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D2797 – Standard Test Method Technical Guide
📐 Scope, Significance, and Essential Apparatus
ASTM D2797/D2797M-21a outlines the standard laboratory procedures for preparing granular samples of bituminous coal and anthracite for examination under a microscope using reflected light illumination. This practice is specifically designed to produce high-quality polished briquettes suitable for advanced microscopical analyses. Per Section 1.1, it applies exclusively to granular samples and explicitly does not cover the preparation of oriented blocks of coal.
The significance of this practice, highlighted in Section 4.1, lies in its ability to create flat, scratch-free surfaces on briquettes containing particles that are representative of the original gross sample. These prepared specimens serve as the foundation for Test Method D2798 (microscopical determination of vitrinite reflectance) and Test Method D2799 (microscopical determination of maceral composition).
Proper selection of apparatus is critical for compliance. The standard specifies precise riffling and sieving equipment to ensure the sample remains statistically representative and correctly sized:
| 🟦 Component | 📐 Specification | 🎯 Requirement |
|---|---|---|
| Final Grinding Sieve | U.S. Standard No. 20 | 850 µm |
| Intermediate Sieve | U.S. Standard No. 4 | 4.75 mm |
| Coarse Riffle Sampler | Number of Divisions (Width) | ≥ 12 (12.7 to 19.1 mm) |
| Fine Riffle Sampler | Number of Divisions (Width) | ≥ 12 (3.2 to 6.4 mm) |
💡 Best Practice: Always verify that your sieves comply with Specification E11. The coarse riffle must have divisions between 12.7 mm [½ in.] and 19.1 mm [¾ in.], while the fine riffle divisions must be between 3.2 mm [⅛ in.] and 6.4 mm [¼ in.] to guarantee statistical representativeness during the splitting process.
⚙️ Sample Preparation and Briquetting Procedure
The core procedure, summarized in Section 3.1, involves crushing a representative sample to a specified particle size, air-drying it, mixing it with a binder, and forming it into a briquet. The briquet is then polished to a flat, scratch-free surface suitable for examination by vertical illumination.
Section 5.1 specifies the equipment requirements: a grinder, pulverizer, or mill capable of reducing a 250 g sample that already passes a 4.75 mm [No. 4] sieve until all of it passes an 850 µm [No. 20] U.S. Standard Sieve. This specific particle size distribution is critical for achieving a densely packed briquet with minimal void space and a homogeneous surface.
After crushing and air-drying, the sample is mixed with an appropriate binder. This mixture is then placed in a mold and pressed under force to form a solid briquet. The polishing stage requires sequential abrasion using progressively finer grits. The final polish must be of sufficient quality to reveal the microscopic organic components without introducing artifacts like “relief” between hard and soft macerals, or scratches that scatter light and degrade image quality.
⚠️ Critical Quality Factor: The polished surface must be completely flat and free of scratches. Achieving a “relief-free” surface is paramount; differential polishing of hard versus soft macerals will lead to inaccurate results in both vitrinite reflectance (D2798) and maceral composition (D2799) analyses. The entire preparation methodology outlined in this standard is designed specifically to mitigate these common pitfalls.
📊 Key Measured Properties and Referenced Documents
Samples prepared under D2797 are the foundation for several critical microscopical analyses. The standard is firmly integrated into the ASTM coal and coke testing suite, referencing several other key standards for sample collection, preparation, and measurement. The following table summarizes the key applications and their interrelation:
| 📊 Standard | ⚡ Application / Measured Property | 📌 Relevance to D2797 |
|---|---|---|
| D2798 | Vitrinite Reflectance | Measurement procedure performed directly on the polished specimen |
| D2799 | Maceral Composition (Vol. %) | Point counting procedure performed directly on the polished specimen |
| D2013 | Preparing Coal Samples for Analysis | General preparation and crushing procedures conducted prior to D2797 |
| D2234/D2234M | Collection of a Gross Sample | Specifies the original sample collection methodology |
| E11 | Woven Wire Test Sieve Cloth | Technical specifications for the sieves used in the sizing process |
Compliance with these interconnected standards ensures the analytical chain—from gross sample collection (D2234) to final microscopical determination (D2798 or D2799)—is performed with the highest level of technical rigor and reproducibility. Users should always consult the latest versions of these referenced documents.
❓ Frequently Asked Questions
🔍 What types of coal samples are covered by this practice?
This practice specifically covers the preparation of granular samples of bituminous coal and anthracite. Per Section 1.1, it does not apply to the preparation of oriented blocks of coal.
💡 What is the target particle size for the final briquet?
Per Section 5.1, the sample must be crushed so that all particles pass an 850 µm [No. 20] U.S. Standard Sieve, starting from a sample that already passes a 4.75 mm [No. 4] sieve.
⚡ Why must the polished surface be scratch-free and perfectly flat?
A scratch-free and perfectly flat surface is mandatory for accurate microscopical analysis. Scratches scatter incident light and interfere with the precise measurement of vitrinite reflectance (Test Method D2798). A flat surface prevents “relief” (differential polishing) which can lead to particle misidentification during maceral analysis (Test Method D2799).
📌 How does the standard ensure the final briquet is representative of the whole sample?
The standard mandates the use of specific riffling equipment (Sections 5.2 and 5.3) with precise division dimensions (12.7–19.1 mm for coarse, 3.2–6.4 mm for fine) and a minimum of twelve divisions. This mechanical splitting, combined with the prescribed crushing procedure, guarantees that the small briquet sample is statistically representative of the entire gross sample.