FIELD OF THE INVENTION:
The present invention relates to development of a process for
production of chromium Nuggets. More particularly the present
invention relates to development of a process for production of chromium nuggets by low temperature reduction of preoxidized chromites ore or chromite concentrates.
BACKGROUND OF THE INVENTION:
In any integrated metal alloy manufacturing organization, high carbon Ferro chromium is commonly produced by smelting-reduction route. This is highly energy intensive and requires an imported low-ash coke as a reducing agent. Low-ash coke and electricity are both expensive resources. Therefore, a novel process route is developed by reduction of pre-oxidized chromite ore or chromite cocentrates using a coal as reductant for producing a product with 50-64% metallic chromium in the form of Ferro chromium or chromium nuggets.

PRIOR ART
There are five primary processes that are currently in use for the production of ferrochrome. These include conventional process with open or semi-closed 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 of ferrochrome production, a mixture of chrome ore, reductants and flux is fed cold with minimum pre-processing 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 ferrochrome 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 ferrochrome metal and slag are then tapped for further processing. 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 chromium metal to slag upto 10% by weight. Outokumpu process: Fine chrome ore is wet milled and then pelletized using a binder such as bentonite. The pellets are then sintered and then air cooled and stockpiled. The pellets together with fluxes and reductants may be heated in a pre-heater located above the furnace. 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 advantage 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 or double solid or hollow 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 coal 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, pelletized 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 to produce ferrochrome metal and slag. 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 prior art processes developed in the past mainly included pre¬reduction or preheating of agglomerates in rotary kilns or fluidized bed followed by smelt reduction of the preheated or pre-reduced charge in submerged arc furnace or converter type furnace. For example, an improved process for reduction of iron-containing chrome ores is disclosed in patent application number US4772316, wherein mixture of chrome ore, coal and slag former (quartz sand) is reduced in a rotary furnace in CO containing atmosphere for 8 hours and the reduced product obtained from the rotary furnace is subsequently

melted to produce Ferrochrome. A reduction level of about 95% was achieved for iron and chromium in rotary furnace by heating the mixture in different zones of rotary furnace. The process was claimed to have low coal consumption and eliminated the variations in reduction level in rotary furnace. Coals containing high volatile matter (VM >20.0%) were introduced into the rotary furnace only through the outer nozzle in the burner. The coal introduced through the burner ensures continuous layer of reducing or inert gas on the furnace charge thereby prevents the reoxidation of the reduced product. In patent application number US4981510, a process for producing ferrochrome with carbon content in the range of 0.02 to 10 % is disclosed wherein the iron containing chrome ore is mixed with coal in 1:0.4 to 1:2 proportion and slag former containing CaO, MgO or Al203 and Si02 such as limestone and dolomite. The reduction process is carried out in rotary furnace for 20 to 240 minutes in CO containing atmosphere at temperature in the range of 1480 °C to 1580 °C. The reduction product is then mixed with additive in a water cooled rotary drum in order to make use of the heat loss from reduced product to the additive material and cool the entire reduced product. Since the mixing and cooling process takes place in rotary drum where continuous mixing action is present, the formation of large agglomerates of reduced product is prevented. The mixture of reduced product and additive (dolomite or limestone) is cooled to 600 °C to

1000 °C and hot charged in melting furnace, wherein 1600 to 1700 °C temperature and basicity (CaO+MgO/Si02) is maintained greater than 1.1 in order to obtain the ferrochrome product. JP58022356 discloses a method of reduction of chrome ore in rotary kiln wherein mixture of chrome ore, slag forming agent and carbonaceous agent are reduced in rotary kiln which is provided with lining of magnesia type basic refractory in order to avoid the wear of refractories. The coal is burnt by blowing oxygen to melt and reduce the ore. A pre¬reduction and melt reduction method is disclosed in patent application number JP59113158, wherein chrome ore is pre-reduced in rotary kiln and 90% of iron oxide in the ore is reduced. The pre-reduced material is charged into top and bottom blown converter type melting and reducing furnace and the method is claimed to be advantageous over conventional electric arc furnace. In patent application number JP59085840, a tilted apparatus essentially composed of rotary furnace is disclosed wherein the preheated chrome ore, reducing agent and slag forming agents are smelt-reduced to produce ferrochrome. The exhaust gases from the tilted apparatus are used to preheat the chrome ore in rotary kiln. In patent application number DE3713883, a process for producing ferrochrome by thermal reduction of pellets in rotary kiln and subsequent smelting of the reduced pellets in melting furnace is disclosed wherein the iron containing chrome ore with coal and pelletizing agent starch, Si02 and a base especially NaOH are

