Promoting the Effective Utilization of Limonitic Nickel Laterite by the Optimization of (MgO + Al₂O₃)/SiO₂ Mass Ratio During Sintering

Abstract

Limonitic nickel laterite always contains abundant high smelting components such as Al₂O₃ and MgO, which is extremely adverse to sintering. To weaken the negative effect of the abundant high smelting components, in this study, sinter pot tests of limonitic nickel laterite were conducted for better sintering performance by the optimization of (MgO + Al₂O₃)/SiO₂ mass ratio at the same basicity. The relevant mechanism was revealed by the theoretical analysis and the measurement of chemistry and mineralogy of product sinter. At the optimum (MgO + Al₂O₃)/SiO₂ mass ratio of 1.10, better sintering performance of limonitic nickel laterite can be achieved with tumble index and productivity increased by 8.22% and 7.22%, respectively, and solid fuel rate reduced by 7.41% compared with that of base case. Metallurgical performance of product sinter is excellent with reduction index (RI) and reduction degradation index (RDI_{+3.15 mm}) of as high as 83.66% and 95.64%, respectively. In this case, liquid phase volume and fluidity are both significantly improved and nicely suitable for sintering. The optimization of (MgO + Al₂O₃)/SiO₂ mass ratio contributes to the homogenization of liquid phase and the generation of tighter sinter microstructure with the obvious reduction of sinter porosity, the more formation of silico-ferrite of calcium and alumina (SFCA) and the improvement of bonding efficacy of solid phase by liquid phase. In the follow-up study, (MgO + Al₂O₃)/SiO₂ mass ratio is considered to be adjusted by adding another type of nickel laterite with relatively lower MgO and Al₂O₃ contents for further improving limonitic nickel laterite sintering and eventually achieving its effective utilization.

1. Introduction

The demand for stainless steel worldwide is increasing strongly while its major nickel source, i.e., nickel sulfide ore, is gradually depleted due to the excessive exploitation and poor exploration results.^1,2,3) Thus, nickel laterite accounting for 70% of the total nickel resources is regarded as one of good alternatives. Nickel laterite is generally divided into two types including saprolite and limonite.^4,5,6,7) Limonitic nickel laterite is considerably abundant with 60% share of the total nickel laterite resources. It is essential for stainless steel industry to effectively utilize limonitic nickel laterite resources.

As confirmed in the previous investigations,^8,9,10,11) limonitic nickel laterite is more suitable for the sintering-blast furnace (BF) process rather than Rotary Kiln-Electric Furnace (RKEF) process because of its relatively higher iron and lower nickel grades. However, sinter indices of limonitic nickel laterite are extremely poorer with solid fuel rate of over 140 kg/t, tumble index and productivity of below 50% and 1.0 t·m⁻²·h⁻¹, respectively, due to the excessively higher crystal water content and massive high smelting components such as Cr₂O₃, Al₂O₃ and MgO, which are far from the demand of blast furnace production.^12,13,14) Thus, sintering performance of limonitic nickel laterite must be effectively improved.

Combined with the relevant studies,^{15,16,17,18,19,20,21)} Al₂O₃ and MgO possess higher melting points and their increased contents lead to the higher formation temperature of primary liquid phase and the gradual deterioration of liquid phase fluidity during sintering. This would further result in the substantial increase of solid fuel consumption for the requirement of higher sintering temperature and the poor sinter consolidation due to the obviously weakened bonding ability of liquid phase. On the other hand, SiO₂ is favourable to the improvement of liquid phase fluidity and the reduction of the melting point of liquid phase, which contributes to weakening the adverse effect of Al₂O₃ and MgO on sintering process.^22,23,24,25) During ordinary iron ore sintering, the Al₂O₃/SiO₂ mass ratio and sinter basicity (CaO/SiO₂) are generally taken into consideration due to that the MgO content of ordinary iron ores is always very low (<1%), which have been confirmed having great influence on the liquid phase formation and sinter consolidation.^26,27,28,29) Nevertheless, limonitic nickel laterite always simultaneously contains high Al₂O₃ and MgO contents. In order to improve the sintering performance of limonitic nickel laterite, the (MgO + Al₂O₃)/SiO₂ mass ratio should be optimized. However, few literatures have been found on this issue yet.

