FIELD OF THE INVENTION

The present invention relates to a method for direct reduction of oxidized chromite ore fines composite agglomerates in a tunnel kiln using carbonaceous reductant for production of reduced chromite product/ - agglomerates applicable in Ferrochrome or charge chrome production. The invention further relates to a slag with a novel slag chemistry for reduction of oxidized chromite one fines in the tunnel kiln. The invention also relates to a method of using carbonaceous layer on the tunnel kiln car surface for making the reduction process more effective in the tunnel kiln. The invention still further relates to a novel method of partial recycling of the cooled tunnel kiln exhaust gas into the cooling zone of the tunnel kiln for quenching/cooling of reduced product/agglomerate to prevent re-oxidation of the reduced product and discharge the reduced product at temperature less than 500°C temperatures from the tunnel kiln.

BACKGROUND OF THE INVENTION

Chromite ore is the prime raw material for production of ferrochrome using a submerged arc furnace (SAF) process. Globally, most of the ferrochrome is produced by submerged arc furnace route except a small fraction, produced in the DC are furnaces route. The smelting reduction of chromite ores is carried out in the submerged arc furnaces using coke as the reductant and quartzite as the flux. Chromite ores in the form of lump are smelted in the SAF directly, whereas the chromite ore fines in the size range of 0 to 4.0 mm are agglomerated to pellets or briquettes. Chromite ore fines are conventionally agglomerated to pellets without any prior heat treatment of ore fines. These agglomerates are then subjected to sintering process in a vertical shaft furnace or a horizontal steel belt sintering furnace prior to smelting in the submerged arc furnace.

According to the prior art, five primary processes are used for production of ferrochrome. These processes include conventional process with open or semiclosed submerged arc furnaces, the conventional process with closed submerged arc furnace, the Outokumpu process, the DC Arc route and the Premus process. Each of these processes are discussed briefly below.

Conventional Process with Open or Semiclosed Submerged Arc Furnaces

In the conventional process, a mixture of chrome ore, reductants and flux is fed cold with minimum preprocessing directly into open/semiclosed type submerged arc furnaces. The furnace off -gases are cleaned in a bag plant before being vented into the atmosphere. The metal and slag are then tapped from the furnace for further processing. The primary advantage of this process is that it requires lowest capital investment and is very flexible in terms of raw materials that can be used in the process. The main disadvantage of this process is that it is increasingly being perceived as being less environmentally friendly than other available processes and it has the lowest efficiencies.

Conventional process with closed submerged arc furnace:

In conventional process with closed submerged arc furnaces, a mixture of sintered pellets, lumpy chromite ores, coke as reductant and flux are fed to closed submerged arc furnaces. The furnace off-gas is cleaned using wet scrubbers in the gas cleaning plant. The metal and slag are then tapped for further process. The primary advantage of closed submerged arc furnace is that it is energy efficient process as compared to open or semiclosed furnaces. The main disadvantages of the process are that, the energy consumptions are still higher up to 4500 kWh/t of metal, requires costly metallurgical coke and high losses of metal to slag upto 10% by weight.

Outokumpu process

Fine chrome ore is wet milled and then palletized using a binder such as bentonite. The pellets are then sintered and then air cooled and stockpiled. The pellets together with fluxes may be heated in a pre-heater located above the furnace bins. Reductants consisting of coke and char, are added to the (preheated) raw materials and fed into closed submerged arc furnaces. The furnace off-gas is cleaned in wet scrubbers and used as an energy source in the sintering and preheating processes. The main advantages of this process is that the sintered pellets and preheating of the charge to the submerged arc furnaces results in reduced specific energy consumptions and improved chromium recoveries.

DC arc furnace

The furnace uses a single solid carbon electrode and produces a DC arc to an anode in the bottom of the furnace. The arc is normally an open or semi-submerged one. Raw materials can be charged either directly into the furnace, or by using a hollow electrode. The primary advantage of this process is that the process utilizes any of the available raw materials including 100% chromite fines with minimum or no pre-processing, thus eliminating the need for an expensive agglomeration plant. Inferior grades of reductants like cola and anthracite can be used in this process, which is also considered an additional advantage of the process.

