Study on the control of high ore pass dust pollution by pre-injection foam dedusting technology in the ore bin

The impact airflow generated by ore unloading in the chute raises the dust carried by the ore itself and the floating dust, and then, the dust raised enters the roadway with the airflow and pollutes the environment. In order to minimize the amount of dust entering the roadway and reduce the pollution of unloading dust, we conducted an experimental study of selection of best foam formula and pre-injection foam dust dedusting technology in ore bin. It was found that the optimal foaming formula was 1.0% sodium dodecyl benzene sulfonate (SDBS) + 0.5% sodium dodecyl sulfate (SDS) + (0.2 ~ 0.4%) sodium carboxymethyl cellulose CMC-Na and coconut oil monoethanolamide (CMEA) by the compound experiment using two evaluation criteria of initial foaming amount and foam defoaming rate. When the air pressure is 0.7 MPa, the foaming rate of the foam generator is proportional to the gas and liquid flow rate and the best foaming gas and liquid flow ratio is 27.8. Under this circumstance, the foaming rate of the foaming formula is 500 l/min. When the height of foam is controlled at 15 cm, the effect of foam dust removal is the best. The dust emission rate from the foam to the fourth level can reach 60%, and the dust fall rate of the third level is 28%, which effectively reduces the dust production and relieves the pressure of the spray hole dust fall at the wellhead.


