Boron mud is the residue in the pyrogenic process producing borax or boric acid with the raw material of szaibelyite or ludwigite in chemical factories, which mainly composed of MgO and SiO2, some Fe2O3 and B2O3 and little CaO and Al2O3. Over the years, boron mud is piled up in the open air resulting to severe farmland destruction, environment pollution and manpower and material resource consumption [1, 2, 3, 4]. According to incomplete statistics observation, millions of tons of boron mud are generated around boron industry developed regions, especially in America, Turkey and China, and the accumulative amount of boron mud is up to 17 million tons in China’s Liaoning province. It costs abundant vigor and funds of local government and companies to the treatment of boron mud discharge and pollution [5, 6]. Over the years, boron mud has been processed mainly as building materials, ceramic additives, agricultural fertilizers, et al. But the treatment amount is quite low and the economic benefits are quite poor [7, 8, 9]. Therefore, it is of great importance to explore efficient utilization methods for boron mud.
Meanwhile, during pellets producing process in China using coarser iron ore, more bentonite is added into raw materials to satisfy the pellets production requirements [10, 11]. Due to the poor quality of bentonite, the addition content is usually 1.2~1.5 %, and even up to 4.0 % in some companies in China, which is much higher compared to advanced production companies overseas (less than 1.0 %). The residue of bentonite after roasting mainly contains SiO2 and Al2O3, leading to a 0.4~0.6 % decrease in pellets iron content for every 1.0 % increase in bentonite content of raw materials. In addition, coke ratio and slag amount in blast furnace increase as well [12, 13, 14]. Therefore, the reduction of bentonite content, optimization of pellet production and development of circular economy become major issues to be addressed in ironmaking process.
There were many researches about the effects of additives on pellets properties. The correlative data indicated that, by adding boron-bearing additive, pellet comprehensive strength was improved obviously, and the required bentonite content was lessened [15, 16]; with more Mg-bearing additive in pellets materials, high temperature metallurgical properties of pellets were improved [17, 18]. Since boron mud contained both B2O3 and MgO, it could be a good additive in pellet process. Based on previous researches about born mud additive and the cooperation with an steel company in northeastern China, this paper investigated the feasibility of boron mud used as pellets additive to the research aims in realizing the comprehensive utilization of boron mud, the decrease of bentonite content in pellets and the reduction of pellets production cost of this company [19, 20]. With systematic and innovative analyses, this paper provided a reference for oxidized pellets production practice and boron-bearing wastes treatment methods. In view of strong production ability of iron and steel industry, the treatment amount of boron mud can be considerable when used as pellets additive. It can promote the development of circular economy with remarkable social benefits and economic benefits, which are of great importance for iron and steel industry, boron industry and environment protection.
Considering both beneficial compositions and harmful compositions for pellets, one typical kind of boron mud was selected from four kinds of boron mud from China’s Dandong region in this study. The chemical compositions (mass content) and particle size (volume content) of iron ore concentrate generated in a steel company in Northeastern China, boron mud selected, and bentonite used in this experiment were listed in Table 1. The TFe content in iron ore concentrate was 65.76 %, B2O3 content in boron mud was 3.80 %, MgO content in boron mud was 35.94 %, and SiO2 content in bentonite was 63.00 %.
During the preparation of green balls, experimental materials were weighed with METTLER PM4000 electronic balance of accuracy 0.5 g, and were pelletized with Φ1,000 mm disc balling machine. Acceptable pellets, with moisture mass content about 8 % and Φ12~14 mm, were tested on shatter strength, comprehensive strength and moisture content of green balls. After dried in an oven, the balls were preheated and then roasted at the temperature of 1,523 K for 20 min in the furnace under the oxygen-enriched atmosphere. Pellets after roasting were tested to investigate comprehensive strength according to GB/T 14201-93 and reduction swelling according to GB/T 13240-91.
Table 2 showed the experimental scheme of this study, which contained three associated series of experiments. In Series 1, with bentonite content of fixed value 1.0 % in pellets materials, boron mud content changed from 0.0 % to 2.0 % to investigate the effects of boron mud on pellets properties and determined the appropriate addition content of boron mud. In Series 2, based on the appropriate boron mud content obtained in Series 1, bentonite content was changed from 1.0 % to 0.1 % to study the feasibility of boron mud lessening bentonite in pellets. Based on the two series of experiments and with the purpose of further reducing the bentonite content and improving the reduction swelling of pellets, Series 3 experiments, adding combination additive made up of boron mud and boron-rich materials instead of boron mud, were carried out. It should be clarified that in this study, bentonite, boron mud and combination additive were extra added to materials on the basis of iron ore concentrate content 100 %.
