High Electrical Resistant Materials E-Book

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Beschreibung

HIGH ELECTRICAL RESISTANT MATERIALS The book describes how the utilization of high-carbon slag/pond ash/fly ash for making value-added ceramics is useful for the electrical sectors. Since waste materials are currently endangering our environment, ways of utilizing them have become a global challenge. Currently, R&D work is being carried out to utilize these materials for producing value-added products. This book details the investigations to utilize fly ash (FA) and pond ash (PA) - both waste materials from thermal power plants - with high-carbon ferrochrome (HCFC) slag (by-product of the ferrochrome industry), for producing a novel material for ceramics. Kaolin/K-feldspar is mixed with PA/HCFC slag to produce ceramics with the formation of mullite. The FA/PA/HCFC slag-based ceramics can replace porcelain-based ceramics, and some permanent ceramic structures can be constructed with such wastes. Properties and structures made with ceramics are found to be comparable with those made with porcelain-based ceramics. Performances of these materials above ambient temperature have been evaluated and results indicate the possible replacement of porcelain with these newly invented ceramics. Audience The book will be used by electrical and civil engineers in the electrical, construction, and ceramic industries as well as the industrial waste sector. Researchers in materials science, structural, civil and electrical engineering, environmental science, and ceramic engineering, will also have high interest.

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High Electrical Resistant Materials

Ferrochrome Slag Resource Ceramics

Muktikanta Panigrahi

Ratan Indu Ganguly

and

Radha Raman Dash

This edition first published 2024 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA© 2024 Scrivener Publishing LLCFor more information about Scrivener publications please visit www.scrivenerpublishing.com.

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Library of Congress Cataloging-in-Publication Data

ISBN 978-1-394-23125-6

Cover image: Pixabay.ComCover design by Russell Richardson

Preface

Since waste materials are currently endangering our environment, ways of utilizing them have become a global challenge. Currently, R&D work is being carried out to utilize these materials for producing value-added products. In the present investigation, an effort has been made to utilize fly ash (FA) and pond ash (PA) (waste materials from thermal power plants) and high-carbon ferrochrome (HCFC) slag (by-product of the ferrochrome industry) for producing a novel material called ceramics. Kaolin/K-feldspar is mixed with PA/HCFC slag to produce ceramics with formation of mullite.

The FA/PA/HCFC slag-based ceramics can replace porcelain-based ceramics, and some permanent ceramic structures can be constructed with such wastes. Thus, it is no wonder that scientists are trying to develop ceramics utilizing waste materials.

The present text will highlight the mechanism involved in the formation of ceramics from FA/PA/HCFC slag. This will perhaps be the first attempt to use FA/PA/HCFC slag for developing ceramics. Properties and structures made with ceramics are found to be comparable with those made with porcelain-based ceramics. Systematic investigations are currently being carried out to develop new ceramic products using these FA/PA/HCFC slag materials. Performances of these materials above ambient temperature are being evaluated, and results indicate the possible replacement of porcelain with these newly invented ceramics.

The Authors

December 2023

1Fundamentals of Ferrochrome (FeCr) Alloy and Its Slag

Muktikanta Panigrahi1*, Ratan Indu Ganguly2 and Radha Raman Dash3

1Department of Materials Science, Maharaja Sriram Chandra Bhanja Deo University, Keonjhar Campus, Odisha, India

2Department of Metallurgical Engineering, National Institute of Technology, Rourkela, Odisha, India

3National Metallurgical Laboratory (NML), Jamshedpur, Jharkhand, India

Abstract

The manufacture of alloy steel consumes chromium in large quantities. Therefore, there is an ever-increasing demand for chromium. Manufacturers use chromium in the alloy steel used for tool steel, high-speed steel, stainless steel, and ball-bearing steel. In addition, nichrome (Ni-Cr) alloy is used as a heating element for electric furnaces.

Chromium is a ferrite stabilizer and a strong carbide former. Addition of chromium improves strength, oxidation resistance, and corrosion resistance properties. Therefore, the global production of chromium has gained importance over the years.

Chromium is not obtained in its free state; therefore, chromium ore is reduced by normal smelting process. Carbothermal reduction of chromium ore enables production of ferrochrome. Usually, an electrical furnace is used for production of chromium, which consumes high electric energy. As a result, the cost of ferrochrome production is enhanced. During the production, chromium is lost through formation of slag. Hence, ferrochrome slag is thought to be a useful raw material for production of high temperature insulation bricks, construction work using geopolymer made from chromium slag, etc. Therefore, R&D scientists are trying to find suitable uses for other applications.

