Application of SAP to Improve the Handling Properties of Iron Ore Tailings of High Cohesiveness: Could a Reagent Help the Decommissioning Process of a Dam?

Abstract

This work aims to evaluate the use of a superabsorbent polymer (SAP) to provide improvements in the handling properties of iron ore tailings (IOT). The material studied came from the magnetic separation reprocessing of the material discarded at the Gelado Dam, located in Serra dos Carajás in the state of Pará, Brazil. While the concentrate presents reasonable handling conditions, the tailings, with 61.5% iron, 15% moisture, and 39% of the mass, have high cohesiveness and adhesiveness due to their fine nature and the climatic conditions of the Amazon rainforest. However, the tailings can still be considered a product as long as the handling and transportation logistics are feasible. Thus, studies with an SAP and IOT were carried out in a bench rotating drum to promote mixing between them, and the main variables studied were the SAP dosage and the required contact time. The improvement in the physical properties of the IOT were evaluated considering the Hausner ratio, Carr index, Jenike’s flow function index, Atterberg limits, and chute angle. The superabsorbent polymer promoted a significant improvement in the state of consistency of the material, and the best performance was obtained with a dosage of 1000 g t⁻¹. As long as a suitable contact condition was promoted, a contact time of 1 min was enough to achieve the expected benefits. After dosing with the superabsorbent polymer, the material’s handling classification changed from ‘cohesive’ to ‘easy flow’, and the chute angle was reduced from 90° to levels below 60°. It was concluded that the application of the superabsorbent polymer has the potential to improve the fluidity of the material discarded in the magnetic concentration operation, allowing it to be handled throughout the production and transportation chain. The SAP appears to be an important additive for the full use of the material present in the dam (100% recovery), with both economic and socio-environmental benefits.

1. Introduction

The handling properties of mineral resources are essential in logistics to serve the consumer market. They are also important for the appropriate disposal of materials discarded as tailings. In conditions of high cohesiveness and adhesiveness, it is essential to seek solutions that improve the material’s properties. In bulk solids handling, ‘cohesion’ is defined as the bulk material sticking to itself, and ‘adhesion’ is generally defined as the action or process of adhering to a surface or object [1,2].

The fluidity of most bulk solids and ores is affected by the following characteristics and parameters: the chemical and mineralogical nature of the material, particle size distribution, moisture content, storage time, consolidation pressure, weather conditions, and presence of oils, clays, talcs, and/or additives [3,4]. In general, increasing the percentage of moisture leads to a more cohesive and adhesive behaviour, which causes difficulty in handling [5].

One possibility for improving ore flow properties is the use of superabsorbent polymers (SAPs). SAPs are cross-linked polymer networks made up of water-soluble base elements. SAPs are composed of ionic or non-ionic monomers, and are characterised by a low crosslinking density, which results in a high fluid absorption capacity (up to 1000 times their own weight). There are three main classes of SAP: (a) cross-linked polyacrylates and polyacrylamides; (b) cellulose- or starch-acrylonitrile graft copolymers; (c) cross-linked maleic anhydride copolymers. Efforts are being made to develop highly efficient absorbents based on natural products and also composites, such as clay-based polymer composites. All of them present huge imbibing mechanisms that physically entrap water via diffusion and capillary forces in their macro-porous structure [6,7].

Researchers have sought to use superabsorbent polymers with the aim of improving the handling properties of materials in mineral processing (

Table 1. Previous work concerning the use of SAPs in mineral processing.

