SMART DOG MININGTM

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An Introduction to Tables

Tables fall into the general class of flowing film concentrators were the primary means of separation is a flowing film combined with stratification. They utilize the principal of flowing film separation, and combine this with bed stratification to enhance recovery and increase capacity.  The general principal is a feed stream a rate where the coarse heavy particles settle to form a bed which then aids the concentration of finer heavy particles.  The lighter particles are then carried away by the flow of water. For more information on how tables work see SDM an Introduction to the Flowing Film Concentration .  Tables as they are currently used (with a differential motion) were first introduced in 1890 by Arthur Wilfley, but derive from earlier bumping tables developed in the early 1800’s. 

 The term table was devised because the basic configuration is a quadrilateral, generally of a trapezoidal form and look a lot like a table.  Feed is in one corner, with wash water added along that same long edge.  The heavy particles discharge along a short edge, opposite of the feed, and the light particles travel across the table

Figure 1: Typical Table

 

TYPICAL CIRCUIT

Tables are normally used as part of a larger plant where they treat the finer particle size range or as a secondary cleaning circuit, processing a jig concentrate.

Figure 2 : Typical Table Circuit

Tables are usually used in sets of two or more and the circuit consists of a feed distributor, the tables, a wash water circuit, and a heavy and flight fraction collection system.  Some multi stage table installation have been used for separating different products, but generally tables are used in a single stage. 

 EQUIPMENT DESCRIPTION

Tables are used for treating material less than 1½  inch (75 mm), and primarily ½ inch (12 mm) x 28 mesh (0.6 mm) material.  Tables have been used down to 200 mesh (75 microns) successfully.  The most generally accepted explanation of the action of a concentrating table is that as the material to be treated is fanned out over the table deck by the differential motion and gravitational flow, the particles become stratified in layers behind the riffles. This stratification is followed by the removal of successive layers from the top downward by cross-flowing water as the stratified bed travels toward the outer end of the table.  The cross-flowing water is made up partly of water introduced with the feed and partly of wash water fed separately through troughs along the upper side of the table.  The progressive removal of material from the top toward the bottom of the bed is the result of the taper of the table riffles toward their outer end, which allows successively deeper layer of material to be carried away by the cross-flowing water as the outer end of the table is approached.  By the time the end of the table is reached, only a thin layer, probably not thicker than one or two particles, remains on the surface of the deck, this being finally discharged over the end of the table. 

Tables come in many different configurations, primarily in deck design (riffling pattern).   Other than that the main difference is in the number of decks.  Mineral jigs have traditionally been single deck units, while coal tables have used multi-deck arrangements.   Single deck tables are commonly floor mounted, while multi-deck tables are usually suspended from over head structure.

Figure 3 – Single Deck Table

 

 Figure 4 – Double Deck Table

 Figure 5 – Triple Deck Table

TABLE OPERATION

The most used adjustment factors are table slope, stroke frequency and stroke length.  Additionally the amount of water, both with the feed and as wash water are important.

 Slope

The table tilt (cross slope) is the inclination of the table from the feed toward the long side. This slope should be set to the minimum at which it is possible to obtain good distribution of material on the table deck. An increase in slope is required when the bed is thick or sluggish or when there is insufficient water available. An increase in the slope tends to carry more material to the long side of the table (lighter material).

 Stroke

The length and frequency of stroke are interdependent variables. With coarse feeds, a long stroke and slow speed is used, whereas short strokes and higher speeds are used for fine sands and slimes. Normal operating ranges are from 230 to 285 rpm and 1 1/4"  to 3/4" stroke for coarse feeds and  285 to 325 rpm and 3/4" to 1/8" stroke for finer material. Speeds and strokes for tabling coal are about the same as for minerals.  A longer stroke moves the heavy particles to the discharge end of the table more rapidly, but more water is required.  lncreasing the length of the stroke requires a decrease in the frequency of strokes and vice versa to maintain the same speed of heavy particle travel. When the differences in specific gravity between ore and associated impurities are small, the stroke length must be shortened.  Common starting points are 275 strokes/minute with a 1” stroke length.  They are then adjusted, along with wash water to get the desired heavy fraction line to report at the desired location along the short edge of the deck.  The higher operating speeds of modern tables result in greater mobility, greater capacity, and increased operating efficiency.

 Water Use

Water consumption is dependent on the size of feed and type of operation; i.e., roughing or cleaning. In roughing 25% by weight water is required. In cleaning, more water is used, ranging up to 20% by weight water. Slime ore treatment water requirements are from 20% to 25% by weight water.

 The water required for coal tabling is tabulated as follows:

Table 1  – Water Requirements

 Feed top size            % by Weight (water)

   3/4"                         30% to 40%

   1/4"                          50%

   28 mesh                  30% to 40%

 For coarse coal, the additional water is necessary to assure that the coarse coal is washed out of the refuse bed into the clean coal product. The additional water is required when processing fine-size coal to reduce the viscosity of the feed slurry so that normal stratification can take place.

 A portion of the water is added directly to the table from a trough along the upper side (Fig. 1). This "dressing" water is necessary to assure complete mobility of the material on the table. Dressing water amounts to about 25% of the total water used in wet tabling.

 EQUIPMENT SIZING

Tables are sized on the feed tonnage per unit of deck area.  Different types of feed will require different sizing criteria.  Specifically coal tables have different sizing criteria than mineral tables.   This is most often due to the size range of feed.  Coal tables are commonly fed a coarser feed than mineral tables (a general observation is that coal is cleaned at coarser sizes than minerals, primarily due to liberation sizes).

 Table 2 – Table Capacitates per Deck

Coal

Feed Size

T/hr Per Deck

3/4" x 28 Mesh

15

3/8" or 1/4" x 28 Mesh

12.5

1/8" x 28 Mesh

10

28 Mesh x 0

5

Minerals

T/hr Per Deck

> 3 mm (Sand)

>5

3 mm x 1 mm

5

1mm x 100 micron

3

100 micron x 0

2

< 100 micron

1

 EXAMPLE

The following is an example of sizing and selecting a equipment name.  It is included for reference only.   In actual practice many different factors can cause the specific selection to change.

 Conditions:

50 t/hr of raw coal

1/4" x 28 mesh (6.5 mm x 0.6 mm)

1.50 separating gravity

80% (at 1.50 Sp.Gr.) reporting to clean coal.

From table 2, 1/4" x 28 mesh coal is 12.5 t/hr per deck, so 50 t/hr would require four (4) decks.  This could be handled with two (2) double deck units.

 From Table 1, the water requirements would be 1200 g/min with 350 of it added as dressing water.

 

MIke Albrecht, P.E.

o   40+ years’ experience in the mining industry with strong mineral processing experience in Precious metals, copper, industrial minerals, coal, and phosphate

o   Operational experience in precious metals, coal, and phosphate plus in petrochemicals.

o   Extensive experience studies and feasibility in the US and international (United States, Canada, Mexico, Ecuador, Columbia, Venezuela, Chile, China, India, Indonesia, and Greece).