Ore material handling

The handling and storage of ore materials (Bulk solids) constitutes a major function in all Mineral Beneficiation plants. The handling of Bulk solids is required between each processing step in a plant, and often as part of a processing step. Surge capacity or storage is also required, both ahead of the plant and within it. As the costs of storage and handling systems in Beneficiation plants are substantial, the storage and handling facilities should be designed and operated with a view to obtaining maximum reliability, efficiency and economy. Bulk solids consist essentially of many particles or granules of different sizes (and possibly different chemical compositions and densities) randomly grouped together to form of a bulk. The nature of such a material – that is, its appearance, its feel, the way it behaves in various circumstances, and so on – is thus dependent upon many factors, but principally upon the size, shape and density of the constituent particles. The nature of bulk solids, described in terms of appropriate characteristics is an essential consideration when designing or selecting equipment for its handling or storage. Size, Shape and Surface area of the particles, Density of the particles, Bulk density, Compressibility, Cohesion and Adhesion, Angle of repose, Angle of fall, Angle of difference, Angle of spatula, Angle of surcharge, Angle of slide, Tackiness, Abrasion, Corrosion, Friability, Dispersability, Hygroscopicity and Moisture content, are some of the important properties of bulk solids to be considered while handling them. Storage and transport systems for bulk materials, slurries and water are inevitable for beneficiation plants.

1.PROPERTIES OF BULK SOLIDS

1.1 Size of the particles

Various terms like coarse, small, fine and superfine, are used to give a qualitative indication of the size of the particles constituting a bulk solid. The word ‘size’ is used loosely to mean some sort of average dimension across the particle.

There are many methods of determining the particle size distribution of bulk solids. The most popular (and cheapest) method of particle size analysis, especially with relatively coarse materials, is sieving. The sieve analysis method is main method for particles determining. Sedimentation, elutriation, Optical microscopy, Coulter counter and Laser diffraction spectrometry are some other methods.

1.2 Shape of the particles

Experience has shown that the shape of the constituent particles in a bulk solid is an important characteristic as it has a significant influence on their packing and flow behaviour. Flaky, needle shaped, bulky, rounded, angular, etc. are the terms used to indicate the shape of the particles qualitatively. Many attempts have been made to define the shape of non-spherical particles to indicate the extent to which particles differ from the sphere. Sphericity and the shape factor are the two terms used for the quantitative representation of the shape of the particles.

1.3 Surface area of the particles

The specific surface is defined as the sum of the surface areas of all particles contained in unit mass of the material. The surface area of certain finely divided bulk materials is of considerable importance during the processing and use of these materials. Various techniques have therefore been devised to measure this property. The most common type of instrument for measuring the surface area of powders and particulate materials

is the permeameter.

1.4 Particle density (Density of the bulk solids)

The density of the particle is defined as the mass of the particle per unit volume. The ratio of the density of the particle to the density of water is defined as specific gravity. For a bulk material (bulk solids), the average particle density can be determined by dividing the mass of the material (solids) with the true volume occupied by the particles (not including the voids).

1.5 Bulk density

The bulk solids are really a combination of particles and space, the fraction of the total volume not occupied by the particles is referred as the ‘voidage’ or ‘void fraction’. Sometimes the term ‘porosity’ is applied to bulk solids to mean the same as ‘voidage’. The particle porosity can be defined as the ratio of the volume of pores

within a particle to the volume of the particle (inclusive of pores).

A quantity of particulate or granular material will have an apparent density, usually termed as ‘bulk density’, which can be defined as the mass of the material divided by its total volume (particles and voids). Three kinds of bulk density that apply to materials handling calculations are (1) Aerated density (2) Packed density (3)Dynamic

or Working density.

1.6 Cohesion and adhesion

Cohesion is defined as the molecular attraction by which particles of a body or material are united or held together. When the forces of attraction (cohesion) are low, the bulk material can be made to flow easily under the influence of gravity with the particles moving as individuals relative to one another. Dry sand is a familiar example of free-flowing bulk solids. However, high inter-particle cohesive forces may be caused by moisture or electrostatic charging in fine materials and tend to form agglomerates so that the material flows in an erratic manner as lumps.

Adhesion is the sticking together or adhering of substances in contact with each other. Cohesion is internal, adhesion is external. Adhesion describes the tendency of solid particles to ‘stick’ to a containing surface, such as a wall of a hopper or the side and bottom surfaces of a channel or chute. An extreme example of this is kaolin clay, which is so tacky that it will stick to a wall when thrown against it. This can create unusual problems in moving this material from storage. Adhesive materials tend to bridge in storage and thus require external assistance.

