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Materials Transactions, Vol. 57, No. 9 (2016) pp. 1560 to 1566 ©2016 The Japan Institute of and Materials

Optimal Combination of and Reduction Conditions as well as Na2SO4 Additive for Carbothermic Reduction of

Guan-Jhou Chen1, Jia-Shyan Shiau2, Shih-Hsien Liu2 and Weng-Sing Hwang1,*

1Department of Materials Science and Engineering, National Cheng Kung University, No.1, University Road, Tainan City 701, Taiwan, R.O.C. 2Iron Making Process Development Section, & Research & Development Dept., China Steel Corporation, 1 Chung Kang Road Hsiao Kang, Kaohsiung 81233, Taiwan, R.O.C.

The optimal carbothermic reduction parameters of limonite ore and the in uence of calcining temperature on these were investigated. In addition, the impact of the addition of Na2SO4 on limonite ore which had been calcined before reduction at optimal carbothermic conditions was also investigated. XRD analysis, BET-specic surface area analysis, bromine methyl alcohol solution analysis, and chemical compositional analysis were used in order to obtain the associated parameters. The best grade and recovery rate of the 673 K-calcined limonite ore can reach >30 mass% and 90.2 mass%, respectively, when the reduction temperature is 1373 K, with a reduction time of 30 min, and a - ratio of 0.6. This is because the 673 K-calcined limonite 2 have the highest specic surface area of 46.8 m /g with pores in the size of 29.7 Å. The addition of Na2SO4 by 5 mass% resulted in the best nickel grade of >30 mass% and the best recovery rate of 93.8 mass% at the same reduction temperature, time and carbon-oxygen ratio. [doi:10.2320/matertrans.M2016072]

(Received March 4, 2016; Accepted June 27, 2016; Published August 25, 2016)

Keywords: limonite, calcination, carbothermic reduction, additive

1. Introduction and high pressure acid (HPAL). Although the recov- ery rate is high, this process is now used less than previous Nickel (Ni) is a ferromagnetic element with a high melting days because of its low economic benets and the large point at 1728 K and thermal endurance, excellent corro- amount of toxic waste it produces, thus causing serious pollu- sion-resistance and antioxidation, good mechanical proper- tion if piled in unsheltered conditions. The technol- ties, superior malleability and chemical stability. Its major ogies include a rotary -electric furnace (RKEF) process, a application is for the smelting of steel and iron, especially for sintering- suldation smelting process and a sin- the manufacturing of stainless steel, which accounts for 61% tering-blast furnace smelting process. The industrial applica- of the total consumption of metallic nickel.1–5) The nickel that tion of hybrid hydrometallurgical-re rening has been explored in the world are approximately 160 million can be found in the Nippon Yakin Company’s Oheyama tons, of which 30–40% is nickel sulde ore and 60–70% is smelting plant. Laterite nickel ore has many applications, and nickel ore. Since the technology used to extract nickel can be used to produce intermediate products like nickel ox- from the nickel sulde ore is more mature, 60–70% of nickel ide, nickel- or nickel iron , of which the nickel-sul- production comes from this. However, the amount of nickel fur and nickel oxide can be used by nickel reneries to solve sulde ore which can be exploited is limited, and the reserves the problem of insufcient nickel sulde. A nickel-iron alloy of nickel oxide ore are still rather high and the cost is can also be used as a substitute for pure nickel addition in the relatively low. As such, the use of nickel oxide ore has be- manufacturing of stainless steel and low-nickel alloy come an important issue. steel.11–15) An oxidized-type of laterite nickel ore is formed from the According to the Gibbs free energy of the Ellingham dia- mass nickel-containing serpentine in tropical or subtropical gram, the lower the coordinate in the diagram, the lower and zones through long-term weathering and erosion effects. It is more stable the free energy, which indicates that the in an oxidized-type of silicate with a loose clay soil ap- the lower part of the diagram will stably exist, and thus be pearance containing hydrous oxides of iron, aluminum and difcult to reduce. In contrast, in the upper range of the Ell- silicon. The mineral vein of laterite nickel ore can generally ingham diagram the existence of oxides causes higher free be divided into two layers: the upper layer is limonite-type energy in the reaction, which then destabilizes the oxides, laterite nickel ore, which contains more iron and less nickel making them more readily reduced. This concept can be ap- (about 0.8–1.5%); the lower layer is saprolite-type magne- plied to the selected reduction of oxides, where manufactur- sium silicate nickel ore, which contains less iron and more ers can allow certain oxides to be reduced by controlling the nickel, and the nickel content is about 1.5–2.5%.6–10) temperature and reducing atmosphere, based on the associat- At present, there are three categories of rening technolo- ed . By applying this mechanism to later- gies for nickel: the hydrometallurgical process, re-rening ite nickel ore in a carbothermic reduction experiment, the re- process and hybrid hydrometallurgical-re rening process. duction of the nickel oxide will take place before the iron Two types of hydrometallurgical processes are used in indus- oxide starts to be reduced. In addition, by adjusting the trial production, reduction - leaching (AL) amount of reductant that is added, as well as the reduction temperature and reduction time, one can control the reduction * Corresponding author, E-mail: [email protected] level of selected oxides. Optimal Combination of Calcination and Reduction Conditions as well as Na2SO4 Additive for Carbothermic Reduction of Limonite Ore 1561