Introduction
Multi-level high ore pass is common in mental mines, which provides great convenience for ore transportation (Wang et al. 2019d). The raw rock extracted from the mine is crushed for the first time and then put into the ore pass transportation system through the mouth of the ore pass. Since the transport of ore passes into the ore bunker by gravity, the airflow in the pass is compressed rapidly during the free fall of a large amount of ore. This process breaks the stability of the airflow (Paluchamy et al. 2021). With the influence of shear airflow, the fine dust in the ore and floating dust in the structure of the ore pass are raised quickly, and a large amount of dust enters the middle section of the lower part of the unloading port along with the airflow, leading to the problem of dust pollution in the mine (Wang et al. 2020c). At the same time, with the increase of mining depth, the ore unloading gap gradually increases, and dust pollution problem is also gradually intensified (Wang et al. 2020d).
In the process of unloading from the ore pass, the fine dust produced is suspended in the air and not easily settled (Chen et al. 2022). The heavy metals attached to the particles will enter the soil, rivers, and biota with precipitation, which will cause soil heavy metal pollution and ecosystem destruction (Sultana et al. 2022). Respiratory dust particles are easily inhaled into the respiratory system of miners and cause fibrotic lesions in human alveolar tissue, leading to pneumoconiosis (Yang et al. 2021). The corrosive effect of dust particles will shorten the service life of lubricants in machinery and equipment, and increase their maintenance costs due to excessive wear or premature failure (Zhou et al. 2022). Moreover, excessively high dust concentration will affect the visibility of ore transport vehicles and increase the probability of traffic accidents (Wang and Jiang 2021). These hazards seriously threaten the safety of production and the health of mine operators. Therefore, it is very important to put forward an efficient dust removal measure to suppress dust pollution caused by mine unloading in the ore pass.
Up to now, many of dust pollution control of the multi-middle section high ore pass mine researches have focused on wellhead sealing and spray dust reduction of pass connecting roadway. In 1995, Xiang (1995) analyzed the calculation formula of impact air volume studied by professor Wang Yingmin of Northeastern University who proposed measures to control impact airflow from the aspects of unloading amount, unloading height, and unloading section, so as to prevent dust from entering the roadway to pollute the working environment. In 2000, according to the theory of dust removal and purification, Wang et al. (2000) raised that a parallel dustproof pressure relief well should be dug near the main ore pass to connect the dustproof pressure relief well with the main ore pass to form a dustproof pressure relief ore pass system to control the dust pollution in the ore pass. In 2008, aiming at the dust production characteristics of the ore-unloading working space in metal mines, Wu (2008) designed a timing spray dust suppression system for the oreunloading working space. In 2013, Yang and LV (2013) proposed a comprehensive dustproof measure for ore pass, namely that a fine water spray system was installed at the joint between the ore unloading chamber and the branch ore pass to reduce dust, and at the same time, a closed chain dustproof baffle was installed at each branch ore pass to seal and block the dust. In 2017, Wang (2017) used numerical simulation to analyze the law of wind pressure at the unloading mouth and dust concentration in the ore pass. According to the law of dust migration, he proposed a gas-water spray dust removal scheme. In 2017, Li et al. (2017) aimed at the problem that the dust production purification effect and purification capacity of high ore pass could not coexist. After studying the influence of liquid droplet inertia on dust deposition, they designed the atomization and dust removal device of high ore pass with large air volume. In 2019, Wang et al. (2019d) analyzed the shielding effect of a high-pressure air curtain on the mine dust carried by the impact airflow of unloading on the basis of CFD-DEM, determined the time when the air curtain blocked the continuous diffusion of dust and proposed a joint dust suppression scheme using high-pressure air curtain shielding and gas water spray.
On the whole, dust pollution control of the multi-middle section high ore pass is categorized into external measures and internal measures. Although the abovementioned dust suppression schemes which belong to the former, such as air curtain and spray, can control dust pollution to a certain extent, there are some limitations in the field application. On the one hand, the nozzles of gas water spray are often blocked because of the impurities in the water supply network (Wei et al. 2020). On the other hand, the back and forth movement of the transport vehicles tends to make the air supply ducts leak . These factors often lead to ineffective solutions for external dust removal and still large amounts of mine dust entering the working environment. This problem can be well solved by adopting foam dust removal technology inside the high ore pass. Foam dust removal refers to mixing foaming agent and aqueous solution according to a certain proportion to form foaming agent solution (Wang et al. 2020a), introducing the foaming agent solution and air into the foaming device to foam, and injecting foam into the dust source through the foam nozzles to realize the suppression and sedimentation of dust (Chen 2018, Weaire andHutzler 1999. The pre-injected foam in the bin at the bottom of the high ore pass enables humidity, interception, and adhesion of the dust carried by the impact airflow, which reduces the amount of dust produced from the source. As for foam dust suppression technology, the research mainly focuses on performance parameter tests of foam formulation and the stability of the foam. Golemanov et al. (2008) focused on the process of bubble breakup in steadily sheared foams at constant shear rate or constant shear stress. Carey and Stubenrauch (2009) studied the foamability, foam stability, and the liquid volume of aqueous foams stabilized by dodecyltrimethylammonium bromide (C 12 TAB). Boos et al. (2013) investigated the influence of surfactant structure and composition on foam volume, liquid fraction, bubble size, and bubble size distribution. Wang et al. (2017a, b) analyzed the effects of foaming agent concentration and temperature on foam stability. Xu et al. (2019Xu et al. ( , 2017 (Petkova et al. 2021). However, the foam formation mechanism and dust reduction mechanism are still unclear, and the measurement index of foam dust removal formulation is also unclear, which tends to cause blindness in foam preparation (Wang et al. 2019b). Simultaneously, foam dust removal technology in underground engineering fields, such as mines and tunnels, often needs to spray large amounts of foam (Wang et al. 2019a;Yan et al. 2020), which easily leads to unnecessarily high costs and seriously hinders the development of foam dust suppression technology.
To sum up, there are quite a lot of research findings on dust pollution control by means of sealing and spraying dust reduction on the wellhead of ore pass, while the studies on dust isolation in the ore bin do not provide a clear and consistent answer. In this paper, the performance measurement index of foam dust removal formulation is clarified from the perspective of formation mechanism of foam and the dust reduction model of foam. Moreover, the feasibility of pre-injected foam dedusting technology in the ore bin was verified by similar experiments on dust reduction in high ore pass. The practical contribution for this study is to address the environmental pollution caused by unloading dust at the source and relieve the pressure of spray dust removal outside the wellhead.

Mechanism of foam formation
Foam is a kind of gas-liquid dispersion medium, in which gas is the dispersed phase (discontinuous phase) and liquid is the dispersion medium (continuous phase). The foam is formed by the accumulation of bubbles on many sides. As shown in Fig. 1, the intersection between bubbles in the foam is defined as the plateau boundary (Wei et al. 2020). Suppose the radius of the bubble is R, the pressure inside the bubble is p A , the pressure outside the bubble is p P , and the surface tension of the vacuolar membrane is σ. As the volume of bubbles increases dV, the surface area increases dA. Without considering other external forces, according to the first law of thermodynamics, there are (Jiang 1990).
If the shape of the foam is spherical, there are According to formula (1) and formula (2), there are It is well known that bubbles are convex, and there will be Δp > 0 . It can be seen from formula (3) that the pressure at point P in the liquid film is less than point A, so the liquid film automatically flows from point A to point P, and the liquid film gradually becomes thinner, which is the process of foam drainage. When the liquid film becomes thinner to a certain extent, it will lead to the rupture of the film and the destruction of the foam. Pure liquid cannot form stable foam, and some surface-active agents (foam stabilizers) are often added to the foam formulation, which can reduce the surface tension σ of the liquid film to relieve the time of foam drainage and make the generated foam more durable.

