Beneficiation process reengineering under the background of green development-- take the full utilization of a low-grade lead-zinc ore as an example

A zero-waste mine is an advanced mine development and resource utilization strategy that aims to encourage the reduction of mine solid waste at source and the utilization of all its components. A low-grade lead-zinc ore in Guangxi, China, contains valuable components like galena, sphalerite, pyrite, barite, etc. and the gangue mineral is primarily dolomite. This paper proposes a flow of “waste pre-discarding by dense medium cyclone - sequential priority flotation of lead and zinc recovery - flotation and gravity combined process for barite recovery” to realize full component utilization. In this technical approach, 38.81% of the waste rock that can be utilized as concrete aggregate was removed from the feed ore through pre-discarding prior to grinding, after that recovers lead and zinc by the sequential priority flotation method with yield of Pb concentrate and Zn concentrate at 7.61%. In order to more efficiently recover barite and pyrite from lead-zinc flotation tailings, the gravity equipment of reflux classifier (RC) was used, which obtained barite concentrate with a yield of 7.97% and pyrite concentrate with a yield of 2.28%. The remaining yield of 43.34% tailings can be used for filling after the recovery of barite concentrate. Utilizing waste rock, lead, zinc, barite, and other resources might help the mine achieve the dynamic balance of “mining-discarding-beneficiation-filling” and realize zero tailings discharge.


Introduction
The lead-zinc sulfide ores in China are typically characterized by large deposit scale, low valuable components, and challenging comprehensive recovery of associated resources.The exploitation of these resources often results in the generation of significant amounts of tailings.As China places increasing emphasis on safety, environmental concerns, and the conservation of water resources, the admission criteria for tailings ponds have undergone significant enhancements, resulting in a general reduction rather than an increase in the number of tailings ponds.Therefore, the issue of tailings disposal has become a key constraint on the survival and development of mines.A low-grade lead-zinc ore in Guangxi, China, currently only recovers lead and zinc through the sequential priority flotation method, despite containing valuable components such as barite, pyrite, etc., and with the primary gangue mineral being dolomite.This paper is dedicated to the development of technology for the full utilization of all components in this ore, aiming to achieve the goal of a zero-waste mine.
A zero-waste mine is an advanced mine development and resource utilization strategy that aims to reduce mine solid waste at its source and maximize the utilization of all components.In recent years, pre-discarding technology prior to grinding has garnered increasing attention due to its ability to reduce solid waste generation in mines from the source.The application of dense medium separation technology in nonferrous and nonmetal mineral processing is gradually on the rise (Lv et al., 2019;Luo and Tian, 2004;Zhou and Yu, 2019;Luo et al., 2015).When valuable minerals in low-grade leadzinc sulfide ores are embedded within aggregates, a significant amount of individual gangue materials is produced after intermediate crushing.These gangue materials can be efficiently removed in advance through dense medium separation technology to prevent them from entering the grinding operation.Based on the characteristics of the waste rocks generated by pre-discarding, the development of resource utilization technologies capable of absorbing a large quantity of waste rocks may significantly extend the service life of the tailings pond.Considering the recovery characteristics of various accompanying elements, developing beneficiation techniques that do not affect the recovery of the main target minerals is another way to address the current dilemma.Currently, in the recovery of barite from sulphide ore flotation tailings, most existing research has primarily employed flotation methods (Xiao and Dong, 2017;Zhang, 2017;Cui et al., 2011;Liu et al., 2019).However, the flotation method can lead to mutual interference between sulphide ore and non-sulphide ore backwater, limiting the application range of hydrophobic barite concentrate and presenting other challenges.Therefore, there is an urgent need to develop non-flotation recovery technologies.
By incorporating techniques such as multistage waste pre-discarding and the comprehensive recovery of co-associated resources, this paper reconstructs the complete component utilization process for the comprehensive utilization of waste rock, lead, zinc, barite, and other resources.This approach is anticipated to facilitate the sustainable development of the mine.

Samples
The low-grade lead-zinc ore used for this investigation was obtained from Guangxi province, China.The chemical composition of the feed ore is provided in Table 1.As shown in Table 1, the feed sample contains 0.84% Pb and 3.56% Zn, which are the main target elements for recovery.Additionally, the content of BaSO4 is 17.09%, making it suitable for comprehensive recovery.The mineralogical composition of the sample, as determined by QEMSCAM, is presented in Table 2. Dolomite is the primary gangue mineral, constituting 61.44% of the sample, and is rarely associated with sulfide minerals.