Hence, on account of the characterization of limonitic nickel laterite, sinter pot tests with different (MgO + Al₂O₃)/SiO₂ mass ratios were systematically conducted at the same sinter basicity and the relevant mechanism was demonstrated via the theoretical analysis and the measurement of chemistry and mineralogy of product sinter for revealing the influence of (MgO + Al₂O₃)/SiO₂ mass ratio on limonitic nickel laterite sintering.

2. Materials and Methods

2.1. Raw Materials

Tables 1 and 2 present the chemical compositions and size distributions of the adopted raw materials, respectively. Limonitic nickel laterite is imported from Philippines and contains 1.09% NiO and 3.45% Cr₂O₃, which is characterized by much lower iron grade and higher contents of Al₂O₃ and MgO compared with ordinary iron ores. In addition, its LOI (Loss on ignition) is as high as 12.49%. These factors would be adverse to limonitic nickel laterite sintering.^16,17) Ferronickel tailing as a solid waste produced from the direct reduction-magnetic separation process of low grade nickel laterite is used to substitute for the conventional sinter fluxes due to the similar chemical compositions. Quartz and burnt lime are adopted to adjust (MgO + Al₂O₃)/SiO₂ mass ratio and basicity of sinter, respectively. Anthracite as the only solid fuel can provide sufficient heat for sintering. The low contents of phosphorus and sulfur contribute to the reduction of stainless steel production costs. In addition, limonitic nickel laterite is identified as a typical ore for sintering with +5 mm and 1–3 mm fractions contents of 16.16% and 44.78%, respectively. The particle contents of sinter fluxes and solid fuel passing 3 mm are all over 70%, which is perfectly suitable for sintering.

Table 1. Chemical compositions of the adopted raw materials (wt-%).

Samples	Fetotal	NiO	Cr2O3	SiO2	CaO	Al2O3	MgO	P	S	LOIa)
Limonitic nickel laterite	45.09	1.09	3.45	5.70	0.12	4.50	5.58	0.001	0.011	12.49
Ferronickel tailing	5.82	0.65	0.86	44.08	1.04	1.34	31.98	0.001	0.005	6.84
Quartz	0.00	0.00	0.00	99.99	0.00	0.00	0.00	0.000	0.000	0.00
Burnt lime	0.22	0.00	0.00	4.20	82.07	1.18	1.81	0.009	0.091	10.42
Anthracite	0.75	0.00	0.00	7.46	0.31	3.7	0.28	0.009	0.091	85.64

^a)  LOI: Loss on ignition of dried samples at 1000°C in air atmosphere.

Table 2. Size distributions of the adopted raw materials (wt-%).

Size/mm	+10	8–10	6.3–8	5–6.3	3–5	1–3	0.5–1	0.25–0.5	0.15–0.25	−0.15
Limonitic nickel laterite	2.80	3.30	4.06	6.00	22.35	44.78	6.43	5.83	1.17	3.28
Ferronickel tailing	0.00	0.00	0.00	0.01	1.62	3.56	17.75	25.78	8.08	43.20
Quartz	0.00	0.00	0.00	0.00	0.00	1.16	10.32	21.36	10.27	56.89
Burnt lime	0.00	0.00	0.63	1.75	21.27	20.11	5.19	2.96	0.81	47.29
Anthracite	1.68	1.58	1.70	3.70	18.08	15.23	15.46	14.10	5.22	23.25

Figure 1 describes the mineralogy of limonitic nickel laterite. Combined with Figs. 1(1#) and 1(2#), limonitic nickel laterite mainly consists of goethite, hematite, maghemite, Cr-spinel, enstatite and stishovite. In limonitic nickel laterite, alumina mainly occurs in iron-bearing minerals by the substitution Al³⁺ ions for Fe³⁺ ions, and silicon mostly exists in enstatite and stishovite whereas magnesium is primarily in form of Cr-spinel and enstatite. The occurrence of major elements including Fe, Al, Mg and Si is quite different from that of ordinary iron ores, which would have significant influence on limonitic nickel laterite sintering.