In this chapter, the fundamentals of ferrochrome slag chemistry are discussed. Results obtained by different workers are cited and thermodynamics models used by different groups of researchers are briefly discussed. Physicochemical properties are also described which may be helpful for future R&D work. Important consideration is also given to environmental issues. Chromium is present in two ionic forms, i.e., trivalent and hexavalent chromium, with the hexavalent form being detrimental to human health. Therefore, this chapter also discusses how unutilized chromium slag pollutes the environment, making it a human health hazard.

Keywords: Chromite ore, ferrochrome production process, ferroalloys, slags, environmental aspects of slag, compositions of slag, thermodynamics of slags, oxidation state of slag, slag properties (i.e., viscosity, electrical resistivity)

1.1 Introduction

Industrialization is a process of socioeconomic development. However, industrial activities are associated with generation of wastes. Production and dumping of wastes pollute our environment as well as water due to the leaching effect when these wastes are submerged in water [1, 2]. Hence, appropriate actions are needed to safeguard atmospheric pollution and subsurface contamination. In recent times, increased construction activities that have arisen due to industrial and population growth have resulted in the scarcity of available conventional construction materials. Hence, nonhazardous wastes are often utilized for construction work [3, 4]. This will reduce waste disposal problems and the costs associated with construction work.

Ferrochrome slag is such a waste material which is obtained from ferrochromium (FeCr) industries [5]. The FeCr alloy is used in the stainless-steel manufacturing process [6]. Chromium in stainless steel improves properties such as corrosion/oxidation resistance, hardness, tensile strength at elevated temperatures, wear and abrasion resistance, etc. [7].

The FeCr alloy is manufactured from its ore by reducing it with coke. The process is carried out at 1,500 °C in an electric arc furnace. Molten slag and metal are slowly tapped in a mold and subsequently cooled in air to enable formation of stable, dense, crystalline phase [8].

The main constituents of FeCr slag are SiO2, Al2O3, FeO, Cr2O3, CaO and MgO [9]. Slag products are crushed and ground into sizes ranging between 6.3 mm and 300 mm. Hence, with increased production of ferrochrome, FeCr slag production (HC FeCr, 52% Cr) has increased.

Mineral chromite with chemical composition FeCr2O4 (ferrous chromic oxide), is a submetallic mineral belonging to the spinel group (with a generic formula of R+2O.R+3O4). It is the only economic mineral mined for chromium production. Because of the high heat stability of chromite, it is also used as a refractory material for high temperature vessels such as furnaces [10]. Two main products are achieved from refining of chromite: ferrochromium and metallic chromium [10]. The smelting operation must be carried out with chromite ore in order to produce the two products mentioned above. One of the major problems encountered during chromite smelting is the issue of energy consumption (i.e., electricity). A large amount of energy is required to smelt chromite to produce ferrochromium or metallic chromium. According to Keesara [11], approx. 4,000 kWh of energy per ton material weight is required for smelting of chromite. This is due to the high melting temperature of chromium. Regarding economic benefits, revenue generated from a ferrochromium plant is dependent on Cr/Fe ratio [12]. The higher the ratio, the higher the revenue earned by the plant. Thus, the economy of the total process depends largely on quality of ore, cost of energy, transportation cost, and others. Hence, one has to consider these factors before venturing into or manufacturing chromium from available ore. During chromite smelting, energy requirement and its cost depend to a large extent on the technology used in smelting the ore. New technologies have been developed to reduce the energy required for smelting chromite and producing ferrochrome. These technologies, together with the conventional chromite smelting technique, are subsequently discussed in this chapter.

1.2 Chromite Smelting Technologies

Production of ferrochrome involves four primary processes: Conventional smelting process, Outokumpu process, DC-arc process, and Premus process. The aim of these processes is to smelt chromite ore in a submerged electric arc furnace. These technologies are discussed below.

1.2.1 Conventional Smelting Process

Traditional chromite smelting technology involves charging chromite ore into a submerged electric arc furnace (Figure 1.1). Coke and coal are used as reductants. Quartzite is used as a fluxing material which removes undesirable oxides in the form of slags. Metal/ferrochrome and slag produced are tapped from the furnace for further processing. According to Naiker [13], the primary advantages of the conventional smelting process are the low capital investment incurred and the flexibility offered in choosing raw materials in the production process. However, in terms of energy requirement, the process is not efficient as it is an energy-intensive process, requiring up to 4,000 kWh per ton of material produced. The conventional smelting process requires about 1.0 indexed energy cost per ton of ferrochrome alloy produced.

Figure 1.1 Schematic diagram of smelting using submerged electric arc furnace (EAF) [14].

