Analysis of bioleaching characteristics and multi-element dissolution behavior of complex zinc ores

In order to recover low-grade complex zinc ore in a reasonable way, this study adopts bioleaching method to study it. The ore samples contain 1.52%, 2.03% and 14.4% zinc, respectively, which occurs in the form of sphalerite. Other major minerals include pyrite, galena, quartz and mica. The inoculation of the domesticated strain was basically free of adaptation period, and the cell concentration could be rapidly increased after a short decrease. The leaching extent of zinc increased continuously, while the leaching rate decreased gradually. After the bioleaching process, sliver, lead and iron were mainly present in the residue phase. X-ray diffraction spectroscopy analysis showed that sphalerite, galena and pyrite were dissolved, and the latter two further precipitated to produce PbSO4 and jarosite. In addition, the dissolution of calcium compounds can lead to the formation of gypsum precipitation. These precipitates covered the fresh ore surface may hinder the further bioleaching process. The Exponential model was used to simulate the bioleaching process, and it was found that the fit coefficients were all greater than 0.98, and a reasonable leaching cycle was further discussed. The results provide a good basis for the economic and environmentally friendly recovery of low-grade complex zinc ores.


Introduction
Zinc is abundant in the earth's crust and has good corrosion resistance.It is widely used in electroplating, alloys, dry cell batteries, rubber, paints, printing, enamelling, fibers, medicine, and other industries.Especially, it has a critial position in the production of batteries (battery surface) [1,2]。With the development of the mining industry, rich and easy-to-handle zinc ore are decreasing, but the demand for zinc is increasing day by day.As a major producer and consumer of zinc resources, China produced 6.29 million tons of refined zinc in 2022, while the demand exceeds 7 million tons.With the expansion of zinc production, the conflict between supply and demand becomes more prominent.Therefore, the abundant low-grade complex zinc ores must be exploited [3,4].Traditional pyrometallurgical and hydrometallurgical processes, although effective, are energy intensive, expensive, prone to secondary pollution, and unsuitable for treating low-grade ores.Bioleaching technology has the advantages of mild condition, no secondary pollution, low production cost, and environmental friendliness.Thus, it has economic advantages and promoting value in treating lowgrade complex ores and refractory ores [5,6,7].
Bioleaching is a process that uses the oxidation/reduction properties or metabolites of microorganisms themselves to separate useful components from the original material in a soluble or precipitated form.It has been applied in commercial production for bioleaching of copper and uranium ores, as well as in the bio-oxidation of complex refractory gold ores [8,9].As the bioleaching mechanism is continuously researched and explored in depth, the technology has also shown good prospects for the extraction of other metals such as zinc, cobalt, nickel, molybdenum, and manganese [10,11].The bioleaching process of zinc ore was proposed by Billiton in 1998, known as the BioZINC process, and a bioleaching pilot plant was established at the Pering zinc mine in South Africa in 2000 [12].In the case of sphalerite, the bioleaching mechanism is shown in Eqs.1-4 [13,14].Shake flask experiments showed that the extraction of zinc from polymetallic sulfide ores could reach 97.1% after 10 days of bioleaching [15].Some scholars used bioleaching-sulfide precipitation method to remove metals from lead-zinc ore tailings.The results showed that the zinc bioleaching rate was about 97.38% after 50 days at 10% pulp density, which obeyed the shrinking core model [16].Bioleaching experiments using mixed acidic Thiobacillus found that the redox potential was higher than that of the chemical control method [17].It can be seen that the bioleaching method is suitable for the treatment of zinc sulfide ores.
In order to better provide guidance for the rational development of low-grade complex zinc ores, this study explores the feasibility of bioleaching technology with three zinc ores as research targets.At the same time, the evolution of system characteristics and elemental leaching patterns during the bioleaching process were analyzed.The results lay the foundation for the bioleaching application of similar ores.

Ore sample
The complex zinc ores were taken from Inner Mongolia, China, and defined as a, b and c ores in ascending order of zinc content.The main elements of these ores are shown in Table 1.The a and b low-grade complex zinc ores containing 1.52% and 2.03% zinc, respectively, while the medium-grade complex c ore containing 14.40% zinc.In addition, they contains small amounts of silver, lead and iron.The results of X-ray diffraction spectroscopy (XRD) analysis (see "Results and Discussion" section) show that the main minerals are sphalerite, galena, pyrite, quartz, muscovite, and black zincmanganese ore.Zinc and lead are mainly in the form of sulfides, and quartz is the main gangue.The three zinc ores were first crushed and then ball milled for 20 min to obtain three ore samples with the particle sizes shown in Figure 1.It can be seen that the particle size less than 40 μm accounted for more than 80% in all three ores.Their surface areas were 0.91 m 2 /g, 1.25 m 2 /g and 1.2 m 2 /g, respectively.The particle sizes at 90% of the ore weight were 18.59 μm, 84.04 μm and 67.01 μm, respectively.

