An improved process for production of high carbon ferrochrome (HCFeCr) and charge chrome with the use of a new type of chromite ore agglomerates.

FIELD OF INVENTION

The invention relates to an improved process for production of High Carbon Ferrochrome (HCFeCr) and Charge Chrome with the use of a new type of chromite ore agglomerates. More particularly it relates to an improved process for production of briquettes from chrome ore fines and concentrates.

BACKGROUND OF INVENTION

The widely applied route for manufacturing HCFeCr/Charge Chrome has always remained the carbotheπnic reduction (smelting) of chromite ores in electrically operated Submerged Arc Furnaces (SAF). For easier operation and lower capital cost, these SAFs were always made as semi-closed or open at the top. But in such furnaces, the carbon monoxide and other gases generated during the smelting process, always escape through the open part at the top, even with most efficient designs of the gas cleaning plants. Therefore, with increasing stringencies on environment friendly operations, the designs of the SAFs were changed to closed top types. These furnaces were always top-charged, and tapping is done at predetermined intervals from the spouts of the furnaces at the bottom. The charge descends and finally reaches the hottest zones at the tips of the three (3) electrodes where the electrical arcs are formed. This is a continuous process, and the hot gases from the arc-zone ascend through the porosities through the descending charge.

In open and semi-closed furnaces, it is easier for the ascending gases to pass through the charge due to lower furnace pressure. But in closed furnaces, the charge porosity plays a very important role to ensure free or unhindered passage of ascending gases to avoid likely cases of explosions. Therefore, in case of closed SAFs, it is mandatory to use a close range of size of the charge instead of a wide size range that would block the inter-particle voids and restrict the passage of gases. Thus, there is a limitation on the maximum percentage of fine ores to be included in the furnace charge. But generation of fines is an obvious outcome of mining operations, and it is extremely expensive to discard the ore fines since these fines also contain chromium metal in oxide forms. Moreover dumping of such fines on continuous basis will also need huge space. Therefore, with increasing use of closed SAFs, it was required to develop processes for agglomeration of the ore fines and concentrates for enabling their usage in closed SAFs. Two such agglomeration processes which became popular are (1) Sintered Pellets, a technology developed by Outokumpu, Finland and (2) Briquetting with lime and molasses as binders, developed on need based approach.

Extensive test work has been conducted for both the options of agglomeration mentioned above, with Indian chrome ore fines and concentrates and the results were not found satisfactory with the conventional modules of the two processes of the agglomerates, hot strengths in particular were the concerns.

Thus there was a necessity to evolve a process of agglomeration of Indian chromites to make agglomerates of adequate hot and cold properties suitable for smelting in closed furnaces, without sacrificing the benefits of closed furnace smelting for production of HCFeCr/Charge Chrome. To cover such a necessity the present work was taken up and the data generated have established that the new process of briquetting with the use of bentonite, a new binder, in addition to lime and molasses, binders conventionally used, can successfully produce HCFeCr/Charge Chrome in closed SAFs.

In the conventional method the Production of HCFeCr /Charge chrome is carried out in two major steps. They are: 1. Agglomeration of fine chromite ore and chrome concentrates

2. Smelting of chrome ore agglomerates (in combination with lumpy chrome ore) in Submerged Arc Furnaces (SAF) to produce HCFeCr / Charge Chrome

Various processes are available for agglomeration, e.g., sintered pellets, briquettes and chrome ore sinters. Different processes of briquetting are also available as characterized by the type of binders used for briquetting. Different designs of SAFs are available, namely open AC SAF, semi-closed SAF and closed SAF. Besides AC furnaces, DC operated smelting furnaces are also in use for commercial production of ferrochrome.