Results and discussion
Effects of boron mud on pellets properties
Effects of boron mud on green balls properties
At the same content of bentonite 1.0 %, with boron mud additives content increasing, shatter strength of green balls was 3.2 times, 3.1 times, 3.2 times, 3.3 times and 3.2 times, respectively, which was nearly at the same level. The comprehensive strength of green balls was 11.2 N, 12.8 N, 13.2 N, 14.1 N and 14.2 N, respectively, which had little degree improvement. As a whole, effects of boron mud on green balls properties were turned out to be small.
Effects of boron mud on comprehensive strength of pellets after roasting
The comprehensive strength of pellets after roasting at the temperature of 1,523 K was shown in Figure 1. With boron mud content increasing, the comprehensive strength of pellets after roasting improved remarkably, wherein with boron mud content 2.0 %, comprehensive strength of pellets was up to a high value of 4,438 N compared to a value of 3,059 N of pellets with no boron mud. In addition, when boron mud content increased from 0.0 % to 1.0 %, the growth rate of pellets comprehensive strength was greater than that of pellets with boron mud content increasing from 1.0 % to 2.0 %. Therefore, boron mud contributed to the improvement of comprehensive strength of pellets after roasting, and with its content increasing, the growth rate was weakened. To satisfy the blast furnace requirements and enhance energy efficiency, appropriate content of boron mud was selected to be 1.0 % in the experiment when bentonite content was 1.0 %.
Pellets in this study were acid and oxidized pellets, the main form of solid phase consolidation in which was hematite recrystallization connection, and the main mineral composition of which was hematite. During the process of pellets roasted to consolidation, comprehensive strength of pellets after roasting was subject to its internal microstructure. Figure 2 showed the SEM micro-morphology of iron ore pellets after roasting at 1,523 K and EDS analyses of relevant points. Without boron mud additive, there were various high-melting-point silicate minerals between hematite phases, such as forsterite. With boron mud additive, binding phases containing elements, such as B, Mg, Fe, O, et al., appeared between hematite phases internal pellets. The lower melting point slag phases formed easily with existence of B2O3 and other basic oxides, resulting to an earlier and more formation of liquid binding phase at the same temperature. The well dispersity and solubility of B2O3 was a supplementary and indirect factor to promote the phase contract and reaction in its favor. Therefore, as boron mud additive content increased in materials, the comprehensive strength of pellets after roasting increased. Since excessive liquid binding phase blocked direct contact of solid phase particles and caused cementation among pellets deteriorating layer aeration, with boron mud content more than 1.0 %, the growth rate of pellets comprehensive strength slowed at some level.
Effects of boron mud on reduction swelling of pellets
The reduction swelling of pellets by adding boron mud content 0.0 %, 0.5 %, 1.0 %, 1.5 %, 2.0 %, respectively were tested and the results were shown in Figure 3. With bentonite content 1.0 % and boron mud changed in content, reduction swelling indices of all pellets samples were on a normal scale (<20 %). In addition, both reduction swelling indices and comprehensive strength of pellets after cooling had a small change with boron mud content varying from 0.0 % to 2.0 %. Consequently, boron mud had a small effect on reduction swelling of pellets and the pellets properties met the requirements of blast furnace.
In this paper, analyses of SEM and EDS (Figure 4) of pellets samples with different boron mud content were carried out, in order to explore relevant mechanism of how boron mud affect pellets reduction swelling indices. Boron mud contained more SiO2, MgO and B2O3, in which MgO can be combined into magnesium ferrite in stable state during roasting process, and then combined into solid solution of MgO and FexO. Mg2+ can distribute uniformly in wustite, resulting to the small change in lattice and swelling stress reduction during transition process of Fe2O3 to Fe3O4. Therefore, volume swelling of pellets decreased. On the other hand, with more binding phase in the internal pellets, the binding phase combined with hematite grains can overcome phase transformation stress and prevent the more generation of cracks and pores, leading to lower pellet reduction swelling indices. However, boron mud contained some content of alkali metal K and Na, which led to the nonuniform reaction rate, and promoted crystal transition of Fe2O3 to Fe3O4. This changed reaction stress during reduction process, resulting to the appearance of fibrous iron and aggravation of volume swelling.
On basis of the above analysis, the positive effect of boron mud on pellets reduction swelling merely compensated the negative effect, therefore, volume swelling of pellets had small changes.
Feasibility of boron mud lessening bentonite content in pellets
For the purpose of investigating the feasibility of boron mud lessening bentonite content during pelletizing process, based on the above study, appropriate boron mud content in materials was set as 1.0 %. Experiments of lessening bentonite content were carried out, wherein bentonite content was 1.0 %, 0.8 %, 0.5 %, 0.3 %, 0.2 %, and 0.1 %, respectively.
The test results of green balls properties showed that, with boron mud content 1.0 %, as bentonite content decreased, shatter strength changed in a small range of 3.0~3.3 times, and comprehensive strength changed in a small range of 12.5~13.6 N. Green balls properties can meet the requirements of subsequent process.
