Since our company was founded in 1992, our seed-processing equipment has undergone numerous upgrades and generational changes. Here, we will discuss the most popular combined sorting machines in China—those that integrate air‑screening and gravity‑separation functions—highlighting their key advantages and disadvantages, and offering guidance on how users can select the appropriate model.

I. Features of the Double-Deck Sorting Machine

At present, double‑stage grain graders are quite popular in China, owing to their high processing capacity, compact footprint, reduced labor requirements, and excellent energy efficiency. As a result, they are widely favored by seed companies and grain‑procurement enterprises.

II. Main Structure of the Compound Sorting Machine

The double‑stage sorting machine primarily consists of an elevator, dust‑removal equipment, an air‑classification section, a specific‑gravity separation section, and a vibratory screening section. Certain models can also be equipped with additional devices such as a wheat huller, a rice awn‑removal machine, and bag‑type dust collectors.

1. Air‑classification section: The main methods are horizontal air screens and vertical air screens. Due to its structural design, the horizontal air screen can perform a secondary air classification; some high‑quality material inadvertently drawn in during the first pass can be retained through secondary settling. Consequently, the recovery rate of the horizontal air screen is superior to that of the vertical air screen.

Based on the light‑impurity removal performance of air screens: single‑stage air screen < horizontal air screen = Dual air sieve

2. Gravity‑separation section: Currently, the mainstream double‑stage graders primarily employ air‑blown gravity tables, which are further categorized into fan‑type and blower‑type models. Fan‑type gravity tables deliver a higher airflow volume, making them well suited for processing materials with relatively high bulk densities, such as corn and soybeans, whereas blower‑type gravity tables have a comparatively lower airflow and are mainly used for lighter‑density materials like sesame, flaxseed, and rice seeds. In most double‑stage graders, the gravity table surface is equipped with fish‑scale screens; to process different materials, it is necessary to replace the screen with one of the appropriate drum height.

3. Vibration Screening Section: Most combined graders use vibration screens driven by vibration motors, with only a small number of models employing motor‑driven crank‑and‑connecting‑rod mechanisms. We recommend purchasing vibration screens equipped with vibration motors, as they offer adjustable vibration frequency, stable amplitude, and a low failure rate. The screen panels are typically made of perforated galvanized steel, and screening is facilitated by rubber balls.

III. The main drawbacks of the duplex sorting machine

1. Complex structure: Due to the integration of the functions of a wind‑sieve cleaner and a density separator, the equipment has a relatively complex design. Installation and commissioning must be carried out by qualified professionals; otherwise, improper installation or adjustment may lead to imbalances in the drive components and inaccurate airflow regulation, thereby compromising screening efficiency, yield, and the equipment’s service life.

2. Simplified gravity table: Compared with traditional positive‑pressure gravity concentrators, duplex concentrators, due to their structural constraints, employ a simplified gravity table. In terms of air‑flow regulation and table surface area, they cannot be compared with conventional positive‑pressure gravity separators.

IV. Main application areas of the duplex sorting machine

At present, double‑stage graders are primarily used in the grain procurement sector. Due to their high processing capacity and relatively modest requirements for grain cleanliness, they are particularly well suited to this industry; after passing through the double‑stage grader, the grain can either be stored or packaged for sale.

As for the seed‑processing industry, due to the simplification of the specific‑gravity separation stage, its performance in removing damaged seeds—such as insect‑bored, moldy, diseased, or underdeveloped grains—is only moderate. It is best suited for users with relatively low requirements for specific‑gravity impurities, those working with fairly clean raw material, or those who use a multi‑stage cleaner as their primary sorting step.

