Study on strengthening of sulfide tailings high-efficiency solid-liquid separation using water reducing admixtures

The global focus on refined mineral processing and environmental protection has led to stricter requirements for tailings disposal processes. The complex composition and chemical environment of tailings pose challenges in sedimentation and pipeline transportation. This research developed a new additive that is suitable for disposing of copper-lead-zinc sulfide tailings. This additive not only improves sedimentation efficiency and concentration, but also enhances the flowability of high-concentration slurry. This study investigated the effects of four typical water reducing admixtures on the settling and rheological properties of tailings. However, the combination of lignin/PCE and NPAM had a synergistic effect, further improving settling and rheological performance. The addition of PCE increased settling rate by over 2 times, increased sediment concentration by 6.71%, and significantly reduced slurry viscosity and yield stress. The study also used FBRM to analyze the changes in particle/floc size distribution over time. The results showed that the addition of PCE captured more fine particles during flocculation and formed larger flocs. These findings can be used as a reference for the use and optimization of additives in tailings disposal engineering.


Introduction
Copper, lead, and zinc ore are a very important and highly consumed commodity.The processing of copper-lead-zinc ore typically involves wet processing methods to separate them from tailings, resulting in a large amount of sulfide tailings.Based on statistical data [1], China's annual production of tailings has surpassed 1.2 billion tons, exhibiting a consistent upward trend, as shown in Figure 1.In 2020, the production of iron tailings and copper tailings constituted 42 % and 26 % of the total tailings production in China, respectively.The disposal of tailings poses significant challenges due to the complex mineral composition and solution chemistry [2].Firstly, the fine particle size and high clay content of tailings can affect the efficiency of settling.Secondly, particle interactions, chemical additives, and rheological behavior in tailings slurry can also pose significant obstacles to efficient pumping and pipeline transportation.Besides, the addition of flocculants can lead to the free water be wrapped into flocs and increase the volume of sediment, occupying more tailings storage capacity.The rheological and chemical/interface properties of tailings slurry are crucial parameters for thickened tailings transportation.The rheological properties of the slurry are influenced by various factors, including (but not limited to) slurry solids content, particle shape and size distribution, particle surface properties, additives, pH, and ionic strength [6,7].A proper particle size distribution can allow more fine particles to fill the void spaces between large particles, achieving the minimum viscosity at high solid concentrations [8,9].The surface properties of particles are closely related to the rheological behavior of the slurry, such as the anisotropic structure in clay minerals is accompanied by an increase in viscosity and yield stress [10,11].Ionic strength, pH, and additives are the main methods for controlling the rheological behavior of mineral slurry, applied in engineering and industrial processes such as cement and concrete manufacturing, ceramic production, mineral flotation, tailings slurry, and coal-water slurry transportation.The essence of these methods is to control the rheological behavior of the slurry by adjusting the interparticle interaction forces.Ionic strength and pH can control the double-layer electrostatic repulsion, while suitable surfactants and high molecular weight adsorbing polymers/electrolytes can manipulate hydrophobic forces, spatial hindrance, and bridging forces [7,12].
Water reducing admixtures (WRAs), also known as plasticizers and high-efficiency water reducers, are commonly used additives in the production of high-strength concrete.Their purpose is to reduce the required water content and void volume in the concrete, while maintaining its compressive strength [13].At the same time, they increase the fluidity of the concrete, making it easier for construction and pouring [14].Improving the compactness and fluidity of the slurry is also an urgent requirement for tailings slurry pipeline transportation.Therefore, the compatibility of tailings with water reducing admixtures is also one of the focuses of this study.Lignosulfonates, naphthalene sulfonates, and polycarboxylate-based admixtures are well-known reagents that can reduce water content by 5-30% while considering slump and rheology [15,16].
This study aims to investigate the effects of flocculants and WRAs on the settling and rheological behavior of copper-lead-zinc tailings, and to find a combination of additives suitable for tailings disposal, to improve the settling efficiency and sediment concentration of tailings and improve the rheological performance of the slurry.The present study investigated the impact of flocculants and water reducing admixtures (WRAs) on the settling performance of tailings through the measurement of settling rate and sediment concentration of the slurry.Additionally, the rheological properties of the sediment were examined by measuring the shear stress of tailings sediment at various shear rates, in order to assess the effects of flocculants and WRAs.Furthermore, the growth behavior of flocs in tailings was analyzed by monitoring the chord length distribution of particles/flocs over time using FBRM technology, with a specific focus on the influence of WRAs.

