Production of Ferroalloys 1.IntroductionFerroalloys are master alloys containin elements that are more less soluble in molten iron and that improve the properties of iron and steel.These alloys usually contain a significant amount of iron.Ferroalloys have been used for the last 100 years,principally in the production of the cast iron and steel.As additives,they give iron and steel improved properties,especially increased tensile strength,wear resistance,and corrosison resistance.These effects come about through one or more of the following: A change in the chemical composition of the steel The removal or the tying up of harmful impurities such as oxygen,nitrogen,sulfur,or hydrogen A change in the nature of the solidification,for example,upon inoculation

Ferroalloys are also used as starting materials in the preparation of chemicals and pure metal;as reducing agents ;as alloying elements in nonferrous alloys;as starting materials for special products such as amorphous metals.Table.1 lists some of commercial ferroalloys. Production.Generally,ferroalloys are prepared by direct reduction of oxidic ores or concentrates with carbon(carbothermic),silicon(silicothermic),or aluminum(aluminothermic).However,roasted ores(ferromolybdenum) and pure technical-grade oxides(ferroboron)are also used as starting materials. The large-volume ferroalloys-ferrosilicon,ferromangenese,ferrochromium-are prepared continuously by carbothermic reduction in large,submerged-arc furnaces.The carbon is provided by coke.Ferromanganese is also produced in blast furnaces.Both ferromanganese and ferrochromium,because of their affinity for carbon,contain high concentrations of carbon(7-8%).In contrast,ferrosilicon contains little carbon;in fact the carbon concentration decreases as the silison concentration is increased,e.g,ferrosilicon containing 25% Si contains 1%C ,while containing 75 % Si contains only 0.1% C. Low- carbon ferromanganese (0.1-2% C) or ferrochromium (0.02-2% C) is produced in the following way: a silicon containing alloy,ferrosiliconmanganese or ferrosilicochromium,is made by carbothermic reduction of an appropriate ore and quartzite in a submerged-arc furnace.Then this silicon alloy is used for the silicothermic reduction of an appropriate ore in an electric-arc refining furnace.Oxygen is seldom used to reduce the carbon content. Ferrophosphorus is a by-product of the carbothermic production of phosphorous.Ferrotungsten is produced carbothermically in small,high power electric furnaces.Ferroboron is produced from carbon,iron and boron oxide or boric acid in oneand three phase electiric furnaces.Zirconiumcontaining ferrosilicon is made carbothermically in electric furnaces from zircon(ZrSiO4) or baddeleyite(ZrO2);ferrosilicon,withe either 75 and 90 % Si is added.

Table.1 Silicothermic reduction,with the addition os some aluminum,is used to prepare ferromolybdenum in refractory-lines reaction vessels.The energy released in the reaction is adequate to melt both metal and slag,and the solidified block of ferromolybdenum can be recovered easily.ferronickel is also mainly produced by silicothermic reduction.Aluminothermic reduction is the method of choice for the production of ferrovanadium.The starting materials are vanadium(V)oxide,aluminum,and iron turnings,chips,stampings,or nasil bits.the reaction is carried out in refractory-lines reaction vessels.The reaction produces enough heat to melt both metal and slag.In fact ,inert materias must be added to keep the peak temperature lower.Ferrotitanium and ferroboron are also produced aluminothermically.

The production and use of titanium as a large-volume construction material yields enough scrap for ferrotitanium containing 70% Ti to be made by melting iron and titanium scrap together.Ferrozirconium containing 80-85% Zr is also made by melting metal scrap and iron together.Ferroselenium is produced by the exothermic reaction of iron and selenium powders. The production of ferroalloys is of great economic importance,especially in countries that have the raw materials and the energy supply.Alloy steel constitute an ever growing fraction of total steel production because of their superior properties. Enviromental Aspects.During the production of ferroalloys,the emission of undesirable substance into enviroment must be kept under control.