1.7 Angle of repose

Angle of repose is defined as the included angle formed between the edge of a cone shaped pile formed by dropping the material from a given elevation (6.8 cm – according to the standard) and the horizontal (Figure1).

Figure 1 Angle of repose

Angle of repose gives a direct indication of how free flowing the material will be.Angle of repose is sensitive to the condition of the supporting surface, the smoother the surface the smaller the angle. Moisture tends to increase the angle of repose. Bulk solids with an Angle of Repose between 25° and 35° are generally considered free flowing. Table 1 shows flowability character based on angle of repose.

Table1 Flowability character

1.8 Angle of fall

When a material lies in a pile at rest, it has a specific Angle of Repose. If the supporting surface experiences vibrations, impacts or other movement, the material on the sloped sides of the pile will dislodge and flow down the slope. The new Angle of Repose that forms is referred to as the Angle of Fall. After measuring the angle of repose, the cone shaped pile of material is jarred by dropping a weight near it. The pile will fall resulting in a new, shallower angle with the horizontal. This new angle of repose is measured as angle of fall.

The way the pile falls is of special interest. If particles fall and spread out along the slope of the pile, only the degree of flowability is indicated. If the entire pile collapses, it indicates that the material contained entrained air and is prone to flushing.

1.9 Angle of difference

The angle of difference is the difference between the angle of repose and the angle of fall. The greater the angle of difference (between angles of repose and fall) the more free flowing the material is. It is an indirect measure of fluidity, surface area and cohesion.

1.10 Angle of spatula

The Angle of Spatula provides an indication of the internal friction between particles. It is determined by inserting a flat blade into a pile of granular material and lifting vertically. The new Angle of Repose, which the material forms relative to the blade surface, is known as the Angle of Spatula. Generally, bulk solids with an Angle of  Spatula less than approximately 40° are considered as free flowing. A highly flowable material will have an acute angle of spatula.

1.11 Angle of surcharge

The Angle of Surcharge is defined as the included angle formed between the edge of a cone shaped material and the horizontal when the material is at rest on a moving surface, such as a conveyor belt. This angle is usually 5° to 15° less than the angle of repose, though in some materials it may be as much as 20° less. The angle of surcharge is often called the dynamic angle of repose.

One yardstick to measure the flowability of a particular material is by the angle of repose and angle of surcharge. The inter-relationship is shown in Table2.

Table 2 Inter-relationship between angle of repose, surcharge and flowability.

1.14 Abrasion

Abrasion means scraping off or wearing away. Knowing a material’s abrasiveness is important in the proper design of equipment to protect against wear. The materials like coke and foundry sand will wear hoppers, chutes, screw feeders and conveyors, and pneumatic handling systems. Hardened steels, wear resistant liners and high density plastics must be considered for contact materials in such cases.

1.15 Corrosion

Corrosion can be defined as the deterioration of the material and its properties due to chemical or electrochemical reaction between a material, usually a metal, and its environment. Most metals corrode on contact with water (and moisture in the air), acids, bases, salts, oils, and other solid and liquid chemicals. When corrosive materials are processed, they must be handled in equipment with contact surfaces of alloy steel, special plastics or coated with corrosion resistant paint.

1.16 Friability

Friability means the breaking down of the particles in to smaller pieces. If it is undesirable to have any breakdown of the product in the process, as in coal, it is mandatory to use equipment whose design or performance will prevent such breakage.

1.17 Dispersibility

Dispersibility is the basic property that causes a material to flood or to produce dustiness in the surroundings. Dispersibility indicates the dusting and flushing characteristics of a material. It is a measure of the propensity for a granular material to form dust and thus lose mass to the surrounding air. Dispersible materials are generally of low bulk density and fine particle size, which causes them to behave more like a gas or a liquid than a solid. Dispersibility and floodability are interrelated. Materials with a dispersibility rating of more than 50% are very floodable and are likely to flush from a storage bin unless measures are taken to prevent this occurrence.

1.18 Moisture content and hygroscopicity

Materials that have an inherently high percentage of moisture may pose a handling problem. Free moisture, surface moisture or combined moisture as in wet ores cause the problems of sticking and poor flow. Generally, free moisture over 5–10% is considered risky. Particles of high porosity pick up and retain moisture and pose problems. Likewise, those materials that are hygroscopic or absorb moisture will cake and refuse to flow.