Mathematical model of foam coalescence
Foams are different in size at the beginning. More importantly, at this time, the foam is in a state of high surface free energy and extremely unstable, and there will be coalescence between foams. The process of coalescence of large foam into small foam will reduce the surface-free energy of foam, thus forming a more stable foam. For the convenience of research, two adjacent foams are taken as research objects, and a foam coalescence model is established, as shown in Fig. 2. The assumptions about the model are made as below (Ren 2009, Ren 2013): 1. The two foam contact surfaces are parallel flat plates, and gravity is not considered. 2. The liquid forming the liquid film is axisymmetric when flowing. 3. Ignore the other movements of the liquid forming the liquid film, only consider the radial movement, and the interface flows completely. 4. The flow field distribution of the fluid in the membrane is flat. 5. The fluid is a Newtonian fluid, which is incompressible and its viscosity value is certain. 6. The liquid film thinning rate is independent of radius r, and the liquid film pressure change is independent of z.
Therefore, when the two foams coalesce, the differential equation when only radial momentum is considered is where ρ represents the mixed-phase density; t represents time; μ r and μ θ represent the velocity components in the spherical coordinate system; τ rθ , τ θθ , and τ rz represent the stress tensor components in the spherical coordinate system. According to hypothesis 2 and hypothesis 4, formula (4) can be simplified as follows: For the flow field distribution of a flat plate, the influence of viscous force can be neglected, so the formula (5) can be further simplified as The formula (6) is integrated along with the film thickness (z-direction), so there is where, Because the liquid film is a parallel plate, the continuity equation of the two foam coalescence processes is Substituting formula (9) into formula (6), there is Simplifying formula (10) and integrating it along the radial direction, and there is Therefore, by combining formula (3), the kinetic equation of spherical foam coalescence can be obtained as follows: As can be seen from formula (12), at the initial stage, the gas is dispersed in the liquid to form foams of different sizes. Since the newly formative foam is in a state of high surface free energy, the whole system is still in an unbalanced stage, at which time the foam will coalesce spontaneously. Large foams will amalgamate small foams, i.e., unstable foams consolidate with each other. In this way, the foam system reduces the surface free energy as much as possible to reach a relatively balanced state system and form a relatively stable foam.

Mathematical model of foam dust suppression mechanism
In the process of dust production during ore unloading from the multi-middle section high ore pass, one part of the dust directly enters the contact lane from the pass shaft, and the other part enters the ore bin with the ore. Pre-injection foam dedusting technology in the ore bin refers to pre-injecting foam generated by a foam generator into the ore bin before ore unloading in high ore pass, so as to wet, cover, and block the floating dust. Compared with spray dust suppression, the foam has favorable isolation, adhesion, and wettability (Chen et al. 2015;Lu et al. 2017), and has stronger wetting and covering effect on mine dust (Chen 2018). The theory of foam dust removal was analyzed from three aspects of foam wetting, interception and covering, and adhesion, by which the performance parameters of the optimal foam formula are determined.
The mechanism of foam wetting The process of dust wetting can be considered as the conversion of dust particles in contact with air to dust particles in contact with air and liquid at the same time (Chen et al. 2008). As shown in Fig. 3, the wetting of the dust by the foam is mainly manifested by the liquid film of the foam covering the surface of the dust particles evenly, when the foam is in contact with the surface of the dust particles. According to the Young equation, the wetting of the foam (Yang 2012) can be expressed by the contact angle θ in formula (13).
Whether the process can proceed spontaneously depends mainly on the difference value of free energy when the contact between mineral dust and the air is converted into the contact between mineral dust and liquid (Churaev 1995): where pl represents free energy of solid and liquid interface per unit area, pg represents free energy of solid and gas interface per unit area, and lg represents free energy of liquid and gas interface per unit area. The adhesive work W a is defined as the work done to pull apart the liquid-solid interface per unit cross-sectional area. The larger the value of W a , the stronger the adhesive properties of the liquid.
Substituting formula (13) into formula (14), and there is For the dust produced by ore unloading in a high ore pass, the free energy of the solid and gas interface pg of the dust particles is a fixed value. As can be seen from formula (15), there is a negative correlation between adhesive work W a and pl . The main reason for hydrophobic mineral dust is that the surface tension pl of mineral dust particles is too large when they contact with water. The foaming agent in the foam solution contains both hydrophilic and hydrophobic groups, which can make the mineral dust form a hydration film when it comes into contact with the solution. The hydrated film can effectively reduce the surface tension pl and contact angle θ, and quickly change the hydrophobicity of mineral dust. The macroscopic performance is that the dust is wet and finally settles down. Therefore, foaming materials with high wettability can reduce the contact angle with mineral dust particles and improve the dust-catching effect of foam.

