquid sulphur dioxide and sulphuric acid via a low-temperature cryogenic process</b>]]> The paper describes the design options available for the simultaneous production of liquid sulphur dioxide, via cryogenic condensation, and sulphuric acid in a sulphur-burning sulphuric acid plant. The impact of the operating conditions of the cryogenic condensation on plant capital and operating costs are discussed, with the main focus on the optimization of the most important economic and operative drivers of the non-ferrous mining industry, such as energy efficiency, reliability, and availability. A case history relevant to an integrated sulphur-burning plant producing liquid sulphur dioxide and sulphuric acid, recently started up in the Democratic Republic of Congo, is described. <![CDATA[<b>Fluorinated melt-extruded thermoplastic linings in off-gas pipelines of sulphuric acid production plants and in tailor-made equipment for sulphur oxide and sulphuric acid handling</b>]]> Gases containing sulphuric acid and sulphur oxide (SO2 , SO3 ) can lead to severe corrosion problems, in particular in high concentrations combined with water vapour. If acid condensation takes place, this can lead to the extremely destructive dew-point corrosion. In these cases, even materials like titanium and stainless steels can be attacked in the aggressive environments, or suffer variable but significant degrees of material loss by surface degradation. An excellent option for solving the corrosion problems induced by sulphuric acid and sulphur oxide is linings made from fluorinated melt-extruded thermoplastics. Those can be divided into partially fluorinated polymers like polyvinylidenefluoride (PVDF) and ethylene-chlorotrifluoroethylene (ECTFE), and fully fluorinated materials like tetrafluoroethylene-hexafluoropropylene (FEP) and tetrafluoroethylene-perfluoroalkylvinylether (PFA). The latter have a much higher temperature and chemical resistance than partially fluorinated thermoplastics, but share their outstanding processing properties. Fully fluorinated melt-extruded thermoplastics extend the application range of the lining solutions remarkably. The linings can be installed in sheets, tanks and reactors for liquid sulphuric acid, as well as in scrubbers and pipelines for the conversion and transport of sulphur oxide and sulphuric acid gases and fumes, including the shut-off valves for flow regulation. An introduction to and comparison of the lining materials and installation methods will be presented, illustrated by means of selected relevant applications. Application ranges for partially and fully fluorinated melt-extruded thermoplastics will be discussed against the background of the chemical compounds and temperatures involved. Case studies of two recently installed and successfully running pipelines in sulphuric acid producing metallurgical plants will be shown in detail. One is a fibre-reinforced plastic (FRP) reinforced FEP liner pipe for off-gases containing droplets of sulphuric acid. The other is FEP-lined blast spools made from stainless steel, also installed in gas streams containing sulphuric acid. Fabrication methods for transition forms, over-dimensional pipes, and other tailor-made parts will be explained, and relevant quality standards and best practices will be discussed. <![CDATA[<b>Recent advances in refractories - aluminate binders and calcium aluminate bonded high-performance monolithic castables</b>]]> Monolithic refractory castables, bonded with calcium aluminate cements (CACs), have a rich history of nearly 90 years. They have evolved from high-cement conventional formulations to the era of reduced-cement castable systems that can be placed using a variety of techniques such as casting, gunning, self-flow, pumping, and shotcreting. The selection of calcium aluminate binders must be considered carefully in the quest to optimize performance and installation success of refractory monolithics. The paper discusses recent advances in aluminate binders for high-performance castables. A specific focus will be the key issues that impact the reliability and predictability of placing properties. The impact of formulation parameters such as the type of calcium aluminate, additive type and role upon final durability is also assessed. New types of aluminate-based binder systems offering enhanced usage characteristics relevant to high-temperature thermomechanical, abrasion, and corrosion resistance are introduced and their merits compared to existing systems. Future perspectives are discussed, along with possible developments. Conclusions are drawn as to the merits of each type of system and the specific advantages that castable systems possess. <![CDATA[<b>Sidewall design for improved lining life in a PGM smelting furnace</b>]]> The matte/slag tidal zone in a nickel-copper-iron-sulphide matte smelting furnace is subject to severe process conditions that can lead to premature failure of the lining. The MgO-FeO-SiO2 slag is aggressive to refractory bricks, while the Ni-Cu-Fe-S matte is aggressive to water-cooled copper cooling elements, and can penetrate and sulphidize refractory bricks. A novel approach to the lining design was taken at a PGM smelting furnace, where an indirectly-cooled graphite ring was applied in the matte/slag tidal zone, with refractory bricks on the hot face. The graphite was cooled by the copper cooling elements in the slag zone, obviating the need for copper cooling elements in the tidal zone. In this paper the effectiveness of different lining designs utilizing varying degrees of cooling in the slag zone and slag/matte tidal zones is compared. In comparison to water-cooled copper in various configurations, cooled graphite shows promise as an indirect refractory cooling system in the tidal zone area of furnaces. <![CDATA[<b>Factors affecting the lifespan of cast refractory linings: a general overview</b>]]> Many factors, from engineering and design to supply, installation, and operation affect the service lifespan of cast refractory linings. If a shortcoming occurs at any of these stages, the lining lifespan can be drastically reduced. Although not every aspect that can affect a cast refractory lining's lifespan is outlined here, a few key points are highlighted, with an emphasis on the various installation parameters. <![CDATA[<b>Fundamentals of designing refractory linings for