Tailings and reagents
The tailings samples used in this study were obtained from a copper-lead-zinc sulfide mine located in Jiangxi, China.The main mineral phases in the tailings are quartz, illite, mica, chlorite, and feldspar, with small amounts of pyrite and calcite.The particle size distribution of the tailings was tested using a laser diffraction particle size analyzer (Malvern Instruments Ltd., Worcestershire, UK), with a d 50 of 18.75 μm.The d 10 and d 90 were 3.92 μm and 69.96 μm, respectively.The tailings disposal reagents used in this study include flocculants and water reducing admixtures (WRAs).The flocculants are divided into cationic polyacrylamide (CPAM), anionic polyacrylamide (APAM), and non-ionic polyacrylamide (NPAM), with molecular weights around 10-11 million.Four different WRAs were used in the study, including sulphonated melamine formaldehyde (SMF), polynapthalene sulfonate/sulphonated naphthalene formaldehyde (SNF), lignin, and polycarboxylic ether (PCE).The molecular structure of water reducing admixtures are shown in Table 1.

Sedimentation experiments
The sample prepared for the settling test was a copper-lead-zinc tailings slurry with a solid concentration of 25 wt%.First, 100 mL of the slurry was dispersed by vigorous stirring.The required amount of water reducing agent and flocculant was added at 200 rpm, with a dosing interval of 2 min.Then, the slurry was transferred to a 100 mL graduated cylinder and inverted 10 times.After completion, the mudline height was recorded at regular time intervals.The settling velocity can be determined by the slope of the mudline height versus settling time.The sediment concentration can be calculated based on the mudline height after 30 min of settling.

Rheological measurements
The rheological properties of the copper-lead-zinc tailings sediment were determined using a rheometer (Anton Paar MCR 102, Austria).The sample used for the rheological test was the sediment obtained after 1 h of settling.40 mL of the sediment from the bottom was transferred to the rheometer cup for measurement.The testing procedure consists of three stages as follows: (1) Pre-shearing stage.To homogenize the sample, the slurry was stirred at 200 s -1 for 60 s, followed by a 10 s rest.
(2) Up shear rate stage.Rheological data was measured starting from a shear rate of 1 s -1 and linearly increasing to 500 s -1 .
(3) Down shear rate stage.The shear rate was linearly decreased from 500 s -1 to 1 s -1 , and the test was ended.
The calculation of yield stress was obtained based on the Herschel-Bulkley model [17,18], with the following equation: where  is the shear stress (Pa),  0 is the yield stress (Pa),  is the consistency parameter (Pa‧s n ), ̇ is the shear rate (s -1 ), and  is the flow behavior index.
The shear stress forms a hysteresis loop with up shear rate and down shear rate.The area of the thixotropic loop, also known as the hysteresis area, represents the change in rheological properties of the fluid [19].The hysteresis area can be expressed by the following equation [20,21]: where  is the area of thixotropic loop (Pa/s),  1 is the up apparent viscosity (Pa‧s),  2 is the down apparent viscosity (Pa‧s), and ̇ is the shear rate (s -1 ).

FBRM measurements
FBRM were conducted using a focused beam reflectance measurement probe (G400 with PI-14/206 probe, Mettler-Toledo LLC, USA) to monitor in real-time the changes in particle chord length and counts in the copper-lead-zinc tailings slurry after the addition of admixtures.The changes in chord length distribution obtained reflect the growth and evolution of flocs [22].The sample prepared for FBRM was a 5 wt% tailings slurry.Firstly, 250 mL of the slurry was predispersed by vigorous stirring for 5 min.The shear rate was set at 200 rpm, and FBRM started collecting data at a scanning rate of 2 s.During the test, the required amount of water reducing admixtures and flocculant were added at relative times of 3 min and 4 min, respectively.The test ended at a relative time of 15 min.