Methodology
The current process at the lead-zinc mine only involves the sequential priority flotation method for recovering lead and zinc.The yield of lead and zinc concentrates is approximately 8%, with 45% of the tailings can be used for filling.However, the remaining 47% of tailings remain untreated.The aim of reengineering the beneficiation process is to address the issue of disposing of this portion of tailings.
The main valuable minerals in the ore feed are galena (with a density ranging from 7.4 to 7.6 g/cm 3 ), sphalerite (with a density of 3.5 to 4.2 g/cm 3 ), barite (with a density of 4.0 to 4.6 g/cm 3 ), and pyrite (with a density of 5.1 g/cm 3 ), all falling within a density range of 3.5 to 7.6 g/cm 3 .In contrast, the primary gangue minerals have a density of 2.8 to 2.9 g/cm 3 .The significant density difference between valuable minerals and gangue minerals allows for the effective removal of gangue minerals prior to grinding through the application of dense medium separation technology.The use of dense medium cyclones in the processing of nonferrous and nonmetallic minerals is steadily increasing.This technology offers advantages such as high separation precision, a wide range of particle sizes, efficient gangue removal in the pre-discarding stage, and a sewage-free separation process.In this experiment, dense medium cyclones will be employed for pre-discarding operations to eliminate the majority of the dolomite gangue.The use of this gangue as concrete aggregate will substantially reduce the volume of tailings that pose treatment challenges.
Recycling a portion of barite from the lead-zinc flotation tailings and enriching it to meet marketable standards represent an additional strategy for dealing with challenging-to-dispose tailings.However, the value of barite is relatively low, and its comprehensive recovery should not interfere with the main lead and zinc recovery process.In light of this prerequisite, it becomes imperative to explore the development of a cost-effective, gravity-based process with minimal impact on process water circulation.Furthermore, given that the value of lead in the ore is significantly lower than that of zinc, minimizing zinc losses during the lead recovery process is another prerequisite for process reengineering.Therefore, maintaining the existing sequential priority process remains the preferred approach for lead and zinc recovery.
Based on these considerations, the full component utilization technology route for this lead-zinc ore was developed, as shown in Figure 1.This innovative technology comprises three main stages: prediscarding before grinding, lead-zinc flotation, and the recovery of barite and pyrite from flotation tailings.The flotation feed consists of two parts: the pre-discarding concentrate and the -0.5mm size fraction of the raw ore (mixed proportionally by yield).The mixed feed sample was crushed, then rotary-split into 1 kg portions, and subsequently wet grinded to achieve a 50% solids content with 70% passing through a 74 μm sieve in a mild steel ball mill.The resulting slurry was transferred to a 3-litre flotation cell for roughing and scavenging, and the pulp concentration was adjusted to 27%.Multiple flotation cells with different volumes were used for cleaning.The impeller speed was set to 1750 rpm before reagent addition and flotation.The flotation reagents involved in this study are all industrial grade.After the flotation process, the products were collected, dried, and weighed.Finally, the recovery rate was calculated based on the dry weights and grades of the products.

Reflux classifier tests.
This study employs a laboratory-scale reflux classifier, as illustrated in Figure 3, for preconcentrating barite in lead-zinc flotation tailings.The reflux classifier, recognized as an innovative pre-discarding technology (Chu et al., 2019;Liu et al., 2021;Zeng et al., 2017), was utilized to reduce throughput and enhance the separation efficiency in the subsequent "reverse flotation for desulfurization -shaking table for cleaning" process.As depicted in Figure 3, prior to commencing the experiment, a 15 kg ore sample is prepared to achieve a specific slurry mass concentration.At the outset of the test, the ⑦ -agitator motor is activated, adjusted to the required test speed, and the ⑤ -pump is set to deliver water at the desired test flow rate.The adjusted pulp is then transferred to the ① -agitation tank for thorough mixing, ensuring even feeding into the sorting column.The valve of the ② -agitation tank is opened, and the pulp flow rates of both the ③ -feeding pump and the ⑥ -underflow pump are adjusted.After 45 minutes of stable sorting, samples are collected from both the overflow and underflow.Each sampling duration lasts 60 seconds, repeated three times under identical conditions, with a 24-minute interval between each sampling cycle.Subsequent to drying, the samples are weighed, analyzed for quality and grade, and various performance indicators such as yield, recovery rate, and sorting efficiency are calculated.The shaking table, Ly-1100x500, was used to conduct a cleaning test on the lead-zinc tailings after preconcentration and desulfurization in order to obtain a high-grade barite concentrate.