Following options are available in terms of the routes for agglomeration and smelting:

1. Agglomeration processes viz., sintered pellets, briquettes and chrome ore sinters

2. Rotary furnace based pre- reduction of chrome ore

3. Open AC SAF / semi closed / closed AC SAF

4. DC Plasma Arc Smelting Furnace Different Forms of Agglomerates

Outokumpu Technology

In South Africa, the most popular and widely accepted form of chrome ore agglomerates is sintered pellets manufactured by the process developed by Outokumpu Engineering and Contracting (OEC), Finland. OEC is known to be a pioneer in the field of agglomeration of chrome ore fines. Besides South Africa, Outokumpu also have their own plant for large scale manufacturing of charge chrome in Tornio, Finland, where they are using sintered pellets manufactured according to their technology.

The process has four (4) distinct stages, namely,

1. GRINDING

2. FILTRATION

3. PELLETISATION

4. SINTERING

The raw ore fines/concentrates are ground to fine size by wet grinding in ball mills. The wet slurry is pumped to special filters for dewatering. The dewatered cake is then mixed with bentonite and pelletised in drum pelletiser. The green pellets thus formed, are finally sintered in steel belt sintering furnace. The product is called the sintered pellets which can be charged into the SAFs for smelting to produce HCFeCr/Charge Chrome.

The process is very widely used and working satisfactorily in Finland and South Africa. The advantages are lower power consumption, stable furnace operation, higher chromium recovery and higher plant availability, when compared to conventional ferrochrome smelting processes. However, the process has been so far used for smelting chromites of Finland and South Africa origin. The two rounds of tests carried out by TSKZN in the Pilot Plant of Outokumpu in Pori in Finland on the suitability of production of sintered pellets from Sukinda chromites suggested that several modifications of plant design and process features would be necessary to produce sintered pellets successfully from Indian chromite ores. These modifications along with the driving factors are presented in the Table below.

In addition to the modifications, the cost of the technology and the plant as offered by Outokumpu is unfavorably high, pushing up the cost of production on the high side. Briquetting Processes

Briquettes bonded with hydrated lime and molasses are also in wide use as far as production of HCFeCr is concerned in India. Brquetting process is relatively simple in terms of design, engineering and operation, when compared to sintered pellet making operation by Outokumpu process. Briquetting Plants are also relatively inexpensive.

The raw materials for briquettes are chrome ore fines and concentrates together with suitable binding materials. The chrome ore is screened and dried. After adding binders and water, the mixture is mixed homogeneously in mixers. The discharge from the mixer is fed into a briquetting press and compacted into briquettes. Molasses, hydrated lime, sodium silicate, slag, cement and silica fumes are the common binders used in briquetting.

It needs to be mentioned here that although briquetting is a popular agglomeration process for the Indian chrome producers, they are all semi- closed furnace plants. TSKZN, therefore, conducted well-planned test work for optimization of briquetting recipe, as well as smelting operation in closed SAF. Open / Semi Cloed / Closed AC SAF

The submerged arc furnaces include three-phase AC operation with three SOderberg electrodes and energy generation by electric resistance heating. Close control of the charge composition and its sizing, as well as the slag composition are essential for effective operation of the submerged arc furnaces.

There are three types of Submerged Arc Furnaces, namely, (i) open furnace, (ii) semi-closed furnace, and (iii) closed furnace. Open and semi-closed furnaces being open at the top, emit a large part of the exit gas from the furnace, into the atmosphere. These furnaces can no longer be considered acceptable due to increasingly stringent environmental norms. For the closed SAFs, 100% furnace off-gas is sucked through the ducts and cleaned in the wet gas cleaning plants by scrubbers, before the gas is let out in the atmosphere, and are thus more environment friendly.

Besides they also offer several advantages like lower specific power consumption, higher chrome yield if operated with properly agglomerated feeds. Moreover they also offer the potential of co-generation of power, which also attracts additional earnings through 'carbon credits' which is becoming more and more popular globally. Various Smelting Processes

Prereduction based Process

In the Showa Denko process (SRC process), chrome ore fines are milled in a rod mill, pelletised with coke as reductant, dried in a traveling grate kiln, and fired in a rotary kiln to approximately 1400 ⁰ C. The kiln is heated by a pulverized coal/CO/oil burner. The pellets achieve approximately 60% metallization of chromium and about 90% metallization of iron. Reduced pellets discharged from the rotary kiln are stored in a completely sealed surge hopper designed to prevent re-oxidation. Reduced hot pellets are discharged from the surge hopper and supplied to the electric furnace after being combined with coke and flux through a feed bin.

