Research Progress on Castor Harvesting Technology and Equipment

Abstract

The harvesting of castor is highly seasonal and labor-intensive, necessitating a reliance on mature mechanical harvesting equipment. Castor harvesting machinery is a weak link in the castor industry chain, severely constraining the development of China’s castor industry. This article elaborates on the current status of the castor industry and the harvesting modes, which mainly include combined harvesting and segmented harvesting. It systematically summarizes the harvesting platforms, cleaning technologies, and shelling technologies of castor harvesting machinery in China and internationally. It analyzes the main structural forms and working principles of the harvesting platforms of castor harvesters. The cleaning technologies and different types of shelling technologies of harvesters are also analyzed and summarized. Finally, this article identifies the existing problems in castor harvesting in China and provides an outlook on future development trends. It suggests that China’s castor cultivation will strengthen the integration of agricultural machinery and agronomy, while harvesting will develop in the direction of combined harvesting. In the future, the focus will be on the development of high-efficiency harvesting, specialized low-loss shelling machines, and breakthroughs in key core technologies to promote the development of mechanized castor harvesting technology in China.

1. Introduction

Castor, also known as red hemp, eight hemp seeds, large hemp seeds, etc., is an annual or perennial herbaceous plant of the Euphorbiaceae family [1]. The castor seeds contain a high amount of castor oil, which is the only renewable vegetable oil found in nature that can replace petroleum products [2,3,4]. Castor oil can produce 175 derivative products, widely used in aviation, chemical, automotive, fragrance, and pharmaceutical fields [5,6]. Castor oil is an industrial vegetable oil source with comprehensive development potential. With the development of new technologies and processes in the petrochemical industry, people’s dependence on and demand for castor oil will continue to increase [7,8]. Castor has a high added value; castor leaves can be used for sericulture, and the stems can be used to make boards and paper [9,10,11]. The roots, stems, and leaves of castor can all be used as medicine; modern medical research shows that ricin is an important anti-cancer substance [12,13,14]. Castor meal can be used as a high-quality green fertilizer, and after detoxification, it can be used as a green high-protein feed [15]. At the same time, the extract from castor oil, ricin, can be made into biopesticides with good effects [16,17]. Castor has strong advantages such as salt-alkali resistance, drought tolerance, poor soil tolerance, and strong adaptability, which can be used for saline-alkali land restoration. Additionally, castor has a high tolerance to heavy metals like copper and arsenic, making it useful for the remediation of heavy metal pollution [18,19,20].

The production of castor mainly includes the stages of land preparation, sowing, pest control, field harvesting, and threshing on the site. Among these, land preparation and pest control can use general machinery for field crops, while other operations require the improvement of similar machines or the design of machinery specifically for castor. Currently, there is extensive and mature research on tillage machinery and pest control machinery. The development of plant protection drones is rapid and widely promoted and used, but there is less research on machinery specifically for castor production. The castor seeder mainly draws on corn seeding technology, as the seed size is close to that of castor seeds. It is then designed according to different varieties and regional planting row spacing to achieve the seeding operation for castor. The main machinery for castor harvesting includes castor combine harvesters, castor fruit harvesters, and castor shelling machines.

The harvest season of castor is highly seasonal, and manual harvesting has the disadvantages of a high labor intensity, low efficiency, high waste rate, and poor working environment. Mechanized harvesting has always been a core issue that needs to be tackled urgently in the industry chain. At present, the development level of castor harvesting technology in China is relatively low, which seriously restricts the improvement of the mechanization level of castor in China [21,22,23]. This paper systematically analyzes the harvesting modes of castor and the current status of their mechanization development, and it summarizes the achievements, development experience, and existing problems of castor mechanization technology in China and abroad, in order to promote the scientific and technological progress and industrial development of the castor mechanization industry in China.

2. Current Status and Harvesting Operation Modes of the Castor Industry

2.1. Castor Cultivation Area and Output Value

Castor cultivation has a long history, originating in Africa and later spreading to Asia, Europe, and North and South America. China has a history of cultivating castor for 1500 years [24,25,26]. There are over 40 countries in the world that cultivate castor, with a global planting area of approximately 1.6 million hectares and an annual production of about 1 million tons of castor seeds. As a major consumer of castor beans, China’s domestic production is far from meeting its production needs. However, with the rapid development of industrial technology, the gap between total demand and total supply in the domestic castor market is increasingly widening. According to data from China Customs in recent years, China mainly imports about 250,000 tons of castor oil from countries such as India, Thailand, and Japan, with over 95% of imports coming from India. The data in Figure 1 shows that the quantity of castor oil imports has shown a relatively stable and slightly upward trend from 2015 to 2023 [27]. In summary, China’s imports of castor oil and its derivatives are almost monopolized by India, making China’s castor oil imports highly dependent on India, which is not conducive to the development of China’s castor industry. It is evident that the supply of domestic castor beans and castor-related products far from meets the demand, and based on the trend, it can be inferred that the demand for imports of castor-related products is likely to continue to rise in the future.