Results of sieve analysis
Under laboratory conditions, sieve analysis was performed on the raw ore crushed to below 15mm, and the results are shown in Table 3.In the sieve analysis, the -0.5mm fraction exhibited a BaSO4 content of 28.52%, with a BaSO4 distribution rate of 37.71%.Compared to the +0.5mm fraction, the -0.5mm fraction had a higher BaSO4 content but lower Pb and Zn content.

Results of pre-discarding tests by dense medium cyclone
Perform pre-discarding tests on the +0.5mm fraction of raw ore.The results of these tests at different specific gravities of the dense medium are presented in Figure 4.According to the results, the discard rate increases with the increase of the dense medium specific gravity, leading to a corresponding increase in the recovery rate of Pb and Zn lost in the waste rock.It is considered appropriate to conduct pre-discarding tests when the specific gravity of the dense medium is set to approximately 2.55.Under this condition, the discard rate of the pre-discarding process is approximately 50.16%, and the yield of waste rock to the raw ore is 38.81%.The Pb+Zn grade in the waste rock is 0.31%, with the recovery rates of Pb and Zn in the process being 3.16% and 3.38%, respectively.The flotation feed for lead and zinc recovery consists of two sample components: the -0.5mm raw ore and the concentrate from the pre-discarding test.Pre-discarding tests are conducted under optimal dense medium specific gravity conditions to obtain the feed for subsequent flotation tests.Through pre-discarding, waste rock with a yield of 38.81% can be removed from the raw ore before the grinding process, significantly reducing the consumption of milling energy and flotation reagents during the subsequent lead-zinc recovery process.Table 4 presents the grade and recovery indicators of the flotation feed, which has a yield of 61.19%, and the Pb and Zn recovery rates are both above 97%, while the BaSO4 recovery rate exceeds 98%.Conduct locked circuit flotation tests on the flotation feed presented in Table 4, following the process flowsheet for "lead-zinc sequential priority flotation" as illustrated in Figure 5, and the results are shown in Table 5.The flotation process and reagent regimen for the flotation feed obtained after prediscarding are essentially identical to those used for direct flotation of the raw ore.
The experimental results indicate that even after pre-discarding the flotation feed that has lost some of the lead and zinc minerals, the lead and zinc concentrates can still achieve high recovery rates for Pb and Zn (64.17% and 92.77%, respectively).While, the recovery rates of Pb and Zn to the raw ore are 62.55% and 92.24%, respectively, which are essentially consistent with the indicators obtained through direct flotation of the raw ore.

Results of barite recovery by combined technology of flotation-gravity separation
A novel technology has been developed for the recovery of barite from lead-zinc flotation tailings, as depicted in Figure 6.This innovative approach incorporates a reflux classifier (RC) for preconcentration, which reduces the throughput of subsequent operations such as "reverse flotation for desulfurization" and "shaking table for cleaning", thereby shortening the barite recovery process.Due to the fine particle size of the lead-zinc flotation tailings, a hydrocyclone is employed for desliming to ensure that the ore sample entering the reflux classifier meets equipment requirements.Following preconcentration, reverse flotation is conducted to effectively remove pyrite, which shares a similar density with barite, from the preconcentrated concentrate, referred to as RC concentrate.The desulfurization process utilizes oxalic acid as an activator and butyl xanthate as a collector.The resulting sulfur concentrate can be sold or disposed of, and the backwater generated during desulfurization closely resembles that of the lead-zinc flotation process.Finally, the rough concentrate obtained after desulfurization undergoes multi-stage shaking table cleaning to yield qualified barite concentrate.
Preconcentration is a crucial operation in the new barite recovery technology; therefore, the effects of barite preconcentration using the reflux classifier were investigated under different concentrate operation yields, as shown in Figure 7.The results indicate that when the RC concentrate operation yield is 45.54%, both the BaSO4 grade and operation recovery rate of the RC concentrate are favorable, measuring 48.77% and 72.91%, respectively.Under this condition, subsequent tests involving "reverse flotation for desulfurization" and "shaking table for cleaning" were conducted.Finally, as shown in Table 6, through the combined flotation-gravity separation process, a barite concentrate with a BaSO4 grade of 93.20% and a BaSO4 recovery of 44.99% was obtained.In addition, a sulfur concentrate with an Fe grade of 37.02% and a S grade of 42.31% was also obtained.*Due to the presence of barite, it is more appropriate to use the content of Fe to characterize the content of pyrite.