1.2.2 Outokumpu Process

The Outokumpu process involves grinding and pelletizing of ore fines, followed by sintering of green pellets and preheating before smelting [15, 16]. According to Goel [16], the ore and coke fines are normally wet-ground to about 35 percent under 37 microns (400 meshes) and are then pelletized to approximately +15 mm size. Figure 1.2 shows the flow sheet of the Outokumpu process. As seen in Figure 1.2, the preheating operation is done mainly in a rotary kiln and the energy required for sintering and preheating the pellets comes from CO gas generated in a submerged arc furnace. In the Outokumpu process, chromite ore is partly reduced in the rotary kiln during preheating, thereby reducing the amount of electric energy required for final smelting of the ore to ferrochrome.

1.2.3 DC-Arc Process

DC arc furnaces use a single solid carbon electrode as the cathode and produce a DC arc to an anode in the bottom of the furnace (Figure 1.3). The arc is normally an open or semi-submerged one [18]. Raw materials can be charged either directly into the furnace or by using a hollow electrode. The DC arc route is designed mainly to overcome the problem of ore fines encountered in the conventional chromite smelting process. Therefore, in order to use this technology for producing ferrochrome, a compromise between coke consumption and energy requirement is necessary.

Figure 1.2 Outokumpu process [17].

Figure 1.3 DC-arc process [19].

1.2.4 Premus Process

The Premus process is being used at the Lion ferrochrome plant in South Africa. It uses Xstrata’s proprietary technology. This process is sophisticated, competitive and technologically advanced for the production of ferrochrome from chromite ore [17]. The technology involves three stages: sintering, pre-reduction, and smelting. Energy reduction is as observed in the Outokumpu process. Energy reduction is achieved in the pre-reduction stage of the process. As can be seen in Figure 1.4, chromite pellets from the sintering stage are pre-reduced in a rotary kiln by a roasting operation before being charged in the closed submerged arc furnace for final smelting. The pre-reduction process results in the reduction of energy required for closed submerged arc furnace during final smelting of ore to ferrochromium. It is worth noting that the energy used in the pre-reduction stage is obtained from hot gas generated in the closed submerged arc furnace and also from coal pulverization, and thus is not paid for. With this technology, Lion ferrochromium is now in the forefront of chromite smelting in South Africa.

Figure 1.4 Schematic illustration of Premus process [13].

Premus technology is the least expensive and most energy-efficient ferrochrome technology. The technology also uses low-cost reductant material (anthracite), and thus coke consumption is low in Premus technology.

Another important feature of this technology, which is also a part of the DC-arc process and Outokumpu process, is that it is designed for smelting fine chromite ore. The conventional smelting process is designed to handle only lumpy ores. This is ideal for production of ferrochromium as the fine ores tend to form a sintered layer at the top of the charge. Thus, using fine ores sometimes causes a technical problem, such as preventing gas from escaping, thus posing operating difficulties [12]. Due to this problem, fine chromite ores have low economic value. However, with the development of Premus technology, DC-arc process, and Outokumpu process, fine chromite ores can now be smelted. These technologies have been designed to convert the fine ores into pellets during the pre-reduction stage. These technologies can therefore be used to generate more revenue in localities having a large amount of fine chromite ore.

1.3 Ferrochrome Production

Ferrochrome is produced by carbothermic reduction of iron magnesium chromium oxide at high temperature. Coke and coal are used as reducing agents. The reduction process is carried out in an electric arc furnace. The arc generates temperature around 2800 °C. For this purpose, the electricity consumed is high and consequently cost increases [20].

1.4 Chemical and Physical Properties of Ferrochrome

Ferrochrome or ferrochromium (FeCr) is a type of ferroalloy. Usually, such alloys contain chromium (50 to 70% by weight) and iron [22, 23]. Physical and chemical properties of ferrochrome are listed in Table 1.1.

Globally, large quantities of domestic chromite resources are available in South Africa, Kazakhstan and India. Stainless steel manufacturers are the largest consumer of chromium and ferrochrome. FeCr production (High Carbon FeCr, 52% Cr) has now reached the level of 5 M ton/year [24].

Chromite ore is ferrous chromic oxide (FeCr2O4). It belongs to spinel group with a generic formula of R+2O.R+3O4. FeCr alloy is produced from chromite ore. Because of its high heat stability, it is used as a refractory liner of an arc furnace vessel.

Table 1.1 Physical and chemical properties of ferrochrome [21].

S.no.

Physical properties

Physical property value

1

Atomic Mass

52.0

2

Density

7.15 g/cm

3

3

Melting Point

1900°C

4

Boiling Point

2642 °C

5

Appearance

Ferrochrome is available in a variety of forms, including small crystals, lumps and granules as well as in powder form.

6

Color

Color varies from dark gray to light gray.

7

Odor

Odorless; can be dangerous when inhaled.

