Barite pre-enrichment experiments before and after breaking gum-washingHigh-density waste drilling fluid is a sol-gel suspension mixed system composed of oil and water as the dispersing medium, clay, viscosity builder, filter loss reduction agent, a variety of inorganic salts and aggravating agent as the dispersing phase, and must be subject to a certain amount of gel-breaking treatment before flotation19. Therefore, in this study, a clean and strong oxidizing gum breaker YJ-I was added to firstly break the gums of high-density waste drilling fluids and subsequently enrich the barite by high-speed washing. The effect of the secondary enrichment of breaking gum-washing on the mixed solid-phase density of recovered barite is shown in (Table 2). The results show that, under the experimental conditions of a rotational speed of 1000r/min, a gum breaker amount of 5‰, and a solid–liquid ratio of 1:2, primary enrichment enhances the mixed solid-phase density of barite by approximately 11% points. Furthermore, the mixed solid-phase density of barite before and after secondary enrichment increases by about 13.8% points, highlighting the significant purification effect of secondary enrichment.Table 2 The effect of breaking gum-washing on barite recovery from high-density waste drilling fluids.To further validate the efficacy of secondary enrichment on barite purification, the morphological attributes of the recovered solid-phase barite were examined both before and after secondary enrichment, utilizing the aforementioned conditions. Figure 3 illustrates the morphological changes observed in the solid-phase barite samples. Initially, the dry sample of the original waste drilling fluid exhibited a black hue with coarse particles, predominantly forming agglomerates (Fig. 3a)20. Following primary water washing, there was a noticeable enhancement in both the color and particle size of the mixed solid phase of barite, rendering it light black in color (Fig. 3b). Subsequent secondary water washing further refined the particle size, resulting in the mixed solid phase of barite exhibiting a light black hue with a hint of pale white coloration, albeit with evident small agglomerates on the surface (Fig. 3c). The alteration in color and particle size of the mixed solid phase of barite can be attributed to the high-speed water washing process, which interacts with the low-speed water. The enhancement in both color and particle size observed in the recovered barite mixed solid phase could potentially stem from the substantial density disparity between barite and the low-density solid phase during the high-speed water washing process. Barite was mainly distributed in large quantities in the outer layer near the wall, whereas the low-density solid phase is primarily concentrated in the inner layer. Upon cessation of spiral cleaner rotation, the low-density solid phase, owing to differing settling velocities, becomes entrapped between the uppermost water layer and the lowermost barite layer, thereby facilitating effective separation. As the proportion of low-density solid phase diminishes within the barite layer, the color gradually transitions to white. With successive washings, the progressive reduction in low-density solid phase content culminates in continuous improvement in barite purity, resulting in increasingly white chromaticity and finer particle sizes21.Fig. 3Comparison of recovered barite before and after broken rubber washing; (a) Dry sample of raw waste drilling fluid; (b) barite after primary enrichment; (c) barite after secondary enrichment.The results suggest that secondary enrichment is indeed more efficient in purifying barite. However, as an independent physical method, it faces challenges in achieving optimal barite separation efficiently. With an increase in the number of enrichment cycles, despite resulting in a slight improvement in barite purity, there ensued notable wastage of resources and human labor, alongside a substantial deterioration in the actual quality of the recovered barite. Thereby augmenting the method’s uncertainty22. Consequently, further investigation into subsequent chemical processes is imperative.Barite flotation experiment in drilling waste fluid before and after microwave-ultrasonic pretreatmentEffect of concentration of SDS and pH value on barite flotation recovery and barite qualityUnder the conditions of ultrasonic-microwave pretreatment and the addition of 0.07% SDS, the flotation performance was investigated across various collector concentrations and pH values. The results are shown in (Fig. 4a,b).Fig. 4(a) Effect of various SDS concentrations on the flotation recovery of barite; (b) Effect of various pH values on the flotation recovery of barite; (c) Effect of various microwave time on the flotation recovery of barite; (d) Effect of various microwave power on the flotation recovery of barite; (e) Effect of various ultrasonic time on the flotation recovery of barite.Figure 4a illustrates the impact of SDS concentration on the flotation performance of barite. Observation