Contribution to the Optimization of the Smelting Reduction of Nickeliferous Laterites, Based on the Recent Industrial Experience ^†

Abstract

The Greek ferronickel industry faces the challenge of restructuring and reoperation in the future, given that privatization is in progress, including the acquisition of the company’s assets for a joint venture enterprise. Within this framework, the current paper aims to contribute to the discussion about developing a new management strategy for the optimization of the pyrometallurgical process, focusing on the critical step of smelting reduction. Based mainly on industrial experience, factors which critically affect the safety and cost-effectiveness of smelting reduction are detected and presented, also by means of case studies, being classified as follows: (i) optimal raw materials’ feed management, including laterite ores (domestic or not), solid fuels and electrode paste; (ii) focus on preventive maintenance management. Substantial increase in the facilities’ operational index and cost saving is obtained; (iii) modern human resources management strategy, enhancing evaluation indicators’ use, education culture, process standardization and tacit—explicit knowledge management. Their economic footprint is discussed.

1. Introduction

Ferronickel (Fe-Ni, Ni: 20–40%), constitutes the predominant Nickel Class II category product apart from nickel pig iron (NPI), which has met an outstanding demand growth since 2006, with widespread production in China and Indonesia. Nickel demand is predominantly driven by the stainless steel industry, accounting for almost 70% of the total primary nickel consumption [1]. Furthermore, the primary nickel production (including Nickel Class I products) reached 3,020,000 TN in 2022, presenting an outstanding increase within the last three years, and is expected to surpass 3,370,000 TN by the end of 2023 [2]. It should also be taken for granted that within the same period of time, global industry faced the significantly negative challenges of the COVID-19 pandemic and the energy crisis. Nevertheless, the primary nickel production trend in China and Indonesia is opposite in comparison with nickel production in Europe. Moreover, Europe’s smelter Fe-Ni production in 2022 faced an enormous 57% decline, which can be safely attributed to soaring energy costs.

The forecast for the significant increase in nickel demand for usage in the battery sector within the next 15 years challenges European governments to motivate nickel producers with more viable energy supply prices in order to remain competitive in the Class II nickel production market. Moreover, producers are challenged to seriously invest in R&D costs for enhancing greenfield projects for the production of nickel matte or intermediate products, such as nickel sulfate or mixed hydroxide precipitate (MHP).

Since 2010, the operation of the Greek ferronickel industry LARCO General Mining and Metallurgical Company, has passed through many stages. The period 2011–2014, when the LME nickel prices fluctuated between USD 16,000 and 28,000/TN, LARCO managed to have annual production over 18,000 Tn of nickel for three years, being historically among the best production performances. Key parameters in the subsequent period, such as the LME low nickel price, which resulted in cuts in maintenance and ore mining expenses, the exclusion from bank loans due to conviction by the European Court for receiving in the past incompatible state aid, and structural and management malfunctions resulted in prohibitive operational costs. The extremely high cost of electrical energy (>EUR 200/MWh) during the last two years was the ‘deathblow’ for the industrial operation, which finally stopped on 30 July 2022. In the meantime, since 28 February 2020, LARCO has been placed under a special administration regime in accordance with the provisions of Article 21 of Law 4664/2020. Furthermore, the Greek ferronickel industry faces the challenge of restructuring and reoperation in the future, given that a privatization procedure is in progress, being in the stage of the acquisition of the company’s assets for a joint venture enterprise after its participation in a relative international open tender.

2. Pyrometallurgical Processing of Greek Laterite Ores

Nickel is extracted pyrometallurgically from domestic laterite (oxidized) ores via the LARCO metallurgical process [3,4]. The domestic nickeliferous laterite ores are Evia island ore (Ore A), Lokrida ore (Ore B) and Kastoria ore (Ore C). Based on the classification according to their Fe and MgO % content [3], all the different deposits of Ore B are typical B1 type of lemonites (Fe > 32%, MgO < 10%). The various deposits of Ore A can be classified among B1 and B2 type of lemonites (25% < Fe < 32%, MgO < 10%). On the contrary, Ore C clearly belongs to intermediate-type C2 laterite ore (12% < Fe < 25%, 10% < MgO < 25%). Temporarily in the past, imported high-grade Ni ores were used to enrich the metallurgical mixture fed in the rotary kilns (R/Ks). Indonesian and Guatemalan laterite ores (Ni% > 1.6 on a dry basis) were also typical intermediate-type C2 ores, while Turkish laterite ores (Ore T) were typical B2 types of lemonites, having a significantly higher Ni grade in comparison with the Greek ores A&B (Ni% > 1.2 on a dry basis).