Premus Technology

Fine chrome ore, bentonite and a reductant such as anthracite fines are dry milled, palletized and preheated before being fed into rotary kilns where partial pre-reduction of the chromium oxidie and iron oxides take place. The metallized pellets are then hot charged into closed submerged arc furnaces. The furnace off-gas is cleaned in ventury scrubber and used throughout the plant as an energy source. Initial capital costs for this process are high and the level of operational control required to ensure smooth operation of the process is also very high.

The above processes manly included pre-reduction or preheating of agglomerates in rotary kilns. The advantage was seen in reducing the cost of electrical energy. Chromium recoveries are improved and cost smelting is reduced by these pretreatment methods. The steel belt sintering furnace process developed by Outokumpu offered the reduced energy consumptions and emissions and was advantageous to harden pellets in order to withstand handling. The preheating of smelting charge by burning carbon monoxide in shaft furnace up to a temperature of 700°C, removes moisture and volatiles, which gave major savings in electricity and increased the capacity of the smelting furnaces by 20%. The best metal recoveries were reached with agglomerated chromites from the materials. Optimal smelting furnace operation was achieved by the right charge mix, depending on the raw materials. The preheating and pre-reduction methods have improved the performance of SAF process; however none of it could replace the SAF process completely. It is worthwhile to mention that, most of these methods for ferrochrome production included the pre-treatment such as preheating or pre-reduction of the agglomerates obtained either by agglomeration of chromite ores such as for sintering or composite agglomerates obtained from agglomeration of chromite ores, reductant, flux and binder. However it should be noted that, none of these processes (pre-heating, pre-reduction) have used the tunnel kiln reduction process or process based on oxidized chromite ore fines which are obtained typically by pretreatment of chromite ore fines such as oxidation of chromite ore fines prior to its agglomeration. The oxidation process of chromite ore fines ensures a uniform degree of oxidation across the oxidized chromite ore particles which help in enhancing the reduction reactions during subsequent reduction process of oxidized chromite ore fines. The prior art technologies are designed for processing of FeO-rich chromite ores for producing the change chrome with relatively lower chromium content, varying between 50 to 55 percent. On the other hand, the MgO- rich chromite ores are highly refractory in nature and therefore needs a different approach. The key problems with SAF process used for production of high carbon ferrochrome are (a) highly energy intensive process consuming about 2600-4500 kWh/t of FeCr (b) high Metallurgical coke consumption of 500-700 kg/t (c) requires high quality feed material with Cr ₂ 0 ₃ greater than 48 percent (d) about 10 percent (by weight) Cr ₂ 0 ₃ loss in slag (e) large volumes of slag (Slag: Metal ratio in excess of 1.0) (f) environmental problems with respect to C0 ₂ emissions and slag. Therefore, these problems needs to be considered while developing a novel technology, which further contemplating use of low cost materials, efficient and effective use of secondary energy produced, increase in process efficiency, minimize the operating costs and improve the working environment.