2.Carbothermic and Metallothermic ProcessesCarbothermic processes are mainly used for the large scale production of ferrosilicon,ferromanganese,ferrochromium,ferronickel and ferrotungsten.Ferrophosphorus is a byproduct of elemental phosphorus production by the carbothermic reduction of phosphate rock.Carbothermic processes for the production of ferroboron,ferrotitanium,ferrovanadium,and ferromolybdenum have been largely replaced by metallothermic processes,mainly aluminothermic and silicothermic. 2.1 General Considerations 2.1.1 Introduction The usual method of reduction by carbon is inapplicable to the reduction of refractory oxides and ores.The products commonly covered under this title are usually required to be low in carbon and they are avid carbide formers.Also ,no conventional refractories would stand up to the long reduction times at the high temperatures required.For such products,advantage is taken of the high temperatures achieved and rapidity of metallothermic reactions using reactive metals,notably Al. Early examples of such reactions are BREZELIUSreduction of K2TaF7 by Na and WHLERs reduciton of AlCl3 by Na,n 1825 and 1828,respectively.However,the father of aluminothermic processes was undoubtedly GOLDSCHMIDT who ,in 1898,described the production of low-carbon ,high melting point metals,without extraneous heat ,by feeding an exothermic mix into an already ignited first portion. The term aluminothermic processescan cover a wide field.For the present purpose it is taken,as is genereally understood,to be the manufacture,by Al reduction of refractory oxides or ores ,of metals and alloys mainly used in the steel and superally are produced aluminosilicothermically. The main alloys under discussion are FeB,FeMo,FeNb,FeTi,FeV and FeW.There are many minor variations,such as CrC,where superior purity or the absence of Fe is required for alloys used in superalloy production,and FeNb from NB2O5 instead of ore,where extra purity is required,again for superalloys. Aluminothermic Mn,formerly of importance ,has been superseded by electrolytic Mn and aluminothermic production of FeTi has largelt given way to induction furnace melting of Fe and Ti scraps.The latter method also now substitutes for the former ,dangerous,highly exothermic production of 55-60% TiAl and 55-60% ZrAl.When possible,combined electroaluminothermic methods can be used.

Becaused most basic reactions require extra heat this is provided by Al and a vigorous oxidant.Aluminum is an expensive fuel.At an Al powder price of $1400/t and a small-scle electricity cos of 4.9C/kWh the heating effect of Al combustion costs about three times that of electricity. Although a minor proportion of production,some FeB,FeNb,FeTi,and FeV is produced in the arc furnace.In view of the small tonnages involved,such productions usually occur only where an arc furnace exists for other purposes. Apart from the necessity of metallothermic reduction advantages of the process as compared to conventional reduction processes are; Very rapid reaction with less heat loss by radiation and convection Mush less gas volume Easy accommodation of diverse productions Small plant investment

A disadvantage of metallothermic production is that no refining of the metal is possible.It is important to leave the slag in situ to allow metal drops to settle into the regulus,so that one is faced with a large depth of slag quickly crusting over.Alternatively,if the slag is tapped ,requiring subsquent ore dressing processes to recover included metal,the metal regulus quickly becomes sluggish.Therefore the composition of the miz must be regulated very carefully to pruduce the optimum combination of oxgen and reactant metal levels in the product,and all ingredients must be low in harmful impurities.Obvious ones are C,S and P.Other impurities in the various ores used for different products are As,Sn,Pb,Sb and Cu. 2.1.2 Basic Metallurgy A metallothermic reduction of a metal compound is possible when the reductant metal has a greater affinity for the nonmetal element of the compound than the desired metal.In various branches of metallurgy the nonmetal may be halogen,sulfur,or oxygen.Aluminothermic processes,as previously defined,depend upon the high affinity of Al for oxygen as compared to that of many other metals. Fundamental Reactions.

Figure.1:Change in free energy of oxide formation reactions Although Ca and Mg may be the most suitable reactants,they are not used in the reduction.Their basic costs are high compared to Al and Si and 16 parts by weight O require 40 parts Ca,24 parts Mg ,18 parts Al,or 14 parts Si. In silicothermic reactions,a proportion of Al is usually used to increase the exothermicity particularly,because the products,e.g,FeW and FeMo,have such high melting points.

3.FerrosiliconThe term ferrosilicon refers to iron-silicon alloys with Si contents of 8-95%. Table.2: lists the composition of commercial ferrosilicons.

