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Quality inspection sampling
Sampling
I. Professional Terminology
Grade - The ratio of the weight of metal or metallic compound contained in the material to the weight of the material.
Sample - A sample used to determine the moisture content, chemical composition, etc. of the material to be inspected, divided into moisture samples and component samples.
Portion sample - A sample of a certain mass obtained from the material to be inspected in a single action (e.g., the sample obtained each time a sampling rod is used is a portion sample)
Sub-sample - A sample composed of partial portion samples
Original sample - A sample composed of all portion samples (referred to as the original sample in daily work)
II. What is sampling?
Sampling refers to the use of scientific methods to randomly select a small portion of the target object that can represent the properties of the entire material. This includes calculation and probability design to ensure that the probability of obtaining each component and each location is the same.
Popular explanation: Taking a small number of items or materials as samples from a large quantity.
III. What is sample preparation?
According to the general rules, the process of further crushing and dividing the original sample to meet the requirements of the sample for analysis.
IV. Sampling Safety Operating Procedures
1. Before sampling, understand the composition, physical and chemical properties of the sample, whether it is toxic and flammable; the temperature and pressure of the medium, etc.
2. According to the sample situation, determine the sampling method and safety protection measures, be familiar with and understand the sampling point and surrounding conditions, and do a good job of safety confirmation. If there are safety hazards, stop work and report immediately.
3. Before sampling, you must wear personal protective equipment and protective equipment as required.
V. The Importance of Sampling
The risks of sampling and analysis testing coexist. Errors in analysis can be quickly noticed and corrected, while sampling errors often cannot be corrected due to time constraints, resulting in huge losses. Usually, the samples we take are only a few tens of thousands or even hundreds of thousands of a batch of materials, and the samples sent to the laboratory are only a few tens of grams, while the amount of an element determined is only a few grams or even a few tenths of a gram.
If our samplers consider randomness as casual and careless, then for the sample preparers and testers, the "sample" handed to them is not a sample of the material taken.
According to statistics, the proportion of errors caused by sampling, sample preparation, and testing is as follows:
VI. Sampling Technical Operating Procedures
(I) Preparation before sampling
1. After receiving the sampling notice, fill out the sampling transfer slip and sample label, go to the sampling site with the supervisor, verify the material name, shipping unit, batch, number of vehicles, and check whether the goods have surface water. If so, notify the buyer to drain the water before sampling and weighing.
2. Visually inspect the uniformity of the goods and determine whether there are any abnormalities in the material.
3. Confirm the sampling batch and determine the sampling method. Generally, the maximum inspection batch for loose materials shall not exceed 200 tons, and the maximum inspection batch for high-value materials shall not exceed 100 tons.
4. Prepare tools: sampling rod, covered stainless steel bucket, chisel, sledgehammer sleeve, umbrella for rainy days, sample box, special sample bag, lock buckle, lock, sample shovel, etc., and ensure a detailed inspection and cleaning of the sampling equipment before sampling.
(II) Sampling of loose powdery materials
1. In principle, water is removed from the vehicle, and component samples are combined. In special cases (such as heavy rain, exposure to the sun, no forklift for turning piles, etc.), moisture samples can be taken first, and then component samples.
2. Determine the minimum number of portion samples based on the weight and color consistency of each batch of materials. When the colors are consistent, the number of portion samples per batch shall not be less than 36, and the number of portion samples per vehicle shall not be less than 12; for high-gold and high-silver materials or uneven materials, the number of portion samples per batch shall not be less than 48, and the number of portion samples per vehicle shall not be less than 18. The materials taken must be turned over before sampling, and the height of the turned materials shall not exceed 800 mm.
3. Uniformly distribute points, vertically poke until the bottom. According to the clearing situation, if the bottom is contaminated, the material about 20 mm at the bottom of the sampling rod should be removed.
4. After sampling, put in the written sample label, immediately lock the sample bag with a lock buckle on site, then put it in the sample box, lock and seal it for preservation, and fill out the sampling transfer slip.
(III) Sampling of bagged powdery materials
1. In principle, moisture samples are taken before unpacking, and component samples are taken after unpacking.
2. Moisture sample taking
(1) For 25 kg/bag (small bags), the bag drawing method is used, and at least 1/10 is drawn, one point is poked per bag, diagonally inserted, and drawn while unloading.
(2) For ton bags, take one sample per bag, use a long insertion rod to diagonally poke to the bottom, or take three points evenly from the top, middle, and bottom. (Note: If there are lumps, unevenness, or other abnormal situations, the component sample must be taken after unpacking.)
3. Component sample taking
After all the materials are unpacked, determine the number of portion samples based on the weight and color uniformity of each batch of materials. When the colors are consistent, the number of portion samples per batch shall not be less than 36, and the number of portion samples per vehicle shall not be less than 12. For high-gold and silver or uneven materials, the number of portion samples per batch and per vehicle shall not be less than 18. If the taken materials are visually uneven, they must be turned over before sampling.
Sampling method using a pointed hollow stainless steel tube
This method uses a 20mm~30mm diameter pointed hollow stainless steel tube. The sharp end is inserted into the packaging container (usually a woven bag or paper bag) to collect samples. When sampling, the cross-section of the hollow tube faces downwards. When inserting into the woven bag, the sampling depth should be no less than half the depth of the product. Then, rotate the cross-section of the hollow tube upwards and pull it out. All samples in the hollow tube groove must be scraped into the sample strip. Do not scratch the inner membrane of the sample strip during operation.
