Advanced sand processing plants

In cases where the goal is to produce a basic concrete sand, classic equipment such as a classifying tank and screws are often employed. These are pieces of equipment that are familiar to most and in many eyes, the very essence of a sand plant.

A newer trend however is to move away from some of this conventional technology in two directions, somewhat simpler and somewhat more complex.

The first of these changes is combination of cyclone and dewatering screen technology, long a mainstay in coal production, has exploded on to the scene as a very cost effective and efficient substitute to cases where well graded albeit fine deposits were worked with water scalping tanks and screws. These compact plants not only represent lower capital cost than conventional plants but can handle very high tonnages while producing exceptionally dry washed sands. Most complete plants are skid mounted, easily erected and can be moved quite readily.

In many instances however, the operator must deal with declining pit quality, dwindling reserves, and increased pressure to perform by consistently producing to more exacting specifications. These forces are market, environmental, and often, in the cases of vertically integrated plants, internally driven. Additionally, those who are fortunate enough to be processing sands that have other exceptional qualities may seriously wish to consider production of specialty sands such as glass, frac, or foundry grade products. The common theme in meeting these challenges is better control on classification.

By splitting the feed into a number of discrete, well defined, size fractions, the producer can sell the fractions "as is" or blend them as necessary in a manner so that every grain of sand report to the right product at the right moment. This style of plant is termed a Recipe Sand Plant. Quarries employ this same theme when dealing with coarse aggregates. Their classifier of choice, the vibratory screen, is very efficient and allows them to segregate their materials into precise size fractions for blending. With sand sized particles the process of making precise cuts is similarly available with a special hydraulic classifier based on rising current and hindered settling. These devices, referred to as Hydrosizers, Dense Media Separators, T-Types etc. have long been used is industrial sand production to effect sharp cuts in the size range between 0.5mm and 0.1mm.

Unlike the classification tank, these rising current classifiers produce a single precise cut. The fine product exits via the overflow and the coarse product drops by gravity through a control valve at the bottom of the unit. Classification efficiency is maintained by monitoring the density of a suspended bed of sand within the hydrosizer and adjusting the outflow of material through the underflow valve.

The Dutch pioneered this style of plant in Holland's aggregate industry as early as 1980. It has since become the defacto method for production of sands in the Netherlands and is both widely accepted and practiced throughout Europe. A general flowsheet for such a plant is shown below. Absent from the diagram are the classic screws and classifying tanks.

The feed sand is first treated through a Hydrosizer which is configured to produce a 0.5mm separation. The coarse product from this unit is dropped onto a double deck vibrating screen which further fractionates the sand into a 4 x 1.5mm and a 1.5mm X 0.5mm product. These sand fractions are transported from the plant via conveyor to storage silos. The minus 0.5mm sand that overflows the first stage of classification is subsequently processed in a second Hydrosizer where a 0.25mm cut is made. The two fractions leaving this Hydrosizer are processed on dewatering screens and similarly transported to storage silos.

Sands from the storage silos can be discharged individually if necessary but most often these sands are blended based on the specific product to be produced. The actual size distributions within each fraction are checked periodically and entered into database. Blending software assists the operator in determining how much of each fraction is required to satisfy the current order. All of the individual sands are dropped onto a single belt and conveyed to a radial stacker that is repositioned each time a change in specification is made. It is also not uncommon to have a single truck load out to accommodate smaller orders for very specific sands. The end result of this is that the producer can guarantee size distributions within several percentage points time and time again. Because of the product consistency, concrete producers and prefabricated concrete structure manufacturers have claimed as much as 3 to 5% savings in cement and in one case up to 20% less in rejected prefab items. One US producer has related that the addition of a recipe plant which took place during a longer term contract not only relieved them from incurring close to $100,000 in penalties due to out of spec sand but gained them over $70,000 in bonuses simply due to their ability to produce a consistent sand that was not possible with their classification tanks.

One of the big benefits to the operators of these plants is that they have known sand fractions in storage. This allows them to make conscious decisions as to where those sand grains will be used. Waste of material due to misplacement is minimized and profitability is maximized.

As far as the actual production of a sand product is concerned, these various plant styles represent the basics. Other unit operations to deal with local matters such as organic contamination, the presence of incompetent materials, heavy sticky clays etc. must be dealt with on a case by case basis.