The present invention relates generally to an apparatus for preparing samples from a flow of bulk material, for the purposes of determining representative chemical and/or physical characteristics of the sampled material.
In one form of apparatus, the sample is taken from the flow of bulk material by way of a suction conduit, in the form of a mix of fine and coarser particles of dust. The apparatus has grading means disposed within at least one housing for separating from the dust mix the grain fraction which is required for performing a representative analysis operation. Thus, German laid-open application (DE-OS) No. 29 07 513 and German patent specification No. 3 616 218 provide that a representative sample for the subsequent analysis operation can be obtained from bulk material by removal by a suction effect of a given fraction from the dust which is produced during the operation of transporting the bulk material. An important consideration is for the sample to be representative of the whole of the material from which it is drawn, and that requirement can only be met if the procedure involves using a specific grain size range which can be referred to as the relevant fraction. The composition of the relevant fraction must therefore correspond to the average composition of the whole of all fractions of the bulk material to be sampled. For more detailed consideration of the factors involved in that situation, reference may be expressly made to the two German specifications referred to above.
European patent specification No. 0 043 826 discloses a sampling apparatus provided with a suction device with which a sample of dust is firstly sucked in through a preliminary intake filter which is of a predetermined mesh size. The intake filter forms the end of a cylindrical housing within which is disposed a sieve or strainer which is of a hollow cylindrical configuration and the length and diameter of which are so matched to the mesh size thereof that all particles of dust which are larger than the mesh size of the hollow strainer are passed to a dust collecting container. Outside the hollow strainer, the housing accommodating same is connected to a suction conduit so that the reduced pressure which is produced thereby in the interior of the housing sucks away the particles of dust which are smaller than the mesh size of the hollow strainer. That apparatus is used for taking samples for determining the content of magnetite and phosphorus in iron ores.
A disadvantage of that apparatus is that the intake filter can easily become blocked and clogged, especially as the operative filter area of the intake filter is relatively small as, for reasons of space, the diameter of the housing within which the hollow-cylindrical strainer is disposed is to be as small as reasonably possible. Particularly when the bulk material from which samples are to be taken has a high moisture content, as is the case for example when dealing with raw brown coal or lignite, the intake filter which is of a generally flat configuration very quickly becomes blocked. Another disadvantage of that apparatus is that it can very easily suffer from damage or at any event is subject to a considerable amount of wear as, in operating for example a mine and having regard to the large flows of material which have to be handled in such a situation, it is inevitable that, even if the apparatus is positioned outside the actual flows of material from which samples are to be taken, the apparatus and in particular the region of the intake filter which is associated with the opening through which the dust forming the sample passes into the apparatus is struck by constituents of the flow of bulk material, so that damage is inevitably caused to the grading device, thus giving rise to interruptions in operation of the installation generally.