Fig. 1.

Mineralogy of limonitic nickel laterite (1#-X-ray diffraction patterns; 2#-The micrograph under scanning electron microscopy (SEM)). (Online version in color.)

2.2. Experimental Procedure

During sintering, limonitic nickel laterite and anthracite were adopted as the only iron-bearing raw material and solid fuel, respectively. Burn lime not only played a role of binder to improve granulation, but also was used to maintain sinter basicity at 1.4, which has been proved the optimum vaule for limonitic nickel laterite sintering.¹²⁾ The (MgO + Al₂O₃)/SiO₂ mass ratio of sinter varied from 0.70–1.50 by adjusting the quartz proportion in sinter mixture. Sinter pot tests were conducted in a pilot scale pot with 200 mm in diameter and 1000 mm in height and the test flowsheet included proportioning, mixing, granulation, ignition, sintering, cooling, crushing, dropping, sieving and quality testing of product sinter. The detailed methods were fully consistent with the descriptions of the previous investigation.¹²⁾

With the optimization of sinter mixture moisture and anthracite dosage, the optimum sinter indices of limonitic nickel laterite were obtained at different (MgO + Al₂O₃)/SiO₂ mass ratios as the return fines balance was achieved and the ratios of raw materials and the design chemical compositions are shown in Table 3. Then, the actual chemical compositions, metallurgical performance, including reduction degradation index (RDI) and reduction index (RI), and the mineralogy of product sinter were determined according to the presentations of the earlier studies.^13,30) The variations of the theoretical volume and viscosity of liquid phase with (MgO + Al₂O₃)/SiO₂ mass ratio were analyzed via the sectors of Equilib and Viscosity in FactSage-8.0 software.

Table 3. Blending conditions of raw materials and design chemical compositions.

(MgO+Al2O3)/SiO2 mass ratio	Sample ratios/(wt-%)						Design chemical compositions/(wt-%)			Sinter basicity
	Limonitic nickel laterite	Ferronickel tailing	Quartz	Burnt lime	Anthracite	Return fines	SiO2	Al2O3	MgO
1.50	53.60	1.50	0.00	7.40	7.50	30.00	7.64	4.86	6.61	1.40
1.30	51.37	1.50	0.98	8.85	7.30	30.00	8.62	4.73	6.51	1.40
1.10	48.29	1.50	2.31	10.90	7.00	30.00	9.95	4.55	6.36	1.40
0.90	43.44	1.50	4.06	13.50	7.50	30.00	11.69	4.31	6.17	1.40
0.70	36.79	1.50	6.51	17.20	8.00	30.00	14.18	4.01	5.85	1.40

3. Results

3.1. Influence of (MgO + Al₂O₃)/SiO₂ Mass Ratio on Sinter Indices

Figure 2 indicates the variations of sinter indices of limonitic nickel laterite with (MgO + Al₂O₃)/SiO₂ mass ratio. The base value of (MgO + Al₂O₃)/SiO₂ mass ratio is 1.50 without the addition of any quartz. As (MgO + Al₂O₃)/SiO₂ mass ratio of product sinter is decreased from 1.50 to 1.10, tumble index and productivity of product sinter are increased from 45.87% and 0.97 t·m⁻²·h⁻¹ to 49.64% and 1.04 t·m⁻²·h⁻¹, respectively, whereas solid fuel rate is reduced from 140.52 kg/t to 130.11 kg/t. Sintering performance of limonitic nickel laterite is substantially improved by adjusting (MgO + Al₂O₃)/SiO₂ mass ratio. However, the further reduction of (MgO + Al₂O₃)/SiO₂ mass ratio to 0.70 is adverse to limonitic nickel laterite sintering with tumble index and productivity lowered to 43.29% and 0.91 t·m⁻²·h⁻¹, respectively, and solid fuel rate increased to 149.97 kg/t. Thus, the appropriate reduction of (MgO + Al₂O₃)/SiO₂ mass ratio of product sinter contributes to the improvement of sintering performance of limonitic nickel laterite. Overall, the optimum (MgO + Al₂O₃)/SiO₂ mass ratio for limonitic nickel laterite sintering is recommended at 1.10. Better sinter indices of limonitic nickel laterite can be obtained with tumble index and productivity increased by 8.22% and 7.22%, respectively, and solid fuel rate reduced by 7.41% compared to that of base case.

