Characterization and stabilization of iron ore suspension and influence of the mixture of natural additive Sapindus mukorossi and SDS on the slurryability

Abstract The traditional way of transporting iron ore from mine to end-use plants often causes the industry significant economic hardship and degradation of the ecological system. Hydrotransportation of slurry through a pipeline is a remarkably viable strategy for effectively transporting iron ore in terms of economics, dependability, and safety, with minimal environmental impact. In the present investigation, a blend of bimodal samples is prepared by mixing medium and coarse particles of Indian iron ore of size 38–75 (I-2) and 75–106 (I-3) µm, respectively, with finer particles of size <38 µm (I-1) in 10–40% (by weight) proportion. Rheological characterization of the blend samples is done to finalize the optimum particle size. To enhance the flowability and stability of the concentrated iron ore samples, they are amalgamated with a mixture of plant-based nonionic saponin extracted from Sapindus mukorossi (Ritha) and the anionic sodium dodecyl sulfate (SDS) surfactants. The correct proportion of SDS in saponin is obtained through a thorough analysis of critical micellar concentration, zeta potential, and rheological parameters. Finally, the stability and slurryability of iron ore suspension with a Saponin–SDS mixture are established through rheological properties, dispersant concentration, and stabilization mechanisms. It is found that the optimum proportion of a mixture of saponin–SDS significantly improved the slurryability and stability of iron ore suspension and the maximum iron ore loading was enhanced by up to 81% by weight. The head loss and specific energy consumption analysis successfully evidence the significance of the surfactants in transporting the slurry through pipelines. Graphical Abstract Interaction of Saponin-SDS mixture at the iron ore water interface


Introduction
Global steel consumption is expected to reach 2000 Mt by 2021.The Indian steel and iron sector have expanded remarkably during the last 20 years.India produced 118 million tonnes in 2021, and this production impulse is expected to assist India in meeting its objective of 500 Mt of generation capacity in the upcoming 25 years, as per the World Steel Association (2021).10] High concentration and low viscosity are the slurry's primary driving factors during hydrotransportation.High concentration leads to high viscosity in a slurry that causes trouble in pipeline transportation due to the solid particles' inter-particle attraction. [11,12][15][16][17] Therefore, choosing the right additives should be acknowledged as one of the most crucial elements in preparing slurries for transportation.
In the past, different aspects of viscosity modifiers have been studied on solid-liquid suspension.It was observed that some chemical additives improved the slurryability and allowed transportation at considerably higher concentrations while consuming lesser water. [18]Rheological behavior of hematite aqueous suspension containing volume concentrations up to 40% having particle size (<45 mm) was examined in the presence of lignosulfonate dispersant, and observed that suspension slurryability noticeably improved with the additive. [19]Chand et al. [20] studied the performance of centrifugal slurry pumps with and without the drag-reducing polymer guar gum.They tested several solid materials, such as coal dust, iron ore, sand, and fly ash, at concentrations of up to 26% (by volume).They noticed that polymer additions enhanced the pump head and efficiency considerably.Mabuza et al. [21] reported that the polymeric surface-active chemical agents promote the fluidity of dense medium ferrosilicon and magnetite slurry suspensions.Nsib et al. [22] in their study highlighted that the hematite suspensions with sodium polymethacrylate additive in a medium alkaline offered steric stabilization. Lee et al. [23] reported the effect of sodium dodecylbenzene sulfonate on Fe 3 O 4 -water suspension and remarked that dispersant adsorption on magnetite suspension occurs above and below the isoelectric point of pH value.
[26][27] Different chemical dispersants like sodium silicate (SS), ethylenediamine-tetraacetatic-acid (EDTA), sodium pyrophosphate (SPP), and sodium hexametaphosphate (SHMP) were employed on ultra-fine iron ore slurry to enhance the flow behavior. [28]Marcos and Antonio [29] examined the effect of twenty different additives on the rheological behavior of concentrated iron-ore suspension.They reported that these chemical additives promoted the reduction of the fluid plastic viscosity and the consistency index.Melorie and Kaushal [30] studied the effect of a few additives, like quick lime, sodium hexametaphosphate, Acti-Gel, and, hydrated lime, to inspect the flow behavior of iron ore suspension and reported that sodium hexametaphosphate considerably reduced the viscosity of iron ore suspension in the solid loading range of 18.8-25.8%by volume.Senapati et al. [31] in their study, analyzed the effect of bio-additives obtained from Bellyache bush (Jatropha gossypifolia Linn) and Indian spinach (Basella alba) on iron ore slurry having a solid concentration of 60-75% by weight and reported that bio-additives improved the fluid mobility and stability.Behari et al. [32] inspected the effect of Sapindus laurifolia on the hematite iron ore slurry and observed the maximum suspension stability at saponin CMC value.
[35][36][37] Mixture surfactant in the slurry plays a crucial role in maintaining stability with high solid loading.[40][41][42][43] Surfactant mixtures are cheaper, more abundant, environmentally friendly, and perform more effectively than solo surfactants in many applications. [44,45]Synergetic interactions between surfactants boost performance.Synergistic surfactant blends minimize surfactant consumption, expense, and environmental effect.[54] However, synergism promotes the formation of micelles at an early stage, increasing the viscosity of the slurry, [55][56][57] which would be unfavorable to pipeline transportation.Antagonism decreases the mixture's surface activity, [58,59] which lowers viscosity and mineral adsorption density.Therefore, it is crucial to preserve the adequate equilibrium of interfacial adsorption and viscosity and establish the procedure for optimizing the effectiveness of a surfactant combination in stabilizing concentrated iron orewater slurry.The surfactant mixture with bimodal iron ore could reduce energy as well as expenses during slurry transportation, storage, and shipment.
The concentration of solid and the distribution of the particle sizes have a significant impact on the rheological behavior of slurry. [60]Investigation revealed that the particle size distribution (PSD) and particle size significantly influence the iron orewater slurry (IWS) rheological behavior.Due to the gap between the particles in a monomodal distribution, less possibility of obtaining a high concentration. [61]The iron ore particle spacing should be lowered due to the particle size.So, the bimodal approach is essential for using iron ore particles from two different fractions to increase iron ore loading.As the packing efficiency improves at constant solid concentration, less water is needed to fill the interparticle gaps.It increases the amount of water that may separate the particles further, lowering the viscosity.The preparation of highly concentrated IWS using a bimodal distribution of Indian-origin iron ore with a combination of biological and synthetic additives has received relatively little attention.
Many works have been done on iron ore slurry but separately employing chemical and natural additives.However, limited research has been carried out using a combination of natural and chemical additives on iron ore slurry.Also, the mechanism behind the adsorption of surfactant mixture on iron ore has been rarely addressed.So, it is required to investigate the synergetic interaction of natural surfactants in conjunction with chemical additives to prepare high-concentration iron ore slurry.In the current article, an effort is made to inspect the sustainability of creating a blend of commercially available synthetic surfactants sodium dodecyl sulfate (SDS) and natural surfactant saponin extracted from Sapindus mukorossi (ritha) in highly concentrated bimodal iron ore slurry for the first time.Foremost, an optimization study on the particle size of the bimodal iron ore sample based on rheological parameters is carried out.Then, surface tension and zeta tests are assessed to find the particle interaction and surfactant activities at various proportions of additive mixture concentrations.Finally, head loss and specific energy consumption are evaluated to evidence the surfactants' relevance.

