Research and Experiment on the Removal Mechanism of Light Impurities of the Residual Mulch Film Recovery Machine

Abstract

Aiming at the problem of high impurity rate in the recycled residual film, combined with the existing installation (4JMLE-210 agricultural residual film recycling machine), the removal mechanism of light impurities on the film surface was analyzed. The statics and kinematics analysis of light impurity particles in different spatial positions were carried out to determine the conditions for the movement of impurity particles. By analyzing critical conditions, such as ideal collision and throwing capacity, the structural dimensions of the straight pipe section and its outlet section were determined. Using Origin 2018 software, the movement track of the impurity particles left from the upper and lower limit positions and the ideal curve of the throwing arc were plotted, and the trapezoidal section was determined at the outlet of the throwing arc section. Finally, trial-produce prototype, and a field test was carried out on the performance of the machine by selecting the impurity rate in the recovered residual film as the test index. The results showed that when the forward speed of the machine and the rotating speed of the cutter roll were in the range of 5.4–5.8 km/h and 1440–1460 r·min⁻¹, the light impurity rate and working efficiency could keep a good balance. The light impurity rate in the recovered residual film was between 10.9% and 31.4%, and the average light impurity rate was around 18.7%, which met the design and application requirements.

1. Introduction

Plastic film mulching is an important agricultural production technology that has the functions of maintaining fertilizer, increasing temperature, and reducing crop diseases [1,2]. It was introduced into China in the late 1970s, mainly applied in Xinjiang, Gansu, and other arid areas in the north [3]. After decades of theoretical research and production practice, plastic mulch has developed into an important means of production in modern agriculture, making important contributions to the improvement of crop yield and economic benefits [4]. However, plastic mulch films are most commonly made of low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE), materials which do not readily degrade in soil [5]. Due to long-term film-mulched planting, the problem of residual film pollution in farmland has become increasingly prominent, and there is a strong dependence on it, which has further affected farmland cultivation and agricultural sustainable development [6,7,8]. In recent years, scholars in the domestic and overseas have done a lot of research on Residual Film Recovery Machine (RFRM) technology. Since the thickness plastic film is greater than 0.020 mm and the film has strong damage resistance in the foreign countries [9], the structure of foreign RFRM is relatively simple [10,11,12,13], and includes the following products: PR2500 Double Reel Plastic Mulch Lifter-Wrapper, PMR-01 Plastic Mulch Retriever, PS614 Two-Row Plastic Mulch Retriever, Challenger Model, 1800 Plastic Mulch Lifter, etc. The better-quality mulch film remains mostly intact after use, is mainly collected easily, and nearly no residual plastic film mulch is left in farmland soil after mechanized recovery. In contrast, the plastic film thickness used in China is generally less than 0.008 mm [14], and the structure of domestic residual film recycling machine is relatively complex. Furthermore, the research focus of domestic scholars mainly focuses on “how to pick up the residual film [15,16,17]”, “how to demount the residual film [18,19]”, “how to convey the residual film [20,21,22]”, etc. Above all, mechanized recovery of mulching film is the most direct and effective means for solving the current problem of residual film [23,24]. However, it ignores the reuse of the residual film after recycling, leading to a high impurity rate in the collected residual film. It is not conducive to the reuse of residual film if the impurity rate in the recovered residual film is too high. Thus, impurity removal is the key technology that must be solved by mechanized recovery of residual film.

The research on residual film–impurity separation (RFIS) mainly focuses on the process of recycling the residual film [25], the residual film–impurity mixture (RFIM) after recycling, and the research on the mechanism of impurity removal mainly focuses on the latter. Here, the impurities in RFIM mainly refer to light impurities. Shi et al. [26] realized RFIS by utilizing the difference in suspending velocity and settlement law of different materials in an air medium, and thus the drum–sieve-typed air separation device for RFIS was designed. Kang et al. [27] analyzed the movement law of RFIM in the drum sieve. Peng et al. [28] studied RFIS theory based on the existing drum–sieve-typed air separation device. Based on the analysis of material properties and water washing separation theory, He et al. [29] proposed a solution to achieve RFIS using liquid phase water as a medium. Li et al. [30] proposed a washing separation and cleaning method based on impeller drive and jet impact, and designed and developed a washing separation device for RFIS, etc. However, RFIS after recycling increases the process, prolongs the treatment period of residual film recovery and utilization (RFRU), reduces the work efficiency in the later stage of RFRU, and increases the cost of RFRU to a certain extent.

