Evaluating the Behavior of Bauxite Tailings Dewatering in Decanter Centrifuges

Abstract

Depending on the ore quality, a washing process can be conducted with the bauxite, which basically consists of scrubbing the ore and screening in order to increase the available alumina grade, i.e., the alumina extractable using the Bayer Process, and reduce the impurity content. Tailings are usually disposed of in a tailings dam in the form of a slurry, which is a mixture of solid particles and liquid, consisting mainly of ultra-fine kaolinite, making the dewatering operation challenging. To reduce the environmental impact, mining companies are studying alternative methods to dewater the tailings, and different dewatering methods are available worldwide. The use of new technologies to dewater the tailings has contributed to facing the challenges of achieving sustainable development with their disposal. The decanter centrifuges are already an option for operations for the Canadian oil sands, gold ore in Peru, and nickel in New Caledonia; they are also being tested for iron ore in Brazil. In the present work, bauxite dewatering using the decanter centrifuge was evaluated to understand more about the behavior of these materials and to investigate the effects of various process parameters on the solid recovery and solid content of the flows, using three different kinds of equipment. The results indicated that decanter centrifuges can be used to achieve a high concentration of solids in the cake, with values ranging from 60% to 80% solids per weight and a great clarification in the liquid phase (centrate) from 0 to 6% solids per weight, values which mean the solid phase is suitable for reutilization in the processing circuit. Additionally, the present work provides a better understanding of how different solid contents feed can affect the behavior of the equipment.

1. Introduction

About 8% of the Earth’s crust comprises aluminum, the most abundant metal in the world. It is considered a metal of the future due to its strategic significance, and bauxite is the only ore commercially used for extracting aluminum metal [1].

The world’s bauxite reserve is around 33.4 billion metric tons. Brazil accounts for 3.5 billion metric tons of these reserves, of which 95% is metallurgical bauxite. Around 95% of production is used to obtain metallic aluminum, through the Bayer process, followed by the Hall–Héroult process [2].

Bauxite consists of hydrated aluminum oxide in the mineral form of gibbsite, bohemite, and diaspore, with a significant number of impurities such as silica, iron, titanium, and other minerals in smaller proportions. After bauxite mining, depending on the ore quality, a washing process can be conducted, in which the bauxite ore is scrubbed and washed in order to increase its available alumina grade, i.e., the alumina extractable using the Bayer Process, and reduce the impurity content. Among the impurities, the most harmful for the Bayer Process is the reactive silica, i.e., the mineral which increases caustic soda consumption, increasing production costs and causing environmental concerns due to the generation of large volumes of tailings [1,2,3,4].

According to Hansen et al. [5], raw materials are vital for economic development and the well-being of populations, and society is therefore dependent on their abundant supply. Aluminum production has a range of impacts, including environmental impacts such as greenhouse gas emissions, huge energy consumption, dust pollution, and changes in the landscape and neighborhoods due to the management of the tailings produced in the process. Figure 1 shows the number of academic publications about different types of impact, with environmental impacts clearly constituting the greatest number of publications, over the 40-year period covered by the search, from 1983 until 2023.

Bauxite tailings are usually disposed of in tailings ponds in form of a slurry, which is a mixture of solid particles and liquid; this not only increases the economic burden of the alumina production enterprises, but also occupies large amounts of land, posing a serious threat to the surrounding air, soil, and water resources, and may cause leakage accidents at any time, which are one of the greatest sources of environmental impacts concerning bauxite processing. To mitigate these environmental impacts of the aluminum industry, several mining companies are studying alternative methods to dewater the tailings prior to their disposal or dry stacking them, but they are also studying methods for tailing thickening, tailing storage, and tailing utilization [4,6].

This new approach is very positive in terms of minimizing the intensive demand for large dams or ponds and reducing their environmental impacts. The methods used for disposing of the tailings have been developed due to environmental pressures, which are becoming more serious with the increasing exploration of lower-grade deposits [7,8,9]. The use of new technologies for dewatering the tailings has contributed to facing these challenges and achieving sustainable development with their disposal [10,11] Centrifuges, specifically the decanter type, are a versatile piece of equipment, with the advantages of continuous production and high dewatering efficiency [12,13].