Source of iron ore and sample preparation
Iron ore is obtained from amalgam steel private limited (India) and used in experimental investigation.The raw sample is crushed utilizing a laboratory-size ball mill crusher.The pulverized iron ore sample is dehumidified in an electric oven for 2 h at 100 � C. The desired size of iron ore particle is obtained using a mechanical sieve shaker.Iron ore with three different particle size ranges, that is, <38, 38-75, 75-106 mm, namely I-1, I-2, and I-3 , are used to prepare bi-modal samples.Iron ore particles I-2 and I-3 are added in 10-40% (by weight) proportion into finer particles I-1 to prepare eight bi-modal samples I-4, I-5, I-6, I-7, and I-8, I-9, I-10, and I-11, respectively.The effective particle size of iron ore samples is represented in supporting information S5.

Physicochemical properties of iron ore sample
The PSD of the iron ore sample has been calculated using the particle size analyzer (Anton Paar PSA 1190, Austria).The PSD of the raw iron ore sample is represented in supporting information S1.The PSD findings reveal that particle size in the sample is smaller than 106 mm in diameter, with 70.34% of particles finer than 75 lm and 51.81% of particles less than 53 lm, and 34.56% of particles below 38 lm.The weighted mean particle diameter of the iron ore sample is 59.92 lm.Raw iron ore sample particle sizes at d10, d50, d70, and d90 are 10.67, 51.25, 74.86, and 92.32 l m, respectively.The substance's specific gravity is a major factor that resolves the suspension's Maximum Static Settled Concentration (MSSC). [62,63]The iron ore sample's specific gravity is determined using the pycnometer, and the sample has a specific gravity of �4.7.The gravitational method is used to determine the MSSC of the sample at different solid loading.The MSSC of the sample is 84.72% by weight at a 75% concentration of iron ore sample.The settling time for the solution to attain a static level is 25-30 min.After that, minor changes are observed in measuring cylinder level.The chemical properties of mineral suspension have a critical role in stability.The mineral's pH value influences the stability of the suspension. [64,65]A digital pH meter quantifies the pH value of slurry at various solid loading and surfactant concentrations.

