Sample preparation method and Laboratory sampling procedures involve either:
- Coning and Quartering; or
- Riffling Method.
Coning and Quartering for sample preparation techniques/method
The method which is used for sampling large quantities of material say 20kg, consists of
pouring or forming the material into a conical heap upon a solid surface (e.g. a steel plate)
and relying on radial symmetry to give four quarters when the heap is divided by a cross.
Two opposite corners are taken as the sample the other two set aside. The portions chosen
may be further reduced by a repetition of the process, until the required size of sample is
obtained.
Operator skill defines the accuracy of this form of sampling.
- Procedure of Coning and Quartering an approximate 50kg sample
- Starting sample weight (approximately 50 kg)
- Set up adjacent to work area.
- Clean steel plate.
- Spread out sample and mix thoroughly into conical heap.
- Quarter.
- Repeat quartering.
- Bag sample – replace container to storage with excess sample.
It is expected that steps 1 – 6 should take less than 30 minutes.
The Riffle Splitter in sample preparation techniques/method
This sample splitter is an open V-shaped container under which a series of chutes are at
right angles to the long axis, giving a series of rectangular slots of equal area. These
alternatively feed two collection trays. The sample whose particular size allows free
movement through the slots (the largest particle being one-third the riffle opening) is poured
into the feeder and becomes split into equal portions. After repeated cycles the desired
sample size is obtained.
Procedure for the Riffle.
- Set up sample and riffle, ensuring that the riffle is initially clean.
- Riffle once.
- Repeat riffling, discarding every alternate sample.
- Bag sample and label – store excess sample.
- Clean riffle.
The above sequence of steps should take 15 minutes.
Another nice image from http://civilblog.org/2015/01/06/how-to-reduce-gross-aggregate-sample-to-test-sample-by-quartering-or-coning-method/
As an alternative, the gross sample can be reduced by using a mechanic quartering device, such as a sample divider or riffler. Riffle dividers are particularly useful with large samples which are normally more difficult to sub-sample. They arc available in many sizes ranging from bench to floor-mounted models.
A riffler can be used to divide a sample into two approximate equal parts. The distance between the slots can vary and should be at least that times the size of the largest particle in the lot. The material to be sub-divided poured into the top of the box or a feeder and the sample is divided longitudinal: and emerges as two equal portions. The procedure of dividing is repeated, discarding the portions from alternate slots, until a portion of suitable size obtained for analysis.
Each riffling stage produces some dust which may cause losses of sample constituents. The amount of dust produced depends on the nature of the material. Care should be taken to include any residual material retained in the slots.