In this paper, from the point of view of a straw-crushing returning device, the mechanism of RFIS on the residual film surface before recycling the residual film was studied. Due to the removal of light impurities on the residual film surface, this process involved a front-mounted straw-crushing, throwing, and returning device, which was one part of the 4JMLE-210 agricultural residual film recycling machine. That is, the RFIS was realized at the same time as straw crushing. The static and kinematic analysis of light impurity particles in different space positions were carried out, and the conditions of impurity particles’ movement were determined. Meanwhile, theoretical analysis was carried out on the throwing shell to determine its specific design parameters. Finally, the performance test of removing light impurities on the film surface in the field was carried out.

2. Materials and Methods

2.1. 4JMLE-210 Agricultural Residual Film Recycling Machine

The 4JMLE-210 agricultural residual film recycling machine was mainly composed of a straw-crushing and throwing unit, a straw-conveying unit, a residual film winding-up unit, a residual film picking-up unit, etc., as shown in Figure 1. Before the recovery of the residual film, straw should be crushed and thrown out, and the light impurities on the film surface should be removed to create good conditions for the subsequent recovery of residual film. More studies have been conducted on straw crushing and the recovery of residual film, but there are few reports on the removal of light impurities. Therefore, this study concentrated on “how to remove the light impurities on the surface of the film while straw crushing”.

2.2. Light Impurity Removal Device

In this paper, the removal of light impurities on the residual film surface involves a front-mounted straw-crushing, throwing, and returning device, which was mainly composed of a drive shaft, belt drive system, tensioning mechanism, throwing shell, post-pressing film roller, moving knife set, front pressing film roller, fixed knife, and crushing shell, as shown in Figure 2.

2.3. Working Principle of the Device and Removal Process of Light Impurities

When working, the tractor is powered through the gear box, the belt transmission drives the knife roller rotation, and the cut straw enters the crushing chamber under the action of moving knife combination, where it is crushed into small segments or fibers under the comprehensive action of cutting, tearing, and rubbing with the combination of the moving fixed knife. At the same time, the high-speed rotating moving knife combination works on the gas below the crushing chamber, so that the air velocity is increased, the pressure is decreased, and the negative pressure area is formed, which could suck up the light impurities on the film surface. This happens together with the crushed straw under the joint action of airflow and centrifugal force evenly spread in the rear of the machine.

The process of removing light impurities could be regarded as a pneumatic conveying process. This process consists of three parts, as shown in Figure 3, which involved the upward movement changing from position 1 to position 2, after which the light impurities left the film surface before entering the crushed shell. Then, the circular movement changed from position 3 to position 4 after the light impurities entered the crushed shell, and the stant cast movement after the light impurities were removed changed to position 5, then entered the throwing shell. Finally, impurities were thrown out of the outlet along with the airflow.

3. Kinetic Analysis of Light Impurity

The removal of light impurities belonged to the “gas–solid” two-phase flow problem. Air belonged to the continuous phase, while light impurities, such as side branch, cotton, residual leaf, and cotton shell, belonged to the granular phase [31]. The simplest model of “gas–solid” two-phase flow was the single-particle dynamics model, and the results obtained from the analysis of single-particle dynamics could be regarded as the basic phenomena in the actual “gas–solid” two-phase flow [32].

