Abstract: Ferrochrome alloy is mainly produced in submerged-arc furnaces. The reductants used in ferrochrome production include coke, anthracite and char. During ferrochrome production reductants are primarily added for reduction to take place. However it is still questionable as to what happens during times when the carbon charged is not enough, is stoichiometric or excess than the required amount. This work investigated phase formation and phase changes at different reducing temperatures with stoichiometric amount of reductants with stoichiometric amount of reductant. Experiments were conducted at 1200oC, 1300oC and 1550oC. Anthracite and coke were used in this investigation. A comparison of reaction steps, the extent of reactions as well as the mechanisms at the same temperature and soaking time, when coke and anthracite were used are discussed. The characterization was conducted using different analytical techniques to identify the changes in morphologies of the raw materials against the temperature as well as the phases that formed

Kalenga, M.

Mosoma, W.

Pan, X.

English

University of Johannesburg Institutional Repository

1  The characterization and effects of stoichiometric reductants in ferrochromium production from 1200oC to 1550oC  M. Kalenga, W. Mosoma and X. Pan University of Johannesburg, Doornfontein, 2028, South Africa michelk@uj.ac.za (corresponding author); xpan@uj.ac.za; wmosoma@gmail.com Abstract  Ferrochrome alloy is mainly produced in submerged-arc furnaces. The reductants used in ferrochrome production include coke, anthracite and char. During ferrochrome production reductants are primarily added for reduction to take place. However it is still questionable as to what happens during times when the carbon charged is not enough, is stoichiometric or excess than the required amount. This work investigated phase formation and phase changes at different reducing temperatures with stoichiometric amount of reductants with stoichiometric amount of reductant. Experiments were conducted at 1200oC, 1300oC and 1550oC. Anthracite and coke were used in this investigation. A comparison of reaction steps, the extent of reactions as well as the mechanisms at the same temperature and soaking time, when coke and anthracite were used are discussed. The characterization was conducted using different analytical techniques to identify the changes in morphologies of the raw materials against the temperature as well as the phases that formed.   Key words: Stoichiometric reductants, characterization, ferrochrome  1 Introduction South African chromite ores reserves are all located in the Bushveld complex. Submerged-arc furnaces are most commonly used to smelt chromite ores into ferrochrome by using suitable carbonaceous reductants such as coke, anthracite and char. The operation of submerged arc furnaces for ferrochrome production are very complex in both physical and chemical aspects (1). The complexity of feed structure as well as electrical-thermal-chemical interactions in the furnace result in a wide variations of temperature distribution. The control of the furnace is therefore often difficult due to the complex physical and chemical situations in the furnace (2, 3). Due to the complexity and heterogeneity of carbonaceous materials, the choice of appropriate reductant has become difficult and is based on many parameters with the quality of the products being one of them (4). It therefore becomes important to understand the steps and extend of reactions when a change in chemical composition of the reductant is remarkable. An investigation on coke use for ferromanganese smelting was conducted and more reactive coke was said to enabling Boudouard-controlled reactions effective in higher part of the shaft (5), therefore impacting the phases that form. But in ferrochrome smelting, very little reduction occurs at temperatures below 1200oC, while at 1500oC and above most of chromites are easily reduced. It has been also shown that between 1200oC, and 1500oC there is specific discrimination and that at 1400oC particularly a better discrimination was observed (6). During ferrochrome production, the role of gaseous reduction reactions should not be overlooked. Further, some investigations have shown inconsistent kinetics of reduction by solid carbon and was theoretically explained by the participation of an intermediate gas 2  phase, carbon monoxide (7). A number of investigations have been conducted on the behaviour of many reductants used in different smelting processes. However, chemical compositions differ from one reductant to another depending on the mine where it was sourced from. The chemical composition therefore leads to differences in the behaviour of reductants. The impact of this would be seen on the reactions extent and phases that form. Studies have shown that anthracite has the highest reactivity