Influence of Thermal Stability of Quartz and the Particle Size Distribution of Burden Materials on the Process of Electrothermal Smelting of Metallurgical Silicon
The article describes the thermal stability of quartz deposits in Kazakhstan-Aktas and Sarykul. It is established that the quartz of the Aktas deposit is a raw material with low thermal stability, and quartz is of medium-durability at Sarykul deposit. The complex influence of the thermal stability of this raw material on the technological parameters of smelting of technical silicon under experimental industrial conditions of electrothermal melting in a furnace with a capacity of 30 MVA was studied at LLP “Tau-Ken-Temir” (Kazakhstan). A new method for optimizing the smelting of commercial silicon has been developed, even when using raw materials with low thermal stability. The main idea of the method is that the less thermally stable quartz is used, the more the initial size of its pieces should be. Optimum values of the average size of pieces of quartz-Aktas(85-95 mm) and Sarykul-(30-40 mm) are obtained.
Heat–induced changes in soil properties: fires as cause for remobilization of chemical elements
Abstract Exposure of soil constituents to elevated temperatures during wildfire can significantly affect their properties and consequently, increase the mobility of the bound contaminants. To estimate the potential of wildfires to influence metal remobilization from the burned soil due to the changes in cation exchange capacity (CEC) after organic matter combustion and mineral alteration and degradation, changes in soil properties after exposure to different temperatures was investigated. This was accomplished through analysis of geochemical, mineralogical and surface physicochemical properties of a soil sample exposed to different temperatures in a laboratory. Heating the soil sample at 200 °C, 500 °C and 850 °C resulted in an increase in pH (from 5.9 to 12.3), decrease in cation exchange capacity (from 47.2 to 7.3 cmol+kg−1) and changes in the specific surface area (observed only at 500 °C), that are associated with structural modifications of clay minerals and ferromagnetic minerals. Extraction analysis showed the increase in the concentration of almost all analysed elements (Al, Cd, Co, Cr, Fe, Mn and Zn) in soil eluates. The observed increase, following high– temperature heating (500 °C and 850 °C), was as much as 15 times higher (e.g., Al), compared to the native soil sample (25 °C). This strongly indicates that wildfire can act as a trigger for remobilization of heavy metals.
NEW TRENDS IN SUBMERGED ARC FURNACE TECHNOLOGY
During the last 100 years, the submerged arc furnace has been one of the most amazing diversified melting units. These furnaces have found their application in more than 20 different main industrial areas such as iron, ferro alloy, chemical industry, lead, zinc, copper, refractory, titanium oxide, recycling, phosphorus etc. Such an evolution was only possible because of tremendous efforts in research and development and due to the large range of design solutions. The worldwide strong competition also forced the industry and plant suppliers to develop highly efficient, economically balanced units in optimized sizes and with long lifetimes, to adapt to the market conditions. Regarding the environment, the plants improved significantly in terms of water treatment plants and off-gas treatments as well as on site recycling units. This paper will give a short overview regarding the history of the furnace and the latest developments in today’s main application fields: ferro alloys, non-ferrous, special application and recycling. In addition new activities in research and developments are briefly discussed. 1. THE HISTORY OF SUBMERGED ARC FURNACE TECHNOLOGY With the development of the dynamo machine by Werner von Siemens, one of the most recognizable developments started: the industrial utilization of electrical energy. The increasing demand for ferro alloy and deoxidation agents in steel making in the beginning of the 20 century led to the development of the first furnaces. DEMAG, for the last two centuries a major supplier for the iron and steel industry started with the construction of the first submerged arc furnace in 1905 [1]. The 1.5 MVA unit was installed in Horst Ruhr/Germany for the production of calcium carbide and was successfully commissioned in 1906. The “evolution” and the major milestones of the unit will be described in more detail. Development of large electrode systems, advanced transformer technology and new furnace construction principles allowed the design of large-capacity rectangular SAF with dimensions of up to 36 m in length and 17 meters in width [2]. Figure 1 shows an example of one of the first closed furnaces for pig iron production, which was installed in Germany. From a technological point of view even bigger units are possible but their economical feasibility has to be checked carefully. New developments are recognized for processes applying increased slag bath operation in the construction of highly efficient units with a high power density [3]. The higher power density in the furnace causes higher heat fluxes through the sidewall which necessitates the application of new cooling concepts. The first application of such a system had been successfully commissioned at the beginning of this century (see Figure 2). The picture below shows the first industrial application of horizontally placed water-cooled copper elements in the side wall of a closed pig iron furnace with a round shell. Figure 1. Pig iron furnace from the 1950s. Figure 2. Modern pig iron furnace. The control systems also went through a significant evolution during the last decades (see Table 1). Evolution of controlling systems for SAF 1906 Manually controlled 1955 Electr. balanced beam controller 1960 Moving-coil controller 1965 AEG transductor controller 1975 AEG transistor controller 1978 Siemens transistor controller 1985 H&B controller 1995 Software controller Since the end of the 1950s SMS Demag SAF’s have been equipped with electrode controllers. Today’s advanced submerged arc furnaces make use of software controllers. The DC application for submerged arc furnace technology shows world-wide an increasing market share in niche areas especially in some ferro-alloys, slag cleaning and TiO2 production units. Nevertheless, the majority of submerged arc furnaces are and will remain AC-technology-based. The next chapter will emphasize current developments and recent projects in the area of ferro alloys, nonferrous and special applications. 