ISO 17025 provides general requirements for Sampling from the management system perspective if a laboratory is performing the sample. The objective is to obtain the required item for testing and retain necessary records. The laboratory needs a sampling method that documents the selection of sites and or samples or sites, a plan, and the preparation and treatment of a sample for the laboratory. In the case of coal sampling, it is not likely the laboratory would be responsible for sampling. Coal sampling is very complex and is and the samples seldom truly represent the whole amount. For this reason, there are standards (international and national) that provide guidelines under different conditions.
Depending on where you are based, I suggest you look at the national bodies for that sector. There are some published guidelines, along with ISO standards which fall under the ISO catalog ICS (International Classification for Standards) 73.040 Coals Including lignites. For Example, standard ISO/DIS 13909-1 Coal and coke Mechanical sampling
To assist you in meeting the ISO 17025 QMS requirements, in conjunction with the ISO standards on coal sampling, have a look at the toolkit https://advisera.co/ISO17025Toolkit where there are there is a procedure to guide you (Sampling procedure) and forms for a sampling plan and sampling report. Also, have a look at the Clause-by-clause explanation of ISO 17025:2017 https://advisera.co/17025ClauseByClause