Sampling procedures

 Introduction

Variance can be reduced by using well planned sampling procedures with as thorough and meticulous extraction of samples as possible. The sample location is surveyed to the nearest survey point and cleaned by either chipping or washing from the rock face all oxidized and introduced material so as to expose fresh rock. A sampling team usually consists of two people, the senior of whom maintains the sampling notebook. At each location a visual description of the rock is made, noting the main rock types and minerals present, percentage of potential valuable mineral(s), rock alteration, other points of interest, and who is taking the sample. Obviously, each sample has a unique number – a safe practice is to use numbered aluminum sample tags with one placed inside the plastic sample bag and the other attached by thin wire about its neck. Any collection device, whether a scoop, shovel, or pipe, must obey the three times the maximum particle size rule, i.e. the width, length, and depth must be at least three times the maximum particle size. This is defined as the largest screen size which retains 5% of the material. There are three hand sampling techniques, namely channel, chip, and grab sampling.

Channel sampling

In mineral exploration, a channel is cut in an outcrop, usually the same diameter as the core being collected, to maintain the volume variance relationships. It is cut using a hammer and chisel or a circular saw, across the strike of the mineralisation (see Fig. 5.3). As the material is cut it is allowed to fall to the floor on to a plastic sheet or sample tray from which it is collected and bagged. Samples are normally 0.5–5 kg in weight, mostly 1–2 kg, and each is rarely taken over 2 m or so in length, but should match the core sample length. There is no point in oversampling and sample spacing perhaps can be determined from the range (a) (section 10.4.1) derived from a study of a semi-variogram.

A mineral deposit can often be resolved into distinct and separate types of mineralisation. Sampling these different types as separate entities rather than as one large sample can reduce natural variation and thus the variance, and keep sample weights to a minimum. This stratified sampling is also of value where the separate types require different mineral processing techniques due to variation in either grain size or mineralogy. Such sample data are then more useful for mine planning. Such samples can be recombined statistically into one composite result for the whole of the mineralisation.

In well-jointed or well-bedded rocks collection by hammering the rock face presents difficulties in that adjacent nonsample material will fall with the material being cut but this must be rejected. Additionally, it is important that a representative collection of rock masses is collected. Frequently, mineral values of interest occur in altered rocks which may well be more friable than adjacent, harder, unmineralized ground, and overcollection of this easily cut material will provide a biased sample.

Channel sampling provides the best technique of delimiting and extracting a sample and, consequently, provides the smallest possible contribution to the total error. The two techniques which follow are less rigorous, and more error prone, but less expensive.

Chip sampling

Chip samples are obtained by collecting rock particles chipped from a surface, either along a line or over an area. In an established mine, rock chips from blastholes are sampled using scoops, channels, or pipes pushed into the heap. With a large database of chip samples, statistical correlation between core and chip samples may establish a correction factor for chip sampling results and thus reduce error. Chip sampling is used also as an inexpensive reconnaissance tool to see if mineralisation is of sufficient interest to warrant the more expensive channel sampling.

Grab sampling

In this case the samples of mineralized rock are not taken in place, as are channel and chip samples, but consist of already broken material. Representative handfuls or shovelfuls of broken rock are picked at random at some convenient location and these form the sample. It is a low cost and rapid method and best used where the mineralized rock has a low variance and mineralisation and waste break into particles of about the same size. It is particularly useful as a means of quality control of mineralisation at strategic sampling points such as stope outlets and in an open pit (Annels 1991).

 

Channel Sampling

Channel sampling is suitable for uniformly distributed mineralization in the form of veins, stringers and disseminations. The sampling is performed by cutting of channels across mineralized body in fresh surface exposures or underground mine workings, such as face, walls and roof. The area is cleaned to remove the dusts, dirt, slimes and soluble salts by any of the three processes. These are washing with hosepipe (air/water) or scrubbing with a stiff brush or by chipping of outer part of the rocks to smoothen the sampling face. A linear horizontal channel is cut between two marked lines at a uniform width and depth. The width is generally 5-10 cm at a depth of 1-2 mm. The length of sample varies depending on the variation in mineralization across the orebody. But in practice, the length is kept at uniform unit between 1 and 2 m within the mineralization. The uniform length of sample eased the statistical applications. The tools are a hammer and sharp chisel (a pointed stub of drill steel) or a pneumatic hammer with pointed or chisel bit. While the sampler cuts the channel, a second person collects the chips, fragments and fines in a clean box, sack or on a canvas sheet spread on the floor. A sample of 1 m length will weigh around 1-2 kg.