The decanter centrifuge has become a major processing tool in a wide range of liquid/solid separation applications. It is a piece of dewatering equipment, the main characteristic of which is the use of centrifugal force, which acts on solid particles with an intensity much greater than the gravitational force and that can be multiplied by increasing the rotation speed [12,14]. Figure 2 shows a schematic view of the decanter centrifuge. The products are a high solid content, a “cake”, and a liquid product, named “centrate”.

According to Gleiss and Nirschl [15], the efficiency of a decanter centrifuge is largely connected with the centrifugal force, feed rate, and weir diameter, as well as the characteristics of the solid particles themselves. As decanter centrifuges can be easily adjusted to different operation requirements, they are used for a wide variety of applications and different industrial branches, such as wastewater treatment, the refining and dewatering of petrochemicals, mineral processing, and vegetable oil and dairy processing [16].

For a successful scale-up to industrial-scale, experiments on a pilot scale are recommended to determine the correct transfer function and hence map the non-linearity of the system in decanter centrifuges. This has the disadvantage that a significant number of experiments is often necessary, which implies a high energy demand, personnel, and cost expenditure [17].

In a tailings dam, solids are settled using sedimentation, so that the force on the particles is the Earth’s natural gravity. However, when the percentage of particles below 37 µm in the tailings is about 90%–100% and the percentage of solids (volume basis) in the slurry is high, the settling rate is very low due to the high viscosity of this kind of slurry. In these cases, prior dewatering becomes an interesting option. Bauxite tailings fit in this case, given that in their processing, all the fine fraction is considered tailings, and the goal of this research is to gain a better understanding of the segregation and settling behavior of bauxite tailings [18,19].

This study aims to evaluate the results of decanter centrifuges for dewatering bauxite tailings from the bauxite deposits in the southeastern region of Minas Gerais State, Brazil, with the intention to develop and to optimize a dewatering system for the bauxite tailings. To describe its geological formation in brief, the ore is from the NE-trending belt that begins in São João Nepomuceno and continues to Espera Feliz, with interruptions mainly in the valleys of rivers Pomba and Muriaé [2].

The present study is relevant not only due to the increasing governmental and social pressures regarding tailings dams, but also due to environmental incentives for using alternative methods for dewatering tailings and, furthermore, mitigating a scarcity of literature on the topics of dewatering bauxite tailings and the use of decanter centrifuges for dewatering tailings.

2. Materials and Methods

2.1. Solids’ Characterization

Technological characterization is essential to understand the behavior of the bauxite tailings. Particle size distribution analysis was performed, using wet sieving and laser diffraction. Solid specific weight was measured using water pycnometry and chemical analysis carried out using X-ray fluorescence.

2.2. Spin Tests

Laboratory-scale batch sedimentation centrifuges are commonly used to predict material behavior and develop material functions describing separation-related properties such as sedimentation, sediment build-up and sediment transport. These is a simple and efficient laboratory test widely used to identify the applicability of centrifuges to the slurry. The influence of the material on the settling behavior must be considered to predict the clarification process [20,21]. The centrifugal spinning tests were performed with and without the addition of flocculant.

The laboratory centrifuge used is shown in Figure 3, consisting of a swing type bucket capable of simultaneously holding four 100 mL samples. The samples were subjected to an angular velocity of 4500 rpm.

The polymer used for flocculation can be anionic or cationic to neutralize the surface charges on these fine particles to build larger flocculated solids [22]. Aiming to evaluate the efficiency of flocculants in the dewatering process, cationic and anionic polymers were previously tested in different dosages, flocculant concentration 0.1% t and pH around 5.5. The best results were from the anionic polymer, dosage in the range of 80 ppm. For testing purposes, the chemical aid used was SUPERFLOC A-130 HMW, supplied by Kemira brand, Brazil.

2.3. Pilot Tests

Experimental trials were performed with tailings from the bauxite washing plant in three decanter centrifuges of different manufacturers and sizes. The separation of the phases depends on decanter design and different operating variables, including the centrifugal force exerted on the feed, the pond depth, the differential speed, and the flow rate [23,24].