8

Solubility

The alloy is not soluble in water.

9

Combustibility

The dust particles of this chemical alloy are combustible.

Ferrochrome is collected from the furnace at regular intervals. Usually, slag and molten metal are tapped where molten metal is separated in molten condition and is chilled in a metal mold. The molten ferrochrome is then solidified in large castings and is processed further.

On the basis of carbon content, ferrochrome alloy is classified into two categories: low carbon ferrochrome alloy and high carbon ferrochrome alloy.

1.4.1 Characteristics of Ferrochrome

It is a steel-gray and hard metal.

Chromite ore minerals belong to the spinel group.

Cr is a carbide-forming element and forms Cr

4

C

3

/Cr

23

C

6

in stainless steel.

Cr is a ferrite stabilizer and makes the alloy stable at high temperature.

Cr in stainless steel improves corrosion-oxidation resistance.

Cr improves stress corrosion property in stainless steel.

Metallic chrome products or super alloys or other special melting metallic products are produced using chrome alloy.

1.5 Ferrochrome Uses

Most of the ferrochrome produced worldwide is used in manufacturing of stainless steel. Chromium content stainless steel varies between 8 to 29% FeCr. Chromium content stainless steel provides resistance to corrosion. Ferrochrome is indispensable in manufacturing of ball-bearing steels, tool steels and other alloy steels [25]. Apart from making stainless steel, low-carbon ferrochrome is also used in the manufacturing of acid-resistant steels [26].

Ferrochromium alloy picks up carbon during carbothermal reduction. Since Cr and Fe have affinity for carbon, they pick up carbon. The amount of carbon in alloy depends on operating conditions, i.e., oxygen potential and temperature. Two types of ferrochrome alloy are produced [27, 28];

Low-carbon ferrochrome alloy

High-carbon ferrochrome alloy.

1.5.1 Low-Carbon Ferrochrome Alloy

An optical photograph of low-carbon ferrochrome is shown in Figure 1.5. Low-carbon ferrochrome is a ferrochrome having carbon content below 0.25. This is used as raw materials for producing silicon chromium alloy, chromite and lime. Low-carbon ferrochrome chromium (LC FeCr) is used in austenitic stainless steel and superalloy. Due to high quality, it is a reliable and economical alternative to metallic alloy chromium for superalloy production [29].

Figure 1.5 Optical photograph of low-carbon ferrochrome [29].

1.5.1.1 Low-Carbon Ferrochrome – Chemical Analysis

Ferrochrome is an alloy of chromium and iron containing 50–70% chromium by weight. Addition of chromium improves the strength and yield point of steel and reduces elongation insignificantly. The presence of chromium in carbon steels improves hardness and wear resistance. The chemical composition of low-carbon ferrochrome is indicated in Table 1.2.

1.5.2 Problems of Carbon in FeCr

Ferrochrome production is done by chromite reduction with carbon (coke) in a submerged arc furnace. Ferrochrome product is basically carbon-saturated iron-chromium alloy with 6–8 wt% carbon. Carbon forms solid chromium carbide and complex iron-chromium carbides. The higher percentage of carbon in ferrochrome creates several problems.

Low-carbon alloy for making austenitic stainless steel, carbon contains less than 0.1% (0.02–0.05%). Excess carbon causes “sensitization” problems at about 810 K. At sensitization temperature, excess carbon combines with chromium to form chromium carbide (Cr23C6). At this temperature, carbide is insoluble and precipitates at the grain boundaries. At grain boundaries, chromium has inadequate corrosion resistance. Thus, carbon is indirectly responsible for corrosion, and hence weakens the material. A similar problem is observed in welded structures [28, 31].

Table 1.2 Chemical composition of low-carbon ferrochrome (LCFeCr) [30].

LCFeCr

Low-carbon ferrochrome (60%)

Low-carbon ferrochrome (70%)

Cr

65% Min

70% Min

C

0.03, 0.06, 0.10, 0.15

0.03, 0.05, 0.06, 0.08, 0.10, 0.15

Si

1% Max

1% Max

S

0.03% Max

0.03% Max

P

0.035% Max

0.035% Max

Size

10-50 mm - Min 90% or as required by end user

In austenitic stainless steels, the chromium to carbon ratio is more than 150, whereas in high-carbon ferrochrome steel it is about 10. High-carbon ferrochrome stainless steel inadvertently increases carbon content. Hence, it is essential to remove carbon from the parent source (i.e., high-carbon ferrochrome). To improve the chemical and mechanical properties of steels, minimizing the concentration of the detrimental and undesirable element (i.e., carbon) from steel is necessary.