The mechanism of foam interception and covering
Dust particles move through the air. When the wind flow carrying dust passes through the foam, the obstructing wind flow effect of the foam causes a winding motion of dust particles within the local area of the foam. As shown in Fig. 4, when dust particles flow through the foam, and the minimum distance between the particle trajectory and the foam surface is equal to the particle radius r p , the dust has the possibility of being caught by the foam, that is to say, the interception of the foam (Huang et al. 2008). When the dust moves in a straight line with the airflow, it may be intercepted by the foam if the dust trajectory is within the critical trajectory.
Assuming that the foam is an ideal sphere with a diameter of d f = 2a and that the airflow around the sphere is a potential flow, then the flow function expressed in spherical coordinates is (Huang et al. 2008) The velocity component can be obtained as follows: On the surface of the ball, there are u R = 0 and u = − 3 2 v 0 sin , Under the condition of potential flow, the interception efficiency around the sphere is The diameter of dust particles in the pass is much smaller than the diameter of foam, so it satisfies the formula (20). On the macro level, the interception effect is manifested as the coverage performance of foam dust removal. When n foams at a distance of h < a act on the dust source at the same time, it will be possible to intercept dust within 4nh range of the foam, as shown in Fig. 5. When the floating dust in the ore chamber of the ore pass is covered by a large amount of foam, the dust is mostly trapped.

The mechanism of foam adhesion
When relative motion is existing between the foam and dust at a certain speed, the dust is captured by the foam after the collision. Due to the gradual increase in mass after the accumulation of foam and dust, the liquid film on the upper surface of the foam gradually thins until it bursts due to gravity, and many small pieces of foam coated with dust fall to the ground (Wang et al. 2019c), as shown in Fig. 6.
The effect of adhesion can be expressed by adhesive force: Considering that the adhesive force of foam is influenced by the physical and chemical properties, viscosity, humidity, PH value, and the nature of dust itself of each component in the foaming agent solution, it can be expressed by experience as where F a represents the adhesive force, k represents the adhesion coefficient of foam surface, d p represents particle diameter (μm), and RH represents the relative humidity of foam (%).
The foam dust removal process is usually not the result of a single effect, and each effect plays a role, but the whole process is generally dominated by two or more effects. Foams can be seen as composed of single foam n c , so total dust removal efficiency η is the sum of a single dust removal efficiency. When the dust removal efficiency of single foam remains unchanged, the total dust removal efficiency of foam increases with the number of foams. It is the premise of a good dust removal effect to produce a large amount of foam that is durable and has the function of wetting and collecting dust. When the foams are pre-injected in a semi-confined ore pass, there will always be foam wetting, foam interception, foam covering, and foam adhesion. Therefore, the best foaming formula can be measured and optimized from the two characteristics of foaming property and the stability of the foam. At the same time, the feasibility of pre-foaming technology in the ore bin has been verified by designing dust reduction tests in the ore pass.

Establishment of experimental platform for dust removal of ore pass
Based on the similarity criterion of the model of the main pass in an iron ore mine in Anhui province, the similar model of ore pass is established on the foundation of the ratio of the size of the similar model to the actual size of 1:25 (Wang et al. 2020a). The size of the established similar model: the height of the ore pass is 3.6 m, the height of each contract roadway is 0.80 m, the diameter of the ore pass is 0.14 m, the diameter of the ore bin is 0.20 m, the diameter of the slant ore pass is 0.12 m (the angle between the slant ore pass and the ore pass is 35°), the length of the contact roadway is 0.60 m, the width of the contact roadway is 0.16 m, and the height of the contact roadway is 0.18 m. The testing equipment of the unloading experiment platform includes a microcomputer laser dust monitor (LD-5C) and multi-parameter wind speed monitor (VC-9565), which can realize real-time monitoring and recording of dust concentration and airflow changes at the wellhead during the unloading process. The established experimental platform for ore discharge through the pass is shown in Fig. 7. The dust concentration variation monitoring points are located at the mouth of each midsection ore pass. After the experimental ore discharge meets the similarity criteria, the single ore discharge is 1.28 kg and the discharge flow is 0.64 kg/s.