Clarification:
1. Why should the cross-section of the hollow tube point downwards when inserting into the woven bag during sampling?
This is to prevent dust and impurities on the surface of the product packaging from falling into the hollow tube groove, causing incorrect sampling results.
2. Why take component samples after unpacking?
This is to prevent uneven goods in the packaging and inconsistent packaging weights from causing incomplete sampling representation.
Supplementary condition: For materials of the same batch that are relatively uniform, free of agglomeration, and have basically consistent weight per bag upon entry into the warehouse, water and component samples can be taken together. The sampling points should be no less than the amount of water sample taken, and the amount at each point should be basically uniform.
(IV) Sampling of antimony ore and various antimony-containing lump ores:
Different sampling methods are used for antimony ore depending on the purchased tonnage. For smaller tonnage, after crushing, methods such as groove sampling, point sampling, and riffle sampling can be used. For larger tonnage, sampling is done during loading, such as taking samples from each scoop during loading with a forklift.
Precautions: Before sampling, observe the appearance, color, dryness, coarseness, and distribution of antimony ore in the same batch of raw materials. Samples should be taken according to the color, dryness, coarseness, and proportion.
For small tonnage purchases, the crushed antimony ore should be mixed evenly, generally no less than three times. After leveling, a groove line is drawn along the diagonal of the flat antimony ore. The sample from the selected groove line is used as the original sample for chemical analysis. Alternatively, points (i.e., markings) are evenly distributed on the leveled antimony ore according to the size. The sample from the selected points is used as the original sample for chemical analysis. If conditions permit, the quartering method can be used directly for reduction. Each reduction of 1-2 times requires crushing and mixing of the sample. Each reduced sample must ensure that the coarseness is basically consistent, and each sample amount is basically equal.
For large tonnage, samples are taken from each scoop. The sampling quantity should reach 5%~10% of the scoop. When sampling, refer to the color, dryness, and coarseness of the antimony ore and sample according to a reasonable proportion. Alternatively, a small scoop can be drawn from multiple scoops to reduce the sample amount before crushing and reduction. Subsequent specific operation steps are the same.
Supplementary knowledge: If conditions permit, it is best to use the riffle sampling method for sample division. The groove sampling and point sampling methods are difficult to obtain a suitable coarse-fine ratio, especially for materials with inconsistent coarse-fine grades, which will seriously lead to deviations between the sample and the actual material.
(V) Antimony Ingot Sampling:
1. Before sampling, the approximate quality of the antimony ingots must be understood. Different batches of antimony ingots must be sampled separately. In the same batch, if the appearance or color of the antimony ingots is inconsistent, questions should be raised to the supplier. Samples should be taken according to the different colors and proportions. In principle, each batch should not exceed 60 tons.
2. Sampling ratio: For less than 20 pieces, all must be taken. For more than 20 pieces but less than 40 pieces, it is advisable to take 50%. For 40 to 100 pieces, it is advisable to take 30%. For 100 to 400 pieces, it is advisable to take 25%. For 400 to 1000, it is advisable to take 20%. For more than 1000, take 10%. When sampling, it must be sampled evenly at a certain interval. The sampled antimony ingots are used as the original samples for chemical analysis.
3. Understand the antimony ingot production process and forming situation to judge the distribution of various metals, and adopt appropriate sampling methods according to different production processes.
4. Before formal sampling, carefully check whether the sampling equipment is clean and washed.
5. Sampling methods:
(1) Drilling sampling method: This method uses a hand-held drill sampler with a drill bit diameter of 6mm~8mm. No lubricant is used when drilling. Choose an appropriate feed rate (to avoid excessive heat and oxidation). Drill holes alternately on the top and bottom surfaces. The drilling positions are specified as three points at equal distances on the diagonal line of the top or bottom surface. The drilling depth should be no less than half the thickness of the antimony ingot, and it is best to drill through. The collected drill shavings are the original samples.
(2) For large antimony ingots, three or more points are evenly distributed on the diagonal line, and all drill shavings are collected after drilling through.
(3) For bucket-shaped antimony ingots, the distribution of this type of antimony ingot is more complex, especially for gold and silver-containing lead-antimony-bismuth-copper alloys.
Method: Drilling sampling method. Three points are evenly distributed on the side, drilling to half the width of the alloy (generally used for sampling alloys where antimony or lead elements are greater than 90%).
Sawing groove sampling method: This method uses a saw to reciprocate and move to separate some shavings from the matrix to obtain samples (generally used for antimony-lead alloy sampling). When using a saw, the saw blade should be kept parallel to the sawed section, and the saw should be pulled evenly (when antimony-lead alloy is formed, the denser lead element will relatively concentrate and settle in the lower part of the antimony block, and the less dense antimony element will relatively concentrate and float in the upper part of the antimony block, so the inclination of the saw will affect the accuracy of the sampling results). The sawing groove depth should be no less than half the width of the antimony ingot.
Fluxing pot melting method: Put the extracted alloy into a clean pot at one time, heat it to a molten state, stir it evenly with a stirring rod, use a clean stainless steel spoon to evenly collect samples in a cast iron mold, cool it, and then collect all the drill shavings by using an electric drill with a positive and reverse drilling method.
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