since its activation energies are the lowest compared to other reductants (8). Recent simulations investigations have shown that there was no direct and clear correlation between fixed carbon and energy consumption among different South African reductants (9). This led to a conclusion that the choice of carbon material should not solely be based on chemical parameters (10). The current investigation focused on comparing the behaviour of anthracite and coke, with stoichiometric amounts used and the sequences of phases that could form; from 1200oC to 1550oC. 2. Raw materials Table 2.1: Chemical composition of reductants Reductants           Anthracite       Coke  Cr2O3  0.019  0.078 FeO  1.38       1.38 SiO2  11.70  8.45 CaO   0.57  0.56 Reductants          Anthracite       Coke  MgO  0.21  1.28 Al2O3  3.98  1.28 P  0.014  0.004 S        0.97       0.002 Fixed‐C  71.38     77.41 Table 2.1 above shows the percentage of each oxide as well as the elemental percentages in the reductants. The reductants had fixed carbon was 71% and 77% for anthracite and coke respectively. The phosphorus was low but sulphur content was high in anthracite. Table 2.2: Chemical composition of the ores Ore        Small Lumpy      Pellets Cr2O3  34.10  41.0 FeO  20.00       24.0 SiO2  16.00  6.63 CaO   2.00  0.5 Al2O3  12.59  9.3 P  0.004  0.002 S        0.02         0.02 Cr/Fe ratio  1.5  1.5 Basicity (CaO/SiO2)                  0.13  0.07  Table 2.2 shows the chemical composition of different chrome ores used in this investigation. It can be seen from the XRF results that for both small lumpy and pellets the percentage of Cr2O3 was below 50% but above 40% for the pellets and below 40% for the small lumpy.  The FeO was below 25% whereas chrome-iron ratio was 1.5. This confirmed the assertion of an investigation 3  conducted previously, which stated that the South African chromite ores generally contain a maximum of 45% Cr2O3 and a chrome to iron ratio of about 1.6:1 (11). The basicity of both lumpy ores and pellets was far below 1. This showed that the chrome raw material used was therefore acidic. 3. Experimental set-up and procedure The experiments were carried out in a vertical alumina tube furnace. Graphite crucible placed in a silica crucible to prevent any spillages in case the graphite crucible reacts with the feed and got perforated. The raw materials received from ASA metals were small lumpy ores and pellets. The samples were crushed and pulverized to prepare for XRD and microscope analysis as well as the improvement of the reactivity of the feed. The crushed chromite ore was sieved to different sizes. Quartz was also crashed using jaw crusher and cone crusher, then sieved according to different sizes. The reductants used included coke and anthracite and were crushed and grinded and sieved to different sizes. A portion of each reductant was further pulverised for chemical composition analysis. Each material was weighed accordingly and blended then charged into the graphite crucibles. When the temperature aimed was reached, the samples were soaked for 90 minutes. The heating rate of the furnace was 5oC per minute. The samples heated below 1500oC were immediately quenched in water placed in a bucket next to the tube furnace just after it was removed from the furnace. The aim was to freeze the phases and prevent phase changes which have formed at those respective temperatures. However, samples heated above 1500oC were not quenched since the phases formed at that temperature were stable thermodynamically irrespective of time taken for the cooling. The products were subsequently crushed and mounted in a polished section of epoxy resin for microscope analysis and pulverized for XRD analysis to identify different phases that have formed at different temperatures for the different reductants and compared thereafter. Some pics were very close to each other such that they seemed to be one pic representing many phases. To avoid this, pics that were close to each other were decomposed to fully identify the different phases that formed. 4. Results and discussion 4.1 Properties of reductants  The behaviour of reductants was studied at various temperatures, namely 1200oC, 1300oC and 1550oC. The behaviour studies at these temperatures were recorded by comparing the behaviour of coke and anthracite. X-ray diffraction analysis and optical microscope examination results for 4  each temperature are shown below for the respective reductant.  