The experimental device and material for foam generation
Waring Blender method was used to determine the foaming formula. The experimental instruments used in the stirring method included digital constant temperature water bath (HH-2), touch screen wet coagulation test agitator (MY3000), electronic balance, measuring cylinder, and beaker. As an auxiliary device for dust removal experiments, foam equipment provides foam for ore bunker (Jiang et al.  Fig. 8. The necessary preparation materials of foam can be divided into two categories according to their properties: foaming agent and stabilizing agent. The agent material selected should meet the following conditions. Firstly, the agents solution can adsorb at the gas-liquid and gas-solid interfaces, which can significantly reduce the indicated tension of the solution. Secondly, the pharmaceutical material can change the hydrophobicity of the mineral dust particles and reduce the contact angle with the mineral dust particles. Finally, the agents will not affect the subsequent beneficiation and smelting of metal mines. More importantly, the agents are cost-effective, environment-friendly, degradable, and widely available. On the basis of the existing research (Ni et al. 2019;Xie et al. 2020), five foaming agents (sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS), sodium alpha-olefin sulfonate (AOS), dodecyl dimethyl betaine (BS-12), sodium oxyethylene ether sodium sulfate (AES)) and three foam stabilizers (anionic polyacrylamide (PAM), sodium carboxymethyl cellulose CMC-Na, and coconut oil monoethanolamide (CMEA)) were selected. In summary, the material details are provided in Table 1.

Single-foaming agent foaming experiments and optimization
The single-agent foaming experiments of the five foaming agents were carried out by using Waring Blender method (Chen et al. 2015). Firstly, according to the characteristics of each foaming agent, 100 ml of foaming agent solutions with different mass concentrations was prepared. Secondly, stir at the speed of 1000 r/min for 1 min, immediately pour the foam into the measuring cylinder after stopping the rotation, and obtain the initial foam volume. Then, the foam volume after 5 min was recorded. Three sets of experiments were repeated for each blowing agent, and an average of the foam volumes was calculated.
The results of the single-agent foaming experiment are shown in Fig. 9. In the figures, the foaming property of the foaming agent is characterized by the initial foaming volume, and the stability of the foaming agent is characterized by the average rate of foam disappearance in 5 min. It could be seen from the standard deviation in Fig. 9 that the dispersion degree of experimental data was small, and the standard deviation of foam volume of initial and foam volume after 5 min was basically within 10%, so the obtained data were considered to be stable and reliable.  As shown in Fig. 9, with the increase of the mass concentration of the solution, the initial foam volume of a single blowing agent firstly increases and then decreases slowly. The average rate of foam disappearance of different foaming agents varies, as well. From the theoretical analysis of adsorption (Wang et al. 2019c), it can be seen that when the concentration of foaming agents is low, the surface adsorption is sparse. With the increase of the mass concentration Fig. 9 The foaming volume of single substance of the foaming agent, the surface adsorption becomes denser and denser, the surface tension decreases, and the foaming volume increases rapidly. When the concentration increased to the critical concentration of the beam glue (Williams et al. 1955), the surface adsorption reached saturation, and the increase of the concentration did not increase the surface adsorption amount.
Overall, on the one hand, while the average rate of foam disappearance of AOS at a concentration of 0.5% was the lowest at 5.34 ml/min, the foaming property of each concentration was not ideal, and the initial foaming volume was less than 510 ml; on the other hand, the foaming volume of BS-12 was much smaller than the other four reagents, and the average defoaming rate was greater than 40 ml/min. As a result, according to the initial foaming volume and the average rate of foam disappearance, the foaming agents with favorable foaming performance were SDS, SDBS, and AES, and the optimal mass concentration was 0.5%, 0.3%, and 0.5%, respectively.

