Large vibrating screens represent a unique challenge for Manufacturers, Plant Designers, and Plant Operators. The inherent mode of operation for vibrating screens is self-destructive. More often than Manufacturers admit, Designers plan for, or Operators staff for, a vibrating screen succeeds and self-destructs. This is a problem. It can magnify with larger vibrating screens.
Vibrating screen structures are subjected to nearly 250 million fatigue cycles in an operating year. The design and construction of these structures are critical in achieving reliable screen performance. Regardless of screen size, the maxims for design continue to be:
A screen design meeting these criteria yields the lowest cost per ton performance. Large screen technology is evolving more scientifically than did the development of small screen technology. As vibrating screen designs increase beyond six foot widths, reliable designs result from sophisticated engineering methods and manufacturing techniques. In addition, large screen technology amplifies the direct relationship of production cost and reliability.
Static Stresses: At rest, motionless, a vibrating screen structure is subjected to the force of gravity, at a minimum. A vibrating screen must first support its own weight. Other motionless stresses are present in the structure as a result of cutting, bending, welding, burning, drilling, assembly, tolerancing, and manufacturing variances. Quite simply, these stresses exist whether or not the screen is operating.
The second step in FEA can be considered the construction of structural loads. These include the imposition of static, dynamic, material, and fatigue conditions on the mathematical model, which approximates the load conditions. An example would be to describe a structural misalignment and the forces input co bolt up this structure through the misalignment.
Reliable vibrating screen designs are dependent upon the proper marriage of a firms manufacturing capabilities and the requirements of the design. It is not reasonable to expect that closely toleranced airframes will be successfully produced in a metal-bending job shop. As design safety factors narrow on larger screens, manufacturing techniques evolve which minimize production variables. Design tolerancing is necessarily compatible with manufacturing accuracy.
Residual metal working stress is the left-over stress in metal when melted or formed into a shape. It is a result of a materials resistance to change shape. Stress concentration sites are more commonly termed notches or stress risers. These areas are not stresses, but sharp geometric transitions or reversals in a structure. Stress loads focus their effect on a structure at these sites. Experience has proven that the methods and procedures of structural assembly can result in preloading screen bodies with excessive static stresses. The scope of this discussion is limited to the discussion of welding, forming, and bolting as they relate to conditions described above.
The side plate of a vibrating screen literally bristles with fasteners. Multi-shift production facilities, as well as maintenance crews, quickly realize the merits of this system. Unlike conventional threaded fasteners, swaged bolts exhibit a distinctly different physical appearance when installed versus loosely installed. The guess-work and wasted efforts to repeatedly insure all bolts are properly torqued are eliminated. A second-shift assembler need not consult with his first-shift counter-part regarding loose or torqued bolts . Sound maintenance practice precludes the reuse of major structural fasteners. A huck-type fastener is destroyed during removal. Normal threaded fasteners depend on proper installation torques to achieve the optimum clamping force. Registered torque wrench values may not be indicative of the true values due to the effects of thread lubrication and frictional force of the fastener face on the bolting surface. Swaged fasteners are installed strictly in tension at an optimum preset tensile load. The positive clamping values are reliably consistent. Installation error is minimal. Replaceable, non-structural components may be installed with conventional fasteners.
Anticipated operating and maintenance costs over the productive life of a processing plant design significantly influence the go or no-go decision to build the plant. Large vibrating screens can both add to and reduce the magnitude of these costs. Plant designers must examine the serviceability of these large units. This includes the complexity of installation, start-up, routine maintenance, major repairs, and operating instrumentation. In assessing these costs, the likely condition exists somewhere between the extreme of a screen leaping momentarily out of position long enough to repair itself and swarms of mechanics covering the unit like bees on honey over several production-robbing shifts.
As larger vibrating screens are used, their size will exceed cost-effective shipping limits fully assembled. Screen manufacturers will join the ranks of other major equipment suppliers in on-site assembly and testing of these units. The incremental costs associated with these efforts must be considered in evaluating the plant construction and start-up costs.
