FI126945B - Procedure for Performing an Improved Process for Producing High Carbon Ferric Chromium (HCFeCr) and Charge Chrome (Charge Chrome) - Google Patents

Description

A process for carrying out an improved process for the preparation of high carbon ferrochrome (HCFeCr) and batch chromium (Charge Chrome)

Field of the Invention

The present invention relates to an improved process for the preparation of high carbon ferrochrome (HCFeCr) and "Charge Chrome" ferrochrome (hereinafter: batch chromium) using novel types of chromite ore agglomerates. More particularly, the invention relates to an improved process for making briquettes from fine chromium ore particles and concentrates.

Background of the Invention

The widely used procedure for the preparation of HCFeCrm / batch chromium has long been the carbon thermal reduction (smelting) of chromite ores in electrically operated submerged arc furnaces (SAF). For ease of operation and to reduce the cost of capital, these SAF blast furnaces have always been made from above to semi-closed or open. However, in such blast furnaces, carbon monoxide and other gases generated during the smelting process always escape through the open portion at the top, even when the most efficient gas purification plants are used. As a result, the ever-tightening requirements for the environmental friendliness of the operation have changed the implementations of the SAF blast furnace to a closed type. These blast furnaces are always filled from above and the contents are discharged at predetermined intervals from the outlet chutes at the bottom of the blast furnaces. The defrost batch depresses and eventually reaches the hottest zones at the three (3) electrode tips where the electric arcs are formed. This is a continuous process and the hot gases from the arc zone rise upwards through the pores in the melting batch.

In open and semi-closed blast furnaces, the rising gases may pass more easily through the smelting batch due to the lower pressure in the blast furnace. However, in closed blast furnaces, the porosity of the smelting batch plays a very important role in ensuring the free or unhindered passage of the ascending gases in order to avoid probable explosions. Thus, in the case of closed SAF blast furnaces, it is necessary to use a narrow size range of melting batch instead of a wide size range, which could block the void between particles and limit the flow of gases. Thus, the maximum percentage of fine ore particles contained in the blast furnace batch is limited. However, the formation of fine particles is an obvious consequence of the extraction operations, and the separation of fine ore particles is extremely expensive because these fine ore particles also contain chromium metal in oxide forms. In addition, there is also a huge amount of space required for the continuous disposal of such fine particles in a landfill. Thus, as the use of closed SAF blast furnaces increased, it was necessary to develop methods for agglomerating fine ore particles and concentrates for use in closed SAF blast furnaces. Two such agglomeration processes that have become popular are (1) sintered pellets, a technology developed by Outokumpu, Finland, and (2) briquette production using lime and molasses as a binder-based approach.

Considerable experimental work has been carried out on both alternative ag-agglomerations using fine particles and concentrates of the Indian chromite ore, and the results were not found satisfactory with the conventional modules of the two processes of the agglomerates, in particular the heat strength. Thus, it was necessary to provide a method for agglomerating Indian chromites to obtain agglomerates having sufficient hot and cold properties and suitable for melting in solid blast furnaces without sacrificing the advantages of melting in a closed blast furnace to produce HCFeCrm / batch chromium. To meet this urgent need, the work presented below has been carried out and the data obtained show that this novel briquetting method using bentonite as a new binder in addition to the conventionally used lime and molasses can successfully produce HCFeCn / batch chromium in closed SAF blast furnaces.

In the conventional manufacturing process, the production of HCFeCrm / batch chromium takes place in two main steps. They are: 1. Agglomeration of fine chromite ore and chromium concentrates 2. Smelting of chromium ore agglomerates (together with block chromium ore) in submerged arc furnaces (SAF) to produce HCFeCrm / charge chromium.

Various methods are available for agglomeration, such as sintered pellets, briquettes and chromium ore sinter. Similarly, various briquetting methods are available which are characterized by the type of binders used to make the briquettes. Various implementations of SAF blast furnaces are available, such as open AC SAF, semi-SAF and closed SAF. In addition to AC furnaces, DC furnaces for commercial ferrochrome production are also in use.