Fig. 2.

Variations of sintering performance of limonitic nickel laterite with (MgO + Al₂O₃)/SiO₂ mass ratio (1.4 basicity).

3.2. Metallurgical Performance of Product Sinter

Table 4 compares the metallurgical performance of product sinter at different (MgO + Al₂O₃)/SiO₂ mass ratios. As (MgO + Al₂O₃)/SiO₂ mass ratio is reduced from 1.50 to 1.10, RI of product sinter is improved from 81.34% to 83.66%. In addition, RDI of product sinter is excellent regardless of (MgO + Al₂O₃)/SiO₂ mass ratio. RDI_{+6.3 mm} and RDI_{+3.15 mm} of product sinter are over 90% and 95%, respectively, and RDI_{−0.5 mm} is as low as no more than 2%. The appropriate reduction of (MgO + Al₂O₃)/SiO₂ mass ratio leads to better metallurgical performance of product sinter, which is more favorable for blast furnace production.

Table 4. Metallurgical performance of product sinter (wt-%).

(MgO + Al2O3)/SiO2 mass ratio	RI/(wt-%)	RDI/(wt-%)
		RDI+6.3 mm	RDI+3.15 mm	RDI−0.5 mm
1.50 (Base case)	81.34	92.95	96.65	1.08
1.10 (Optimum)	83.66	91.71	95.64	1.47

4. Discussion

4.1. Theoretical Analysis

Figure 3 illustrates the variations of the theoretical volume and viscosity of liquid phase with (MgO + Al₂O₃)/SiO₂ mass ratios during limonitic nickel laterite sintering at the optimum basicity of 1.4.¹²⁾ As shown in Fig. 3(1#), liquid phase volume is increased with sintering temperature. In addition, the decrease of (MgO + Al₂O₃)/SiO₂ mass ratio contributes to the formation of liquid phase. As confirmed in the previous studies,^24,31) the suitable liquid phase volume for sintering is about 40–50% and sintering temperature of limonitic nickel laterite is generally kept at 1300–1350°C. When (MgO + Al₂O₃)/SiO₂ mass ratio is as high as 1.50, solid phase cannot be effectively bonded by liquid phase due to the limited liquid phase volume during sintering, eventually leading to loose sinter microstructure and poor sinter strength. As (MgO + Al₂O₃)/SiO₂ mass ratio is reduced to below 1.10, liquid phase volume is so high that the permeability of high temperature zone would be greatly deteriorated and cannot satisfy the requirement of limonitic nickel laterite sintering due to the excessive diffusion of liquid phase. In order to obtain the suitable liquid phase volume, (MgO + Al₂O₃)/SiO₂ mass ratio should be maintained at about 1.10.

Fig. 3.

Influence of (MgO + Al₂O₃)/SiO₂ mass ratio on the theoretical volume (1#) and viscosity (2#) of liquid phase at the basicity of 1.4 during limonitic nickel laterite sintering (MS- (MgO + Al₂O₃)/SiO₂ mass ratio). (Online version in color.)