For scale-up purposes, tests should be performed in a pilot plant through the steps seen in Figure 4.

The pilot plant is composed of the following main equipment:

• A sizer-type crusher, with an opening of 50 mm;
• A slurry chute that uses high-pressure jet technology to promote disaggregation of the clay particles on the bauxite particles;
• A sieve with 1 mm screen;
• Decanter centrifuge.

The three decanter centrifuges tested had different operational variables, such as bowl rotating speed, differential speed, and total length, as shown in

Table 1. Geometries of centrifuges tested in the present work.

Centrifuge	Bowl Diameter (mm)	Bowl Length (mm)
1	620	2800
2	457	1372
3	353	1445

.

Samples of feed, cake and centrate were collected at the points illustrated in Figure 5.

The decanter centrifuge feed flow (point #1) is essential for closing the mass balance. Information on flow rates and densities at this point are essential and recorded at each aliquot collection. For sampling the flow, the following protocol was followed:

• Sample the slurry at the feed of the centrifuge, taking 1 aliquot of 2.5 L at every 20 min of operation, with at least 3 aliquots per test;
• Minimum operating time of 1 h straight per condition.

Collecting the cake sample (point #3) is the most difficult step to carry out, and the accuracy of representation in this experiment is a major uncertainty in this study. For sampling the flow, the following protocol was followed:

• Sample at the top of the dewatered clay pile to obtain an aliquot of 10 kg every 20 min of operation, with at least 3 aliquots per test;
• Minimum operating time of 1 h straight per condition.

Finally, the centrate flow sample (point #2) is another flow essential for closing the mass balance. Information on flow rates and densities at this point are essential and recorded at each aliquot collection. For sampling the flow, the following protocol was followed:

• Sample slurry at the clarified pumping outlet, obtain an aliquot of 7 L every 20 min of operation, with at least 3 aliquots per test;
• Minimum operating time of 1 h straight per condition.

For determining the density of the slurry, the Marcy scale was used based on Equation (1). Densities of the ore and liquid were assumed to be 2.5 g/cm³ and 1.0 g/cm³, respectively.

$$Solidscontent\left(\%\right)=\frac{\left[Ds\times \left(Dp-1\right)\right]}{\left[\left(Ds-1\right)\times Dp\right]}$$

(1)

• Ds = solid (ore) density;
• Dp = liquid density.

The Marcy scale is composed of a scale and a stainless-steel container with a volume of 1000 cm³ (Figure 6), and is a practical and widely used piece of equipment in the operations of mineral processing plants, being used to measuring the density of slurry, solids, and liquids with quick readings, without the need to use graphs or abacus, or perform calculations [25,26].

Centrifugal force is the most obvious parameter that comes to mind when considering the action of a centrifuge. The maximum centrifugal acceleration developed inside a centrifuge is a function of its radius and angular rotational speed. The term G-force or G-value is more commonly used instead of acceleration. The G-force is defined as the multiple of the gravitational constant obtained in the centrifuge, and a formula for approximating the G-force at the bowl periphery is in Equation (1) [27].

$$G=\frac{n^2\times D_b}{1800}$$

(2)

• G = G-force (N)
• n = bowl speed (rpm)
• D_b = Inner bowl diameter (m)

3. Results and Discussion

3.1. Technological Characterization of the Ore

The material fed to the pilot plant in the study was tailings from the bauxite processing that present a fine particle size distribution, with a d₉₅ of 547 µm and 10% of the material below 1 µm, as shown in Figure 7.

The solid’s specific weight was 2.842 g/mL and the chemical analysis of the material showed that it contained 31.7% Al₂O₃, 22.1% SiO₂, 23.4% Fe₂O₃ and 7.5% TiO₂.

3.2. Solid’s Settling

A common approach to studying sedimentation behavior is to use a graduated cylinder or column filled with the suspension to be studied [28]. The objective of determining the solid’s settling is to quickly determine the amount of slurry. For this purpose, 1 L of slurry (feed and centrate flows) was transferred to a graduated cylinder and left to settle for 30 min; the volume of the sediment was then recorded, as shown in Figure 8. The result for the feed was about 30% sludge, with solid particles but also a certain portion of water.