3.1. Statics Analysis of Impurity Particle in Air Flow

There were many kinds of forces on impurity particles in the airflow [33], but the main factors that had a significant influence on the particle movement were gravity F_g, buoyancy F_f, and airflow drag force F_D. [34]. False mass effect (force), Saffman force, Magnus force, etc., which had little influence on particle movement, were ignored [35]. In the force analysis, only F_g, F_f, and F_D were analyzed. In order to study motion, the size of irregular impurity particles was expressed by equivalent diameter in this paper. The following relationship existed [36,37]:

$$\{\begin{array}{l}{F}_g=m_p\cdot g=\frac{1}{6}\cdot \pi \cdot d_p^3\cdot {\rho }_p\cdot g\\ F_f=\frac{1}{6}\cdot \pi \cdot d_p^3\cdot {\rho }_a\cdot g\\ F_D=\frac{1}{8}\cdot \pi \cdot C_D\cdot d_p^2\cdot {\rho }_a\cdot {\left(v_a-v_p\right)}^2\end{array}$$

(1)

where m_p was the equivalent mass of impurity particles per kg; ρ_p was the density of impurity particles, kg/m³; d_p was equivalent diameter of impurity particle, m; ρ_a was the density of air, kg/m³ (standard conditions, air density was about 1.29 kg/m³); v_a was the airflow velocity, m/s; v_p was the velocity of impurity particles, m/s; C_D was a dimensionless coefficient which depended on the shape of impurity, orientation in the flow. and Reynolds number, Re. After cotton harvest, the density of light impurities on the film surface was measured. The densities of the side branch, cotton shell, cotton, and cotton leaf were 282.23 kg/m³, 458.57 kg/m³, 337.98 kg/m³, and 410.4 kg/m³, respectively. Obviously, the density of light impurity ρ_p was much higher than the density of air ρ_a, and F_g was greater than F_f. Thus, the buoyant force F_f can be ignored.

In addition, according to Equation (1), it was not difficult to find that the drag force F_D occurred only when ∆v ≠ 0 (∆v = v_a − v_p), that is, when the movement velocity of the impurity particle was the same as the airflow velocity, the drag force F_D disappeared. Assuming that the shape of the impurity particles did not change, F_D only related to (v_a − v_p)². Furthermore, the aerodynamic characteristics of impurity particles could be expressed by the numerical value of their suspension velocity [37]. According to the suspension velocity v_f, (m/s) referred to the air velocity of an impurity particle under the action of vertical air flow when the force of air flow on the impurity particle was equal to its own gravity and remained in balance [38], namely:

$$F_D=\frac{m_p\cdot g}{v_f^2}\cdot {\left(v_a-v_p\right)}^2=\frac{F_g}{v_f^2}\cdot {\left(v_a-v_p\right)}^2$$

(2)

3.2. Kinematic Analysis of Light Impurity Particle in Air Flow

3.2.1. Analysis of Upward Movement of Light Impurity Particle

Assuming that the machine remained static and the impurity particle on the film surface moved towards the machine at a certain velocity v₀, the initial velocity v_p of the impurity particle was equal to v₀ and opposite to v_a. Due to the complexity of the flow field formed by the high-speed rotating knife roller, the force properties of the impurity particle in the x-axis and z-axis directions were the same, and they were only affected by the drag force of the air. In order to facilitate the study of the motion characteristics of the impurity particle, the drag force in the z-axis direction was ignored, and the impurity particle’s motion was approximate to the two-dimensional motion in the xy plane. The mechanical analysis of light impurity on the film surface is shown in Figure 4. The impurity particle was in the high-speed flow field area, and was subjected to the drag force F_D, and the included angle between the airflow direction and the horizontal direction was α, and also subjected to its own gravity F_g and supporting force F_N, and the airflow velocity direction was the same as the drag force F_D.

