Coupled Preparation of Ferronickel and Cementitious Material from Laterite Nickel Ores

Nickel slags can be produced through ferronickel preparation by the pyrometallurgical processing of laterite nickel ores; however, such techniques are underutilized at present, and serious environmental problems arise from the stockpiling of such nickel ores. In this study, a modification to the process of ferronickel preparation by the direct reduction of carbon bases in laterite nickel ores is proposed. The gangue from the ore is used as a raw material to prepare a cementitious material, with the main components of tricalcium silicate and tricalcium aluminate. By using FactSage software, thermodynamic calculations are performed to analyze the reduction of nickel and iron and the effect of reduction on the formation of tricalcium silicate and tricalcium aluminate. The feasibility of a coupled process to prepare ferronickel and cementitious materials by the direct reduction of laterite nickel ore and gangue calcination, respectively, is discussed under varying thermodynamic conditions. Different warming strategies are applied to experimentally verify the coupled reactions. The coupled preparation of ferronickel and cementitious materials with calcium silicate and calcium aluminate as the main phases in the same experimental process is realized.


Introduction
Nickel (Ni) is a transition element that exhibits a mixture of ferrous and nonferrous metal properties, and is used in many industries, for example for making stainless steel, superalloy, non-ferrous alloy, superalloy, coin, and batteries [1][2][3][4][5]. World nickel reserves are abundant [6]. Identified land-based resources averaging 1% nickel or greater contain at least 130 million tons of nickel, with about 60% in laterites and 40% in sulfide deposits. In 2019, nickel resources are divided into two types of ores: sulfide and laterite, of which sulfide ore accounts for about 30% and laterite nickel ore accounts for the rest [7][8][9]. In China, about 70% of nickel reserves are concentrated in Gansu Province, next to Canada's Sadbury nickel, Asia rank the first and world rank the second, followed by Xinjiang, Yunnan, Jilin, and other places [10][11][12]. Before 2000, because of available upgrading methods, availability of high-grade ore, and low energy consumption in the extraction process, nickel was mainly produced from sulfide (60%), while laterite needed comprehensive treatment to extract nickel metal, resulting in a higher processing cost of laterite than nickel sulfide [13][14][15]. In recent years, with the rapid development of our national economy, the consumption of nickel is increasing day by day. At the same time, the total amount and grade of nickel sulfide ore resources shows a downward trend, and laterite is becoming increasingly important in the future. Therefore, pursuers begin to improve the nickel recovery technology of laterite nickel ore [16,17]. Laterite nickel ore has low exploration cost, mature technology, and broad development prospects. Therefore, nickel oxide mining is listed as a key project in China's "medium and long term scientific development and regulation of non-ferrous metal industry (2006-2020)" (draft for comments) [18,19].
The nickel content of laterite ores is very low, and special treatments are needed to improve the nickel content of these ores, such as physical separation techniques, but the complexity of laterite nickels makes it difficult to achieve any significant breakthroughs [20][21][22][23][24][25]. Thermal modification of laterite ore by carbonization, dehydration, and reduction technologies has been shown to significantly improve nickel extraction in the acid leaching stage [26][27][28][29][30][31][32][33]. Pyrometallurgy is the main process route for nickel extraction from saprolite ore, and there are two commonly used laterite nickel smelting methods [34,35]. One is the rotary kiln dry-electric furnace reductive smelting. The laterite nickel ore is placed in the rotary kiln for drying intervention reduction at a temperature of 650-800 • C, and then placed in the reducing furnace at 1550-1600 • C for smelting and sorting to obtain coarse nickel iron. Nickel-iron alloys containing more than 25% nickel can be produced after further refining [36]. The second method is rotary kiln direct reduction. After drying, crushing, and sieving, the raw ore is mixed with lime and pulverized coal in proportion to the pellet. Drying and high temperature reduction produce spongy nickel-iron alloys and slag mixtures. After cooling, grinding, screening, and magnetic separation, the coarse nickel-iron particles were obtained. However, there are some problems in laterite nickel mining technology. In the process of rotary kiln drying-furnace reduction smelting, the melting temperature is high, the power consumption is large, the overall energy consumption is large, the slag production is large, the direct use failure is serious. Due to the direct reduction of rotary kiln, tailings yield is large and slag cannot be directly used, which increases the cost of subsequent treatment of nickel slag [37][38][39][40][41][42][43][44][45]. However, in the process of fire smelting of nickel-iron from laterite, the gangue components in the raw ore are comprehensively treated and applied to high value-added materials, so as to realize the green metallurgical utilization of laterite nickel, which has the characteristics of low cost, high added value, and short flowing time. There are great similarities between technology and equipment in traditional rotary kiln production of direct reducing iron and cement clinker production. First of all, the main equipment is the same rotary kiln, and the raw materials for the production of cement clinker are similar to the gangue composition in iron ore. Secondly, their main production processes are the same. All raw materials must be processed by the process of abrasive mixing-roasting-abrasive, and the roasting temperature is relatively close. The difference is that in the production process of direct reduced iron, it is necessary to separate the direct reduced iron from the gangue components after grinding by means of magnetic separation, while the purpose of cement clinker abrasive is to control the size of cementing materials. In the traditional direct reduced iron production process, gangue formation consumes a lot of energy in each process, but it cannot form high value-added products in the end. Instead, it is treated as solid waste.
Due to the correlation between the two production processes, we proposed the idea of coupling the process of preparing direct reduced iron with the process of preparing cementitious materials. The purpose of the experiment is to design a reaction coupling process. In the process of preparing direct reduced iron or nickel-iron alloy, it can also make the gangue component in the ore react with the CaO flux to produce the cementitious material. In a material flow and energy flow process, two different valuable products of nickel-iron alloy and cementitious material can be obtained without producing solid waste. Compared with the traditional process, the new process has the characteristics of high energy efficiency, high resource efficiency, and no solid waste. In this study, while preparing nickel iron from carbon base of laterite nickel ore by direct reduction, the gelling materials with silicate tricalcium silicate (C 3 S) and aluminate tricalcium (C 3 A) as main components were prepared from coal gangue. The coupling of these two reaction processes was investigated experimentally.