prepared and reduced in rotary kiln for two to five hours at 1200 °C to 1400 °C to carry out the pre-reduction of chrome ore. The pre-reduced product is then hot charged into a melting furnace at 1600 °C to 1700 °C to produce ferrochrome. A process to produce ferrochromium from chrome ore is disclosed in patent application number US4629506, wherein mixture of chrome ore, coal and slag forming constituents are reduced in rotary kiln at 1480 °C to 1580 °C in CO containing atmosphere for 20 to 240 minutes. The Si02 is added to rotary kiln only when the mixture reaches a temperature of 1200 °C. The pre-reduced ore obtained is cooled to less than 700 °C, crushed to less than 25 mm size and separated by density or magnetic separation in to coal containing fraction to recycle to rotary kiln and other fraction containing slag and metal crushed to less than 0.5mm size and melted in a melting furnace at 1700 °C by blowing oxygen and coal to produce ferrochromium. The process minimizes the gangue content to be smelted in melting furnace. A method for preheating and pre-reduciton of chrome ore is disclosed in patent application number JP63216910, wherein pre-heated chrome ore is pre-reduced in fluidized bed furnace by using coal. The char obtained after reduction is separated from pre-reduced ore, crushed and used to preheat the chrome ore by burning the char outside fluidized bed furnace. The ash obtained during preheating of chrome ore is separated from pre-heated ore and pre¬heated ore is fed to fluidized bed furnace for pre-reduction. In patent

application JP63216934, a process for reduction of chromium ore is disclosed wherein powdered chrome ore and high temperature char are charged to preheating fluidized bed furnace where oxygen or air and high temperature reducing gas obtained from another reducing fluidized furnace are fed. The preheated chrome ore obtained is charged to reducing fluidized furnace with additional coal for carrying out reduction of ore. Preheater is used for reduction furnace to heat the reduction gas and the char obtained from the reduction furnace is separated in a separator. The reduced ore with small quantity of char is melted in a melting reduction furnace and the separated char is used as fuel in the preheating fluidized bed furnace. The process is claimed to be useful in recovering the heat in the process and also to eliminate the need for peptization of chrome ore. JP57032351 discloses a method for enhanced pre-reduction of chromium and iron oxide in chrome ore to carbides in solid phase by heating ore with liquid or solid reducing agent in fluidized bed reactor. The internal temperature in fluidized bed is maintained in the range of 1000 °C to 1300 °C to convert chromium oxide in ore to Cr4C and FeO in ore to Fe3C. The reducing gas CO and H2 along with inert N2 is blown in fluidized bed furnace from bottom and heavy oil or solid carbon reducing agent such as slack coal is fed from the top. The patent claims to have produced product in the form of carbides of iron and chromium wherein the preliminary reduction of 50 to 60% is achieved.

In patent application number JP59113131, a method for refining of ferrochromium and effective use of slag formed is disclosed wherein the chrome ore powder is reduced in solid state in rotary kiln using coal or coke reductant and subsequently melt-reduced by coal in converter type melting furnace. The slag obtained after melt-reduction is separated from alloy, disintegrated, pulverized and magnetically screened so that very low chromium (0.6%) containing slag is used as soil-reforming agent and or additive to cement. A process for producing high carbon chromium iron is disclosed in patent application number CN1673403 wherein chrome ore powder, iron ore powder, coke or coal powder, lime & silica are mixed, pressed into blocks or pellets and then reduced at 1350 °C to 1500 °C to produced reduced product which is pulverized naturally during the cooling process. The product is further separated by sieving to obtain granulized ferrochrome product. A method of reducing metal oxide in rotary hearth furnace is disclosed in patent application number US5567224, wherein a layer of mixture made from pulverized metal oxide and pulverized reductant is placed on the hearth surface and heated up to 1200 °C by radiation heat from an oxidizing flame generated from stationary burner by burning a fuel. A second layer of reductant is placed on the preheated prereduced layer of mixture to prevent the reoxidation of the metalized product and heated further to desired reduction temperature of 1350 °C for further reduction of iron and