According to the description of Fig. 3(2#), liquid phase viscosity is gradually reduced with sintering temperature while it is increased with (MgO + Al₂O₃)/SiO₂ mass ratio due to the more formation of high smelting minerals. As (MgO + Al₂O₃)/SiO₂ mass ratio exceeds 1.10, liquid phase viscosity is always higher than 0.50 and even over 1.00 at the sintering temperature of 1300–1350°C. It is too high for sintering process, which would lead to the uneven diffusion and the deterioration of bonding ability of liquid phase. Eventually, poor sinter consolidation and low sinter strength are obtained at the high liquid phase viscosity. When (MgO + Al₂O₃)/SiO₂ mass ratio is no more than 0.90, liquid phase viscosity is so low that the permeability of sinter bed would be deteriorated due to the blockage of sinter pores and the thickness of bond layer of liquid bonding phase may be reduced resulting in the formation of highly brittle sinter, which is extremely adverse to sintering process. Thus, the liquid phase viscosity must be appropriate for sintering process. Overall, the optimum (MgO + Al₂O₃)/SiO₂ mass ratio of product sinter should be recommended at 1.10, which can simultaneously ensure the suitable liquid phase volume and viscosity during sintering. This is exactly consistent with the results of Fig. 2.

4.2. Chemistry of Product Sinter

Table 5 demonstrates the actual chemical compositions of product sinter at different (MgO + Al₂O₃)/SiO₂ mass ratios. As (MgO + Al₂O₃)/SiO₂ mass ratio is adjusted from 1.50 to 1.10, the total iron content of product sinter is reduced to some extent with the addition of quartz. As indicated in Fig. 3, the optimization of (MgO + Al₂O₃)/SiO₂ mass ratio contributes to the formation of liquid phase including silico–ferrite of calcium and alumina (SFCA) and the reduction of liquid phase viscosity. Solid fuel consumption would be decreased due to the reduction of formation temperature of liquid phase. Consequently, the FeO content of product sinter is lowered from 21.15% to 18.53%. Thus, RI of product sinter can be improved with the optimization of (MgO + Al₂O₃)/SiO₂ mass ratio. In addition, the FeO content of product sinter is still as high as 18.53% even at the suitable (MgO + Al₂O₃)/SiO₂ mass ratio. Hence, RDI of product sinter is excellent due to the rare generation of internal stress and cracks during low temperature reduction.^32,33)

Table 5. Actual chemical compositions of product sinter.

Mass ratios				Chemical compositions/(wt-%)
(MgO + Al2O3)/SiO2	Al2O3/SiO2	CaO/SiO2	MgO/SiO2	Fetotal	FeO	NiO	Cr2O3	SiO2	CaO	Al2O3	MgO
1.50 (Base case)	0.64	1.40	0.86	43.95	21.15	1.08	3.36	7.69	10.79	4.89	6.65
1.10 (Optimum)	0.46	1.40	0.64	40.46	18.53	0.99	3.1	9.93	13.93	4.57	6.35

As proved in the previous investigations,^19,26,27,28) the optimum sinter basicity (CaO/SiO₂ mass ratio) of ordinary iron ores is generally over 1.8 and the suitable Al₂O₃/SiO₂ mass ratio is about 0.30–0.50. By contrast, the appropriate sinter basicity and (MgO + Al₂O₃/SiO₂) mass ratio during limonitic nickel laterite sintering are 1.40 and 1.10, respectively, whereas the Al₂O₃/SiO₂ and MgO/SiO₂ mass ratios are 0.46 and 0.64, respectively. Obviously, the suitable sinter basicity of limonitic nickel laterite and ordinary iron ores is very different while the suitable Al₂O₃/SiO₂ mass ratios are basically similar regardless of the MgO/SiO₂ mass ratio. This indicates that the mineralization process of limonitic nickel laterite sintering is more complicated and very different from that of ordinary iron ore sintering.

