2018 年 58 巻 6 号 p. 999-1006
Global iron and steel making industries generate immense number of by-products, among which few are already being utilized for the generation of high value products, while few are still struggling to gather a good market space. Waste utilization is the major concern for every industrial maker. In this context, iron powder which holds immense market in applications like powder metallurgy (PM) parts, welding electrodes, advanced oxidative agents for water treatment, food fortifying agents, metal injection moulding, catalysts, fuels etc., can now be a novel outcome from steel industry by-products. Iron powder can be manufactured by various techniques like reduction, atomization, electrolysis, etc., resulting in fine particles of various morphologies ranging from spherical to irregular shapes and low to high purity. The application of these iron powders is depended on the process, quality and purity of the desired product.
Manufacturing industries tend to generate large amounts of wastes in solid, liquid and gaseous forms. Among these industries, giants like Iron and Steel manufacturers stands chief due to numerous unit processes and operations creating air, water, land and noise pollution. An integrated steel plant typically consists of various sections like, raw material handling, iron making, steel making and product development. Each of these sections comprises of a sequence of operations running day long, throughout the year consuming a lot of energy and promoting continuous emissions.1) India stands third among the global nations in the production of crude steel with domestic production capacity about 121.97 Million tons per annum. Presently about 0.72 tons of solid wastes per ton of crude steel is being generated out of which 0.67 tons is being utilized in one way or the other.2) Thus, reducing the gap between generated and utilized waste is very challenging. With increase in global concern, the apex agenda of every integrated steel plant shifted from increasing production to decreasing carbon foot print and energy consumption. The concept of targeting zero waste in steel industry with mottos like 4R’s (Reduce, Recycle, Reuse and Restore), sustainability and life cycle assessment has become a part of every steel maker.3,4,5,6)
Reviews on steel industry wastes, especially solid waste, their management and utilization are already reported.7,8,9,10,11) Few authors also reviewed energy recovery and energy efficiency in steel industries by use of effective technologies.12,13) No literature review is reported till date focusing the utilization of steel industry wastes for synthesis of iron powders and their mechanism of synthesis. The current review emphasizes on a novel and promising product for the future, the Iron powder, its prospects, applications and synthesis from various steel industry by-products.
Integrated steel plants produce many solid, liquid and gaseous by-products which need careful attention for further processing and usage. Major by-products generated in the steel industry are tabulated in Table 1.
Apart from the tabulated ones, materials such as sinter dust/sludge, coal tar sludge, Kiln dust, fly ash, ESP dust, refractory waste are also being generated and utilized.14,15) The nature and composition of the tabulated by-products depends on their source of generation. Day in and day out, researches throughout the globe are on board in generating and implementing novel ideas, to optimize the utilization of generated by-products, benefiting the iron and steel makers. One among such ideas is the synthesis of iron powders from by-products or wastes obtained from integrated steel making operations. Powdered form of iron is of high demand and attracts spectrum of applications. Information of iron powders, their synthesis and applications is illustrated below.
Iron powder metallurgy has a rich heritage under the branch of powder metallurgy.16) Iron powders also termed as zero valent iron (ZVI) powders are basically manufactured under three branches namely physical, chemical and mechanical processes.17) Image illustrating various methods involved in each of the process is shown in the Fig. 1. Since the inception of the industrial production in 1939, iron powder manufacturing technology transformed decade by decade and in the present day only few process like reduction, atomization, carbonyl, and electrolytic synthesis have gained industrial acceptance.18) Chemical reduction technique refers to the solid phase reduction of iron oxide either by solid (coal) or gaseous (H2, CO) reducing agents. Atomization techniques on other hand deals with the atomization of liquid metal using fluid streams followed by gaseous reduction to give iron powder. A German manufacturer, BSAF, 1925, invented the carbonyl decomposition method of producing iron powders and till date leads the industrial sector. Electrolytic method involves cathodic deposition of fine iron powder, resulting from electrolysis of iron bearing raw material in presence of suitable electrolyte. Pictographic representation of the process of prominent iron powder synthesis techniques is shown in the Fig. 2. The morphology of iron powders depends on the choice of manufacturing technique, wherein the choice of manufacturing technique is dependent on the required property and targeted application of iron powder. Morphology of iron powders made from various techniques is given in Fig. 3.
Iron powder manufacturing techniques.
Process steps involved in prominent iron powder synthesis techniques.
Morphology of iron powders synthesized from various techniques.
The applications of iron powders are branched into various areas like powder metallurgy parts,19) welding electrodes, magnetic materials,20,21,72) electrochemical parts,22) alkaline batteries,23,24,25,26,27,28) food fortification,29) environmental remediation,30,31,32) industrial filters and purifies,33) etc. Extremely modern and futuristic application of iron powders is targeted to use them as clean recyclable metal fuel.34,35) Iron powder acts as an alternate to fossil fuels, for heat and power generation with low carbon emissions.