2.2. Castor Harvesting Methods

Currently, there are many castor varieties cultivated in China, including both annual and perennial types. The annual varieties are suitable for mechanized harvesting. Castor plants have unique characteristics: When most of the plant is mature, the fruits at the top may still be immature or flowering. This places high demands on the performance of harvesting machines. Additionally, a significant amount of high-moisture-content straw entering the cleaning device of the harvester can easily cause blockages. Hence, selecting a reasonable harvesting time and improving the consistency of castor bean maturity is crucial for the quality of the harvesting operation. Pre-harvest leaf stripping and ripening are vital [28]. In China, there is a significant height variation in castor stems at the time of harvest. Tall castor can reach about five meters, which is not suitable for mechanized harvesting. Currently, varieties with medium and short stems below two meters can achieve mechanized harvesting. Castor harvesting is mainly divided into combined harvesting operations and staged harvesting operations [29].

2.2.1. Combined Harvesting

The castor combine harvester, also known as the castor seed harvester, refers to a mechanical harvesting method that completes the entire process of harvesting castor seeds, including fruit harvesting, transportation, hulling, and cleaning, all in one. Currently, there have been no relevant reports domestically, while abroad, roller-brush-type castor seed harvesters have been developed. The seed harvesting process involves combing the castor pods off the plants and transporting them via a conveyance system to the rear for hulling and cleaning, and the harvested castor seeds are sent to a collection box, leaving the castor plants intact in the field, thereby achieving the operation of castor seed harvesting.

2.2.2. Segmented Harvesting

Segmented harvesting refers to the operational process of completing the harvesting, transporting, cleaning, and dehulling of castor fruit using two or more machines. It is mainly divided into two operating scenarios: field and site. First, the harvesting of castor fruit is completed in the field, then transported to the site for stacking and curing, and finally, a dehulling and cleaning machine is used for the dehulling and cleaning operations at the site to obtain castor beans, achieving the harvesting of castor beans (as shown in Figure 2). According to the structure of the castor fruit harvesting platform, it can be mainly divided into two categories: cutting type and roller brushing type [30]. The cutting-type harvesting involves cutting off the castor plants, then transporting them to the rear threshing device for threshing, followed by a cleaning device for cleaning, and finally, transporting the cleaned castor fruit to the collection box. The roller brushing-type harvesting involves brushing the castor fruit off the plants and transporting them to the collection box, leaving the castor plants in the field. The castor fruit harvested in the field is transported to the location. Depending on the maturity and moisture content of the castor fruit, if they meet the shelling requirements, they can be processed directly using a castor sheller on-site. If they do not meet the shelling purification conditions, they need to be cured on-site for a period of time until they meet the shelling requirements, before the on-site shelling operation can be conducted [31,32].

3. Current Research Status of Castor Harvesting Technology

In recent years, with the continuous expansion of castor planting areas and the rising labor costs, relevant research institutes and enterprises have independently innovated and developed some castor harvesters and shellers for production, solving the dilemma of no machinery available in castor production [33].

3.1. Current Research Status of Castor Harvesting Technology Abroad

There is little information available internationally on castor harvesting machinery. Through searches, it was found that there are patents in the United States related to a castor combine harvester. This machine can perform multiple processes such as harvesting, transporting, dehulling, and screening separation, achieving a high level of mechanization for castor bean harvesting. However, due to the poor consistency of castor bean maturity, both the cleaning rate and damage rate are relatively high, indicating that further optimization is needed [33]. D.S. Verkrop and others in the United States designed a castor harvester with a cage-like structure for the cutting platform. Bristles are installed on both sides of the channel where the castor plants enter the cutting platform to reduce castor loss. A rotating striker is installed at the center bottom of the cutting platform, which beats the castor stems that enter the narrow slots under the action of centrifugal force, causing the castor fruit on the inflorescence to fall from the plant into the cage and be concentrated in a longitudinally extended spiral conveyor. The fruits are then transported by a screw conveyor to a scraper conveyor. However, the operating efficiency of this machine is relatively low, making it unsuitable for large-scale harvesting operations [34,35].

Brazil has a large area of castor cultivation, leading to a well-developed production machinery for castor, with a high degree of mechanization in harvesting. As shown in Figure 3, Brazil has developed a castor harvester that offers high operational efficiency, completing the harvest of eight rows at once, and capable of executing processes such as harvesting, transporting, dehulling, and sorting. Each row is equipped with a pair of horizontally installed brush rollers that rotate in opposite directions. The speed difference between the two brush rollers brushes the castor fruit down, which is then transported via a conveyor belt and transverse auger to the rear for dehulling and sorting, ultimately delivering the castor seeds to a collection box. However, the horizontally installed brush rollers can lead to issues, such as collisions that cause damage to the harvesting platform and push rods during the harvesting process, and this machine is only suitable for low-stalk castor varieties [36]. The Italian researchers Stefanoni et al. conducted trials on castor harvest with a combined harvesting machine equipped with both grain cutting platforms and sunflower cutting platforms. A New Holland CX8060 (New Holland, PA, USA) combine harvester was used throughout the test. During the harvesting operation, the forward speed of the harvester was 2.3 km/h, and the parameters of the cleaning shoe were set to reasonable values. The experimental results confirmed the hypothesis that using a sunflower header for mechanical harvesting of castor seeds would reduce seed loss. In fact, the sunflower header showed significantly lower header collision losses compared to the grain header [37]. Researchers from Greece and Italy conducted trials at the farm of Thessaly University in central Greece using a New Holland CX780 combine harvester paired with a 17 V-type grain harvesting platform that is 5.10 m wide, specifically for low-stalk castor plants, as shown in Figure 4. When setting the sieve openings in the upper and lower sieves to 17 mm and 10 mm, respectively, and the fan speed to 800 r.p.m., with the threshing drum rotation speed set at 400 r.p.m. and the clearance value at 10 mm, the harvesting operation was most effective. However, the results showed that the collision losses from the combine harvester’s harvesting platform were significant and cannot be ignored [38].