With the addition of reductants, and with the appropriate to meet industrial standards: 15–16 mass% for nickel grade control of temperature and time, the carbothermic reduction and 85 mass% for the recovery rate. process of laterite nickel ore can be conducted in order to extract the phases of nickel and iron from the mineral 2. Experimental Procedure phase.16–20) Many studies on the thermal analysis of laterite nickel ore report that the free is removed at tempera- The experimental ow chart of this research is shown in tures between 333 K and 383 K, dehydration occurs at 553 K, Fig. 1. The rst step was to prepare the samples for the exper- the serpentine phase transforms into enstatite and olivine at iment. In order to conduct the calcination experiment, Indo- 898 K, and the recrystallization reaction happens at 1093 K.21) nesian limonite-type laterite nickel ores undergone heat treat- Zevgolis et al.22) pointed out that a higher specic surface ment at 673, 873 and 1173 K, each for 2 h, in advance. The area is helpful to the carbothermic reduction of laterite nickel ores and reductants (graphite powder) were then separately ore, and the limonite-type laterite nickel ore has a lower re- ground into ne powders, which could pass through a 200- duction rate. As indicated by previous study,23) the mineral mesh sieve, and these were dried at 378 K until they had no phase of limonite-type laterite nickel ore could be simplied free water. Next, the powders were mixed based on the calcu- by calcining at 1173 K. Therefore, after being calcined in ad- lated carbon-oxygen ratio (C/O), and the additive (Na2SO4) vance, the following carbothermic reduction process (at a re- and binding agent (Bentonite) were then added. Finally, the action temperature of 1373 K) could enhance the nickel grade powders were manually rolled into small pellets with a diam- of the laterite nickel ore, accompanied by a lower recovery eter of about 12 mm. rate. Yang et al.24) proposed that garnieritic-type laterite nick- The procedures of the high temperature carbothermic re- el ore has a phase transformation at the calcining temperature duction experiment are as follows: Let the inert gas (N2, of 873–973 K, and the accompanying increase in surface area 500 cc/min) ow into the muf e furnace; start the furnace and the generation of ne pores are also helpful to the carbo- and let the temperature rise to 1373 K. Put the sample pellets thermic reduction. At a calcining temperature of 973–1123 K, into the furnace; conduct the carbothermic reduction process laterite nickel ore will start its recrystallization, reducing its and then, take the sample out of the furnace after the preset specic surface area and generating bigger pores, which are reaction time is completed. Place the hot sample into a nitro- detrimental to the following carbothermic reduction. There- gen- owing cooling box until it is chilled to room tempera- fore, for garnieritic-type laterite nickel ore, the calcining tem- ture. A schematic diagram of the furnace is shown as Fig. 2. perature benecial to the carbothermic reduction is 873– The optimal carbothermic reaction parameters with the 973 K. best nickel grade and recovery rate were investigated through salts or other alkaline additives can react with min- the carbothermic reduction process of 1173 K-calcined limo- erals containing aluminum oxide and silicates, thus forming sodium aluminate or sodium aluminum silicates. Moreover, the sodium salts can intensify the reduction of iron oxides, resulting in the separation of iron from aluminum and sili- 25) 26,27) con. As indicated by Li et al., the addition of Na2SO4 to the carbon-bearing pellets of laterite nickel ore is benecial to the reduction of nickel, resulting in a better nickel grade and recovery rate. Sun et al.28) studied the in uence of four additives (Na2SO4, Na2CO3, CaCO3 and CaSO4) on the car- bothermic reduction of low-nickel high-iron laterite nickel ore. Their results showed that Na2SO4 suppressed the reduc- Fig. 1 Flow chart of experiment. tion of iron, and that the dissolution of Na2SO4 could gener- ate sulfur which would reduce the surface energy and melting point of the nickel-iron pellets, prompting the growth of the pellets and inducing their separation from other oxide-phase compositions. After being dissolved, Na2SO4 could generate alkaline oxides and sulfur. The sulfur would then react with iron to produce iron sulde, suppressing the reduction of iron. Meanwhile, the alkaline oxides could improve the reduction effect on the carbothermic reduction. By studying the reduction of 1173 K-calcined limo- nite-type laterite nickel ore under different reaction times and carbon-oxygen ratios, this study has obtained the optimal car- bothermic reduction conditions. The in uence of calcining temperatures (673, 873 and 1173 K) on the ore’s nickel grade and recovery rate when it was reduced under xed carbother- mic reduction conditions was also examined. Since the added Na2SO4 can improve the nickel grade and recovery rate of laterite nickel ore, this research studied the optimal addition level that could yield the best nickel grade and recovery rate Fig. 2 Schematic diagram of furnace. 1562 G.-J. Chen, J.-S. Shiau, S.-H. Liu and W.-S. Hwang

