Study on bioleaching behavior of sphalerite from lead concentrate in agitator tank

Sphalerite which is highly continuous with galena cannot be effectively separated by flotation, the extraction of zinc from sphalerite in bioleaching process has become an alternative method. By testing the initial iron concentration, pulp density, grinding fineness and other parameters as well as monitoring pH, potential and dissolved oxygen, the zinc content of the lead concentrate decreased from 8.41% to less than 1% after bioleaching in 8L agitator tank. The initial concentration of ferric is the most significant factor affecting the zinc extraction efficiency of sphalerite, and when the concentration of ferric ions reaches 9g/L, acceptable leaching yield can be achieved.


Introduction
As a nonferrous metal, zinc is an important resource necessary for modern industry.It ranks fourth among all metals in terms of world production after iron, aluminum, and copper.Zinc sulfide ore is the most important zinc-producing mineral in the world and it often coexists with galena.In many cases the crushed ore particles are not sufficiently small to permit liberation of the desired mineral so that separation can be achieved.The process of utilized lead-zinc minerals gives rise to waste of resources and affects the subsequent smelting process.It also induces tremendous pressure to production efficiency and ecological environment.Due to the complex intergrow relationship between sphalerite and galena, the particle size of lead-zinc mineral is ground relatively fine.For the purpose of separating lead and zinc, the raw ore often needs to be ground to over 90% of the ore particle size through the 0.074mm sieve.Nowadays the flotation process of lead-zinc is becoming more difficult and costlier than before.As lead and zinc unevenly distributed in gangue with a wide range of particle size, the lead concentrate produced by flotation has a high content of zinc and the yield of lead concentrate is low.With industrial development promoting, the bioleaching technology, as a green and low-carbon hydrometallurgy technology, meets the current environmental protection policy requirements.Bioleaching of zinc sulfide minerals can significantly increase the recovery of zinc, especially in terms of extracting metals from low-grade sulfide deposits.Thereby it shows advantages on the economic [1].
Through selective adsorption and biological oxidation of minerals, bacterial leaching can effectively solve the problem of encapsulation and make sulfide minerals be selectively dissolved [2,3].With the valuable metals in the minerals mobilized to solution, microbial can accelerate the oxidation from ferrous to ferric and promote to change S 2− in sulfides into sulfur or sulfate [4,5].Metal sulfides are conductors, semiconductors, or insulators in which both sulfur and metal atoms are bound to the lattice [6].Both molecular orbital theory and valence band theory indicate that atoms and molecular orbitals form electron bands with different energy levels.The valence band generated by the zinc sulfide orbital is an acid-soluble "polysulfide" pathway.In sphalerite, both of H + and Fe 3+ remove valence band electrons, resulting in the fracture of zinc-sulfur bond [7].Microorganisms in the oxidation of these intermediate also plays a crucial role in the process, and turn them into sulfate and H + [8].Many literatures indicted that zinc sulfide is acid soluble metal sulfide, and its bioleaching follows polysulfide way: by the ore leaching bacteria, the electron on valence band of metal sulfides was combined with H + and Fe 3+ , releasing the sulfur compounds, which is turned into sulfate by ferric and O2 oxidation [9,10].The investigate focuses on selective bioleaching of zinc in agitator tank which contains in lead concentrate produced by traditional flotation.The possibility of selective bioleaching of zinc from zincbearing lead concentrate is determined by the investigation of effects.It provides the new approach to solve the beneficial recovery problems of complex lead-zinc ore.

Ore samples
One sample (S1) is lead concentrate which came from a flotation concentrator in Yunnan province.The particle size of the lead concentrate is 90% under 0.074mm screen.The other sample (S2) is lead concentrate after finely ground to a particle size (over 90%) of 0.038 mm.Prior to use, both of the lead concentrate was washed with dilute sulfuric acid (pH=3) to remove the surface-oxidized deposits and the residue of flotation agent, rinsed repeatedly with distilled water and dried at 70 o C for 4 h.The elemental composition was determined after mineral dissolution in aqueous solution by inductively coupled plasma -optical emission spectroscopy (ICP-OES) measurements.Mineral liberation analysis system (MLA)combined with scanning electron microscopy (SEM)were employed for the mineralogical identification of the ore samples.residues after bioleaching were coldinlaid with epoxy resin and observed by SEM after polished.