Settling performance of tailings
3.1.1.Effects of ionic flocculants on settleability of tailings.Due to the presence of charges on the surface of tailings particles and also ions in the slurry solution, the addition of different ionic flocculants can change the interaction between particles and alter the stability of the slurry.Therefore, the settling behavior of copper-lead-zinc tailings with different ionic flocculants was investigated.The molecular weight of the flocculants was around 10-11 million, and the dosage was fixed at 60 g/t.
Figure 2 shows the effect of different ionic flocculants on the settling rate and sediment concentration of tailings.The mudline height vs. settling time curve indicated that the addition of CPAM had the slowest settling rate on tailings slurry, while adding NPAM had the most significant settling rate.During the mudline height decreased from 100% to 60%, it tended to decrease linearly with settling time for all three flocculants.Below 60%, it entered a slow settling stage.As shown in Figure 2(b), the sediment concentration after 30 min of settling was calculated, and the sediment concentrations of copper-lead-zinc tailings for adding CPAM, APAM, and NPAM were 43.34 %, 44.59 %, and 46.11 %, respectively.In the slurry, at the same solid mass, the higher the sediment concentration, the smaller the sediment volume.Therefore, the sediment formed after flocculation with NPAM has the smallest sediment volume.Thus, NPAM in the flocculant significantly improved the settling performance of copper tailings slurry.

Effects of WRAs on settleability of tailings.
In order to further improve the settling rate and sediment concentration of copper tailings, water reducing admixtures were used as surfactants in tailings disposal.The wettability or/and electrical property of particle surfaces could be regulated through the functional groups of WRAs.Therefore, the effects of different types of WRAs on the settling performance of tailings were investigated.
As shown in Figure 3(a), compared to the reference curve with only NPAM added, the addition of SMF and SNF hindered the flocculation and settling of tailings, while lignin and PCE further improved the settling rate, with PCE showing the highest settling rate.Although lignin improved the settling rate of tailings, it did not further concentrate the sediment concentration, with sediment concentration similar to that with only NPAM added, at 46.51% and 46.15% respectively.However, the synergistic effect of PCE and NPAM increased the sediment concentration to 50.68%, which was 4.53% higher than that with only NPAM added.Therefore, PCE has a dual effect of improving the settling rate and sediment concentration of tailings and is an ideal agent for solid-liquid separation of tailings among the water reducing admixtures.

Effects of WRAs dosages on settleability of tailings.
To investigate the effect of dosage of WRAs on the settling rate of tailings, settling experiments were conducted in 25 wt% tailings slurry with different PCE dosages (0, 150, 300, 450 g/t) and a fixed NPAM dosage of 60 g/t. Figure 4(a) shows that the settling rate of copper-lead-zinc tailings slurry significantly increased with the PCE dosage.When the mudline height decreased to 60%, the settling times for only adding NPAM, and for adding PCE 450 g/t and NPAM were 10.5 min and 4.7 min, respectively, indicating a more than 2 times increase in settling rate.Figure 4(b) shows the sediment concentration after 30 min of slurry settling for different PCE dosages.As the PCE dosage increased from 0 to 450 g/t, the sediment concentration increased from 45.92% to 52.63%, representing a 6.71% increase.These results indicate that increasing the dosage of PCE was beneficial for improving sediment concentration and reducing sediment volume.Therefore, both the settling rate and sediment concentration of tailings significantly increased with the increase in PCE dosage.