Then, the drag force F_D was decomposed, and the Y-axis component was F_D in α, and the X-axis component was F_D·cosα. The conditions to be met when the impurity particle left from the ground (F_N = 0, f = 0) was:

$$\{\begin{array}{l}{F}_D\cdot \cos \alpha \ge f=\mu \cdot \left(F_g-F_D\cdot \sin \alpha \right)\\ \\ F_D\cdot \sin \alpha \ge F_g\end{array}$$

(3)

Substituting Equations (1) and (2) into Equation (3) above, the following could be obtained:

$$\{\begin{array}{l}\left(v_a+v_0\right)\cdot \sqrt{\frac{\cos \alpha }{\mu }+\sin \alpha }\ge v_f\\ \left(v_a+v_0\right)\cdot \sqrt{\sin \alpha }\ge v_f\end{array}$$

(4)

According to Newton’s second law:

$$\{\begin{array}{l}{m}_p\cdot {\alpha }_{xi}=F_D\cdot \cos {\alpha }_i\\ \\ m_p\cdot {\alpha }_{yi}=F_D\cdot \sin {\alpha }_i-F_g\end{array}$$

(5)

Then, substituted Equation (2) into Equation (5), and the following can be obtained:

$$\{\begin{array}{l}{a}_{xi}=\frac{g}{v_f^2}\cdot {\left(v_{ai}-v_{pi}\right)}^2\cdot \cos {\alpha }_i\\ a_{yi}=\frac{g}{v_f^2}\cdot {\left(v_{ai}-v_{pi}\right)}^2\cdot \sin {\alpha }_i-g\end{array}$$

(6)

In the formula, i refers to different positions, a_xi and a_yi are the acceleration of impurity particle in the x and y axes in the airflow respectively, m/s²; and v_pi and v_ai are impurity particle velocity and airflow velocity respectively, m/s. α_i was the included angle between the airflow velocity direction and the negative direction of x-axis°. According to Equation (6), it was not difficult to find that the impurity particle moved in a variable acceleration curve with decreasing acceleration in the airflow until the velocity of the impurity particle was the same as that of the airflow, and the drag force F_D disappeared. At this time, the particle was only affected by gravity, and the particle decelerated. When the particle velocity and airflow velocity were not equal, the drag force of airflow on the particle was generated, and the particle accelerated. Therefore, the movement of particle in the airflow was a complex process of accelerating, decelerating, and accelerating again.

3.2.2. Kinematic Analysis of Impurity Particle after Entering Crushing Chamber

When the impurity particle entered the crushing chamber, it would collide with the wall and move in a circle under the action of centrifugal force. The centrifugal movement of impurity particle was analyzed, and the present typical separation theory of rotation was adopted. The assumptions were as follows [39]:

(1) The airflow was passing through the curved wall of the crushing shell, and the tangential velocity of each point was constant;

(2) The interaction between particles was not considered;

(3) The impact of crushing and throwing shells was not considered.

The force analysis of when the impurity particle entered the position 3 of the crushed shell is shown in Figure 5.

Furthermore, they were mainly affected by gravity F_g, drag force F_D, and centrifugal force F_r. The direction of drag force F_D was perpendicular to the direction of centrifugal force. When there was no relative movement between impurity particle and air flow, the drag force F_D would disappear. The impurity particle was only affected by gravity F_g and centrifugal force F_r, which could be expressed as:

$$F_r=m_p\cdot \frac{v_p^2}{R_p}$$

(7)

According to the preliminary design, the cutting speed v ≥ 48 m/s should be made to meet the requirements of unsupported cutting [40]. When R_p = R = 265 mm and v_p = 48 m/s, substitute into Equation (7) above, the following exists:

$$F_r=m_p\cdot \frac{v_p^2}{R_p}>>F_g=m_p\cdot g$$

(8)

Therefore, gravity F_g could be ignored. In this process, centrifugal force F_r played a key role, and the impurity particle would move rapidly along the inner wall of the shell.

When the impurity particle moved to position 5, gravity F_g and drag force F_D could also be ignored. Under the action of centrifugal force F_r, the particle moved into the throwing shell, and the speed of the airflow started to decrease. Due to inertia, the particle velocity was greater than the airflow velocity, so the resulting drag impeded the particle’s motion. The impurity particle entered the throwing shell with a high kinetic energy, and then violently reacted with friction with the air. In a short period of time, the particle moved in a variable deceleration curve with decreasing acceleration, and decelerated to the air speed at a very short distance [41,42]. The drag force disappeared, and the particle was subjected only to gravity, which performed an oblique projectile motion. Finally, the particle was thrown out of the outlet along with the airflow.