3.2. Current Research Status of Castor Harvesting Machinery in China

The degree of mechanization in castor production is not high in China, and the research on mechanized harvesting technology and equipment for castor started relatively late. The mechanization harvesting technology is not mature enough, and most of the existing research on castor harvesting machinery is still at the theoretical design and prototype testing stage. In many regions, castor harvesting relies mainly on manual labor, leading to high labor intensity and low efficiency.

In China, the castor harvesters are mainly categorized by harvesting methods into the roller brush type, cutting type, and other structural forms. As shown in Figure 5, the 4BZ-4 roller brush-type castor bean harvester developed by the Nanjing Research Institute of Agricultural Mechanization under the Ministry of Agriculture and Rural Affairs features a self-propelled chassis with 80 kW power supply and four harvesting units, allowing it to harvest four rows at a time. It is mainly suitable for harvesting castor fruit planted with an equal row spacing, which generally requires a planting row distance of 70–90 cm, a minimum castor fruit growth height of 25 cm, and typically a plant height of no more than 1.5 m. The operational process of the brush roller-type castor harvesters mainly includes the rotation of the flexible bristle rollers within the harvesting unit, which generates a speed difference with the castor fruit, causing them to be stripped from the plants, followed by transportation via a stirring auger to a scraper conveyor channel, and then transport by air to a collection box at the back. All components of this machine utilize a modular design, with a universal chassis, and the harvesting unit can be adjusted within a range of 70–90 cm row spacing according to different planting practices. The brush roller group and transverse conveyor also adopt a modular design, facilitating quick maintenance and assembly. This machine has advantages such as a simple structure, ease of operation, convenient maintenance, and good economic efficiency, but still faces issues like a slow working speed and impurity content [39,40]. The team led by Hou Junming at Shenyang Agricultural University designed a comb-brush-type castor fruit harvesting device (as shown in Figure 6). They established a mechanical model of fruit–plate interaction and a finite element model to analyze the collision damage mechanism of the fruit, concluding that the maximum contact stress during fruit collisions is directly proportional to the collision speed, equivalent elastic modulus, and the mass of the castor fruit, while inversely proportional to the equivalent radius of curvature. They also determined the maximum destructive force of the castor shell and the speed value of the fruit. A discrete element model of harvestable castor branches was established, and parameter calibration for the force connecting the fruit and peduncle was conducted. The comb finger number, forward speed of the machine, and rotational speed of the harvesting rollers were used as experimental factors, with the harvesting rate and damage rate as experimental indicators for orthogonal experiments. Through multi-objective optimization, the optimal combination was identified and verified through experiments, revealing that the actual motion law fitting curve is consistent with the simulation fitting trend [41].

At the same time, some domestic companies have designed harvesting platforms for castor fruit, which can be attached to corn, soybean, or other grain combine harvesters, utilizing the fruit-harvesting and cleaning devices of the combine harvester for harvesting operations. A certain agricultural machinery processing company in Ulanhot, Inner Mongolia, has developed a cutting full-feeding cutter platform and harvester based on a soybean harvester (as shown in Figure 7). This utilizes a chain cutter platform to transport the cut castor plants via a conveying auger into a threshing drum, with the sieve plates of the threshing drum redesigned according to the characteristics of the castor fruits [34]. Northeast Agricultural University, in conjunction with the Nanjing Agricultural Machinery Institute, has designed a disc cutting castor harvesting device (as shown in Figure 8). This device can be fitted to rice or corn combine harvesters and cuts and separates castor plants using dual disc blades, with harvesting completed through the cleaning process of the harvester. Experimental research has been conducted using cutting height difference and harvesting loss rate as evaluation indicators, with the disc structure, disc rotation speed, and forward speed as experimental factors for a three-factor, three-level orthogonal experiment to optimize relevant parameters. Field verification tests showed that with a wave-shaped disc structure, a disc rotation speed of 600 r/min, and a forward speed of 1.1 m/s, the height difference in the average cutting stubble was 0.85 mm, and the average harvesting loss rate was 3.13%. The cutting process was stable, with a low loss rate, demonstrating good adaptability to farming practices and meeting the field operation requirements for castor harvesting [42,43]. The 4ZB-6 self-propelled castor harvester jointly developed by the Chinese Academy of Agricultural Machinery Sciences and Xinjiang ZhongShou Agricultural Machinery Co., Ltd. employs a brush harvesting device on the cutter platform to reduce the breakage of castor fruits, achieving complete separation of stems and castor fruits; the harvesting drum uses a flexible structural material to ensure a low breakage rate during threshing. By replacing key working components, the machine can also harvest other crops like wheat and corn, enabling multipurpose use, reducing costs, and improving economic efficiency [33]. The above research on harvesters primarily focused on the design of castor fruit harvesting platforms for soybean, rice/wheat, and corn combine harvesters and conducted relevant parameter experimental studies. However, castor plants and fruits exhibit different characteristics and physical dimensions, leading to significant losses and high breakage rates during subsequent cleaning operations; thus, further optimization of machine structural parameters is needed.