4.1.1 Behaviour of Anthracite  The following graphs show the XRD at different temperatures and from 1 to 8 give information the decomposed picks showing phases that formed when anthracite was used at different temperatures:                                            From the graphs above the following could be observed: at 1200oC, anthracite showed low levels of reductions. From the qualitative analysis it could be noted that the main phases were Fe3O4, high oxides of chromium, iron carbides that start to form as well as chromium carbides. The presence of these phases simply showed the start of reduction chromite and spinel present in the samples. The formation of iron carbide at this stage showed that some iron was being reduced which then combined with the carbon through diffusion. However, in the presence of Cr2O3 and Fe3O4 made the carbides metastable. Since there was no liquid present or forming at this stage, the interface present was solid-solid interface hence solid-solid reduction.  5        Figure 4.1b: microstructure at 1200oC On the in Figure 4.1b which represent the microstructure at 1200oC, the dark area A represents the matrix while the grey area B is the unreduced spinel. The picture also depicts secondary spinel C.       Figure 4.2b: microstructure at 1300oC Figure 4.2b shows the microstructure of chromite reduced by anthracite at 1300oC. The white dots represent the metal phase, light grey area represents the partially reduced chromite and dark grey area represents the gangue phases. At 1300oC, anthracite shows more reduction of chromium oxide into chromium, chromium carbides started forming and more iron carbides appearing.  This was explained by the fact that the temperature has increased which implied an increase in the diffusion rate as a consequence. This allowed a faster diffusion of carbon. These carbides then acted as carbon carriers to ensure further reduction of the chromium oxide into chromium. The most probable reaction that translated the above have been previously reported to have followed the equation:    Cr2O3 + Fe3C = (Fe-Cr)7C3 + CO  (12)  Phases found to have formed at 1300oC were Fe3O4 that remained, Fe7C3, FeC, Fe2C, Fe5SiC, FeCr, SiC and Fe5C2 in Figures 1 to 8. At the same time it could be seen  that fayalite started to form which was an indication of the start of slag forming.                                                          Figure 4.3b: microstructure at 1550oC Figure 4.3b shows a microstructure of chromite ore reduced by anthracite at 1550oC. The dark area being the slag and the white part being the ferrochromium. From the picture it can be seen that there is some metal in the slag which can be an indication of reduction of oxides into metal. Figures 1 to 8 have clearly shown that at 1550oC the metal FeCr was produced and more carbides, namely Fe2C, Cr3C2, Fe7C3, Fe3C, Cr7C3, SiC, FeSiC and Fe5SiC have formed. Also, it could be seen that the slag formed thus the presence of fayalite as well as of a spinel phase.  The presence of chromium 6  carbides such as Cr3C2 and Cr7C3 were a good indication of solid state reduction of the ore. Similar results were found in previous research works (13). 4.4.3 Behaviour of coke   The following XRD results showed the behaviour of coke at different temperatures. On all the Figures the bottom, middle and upper lines showed the phases that formed at 1200oC, 1300oC and 1550oC respectively.  0 10 20 30 40 50 60 70 80 901200OC1300OC8867 87676555444333222111550OCIntensity (cps)2-Theta (degree)1a: Fe5SiCb: Cr23C6c: Fe7C3d: SiO2e: CrO2 a b cd eCoke                   From the graphs above the following could be observed: At 1200oC, coke also showed low level of reductions with the presence spinel phase. However unlike with anthracite, with the use of coke more carbides formed. This was an indication that more reduction place than 7  when anthracite was used. Fe7C3, Cr23C6 were common carbides formed at 1200oC when both anthracite and coke were used. However, additionally to the above mentioned common carbides, more carbides namely Fe5C2, Fe7C3, SiC, CrC and Cr3C2 formed 1200oC when coke was used. Further, it was observed that SiC, Fe7C3 and Cr3C2 already formed at 1200oC when coke was used but could only start to appear at 1300oC with the use of anthracite. However, the presence of carbides was a good indication of solid state reduction of the ore when both reductants were used. However some spinel are partially reduced to form carbides. Figure 2 showed the presence of chromium phases at 1300oC.       Figure 4.4b microstructure at 1300oC Figure 4.4b above shows the microstructure at 1300oC when coke was used. Partially reduced spinel (PRS) phase, the slag that started to form as well as shiny metal drops (M) were depicted. Although for anthracite and coke metal drops could be depicted at 1300oC, the amount and size of metal drops when coke was used were more than with anthracite use.        