4. Design and Parameter Determination of Throwing Shell

The throwing shell was mainly composed of a straight pipe section and an arc section. The main function of the straight pipe section was to change the airflow speed by changing the size of the outlet section, and the main function of the arc section was to change the airflow direction.

4.1. Design of Straight Pipe Section

The straight pipe section was directly connected with the crushing shell, as shown in Figure 6. The inlet size, l_AB, of the straight pipe section was the operating width (2100 mm) of the device. When the impurity particle had a certain velocity, it was inevitable that it would collide with the straight pipe section. Assuming that the impurity particle had an ideal collision with point B at a certain velocity, v_t at a certain velocity was thrown out from point D at the outlet of the pipeline, ignoring the influence of the flow field on the straw temporarily. It was assumed that the angle between the impurity particle velocity v₀ and the pipe edge line BC was θ, FB⊥BC, HB⊥AB, DE⊥AB. Then, the following relationship existed:

$$\{\begin{array}{l}\angle CBH=\angle CBD=\angle ADE=\theta \\ \angle EDB=\angle DBH=2\theta \\ l_{AE}=l_{DE}\cdot \tan \theta \\ l_{BE}=l_{DE}\cdot \tan 2\theta \\ l_{DE}=\frac{l_{AB}}{\tan \theta +\tan 2\theta }\\ l_{DC}=l_{AB}-2l_{AE}\end{array}$$

(9)

According to Equation (9) above, when θ = 30°, l_DE had a critical value of 909.3 mm. In this paper, integer l_DE was designed as 910 mm. In order to meet the requirement that impurity particles were thrown from point D, the minimum value of l_DC at this time was 1050 mm. If the outlet section size was too small, under the same production conditions, the friction loss between the impurity particles would increase, and the friction loss between the impurity and the inner wall of the shell would also increase, which would thereby increase the energy loss of unit mass impurity. The filling coefficient K was used to represent the filling of impurities as thrown [40]:

$$K=\frac{Q}{{\rho }_z{v}_{p}A}$$

(10)

In the formula, Q was the mass of impurity particles discharged at the outlet of section per unit time, kg/s; A was the outlet section area, m²; ρ_z was the impurity particle density, kg/m³; and v_p was the average velocity of impurity particles passing through the outlet section, m/s. The value of K ranged from 0.02 to 0.09. On the premise of satisfying the throwing speed, the impurity particles could be thrown smoothly, taking K_min = 0.02. According to the field data collection in the early stage, the average value of straw and light impurities collected per unit length on the surface of film mulch was 2.74 kg/m. According to the designed operation efficiency of the machine, the quality of impurities particles thrown out of the unit time section was calculated as Q = 7.63 kg. Combined with a light impurity density of 3.1, the impurity particle density ρ_z = 459 kg/m³ was obtained by weighting. The minimum vertical height h_min = 0.23 m for the impurity particles thrown out from the straight pipe section, v_p ≥ $\sqrt{2}$v_pmin = 3.03 m/s could be obtained according to v_min² = 2gh_min. According to Equation (10), when the outlet section area A = 0.32 m², as shown in Figure 7, the S_CDIJ = l_CD·l_DI = 0.32 m²; l_DI ≤ l_AL = 250 mm, when l_CD = 1350 mm, l_DI = 239 mm, integer designed l_DI = 235 mm.

4.2. Design of Throwing Arc Section

According to the design idea of Section 4.1’s straight pipe section, the entrance of the arc section was the outlet of the straight pipe section, as shown in Figure 8. Then, l_OM = 235 mm, the x and y rectangular coordinate systems were established with point O as the origin of coordinates, and the included angle between OM and y axis was 45°. Thus, the movement trajectory of impurity thrown by the upper and lower limit positions (red and blue dotted lines) could be obtained, respectively.