Tunnel kilns are traditionally used for heat treatment of refractory bricks or calcinations of materials. For example, a calcinations process of chrome ore and lime mixture bricks in the tunnel kiln at 1400°C is disclosed in GB496890 to produce chromium chemicals such as calcium chromate and calcium bichromate. The calcium bricks of chrome ore-lime mixture obtained from the tunnel kiln are ground further and boiled with water in presence of air to remove C0 ₂ and produce calcium carbonate precipitate and calcium chromate solution. US3295954 discloses a process for alkaline roasting of chromium ore with Na ₂ 0 is disclosed wherein molding of ore and soda mixture is heated in tunnel kiln at 1100°C to obtain improved chromium yield. A method for reduction of chromium ore in tunnel kiln is disclosed in JP62256938, wherein the lump (molding) of mixture of ore, carbonaceous reducing agent and binder is provided with a protective film of Cr ₂ 0 ₂ SiC and fly ash on the surface for preventing intrusions of oxidizable combustion gas into the molding during reduction between 1200 to 1500°C to obtain high chromium reduction. JP62243723 describes a method for reduction of chromite wherein the mixture of chrome ore, coke reductant and binder (carboxymethyl cellulose) is filled in a ceramic (aluminous) vessel to form green compact of about 10 to 200mm diameter inside the vessel. The ceramic vessels are then placed into a tunnel kiln of about 30 to 100 m length for atleast 30 minutes in a temperature range of 1400 to 1500°C for obtaining about 90% reduction. A new smoke pollution device for tunnel kiln is disclosed in US5603615, which prevents the discharge of pollutants such as sulphides, steam and ashes to atmosphere. A tunnel kiln without exterior combustion chamber for producing coal based direct reduced iron is disclosed in CN1804049. According to the cited invention the width of the tunnel kiln is enlarged upto more than 10 m and single kiln production capacity up to O.lmtpa and more than 0.5mtpa using suitable product quality and 1050 to 1300°C baking temperature can be achieved. CN101497933 discloses a method for rapidly and directly reducing iron/hematite ore or limonites into ferrous powder. The hematite ore is crushed, washed and subjected to magnetic separation. The ore is mixed with coal dust, lime and calcium chloride (CaCI ₂ ) and binder. The mixture is pressed into blanks, dried and stacked on cars to send in the tunnel kiln. The blanks are subjected to water quenching to separate iron powder and slag automatically. CN101113488 discloses a method for comprehensive utilization of V-Ti- bearing iron ore concentrate to produce iron powder and vanadium pentaoxide compounds by using tunnel kiln reduction-grinding-separation. High temperature metal recovery process is disclosed in US2003047035, for particulate metal containing dust such as electric arc furnace dust wherein metallic values are recovered from the dust using a tunnel kiln. The mixture of metal containing dust alongwith carbonaceous fines is heated in a moving bed horizontal tunnel furnace to release volatile and alkali metals in gaseous product principally zinc and iron. The zinc and iron are separated from gases as the process product. A process and apparatus for producing direct reduced iron is disclosed in CN114708, wherein oxide of iron as raw material is mixed with bituminite, lime, coal and nutshell as an additive. The mixture is fed in to a material column reactor which is passed through externally heated tunnel kiln for 30 to 38 hours at about 950°C. The process is used for direct reduction of iron-containing raw material in solid state into metal iron. CN 1142541, discloses a coal based direct reduction process of iron containing material to produce sponge iron using a tunnel kiln. The mixture of iron ore, coal is fed to a metallic reactor and claimed in slope-type tunnel kiln to produce sponge iron in the metallic reactors.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to propose a method for direct reduction of oxidized chromite ore fines (0 to 4.0mm) composite agglomerates in a tunnel kiln to produce reduced chromite product/agglomerate to use in ferrochrome or charge chrome production.

Another object of the invention is to propose a method for direct reduction of oxidized chromite ore fines (0 to 4.0mm) composite agglomerates in a tunnel kiln to produce reduced chromite product/agglomerate to use in ferrochrome or charge chrome production, which uses oxidized chromite ore fines (0 to 4.0mm) obtained by heat treatment of one of ground run of mine chromite ore (0 to 4.0mm) and a beneficiated product of run of mine chromite ore (0 to 4.0mm). A further object of the invention is to propose a method for direct reduction of oxidized chromite ore fines (0 to 4.0mm) composite agglomerates in a tunnel kiln to produce reduce chromite product/agglomerate to use in ferrochrome or charge chrome production, which uses slag with improved slag chemistry for reduction of oxidized chromite ore fines.

A still further object of the invention is to propose a method for direct reduction of oxidized chromite ore fines (0 to 4.0mm) composite agglomerates in a tunnel kiln to produce reduce chromite product/agglomerate to use in ferrochrome or charge chrome production, in which the reduction process of composite agglomerates is carried- out in a tunnel kiln, using a carbonaceous agent layer on the surface of kiln cars in order to maintain a highly reducing atmosphere in the immediate vicinity of oxidized chromite ore fines composite agglomerates during reduction process in the tunnel kiln.

Yet further object of the invention is to propose a method for direct reduction of oxidized chromite ore fines (0 to 4.0mm) composite agglomerates in a tunnel kiln to produce reduce chromite product/agglomerate to use in ferrochrome or charge chrome production, in which a zone wise temperature profile (distribution) is maintained in the tunnel kiln across its length for efficient reduction of oxidized chromite ore fines composite agglomerates.