The use of larger vibrating screens results in the dependence of a larger percentage of total plant production on each unit. It is imperative that plant operators maximize the production availability of large screens. This effort is enhanced by carefully planned operating and maintenance procedures. Since volumes have been published on efficient and successful preventative maintenance programs, this discussion will not deal with that topic. There are several suggestions that can be made to help potential big screen users better position themselves to react to the service requirements of these units.
As trite as it sounds, talk to potential screen suppliers specifically about the service requirements of their screens. Determine how recently a manufacturer has entered the wide screen market. Was this entry preceded by years of research and testing? There are generally two major shortfalls in a hastily planned new product introduction. Invariably, replacement parts availability is a problem. Second is the frustrating response to a frantic maintenance question, The only guy who knows that unit is on an island in Indonesia. Solidly planned programs will have organizational depth.
The labor pains, which have normally accompanied the birth of new vibrating screen designs, have been no less severe with the gradual introduction of large, high-capacity screens. More difficulty would have been encountered without the aid of advanced engineering and manufacturing techniques.
The development of vibrating screens over the last century has seen many variations to suit the exacting requirements of industry. Indeed, as each year passes, industry has presented the challenge to screen manufacturers of supplying larger machines than those used in the past and the question is often posed what is the maximum limit?
Innovations introduced such as bouncing ball decks, heated decks, tri-sloped and bi-sloped decks and pool washing features have all sought to achieve improved anti-blinding results and improved capacity for a given screening efficiency. Although the benefits achieved by the inclusion of these features were shown in some cases to be beneficial, the application of good throw in conjunction with the required G force in the operation of the screen has proven in screen performance today, to provide maximum screening efficiency and capacity. The importance of good throw is often overlooked and should be the first consideration when wishing to maximize screen capacity.
For a straight line motion screen the throw is the distance between the extremities of motion. For a circular motion screen, the throw is measured across the diameter of motion but if the screen has an oval motion, throw is measured by taking the mean of the major and minor axes.
The throw which is specified for a particular application is determined on a screen body eccentric weight basis and normally does not take into allowance the load of material which will be handled by the vibrating screen.
Therefore it is imperative that the live weight of the vibrating screen is sufficient to maintain, within reason, the throw which has been originally specified so as to effectively handle the loads being fed to the screen.
The above comments relate essentially to a dry screening application but in wet applications where metalliferous pulp is received on the screen, the benefits of a large throw in terms of increased screen capacity have been demonstrated in commercial practice. The ideal machine for receiving pulp for wet screening or desliming, dewatering etc. is a horizontal screen. Among other reasons, the horizontal screen provides the benefit of long retention time for handling the pulp. Also the straight line motion provided with good throw imparts a positive breaking of surface tension present between the pulp and the screen deck within the apertures.
The inclusion of large vibrating screens in the design of new plants by planning engineers and metallurgists responsible for such work, particularly where large associated equipment is available, is inevitable and is in fact a progression of size we have witnessed over the years.
We should remind ourselves that size progression could not proceed without the accumulation of experience in screen body design, in application knowledge, improved quality of manufacture and refinements of mechanism design with regard to achieving improved bearing life which allows the use of a good G force.
As referenced previously G force and throw are interrelated and therefore with the good G forces available today in the modern vibrating screens, the way is clear to taking full opportunity of increasing throw to handle the high tonnages which can be expected and are currently experienced on large vibrating screens.
Where abrasion of the screen deck surface is severe as in most metalliferous mining applications, and the separation sizes are in the order of mm to 50 mm aperture sizes, polyurethane screen panels are now in common use because of their excellent resistance to wear. The trend in the use of polyurethane panels in the metalliferous mining industry is quite definite and in fact in the major mining operations in Australia at least, the use of polyurethane screening panels is firmly established.
With reference to metalliferous tailings the need for dewatering presents a new dimension. The amount of tailings produced is very much greater since some 98-99% of mined ore is rejected in tailings form compared with varying amount of 3 to 5% rejected in a coal washing operation. Furthermore with dewatering of metalliferous tailings, using equipment as mostly used in coal washing would present maintenance problems because of the more abrasive nature of the tailings and therefore for that reason it is customary to discharge all metalliferous tailings slurry to a dam.