of Fig. 4a reveals a gradual increase in both barite recovery and mixed solid phase density with rising SDS concentration. The peak values for both barite flotation recovery and mixed solid phase density occurred at an SDS concentration of 0.05%, reaching 80.4% and 4.232 g/cm3, respectively. A slight decline in barite recovery was observed as SDS concentration continued to increase, which may be due to the non-selective trapping of SDS, and too much SDS will improve the wettability of barite and low-density solid phase at the same time, making them both more hydrophobic, so that part of barite was flown to the surface with the low-density solid phase, which resulted in the decrease of barite recovery and density23.Figure 4b illustrates the impact of pH variations on the flotation performance of barite. As evident from Fig. 4b, both barite recovery and the density of the barite mixed solid phase escalated with rising pH levels. At a pH value of 8.0, both barite flotation recovery and the density of the barite mixed solid phase peaked at 80.4% and 4.232 g/cm3, respectively. With further increases in pH, there was a slight decline observed in both barite recovery and the density of the barite mixed solid phase. This phenomenon may be attributed to the increased presence of OH- ions in the slurry at higher pH levels, leading to enhanced adsorption of OH- ions on the surface of the low-density solid phase. Consequently, this enhances the hydrophilicity of the surface of the low-density solid phase, thereby hindering the adsorption of trapping anions24.Effect of time and microwave power on barite flotation recovery and barite qualityMicrowaves are a form of electromagnetic radiation characterized by frequencies ranging from 300 MHz to 300 GHz25,26. Due to their properties of penetration, selective heating, low thermal inertia and absorption, microwaves find widespread applications in substance heating, microwave extraction, microwave grinding and microwave floatability. Yu investigated the impact of microwaves on the interaction between deionized water, slurry and the trapping mechanism in fluorite flotation. The study showed that microwaves can enhance mineral flotation recovery through the trapping mechanism. Consequently, the microwave pretreatment process was incorporated to explore the potential enhancement of barite recovery and density. The results are shown in (Fig. 4c-d).The results showed that after the microwave pretreatment, the flotation indexes of barite were all increased to a certain extent, and the recovery rate of barite was higher than 78% under the microwave time of 20–60 s and the microwave power of 200–800 W, and the density of recovered barite was higher than 4.200 g/cm3, which indicated that the microwave treatment could have a certain promotional effect on the flotation27. In the range of microwave time from 20 to 60 s, the flotation indexes increased rapidly when the microwave time was increased from 20 to 30 s. However, when the microwave time continued to be increased after 30 s, the barite recovery indexes showed a small fluctuation, and thereafter no further change. With the increase of microwave power, the barite recovery index increased rapidly at 200–400 W, but showed a small decrease as the microwave power continued to increase. This trend can be attributed to the coexistence of barite and low-density solid phase in high-density waste drilling fluid. Increasing the microwave time and intensity can increase the specific surface area of barite to activate flotation and partially desorb these two phases to improve flotation. However, it cannot be completely dissociated, and too long microwave time and too high microwave intensity will lead to partially dissociated tiny barite and low-density solid-phase mixtures can be easily brought to the liquid surface by bubbles for simultaneous flotation due to their small mass and volume, resulting in low flotation efficiency. Therefore, reasonable adjustment of microwave treatment time and intensity to control the particle size of the treated mixed solid phase is the key to promote flotation. The peak recovery of barite (recovery = 80.4%, density = 4.232 g/cm3) was achieved at a microwave time of 30 s and a microwave power of 400 W, respectively.Effect of microwave-ultrasonic synergistic pretreatment on barite flotation recovery and barite qualityUltrasound is a technique that employs acoustic energy to uniformly disperse particles in a liquid. Typically utilizing ultrasonic frequencies (greater than 20 kHz), it can be conducted using either an ultrasonic bath or an ultrasonic probe. Ultrasonic waves induce the formation of vacuum bubbles (voids) within the liquid, which subsequently expand and implode with considerable force, a phenomenon known as cavitation. This process yields various effects including emulsification, dispersion, particle size reduction, and homogenization28. This section delves into investigating the impact of ultrasonic treatment duration on barite flotation, conducted under the optimal conditions delineated