Nickel is an important strategic metal, widely used in the field of critical materials and high-tech stainless steel, high temperature alloys, fuel cells.
Currently, nickel sulfide ore resources are depleting, while accounting for 70% of nickel reserves of nickel oxide ore (laterite nickel ore) abundant resources, low cost exploration and mining, to produce nickel oxide, nickel matte, nickel, iron and other intermediate products. The utilization ratio of laterite nickel ore resources has accounted for more than 40% of the world's nickel production, and it is on the rise.
The process of producing nickel metal from nickel oxide ore is divided into fire method and wet method. The wet process has the problems of complicated process, long process, low recovery rate and high requirements on equipment, and is more suitable for treating laterite ore with low nickel and low magnesium content. The fire process has a blast furnace smelting method and a rotary kiln-electric furnace reduction smelting method (RKEF). With the demand for nickel-based raw materials in steel mills and the need for environmental protection, blast furnace smelting has gradually been eliminated. Although electric furnace smelting has the disadvantage of high energy consumption, it can process raw materials containing many refractory materials, high metal recovery rate, low furnace gas volume and low dust content, easy production control, and simultaneous recovery of nickel and iron. Nickel-iron alloy can directly replace electrolytic nickel, and it is used as a steel-making nickel element additive for stainless steel production, which has strong cost and competitive advantage. Therefore, the reduction and smelting of electric furnace to produce ferronickel is the most effective method for treating high-silica-high-magnesium laterite nickel ore.
In this study, the direct reduction smelting process of electric furnace was used to treat ferronickel in laterite nickel ore. The related influencing factors and their mechanism were discussed, and the smelting process parameters were optimized.
First, the test
(1) Test materials
The test laterite nickel ore has high content of MgO, SiO 2 and Ni, and is low in iron and cobalt . It belongs to the typical silicon-magnesium-nickel ore. The nickel grade is 1.99%, Ni/Fe=0.14, SiO 2 /Mg0=2.58. The fire process is used to produce stainless steel ferronickel. Mineral Composition of nickel laterite mainly iron enstatite (Ca 0.02 Fe 0.35 Mg 1.63 Si 2 O 6), tridymite (SiO 2) and diopside (CaMgSi 2 O 6) (FIG. 1). The reducing agent is coke breeze, and the fixed carbon component of the coke powder is 80.49%. The flux is limestone containing 50.65% CaO.
Figure 1 Raw ore X-ray diffraction pattern
(2) Test methods
The laterite nickel ore is dried, crushed, ground and mixed with a reducing agent, a flux, a binder, and water, then granulated and dried to control the granulating pellet diameter to be about 1 cm, and the drying temperature is 200 °C. The dried pellets are placed in a magnesite crucible, heated and smelted in an electric furnace, heated to a melting temperature, and then kept warm for a certain period of time, and then naturally cooled to room temperature to obtain a slag and a nickel-iron alloy which are separated up and down.
(3) Analysis and testing
The raw mineral composition was analyzed by XRD (Siemens D5000). Inconel nickel, and iron grade S, P content respectively dimethylglyoxime gravimetric method (GB / T223.25-1994), Chin chloride - Potassium weight chromium titration (GB / T8638.6-1988), Combustion infrared absorption spectroscopy (GB/T8647.8-2006), phosphorus molybdenum blue spectrophotometry (GB/T 8647.4-2006) for analysis.
Second, the results and discussion
(1) Effect of coke powder ratio on smelting and its mechanism
In the melting point of laterite nickel ore (1600～1700K), the stability of oxides in minerals is CaO>SiO 2 >Fe 2 O 3 >CoO>NiO. The smaller the stability, the easier it is to reduce. Therefore, in laterite nickel ore Reduction ability of each oxide: NiO>CoO>Fe 2 O 3 >SiO 2 >CaO. In order to improve the quality of ferronickel products, the principle of selective reduction in smelting ferronickel is adopted: by controlling the reduction conditions, the nickel oxide is reduced to metal as much as possible, while the high valence Fe 2 O 3 is partially reduced to metal and the rest is reduced to FeO. Or Fe 3 O 4 for slagging, so as to achieve the purpose of producing high nickel iron alloy.
The amount of iron reduced is controlled by the amount of reducing agent coke powder added. The ratio of fixed flux was 10%, the condition of melting at 1550 °C for 50 min was unchanged, the amount of coke powder was changed, and the effect of coke breeze ratio on nickel iron grade and metal recovery was investigated. The results are shown in Fig. 2 and Fig. 3, respectively. . It can be seen from Fig. 2 and Fig. 3 that when the proportion of coke powder is above 5%, the grade of nickel gradually decreases with the increase of the proportion of coke powder, and the recovery rate of nickel, drill and iron gradually increases.
Figure 2 Effect of coke powder ratio on nickel grade
Figure 3 Effect of coke powder ratio on metal recovery
As previously analyzed, the reduction ability of each oxide in the laterite nickel ore in a reducing atmosphere is NiO>FenO. When the amount of coke powder is small, Ni is preferentially reduced than iron, so the grade of nickel in the alloy is high. As the proportion of coke powder increases, more nickel, diamond, and iron oxides are reduced, and metal recovery rate increases. When the coke powder ratio is >11, the recovery rate of nickel is almost unchanged, but a large amount of iron and diamond are reduced, which causes the nickel grade in the alloy to decrease, affecting the quality of the nickel-iron alloy product. Therefore, this experiment selected the best coke powder ratio of 11%.