The following alternatives are available for agglomeration and smelting processes: 1. Agglomeration processes, ie sintered pellets, briquettes and chromium ore sinter 2. Rotary blast furnace pre-reduction

3. Open AC SAF (AC SAF) / Semiconductor / Closed AC SAF 4. Direct current (DC) and plasma arc melting furnace

Different forms of agglomerates Outokumpu technology

The most popular and widely accepted form of chromium ore agglomerates in South Africa is sintered pellets made by a process developed by Outokumpu Engineering and Contracting (OEC), Finland. OEC is known as a pioneer in the field of agglomeration of fine chromium ore particles. In addition to South Africa, Outokumpu also has its own facility for large-scale batch chromium production in Tornio, Finland, which uses sintered pellets manufactured in accordance with the company's technology. There are four (4) separate steps in this process, which are: 1. Grinding 2. Filtration 3. Pelletizing 4. Sintering

The fine particles / concentrates of crude ore are ground to fine size by wet milling in ball mills. The wet sludge is pumped into special filters for dewatering. The cake obtained by removing water is then mixed with bentonite and pelleted in a drum pelletizer. The green pellets thus formed are eventually sintered in a steel belt sintering furnace. The product is called sintered pellets that can be placed in SAF blast furnaces for melting HCFeCrm / batch chromium. This process is used extensively and works satisfactorily in Finland and South Africa. The advantages include lower power consumption, stable blast furnace operation, higher chromium recovery and higher plant availability compared to conventional ferrochrome smelting processes. However, the process has hitherto been used to melt chromites originating in Finland and South Africa. Two sets of tests carried out by TSKZN at Outokumpu's test facility in Pori, Finland, to determine the suitability for producing sintered pellets from Sukinda chromites indicate that several modifications to the design and process features of the plant are necessary for the successful production of sintered pellets. These modifications and the driving factors are presented in the table below.

In addition to these modifications, Outokumpu's technology and plant costs are unfavorably high, which drives up production costs.

Briquette manufacturing processes

Briquettes bound with slaked lime and molasses are also widely used in the production of HCFeCrm / batch chromium in India. The briquette manufacturing process is relatively simple in terms of design, engineering and operation compared to Outokumpu's production of sintered pellets. Briquetting plants are relatively cheap.

The raw materials for the briquettes are fine chromium ore particles and concentrates together with suitable binding materials. The chromium ore is screened and dried. After the addition of binders and water, the mixture is blended homogeneously in mixing equipment. The material obtained from the mixing device is fed to a briquetting press and pressed into briquettes. Molasses, slaked lime, sodium silicate, slag, cement and fine silica are common binders used in briquetting.

It should be mentioned here that although briquetting is a popular agglomeration process among Indian chromium producers, they are all semi-blast furnace plants. Consequently, TSKZN carried out a well planned experimental work to optimize the briquette production and the smelting operation in the closed SAF blast furnace.

Open / Semi-Red / Solid AC SAF

In submerged arc furnaces, three-phase alternating current operation is carried out using three Söderberg electrodes and energy generation by heating. Close monitoring of the composition of the smelting batch and its size as well as of the slag composition is essential for the efficient operation of submerged arc furnaces.

There are three types of submerged arc furnaces, namely (i) open blast furnace, (ii) semi-blast furnace and (iii) closed blast furnace. The open blast furnace and the semi-blast furnace are open from above, so that a large part of the exhaust gas is emitted from the blast furnace and released into the atmosphere. These blast furnaces can no longer be considered acceptable due to ever-increasing environmental standards. In the case of closed S AF blast furnaces, 100% of the blast furnace exhaust gas is sucked through conduits and cleaned in scrubbing gas scrubbers before being vented to the atmosphere and thus more environmentally friendly. In addition, they also provide a number of advantages, such as lower power specific consumption, higher chromium yield if operated with properly agglomerated feeds. In addition, they also provide the opportunity for simultaneous power generation, with the added benefit of "carbon credits", which are becoming increasingly popular worldwide.