Rheological performance of tailings
To investigate the differences in rheological behavior of the slurry under different reagent conditions during pipeline transportation, rheological performance tests were conducted using a rheometer, as shown in Figure 5.As the shear rate increased linearly from 1 s -1 to 500 s -1 , the shear stress of the slurry was recorded, and an ascending curve was formed.Subsequently, the test results formed a descending curve as the shear rate decreased linearly from 500 s -1 to 1 s -1 .Finally, these two curves formed a thixotropic loop, and the area between the curves was taken as the hysteresis area.Based on the functional relationship between shear rate and shear stress, it can be observed that the samples were all non-Newtonian fluids.According to the Herschel-Bulkley model, the shear stress at shear rate approaching 0 approximates the yield stress, which is the critical stress value for the fluid to start flowing.The results showed that the fluid with only NPAM added had the highest yield stress value, which was 38.21 Pa.The yield stress values for the samples with no additive and with lignin and NPAM added were close, at 13.54 Pa and 11.76 Pa, respectively.The slurry with PCE and NPAM added had the lowest yield stress value, which was 3.15 Pa.Therefore, the addition of PCE facilitated the initiation of slurry flow.the thixotropy of the slurry and cause changes in slurry properties during pipeline transportation due to shear.The addition of lignin and PCE both reduced the high thixotropy caused by flocculants.The lowest hysteresis area (887.2Pa/s) was achieved when 450 g/t PCE and 60 g/t NPAM are added, which was approximately 6.5 times lower than the hysteresis area (5782.5 Pa/s) when only NPAM was added.Thereby, the addition of PCE enhanced the stability of the slurry during pipeline transportation.Figure 6(b) shows the variation of apparent viscosity of tailings slurry with shear rate under different reagents addition.The results indicated that the addition of flocculants significantly increased the apparent viscosity of the slurry and reduced its flowability.However, the addition of WRAs such as lignin and PCE can reduce the apparent viscosity of the slurry, even lower than that of the original slurry.The synergistic effect of PCE and NPAM reduced the apparent viscosity of the slurry to the lowest level, making the slurry highly flowability, which is beneficial for improving the efficiency of slurry transportation and reducing transportation costs.

Floc formation of tailings
The growth behavior of flocs in copper-lead-zinc tailings particles with various dosages of PCE was investigated by monitoring the changes in particle/floc size (chord length) and count in real-time using FBRM.Firstly, 250 mL of 5 wt% tailings slurry was pre-dispersed by vigorous stirring at a shear rate of 500 rpm for 5 min.Then, the shear rate was reduced to 200 rpm and FBRM data recording was started.Various dosages of PCE and a fixed amount of NPAM were added at relative times of 3 and 4 minutes, respectively.And the shear rate was maintained at 200 rpm until the end of the test at relative time 15 min.Figure 7 shows the chord length distribution of tailings particles/flocs at different time points with various dosages of PCE.
By analyzing the changes in chord length distribution at different times, the growth behavior of flocs formed by particles in the slurry was studied.The chord length distribution of tailings particles/flocs at different time points with different dosages of PCE showed some similarities.Within the relative time of 0 -5 min, the count of particles in the range of 10 -50 μm decreased, and > 50 μm flocs began to form gradually.Within the relative time of 5 -10 min, the count of < 10 μm particles decreased significantly, while the count of flocs > 50 μm flocs increased, and the flocs were mainly concentrated in the range of 100 -110 μm.Within the relative time of 10 -15 min, the count of fine particles (< 10 μm) decreased, while the count of > 50 μm flocs increased significantly and the distribution peak shifted to the right.Therefore, the growth behavior of flocs may involve the bridging of relatively larger particles (10 -50 μm) followed by the gradual capture of fine particles (< 10 μm).Comparison of the differences in particle/floc chord length distribution at different PCE dosages at the same relative time, the effect of PCE dosage on floc growth behavior was studied.At relative time of 0 min, the chord length distribution of particles at different PCE dosages was basically consistent, indicating that the samples were well dispersed and had similar particle size and number.At relative time of 5 min, with the condition of only adding NPAM, the count of > 50 μm flocs was the highest and decreased with increasing PCE dosage.However, as the PCE dosage increases, the peak of chord length distribution for > 50 μm flocs shifted to the right, indicating the particles formed to larger flocs.At relative time of 10 min, with the increase of PCE dosage, < 10 μm particles showed a decreasing trend, while the number and size of > 10 μm particles increased.At relative time of 15 min, there were significant differences in the chord length distribution results.With the increase of PCE dosage, the number and size of fine particles (< 10 μm) decreased significantly, indicating that more fine particles were captured by the flocs.At the same time, the number and size of > 50 μm flocs increased significantly.Therefore, the PCE additive has the ability to capture more fine particles in tailings and can efficiently flocculate, forming more and larger flocs.In terms of tailings settling, due to the effect of PCE, more fine particles can be captured and formed larger and denser flocs during flocculation, which was beneficial for the rapid settling of tailings and the reduction of sediment volume.In terms of tailings slurry pipeline transportation, the bridging effect of flocculants increased the interaction between particles, resulting in increased internal friction of the slurry during flow.The PCE additive could reduce the number of fine particles in the slurry, while forming larger flocs to reduce the specific surface area, thereby reducing the internal friction of the slurry, and improving its flowability.