Results of the complete flowsheet test
By combining waste pre-discarding technology, comprehensive recovery technology for tailings and the existing lead-zinc flotation technology, a new full component utilization process has been developed, referred to as the "waste pre-discarding by dense medium cyclone -sequential priority flotation of lead and zinc recovery -flotation and gravity combined process for barite recovery".The test results obtained using this new process are shown in Table 7.
The results show that waste rock with yield of 38.81% can be removed from the feeding through pre-discarding prior to grinding, while the overall yield of Pb concentrate and Zn concentrate remains at 7.61%, and the recovery rates of Pb and Zn are essentially the same as those achieved through direct flotation.After recovering a barite concentrate with a yield of 7.97% and a sulfur concentrate with a yield of 2.28% from the lead-zinc flotation tailings, the remaining tailings with a yield of 43.34% can be utilized for filling.
Additionally, research was conducted on the utilization of waste rock as a construction material.The results demonstrated that waste rock can be used as concrete aggregate to produce C30, C35, C40, and C45 concrete.The slump, expansion, density, and compressive strength of the concrete at different ages all met the corresponding strength grade requirements for concrete.

Conclusion
This paper reconstructed a full component utilization process flow of "waste pre-discarding by dense medium cyclone -sequential priority flotation of lead and zinc recovery -flotation and gravity combined process for barite recovery" by adding techniques like multistage waste pre-discarding and comprehensive recovery of co-associated resources.
In this technical approach, 38.81% of the waste rock that can be utilized as concrete aggregate was removed from the feed through pre-discarding prior to grinding, reducing the consumption of milling energy and flotation reagents in the subsequent lead-zinc flotation process.The Pb concentrate and Zn concentrate are obtained with an overall yield of 7.61% through the sequential priority flotation method.A simplified process for recovering barite from lead-zinc flotation tailings has been developed, which can recover barite concentrate with the yield of 7.97% and sulfur concentrate with the yield of 2.28%.The remaining yield of 43.34% tailings can be consumed through filling.The utilization of waste rock, lead, zinc, barite, and other resources may help the mine achieve a dynamic balance in the "mining-discarding-beneficiation-filling" cycle and realize zero tailings discharge.

Figure 1 .
Figure 1.Technology route for full component utilization of the lead-zinc ore 2.2.1.Pre-discarding tests by dense medium cyclone.Pre-discarding tests by dense medium cyclone were conducted on the size fraction -15+0.5 mm of raw ore to remove gangue minerals, primarily dolomite, from the low-grade lead-zinc ores.The equipment flowsheet for the dense medium cyclone system is shown in Figure 2, and the pilot-scale cyclone used in the experiment was manufactured by Weihai Haiwang, China.

Figure 2 .
Figure 2. Equipment flowsheet for pre-discarding tests2.2.2.Lead-zinc flotation tests.For the recovery of lead and zinc, a sequential priority flotation process is adopted, and corresponding closed-circuit tests are conducted.As a result, the flotation test comprises various stages including roughing, scavenging, and cleaning.The flotation tests were conducted using an XFG series flotation machine (Jilin Prospecting Machinery Factory, China) equipped with plexiglass flotation cells of various volumes to accommodate different flotation operations.The flotation feed consists of two parts: the pre-discarding concentrate and the -0.5mm size fraction of the raw ore (mixed proportionally by yield).The mixed feed sample was crushed, then rotary-split into 1 kg portions, and subsequently wet grinded to achieve a 50% solids content with 70% passing through a 74 μm sieve in a mild steel ball mill.The resulting slurry was transferred to a 3-litre flotation cell for roughing and scavenging, and the pulp concentration was adjusted to 27%.Multiple flotation cells with different volumes were used for cleaning.The impeller speed was set to 1750 rpm before reagent addition and flotation.The flotation reagents involved in this study are all industrial grade.After the flotation process, the products were collected, dried, and weighed.Finally, the recovery rate was calculated based on the dry weights and grades of the products.

①Figure 3 .
Figure 3. Operation process diagram of reflux classifier 2.2.4.Shaking tabling tests.The shaking table, Ly-1100x500, was used to conduct a cleaning test on the lead-zinc tailings after preconcentration and desulfurization in order to obtain a high-grade barite concentrate.

Figure 4 .
Figure 4. Pre-discarding tests results at different dense medium specific gravities

Table 1 .
Chemical composition of the feed sample (mass fraction, %)

Table 3 .
Results of sieve analysis for raw ore(-15mm)

Table 4 .
Indicators of the flotation feed obtained after pre-discarding 3.3.Results of flotation test for lead and zinc recoveryFigure 5.The flowsheet of locked circuit flotation tests

Table 5 .
The results of locked circuit flotation tests

Table 6 .
The results of flotation-gravity separation combined tests

Table 7 .
The results of the complete flowsheet test