Material Analysis and Burdening
In the experiment, X-ray diffraction (XRD) analysis and fluorescence spectroscopy chemical component analysis were performed on the raw laterite nickel ore (Jinchuan Group Ltd., South Halmahera, Indonesia). The X-ray diffractometer used for detection is the Empyrean type with the scanning rate of 8 • /min from PANalytical B.V (Almelo, Netherlands). The XRD analysis result from the laterite nickel ore is shown in Figure 1, and the chemical components determined through fluorescence spectroscopy are listed in Table 1.  The figure shows that the main phases of the laterite nickel ore are FeO(OH) (Goethite), Al(OH) 3 (Gibbsite), and Fe 2 O 3 . The XRD peaks for the SiO 2 phase are comparatively lower, and those for other minerals with calcium, magnesium, and nickel are not obvious. The laterite nickel ore thus has FeO(OH), Al(OH) 3 , and Fe 2 O 3 as the main components.
According to XRD analysis and fluorescence spectroscopy, in the laterite nickel ore, FeO(OH) and Fe 2 O 3 are the main phases. Among them, the content of Fe 2 O 3 reaches 46.26%, the content of SiO 2 is 3.30%, and the content of Al(OH) 3 is equivalent to Al 2 O 3 , which accounts for 8.77% of the total mass, while the content of Ca, Mg, Ni, and other elements is less. Based on these characteristics of phase composition, through thermodynamic analysis and calculation, the process coupling for the reduction of nickel and iron and the formation of C 3 S, C 3 A, and other cementitious materials via the reaction of CaO with SiO 2 or Al 2 O 3 is calculated. Other elements with lower contents are not considered in the thermodynamic analysis.
Based on the above analysis, the mix ratio of reactants in the raw material is calculated first. The principle for burdening mainly includes two aspects: (1) The addition of reducing agent must ensure the complete reduction of nickel and iron; (2) the added amount of CaO must be sufficient to fully react with SiO 2 and Al 2 O 3 to produce C 3 S and C 3 A. According to these calculations, the ratio of laterite nickel ore and raw materials for reduction roasting is shown in Table 2. All the raw materials are ground to particles of less than 200 mesh in size. Burdening was prepared in conformity with the calculated ratio and reaction samples were made by fully blending. The thermodynamic analysis and calculation were performed based on this sample's ratio of raw materials.