Morphology of iron ore sample
The mineralogical characteristics of the iron ores differ from mine to mine and influence the slurryability of suspensions.
[68] The morphology of procured iron ore samples is analyzed by SEM and EDX techniques (Model: JEOL 6510LV, Japan).SEM images of the iron ore at 100� magnification are shown in S2 (a).It reveals that iron ore particles have fibrous and granular shape microstructures.S2 (b) depicts the EDX spectrograph of the iron ore sample.The elements contained in the sample are identified quantitatively using distinctive pick spots on the spectrograph.EDX confirms that iron (Fe) is the main constituent in the sample.The XRD (SmartLab Studio II Rigaku, Japan) analysis determines a substance's crystalline phases, revealing its mineral composition.S2 (c) display the XRD diffractogram of the iron ore sample.XRD analysis confirmed the presence of hematite, goethite, kaolinite, and gibbsite minerals in the sample.However, Scherrer's calculation reviled that the average crystallite size is nearing 349.41 nm.

Sources of additives
Sapindus mukorossi fruits are collected from the eastern coastal region of India.Sapindus mukorossi is commonly known as soapberries or ritha.These plants are abundantly grown in the coastal region throughout India's north and eastern states.The Sapindus mukorossi fruits are dried, powdered, and dissolved in the water at a ratio of 1:9.The magnetic stirrer agitates the additive mixer for 4 h at 1000 rpm.The supernatant solution is then centrifuged for 1 h using a centrifugation machine.Because no chemicals are employed in the preceding method, the liquid solution is filtered and collected, resulting in an aqueous saponin extract.The industrial surfactant sodium dodecyl sulfate (C 12 H 25 OSO 3 Na, anionic surfactant) is purchased from Merck India Ltd. and utilized exactly as is.The structure of saponin extracted from Sapindus mukorossi and commercial surfactant Sodium Dodecyl Sulfate (SDS) is shown in Figure 1.

Determination of CMC
A surface tensiometer (DCAT-21, Dataphysics, Germany) is used to analyze the surface tension of the aqueous extract's saponin at different concentrations.The Wilhelmy plate method is used to evaluate surface tension at different fractions of surfactants.The relation between the aqueous phase's surface tension and saponin content is presented in Supporting information S3.The nature of the curve demonstrates a rapid decline in surface tension when the dispersant dosage is increased from 0.001 to 0.018 g/cc.The surface tension of pure water is (72.7460.2mN/m) and reaches a minimum saturation value of (40.6260.2mN/m) at a saponin concentration of 0.018 g/cc (1.8%wt.).This surfactant concentration is CMC of aqueous extract saponin.The effect of chemical surfactant SDS on the surface tension of the aqueous saponin system is examined by varying the fraction of SDS from 0.05 to 0.6 wt% of CMC of aqueous extract saponin.The surface tension reaches its lowest point at 0.4 weight fractions of SDS, as shown in Figure 2, after that, the surface tension rises as the weight fraction of SDS increases.It is perceived from the graph that surfaces tension is lowest in the presence of surfactants (SDS:saponin) mixes at 40:60 ratios.That is the optimum proportion of surfactant mixtures.Every surface tension experiment is performed thrice, and the mean of the data is finally considered.The reproducibility of the surface tension value is within experimental error (60.2 mN/m).

Measurement of rheological characteristics of sample slurry
Bi Instrument measurement ambiguity and equipment geometry accuracy produce experimental data errors. [69]To minimize experimental errors (within 61%), all shear stress and viscosity measurements were carried out at a constant temperature and repeated three times.The overall uncertainty involved in the experimental measurement is found to be 3.26%.

Fitting into the rheological properties
Rheological model fitting is a technique that provides information about the behavior of fluids, and to check the fit, two different models (Herschel-Bulkley and Bingham Plastic model) are applied to experimental data.The general description of rheological models is described below.The Herschel-Bulkley model, frequently referred to as the yield power-law model, describes how some viscous fluids behave; specifically, those with yield stress and shear stress at higher shear rates behave like a power-law fluid.
Bingham Plastic model presents the rheological features of slurry with linear rheological profiles and yield stress.
The yield stress is represented by s y , while c denotes the shear strain rate, and the model parameters k and n are the flow behavior index and consistency index, respectively.