2 STORAGE

The following are the main purposes of ore storage:

1 To receive ore intermittently and deliver it smoothly.

2 To accumulate mill products for intermittent disposal.

3 To maintain an adequate tonnage of ore for treatment.

4 To smooth out irregularities in working (surge storage).

5 To facilitate balanced blending of dissimilar minerals

Storage is accomplished in stock piles, bins, tanks and ponds.

Stockpiling is defined as the storage of dry bulk material in piles on the ground and is adopted to store coarse material of low value at outdoors, especially if the duration of storage is extensive. Stockpiles are formed on a concrete or earthen pad. Fixed, traveling and radial stackers, tripper and shuttle conveyors are used to form stockpiles.

Cyclic machines such as grab bucket, dragline and shovel, continuous machines such as drum, rake and bucket wheel are used for reclaiming the stock piles. Stacker/re-claimers are also available to perform both functions.

Bins, Silos and Bunkers are used for dry or filtered materials where the storage period is short. A bin is a container of cylinder and hopper sections for bulk solids with one or more outlets for withdrawal either by gravity alone or by gravity assisted by flow promoting devices such as feeders. Silos, bunkers and other specialized terms used in various industries are also bins. A Silo has large cylindrical section whereas a Bunker has short cylindrical section.

Tanks, usually with an agitator, are employed for storing pulps and slurries.

Ponds and Dams are used for storing water and tailing slurries.

3 CONVEYING

The ore is transported by wagon, truck, rope haulage, aerial tramway etc., from mine to beneficiation plant. Within the mill, the ore is transported by gravity and by conveyors. Gravity transport is the flow of material in which actuating force is gravity and is carried out in chutes and launders. Chutes are steeply inclined troughs of rectangular sections for the gravity transport of dry solids. Launders are gently sloping troughs of rectangular, triangular or semicircular sections for the gravity transport of suspensions of ore or mineral.

Conveyors are used to transport the ore when the horizontal distance is relatively short. There is no arbitrary limit beyond which a conveyor system cannot be used. Travel may be horizontal or inclined either up or down, the maximum practicable inclination varies with the type of conveyor and the nature of materials.

A Belt conveyor, the most versatile and widely used type, consists of an endless belt running around head and tail pulleys and resting on various kinds of idlers at intervals along both its upper and return runs. Drive is generally through the head pulley

Pan conveyors are similar to belt conveyors in method of drive and support, but differ in that the carrying surface consists of a series of articulated plates or shallow pans supported on rollers and tied together by pins.

A Vibrating conveyor consists of a material-transporting trough driven by a vibrator.

A Flight conveyor is essentially a trough through which a series of scrapers attached to chain or rope is drawn.

An En masse conveyor conveys materials by causing them to flow in a compact and unbroken stream through a conduit.

A Screw conveyor consists of a spiral blade attached to a revolving shaft which pushes the material along the bottom of a semicircular trough.

A Bucket conveyor consists of an endless belt or chain with buckets attached, running upon two pulleys or sprocket wheels, one above and one below. The belt or chain travels vertically or at a steep angle.

Skip hoists are widely used to haul ore from underground mines and to elevate coarse bulk materials over a limited distance. They provide an intermittent flow of material.

Pneumatic and Slurry transport are the methods of transporting material using air and water as the transporting medium respectively. Material to be transported is usually in powdered form and is transported through a pipeline system by utilizing the kinetic energy of the fluid (air/water) to move the material. This kinetic energy to the fluid is imparted by a blower or vacuum pump in the case of pneumatic transport and by a reciprocating or centrifugal pump in the case of slurry transport.

4 FEEDERS

Feeding is essentially a conveying operation in which the distance traveled is short and in which close regulation of the rate of passage is required. The feeder is installed at the outlet of a bin, silo or bunker. The different types of feeders are chain feeder, apron feeder, pan feeder, belt feeder, roller feeder, rotary feeder, reciprocating-plate feeder, plunger feeder, revolving-disk feeder, and vibrating feeders.

5 DISPOSAL OF PRODUCTS

The concentrated product from a beneficiation plant is transported through wagons, trucks, conveyors, and pipelines for further processing. The methods for disposal of tailings include discharge of tailing into rivers and streams, dumping of coarse tailings on to land, re-using tailings for some purpose, filling of mined-out areas etc. The disposal of tailings is a major environmental problem. It leads to pollution which is of major concern to the public and government. Uncontrolled discharge of tailings is no longer tolerated. One has to follow environmental legislation laid down by the government before the tailings are disposed of.

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