According to the kinematics equation:

$$\{\begin{array}{l}x=x_0+v_x\cdot t\\ y=y_0+v_y\cdot t-\frac{1}{2}\cdot g\cdot t^2\end{array}$$

(11)

Therefore:

$$y=y_0+v_y\cdot \frac{x-x_0}{v_x}-\frac{1}{2}\cdot g\cdot {\left(\frac{x-x_0}{v_x}\right)}^2$$

(12)

In the formula, x₀ and y₀ are the initial horizontal and vertical coordinates thrown by the impurity particle, respectively. Then, the movement trajectory of impurity particle thrown from the upper and lower limit positions of point M and point O is:

$$\{\begin{array}{l}y=y_M+v_y\cdot \frac{x-x_M}{v_x}-\frac{1}{2}\cdot g\cdot {\left(\frac{x-x_M}{v_x}\right)}^2\\ y=y_O+v_y\cdot \frac{x-x_O}{v_x}-\frac{1}{2}\cdot g\cdot {\left(\frac{x-x_O}{v_x}\right)}^2\end{array}$$

(13)

In the Equation (13) above:

$$\{\begin{array}{l}{x}_O=y_O=0\\ x_M=-\frac{235}{\sqrt{2}}\mathrm{mm}\\ y_M=\frac{235}{\sqrt{2}}\mathrm{mm}\\ v_x=v_y=\frac{v_p}{\sqrt{2}}=\frac{3.03}{\sqrt{2}}\mathrm{m}/\mathrm{s}\end{array}$$

(14)

In order to reduce the collision between impurity particle and the upper or lower inner walls of the shell of the arc section, the radius of the arc section (1/8 arc) designed to be the tangent to point M was R_u, and the radius of the arc section connected with point O was R_d (R_d = R_u − 235), designed at the lower edge. Let the coordinates of the central point O’ of the arc be (x_O’, y_O’), as shown in Figure 7:

$$\{\begin{array}{l}{x}_{O’}=\frac{R_d}{\sqrt{2}}\\ y_{O’}=-\frac{R_d}{\sqrt{2}}\end{array}$$

(15)

Then, the equations of the circle with point O’ as the center, R_u and R_d as the radius are:

$$\{\begin{array}{l}{\left(x-x_{O’}\right)}^2+{\left(y-y_{O’}\right)}^2=R_u^2\\ {\left(x-x_{O’}\right)}^2+{\left(y-y_{O’}\right)}^2=R_d^2\\ R_u-R_d=235\mathrm{mm}\end{array}$$

(16)

Here:

$$l_{NP}=l_{O’P}-l_{O’N}=R_d-\frac{R_d}{\sqrt{2}}\ge h_{min}$$

(17)

In addition, letting h_min = 0.23 m, plugging it into Equation (17), R_d ≥ 785 mm; the integer designed R_d = 800 mm, R_u = 1035 mm; R_d = 800 mm was substituted into Equation (17), as l_NP = 234 mm > h_min, to meet the design requirements. According to the above data, the movement track and ideal curve of impurity particle in the throwing arc section were drawn. As shown in Figure 9, there was almost no collision between the impurity particles and the inner wall of the shell during the throwing process, which met the design requirements.

4.3. Design of Outlet Section of Throwing Arc Section

In order to make the light impurities dropped smoothly, it was necessary to accelerate the impurities twice and reduce the outlet section area of the arc section. According to the structural design requirements of the machine, the horizontal displacement of impurity particles l was not less than 1 m. According to Equation (18), v_t > 4.6 m/s; combined with Equation (10), the section area at the outlet of the throwing arc section was no more than 0.21 m². Furthermore, the exit section height h was 235 mm, the section length could be 902.3 mm, taking the integer design as 900 mm.

$$l=v_{\mathrm{t}}\cdot \Delta t=v_{\mathrm{t}}\cdot \sqrt{\frac{2h}{g}}$$

(18)