The screen-cyclone system relies on the blinding tendency of the screen deck apertures for its success, using either stainless steel wedgewire or polyurethane deck panels in conjunction with the use of cross dams spaced every 120 cm along the deck surface. When considering the screen-cyclone system it is important to appreciate that the screen function is not one of separation at a given aperture size but bleeding of water through the restricted deck apertures caused by the semi blinding condition. That is, if the deck apertures were to remain completely free of blinding, which is not the case, practically all of the tailings would pass through the apertures in the first pass and would not allow the system to function.
The underflow from the primary cyclones should be deposited on the horizontal section of the screen deck at the feed end where the maximum of water should be removed with the assistance of an additional section of wedgewire located on a 45 inclined back plate to remove free water that has accumulated on top of the bed of slurry most solids having stratified to the deck surface. The underflow should be evenly distributed across the width of the screen at minimum velocity, so as to allow the full benefit of stratification provided by the screen.
The actual results from the initial test run taken on the pilot plant installed at Philex Mining Corporation, Philippines in March, 1980 are as follows using a gravitated flow of tailing slurry from the concentrator.
The problems involved in installing, maintaining, and operating large vibrating screens have been summarized and discussed, based on a survey of current use of such screens in selected North American mineral processing applications. Practical, effective solutions for the more serious common problems are described, along with some recommendations on design practice for specifying, selecting, and installing large screens.
In order to properly assess the information gathered through the survey questionnaire, the results pertaining to each group of applications will be presented and discussed separately in the following section. The small number of installations actually surveyed makes any rigorous statistical interpretation of the data difficult, therefore the information is presented in a generalized fashion. Notwithstanding the small sample of operations as compared to the total number of such large screen installations around the world, the results are felt to fairly represent typical operating, maintenance and installation problems and practices in the sectors of the mineral processing industry the survey covered.
The results reported in this section refer to inclined vibrating screens used in conventional crushing and screening plants. Four operations replied to the survey questionnaire, all four are medium sized producers, primarily of copper concentrate, some with significant by-product production of Mo or Ag. Daily throughputs range from 5,300 tons to 38,000 tons.
The major problem areas reported by the users of these screens were bearing failure and replacement and side plate cracking. The minor problems reported were loose bolts, seals and routine wear items such as cloth and liner changes. Reported availability of the screens ranged from 92-96%. At one operation, the crushing and screening plant is oversized and operates only one shift per day, therefore downtime for maintenance is readily available and actual availability was not reported.
The maintenance of large vibrating screens in conventional crushing applications would normally consist of the regular replacement of wear parts, such as liners and screen cloths, as well as regular lubrication of the bearings and other moving parts as recommended by the manufacturer of the particular screens in use.
The operations with large horizontal vibrating screen installations replying to the survey questionnaire were Syncrude Canada Ltd., Climax Molybdenun (Henderson Operations), Quintana Minerals and Fording Coal Ltd. As previously noted, the screen applications at these operations are all basically very similar, involving wet screening of relatively large tonnages of slurry feed.
The major problem areas with these screen installations once again include bearing failure and side plate cracking in three out of the four installations. The fourth installation, Henderson, reported major problems with the mounting springs and feed lip both of which have presently been rectified to the point where only minimal unscheduled downtime occurs.
The major problems associated with the horizontal screens were with bearings and side plate cracking, and were evident soon after commissioning. Major efforts were undertaken at all the operations to correct the serious problems.
Large vibrating screens are normally selected for applications where multiple screens would be more costly to purchase and install. There have been a considerable number of large screen installations in a variety of mineral processing applications, therefore a considerable amount of operating data with respect to the screen components and performance has been gathered. From the plant designers viewpoint the design of a screen installation should consider the following areas:
The design of a large vibrating screen installation requires close attention to not only the screen itself, but also to the ancillary structures, maintenance procedures and personnel comfort and protection.
Large vibrating screens represent a considerable investment in equipment alone. In addition the loss due to interrupted production should one of these units go out of service can be economically much more severe. As plant tonnages have risen and larger equipment has been utilized in single trains or a small number of multiple trains, the risk of having a single large screen down for any length of time has become too great to ignore.