This process of smelting can lower the specific power consumption by approximately 30-35%. But the shortcomings are high capital investment, accretion in kiln, poor plant availability and very high plant maintenance cost, which offset the advantages otherwise received from pre-reduction. Moreover the risk of re-oxidation is also a critical issue, as that can neutralize the benefits of prereduction to a large extent. There were only two plants in the world and both these plants are reported to have closed down.

Sintered Pellet based Process

Sintered pellets (by Outokumpu process) are mostly smelted in closed furnace to produce charge chrome / HCFeCr. This process has established itself in terms of excellent plant operation in Finland as well as South Africa. Several advantages including lower power consumption, higher chromium recovery, higher plant availability etc. have made this process highly popular. The cost of this plant is significantly high and is considered as a disadvantage.

DC Plasma Arc Smelting Furnace

The DC arc furnace includes a single central hollow graphite electrode. It is of closed top design. Unlike the Submerged Arc Furnaces, the arcs are open. It operates with ore fines/concentrates without prior agglomeration and lower grade low cost reductants like coal, char and anthracites can be used. For a given grade of input ores, this process is also known to present (i) higher %Cr (chromium) in product, (ii) higher Cr recovery, (iii) lower Phosphorus and Silicon in product, and (iv) lower slag/metal ratio. The most significant advantage of DC Plasma process route is the complete elimination of agglomeration plant, and potential for using low cost reductants. But, due to the open bath condition, the specific power consumption for DC Plasma Furnace is higher than AC SAF. Further, the operating process is complicated and, therefore, the availability of skilled manpower to operate DC furnaces is quite restricted. For these disadvantages associated with the DC process, the total number of operating installations have not gone beyond two. Both these installations are working in South Africa.

Preheating

The specific consumption of electric power for ferrochromium smelting can be reduced by preheating the feed materials. This is possible only for smelting furnaces of closed top design. Thus preheaters can be located at the top of the closed AC SAFs and DC Smelting Furnaces, where the charge is preheated up to a temperature of 600 - 700 ⁰ C with the help of the exit gas from the closed furnace, rich in carbon monoxide.

OBJECT OF INVENTION An object of this invention ^" is to provide a process for agglomeration of chromite ore fines and concentrates in all possible proportions to produce agglomerates of satisfactorily high hot strength to ensure minimum generation of fines during smelting in closed SAFs.

A further object of this invention is to provide a process for agglomeration of chromite ore fines and concentrates in all possible proportions to produce agglomerates of satisfactory cold strength to ensure minimum generation of fines during handling and storage of briquettes.

A still further object of this invention is to provide a process for agglomeration of chromite ore fines and concentrates in all possible proportions, relatively simple in terms of design, engineering and operation, when compared to conventional sintered pellet making process.

A still further object of this invention is to provide a process for agglomeration of chromite ore fines and concentrates in all possible proportions, with bentonite with or without swelling, in addition to the two other binders, namely molasses and hydrated lime.

A still further object of this invention is to provide a process for agglomeration of chromite ore fines and concentrates in all possible proportions, with or without heat hardening of the briquettes.

A still further object of this invention is to provide a clean process for agglomeration of chromite ore fines and concentrates in all possible proportions to produce agglomerates without any need for high temperature curing, thus ensuring minimal generation of hexavalent chromium (Cr ⁺⁶ ).

A further object of this invention is to provide a clean process for smelting of briquettes of chromite ore fines and concentrates in all possible proportions, in combination with lumpy ore in all possible proportions, where applicable environmental norms can be achieved.

Yet another object of this invention is to provide a process for ensuring stable smelting of agglomerates made from chromite ore fines and concentrates in all possible proportions in closed SAFs without any untoward incidents.