Domestic researchers have innovated in harvesting methods. Zhao Huayang and others from Inner Mongolia University for Nationalities have developed an innovative harvesting method for castor fruit. Based on the characteristics that castor stems are thick and hard and thus difficult to break, while the fruit easily falls off, they designed a vibratory harvesting machine (as shown in Figure 9) that utilizes vibration to separate the fruit from the plants. This method allows for the quick and effective separation of mature fruit. It primarily employs an upper and lower vibratory harvesting mechanism: the upper section features a fork-type device for flicking the fruit off, while the lower section is equipped with a reciprocating shaking device. During harvesting operations, both the fork-type flicking device and the reciprocating shaking device act on the castor plants, striking them to cause the fruit to detach and fall into the collection and conveyance system below. The relative positions of the reciprocating vibrating device and the upper fork-type flicking device remain unchanged and are driven by a hydraulic cylinder, allowing for angle adjustment of the harvesting fork. Additionally, a cutting and conveying device is installed to cut the castor plants and transport the stalks to the rear end. The machine is currently still in the experimental research stage, and further optimizations are needed for production application [44]. The team led by Li Changhe from Qingdao University of Technology has improved and innovated the castor harvesting method. They designed a comb-tooth fruit harvesting and fixed-length, quantitative bundling combined harvesting machine for castor fruit to achieve fully automated separation of castor fruit and stalks and collection of castor fruit. The harvester employs the principle of imitating human hands and has improved the design by combining a grain comb wheel with a circular concave roller to create a comb-tooth harvester for castor fruit. This device integrates functions such as casting seed harvesting, separation, cleaning, fixed-length segment cutting of stalks, quantitative compression, and automatic bundling. Currently, there are no prototypes for field validation, and the harvesting effect remains unclear [45].

In summary, current research on castor harvesting technology both domestically and internationally mainly focuses on the methods of castor harvesting operations, the structural forms of castor harvesting devices, and operational parameters. There is a lack of research on the mechanical properties of castor plants and the interaction processes between the plants and the harvesting components. To further improve the operational performance of castor harvesters, enhance harvesting efficiency, and reduce harvesting losses, it is necessary to strengthen theoretical research on the principles of collision losses during the harvesting process, the mechanical characteristics of castor plants and harvesting devices, the movement patterns of castor fruits after harvesting, and the methods of cleaning and sorting castor after harvesting. Additionally, there needs to be an integration of agricultural machinery and agronomy to improve the maturity consistency of castor and the leaf shedding effect, and to reduce the rate of natural fruit drop, thus promoting the development of castor harvesting technology. Further research is also required on production technology models for large-scale production and mechanical harvesting operations.

3.3. Comparison and Analysis of Domestic and Foreign Equipment

As shown in

Table 1. Comparison of domestic and foreign castor harvesters.

Equipment Name	Harvest Mode	Machine Maturity	Harvest Header Type	Advantages	Disadvantages
Brazilian Roller Brush Type Castor Harvesting Machine [36]	Combined harvesting	Relatively mature	Roller brush type	Grain harvesting; high degree of mechanization; high operation efficiency.	Suitable for dwarf varieties; collision loss is significant.
4BZ-4 Roller Brush Type Harvester of Castor Fruit [40]	Staggered harvesting	Sample trial production	Roller brush type	Variable row spacing harvesting; economic efficiency.	Low operation efficiency; high impurity rate.
Cutting-type castor harvester [34]	Staggered harvesting	Sample trial production	Cutting type	High operation efficiency; low impurity rate.	High loss rate; sowing time delayed.
Disc-cutting harvesting device of castor harvester [42]	Staggered harvesting	Experimental research phase	Cutting type	Low loss rate; the stubble is neat.	Low harvesting efficiency; suitable for dwarfs.
Impact-type castor harvester [44]	Staggered harvesting	Experimental research phase	Impact type	Achievable high-stakes harvest; the damage ratio is low.	Low operation efficiency; low research maturity.

, a comparative analysis of the current research statuses of castor harvesting machinery in China and abroad shows that the main types of harvester headers for castor harvesters include roller brush, cutting, and vibrating structures. The research on castor harvesting machinery in China started relatively late, with only a small number of models suitable for actual castor production, while most of the castor harvesters are still in the experimental research stage. Compared with foreign castor combine harvesters, there is still a certain gap in harvesting operational efficiency, intensification level, and harvesting efficiency. However, foreign castor combine harvesters can only harvest short-stalk castor and have issues such as significant collision losses. To promote the development of castor harvesting machinery and equipment in China, future efforts need to strengthen research on key component materials and structural optimization of castor harvesting platforms, castor threshing methods, and shelling and cleaning structures and parameters, providing technical support for improving the operational performance of castor harvesting equipment in China.

