The determination of the caking index (G value) of coal is a key indicator for evaluating the coking performance of coal used for coke making. The operational specifications of the sieving step directly affect the accuracy and reproducibility of the results. Below are the precautions that require special attention regarding the sieving steps during caking index determination:

I. Preparation Before Sieving

1. Equipment Selection: Standard sieves specified by national standards must be used. Specifically:

   • Round-hole sieve: Aperture 1 mm. Used for the preliminary removal of coke buttons from the crucible.

   • Mechanical sieve shaker: Amplitude, frequency, and time must comply with standards (typically about 130 revolutions per minute, amplitude about 40 mm, sieving for 5 minutes). This is crucial for ensuring result comparability.

   • Square-hole sieves: One each with apertures of 0.5 mm and 0.25 mm. Used for final separation.

2. Equipment Inspection: The sieve mesh should be intact, without damage or deformation. Before use, gently tap the sieve frame to check for clogged sieve openings.

3. Environment Preparation: The workbench should be stable to avoid vibration interference. Place a suitable sample collection pan underneath.NJ-6 Fully automatic Bonding Index Tester

II. Key Precautions During the Sieving Process

1. Handle with Care to Protect Coke Buttons:

   • After removing the crucible from the muffle furnace, allow it to cool to room temperature before handling.

   • Gently pour the coke buttons from the crucible onto the 1 mm round-hole sieve. Avoid forceful pouring, tapping, or crushing the coke buttons, as this will damage their inherent strength, leading to an erroneously low mass for the ">1 mm" portion and a偏低 (low) G value.

2. Correct Technique for the "Drumming" Operation:

   • (Key Step) Using a soft-bristled brush with a handle or a small soft cloth, repeatedly and gently wipe/brush the coke buttons on the 1 mm sieve (i.e., the ">1 mm" portion). The purpose is to brush off the powder adhering to the coke buttons due to expansion, allowing it to pass through the 1 mm sieve openings.

   • The key movement is "wiping/brushing," not "poking" or "pressing." Continue until no visibly obvious particles are observed on the black paper placed beneath for the sievings.

   • This step greatly impacts the results. Excessive force will abrade the coke buttons, while insufficient brushing will leave powder inadequately separated, both leading to errors.

3. Standardized Operation of Mechanical Sieving:

   • Carefully transfer all coke buttons obtained on the 1 mm sieve (the >1 mm portion) to the sieve set already stacked (from top to bottom: 0.5 mm sieve, 0.25 mm sieve, bottom pan) on the sieve shaker.

   • Cover the sieve set properly and secure or fasten it to prevent material from leaking during sieving.

   • Set the accurate sieving time (typically 5 minutes) and start the sieve shaker. Do not manually intervene or stop during the process.

4. Precise Collection and Weighing of Sieve Oversize:

   • After sieving, collect the oversize material from each sieve layer separately.

   • The most crucial is the >0.5 mm portion (i.e., the residue on the 0.5 mm sieve): This is the core data for calculating the G value (recorded as m1). Use a soft brush to carefully transfer all the coke residue on the 0.5 mm sieve (including particles stuck in the sieve openings) to the weighing dish, ensuring no loss.

   • The >0.25 mm portion (i.e., the residue on the 0.25 mm sieve, recorded as m2) must also be completely collected for auxiliary calculation and verification.

   • Calibrate the balance before weighing. Weigh to a precision of 0.001 g.

III. Calculation Formula and Notes on Result Evaluation

1. Calculation Formula:
   Caking Index G = 10 + (30 * m1 + 70 * m2) / m
   Where: m1 = mass of >0.5 mm portion (g), m2 = mass of 0.25 mm~0.5 mm portion (g), m = total coke residue mass (>0.25 mm portion, i.e., m1 + m2).

2. Judging Result Validity:

   • Requirement for m value: The total coke residue mass m must fall within a specified range (typically related to the coal sample mass, generally around 1.5~2.5 g, refer to the specific standard). Otherwise, the test is invalid.

   • Special provisions for G < 18: If the G value calculated by the above formula is less than 18, a supplementary test is required. The blending ratio and method for the supplementary test are different (adding a specific proportion of专用 anthracite). The sieving operation is the same, but a specialized calculation formula is used.

   • Allowable deviation for parallel tests: The difference between two parallel determinations of the same coal sample must not exceed the national standard specification (typically a difference of 1 when G ≤ 18, and a difference of 2 when G > 18). Otherwise, re-determination is required.

Summary of Key Points:

• Gentle technique is core to protecting the integrity of the coke buttons.

• The "drumming" wipe/brush operation is a key step requiring proficiency and standardization.

• Mechanical sieving time must be accurately fixed.

• Completely collect coke residue of each size fraction and weigh precisely.

• Judge whether results are valid, and note the special treatment methods for low G value coals.

Strict adherence to the detailed specifications in the National Standard "GB/T 5447-2014 Method for Determination of Caking Index of Bituminous Coal" is the fundamental guarantee for obtaining accurate and reliable data. Consistency in operational details is particularly important in laboratory accreditation or comparative experiments.

*This article is shared by the coal testing instrument manufacturer Hebi Huanuo Coal Testing Instruments, for reference only. If you have any questions, please leave a comment or send us a private message! Follow the WeChat public account "Hebi Huanuo Coal Testing Instruments" to inquire about more coal testing knowledge. 86-13839220101*