3.3. Laboratory Spin Tests

The objective of the laboratory spin tests was to determine the limit of the mechanical dewatering and to compare this value with the pilot-scale results. The sample was spun in a lab scale centrifuge at a rotational speed enough to obtain representative G-forces for full-scale operation.

After performing the tests (Figure 9), a high rate of sedimentation was observed in the sample. This indicates that the centrifugal decanter can operate satisfactorily in this application.

According to Jackson [29], the application of solid bowl decanter centrifuges to solid/liquid separation processes often entails the introduction of flocculating or coagulating reagents to assist the settling of suspension. When the polymer solution is applied, there is excellent clarification and solid capture from the centrifugation process, making it even better.

3.4. Pilot Tests

In the tests, the decanter centrifuge was started up, all process parameters set, and then rotational speeds adjusted. The solids mass fraction of the feed, the discharged cake, and the liquid centrate were determined. To remove the settled solids, a scroll rotates inside the rotor, with a differential speed towards the rotor, and picks up the accumulated solids. The solids are transported up the drainage zone, out of the liquid and up the dry beach, before being discharged [7,30]. Testing was conducted to evaluate the effects of solid’s content, cake moisture, solid recovery, and unit capacity of each piece of equipment.

There are several variables that the operator can adjust, including bowl rotating speed, the differential speed between the bowl and screw conveyor, and the feed flow, and all these variables can affect the performance of the decanter centrifuge [24,31]. These process parameters, such as centrifugal force, differential speed, and bowl speed are shown in

Table 2. Operative variables tested in the present work.

Centrifuge	G-Force (G)	Differential Speed (rpm)	Bowl Speed (rpm)	Feed Flow (m³/h)
1	1500	12 to 23	1400	50
2	2000	10	2800	7
3	2700	17	3700	6.6

. The feed throughput in centrifuge 1 was higher because of the geometry of centrifuge, as presented in the Table 1.

The solid content of the samples was determined through the slurry density. For this, liquid centrate and cake samples were taken directly from the liquid centrate exit and cake pile. The measured value for the cake solid’s mass fraction represents a mean value for the entire discharged sediment layer. After the tests, the value for each operational day of decanter centrifuge 1 is shown in Figure 10.

For the second test, the same initial parameters were adjusted, and the same sampling protocol was used. Figure 11 shows the results for centrifuge 2, presenting lower results for the cake solid’s content, and a wider range in the results.

The same happens with centrifuge 3, and the results are shown in Figure 12.

As observed, the higher the percentage of solids in the feed, the higher the percentage of solids in the cake, but the same occurs with the percentage of solids in the centrate.

Table 3. Mean values for each tested centrifuge.

Centrifuge	Feed Solid Content (%)	Cake Solid Content (%)	Centrate Solid Content (%)	Recovery (%)
1	12.5	81.7	0.46	97.7
2	6.6	65.6	1.64	77.6
3	12.0	77.2	0.27	98.1

summarizes the results.

The experimental results allowed us to obtain the relation between both decanter parameters and the machine throughput to compare the three decanter centrifuges when fed with the same material from the same process. The increase in solid content of the feed affected the separation efficiency regularly, and high or low capacity also had a certain impact on the separation efficiency [32].

In optimal conditions, a higher rotation speed produces higher centrifugal force for improving the settling of suspended solids in the liquid pool and the subsequent dewatering of the cake formed by the sediment. The consequence is lower cake moisture and/or a lower solids content in the centrate [24,30].

However, the bauxite processing stage in a pilot plant is susceptible to variation, which results in a range of the solid content in feed, which influences the results. Evaluating the concentration of solids in the feed during the three tests, variations in value over time were noticed (Figure 13). The feed’s solid content ranged from 4.1% to 21.7%. This variation was expected and positive for the analysis used.

Furthermore, each piece of equipment had a different capacity, and the concentration of solids in the feed had to be adjusted to accommodate process interruptions.