4.3. Mineralogy of Product Sinter

Figure 4 shows the microstructure of product sinter of limonitic nickel laterite under optical microscopy. At the (MgO + Al₂O₃)/SiO₂ mass ratio of the base value (i.e., 1.50), product sinter is characterized by large thin-wall pores and sinter pores are always interconnected with sinter porosity of as high as 48.92% (Table 6). This highly porous sinter microstructure leads to much poorer sinter strength while it contributes to the higher RI of product sinter due to the easier diffusion of reducing gas. With the optimization of (MgO + Al₂O₃)/SiO₂ mass ratio, the pore size and interconnectivity are both obviously decreased and sinter porosity is significantly lowered to 35.63% at the (MgO + Al₂O₃)/SiO₂ mass ratio of 1.10 due to the more formation of liquid phase and the improvement of liquid phase fluidity. Thus, better sinter indices of limonitic nickel laterite can be obtained at the suitable (MgO + Al₂O₃)/SiO₂ mass ratio.

Fig. 4.

Microstructure of product sinter under optical microscopy (A and B represent the (MgO + Al₂O₃)/SiO₂ mass ratios of 1.50 and 1.10, respectively; 1# and 2# are the optical micrographs with the magnifications of 50× and 200×, respectively; H-Hercynite, K-Eutectic olivine, SFCA-Silico-ferrite of calcium and alumina, C-Cr-spinel, P-Pore, LP-Large thin-wall pore, R-Resin). (Online version in color.)

Table 6. Porosity of product sinter (area-%).

(MgO + Al2O3)/SiO2 mass ratio	1.50 (Base case)	1.10 (Optimum)
Porosity/(area-%)	48.92	35.63

Combined with Table 7 and Figs. 4–5, solid phases in product sinter of limonitic nickel laterite mainly include hercynite, a small amount of Cr-spinel and nickel-ferric spinel whereas liquid bonding phases contain eutectic olivine and SFCA. Hercynite as the major solid phase exhibits granular, tabular and dispersive textures (Fig. 5) and is identified as (Fe, Mg)Fe₂O₄, Fe(Fe, Al)₂O₄ and (Fe, Mg)·(Fe, Al)₂O₄, respectively (Table 7), which is formed by the partial substitution of Al³⁺ or Mg²⁺ ions for Fe³⁺ or Fe²⁺ ions.³⁴⁾ SFCA acts as the most desirable bonding phase for sintering and mainly exists in acicular, dendritic and tabular textures (Figs. 4 and 5). In addition, eutectic olivine is generated by the eutectic reaction of kirschsteinite (CaO·FeO·SiO₂), monticellite (CaO·MgO·SiO₂) and fayalite (2FeO·SiO₂) with hercynite in different proportions.^12,13,31)

Table 7. EDS analysis results for areas in Fig. 5.

Area No.	Elemental compositions/(atomic conc, %)								Mineral phases
	Fe	Cr	Ni	Mg	Al	Si	Ca	O
A-1	32.26	0.18	0.11	3.49	5.21	0.19	0.06	58.50	(Fe, Mg)·(Fe, Al)2O4
A-2	12.89	0.12	0.07	6.21	0.33	13.87	14.31	52.20	CaO·(Fe, Mg)Fe2O4·SiO2
A-3	36.21	0.15	0.10	5.34	0.67	0.25	0.13	57.15	(Fe, Mg)Fe2O4
A-4	13.28	0.18	0.14	0.73	5.90	14.26	13.95	51.56	CaO·FeAl2O4·SiO2
A-5	33.82	0.22	0.13	0.55	3.62	5.01	6.11	50.54	SFCA
A-6	31.06	0.37	0.09	0.63	4.88	4.79	7.51	50.67	SFCA
A-7	29.36	0.19	0.12	0.53	6.29	5.35	7.96	50.20	SFCA
A-8	20.35	13.61	0.11	3.88	5.72	0.27	0.36	55.70	(Fe, Mg)·(Cr, Fe, Al)2O4
A-9	28.86	0.38	16.67	0.42	0.39	0.33	0.25	52.70	NiFe2O4
A-10	36.27	0.16	0.06	0.58	6.69	0.45	0.37	55.42	Fe(Fe, Al)2O4
A-11	12.24	0.21	0.15	4.35	4.71	13.56	13.88	50.90	CaO·(Fe, Mg)Al2O4·SiO2
B-1	18.57	16.81	0.05	4.75	7.37	0.45	0.28	51.72	(Fe, Mg)·(Cr, Fe, Al)2O4
B-2	11.34	0.29	0.14	5.23	4.46	16.78	12.63	49.13	CaO·(Fe, Mg)Al2O4·SiO2
B-3	35.38	0.35	0.16	5.19	0.54	0.28	0.11	57.99	(Fe, Mg)Fe2O4
B-4	31.42	0.23	0.08	3.65	4.83	0.39	0.24	59.16	(Fe, Mg)·(Fe, Al)2O4
B-5	26.39	0.42	16.27	0.61	0.55	0.23	0.31	55.22	NiFe2O4
B-6	35.13	0.25	0.06	0.84	6.34	0.65	0.44	56.29	Fe(Fe, Al)2O4
B-7	29.54	0.32	0.17	0.26	6.13	5.56	7.83	50.19	SFCA
B-8	31.37	0.28	0.20	0.51	4.72	5.03	7.35	50.54	SFCA
B-9	34.16	0.36	0.04	0.44	3.57	4.93	6.27	50.23	SFCA