Various melting processes

Pre-reduction process

In the Showa Denko process (SDC process), the fine chromium ore particles are ground in a rod mill, pelletized using coke as a reducing agent, dried in a chain oven and superheated in a rotary kiln to about 1400 ° C. The furnace is heated with a pulverized coal / CO / oil burner. In these pellets, the metalization achieved by chromium is about 60% and the metalization achieved by iron is about 90%. The reduced pellets exiting the rotary kiln are stored in a perfectly sealed equalization tank designed to prevent re-oxidation. The reduced hot pellets are removed from the equalization tank and fed to the electric furnace after being combined with the coke and the melt through a feed pan.

The defrosting process can reduce specific power consumption by about 30-35%. However, the disadvantages include high investment costs, increased furnace size, poor plant availability and extremely high plant maintenance costs, which negate the benefits of otherwise pre-reduction. In addition, the risk of reoxidation is also a critical consideration, as it can negate much of the benefits of pre-reduction.

There have been only two facilities in the world and both have been declared closed.

A process based on sintered pellet

The sintered pellets (by the Outokumpu process) are most often melted in a closed blast furnace to produce batch chromium / HCFeCl. This process is well established in Finland and South Africa for the excellent operation of the plant. Many benefits, including reduced power consumption, higher chromium recovery, improved plant availability, etc. have made this process very popular. The cost of this plant is considerable and is considered a disadvantage. DC plasma arc melting furnace

The DC arc furnace includes a single central hollow graphite electrode Rodi. Its implementation is of the top closed type. Unlike submerged arc furnaces, the arches are open. It uses fine ore particles / concentrates without prior agglomeration and low quality inexpensive reducing agents such as coal, carbonized material and anthracite can be used. For a given quality of the ores fed, this process is also known to involve (i) a higher percentage of chromium in the Cr product, (ii) a higher chromium Cr recovery, (iii) less phosphorus and silicon in the product and (iv) a lower slag / metal ratio.

The major advantage of the DC plasma route is the complete elimination of the agglomeration plant and the possibility of using cheap reducing agents. However, due to open bath conditions, the specific power consumption in the DC plasma furnace is higher than in the AC SAF blast furnace. In addition, the operation process is complex and thus the availability of skilled personnel to operate DC blast furnaces is very limited. Due to these drawbacks associated with the DC process, the total number of operating plants has not risen to two. Both of these facilities operate in South Africa.

Preheating The specific power consumption for melting ferrochrome can be reduced by preheating the feed materials. This is only possible in the case of melting furnaces of the top closed type. Thus, preheaters can be placed on closed AC SAF blast furnaces and DC smelting furnaces, in which the preheaters are preheated to a temperature of up to 600-700 ° C by means of a high carbon monoxide exhaust gas from the closed furnace.

OBJECTIVE OF THE INVENTION It is an object of the present invention to provide a process for agglomerating fine particles and concentrates of chromite ore in all possible proportions to produce agglomerates of satisfactory high heat strength so as to ensure minimal formation of fine particles during melting in closed SAF blast furnaces. It is another object of the present invention to provide a process for agglomerating fine particles and concentrates of chromite ore in all possible proportions to produce agglomerates of satisfactory cold strength so as to ensure minimal formation of fine particles during briquette handling and storage. Yet another object of the present invention is to provide a process for agglomerating fine particles and concentrates of chromite ore in all possible proportions, which is relatively simple in design, engineering and operation as compared to the conventional process for making sintered pellets. Yet another object of the present invention is to provide a process for agglomerating fine particles and concentrates of chromite ore in all possible proportions using, in addition to two other binders, molasses and slaked lime, with or without swelling of bentonite. Yet another object of the present invention is to provide a process for agglomerating fine particles and concentrates of chromite ore in all possible portions, wherein the briquettes are heat-cured or non-cured. Yet another object of the present invention is to provide a pure process for agglomerating fine particles and concentrates of chromite ore in all possible proportions to produce agglomerates which do not require any high temperature curing, which guarantees a minimum production of hexavalent chromium (Cr6 +). Yet another object of the present invention is to provide a clean process for melting briquettes made of fine particles and concentrates of fine chromite ore in all possible proportions in combination with boulder-like ore in all possible proportions, in accordance with the relevant environmental standards. Yet another object of the present invention is to provide a process for guaranteeing the stable melting of agglomerates of fine particles and concentrates of fine chromite ore in closed SAF blast furnaces without any adverse effects. It is another object of the present invention to provide a method for satisfactorily melting chromium ore agglomerates of fine particles and concentrates of chromite ore in semi-semicircular or open submerged arc furnaces in addition to solid submerged arc furnaces. It is another object of the present invention to provide a process for melting all possible portions of chromium ore chromium ore chromium agglomerates in closed SAF blast furnaces for the production of HCFeCr / batch chromium having process parameters such as chromium recovery, metallic chromium, the percentage of slag, total metal chemistry, and specific power consumption are comparable to conventional sintered pellet based activities. It is a further object of the present invention to provide a process for melting chromium ore agglomerates made of fine particles and concentrates of chromite ore in all possible parts in the production of HCFeCr / batch chromium, which is internationally recognized by designers and suppliers of solid SAF blast furnaces. It is another object of the present invention to provide a process for agglomerating fine particles and concentrates of chromite ore in all possible proportions, so that the cost of capital is much lower than in the case of conventional sintered pellets, resulting in reduced manufacturing costs. Another object of the present invention is to provide a process for agglomerating fine particles and concentrates of chromite ore in all possible proportions to produce agglomerates which can be used in all types of melt batch preheating furnaces fitted with a closed submerged arc furnace.