It is important to mention that test with equipment 2 was affected by external instabilities in the plant, which made difficult to reach a steady state, making its results worse than expected. Nevertheless, it achieved a recovery above 75% and low solid content in centrate, making the test valid for information purposes.

The objective of the solid dewatered phase, the cake, is to reach a solid concentration that enables its handleability. The handleability and disposability of these cakes are greatly influenced by the selection of chemical aids (such as polymer flocculants) and the mechanical dewatering technique employed [33]. An example of the dewatered material is seen in Figure 14.

Comparing the results of the cake quality among the tests on the three pieces of equipment (Figure 15) and considering that the design characteristics and operational parameters can influence these results, a solid content in the cake was reached that was enough to enable the solids’ handling and transportation to tailing storage facilities using haul trucks or conveyors. For the proper handling and safe disposal of the dewatered fine tailings, further investigation on the rheological properties of samples is recommended [33,34].

The liquid phase, the centrate, presented a low solid concentration in all tests, as shown in Figure 16. Even in tests with equipment 2 reached a peak value of 6%. It is important to mention that the performance can still be enhanced, considering that the tests were carried out without the addition of any chemical aids, and this addition would reduce the solid content in the flow. Therefore, the results enable the statement that, in an industrial operation, the liquid phase has a solid content low enough to be reused in other stages of the process, saving the resource of water, which is scarce at mine sites.

Overall, a great solid recovery was observed in the dewatering via the decanter centrifuge, reaching values near 100% in optimal conditions. The recovery achieved for each of the test conditions can be observed in the Figure 17.

The transfer from the pilot to industrial scale is usually only possible for geometrically similar machines in combination with decades of experience of the manufacturers [21]; since the existing models usually do not cover all variables, there are often deviations between predictions and the real process behavior. Thus, when it is possible, is important test under real conditions.

The successful combination of chemical reagents and mechanical forces generated using solid bowl centrifugation would revolutionize the bauxite tailings disposal methods, contributing to safer, eco-friendly, and sustainable mining production [35].

4. Conclusions

This work contributed to decreasing the scarcity of references on dewatering bauxite tailings using a decanter centrifuge; it also showed that high solid contents in the cake can be achieved with different centrifuges with different operating parameters. Pilot tests were performed with previously optimized parameters defined by bench scale tests (G-force and others).

The bauxite tailing characterization reveals that it has a fine particle size distribution, with d₅₀ of 45 µm and d₉₅ of 547 µm, 10% of the material below 1 µm, a size distribution that can interfere with the dewatering process due to the interfacial forces predominant in this size fraction.

The spin test results indicated the applicability of the decanter centrifuge bauxite tailings prior to the pilot tests. It also indicated that polymer has low influence on the liquid centrate results, but it can be used if industrial performance requires it.

The pilot plant studies showed that it is possible to obtain a cake with solid contents above 70%, with solids’ recovery near 100%, enough to ensure the solids’ handling, and it has been constated that it is possible to obtain a centrate (clarified liquid) with solid contents low enough to enable the reutilization of the liquid phase in the processing circuit. Furthermore, it also enabled to determine operational parameters for the plant design.

These results indicate that decanter centrifuges can be used to dewater bauxite tailings, even without adding any kind of polymer. With these results, it is possible to apply the development and optimization of a dewatering system to bauxite tailings from the aforementioned mine.

The research also explores how different solid contents affect the settling behaviors of tailings suspensions. Overall, this work contributes to an improved understanding of the fundamentals of bauxite tailings’ dewatering, potentially benefiting the development of new technologies and minimizing environmental impacts.

Although ready for disposal, the tailing cakes obtained after the dewatering stage were still difficult to handle and required considerable management. Therefore, the proper handling and safe disposal of dewatered fine tailings can be clarified in further studies, focusing on both technical and operational considerations.

Yield stress and stickiness are the main parameters reflecting the disposability and handleability of the tailings, respectively. Further studies can focus on the effect of chemical usage and dosage, as well as other operating parameters of the centrifuge (i.e., differential rate).