Another object of the present invention is to propose a method for direct reduction of oxidized chromite ore fines (0 to 4.0mm) composite agglomerates in a tunnel kiln to produce reduce chromite product/agglomerate to use in ferrochrome or charge chrome production, in which the an improved tunnel kiln for carrying out an efficient reduction of composite agglomerates made from oxidized chromite ore fines is used.

SUMMARY OF THE INVENTION

According to the present invention, a method is developed for direct reduction of oxidized chromite ore fines composite agglomerates in the tunnel kiln using carbonaceous reductant such as coal. The oxidized chromite ore fines are obtained by heat treatment of the chromite ore fines prior to its agglomeration. This heat treatment in order to obtain oxidized chromite ore fines, is carried out in a tunnel kiln or a rotary kiln at temperature of less than 1100°C with residence time in the range of 30 to 300 minutes in presence of air in order to oxidize the FeO present in chrome spinel to Fe-sequioxide (Fe ₂ 0 ₃ ). The Fe- sequioxide phase formed during oxidation process enables enhancing the subsequent reduction process of oxidized chromite ore fines composite agglomerates in the tunnel kiln. The oxidized chromite ore fines in the size range of 0 to 4.0 mm are mixed with carbonaceous reductant, quartz or quartzite and lime as the slag formers and bentonite as the binder. The reductant, slag formers and binder are added in predetermined ratio as per the designed slag chemistry to form composite agglomerates for achieving enhanced reduction of oxidized chromite ore fines and formation of low temperature slag during reduction. These composite agglomerates are dried and then fed to a reduction process in the tunnel kiln. In tunnel kiln reduction process, a layer of carbonaceous agent such as coal is placed on the surface of the kiln cars which allows maintaining a highly reducing atmosphere in the immediate vicinity of the composite agglomerates subjected to reduction process. The tunnel kiln used in the present invention is a closed type and air required for the combustion of the fuel combustion as well as the excess air required for post-combustion of the reaction gas typically CO, is supplied only in the firing/combustion zone through externally fired burners or additional air input below the burners. Thus, the operating principle of the tunnel kiln in the present invention is different than that of the conventional kilns for firing refractory bricks, where additional air is blown inside the kiln at entry, exit and immediately after the firing zone (i.e. rapid cooling) in order to maintain proper temperature profile as well as gas flow inside the kiln. One of the important features of the present invention is that the proposed tunnel kiln makes use of partially cooled tunnel kiln exhaust gas for cooling/quenching of the reduced product inside the tunnel kiln cooling zone, which achieves the cooling of the reduced product/agglomerate to less than 500°C by preventing reoxidation of the reduced product. This feature eliminates the need of quenching the reduced product in water outside the tunnel kiln and also prevents the disintegration of reduced product which usually occurs in water quenching process. The reduced product/agglomerates obtained from the tunnel kiln reduction process have high metallization of iron and chromium. These reduced chromite agglomerates can be used in ferrochrome or change chrome production at a lower energy consumption in ferrochrome or charge chrome production, and further < improves the productivity of the submerged arc furnaces.

The present invention results in production of reduced chromite product/ gglomerates with a high degree of iron and chromium metallization which can reduce the energy and coke consumption in ferrochrome or charge chrome production. The reduced chromite agglomerates are obtained by reduction of oxidized chromite ore fines composite agglomerates in a tunnel kiln.

According to the invention, the heat treatment of ground run of mine chromite ore fines is carried out in a tunnel kiln or a rotary kiln in presence of air and typically at temperature upto 1100°C to 30 to 300 minutes duration in order to break the chrome spinel in chromite ore fine particles due to oxidation and conversion of FeO present in chrome spinel to Fe-sequioxide lamellae on the surface'of chromite ore particles.

According to the invention, an improved slag chemistry is obtained by using proper mixture ratio of carbonaceous reductant, slag formers such as quartz or quartzite and calcined lime or hydrated lime in order to form low temperature slag for enhancing the reduction of oxidized chromite ore fines in tunnel kiln. The agglomerates made from proper mixture ratio of oxidized chromite ore fines, slag formers such as quartz or quartzite and lime alongwith bentonite binder are termed as composite agglomerates.