Thermodynamic Analysis of Coupling Reaction
In the reduction roasting process, chemical reactions including the decomposition of FeO(OH) (Goethite) and Al(OH) 3 (Gibbsite), reduction of nickel and iron, reaction of iron oxides with CaO to form calcium ferrite, reaction of CaO with Al 2 O 3 to form CaAl 2 O 4 (CA) and Ca 3 Al 2 O 6 (C 3 A), reaction of CaO with SiO 2 to create Ca 3 SiO 5 (C 3 S, alite) and Ca 2 SiO 4 (C 2 S, Belite), and reaction of CaO with C 2 S to create C 3 S may occur. FeO(OH) and Al(OH) 3 can decompose into Fe 2 O 3 and Al 2 O 3 after heating, respectively; thus, Fe 2 O 3 and Al 2 O 3 are used to replace the original phases in thermodynamic analysis and calculation, following the relevant chemical reaction Equations (1)- (9). Using the Reaction model in FactSage software (FactSage 6.3, Thermfact/CRCT, Montreal, Canada and GTT-Technologies, Aachen, Germany), the possible standard Gibbs free energy produced by the reactions among the main phases during reduction roasting is calculated at intervals of 10 • C between 0 • C and 2000 • C, and the relationship of temperature and standard Gibbs free energy ∆G for each equation is drawn via Origin (Origin 2019, OriginLab, Northampton, MA, USA) as shown in Figure 2 [46].
3CaO + SiO 2 →Ca 3 SiO 5 CaO + Ca 2 SiO 4 →Ca 3 SiO 5 (9) As shown in Figure 2a, in the temperature range 500-1450 • C, Equations (1), (3), and (8) proceed spontaneously to realize the reduction of nickel and the production of CA, C 3 A, C 2 S, C 3 S, and Ca 2 Fe 2 O 5 . In Equation (2), the temperature at the beginning of iron reduction is 653 • C. At temperatures below 780 • C, the reaction of Equation (3) consumes CaO and Fe 2 O 3 and then produces Ca 2 Fe 2 O 5 ; because the reaction proceeds more easily than iron reduction in Equation (2), this temperature regime is unfavorable for iron reduction. At temperatures above 780 • C, the reaction of Equation (2) proceeds before that of Equation (3); Fe 2 O 3 is reduced to iron and the temperature for Fe 2 O 3 reduction should be higher than 780 • C, under which nickel reduction can also occur.
Through the comparison of the ∆G-T curves of Equations (4)- (6) in Figure 2a, when the temperature is above 500 • C, CaO and Al 2 O 3 react to produce C 3 A more easily. The product of CA also reacts further with CaO to produce C 3 A; therefore, at high temperatures, the reaction of CaO with Al 2 O 3 finally produces the C 3 A phase.
Through the comparison of the ∆G-T curves of Equations (7) and (8) in Figure 2a, at 1300 • C, the reaction given by Equation (7) proceeds more easily compared to that given by Equation (8) to produce C 2 S. At temperatures above 1300 • C, C 3 S is more easily produced. From Figure 2b, only at temperatures between 1300 • C and 1800 • C does the reaction of Equation (9) occur spontaneously; then, C 2 Se produced early in the reaction reacts with calcium oxide to generate C 3 S. Combining Figure 2a,b, to acquire a cementitious material with C 3 A and C 3 S as the main phases, the temperature must be between 1300 • C and 1800 • C.