The effects of lessening bentonite content on comprehensive strength after roasting and reduction swelling indices were shown in Figure 5. The comparison of chemical compositions of pellets added boron mud and base pellet (made of the iron concentrate mass content 100 %, bentonite mass content 1.0 % and boron mud mass content 0.0 %) was listed in Table 3. The results showed that:
When roasting temperature was 1,523 K, as bentonite content in pellets materials decreased, the comprehensive strength of pellets after roasting decreased, and reduction swelling indices of pellets increased.
When boron mud content was 1.0 %, the comprehensive strength of pellets after roasting with bentonite content decreased to 0.3 %, which was similar to that of base pellet. Taken into consideration of production efficiency and pellets properties, it was suggested that the appropriate addition content of boron mud was 1.0 % and bentonite content can be decreased to 0.3 % accordingly.
With boron mud content 1.0 %, the reduction swelling indices of pellets with bentonite content decreased to 0.3 % was 16.10 %, which was slightly lower than that of base pellet. Pellets properties can meet the requirements of blast furnace process.
With boron mud content 1.0 %, the iron content of pellets with bentonite content 0.3 % was 63.25 %, which was slightly higher than the value 62.90 % of base pellet. It made for furnace operation, improvement of smelting intensity and reduction of coke rate in blast furnace.
In current iron and steel industry, if the bentonite content is decreased from 1.0 % to 0.3 % by adding 1.0 % boron mud additive, it is estimated that an steel company with annual output of oxidized pellets 10 million tons, will dispose 120 000 tons boron mud and meanwhile create 30 million yuan economic benefits only from the perspective of raw materials cost every year. In addition, through developing boron mud as pellets additive, it can alleviate environmental pressures and create new job opportunities.
Modification of boron mud additive
Through the above two series of experiments, by adding 1.0 % boron mud additive, bentonite content can be decreased from 1.0 % to 0.3 %. If further reduce the bentonite content to 0.2 %, the reduction swelling indices would be 16.23 %, which was nearly the same with the base pellet (16.31 %). Therefore, based on the two series of experiments and with the purpose of further reducing the bentonite content and improving the reduction swelling of pellets, Series 3 experiments, adding combination additive made up of boron mud and boron-rich materials instead of boron mud, were carried out. The ratio of boron mud content and boron-rich materials content in combination additive was 1:1, and the chemical composition was listed in Table 4, wherein the boron content increased obviously, magnesium content increased slightly, kalium and sodium content decreased obviously, compared with boron mud. Other pellets materials, such as iron ore concentrate and bentonite, were the same with the materials used in the above experiments. Considering that quality of combination additive was better than boron mud, combined with previous experiments results, precondition of this series of experiments was bentonite content decrease to 0.2 %, and combination additive content was 2.0 %, 1.5 %, 1.0 %, 0.8 %, respectively.
The results of green balls properties showed that, with bentonite content 0.2 %, as combination boron mud additive content decreased, shatter strength was 3.9 times, 3.7 times, 3.6 times and 3.3 times, respectively. Comprehensive strength was 13.5 N, 13.4 N, 12.8 N and 12.5 N, respectively. Green balls properties can meet the requirements of subsequent process.
The effects of combination additive on comprehensive strength of pellets after roasting at 1,523 K were shown in Figure 6. As combination additive content decreased, comprehensive strength after roasting decreased as well, and with combination additive content 0.8 %, comprehensive strength was 3,019 N, which was very close to that of base pellet (3,059 N). The analyses of SEM (Figure 7) showed that, compared with base pellet, structures internal pellets with combination additive were more compact, and binding phases distributed more equally. With low melting point, B2O3 in combination additive formed low-melting-point complex compounds or solid solution with many oxides, and produced liquid core. It can promote rearrangement of particles to high density, and promote activation of crystal lattice to reduction activation energy. Therefore, with combination additive, bentonite content can be lessened to 0.2 %, while the pellets comprehensive strength was still close with base pellet.
The effects of combination additive on reduction swelling indices of pellets were shown in Figure 8. As combination additive content decreased, reduction swelling indices of pellets decreased as well, and with combination additive content 0.8 %, reduction swelling indices of pellets was 14.70 %, which was lower than that of base pellet(16.31 %).
Within certain limits, boron mud additive improved the comprehensive strength of pellets after roasting, and pellets reduction swelling indices were more or less steady. Appropriate addition content of boron mud was 1.0 %.
When boron mud content was 1.0 %, bentonite content can be decreased to 0.3 %, and pellets properties were close to that of base pellet.
With combination boron mud additive content 0.8 %, bentonite content can be further decreased to 0.2 % and reduction swelling indices of pellets decreased obviously.
Used as pellets additive, boron mud secondary resource can be utilized comprehensively and efficiently, which has a broad prospect with considerable economic benefits and social benefits.
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About the article
Published Online: 2016-09-24
Published in Print: 2017-07-26
This work is supported by Ph. D. Programs Foundation of Ministry of Education of China (No. 20100042110004) and Fundamental Research Funds for the Central University (No. N090502004 and N140206003).