Table 1 Chemical analysis of limonite ores calcined at different tempera- Table 2 Chemical composition of bentonite, unit: mass%. tures, unit: mass%. Elemental P S CaO MgO Al2O3 SiO2 K2O Na2O Calcining Ni Co CaO MgO Al O SiO Fe O FeO TFe Bentonite 0.03 0.37 4.63 1.25 12.9 62.8 0.62 1.69 Temp. (T/K) 2 3 2 2 3 RT 1.18 0.16 0.15 4.41 4.50 10.2 59.7 23.4 41.1 673 1.41 0.13 0.25 5.64 4.78 11.1 59.3 3.72 44.4 873 1.37 0.13 0.26 6.10 5.17 11.9 67.2 0.37 47.3 1173 1.28 0.01 0.20 6.11 5.24 11.9 70.0 0.12 49.0

nite ore while its reaction time varied between 10 to 40 min and the carbon-oxygen ratio ranged between 0.4 and 0.9. In addition, the in uence of different calcining temperatures (673, 873, and 1173 K) on the nickel grade and recovery rate under xed carbothermic reduction conditions was also probed. The impact of the addition of Na2SO4 (5, 10 mass%) on the carbothermic reduction of limonite ore calcined at dif- ferent temperatures (673, 873 and 1173 K) was studied. Analysis of the iron samples was carried out as follow: 1. Compound analysis of the reduction products through the X-ray diffraction method; 2. Analysis of the nickel content by bromine methyl alcohol solution analysis;23) 3. Chemical analysis of the content of each element by the Inductively Coupled Plasma (ICP-OES) method to calculate the nickel grade and recovery rate; 4. Measurement of the specic sur- Fig. 3 XRD analysis results of limonite ore (a) calcined at 673 K and (b) face area of the pellets and the average size of their pores calcined at 873 K. before reduction through BET specic surface area analysis (using a Surface Area & Mesopore Analyzer). The nickel grade and recovery rate were calculated using the following equations: H2O). When the temperature reached 794 K, the crystal water was removed, the goethite (including limonite rich in iron M (%) Ni (%) = Ni 100 (1) phase) was pyrolyzed into magnetite and the serpentine was grade M (%) + M (%) Ni Fe × pyrolyzed into olivine and enstatite. Recrystallization took place at 1059 K. M Ni (%) = Ni 100 (2) Figures 3(a) and 3(b) show the XRD analysis results of li- recovery T Ni × monite ore calcined at 673 K and 873 K, respectively. Fig- MNi: Content of metal Ni ure 3(a) indicates that after having been calcined at 673 K for MFe: Content of metal Fe 2 h the ore contains potassium silicate TNi: Total content of Ni (K2MgSiO4), calcium silicate (Ca2SiO4), hematite (α-Fe2O3), For chemical analysis, the major elements of limonite ore iron oxide (Fe21.333O32), quartz (α-SiO2) and magnetite calcined under different temperatures are shown in Table 1. (Fe3O4). As shown in Fig. 3(b), the major phases of limonite The contents of nickel, Fe2O3, and FeO in the uncalcined li- ore calcined at 873 K for 2 h are hematite (α-Fe2O3), iron ox- 23) monite ore were 1.18 mass%, 59.7 mass% and 23.4 mass%, ide (Fe21.333O32) and quartz (α-SiO2). Previous study respectively. For the 673 K-calcined limonite ore, the con- showed that when limonite ore is calcined at 1173 K for 2 h, tents of nickel, Fe2O3 and FeO were 1.41 mass%, 59.3 mass% the major phases are hematite (α-Fe2O3) and magnetite and 3.72 mass%, respectively. For the 873 K-calcined limo- (Fe3O4). nite ore, the contents of nickel, Fe2O3 and FeO were During calcination at 673 K, it was found that silicon diox- 1.37 mass%, 67.2 mass% and 0.37 mass%, respectively. For ide combined with other elements to form extra compounds. the 1173 K-calcined limonite ore, the contents of nickel, However, this was not the case when the calcining tempera- Fe2O3 and FeO were 1.28 mass%, 70.0 mass% and ture was 873 K and above. As the calcining temperature rose, 0.12 mass%, respectively. the phase components of the limonite ore became simpler, As the temperature rose, the iron oxide phase was oxidized and the extra phases disappeared. After calcination, the iron into Fe2O3. After calcination, the mass fractions of nickel and oxides of the limonite ore were oxidized into Fe2O3 and iron increased because of the removal of water and other im- Fe3O4, which were more difcult to be reduced to an iron purities. Table 2 shows the chemical composition of benton- phase. ite, which is used as the binding agent for pelleting. Previous report20) showed that these phenomena can be in- terpreted as follows. The free water was removed at 388 K. Dehydroxylation occurred at 657 K (2αFeOOH→αFe2O3+ Optimal Combination of Calcination and Reduction Conditions as well as Na2SO4 Additive for Carbothermic Reduction of Limonite Ore 1563