4. Current Research Status of Cleaning Technology for Harvesters

Currently, research on castor harvesting technology both domestically and internationally mainly focuses on harvesting operation methods, harvesting principles, and reducing harvesting losses. There is relatively little research on the post-harvest cleaning technology for castor fruit, but the cleaning device is one of the key components of the harvester, and its performance directly affects the overall operational performance of the harvester. The impurity rate and loss rate of the seeds after cleaning are also important indicators for measuring the operational quality of the harvester. Therefore, there is considerable research both domestically and internationally on the cleaning devices of harvesters [46]. The cleaning devices can be mainly divided into two categories based on their working principle: airflow type and wind screen type. The airflow type mainly includes the air suction type, air blowing type, and cyclone separation type, which primarily rely on the aerodynamic characteristics of the materials for cleaning [47]. The wind screen type mainly relies on the combination of airflow and screens for cleaning, with fan types including centrifugal fans and cross-flow fans, and screen types including scaled screens, perforated screens, and woven screens. The wind screen cleaning method is currently the most widely used cleaning method in grain combine harvesters both domestically and internationally [48,49]. It utilizes the airflow generated by the fan and the reciprocating vibration of the vibrating screen to perform a compound effect on the drum ejected materials, clearing the impurities out of the machine while retaining clean seeds [50,51]. The operational performance of the wind screen cleaning device is a key reflection of the quality of the overall machine operation. Scholars both domestically and internationally have conducted extensive research on this key technical device [52]. The Dyna-Fl cleaning system used in the S series combine harvesters from John Deere Company (as shown in Figure 10) has low air intake loss and uniform lateral airflow distribution at the outlet, increasing the screening area and enhancing the material handling capacity, improving the diffusion stratification effect of the ejected materials [53]. The cleaning device equipped on the LEXION series combine harvesters from Germany’s CLASS Company has structures such as dual air outlets, multi-channel centrifugal fans, three-layer vibrating screens, and shaking plates, as shown in Figure 11. The three turbo-style dual air outlets and multi-channel centrifugal fans are used in series, which provide advantages such as a stable airflow, slow airflow attenuation, and uniform lateral airflow. The modified model is also equipped with shaking plates and a self-leveling system for the vibrating screen, significantly improving the cleaning performance [54].

In recent years, scholars both domestically and abroad have conducted some optimization of the structure and parameters of the cleaning system of harvesters. GabiM and others used CFD simulation calculations to study the complex flow and working principle of the airflow within the axial fan, analyzing and optimizing the structure of the axial fan, particularly the influence of different vortex locations on the airflow field of a six-blade centrifugal fan with dual outlets. The research shows that the position of the vortex wall plays a key role in the performance of the fan, and by changing the position of the vortex, a more uniform airflow distribution can be achieved [55]. Lu Rong and others designed a three-channel cross-flow air suction cleaning device, analyzing the force and motion trajectory of peanut pods in a horizontal cross-flow suction air field. Based on this, they conducted structural and parameter design of key components of the cleaning device, and optimized key parameters, with the loss rate and impurity content as evaluation indicators. This research fills the gap in the domestic peanut shelling and cleaning devices for small areas [56,57]. Yu Zhaoyang and others designed a cleaning mechanism for a tangential-flow and whole-feed peanut combine harvester that addresses the challenges of inadequate separation between fruit and impurities, high loss rates, and blockage of sieve surfaces (as shown in Figure 12). They studied the relative motion of materials on the sieve surface, determined the theoretical range of the main motion parameters of the vibrating sieve, and optimized parameters such as the main fan speed, sieve amplitude, and vibration frequency [58]. Liang Zhenwei and others proposed a dual-outlet, multi-channel centrifugal fan, dual-layer vibrating screen, and back conveying device to develop a high-performance, dual outlet, multi-channel cleaning device (as shown in Figure 13). The research investigates the cleaning performance and airflow field distribution rules under different working conditions, elucidating the ideal airflow field distribution model inside the dual-outlet, multi-channel cleaning chamber. By taking parameters such as the fan speed, air-dividing plate inclination angle, and sieve opening size as variables, experimental studies are conducted on the distribution rules of grain loss at the tail end of the air-screen cleaning device, and a mathematical model for monitoring cleaning loss grain quantity is established to achieve real-time monitoring of cleaning loss grains [59]. Leng Jun and others determined the wind speed distribution on the screen surface of the cleaning device through field trials, and they optimized the structure of the cleaning device using CFD simulation, improving the uniformity and symmetry of the wind speed distribution on the upper screen surface. The wind speed at each measurement point on the screen surface was increased by an average of 2 m/s compared to the conditions before optimization [60]. Dai Fei and others proposed an operation mode of “first airflow cleaning, then wind screening” for the various component types of sesame threshing materials, formulated the process flow for separating and cleaning sesame threshing materials, and designed a double-airflow, wind-screening, sesame-threshing material separation and cleaning machine (as shown in Figure 14). Under the optimized parameters from the experiments, the seed cleanliness rate of this machine reached 97.16%, with a total loss rate of impurities at 1.12% [61,62]. Wan Xingyu and others addressed the issue of increased loss of seed during separation and the higher impurity content caused by dead zones in the airflow field of the rapeseed combine harvester’s cyclone separation system (as shown in Figure 15). They analyzed the impact of dead zones on the movement of rapeseed grains and proposed a bump-type turbulence separation and cleaning device. They conducted bench tests and field trials to validate their findings, providing references for the design and optimization of cleaning devices in combine harvesters [63,64]. Yuan Jiacheng and others designed a modular drum screen-type cleaning device that removes impurities from materials after secondary screening and cyclone separation operations. Based on kinematics and dynamics, they analyzed the structural and operational parameter ranges of the material-lifting spiral conveyor and screening device, optimized the parameters, and determined the best parameter combination, which can improve the cleaning rate by 4.38% [65]. Wang Hanhao and others proposed the requirements for the air screening and cleaning of regenerated rice materials, and they improved the cleaning device of rice combine harvesters. They adopted a six-blade centrifugal fan as the cleaning fan, and for the vibrating screen, they used louver screens with flat, unpressed sheet structures as the screen pieces. The improved cleaning device effectively utilized airflow to disperse and layer the materials, enhancing the screening efficiency of rice grains [66].