A further object of this invention is to provide a process for smelting of chrome ore agglomerates made from chromite ore fines and concentrates in all possible proportions in semi-closed or open submerged are furnaces satisfactorily, in addition to the closed submerged arc furnaces. A further object of this invention is to provide a process for smelting of chrome ore agglomerates made from chromite ore fines and concentrates in all possible proportions in closed SAFs to produce HCFeCr/Charge Chrome with the operating parameters like chromium recovery, chromium in metal, slag/metal ratio, % Cr ₂ O ₃ in slag, overall metal chemistry and specific electric power consumption, comparable with the conventional sintered pellet based operation.

Yet another object of this invention is to provide a process for smelting of chrome ore agglomerates made from chromite ore fines and concentrates in all possible proportions in closed SAFs to produce HCFeCr/Charge Chrome, acceptable to the designers and suppliers of closed SAFs of international reputes.

A further object of this invention is to provide a process for agglomeration of chromite ore fines and concentrates in all possible proportions with a capital cost much lower than the conventional sintered pellets, resulting in lower cost of production.

A further object of this invention is to provide a process for agglomeration of chromite ore fines and concentrates in all possible proportions to produce agglomerates those can be used in any type of Charge Preheating Kiln fitted with a closed submerged arc furnace.

DESCRIPTION OF THE INVENTION

Briquetting (Stage T)

Friable chrome ore and concentrate was taken for briquetting and to increase the hot strengths of the briquettes, it was decided to include bentonite in the test work as a binder in addition to molasses and hydrated lime conventionally used for briquetting.

Friable ore and concentrate were mixed in the required proportions and dried to give a moisture content of 0.2-0.5%. After drying, bentonite followed by molasses and then water and lime (when required), were added in the required amounts. Moisture was maintained below a certain limit in the feed for all the tests. The mix prepared in this manner was then sent for briquetting and during briquetting, the briquetting force as well as the speed of the briquetting rolls were varied. In all cases, the acceptable portion of briquettes produced was subjected to shatter/compression tests: (a) immediately after manufacture, (b) following curing for 24 hrs at normal room temperature conditions, (c) after exposure to heat hardening at 800 ⁰ C and 1000 ⁰ C for 1 hr; in some special tests, the exposure time was changed to 90/105 mins. Thermo-stability tests were conducted on good quality briquettes from different feed combinations by exposing the briquettes (covered with a layer of coke breeze, but with no weight on the briquettes) to 400, 600, 800, 1000, 1200 and 1350°c for half an hour, and immediately assessing their compressive strength values at that temperature (approx. 30- 5O ⁰ C drop in temperature occurred when the samples were taken out of the furnace and before the compressive strength could be measured). The complete process of briquetting is shown in Fig 1.

Results

The blends tried were: (a) 70% friable ore + 30% concentrate, (b) 50% friable ore + 50% concentrate, (c) 30% friable ore + 70% concentrate, and (d) 100% concentrate alone.

The results obtained indicated the following:

• All proportions of friable ore and concentrate can be briquetted.

• Briquetting was possible with a reasonably low force. Higher pressures can be used without any detrimental effects on the briquette quality. • The optimum speed of the rolls for good quality briquettes was established.

• The binder combination in terms of lime, molasses and bentonite was optimised.

• Curing the green briquettes for 24 hrs increased the compressive strength of the briquettes. However, no perceptible change occurred as far as the shatter strength was concerned.

• Swelling of the bentonite before making the briquettes (as suggested by Koppern) did not show any positive impact.

• Heat-hardening reduced the shatter strength of the cured briquettes, but it had a positive impact on the compressive strength, as the temperature and holding time were increased.

• For all the combinations tried, the shatter strength values of the briquettes on a comparative scale were as follows:

1. Green shatter strength (% + 20 mm immediately after making) : 1.00

2. Shatter strength after 24 hr curing at room temperature : 1.05

3. Shatter strength after heat hardening: 0.90

• The corresponding compressive strengths on a comparative scale were:

1. Green compressive strength : 1.00 2. Compressive strength after 24 hr curing at room temperature : 1.75

3. Compressive strength after heat hardening: 2.15

• All the combinations of friable ore and concentrate appeared to produce briquettes with adequate thermal stability. The thermal stability appears to be so good that heat treatment of room temperature cured briquettes may not be necessary at all.