in Sect. “Effect of time and microwave power on barite flotation recovery and barite quality”, with the results presented in (Fig. 4e).The results indicate a substantial increase in both barite recovery and the density of the mixed solid phase of barite with prolonged ultrasonic treatment time, suggesting the promotional impact of ultrasonic pretreatment on flotation. Upon increasing the ultrasonic treatment time from 0 to 5 min, the optimal flotation performance was attained (recovery = 81.5%, density = 4.238 g/cm3), with minimal influence observed on the flotation performance with further increases in ultrasound duration.A detailed study of the images depicting barite recovery after 0, 5, and 10 min of ultrasonication reveals that the barite sample obtained post-ultrasonication exhibits a powdery consistency, with scant evidence of prominent lumps or solid chunks on the surface layer. This observation suggests that ultrasonication facilitates a more thorough separation of the mixed solid phase of barite. Furthermore, upon comparing the barite recovered after 5 and 10 min of ultrasound treatment, minimal disparity in the quality of the recovered barite between the two durations is observed.The above trend can be attributed to the increase in the adsorption capacity of barite by ultrasonic pretreatment, which increases the adsorption rate of barite to flotation chemicals. Furthermore, given that the mixed solid phase of barite post-microwave treatment assumes an irregular lumpy form characterized by heightened micropores and cracks, the cavitation bubbles generated during ultrasound can infiltrate these irregular lumps, inducing their fragmentation and disintegration29. This phenomenon facilitates more effective removal of the low-density solid phase in barite and enhances the efficacy of flotation chemicals. The separation process gradually reaches completion with increasing ultrasonic treatment duration. As pretreatment entails a dual process resulting in the refinement of the barite mixture, reducing the duration of ultrasonic treatment achieves both removal effects. Subsequent prolongation of ultrasonic treatment time yields marginal improvements in separation efficiency. As depicted in (Fig. 5e), the optimal recovery (recovery = 81.5%, density = 4.238 g/cm3) was attained with an ultrasonic treatment duration of 5 min.Fig. 5(a–f) SEM images of recovered barite mixed concentrate under different treatment conditions; secondary enrichment (a-5 and d-20 μm), secondary enrichment + microwave (b-5 and e-20 μm), secondary enrichment + microwave + ultrasonic (c-5 and f-20 μm); (g) Adsorption capacities of SDS under different concentrations on the barite surfaces; (h) Zeta potentials of barite as a function of various pH and changes in contact angle of barite before and after the action of GG and SDS.Analysis of flotation mechanism before and after ultrasonic-microwave pretreatmentComparison of SEM images of barite surface before and after different pretreatmentsThe SEM images of the barite surface after various pretreatments are shown in (Fig. 5a–f). Following washing, a higher presence of irregular particles adheres to the barite surface (Fig. 5a). The overall morphology exhibits a relatively rough and lumpy distribution (Fig. 5d), with noticeable rises and pits. Nevertheless, the overall purity is increased, suggesting that water washing enables a comparatively pure separation between barite and the low-density solid phase, albeit with limited separation effectiveness30. During the removal process, the barite and the low-density solid phase collide with each other at high speeds, resulting in deep and shallow pits and cracks in the recovered barite mixed solid phase. Hence, a single high-speed water washing pretreatment alone remains insufficient to achieve complete separation, necessitating the implementation of additional auxiliary processes.After microwave treatment, irregular cracks appeared on the surface of barite (Fig. 5b), and some projections were partially dislodged to form small spheres (Fig. 5e) with a slight decrease in diameter. This phenomenon is due to the barite mixed solid phase in the microwave electromagnetic field, due to its own magnetic loss and dielectric loss, the absorption of electromagnetic energy into internal energy, inducing the internal thermal effect of its own particles, so that the particles are rapidly subjected to thermal expansion, and the release of its thermal stresses promotes the growth and expansion of the cracks on the surface of the barite mixed solid phase as well as the interfacial friction and fragmentation between its different structures, which promotes the desorption of the low-density solid phase31.The density and smoothness of the barite surface increased after ultrasonic treatment (Fig. 5c), while the diameter decreased. This is because the ultrasonic pretreatment on the one hand can destroy the adsorption of low-density solid phase on the surface of barite, on the other hand, can cause fatigue damage to the low-density solid phase and be