In summary, the research on cleaning and selection devices by domestic and foreign scholars mainly focuses on the motion analysis of screening devices, control of operational parameters of cleaning devices, and optimization of cleaning parameters. At the same time, simulation software is used to analyze the working process of cleaning and selection devices, establish simulation models for these devices, and researchers have conducted various theoretical studies on the airflow distribution in the cleaning fan and chamber using simulation software. Therefore, in response to the lack of research on a cleaning and impurity removal device of castor harvesters, as well as issues like the easy breakage of castor seeds and poor cleaning performance during the cleaning process, this study will utilize research methods and theoretical approaches related to the cleaning and separation of grains and other materials to develop a castor bean separation and cleaning device. This aims to improve the performance of cleaning and impurity removal in castor harvesters and provide technical support for the research on high-performance castor harvesters.

5. Current Research Status of Castor Fruit Dehulling Technology

The dehulling technology of castor fruit is an important part of the mechanized harvesting of castor. However, there has been little research on castor fruit dehulling technology, both domestically and internationally. Existing dehulling equipment has various issues such as poor applicability, a high seed damage rate, high impurity content, and low screening efficiency. Therefore, studying the current state of dehulling equipment in domestic and international contexts and analyzing the advantages and disadvantages of existing dehulling equipment is of great significance for the development of castor fruit dehulling technology. Research on agricultural material dehulling and cleaning machines has a long history, mainly focusing on the dehulling and cleaning machines for typical economic crops such as peanuts, corn, castor fruit, walnuts, rubber fruits, cashews, longan, and camellia seeds. For the separation of shells and seeds after dehulling, methods include air separation, water separation, and screening. Depending on the physical characteristics of the fruit and the shell, appropriate screening methods are chosen when designing cleaning devices [67].

Castor oil is the main product of castor, thus the castor fruit harvested in the field usually needs to undergo shelling and cleaning processes to obtain clean castor seeds. Common castor shelling machines are divided into two categories: friction type and impact type. Wang Lijie carried out structural improvements and tests on the 6BBS-50-type castor shelling machine (as shown in Figure 16), creating impacts through the rotation of the shelling components, where the shelling process is completed by the combination of the impeller scraper and concave plate. A vibrating screen and blower separate the seeds from the chaff, resolving the contradiction between the seed clean rate and the breakage rate [68]. Hou Junming and others designed a double conical shelling and cleaning machine for castor fruit (as shown in Figure 17). The designed shelling device has an asymmetric double conical structure for the inner shell, and the researchers utilized ADAMS finite element to perform a kinematic simulation of different shelling stages of the castor pods. Under optimal parameters, the test results showed a clean rate of 92.03% and a breakage rate of 3.10% [69,70]. Yang Yong and others established a dynamic model for the frontal collision of castor pods and shelling components based on the classical Hertz contact theory combined with the characteristics of the castor pods, and through finite element simulation, derived the optimal shelling conditions during collision, while validating with related mechanical characteristic tests and impact tests [71,72]. He Tao and others designed a squeezing-type shelling and cleaning device based on the physical and mechanical properties of castor pods and the suspension coefficient of their components, with the drum speed, upper shelling drum discharge gap, and lower shelling drum discharge gap as experimental factors, and the clean rate and breakage rate as experimental indicators for parameter optimization [73,74]. Cao Yuhua and others developed a spiral groove-type castor pod shelling device (as shown in Figure 18), improved the design, and performed a mechanical analysis on key components, establishing a mathematical model of force and deformation; operations were conducted using simulation software to optimize and determine the best drum speed, drum differential speed, and drum gap parameters [75,76]. Cheng Xinxin and others designed a rolling and rubbing-type castor shelling machine (as shown in Figure 19), performing theoretical analyses on the parameters of key components, establishing a three-dimensional model of the shelling components, and conducting modal simulation analysis to explore the influences of various factors on experimental indicators and the interrelations and significance levels among these factors [77,78]. Yao Liangliang and others designed an air-sifting cleaning device, establishing kinematic and dynamic equations for its castor pod shelling. Using simulation software, they optimized the dynamic equations and analyzed the effects of the initial speed, initial directional angle of the shelling material, airflow speed, and airflow directional angle on the displacement of the shelling material in the airflow field, arriving at the optimized solutions for the influencing factors of the shelling material [79].