The domestic laterite ore material fed in the Greek Fe-Ni industry can be characterized as unique given that high iron content in the E/F slag (Fe > 28%) results in the need for slag superheating in order to recover Ni in the alloy. Thus, the smelting-reduction process with open-bath submerged-arc E/F operation is the only technically feasible method. Moreover, the critical parameter of the SiO₂/MgO (S/M) content of the Greek laterites (as well as the slag produced) is often greater than 7, while it does not exceed 3 in the case of all the other nickel smelters. The LARCO method is also classified in the low-iron reduction category [5].

3. Critical Parameters for the Optimization of the Smelting-Reduction Step

It is clearly deduced by the industrial experience concerning open-bath submerged-arc E/Fs that the factors which constitute the prerequisite for a safe and cost-effective metallurgical process are presented in Figure 1.

3.1. Laterite Ores

The Ni % content of the laterite ore mixture (laterite mixture—L.M.) fed in the R/K-E/F system of the metallurgical plant is no doubt the most critical factor affecting the recovery rate and the productivity of the process. It is clear form Figure 2 that the Ni grade of the L.M. on a dry basis after 2014, fell gradually in 2019 (the last year of operation of all the metallurgical equipment of the plant) to a level (0.91%) that marginally can be characterized as economic [3] or viable in literature. That was one of the main reasons why the E/F annual production decreased from 17,554 to 11,137 TN.

Additionally, another vital parameter for the final result of smelting reduction is the (Fe-SiO₂) % content of L.M. (the decreasing progress of which is seen in Figure 2) and the corresponding relation in the E/F slag. The aforementioned is important since it mainly affects the viscosity of the E/F slag, which is related to the intensity of the slag foaming phenomena and the E/F power losses.

The operational index (O.I. %) of the E/F equipment constitutes also a very critical index of the economic viability of the process since it takes into consideration both of the following parameters: (i) time of equipment operation (Uptime) and (ii) % utilization of the E/F’s maximum power capability (Speedfactor). Moreover, it is deduced that an E/F O.I. > 70%, which is a prerequisite for high Ni production, is obtained, having ensured an average Fe-SiO₂ (%) content > −4.5 in laterite feed, as seen in Figure 3.

In

Table 1. Typical laterite mixtures examined in correlation with E/F operation.

Laterite Mixture (L.M.)	Ni (%)	Fe-SiO2 (%)	SiO2/MgO (%)
L.M.1: Ore A–Ore B–Ore C (%) = 65–25–10 (%)	1.04	2.4	7.3
L.M.2: Ore A–Ore B–Ore G * (%) = 45–25–30 (%)	1.17	−4.5	3.8
L.M.3: Ore A–Ore B–Ore C (%) = 50–25–25 (%)	0.96	−7.1	4.8
L.M.4: Ore A–Ore B–Ore C (%) = 45–25–30 (%)	1.22	−1.3	3.6
L.M.5: Ore A–Ore B–Ore C (%) = 45–25–30 (%)	1.25	−0.4	3.3
L.M.6: Ore A–Ore B–Ore D *–Ore G (%) = 45–15–10–30 (%)	1.26	1.22	3.8
L.M.7: Ore A–Ore B–Ore T *–Ore G (%) = 40–15–20–25 (%)	1.26	−1.7	4.6

* Ore D: Albanian ferrous ore, Ore G: Guatemalan ore, Ore T: Turkish ore.