To discuss the reactant phase compositions under high temperatures when iron is not fully reduced, the phase model in FactSage software is used to determine the phase diagram for the quaternary oxide system of CaO-SiO 2 -Al 2 O 3 -Fe 2 O 3 at 1350°C. Supposing that unreduced iron comprises 10% of the total mass of this quaternary oxide system, the resulting phase diagram is as shown in Figure 3. The red line in Figure 3 indicates the raw material ratio of Al 2 O 3 /SiO 2 = 2.66, equal to that in the laterite nickel ore. The phase composition of the quaternary oxide system varies along the red line as the CaO content changes. For a continuous increase in CaO content, the phase composition of the quaternary oxide system also changes correspondingly. For lower CaO contents (area 4 in the figure), the phase composition includes CA, Ca 2 SiO 4 , Ca 2 Al 2 SiO 7 , and Ca 2 Fe 2 O 5 . As the CaO content increases (area 3), CaO reacts with Ca 3 Al 2 SiO 7 to produce C 3 A and C 2 S, and the Ca 3 Al 2 SiO 7 phase disappears. With further increases in CaO content (area 2), sufficient CaO reacts with CA to form C 3 A and consume CA; afterward, CaO reacts with C 2 S to generate C 3 S. When the CaO content reaches area 1, C 2 S is fully consumed, for it reacts completely with CaO to produce C 3 S. Additional CaO exists in the free state.
In addition, Fe 2 O 3 reacts with CaO at 1350 • C to produce Ca 2 Fe 2 O 5 , which coexists with CA, C 3 A, C 2 S, and C 3 S (areas 1-4) as impurities in cementitious materials. Thus, Fe 2 O 3 should be reduced to iron to avoid yielding Ca 2 Fe 2 O 5 . Figure 3 indicates that a sufficiently high CaO content (areas 1-3) can ensure the coexistence of silicate and aluminate systems independently under high temperatures. To further explore the interaction relationship of reduced iron with the silicate and aluminate systems, FactSage is used to determine the two ternary diagrams of Fe-CaO-Al 2 O 3 and Fe-CaO-SiO 2 . Iron is set to comprise 50% of the whole, as shown in Figures 4 and 5.   are the composition points of CaAl 2 O 4 and Ca 3 AlO 6 , respectively. In conclusion, when cementitious materials with the main phase of C 3 A are prepared, the effective mass of CaO combined with Al 2 O 3 should meet the proportion of CaO/Al 2 O 3 = 1.65 in the blending process, and the roasting temperature should be between 237 • C and 1540 • C. Figure 5 indicates that C 3 S is also unstable outside of the temperature range 1300 • C to 1800 • C. Stable C 3 S is produced by the reaction of aluminum oxide with C 2 S (areas 2-4) within this range. When the reaction temperature is lower than 1300 • C or higher than 1800 • C (areas 1 and 5), C 3 S is decomposed to C 2 S and free-state CaO, consistent with the thermodynamic calculations shown in Figure 2b. Meanwhile, the effect of reduced iron on the CaO-SiO 2 system is not significant, indicating that these three phases can coexist. At temperatures 1300 • C-1800 • C and lower CaO contents (CaO/SiO 2 < 1.40), neither C 3 S nor C 2 S is found in the system. The reaction of CaO with SiO 2 mainly produces CaSiO 3 and Ca 3 Si 2 O 7 . When the ratio of CaO/SiO 2 is between 1.40 and 1.86, C 2 S exists in the system (areas 7 and 8); when the ratio range is 1.86 to 2.79, C 2 S and C 3 S can coexist in the system (areas 3 and 4); when the ratio range exceeds 2.79, C 3 S, free-state CaO, and coexistent Fe (area 2) are the main phases, meaning that CaO/SiO 2 = 1.40 and CaO/SiO 2 = 1.86 are the composition points of Ca 3 Si 2 O 7 and C 2 S, respectively. In conclusion, when the cementitious materials with the main phase of C 3 S are prepared, the effective mass of CaO combined with SiO 2 should meet the proportion CaO/SiO 2 = 2.79 in the blending process, and the roasting temperature should be between 1300 • C and 1800 • C.