FeSi 75 is commercially the most important,although FeSi45 is still produced in large quantities for the North American market. Physical Properties. The Fe-Si phase diagram shows the existence of four compounds;Fe2Si,Fe5Si3 and FeSi and FeSi2.Commerical alloys differ from the compounds shown in the phase diagream with redarded to theri stoichiometry. Raw Materials.Pure quartzes with SiO2 content >98% are used for the production of FeSi45 to FeSi90.Such quartzes occur naturally as pebbles or rock.Quartz sands are processed into briquettes,in which the reducing coal required for quasi-self-fluxing burden can be incorporated.For good gas flow through the furnace,the starting materials should be free of fine particles,which also requires that they remain dimensionally stable on heating and do not decrepitate too early.Depending on the location,high-quality iron ore or scrap iron is used as the iron source.In industrialized countries,washed and dried unalloyed steel turnings are generally used. Flaming coals with low ash content,metallurgical coke,high-temperature brown coal coke,petroleum coke,brown coal briquettes,or wood chips are employed as reducing agent,depending on availability. Production.Ferrosilicon is produced in three phase,submerged arc furnaces with power consumptions of 10-70MW,corresponding to annual capacity between 9x103 and 60x103 t per furnace.

Figure.2:Submerged arc furnace: a) furnace casing with lining,b)electrodes,c)transformers,d)secondary power supply,e)raw material bunker,f)charging pipes,g)stoker machine,h)burning out unit,i)tapping-off ladle

With few exceptions,production is carried out in open furnaces,which allow the burden surface to be poked with stoker machines to prevent crust formation and thus maintain uniform gas flow through the furnace.In the modern furnaces the furnace casing can be rotated to reduce encrustation in the lower areas. Closed furnaces must be built with a rotating furnace vessel to aviod sintering of the burden,since stoker machines can no longer be used.Closed furnaces operate more efficiently because the carbon monoxide gas formed can be used.However,this requires a more eleborate construction of the furnace:with rotatable herath and extensive dedusting of the hot off-gases.

Reactions in the furnace occur according to the simplified scheme: SiO2+2C Fe+Si Si+2CO FeSi

Side reactions also occur that result in a lower yield of the desired product,especially when insufficient carbon is used: SiO2+C SiO2+Si SiO+CO 2SiO

Gaseous SiO is oxidized by atmospheric oxygen at the burden surface to give SiO2 dust ,which is carried out of the furnace with off-gas. An excess carbon leads to the formation of SiC,which also lowers the yield. Heating the batch to the reaction temperature of up to 1800 C is achieved mainly by electrical energy.The energy uptake of the batch depends on the so-called hearth resistance,whereby the conductivity of the FeSi burden is provided mainly by the coal.Thus,changing the type of coal influences the resisrance.The grain size also plays a role : the smallet the grain,the higher isthe resistance.the hearth resistance automatically regulates the depth of the insertion of the Sderberg electrodes and thus the power consumption of the furnace.A uniformly high hearth resistance and low electrodes are desirable. The liquid alloy,that accumlates in the hearth is tapped off to pouring ladles at regular intervals and poured into shallow moulds. Ferrosilicon production is a slag-free process,which means that all the impurities present in the raw materials are transferred to the product.To obtain high purities the alloy must be purified by further treatment outside the furnace. Aluminum and calcium impurities are removed by oxidation:

By injection of gaseous oxygen through immersed lances or through nozzles or sparging blocks in the baseof the pouring ladle By treatment with oxidizing siliceous slag which can be stirred or blown in

Uses.Approximately 75 % of the FeSi produced is used in the steel industry,where a requirement of 3-3.5kg of FeSi75 per tonne of steel can be considered normal.For melting almost all grades of steel,silicon is added as deoxidizer and alloying agent.Silicon binds oxygen dissolved in the steel melts,leading to noncritical concentrations.To increase this effect,FeSi is generally added together with other deoxidizers such as aluminum,calcium and manganese. Silicon that is not consumed in deoxidation dissolves as an alloying element in the steel and increases its strenght and yield point.More highly alloyed grades include tool steels,in which silicon improves the hardenability and wear resistance;hot-work steels that contain silicon for better tempering properties.