Ore	How It Was Used	Key Observations	Reference
Coal fines	Direct method	It was possible to dewater fine coal from a moisture content of 29.4% to 12–14% using a pH-sensitive SAP (2 wt.%) within a 4 h contact time.	Dzinomwa et al. (1997) [8]
Coal slurry	Indirect method (in sachets)	Different coal moisture levels and SAP dosages were applied. Under one condition, e.g., the moisture content of the coal slurry dropped from 30% to 7% with an SAP dosage of 2 wt.%. Thermal and pH-induced regeneration procedures were investigated. The methods can be effective, but they still present economic and technical issues to be addressed.	Peer and Venter (2003) [9]
Tailings from oil sand (<0.1 mm)	Direct and indirect method (in sachets)	Rapid and significant dewatering, as well as an increase in the undrained shear strength and solid content of mature fine tailings, can be achieved using SAP dewatering and densification methods (direct or indirect). Better performance with regard to dewatering and an increase in strength are obtained with the indirect method. The SAP dosages applied varied from 1 to 3 wt.%, with a contact time on the order of days.	Farkish and Fall (2013) [10]
Three types of low-grade nickel laterite (<0.1 mm)	Both direct addition mode and sealed in a water permeable polyester bag	The direct addition method was not successful due to the attachment of the laterite particles to the surface of the swollen SAP granules and the retrieval of water from the superabsorbent hydrogel. The SAP provided a favourable dewatering. The best performance was achieved at dosages of 2% to 3% within an 8 h contact time.	Joseph-Soly et al. (2016) [11]
Mine tailings (<0.3 mm)	Both direct addition mode and in geotextile bags	The preliminary results showed that an SAP volume dosage of 10 to 13 kg m−3 of slurry resulted in a final solids mass concentration (Cw%_final) ≥ 70%. The occurrence of a gel-blocking phenomenon was observed.	Sahi et al. (2019) [12]
Iron ore (<1 mm)	Direct addition mode	Flow function curves for 12% moisture iron ore showed the change in the flow regime from cohesive to easy flow in the presence of the SAP. A dosage of 100 g ton−1 and retention time of 30 min were obtained and incorporated during plant trials.	Srikakulapu et al. (2020) [13]
Lump iron ore (<40 mm)	Direct addition mode	Iron ore changed its flow behaviour from the poor flow to the easy flow regime with SAP dosages of 1200 g t−1, 1400 g t−1, and 1800 g t−1 for moisture contents of 8%, 10%, and 12%, respectively. A 40% increase in production was achieved during trials and a 24% increase in long-term implementation of SAP during monsoons by improving the fluidity of LD iron ore.	Srikakulapu et al. (2021) [14]
Iron ore tailings	Direct addition mode	SAP was studied comparatively with quicklime as agents for water removal and compaction of iron ore tailings for dry stacking disposal. SAP did not improved the compaction properties of the IOT and lime did.	Rissoli et al. (2024) [15]

). Dzinomwa et al. (1997) were perhaps the first to exploit the concept of dewatering in mineral processing with the aid of superabsorbent polymers [8]. They suggested a process with three main stages: (a) contact between superabsorbent polymers and high-moisture fine coal; (b) the separation of dried fine coal from superabsorbent polymers by screening; and (c) the regeneration of the used superabsorbent polymer, taking advantage of its response to changes in conditions such as the pH, temperature, or electric field. Since then, studies have presented direct and indirect methods. Indirect methods, where the SAP is packaged, for instance, in sachets, provide benefits for removing the polymer, its regeneration, and its return to the process [9,10,11,12]. The direct application of the SAP to wet ore was also considered, as it provides improvements in production trials [13,14]. The superabsorbent was also tested for dry stacking of filtered iron ore tailings, but did not improve the material’s compaction properties. There was a moisture transfer from the mineral phase to the polymeric phase, but this did not reduce the humidity of the stack as a whole [15]. In short, the studies converge in stating that the SAP helps in the handling and flow properties of the materials, but its direct application for compaction is ineffective.

A critical case of an ore with high cohesiveness and adhesiveness occurs at the Gelado Dam, located in Serra dos Carajás in the state of Pará, Brazil; it contains 140 million tons of iron ore fines with an iron content of 63% (Figure 1). A project to restore the Gelado Dam has begun, and the material is being reprocessed; this includes dredging, protective screening, pipeline transportation, magnetic concentration, and filtration stages (Figure 2). While the product of magnetic concentration has properties that allow it to be transported in the context of conventional logistics, the tailings, with approximately 61.5% +/−0.8% iron on a dry basis and a higher percentage of fines and moisture, have a sticky behaviour that impairs handling. This material cannot be handled without some kind of assistance, as it tends to cause blockages and consequently stoppages in the production system. Furthermore, the waste has an iron content that would allow it to be commercialised if its handling conditions are improved, which would allow the full recovery of the material deposited in the Gelado Dam. The return of tailings to the dam implies the need for the continuous maintenance and monitoring of the disposal structure in the Amazon forest.

