Journal of Materials Research and Technology Journal of Materials Research and Technology
Original Article
Agglomeration behaviour of steel plants solid waste and its effect on sintering performance
Prince Kumar Singha, Kumar Avala Lavab,, , Pravan Kumar Katiyara, Rita Mauryaa
a Department of MSE, Indian Institute of Technology, Kanpur 208016, India
b Department of MME, V.S.S. University of Technology, Burla 768018, India
Received 29 April 2016, Accepted 26 November 2016
Abstract

Recycling has been the fascinating topic among the researchers for all times. The present study shows the recycling of steel plant's solid wastes as blast furnace flue dust and sludge towards agglomeration and their use in the production of sinter. These wastes consist of metal oxides and coke fines as a valuable material with some alkali oxides. Using these wastes as it is in the form of fines exacerbate the further processing. Pellets of these wastes are prepared with three types of binders as molasses, dextrin and bentonite. The result reveals that properties as compressive strength, shatter strength, are better in the case of bentonite binder having the productivity of the disc pelletizer machine as 75. After that, these macro pellets used for sintering with iron ore and other ingredients in pot type, down draft laboratory grade sintering machine, which shows very high productivity and good mechanical properties of the sinter as well. The microstructural analysis reveals the presence of re-oxidized hematite and a little bit of a magnetite phase with some slag phases, which confirmed later by XRD analysis. Results also show the decrease in coke rate, i.e. coke consumption to produce sinter and at the same time, this process is highly eco-friendly.

Keywords
Blast furnace flue dust and sludge, Pellets, Sintering, Waste utilization, Recycling, XRD
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1Introduction

Blast furnace sludge and blast furnace flue dust are the hazardous metallurgical waste generated in the iron making plants [1]. The flue dust and sludge is a mixture of oxides expelled, whose major components are iron oxides and coke fines. It also contains silicon, calcium, magnesium and other minor elemental oxides in lesser amounts. The recovery of these valuable metals and carbon from this flue dust and sludge becomes very important, due to the increase in the price of coke breeze and the decrease of the primary resources of the raw materials. Moreover, it makes the environment safer by decreasing pollution.

The fact that it is not possible to recycle this dust and sludge directly or to reject it as landfill because it will contaminate the soil badly, so it is necessary to consider the recovery of the valuable elements contained in it and to obtain a non-hazardous residue that can be stored without problem or can be used in agglomeration units in iron-making industries [2,3]. Although utilization of these wastes as it is from the plant for the production of sinters is a recirculation technique [4] but at the same time, it adversely affects the property of sinter, which is not up to the mark for use in blast furnace. In this study, we made the pellets [5–8] from the blast furnace waste and then used these pellets for preparing of sinter, which results in sintering properties such as sinter strength increases with using the waste made pellets, at the same time the productivity of the sinter machine also increases [9–11]. It also decreases the fuel rate in the sintering process [12]. The results were found that the presence of bentonite binder up to 4% provides the shatter index of the pellets less than 20, whereas molasses and dextrin gave the significantly high shatter index means very low strength. Hence, the 4% bentonite binder gave the optimum condition for the pelletizing machine. Thereafter, these pellets are used for sintering results in a decrease of coke consumption from 60 to 30kg/ton of sinter, utilizes 25% of waste. At the same time, this much of waste addition also decreases the lime addition to maintaining the basicity. The productivity of sintering machine was found to increase up to 5.2 at 27% waste for the basicity of the sinter as 2.2.

2Materials and methods2.1Material used for pelletization

The raw materials (blast furnace flue dust and sludge) were used for pelletization, obtained from the Durgapur Steel Plant (India). The chemical composition and sieve analysis are shown in Tables 1–3.

Table 1.

Chemical composition of blast furnace sludge.

Element  Weight %, by mass 
Fe  30.82 
CaO  8.95 
SiO2  11.59 
MgO  3.83 
Al2O3  3.6 
Na20.09 
K20.29 
Table 2.