Experiment of compounding foaming agent monomer
Pairwise compounding experiments of the three preferred blowing agents were performed to determine the effect of a component on the foaming properties. The mass concentration of the compounding agent was, respectively, set to be 0.3, 0.5, 1.0, 1.5, and 2.0 times of the agent to be compounded. The Waring Blender stirring method was used to repeat the experiments in three groups and calculate the average volume of the foam. The results of the foaming agent compounding experiment are shown in Fig. 10.
It can be observed from Fig. 10 that.
1. With the increase of c (SDS) /c (SDBS) , the initial foam volume shows a trend of sharp increase and then slowly decrease. This was because both SDS and SDBS were anionic forming agents (Wang et al. 2017d). When the concentration of their combination was lower, the active groups interacted to reduce the surface tension Fig. 10 Experimental results of blowing agent compounding and increase the foaming volume. However, as the concentration increases, the synergistic effect reaches equilibrium (Wei et al. 2020), the surface tension no longer decreases and the foaming volume reaches a maximum. As the concentration increases further, the foam volume decreases due to the inhibitory effect of too many free radicals on the foam. 2. With the increase of c (AES) /c (SDBS) , the initial foam volume shows a trend of rapid increase at first and then tends to be stable. The adsorption layer produced by AES was repelled by the same charge, so the molecules were not arranged closely enough and the cross-sectional area of the molecules was large. After the addition of SDBS, due to the hydrophobic effect and the generated dipole-ion interaction (Wang et al. 2019a), it is easy to be inserted into the loose adsorption layer, reducing the repulsion of the same charge, and increasing the density of the hydrophobic chain, so the molecules are arranged more closely, resulting in the reduction of the surface tension and the increase of the foaming volume. When the concentration went up to a definite value, the synergistic effect no longer produced effects and the foam volume also tended to be stable. 3. The significance value of the initial foam volume to the compounding concentration ratio of c (AES) /c (SDS) was P = 0.496 > 0.05, indicating that there was no significant relationship difference between them. In addition, when the compounding concentration ratio was 1:1, i.e., the mass concentration of both AES and SDS was 0.5%. There was the maximum initial foaming volume of 547 ml, and the foam volume after 5 min was 492 ml, with an average defoaming rate of 11.2 ml/ min.
When c (SDS) /c (SDBS) = 0.5, the initial foam volume of this group of compound experiments is much higher than that of other groups of compound experiments, and the foaming effect is also higher than that of a single foaming agent. To determine the optimal content of the composite foaming agent of SDS-SDBS, in view of the condition that c (SDS) /c (SDBS) = 0.5, the experiments were conducted with the mass concentration of the composite foaming agent set at 0.5 ~ 3%, and the concentration interval at 0.5%, respectively. The experimental results are shown in Fig. 11. It could be seen from the figure that the initial foam volume firstly increased sharply and then basically stabilized with the increase of the concentration of the compounded foaming agent. To ensure the optimal foaming effect and economy, the optimal mass concentration of the SDS-SDBS compound foaming agent was 1.5%, under which c (SDS) was 0.5% and c (SDBS) was 1%.

Experiment of compound foaming agent and foam stabilizer
When the mass concentration of the SDS-SDBS combined blowing agent was 1.5%, the optimal initial foam volume was 622 ml, but the average defoaming rate was as high as 19.72 ml/ min within 5 min, and the stability was weaker than that of the single foaming agent. Therefore, for the purpose of improving the stability of the compounded foaming agent, three foam stabilizers, namely, PAM, CMC-Na, and CMEA, were added for the optimization experiment. Experiments were performed by setting the mass concentration of foam stabilizer to be 0 ~ 1%, with the concentration interval of 0.2% and the average volume of foam disappearance during 5 min as shown in Fig. 12. The foam-stabilizing effect of CMEA is better. With the increase of CMEA content, the foam-stabilizing effect firstly increases and then decreases, and the optimal mass concentration range is 0.2 ~ 0.4%. Therefore, the optimal foaming scheme was determined through the performance test of foaming agent formulation: c (SDS) = 0.5%, c (SDBS) = 1%, c (CMEA) = 0.2 ~ 0.4%.