Accordingly, the challenge in using this tailing as a product is their high cohesion/adhesion, which tends to cause handling problems throughout the logistics chain. Given this condition, we studied the application of an SAP as a means of reducing the cohesiveness of the material. The studies with the SAP and iron ore tailings (IOT) were carried out in a bench rotating drum to promote mixing between them, and the main variables studied were the SAP dosage and the required contact time. The improvements in the physical properties of the IOT were evaluated using the Hausner ratio, Carr index, Jenike’s flow function index, Atterberg limits, and chute angle. The results attained are discussed in terms of the socio-technological benefits.

2. Materials and Methods

2.1. Iron Ore Tailings (IOT) and Superabsorbent Polymer (SAP)

The studied IOT were derived from the Gelado dam, Pará, Brazil (X 595,637,50, Y 9,337,672,21, Z 221—SIRGAS 2000). The sampled material was the filtered tailings of magnetic concentration stage, without the addition of any chemicals.

The IOT was physically and chemically characterized. The surface area was determined using a manual Blaine air permeability apparatus, following the guidelines of the Brazilian Standard NBR 16,372 [16]. The particle size distribution was obtained by laser diffraction using CILAS 1180 particle size analyzer. Chemical characterization was carried out by X-ray fluorescence (XRF) and X-ray diffraction (XRD). X-ray fluorescence (XRF) was obtained by spectrometer X-ray fluorescence Rigaku, model RIX 2000. X-ray diffraction was carried out in a Siemens (Bruker AXS, Billerica, Massachusetts, United States) X-ray diffractometer, model D-5000 (θ–2θ), equipped with a fixed Cu anode tube, operating at 40 kV and 25 mA, with an incident radiation of 1.5406 Å. The angular range analyzed was from 3° to 80° 2θ with a step size of 0.02°/3 s using divergence and anti-scattering slits of 2 mm and 0.2 mm in the detector. Petrographic analysis was carried out through circularly polarized light microscopy and image analysis [17]. Moisture was determined by measuring the loss in weight of the sample on heating at 105 °C following the Procedure ISO 3087 [18].

A sodium poly-acrylate-based superabsorbent polymer (SAP), sold in the particle size range between 0.2 and 1.0 mm and with an absorption capacity of 150 g of water per g of dry SAP, was used.

2.2. IOT and SAP Mixing

Interaction studies between the IOT and SAP were carried out on a rotating drum with a length of 41 cm and a diameter of 30 cm at a rotation of 50 rpm. The rotation speed of 50 rpm was chosen because it promoted the “cataracting” flow regime inside the drum. The amount of IOT, coming from the filter press with 15% moisture, was 5 kg in each batch. The variables studied were the SAP dosage and conditioning time. Figure 3 shows images of the rotating drum, the IOT, and the SAP.

To evaluate the effect of the SAP on the handleability properties of the IOT, compressibility tests, cohesive strength assays, Atterberg limits tests, and shuttle angle measurements were carried out. These same assays were carried out with the sample without applying the reagent for comparative purposes.

3. Hausner Ratio and Carr Compressibility Index

An indication that the bulk solid has developed cohesiveness is the significant reduction in its apparent density, and this can be measured by the Hausner ratio and Carr compressibility index (Equations (1) and (2)). The Carr index is a measure of a solid’s bridge strength and stability, and the Hausner ratio is a measure of interparticulate friction [19]:

$$\mathrm{H}\mathrm{a}\mathrm{u}\mathrm{s}\mathrm{n}\mathrm{e}\mathrm{r} \mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}={\displaystyle \frac{{\mathsf{\rho }}_{\mathrm{t}}}{{\mathsf{\rho }}_{\mathrm{b}}}},$$

(1)

$$\mathrm{C}\mathrm{a}\mathrm{r}\mathrm{r} \mathrm{i}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{x}={\displaystyle \frac{\left({\mathsf{\rho }}_{\mathrm{t}}-{\mathsf{\rho }}_{\mathrm{b}}\right)}{{\mathsf{\rho }}_{\mathrm{t}}}}\times 100,$$

(2)

where

${\mathsf{\rho }}_{\mathrm{t}}$—the tapped density;

${\mathsf{\rho }}_{\mathrm{b}}$—the bulk density.