Fig. 5.

Microstructure of product sinter under SEM (A and B represent the (MgO + Al₂O₃)/SiO₂ mass ratios of 1.50 and 1.10, respectively; 1# and 2# are the SEM micrographs in different areas). (Online version in color.)

As (MgO+Al₂O₃)/SiO₂ mass ratio is 1.50, eutectic olivine presents three forms such as CaO·(Fe, Mg)Al₂O₄·SiO₂, CaO·FeAl₂O₄·SiO₂ and CaO·(Fe, Mg)Fe₂O₄·SiO₂ (Figs. 5(A-1#) and 5(A-2#)), and SCFA is always formed at the edge of pores with limited amount (Figs. 4(A-1#) and 5(A-2#)). Furthermore, hercynite is characterized by small size, scattered distribution and low interconnection degree and cannot be adequately bonded by liquid bonding phases such as eutectic olivine and SFCA (Figs. 5(A-1#) and 5(A-2#)). Eventually, loose structure of product sinter is formed, leading to poor sinter strength of limonitic nickel laterite. With (MgO + Al₂O₃)/SiO₂ mass ratio adjusted to 1.10, eutectic olivine is converted into one type, i.e., CaO·(Fe, Mg)Al₂O₄·SiO₂ (Fig. 5(B-1#) and Table 7), due to the appropriate reduction of liquid phase viscosity, which is beneficial to the homogenization of liquid phase during sintering. SFCA is extended to the interior of product sinter along the edge of the pores with obviously increased amount (Figs. 4(B-1#) and 5(B-2#)). The improvement of liquid phase fluidity contributes to the diffusion of particles during sintering and then the aggregation and growth of hercynite grains (Figs. 5(B-1#) and 5(B-2#)). Hercynite is more commendably wetted by eutectic olivine and SFCA, leading to the densification of sinter microstructure. Thus, sinter strength of limonitic nickel laterite is substantially improved with the optimization of (MgO + Al₂O₃)/SiO₂ mass ratio.

Table 8 summarizes the mineral compositions of product sinter at different (MgO + Al₂O₃)/SiO₂ mass ratios. As (MgO + Al₂O₃)/SiO₂ mass ratio of product sinter is reduced from 1.50 to 1.10, the amount of SFCA is increased from 8.78% to 16.72% and the total amount of liquid phase is increased from 35.21% to 42.32%. The optimization of (MgO + Al₂O₃)/SiO₂ mass ratio actually promotes the formation of liquid phase, which is perfectly in accordance with the results of theoretical analysis in Fig. 3. Furthermore, with the significant reduction of sinter porosity, the homogenization of liquid phase and the formation of tighter sinter microstructure, much better sinter strength and productivity can be obtained at the optimum (MgO + Al₂O₃)/SiO₂ mass ratio of 1.10. In addition, lower solid fuel consumption should be able to satisfy the requirement of limonitic nickel laterite sintering due to the effective reduction of melting point and viscosity of liquid phase. Thus, the proper decrease of (MgO + Al₂O₃)/SiO₂ mass ratio indeed contributes to better sinter performance of limonitic nickel laterite. Although sinter porosity is decreased, the more formation of SFCA still promotes the improvement of RI of product sinter due to that SFCA possesses much better reducibility than other liquid phases. Consequently, metallurgical performance of product sinter is excellent at the optimum (MgO + Al₂O₃)/SiO₂ mass ratio. However, the addition of quartz would reduce the total iron and nickel contents of product sinter to some extent and increase the slag emission during blast furnace production. In the follow-up study, it is considered that (MgO + Al₂O₃)/SiO₂ mass ratio can be optimized by adding another type of nickel laterite with relatively lower MgO and Al₂O₃ contents for much better sinter indices and more effective utilization.

