Technology: Dry Processing
Production processes in quarries can be broadly divided into two categories: the dry production of
crushed rock aggregate and the wet production of
sand and
gravel. However, increasingly there is a crossover of production practice; the most significant being the adoption of
washing plant processes in crushed rock production to reclaim usable stone from
scalpings. Dry processes have made little inroads into sand and gravel operations, aside from the use of
crushers to process oversize material into saleable crushed rock aggregate. Due to the pressure on water resources and the advent of time-limited
abstraction licences a consideration of dry alternatives for
fines removal from sand and gravel is appropriate.
Drying
Drying is the process of reducing the moisture content of material by the application of heat to evaporate all or part of the water. This is carried out to facilitate further
processing of
feed material, to improve the handling of products and reduce the transportation costs. Drying is an essential operation in the chemical, agricultural, biotechnology, food, polymer, ceramics, pharmaceutical, pulp and paper, mineral processing and wood processing industries. It is an energy-intensive, expensive, operation due to the high latent heat of vaporisation of water and the inherent inefficiency of using hot air as a drying medium.
Key finding
In the developed world, industrial drying operations account for anything between 10 and 25% of national energy consumption. The major costs for dryers are in their operation rather than their capital costs.
Drying occurs by the transfer of heat to the wet feedstock; the most common is by convection (over 85%), other types include the use of conduction, radiation or electromagnetic fields. Convective heating is the focus for this brief review. The first stage of drying is the removal of free, surface, or adsorbed water; the second stage is the removal of the residual, absorbed water. Heat is supplied to the boundary of the material and diffuses into the solid by conduction. Water travels to the boundary by either liquid or gaseous diffusion and is removed by the surrounding air. The residence time of material within a dryer is dependent on the rate of diffusion of water from the core to the surface of the material.
Dryers can be classified by the following criteria:
- mode of operation: batch or continuous;
- heat input type: convection, conduction, radiation or electromagnetic;
- state of material in dryer: stationary or moving/ agitated/ dispersed;
- operating pressure: atmospheric or vacuum;
- drying medium: air, superheated steam or flue gases;
- drying temperature: below or above water boiling, below water freezing;
- relative motion between drying medium and material: co-current or counter current;
- number of stages: single or multiple;
- residence time: short (<1 minute), medium (1 to 60 minutes) or long (>60 minutes).
In the quarrying industry, convective, direct-heat, continuous dryers are the type most commonly used e.g.
rotary dryers and
fluidised bed dryers. These are primarily used to dry
asphalt plant raw material. Dryers incorporate feeding and material handling equipment, a combustion system, fuel-handling equipment and
dust collection and may also include a cooling system.
The hot air is produced by indirect-heating or direct-firing; temperatures are typically in the range 100 to 200o C, with temperatures as high as 450 to 550o C in some dryers. The evaporated moisture is carried away by the drying medium. These dryers have a relatively low thermal efficiency; a significant proportion of energy is lost in the dryer exhaust and there is no cost-effective means of recovery.
Indirect dryers supply heat to the material by conduction; the heat transfer medium is typically contained in an outer shell.
Dryers are not normally insulated, unlike kilns, which are lined with refractory bricks that protect the mechanical parts from the high temperatures used during
processing. A key issue in thermal processing is the input in calories per tonne processed.
Key finding
Systems for recovery of heat are used in kilns but not with dryers; this leads to poor thermal efficiency and a perception that drying is a prohibitively expensive process.
The efficiency of the dryer is a function of the differential between the inlet and outlet exhaust gas temperatures; in rotary dryers it is also influenced by the design of the flights and the speed of rotation of the dryer.
Rotary (cascade) dryers
Rotary dryers are commonly used in the minerals industry to dry a range of commodities including clay, gypsum, kaolin, limestone, mineral sand, potash and silica sand (Photo 54). They consist of a relatively long cylindrical shell, which ranges from 0.6 to 5 m in diameter and from 5 to 30 m long, supported by two riding rings running on a set of rollers (rotation speed up to 25 rpm). They are slightly inclined from the horizontal; the slope enables material to move from the
feed to the discharge end under gravity. They are suitable for a wide range of materials with varying size and composition. The feed rate ranges from less than 1 tonne to 500 tonnes per hour. Internal lifters, or flights, are used to lift, distribute and transport the material. This produces a shower, or cascade, of wet feed material through the hot gas stream, which promotes evaporation of the moisture and breaks up lumps to produce a more uniformly dried material. The hot air is introduced either at the feed end such that it moves in the same direction as the material (co-current) or at the discharge end such that it moves in the opposite direction (counter-current). The co-current direct-heat rotary dryer is the most common; wet material is in contact with the hot gas stream as its highest temperature, which causes rapid evaporation of surface moisture. As it progresses through the dryer, heat energy is lost to the material it is drying and it leaves the dryer at a comparatively low temperature. In co-current dryers, the initial heat transfer is high, which causes a considerable drop in temperature thus preventing overheating of the material and the dryer itself. The final dried product is discharged with the gas stream at its lowest temperature, which ensures the moisture content can be readily controlled. Counter current dryers are more suitable for material that must be dried to very low moisture contents or where the last traces of moisture are difficult to remove.