that follows, typical laterite mixtures of both domestic and foreign ores are presented in order to assess the effect of the raw materials’ qualitative and quantitative characteristics on smelting reduction. Based on industrial operation, S/M ≥ 3.5 and (Fe-SiO₂) % ≥ −4.5 in the slag are critical for the operational stability. Moreover, a lower S/M ratio increases the slag melting point in values that render the open-bath operation forbidden and the energy consumption index KWh/Tn of calcine even higher than 600, in comparison with a mean value of 450–460 applied for smelting reduction of domestic ores. The participation of an intermediate-type ore, domestic (Ore C) or not, more than 35–40% in the laterite mixture, results in S/M < 3. Apart from that, the Fe and SiO₂ (%) content of the remaining 60–65% of the L.M. is critical in order for the viscosity of the produced slag to be such that foaming phenomena are controllable and do not result in low E/F power operation for safety management issues.

L.M. 1 and L.M. 3 constitute typical examples of ore blending that corresponded to a stable and unstable operation in Greek Fe-Ni industry, respectively. L.M. 1 resulted in the production of an industrial slag (FeO = 40.14%, SiO₂ = 35.86%) with a viscosity of 68.45 poise in 1300 °C, calculated based on the Urbain model. On the contrary, an industrial slag (FeO = 31.9%, SiO₂ = 43.81%) produced by an ore mixture close to L.M. 3 with a viscosity of 138,0 poise, caused an increase in the energy consumption index Mwh/Tn Ni due to serious operational problems and a decrease in the O.I. L.M. 2 is a typical example of blending domestic lemonitic-type ores with intermediate-type laterite ore of Guatemalan origin (Ore G, Ni% = 1.79 on a dry basis). During the period of time in which such a high Ni grade ore participated in the L.M. up to 30% by weight, accompanied by a domestic ore blend with the appropriate Fe-SiO₂ relationship, the operation was generally stable, with no intensive slag foaming phenomena, low energy consumption indexes and high recovery rate, indicating that this should always be the strategic operational plan for the Greek Fe-Ni industry.

Based on the records of a quite recent database of various proved deposits of the Greek mines on Evia island and Ag. Ioannis, theoretical laterite mixtures were created, only with the participation of domestic deposits (L.M. 4 and 5), as well as with the participation of imported ores (L.M. 6 and 7). Ore G, Turkish ore (Ore T, lemonitic type) and Albanian ore (Ore D, Ni = 0.97%, Fe = 35.3%, SiO₂ = 21.0%) [6] were used. In all cases, as seen in Table 1, the index values for obtaining the optimal laterite’s feed management Fe-SiO₂ (%) and S/M, are not lower than the critical values −4,5 and 3, respectively. Moreover, the highest possible Ni grade obtained in such a way is approximately 19% higher (1.26%) compared with a typical annual average of the period 2011–2014 (1.04–1.06%).

In Figure 4a–c, the phase diagrams created via the equilibrium module in FactSage7.0 professional software are presented, mainly in order to determine the melting points of typical industrial slags produced by laterite mixtures similar to those of Table 1. The aforementioned module uses the “Gibbs energy minimization” principle to find the phases that exist in equilibrium with liquid ferronickel slag and their respective compositions and amounts, based on the thermochemical database of the software, under a certain range of constraints (i.e., temperature, pressure, composition, etc.). It is noted that the effect of the refractory Cr-bearing spinel mineral phases has not been taken into consideration for the calculation of the melting points in the concentrated chart of Figure 4d. The temperature determined each time as the ‘melting point’ is the liquidus temperature at which all the mineral phases of the industrial slags are melted, apart from the Cr-bearing solid spinel minerals that still co-exist, even at 1600 °C.

In Figure 4a, the phase diagram of a slag produced by only domestic laterite feed with S/M 7.23 (Figure 4a) can be seen. In Figure 4b,c, the phase diagrams of slags produced by partial competition (30% in Figure 4b, S/M = 4.18) or total competition (100% in Figure 4c, S/M = 2.1) of intermediate-type foreign laterite (Ore G) in the L.M. are presented. It is clear that in the phase diagram of Figure 4c, the mineral phases orthopyroxene, clinopyroxene (Mg, Fe)₂Si₂O₆ and olivine (Mg, Fe)₂SiO₄ co-exist in equilibrium with liquid slag at significantly higher temperatures and in higher contents in comparison with the other two phase diagrams. Moreover, when the S/M ratio is decreased to levels lower than three, as seen in Figure 4d, the melting point of the slag is increased up to 1432 °C, in comparison with 1220 °C in Figure 4a.