hydrometallurgy autoclaves</b>]]> Pressure hydrometallurgy operations require vessels to be lined with an impermeable membrane for corrosion protection and one or more courses of refractory or ceramic brick. Examples of unit operations that utilize composite lining systems include pressure oxidation autoclaves, sulphide precipitation autoclaves, chloride leach reactors, flash vessels, cyclone separators, and direct contact condensers (heater vessels and quench vessels). The refractory lining must satisfy multiple requirements: it must thermally insulate the membrane from process fluid, be structurally stable, provide erosion resistance, be chemically compatible with process fluid, and provide an economic service life. New hydrometallurgical processes are pushing the pressure, and temperature with each new generation of plants. A fundamental understanding of all factors affecting the mechanical stability of the lining system is essential as lining designs move further away from the industry's experience base. The method of designing a refractory lined vessel is examined taking into account the impact of irreversible chemical swell, operating factors, design factors, and installation factors. The effect of geometry and incorporating additional degrees of freedom to the analysis is explored using two-dimensional and three-dimensional finite element analysis (FEA). The effect of these additional degrees of freedom on the results of the one-dimensional model are discussed. <![CDATA[<b>Merits of using andalusite-based refractories compared to bauxite-based refractories</b>]]> Historically bauxite-based refractories have been used in applications where andalusite-based refractories could work. Bauxite-based refractories were chosen over andalusite-based refractories mainly because of the availability of low-cost Chinese bauxite and also because many furnaces were designed by international companies that cannot easily access high-quality products. Currently, the availability of low-cost bauxite is under threat as a result of high export duties and tariffs as well as restrictions on the amount of material that China allows to be exported. South Africa is a major producer of andalusite, and this guarantees stability with respect to both availability and price. Refractories based on andalusite have an added economic advantage over bauxite-based refractories in that unlike bauxite, which requires high-temperature calcination before use, andalusite is used in its raw state. This paper constitutes a literature study of the merits, both technical and economic, of using refractories based on andalusite compared to bauxite-based refractories. <![CDATA[<b>Low-cement chrome-oxide-free castable for use in ironmaking rotary kilns</b>]]> In the Evraz Highveld production process, iron ore is prereduced in rotary kilns prior to charging into submerged arc furnaces (SAFs). Owing to the highly aggressive slags that are formed within the combustion zones of these kilns, chrome-oxide-containing low-cement castables have been used for some years in the kiln lining. Due to concerns around the potentially harmful effects should Cr(V1) be present in the spent linings, Highveld and its suppliers of kiln castables devoted time to developing alternative chrome-free lining materials. Such materials needed to be cost-effective and ensure good performance and improved kiln availability. This paper discusses all the parameters surrounding this development. <![CDATA[<b>Multi-mineral cut-off grade optimization by grid search</b>]]> Orebodies that contain more than one economically important mineral are generally evaluated by parametric cut-off grades. This approach often leads to mis-valuation of mineral deposits because cut-off grades are not based on the grades of each individual mineral and, because of the parametric formulation, are only indirectly related to the individual grade distributions. The only realistic approach is a formulation that accounts separately for each component mineral. The grid search method can be used as a means of multi-mineral cut-off grade optimization in this context. This paper describes the use of the grid search method in cut-off grade optimization for multi-mineral deposits. The authors introduce the general concepts of the method and formulate its application to cut-off grade optimization; they describe a software implementation of the method that can accommodate cut-off grade optimization for mineral deposits that contain up to three economic minerals. <![CDATA[<b>The manganese ferroalloys industry in southern Africa</b>]]> Southern Africa has a history rich in the pyrometallurgical processing of ores. Two of the alloys produced are high-carbon ferromanganese (74-83 per cent Mn) and silicomanganese (59-67 per cent Mn and 14-31 per cent Si). The largest land-based manganese ore deposit in the world is based in the Northern Cape Province of South Africa. In 2010, 6.3 Mt of ore from the deposit was exported, mainly to China, Norway, Japan, and India, and an estimated 1.1 Mt beneficiated locally to produce manganese ferroalloys. In southern Africa the producers of manganese ferroalloys are Metalloys and Assmang, and of silicomanganese Transalloys, Mogale Alloys, and Match Corporation. Submerged arc furnace technology is utilized in the production of high-carbon ferromanganese (HCFeMn) and silicoman-ganese, applying the discard slag practice in both instances. The carbon content of HCFeMn is reduced using converter technology and that of SiMn using the Perrin process. Ninety per cent of the world's manganese ferroalloys production is consumed by the steel industry. The consumption (and production) of manganese ferroalloys therefore closely follows worldwide steel production. From 2001 to 2010, the doubling of the worldwide production of manganese ferroalloys can be attributed to an increase in production capacity in Asia and Oceania, with the largest increase in the production of SiMn. Manganese ferroalloys produced in Asia and Oceania are mainly for the local market, with North America, Europe, Russia, and Turkey the major markets for alloys from southern Africa. The major challenges faced by the southern African manganese ferroalloy producers are increasing electricity tariffs and productivity of labour, which require a concerted effort by all parties involved to achieve a satisfactory solution.