Description of the Invention

Briquetting (Phase I)

Fragile chromium ore and concentrate were used in the manufacture of briquettes, and in order to increase the thermal strength of the briquettes, it was decided to include bentonite as a binder in addition to the molasses and slaked lime commonly used in briquetting.

The brittle ore and concentrate were mixed in the required proportions and dried to a moisture content of 0.2-0.5%. After drying, the required amounts of bentonite and then molasses and water and lime were added (if necessary). The moisture in the feed was kept below a certain value in all experiments. The mixture prepared in this way was then transmitted to the briquetting and during the briquetting process the force used for briquetting and the speed of the briquetting rolls were varied. In all cases, an acceptable proportion of the briquettes produced were subjected to crushing / pressing tests: (a) immediately after preparation, (b) after curing for 24 hours at normal room temperature, (c) after being subjected to heat curing at 800 ° C: and 1000 ° C for 1 hour; in some specific experiments, the exposure time was changed to 90/105 minutes. Thermal stability tests were conducted on high quality briquettes from various feed combinations by exposing the briquettes (which were covered with a layer of coke dust but without using weight on the briquettes) to 400, 600, 800, 1000, 1200 and 1350 ° C for half an hour and immediately determining their compressive strength (the temperature dropped approximately 30-50 ° C when the samples were taken out of the blast furnace and before the compression strength could be measured).

Score

The mixtures used in the experiment were: (a) 70% brittle ore + 30% concentrate, (b) 50% brittle ore + 50% concentrate, (c) 30% brittle ore + 70% concentrate, and (d) 100% pure concentrate.

The results obtained showed the following:

All portions of brittle ore and concentrate can be briquetted.

Briquetting was possible with moderate force. Higher pressures can be applied without adversely affecting the quality of the briquette.

The optimum speed of the rolls for producing good quality briquettes was found.

The binder combination was optimized for lime, molasses and bentonite.

Hardening of the green briquettes for 24 hours increased the compressive strength of the briquettes. However, no appreciable change was observed when looking at crushing strength.

Bentonite swelling prior to the manufacture of briquettes (as suggested by Koppern) showed no positive effect. Heat curing reduced the crushing strength of the hardened briquettes but had a positive effect on the compression strength as the temperature and residence time were increased.

For all tested combinations, the briquetting strength values of the briquettes were as follows: 1. Green crushing strength (% + 20 mm immediately after manufacture): 1.00 2. Crushing strength after curing at room temperature for 24 hours: 1.05 3. Crushing strength at heat curing after: 0.90.

Corresponding compression strengths on the reference scale were: 1. Green compression strength: 1.00 2. Compressive strength after curing for 24 hours at room temperature: 1.75 3. Compressive strength after curing: 2.15.