According to the invention, the tunnel kiln has a length of about 20m which is much less as compared to conventional refractory bricks application kiln which are usually greater than about 30m long. The new design of the kiln uses partially cooled tunnel kiln exhaust gas for quenching/ cooling of the reduced agglomerates in the cooling zone of the kiln such that the reduced product/agglomerates are discharged at less than 500°C temperatures from the tunnel kiln. This feature helps to achieve the cooling of the reduced product and prevent its re-oxidation. Use of partially cooled tunnel kiln exhaust gas for cooling also eliminates the need for water quenching of the reduced product outside the reduction equipment which is usually applied for cooling of direct reduction processes product.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

The invention is explained in greater details wit the accompanying drawings:

Figure 1 shows a typical size analysis of Mgo-rich chromite ore fines obtained either by girding run of mine chromite ore or by beneficiation of run of mine chromite ore

Figure 2 present a comparison of optical micrograph of chromite ore particles before and after oxidation in a tunnel kiln or a rotary kiln, the microstructure of chromite ore particles after oxidation exhibiting an exolved lamellae of Fe-sequioxide developed on the surface of chromite grains due to oxidation process. Figure 3 shows a process for reduction of composite agglomerates in the tunnel kiln emphasizing placing of the carbonaceous layer on the surface of trolley or car, and the physical state of the composite agglomerates before reduction in the tunnel kiln and after reduction in tunnel kiln.

Figure 4 shows a schematic diagram of tunnel kiln reduction process for reduction of composite agglomerates made from oxidized chromite ore fines consisting of typical temperature profile and residence/cycle time of composite chromite agglomerates in the tunnel kiln, which particularly shows the zone-wise temperature profile in the tunnel kiln across the length of the kiln, the cumulative residence time of the composite agglomerates placed on the cars for total reduction cycle in the tunnel kiln.

Figure 5 shows a schematic diagram of a tunnel kiln process showing a method for cooling reduced product/agglomerate inside a tunnel kiln for prevention of re-oxidation of the reduced product by using a part of the tunnel kiln exhaust gas cooled to 100 to 200°C before injecting it into the cooling zone of the tunnel kiln.

Figure 6 shows typical micrographs of reduced product/agglomerates (pellet and briquette which are typically pillow shape agglomerates) obtained by reduction of composite chromite agglomerates in a tunnel kiln reduction process, the micrograph exhibiting three phases namely reduced Fe-Cr metal, slag and partially reduced chromite particles.

Figure 7 shows a size analysis of FeO-rich chromite ore as an additional example of oxidized chromite ores used for reduction in the tunnel kiln.

Figure 8 shows optical micrograph of oxidized FeO-rich chromite ore particles, the micrograph exhibiting an exolved Fe ₂ 0 ₃ phase on the surface of oxidized FeO-rich chromite ore particle.

Figure 9 shows micrographs of reduced product/agglomerates obtained by reduction of oxidized FeO-rich chromite ore composite agglomerates in a tunnel kiln showing reduced metal (Fe-Cr metal), slag and partially reduced chromite partices. DETAILED DESCRIPTION OF THE INVENTION

Chromite ore fines (0 to 4.0mm) are typically obtained by grinding the run of mine chromite ore to fine size (less than 4.0mm). The typical particle size analsysis of Mgo-rich ore fines is shown in Figure 1. It can be seen that, the d80 (80% of particles having average diameter) of chromite particle is in the range of 80 to 90 pm. The oxidized chromite ore fines used in the present invention to form composite agglomerates were obtained by heat treatment of chromite ore fines (0 to 4.0mm) prior to its agglomeration. This heat treatment in order to obtained oxidized chromite ore fines, is typically carried out in tunnel kiln or rotary kiln at temperatures of less than 100°C for residence time in the range of 30 to 300 minutes typically in presence of air in order to oxidized the FeO present in chrome spinel to Fe-sequioxide (Fe ₂ 0 ₃ ). The typical optical micrograph of chromite ore particles before and after oxidation process is shown in Figure 2. The optical micrograph of chromite ore particles after oxidation process (Figure 2b) shows the Widmanstatten structure of bright lines on chromite mineral. It can be seen that, the Fe-sequioxide phase has exolved on the surface of chromite particles. During oxidization reaction, the FeO present in chrome spinel gets oxidized to Fe ₂ 0 ₃ as exolved Fe-sequioxide phase and in turn cation vacancies are generated in the spinel lattice. In an oxidizing atmosphere, in addition to the intrinsic driving force for phase transformation, the imposed oxygen chemical potential promotes the diffusion of Fe ²⁺ ions from the core of chromite grain towards the solid-gas interface on the surface of the grain. This outward diffusion of Fe ²⁺ cations and oxidization to Fe ³⁺ cations takes place via following reactions: - ^l 0 ₂ -^→0 ₀ ² - + V _aV+ 2.h' [1]