In Figure 5, the melting points of typical industrial slags, defined as solidus temperatures, are depicted by the use of FactSage7.0 software and the phase diagram module in a ternary system FeO-MgO-SiO₂.

It is noted that in the plotted area G are depicted the melting points of typical industrial slags (FL_A and FL_B, solidus temperature 1256 °C, with S/M 5 and 7, respectively), produced by different blend ratios of domestic laterite ores. Moreover, the melting point with the code name MEG is also depicted (solidus temperature 1380 °C, with S/M 2), which corresponds to the slag produced by a 100% foreign (Ore G) ore feed. It is verified that the trend of the low Fe content and the high MgO and SiO₂ content of ferronickel slags produced by the total participation of intermediate-type laterite ores (like Ore G) in the L.M. results in an increase in the slag melting temperature by almost 200 °C. A critical parameter for this is the remarkable presence of the olivine mineral phase in such a case, and especially forsterite (Mg₂SiO₄). On the contrary, the remarkable presence of the Fe-bearing spinel mineral phases (such as MgFe₂O₄) and magnesiowustite (Mg, Fe)O is a critical parameter for the low-melting-point ferronickel slags produced by domestic laterite ores.

In Figure 6a, the correlation between the basicity of the same industrial slags and their viscosity calculated by the Urbain model is depicted. Moreover, an increase in the viscosity of a low basicity slag at levels higher than 110 poise, close to that of the slag produced by the L.M 3 fed in the R/K-E/F system, with an Fe-SiO₂ (%) ratio lower than −7, corresponds to intensive foaming phenomena, low operational index and high energy consumption.

Furthermore, linear fitting of data from the Greek industrial operation database concerning Ni (%) of L.M., O.I. (%) and ΜWh/Tn Ni can lead to a quite conservative prediction for obtaining an energy consumption index of 58–59 MWh/Tn Ni, for an average 1.26% Ni in L.M. Taking as the base of the consideration that (i) processing of more than 2,350,000 Tn of calcine annually by the R/K–E/F system is impossible and that (ii) the cost of the electrical energy is approximately 30% of the total cost of nickel extraction, assuming that the energy prices will at least come back to the levels of 2021 (70 Euros/MWh), an annual plant Ni production of 7000–8000 Tn is regarded as realistic for operation of two E/Fs, just for the potential beginning of the reoperation period of the Greek Fe-Ni industry. In such a case, a selling price of USD 16,500 /Tn of Ni could be very close to the marginal operational cost of the industry.

An increase in the FeO content of the slag as well as a decrease in the slag viscosity result in a considerable decrease in intensive slag foaming phenomena [7,8] and a series of operational problems. This can also be clearly seen in Figure 6b, where the evolution of the ratio energy consumed/energy loss due to operational problems caused by the poor quality of the laterite feed (Ni grade, Fe-SiO₂ %) is presented. The loss of income due to the energy losses caused by such operational problems was almost EUR 22,000,000 in 2019. The serious operational problems that the Greek Fe-Ni industry faced, affecting both the collapse of the E/F O.I. and the safety issues of personnel and equipment due to the poor laterite feed quality, is briefly presented by means of case studies.

Uncontrolled E/F Operational Case Studies

A typical case study of an extremely unstable E/F operation, due to the very bad quality of the laterite feed, is presented in Figure 7. More precisely, the fluctuations and the violent decrease in the Fe-SiO₂ (%) relationship concerning Ore A of the L.M. just a few days before the corresponding incident resulted in (i) intensive slag foaming phenomena, current fluctuations and power loss due to intensive electrode consumption; (ii) very high flames around the freeboard surface; (iii) very high slag viscosity that renders slag tapping very difficult; and (iv) inability to maintain calcine sidewall protection because of the high slag level inside the E/F. The aforementioned serious operational problems finally resulted in an uncontrolled metal tapping from the E/F sidewall shell the second day, which caused, in addition to the safety issues, a big decrease in the plant O.I. as well as increased cost of maintenance.