Analysis of zeta potential
Zeta or electrokinetic potential is the electrostatic attraction or repulsion of charged particles in an emulsion or slurry.The electrophoretic mobility of charged particles is the measure of zeta potential. It is also used for forecasting the optimum quantity of dispersed phase and dispersion medium in colloidal suspension.Zeta potential for slurry-additive mixture is obtained using a particle size analyzer (Litesizer 500 Anton Paar, Austria).Samples are prepared by mixing 1 g of iron ore sample in 100 mL of deionized water.The slurry solution is well mixed by a magnetic stirrer at room temperature for 15 min.The experiment is performed thrice with 1 mL of this mixed slurry sample and considers the average.The reproducibility of the zeta potential value is within experimental error (61 mV).

Head loss (HL) and specific energy consumption (SEC) in slurry pipeline of iron ore
The HL is evaluated for the slurry pipeline by utilizing rheological properties.The rheological variables like yield stress, flow index, and consistency index are employed to assess the HL for the projected slurry pipeline transportation system.As per rheological analysis, all slurry samples are governed by the power-law model.Dodge and Metzner [72] proposed a correlation to determine the HL of yield pseudoplastic fluid based on local power-law assumption as given below where K 0 is the Local power law index, D pipe diameter (100 mm), q m Slurry density (kg/m 3 ), and L designates pipe length (m).
The SEC is an essential measurement for the transportation of solid substances. [73]The SEC is anticipated to inspect the economic significance of the surfactants.SEC measures the energy needed to transport a unit amount of material over a distance.Hydraulic power involved by the pump to transport one tonne of solid substance via a one-kilometer pipeline is stated as: where P h is hydraulic power (kW), F s flow rate of solidliquid (tonne/h), C w concentration by weight %, and DP head loss (mWc).

Surface activity of SDS-saponin on bi-modal IWS
The tendency of the surfactants to preferentially bind to the iron surface plays a significant role in stabilizing the bimodal IWS.Consequently, the potential for the addition to become adsorbed onto the ore surface decreases as the additive becomes more at ease in solution.Investigation of the behavior of additive combinations in solutions must thus be conducted.Since one of the key characteristics of surfactants in solution is their ability to reduce surface or interfacial tension, the behavior of mixture systems has been examined by assessing the surface tension of the mixtures as the quantity of SDS in the solution is gradually varied, as shown in Figure 2. The saponin surfactants, which are in equilibrium with their monomeric surfactants/micelles in bulk solution, form a monolayer film across the air-water interface.The surface tension of the solution steadily decreases as the amount of SDS in the mixes rises.It could result from saponin preferentially partitioning to the interface in place of ionic surfactants, the interface forming a mixed monolayer of saponin and ionic surfactants or various SDS-Saponin interactions that are prevalent in the bulk solution that increase the surface activity of the solution. [27,38]The graph shows that the surface tension of aqueous extract saponin is (40.6260.2mN/m) and decreases as the saponin concentration decreases with the constant addition of SDS.Surface tension lowers to (33.6260.2mN/m) when 40% SDS is introduced at 60% saponin concentration, and surface tension steadily rises as SDS concentration further increases.The optimum surfactant mixture ratio is obtained by combining 60% Sapindus mukorossi saponin and 40% SDS.Since the surface activity is highest at the lowest surface tension value, a saponin-SDS mixture ratio of 60:40% stabilizes iron ore water slurry.

Optimum particle size
Particle size and size distribution are crucial factors for preparing suspensions with high concentrations of minerals or ores.The rheological parameters are affected by particle size variation, which also alters the suspension's flow behavior.As a result, determining the particle size is crucial when taking rheological measurements. [74]Figure 3 depicts the relationship between apparent viscosity and shear rate for the three representative iron ore samples (I-1, I-2, and I-3) at a slurry concentration of 60% by weight.The graph shows the diminishing trend in the apparent viscosities of the three IWS samples with increasing shear rates and particle sizes.Higher apparent viscosity is observed in iron ore slurries with smaller particle sizes than coarser particles. [60]Sample I-3 showed the lowest apparent viscosity values in the examined range of shear rates.The big gaps in the coarse particle size allow water to flow through them and increase the fluidity of the slurry. [75,76]Therefore, the current investigation has selected a bimodal distribution of the iron ore sample, a combination of fine, medium, and coarse fractions of particle sizes.