Chromium metal is reactive towards carbon by simple oxidation to form chromium oxide. Such practice cannot be used for removing carbon. This is a major problem in the case of most ferroalloys [32].

Conventional decarburization techniques are used to remove carbon from FeCr. There are many disadvantages to these techniques such as high refractory consumption, poor metal recovery due to losses of chromium in slag, etc. However, nonconventional decarburization techniques are not used. Conventional decarburization methods are described below.

1.5.2.1 Refining of Ferrochrome by Chromium Ore

Elyutin [32] assumed that liquid high-carbon ferrochrome is a solution of chromium and carbon which exists in the form of carbide (i.e., Cr7C3). This process is used to determine the thermodynamic characteristic. Decarburization reaction is written as (Eq. 1.1):

(1.1)

Difficulties encountered in the decarburization process are due to the high melting point and viscosity of the high-chromium refining slag. This prevents the growth of chromic oxide in the slag. Hence, there is a limited possibility of increasing the oxidizing capacity of slag. Decarburization process is conducted at a temperature of 2175 K; 2% of the carbon in the alloy is decarburized. Decarburization of low carbon grade material is feasible at a temperature higher than 2375 K. Thus, production of low-carbon ferrochrome by this process is very difficult.

1.5.2.2 Refining of Ferrochrome by Blowing Oxygen

In this process, carbon is removed by blowing O2 through molten ferrochrome in a converter. It is a more expensive process. Elyutin [32] brought carbon below 1% using air as an oxidizer. This method did not give satisfactory results because of insufficient bath temperature (i.e., 1935–1985 K).

1.5.2.3 Refining of Ferrochrome in Presence of Silicon

The process consists of refining ferrochrome by oxygen in the presence of silica under high vacuum. Since the reaction is carried out, gaseous product is formed. The disadvantages of the process are contamination of alloy with unreacted silica, high silicon content of product and lengthy production time.

1.5.2.4 Silicothermic Process (Production of Low-Carbon Ferrochrome)

Increasing silicon content in an alloy decreases solubility of carbon. Low-carbon ferrochrome is prepared by reduction of chromite using silicon as reducing agent and silico-chrome is formed. Smelting is done in a shaft type of electric furnace lined with magnesia [33]. Size of chromite ore should be less than 50 mm and size of ferrochrome should be less than 10 to 15 mm. Also, chromite ore contains less phosphorus and sulfur. Ferrosilicon chrome is free from slag inclusions. This is because slag contains entrapped carbon in silicon carbide particles. Use of higher voltage minimizes carbon pick up from the electrodes. The main advantage of the process is that it can produce an alloy with carbon less than 0.03%.

1.5.2.5 Production of Carbon-Free Ferrochrome (Aluminothermic Process)

In this technique, carbon-free ferrochrome is produced on a small scale. The reduction reaction (Eq. 1.2) proceeds as follows;

(1.2)

The amount of heat produced per unit mass of reactants is 2649 kJ/kg, which is sufficiently higher [34]. Aluminothermic reduction will be easier in comparison to the silicothermic process.

1.5.3 Non-Conventional Techniques

Non-conventional techniques (AOD and VOD) have been adopted for the stainless-steel manufacturing process since several difficulties are encountered by conventional decarburization methods.

1.5.3.1 Decarburization of Solid Ferrochrome

0.01 to 0.03 % carbon content ferrochrome is obtained from high-carbon ferrochrome (6–8% carbon) by vacuum heat treatment process with an oxidant (e.g., iron oxide). A distinctive composition of ferrochrome of 60–70% Cr, 0.01–0.03% C is obtained by this process.

1.5.3.2 Decarburization Using Oxidizing Gas Mixture

(a) Water Vapor

Oxidation of carbon from ferrochrome takes place according to the reaction shown in Eq. 1.3. Shimizu [35] carried out the experiment using water and calcium compound to form dense ferrochrome. Actual mechanism of decarburization is still not clear. This is because of the endothermic nature of the reaction (Eq. 1.3) and requirement of high vacuum.

(1.3)

(b) Carbon Dioxide

In this method, decarburization is carried out by subjecting ferrochrome pallet to carbon dioxide flow. Decarburization proceeds by the following reaction (Eq. 1.4):

(1.4)

It is basically a solution reaction, which is endothermic. Thermodynamically high temperature and low partial pressure of CO can help make it feasible [36].

1.5.3.3 Production of Low-Carbon Ferrochrome from Chromite Ore

Other than decarburization of high-carbon ferrochrome, low-carbon ferrochrome material is processed by reduction of chromite ore through solid carbon (i.e., graphite) or H2 to form metallic chromium or an alloy of iron and chromium in solid state.

Chu [37] has studied reduction experiments with H2 of chromite ore. Rankin [38] has reduced chromite ore by CO. He analyzed the diffraction pattern of the product at various stages and at different temperatures.