Experimental process
This study was carried out in a 500 mL erlenmeyer flask with a total solution volume of 300 mL.The reagents other than FeSO4•7H2O were weighed into the erlenmeyer flask, 270 mL of distilled water was added, and the pH was adjusted to about 1.8.Then FeSO4•7H2O was weighed into it and the pH was adjusted to about 1.8 again.Finally, 30 mL of bacterial solution was added with a cell concentration of 20×10 7 /mL.The erlenmeyer flasks were put into a thermostatic shaker at a speed of 170 rpm and a temperature of 45℃.When the potential (Eh) of the solution reached 600 mV or more, 30 g of ore sample were added and started bioleaching.

Analytical method
During the experiment, the supernatant in the erlenmeyer flask was periodically taken for ion concentration detection.At the end of the experiment, the slurry was filtered to obtain the filtrate and residue.The residue was washed and dried for further detection.The sulfur content in the residue was obtained by combustion iodometric method, and the other elemental contents were detected by atomic absorption spectrometry (AAS).A LeiCi PHBJ-260 portable pH meter was used for Eh and pH determination.The Fe 2+ and total Fe concentrations were determined by potassium dichromate titration.The XRD of the ore samples and residues were obtained by XRD-7000 X-ray diffractometer from Shimadzu, Japan.The cell concentration was obtained by the cell counting plate method.

Process Fitting
By observing the trend of zinc leaching rate, the exponential model (Eq.5) was assumed to be satisfied.The bioleaching data were fitted based on this model to evaluate the bioleaching process.

Exponential model:
x R e A y y     0 0 (5) where y is the dependent variable, denoting the zinc bioleaching rate; x is the independent variable, denoting time; y 0 , A, and R 0 are all model parameters.

Variation of bioleaching parameters
During the experiment process, the change of cell concentration, Eh, pH, Fe 2+ concentration, Zn 2+ concentration and zinc leaching rate are shown in Figure 2. Due to environmental stress, some microorganisms died after inoculation.The remaining microorganisms rapidly adapted to the new environment and repopulated rapidly again.The Eh is related to the Fe 3+ /Fe 2+ magnitude, which first decreased and then increased during the bioleaching process.This is due to the rapid oxidation of sulfide ore (Eq.2) by Fe 3+ at the initial stage, and the Fe 3+ /Fe 2+ value decreases.Under the action of microorganisms, Fe 2+ is re-oxidized to Fe 3+ , resulting in an increase in Eh. pH increased rapidly in the initial stage probably due to acid consumption by oxides on the ore surface.With the acid production by bio-oxidation reaction (Eq.3), the pH is in a state of gradual decrease or dynamic equilibrium.The trend of Fe 2+ change is consistent with pH, microorganisms quickly oxidize Fe 2+ to Fe 3+ after adapting to the environment, resulting in a gradual decrease in Fe 2+ concentration.Sphalerite is continuously dissolved under microbial catalytic oxidation, and finally all zinc basically enters the solution.According to Figure 2, the various indicators in the bioleaching process of a and b ore are superior to that of c ore.Specifically, the zinc bioleaching rate of b ore was close to 90% after 3 days of leaching.This may be due to the higher amount of lead and iron associated with the high-grade c ore, which entered the solution and inhibited the microbial activity, thus attenuating the bacterial corrosion of sphalerite [18].The higher sulfur content in b ore serves as a microbial nutrient substrate to provide energy for biochemical processes, thus achieving better microbial activity and leaching effect.

Distribution rate of major metal elements
After the bioleaching, the slurry was filtered and cleaned to obtain the leaching residue.The elemental content was measured, and the leaching rate of metal elements in the three ore samples was calculated, which are shown in Figure 3.It can be seen that the bioleaching efficiency of a ore is the best, while ore sample c has the worst leaching effect.The particle size of ore samples b and c is relatively large, and the complete dissolution of zinc cannot be achieved within a limited time.When reaching a stable point, it is the initial point for achieving maximum economic benefits.Due to the high zinc content in the sample c, it takes 15 days to achieve a high leaching rate.We cannot disregard the slow rate of bioleaching when compared to typical pyrometallurgical and hydrometallurgical processes.To speed bioleaching, strategies such as bio-domestication, genetic engineering, parameter optimization, process control, and additive enhancement must be further investigated [6,19].Most of the other metal elements remained in the residue phase, including 89%-98% silver, more than 99% lead, and 93%-96% iron.However, their presence in the residue phase does not mean that they were not leached.Element silver is generally insoluble in acidic solutions and requires cyanidation for recovery.However, some silver in the ore sample entered the liquid phase, which could be the complexation of organic acids secreted by microorganisms (Eq.6) [20].Lead exists in the ore in the form of galena (PbS), and bio-oxidation makes it enter the solution as Pb 2+ (Eq.7), but it then precipitated with SO4 2-to form PbSO4 and enters the residue phase (Eq.8).Elemental Fe enters the solution by bio-oxidation (Eq.9) and is then oxidized to Fe 3+ and produces jarosite (Eqs.4 and 10).Therefore, the leaching rate of iron is not high.PbSO4 and jarosite may cover the surface of the sulfide minerals and prevent further bioleaching [21].(10)