Establishing the optimal coarse to fine proportion
The shear rate and apparent viscosity of blended samples for various fine-to-coarse ratios at a slurry concentration of 70% by wt.
are shown in Figure 4, According to graph 4(a), slurries with a fine to coarse ratio of 60:40 (I-7) have less viscosity than those with a fine to coarse ratio of 70:30 (I-6), 80:20 (I-5), and 90:10 (I-4).In contrast, graph 4(b) shows that slurries with a ratio of 70:30 (I-10) have low viscosity than those that have a ratio of 90:10 (I-8), 80:20 (I-9), and 60:40 (I-11).The reduction in apparent viscosity is due to the breakdown of the complex structure of the finer particulate slurry.The coarse iron ore particles take place among the fine particles, breaking the complex structure of fine particulate slurry.As a result, the particle's movement over one another becomes more accessible, and they are no longer as complex structures as previously. [61,77]As a result, the apparent viscosity of the I-10 sample is lower than all the bimodal samples.Therefore, we chose the best particle size for the rheology of sample I10, with a fine to coarse ratio of 70:30.

Effect of mixture additive on the apparent viscosity
The effect of a mixture of the two surfactants on the highly concentrated bi-modal IWS has been examined by varying   the surfactant concentration from 0.1 to 1.6% wt.While keeping the fraction of additive constant saponin:SDS (60:40:: w:w).Figure 5 shows the correlation between viscosity and fraction of surfactant mixture for the discrete value of iron ore concentration.The iron ore concentration varies from 75 to 80% (by wt.) while maintaining the constant surfactants ratio Saponin: SDS (60:40:: w:w) at a shear rate of 103 1/s.Introducing additives in the slurry concentrate reduces the apparent suspension viscosity significantly as the additive mixture increases from 0.1 to 1% by wt.Further addition of surfactant increases the viscosity of the suspension.Maximum fluidity (minimum apparent viscosity) is obtained at the 1 wt.% of mixture surfactant.The viscosity of bi-modal IWS may be affected by changes in the bulk phase's viscosity and by changes in the steric/electrostatic interactions between the adsorbed surfactants at the iron ore-water interface.The presence of minima in the viscosity of the saponin:SDS system may be caused by variations in the interactions between the surfactants in the solution, which might affect how the mixture's surface behaves.Due to its hydrophobic nature, the surfactant has the propensity to isolate its nonpolar moiety from an aqueous solution and then adsorbs at the surfaces through effective contact. [78,79][82] The dispersant's ability to bind the iron ore surface is critical to stabilizing the slurry.The hydrocarbon chains of both ionic surfactants in the solution can also interact with it synergistically through its nonpolar moiety.The behavior of nonionic surfactants with high oxygen atom counts in their hydrophilic heads is similar to that of anionic surfactants. [83,84]The minimum viscosity at a constant surfactant ratio at all iron ore concentrations shows that the variance causes the minimum viscosity in surfactant interactions in solution and at iron ore-water interfaces.Sapindus mukorossi saponin and SDS mixture in the ratio 60:40 having 1 wt.% could be a surfactant substitute in the pipeline transportation system.

Maximum iron ore loading
The solid-liquid slurry can be loaded with the most solids by adding 1 wt.% of the additive mixture.Maximum iron ore loading from Sapindus mukorossi is 74% (wt.%). [32]ccording to the literature, 1% weight of addition is employed to maintain the stability of mineral water slurry. [38,39,42]In this work, bi-modal IWS stabilization is accomplished using 1% of a mixture surfactant.In the presence of saponin and SDS (saponin:SDS ¼ 60:40), Figure 6 illustrates the variation in apparent viscosity with the shear rate for various concentrations, that is, 75, 78, 80, and 81%.The apparent viscosity of these iron ore samples at a 306 1/s shear rate is 17.5, 63.06, 89.8, and 109.78mPa.s, respectively.As previously reported, [85,86] the apparent viscosity of the slurry increases as concentration gradually rises.It is due to increased layer interaction caused by increased particle-particle contact in the concentrated slurry.Therefore, perceived viscosity rises.All slurries have apparent viscosities within 1000 mPa.s at 51.8 1/s shear rate in the presence of additives mixes.The threshold for tolerance at these ratios is established by further increasing the iron ore loading since the apparent viscosity is substantially lower.Blended systems are deemed unfavorable as additives in excess of 81 wt.% of iron ore.