 Photo 54. Rotary dryer
Fluidised bed dryers
Fluidised bed dryers consist of a bed of fine-grained solids, which is fluidised by the upward passage of hot air. The bed has the appearance of a vigorously boiling liquid and it takes on some distinctly fluid-like properties; this means that fluidised bed dryers operate with few moving parts. The hot air is supplied to the bed through a perforated distributor plate and flows through the bed at a flow rate sufficient to support the weight of the particles in a fluidised state. The flow rate is such that only the very fine-grained material is removed by the
dust collection system. The boiling appearance is caused by the formation and collapse of air bubbles within the fluidised bed. Fluidisation ensures uniform temperature conditions with a high heat flow, thorough mixing and product consistency, removal of dust (from material such as
quarry fines) and avoids overheating of the material. Conveying of the material through the dryer is either by vibration or a low frequency, high-amplitude shaker mechanism; a high mass transfer is achieved aided by the fluidising air. Fluidised bed dryers have a throughput capacity up to 300 tph. The optimal particle-size range is 250 mm to 1 mm, with a maximum of 6 mm. They have found increasing use in sand processing, particularly in the drying of sands for use in
Dry silo mortar.
Indirect dryers
Indirect dryers are used where the material to be dried cannot come into direct contact with the hot air or other
drying media. Indirect heating avoids
contamination of the material. They are also used where the material is very fine grained or of low density. Steam tube dryers have tubes running the length of the rotary drying chamber, indirect fired rotary kilns have an outer shell for the drying medium and indirect fluidised bed dryers use steam tubes in the material bed. Removal of moisture is by use of a vacuum or a small purge of air or inert gas.
Air classification
Air classification is a process used to separate material according to its particle equivalent diameter (controlled by its density, volume and surface characteristics) using a flow of air (Photos 55 & 56). It is an approximate sizing process
352 ordinarily used to separate coarser from finer material; the size at which separation occurs is known as the cut point. This is an alternative to
screening which is the standard means of sizing material; however it is inefficient below 250 micron, especially for dry material. In the
sand and
gravel industry, size
classification of fine material is typically carried out using wet separators such as screw and bucket-wheel classifiers,
hydrocyclones and other hydraulic classifiers. For further details on washing plants
click here. Air classification is used in the chemical and agricultural industries to grade granular materials. In the mineral industry, it is used for sizing powders with cut points in the range 5 to 100 micron
353; mineral industry commodities processed using air classification include calcium carbonate, cement, diatomite, feldspar, gypsum, kaolin, lime, mica, perlite, phosphates, silica sand and talc.
 Photo 55. Laboratory air classifier
 Photo 56. Pilot-scale air classifier
Particles introduced into a rising air current are carried upward or drop downward; the main factors are
particle size and density. The airflow velocity required for a separation can be calculated from Stokes Law; this can be used to determine the velocity of air (otherwise referred to as the air drag force) required to carry away particles finer than the cut point. If the drag force exceeds the opposing gravitational force the particle is entrained in the airflow and reports to the fines product; if the drag force is lower, the particle reports to the coarse product. In theory, where the gravitational force is the same as the drag force, particles at the cut point size will be suspended indefinitely in the separator. In practice, there is a 50% likelihood that these particles report either to the coarse or fine product. A more appropriate definition of the separation cut point is the particle size where the gravitational (plus centrifugal) force is the same as the opposing drag force. This simple grading of particles using an upward flowing streams of air is known as
elutriation. This process does have its limitations and is only effective for particles (density 2.7 g/cm3 ) in the size range 10 to 60 micron. Centrifugal forces can be used to enable separations at cut points below 10 micron. This supplements the gravitational force to overcome the effect of the drag force on fine particles; this is the basis for the operation of
dynamic air classifiers.