V. The Development History of Our Company’s Double-Deck Grading Machine

As a leading manufacturer in the seed-processing equipment industry, our company has been operating since 2008. Since the launch of the first-generation duplex sorter in [year], the duplex sorter has evolved into a series‑based, modular product line, with over 30 models available. The remaining models can largely meet the needs of most users. The main representative products are as follows:

  First-generation compound sorting machine:

5XFZ-5BX/7.5B/10B Type: Single vertical air screen+ Specific gravity table+ Vibrating screen

Second-generation double‑deck sorting machine:

5XFZ-7.5/10 Type: Horizontal air screen+ Specific gravity table+ Vibrating screen

Compared with the first-generation model, the primary upgrades include replacing the vertical air screen with a horizontal one and extending the length of the vibrating screen to enhance screening efficiency, while also integrating a wheat hulling machine and a rice awn‑removal machine.

Third-generation duplex sorting machine:

5XFZ-25G Type: Double vertical air sieve + Vibrating screen+ Specific gravity sieve

Primarily, the processing flow has been optimized: air separation (first stage) — Filter-- Air separation (secondary) -- Gravity separation—screening (grading). This makes the multi‑stage cleaner better aligned with the impurity‑removal standards for seed and grain‑type particulate materials.

Fourth-generation composite sorting machine:

It primarily achieves high throughput and modularity based on the third-generation model. Representative products include the 5XFZ-150. Series, 180 Series and 200 Series, among others. Users can freely combine and customize the gravity table, vibrating screen, and air‑separation components according to their specific needs. Furthermore, to address the limitations of conventional double‑stage graders in removing density‑based impurities, a dual‑gravity model has been introduced, further enhancing the screening purity of the double‑stage grader and meeting stringent seed‑sorting cleanliness requirements.

At present, our company continues to devote substantial R&D resources to our double‑deck sorting machines, maintaining a pace of developing one generation for production, piloting another, and conducting preliminary research on a third, with 2–3 new models launched onto the market each year. A new model of duplex sorting machine has been introduced to meet the needs of a wide range of users.

VI. How to Choose a Duplex Sorting Machine

1. According to one’s own processing volume requirements

The mainstream multi‑spindle precision machining capabilities range from 5 to 30. ton/ Time waits for no one; users should select the appropriate machine model based on their actual processing volume requirements.

2. According to the type of material

Different models of double‑stage graders are suited to different types of crops, so users should select the most appropriate machine based on the specific materials they process. Some manufacturers have limited R&D capabilities and offer a narrow product range, attempting to cover all applications with a single model—this approach is unscientific. At best, it leads to unnecessary expenditure; at worst, it compromises processing performance.

3. According to the original purity of the material

When the impurity content is high, more processing steps are required, such as double‑density separation and dual‑vibrating screens. Conversely, a simpler, single‑stage refining machine will suffice.

4. Based on the available funding

Taking our company’s double‑deck sorting machine series as an example, the price per unit ranges from 4… Ten to the minus nine Ranging from several thousand yuan, users can choose the model that offers the best value for money based on their budget.

5. Equipment's external dimensions

There are many models of double‑deck sorting machines, and their external dimensions vary considerably. Users should take into account the dimensions of the factory building and the size of the warehouse doorways when selecting equipment, to ensure that the chosen machine can be accommodated in the appropriate location within the facility.

For overseas customers, who rely on container shipping, this issue must also be taken into account; indiscriminate equipment selection could result in the need to disassemble numerous components during shipment or render the goods unshippable.

6. Choosing a Manufacturer

There are many manufacturers of double‑stage grain graders, but the market is a mixed bag, with widely varying levels of quality. Therefore, we recommend that buyers consult friends who already use such machines or visit the manufacturers in person to assess their offerings. Large, established producers—whether in terms of equipment quality control or after‑sales service—far outstrip smaller companies and small workshops.

In summary, at this stage, the adoption of double‑stage grain sorting machines is an inevitable trend in China’s grain processing industry. We will continue to refine our existing products and develop new ones, contributing our part to the advancement of agriculture.

Shijiazhuang Sanli Grain Machinery Co., Ltd. and its subsidiary, Qintong Import & Export Co., Ltd., warmly welcome colleagues from the seed industry across China and around the world, as well as all our friends, to visit and offer their guidance. We also invite you to come to Shijiazhuang for interviews and to be our guests.