Coal sampling methods
The better is the quality, higher is the price of a coal. Coal sampling and analysis determine whether the coal can be sold as a cocking coal, metallurgical coal, prime coal or lower grade coal. Many times the customers specify a particular quality limit for buying coal. Coal sampling and analysis is necessary to check for the quality of the coal seams to be mined or under production.
What to look for? Sulfur content and Ash content!
Sampling of coal is very important for a coal reserve evaluation. Sampling is required for various reasons such as:
- Part of mine development
- Part of reserve evaluation and delineation of the mine projections.
- Part of quality check of the coal seam.
- Also required as a part of mine permit application.
- Required for SEC filing in the USA.
Type of coal samples –
- In-situ samples: Samples from coal outcrop, exposed seam in opencast or deep mine, cores or chips from drill holes.
- Grab Samples: Often the coal sample is not representative of the entire coal seam. One of the least reliable coal sampling method.
- Channel Samples: One of the best sampling methods for coal. When the coal sample is collected from an outcrop, the exposed area should be cleaned to avoid the weathered exposed coal surface. Normally a small is box cut is made at the coal outcrop exposing the entire thickness of the coal seam. For a relatively thin seam only one coal section is recommended. However, if the seam is really thick, two or more coal sections may be necessary to sample entire seam. The channel sample should be cut perpendicular to the bedding plane and stored in plastic bags. The sampling of the full seam provide overall quality of the seam including all boney and mineral matters within coal. Sometimes a more detailed analysis is required where the coal seams are collected separately from bone and rock partings. However the bone and rock partings are collected separately. Then the position and thickness of the coal – bone – rock samples are recorded and sent to the lab for detailed analysis. This type of channel sampling is knows as Channel Ply sampling. At many times during ply sampling a small portion of the roof and floor rock of the coal seam are also included in the coal analysis to allow “out-of-seam-dilution“.
- Location of sample collection – should be marked on a map, if GPS available, record location also.
- Take photograph of the sample location – assign a photograph number and write it down on the field map.
- Locate any reference point – river, house etc that can be identified using a aerial topo or on a general grid map.
- Record dip and strike of the coal seam contact with roof and floor.
- Record any fractures in the roof or floor strata.
- Hand draw the coal section and mark the various intervals of ply sample collection.
- Describe each ply interval.
- Seal all samples immediately after collecting in a plastic bag to avoid oxidation, label all sample bags; use tags if necessary.
- Before sending the samples to the lab, mention the type of analysis you may like.
- Pillar Samples: In some special occasions the “strength” of the coal becomes important mainly in the underground mining. A large block of undisturbed coal is usually samples carefully from some specific areas of potential problems or areas with known problems. Sampling scheme is similar to Channel sampling.
- Core samples: Core sampling is mainly a part of the exploration and reserve evaluation stage. This is however very important for the development of a future mine. A geologist is usually assigned to supervise a drilling program. Coal samples are collected in wooden boxed carefully in the field if not sampled at the field. Most of the time an e-log is prepared for each completed hole in recent time. A geologist check the e-log for the coal thickness and adjust the “core recovery” for the collected coal seam. Core recovery is usually >90% for the coal seams however a poor core recovery is possible if the driller is not very much experienced with coal drilling. Sampling method is similar to Channel Ply sampling. On the logging sheet, the depths of each ply sample intervals are recorded before sending to the lab. Total seam, Total coal and the % of coal recovery are also recorded on the sampling tags as a note to the coal analytical lab.
- Cutting/Chip samples: This is a much less accurate sampling scheme than the core sampling. Cuttings are generated by rotary type drilling where no core is recovered except chips. Air flush or mud-flush rotary drilling is a much faster drilling and mostly used for gas wells. This king of sampling can only give us a very general analysis of the coal. It is very difficult to collect samples and most of the time we have lots of impurities mixed in it. Also, the exact depth of coal can not be accurately recorded unless generated from a geophysical log after drilling is completed.
- Bulk Samples: Bulk samples are collected mainly for larger scale tests, to check swelling properties of various coal seams, to rank coal by High Pressure coals and low pressure coals and so on.
- Non in-situ: Sample from coal stockpile, coal transport containers etc. They are often not representative of the coal seam that is mined. They are also may be diluted with “out-of-seam” products from the roof and floor strata. Sometime the stockpile contain a mixture of two or more seams to meet certain quality standard specified by the client. Very rarely geologists are asked to perform this type of sampling.
Reference: For more detailed coal analysis technique please refer to “Handbook of Practical Coal Geology” by Larry Thomas.