stripped, the vibration of the gas-type bubbles on the surface of the barite scrubbing, once the low-density solid-phase attached to the surface of the seam can be drilled, the bubbles immediately drilling vibration to make the low-density solid-phase detachment32. Ultrasound propagation in the cleaning fluid will produce positive and negative alternating acoustic pressure, forming a jets, impacting the cleaning parts, while due to the nonlinear effect will produce acoustic and micro-acoustic flow, and ultrasonic cavitation at the interface of solids and liquids will produce high-speed micro-jets, all of these roles, able to destroy the low-density solid phase, removing or weakening the boundary layer, increasing the agitation, diffusion, resulting in the microwave treatment process of the adhering residual solid phase shedding, which improves the purity (Fig. 5f)33.Determination of the adsorption of trapping agent on barite surfaceInvestigating the adsorption capacity of SDS on the barite surface can provide insights into the mechanisms underlying traps and inhibitors affecting both barite and the low-density solid phase. The results are shown in (Fig. 5g).The observed amount of SDS adsorbed on the barite surface amounted to 1.546 mg/g upon addition of SDS without the inhibitor GG. Furthermore, saturation of adsorption on the barite surface was attained as the SDS concentration escalated from 0.03 to 0.05%, with subsequent increments in SDS concentration yielding minimal alterations in adsorption levels34,35. Owing to the fragile Ba–O bonds within the barite structure, facile breakage occurs, facilitating the chemical adsorption of Ba2+ ions and dodecyl sulfate on the barite surface. This process leads to the formation of relatively stable barium dodecyl sulphate compounds on the barite surface with alkyl orientated towards the surrounding medium. Consequently, the hydrophobicity of the barite surface is enhanced, promoting its flotation upwards36. Analogously, metal ions present on the surface of the low-density solid phase can form stable compounds with dodecyl sulfate, resulting in the inefficient separation of barite from the low-density solid phase. Thus, the introduction of a selective inhibitor is imperative to shield the barite. The environmentally friendly selective inhibitor GG harbors a plethora of reactive functional groups, including carboxyl and hydroxyl groups37. Given that these reactive functional groups readily interact with multivalent metal sites on the mineral surface. Upon addition of GG, the ionized Ba2+ ions on the barite surface exhibit a pronounced affinity for binding with the active functional groups (–COO and –OH), resulting in robust chemisorption of GG onto barite. This phenomenon effectively impedes the subsequent adsorption of SDS on the barite surface, leading to a notable decrease in the floatability of barite and facilitating the floatability-based separation of barite from the low-density solid phase. Thus, this further corroborates the selective inhibition of barite by GG, underscoring its indispensable role in this flotation experiment.Zeta potential analysis before and after microwave-ultrasound pretreatmentZeta potential changes in minerals during the flotation process are often associated with reagent adsorption38. Therefore, zeta potential measurements were performed to elucidate the adsorption behaviors of reagents on barite surfaces. Figure 5h illustrates the alterations in surface charge experienced by barite following interaction with various reagents. As depicted, the zeta potential of barite exhibits a negative charge. Following treatment with GG, the zeta potential of barite consistently shifted towards negativity across the pH range examined, suggesting adsorption of GG onto the barite surface39. At pH = 8.0, the zeta potential of barite exhibited a negative shift of 20.0 mV, indicative of robust adsorption of GG onto the barite surface. Conversely, at pH = 7.0 or pH > 8.0, the change in zeta potential of barite ranged from 12.3 to 18.7 mV, suggesting that the adsorption of GG onto barite is most pronounced at pH = 8. The zeta potential of barite exhibited minimal change following the subsequent addition of SDS, with a shift of < 3.5 mV from pH 7.0 to 11.0, indicative of the pronounced hindrance imposed by prior GG adsorption on the subsequent SDS adsorption onto the barite surface40. Consequently, the zeta potential findings suggest that GG significantly obstructs the adsorption of SDS onto the barite surface.Surface wettability of barite under different agent treatments before and after microwave reactionThe surface wettability of minerals plays a crucial role in their interaction with air bubbles, thereby directly influencing flotation performance41,42. Typically, surface wettability is assessed through the measurement of mineral contact angles, with higher contact angles generally indicative of greater floatability. In this study, the surface wettability of barite was assessed under varying agent conditions both before and after microwave