Effect of solid concentration on apparent viscosity
Here, the impact of solid concentration on the apparent viscosity of the suspension at a shear rate of 306 1/s is plotted (refer Figure 7).The solid concentration varies from 70 to 81% (by wt.) for the measurement of apparent viscosity.The experimental outcome shows that the apparent viscosity of sample suspension upsurges with solid loading.Slurries with solid loading ranging from 70 to 81% have excellent fluidity with the mixture additive.Whereas without additive bimodel slurry sample up to 78%, fluidity diminishes as the  solid percentage rises from 78 to 81% due to the high viscosity.The apparent viscosity for 80 and 81% bimodal iron ore loading are 957.35 and 1077.6 mPa.S at a shear rate of 51.8 1/s.The slurries exhibit poor flow characteristics as the solid loading is raised beyond 78%.It happens due to the reduction in water volume percentage in the slurry suspension results in increased particle contacts.Also, at high solid concentrations, particle agglomeration occurs due to the intramolecular force of attraction between iron ore particles. [85]

Effect of shear stress and shear rate on bi-modal iron ore suspension
The variation of shear stress with shear rate is used to identify the rheological characteristics of slurry or fluid.The shear stress and shear strain relationship, often known as the rheogram or flow curve, broadly describes the flow properties of a moving slurry.Concentrated mineral suspensions frequently exhibit non-Newtonian flow behavior because they have yield stress that must be overcome before flow occurs.This yield stress value, known as initial shear stress, is a crucial factor in the hydrotransportation of slurry through pipelines.The shear rate must be used to counteract this yield stress when shear is introduced during pumping.The flow behavior of sample slurry I-10 with 1% additive mixture (Saponin: SDS:: 60:40 w/w) is investigated at distinct concentrations, that is, 75, 78, 80, and 81%, with a variation of shear rate 204-498 1/s. Figure 8 illustrates the relation between shear stress and the shear rate of the bimodal IWS sample with optimum surfactant mixture at various solid loading.The shear stress rises nonlinearly with increasing shear rate at all solid loadings, indicating non-Newtonian flow behavior with minor yield stress.The Herschel-Bulkley model may model curves with yield stress followed by power law at high shear rates.Melorie and Kaushal [30] have observed similar trends.Adding a surfactant mixture improves the fluidity of the highly concentrated bi-modal iron ore suspension.Also, the solid loading characteristic improves in the presence of the additive solution.

Effect of surfactant solution on yield stress
Yield stress measures the material structure's strength and is defined as the least stress necessary for the material to flow.Therefore, examining the yield stress of the aqueous solid suspension is important.The yield stress is quantitatively evaluated using the regression technique.The investigation for ore-additive mixture slurry is performed in the presence of a 1% additive mixture (saponin:SDS:: 60:40 w/w).The impact of surfactant mixture solution on the yield stress of bi-modal IWS for 75, 78, 80, and 81% solid concentration is depicted in Supporting information S4.Graph shows yielding of additive mixture slurry is much lower than without surfactant mixture slurry.The yield stress and other different rheological parameters of the I-10 sample at different solid loadings are given in supporting information S6 and S7.It can be observed from the graph addition of additives results in a decrease in yield stress in all performed observations.This reduction in yield stress may be due to the breakdown of aggregation among the iron ore particles. [61]

Analysis of zeta potential
Zeta potential is the electrostatic repulsion or attraction among charge carriers in an emulsion or suspension. [70,71]ere, the electrophoretic mobility of particles is measured and analyzed.Zeta potential magnitude can be used to optimize suspension and emulsion compositions.The surface properties of metal oxides are pH dependent.The surface charge on iron ore particles is measured by maintaining them in an electrolytic cell and quantifying the amount of attracted or repulsive interaction between the ore particles.Figure 9 depicts the impact of pH concentrations on the zeta potential of aqueous iron ore suspension.The zeta potential of the solution without additives is approximately (À 16.261 mV).The  magnitude of the zeta value in the presence of saponin drops from around (þ12.161 mV) to (À 44.661 mV) and with SDSsaponin solution from (þ20.861 mV) to (À 116.461 mV) as pH intensifications from 3 to 11. Maximum iron ore particle aggregation is anticipated at a lower pH level and minimum at a higher pH value.The bulk water molecules are exposed to the adsorbed layer of saponin and SDS molecules.The surfactant ionic heads are hydrated and connected to their counterions.The degree of surfactant aggregate dissociation and the rivalry between sodium and hydroxyl ions for accessible locations at the interface of the surfactant aggregates will determine how pH will affect ionic surfactants.Because of the common ion effect, the surfactant head group is impacted as the concentration of NaOH increases. [36,38]Therefore, the exquisite negative charges acquired on the uninhabited site of iron ore surfaces resulted from polar groups (carboxyl) and (hydroxyl) attached to the iron ore surface, associated with crossed links.The polar groups on saponin molecules are predominantly responsible for decreased zeta potential with a rise in pH value.Increased negative ions emerge on the surfaces of iron ore particles as the pH of the solution rises, leading to more surface hydroxylation and a greater degree of electrostatic repulsion between the ore particles.The decreases in zeta potential confirmed that the stabilization of bi-modal IWS suspension is related to the electrostatic repulsions provided by iron ore particles.