Chemical composition of blast furnace flue dust.

Element  Weight %, by mass 
Fe  39.92 
CaO  6.28 
SiO2  6.95 
MgO  2.01 
Al2O3  4.0 
Na20.51 
K20.29 
Table 3.

Sieve analysis of blast furnace ferruginous waste material.

Sieve size (μm)  Weight % of flue dust  Weight % of sludge 
1000  20  30 
500  10  25 
300  20  25 
150  30  20 
75  10  – 

The waste sample shows the abnormal accumulation of iron as well as very fine carbon. At the same time, these also contain a high percentage of alkali, so it cannot be reused directly in the sintering process. The optical emission spectroscopy studies (OES) of these samples indicated that iron is present in very high amounts in comparison with the other elements. The X-ray diffraction (XRD) study shows the associated phases of iron metal, gehlenite (Ca2Al2SiO7), magnetite, hematite, quartz and wustite in order of abundance. The XRD pattern of flue dust and sludge is shown in Figs. 1 and 2.

Fig. 1.
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X-ray diffraction pattern of blast furnace flue dust.

Fig. 2.
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X-ray diffraction pattern of blast furnace sludge.

A number of organic and inorganic binder materials used as an additive in agglomeration technique are being reported in the literature. Here bentonite an inorganic, whereas molasses and dextrin (organic binders) are used for pelletization. The chemical compositions of bentonite and molasses binder are listed in Tables 4 and 5.

Table 4.

Chemical composition of bentonite binder.

Element  Weight %, by mass 
Al2O3  20.27 
Fe2O3  9.08 
TiO2  0.78 
SiO2  54.82 
CaO  2.10 
MgO  3.02 
Na21.31 
K20.06 
Table 5.

Chemical composition of molasses.

Constituent  Weight %, by mass 
Organic material  72.7 
Crude protein  0.06 
Saccharose  49.3 
Nitrogen  1.6 
Water  20 
Ash  7.3 
2.1.1Dextrin

Dextrin is a family of polysaccharides, which are obtained as an intermediate by-product of the breakdown of starches.

2.2Experimental set up and methods

A detailed flow sheet of recycling of blast furnace sludge and flue dust is given in Fig. 14. A disc pelletizer was used in the formation of pellets having 56cm diameter. The angle of inclination of disc pelletizer was set to 37.10° and the rotating speed of the disc was 28rpm with the residence time of 10min. The pelletization was carried out by taking the aforesaid binders in different proportions. Three categories of pellets were prepared: (i) flue dust only, (ii) sludge only, and (iii) mixture of both.

Raw materials mixed with the binder were fed to the pelletizing machine. The predetermined amount of water was spread onto the rolling bed of material in the disc pelletizer. The machine was put on and the spherical pellets were allowed to form. A scraper was used to wash out the material sticking onto the disc. At the end of the experiments, the pellets were collected and indurated. The green pellets were dried in air for two days to assure the complete evaporation of moisture. Bentonite made pellets were heated up to 300°C for 1h and then isothermally indurated at 900°C for 1h. Dextrin and molasses made pellets were heated in an air oven at 150°C for 1h. At the end of drying/induration, the samples were screened to collect the fraction of less than 5mm, which is also a measure of the productivity of the pellet.

These indurated pellets were used in the sintering with iron ore and other ingredients like limestone, coke breeze and dolomite in the laboratory grade sinter machine. A real view of the top burning bed looks as Fig. 3. Productivity is also calculated in case of sintering by taking +10mm sinter size.

Fig. 3.
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Top burning bed of sinter of laboratory grade sintering machine.

2.3Charge calculation for sintering

Basicity (CaO/SiO2) is being taken as the basis for charge calculation and was fixed as 1.5, 1.73 and 2.2. The amount of coke breeze and iron ore was fixed to 300g and 3kg respectively, whereas the other raw material mixes are calculated for aforesaid basicity of sinter. The sinters produced in experiments have been graded as S1, S2, S3, …, S15 by varying limestone fines, i.e. flux and the amount of waste made pellets which are quoted in Table 6.

