Stopped belt sampling

Coal sampling is an important part of the process control in a coal preparation plant. 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 rontine sample is taken at a set frequency, either over a period of time or per shipment. Coal sampling consists of several types of sampling devices snch as a crosscut sampler to mimic the stop belt sample (Speight, 2005) which designate the manner in which coal must be sampled. A crosscut sampler mounts directly on top of the conveyor belt. The falling stream sampler is placed at the head section of the belt. 

Systematic spacing of increments collected from a stopped belt is accepted universally as the reference method of sampling that is intrinsically bias-free. Stationary sampling, that is, sampling coal at rest in piles, or in transit in trucks, railcars, barges, and ships, suffers decreased reliability to an indeterminate degree. 

Sampling should be made preferably from a moving stream (this applies to both powders and suspensions) but powder on a stopped belt can be sampled. 

Manual sampling is usually performed in low capacity handling and when the top size of the material is low. It can be done from a falling stream, from a stopped belt or from a stationary pile or hopper. 

The alternative method of sample extrac tion is termed the ctoss-sti eam samphng method, or ci o.s.s-belt when used in conjunc tiou with a belt conveyor. Sample extraction typically take place with a belt conveyor in motion. However, with a rotary table-feeder conveyor, extractions are made with the table stopped. A cutter can perform extractions by this means from a machined flat surface with neghgible... 

Extractions performed with the conveyor stopped allow more assured accuracy by the certainty of including fines in the sample increment. Sampler design to extract increments from a flat belt or rotaiw table sampler while the conveyor is stopped minimizes potential for residual fine particles remaining on the conveyor surface in carrying out extractions. See Fig. 19-6 for rotary table sampler extraction diagram. 

When solution must be pumped, consideration should be given to use of holding tanks between the dry feed system and feed pumps, and the solution water supply should be controlled to prevent excessive dilution. The dry feeders may be started and stopped by tank level probes. Variable-control metering pumps can then transfer the alum stock solution to the point of application without further dilution. Means should be provided for calibration of the chemical feeders. Volumetric feeders may be mounted on platform scales. Belt feeders should include a sample chute and box to catch samples for checking actual delivery with set delivery. Gravimetric feeders are usually furnished with totalizers only. Remote instrumentation is frequently used with gravimetric equipment, but seldom used with volumetric equipment. 

Stream sampling and flow sampling are terms usually reserved for the collection of sample increments from a free-falling stream of coal as opposed to the collection of increments from a motionless (stopped) conveyor belt. Coal that passes from one belt to another at an angle tends to become segregated because... 

When a sample is to be collected from a conveyor belt, the best position for collecting the increments is where the material falls in a stream from the end of the belt. If access at such a point is not possible the sample must be collected from the belt. The whole of the powder on a short length of the belt must be collected. It must be borne in mind that the particles at the edge of the belt may be different to those at the center, and particles at the top of the bed may not be the same as those at the bottom. If the belt can be stopped, inserting into the stream a frame consisting of two parallel plates shaped to fit the belt may collect the sample, the whole of the material between the plates is then swept out. A scoop, such as the one shown in Figure 1.7, can be used to scoop out an increment, but this operation can be hazardous if the belt is moving. 

Using a conveyor belt, the belt is stopped and a sample is taken by hand or an automatic or semiautomatic device. A suitable size for sampling can be designated by markings on the conveyor belt and all material within markings was collected. If an automatic system is used, a mechanically operated head moves across the material at a preset interval and moves a sample into the collection point. 

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