Modeling of the rheological behaviour of bimodal IWS
The dynamic viscosity and yield stress of bi-modal IWS may be determined by modeling the rheological profile using two rheological models.The Herschel Buckley and Bingham plastic models are applied to the experimental rheological data to accomplish the fitting.The flow characteristics of the bimodal IWS suspension at a solid concentration of 75, 78, 80, and 81% by wt. are analyzed using an empirical model fitting.
Each regression model's parameters are determined to represent the experimental curve's trend properly.Figure 10 represents the model fitting to the rheological data of iron ore suspension of sample I-10 in the presence of a 1% additive mixture.The R 2 values for all samples ranged from 0.98187 to 0.92106 for the Bingham model and from 0.99792 to 0.99248 for the Herschel-Bulkley model, respectively.Thus, at all solid concentrations with the additive mixture, the variation of the rheological data follows a power law with yield stress and is most accurately described by the Herschel-Bulkley model.Iron ore slurry with various chemical additives shows a similar trend as observed by Melorie et al. [30] The rheological parameters of these models are listed in supporting information S6 & S7.The values of flow behavior index n for all data sets are less than 1, indicating that the samples show yield shear thinning characteristics.Further, it exhibits an increased degree of shear thinning as the solid loading increase.Additionally, it should be stated that K's value is approximately proportionate to the increased apparent viscosity with iron ore loading.The Herschel Buckley models are a R 2 value that is close to 1, as seen in S6.As a result, it is the most appropriate model that demonstrates the rheological profile of bi-modal IWS suspension at highly concentrated loading and best fit at all measured slurry concentrations.

Stabilization mechanism
An iron ore particle's surface is primarily amphipathic.In the current investigation, iron ore exhibits hydrophilic nature.When the nonionic surfactant saponin is present alone in bi-modal IWS, it binds to the polar portion of the iron ore surface through its polar moiety.This division of the surfactants eliminates the unfavorable energy interactions that contribute to the breakdown of the hydrocarbon chain and lowers the interfacial tension between iron and water.The amount of nonionic surfactant adsorption on the iron surface in the presence of SDS depends on how the nonionic surfactants interact with SDS in bulk solution and at the solid surface (partitioning effect).Compared to nonionic surfactants, SDS, an ionic surfactant (with a charged head group), is more likely to bind to the interface (with uncharged head groups).SDS would, therefore, partition to the interface and either take the place of the nonionic surfactant or create a mixed monolayer there. [38,39]The surface activity of the mixes grows until a solution ratio of 60:40 (w/w) for the saponin-SDS system, according to the measurement of surface tension at several variable ratios of saponin with SDS (Figure 2).Therefore, at these ratios, the interfacial adsorption would be at its highest.Beyond these ratios, SDS/saponin does not partition to the iron ore surface, most likely because the development of micelles or hydrophobic complexes in the solution constrains the number of surfactant monomers.The decreases in zeta potential confirmed that iron ore suspension stabilization is related to the electrostatic repulsions provided by iron ore particles.The intensity of the surface charge on the iron ore influences the apparent viscosity, yield stress, and slurry stability.The strong electrostatic repulsion between the solid colloidal particles prevents aggregation or coagulation and keeps the solid particles well-dispersed.Figure 11 depicts the interaction of saponin-SDS surfactant at the iron ore water interface.The slurry stabilization behavior of SDS-saponin may be related to the distinct sort of head-tail interaction that occurs within the mixture during the formation of the adsorbed layer at the iron ore-water interface.Saponin is composed of two parts: triterpenes or steroids, known as aglycon (hydrophobic), and saccharides, known as glycon (hydrophilic).The structure of the saponin is such that the hydrophobic tail of the glycon (saccharides skeleton) is exposed when it is bonded to the surface of the iron ore.
The SDS hydrocarbon chain can bond to the hydrophobic tail.Due to the similar hydrocarbon chain length of the nonpolar moiety, saponin's triterpene rings and SDS may bond efficiently.Strong hydrophilic binds promote IWS stabilization in SDS-saponin mixture system.

Estimation of head loss in slurry pipeline transportation of iron ore
The head loss and SEC behavior of the iron ore suspension are demonstrated in Figure 12 with and without adding the optimum proportion additive mixture, that is, 1% by wt. in the sample slurry I-10.The HL and SEC are examined by varying the flow velocity from 2 to 5 m/s at a solid loading of 80% by weight.The HL and SEC of the bi-modal IWS rise with the flow velocity of the slurry.Maximum HL and SEC are observed at 80% concentration and a velocity of 5 m/s.Despite the combination of additives, the HL and SEC are seen to rise with flow velocity, but it has a considerably much lower HL and SEC than without additives.The HL and SEC significantly reduce with an additive mixture, which implies that the power required to overcome slurry head loss will be reduced by almost 72% for the proposed pipelines.As a result, iron ore can be conveyed through the pipeline system at lower power consumption and reduced transportation costs.

