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Design, Fabrication, and Characterization of an Indigenously Fabricated Prototype Transferred Arc Plasma Furnace Arup Kumar Mandal , Raj Kumar Dishwar, and Om Prakash Sinha
Abstract— The extraction of metals like Al and Si from bottom ash having alumina and silica requires high temperature for their reduction. Only transferred arc plasma (TAP) furnace could meet this requirement. The TAP furnace was fabricated indigenously for performing experiments in laboratory based on the information available in the literature, related to the furnace design. The furnace was fully characterized for its smooth functioning by observing parameters like arc length, energy, and electrode consumption as well as the generation of sound level. It was found that the required temperature could be achieved by varying arc length, arc current, power rating as well as plasma gases. Arc length could be adjusted by varying power rating and changing voltage for required heat distribution above the melt. The accessories like the charge feeding system, gas purification and its flow system, gas exit system, the pouring system were worked satisfactorily. Sound level could be controlled by controlling arc current with the plasma gas. The thickness of lining was found sufficient to hold liquid metal either in graphite or magnesite crucible. Increased melt temperature (∼200 °C) and ∼10-dB lower sound level could be achieved by using nitrogen plasma as compared to the normal arc. But, hydrogen plasma produces more ∼100 °C melt temperature, and ∼5-dB sounds as compared to nitrogen plasma. Based on the results of the operational trial, it could be concluded that the TAP furnace was found to be a suitable tool for melting 2-kg iron and its various attachment were also found to be trouble-free operation. Index Terms— Arc furnace design, furnace fabrication, graphite and magnesite, hollow electrode, smelting reduction, transferred arc plasma (TAP).
I. I NTRODUCTION
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IGH-HEAT energy is required for smelting reduction of industrial solid wastes containing primarily stable oxides like silicon dioxide and aluminum oxide [1]. Plasma is a useful tool for the generation of considerable heat, enough to get the temperature from 4727 °C to 7727 °C. It is beneficial to melt refractory metals and alloys [2]–[5]. Application of plasma smelting toward mineral extraction such as reduction of chromite and other highly stable oxides has been gaining popularity day by day [6]–[8]. The quantum of heat generation Manuscript received September 13, 2017; revised January 29, 2018; accepted March 14, 2018. The review of this paper was arranged by Senior Editor S. J. Gitomer. (Corresponding author: Arup Kumar Mandal.) The authors are with the Department of Metallurgical Engineering, IIT(BHU) Varanasi, Varanasi 221005, India (e-mail:
[email protected];
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPS.2018.2817234
in plasma arc depends on types of current, current density, and the ionization behavior of gasses [9]. AC and DC, both types of current, are used for generation of normal and plasma arc [9]–[11]. Many researchers studied the heat distribution in several arc systems and showed that 72% of the whole heat of dc arc is transferred into melt, whereas only 62% in case of ac. In laboratory experiments, dc plasma is widely used for its simplicity, safety, and enhanced heat input to the metals [12], [13]. The choice of the plasma generating gas depends upon its necessity during the process and their chemical impact on the melt/work piece and electrodes [14]. Nitrogen plasma is used for creating a neutral atmosphere around the melt/work piece. Sometimes it is used for replacing nickel by absorbing inside the melt during the production of austenitic stainless steel due to cheaper and readily available as a substitute of nickel [15], [16]. In case of smelting reduction studies, nitrogen plasma is widely used to minimize the reoxidation of metals [17]. Nontransferred plasma is widely used for surface modification of metals and waste treatment [18]–[21]. Electric steelmakers accept electrically generated gas plasma in the form of an arc produced between graphite electrode and charge materials, known as transferred arc plasma (TAP) [22], [23]. In case of non-TAP, the arc is generated between the electrodes; therefore, the heat absorption by metal is less. The transferred arc provides higher current compared to nontransferred arc, caused by the additional heat transfer toward the metal [24], [25]. For that reason, the transferred arc is usually used for metal and alloy melting [14]. In early ages, plasma arc was generated in dc furnaces, however nowadays, three phase ac plasma furnace is feasible [10], [24], [25]. In 1991, Takuma developed a 300-kW dc plasma furnace having 7.2 T/day production capacity for using melting of residue and conducted some primary melting trails since 1993. In 1998, new demonstration plants of 1710-kW capacities with 25 T/day output were designed and trialed [9], [23]. Modeling and simulation of plasma spray characteristics, arc root rotation, and temperature profile work is done mainly for nontransfer arc plasma [21], [26]–[28]. The work on the prototype TAP for smelting reduction studies in laboratory scale is however not found elsewhere in the literature. Hence, the primary objective of this paper is to fabricate a laboratory scale TAP furnace for smelting reduction studies and also examine its proper functioning concerning different parameters like meltdown time, arc length,