treatment, with the results delineated in (Fig. 5).The results revealed that the contact angles of barite prior to and following microwave treatment were 86.7 and 71.3°, respectively. Following GG treatment, there was a notable decrease in the contact angle of barite, decreasing from 86.7° and 71.3 to 71.2° and 65.2° before and after microwave treatment, respectively. This observation suggests effective adsorption of GG molecules onto the barite surface, resulting in a discernible reduction in barite floatability43. Subsequently, SDS was introduced, and the contact angle of barite remained nearly unchanged before and after microwave treatment. This result suggests that the presence of GG significantly influences the subsequent attachment of SDS onto the barite surface, likely attributed to the enhanced chemical adhesion of GG onto the barite surface, potentially in the form of Ba-OH or Ba-COOH44. This phenomenon could explain the reduced floatability of barite with a high recovery rate. Consequently, GG demonstrated enhanced adhesion to barite, leading to a substantial hindrance in the adsorption of SDS onto the barite surface.Analysis of the mechanism of promoting barite flotation under water-washing microwave and ultrasonic pretreatment conditionsIn order to have a clearer understanding of the mechanism of reagents’ action on the mineral surface in pretreatment and flotation, the whole purification mechanism of barite was demonstrated as in (Fig. 6). The clean gum breaker YJ-I through oxidation can firstly destroy the colloidal structure in the drilling fluid, reduce the viscosity of the drilling fluid, and enhance the fluidity of the drilling fluid in the subsequent high-speed water washing45. Due to the water dilution and centrifugal force, centrifugal process of barite solid phase most of the slurry settled in the lower part of the wall near the machine position, when the machine stops rotating, the slurry inside the machine at this time is divided into three layers, from top to bottom, respectively, for the water layer, low-density solid phase mixing layer, barite mixing layer, and remove the uppermost layer of the water layer, excluding the surface layer of the low-density solid phase mixing layer, at this time a more pure barite mixed solid phase can be obtained46. According to the above purification process and adjusting the number of washing times, purer barite mixed solid phase can be obtained.Fig. 6Mechanism of barite recovery based on the whole process of washing-microwave-ultrasonic pretreatment and flotation.Based on the close combination of barite and the lumpy solid formed by the low-density solid phase, it is difficult to carry out effective separation by direct flotation. Therefore, microwave pretreatment was introduced, and the mixed solid phase of barite under microwave radiation showed graded thermal response characteristics, in the main stage of thermal response of barite, a large amount of moisture evaporates from barite, generating gas pressure, causing pore expansion and crack extension; in the main stage of thermal response of externally adhered low-density solid phase, the low-density solid phase absorbs the microwave energy, resulting in the removal of low-density solid phase, and the cracks are extended and through the pores, and the moisture and the migration of the low-density solid phase leads to a large number of pore and fissure generation, and provides a channel for the removal of moisture and the desorption of the low-density solid phase. As a result, desorption between barite and low-density solid phase is formed47.Subsequently, under the influence of ultrasonic waves, the minuscule bubbles lodged within the pores and fissures of both barite and the low-density solid phase in the slurry swiftly collapse.The local instantaneous temperature increase and pressure change are induced, which help to break the adsorption chemical bond between barite and low-density solid phase particles, thus promoting the dissociation between the two and exposing more sites48. Simultaneously, ultrasonic waves create feeble vortices and eddies in the slurry, generating miniature water currents. These currents expedite the movement of surfactant molecules towards the solid–liquid interface, which enabling faster adsorption and subsequent separation from the surface of the low-density solid phase.Following a comprehensive pretreatment involving water washing, microwave irradiation, and ultrasonic treatment, the residual low-density solid phase adhering to the barite surface was entirely dislodged, thereby establishing an optimal flotation environment49. Due to the selective inhibition exerted by GG. It interacts with Ba2+ ions on the barite surface through hydrophilic functional groups (–COOH, –OH), forming a protective capping layer which hinders the direct interaction between SDS and the barite surface, effectively shielding the active sites on the barite surface. Consequently, the collector fails to efficiently adsorb onto the barite, thereby enhancing the flotation efficiency.