The flow characteristics ranges are the following:

Hausner ratio: excellent—1.00 to 1.11, good—1.12 to 1.18, fair—1.19 to 1.25, passable—1.26 to 1.34, poor—1.35 to 1.45, very poor—1.46 to 1.59, and very, very poor—>1.60.

Carr index (%): excellent—≤10, good—11 to 15, fair—16 to 20, passable—21 to 25, poor—26 to 31, very poor—32 to 37, and very, very poor—>38.

4. Uniaxial Compression Test

Jenike’s flow function and the corresponding flow index represent a widely accepted industry standard for the measurement of flowability [20]. Jenike’s flow function index (ff) was calculated to predict the ease of material flow. It was defined as a dimensionless ratio of the major principal consolidation stress (σ₁) to the unconfined failure yield stress (σ_c), and it was measured under different levels of consolidation stresses, as described in Equation (3):

$$\mathrm{f}\mathrm{f}={\displaystyle \frac{{\mathsf{\sigma }}_1}{{\mathsf{\sigma }}_{\mathrm{c}}}},$$

(3)

where

${\mathsf{\sigma }}_1$—the major consolidation stress;

${\mathsf{\sigma }}_c$—the unconfined failure yield stress.

The Jenike classification of the solid flowability byflow index (ff) is as follows: ‘non-flowing’—<1, ‘very cohesive’—1 to 2, ‘cohesive’—2 to 4, ‘easy flow’—4 to 10, and ‘free flow’—>10.

5. Atterberg Limits

The Atterberg limits were measured following the procedure ASTM D4318 [21]. In liquid limit determination, the water content versus the number of drops of the Casagrande cup was plotted. Tests were conducted from ‘wet to dry (from low blow count to high blow count)’ by allowing the soil samples to gradually dry at room temperature, frequently mixing the soil to obtain a uniformly dried sample. The plastic limit was determined using the ‘hand rolling method’.

In this study, the moisture content (MC, %) was determined on a wet basis by drying the samples at 105 °C in an oven for 48 h or until the dry mass was no longer changing with drying, following the equation

$$\mathrm{M}\mathrm{C}\left(\mathrm{\%}\right)={\displaystyle \frac{\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s} \mathrm{i}\mathrm{n} \mathrm{m}\mathrm{o}\mathrm{i}\mathrm{s}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{e} \left(\mathrm{g}\right)}{\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e} \left(\mathrm{g}\right)}}\times 100,$$

where

loss in moisture—the initial weight of the sample (g)—the final weight of the sample (g);

initial weight—the wet or original weight of the sample before drying (g);

final weight—the weight of the sample after drying (g).

6. Chute Angle

The chute angle is also an important parameter used in the study of the flow properties of solids. In practice, it represents the angle at which the wall surface needs to be inclined for the material to slide. The test consists of evaluating the angle necessary for a grain sample to begin flowing on a flat surface. The sample is subjected to different contact pressures against the surface, and the surface angle is increased until the material flows. The flat surface used to perform the test was CDP—4666 steel. The minimum angle is also called the ‘kick angle’ and, for a given surface, it is a function of the pressure applied on the wall. The determination of the minimum slope to maintain the flow of materials on a flat surface, also called the ‘kick angle’, allows the correct design of transfer chutes between belt conveyors [22].

7. Results and Discussion

The IOT specimen resulting from the process of magnetic concentration used in this investigation presented a specific surface area of 7802 cm² g⁻¹ and particle sizes ranging from 0.1 to 82 μm, with a D₅₀ of 3.0 µm. The tailings are extremely fine, with 80% of the material being below 10 µm. It is known that cohesiveness depends on multiple elements such as genetic, chemical, mineralogical characteristic, as well as past stress conditions of the material. Typically, a geological material is classified as cohesive if the amount of fines (silt and clay-sized material) exceeds 50% by weight, which means 50% by weight of particles below 75 µm [23]. In the present case, the amount is approximately 95% by weight of particles finer than 75 µm (Figure 4). There is no other iron ore product in the world being handled from the mine to the port with this particle size.