Figure 4.4b microstructure at 1550oC Figure 4.4b shows the phases present at 1550oC when coke was used. It can be seen that coke was more effective than both anthracite at 1550oC.  5. Conclusions At 1200oC: Anthracite showed lower levels of reductions with dominant phases being spinel, iron carbides and chromium carbides in small amounts. Although the presence of these phases was confirmed, it was observed that there was still some chromite and hematite unreduced. The formation of iron carbide at this stage also showed that some iron was being reduced which then combined with the carbon through diffusion. At 1300°C: Anthracite showed reduction of chromium oxide into chromium. This was attributed to diffusion with then increase of temperature. In other words carbon diffused much more with increasing temperature, thereby forming more chromium and iron carbides, which then acted as carbon carriers leading to further reduction of the chromium oxide into chromium. The results showed spinel phase, silicon carbide and iron carbides phases. Based on the fact that 8  carbides generally act as carbon carriers it could therefore be believed that with increased temperature, oxides were expected to be reduced by these carbides thus leading to more metal. With the use of coke, it was observed that the amount of chromium increased and partially reduced spinel surrounded by secondary spinel. Also, more chrome metal was detected.  At 1550oC: With the use of anthracite, it was observed some metal was present disseminated in the slag which assumingly was an indication of reduction of oxides into metal, FeCr phases were present. Also iron carbide, chromium carbide and chromium as a metal were present. With the use of coke, it was observed that amongst the phases formed FeCr was present. From the results found in this investigations, it could be seen and concluded that coke was more effective reductant than anthracite at 1823K (1550oC). Further, from the chemical analysis in Table 2.1, the fixed-C was higher in coke than anthracite, this is a major contributing factor in the reductions studied in this investigation.  6. REFERENCES [1] Y. Xiao; Y. Yang; L. Holappa and R. Boom:”Microstructure changes of chromite reduced with CO gas”, INFACON XI proceedings, 18-21 February 2007, New Delhi, India, pp.133-144 [2] A. Bhalla (2012):” The reducibility of chromite ore and reactivity of carbonaceous   reductants”, Witwatersrand University submission for Master’s Degree, Johannesburg: pp.34-35 [3] Y. Yang, Y. Xiao and M.A. Reuter:”Analysis of Transport Phenomena in Submerged arc furnace for ferrochrome production”, Tenth International Ferroalloys Congress proceedings, 1-4 February 2004, Cape Town, South Africa, pp. 15- 25 [4] V. Sahajwalla, M. Dubikova and R. Khanna:” Reductant characterization and selection: Implications for ferroalloys processing”, Tenth International Ferroalloys Congress proceedings, 1-4 February 2004, Cape Town, South Africa, pp. 351- 362  [5] S. Kamalpour and W.J. Rankin:” The behaviour of coke in submerged arc furnace smelting of ferromanganese”, Tenth International Ferroalloys Congress proceedings, 1-4 February 2004, Cape Town, South Africa, pp.381-391.  [6] A. Barnes & R. Eric:” The relative irreducibilities of chromite ores and relative reactivity of carbonaceous reductants”, Department of Metallurgy and Materials, Engineering, University of the Witwatersrand, South Africa. [7] A.V. Roschin; V.N. Karnoukhov; V.E Roschin & N.V Malkov:”New findings in the Research of Solid Phase Reactions in the Chromite Ore Reduction Processes”, Tenth International Ferroalloys Congress proceedings, 1-4 February 2004, Cape Town, South Africa, pp.333-342.  [8] G. Makhoba and E.R. Hurman:” Reductant characterisation in ferroalloys: aspects of submerged arc furnace smelting of ferrochrome and reductant selection. INFACON XII proceedings, June 6-9, 2010, Helsinki, Finland, pp.359-366. [9] X. Pan:” Effect of south African reductants on ferrochrome production”, Fourteenth International Ferroalloys Congress 9  proceedings, 31 May-4th June 2015, Kiev, Ukraine, pp.382-389.  [10] R. Falcon; V. du Cann; D. Comins; R. Erasmus; P. den Goed and A. Luckos:”The Characterisation of carbon reductants in the metallurgical industry-Case study”, Tenth international ferroalloys congress proceedings, 1-4 February 2004, Cape Town, South Africa, pp.363-376.  [11] J. Basson, T.R. Curr and W.A. Gericke:” South Africa's ferro alloys industry - present status and future outlook. Innovations in ferro-alloy industry”. South Africa: Witbank, INFACON VIII. [12] Alain, V. (2011), Extractive Metallurgy 2: Metallurgical reaction processes. London: United Kingdom, John Wiley and Sons, Inc.  [13] M.H. Farjadi & J. Azari:”Effect of Chrome Ore Quality on Ferrochrome Production Efficiency”, INFACON X proceedings, 1-4 February 2004, Cape Town, South Africa, pp.103 – 107   

The characterization and effects of stoichiometric reductants in ferrochromium production from 1200oC to 1550oC

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The characterization and effects of stoichiometric reductants in ferrochromium production from 1200oC to 1550oC

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