Yakubu AU designed a friction shelling machine and experimentally studied the optimal concave gap and roller speed for three types of drum materials: metal, rubber, and wood. Statistical analysis was conducted on the data for shelling efficiency, impurity cleaning efficiency, grain damage, and loss rate under optimal operating parameters [80]. Gbabo A and others developed a castor fruit shelling machine using air selection cleaning methods and performed performance tests (as shown in Figure 20). The experimental study evaluated the machine’s shelling efficiency, cleaning effect, and loss rate at different moisture contents, with results indicating the best operating effect was under 6% moisture [81]. Gatmen NC and others analyzed the physical and mechanical properties of castor fruit to address the shelling requirements for Brazilian varieties. They studied shelling operation quality under different shelling drum speed ranges (490–510, 590–610, 690–710 rpm), finding that optimal separation loss, carryover loss, total loss, mechanical damage to seeds, and shelling efficiency occurred at drum speeds above 590 rpm [82]. Michal Petru and others used the finite element method to experimentally analyze the performance of immature, mature, and overripe castor seeds under linear quasi-static compression. The results indicated that the behavior of the castor seeds during the viscoelastic phase is related to their oil content [83]. Sebastian Romuli and others used an improved grinding machine to group castor fruit by different diameters and analyzed their impact on the shelling rate and energy consumption using the response surface methodology (as shown in Figure 21). The experimental results showed that pre-grouping castor fruit can significantly improve the cleaning rate, achieving a shelling rate of about 85% under optimal conditions [84]. Bo Yuan Lim and others designed a roller extrusion-type castor seed shelling machine, which consists of a crushing phase and a shelling phase. Each pair of rollers forms an extrusion mechanism, equipped with an air selection device afterward. Through experimental research to optimize machine parameters, results showed that in the crushing phase, with a roller gap of 10.5 mm and wind speed of 10 m/s, the separation rate reached 94.6%; meanwhile, in the shelling phase, with a roller gap of 6.0 mm and wind speed of 7.5 m/s, the shelling rate was 97.7%, with a loss rate of 2.4% [85].

In summary, the research on the castor fruit shelling technology by domestic and foreign scholars mainly focuses on the structural form of components and kinematic analysis, motion analysis of screening devices, and parameter optimization. Therefore, it is necessary to strengthen the research on castor fruit shelling devices using modern design methods and technologies, further optimize the shelling performance, improve shelling efficiency, and promote the development of the castor industry.

6. Problems and Development Suggestions for the Mechanized Harvesting of Castor

6.1. The Problems Faced by Mechanized Harvesting of Castor

In recent years, with the increasing demand for castor oil, the castor industry has been continuously developing and expanding. However, the population engaged in agricultural production is decreasing and labor costs are rising. Therefore, the mechanization of castor production is an inevitable trend for the development of the castor industry. Currently, the cultivation modes of castor in China are diverse, and the varieties of castor are not suitable for mechanical harvesting. Additionally, the agricultural machinery and agronomy are not well coordinated, and there are no established standards for castor mechanized production. Considering the actual situation of castor production in China, the development of mechanized castor production faces the following challenges.

(1) The integration of agricultural machinery and agronomy in castor harvesting is poor, and there is a lack of standards for mechanized production operations. The degree of integration between agricultural machinery and agronomy affects the quality of castor harvesting operations. Due to the wide distribution of castor cultivation, the variety of castor species, and the diversity of cultivation modes in China, the effective application of castor harvesting technology is also restricted. Moreover, issues related to supporting power, equipment selection, field management, match of stubble, and straw treatment during large-scale production still require further research. It is necessary to propose suitable production technology models for the mechanized operation of large-scale castor production and establish standards for castor machinery operations.

(2) There is a lack of high-performance castor harvesting and shelling equipment. Currently, the harvesting equipment for castor in China is mainly composed of segmental harvesting machines for castor fruit and field shelling machines, which lack a highly integrated harvesting machine that can achieve the functions of harvesting, transporting, shelling, and cleaning separation of castor seeds. At the same time, the existing castor fruit shelling machines have low efficiency, and the shelling operation efficiency needs to be further improved. It is necessary to develop castor shelling equipment witha large feeding capacity and low damage rate, which can realize shelling operations in the field. Efficient shelling equipment can shorten the harvesting time of castor and improve the level of mechanical harvesting operations.

(3) The level of automation in castor harvesting machinery is relatively low. The development of castor harvesting machinery in China started late and is currently still in the initial stage of mechanization, transitioning from ‘non-existence to existence’. Most castor harvesting operations adopt row-specific harvesting, making the application of technologies such as automatic row alignment, autonomous navigation, and automated control highly significant. Meanwhile, the research and application of related intelligent technologies play a crucial role in reducing the loss rate of castor harvesting and improving operational comfort.