Table 8. Mineral compositions of product sinter (area-%).

(MgO + Al2O3)/SiO2 mass ratio	Solid phases/(area-%)					Liquid phases/(area-%)
	Hercynitea)			Cr-spinel	Nickel-ferric spinel	Eutectic olivine phasesb)			SFCA
	H	H-1	H-2			K	K-1	K-2
1.50 (Base case)	17.14	20.81	23.24	2.55	1.05	17.23	2.13	7.07	8.78
1.10 (Optimum)	19.36	16.64	18.28	2.39	1.01	25.60	–	–	16.72

^a)  H-(Fe, Mg)·(Fe, Al)₂O₄, H-1-Fe(Fe, Al)₂O₄, H-2-(Fe, Mg)Fe₂O₄;

^b)  K–CaO·(Fe, Mg) Al₂O₄·SiO₂, K-1-CaO·FeAl₂O₄·SiO₂, K-2-CaO·(Fe, Mg)Fe₂O₄·SiO₂.

5. Conclusions

(1) At the optimum (MgO + Al₂O₃)/SiO₂ mass ratio of product sinter of 1.10, tumble index and productivity are increased by 8.22% and 7.22%, respectively, and solid fuel rate is reduced by 7.41% compared with that of base case. Sintering performance of limonitic nickel laterite is significantly improved with the optimization of (MgO + Al₂O₃)/SiO₂ mass ratio of product sinter. Metallurgical performance of product sinter is excellent with RI and RDI_{+3.15 mm} of as high as 83.66% and 95.64%, respectively.

(2) The reduction of (MgO + Al₂O₃)/SiO₂ mass ratio of product sinter promotes the formation of liquid phase and the improvement of liquid phase fluidity. As (MgO + Al₂O₃)/SiO₂ mass ratio of product sinter is maintained at the optimum value of 1.10, liquid phase volume and viscosity can both be suitable for sintering. In addition, the high FeO content of product sinter of 18.53% leads to excellent RDI of product sinter.

(3) The optimization of (MgO + Al₂O₃)/SiO₂ mass ratio of product sinter contributes to the obvious reduction of sinter porosity, the more formation of SFCA and the improvement of bonding efficacy of solid phase by liquid phase. Eventually, the homogenization of liquid phase and tighter sinter microstructure are achieved. Thus, better sinter performance of limonitic nickel laterite and higher RI of product sinter can be obtained.

(4) The total iron and nickel contents of product sinter are lowered to some extent with the reduction of (MgO + Al₂O₃)/SiO₂ mass ratio via the addition of some quartz in the sinter mixture. In order to achieve further improvement of sinter performance and more effective utilization of limonitic nickel laterite, it is considered to adjust (MgO + Al₂O₃)/SiO₂ mass ratio by blending this limonitic nickel laterite with another type of nickel laterite containing relatively lower MgO and Al₂O₃ contents in the next research project.

Acknowledgements

Financial support from the Major Project of Master Alloy Manufacture for Heat Resistant Stainless Steel Production No. (AA18242003) funded by the Provincial Government of Guangxi Zhuang Autonomous District, is sincerely acknowledged.

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