2Fe ²⁺ ^→ 2Fe ³⁺ +2.e ^' [2]

2Fe ²⁺ + 0 ₂ ^→ 2Fe ³⁺ + 0 ^~2 + V _cal [3]

Where 0 ° ^2~ represents oxygen anions on the cubic closed packed lattice, V ^cat is cation vacancy, " w is hole, and ^e is electron. Oxidation of chromite ore also results in opening up of the spinel structure. The optical micrograph of chromite ore particles after oxidation process is shown in Fig. 2b, which shows the Widmanstatten structure of bright lines on chromite mineral. It can be seen that, the Fe-sequioxide phase has exolved on the surface on the chromite particles. The EDX analysis of oxidized chromite particles has indicated that the bright lines are iron rich phases and the matrix is magnesium rich phase. It can be concluded from above that iron is precipitating from the matrix on {111} crystallographic planes. The oxidation of chromite resulting in Widmanstatten structure is in accordance with established theory of oxidation and precipitation of sequioxide phase which states that preferential crystallographic orientation of Widmanstatten lamellae was along the {111} plane of spinel matrix phase. The {111} planes of spinel and the {001} planes of sequioxide have a similar close packing arrangement of oxygen ions, which account for the common orientation of sequioxide lamellae along the {111} plane of the spinel matrix. Tapered terminals develop at the intersection of two or more lamellae, which are indicative of a diffusion-controlled process. This newly formed Fe-sequioxide phase on chromite particles along with the vacancies generated during oxidation improves the reactivity of chromite ore during reduction. This Fe-sequioxide phase formed during oxidation process helps in enhancing the subsequent reduction process of oxidized chromite ore fines composite agglomerates in tunnel kiln. The oxidized chromite ore fines in the size range of 0 to 4.0 m are mixed with powdered carbonaceous reductant such as coal, powdered quartz or quartzite and powdered lime such as calcined lime or hydrated lime as slag formers and bentonite as binder. The typical chemical composition of different raw materials such as coal (reductant), quartz, calined lime or hydrated lime (slag formers) and bentonite (binder) used in the present invention is shown in Table 1. The reductant, slag formers and binder are added in predetermined ratio as per the specially designed slag chemistry to form composite chromite agglomerates for achieving enhanced reduction of oxidized chromite ore fines and formation of low temperature slag during reduction. The typical chemical composition of composite agglomerates used in the tunnel kiln reduction process obtained after mixing and agglomerating predetermined ratio of raw materials in order to achieve special slag chemistry is also shown in Table 1. The formation of low temperature slag during reduction in tunnel kiln is achieved with the help of lowering the slag liquids due to use of lime as slag former and also due to reduction of Fe-sequioxide (Fe ₂ 0 ₃ ) phase in oxidized chromite ore fines formed during oxidation of FeO at low temperatures. It is worthwhile to mention that, the formation of Fe-sequioxide phase lamellae is uniform across the chromite grain surface due to selection of finer (0 to 4.0 mm) oxidized chromite ore fines which ensures that maximum iron present in oxidized ore is available as a separate the FeO has to come to travel to the particle surface and then reduce while is not an effective reduction process. Since the iron is present as separate exolved Fe-sequioxide (Fe ₂ 0 ₃ ) phase in oxidized chromite ore fines, it reduces quickly to FeO at low temperature during reduction. This newly formed FeO acts as slag forming agent and enhances the diffusion of iron and chromium ions in slag. One of the important features of the newly formed FeO is that it further reduces to Fe ₃ C (iron carbides) at low temperatures and the carbon present in iron carbides helps in reduction of chromium oxides thus further enhancing the reduction process. The composite agglomerates obtained from mixing and agglomerating the oxidized chromite ore fines, slag formers and binder can promote formation of spherical pellets or briguettes or lump or any other form of agglomerates. These composite agglomerates are then subjected drying process for 30 to 120 minutes in the temperature range of 100°C to 200°C. The drying process is carried out in continuously moving steel grate belt dryer which typically uses hot air from hot air generator as drying medium. The dried composite chromite agglomerates are then fed to reduction process in tunnel kiln. In tunnel kiln reduction process, a layer of carbonaceous agent such as coal is placed on the surface of the kiln cars which helps in maintaining highly reducing atmosphere in the immediate vicinity of the composite agglomerates subjected to reduction process. Figure 3a shows the method of placing the carbonaceous layer on the surface of tunnel kiln car and Table 1 shows typical chemical composition of the carbonaceous layer agent such as coal. The carbonaceous layer is sprayed/placed on the cars or surface of the trolley such that each car/trolley have atleast 100 g or upto 5000 g of additional coal on each car/trolley surface apart from coal present in the dried composite agglomerates. The dried composite agglomerates are then laid on the carbonaceous layer on the surface of car/trolley such that a typical thickness of agglomerate layer is about 30 to 40mm on the carbonaceous layer is ensured. The feeding of composite agglomerates on the surface of car or trolley is carried out using a vibrating hopper feeder arrangement supported with a leveler which ensures the desired thickness of composite agglomerates layer on the surface of each car. The composite agglomerates laid on a typically trolley of tunnel kiln car before subjecting to reduction process is shown in Figure 3b. The tunnel kiln used in the present invention is closed type and air required for fuel combustion as well as excess air required for post-combustion of reaction gas typically CO, is supplied only in the firing/combustion zone through externally fired burners or additional air input below the burners. Thus, the operating principle of the tunnel kiln in present invention is different unlike the conventional refractory brick application kilns where additional air is blown inside at entry, exit and immediately after the firing zone (i.e., raid cooling of product) in order to maintain proper temperature profile as well as gas flow inside the kiln.