XRD analysis
The ore samples and residues were washed and dried for XRD analysis, and the results are shown in Figure 4.It can be seen that non-metallic minerals are the main components of the ore, including quartz and mica.Metal minerals occupy a relatively low peaks and mainly include sphalerite, pyrite and galena.The peaks of sulfide and quartz in sample c with high metal content basically overlap.In addition, chalcophanite peaks can be detected in the b and c samples.Bioleaching causes the dissolution of metal minerals and the corresponding peaks disappeared.Gypsum may be caused by the dissolution of a small amount of calcium compounds in the ore, and it can also act as a passivation substance to hinder the further bioleaching process.To avoid passivation of gypsum precipitation, a pre-acidification step is performed prior to microbial inoculation to eliminate most of the calcium compounds in practice, such as calcite [22,23].

Process fitting
The inoculated strain underwent a long-term domestication process and started leaching with little adaptation period.Based on the leaching trend of zinc, the Exponential model was selected and the bioleaching process was fitted using the software Origin 9.1, and the results are shown in Figure 5.According to the fitting results, correlation coefficient (R 2 ) of the three ores are greater than 0.98 during the leaching cycle, which means that the sphalerite bioleaching is consistent with the Exponential model.From the three fitted equations, the leaching time was extended by 5 days for each of the three ores, and the Zn leaching rates were obtained as 100.81%,92.56% and 96.96%, respectively.The leaching rate of the ore sample a exceeded 100%, indicating that the model were no longer applicable beyond this experimental cycle.The increase in leaching rate was minimal when the leaching time was increased for the ore sample c.Therefore, the leaching cycle was set to 15 days as the most appropriate, which not only ensures the effective recovery of metals, but also controls the time cost.

Conclusion
(1) The zinc ores used in this study contain 1.52%, 2.03% and 14.4% zinc, respectively, and are all occurrence in the form of sphalerite.Other major minerals include pyrite, galena, quartz and mica.
(3) Pb 2+ and Fe 3+ further precipitated to produce PbSO4 and jarosite.Ca 2+ generated by the dissolution of calcium compounds combined with SO4 2-to produce gypsum.(4) The Exponential model can simulate this bioleaching process with coefficients greater than 0.98.
(5) While bioleaching can produce ecologically benign and effective metal resource extraction, there are still issues with long cycle duration and passivation.Further study in strain cultivation, genetic engineering, and process control should be conducted in the future.

Figure 1 .
Figure 1.Particle size distribution of finely ground zinc ore samples.

Figure 2 .
Figure 2. Variation curve of parameters of ore bioleaching process.According to Figure2, the various indicators in the bioleaching process of a and b ore are superior to that of c ore.Specifically, the zinc bioleaching rate of b ore was close to 90% after 3 days of leaching.This may be due to the higher amount of lead and iron associated with the high-grade c ore, which entered the solution and inhibited the microbial activity, thus attenuating the bacterial corrosion of sphalerite[18].The higher sulfur content in b ore serves as a microbial nutrient substrate to provide energy for biochemical processes, thus achieving better microbial activity and leaching effect.

Figure 3 .
Figure 3. Distribution of metal elements in the ore after bioleaching.Most of the other metal elements remained in the residue phase, including 89%-98% silver, more than 99% lead, and 93%-96% iron.However, their presence in the residue phase does not mean that they were not leached.Element silver is generally insoluble in acidic solutions and requires cyanidation for recovery.However, some silver in the ore sample entered the liquid phase, which could be the complexation of organic acids secreted by microorganisms (Eq.6)[20].Lead exists in the ore in the form of galena (PbS), and bio-oxidation makes it enter the solution as Pb 2+ (Eq.7), but it then precipitated with SO4 2-to form PbSO4 and enters the residue phase (Eq.8).Elemental Fe enters the solution by bio-oxidation (Eq.9) and is then oxidized to Fe 3+ and produces jarosite (Eqs.4 and 10).Therefore, the leaching rate of iron is not high.PbSO4 and jarosite may cover the surface of the sulfide minerals and prevent further bioleaching[21].

Figure 4 .
Figure 4. XRD analysis results of ore and residue.

Figure 5 .
Figure 5. Exponential model fitting results for zinc bioleaching rate.According to the fitting results, correlation coefficient (R 2 ) of the three ores are greater than 0.98 during the leaching cycle, which means that the sphalerite bioleaching is consistent with the Exponential model.From the three fitted equations, the leaching time was extended by 5 days for each of the three ores, and the Zn leaching rates were obtained as 100.81%,92.56% and 96.96%, respectively.The leaching rate of the ore sample a exceeded 100%, indicating that the model were no longer applicable beyond this experimental cycle.The increase in leaching rate was minimal when the leaching time was increased for the ore sample c.Therefore, the leaching cycle was set to 15 days as the most appropriate, which not only ensures the effective recovery of metals, but also controls the time cost.

Table 1 .
Main element content in the three zinc ores.