With powdered carbon as the reducing agent, laterite nickel ore as the raw material, and CaO as the additive, the theoretical analysis results of the changes in the phase composition of the reactants during heating are calculated under a protective CO atmosphere, by the equipment module in the FactSage software. As shown in Table 2, the reactant proportion was the same. Figure 6 shows the calculation result of the theoretical reaction product, which analyzes the phase change of the system from 500 • C to 1500 • C in a temperature interval of 50 • C. It can be seen from Figure 6 that in the reaction in the low temperature zone (<569 • C), the phases formed are mainly carbon, pyroxene, CaCO 3 , Fe 2 O 3 , nickel, and partly reduced iron. When the temperature exceeds 569 • C, a liquid iron-nickel phase is formed with a high content of nickel; the ratio of this liquid alloy phase increases with increasing of temperature. At temperatures above 653 • C, most of the Fe 2 O 3 and carbon reactants are consumed to form iron. For temperatures above 787 • C, CaCO 3 is decomposed into CaO, part of which reacts with SiO 2 and Al 2 O 3 to generate C 2 S and some Ca 3 Al 2 O 6 , while the pyroxene is consumed. When the reaction temperature is between 1093 • C and 1300 • C, there is no obvious change in each phase, and the main components of liquid iron-nickel alloy, iron, CaO, Ca 3 Al 2 O 6 , and C 2 S. However, at temperatures above 1300 • C, C 3 S is formed by the reaction of CaO and C 2 S. Under high-temperature reaction, the final products are liquid iron-nickel alloy, Ca 3 Al 2 O 6 , and C 3 S.
According to the above-mentioned thermodynamic analysis, it is feasible to prepare a cementitious material of C 3 A and C 3 S by the reduction roasting of laterite nickel ore after the appropriate proportioning. However, in order to ensure the sufficient reduction of nickel and iron as well as the effective production of C 3 A and C 3 S, the roasting temperature must be controlled by stages.
The reduction temperature of Fe 2 O 3 should be controlled at 780-1220 • C since Fe 2 O 3 reacts with CaO to form Ca 2 Fe 2 O 5 which affects iron reduction. Thus, in order to ensure complete iron reduction and prevent the formation of low-melting-point liquid fayalite (which melts at 1220 • C) by Fe 2 O 3 and SiO 2 , this temperature should be maintained for a certain period of time. Under these conditions, C 2 S and a small amount of Ca 3 Al 2 O 6 can be produced. After the sufficient reduction of iron, the temperature is increased again and then maintained at 1300-1540 • C (the temperature range for forming stable existent C 3 S is 1300-1800 • C, that for the decomposition of pyroxene is 1093 • C, and that for stable existent C 3 A is 237-1540 • C). In this regime, the formation of C 3 S and the reaction of pyroxene after decomposition with CaO to form Ca 3 Al 2 O 6 are accomplished. Finally, the samples are cooled by fast cooling; tricalcium silicate is metastable at normal temperatures with this treatment.