The tunnel kiln used in the present invention has four major temperature zones across the length of tunnel kiln. These temperature zones starting from entry are drying, preheating, firing and cooling zone. The schematic of typical tunnel kiln reduction process used in the present invention is shown in figure 4. The typical temperatures maintained in the four zones of tunnel kiln during reduction process are, less than or equal to 800°C in drying zone, less than 1000°C in drying zone, less than 1000°C in preheating, less than or equal to 1500°C in firing zone and less than or equal to 500°C in cooling zone.

It is worthwhile to mention here that, the temperature in different zones or temperature distribution across the tunnel kiln length are only indicative and slight variations (±200°C) in temperatures can occur across the length of tunnel kiln. The tunnel kiln cars having composite agglomerates placed on it, travels through all the four temperature zones of tunnel kiln such that the total residence/ cycle time (in to out) of composite agglomerates is maintained typically of about 75 minutes within kiln however this residence or cycle time may not be sacrosanct and can vary depending on the properties of the composite agglomerates subjected to, reduction process using the disclosed method in present invention. The indicative cumulative residence time or cycle time distribution for composite agglomerates within kiln is also shown in Figure 4. The reduced product or agglomerates are discharged from tunnel kiln without its re-oxidation at temperatures less than 500°C while exiting from tunnel kiln. This fairly rapid quenching or cooling of the reduced product without re-oxidation is achieved using partially cooled tunnel kiln exhaust gas for rapid quenching or cooling of the reduced product in the cooling zone of tunnel kiln. One of the important features of the present invention is that the proposed tunnel kiln makes use of part of tunnel kiln exhaust gas which is cooled to about 100 to 200°C in gas cooling unit outside tunnel kiln. Figure 5 shows the schematic of typical tunnel kiln arrangement used for achieving the cooling of the reduced product or agglomerate to less than 500°C while exit from tunnel kiln. The tunnel kiln exhaust gas consists of C0 ₂ and N ₂ (nitrogen) as a major constituent since most of the CO generated during reduction reactions is burned inside the firing or combustion zone to convert to C0 ₂ by use of excess air. Therefore, the tunnel kiln exhaust gas containing C0 ₂ and N ₂ as major gas components is partly diverted to a gas cooling unit for cooling the tunnel kiln exhaust gas to about 100 to 200°C. This cooled gas is injected into cooling zone of tunnel furnace for rapid quenching or cooling of the reduced product/agglomerate arriving from the firing/reduction zone of tunnel kiln. The rapid quenching of the reduced product using the cooled tunnel kiln exhaust gas prevents the re-oxidation of the reduced product and also ensures rapid cooling of the reduced product to less than 500°C. The reduced product is cooled to less than 500°C within the cooling zone and cars are taken out of the kiln since the reduced chromite product or agglomerates does not re-oxidize due to exposure to atmospheric air at temperature below 500°C. This feature eliminates the need of quenching the reduced product in water outside the tunnel kiln and also prevents the disintegration of reduced product which usually occurs in water quenching process used in