Foundries consume almost 25%of the ferrosilicon produced worldwide.Normal cast iron contains 2-3% silicon for improved precipitation of graphite and increase strenght. Economic Aspects.CIS and China are important producers ,followed by Norway,the United States and Brazil.

4.FerromanganeseA number of manganese-containing ferroalloys are manufactured which are used largely in the mild steel,foundry,and stainless steel industries.The names and typical compositions of these alloys are given in table.3:

Generally,high-carbon ferromanganese and silicomanganese are produced from a blend of manganese-containing ores,and in the case of silicomanganese,slags and silica are added.Ferromanganese can be produced in either electric submerged furnaces or blast furnaces,although only four blast furnace producers exist in the westren world,whereas silicomanganese is largely produced in submerged arc furnaces..High carbon ferromanganese can be converted to medium carbon manganese by an oxygen blowing process,and silicomanganese can be further refined into medium-or low-carbon ferromanganese as well as manganese metal(Figure.3)

4.1.High Carbon Ferromanganese

4.1.1 Production of Ferromanganese in Blast Furnaces Ferromanganese can be produced in blast furnaces in a manner similiar to pig iron;however ,in the western world only four producers employ this method.These are Thyssen Stahl(Germany),BSC Cleveland(United Kingdom),SFPO(France) and Mizushima(Japan).The choice of the use of blast furnaces over electric furnaces is based one the reletive price of coke and electricity.The product produced from blast furnaces generally contains 76%Mn and 16%Fe. Raw Material Selection and Pretreatment. The raw materials required for the production of high carbon ferromanganese are manganese ores,Fluxes such as limestone,dolomite,or silica ,and solid fuels and reductants such as coke. In order to produce ferromanganese of the required grade a single ore is seldom suitable because the desired Mn/Fe ratio of the charge determines the Mn content of the final product.Ores from various sources are therefore blended to achieve the ideal ratio and to limit the contents of the deleterious components silica,alumina,and,phosphorus in the raw material mix.The raw material is crushed and screened to 5-30mm.alternatively,sintered or pelletized fine ore can be used.Some deletetious components can be partially removed from the ore prior to melting by dense medium separation or flotation.Slaging components can be added to the sintered or pelletized ore,which results in cost savings in the blast furnaces. Blast Furnace Operation.In comparison to iron making ,high gas temperatures are required in ferromanganese production because of manganese(II)oxide takes place at a higher temperature than is required for the reduction of wustite.This is achieved by oxygen enrichment of the hot blast or ,in the case of SFPO,by heating the blast with nontransferred arc plasma torches.The plasma arc increases the flame temperature from 2200to2800C and considerably reduces the coke consumption,which usually ranges from 1270 to 2000kg/t. The recovery of manganese in the alloy is usually 75-85%.This is influenced by the MnO content ofthe slag ,the slag to metal ratio,and losses in the flue gases.The MnO content of the slag is highly dependent on the basicity ratio(CaO+MgO)/SiO2

Figure.4:Variation of equilibrium MnO content of slag for high-carbon ferromanganese and silicomanganese production

The Reduction Process in the Blast Furnace .The reduction of the higher manganese oxides to manganese (II) oxide takes place in the upper zone of the shaft .These generally occur below 900Cand are indirect.The reactions are exothermic,and the heat generated causes high top temperatures and necessitates water cooling of the furnace top. The reduction of manganese(II) oxide; MnO+C Mn+CO

is highly endothermic,in contrast to the weakly endothermic reduction of wustite.This requires higher temperatures and, consequently,higher coke rates are required for the smelting of ferromanganese in blast furnaces.

4.1.2.Production Of Ferromanganese in Electric Arc FurnacesThe majority of producers of ferromanganese in the Western world use submerged arc electric furnaces.Altough electric arc furnaces have lower capacities than blast furnaces they have the advantage that the heat requirement is provided by electricity,and coke and coal are added to the feed only as reductants.Consequently the coke consumption is lower in electric furnaces than in blast furnaces,which is a considerable advantage in the light of dramatically increasing coke prices.An addtional advantage is that the process is not entirely dependent on high strenght coke ,enlike blast furnaces,and a portion of the carbon requirement can be supplied in the form of coal. In modern electric arc furnaces the raw material is usually fed by gravity from bunkers above the furnace .Fresh burden therefore automatically enters the furnace as the raw materials ar melted and slag and metal are removed from the system.

