Sampling of coal is an important part of the process control in a coal preparation part, as well as elsewhere in the bulk handling process.
A grab sample is a one-off sample of the coal at a point in the process stream, and tends not to be very representative. A routine sample is taken at a set frequency, either over a period of time or per shipment.
Coal sampling can be carried out using several types of sampling devices. A ‘cross cut’ sampler to mimic the ‘stop belt’ sample according to ASTM. ASTM is the standard in which coal must be sampled. A cross cut sampler mounts directly on top of the conveyor belt. The falling stream sampler is placed at the head section of the belt. There are several points in the wash plant that many coal operations choose to sample: the raw coal, before it enters the plant; the refuse, to see what the plant missed; then the clean coal, to see exaclty what is being shipped. The sampler is set according to tonnes per hour, feet per minute and top size of the product on the actual belt. A sample is taken then crushed, then sub-sampled and returned to the main belt. The sample is sent to an independent laboratory for testing where the results will be shared with the buyer as well as the supplier. The buyer in many cases will also sample the coal again once it is received to double check the results. Continuous measurement of ash, moisture, kCal (BTU), sulfur Fe, Ca, Na, and other element constituents of the coal are reported by cross belt elemental analyzers. This information can be calibrated
periodically to the lab data per ASTM methods. Electricity production from coal is a vital source of energy in
many parts of the world including the United States,Australia, China, Indonesia and India. Power stations buy coal from various suppliers. Supply contracts are based not only on quantity but also equally importantly on quality. Quality parameters may include ash, moisture, specific energy, sulphur, nitrogen and other components. Correct sampling is essential to ensure that contracted coal specifications are accurately monitored.
Without correct sampling, the buyer or seller of the coal may be unfairly penalized through over or under payment due to the analysis of inaccurate or biased samples.
Mechanical sampling is the most reliable and safe method of accurately assessing composition of large volumes of bulk materials due to the difficulties posed by high capacity streams, sheer volume, labour costs and stringent contract specifications.
Correct sampling for commercial transactions is not the only reason coal is sampled. Generally, commercial sampling is done prior to or as the coal passes onto the power station site. Such sampling and analysis can be used to blend different coals to produce a fuel of a particular quality.
Sampling is also done immediately before combustion. Knowledge of a fuel’s composition is used to optimize operating conditions for combustion and hence maximize boiler efficiencies and promote environmental sustainability by minimizing hazardous emissions.
Wheat dockage testing by SGS
Determining the quality of wheat beings with running standardize tests to determine the wheat grade. It is important to use the correct equipment that is properly calibrated in wheat grading. SGS Agricultural Trade Services ensures the use of proper equipment calibrated according to USDA/Federal Grain Inspection Service (FGIS) standards.
Some grade-determining factors include: test weight, foreign material, heat damage, shrunken and broken kernels, total damaged kernels, and defects. The other required factors which are non-grade-determining are wheat class, moisture, and dockage.
Dockage is defined as wheat husks, straw, sticks, and other grains such as corn, soyabeans, and sunflower seeds. Dockage is tested on samples of 1,000 grams. Using a Borner Divider, the sample is split to 1,000 grams.
The removal of dockage from wheat is very important in grading. Dockage is the amount of product that will be removed from the whole cargo. For example: If your contract calls for US No 2 or better with .0.5% dockage, and after removal of dockage from your sample you find out you have 0.9% dockage, you have lost an additional 0.4% of your cargo.
Dockage testing is done by the use of a Carter-Day Dockage
Tester. The dockage tester must be set up with the proper sieves and riddles. For Hard Red Winter wheat you will need a number two riddle in the top position in front of the tester and two number two sieves in the lower two slots from the rear of the tester. The air adjustment is set to four and feed control to six. After turning the tester on, the 1,000 gram sample of wheat is poured in the hopper at the top rear of tester.
After the sample has cleared all the sieves, the tester is turned off and all air dockage is removed. Once removed, dockage is poured into a designated pan. Next, the triangular pan at the bottom of the tester is removed. This pan is checked for any insects and combined with the air dockage.
Next the other triangular pan from top front of the tester is removed and poured in with the contents in the dockage pan. All of the contents of the dockage pan are weighted and a percentage is figured, determining dockage content. The bottom front triangular pan is removed and the contents are poured back into the wheat sample. The tester is then cleaned before the next sample.
SGS Agricultural Trade Services in the USA is set up to perform complete grading and analytical testing of wheat and other whole grains.
Stewart Group acquired by ALS Group
In July 2011 Stewart Group entered into an unconditional agreement to be acquired by ALS Group, the testing services business of Campbell Brothers Limited, for an enterprise value of £146 million.
Global business Stewart Group provides geochemical, metallurgical and inspection services to the mining and processing industries worldwide. Employing approximately 900 staff, it operates from 26 laboratories and offices throughout North America, Africa, Asia and Europe.
With an outstanding reputation, built over 40 years, the company operates three main global divisions: Geochemical and Assay, Mineralogy and Metallurgy, and Inspection and Analysis.
The Inspection and Analysis Division provides inspection and analytical services to the coal, metals and mineral trading markets through offices and labs in the UK, Netherlands, Belgium, Germany, Mongolia, Kyrgyzstan and southern parts of Africa.
John Notman-Watt, managing director of Stewart Group, said: “I would like to thank CBPE Capital LLP for their support in the development of Stewart Group over the last five years. The acquisition by ALS is an excellent outcome for Stewart Group and its employees. The strategic fit between ALS and Stewart Group will accelerate the development of our combined integrated service proposition enabling us to provide a wider service offering to customers on a global scale and we are all looking forward to the future with ALS.”
Sampling based on modern sampling theory
Alex Stewart International employs more than 1,200 people in over 45 countries and operates 17 laboratories, write Mr. Graham Stewart and Mr. Kozo Matsumoto, Alex Stewart International Corporation. Alex Stewart International provides its customers with independent verification of the quantity and quality of bulk commodity shipments, as well as geochemical analysis of mining samples for exploration projects. Core business services are for large international commodity trading
companies within the agriculture, metals and mineral and oils and petroleum industries.
As a service company, one of Alex Stewart International’s core business areas is the supervision of weighing and sampling of base-metal concentrates, where we consider it most important to rely on modern sampling theory, not on rule-of- thumb.
Total sampling variance is defined as:
- Primary variance of primary increments divided by the number of primary increments;
- + sample preparation variance + analytical variance divided by the number of replicate analyses.
In order to design any sampling scheme, first it is important to decide the target value of the total sampling variance.
Then a decision must be made on: primary variance of primary increments; sample preparation variance; and analysis variance.Here, in fact in modern theory, there are two schools of thought;
- P. Gy’s theory using the concept of variogram; and
- traditional theory stemmed from some of JIS standards.
Traditional sampling theory is based on the assumptions that each sampling point is statistically independent of all others and that analysis results at each point are subject to a normal
standard distribution. If this is not deemed to be the case, then
it is better to follow Gy’s theory. Following Gy’s theory, the required
number of sample increments for each lot tends to be much less than the one established by traditional theory. However, at Alex Stewart International, it is believed that the truth lies in-between both theories and that considerable experience in actual shipments is vital.
Also, in actual shipments, correct determination of moisture content of any bulk cargo is extremely important. This point would seem self-evident, but in reality, contrary to expectation, it is rarely carried out at the loading point. Alex Stewart has considerable experience in this throughout the world.
Lastly, it should be emphasized that normally taking many samples will not resolve any sampling issues. Too many samples can create other serious difficulties in sample reduction. This issue will also have to be resolved based on theory and experience.