Centrifugal separators can impart a force 500 to 2000 times greater than achievable using gravitational force alone (Buell, 2006).
A
classifier sizes particles according to their settling velocities in air; several factors affect particle settling velocities other than particle size. The density can cause small particles to behave as larger particles; a 53 micron particle with a density of 4 g/cm3 will behave in the same manner as a 75 micron particle with a density of 2 g/cm3. Particles with a high porosity will have low apparent density and this has the effect of increasing the effective cut point size. Particle shape also affects classifier performance, especially when it deviates significantly from a spherical form.Flaky particles will tend to report to the fine product due to their large surface area.
Key finding
Material dispersion is critical for efficient separation; particle agglomeration results in the misplacement of fines into the coarse product. High moisture content is the chief cause; it should be less than 0.51% to avoid this problem. The airflow can be heated to enable drying of material during classification.
There are two categories of air classifier;
static (gravitational) and
dynamic (centrifugal or mechanical). These use the separation principles of counterflow or crossflow; this refers to the passage of the material, which is either opposite (counterflow) or across (crossflow) the main airflow. In gravitational-counterflow separators, particles experience the downward pull of gravity and the uplift due to airflow. In gravitational-crossflow separators, horizontal airflow carries particles until they drop out or are carried through the outlet. Particles are graded within the separating chamber with coarse particles close to the inlet and finer particles closer to the outlet. In centrifugal-counterflow separators, airflow is fed tangentially into a cylindrical or cone-shaped chamber forming a vortex. Coarse particles are thrown outward and migrate to the outlet at the base. Fine particles are entrained in the airflow and migrate to a central outlet. In centrifugal-crossflow separators, an air vortex is created in a cylindrical chamber with the inlet and outlet placed on opposite sides of the chamber. Coarse particles report to the lower outlet and fine particles are entrained in the airflow and migrate to an upper outlet.
Static gravitational classifiers
These consist of air flowing through a separating chamber with product outlets for coarse and fine products; there are no moving parts in the separation chamber. They are typically limited to coarse
classification with cut points in the range 212 micron to 1.7 mm, although this can be extended to 75 micron. Early classifiers consisted of vertical chambers with an upward moving airflow or winnowing machines that use the gravitational-crossflow principle; however these suffered from poor separation efficiency. Cascade air classifiers are a development of the vertical classifiers, with varieties such as the zig-zag and shelf classifier. In these, separation efficiency is improved by disrupting the flow of material as it falls through the chamber; air vortexes in the chamber improve the separation.
Fluidised bed classifiers employ the gravitational-counterflow principle; a fluidised state is created by forcing air up through a bed of
feed material and fines are removed in the airflow. Coarse particles remain in the bed of the separator and are removed through the outlet. Fluidised bed classifiers have higher recoveries of fines than other classifiers; this may be a function of the longer residence time in the separator. They also have the sharpest separation of the static classifiers; cut points are achievable in the size range 50 micron to 1 mm.
Dynamic classifiers
These usually consist of cyclones (conical separation chambers) either with or without the assistance of mechanical rotors. They generally employ the centrifugal-counterflow principle; some air classifiers of this type employ a combination of both gravitational and centrifugal separation. They enable finer separations than static classifiers, with a greater degree of cut point control and higher recoveries. Classifiers employing centrifugal force can achieve separation cut points in the range 5 to 100 micron (sub-micron sizes with some classifiers). Dynamic air classifiers are often integrated into dry
grinding mills. The efficiency of dynamic air classifiers is influenced by different factors such as centrifugal force, drag factor, particle concentration and air flow conditions (including the inlet and outlet areas).
Vortex, or spiral, air classifiers usually consist of single or double cones; stationary inclined vanes or adjustable blades are often used to create a vortex in the airflow. The feed is entrained in the airflow and introduced into the separator via a tangential inlet into the top of the chamber or an inlet at the base of the chamber. Single cones are used for coarse
classification whereas double cones can be used to remove material finer than 75 micron. Rotor classifiers contain rotating blades that create cyclonic air circulation within the separator; these are mounted on vertical or horizontal shafts. The speed of rotation and airflow velocity are the main process factors. Circulating air classifiers are widely used in the cement industry; these consist of a double-cone separating chamber. They have a high volume throughput (up to 800 tph) although controlling the desired cut point is difficult.