• To find out whether this type of thermal stability would also be true for heat hardened briquettes, similar thermal stability tests were conducted with heat hardened briquette samples. These results were analysed and compared with the thermal stability of the room temperature cured briquettes. In general the thermostability of the heat hardened briquettes were observed to be lower, compared to the thrmostability of naturally cured briquettes. More precisely, while no significant differences were noted between the hot strengths of naturally cured and heat hardened briquettes in the temperature range 400 ⁰ C - 1000 ⁰ C, progressively increasing strengths were noted for naturally cured briquettes in the range 1000 ⁰ C - 135O ⁰ C (which is not true for heat hardened briquettes). In view of this it was observed that no benefits commensurate with the investment associated with the high temperature heat-hardening facilities, appeared to be achievable. But depending on the grade of feed ores, heat hardening may be adopted if substantial improvements are noted, on a case-specific basis. Smelting of Briquettes (Stage II)

As there was no concrete evidence of using briquettes for closed Submerged Arc Furnaces (SAF) extensively. It was, therefore, necessary to conduct a trial at the 30 MVA closed SAF Smelting Plant at the Ferro Alloys Plant (FAP) for smelting of chrome ore briquettes in a closed SAF on plant scale for a reasonably long period.

Bentonite and molasses were used as the binders for making the briquettes.

The salient features of the smelting test work are described below.

Trial Period

The total duration of the smelting trial was 27 days including a ramp-up period of 9 days. On the Day 1 of the trial period, the trial was started with 30% briquette in the furnace charge, accompanied by a blend of chrome ore pellet and 30% hard lumpy ore. The proportion of briquettes was increased in steps of 10% (at the expense of pellets) keeping hard lumpy fixed at 30%. On Day 9, a furnace charge composition of 70% briquette and 30% hard lumpy was reached, and accordingly Day 10 is considered as the start of the stabilised period of the trial, for which all data have been taken as the basis for operation of furnace with 70% briquette. The same charge composition (70% briquette and 30% hard lumpy) was maintained throughout the remaining period of the trial till Day 27.

Environmental Perspective

Collection and capture of all necessary and relevant data during the trial was of paramount importance, since it was necessary to transfer these data to the Environmental Consultant, CSIR, engaged in the ongoing Environmental Impact Assessment (EIA) study at Richards Bay in South Africa. From the viewpoint of credibility, these data need to be furnished by the prospective Technology and Equipment Suppliers who had already participated in the process of bidding and participation of Public Hearing in the EIA study in Richards Bay. A comprehensive list of the parameters related with the environmental issues were, therefore, sent to these bidders, and their suggestions regarding the modification of the proposed format for capture of environmental data, were taken into account to finalise the format that was to be used during the conduct of the trial at Bamnipal. The final format as acceptable to the bidders was used for capture of environmental data. All aspects including dust generation, characteristics of solid, liquid and gaseous effluents, and noise levels during the trial were included in the format. Furnace Response

During the entire period of the trial, the furnace has shown absolutely stable operation. No untoward incidents like accretion / build-up, electrical irregularities, over-pressure, trouble in furnace tapping etc., were experienced in operation of the furnace. The dust observed in the furnace was mostly dry dust - and not hot sticky dust - therefore, the chances of significant build-up were not there. Visual inspection of furnace interior has also proved this. It has been established (in line with the objective of the trial) that 70% briquette along with 30% lumpy ore can be successfully smelted in a closed Submerged Arc Furnace (SAF), to produce Ferro Chrome (HCFeCr). Extensive data on operational as well as environmental parameters have been collected on a regular and consistent basis for the entire period of the trial. These data have been critically analysed, and there is clear evidence that the operation with 70% briquette is feasible.