3.2. Solid Fuels

The stable quality of the solid fuels used for roasting reduction in the Greek Fe-Ni industry plays very important role in the final result of the smelting-reduction step [9].

Within the examined period of time of the case study, the decision taken temporarily for the supply of the plant with spot shiploads of ’hard coal’, with volatile content <25%, and ‘soft coal’, with volatile content >35%, resulted in the inability to manage the optimum volatile combustion along the R/Ks as well as operational problems for the E/Fs. The most typical case study was that of an E/F that after almost a month of being fed with calcine of lower than the typical temperature (<700 °C), with excessive content of C^fix, its O.I. was dramatically decreased to lower than 59% on a three-month basis (almost 30% lower than the annual average). Moreover, intensive slag foaming and reduction phenomena finally led to such an uncontrolled slag height inside the E/F that all the freeboard was covered by slag.

Solid fuels’ granulometry was also proved by the industrial experience that is critical for the result of the smelting-reduction step. In Figure 8, we can see that at periods of utilization in the R/Ks of spot shiploads of coal with −1 mm (%) grain size higher than 25%, instead of 16–20% when the best operational results were achieved, the E/F recovery rate was significantly decreased by almost 12%. This can be attributed to the fact that more fine-grained coal in the R/Ks, due to its easier combustion, causes a higher-than-required exit temperature of the calcine fed in the E/Fs. The former results in significantly higher Ni% grade in the E/F alloy phase (>14% on average), which is intimately related with lower recovery rates.

3.3. Electrode Paste Management

Management of a stable operation of self-baking Söderberg electrodes has been proved by the industrial experience to be one of the most critical parameters for an optimum result in the smelting-reduction step. Thus, avoiding electrodes hard breakages, soft breakages, high electrode consumption that causes E/F power losses and maintaining a stable molten paste level between 2 and 3 m undoubtedly favor E/F O.I. and productivity. Laterite ore quality by means of the Fe-SiO₂ relationship also affects positively electrode paste special consumption since a decrease in slag viscosity and, more precisely, an increase in Fe content render the slag more conductive, resulting in less deep electrode immersion to molten bath. A decrease of 1.5 Kg/MWh of paste consumption, which has been achieved in the Greek Fe-Ni industry since 2010, has caused an annual saving of almost EUR 2,000,000.

Moreover, a decrease in the molten paste level to extremely low values (<50 cm) can cause a reverse of the baking zone under the contact clamps and may result in soft breakage. In Figure 9, two case studies are presented, which both resulted in very dangerous accumulation of alloy inside the E/Fs that could not be extracted due to the fact that there were continuous Fe-Ni leakages in the slag. In cases like these, since in Greek industry there are no air fans installed in the electrodes with a heater for blowing hot air and melting the non-baked solid paste, the technicians are urged to stop the electrode slipping rate till the molten paste level gradually starts to increase. In one of the two case studies presented, accumulation of alloy in E/F in correlation with the bad quality of the laterite feed resulted in uncontrolled metal tapping from the E/F sidewall shell. Therefore, the great importance of installing in each E/F electrode fans with a heater of 0–36 KW in two–three steps, something which constitutes a low-cost solution to prevent very difficult situations, is easily understood.