The mineralogical composition, obtained by X-ray diffraction (XRD), mainly consisted of hematite and goethite. Gibbsite and quartz were also identified in smaller proportions. This is in line with the mineralogical distribution attained optical microscopy and image analysis (

Table 2. Volumetric composition of the reject from magnetic concentration from Gelado dam attained by optical microscopy and image analysis.

Minerals	% (in Volume)
Hematite-lamellar	53.9
Hematite-granular	22.2
Hematite-lobular	4.4
Martite	0.4
Goethite	16.0
Quartz	1.5
Gibbsite	1.3
Other	0.3

). X-ray fluorescence analyses yielded the following composition on a dry basis: Fe—61.5%, SiO₂—2.86%, P—0.083%, Al₂O₃—3.68%, Mn—0.977%, TiO₂—0.265, CaO—0.013%, MgO—0.174%, and ignition loss—3.89%. The moisture of the material from the filter was 15%, with a possible variation of +/−1.5%.

The compressibility properties of the IOT with the presence of different dosages of the SAP were evaluated using the Hausner ratio and Carr index parameters. For both the Hausner ratio and Carr index, the smaller the difference between the compacted density and the apparent density, the better the flow characteristics. From the results presented in

Table 3. Effect of SAP dosage on the Hausner ratio and Carr index with a mixing time of 1 min.

Mixing Time	Bulk Density (kg m−3)	Tapped Density (kg m−3)	Hausner Ratio	Carr Index
0 (g t−1)	1340	2418	1.80	44.6
500 (g t−1)	1300	2354	1.81	44.8
1000 (g t−1)	1400	2300	1.64	39.1
1500 (g t−1)	1290	2197	1.70	41.3

, it appears that better results were obtained with a dosage of 1000 g t⁻¹. However, it can be seen that even under the best conditions (Hausner ratio of 1.64 and Carr index of 39.1), the flow characteristics are classified as very poor. These properties do not seem to portray reality well, since the cohesive strength clearly showed a decrease in handling when the reagent was added. Furthermore, the reduction in index values with SAP was very small. Both criteria are based on changes in density due to compaction, which corroborates the results obtained by Rissoli et al. [15].

Table 4. Effect of mixing time on the Hausner ratio and Carr index using 1000 g t⁻¹ of SAP.

SAP Dosage	Bulk Density (kg m−3)	Tapped Density (kg m−3)	Hausner Ratio	Carr Index
1 (min)	1400	2300	1.64	39.1
5 (min)	1380	2390	1.73	42.3
10 (min)	1340	2349	1.75	43.0

presents the results attained with a dosage of 1000 g t⁻¹ and different SAP/IOT contact times. Even with the low accuracy of the test, the absorption seems to present a good result after just 1 min.

Table 5. Effect of the SAP dosage on Jenike’s flow function index (ff) with a mixing time of 1 min.

SAP Dosage	Parameters for Determination Jenike’s Flow Function Index (ff)
	σ1 (kP)	σc (kPa)	ff	σ1 (kPA)	σc (kPa)	ff	σ1 (kPa)	σc (kPa)	ff
0 (g t−1)	105	43.7	2.4	185	51.4	3.6	260	55.3	4.7
500 (g t−1)	105	35.0	3.0	185	46.3	4.0	260	53.1	4.9
1000 (g t−1)	105	31.8	3.3	185	36.3	5.1	260	44.1	5.9
1500 (g t−1)	105	33.8	3.1	185	39.4	4.7	260	46.4	5.6

shows the effect of the SAP dosage on Jenike’s flow function index (ff) with three different consolidation pressures (σ1). In fact, this is a recommended parameter for starting solid flow studies and the higher the ff ratio, the better the flow of a bulk solid [4]. As expected, an increase in the consolidation stress (σ1) caused an increase, albeit to a lesser extent, in the unconfined failure yield stress (σc). Since ff is obtained by σ1/σc, the values of ff increased with the increase in σ1. The tests were carried out with different SAP dosages, with an interaction time between the ore and the superabsorbent polymer of 1 min. Clearly, the best results were obtained with the dosages of 1000 and 1500 g t⁻¹ for the three values of σ1 applied. The application of the SAP changed the material behaviour from cohesive to easy flow. The interaction time of the superabsorbent polymer with the ore was also evaluated (

Table 6. Effect of the mixing time on Jenike’s flow function index (ff) using 1000 g t⁻¹ of SAP.