All combinations of brittle ore and concentrate appeared to produce briquettes with appropriate thermal stability. The thermal stability seems to be so good that heat treatment of room-cured briquettes may not be necessary.

In order to determine whether this type of thermal stability is also true for heat-cured briquettes, similar thermal stability tests were conducted on heat-cured briquette samples. The results were analyzed and compared with the thermal stability of the briquettes cured at room temperature. In general, the thermal stability of heat-cured briquettes was found to be lower compared to that of naturally cured briquettes. Specifically, although no significant differences were found between the heat strengths of naturally cured briquettes and heat cured briquettes in the temperature range of 400-1000 ° C, gradually increasing strengths were found for naturally cured briquettes in the range of 1000-1350 ° C (which is not the case for heat cured briquettes). In view of this, it was found that the benefits did not appear to be achievable given the cost of the equipment for high temperature curing. However, depending on the quality of the feed ores, thermal curing may be used if significant improvements are observed in the particular case.

Briquetting of the briquettes

Because there was no concrete evidence of widespread use of briquettes in closed submerged arc furnaces (SÄF). Thus, it was necessary to conduct an experiment in a closed 30 MVA smelting plant in a ferroalloy plant (FAP) for melting chromium ore briquettes in a closed SAF blast furnace on a scale for a moderate period of time.

Bentonite and molasses were used as binders in the manufacture of briquettes. The main features of this melting test are described below.

The test cycle

The total duration of the thawing test was 27 days including the 9 day start-up phase. On day 1 of the test period, the experiment was started with a blast furnace charge containing a mixture of 30% briquettes and chromium ore pellets and 30% hard block ore. The share of briquettes was increased by 10% (at the expense of pellets), keeping the share of hard blocks at 30%. By day 9, the blast furnace charge composition had been reached at 70% briquettes and 30% hard boulders, and thus day 10 is considered to be the beginning of a stable period of the experiment, for which a stable period is taken as the basis for 70% briquette operation. The same composition of the charge (70% briquettes and 30% hard boulders) was maintained for the remainder of the experiment until 27th day.

environmental considerations

Collecting and retrieving all the necessary and relevant information during the trial was extremely important because it was necessary to transfer this information to the environmental consultant, CSIR, which is participating in an ongoing Environmental Impact Assessment (EIA) in Richards Bay, South Africa. For the sake of credibility, this information must be obtained from potential technology and hardware vendors who had already participated in the bidding process and participated in the public consultation on Richards Bay related to the EIA study. Therefore, a comprehensive list of environmental parameters was sent to these bidders and their suggestions for modifying the proposed procedure for obtaining environmental information were taken into consideration to complete this procedure, which was used to conduct the experiment in Bamnipal. The final procedure approved by the tenderers was used to obtain the environmental information. All features were included in this procedure, including dust generation, solids characteristics, liquid and gaseous effluents, and noise during the test.

Blast furnace response

The performance of the blast furnace has been found to be absolutely stable throughout the trial period. No unfavorable events such as buildup / accumulation, electrical irregularities, overpressure, blast furnace emptying problems were observed in the operation of the blast furnace, etc. The dust detected in the blast furnace was mainly dry dust - not hot sticky dust - and thus there was no possibility of significant buildup. This was also demonstrated by a visual inspection of the blast furnace interior. Thus, it was established with certainty (consistent with the purpose of the experiment) that 70% of the briquettes and 30% of the boulders can be successfully melted in one solid submerged arc furnace (SAF) to produce ferrochrome (HCFeCr). Comprehensive information on operational and environmental parameters has been collected regularly and consistently throughout the trial period. This data has been critically analyzed and there is clear evidence that operation with 70% briquettes is viable.

Analysis of operating parameters

During the trial period, the blast furnace could be operated in a relatively uninterrupted manner. The main operating parameters of the blast furnace are found to remain very uniform. The interdependencies between the parameters have only reinforced the expected logical dependencies based on the very stable nature of the blast furnace operation in the production of ferrochrome / batch chromium.