3. Results and Discussion lot of carbon remained in the pellets after reduction for 30 min; therefore, further investigation regarding the best car- 3.1 Optimal reduction conditions versus reduction re- bon-oxygen ratio was needed to nd the optimal reduction sults of 1173 K-calcined limonite ores parameters. Previous study indicated that the best reduction tempera- Table 4 shows the chemical analysis results of calcined ture was 1373 K for calcined limonite ore.23) However, this (1173 K, 2 h) limonite ore which was reduced for 30 min at earlier research focused on the optimal carbothermic reduc- 1373 K under different carbon-oxygen ratios. From the table, tion parameters for limonite-type laterite nickel ore by exam- it is noted that as the carbon-oxygen ratio went up, the con- ining reactions under various reduction times and carbon-ox- tents of the metallic iron and nickel also increased from ygen ratios. Table 3 shows the chemical composition of limo- 0.18 mass% to 7.46 mass% and from 0.76 mass% to nite ore which was calcined at 1173 K for 2 h in advance, and 1.01 mass%, respectively. With the increase in carbon-oxy- then reduced for different reduction times under a tempera- gen ratio, the content of carbon increased from 5.12 mass% to ture of 1373 K with a carbon-oxygen ratio of 0.7. The results 9.04 mass%. Figure 5 shows a bar diagram of the nickel indicate that the content of nickel and iron increases as the grade and recovery rate of calcined (1173 K, 2 h) limonite ore reduction time is prolonged; the iron content increased from which was reduced for 30 min at 1373 K under different car- 0 to 3.51 mass%, and the nickel content increased from 0.25 bon-oxygen ratios. to 0.98 mass%. The results shows a declining trend for the nickel grade During the reduction process, the carbon combined with from 80.85 mass% to 11.9 mass%, accompanied by an in- iron oxide and nickel oxide to form a metallic iron-nickel crease in the carbon-oxygen ratio, as sufcient carbon con- phase, reducing the reacted content of carbon from 8.30 to tent could reduce the associated content of ore into metallic 5.87. Figure 4 shows a bar diagram displaying the nickel iron, consequently lowering the nickel grade. Along with the grade and recovery rate of the limonite ore which had been rising carbon-oxygen ratio, the nickel recovery rate rst rose calcined at 1173 K for 2 h in advance, and then reduced for from 60.3 mass% to 79.5 mass% and then stayed constant. different reduction times under a temperature of 1373 K with According to Table 4 and Fig. 5, both the best nickel grade a carbon-oxygen ratio of 0.7. As the reduction time was pro- (22.1 mass%) and recovery rate (78.6 mass%) were obtained longed, the nickel grade rst declined from 100 mass% to when the carbon-oxygen ratio was 0.6. 21.8 mass% and then, remained constant. This was because Integrating these discoveries regarding reduction time and metallic iron gradually formed from the reduction process as carbon-oxygen ratio, the best nickel grade (22.1 mass%) and time passed, thus reducing the nickel grade. recovery rate (78.6 mass%) of calcined (1173 K for 2 h) li- The nickel recovery rate rst increased and then declined. monite-type laterite nickel ore could be obtained under a re- As time passed, the rate increased from 18.5 mass% to duction temperature of 1373 K, a reduction time of 30 min 75 mass%, which meant the ore was gradually reduced to me- and a carbon-oxygen ratio of 0.6. In order to meet industrial tallic nickel. According to Table 3 and Fig. 4, a 30-min reduc- standards: a 15–16 mass% for nickel grade and 85 mass% tion time could generate the best nickel grade (20.6 mass%) and recovery rate (75.0 mass%). However, it is noted that a Table 4 C/O ratio vs. chemical composition of calcined (1173 K, 2 h) li- monite ore reduced under 1373 K with C/O ratio of 0.7, unit: mass%.