Further analysis of the effect of the proportion of coke powder on the distribution ratios (Ls and Lp) of S and P in slag and alloy, the results are shown in Figure 4.
Figure 4 Effect of coke powder ratio on the distribution ratio of S and P
It can be seen from Fig. 4 that when the proportion of coke powder increases in the range of 5% to 10%, the distribution ratio of S increases; after the proportion of coke powder exceeds 10%, the distribution ratio of S no longer changes significantly.
According to the molecular structure theory, the desulfurization reaction can be regarded as:
The equilibrium constant of the reaction:
Where: W(S), ω[S] are the mass fraction of sulfur in the slag and the metal melt, respectively, fs and γs are the activity coefficients of sulfur in the metal melt and slag, respectively, and a is the activity of the substance. .
In the absence of additional FeO slagging, the FeO content in the slag is mainly affected by the amount of coke powder. As the proportion of coke powder increases, the content of FeO in the slag decreases. There is a balance between (FeO) in the slag and [FeO] in the alloy. As the proportion of coke powder increases, the decrease in (FeO) leads to a decrease in [FeO], which is favorable for the distribution ratio of IS in the desulfurization reaction. On the other hand, (FeO) can promote the melting of lime. When the (FeO) is reduced to a certain extent, the desulfurization reactant (CaO) activity is lowered, which is unfavorable for desulfurization. These two effects cancel each other out, resulting in the distribution ratio of S being basically not maintained. change. This point can also be verified from the experimental results. In the range of 10% to 17.5% of the coke powder ratio, the distribution ratio of S is stable at about 0.025.
As can be seen from Fig. 4, the distribution ratio of P decreases as the proportion of the coke powder increases in the range of 5% to 17.5%.
The dephosphorization reaction of molecular theory is:
The concentration of 4CaO·P 2 O 5 in the slag is very low, and can be replaced by X(P 2 O 5 ) to obtain the fraction of ester of phosphorus.
Therefore, as the proportion of coke powder increases, the FeO content in the slag decreases, which reduces the dephosphorization ability of the slag.
(2) Influence and mechanism of limestone on smelting
The addition of limestone not only adjusts the alkalinity, but also reduces the melting point and viscosity of the slag, as well as the recovery of the metal and the grade of nickel in the alloy. The fixed coke powder ratio was 11%, and the conditions of melting at 1550 °C for 50 min were unchanged. The effects of flux ratio on the nickel-iron grade and metal recovery were investigated. The results are shown in Fig. 5 and Fig. 6, respectively.
Figure 5 Effect of solvent ratio on nickel grade
Figure 6 Effect of flux ratio on metal recovery
As can be seen from Fig. 5 and Fig. 6, as the flux ratio increases in the range of 6% to 11%, the recovery of nickel, diamond, and iron increases, and the grade of nickel in the alloy decreases. Continue to increase the flux ratio, metal recovery rate decreased.
The addition of a certain flux can improve the performance of the slag, so that the mass transfer of the metal in the slag is sufficient, the separation factor is increased, and the inclusion loss is reduced, so the metal recovery rate is increased. As the amount of limestone added increases, the amount of slag increases and the metal recovery rate decreases. This is because the increase of the amount of slag causes the metal to be partially lost due to mechanical inclusions in the slag; in addition, the CO 2 produced by the decomposition of limestone consumes part of the coke powder, which reduces the reducing atmosphere in the furnace and affects the reduction process. This is consistent with the rule that the nickel-iron grade is high and the metal recovery rate is low when the ratio is low. Therefore, the optimum flux ratio was chosen to be 11%.
The distribution ratios of S and P in the slag and the alloy vary with the flux ratio as shown in Fig. 7. As can be seen from Fig. 7, when the flux ratio is increased in the range of 6% to 40%, the distribution ratio of S and P is increased.
Figure 7 Effect of flux ratio on the distribution ratio of S and P
According to the desulfurization reaction formula (1) and the dephosphorization reaction formula (4), it is known that lime acts as a reactant to promote desulfurization and dephosphorization reactions at the metal-slag interface. In addition, lime also provides Ca 2 + . Since the radius of S 2 - is larger than the radius of O 2 - , Ca 2 + is mainly concentrated around S 2 - , forming a weak ion pair, reducing the activity of S in the slag, thereby improving Promote the transfer of S in the alloy to the slag. It can be seen from formula (3) and formula (5) that as the limestone ratio increases, the S and P distribution ratios increase.
Third, the conclusion
(1) With the increase of the amount of coke powder added, the grade of nickel in the nickel-iron alloy decreases, and the metal recovery rate increases gradually. At the same time, the increase of the amount of coke powder is not conducive to dephosphorization, and has a limited effect on desulfurization;
(2) The addition of proper amount of limestone can improve the properties of the slag, improve the metal recovery rate, and facilitate the desulfurization and dephosphorization process, but excessive limestone increases the slag amount and increases the metal loss;
(III) The optimum process conditions for extracting nickel-iron alloy from laterite nickel ore by electric furnace direct reduction smelting process: 1550 ° C, coke powder ratio 11%, limestone ratio 11%. Under the optimal smelting conditions, a nickel-iron alloy with a nickel grade of 22.82% was obtained. The recovery of nickel was 97.6%, and the distribution ratios of S and P were 0.024 and 0.145, respectively.
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