About 80% of the market share of iron powder applications is swept by PM parts alone, mostly serving various automobile parts and other structural parts. Welding, magnetic and chemical applications follow the listing. Iron materials comprising unalloyed iron, alloyed iron and insulated iron powders, processed by powder metallurgical route are largely used as soft magnets in electromagnetics, catering both alternating current (AC) and direct current (DC) applications. Porous metals made from processing of iron powders have massive application in filtration & separation industrial appliances, air purifiers and deodorizers. Iron powder such as iron-magnetite powder, electrolytic, water atomized, sponge iron powder, carbonyl iron powder in the form of porous iron electrode are used in iron based rechargeable alkaline batteries. Elemental iron powders, which have purity, stability and economic advantage, in par with conventional iron salts and chelates are used for food fortification. Iron powders with coarse and fine particle size and various apparent densities and various iron based alloy powders are being used as semi metallic frictional and breaking materials for more than 50 years. Nano size powders of iron are being used as detoxifying agents for environmental pollutants. Application of iron powder largely depends on the cost of raw material, final properties and compatibility which in turn depends on the choice of manufacturing technique.36) Information regarding the dependency of application on manufacturing technique was tabulated in Table 2.18)
Iron powders need to be evaluated for various chemical, physical and processing characteristics.37) Chemical characteristics include composition and purity; Physical characteristics comprises of size, shape, density, surface area and Processing characteristics include flow rate, density, compressibility, porosity. The chemical composition of the powders can be evaluated by using various techniques like hydrogen loss method, wet chemical analysis, X-ray fluorescence, Inductive coupled plasma (ICP), Emission spectroscopy, Neutron activation analysis, Energy dispersive X-ray analysis (EDAX), Electron probe micro analysis (EPMA) and X-ray Diffraction (XRD) studies. Similarly, there are few surface analysis techniques like Scanning auger electron spectroscopy, X-ray photoelectron spectroscopy (XPS), Secondary ion mass spectroscopy (SIMS) and Ion scattering spectroscopy for evaluation of surface chemical properties. Shape of the samples can be analysed using Optical microscopy, Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and Image analyser whereas the size can be examined using Sieving (> 5 micron), Microscopy [Optical, SEM, TEM (0.001–100 micron)], Sedimentation [sedimentation and decantation, pipette, gravitational, turbidimetry, centrifugal (0.05–50 micron)], Elutriation [elutriation, roller air analyser (5–100 micron)], Permeability [permeability, Fischer sub sieve sizer (0.2–100 micron)], gas adsorption [Bruner–Emmett–Teller (BET)], Electrolytic resistivity [Coulter counter, Electrozone (0.1–2000 micron)], Light scattering (2–100 micron) and Light obscuration(1–900 micron) methods. Surface area of powdered samples is assessed by BET analysis. Flow rate of the samples is estimated using Hall flow meter, Carney funnel and Scott volumeter, whereas apparent density and tap density are measured using hall flow meter and pyconometer respectively. Mercury porosimeter is used for porosity analysis, while rattler and transverse bend test are for green strength analysis. Powders are also analysed for sintering properties like compression, shrinkage, density and porosity in sintering atmospheres.
Global iron powder production is growing in a steady rate with Europe, North America, China and Japan being the leading manufacturers. Information regarding major global producers of iron powders, their capacity and scope is tabulated in Table 3.38) Even though the commercial production of iron powders started in United States and Europe, global centre for production has gradually shifted to China due to more production sites. During the years 2011–16 the iron powder prices were gradually decreased due to advancement in technology and operation. Iron powder manufacturing cost analysis reveals that about 75% of the total cost comprises of raw material cost, whereas other costs like energy, labour, depreciation and accessories combine to account the left 25%. Iron powders from atomization and reduction routes have high demand and occupy 72.07% and 25.21% of global iron powder production market share respectively in the year 2015. Atomized iron powders are costlier than reduced ones. Thus, increasing production, reducing cost and good quality are global challenges in iron powder synthesis.
This fine sized powdered form of ZVI, which occupied handful of market potential, can be a value-added outcome of integrated steel industry. Keen interest is laid by researchers to utilize solid and gaseous steel industry by-products as raw material and reducing gases respectively to synthesize rich iron bearing products like iron powder. This interest is not novel but holds its impression far back in 1951 when researches of Allied chemical and Dye Corporation, USA utilized coke oven gas to convert iron oxide to powdered iron.39) Later, many researches and scientists have documented handful literatures which are shown in the Table 4.
Among the various manufacturing methods of iron powder, chemical reduction technique stands suitable for synthesis of iron powders using steel industry by-products. Numerous research activities are being performed day in and day out to optimize the process parameters to produce pure iron powders using this technique. Reduction of iron oxides is one of the widely studied topics and most of the research is dictated during 1960’s to 1980’s. Depending on the temperature of reduction, iron oxide reduction prefers either two-step or three-step process. Stability of iron oxides with respect to temperature and oxygen content is shown in the Fig. 4.42) This reveals that iron oxide reduction below 570°C happens through a two-step process, which involves conversion of hematite to magnetite and direct conversion of magnetite to iron. On the other hand, reduction above 570°C undergoes a three-step process in which the reduction of magnetite to iron involves intermediate wustite formation. Theoretical reason behind this behaviour is wustite being thermodynamically unstable at temperatures below 570°C, where in it decomposes into magnetite and iron due to eutectoid reaction.45) As an anomaly to the this above stated transition temperature, experimental results reported by pineau and co-workers revealed a drastic change in the activation energy of magnetite to iron reduction step at temperatures 417°C, 415°C and 439°C, when reduced in Hydrogen (H2), Hydrogen- Nitrogen (H2–N2) and Carbon Monoxide (CO) gaseous atmospheres respectively. This behaviour is attributed to intermediate formation of wustite during reduction at temperatures below 570°C, which occurs due to upgrading of crystal structure or recrystallization of iron.48) Similar anomaly is also reported by Guan and co-workers where oxide scale of hot rolled steel is reduced by hydrogen.43) The existence of wustite below 570°C in this case might be due to the rapid cooling of mill scale during its processing creating thermodynamically irreversible condition.