chromium oxides. The total time required for reduction is about one hour and the partially reduced mixture containing Chromium and iron metal units can be used in manufacturing of alloyed iron, alloyed steel or alloyed stainless steel. A method for producing ferrochrome is disclosed in patent application number JP58016053, wherein mixture of chrome ore, reducing agent and fluxes in the form of pellets or briquettes is preheated and prereduced in a rotary furnace such as rotary kiln or shaft furnace or fluidized bed at temperature of 1800 °C. The preheated, prereduced agglomerates are then melted in horizontal axis rotating furnace by addition of coke and oxygen enriched gas is blown at the bottom of the furnace to produce ferrochrome. The reducing gases generated in melt furnace are used as heat source to carry out preheating and prereduction in rotary kiln. The process claims to have 25% less energy consumption compared to the electric smelting operation.
In prior art technologies mentioned above, the advantage was seen in reducing the cost of electrical energy in ferrochrome production. Chromium recoveries were improved and cost of smelting was reduced by the preheating or pre-reduction methods. Optimal smelting furnace operation was achieved by the right charge mix, depending on the raw materials. 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, none of the above processes (pre-heating, pre-reduction) used the pre-reduction process based on oxidized chromite ores which are obtained typically by pretreatment of chromite ore 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 composite agglomerates. The smelt reduction methods based on the smelting of chrome ore in converter type furnaces need oxygen during the smelting processes which are reasonably costly methods. The prior art technologies were also designed for processing of FeO-rich South African chromite ores for producing the charge chrome with relatively lower chromium content, varying between 50 to 55 percent. On the other hand, the MgO-rich Indian chromite
Considering all the above points and suitability of present state-of-the-art technologies for processing chromite ores a novel process is developed in the present invention to produce metallic chrome nuggets having metallic chromium in the range of 50 to 64% by low temperature reduction of pre-oxidized chromite ore or chromite

concentrates. The developed process will require less energy and coke compared to conventional Ferrochrome production processes due to enhanced reduction of pre-oxidized chromite ore or chromite concentrates and use of coal reductant for reduction.

OBJECTS OF THE INVENTION:
It is therefore an object of the present invention to propose a process for production of chromium nuggets by low temperature reduction of pre-oxidized chromites ores or chromite concentrates which eliminates the disadvantages of prior Art.
Another object of the present invention is to propose a process for production of chromium nuggets by low temperature reduction of pre-oxidized chromites ores or chromite concentrates which saves energy.
A further object of the present invention is to propose a process for production of chromium nuggets by low temperature reduction of pre-oxidized chromites ores or chromite concentrates which reduces the cost of production of ferro chrome by 20%.
A still further object of the present invention is to propose a process for production of chromium nuggets by low temperature reduction of pre-oxidized chromites ores or chromite concentrates which reduces coke consumption.
An yet further object of the present inventions is to propose a process for production of chromium nuggets by low temperature reduction of

pre-oxidized chromites ores or chromite concentrates which possess a better reaction surface during further processing in steel melting.
A still another object of the present invention is to propose a process for production of chromium nuggets by low temperature reduction of pre-oxidized chromites ores or chromite concentrates which can be exportable to be used directly in stainless steel manufacturing.
SUMMARY OF THE INVENTION:
The chromites ores or chromite concentrates with Cr: Fe ratio ranging from 1-0 to 3.3 were oxidized at low temperatures (900° C).(The oxidation process was carried out in a oxidation unit in presence of air at temperature of about 900 °CJThe typical examples of oxidation unit can be fluidized bed reactor or multiple hearth furnaces. During oxidation process, the FeO present in the chrome spinel gets oxidized to Fe203 on the surface of chromite grains. The optical micrograph of chromite ore particles before and after oxidation process in an

oxidation unit is shown in Fig. 4. It can be seen from Fig. 4b that, the exolved lamellae of Fe203 appears on the surface of chromite grains which are result of oxidation of FeO in chrome spinel to Fe203/ which occurs on the surface of chromite grains. This oxidation process improves the reactivity of chromite ore during subsequent reduction reaction. It can be seen from Fig. 4b that, the oxidized chromite ore samples show complete oxidation of FeO phase to Fe203. The oxidized chromite ores or chromite concentrates obtained after the oxidation process were mixed with coal, fluxes such as lime and silica and bentonite binder. The mixture was then agglomerated to form pellets using a pelletizer equipment. The pellets obtained were then subjected to reduction process in a reduction unit. The reduction experiments were carried out using the controlled atmosphere high temperature furnace. The typical example of reduction unit can be rotary hearth furnace. The raw materials used along with there composition to prepare pellets for subjecting to reduction process is given in the following tables. All the percentages are expressed by weight.

Table 1 Raw materials and their Composition (wt%)

The experimental studies for reduction were carried out at low temperatures of 1400-J.550°C. The coal redundant is used in 30-50% excess of the stichoimetric carbon requirement for reduction of iron oxide (Fe203) and chrome oxides (Cr203) in the ore. Based on the special slag designed, the flux addition was carried out as quartz addition in the range of 0-10% excess of required for aluminium oxides and magnesium oxide dissolution to slag. The lime addition was done in the rage of 3-10% of the chromite ore or chromite concentrates. Reduction was carried out in the temperature range of

1400-1550°C for 1.5-3.0 hours. The chromite nuggets product is shown in fig 1.
The representative sample microstructure of the product alongwith the phase composition is shown in Fig 2. It can be seen that the metal shows two phases one chromium rich and the other iron rich phase. The presence of chromium in the nugget product is in form of chromium carbides (C7C3) and iron chromium carbides
The Chemistry of the chromium metal, nugget and slag products produced is given in Table 2. The metal nugget diameter ranges from 0.5-25 cm. The metal and slag phase separation is clear which after quenching in water can be separated by physical separation methods.
Table 2 Chemistry of Chromium metal nugget and slag (All percentages given are percent by weight)