As the raw material move down the furnace,the higher oxides of manganese are reduced to MnO by the gas leaving the furnace.The reduction of manganese(II)oxide occurs by the contanct of carbon with the molten oxide in the slag phace.The overall reaction is ; MnO+10/7 C1

/7Mn7C3+CO

The heat required for this endothermic reaction,for heating the burden,and to compensate for heat losses is supplied by the electrical input to the furnace.Heating takes place by the flow of electricity from the tips of the electrodes,which are submerged in the burden,through the burden and slag to the metal,as well as through the flow of electicity between the electrodes.

Figure.5:Layout of an electric arc furnace:a)Charging bins,b)Charging tubes,c)Electrodes,d)Electodes,d)Electrode slipping device,e)Electrode positioning devices,f)Current transmission to electrodes,g)Electodes sealing,h)Furnace transformer,i)Current bus bar system,j)furnace cover,k)furnace shell,l)tap hole,m) furnace bottom cooling,n) refractory material

Raw Materials required for the Manufacture of High Carbon Ferromanganese Manganese ores from different sources vary widely in their contents of manganese,iron,silica,alumina,lime,magnesia,and phosphorus.To produce standard ferromanganese (78%Mn) and a slag containing 30% MnO,the manganese to iron ratio in the charge must be 6:5.Since a single manganese ore of this ratio is seldom available,blending of ores from different sources is common practice to reach the desired manganese to iron ratio and to control the level of deleterious elements,particularly phosphorus. Chemistry of the Process.

Figure.6:a)loosely sintered burden,b)loosely sintered material enriched in carbonaceous reducing agents,c)coke and slag region,d)coke bed,e)coke-enriched layer,f)MnO melt layer,g)ferromanganese alloy layer,h)graphitized and carbon rich layer,i)carbon lining,j)brick lining,k)slag taphole,l)metal tap hole level,m)pieces of electrode

The material descends rapidly down the side of the electrode (a) into the semi-active zone (b),where prereduction of higher manganese oxides to MnO takes place.Thereafter,the material moves into the active zones of the furnace (e,f),where reactions take place between the manganese(II) oxide in the melt and the coke particles in the coke bed: 7MnO+10C Mn7C3+7CO

Eqilibrium between the slag and metal was thought to exist under each electrode,and further from the electrode,layers of unreacted ore and coke were found to be peresent(h).This suggests that heat is concentrated under each electrode.The path of electrical transfer was deduced to be from the electrode tip through the coke bed and into the alloy layer(g) The efficient productin of high-carbon ferromanganese therefore depends on the degree of the reduction of MnO by carbon as well as the prereduction that occurs in the upper region of the furnace.The ratio of CO and CO2 in the off-gas is important and can be used to monitor the condition of the furnace.The higher the CO2 content of the off-gas,the higher is the energy efficiency of the process,because the reducing potential of the gas is being more full utilized.Good operation of the furnace is indicated by a CO2/(CO2+CO) ratio of 0.55.this ratio,as well as the MnO content of the slag,can be used to control the coke rate of the furnace.Undercoking of the furnace is indicated by high MnO Content of the slag and a low CO2 content in the off-gas.

4.2 Production of Medium Carbon Ferromanganese Medium-carbon ferromanganese contains 1-1.5% carbon and has a manganese content of 7585%.Medium-carbon ferromanganese can be produced either by refining high-carbon ferromanganese with oxygen(MOR) or by the silicothermic route,whereby the silicon in silicomanganese is used to reduce additional MnO added as ore slag.The Former process has considerable advantages annd is used by most producers.Figure.7

Figure.7:Process flow sheet comparison for silicothermic reduction and the MOR process Production of Medium-Carbon Ferromanganese by Oxygen Refining of High-Carbon Ferromanganese In the MOR process patented by Union Carbide,high-carbon ferromanganese is decarburized in a similar manner to the steelmaking process in thebasic oxygen furnace.However,several distinctive differences are encountered in the case of ferromanganese: A final temperature of 1750C compared to 1550C Refractory attack is more severe Difficult casting of the final alloy The higher vapor pressure of manganese The higher volume and temperature of the off-gas

