Thermodynamic Processes and Characterisation of Dead Burned Magnesia: A Review

Transcription

1 Thermodynamic Processes and Characterisation of Dead Burned Magnesia: A Review ASHFAQUE AHMED CHOWDHURY 1, M G RASUL and M M K Khan Process Engineering and Light Metal Centre, College of Engineering and Built Environment Faculty of Sciences, Engineering and Health, CQ University Bruce Highway, Rockhampton, Qld 4701 AUSTRALIA 1 a.chowdhury@cqu.edu.au Abstract: - In magnesia plant, annular vertical shaft kilns - high temperature processing units, are used for the production of dead burned Magnesia. Vertical Shaft kilns such as those operating at the reference magnesia plant are known to be efficient. However; the knowledge of thermal processes and physical behaviours inside the operating kiln has not yet been studied in detailed. This is mainly due to the high working temperature inside the kiln which makes temperature and pressure measurements very difficult and expensive or almost impossible. The inability to obtain such information has limited the optimisation of the kiln's process variables. With increasing knowledge and computing power, computational modelling is becoming a readily used resource for improving processes that occurs in a vertical shaft kiln. This study represents the review of the thermodynamic processes of dead burned magnesia production with an integrated view to improving energy efficiency of magnesia production and to achieve environmental sustainability. Key-Words: - Dead-burned magnesia, thermodynamic process, vertical shaft kiln. 1 Introduction Dead-burned magnesia is used almost exclusively for refractory applications in the form of basic bricks and granular refractories. Dead-burned magnesia has the highest melting point of all common refractory oxides and is the most suitable heat containment material for high temperature processes in the steel industry. Basic magnesia bricks are used in furnaces, ladles and secondary refining vessels and in cement and glass making kilns. The vertical shaft kiln is a high temperature processing unit in the production of deadburned magnesia (DBM). Shaft kilns are required to be capable of producing different grades of products, which can be achieved by changing operating parameters of the shaft kiln. Environmental degradation is always present throughout the life cycle of a process industry starting from exploration of raw materials and energy resources to disposal of wastes and end product. Industrial emissions have potential to cause serious contamination of the environment. Emissions in the vicinity of the process industries are already threatening the sustainability of the resource base and having effects on the health of people, most of who are affected but unaware of or hardly have any other choices. The reference magnesia plant has multiple vertical shaft kilns to perform the dead burn process, all of which can have improved efficiency by fine tuning the plant. It is essential to develop a more realistic three dimensional computational technique which is capable of predicting more accurate internal heat profiles of the kiln by taking into account of the interacting physical and chemical laws of combustion, particle gas dynamics and mass and heat transfer, all of which are results of the interaction between natural gas-air combustion and the continuous moving packed bed of briquettes inside a vertical shaft kiln. This study reviews an integrated view to improving energy efficiency of magnesia production and to achieve environmental sustainability. 2 Overview of Magnesia Production Magnesium is the lightest metal commonly used for structural purposes. According to a study by USGC [1], magnesium is in plentiful supply and is widespread globally. Magnesium is the eighth most abundant element in the earth's crust and the third most plentiful element dissolved in seawater. World resources of magnesium are enormous and Mg-bearing brines contain a resource estimated in billions of tonnes [1]. World production of natural magnesite (MgCO 3 ) was 20 million tonnes in Australia was ranked 7 th with 540,000t production of natural magnesite. It is reported in the literature that over 98% of world magnesite production is converted to magnesia (MgO). In 2002, MgO production totalled 7.85 million metric tons (mmt) of which 6.85 mmt came from magnesite (natural magnesia) and 1 mmt from brines/seawater (synthetic magnesia) [2]. In refractories, 75% of the produced magnesia are used for furnace linings in the iron/steel, ISSN: ISBN:

2 nonferrous metals, cement and glass industries. The remaining 25% are used in agriculture, insulation and water purification [3]. The major uses of magnesium are in aluminium alloys (43%), die casting (33%) and steel desulphurisation (10%). The estimated world demand for magnesium in 2007 was around 711 ktonnes and there are good reasons to anticipate that demand will increase very significantly in the near future, particularly in the die casting sector [3]. Because of its low density, a given amount of magnesium can make 1.55 times as many articles as the same amount of aluminium [4]. Norsk Hydro has done research into the emissions savings attributable to the replacement of steel with magnesium components. The research showed that, where 50% of the magnesium is recycled, there is an average emissions savings over the life cycle of a passenger motor vehicle of about 50 kg of CO 2 per kg of magnesium used [5,6]. Every year 110,000mt of dead burned magnesia is produced in Australia and there is a need to increase the efficiency of the methods [7]. 3 Basics of Vertical Shaft Kiln at the Reference Plant Vertical shaft kilns involve two stages of thermal processing and are critical plants in the production of dead burned magnesia (MgO). Raw magnesite (MgCO 3 ) is processed through a multiple hearth furnace and converted into calcined magnesia (clinker) (MgO). Then the clinker is further processed through vertical shaft kiln or electrofusion furnace, in which the clinker undergoes densification or dead burned process. The crystal size of the final product can be used for identifying the temperature at which the briquettes are heated inside the kiln. It has been identified by observation that the minimum temperature at which the green briquettes (kiln's input material) undergo the process of densification is C. The vertical shaft kiln geometrically shaped like a vertical cylinder. During the operation compact briquettes of clinker is fed through the top and onto the existing packed bed which progresses down the vertical shaft body only by the influence of the gravitational force. The packed bed is encountered by the hot gas resulting from combustion. The combustion in the kiln is fuelled by the combination of natural gas and air which injected through 20 fuel injectors annularly mounted around the middle of the shaft. Heat generated from combustion process flow through the packed bed is transferred to the briquettes packed bed via convection and radiation. This increases temperature of the downward moving material causing densification process to occur and turning the clinker into final dead burned product. Prior to discharge the hot dead burned product is cooled by secondary air and water injected from the bottom of the kiln. The performance and stability of the internal process of the kiln is the main influence of the quality of the final dead burned magnesia product which can be measured by the Periclase Crystal Size (PCS and the Bulk Specific Gravity (BSG). By altering the operating setting of the kiln e.g. gas and air injection rate; different grades of the product can be obtained. Most importantly the quality of the final product depends on the gas-solid contact which influences the uniformity of heat transfer. At the reference plant, a packed bed arrangement is used for the process of dead burning in a vertical shaft kiln. The plant utilises a counter current moving bed, which is considerably dense with the air moving between particles at low velocity. The functionality of a vertical moving bed required that the air velocity be low enough so that the terminal velocity of the material is not reached, therefore the bed moves with the aid of gravity in a downward direction. A counter flow moving bed normally utilises large uniform particle size and low air velocity. The operational characteristics are such that it can be applied to large scale processes. It has controllable temperature gradients, and small pressure losses are encountered, although heat exchange in moving bed is not as good as higher levels of fluidisation. Such a heat distribution could result from the kiln geometry and air induction, or particle agglomeration causing poor air circulation [8]. 4 Description of Thermodynamics Processes The primary sources of combustion reaction occur in the shaft kiln are Oxygen (exists in the primary and secondary air) and methane (exists in the injected natural gas). Due to excessive supply of oxygen into the shaft kiln, the type of combustion takes place is complete and lean fuel combustion. Methane takes up about 89% of the total of the combustible components in natural gas. Therefore, only the reaction kinetic of methane and oxygen can be considered as the vital to simulate the performance of the shaft kiln. In addition, the effect of thermal decomposition can be neglected because of the fact that the dissociation and ionisation reaction are limited to only reaction at a higher temperature than that takes place in the kiln. The amount of heat release from combustion is simply the difference between the enthalpy required to form the compound of the products and that of the reactants. The amount of heat released for each type of chemical reactions can be calculated by applying the principle of stoichiometry; this is done through the use of species transport and finite volume chemistry model. The enthalpy of formation varies with ISSN: ISBN:

3 surrounding temperature, which in turns directly affects the amount of heat released from combustion. Heat transfer in the packed bed system of shaft kiln at the reference plant can be considered to consist of the conduction heat transfer in axial and radial direction between the particles in the bed, the convective heat transfer between the bed particles and the flowing gas, the heat transfer due to the effect of the convective mode on the conduction, heat transfer due to the radiation between the bed particles, the flowing gas and bed wall and the particle and the flowing gas, wall to bed heat transfer bed wall and bed particles and bed wall and the flowing medium. Particle to particle heat transfer can be modelled by an equation described by Wen and Chang (1967). The equation allows for particle to particle heat exchange as well as gas to particle exchange. From this set up, it was estimated that 10% to 35% of total heat is transferred by particle to particle transfer. These figures then provide some idea of the transfer that could occur within a particle agglomeration in which gas cannot fully contact the particles. Therefore the remaining heat in other parts of the kiln is transferred by means of gas to particle conduction, convection, and radiation. Heat transferred to the refractory can be represented by the equation described by Kunii and Levenspiel [8] which gives the heat transfer rate taking into account the correct coefficients. Kunii and Levenspiel [8] described that these equations are the effect of scouring and the heat transfer effects caused by gas film thickness at the wall of vessel. Malone and Xu [9] noted that the major form of heat transfer in a fluid is by convection, this creates a large dependence on the motion of the fluid for heat transfer. 4 Modelling of Gas-Solid Flow Raschman and Fedorockova [10] investigated the kinetics of the reaction between dead-burned magnesite and hydrochloric acid with special regard to the rate of chemical dissolution of MgO. The effect of process parameters such as temperature, activity of H+ ions, and particle size and composition of the solid was investigated. It was found that the dissolution of MgO was strongly affected by temperature (from 45 to 75 deg C) and particle size (from 63 to 355 mm), while the effect of composition of the solid was weak. Marsh [11] developed a computational model of calciner furnaces by considering the relevant physical processes including the gas flow, turbulence, flow of solids particles, fuel combustion and the pyrometallurgical process. As all these processes were closely coupled, the interaction between the phases was considered in order to produce realistic simulations of the complex behaviour of the calciner furnace. It was reported that the modelling results greatly enhanced the design process by providing information on calcination rates and particle composition exiting the furnace, interaction between the combustion zone and calcination reaction and identifying high temperature region. Witt et al [12] derived the continuum equation to predict the behaviour of gas and solid particles by averaging particle and gas properties over space and time and demonstrated the application of computational modelling to a number of typical fluidized bed systems. The hydrodynamic model applied the principle of mass and momentum conservation to the individual phase that allowed the continuum equation for volume fraction and velocities. Senegacnik et al [13] discussed some specific problems associated with heat transfer, which occurs during the process of lime burning. Increases in the thickness of the calcinated layer affect the conditions of heat inflow into the sphere. It was found from the viewpoint of increasing the capacity of existing annular shaft kilns that it is an advantageous to increase the heat transfer coefficient from kiln gases to the stones up to the value of 60 W/ (m 2 K) in the incipient part of the calcinations. Martin et al. [14] developed a novel rotary kiln through the knowledge of computer aided finite element modelling to predict the temperature profiles and heat fluxes involving non-linear properties of the exterior insulation materials and internal radiation effects. The process was modelled with the kiln which was not in rotating condition and used heat transfer effects in the forms of conduction, convection and radiation. Heat was considered to flow from the hot interiors through the composite cross section of inner liner, insulation and cladding and then by convection from the external surfaces to the ambient air. Radiation from the external surface was not considered because of an assumption based on the fact that external temperatures that were targeted to be below C. The effects of conduction between the liner and the surface of the aggregate were also considered. However, the effect of conduction into the bulk of the materials was omitted. Also neglected were forced convections, direct radiation and conduction from the hot gases and any dust or ash particles within the free spaces. 3 Characterisation of Magnesia World s magnesia demand will continue to grow due to China s industrialisation. Strong growth in MgO demand found from steel, cement, nickel, copper, cobalt and environmental markets. Roughead [15] reported that reference plant s conservative growth assumption as Total MgO 4.2% pa (330ktpa) Refractory MgO 3.8% pa (220ktpa) ISSN: ISBN:

4 Chemical MgO 5.2% pa (110ktpa) High value refractory MgO 4.3% (70ktpa) High value chemical MgO 15.3% pa (60ktpa) Products of the reference plant are historically produced calcinied, dead burn and electro fused magnesia. In one of the newest mine, magnesite reserves are large, but those are not yet fully characterised. The physical properties of magnesite are similar to earlier mine but significant differences exist with respect to chemistry. A production trial is carried out at the end 2008 to produce high purity dead burn and electro fused materials. The new mining development lease allowed the reference plant 130,000t run of mine (ROM) to be excavated. Ore body predominantly was found above water table therefore it was well suited to dry mining techniques. Magnesite beneficiations were carried out by washing, crushing, screening, heavy media density separation and laser ore sorting. Productions of 12 separate stockpiles of refined magnesite were isolated to allow characterisation of different benches (distance from surface) and nodule size. Each stockpile evaluated to generate magnesite blending strategy. Overall yield of suitable magnesite was found approximately 10% [16]. Fig. 1 represents the Magnesium chemistry and density in the blended magnesite. Fig. 1: Magnesium Chemistry vs Density [16] Magnesites from various areas of test pits were examined using Scanning Electron Microscope. Cryptocrystalline magnesite by definition has small crystals (<5μm). Work correlating magnesite crystallinity and final dead burn grain density were appeared correct. Relationship between crystal size, purity and dead burn properties were obtained and confirmed that required dead-burn density could be achieved with high purity new product [16]. Fig. 2 represents the scanning electron microscopic view of the magnesia. Fig. 2: Scanning Electron Microscope Crystallinity/ Laboratory Vertical Shaft Kiln rediction [16]. The stockpiles were further blended to give target head grade feed. Magnesite were calcined through 17 hearth MHF (temp >1000ºC). Characteristic of both new and old magnesite is that calcine chemistry varies with particle size as shown in Table 1. The materials are screened, tested and the required size fractions were blended for milling and briquetting. Significant additional reductions in silica possible with screening and air swept classifiers. Table 1: Dead Burned Magnesia - Material Properties [16] Source New Synthetic Synthetic Natural MgO Product MgO B MgO A SIO CAO Al 2 O FE 2 O Mn 3 O MgO B 2 O 3 20 ppm 86 ppm 200 ppm 70ppm C:S Ratio Grain Density (g.cm -3 ) Crystal Size (µm) In 2008, dead burn production was carried out for 2 weeks using gas fired vertical shaft kilns. Target properties were defined prior to trial commencing. Operating parameters were adjusted to assess the effect upon grain density and crystal size development. Deadburn magnesia production generated extremely ISSN: ISBN:

5 consistent properties despite significant changes in kiln operating conditions showing a very robust production process. Anderson and Taylor [16] carried out a detailed scanning electron microscopic analysis for the new dead-burned magnesia. Extremely consistent grain to grain microstructure was reported as shown in Fig. 3. Low CaO and SiO 2 content were resulted in very low levels of secondary phase. Vast majority of secondary phase present was highly refractory di-calcium silicate (2130ºC). Fig. 3: Microstructure of the New Product [16] 4 Environmental Assessment Life Cycle Assessment (LCA), a systematic approach to estimate environmental impacts associated with products, processes and services, has become an important tool to investigate the direct and indirect environmental implications of a product system [17]. Life cycle assessment (LCA) is generally used in environmental management for identifying options for environmental improvements of a system in which complete supply chains are considered. To aid the decision-making process, Azapagic and Clift [18] proposed the use of multiobjective optimisation where the system was optimised on a number of environmental objective functions, defined and quantified through the LCA approach. Khan et al [19] focused on the development of process selection and design methodology considering assessment and minimization of risks/impacts of process systems by embedding the LCA principles within a formal process design and decision-making framework. It had implications to process synthesis as it includes environmental objectives together with technology and economics at the design stage to determine cost efficient solutions. Khan et al [19] believed that employing the LCA with a process design and