3.4. Maintenance

Within the nickel price crisis period 2015–2020, tremendous cuts in the maintenance expenses program took place as well, as depicted in Figure 10 that follows. Nonetheless, apart from the objective problem of funding the Greek Fe-Ni industry in the period 2015–2019, there should be a shift from the philosophy of corrective maintenance to the preventive maintenance one being incorporated into a holistic maintenance management system [10]. Moreover, the lack of replacement of very low-cost spare parts, either due to a lack of funding or within the framework of just corrective and no preventive management strategies, resulted in great production loss as well as dangerous operational situations. To further emphasize this in a typical case study, non-replacement of flexible conduits of the E/F transformer within the framework of the shutdown of the furnace a few months before for annual general maintenance resulted in non-uniform conduction of the electrical current to each of the EAF conduct clamps. Thus, permanent overheating of a certain contact clamp resulted in wear of the clamp, water leakage inside the E/F, danger for the safety of personnel and equipment, melting of the steel behind the clamp (Figure 11) and removal of the electrode baking zone considerably higher. Additionally, there was an income loss of EUR 3,500,000 and a 30-day shutdown of the E/F due to the lack of spare parts with a cost of just EUR 5000.

3.5. Human Resources and Knowledge Operationalization

Despite the fact that the Greek Fe-Ni industry has operated for over 70 years, mainly due to structural malfunctions, it is true that there is an absolute need for the application of a new management strategy concerning human resources management and process formalization-standardization. Experience among the technical personnel in general is a prerequisite for the optimum result in the pyrometallurgical industry [11]. Moreover, maintaining the stable operation of units where complex metallurgical processes take place, like E/Fs, is difficult to achieve without a combination of applying a knowledge management system with the standardization of policies and procedures [12]. Within this framework, it is quite essential for planning a new total management strategy for the re-operation of the Greek Fe-Ni industry to realize that it is not possible to obtain a cost-effective and optimum result without seriously emphasizing the following parameters:

• Experience retention of management: loss of experience results in productivity decreases and lack of a sustainable competitive advantage in the Greek Fe-Ni industry. Within this framework, incentives should be given to skilled and experienced workers to remain in metallurgy and to assure successful troubleshooting in difficult-to-handle situations;
• Tacit knowledge transfer through establishing meetings with supervisors, coaching of younger personnel and relationship-building interventions. There is an absolute need for skilled personnel capable of making critical metallurgical decisions;
• Establishment of standardization procedures for every single metallurgical process in order to ensure compatibility, safety, repeatability and quality. There were many cases where the lack of clearly described step-by-step procedures resulted in lost unit operation time, with respective losses in thousands of Euros;
• Introduction of modern ΙΤ systems, for thorough cost-monitoring management;
• Occupational safety education of the personnel, including safety seminars and establishment of modern on-the-job-training techniques.

4. Conclusions

To achieve an optimum and cost-effective productive result for the smelting-reduction step in the Greek ferronickel industry, the application of a long-term strategic plan is absolutely critical, taking seriously into consideration the following:

• Supply the plant with high-Ni-grade (Ni% > 1.8% on a dry basis) imported laterite ores, with a participation of 35–40% to the laterite mixture. The aforementioned, in combination with the stable quality of critical raw materials, such as solid fuels and electrode paste, are the prerequisite for obtaining an operational index of 75% at least and a special energy consumption lower than 60 MWh/Tn Ni. Therefore, an updated detailed study of the proved domestic ore reserves is very important to confirm that a laterite feed with approximately 1.26% Ni content on a dry basis is possible to be practically obtained.
• A (SiO₂/MgO) ratio lower than 3 by the use of foreign laterite ores results in a serious increase in the slag melting point, rendering the open-bath E/F operation very difficult. An (Fe-SiO₂) relationship lower than −5 results in a serious increase in the slag viscosity, causing intensive slag foaming phenomena and operational problems, negatively affecting the productivity of the process.
• Retaining maintenance expenses at a stable level, based on industrial experience and the application of a shift from the philosophy of corrective maintenance to preventive maintenance is very essential. The lack of low-cost spare parts, mainly due to maintenance expense cuts, may result in a dramatic decrease in the E/F’s operational index and can be responsible for the occurrence of incidents that include risks for the personnel and the equipment.
• There is a need for the application of modern HR management tools and techniques, such as a tacit and explicit knowledge transfer management strategy, benchmarking, motivation policy, introduction of modern IT systems for maintenance cost monitoring management, health and safety training programs and non-stop promotion of a safety culture. The aforementioned should be incorporated in a holistic total quality long-term management plan.