Experiments and Results
To verify the accuracy of the above thermodynamic analyses and calculations, coupled experimental studies are conducted under different reaction conditions. Three different control processes are set using three 8 g samples placed in separate Al 2 O 3 crucibles. Each crucible is placed in airtight tube-type resistance furnace with the heating rate set to 10 • C/min. The controlling processes for the three samples are as follows:

1.
No atmospheric protection; the temperature is increased from indoor ambient temperature to 1450 • C and maintained for 1 h before cooling to indoor temperature to obtain product A.

2.
At the flow rate of 60 mL/h under the atmospheric protection of CO, the temperature continues to rise from room temperature to 1000 • C, maintained for 2 h, and then increased to 1450 • C and maintained for 1 h. Product B was obtained after furnace cooling to room temperature. 3.
The period of cooling applied in (2) is changed. The atmosphere conditions and control process of the temperature increase to 1450 • C are the same as those used for obtaining sample B. After maintaining the temperature of 1450 • C for 1 h, by using fast cooling, product C was obtained.
Finally, the obtained reaction samples were examined by XRD and scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDS). The XRD equipment was the same as that used for raw material analysis; the SEM analysis equipment was type VEGA II XMU (TESCAN Company, Brno, Czechia); the EDS analysis equipment was type 7718 (Oxford Instruments, London, UK). The XRD and SEM-EDS analysis results are shown in Figures 7 and 8, respectively.
Iron oxide comprises more than 45% of the total sample mass, and the ferronickel alloy is 40% of the total mass in the theoretical reaction product. Therefore, it can be seen from the XRD pattern that the intensity of the diffraction peaks of iron-nickel alloy and iron oxide is higher than that of other phases.
As can be seen from the XRD analysis results, sample A shows incomplete iron reduction, attributed to the lack of atmosphere protection; iron is present as unreacted Fe 2 O 3 and Ca 2 Fe 2 O 5 after reacting with CaO. The consumption of CaO causes the reduction of CaO/Al 2 O 3 and CaO/SiO 2 . The diffraction peaks of the low calcium phase appear obviously in the XRD pattern of the reaction products, such as CaAl 2 O 4 and Ca 2 SiO 4 . At the same time, the diffraction peaks of Ca 3 Al 2 O 6 have a weak diffraction peak, which is consistent with the analysis results in Figure 3. At 1000 • C for 2h, sample B reacts in a reducing atmosphere to ensure that the nickel-iron is completely reduced to form an iron-nickel alloy, and is then held at 1450 • C for 1 h to form C 3 S. However, as a result of furnace cooling, C 3 S is decomposed into C 2 S and CaO phases. Therefore, the final products of sample B are iron, CaO, Ca 2 SiO 4 , CaAl 2 O 4 , and Ca 3 Al 2 O 6 , which are consistent with the analysis results of Figures 4 and 5. Sample C uses the same heating control method as sample B under the protection of CO to achieve the reduction of nickel-iron to form iron-nickel alloys, C 3 S and C 3 A. C 3 S avoids the large-scale decomposition using rapid cooling. The final products are iron, C 3 S, and Ca 3 Al 2 O 6 , and the decomposition products are CaAl 2 O 4 and Ca 2 SiO 4 , which show obviously stronger XRD peaks than that of Ca 2 SiO 4 in sample B. Simultaneously, the SEM-EDS analysis and scanning of sample C show that the morphology of the product is composed of many iron grains with different grain diameters and the cementing material in them. The distributions of nickel and iron are the same; those of calcium, aluminum, silicon, and oxygen are the same, and clear boundaries are observed between the ferronickel alloy and the cementitious materials. The analytical results of theoretical products shown in Figure 6 are consistent with the above analytical results.

1.
For the preparation of high-temperature coexisting cementitious materials such as ferronickel alloy, C 3 S, C 2 S, etc., it is feasible to reduce and roast laterite nickel ore after proportioning.

2.
It is necessary to control the coupling reaction temperature in stages. The reduction temperature of Fe 2 O 3 is controlled at 780-1220 • C, the formation temperature of C 3 S is controlled at 1300-1800 • C, and the stability of C 3 S is controlled at 237-1540 • C. Fast cooling must be applied in order to obtain a metastable C 3 S phase. 3.
At the reducing atmosphere protection of CO, the first reaction stage is holding at 1000 • C for 2 h, the second reaction stage is holding at 1450 • C for 1 h, and fast cooling is used. In this process, ferronickel alloy, C 3 S, and Ca 3 Al 2 O 6 laterite nickel ore are reduced and roasted, and the decomposition products are Ca 2 SiO 4 and CaAl 2 O 4 to obtain the final products. 4.
The reduction of iron has a significant effect on the formation of C 3 S. When iron is insufficiently reduced, calcium ferrite and other phases are formed in combination with CaO, hindering the formation of C 3 S. The necessary condition for the formation of C 3 S is the sufficient reduction of iron.

Funding:
The work was financially supported by the National Natural Science Foundation of China (51874224) and Project of young talents in basic research of Natural Science in Shaanxi Province (2014JQ7282).

Conflicts of Interest:
The authors declare no conflict of interest.