After the collection of sample from different location coal sample preparation method is same for all collected sample but depends on the testing method which method use for testing means ASTM or IS method, both methods have some different sieves using for preparation of coal samples. Most often, samples are reduced and divided to provide a sample for analysis. The objective of sample preparation is to prepare one or more test sample from the primary increments for subsequent analysis. The sample preparation may involve some or all of the following constituents i.e. coal sample, reduction (crushing), and division, mixing and drying. The preparation helps to increase the sample surface area which enhances the efficiency of chemical reaction and also aids in the homogenization of the sample to ensure that the test sample analyzed is representative of the entire sample (obtaining an unbiased analysis sample). The required mass and particle size of the test sample depends on the analysis to be carried out.
Terminology used in Preparation
Gross sample
When we are collected the sample from each truck or from each wagon of rack as per random chart and this sample collected as a single sample bags which is collected to up to 00:00 hours. There are number of sample bag collected in 24 hours (mine wise segregated). E.g. we are received the 20 no of truck of ukhni mine in 24 hours and we are collected 6 no of sample bags this all sample bag called as gross sample of ukhni mine.
Sample reduction:
Sample reduction is the process by which a coal sample is reduced in particle size without a change in weight. The test sample should be reduced to the particle size specified in the relevant test method and therefore the type of mill should be chosen to ensure that the required size is obtained. Mechanical equipment should be used to reduce the particle size but manual crushing is permitted for the breakage of large material to meet the maximum feed size acceptable to the first stage mill. Mechanical mills that are commonly used include hammer mill and double roll crusher.
Sample Division:
Here the sample is reduced in weight without significant change in particle size. The reduction and division of the gross sample can be carried out either by an online mechanical process, or an offline mechanical or manual process, to a top size and mass suitable for subsequent testing.
Coal sample preparation method:
- Pour the all sample bags particular mine wise or rake wise (i.e. gross sample) on clean metal surface on 50 mm sieve and sieve it and more than 50 mm size of coal remaining on sieve which is crushed by mechanical or manual method.
- Mix well all the sample and reduced the size as per conning and quartering method i.e. cut it to four part and discard the opposite side.
- Use the 12.5 mm sieve and take the Total moisture sample for analysis (IS Method) with referee part and analysis part and 4.45 mm for ASTM method.
- Again reduction in quantity and size up to 3.35mm with the help of jaw crusher and conning quartering method this sample remaining near about 3 kg.
- If sample is too wet or more moisture than its limit then first air dry and then process for next procedure i.e. pulverize it to 212 mic sieve for IS method and 250 mic for ASTM method. Make a 3 part 1 for analysis part and 2 part is for referee part for any dispute in analysis results.
Overview of preparation of coal sample and conning and quartering process:
Reference : IS 436 Part-1