Table 6.

Raw material composition for sintering.

Raw material  Basicity=1.5Basicity=1.73Basicity=2.2
Iron ore fines (kg) 
Coke breeze (g)  300  300  300  300  300  300  300  300  300  300  300  300  300  300  300 
Limestone fines (g)  335  383  400  432  465  400  445  471  490  535  550  660  697  770  843 
Dust+sludge pellets (g)  300  400  600  800  300  400  600  800  300  400  600  800 
I.D. No.  S1  S2  S3  S4  S5  S6  S7  S8  S9  S10  S11  S12  S13  S14  S15 
2.4Productivity of the pelletizing machine

The productivity of the pelletizing machine was calculated as [12]:

2.5Productivity of sintering machine

At the end of sintering experiment, the produced sinter was screened over a sieve of 10mm to determine the productivity of the machine as [12]

S, amount greater than 10mm; H, height of the charge/time of sintering; K, bulk density of the sinter; L, machine constant.

2.6Testing of sinter

The sinter strength was observed by the series of tests as: (i) shatter test, (ii) tumbler test, (iii) abrasion test and (iv) cold crushing strength (CCS) test. CCS test was carried out in a universal testing machine to judge the breakability of sinter.

2.7Reducibility of sinter

Reducibility of the produced sinters was analysed in a reduction furnace by carbon mono oxide and nitrogen gas mixture in the proportion of 30:70 at the rate of 1.50L/min, at temperature 900°C and reduction time 90min. The weight loss during the process is the clear indication of the reducibility of sinters and weight loss was calculated as:

3Results and discussion3.1Strength of the pellets

The physical strength of the pellets is shown in Figs. 4–6, showing the shatter index as the varying binder amounts for different types of pellets. Nearly the pattern of shatter index is same in each type of pellets, but there is a considerable change in the values of shatter index with respect to the type of binders used.

Fig. 4.
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Effect of binder amount on pellets strength: (a) flue dust pellets, (b) blast furnace sludge pellets, (c) sludge and flue dust mixed pellets.

Fig. 5.
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Effect of binders on productivity of machine.

Fig. 6.
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Effect of waste made pellets on productivity of sintering machine.

The presence of bentonite binder shows the maximum strength of pellets as the graphs shown above. Apart from the choice of binder, the amount of the binder is also important, otherwise, it will adversely affect the further processing as well as the economy of the process. Use of four weight percent of the binders shows an optimum value in each case, otherwise, it will be harmful as said above. Bentonite binder shows the best result among the other binders because it has more of inorganic components which provided the bond of greater strength, whereas in the case of the organic cold setting binders have more of organic components and bond formation between organic and inorganic materials cannot be made easily. Therefore, it could clearly analyse that the four weight % bentonite binder provides the maximum strength in comparison to others binder.

3.2Productivity of the pelletizing machine

Fig. 5 illustrates the effect on the amounts of binders on the productivity of the pelletizing machine, it can be seen that the bentonite binder showed the maximum productivity in comparisons to the other same amount of binders. The optimum result was found in the case of 4wt.% bentonite binder which imparts the maximum productivity (greater than 75) of the pelletizing machine in case of mixed pellets.

3.3Influence of waste on productivity of the laboratory grade sintering machine

Fig. 6 illustrates the waste addition behaviour on the productivity of the sinter machine. The productivity increases with waste addition up to 27–29% as well as with basicity. The maximum productivity was found 4.9ton/m2/day at basicity 2.2, whereas the productivity value reduced to 4.2 and 3.9ton/hm2 for the basicity of 1.73 and 1.5, respectively. A slight increment in productivity by basicity was due to waste pellets addition which provides the better permeability of the bed. At the same time, the presence of fixed carbon in micro pellets meets the heat requirement for all the endothermic reaction with externally added coke.