(4) Insufficient emphasis and inadequate policy and financial support. Compared to major crops, many regions do not pay enough attention to the castor industry. There is insufficient promotion of research and development of castor machinery, with relatively low investment in the development of machinery for mechanical harvesting of castor. As an economic crop, castor has significant advantages in salinized land improvement and desert land development, with good economic returns. However, in recent years, local governments have provided inadequate policy and financial support for castor, and agricultural technology promotion departments have not adequately guided the promotion of castor cultivation and harvesting techniques. Farmers also show low interest in the mechanization of castor harvesting.

6.2. Recommendations for the Mechanized Harvesting Development of Castor

With the continuous improvement of China’s agricultural machinery support and the increasing mechanization rate, the key technologies and equipment for mechanized harvesting of castor will encounter new development opportunities, and progress should be made in the following areas in the future.

(1) Promote the deep integration of agricultural machinery and agronomy for the mechanization of castor harvesting. Currently, China’s castor planting operating modes are diverse, with non-uniform planting row spacing, and with significant differences in planting density, plant spacing, and harvesting period, leading to poor consistency in castor bean maturity. This severely hinders the efficiency and quality of mechanized harvesting, increases the difficulty of developing supporting machinery, and is not conducive to the large-scale development of agricultural machinery manufacturing enterprises. To improve the mechanization harvesting rate of castor in China, it is essential to standardize and regulate the castor planting model and propose a castor planting mode suitable for mechanized operations in large-scale production. Additionally, during the mechanized harvesting process of castor, collision losses and natural drop losses are considerable. It is recommended that breeding experts improve castor varieties to cultivate varieties that are less prone to fruit drop and have better maturity consistency, which will help reduce the harvest loss rate of castor beans.

(2) Accelerate the research and development of efficient and low-loss castor fruit harvesting and shelling machines, as well as make breakthroughs in key core technologies. The harvesting devices, shelling components, and cleaning devices of castor harvesters are core components that directly affect the loss rate, impurity rate, and breakage rate during the harvesting of castor beans. We call on others to research and develop mature harvesting technologies for tall-stalk crops such as cotton and sunflower, similar to castor, and optimize the design of key components of castor harvesters. Drawing on the shelling technologies of crops such as peanuts, soybeans, and edible beans, which are similar to castor seeds, optimize the structural parameters of key shelling components. Develop efficient and low-loss shelling machines suitable for castor to meet the actual production needs of castor.

(3) Improve the intelligence and informatization level of castor harvesting equipment. China’s agricultural machinery manufacturing industry still has a certain gap compared to developed countries, especially in the application of special materials in cutting devices and threshing and cleaning devices, electromechanical and hydraulic integration control, various sensor technologies, and image and signal acquisition, among others. Therefore, to create high-end intelligent castor harvesting equipment, it is essential to strengthen research on intelligence and informatization technologies. It is recommended that agricultural universities, research institutes, and high-tech enterprises collaborate in areas such as agricultural machinery, hydraulics, control technology, information technology, and agronomy to overcome the shortcomings of intelligent technology in castor harvesting equipment.

(4) Increase policy support and financial investment and optimize the industrial structure. The government should provide precise support policies based on the needs related to mechanized castor harvesting and increase investment in technological research and development. Encourage institutions, leading enterprises, and research institutes to jointly conduct research and development, complement each other’s advantages, accelerate the transformation of production, education, and research, and expand the industrial scale. Relevant departments should improve the social service system for agricultural machinery and technology and promote the development and growth of agricultural machinery leasing service systems in various regions through special funds and agricultural machinery purchase subsidies. Provide financial services for small-scale operating groups with weak capital and poor risk resistance, expand the castor planting area, optimize the castor industrial structure, and promote the development of the castor industry.

7. Conclusions

With the continuous development of China’s economy and urbanization process, the demand for castor harvesting machinery is increasing, and the performance requirements for the machines are also becoming higher. In the process of castor harvesting, the performance of the fruit harvesting equipment, the cleaning device, and the dehulling and cleaning equipment plays a crucial role in the quality of the castor harvesting operation. Currently, castor harvesting machinery faces issues such as high harvest loss rates, high impurity rates, and significant damage. It is necessary to accelerate the development of related technologies and equipment to reduce the harvest loss rate of castor, improve harvesting efficiency, and enhance the quality of mechanical harvesting operations. Breakthroughs in the technical bottlenecks of mechanized harvesting in the castor industry will promote its development.

This article presents an overview of the current state of the castor industry, with a focus on the research status of castor harvesting technology, cleaning devices of harvesters, and castor fruit dehulling technologies in China and other countries. It summarizes the advantages and disadvantages of different structural forms of castor harvesting technology, analyzes the existing problems in castor harvesting machinery in China, and provides an outlook on future development trends. By accelerating the research and development of efficient and low-loss specialized machinery for castor harvesting and dehulling, as well as breakthroughs in key core technologies, we may enhance the automation, informatization, and intelligence levels of castor harvesting equipment. Promoting the deep integration of agricultural machinery and agronomy in castor harvesting mechanization, and increasing government support and funding while optimizing the industrial structure, will inevitably promote the advancement and development of castor harvesting technology and equipment.