The plane where S_CDIJ was located was the outlet section of the straight pipe section, as shown in Figure 10. When the design outlet cross section was rectangular, C₂J₂⊥I’J’, D₂T₂⊥I’J’; the length of C₂D₂ and J₂T₂ was all 900 mm. However, points C, C₂, J₂, and J (points D, D₂, T₂, and I) were not on the same plane, and the manufacturing difficulty increased. In order to reduce the length of C₂D₂ without changing the S_CDIJ size parameters (1350 mm × 235 mm) of the outlet section of the straight pipe section, points D, I, and D₁ (C, J, C₁) were selected to be co-planar and intersected with the line I’J’ at point T₁ (J₁); then, the outlet section C₁J₁T₁D₁ was trapezoidal. When the length of C₁D₁ was 840 mm, the length of T₁J₁ was 956 mm, and the area of outlet section C₁J₁T₁D₁ was 0.21 m² (no more than 0.21 m²), taking the integer design T₁J₁ as 960 mm.

5. Field Performance Test

5.1. Test Conditions

Field tests on the removal of light impurities on the film surface were carried out at Unit two, Lan-pa-ke-ri-ke Village, Shaya county, Xinjiang province (N, 41°21′03″; E, 82°47′53″) in November 2021. The experimental plot was in a cotton field after cotton harvest, and the cultivation mode was “66 + 10” [43], as shown in Figure 11a. The width of mulch film was 2050 mm, and the thickness of mulch film was 0.01 mm. In order to enhance the pertinence of the experiment, the cotton straw was crushed and packed before the experiment, and pulled out of the cotton field, as shown in Figure 11b, leaving only light impurities on the film surface partially covered with soil, which would fall off by vibration in the process of film was picked up.

The supporting power machine is the Luoyang-1804 wheel tractor with a power output of 106 kW, as shown in Figure 12. The experimental machine was in good condition before the experiment.

5.2. Test Method

As shown in Figure 13a, it was found in the field performance test that most of the light impurities on the film surface were thrown out by the throwing shell. As shown in Figure 13b, very few of them fell off in the field during the process of picking up the residual film and could be ignored, while some of them were picked up together with the residual film. Therefore, the residual film impurity rate was selected as the test index, that is, the smaller the residual film impurity rate, the better the removal effect of light impurities on the film surface. The test area was randomly selected in the operation plot, and the test was carried out on a film with a length of 500 m. The residual film recovery test was repeated six times, and the average of the test results was taken. The calculation formula of residual film light impurity rate was as follows:

$$\eta =\frac{m}{M}\times 100\%$$

(19)

where η is the residual film light impurity rate, %; m is the mass of impurities recovered from the residual film in the test area, kg; M is the total weight (including impurities) of the recovered residual film in the test area, kg.

6. Results and Discussion

6.1. Experimental Results

Through the field test, it was found that if the tractor forward speed is too fast (more than 6 km/h), the light impurities on plastic film surface had no time to enter into the crushing chamber, and may be recycled along with the residue film, which directly results in higher impurity rate of recycled residual film. Furthermore, when the speed of the cutter roller exceeds 1500 RPM, too much broken film was thrown out, indirectly leading to a low recovery rate of film residue. However, the rotating speed of the knife roller should not be too low. If it is, the removal effect of light impurities on the plastic film surface is not good, resulting in more impurities in the recovered residual film.

The above test parameters are the results of repeated tests. For the removal test of light impurities on the plastic film surface, when the forward speed and cutter roll speed were in the range of (5.4–5.8) km/h and (1440–1460) r·min⁻¹, the removal effect of impurities was better. The field test results of removing light impurities on film surface are shown in Figure 14. The light impurity rate in the recovered residual film was between 10.9% and 31.4%, and the average impurity rate was around 18.7%. According to the Grubbs criterion, the experimental data of six groups were all valid, and initially met the design requirements. In addition, the residual film recovery machinery has the impurity separation function, but there were no specific test data, such as in [25]. According to [14], the separation rate of residual plastic film was 83.27%, so the impurity rate was around 16.73%. The average impurity rate was around 18.7% in the revised manuscript. Thus, the results of our experiment are reliable and valid.