The following interdependencies have been assessed:

Effect of feed Cr / Fe ratio on Cr content of HCFeCnn - Carbon & dependence of silicon in HCFeC (Carbon + silicon) & dependence of silicon in HCFeC (Effect of carbon + silicon) on HCFeCn on chromium Cr - Dependence of reducing agent / chromite on silicon in HCFeC

It was found that less than 2% of silicon Si is enabling carbon is still absent. It was also seen that when the metal has less Si, the (C + Si) is also low and, for a correspondingly low combination (C + Si), the Cr% in the metal is high with a given Cr / Fe ratio of the ore fed. Therefore, Si must be kept lower in order to achieve higher chromium content in the alloy. From these trends in the variation of the alloy silicon content with the ratio of reducing agent to chromite, silicon was found to be sensitive to small variations in the feed of the reducing agent, a natural phenomenon in ferro-chromium / batch chromium smelting. Main operating parameters

The main operating parameters evaluated during the trial period are listed below: (1) Blast furnace melting point (2) Fine chromium ore particles: concentrate in briquettes (3) Binder content (4) Cr2C> 3 in briquettes (5) Cr2C> 3 in block ore (6) Cr / Fe in briquette (7) Cr / Fe in block ore (8) Specific consumption of chromites (including briquettes) (9) Chromium recovery (10) Metal chromium range (11) Medium chromium content in metal (12) Slag / metal (based on liquid metal) (13) )% Of Cr203 in slag (14) P in metal (15) Total production (16) Total energy consumption (17) Specific power consumption

Slag Features & loss of chromium

The average compositions of the effluent slag were closely monitored daily. The average concentrations of Cr 2 O 3 and chromium Cr in the slag during the stable period of the experiment were 11.16% 7.64% and the average slag alkalinity 0.96. These figures are acceptable for stable industrial activities.

The range of binders to be displayed is considered to be:

Bentonite - 0.25-10% of ore feed, molasses - 0.25-10% of ore feed. A higher percentage is not used because it makes the feed mixture too sticky and the pockets of the segments clog. Our experience was limited to 4% above which clogging was observed. However, for a grain size sensitivity, a wider range of molasses use might be useful, and thus included. Slaked lime - 0.1-5%. Usually slaked lime is half the amount of molasses used. Grain size is not suggested as a parameter to include. Thus, the process covers fine particles and concentrates of chromium ore of any size / size distribution. Although we have only conducted experiments with Indian and South African chromites, it is not our intention to limit the method to these two feed sources only. Thus, nothing is mentioned about the origin of the chromites to keep the area wide to include chromites of all possible origins.

The phosphorus in the metal does not cause concern

When molasses was used for briquetting ore, there was a legitimate concern that phosphorus was present in the product in excess of an acceptable limit (from me-Lassi phosphorus). The average P content of ferrochrome produced during the experiment was 0.019% compared to 0.020-0.025% P for all major customers. (The specified limit for phosphorus P according to international specifications is 0.030% in most cases.) This indicates that the proposed briquette based route for ferrochrome production is acceptable and will not lead to any exceeding of the specified levels of phosphorus in the product.

The table compares the characteristics of briquettes made from South African concentrates under various briquetting conditions, with Indian concentrate serving as a reference.

The results in the above table show that both the cold and hot strengths of briquettes are improved by the addition of bentonite. The results obtained for batches 12 and 19 show the difference in strength values between bentonite-containing and bentonite-free briquettes. Significant differences can be observed.

Claims (3)

A process for carrying out an improved process for the preparation of high-carbon ferrochrome (HCFeCr) and batch chromium (Charge Chrome), comprising: drying chromium concentrate and fine chromite particles; the dried chromium concentrate and the fine chromite particles are mixed in all possible proportions to form a crude ore feed; subjecting said crude ore feed to a step of mixing said mixture with slaked lime, molasses and bentonite to form a briquettable feed mix; said mixture is compressed into briquettes, characterized in that 0.1 to 5% by weight of slaked lime, 0.25 to 10% molasses and 0.25 to 10% bentonite are mixed as binders. The method of claim 1, wherein the binders are slaked lime, molasses and bentonite. The method of claim 1, wherein said briquettable mixture is compressed to form briquettes and then cured. claim