Table 3 Reduction time vs. chemical composition of calcined (1173 K, 2 h) C/O Fe TNi Ni C limonite ore reduced under 1373 K with C/O ratio of 0.7, unit: mass%. 0.4 0.18 1.26 0.76 5.12 Time (t/min) TFe FeO Fe TNi Ni C 0.5 0.40 1.26 0.84 5.86 10 44.2 23.8 / 1.37 0.25 8.30 0.6 3.49 1.26 0.99 6.08 20 46.0 45.7 0.24 1.39 0.87 7.18 0.7 3.67 1.27 0.95 6.17 30 47.4 53.6 3.67 1.27 0.95 6.17 0.8 4.46 1.27 0.96 7.62 40 47.3 50.9 3.51 1.44 0.98 5.87 0.9 7.46 1.27 1.01 9.04

Fig. 4 Relationship between reduction time and nickel grade and recovery Fig. 5 Relationship between nickel grade and recovery rate of calcined rate of calcined (1173 K, 2 h) limonite ore reduced under 1373 K with (1173 K for 2 h) limonite ore reduced at 1373 K for 30 min with different C/O ratio of 0.7, unit: mass%. C/O ratios. 1564 G.-J. Chen, J.-S. Shiau, S.-H. Liu and W.-S. Hwang

Table 5 Chemical composition results of limonite ore calcined at different temperatures and reduced under optimal reduction conditions, unit: mass%.

Calcining Temp. (T/K) TNi Ni TFe FeO Ni grade Ni recovery 673 1.43 1.29 47.9 46.0 >30 90.2 873 1.44 1.05 47.9 46.2 >30 72.9 1173 1.26 0.99 47.7 52.6 22.1 78.6

Table 6 BET analysis results of limonite ore calcined at different tempera- tures.

Calcining Temp. (T/K) Surface Area (m2/g) Average pore radius (Å) RT 25.5 31.0 673 46.8 29.7 873 30.7 44.9 1173 12.5 61.5

