Beneficiation experiment-ore sampling

A Mineral Beneficiation plant costs thousands of dollars to build and operate. The success of the plant relies on the assays of a few small samples. Representing large ore bodies truly and accurately by a small sample that can be handled in a laboratory is a difficult task. The difficulties arise chiefly in collecting such small samples from the bulk of the material.

The method or operation of taking the small amount of material from the bulk is called Sampling. It is the art of cutting a small portion of material from a large lot. The small amount of material is called Sample and it should be representative of the bulk in all respects (in its physical and chemical properties). More precisely, sampling can be defined as the operation of removing a part, convenient in quantity for analysis, from a whole which is much greater, in such a way that the proportion and distribution of the quality to be tested are the same in both the sample and the whole.

Sampling is a statistical technique based on the theory of probability. The first and most obvious reason for sampling is to acquire information about the ore entering the plant for treatment. The second is to inspect its condition at selected points during its progress through the plant so that comparison can be made between the optimum requirements for efficient treatment and those actually existing, should these not coincide. The third is to disclose recovery and reduce losses.

The prerequisite for the development of a satisfactory flowsheet is the acquisition of a fully representative ore sample, even though, in respect of a new ore-body, this sample may have to be something of a compromise.

A bad sample will result in wastage of all test work and can lead to a completely wrongly designed mill. A sample can be taken from any type of material dry, wet or pulp. But, in each case, the method of sampling and the apparatus necessary for them are different.

A sample is collected from huge lot of dry material in stages. At first, a large quantity sample is collected from a lot, known as primary sample or gross sample, by means of various types of sampling equipment such as mechanical or hand-tool samplers using appropriate sampling methods and techniques.

Such reduced samples are called secondary sample and ternary sample depending upon the number of stages used. Figure 1 shows the stages of sampling. Reduction of this reduced sample to a quantity necessary for analysis, known as final sample or test sample, is called sample preparation. It is the process of reducing the quantity by splitting.

Sample preparation is done by Coning and quartering or by using paper cone splitter, riffle splitter, rotary cone splitter, rotary table splitter or micro splitter. The sampler’s knowledge, experience, judgment and ability are of greater value because instructions cannot cover every point or combination of circumstances encountered on each preparation.

When it is required to collect samples from streams of solids and/or pulps, manual or mechanical sample cutters are employed to cut and withdraw small quantities from a stream of traveling material at predetermined frequencies and speeds to form a gross sample. The sample cutter should travel across the material stream and intersect the stream perpendicular to the flow so that the material from the entire width of the stream is collected. The cutter width should be 3 times the top size of the particle and should travel at constant speed. A common falling ore sampling device is the Vezin sampler. Sampling devices called poppet valves are used for pulp sampling in pipes.These are typically used in pipes where the flow is upward.

The procedure to be adopted for taking a sample and the amount of the sample depends on the size of the original material, particle size of the material, the method of sampling and the purpose for which the sample is taken. Table 1 is one of the early sampling studies that proposed to relate the particle size of the material being sampled to the sample size required for a representative sample.

The basis for the sample for this theory was 100 tons of ore. As one can see, the finer the material being sampled, the smaller the size of sample required. Owing to the statistical fact that the finer particles have many more individual particles per pound than the coarser particles do and, since ore is made up of many different materials the finer particles are much more likely to contain all of the individual elements of the whole sample.

Beneficiation Experiment

Figure 1 Sampling process.

sampling

Table 1 Particle size and minimum weight of the sample.

When a sample is to be taken for chemical analysis to determine the assay value of the ore, the sample should be re-crushed sufficiently between each cutting down of the sample so that the ratio of the diameter of the largest particles to the weight of the sample to be taken shall not exceed a certain safe proportion. It is to be noted that no amount of mixing and careful division can make the sample and reject alike in value when the lot before division contains an uneven number of large high grade ore particles.

In a process plant, or mine, the preferred method of obtaining a sample, is from a moving stream, such as a conveyor belt, a slurry pipe line, or perhaps from a chute that a stream of ore flows through. The material collected each time is called an increment. How the sample is obtained, the number of increments and the size of each increment, will often determine the degree of probability that a sample is indeed representative. The sampling ratio, which is defined as the ratio of the weight of the sample taken by the sampling system to the weight of the lot from which that sample is taken, is the most important indicator of the performance of the sampling system.

When the sample is taken, and subjected to analysis, there exists some chance of error in a single sample. One must take the number of samples to reduce the error and to keep the overall error within the tolerable working limits.

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