Analysis of Operating Parameters

During the trial period, the furnace could be operated in a fairly uninterrupted manner. The furnace key operating parameters are observed to be highly consistent. The inter-relations of the parameters have only established the logical relationships expected out of a very stable furnace operation characteristic of manufacturing of ferrochrome / charge chrome.

The following inter-relationships have been assessed:

Effect of Input Cr/Fe Ratio on Cr Content of HCFeCr

• Relation Between Carbon & Silicon in HCFeCr

• Relationship Between (Carbon+Silicon) & Silicon in HCFeCr Influence Of (Carbon+Silicon) on Cr of HCFeCr

• Relationship Between Reductant/Chromite and Silicon of HCFeCr

It was observed that less than 2% Si is possible, and carbon is still not off. It was also seen that with lower Si in metal, (C+Si) would also be low, and correspondingly for low (C+Si), %Cr in metal will be high for a given Cr/Fe ratio of the input ore. Therefore, Si needs to be maintained to lower side in order to achieve higher chromium in the alloy.

From the trends of the variation of the silicon content in the alloy with the variation of the reductant to chromite ratio, it is observed that the silicon is sensitive to small variation of reductant input, which is a natural phenomenon for smelting of ferrochrome / charge chrome. Key Operating Parameters

The key operating parameters assessed during the trial period, are listed below:

(1) Furnace charge

(2) Chrome ore fines: Concentrate in briquettes

(3) Proportion of binders

(4) Cr ₂ C^ in briquettes

(5) Cr ₂ θ ₃ in lumpy ore

(6) Cr/Fe in briquette

(7) Cr/Fe in lumpy ore

(8) Specific consumption of chromites (including briquettes)

(9) Recovery of Chromium

(10) Range of chromium in metal

(11) Average chromium in metal

(12) Slag/Metal (liquid metal basis)

(13) % Cr ₂ O ₃ in slag

(14) P in metal

(15) Total production

(16) Total energy consumed

(17) Specific consumption of power Slag Characteristics & Chrome Loss

The average compositions of the tapped slag day-wise were closely monitored. The average Cr ₂ O ₃ and Cr content of the slag during the stabilised period of the trial work out to 11.16% 7.64%, and the average basicity of slag was 0.96. These figures are acceptable for a stable industrial operation.

The range of the binders to be shown is being kept as follows: Bentonite-0.25% to 10% of the ore feed, molasses-0.25% to 10% of the ore feed. A higher percentage is not envisaged, as that makes the feed-mix too sticky and pockets of the segments get clogged. Our experience was limited to 4%, beyond which clogging was there. However, for the sensitivity to grain size, a higher band of molasses usage might be applicable, and is, therefore, being incorporated. Hydrated lime -0.1% to 5%. In general, hydrated lime is half of the quantity of molasses used. The grain size is not proposed to be included as a parameter. Therefore, the process will cover chrome ore fines and concentrates of any size/size distribution. Although we have conducted test works only with Indian and South African chromites, we do not propose to restrict the process to only these two sources of feeds. Therefore, no mention will be made about the origin of the chromites in order to keep the domain wide open to include chromites of all possible origins.

No Concern for Phosphorus in Metal

As molasses was used for briquetting the ore, there was a valid concern for the possibility of phosphorus beyond acceptable limit in the product (from the phosphorus of molasses). The average P content of the Ferro Chrome produced during the trial was 0.019% as against a range of 0.020% — 0.025% P desired by all major customers. (The specified limit for P according to international specifications is 0.030% in most of the cases.) This proves that the proposed briquette-based route for Ferro Chrome production will be acceptable and will not overshoot the specified phosphorus levels of the product at all.

The comparison of properties of briquettes made from South Africa concentrates for different briquetting conditions with Indian concentrate as reference is shown in the Table.

The results from the above Table show that both the cold and the hot strengths of the briquettes are improved by addition of bentonite. The results of the Batch Nos. 12 and 19 show the differences of the strength values of bentonite and no-bentonite briquettes. Substantial differences van be noted.