decision-making would yield an optimal design and a best management alternative. Azapagic [20] reviewed the state of the art of methodological development and uses of LCA to process optimisation. In particular, it focused on the application of LCA in process selection, design and optimisation as a tool for identifying clean technologies. The procedures for incorporating into the system optimisation framework the environmental criteria alongside the economic and technical criteria were reviewed and discussed. Bargigli et al [21] explored the thermodynamic efficiency and the environmental sustainability of selected processes that deliver gaseous energy carriers (natural gas, syngas from coal gasification, and hydrogen from steam reforming of natural gas and alkaline electrolysis) by means of a multi-criteria, multiscale approach based on four methods: material flow accounting, energy analysis, exergy analysis, and emergy synthesis. Cherubini et al. [2] reported the findings of the assessment to estimate the Global Warming Potential (GWP) and the Acidification Potential (AP). Results highlighted the relevance in terms of environmental impact of the country where the Mg production process takes place, and clearly identified the dominating Chinese Pidgeon process as the least sustainable Mg production chain, despite its growing diffusion: it had the highest environmental burdens as well as material and energy consumption, and the lowest exergy efficiency. Feng et al [22] showed that the direct emission of fuel combustion in the process is the major contributor to the pollutants emission of magnesium production. Global warming potential and acidification potential made the main contribution to the accumulative environmental impact. The different fuel use strategies in the practice of magnesium production cause much different impacts on the environmental performance. 5 Concluding Remarks During the computation of the performance of a fluidised bed vertical shaft kiln to improve its efficiency, the viscous and inertial resistance are required to be considered. The resistances should be calculated differently for various particle shapes and arrangements. In the shaft kiln, the heat transferred to the material in occurred through conduction, radiation, and convection. A fluidised bed or packed bed such as that used at the reference plant, utilises forced convection as one way to perform heat transfer, which involves the gas and materials within the kiln. Assumptions should be made for the equations, are that it is a Newtonian fluid of constant density and thermal conductivity with viscous dissipation the only generator of thermal energy. Particle agglomeration can result in poor operation of a moving bed because of the change in air path. Agglomeration will reduce the contact area between air / material. An assumption should be made for the degree of agglomeration, which can affect the heat transfer. The packed bed of the reference plant should be analysed as many identical volumes of material, which is a good representation of agglomeration to develop an analysis ISSN: ISBN:

6 of iron-ore processing. Temperature variation within particles should be considered as zero in the development of dynamic and steady state models for the vertical shaft [23]. Nevertheless the temperature variation should be estimated if required by utilising Fourier's conduction [8]. In using this equation at dynamic equilibrium, the temperature difference between centre and surface of spherical particle can be calculated. The outer surface has a temperature change at constant rate of pressure. Heat convection from hot kiln gases should be considered the main mode of heat transfer to the stones. Radiative heat transfer has also significance in the area of the combustion chambers only for the outer layer of the stone bed, which is exposed to the flame heat radiation for a short period of time, while for internal layers the heat is supplied primarily by convection. The life cycle analysis has the potential to overcome many of the problems faced in the conventional approaches and therefore establish a link between the environmental impacts, operation, and economics of a process. It offers an expanded environmental perspective, considering impacts from resource extraction to the end product use and disposal. The LCA relates these effects to the mass and energy flows into, out of, and within a process [24,25]. According to the ISO standards [17], the LCA should assess the potential environmental issues and aspects associated with a product or service by compiling an inventory of relevant inputs and outputs; evaluating the potential environmental impacts associated with those inputs and outputs; and interpreting the results of the inventory and impact phases in relation to the objectives of the study. Therefore, the ability of LCA should focus on both the process operational feasibility and environmental concern along with other attributes. 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