Reaction mechanisms:
The oxidation of FeO in the chromite ore or chromite concentrates
opens the spinel structure which increases the reactivity of chromite
spinel due to formation of vacancies.
The oxidation of the chromite ores also helps in reducing the reduction time. The reduction mechanism of chromite ores or chromite concentrates in absence of lime as flux, generally proceeds as per the following steps. Chromium oxide reacts at 1200 to 1600°C with carbon to form one of the carbides Cr3Cr2, Cr7C3. 3Cr203+ 13C-+2Cr3C2 + 9CO (1150-1200°C) ^ 27CR3C + 5Cr O -> 13Cr C + 15 CO (1200-1600°C) ^
At still higher temperature Cr7C3 reacts with Cr23C6 and finally above 1820 C the Cr metal is favored product. However due to use of lime as fluxing above component the slag formation reactions plays an important role in carrying out the reduction of chrome oxide at lower temperatures. In presence of lime as fluxing component the slag formation reactions take placerat lower temperatures which promotes the reduction by dissolution in slag.

DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWING:
Fig 1: Shows a photo-view of chromium nugget produced after reduction of oxidized chromite ore or chromite concentrates.
Fig 2: Shows a micro-structure of the chromium metal nugget.
Fig 3: Shows a process flow sheet for production of chromium nuggets.
Fig. 4 Shows the optical micrograph of chromite ore particle before and after oxidation process in a oxidation unit. The optical micrograph after oxidation process shows the exolved Fe203 lamealle on the surface of chromite grains indicating oxidation of FeO in chrome spinel to Fe203 phase.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION:
Fig 3- shows a Process flow sheet for the commercial production of the chromium nuggets. The oxidation of the chromite ore or chromite concentrates (1) will be carried out in fluidized bed (2) multiple hearth

furnace or MHF (2) and a hot air (3) blow means also provided there. The oxidized chromite ore or chromite concentrates are fed into storage bin (4), a bin of reductant such as coal (5) and a bin of flux such as silica and lime (6) are provided near the oxidation unit.
The oxidized chromite ores or chromite concentrates are then mixed in a mixer (7) and after mixing with the coal reductant and flux (silica, lime) is fed to a pelletizer (8). The pellets are then fed to a rotary hearth the furnace (9) for carrying out the reduction in it. The metal and slag product obtained from the rotary hearth furnace (9) is fed to a physical separation unit (11) for the separation of chromium metal nuggets (12) and slag (13).

WE CLAIM:
1. A process for production of chromium nuggets by low temperature reduction of preoxidized chromite ore or chromite concentrates comprises:
- oxidizing of chromite ore or chromite concentrate Cr: Fe ration ranging from 1.0 to 3.3 in a furnace at a low temperature of 900°C;
- mixing oxidize chromite ore or chromite concentrate with reductant coal and flux, lime and silica in a mixture (7);
- pelletizing the mix obtained from mixer (7) in a pelletizer (8);
- reducing the pellets in rotary hearth furnace (9) at a temperature 1400°-1550°C to produce chromium nugget and slag;
- separating the chromium nuggets from the slag in a separation unit (11); and
characterized that said process produces a chromium metal nugget having a diameter ranges from 0.5-2.5 cms including a chemical composition by weight%:
Cr 50-64
C 3.0-6.0

Si 0.7-1.0
S 0.01-0.03
P 0.003-0.04
2. The process as claimed in claim 1, wherein metallization of chrome ore is 50-70% is obtained.

ABSTRACT

PROCESS FOR THE PRODUCTION OF CHROMIUM METAL NUGGETS FROM CHROMITE ORES OR CHROMITE
CONCENTRATES
The present invention is provided with a process for production of chromium nuggets by low temperature reduction of preoxidized chromite ore or chromite concentrates comprises oxidizing of chromite ore or chromite concentrate Cr: Fe ration ranging from 1.0 to 3.3 in a furnace at a low temperature of 900°C; mixing oxidize chromite ore or chromite concentrate with reductant coal and flux, lime and silica in a mixture (7); pelletizing the mix obtained from mixer (7) in a pelletizer (8); reducing the pellets in rotary hearth furnace (9) at a temperature 1400°-1550°C to produce chromium nugget and slag; separating the chromium nuggets from the slag in a separation unit (11); and characterized that said process produces a chromium metal nugget having a diameter ranges from 0.5-2.5 cms including a chemical composition by weight% of Cr 50-60, C 3.0-6.0, Si 0.7-1.0, S 0.01-0.03, P 0.003-0.04.