Sundermurti [39] carried out chromite reduction in Stanton Red croft thermo-gravimetric balance under inert environment (i.e., argon gas). Also, he studied the kinetics of the process. Hiroshi [40] reported the fractional reduction of iron and chromium oxide. However, he has not noticed carbide formation.

Synthetic chromite reduction was studied by Sheshadri [41]. Various mechanisms were explored to derive a rate-controlling step. However, he found that these processes are kinetically slow. Additionally, he observed that the separation of metal beads is difficult from unreduced oxide and gangue material.

1.5.3.4 Khalafalla Method

Pahlman [42] used a direct method to produce low-carbon ferrochrome. Khalafalla [43] has suggested a direct method to prepare low-carbon ferrochrome alloys containing carbon less than 2%. In his process, chromite is reduced through electrothermal reduction route. The reduced product contains less than 2% carbon. The process of reduction of chromium pellet is carried out using carbonaceous reductant mixture at a pressure of 0.1 to 1 torr and temperature of 1500 to 1600 K. Reduction of chromite to metallic product is found to proceed via formation of intermediate carbides (i.e., Fe3C + Cr3C2). Soaking the charge for about 20 minutes has given button of low-carbon ferrochrome alloy [43].

1.5.3.5 Other Methods

In low-carbon ferrochrome, efforts are being made for decarburization of carbon in pre-reduced chromium ore by J.E. Stress [40]. Pre-reduced chromium ore reduces power consumption and increases ferrochrome. K. Suzuki has studied decarburization of liquid Fe-Cr-C alloys by rotating Cr2O3 cylinder in the melt [44].

Low-carbon ferrochrome is being produced worldwide by various processes, namely [45]:

Perrin process

Duplex process

Simplex process

Triplex process

Fusion process.

1.5.4 High-Carbon Ferrochrome Alloy

The metallic Cr content of high-carbon ferrochrome alloy is 60–65% with varying amounts of Fe and C. Therefore, quality control of raw materials ensures maximum output and uniform quality of the product. The main components of slag are SiO2, MgO, and Al2O3. Slag also includes oxides of Cr and Fe and calcium [9, 46]. Common phases in slag are glass, spinels (Al2O3–MgO) and forsterite (MgO–SiO2) and a small amount of CaO [47].

Silicon content in conventional high-carbon ferrochrome is normally below 2%. Thermodynamic equilibrium allows flexibility of silicon content (2% and 5%) in the high-carbon ferrochrome alloy. Silicon content of alloy depends on chosen production technology and operating conditions. An optical photograph of high-carbon ferrochrome is shown in Figure 1.6.

Figure 1.6 Optical photograph of high-carbon ferrochrome [48].

Table 1.3 Typical chemical composition of high-carbon ferrochrome [49].

HCFeCr

High-carbon ferrochrome

Cr

60% Min

C

8% Max

Si

3% Max

S

0 . 04% Max

P

0 . 04% Max

Cu

0 . 5% Max

Size

10-150 mm; Min 90% or as required by end user

High-carbon ferrochrome is an intermediate product and is used as a feed material for production of stainless steel. Characteristic features of high-carbon ferrochrome include crushed lumpy product with a silvery metallic appearance and irregular, flaky granulated product with a greenish appearance. The chemical composition of high-carbon ferrochrome is given in Table 1.3.

An optimal slag composition is achieved using a modern process. Factors like temperature and composition of slag, viscosity and electrical conductivity are prime criteria. Major compositions of slag are SiO2, MgO and Al2O3.

Also, slag contains Cr oxide, Fe oxides and Ca oxide. Phases present in slag are glass, spinels and forsterite. Slag chemistry is important to get an efficient amount of ferrochrome.

1.6 Waste Generation During Ferrochrome Production

Chromite ore contains undesirable oxides. These oxides, such as chromium, magnesium, iron, and calcium oxides, are called gaunge materials. During production of FeCr alloy, gangue materials are removed by addition of fluxing agent (i.e., quartzite). Thus, slag generated from the smelting furnace contains MgO, CaO, Cr2O3, SiO2, Al2O3, etc.

Typical reduction reactions in the ferrochrome furnace can be seen in Figure 1.7. Other smelted products are CO gas and FeCr slag. Carbon monoxide (CO) gas is obtained from reduction reactions. It is generated at 700 Nm3/t FeCr. In high temperature, charge materials start to smelt. Materials are not dissolved into metallic ferrochrome phases and primarily silicate phases are formed. Silicate phases are generated in the form of ferrochrome slag. Ferrochrome slag production is 1.2 t / t FeCr. It is mainly in granulated form [50].