3.4Effect of pellets addition on coke rate

Fig. 7 shows the effect of waste addition on coke rate, i.e. consumption of coke (kg) per ton of sinter. It is seen that coke rate decreases with an increase in waste addition. The waste contains fixed carbon in flue dust and sludge based pellets, which reduce the coke consumption during actual sintering. Whereas, the basicity of the sinter was varied in between 1.5 and 2.2 and maintained by lime addition.

Fig. 7.
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Waste addition effect on coke rate.

3.5Effect of percent waste on sintering strength

Sinter strength results are shown in Figs. 8–10, which illustrate the shatter index, tumbler index and abrasion index values of the sinter respectively, as the waste made pellets percentage is increasing at the same time basicity also plays an important role, the strength is good in case of the intermediate value of the basicity, i.e. 1.73. At higher and lower basicity values the product becomes fragile in nature, whereas for intermediate one, it is optimized. The presence of fluxes as constituents (refer Tables 1 and 2) in waste material is responsible for increment in the strength.

Fig. 8.
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Effect of waste on Shatter index.

Fig. 9.
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Effect of waste on Tumbler index.

Fig. 10.
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Effect of waste on abrasion index.

3.6Reducibility behaviour of produced sinter on addition waste

Fig. 11 elaborates the effect of waste on the reducibility of sinters. It can be clearly analysed from the figure that more waste addition is degrading the reducibility or in other words, it just decreases the weight loss during the reduction process. This may be due to the fact that the magnetite (Fe3O4) phase, which is less reducible than hematite (Fe2O3), increases in the sinter mix with the increment in the waste. The sinter having the higher basicity will show the higher losses in weight, although the scenario of weight loss with percentage waste addition is same for every basicity values.

Fig. 11.
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Effect of waste on reducibility of sinter.

3.7Characterization of the pellets and the sintered product

As seen from Fig. 12(a)–(c), photographs of pellets, hot sinter and cold sinter respectively. Whereas, Fig. 12(d) shows the microstructure of the sinter at 200μm magnification. It can be clearly analysed that sinter consists of re-oxidized hematite (Fe3O4) and few magnetite (Fe2O3) phases with slag phase having calcium silicate (CaSiO3) and calcium ferrite (CaFe2O3) which was also confirmed by the X-ray analysis as shown in Fig. 13.

Fig. 12.
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Characterization of the pellets and the sintered product.

Fig. 13.
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X-ray diffraction of iron ore sinter using blast furnace flue dust and sludge pellets.

Fig. 14.
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Flow sheet of recycling of blast furnace sludge and flue dust.

4Conclusions

The studies concluded that the agglomeration properties of the flue dust and sludge fines are more appropriate in combined form in comparison to individual of flue dust and sludge. The pellets having good strength were found in the case of mixed pellets (50% of each, flue dust and sludge) by using bentonite binder with very high productivity value as 75 of disc pelletizer. The most suitable pellets (5–8mm) of wastes (blast furnace flue dust and sludge) are obtained with 4% bentonite binder, effectively used as one of the raw materials for sintering, which confirms the recirculation of wastes of plant efficiently utilized without deteriorating the quality of sinter. Use of wastes made pellets decreases the coke rate in sintering operation due to the presence of some fixed carbon in wastes (flue dust 9.60% and in sludge 12.34%). The productivity of the sintering machine is also increased with the increment in waste addition percentage. The maximum productivity is found as 5ton/m2/day at basicity 2.2 for nearly 27% waste addition to the charge mix. The strength of the sinter found up to the mark, but the only thing which is compromised is reducibility, although it is more important than the other properties, but at the same time waste was recycled so that it is manageable.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

Author expresses his sincere thanks to Mr. S K Dubey, General Manager Blast Furnace division DSP-SAIL for providing materials from plant and his valuable guidance without which this study was not possible. Author also thankful to MME Department's staff of NIT Durgapur for their support.

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