Mixing Time	Parameters for Determination Jenike’s Flow Function Index (ff)
	σ1 (kP)	σc (kPa)	ff	σ1 (kPA)	σc (kPa)	ff	σ1 (kPa)	σc (kPa)	ff
1 (min)	105	31.8	3.3	185	36.3	5.1	260	44.1	5.9
5 (min)	105	35.0	3.0	185	41.1	4.5	260	44.8	5.8
10 (min)	105	33.9	3.1	185	41.1	4.5	260	47.3	5.5

). Definitively, good results have been achieved with an interaction time of 1 min.

Figure 5 shows a graphical view of the consistency limits of the material removed from the dam as a function of the SAP dosage. Without the application of the reagent, the moisture of the plastic limit is 13%, and the liquid limit is 22%. Increasing the superabsorbent dosages results in a gradual change in these limits. Considering the moisture content from the filter press as 15%, it can be seen that without the addition of the SAP, the material is in the plastic range, making handling difficult. Starting from a dosage of 750 g t⁻¹ SAP, the material behaves as a semi-solid, with markedly lower cohesiveness.

Another performance parameter analysed in this study was the kick angle. Tests were carried out with the sample without reagent and with the sample with the superabsorbent polymer under the optimised conditions (dosage of 1000 g t⁻¹ and interaction time of 1 min) at different pressure intensities (Figure 6). There was a significant reduction in the kick angle when the SAP was used, as a consequence of improving the flow properties of the solid. The kick angle dropped from the 74–90° range to the 45–55° range with the addition of the SAP. It is also observed that greater pressure increases the kick angle slightly, with the effect being more pronounced without the addition of the SAP.

The presence of moisture is adverse for the flow characteristics of ores. The presence of ultrafine clay particles and moisture may transform mining materials into a sticky, wet cohesive mass that is difficult to handle. In this and previous works, the effect of the dosage of the superabsorbent polymer on improving the flowability of the solids was evaluated. At different humidity levels, the SAP has proven its effectiveness in adapting the flow behaviours of ores. Comparing results is difficult, as they involve different ores, regions, and SAP application modes. Notably, this work was carried out with an iron ore finer than any other. However, the works of Srikakupapu et al. [13,14], carried out with iron ores and the direct application of an SAP, present a certain similarity.

Two important parameters are the SAP dosage and the interaction time of the reagent with the ore. Concerning the SAP dosage, it is moisture-concentration-dependent. Srikakupapu et al. [13] have indicated that a proper dosage is 100 g t⁻¹ of SAP for iron with 12% moisture, and in [14] it is stated that the proper dosages are 1200, 1400, and 1800 g t⁻¹ for LD iron ore (LD referred to the Linz and Donawitz process) with 8%, 10%, and 12% moisture, respectively. So, in some studies, the values were similar and in others they differed from that one chosen in this work as best, 1000 g t⁻¹ for an ultrafine IOT with 15% moisture.

Regarding the interaction time between the SAP and the ore, although the addition of the SAP had an almost instantaneous effect on improving the fluidity of iron ores, the time needed diverged from that reported in other works. Srikakupapu et al. [13] reported values of 30 min for iron ore, and a time of 24 h was reported for LD iron ore [14]. Our observations indicate that a time of a few minutes is satisfactory when using a rotating drum. Obviously, the characteristics of the materials such as particle size distribution, mineralogy, and specific surface area are different. The addition of SAP to the ore was also distinct. Srikakulapu et al. [13,14] introduced the SAP particles by a pneumatic device to a chute in an industrial plant. In the first paper of Srikakulapu et al. [13], the ore particles had a top size of 1 mm and the contact time among SAP and the iron ore was not investigated as a variable. In the second paper of Srikakulapu et al. [14], the top size was 40 mm and they do investigate the time, starting from 10 min. However, they explain that a longer period of time is necessary, since the coarse particles hinder the migration of the water to the SAP grains.