Figure 1.7 Typical reduction process in the ferrochrome furnace [50].

1.6.1 Ferrochrome Slag

The FeCr Industries generate a huge amount of wastes. These wastes are dumped, which pollute our environment. Due to the leaching effect, underground water is polluted. Therefore, appropriate precautions are to be taken in order to save our contaminated environment. Currently, there is increased construction activity due to industrial and population growth. These activities have resulted in scarcity of conventional construction materials. Hence, it has led to an increase in construction costs.

Some nonhazardous wastes are suitable for use as construction material. This will not only solve the waste disposal problem, but also reduce the cost of construction.

Major components of FeCr slag are SiO2, Al2O3, FeO, Cr2O3, CaO and MgO [50]. A major part of slag is granulated and sizes vary between 6.3 mm to 300 mm.

Temperature of slag and ferrochrome are tapped at 1700 °C and 1600 °C, respectively. Smelting temperature of slag is higher than temperature in metal alloy. This is because metal alloy is heated up in slag liquid phase. Optimum smelting temperature is practically noted between 1680–1720 °C. The slag composition is checked in every melting process. A proportion of quartzite in the charge mix controls the slag composition, which is tested every day by sampling and analysis. Ferrochrome slag composition is SiO2 (30 %), Al2O3 (26 %), MgO (23 %) and CaO (2 %). Chrome and iron contents in the slag are 8% and 4%. Ferrochrome slag is acidic in nature. Its basicity is 0.8. Basicity is calculated by the following formula:

Since the densities of metal alloy and liquid slag are significantly different, slag is easily separated from the reactor. Density of the ferrochrome slag is 2.5–2.8 g/cm3, whereas density of ferrochrome is 6.8 g/cm3. This makes separation of slag easier from liquid alloy. Slag is first tapped in the upper part of the reactor. Liquid alloy is tapped in the lower part of the rector. Hence, separation occurs easily. Viscosity is also important during separation of slag and metal alloy. Viscosity of ferrochrome slag will increase when the SiO2 and Al2O3 content rise. Total chemical composition of slag defines the order of crystallization during the cooling process. The cooling rate has an influence on degree of crystallization. It is crucially important for the operation of the smelting furnace [50].

1.6.2 Ferrochrome Slag Products

Slag produced from a ferrochrome smelter will have different characteristics depending upon composition and mode of cooling of molten slag. Slag is tapped either in ladles or in launders. If it is tapped in ladles, then the slag produced becomes granulated. The overflow from the ladles that flows along the slag launder to a pond is depicted in Figure 1.8.

Figure 1.8 Slag granulation process [50].

Ferroalloy separation from lumpy slag is based on gravity and magnetic separation process. The first stage of slag processing is crushing and screening of feed material to fractions of 0-4 and 4-22 mm. Coarser fractions are handled by heavy media separation (DWP method). A magnetic separator and spiral washing are used for fine-grained material. Recovered slag of 0-4 and 4-22 mm are saleable products. The slag products from the process are aggregates of 0-4, 4-11 and 11-22 mm. Crystallization portion of lumpy slag is higher due to slower air cooling than that of granulated slag. Structure of slag is partly crystalline and partly glassy. Significant phases are amorphous glass, Fe-Mg-Cr-Al-spinels, forsterite, Mg-Al-silicate and metal alloy [50].

1.6.2.1 Petrographic Analysis of Low-Carbon Ferrochrome Slag

Complex processing of industrial wastes is an integral part of development for ferroalloy industries. An example of such an approach is extraction of alumina from ferroalloy slags having high content of aluminum oxide. Fe-Cr-Si (used as a reductant) is used for low-carbon ferrochrome alloy. Fe-Cr-Si may be replaced by less expensive ferrosilicoaluminum (Fe-Si-Al). In such a process, slag is formed having a composition near to the composition of helenite phase (2CaO·Al2O3·SiO2).

Helenite slag containing more than 25% of Al2O3 is a resourceful raw material for production of alumina (construction materials). Petrographic analysis has revealed the presence of different forms of helenite phases. Isolated impregnations of melilite, larnite and vitreous phase are distinctly separable, which is proof of their extraction [51].

Phase compositions of slag used in the studies were determined to be Cao-MgO-SiO2, 2CaO-MgO-2SiO2, 2CaO-Al2O3-SiO2, MgO-Al2O3, MgOCr2O3, FeO-Cr2O3 and 3CaO-MgO-2SiO2 [52, 53]. Petrographic analyses are carried out for polished section of samples using Neophot 21 microscope. Slags are related to microcrystalline types which include helenite and anorthite phases. Sample phases are mostly represented by cryptocrystalline helenite 2CaO·Al2O3·SiO2.