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energy and electrode consumption, sound levels, and lining life. II. E XPERIMENTS A. Design of Various Parts For fabrication of the TAP furnace, different components were selected and fabricated as per calculation and feasibility of working. 1) Selection of Transformers: Selection of required capacity transformer for minimum 2-kg steel melting along with waste material is essential because it supplies the current for generation of heat needed in the melting system. For smelting of 500-g waste material (fly ash/ bottom ash) in 2-kg steel melt at a temperature of 1600 °C required 675-kcal heat energy. Hence, to generate 675-kcal heat energy, nearly 9.8-kVA electrical energy would be needed [29]. Considering energy losses (∼60%) the total energy input should be around 16 kVA [12], [13]. A transformer having a capacity of 30 kVA was selected by keeping a good safety margin from an average working load. A transformer of variable voltage setting (i.e., 77, 81, 85, 89, and 93 V) was procured to study the effect of voltage and current on the behavior of the furnace. 2) Electrode and Holder: The present unit belongs to transferred arc type system which is associated with high-heat flux generation. Most of the material evaporates at the temperature needed to get current density for operating plasma. Therefore, a suitable cooling arrangement is required. The graphite rod is an ultimate choice as an electrode material due to the feasibility of its use without cooling. The diameter of solid graphite electrode depends on current to be carried out and its material which possesses excellent electrical conductivity and high melting point [30]. Eschenbach et al. [25] showed that the heating depends on the electrical resistivity and thermal conductivity of the graphite electrode. After several trials with various diameters, it was found that the lower diameter electrode generates lower arc diameter resulted in the formation of bridge inside the crucible. Increasing the width of the electrode, erosion of inner surface wall occurs adversely due to intense heat generation and arc flaring. The optimum diameter of the electrode was observed 16 mm. The current density of the graphite electrode was calculated for the present case which is nearly 700–800 kA/m2 . Due to smaller diameter crucible and electrode, the current density was more than the conventional standard (200 kA/m2 ) and superior (250 kA/m2 ) grades of graphite electrode for ac operation and standard (350 kA/m2 ) grades for dc operation [2], [25]. A bore of 4-mm diameter and 610 mm long was drilled in the electrode for the flow of plasma gas [Fig. 1(a)]. As the graphite electrode has no cooling arrangement, the electrode will be heated enough, which creates handling problem as well as surface oxidation of graphite. The graphite electrode is consumed continuously. Thus, a continuous downward movement is required during melting operation. Considering that, the half portion of the electrode holder was made of copper, and another half was made of steel, which was attached on the top of the roof having a hand wheel to provide upward and downward electrode movement. To protect the
Fig. 1. Different parts of the fabricated furnace. (a) Solid and hollow graphite electrodes. (b) Electrode holder with cooling and moving arrangement. (c) Furnace body and roof. (d) Electrical connector block with nails and SS rod.
copper from the intense heat, water circulation system was provided for cooling the holder. One point of the power supply of 30-kVA transformer was connected to the electrode holder by flexible copper strip [Fig. 1(b)]. 3) Furnace Body and Roof: Mild steel (MS) sheet of 3-mm thickness was used for fabricating cylindrical shape furnace body having 300-mm diameter and 280-mm height. The top of the furnace was provided with an arched shape swinging lid, made of 5-mm-thick steel plate which could be easily lifted up and closed as required during melting operation. The inside of the MS roof was lined with magnesite to protect from direct heating and also to minimize the heat loss of the chamber. Provisions were made for charging, exit gas, and electrode holder assembly above the ceiling, as shown in Fig. 1(c). 4) High-Temperature Electrical Connector: A graphite block of 120-mm diameter and 80-mm height was used for providing electrical connection to the charge through the bottom of the crucible. It was connected by a 250-mm-long stainless steel rod, screwed into it without any cooling system for providing power. Around 20–25 steel nails (i.e., hard steel wire), 3 mm diameter and 35 mm long were fixed on the top of the graphite block embedded by refractory lining for making electrical connection between melt and graphite block. These arrangements are shown in Fig. 1(d). 5) Lining: As per calculation, the crucible volume requirement for 2-kg steel melting is very less. Considering, the lower density of charge material and slag formation after melting, a crucible diameter of about 80 mm was adopted, which gave a melt depth of about 90 mm. The larger diameter was avoided for higher heat losses, and narrower diameter may cause difficulty of charging with bridge formation during melting operation. Lining material greatly affects the performance of lining life, melt chemistry; therefore, for the design of a furnace, lining material plays a vital role [31].
This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. MANDAL et al.: DESIGN, FABRICATION, AND CHARACTERIZATION OF INDIGENOUSLY FABRICATED PROTOTYPE TAP FURNACE
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Fig. 3. Cross-sectional view of furnace with (a) graphite lining, (b) magnesite lining.
Fig. 2. Steps of making magnesite crucible. (a) MS Shell (body). (b) Bottom lining. (c) Insulation lining with graphite block at bottom. (d) Crucible making by pattern. (e) Green crucible. (f) Crucible after firing.
To sustain high temperature ∼1800 °C–2000 °C, a readymade (available in the market) graphite crucible was used. In industry, magnesite is widely used for lining in the electric arc furnace, for that reason major melting trials were performed by using magnesite lining to understand the reality of industrial-scale practice, considering of such high temperature. B. Fabrication In case of graphite lining, a readymade graphite crucible was purchased from the market and placed at the center of the furnace over a graphite block, and grog powder (crushed firebricks) was rammed between crucible and insulation bricks lining. For making rammed magnesite lining, the combination of magnesite granules (20%, 3–6 mm; 40%, 1–3 mm) and fines (20%,