In the case of the Gelado Dam, the use of the SAP could help overcome the difficulties of transporting the material. Figure 7 shows a schematic drawing of the logistics chain that the material will have to go through, which is quite universal for the intercontinental transportation of iron ores. The handling of the Gelado Dam material begins with detaching the solids from the filter press and sending this material to the product yard, where it will be piled up. From this stockyard, the material will be taken back via the bucket wheel reclaimer, where it will be sent to the rail loading terminal that will supply the ships for transoceanic transport.

The mass of waste generated in the magnetic concentration process of the Gelado dam (per the flowchart depicted in Figure 2) represents a total of 54.6 million metric tons of iron ore with 61.5% Fe. The cost of the superabsorbent reagent is around 3600 USD per metric ton. Considering the processing of 5 million tons of IOT per year and the application of the SAP in a dosage of 1000 g t ⁻¹, the annual cost with the SAP is estimated to be USD 17.8 million. Conversely, this processing requires 6000 worked hours per year, and this reagent is of fundamental importance to the logistics; any failure will result in stoppages. It is estimated that if the total number of stops in a year exceeds 250 h, the drop in production will result in a revenue loss greater than the amount spent with the additive.

Dam disasters in Brazil, especially in the cases of Mariana and Brumadinho, forced the review of procedures and the adoption of much more cautious approaches [24,25]. Safe decommissioning is part of this context, and this process may include different strategies, such as land restoration projects, reforestation, and the usage of the material [26,27]. It is also important to consider that any investments in safety or decommissioning are smaller than the costs involved in the failure of a dam [28].

In the specific case of the Gelado Dam, in addition to the commercial benefits, the proper functioning of these logistics will have an important socio-environmental consequence. The site is located within the Carajás National Forest, an important environmental preservation unit. The waste deposited in this dam occupies an area of around 700 ha, and as the dam decommissioning process progresses, the forest will be free to recover. The Carajás National Forest is open to visitors and is used in activities of environmental education. By implementing the tailings recovery project, it will be possible to increase the visiting space, contributing to education projects with the community.

Finally, the iron ore sector, as well as other branches of mining, is committed to overcoming a set of challenges, including exploring new frontiers [29,30], the development of sustainable technologies [31,32,33,34,35], and the improvement of regulatory actions [36,37,38,39,40,41], which are currently pooled and named as Environmental, Social and Corporate Governance (ESG). With the depletion of the world’s plentiful iron deposits, the use of SAPs can contribute to the use of ultrafine iron ore tailings discarded in dams [42] or reprocessing rejects from the exploration of ferruginous quartz deposits [43].

This study was carried out using a superabsorbent polymer based on sodium polyacrylate, a superabsorbent produced synthetically, mainly due to the need to meet a high production scale. Future research could be directed towards the development of equipment to assist the mineral industry and, perhaps, to the usage of alternative natural superabsorbent materials [6,7].

8. Conclusions

The material studied in this work was the tailings from the magnetic separation reprocessing, followed by a filtration operation, of the material deposited at the Gelado Dam, located in Serra dos Carajás in the state of Pará, Brazil. It was concluded that the application of the superabsorbent polymer has the potential to improve the fluidity of the material, allowing it to be handled throughout the transportation chain. The superabsorbent polymer promoted a significant improvement in the state of consistency of the material, and the best performance was obtained with a dosage of 1000 g t⁻¹. As long as suitable contact conditions were promoted, a contact time of 1 min was enough to achieve the expected benefits. After dosing with the superabsorbent polymer, the material goes from a plastic state to a semi-solid state. In terms of the material’s handling classification, it changed from ‘cohesive’ to ‘easy flow’, and the chute angle was reduced from 90° to levels below 60°. The use of the SAP could be the ‘silver bullet’ that would allow the decommissioning of the Gelado Dam. The benefits are economic, as this will increase the global recovery of iron ore from the Carajás Mine, and will also be a step towards recovering 700 ha in a preservation area inside the Amazon forest biome.