Microbial culture
The bacteria were originally grown with 120 mM ferrous sulfate in a mineral salts solution that contained 3.0 g/L (NH4)2SO4, 0.5 g/L MgSO4•7H2O, 0.01 g/L Ca(NO3)2, 5 mol/LH2SO4, 0.5 g/L K2HPO4 and 0.1 g/L KCl.The pH was adjusted to 1.9 with H2SO4.The cultures were acclimated to grow with 5% (w/v) zinc concentrate in the mineral salt solution through several subcultures before the leaching experiments reported in this paper were conducted.The microbial culture was inoculated (initial cell number 10 6 cells/ml) in the pulp.The bacteria were cultured on a stirred fermentation cylinder with air inflation at 30 o C. The population of culture was identified and characterized with molecular level technique that metagenomic sequencing based on 16S rDNA analysis.

Bioleaching experiment
The bioleaching experiments were carried out at 35℃ in agitator tanks (Maximal volume 8L) containing 6L pulp.The initial pH of the pulp in all agitator tanks was around 1.9, but no pH control was executed during the experiment.All agitator tanks are with aeration and water bath.Inflating volume was adjusted at 3L/min and rotating speed of mixer was set at 300rpm in the experiment.Bioleaching was sampled to analyze the concentration of Zn 2+ , total Fe, pH, redox potential (Eh) and solvent oxygen at regular intervals.The factors and correlative value of experiments were shown by table 1

Elemental analysis by XRF and ICP-OES
The ICP-OES analysis on the composition of lead concentrate used for this work has been shown in table2.The results of the elemental analysis indicate that zinc in lead concentrate affects the quality of lead concentrate and cannot be removed by flotation.Microbe adhere on the mineral particles by the extracellular polymeric substance (EPS) to oxidize the sulfides in zinc-bearing lead concentrate [11].There are many substrates on which Acidiphilium strains act, including S 2-in sulfides, element S, ferrous iron and other materials [12][13][14].The dominant bacteria Acidiphilium oxidize pyrite to release Fe 2+ and produce a large amount of Fe 3+ .Due to the potential difference between sphalerite and galena, sphalerite is more susceptible to be oxidized.As the production of lead sulfate on the surface of galena, Sulfobacillus tends to absorb on the surface of sphalerite and effectively dissolves Zn 2+ into leachate.The function of Leptospirllum is oxidation of ferrous iron.As the main species, they promote the oxidation of sphalerite in the leaching system.With the dissolution of pyrite in zinc and lead concentrate, the formation of jarosite, the consumption of Fe 2+ concentration gradually decreased, Leptospirllum is not suitable for survival on the lack of energy substrate; However, a variety of reduced sulfur, such as elemental sulfur and polysulfides [15,16], produced in the oxidation leaching process of zinc and lead bearing concentrate can be used by Sulfobacillus, so the leaching system is suitable for the growth of Sulfobacillus.Therefore, considering that the extreme acidophilic population, it is the positive factor of the zincselective bioleaching from lead concentrate.The microbial population is suitable for redox reactions with sulfide minerals in acidic conditions, which is beneficial to the dissolution of valuable metal ions from sulfide minerals into solution.It is also the internal reason for leaching selectivity of zinc from zinc-bearing lead concentrate.

Changes of parameters in bioleaching
The changes of pH in each stirring tank are different, and the curves of pH are shown in the figure 2. The pH value of tank 1# is relatively stable.Except for a small range of fluctuations in the early stage, it has been maintained at about 1.7.The trend of the pH value in 4# is basically decline.The pH value of the solution in tanks 2# and 5# showed a trend of increasing first and then decreasing slowly.The pH value of tank 3# fluctuated greatly, which first increased and then decreased, increased at the later stage of leaching.Based on properties of solution, the main reason for the fluctuation of pH value is related to the initial Fe 3+ concentration used in the initial stage of the experiment.It needs to be adjusted to a more suitable pH range by sulfuric acid in time to avoid Fe 3+ hydrolysis caused by pH increase and affect the zinc efficiency of sphalerite.Potential in bioleaching (shown in Figure 3) reflects that the bacteria in the 1# and 3# tanks mainly maintained the concentration ratio of oxidized ions and reduced ions in the early stage of leaching.In the middle of leaching, with the stability of the pH value of the pulp, the adaptability of bacteria to the pulp gradually increases, and the oxidation-reduction potential gradually increases.In the late stage of leaching, the potential began to stabilize above 670 mV (reference electrode Ag/AgCl), indicating that the oxidation of bacteria is extremely significant, and the oxidation of the pulp is suitable for the oxidation of zinc-containing sulfide minerals and the dissolution of zinc.The dissolved oxygen in solution indirectly reflects the transfer rate of the sulfides oxidized by bacteria with oxygen as electron acceptor.The changes of the dissolved oxygen are shown in figure 4. The dissolved oxygen in 1# and 3# tank change greatly, from 4mg/L to less than 0.3mg/L, indicating that bacteria are rapidly consuming the dissolved oxygen in the pulp.Microorganisms use oxygen as an electron acceptor to convert Fe 2+ into Fe 3+ and the rapid dissolution of valuable metals in sulfide minerals is followed by oxygen consuming.The core problem in the lead concentrate selective bioleaching is the growth and generation of bacteria on leaching zinc.Therefore, it is particularly critical that the concentration of bacteria is monitored in the leaching process.The number of bacteria in experiment was shown in figure 5.In the zinc bioleaching, the change trend of the bacterial concentration in each stirring tank was gradually increasing.It indicates that the bacteria provide energy for their own metabolism pathway by oxidizing S 2-in sulfide minerals and the electron transport respiration chain in themself during the leaching process.With rapid growth and reproduction, the bacteria in the 3# and 1# tanks reached 1×10 8 cells/ml within 100 hours.It shows significant adaptability to the zinc leaching of the lead concentrate.During the leaching process, the zinc ions gradually increased with the extension of the leaching time, especially the Zn 2+ concentration in the pulp of 1# tank increased rapidly (shown in figure 6).The zinc concentration had reached 2.4g/L within 120h, which is equivalent to the zinc content of about 17g.By this time 57.75% of the zinc in sphalerite has been dissolved in solution.Although the rate of increase in the Zn 2+ concentration after 120h is slower than that of the previous 120h, the zinc ion can eventually reach 3.7 g/L.The zinc ion concentration in the 3# tank rose slowly within 240h, but after that there was a rapid rise.At the end of the experiment the zinc ion reached 3.6g/L.The zinc ion concentration in the other tanks increased extremely slowly and did not exceed 1g/L until bioleaching finished.

