The invention is in the field of classifying granular materials into several stations by particle-size distribution and reblending the station contents into end products having selected particle-size distributions. As a particular example, the invention relates to granular materials such as sand and gravel and to producing controlled products for end uses such as in particular types of concrete and filter media.
A sand classification plant typically uses feed material mined in nature and extracts from it the constituents suitable for a particular end use. For example, the feed material is segregated by particle size into several stations, each for a respective particle-size distribution, and the contents of selected stations are blended in selected ratios to produce a given controlled product having a selected particle-size distribution. The classification into stations by particle size can be by techniques such as wet or dry screening, hydraulic classification and hydrocyclone separation. Examples of prior art techniques are proposed in U.S. Pat. Nos. 3,114,479; 3,160,321; 3,467,281; 3,913,788 and 4,199,080.
The desirable characteristics of a sand classification plant include optimized use of feed material, so that most of it goes into the controlled product and as little as possible into waste by-product, the ability to produce controlled products of any selected particle-size distribution, close tolerances in the particle-size distribution of the controlled product, minimum manpower and energy demands, high reliability in continuous use in an adverse environment, the ability to use feed material whose nature is difficult to control, etc. While the prior art proposals identified above, as well as other known prior art proposals, evidence a long-standing need to provide these and other desirable characteristics, it is believed that this has not been done in a fully satisfactory way and that much need remains for doing so. Accordingly, the invention is directed to arranging and operating a sand classification plant to enhance the desirable and suppress the undesirable characteristics thereof and to provide features not attained in the known prior art.
In a nonlimiting example of an embodiment of the invention, a material classifier is fed with source material from at least a primary source and has several stations each receiving material constituents characterized by respective particle-size distributions. Sensors detect the presence of material at the respective stations, and valves control the flow of material from the respective stations to at least one controlled material outlet and at least one by-product outlet. Process control means respond to signals from the sensors and to the particle-size needs of at least one controlled product to open or close selected valves at times and for intervals causing material having the needed particle-size range to flow into the controlled material outlet. Means are provided to allow for communication between a plant operator and the process control means.
The classifier can be fed with additional source material from at least one secondary material source. The classification process can be hydraulic, with the help of at least partly recirculated water. In one example the classifier has 12 stations each receiving material characterized by a respective particle-size distribution. Two or more controlled material outlets can be used, each for controlled material having a respective selected particle-size distribution, and one or more by-product outlets can be used, each receiving its respective kind of by-product. The rate of flow from each respective feed source and water source into the classifier can be controlled, e.g. through a suitable material grate opening or a gate or valve arrangement, and the flow from each respective station in the classifier can similarly be controlled, e.g. through a respective valve or gate. The sensors can detect the presence of at least a predetermined quantity of material at each respective station, but the system can alternately or in addition use sensors which provide additional information as to the amount of material present at each respective station. In addition, sensors can be used at the outlets of one or more of the stations to provide an instantaneous measure of the respective particle-size distributions of material coming out of the respective stations. The system can control the absolute and relative feed rates of feed materials, the flow rate of primary water to carry sand through the classifier, the absolute and relative flow rate of water recycled through the classifier, the valve and gate opening times for valves assigned to a particular product (to assure a specified gradation for each product) and the proportional relationship between valve opening times at each station (to control the relative production levels of the products).