The potential application of WRAs on tailings disposal process
The solid-liquid separation of tailings not only needs to achieve rapid settling, but also needs to consider the subsequent process.In the industrial processes of tailings slurry pumping and pipeline transportation, there has always been an unresolved contradiction.Efficient tailings transportation requires high-concentration slurry as a prerequisite.However, increasing the slurry concentration leads to decreased slurry flowability, increased additional energy consumption costs, and even the possibility of pipeline blockage.On the other hand, transporting low-concentration slurry not only has low efficiency but also increases the cost of wastewater treatment.Therefore, to achieve efficient pipeline transportation of high-concentration slurry, external forces need to be added.In this study, two WRAs (PCE and lignin) were introduced into tailings disposal, which adsorbed onto the surface of mineral particles and changed the interaction forces between particles.Firstly, this method promoted the capture of more fine particles by flocs, reducing the void volume in flocs, and forming high-concentration sediment.Secondly, this method also promoted the improvement of slurry flowability, solving the problem of balancing high-concentration slurry and easy flow, and achieving efficient pipeline transportation of slurry.As shown in Figure 8, the potential application of WRAs on tailings disposal process is depicted in the schematic.

Conclusion
Efficient solid-liquid separation of tailings not only requires rapid settling and higher sediment concentration, but also needs to maintain well flowability to save energy consumption costs for pipeline transportation of tailings slurry.In this study, the effects of various flocculants and WRAs on the settling performance and rheological properties of copper-lead-zinc tailings was investigated, and the suitable WRAs for tailings disposal was determined.The results indicated that SMF and SNF were not conducive to the settling and concentration of tailings.However, both lignin and PCE, as water reducing admixtures, contributed to the increase in settling rate and sediment concentration.The addition of PCE had the highest settling efficiency, increasing the settling rate by more than 2 times and the sediment concentration by 6.71 %.In terms of rheological properties, PCE was more effective in regulating the flowability of tailings slurry.After adding 450 g/t PCE, the thixotropic loop area decreased by 6.5 times and the slurry viscosity and yield stress also decreased significantly.The effect of PCE on floc growth behavior in the slurry was studied through FBRM test.The results showed that the assistance of PCE enabled more fine particles to be captured during the flocculation process and formed more and larger flocs.This study confirmed that PCE was a tailings disposal reagent beneficial to settling and pipeline transportation processes, providing potential application prospects for efficient solid-liquid separation of tailings.

Figure 1 .
Figure 1.China's tailings production volume and the proportion of different types of tailings production.

Figure 6 (
Figure 6(a) shows the hysteresis area of tailings slurry under different reagent addition.Flocculants are commonly used additives for solid-liquid separation of fine tailings, which significantly increased

Figure 8 .
Figure 8.The schematic of the potential application of WRAs on tailings disposal process.

Table 1 .
Molecular structure of water reducing admixtures