Analysis of bioleaching residues
As shown in Figure 7, the grade of zinc in tanks 1# and 3# gradually decreased, the final residues of zinc grade in tank 1# could be reduced to less than 0.9%, and the slag of zinc grade in tank 3# also decreased to about 1.2% at the end of the bioleaching.It is not obvious that the zinc grade decreased in tanks 2#, 4# and 5# and the zinc grade of residues were above 7.6% after the experiment.By the examination of leaching residue by SEM-EDS, it was shown that sphalerite could hardly be found.Except for a small amount of jarosite produced, lead compounds, among which the representative one was that the surface of galena was coated with lead sulfate, as shown in Figure 8.
(a1) (a2) (a1: Lead sulfate on leaching residue surface ； a2: Unreacted galena in the core of leaching residue) Because the domesticated bacteria of zinc concentrate have better tolerance and selective adsorption, they act on the surface of zinc lead concentrate in the process of bioleaching and give priority to the release of Zn 2+ .After the release of Zn 2+ , galena is exposed and then covered by lead sulfate produced by oxidation on the mineral surface to prevent further leaching.However, other domesticated bacteria had poor tolerance and selective adsorption, and the oxidation rate of lead sulfate was faster, and the premature production of lead sulfate prevented the further oxidation of ZnS.Therefore, using domesticated bacteria to leach zinc and lead concentrates has certain advantages in selective zinc leaching.

Conclusions
Bacterial leaching of metal values from the substrate is a complex process and is affected by multiple factors.Effects of important parameters including different partial size, different concentration of ferric addition, different pulp density and their interactions on bioleaching of zinc sulfide ores were investigated by regular monitor in 5 agitator tanks.Bioleaching results in agitator tanks were encouraging in that more than 70% of zinc was extracted from in 300h, which is so remarkable to compare with conventional bioleaching.The key factor in zinc recovery during the bioleaching is initial ferric concentration.Higher concentration of ferric ions leads to disintegrate zinc from sphalerite faster than lower ferric concentration.Ferric ion reduces itself to ferrous ion, and promotes the rapid generation and growth of bacteria.

Acknowledgment
The research was funded by the National Natural Science Foundation of China (No: 52074243).

Figure 2 .
Figure 2. Change of pH in each stirring tank.

Figure 3 .
Figure 3. Changes of potential in each stirring tank.

Figure 4 .
Figure 4. Changes of the dissolved oxygen in each stirring tank.

Figure 5 .
Figure 5. Number of bacteria in bioleaching of agitator tank.

Figure 6 .
Figure 6.The changes of Zn 2+ concentration in bioleaching of agitator tank.

Figure 7 .
Figure 7.The changes of zinc grade in bioleaching residues.
The 10th International Conference on Lead and Zinc Processing (Lead-Zinc 2023)

Table 2 .
The results on ICP-OES analysis.
3.1.2Mineralsanalysis by MLAThe mineral ore used in this study mainly consisted of galena and sphalerite.The specific mineral analysis results are shown in table3.

Table 3 .
Mineral composition list by MLA.