Fig. 6 Schematic diagram of specic surface area and pore size. and above for the recovery rate, and to make up for an insuf- cient recovery rate, this research also examined other pa- rameters. From Fig. 3, it is noted that three reactions hap- pened around 657 K, 794 K and 1059 K. Therefore, the cor- face area. Under a calcining temperature of 873 K, phase responding effects of the calcining temperatures of 673 K, transformation of limonite ore began, and its serpentine phase 873 K and 1173 K on the optimal reduction parameters of li- transformed to olivine and enstatite, decreasing the specic monite-type laterite nickel ore were investigated. surface area and slightly enlarging the pore size. Recrystalli- zation started at 1173 K, and a sintering effect occurred in the 3.2 Calcining temperatures versus carbothermic reduc- interior of the pellets, making the pore size bigger and result- tion results ing in a large loss of specic surface area. The whole process According to previous research,26,28) the reduction efcien- is illustrated in Fig. 6. cy of laterite nickel ore calcined at 873–1273 K is good, while According to the reaction formula 2αFeOOH→αFe2O3+ that for limonite-type laterite nickel ore is still not suitable for H2O, the hydrogen cations and crystal water contained in the reduction even after being calcined. Therefore, this research lattice are removed by the temperature effect at 673 K, result- investigated the effects of calcining temperatures (673, 873 ing in more ne gaps among the lattice and increasing the and 1173 K) on the optimal reduction parameters. Table 5 specic surface area, which would be benecial to the spread shows the chemical composition of limonite ore calcined at of gas and eventually speed up the reduction of nickel. A re- three temperatures and reduced under optimal reduction con- duction temperature of 1373 K was too low for the iron com- ditions. The table shows that the metallic nickel content de- pound to be fully reduced into metallic iron. Therefore, the clined from 1.29 mass% to 0.99 mass% as the calcining tem- best nickel grade (>30 mass%) and recovery rate (90.2 mass%) perature increased. Therefore, when the calcined temperature for limonite ore can be obtained when it is calcined at 673 K was 673 K, the best nickel recovery rate was obtained and in advance, and then, reduced under optimal reduction pa- relatively less metallic iron was produced. Meanwhile, the rameters (a reduction temperature of 1373 K, a reduction nickel grade decreased in the range of 22.1–30 mass% as the time of 30 min and a carbon-oxygen ratio of 0.6). In order to calcined temperature increased because the metallic iron was improve limonite ore calcined at 873 K and 1173 K and to gradually reduced from its oxides leading to a decline in the make the nickel grades and recovery rates reach 15–16 mass% nickel grade. The nickel recovery rate rst declined from and 85 mass%, respectively, as required by industrial stan- 90.2 mass% to 72.9 mass% and then rose to 78.6 mass% dards, the previous researchers25–28) proposed the addition of along with the rising temperature. Table 6 shows the results Na2SO4, which could effectively increase the reduction of of the BET analysis of the carbon-containing pellets of limo- metallic nickel. This study thus probed the effects of the addi- nite ore calcined at three temperatures. These indicate that as tion of different amounts of Na2SO4 on the nickel grade and the calcining temperature rose, the specic surface area rst recovery rate of limonite ore which was calcined at different increased from 25.5 m2/g to 46.8 m2/g and then, decreased to temperatures in advance and then reduced under optimal re- 12.5 m2/g; and the average pore size rst decreased from duction parameters. 31.0 Å to 29.7 Å and then, increased to 61.5 Å. The results show that the lattice of the uncalcined limonite 3.3 Impact of different amounts of additives ore whose specic surface area was smaller, contained free Table 7 shows the chemical analysis results of calcined water, crystal water and hydrogen cations. When the calcin- (673, 873 and 1173 K) limonite ore which was reduced under ing temperature was 673 K, dehydroxylation occurred, re- optimal reduction parameters. The results indicate that the moving some crystal water and hydrogen cations, leaving metallic nickel content of 673 K-calcined limonite ore in- pores with similar sizes and thus increasing the specic sur- creased from 1.29 mass% to 1.40 mass%, as the amount of Optimal Combination of Calcination and Reduction Conditions as well as Na2SO4 Additive for Carbothermic Reduction of Limonite Ore 1565

Table 7 Chemical analysis results of limonite ore calcined at different tem- peratures after being reduced under optimal reduction parameters and with additives of various amounts, unit: mass%.