Process performance can be described in relation to the particle-size distribution of the
feed material and the
classification products. Feed material is separated into coarse and fine-grained products at a given cut size. Due to various random factors (such as air turbulence and interparticle collisions) some fines are separated with the coarse product and vice versa. The quality of the products can be defined by the proportion of expected particles, such as the proportion of coarse particles in the coarse product; this is known as fractional cleanness. Correspondingly, the proportion of unwanted particles, such as fines in the coarse product is known as fractional dirtiness. Other process factors include the product yield, which is the mass of a product relative to the feed and the fraction recovery, which is the ratio between the masses of any fraction in a product and in the feed. The latter characterises the separation efficiency; for example, 90% efficiency would relate to 90% of the mass of fines in the feed reporting to the fines product. Improvements in air classifier design and separation efficiency have focused on creating a stable, well-defined airflow, reducing turbulence, eliminating particle collisions, controlling the feed and multiple classification stages.
Industrial up-take of air classification
Air classification has yet to be taken up in any significant way by the UK aggregates industry. It is used by some aggregate companies, for the production of higher-value industrial mineral products, such as mineral fillers,
dry silo mortar and cement. This is particularly the case with
limestone quarries and a few
sandstone quarries. However, the higher value of these products enables the use of higher-cost processing options.
Key finding
The most likely use of air classification in the UK quarrying industry will be for the removal of material finer than 63 microns from fine aggregate or quarry fines to produce manufactured sand. This is already the case in some quarrying operations in the USA. Further details on quarry fines and waste are available here.
The motivation for using air classification, rather than the more traditional wet processing option, will be the increasingly restricted access to water, and the costs of mitigating the environmental impacts associated with its use. Currently, there is a widely held perception that air classification is an expensive process to install and operate, particularly as there is a need to dry material before it can be processed. The future development of more efficient
drying technology will reduce the costs of drying. Further details on Future Technology are available here.
Technology: screening
Screening
Screens play an important role in the operation of almost all mineral processing plants (Photos 57, 58 & 59). The correct selection and design of a
screening system will have an important impact on the efficiency of an operation. Screens can be divided into static and dynamic (vibrating) screens. Vibrating screens are typically employed for applications above 2 mm and therefore are commonly found within crushing plants. Screens are used to remove material that is already fine enough for the next processing step. This reduces the load on the
crusher, enhances reliability by reducing packing on the screen and improves energy efficiency. Avoidance of unnecessary crushing also contributes to
fines minimisation, maximising the production of saleable material . Screening is also used to produce closely sized products.
 Photo 57. Wash jets over wet screen
 Photo 58. Wet screening gravel
 Photo 59. Dry screening crushed rock
Importance of screen vibration
The deck of a screen is vibrated to produce stratification of material on the deck surface. Finer particles pass through the bed and meet the deck surface. Particles much smaller than the screen aperture dimension have a high probability of passing through on any contact with the deck. For particles of dimensions approaching that of the screen aperture the probability of passing through on contact is lower, and a greater number of opportunities (or trials) must be given to improve
screen efficiency. In practice, screening plants are normally designed to operate at 90-95% efficiency.
Screening efficiency
A number of different definitions of screen efficiency have been proposed depending on whether the screen is producing oversize as a product (efficiency of undersize removal) or undersize as a product (efficiency of undersize recovery). Most screen manufacturers use undersize recovery as a measure of screen efficiency. Industrial screens are typically designed to give 90-95% screen efficiency.
Key finding
Inefficient screening can result in the unwanted presence of fines in aggregate products, increased loading on crushers and increased fines production as undersize is recirculated for crushing.
Wet or dry screening?
Wet screening is typically considered when the feed material has a high moisture content (39%), when screening at fine sizes (-5 mm) and when fine particle agglomerates are present such as clay balls. Wet screening can improve screen efficiency by helping to transport fines through the aperture, removing build-up on the screen surface and reducing blinding. The more dilute the feed the more efficient will be the process, with addition of spray water assisting the separation. It can also assist with
dust suppression but may increase corrosion if wire screens are used.
Types of screen
Fixed or stationary screens
A fixed, inclined grid can be used to prevent oversize passing to all or part of a processing plant. An example of a screen that can be operated in stationary mode is a
grizzly screen commonly used to scalp material prior to a
primary crusher. In its simplest form it can be made from stationary bars aligned in the direction of flow. To minimise the problem of clogging the bars may taper with an increasing gap at the discharge end.
Sieve bends are fixed screens that can be used for very fine wet screening operations. The sieve bend has a curved screen composed of horizontal wedge bars. Feed slurry flows tangentially over the screen surface assisting transportation of oversize. The separation size achieved is approximately 50% of the aperture width, a feature that contributes to reduced blinding of the screen surface.