1.6.2.2 Description of Crystalline Phases in Ferrochrome Slag

Different types of phases are present in ferrochrome slag. The actual nature and composition of slag depend on the chemical composition of ore, fluxing agents, and reduction parameters such as oxygen partial pressure, reduction temperature, and residence time in high temperature zone [54]. The melting and crystallization process in ferrochromium slag is very important for understanding the developed crystal phases. Formation of phases affects properties such as strength, refractoriness, bulk density, and chromium stability in slag. Phases present are CaO-A2O3-MgO-SiO2 and MgO-MgO.Al2O3-2MgO.SiO2-2CaO.SiO2 systems.

During the cooling period, crystals of MgO, MgO-spinel, MgO-spinel-forsterite crystallize from the molten slag [55, 56]. Hayhurst [55] has studied spinel-forsterite-anorthite-coerdite system to understand the crystallization process of high-carbon ferrochromium slag during cooling. During cooling of high-carbon ferrochromium slag, spinel and forsterite phases crystallize from solution in liquid phase. Formation of glassy and enstatite phases are due to incomplete peritectic reactions during rapid cooling. In fully crystallized ferrochromium slag, spinel phases are dissolved in silicate solution at 1340 °C. This temperature is reduced to 1280 °C due to the presence of coerdite phase. Presence of Cr2O3 in spinel phase increases smelter temperature. Thus, it is concluded that major phases in ferrochrome slag are chromite spinels, forsterite, enstatite, anorthite; and minor phases are spinel and chromium carbide [55].

Particles of unreduced chromium (PAC) are surrounded by angular spinel of low reflectivity. It contains globules of metallic ferrochromium. Lath-shaped crystals of enstatite are dispersed in glassy matrix. Calcium oxide to silicon dioxide ratio (CaO/SiO2) and magnesium content in ferrochrome slag are two major factors to control the leaching behavior of chromium. The critical Cr-bearing mineralogical phases formed under certain criteria are as follows:

If CaO/SiO

2

ratio is less than 2, then Cr preferably exists in slag as magnesiochromite spinel (MgCr

2

O

4

) phase.

If CaO/SiO

2

ratio is greater than 2, then Cr preferably exists in slag as calcium chromite. Calcium chromite phase is susceptible to leaching by acid and may be oxidized to calcium chromate if exposed to the environment for a long period of time. However, magnesiochromite spinel phase is stable against oxidation and is resistant to dissolution in aqueous media [

57

].

Mutual interference between phases causes interlocking of crystalline phases. Thus, solidified ferrochromium slag is very hard and possesses high refractoriness with enhanced mechanical strength at high temperature. Residual chromium present in ferrochromium slag remains stable and gets immobilized due to its locking with highly stable crystal phases like chromite/magnesium aluminium chromite/magnesiochromite. Therefore, chromium release from slag matrix under normal environmental conditions is inhibited [57].

1.6.2.3 Slag Properties

Slag properties are important since they play a vital role in process control and product quality. Also, they are needed to develop reliable mathematical models of the process and to optimize the efficiency of the process and quality control. Slag compositions have high sulfur capacity (S-capacity), which means maximum amount of transfer of sulfur from metal to slag. This is usually achieved by high-basicity flux. Increased basicity due to addition of CaO will ensure effective removal of sulfur from ore to slag. However, higher basicity will increase the liquidus temperature (Tliq) of slag.

Product quality will occur by adoption of continuous casting (CC) process. Longitudinal cracks occur on the surface of steel and have to be scrubbed off. This process will reduce yield and increase expenditure due to energy consumption for scrubbing. In order to minimize longitudinal cracking on the sample, it is essential to optimize variables such as flux viscosity (η) and break temperature (Tbr) of mold fluxes used in the CC process.

There are several types of mathematical models, namely thermodynamic models (to establish viability of process and optimum conditions), kinetic models (to establish productivity rate), heat and fluid flow models (to improve process design and control), and models of the environmental and financial impact of the process. Reliable property values are principally needed as input data for heat and fluid flow models.

1.6.3 Reactivity of Slag with Container (Metal and Atmosphere)

With an increase in temperatures, reactivity increases; therefore, the possibility of a reaction occurring between two systems accelerates exponentially at very high temperatures. Slags react with container and other contact materials (i.e., metal and atmosphere). The extent of reactivity varies between viscously aggressive (e.g., coal slags and ash) to benign (glasses) depending upon its composition.

Some slags are unstable at high temperatures and reactivity between two or more components in the slag react, thereby continuously changing the composition. The following Eq. 1.5 is an example of ESR slags where fluorides and oxides react:

(1.5)

1.6.3.1 Slag/Container Reactions