FIGURE  Schematic presentation of channel sampling of mineral exposure at surface and underground mine crosscut wall at 1 m interval.

Chip Sampling

If the mineralization is irregularly distributed or disseminated and not easily recognized by necked eye, channel sampling may not be representative. The better alternative practice is to take samples by chipping off fragments of about 1-2 by 1-2 cm size covering the entire surface exposure, underground mine face, wall and roof in a regular grid interval, say 25  25 cm. The area is cleaned before sample cutting. The sampler chips off the fragments with the help of hammer and a pointed chisel. The chips are collected in a clean box or satchel or on a canvas sheet spread on the floor. The weight of samples from a 3  3 m area is generally between 1 and 2 kg. Channel sampling is laborious, tedious, time consuming and expensive compared to chip sampling. Chip sampling is preferred for low-cost fast sampling, identification of mineralized contacts and quick evaluation of grade of the area.

FIGURE  Chip sampling (þ) of wall/face in irregular vein-type deposits such as auriferous quartz veins.

Sludge Sampling

Sludge is the finer coproduct particles of diamond drilling generated by cuttings of rocks between core and outer hole diameter. The core recovery often becomes poor in case of drilling through fractured mineralized zones. In such situation the sludge forms an integral part of sample while passing through the concerned mineralized horizon. It is relevant to recover the maximum portion of sludge in such circumstances. Sludge collection can be done in various ways as suits to the operator. The simplest way is to put a plastic or metallic tub and allow the return water to pass. The cuttings are settled and can form a sample corresponding to drilling interval. The method can be modified by using a large sludge box with three to four longitudinal partitions. The return water can flow in zigzag pattern between the successive partitions so that settling of materials becomes better. Commercially designed sludge cutters with mechanical operation are available. Sludge samples are not incredibly authentic due to contamination between drill runs. But it serves the purpose of existence and to some extent the quality of mineralization in the absence of good core recovery.

 

Grab Sampling

Grab sampling is done at any stage of exploration and more so during the mine production phases for quick approximation of the grade. Hand specimen and smaller size are randomly picked up from loose broken material on outcrop, pits, trenches, mine workings, stope draw points, mine cars, train load shipments, and all shorts of stock piles. Care should be taken to avoid inclusion of any foreign objects like wood, iron pieces, nails and plastics.

 

Muck Sampling

Muck sample is composed of few handheld spade or mechanized shovels full of mineralized fragments and fines collected from the mine face or stope draw points (Fig. 7.29). These samples collected from actual hoisting of mine face are useful for comparing with predicted value of the stope or face blasts. These samples are also compared with cuttings/sludge samples of the Jackhammer and long hole drills. The grade may not match on day-to-day basis. But the average over a period of week, fortnight, month, quarter and annual can be comparable depending on the heterogeneity of the deposit. This also helps to indicate the intrinsic external dilution during mining.

FIGURE  Muck sampling collected from all sides using a handheld spade or mechanized shovel depending on the volume of the sample.

Car Sampling

Car sample is composed of a hand full of broken pieces picked up randomly from every 5th/10th/15th moving car coming out from the underground mine or dumpers/trucks of surface mine or aerial ropeway tubs which transport ore to integrated or third party beneficiation plant and smelters. This is compared between the runof-mine grade and mill head grade for valuation and grade control. The same sampling method is followed for valuation of metal grade, deleterious components and moisture content of concentrate being shipped for the integrated or third party smelter.

 

Bulk Sampling

Bulk sample is composed of large volume of material (100-1000 tonnes) representing all the distinctive characters of entire orebody with respect to metal grade and mineral distribution. The samples are collected from different parts of the stock pile of trial pit of surface mine, trial crosscuts of underground mine and Run of Mine (ROM) ore of regular production. The best collection equipment will be shovels due to handling of large volume. The total material is mixed thoroughly to reduce the heterogeneity. The sample is used for developing beneficiation flow sheet for optimum uses of reagents and maximizing the recovery efficiency.