Conclusions
The current study examined the effect of aqueous-extracted saponin from the ritha mixed with SDS as an additive in highly concentrated bimodal IWS.For this, a bimodal sample I-10, consisting of a 70:30 distribution of fine and coarse particles with sizes of (À 38 mm) and (þ75À 106 mm), is selected after finding an optimized result of the rheological analysis.Consequently, to enhance the slurryability of the suspension, a novel mixture of surfactants (Sapindus mukorossi and SDS) was mixed.Finally, the characteristics of the highly concentrated slurry with surfactant mixture were tested for obtaining enhanced rheological properties.
From the results, it can be concluded that: � The mixture system of surfactants saponin and SDS in the optimum ratio of 60:40 significantly boosted the slurry's overall wettability, as revealed by the reducing tendency of the surface tension from 72.7460.2 to 33.6260.2mN/m.� The stabilizing effect is maximum when the surfactant mixture is 1% by weight concentration, which enhances the dispersing ability and stability of bi-modal IWS.� The optimum surfactant mixture improves the slurryability of bi-modal IWS and enhances iron ore loading up to 81% by weight.� The reduction in zeta potential from (À 16.261 mV) to (À 116.461 mV) proved that bi-modal IWS stabilization is related to electrostatic repulsions offered by iron ore particles.� The Herschel Buckley model is the best fit for describing the flow behavior of bimodal iron ore-water suspension with an additive mixture at high concentrations.� The reduction of head loss and SEC with the application of additive mixture evidence the significance of the surfactants and is a viable choice for hydrotransportion of iron ore at higher concentrations.
Hence, applying the current bimodal distribution with the proposed mixture of surfactants can be used in slurry transportation without changing the existing infrastructure, which can help reduce water and power consumption.
-modal IWS samples with and without additive mixture are investigated experimentally on Modular Compact Rheometer (Anton Paar MCR 92, Austria).The rheometer gives the rheological property of samples at different parameters.The setup consists of a concentric cylinder system with a measuring cup C-CC39/T200/XL (diameter 42.023 mm) and a cylindrical bob B-CC27 (diameter 26.6566 mm and effective length 39.997002 mm).A multifarious slurry sample has been prepared with 100 mL of tap water as a solvent to conduct rheological tests.Rheological measurement tests are performed in the shear rates range of 204-498 1/s, the equivalent order of extent as a slurry pipeline system.Various rheological parameters are monitored for 120 s at a ramp linear input shear rate at a temperature of 30 � C. The measuring cup and tool are cleaned and airdried to minimize the ambiguity in instrument measurement.The experiment is repeated thrice to ensure accurate measurement of the rheological outcomes and consider their average.The rheological properties of the slurries are determined by varying the concentration and distribution of solid particle sizes.Initially, the rheological behavior of bi-modal IWS samples is experimentally examined at a different solid loading range of 75-81% (by wt).These investigations are further expanded for the mixed solution of additives and bimodal IWS samples under similar circumstances.

Figure 2 .
Figure 2. Effect of addition of SDS on the surface tension of aqueous saponin solution.

Figure 3 .
Figure 3.Effect of particle size on the apparent viscosity and shear rate of iron ore slurry at 60% by wt.concentration for samples I-1, I-2, and I-3.

Figure 5 .
Figure 5.Effect of weight fraction of saponin-SDS additive mixture on the apparent viscosity of I-10 iron ore slurry at 103 1/s shear rate.

Figure 6 .
Figure 6.Variation of apparent viscosity with the shear rate in the presence of 1 % additive mixture for sample I-10.

Figure 7 .
Figure 7. Effect of solid loading on the apparent viscosity of slurry I-10 at 306 1/s shear rate.

Figure 9 .
Figure 9.Effect of surfactant mixture fraction on zeta potential of iron ore slurry.

Figure 10 .
Figure 10.Rheological model fitting (a) Herschel-Bulkley (b) Bingham plastic in the presence of 1% additive mixture for sample slurry I-10.

Figure 11 .
Figure 11.Structure of Saponin-SDS at the iron ore water interface.

Figure 12 .
Figure 12.(a) Head loss (b) SEC characteristics of iron ore slurry with and without additive mixture.