direct reduction processes. The reduced product or agglomerates typically obtained after reduction of composite agglomerates are shown in Figure 3b. The reduced product/agglomerates are then removed from the tunnel kiln cars using a physical scrapper or pneumatic hammer for further use of the reduced product in ferrochrome or charge chrome production. The reduced product/agglomerates obtained from tunnel kiln reduction process have high metallization of iron and chromium. The typical chemical composition of reduced product or agglomerates is given in Table 2. The % iron metallization (i.e. ratio of metallic iron to total iron in reduced agglomerate) in reduced product is upto 90% and % chromium metallization (i.e. ratio of metallic chromium to total chromium in reduced agglomerate) in reduced product obtained from the tunnel kiln is as high as 75%. Figure 6 shows the typical micrographs of reduced pellets and reduced briquettes obtained by reduction of composite pellets or briquettes in tunnel kiln. The micrographs shows three phases namely reduced Fe-Cr metal, slag and partially reduced chromite particles in the reduced agglomerates. These reduced chromite agglomerates can be used in ferrochrome or charge production which can result in low energy consumption in ferrochrome or charge chrome production and also improves the productivity of the submerged arc furnaces due to presence of highly metalized iron and chromium in it. The reduced chromite agglomerates can be used in ferrochrome/charge chrome production. The use of reduced agglomerates obtained in the present method, in ferrochrome production by submerged arc furnace process can result in low energy consumption in ferrochrome/charge chrome production and also improves the productivity of the submerged arc furnace. The reduced agglomerates obtained in present invention, can further result in increased chromium content in the ferrochrome/charge chrome product due to its effective smelting properties in submerged arc furnace. Therefore, a new method of direct reduction of oxidized chromite ore fines in tunnel kiln is developed in the present invention. As a additional example of the tunnel kiln reduction process, oxidized Feo-rich chromite ore (0 to 4.0mm) having typ'ical size analysis as shown in Figure 7 and chemical composition as given in Table 3 was also subjected to reduction process in tunnel kiln as described above. The optical micrograph of oxidized FeO-rich chromite ore used in the present invention is shown in Figure 8. It can be seen that, the FeO present in chrome spinel was oxidized to Fe ₂ 0 ₃ as exolved Fe-sequioxide phase which improves the reactivity of ore in subsequent reduction processes. Typical micrographs of reduced product/ agglomerates obtained by reduction of oxidized FeO-rich chromite ore composite agglomerates obtained by reduction of oxidized FeO-rich chromite ore composite agglomerates in tunnel kiln are shown in Figure 9 showing three phases namely reduced metal (Fe-Cr Metal), slag and partially reduced chromite particles. The reduction process of oxidized South African chromite ore composite agglomerates in tunnel kiln was highly effective, and high degree of iron and chromium metallization is achieved in reduction process. Table 1 Typical chemical composition of raw materials for reduction process

Table 2 Typical chemical composition of reduced agglomerates obtained from tunnel kiln reduction process

Table 3 Typical chemical composition of South African chromite ore