6.2. Discussion

Since part of the light impurities on the mulch film surface was thrown out from the throwing shell and returned to the field, while the other part was recycled with the residual film together, and the density of light impurities is much higher than the density of the residual film, here, the impurity rate in the recovered residual film was reported as being 10.9–31.4%. We only reflect the removal effect of impurities on the plastic film surface with the impurity rate in the recovered residual film. If we use η₀ to represent the removal effect of light impurities, the formula is as follows:

$${\eta }_0=\frac{M_0-m}{M_0}\times 100\%$$

(20)

where m (1.56/3.5/1.52/1.43/0.99/1.96) is also the mass of impurities recovered from the residual film in the test area, kg; and M₀ is the mass of all light impurities on the film surface of test section (500 m length), which was about 331.0 kg (data were obtained from previous experiments). Thus, the corresponding value of η₀ is between 98.9% and 99.7%, therefore the removal effect of light impurities on the plastic film surface is good and can meet the expected design requirements.

When the tractor forward speed was too slow, it was equivalent to the longer time for which the crushing chamber stayed on the film surface, which greatly increased the possibility of the residual film being blown up by the wind, causing damage to the film surface and reducing the recovery rate of residual film. If the unit was moving too fast, the impurities on the film surface could not enter the crushing chamber smoothly, resulting in a high impurity rate. From the perspective of theoretical analysis, the faster the speed of the knife roller, the better of light impurity removal effect. However, the high speed of the blade roller increases the flow velocity of the wind field on the film surface. Once the residual film collides with the knife, the film is going to be torn, broken, and thrown out with impurities (the damage will be continuous), greatly increasing the film rate of impurities to be thrown out, and indirectly decreasing the recovery rate of the residual film.

Last, the design of the residual film recovery machine was mainly based on the GB/T 25412-2010 “Residual film recycling machine” and NY/T 1227-2006 “The operating quality of retrieving machines for film residue”. The criteria listed above focus on the rate of hurt seedling (when recycling the residue film in the seedling stage), the rate of residual film recovery, and so on. The criteria include elements such as: the qualified rate of cotton straw crushing should be no less than 85% (the qualified length of cotton straw crushing should be no more than 200 mm), and the stubble height should be no more than 12 cm. In recent years, China has attached great importance to plastic mulch pollution; as such, the residual film recycling machine continues to develop and mature. The problem of residual film recovery gradually evolved into the problem of secondary utilization of the recovered residual film. Faced with the new problem of the separation of the residual film and impurities, there is no specific standard. Thus, what we are doing now is to accumulate data for the formulation of standard; as such, our work is very valuable. We still hope that the standards for impurity content of recycled film will be issued as soon as possible.

7. Conclusions

The removal mechanism of impurities on the film surface could be considered as a pneumatic conveying process. The static and kinematic analysis of light impurity was carried out to determine the conditions of impurity particles’ movement. The straight pipe section and the arc section were designed. The length of the straight pipe section was designed as 910 mm, and its outlet section (1350 mm × 235 mm) were determined. The motion track ideal curve and exit section size of the arc section at the upper and lower limit positions of impurities were all obtained, respectively. Finally, on a trial-produce prototype, the field performance test was carried out. When the unit operating speed and cutter roll speed were kept in the range of (5.4–5.8) km/h and (1440–1460) r·min⁻¹. The impurity content of the recovered residual film was between 10.9% and 31.4%, and the average impurity content was about 18.7%, which met the design and application requirements.

In the future, from the perspective of simulation analysis, the design parameters of key components should be optimized. Subsequent experimental studies should combine the stubble height, residual film recovery rate, and straw crushing pass rate, along with other test indexes, to find the optimal working parameters in order to make full preparations for the transformation of the project achievements.