In the MOR process ,oxygen is blown into the molten high-carbon ferromanganese and the temperature is increased from its tapping value of 1300 to 1750C.The heat required is supplied by the oxidation of manganese to manganese(II) oxide and carbon to carbon monoxide.In the early part of the blowing process,most of the oxygen is consumed by oxidation of manganese,and the temperature of the melt increases from 1300C to 1550C.Hereafter,carbon is rapidly oxidized and the temperature rises to 1650C.Above this temperature ,the rate of carbn removal decreases and manganese is one again oxidized.The process is stopped at 1750C ,which corresponds to a carbon content of 1,3%.Further

reductions in carbon content result in unacceptably high losses of manganese.In the MOR process ,the recovery of manganese is 80% and the distribution of manganese can be broken downs as follows; Alloy MC FeMn Fume formed by vaporization Slag formed by oxidation of Mn Other losses,splashing 80% 13% 5% 2%

The manganese lost in the fume recovered in the gas cleaning plant and is then pelletized and returned to the high-carbon ferromanganese furnace.The slag ,which contains about 65%MnO,is also returned to the high-carbon ferromanganese furnaces. The MOR process has many advantages over the silicothermic process : lower energy consumption ,lower capital investment ,lower production cost ,and greated flexibility. Silicothermic Production of Medium-Carbon Ferromanganese In the silicothermic production of medium-carbon ferromanganese, a high-grade slag or a melt containing manganese ore and lime is contacted with silicomanganese containing 1630.1% silicon.The silicon in the alloy acts as the reducing agent in the process,which reduces the manganese(II) oxide in the melt.Similarly to silicomanganese production,the equilibrum is determined by the reaction: Si+2MnO SiO2+2Mn

The purpose of the lime is to reduce the activity of the SiO2 in the melt,thus forcing the above reaciton as far to the right as possible.The ratio of CaO to SiO2 in the slag should be greater than 1.4 to ensure a sufficient reducing in the activity of SiO 2.The carbon entering in the process in the silicomanganese remains entirely in the metal phase and is therefore found in the product.Thus,to produce a medium-carbon ferromanganese containing 1%C,a silicomanganese containing 20% Si is necessary. The heat produced by the silicothermic reduction is not sufficient to sustain the process;hence it is usually carried out in an electric arc furnace. 4.3.Production of Low-Carbon Ferromanganese Low-carbon ferromanganese contains 76-92% Mn and 0.5-0.75% C.The productin of lowcarbo silicomanganese is not possible by the decarburizastion of high-carbon ferromanganese without incurring extremely high losses of manganese. Use must accordingly be made of a silicothermic reduction process. The process is similar to that used in the silicothermic production of medium-carbon ferromanganese.High purity ores are used and in particular ores containing iron and phosphorus should be avoided.An artifical manganese ore,produced as a high-grade slag,is

particularly suitable because of its low impurity level and because all the manganese is present as MnO.The reduction of the higher oxides of manganese istherefore unnecessary. The operating figures for 1 t of ferromanganese containing 85-92%Mn,0,1%C,and1%Si with a manganese recovery of 75% are: Calcined manganese ore :1250-1350 kg Silicomanganese(32-33%):800-850 kg Quicklime : 1000-1100 kg Electrodes :10-12 kg Electricity :1800-2500k Wh Since the required silicon content of the metal is low ,a slag high in MnO is necessary.The MnO content of the slag can,hoever ,be reduced by the use of a two stage refining operation.In the first stage ,an excess of silicomanganese is maintained and a slag containing 6-8%Mn is teemed and discarded.The second refining stage with manganese ore and lime results in a salg containing 10-14%Mn,which is returned to the silicomanganese furnace. 4.5 Recent Developments and Future Trends In regions with high electricity prices recent developments have concentrated on the saving of energy.This is of particular importance to Japanese producers of manganese alloys that use of the off-gas from the furnace to preheat and mildly prereduce the ore,either in a rotary kiln or in a shaft kiln above the furnace.