2. DEVELOPMENTS AND PROJECTS FOR FERRO ALLOY APPLICATION The ferro alloy industry is directly linked to the steel industry. It is predicted that the total crude steel production will reach an annual rate of almost 800 million ton of crude steel per year by the end of 2010 (see Figure 3). An increase in EAF share of steel making, whereas LD steel making will remain stagnant [4]. The stainless steel production is predicted to increase at an annual rate of 5%. The demand for ferro alloys as a bulk alloy is expected to increase steadily which indicates a solid base for good future market developments. Figure 3. World steel production developments by process route. Figure 4. Metal tap at Minera Loma de Niquel in Venezuela. Originally, the alloying metals required for upgrading steel were exclusively produced in special blast furnaces which results in a pig iron consisting of small to medium contents of silicon, chromium and manganese. Type of the melting and metallurgical equilibrium determined the carbon content of these alloys. The submerged arc furnace is a much better qualified unit to produce ferro alloys with regard to temperature control and adjustment to different reduction potentials. Over the time an independent ferro alloy industry established which meets today’s permanently increasing demands of the steel industry. The majority of ferro alloys is produced by pyrometallurgical smelting taking place in submerged arc furnaces. Other attempts regarding the hydrometallurgical production of ferro alloys such as high pressurized acid leaching processes for nickel production do so far not show the predicted economical and technical benefits. The strong competitiveness of submerged arc furnaces in this sector was mainly achieved by the installation of advanced high power smelting units. 2.1 FeNi-application Today, a modern FeNi-furnaces is characterized by its high efficiency. In general we offer, depending on the requested capacity, round furnace designs or rectangular shaped furnaces. Efficient furnace operation can only be achieved with: • correctly designed SAF • highly efficient energy transfer with arcing mode • proper mining management • controlled calcinations of the SAF feed • sufficient dust recycling • modern furnace operation with skilled personnel, allowing precise slag and temperature control [5] A good example for those conditions is practiced at the plant at Minera Loma de Niquel in Venezuela. The mine is located only 3 km from the site. The ore contains 15-30% moisture, 1.5-1.8% nickel, 15-17% iron and the SiO2/MgO ratio is approx. 1.3. After the ore is processed in a drying and raw material preparation step it is charged for calcination to 2 rotary kilns. The generated calcine is then charged into the 2 45 MVA submerged arc furnaces (see Figure 4). The design of high power smelting units in the ferro nickel application led also to the development of various suitable side wall cooling concepts. A new milestone in terms of cooling was achieved with the development of copper cooling elements for round and rectangular submerged arc furnaces. The concepts will be explained in more detail in the chapter design criteria on the following pages. Such a cooling system is currently being installed at a furnace for Eramet in New Caledonia. It is planned to restart the modified furnace by the beginning of 2004. The principles of this cooling concept had been developed during the last decade and were tested in a 1:1 model. The furnace at Eramet is a 6-electrode rectangular SAF with a transformer rating of 99 MVA and an operating load of 75 MW. This is the largest ferro nickel furnace installed by SMS Demag. The advanced furnace design and the development of a 2100 mm long self-baking electrode will allow even bigger production units. 2.2 FeCr-application FeCr production is carried out either in DC or AC based SAF. The increasing share of fine ores in the mine bodies of newly explored mines might force the ferro chrome producer either to agglomerate the ore or to process it directly as fines. A common way of charging fines to the SAF is via a hollow electrode system (HES), which had first been successfully utilized in the fifties. Since 1993 such a system operates successfully. It is also possible to agglomerate the fine ores by a mini sinter plant or by a cold briquetting unit before charging to a conventional AC-based submerged arc furnace. SMS Demag received the order for two 60 MVA FeCr furnaces in India. The plant combines a cold briquetting line with the submerged arc furnaces and will have a nominal annual production of approx. 150 000 t high carbon ferro chrome. A picture of a mini sinter plant is shown in Figure 5. Figure 5. Mini sinter plant. Figure 6. Rotary hearth furnace/submerged arc furnace combination [7]. Other investigations check the heating and pre-reduction of material to higher metallization levels of the chromium and iron in the material prior to the submerged arc furnace. This technological approach is comparable to several FeNi-processes [6]. A future pre-treatment of the fine ore is currently under investigation in our research laboratory in Italy [7]. The results look promising. Past industrial scale tests at Inmetco and pilot scale tests investigated the option of achieving an acceptable range of pre-heating with pre-reduction of the iron and the chrome fraction in a rotary hearth furnace. Such a unit in combination with an SAF is shown in Figure 6. In principle, ore fines are pelletized together with coal and then heated up in the RHF, where the prereduction is carried out. Hot pre-reduced pellets can then be charged with approx. 1000°C via a hot feeding system to the SAF. 2.3 FeMn-application During the last decades we design and supply several FeMn/SiMn furnaces worldwide. The technology is in a good development stage. Some highlights regarding some process innovations are: At SEAS the largest FeMn-SiMn furnace is operating very successfully. The furnace 