Stability of iron oxides with respect to temperature and oxygen content.
Reduction of iron oxide using solid carbon source or gaseous CO as reducing agent is temperature dependent but deviates from the mechanism shown in the Fig. 4. It proceeds by Eqs. (1), (2), (3), (4) and (5) for temperatures above 570°C.41,55,67,68,75) Reduction using solid carbon as shown in the Eq. (1) is termed as direct reduction, while using CO gas as shown in the Eqs. (3), (4) and (5) is termed as indirect reduction. Iron oxide reduction occurs either by direct carbon source or indirect CO gas obtained from gasification of carbon source by boudouard reaction as shown in the Eq. (2), or by direct supply of CO gas. Boudouard reaction being exothermic, reduction temperatures in the range of 800–1100°C with higher CO composition is good operating regime for iron oxides reduction. For, reduction of iron oxides comprising pure Fe (III) oxides or mixture of Fe (III) and Fe (II) oxides, temperatures below 1000°C are detrimental due to the formation unwanted impurities like carbon, caused by reverse boudouard reaction as shown in the Eq. (6) and cementite (Fe3C) as shown in the Eqs. (7), (8) and (9).41,47,68) On the other hand for oxides comprising majorly Fe (II) oxide, operating temperatures about 750°C and higher CO gas composition promoted good reduction.45) Mondal and co-workers stated the good reduction of Fe (III) oxide to iron occurred with negligible formation of carbon and cementite at operating temperature of 850°C and lower CO composition (<50%), but realized very slow reaction rates. Thus, it is evident from the above stated literature that, higher temperature (> 1000°C) and higher CO composition will give higher reduction of chosen iron oxide. Graphical representation of above stated mechanism of iron oxide reduction using CO gas is elucidated in the Fig. 5(A).74) No proper literature on reduction of iron oxides to iron below 570°C using CO gas as reducing agent is found. Pineau and co-workers stated the reduction of iron oxide (Fe3O4) to iron with CO gas at 439°C with intermediate wustite formation. They also reported the formation of high carbide (Fe3C).48) One may state from the above examples that, reduction of iron oxides at temperatures below 570°C stands ineffective both thermodynamically and practically.
Mechanism of iron formation A) Fe–O–C system, B) Fe–O–H system.
In a similar way with CO gas, reduction with hydrogen is also temperature dependent, but follows the same mechanism shown in the Fig. 4. Reduction of iron oxide below 570°C reduction temperatures, proceeds by Eqs. (10) and (11) and for reduction temperatures above 570°C it proceeds by steps as shown in the Eqs. (10), (12) and (13).46,48) Graphical representation of the above stated mechanism is shown in the Fig. 5(B).74)
Even though the mechanism and features of iron oxide reduction are similar while reduction with H2 and CO gases, they show few significant differences:42) a) Reduction with CO is exothermic while with H2 is endothermic, b) Reduction rate of iron oxides with H2 is higher than with CO, c) compact iron formation is seen in reduction with H2 which is absent with CO, d) Whiskers formation in iron is seen in reduction with H2 which is absent with CO, e) Slow reduction rates at higher degree of reduction is observed in reduction with H2, which is not seen in reduction with CO, f) No formation of unwanted compounds occurs while operating at lower temperatures below 1000°C with H2, whereas impurities like carbon and cementite occurs by reduction with CO. Thus, reduction of iron oxides using hydrogen as reducing agent stands beneficial in terms of ease of operation, low reduction temperatures, low time periods and high purity of product.
Literature shown in the Table 4 reveals that, spray roasted iron powder and mill scale are two major solid by-products which are widely been used to synthesize iron powders. The reduction was majorly performed using hydrogen gas in the temperature range 500–1250°C. Gaseous by-products like CO, Coke oven gas and LD gas are used as reducing media to produce iron powders. Most of the researchers targeted to produce iron either in sponge or powder form of iron. Therefore, it is evident that some of steel industry by-products either in the solid or gaseous forms or combination of both can be a potential raw material or reducing agent for the synthesis of iron powders.
Powder metallurgy has been a challenging and resourceful area of research then, now and ever. Metal powders, especially iron powders, hold a prominent market potential due to their ease in preparation, handling and transportation. In this context, utilization of by-products to generate a value-added product like iron powder will be a potential outcome and can occupy an outsized market.