5.FerrochromiumFerrochromium is a master alloy of iron and chroimium,containing 45-95% Cr and various amounts of iron ,carbon,and,other elements.the ferrochromium alloys are classified by their carbon content; High-carbon ferrochromium with 4-10 % C Medium-carbon ferrochromium with 0,5-4%C Low-carbon ferrochromium with 0,01-0,5%C

The mechanical and chemical properties of steel can be improved by alloying it with ferrochroium.Chromium combined with nickel gives stainless steel excellent chemical resistance. 5.1 Raw Materials The only raw materials for the production of ferrochromium are chromite ores.The mineral chromite has spinel structure and its formula may be written as (Fe2+,Mg)O.(Cr,Al,Fe3+)2O3.A high Cr:Fe ratio is advantageous to produce an alloy with high chromium content. In the production of high-carbon ferrochromium,which is by far the alloy in greatest demand,generally a lumpy type of chromite ore is necessary.The submerged arc smelting of

high-carbon ferrochromium by the direct reduction of carbon in large low-shaft electric furnaces generally requires lumpy chromite ores to allow the reaction zone to the top the furnaces where the burden is continously charged. The reducing agent for chromite is usually carbon in the form of coke;its contents of S and P should be low.Silicon as a reducing agent is used in the fform of ferrosilicochromium or ferrosilicon to produce low-carbon ferrochromium.Fluxing agents,e.g.quartzite or alumina and lime,are charged with the burden for slag formation.In the carbothermic production of ferrosilicochromium,chromite and quartzite are used as the raw materials. 5.2.Production The oxides of iron and chromium present in the chromite can be readily reduced at high temperature with carbon.Because of the tendencey of chroimum to form carbides,a carbon containing alloy is obtained.The oxides can also be reduced with silicon,aluminum,or magnesium.However,only carbothermic and silicothermic reductions are used commercially.The reducibility of chormite depends on its composition.A chromite rich in iron(FeO.Cr2O3) can be reduced by carbon at lower temperature than ore rich in magnesium(MgO. Cr2O3) Carbides with higher carbon content formed initially at lower temperature react at higher teperature with Cr2O3 and form carbides with lower carbon content ; finally,reduction of SiO2 starts at higher temperature.Therefore,production of ferrosilico-chromium alloys requires high temperature. In practice the reactions are somewhat more complicated because iron-containing chromium carbides are formed.In high-carbon ferrochromium,the double carbide(Cr,Fe)7C3 is present.In this compound,two to four Cr atoms can be substituted by iron atoms. FeO.Cr2O3+C Fe+ Cr2O3+CO t=950C

For the Cr2O3 reduction: MgO.Cr2O3+13/3C2

/3Cr2C3+MgO+3CO t=1200C

Because the difference in temperature between these two reactions is slight and because iron also facilitates reduction of chromium oxide ,selective reduction of iron is difficult. In carbothermic reduction process,unreduced oxides from the chromite (MgO,Al2O3) and from the gangue are collected in a slag,which generally contains 30 % SiO2, 30 % MgO and 30% Al2O3.The remaining 10% is composed of Cr2O3,CaO,MnO,And FeO.Control of lag composition is important with respect to melting temperature and fluidity. Low-carbon ferrochromium is produced by the silicothermic reduciton of chromite ore.Silicon is used in the form of ferrosilicochromium,which is produced in submerged arc furnaces by carbon reduction of chromite ore and quartzite.The solubility of carbon in the FeSiCr alloy depends on the silicon content;if the silicon content is higher,the carbon content is lower.The reduction of Cr2O3 by Si is enhanced by addition of lime (CaO),which reduces the activity of SiO2 in the slag.the reduciton may be written as follows:

FeO. Cr2O3+2Si+4CaO

(2Cr+Fe)+2Ca2SiO4

5.3.High-Carbon Ferrochromium High-carbon ferrochromium is produced by direct reduction of chromite ores with carbon (coke,coal,charcoal) in large,three-phase submerged arc furnaces. Submerges arc furnaces work continously as a low-shaft electric furnace,where burden is charges around the self-burning Sderberg Electrodes.These electrodes are deeply immersed in the burden column and discharged to reduce liquid products. The silicon content in high-carbon ferrochromium is dependent on the reduction temperature.High-melting slags lead to higher silicon content in the alloy.A typcial slag composition is 30-33%SiO2,26-28% Al2O3,20-25% MgO,3-7%CaO,and 8-13% Cr2O3 for charge chrome containing 63-67%Cr,5-7 % C and 3-6% Si produced from Transvaal ore or charge chrome containing 63-67%Cr,5-7% C,and 3-6% Si produced from Zimbabwe ore. The lumpiness of the ore and the coke quality are important to maintain a proper submerged arc process.Because of the high coke rate,the coke properties(size,bulk density,volatile matter,and fixed carbon)are mainly responsible for the electrical resistance of the burden. Figure.8 shows the new process deceloped by Outokumpu Oy in Finland for the production of ferrochromium from their own chromite-containing deposit.

Figure.8:Outokumpu Oy high-carbon ferrochromium process. The chromite concentrate is pelletized ,using bentonite as a binder.After sintering in a in a shaft furnace ,the pellets are blended with fluxes and coke.This burden is then preheated in a rotary kiln at 100-1100C and charged to a fully closed 24MV. A submerged arc furnace producing 60 000 t/a of charge chrome (53.5%Cr,7%C,and2.5% Si) and slag (30,2% SiO2,249% MgO,6.8%Cao,25.9 Al2O3,6.8% Cr,and 1.8%Fe);Cr recovery is 84%.The off-gas from the furnace is used as a fuel for the shaft furnace and for heating the kiln.

A further improvement in specific energy consumption was achieved by the SRC process (solid-state reduction of chrome ores),developed by Showa K.K. in Japan.This process is shown in Figure.9

Figure.9:Show a Denko high-carbon ferrochromium The addition of carbon and flux during pelletizing resulted in a reduction of iron oxide and partial reduction of chromium oxide during sintering in a rotary kiln at 1350-1450C.Hot charging a burden containing 60% prereduced pellets in a closed 18MV.A submerged arc furnace required an energy consumption of 200-2100 kWh/t of alloy for an annual production of 50 000 t high-carbon ferrochromium (57-60%Cr,8% C,and 3%Si).This process was also adopted by Johannesburg Consolidated Investment in a 180 000t/a ferrochromium plant.

5.3.2 Medium-Carbon Ferrochromium Medium-carbon ferrochromium with 0.5-4% C can be produced by refining high-carbon ferrochromium or by silicothermic reduction of chromite ores.Batch refining of high-carbon ferrochromium with refractory chromite ores in an electric arc furnace is no longer used because of the high power consumption. Dercaburization of high-carbon ferrochromium in an oxygen-blown converter is more economical,especially on a large scale. Because demand for medium-carbon ferrochromium is small compared to demand for the high-carbon material,the decarburization processes are rarely used.Hoever,reduction of chromite ores with silicon in the form of silicochromium is used and is an economical production method for medium carbon ferrochromium because the low carbon grade can be produced as well. 5.3.3 Low-carbon Ferrochromium Low carbon ferrochromium is produced mainly by silicothermic reduction of chromite ores. In step one ,chromite ore was totally reduced with carbon in an electric arc furnace to an intermediate ferrochromium with 60-65%Cr,5-6%C,and5-7% Si ,and a discard slag.In step two,this intermediate ferrochromium was smelted with quartzite and coke in a slagless process to produced silicochromium.Finally,this silicochromium was used as a reductant in the third step;the crushed alloy was thrown onto the surface of a chrome ore-lime melt in an arc furnace.The tapped ferrochromium contained 70%Cr,1%Si,and 0.03-0.05%C.The slag,which contained 20-25% Cr2O3,was returned to the first step to improve Cr recovery. The direct silicochromium production by carbothermic coreduction of chromite ore and quartzite in large submerged arc furnaces in one step proved more economical than the slagless process.Furthermore,the silicothermic conversion was improved by adding more lime to form a highly basic slag.Thus,ferrochromium with < 1.5%Si was produced with a lean slag in one step.Residual elements,S and N ,were low and