Calcining Content TNi Ni TFe FeO Ni grade Ni recovery Temp. (T/K) 0 1.43 1.29 47.9 46.0 >30 90.2 673 5 1.45 1.36 48.7 49.9 >30 93.8 10 1.44 1.40 48.1 54.3 21.6 97.2 0 1.44 1.05 47.9 46.2 >30 72.9 873 5 1.45 1.37 48.9 52.7 >30 94.5 10 1.46 1.50 48.7 52.0 16.5 100 0 1.26 0.99 47.7 52.6 22.1 78.6 1173 5 1.20 1.26 48.5 53.5 23.1 100 10 1.29 1.48 47.7 45.0 17.4 100

Na2SO4 added rose during the reduction process. For 873 K-calcined limonite ore, the metallic nickel content grew from 1.05 mass% to 1.50 mass% along with the increase of Na2SO4 during the carbothermic reduction process. Further- more, the increase of Na2SO4 during the carbothermic reduc- tion process of 1173 K-calcined limonite ore caused the me- tallic nickel content to rise from 0.99 mass% to 1.48 mass%. Generally speaking, the metallic nickel content of limonite ore (calcined at 673 K, 873 K and 1173 K) grew along with the increase in Na2SO4 in the carbothermic reduction process. Therefore, it is deduced that the addition of Na2SO4 in the reduction process could effectively reduce the ore back to a metallic nickel phase. The results also show that the nickel grade of 673 K-cal- cined limonite ore increased to 21.6 mass% with an increase in the amount of Na2SO4 added during the reduction process. 873 K-calcined limonite ore displayed a similar trend, as its nickel grade increased to 16.5 mass% as the amount of Na2SO4 added increased. However, the 1173 K-calcined li- monite ore presented a different pattern; its nickel grade rst rose from 22.1 mass% to 23.1 mass% and then, descended to 17.4 mass% as the amount of Na2SO4 added increased. When the amount of additive increased, it was easier to reduce the ore to a metallic iron phase, causing the nickel grade to de- cline. After being reduced, the nickel recovery rate of 673 K-cal- cined limonite ore increased from 90.2 mass% to 97.2 mass% as the added amount of Na2SO4 increased. The 873 K-cal- cined limonite ore showed a similar trend; its nickel recovery rate after reduction rose from 72.9 mass% to 100 mass% as the addition of Na2SO4 increased. This was also the case for the 1173 K-calcined limonite ore, whose nickel recovery rate Fig. 7 XRD analysis results of limonite ore after being reduced under opti- grew from 78.6 mass% to 100 mass% with a rise in the mal reduction parameters and with additives of various amounts: (a) cal- cined at 673 K, (b) calcined at 873 K and (c) calcined at 1173 K. amount of Na2SO4 added during the reduction process. It is thus concluded that Na2SO4 could prompt effective displace- ment of the metallic nickel, increasing its content and recov- ery rate.28) er, XRD peaks corresponding to the Fe-phase appeared as the Figure 7 shows the XRD analysis results of limonite ore amount of Na2SO4 increased. Figures 7(b)(c) also show a calcined at different temperatures (673, 873 and 1173 K) and similar pattern in which the peaks corresponding to the metal- then, reduced under optimal reduction parameters and with lic iron increased as the amount of Na2SO4 increased. different amounts of additive. Figure 7(a) indicates that after The results of the experiment show that the addition of being reduced, the major phases of the 673 K-calcined limo- Na2SO4 could aid in the reduction of calcined limonite ore to nite ore were Fe2SiO4, Fe0.9712O and (Fe, Mg)2SiO4. Howev- metallic nickel and iron, increasing the nickel recovery rate. 1566 G.-J. Chen, J.-S. Shiau, S.-H. Liu and W.-S. Hwang

This is because the sodium salts combined with the aluminum 30 min, and the carbon-oxygen ratio is 0.6. and silicon in the mineral phase, forming aluminum-sili- con-sodium salts and displacing the metallic nickel and iron Acknowledgments phases, while the sulfur combined with the iron to form iron sulde, suppressing the reduction of the metallic iron.28) This work was support by the Ministry of Science and From experimental results and the discussion outlined Technology of Taiwan under grants MOST 103-2221-E-006- above, our study found that the optimal reduction conditions 059-MY3 and MOST 103-2622-E-006-037. of 1173 K-calcined limonite ore are 1373 K for the reduction temperature, 30 min for reduction time and 0.6 for the car- REFERENCES bon-oxygen ratio, which contribute to the best nickel grade of 22.1 mass% and recovery rate of 78.6 mass%. Through the 1) Z.C. Cao, T.C. Sun, H.F. Yang, J.J. Wang and X.D. Wu: J. Univ. 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