Measurement experiment of mineral dust discharge in ore pass
The ore enters the transportation system from the slip well to form an ore flow, which falls freely by gravity and is accompanied by a large amount of dust. Through on-site monitoring and similar experimental analysis, it can be known that the dust in the ore pass mainly gushed out from the third and fourth levels when the first middle level was unloaded. The unloading of mineral dust in the pass shaft is mainly reflected in two aspects: the dust carried by the ore is generated by the sheer airflow during the falling process, and the dust attached to the inner wall of the pass shaft is secondary dust generated by the impact of the ore and the airflow. The pre-injection of foam in the ore bin mainly controls the floating dust in the ore bin, so the ore unloading experiment is conducted to determine the dust yield of the floating dust.
In order to distinguish the percentages of different dust sources, the controlled variable method was used to conduct three groups of controlled experiments (α, β, γ) with different ore unloading conditions on the basis of unchanged ore unloading amount and ore unloading flow. The settings of the experimental groups were as follows: experiment α was used as the control group of the other two groups for normal ore unloading; experiment β laid flexible materials at the bottom of the ore bunker to absorb the falling impact energy of ore and prevent the dust from the recoil flow in the ore bunker; experiment γ is the influence of another ore unloading experiment on the dust deposit in the ore bunker after two consecutive ore unloading. The settings of the three experiments and the monitoring of dust concentration are shown in Fig. 13.
The changes in dust concentration in the three groups of control experiments were measured by the dust concentration monitor. The dust concentration under different conditions is shown in Fig. 14. It can be realized from Fig. 14 that the amount of dust released from the shear gas during ore falling is greater than the amount of dust released from the re-collision when the ore falls into the ore bin. The amount of dust produced during ore unloading accounts for 78% of the total amount of dust produced, and the amount of dust produced after the ore falls into the ore bin accounts for 22%. When there is floating dust in the ore bin, the dust production in the ore pass increases obviously after ore unloading again, and the dust production in the ore bin increases by 39% compared Fig. 13 Experimental settings and analysis of dust production in the ore pass with that when there is no floating dust. The use of foam to inhibit the floating dust at the bottom of the ore bin can play a role in reducing the overall production of dust.

Experimental analysis of foam dust removal
Through the control variable method for the unloading experiment, we can know that the dust output of floating dust at the bottom of the warehouse reaches 39%, so it is significant to adopt the pre-injection foam to control the dust in the warehouse. In order to improve the foaming effect and boost the dust removal rate, based on the above formula, the foaming experiment was carried out by using the foam-collecting system in the ore pass. When the foaming equipment is unchanged, the change in gas-water flow has a great influence on the foaming quantity.
In the interest of studying the relationship between foaming quantity and gas-water flow ratio, the control variable method is adopted to select the gas flow as 10, 15, 20, 25, and 30 m 3 /h and the liquid flow as 4, 8, 12, 16, and 20 l/min on the basis of setting the pressure empirical value as 0.7 MPa for monitoring the foaming flow. The relationship between foaming flow and gas-water flow is shown in Fig. 15. When the gas flow reached 30 m 3 /h, the foaming speed increased first and then stabilized with the increase of water flow. When the pressure is constant, the foaming rate decreases with the increase of the gas-water ratio. When the water pressure is constant, the foaming rate increases with the increase of the gas-water ratio. According to data analysis, it is found that when the gas flow is 30 m 3 /h, the liquid flow is 18 l/min, the optimal gas-water ratio is 27.8, and the foaming agent has the best foaming effect.
Based on the optimal foaming gas-water ratio of 27.8 obtained from the above study, the dust control effect was studied by injecting foam of different heights (the height of foam in the ore bunker is 0 cm, 5 cm, 10 cm, 15 cm, and 20 cm) into the ore pass. As the dust returning from the ore bin is mainly reflected in the third and fourth levels of the ore pass during ore unloading at the first middle level, only the dust changes at the third and fourth levels are monitored. The influence of foam of different heights on dust concentration change in the third and fourth middle sections after unloading is shown in Fig. 16.
The ore unloading experiment was carried out after filling different heights of foam in the ore bin of the chute. And the analysis of experimental data shows that with the increase of the foam height in the ore pass bin, the concentration of dust flushed out from the ore chute mouth decreases gradually. From the histogram of dust control efficiency at the wellhead of the third and fourth levels in Fig. 16, it can be shown that the total dust reduction rate of foam in the ore bin for the fourth middle section is 60%, and that for the third middle section is 28%. Among them, the dust reduction effect of respirable dust is lower than that of total dust. The pre-injection of foam in the ore bin controls the dust return amount in the ore bin, weakens the total amount of dust entering the third and fourth levels of the ore pass, and relieves the pressure of spray dust reduction at the ore pass mouth to a greater extent.