Vibrating screens
Vibrating screens have one or more screen decks, mounted one above the other, with each deck having a smaller aperture then the one above it. The whole assembly may be horizontal or inclined from feed to discharge. According to Napier-Munn et al356 vibration is induced by means of eccentric counter-weights on a lateral rotating shaft, eccentrically mounted shafts, or eccentrically counter-weighted motors. The choice of mechanism will control the type of screen motion that causes movement of the particles (both vertical and horizontal) on the screen. Vertical movement helps to dislodge particles that have pegged the apertures, hence increasing the screen area available. Horizontal movement ensures that particles are presented in different positions on the screen surface. The types of motion that are most common (Fig. 2) are:
i) Circular motion with inclined decks. Typical inclination is 20°. Gravity assists with transportation. The stroke and direction of rotation influence screen performance. Commonly used for coarse sizing (Screen Operation animation below).
ii) Low angle linear motion screens. Typical inclination is 0 to 10°. Motion is usually directed at 40 to 45° to the screening surface. Larger throws are necessary to enhance transportation by gravity. Commonly used for fine sizing and washing, where the lower bed depth is an advantage.
 Figure 2. Double deck banana screen
 Figure 3. Screen motions
Mellor
355 provides more detail on the design of vibrating screens that can be further categorised into inclined two-bearing, inclined four bearing and variable ellipse screening units. One hybrid-type screen used in the minerals industry is the multi-angle or banana screen (Figure 3). This has a stepped deck arrangement that is typically angled at 20 to 30° at the feed end and 10 to 15° at the discharge end. The advantage of this design is that fines are quickly removed at the feed end that has a fast flowing low bed depth, while near-size material can be separated at the discharge end with its lower flow-rate and thicker bed depth.
Rotary Screens
Rotary screens (trommels) are screens of perforated steel plate are assembled into a tubular arrangement and rotated. This screen has largely been superceded by the vibrating screen because of its low capacity. It still may be used as an initial
dewatering ring at the discharge end of a washer barrel.
Probability screens
Probability Screen, such as the Mogensen Sizer, uses a multi-deck configuration, with each deck mounted at an increasingly steep slope angle from top to bottom as the aperture size is reduced. Each short deck (3, 5 and 6 deck options are available) which are of equal length, only pass particles that are typically less than 70% of the aperture size. This property helps to reduce the problem of screen pegging. A very high specific throughput, while maintaining acceptable screen efficiency, is claimed by the manufacturer.
The screen deck surface
A range of different screen surfaces is used in practice. These include:
- Woven wire. Offers a high open area and good screen efficiency. Has a high wear rate with abrasive materials.
- Wedge wire. Comprises wedge-shaped parallel members with small separations, usually mounted cross-flow. Is typically used for dewatering or fine separations.
- Punched plate. This is stronger than woven wire giving a longer life. Different aperture shapes are possible.
- Rubber. Decks are moulded with reinforcing (steel wire cables etc.). It gives good wear resistance and has a lower open area than wire screens.
- Polyurethane. Gives good resistance to sliding wear which is particularly important in fine screening and dewatering operations. Modular screen panels are typically used. Relatively easy maintenance panels are light and only worn panels need to be replaced. Open area lower than for steel screens.
Aperture shapes and open area
Screen open area is an important design parameter (Photo 60). It can be defined as the percentage of the screen surface that is aperture. Care should be taken as fixings and borders may result in the screen open area being significantly lower than the individual panel open area. Some of the aperture shapes available are shown in Figure 4.
 Photo 60. Close-up of screen apertures
- Square apertures are the most commonly used offering accurate sizing; good wear life with reasonable open area.
- Round apertures provide a strong deck surface that is used in some heavy-duty applications where crushing and wear is likely. The open area is however lower than for square apertures and the deck surface is more prone to pegging. Rectangular or slotted decks provide a means of increasing the open area of a deck and can reduce the incidence of pegging or blinding.
- Rectangular apertures with flow are ideally suited to screening regular shaped particles, but are not suitable for flaky material where very accurate sizing is important
- Rectangular apertures across flow are suited to applications where pegging is likely to occur.
Direction of screening
 |
|
 |
| a) Square |
b) Round |
|
 |
| c) Rectangular - across flow |
d) Rectangular - with flow |
Figure 4: Common aperture shapes used in comminution circuits (after Napier-Munn
356).
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