Sand production plant

Sand plays a critical role as a construction aggregate and deserves special attention when considering the means of process control. Unlike coarse aggregate where various types of crushers may be used to upgrade mineral quality, sand basically relies on the same techniques to address both mineral quality and sizing. These techniques are called particle exclusion. Whichever size the Producer decides to eliminate for quality reasons obviously also affects the sizing.

1. Natural sand

Good quality natural sand is readily available in many areas and may be easy to obtain and process. As with the gravels that they often accompany, the sand deposits may not have been laid uniformly, meaning a potential change in quality and size is possible. In some deposits, sand found below the water table differs in fines content and quality from that found above the water table. Subsurface drilling, sampling, and testing is necessary to know to what degree and where these differences occur. Standard operating procedures in the Quality Control Plan should address the process if differences in size and quality are encountered, as a uniformly graded product of predictable quality is required to be maintained.
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Stone crushing plant

There are two main stages in a crushing plant, namely crushing and screening.

A. Crushing

The first step of processing begins after the extraction from quarry or pit. Many of these steps also are common to recycled materials, clay, and other manufactured aggregates. The first stage in most operations is the reduction and sizing by crushing. Some operations, however, provide a step prior to crushing called scalping.

1. Scalping

Scalping (Figure 1) most often is used to divert fines at a jaw primary crusher in order to improve crusher efficiency. In this way the very coarse portion is crushed and then recombined with the portion of crusher-run material before further processing. This first step may, however, be an excellent time to improve a deleterious problem. If a deleterious or fines problem exists in the finer fraction of crusher-run material (namely, clay, shale, finely weathered material, etc.) the fall-through of the scalping operation may be totally or partially diverted and wasted, or may be made into a product of lesser quality. In any case, only acceptable amounts, if any, should be returned back into the higher quality product. Consideration of process variables in this early stage may be very important.

2. Primary crushing

In stone quarries or in very “boney” gravel pits, large material usually is reduced in size by either a jaw (Figure 2) or a gyratory crusher. Both types are compression crushers. Although economical, they have the tendency to create thin, elongated particles. Particle shapes sometimes may be a problem for Producers of hot mix asphalt. In some operations impact crushers are used for primary crushing, but they may have a slightly higher cost per ton. Impact crushers may upgrade poor-quality aggregate and increase separation, such as removal of rebar from concrete in recycling operations.

After primary crushing/reduction the resulting aggregate generally is placed in a large “surge” pile where the aggregate may be fed into the secondary operation whenever convenient. Care is always taken when building up and loading out surge piles, as this step may be a major source of segregation of material going to the secondary plant. Variation at this point may affect both mineral quality and gradation.
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Magnetic Rolls and Wet Mineral Separation

The methods to produce a magnetic field which induces differential polarizing forces in mineral particles and the use of this property for the purpose of mineral separation has so far been considered for the operation of separators under dry conditions. However when the feed to the separators is in the form of a slurry, the design of magnetic separators has to be reconsidered, with the basic separating forces already described remaining unaltered. In a wet environment, a low-intensity drum is immersed in a slurry with the internal magnetic segments arranged as shown in Figure 1.

Drum diameters of commercial units vary up to 1200 mm with corresponding lengths up to 3600 mm. The lower portions of the drums are partly submerged in the feed which carry up to 35 % solids.
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Gravity Separation

1. Introduction

Separation by density difference is a process that is as old as recorded history. Separation of gold by density difference dates back to at least 3000 BC as depicted in writings from ancient Egypt. The principle employed in gravity separation goes back further in time to the formation and weathering of the rocks and the releasing of the minerals they contain and the transport of the mineral grains by heavy rains. It is the driving force for the formation of the alluvial deposits of precious metals and gemstones that have been worked since beyond recorded history as they still are today. Archaeological excavations have discovered mineral concentration activities such as the lead–silver concentrating plant in Attica, Greece, dating from 300 to 400 BC. So gravity separation has a long history as a mineral concentration process.

Not all mineral combinations are amenable to this type of concentration technique. To determine the suitability of gravity separation processes to a particular ore type, a concentration criterion is commonly used. A concentration criterion (CC) can be defined as

where SG = specific gravity (or density), and the fluid is typically water or air.
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